Alkaline rocks from the Deccan Large Igneous Province

Alkaline rocks from the Deccan Large Igneous Province:Time–space distribution, petrology, geochemistryand economic aspects

ROHIT PANDEY1, N V CHALAPATHI RAO

1,* , MAHENDRA K SINGH1 and

DEBOJIT TALUKDAR2

1Department of Geology, Institute of Science, Banaras Hindu University, Varanasi 221 005, India.2Geological Survey of India, Bangalore 560 078, India.*Corresponding author. e-mail: [email protected]

MS received 18 November 2021; revised 17 January 2022; accepted 3 February 2022

We present a comprehensive review on the alkaline rocks from the Deccan Large Igneous Province (DLIP)and discuss their (i) temporal and spatial association with the Deccan Traps, (ii) petrography, mineral- andwhole-rock geochemistry (including radiogenic and stable isotopes) and geophysical aspects, and (iii) P–Tdata available on their entrained xenoliths. The alkaline rocks occur in seven sub-provinces, viz., (i) theKachchh, (ii) the Saurashtra, (iii) the Gujarat Central and Chhotaudepur, (iv) the Mumbai–Trombay,(v) the Central Deccan, (vi) the Aravalli, and (vii) the Tethyan Himalayan, with the Brst Bve in associationwith the Deccan Traps. A diverse variety of silica under-saturated to over-saturated alkaline rocks withvaried mineralogical and geochemical compositions have been reported from these sub-provinces. Theseinclude alkali basalt, basanite, carbonatite, ijolite, lamprophyre, leucite, melteigite, mugearite, nephelinite,nepheline syenite, orangeites, alkali pyroxenite, phonolite, tinguaite, etc. Available geochronological dataon the Deccan alkaline rocks reveal a wide duration of the related magmatic activity (124–55 My), andsuggest the presence of pre-, syn- and post-emplacement ages of the DLIP units. Alkaline rocks of the DLIPare hosted by discrete aged lithotypes in a variety of stratigraphic horizons, such as the Deccan Traps,Cretaceous Bagh beds, Jurassic sandstones, Triassic Shrinab sediments, Proterozoic Godhra Granite andunclassiBed gneisses. In a majority of the sub-provinces, intrusions of alkaline rocks are controlled byfractures, rift or lineament systems such as the Kutch rift, the Son–Narmada Tapti rift, etc. Their majormineralogy is dominated by pyroxene, feldspar, amphibole, mica, olivine, nepheline, leucite, sodalite andcarbonate minerals whereas accessory and minor minerals include titanite, apatite, spinel, rutile, pyrite,chalcopyrite, epidote, zircon, pyrochlore, garnet, perovskite and other REE-bearing phases. Geochemicalstudies reveal their sodic to potassic nature, with distinct shoshonitic character for some alkaline rocks.Combined geochemical and isotopic studies highlight the role ofmixedmantle sources ranging from spinel togarnet stability depths and involvement of the lower degrees of partial melting. Source modiBcation bysubduction and crustal contamination is evaluated. Geodynamic implications for the orogenic and anoro-genic signatures found in various occurrences, depth of the lithosphere–asthenosphere boundary, and eco-nomic resources are also examined and future research directions are identiBed.

This article is part of the Topical Collection: Deccan Traps and other Flood Basalt Provinces – Recent Research Trends.

Supplementary materials pertaining to this article are available on the Journal of Earth System Science website (http://www.ias.ac.in/Journals/Journal˙of˙Earth˙System˙Science).

J. Earth Syst. Sci. (2022) 131:108 � Indian Academy of Scienceshttps://doi.org/10.1007/s12040-022-01852-x (0123456789().,-volV)(0123456789().,-volV)

Keywords. Alkaline rocks; Deccan Traps; large igneous province; petrology; geochemistry;geochronology; India.

1. Introduction

Alkaline rocks contain higher concentrations ofNa and K, are deBcient in silica and/or aluminawith respect to these alkali elements and containmodal or normative feldspathoids, alkali pyroxe-nes and alkali amphiboles (Sørensen 1974).Owing to their small volume, and trickynomenclature – due to a great diversiBcation intheir mineralogy – the study of alkaline rocks ischallenging compared to that of the most otherigneous rocks (Fitton and Upton 1987). Despitethis, the study of alkaline rocks remains fasci-nating to the petrologists owing to their deepermantle origin and the xenoliths they entrainwhich constitute the direct proxies of the deeperparts of the continental lithospheric plates. Agreat deal of economic significance has beenattached to the alkaline rocks, as they are sig-nificant repositories of precious minerals as wellas elements like diamond, gold, copper, rareearth elements (REE), etc. (M€uller 2002; Tappeet al. 2018; Hou et al. 2020).The cratons, the mobile belts and the Large

Igneous Provinces (LIPs) from Archaean to Eoceneages in diverse tectonic settings of the Indian shieldhost a variety of alkaline rocks such as alkalibasalt, basanite, carbonatite, ijolite, kimberlite,lamproite, lamprophyre, leucite, melteigite,mugearites, nephelinites, nepheline syenites, oran-geites, alkali pyroxenite, phonolite, and tinguaiterocks (Subba Rao 1971; Krishnamurthy 2019; Paulet al. 2019; Chalapathi Rao et al. 2020). Alkalinemagmatism in LIPs has been considered either asprecursors to Cood basalt volcanism in the form ofearly small-scale melting events or mark the cul-mination of major volcanic episodes (McKenzie1989; Basu et al. 1993). The DLIP represents one ofthe largest manifestations of the continental Coodbasalt magmatism in the world and covers major-ity of the western and a few parts of the centralIndia (Mahoney et al. 1988). It has an aerial extentof 500,000 km2 and bulk of the volcanic rockshave tholeiitic compositions (Krishnamurthy 2020).Vast amount of geochronological data on differentdomains of the DLIP suggest that it encompassed a

short time span of *10 Ma as a product of theReunion plume (Basu et al. 1993; All�egre et al.1999; Chenet et al. 2007; Dongre et al. 2021). TheDLIP has also been linked with the (i) large scaleplate movements (Cande and Stegman 2011; Bui-ter and Torsvik 2014; Condie et al. 2015), (ii)Cretaceous–Tertiary mass extinction events (Bondand Wignall 2014), and (iii) even extra-terrestrialimpacts (Courtillot et al. 2000).Recent reviews on the DLIP (see Dhote et al.

2013; Krishnamurthy 2019, 2020; Manu Prasanthet al. 2019; Kale et al. 2020; Kale and Pande 2022)concern more about the stratigraphy andgeochronology of the lava Cows, tholeiitic basalts,and carbonatites than about a wide variety of theassociated alkaline rocks. In fact, the alkalinerocks associated with the Deccan LIP, especiallyfrom NW India, have been studied over the pastcentury or so. A majority of them have beenemplaced in quasi-parallel deep-seated fault sys-tems of mega-scale rifts, such as the Nar-mada–Son (Gwalani et al. 1993; Chalapathi Raoet al. 2012; Pandey et al. 2019; Basu et al. 2020),the Cambay (Vijayan et al. 2016), the Kutch(Karmalkar et al. 2005; Sen et al. 2009; Pandeyet al. 2018a; Chatterjee 2021), and the west coastrifted margin (Melluso et al. 2002; Dessai andViegas 2010) and carry geochemical imprints ofthe Reunion mantle plume (Sen and Chan-drasekharam 2011). The previous major reviewsaddressing the alkaline magmatism associatedwith the DLIP dates back to almost half a centuryto three decades (Subba Rao 1971; Bose 1980;Sethna 1989), and hence a need for an updatedreview of the studies on these rocks.The objective of this contribution is to provide

a comprehensive appraisal on the diverse alkalinerocks associated with the DLIP, based on theirtime–space distribution, petrology, geochemistry(including isotope geochemistry) and economicsignificance. Occurrences of different crustal aswell as mantle xenoliths entrained in thesealkaline rocks have also been emphasized. Wealso explore their petrogenetic aspects, geody-namic implications, and identify future directionsof research.

108 of 45 J. Earth Syst. Sci. (2022) 131:108

2. Geology, tectonic setting and time–spacedistribution

Occurrence of small-volume alkaline rocks in theDLIP is well established and most of these occur-rences are well known for more than one century(Evans 1901). In a seminal work on the MountGirnar Alkaline complex, Mathur et al. (1926) andChatterjee (1932) have not only discussed theoccurrences and association of various alkalinerocks, but also commented on their origin on thebasis of Beld as well as petrographic observations.Alkaline rocks from the DLIP can be categorizedinto two broad groups, based on their mode ofoccurrence: (i) as alkaline complexes in associationwith other alkaline rocks, e.g., the Amba Dongar,the Mount Girnar, the Mundwara, the Nir–Wandh, the Phenaimata, and the Sarnu–Dandali,and (ii) as discrete occurrences, e.g., Dongargaon,Palanpur (Kutch), Jawhar, Bodhan, Bastarorangeites, etc. (table 1). Incidentally, the tem-poral association of various alkaline rocks showsthem to represent the earliest, post-Deccan,synchronous, and even last phase of igneousactivity in the DLIP (table 1 and referencestherein).Earlier, it was considered that the alkaline

magmatism has been restricted to only the westernand northwestern parts of the DLIP, but recently,the extent of alkaline rocks as well as that of theDeccan Traps has been reported up to the Bastarcraton, central India (Lehmann et al. 2010; Cha-lapathi Rao et al. 2011) and Seychelles (Owen-Smith et al. 2013). The various occurrences ofalkaline rocks from the DLIP with their associatedrock types, mode of occurrence and availableemplacement ages have been compiled in table 1.Based on the locality of occurrence and for easycomprehension, we have divided the alkaline rocksinto seven sub-provinces and their geological mapsare provided in Bgure 1. Each of these sub-pro-vinces is discussed here.

2.1 The Kutch or Kachchh sub-province

Pericratonic rift basin of Kutch (or Kachchh;Bgure 1A) hosts a variety of alkaline rocks,mainly inthe form of lava Cows, dykes, laccoliths and plugsand lithologically represented by alkali basalt, var-ious undersaturated nepheline-bearing rocks, syen-ites and lamprophyres (De 1964; Pande et al. 1988;Das et al. 2007; Ray et al. 2014; Pandey et al. 2018a;

Thakkar et al. 2021). Alkaline rocks from the Kutchbasin have been reported from localities of Bhuj,Nir–Wandh, Palanpur, Pachcham Island, SadaraandMeruda Takkar hill (table 1). The Kutch basin,along with that of the Cambay and the Narmadabasins, was formed during the northward drift ofthe Indian plate after separation from the rest ofGondwanaland during Late Triassic to EarlyJurassic. Tectonic fabric of basin was inherited fromintersecting sets of Precambrian rift and fault sys-tem, which remains active even in recent times(Biswas 1987). Seismogenic agile nature and neo-tectonics of Kutch rift zone were witnessed duringthe devastating 2001 Bhuj earthquake, and havebeen linked to the thinned lithosphere of the domain(Mandal 2011). The basin is bounded by NagarParkar Fault (NPF) and North Kathiawar Fault(NKF) on the northern and southern sides, respec-tively and is also intersected by several intra-basinalstrike faults which have similar E–W trend as of theKutch rift (Biswas 2005). These strike faults create asystem of parallel horst, graben and half-grabenstructures and gave rise to the characteristic geo-morphic expression of Kutch uplifts and stand ashighlands distanced by intervening lows of mud andsalt Cats. The Kutch highlands are also roughlyaligned in EWdirection and have been named as theIsland Belt Uplift (IBU), the Wagad Uplift (WU)and the Kutch Mainland Uplift (KMU) from thenorth to the south, respectively. As a central high-land, the Kutch mainland represents the largestuplift of the rift basin (Biswas 2005).The alkaline magmatism in the Kutch area is

considered to represent the earliest imprint ofReunion plume on the Indian shield (Paul et al.2008; Sen et al. 2009). Whole rock 40Ar–39Ar agesof alkali basalts of Kutch (Bhuj area) havedemonstrated a 64.4–67.7 My eruptive ages (Pandeet al. 1988). In-situ U–Pb age of the perovskitefrom groundmass of Palanpur (Kutch) ultra-maBclamprophyre gave a Lower Cretaceous age of124 ± 4 My (Karmalkar et al. 2014) very likelyrepresenting an earlier magmatic event related tothe Kutch Rift Basin. Various alkaline intrusivesfrom the Kutch yielded an age varying from 75 to61 My (Sen et al. 2016). Lamprophyre fromNir–Wandh area is of 67 My age. Thakkar et al.(2021) tentatively suggested a Precambrian or Pre-Deccan Mesozoic age for nepheline syenite bearingMeruda Takkar hill, northern Kutch. The presenceand eAect of magmatic activities on the Mesozoicsuccessions of the Kutch were described by Bowen(1927) and subsequently by Paul et al. (2008), who

J. Earth Syst. Sci. (2022) 131:108 of 45 108

Table

1.Distribution

ofalkalinerocksin

thevariou

ssub-provincesassociated

withtheDeccanLarge

IgneousProvince.Availableradiom

etricages

andim

portan

treferencesare

also

provided.

Location

Rock

types

Modeofoccurrence

Age(M

a)

Selectedreferences

I.Kach

chhSub-p

rovince

1.Bhuj

Alkalibasalt

LavaCows

64.4

to67.7

Ma

40Ar–

39Ar(W

R)

Pandeet

al.(1988)

2.Nir–W

andhComplex,Kaladongar

(Patcham

Island)

Olivinenephelinite,

olivine

basanite,alkaliolivinebasalt,

gabbro

andlamprophyre

Plug;lavaCow;

dykes

66.86±

0.16to

75.60±

0.19

40Ar–

39Ar(plagioclase

and

hornblende)

Jaitleyet

al.(1987);

Karkare

etal.(1989);

Moitra

(2003);

Sen

etal.(2016)

3.Sadara

(Patcham

Island)

Alkalibasalt

Sill

75.81±

0.55Ma

40Ar–

39Ar(plagioclase)

Rayet

al.(2006);

Paulet

al.(2008);

Sen

etal.(2016)

4.Kuran(P

atcham

Island)

Alkaligabbro

Plug

[65(P

aleomagnetic

dating)

Daset

al.(2007)

5.Palanpur

UltramaBclamprophyre

Dyke

124±

4Ma

(U–Pbperovskite)

Karm

alkaret

al.(2014);

Pandey

etal.(2018a)

6.MerudaTakkarHill

Alkalifeldsparsyeniteand

nephelinesyenite

Plug;dykes

–Thakkaret

al.(2021)

II.Saura

shtraSub-p

rovince

1.MountGirnarComplex

Syenites,nephelinesyenites,

limburgites,undersaturated

olivinedolerite,olivinebasalt

andlamprophyres

Dykes,veins,

and

Cows

56.2

±1.7

to63.6

±1.9

K–Ar(W

R)

64.2

±1.3

to69.1

±1.17Ma

K–Ar(W

R)

65.9

±0.3

to66.1

±0.4

Ma

(mineralseparates:

amphibole

and

biotite)

Evans(1901);Mathuret

al.

(1926);Bose

(1971);

Paulet

al.(1977);

Rathore

etal.(1996);

Sahooet

al.(2020)

2.W

aori

andBardaHills

Felsitesandgranophyres

LavaCow

anddykes

–Dave(1972)

3.Kanesara

Lamprophyre

Dyke

Naushadet

al.(2019)

III.

Gujara

tCentralandChhota

udepurSub-p

rovince

1.Pavagadh–Rajpipla–Hingoria

Alkaliolivinebasalt,

mugearites,

trachyte,basic

alkalinerocks,

carbonatite

LavaCowsanddykes

64.90±

0.80Ma

40Ar–

39Ar

KrishnamurthyandCox(1980);

Greenoughet

al.(1998);Sheth

andMelluso

(2008);

Parisioet

al.(2016)

2.AmbaDongar

Carbonatite,nephelinites,

phonoliticnephelinite

Ringcomplex

65.90±

1.70Ma

40Ar–

39Ar

plagioclase

mineralseparates

65.4

±2.5

U–Pbapatite

Sukheswala

andUdas(1963);

ViladkarandSchidlowski(2000);

Viladkar(2015);

Fosu

etal.(2018);Chandra

etal.

(2019)


Table

1.(C

ontinued.)

Location

Rock

types

Modeof

occurrence

Age(M

a)

Selectedreferences

3.Siriwasan–Dugdha(N

asw

adi)

Carbonatite,trachyte,

tinguites

Dykes

70±

2;Pb–Pb

VeenaKrishnaet

al.(1993);

ViladkarandGittins(2016);

Fosu

etal.(2020)

4.Phenaim

ata

Lamprophyre,alkalibasalt,

carbonatite

Plugsand

dykes

66.60±

0.35to

65.25±

0.29;

40Ar–

39Aramphibole–biotite

separates.

Sukheswala

andSethna(1969);

Parisioet

al.(2016);

BanerjeeandChakrabarti(2018)

5.Panwad–Kawant

Lamprophyres,

tinguaites,

pseudoleucites

Dykes

–Sukheswala

andAvasia(1972)

6.Bakhatgarh–Phulm

ahal

Lamprophyres,

picrobasalt

Dykes

–Randiveet

al.(2005)

7.Chhotaudepur,

Seraphalia,

Piliwat,Moradungri,Jitnagar,

Kanthari,Heran,Hamirpur,

and

Dongargaon

Lamprophyres

Dykes

65.25±

0.29to

55±

240Ar–

39Arbiotite

separatesandRb–Sr

mineralisochrondating

Gwalaniet

al.(1993);Srikarniand

SanjayDas(2005);

ChalapathiRaoet

al.(2012);

Randiveet

al.(2012);

Parisioet

al.

(2016);Pandey

etal.(2019)

8.Chhaktalao

Lamprophyres,

carbonatites

Dykes

–Chawade(1996);Khandelwalet

al.

(1997);Hari

(1998)

9.KadinearCambay

Microsyeniteporphyry

or

alkalisyenite

Dyke

–Raju

etal.(1971)

10.Bodhan

Albitised

basalt

Flow

–Sukheswala

etal.(1964)

IV.M

umbai–Tro

mbaySub-p

rovince

1.Murud–Janjira

Lamprophyre,picrite,

nephelinite,

tephrite,

tephriphonolite,microdiorite,

trachyte,ijolite,and

carbonatite

Dykes

and

veinlets

64.9

±0.8

Rb–Srmineralisochrondating

SethnaandD’Sa(1991);

Melluso

etal.(2002);

Sahuet

al.(2003);

DessaiandViegas(2010)

2.Parol,Basseindistrictand

Trombay

Lamprophyres

Dykes

–Sukheswala

andPoldervaart

(1958);

Sukheswala

andSethna(1962)

3.Mahdawa–Dhulia

Carbonatite

andnephelinite

Dykes

–Santet

al.(1991)

4.Salsette

Island

Trachyte

Dykes

–SubbaRao(1971)andreferences

therein

5.Silvassa

Leucite-basanite,

andtrachyte

Dykes

–SubbaRao(1971)

6.Jawhar

Nephelinesyenite

Dyke

–Sukheswala

andAvasia(1966)

7.Bhir

Alkalibasalt

LavaCow

–Talusani(2010)


Table

1.(C

ontinued.)

Location

Rock

types

Modeofoccurrence

Age(M

a)

Selectedreferences

V.CentralDecc

anSub-P

rovince

1.KodomaliandBehradih

Orangeites(G

roupII

kim

berlites)

Dykes

65.1

±0.4

to62.3

±0.8

40Ar–

39Ar(W

R)andU–Pb

(Perovskite)

Lehmannet

al.(2010);

ChalapathiRaoet

al.(2011)

VI.

Ara

valliSub-p

rovince

1.Mundwara

Lamprophyre,carbonatite,

melteigite

Olivine-gabbro,

feldspathoidalandnon-

feldspathoidalsyeniteand

shonkinite

Plugs,

dykes,andveins

68.53±

0.16;80–110

40Ar–

39Ar(biotite)

Basu

etal.(1993);

Rathore

etal.(1996);

Pandeet

al.(2017);Sharm

aet

al.

(2021a)

2.Sarnu–DandaliandKamthai

(Barm

er)

Syenite,

nephelinite,

phonolite,alkalipyroxenite,

ijolite,carbonatite

and

lamprophyre

Dykes,dykeletsand

veins

88.9–86.8

to66.3

40Ar–

39Ar(biotite

andW

R)

NarayanDaset

al.(1978);

Chandrasekaranet

al.(1990);

Basu

etal.(1993);

BhushanandChandrasekaran

(2002);

Shethet

al.(2017)

Dongre

etal.(2021)

VII.Teth

yanHim

alayanSub-P

rovince

1.Quetta

Lamprophyre

Sill

69.7

±0.2;

40Ar–

39Ar

(biotite;amphibole)

Kerret

al.(2010)


Figure

1.(A

)AgeneralizedregionalgeologicalmapoftheKutcharea(after

Biswas1992)showingthelocationsofvariousalkalinerocksnearKaladongar,Sadara,Palanpurand

IslandBelt.(B

)GeologicalmapofTethyansuture

zoneareain

betweenMuslim

BaghandMesozoic

continentalpassivemargin

(after

Ahmed

andMcC

orm

ick1990;Kerret

al.

2010).

LocationoftheultramaBclamprophyre

sillsis

alsomarked;(C

)LocationoftheBehradih

orangeite

intheMainpurkim

berlite

(orangeite)Beld,Bastarcraton(after

ChalapathiRaoet

al.2011);(D

)GeologicalmapoftheSarnu–Dandaliareashowinglocationofthealkalinecomplex.Kamthaiisalsoshownin

themap(redrawnfrom

Geological

Survey

ofIndia

MapDatabase

on1:50,000scale);

(E)Locationofthevariousalkalinerocksoccurrencesassociatedin

timeandspace

withtheDeccanTrapsin

NW

India

(modiBed

after

Lehmannet

al.2010);(F

)Distributionofalkalinerocksin

theChhotaudepuralkalinesub-province

(after

Gwalaniet

al.1993);AmbaDongar,Phenaim

ata,and

Panwad

sectors

havebeen

shown

along

with

other

locationsofminoralkalinerocks(H

RL:Heran

River

lamprophyres;

KTL:Kanthari

lamprophyres;

PW

L:Panwad

lamprophyres;(G

)Locationofthealkalinerocksin

theSaurashtrasub-province;(H

)GeologicalmapoftheMundwara

alkalinecomplexshowingthelocationofalkalinerocks

(modiBed

after

Subrahmanyam

andLeelanandam

1989).(I)Geologicalsettingofthelamprophyresandother

alkalinerocksin

theMurudJanjira

area(m

odiBed

from

Dessaiand

Viegas2010).


Figure 2. Field photographs of (A) Phonolite with Bne- to medium-grained and porphyritic nature; intruding in alkali syenitefrom Sarnu–Dandali complex (AA/DD/10.2); (B) Carbonatite with leucocratic and brecciated nature from the Mundwaracomplex (MM/C/1); (C) Intrusive relationship of lamprophyre in syenite from the Moradungari area Chhotaudepur sub-province; sharp contact of lamprophyre can be seen (MDL/1); (D) Coarse-grained, leucocratic nepheline syenite from the MountGirnar complex (SK/RDA/N1); (E) Calcite-dominated, leucocratic carbonatite from the Sarnu–Dandali area (SD/C); (F)Megacryst of amphibole ([25 mm in size) in a lamprophyre from the Heran River area (HRL/2); (G)Mela-nephelinites from theSarnu–Dandali showing with a bouldary appearance and spheroidal weathering pattern; (H) Fine grained picrobasalt from theSarnu–Dandali complex; (I) Alkaline dolerite from the Dhosa area, Kutch (KH-1); (J) Behradih orangeite displaying thepresence of megacrystic and microcrystic olivine imparting inequigranular texture (2/47 and 2/40) and (K) REE bearing phasein carbonatite from Sarnu–Dandali complex.


divided the widespread magmatism of the area intothree groups, i.e., (i) tholeiitic basalt, dolerite andgabbroic dykes from Kutch Mainland, (ii) alkalinebasalts in the form plugs from Kutch Mainland,and (iii) the alkaline intrusives including lampro-phyres of Kutch Island Belt. This division wasbased mainly on the spatial distribution and themode of occurrences of these rocks. To reBne theoccurrences of alkaline magmatism from Kutchprovince further, we have divided it in terms oftheir locality, type of alkaline rock, and the modeof occurrence (see table 1). Bouldery, discontinu-ous and mesocratic nature of alkaline dolerite fromthe Dhosa area of the Kutch can be noticed(Bgure 2J).

2.2 The Saurashtra sub-province

The Saurashtra sub-province lies at the north-western part of the DLIP. It is bounded by Kutchrift in the north, Gulf of Cambay and Narmada riftin the east and by Arabian Sea in the south, westand northwest. Nepheline syenites, limburgites,undersaturated olivine dolerite and lamprophyreare the major alkaline rock types which have beenreported from three different localities, viz., (i) theMount Girnar alkaline complex, (ii) the Waori andBarda Hills, and (iii) the Kanesara area. TheMount Girnar, situated in the east of Junagadh isthe most striking geomorphic feature in the plainsof Saurashtra peninsula (Bgure 1G). The MountGirnar has a roughly circular outcrop, whichimparts resemblance to a volcanic vent (Mathuret al. 1926). Magmatism in the Mt. Girnar complexis associated with crustal scale tectonic fabric ofthe area as it is located at the western extension ofEW trending Son–Narmada lineament (Chan-drasekhar et al. 2002). In fact, Mount Girnar hasbeen considered as the centre of Bve radiatingmajor lineaments of western India, viz., (i) theAravalli, (ii) the Great Boundary fault, (iii) theSon–Narmada rift, (iv) the WNW–ESE trendingtroughs of the northern part of western India, and(v) the western continental margin of India(Crawford 1983). Alkaline rocks in the sub-pro-vince occur as dykes, veins and lava Cows. Bose(1973) delineated the sequence of crystallizationof the co-magmatic plutonic rocks consisting ofgabbro-diorite-lamprophyre-syenite. Lamprophyresfrom this complex have yielded 67.3 ± 1.7 My agesbased on K–Ar whole rock dating (Rathore et al.1996). Precise mineral separates (amphibole and

biotite) from the lamprophyres have recently givenCretaceous–Paleogene boundary ages rangingfrom 65.9 ± 0.3 to 66.1 ± 0.4 My (Sahoo et al.2020). Nepheline syenites of the Mount Girnar aremedium- to coarse-grained, leucocratic, hard andcompact in nature (Bgure 2D) and show a closeassociation with the lamprophyres.The Barda hill is a volcanic–subvolcanic–acidic

complex, which has an area of around 400 km2 andis situated 40 km NE of Porbandar town. Theoccurrences of felsites, diorites, trachytes andoccasional pitchstone Cows have been reported byFedden (1884) from Barda–Alech hills and nearbyareas. De and Bhattacharya (1971) provided theBrst detailed map of this area with various lithou-nits, whereas De (1981) has described the variousmagmatic activities. The Kanesara area is locatedin the north-central part of the Saurashtra pro-vince and comprises of basaltic Cows, basanite,picrite, ankaramite, volcano-plutonic complex andlamprophyres (Naushad et al. 2019).

2.3 The Gujarat central and Chhotaudepursub-province

The Gujarat central and Chhotaudepur alkalinesub-province is regarded as the ‘museum of alkalinerocks’ (Gwalani et al. 1993) since it hosts compo-sitionally the most diverse association of alkalinerock types from the DLIP (Bgure 1F). Alkalinerocks from Chhotaudepur sub-province includealkali basalt, alkali syenite, alkali olivine basalt,carbonatite, mugearite, trachyte, nephelinites,phonolitic nephelinite, tinguite, pseudoleucite,picro-basalt, fenite, syenite and all varieties oflamprophyres, viz., alkaline (camptonite, san-naite), calc-alkaline (kersantite, vogesite) andultra-maBc (alnoite, damtjernite) (table 1). TheChhotaudepur sub-province is conBned to thelower reaches of Narmada valley, and tectonicallycontrolled by fault system of Narmada rift, which isconsidered to be an arm of the failed triple junction(Gwalani et al. 1993 and references therein).Alkaline magmatism in the Chhotaudepur sub-province is distributed over [1200 km2, and hasbeen further sub-divided by Gwalani et al. (1993)into Bve sectors, viz., (a) the Amba Dongar, (b) theSiriwasan–Dugdha (Naswadi), (c) the PhenaiMata, (d) the Panwad–Kawant and (e) theBakhatgarh–Phulmahal. We have added a fewmore alkaline rock localities and identiBed furthersectors (e.g., Chakatalo). A complete list of


alkaline rock occurrences from each sector is pro-vided in table 1, and their salient aspects aresummarized as under:

(a) Pavagadh–Rajpipla–Hingoria sector: ThePavagadh bimodal lava sequence, situated justnorth of Narmada rift has an area of 35 km2 andrepresents perfectly horizontal lava Cow of basalticto rhyolitic composition (Hari et al. 2000; Fur-uyama et al. 2001). There are different opinionsregarding the basement of the Pavagadh igneouscomplex, varying from Cretaceous sandstone ofBagh Formation to the Precambrian basementmade up of granites, gneisses, and quartzites(Greenough et al. 1998; Sheth and Melluso 2008).The presence of 12 lava Cows and involvement ofgranitic basement crust in the genesis of rhyolitewas also deduced by these authors. Alkaline rocksof the Pavagadh hold the signatures of astheno-spheric origin, whereas those from the Rajpiplaarea have some lithospheric mantle component(op. cit.). Plagioclase mineral separates from thealkali olivine basalt and rhyolite of the Pavagadhhave given 40Ar–39Ar ages of 66.40 ± 2.80 My and64.90 ± 0.80 My respectively, whereas the plagio-clase from the Rajpipla basalt has yielded 65.90 ±

1.70 My age (Parisio et al. 2016).

(b) Amba Dongar sector: With a spread of 30km2, it is one of the most studied alkaline silicaterocks and carbonatite complexes of the DLIP hasattracted the attention of many petrologists fromall across the world (Sukheswala and Udas 1963;Williams-Jones and Palmer 2002; Doroshkevichet al. 2009). This complex has a peculiar ringstructure (Bgure 1F) and shows a complex intru-sive relationship with the Bagh Sandstone–Deccanbasalt sequence, where nephelinite plugs in theform of conical hills have enveloped the outerperiphery of carbonatite ring structure. Carbon-atite–nephelinite suite in the inner-most domain ofthe complex is present as sub-volcanic diatremeand has fennitized rim of 300 m width (Sukheswalaand Udas 1963; Viladkar and Dulski 1986; Wil-liams-Jones and Palmer 2002; Chandra et al. 2019).Chemical gradation in spatial distribution of fen-nitization ranges from being sodic at the deeperlevel to potassic on shallower level. S€ovite form themajor constituents of carbonatitic melt, which isalso texturally variable due to (i) earlier episodes ofCuid impulses (sovite xenoliths) and (ii) chilledmargins at the contact with the Bagh Sandstones(Doroshkevich et al. 2009). 40Ar–39Ar dating ofphlogopite from the carbonatite and the whole-rock

nephelinite and tinguite suggest co-eval Deccan ageof the Amba Dongar complex (Ray and Ramesh1999; Ray et al. 2000). Recent geochronolgical resultsfrom dating of in situ apatite yielded similar age (65.4± 2.5 My, Fosu et al. 2018).

(c) Siriwasan–Dugdha (Naswadi) sector:Bulk of this sub-province comprises mainly tra-chytic rocks. The Siriwasan carbonatite intrusionsare of minor nature and are present in the form ofsills and s€ovite plugs. Sills are in discordant rela-tionship with the upper part of the Bagh Sand-stone, but are concordant with the basalt cover.Carbonatitic intrusion with some xenoliths has anaerial extent of 11 km with a thickness of 150 m(Viladkar 2015; Viladkar and Gittins 2016).

(d) Phenai Mata sector: Phenai Mata IgneousComplex is a bimodal, plug-like complex which com-prises of tholeiitic (gabbro-anorthosite-granophyre),as well as alkaline end members (Bgure 1F). Kumar(1996) reported intrusions of nephelinite, phonolite,lamprophyres, dolerite, microgranite, granophyre,and gabbro. 40Ar–39Ar ages of the gabbro revealed asynchronous origin of 65 Ma with the Deccan Traps(Basu et al. 1993). Recently, Parisio et al. (2016) pre-sented more robust 40Ar–39Ar ages for the PhenaiMata Complex based on biotite and amphibole min-erals separated from alkali gabbro and nephelinesyenite, and reported a mean age of 66.24 ± 0.37 and66.87± 0.32 My, respectively.

(e) Panwad–Kawant sector: The Panwad andKawant sectors are situated at the north of Heranand Kara river fault zones, respectively. Magma-tism in the area is controlled by these fault zoneswhich is reCected by the dyke trends parallelingthem (Bgure 1F; Viladkar and Avasia 1992).Alkaline magmatism in this sector is representedby the complex dyke system constituting phono-lites and tinguaites in association with a few dykesof sovite and beforsite. This sector also has abun-dant occurrences of lamprophyres, few pseu-doleucites and plugs of nephelinite (Sukheswalaand Avasia 1972; Gwalani et al. 1993). Most of thealkaline rocks occur as dykes of a kilometer scale.Bagh sediments, as well as the Deccan Traps, arethe country rocks and at places, Bagh sedimentsdepict quaquaversal dome-shaped patterns due toplug-like intrusion of the phonolites (Sukheswalaand Avasia 1972).

(f) Bakhatgarh–Phulmahal sector: This sectoris situated towards the NW of the Narmada and


Rampura–Narmada rivers near the ChhaktaloTownship, having an area of 200 km2 with Phul-mahal in the north and Bakhatgarh in the south(Bgure 1F). This sector is represented by a wide-spread abundance of NE trending basic and ultra-basic dykes. Carbonate-rich rocks are intrusive,fracture controlled and infra-trappean (Khandel-wal et al. 1997). Nine lamprophyres dykes havebeen reported from this area from Biswani,Kundwat and Undri villages. A majority of dykesfrom the terrain are E–W trending and locallyswerve in NE–SW and ENE–WSW directions(Randive et al. 2005; Randive 2008). Dimensions ofthese lamprophyres vary from 5 to 25 m in widthand 50 m to 2 km in length. The lamprophyre-associated fenitization eAects have also beenmarked at places.

(g) Chhotaudepur sector: Widespread alkalinemagmatism is represented by this sector. Lam-prophyre and accompanied alkaline magmaticactivity has been reported from Chhotaudepur,Seraphalia, Pipliwat, Moradungri, Jitnagar, Kan-thari, Heran, Hamirpur, and Dongargaon localities(Bgure 1F). This area also lies in the lower reachesof E–W trending Narmada rift zone and the pres-ence of a majority of the E–W trending alkalinerocks from this sector demonstrates their tectoni-cally controlled nature. A variety of alkaline andcalc-alkaline lamprophyres have been reportedfrom this area, which display an intrusive rela-tionship with the Deccan Traps as well as with thePre-Champaner granitic gneisses of Precambrianbasement. Some of the longest known Indian lam-prophyres have been reported from this sector(Gwalani et al. 1993; Chalapathi Rao et al. 2012;Pandey et al. 2019). An Eocene mineral isochronage (55 ± 2.3 Ma; Randive et al. 2012) has beenreported from Rb–Sr dating of Dongargaon lam-prophyre. Parisio et al. (2016) have also dated alamprophyre dyke from Dongargaon area, andobtained 65.25 ± 0.29 My age from its biotiteseparates although exact sample location for thedated lamprophyre has not been provided. Com-prehensive details of each lamprophyre occurrenceand their petrogenesis are illustrated in Pandeyet al. (2019). ENE–WSW trending Moradungarilamprophyre intrudes the syenite and is devoid ofany contact chilled or baked margin (Bgure 2C).The Heran river lamprophyres are characterized bymesocratic to melanocratic nature and large phe-nocrysts and megacrysts ([25 mm) of mica in afeldspathic groundmass (Bgure 2G).

(h) Chhaktalao sector: Alkaline rocks of theChhaktalao area are marked by the lamprophyreand carbonatites. The area is situated at 25 km NEof the Amba Dongar sector. Precambrian meta-morphics forms the basement for Cretaceous Baghsediments, which are in turn overlain by the Dec-can Traps. Lamprophyre and carbonatite of thearea are essentially of post-Deccan age, as theyshare an intrusive relationship with Deccan Traps(Chawade 1996; Hari 1998).

2.4 Mumbai–Trombay sub-province

The Murud–Janjira is situated *65 km south ofMumbai (earlier termed Bombay) and tectonicallyshaped by the Panvel Cexure. The area is distinctfrom the rest of the domains of this alkaline sub-province, as coastal dyke swarms intrude in thicksequence of basalts belonging to Wai sub-group ofDeccan Traps (Bgure 1I; Dessai and Bertrand1995). A vast variety of alkaline rocks and theirentrained crustal as well as mantle xenoliths, hasbeen reported from this area. Dessai and Viegas(2010) have divided the intrusives into 10 groupson the basis of their composition and cross-cuttingrelationship. Alkaline rocks from this domaininclude alkali basalts, picrites, nephelinites,tephrite, tephriphonolites, microdiorites, tra-chytes, carbonatites, ijolites and lamprophyres.Two genetically distinct alkaline suites are distin-guished: (i) sodic/potassic lamprophyres repre-senting primitive, metasomatized and volatile-richmelts and (ii) nephelinites which are dry meltscontaminated by felsic crust. On the basis of con-trasting geochemical and isotopic signatures ofthese two alkaline suites, two different sources withdistinct evolution histories have been traced(Melluso et al. 2002). Xenoliths and megacrystsuites from this sub-province have been reportedfrom alkaline lamprophyres and comprise clinopy-roxenites, granulites of eclogitic aDnity, felsicfeldspar bearing xenoliths, ultramaBc xenoliths,etc. (Dessai 1985; Dessai and Vaselli 1999). Gran-ulites and syenite felsic xenoliths are believed to beassimilation products and preserved cryptic meta-somatic signatures which have substantially mod-iBed the protolith during the subsequent magmaticevents (Dessai and Vaselli 1999; Dessai et al. 2008).Other smaller alkaline intrusions/extrusions

from this sub-province have been reported fromParol, Mahdawa–Dhulia, Salsette Island, Silvassa,Jawhar and Bhir areas. Alkaline rocks comprise


lamprophyres, carbonatites, nephelinites, tra-chytes, leucite-basanites, trachytes, nephelinesyenites and alkali basalts. Locality of each alka-line rock along with their mode of occurrence isprovided in table 1. 40Ar–39Ar ages of amphibolemineral separates from the Murud–Janjira lam-prophyres have yielded 65.2 ± 0.4 My (Hofmannet al. 2000). Rb–Sr isochron dating of the lampro-phyres have provided slight varying but post-Deccan ages of 63–64 My (Knight et al. 2000) and64.9 ± 0.8 My (Sahu et al. 2003). From the40Ar–39Ar ages of the lamprophyres and otherdykes of Murud Janjira area, Hooper et al. (2010)have suggested that Panvel Cexure was eithercontemporaneous or predate the lamprophyricintrusions.

2.5 Central Deccan sub-province

The Bastar craton represents one of the oldestcratonic nuclei of the Indian shield and is boundedby Mahanadi rift in NE and Godavari rift in SW,which separates it from adjoining Singhbhum andDharwar craton, respectively. The presence ofkimberlite, lamproites and other alkaline rocksfrom the Bastar craton is well known, but thesynchronus nature of a few of them with that of theDLIP was discovered much later. The Kodamaliand Behradih orangeite (Group II kimberlite) pipesfrom the Mainpur Beld are two such alkalinemagmatic activities, which mark the presence ofDeccan-related alkaline magmatism in centralIndia, and have been referred to as the CentralDeccan sub-province (Bgure 1C). The MainpurKimberlite Field comprises Bve diamondiferouskimberlites at the Payalikhand East and Paya-likhand west near Payalikhand, Kodomali, Behra-dih and Jangra at the northeastern part of theBastar craton (Lehmann et al. 2010). Based on the40Ar–39Ar ages and U–Pb perovskite dating, theBehradih orangeites are shown to be synchronouswith the DLIP at 65 My (Lehmann et al. 2010).The Kodomali pipe, on the other hand, has beendated to be slightly younger Paleogene age of 61My (Lehmann et al. 2010).The Behradih pipe has an oval shape with

elongated directions in NNW–SSE, while theKodamali pipe strikes in NW direction with surfaceexposures of about 200 m and 200980 m, respec-tively. Both the pipes represent largest kimberliticactivity of the MKF. The Behradih pipe is deeplyweathered and intruded in coarse-gained biotitegranite (Bundeli Granite). Macro- as well as

micro-diamond up to 200 ct have been recoveredfrom Behradih pipe (Newlay and Pashine 1993).The Behradih pipe has been established as oran-geite (Group II kimberlite) (Chalapathi Rao et al.2011). Orangeites were earlier believed to berestricted in Kaapvaal craton, southern Africa,only (Mitchell 1995). Melanocratic nature of Beh-radih orangeites and their typical inequigranularnature can be observed in Bgure 2K.

2.6 Aravalli sub-province

The Aravalli craton in western India is one of theoldest known continental nuclei, with a geologicalhistory spanning more than 3.3 billion years (e.g.,Wiedenbeck et al. 1996). On the eastern front, thePhulad Ophiolite suite of rocks are widely consid-ered as suture between the Mewar–Marwar cratons(Chatterjee et al. 2017), while the western frontshowcase a classic milieu of two large igneousprovinces (LIPs) of two different times: (i) the end-Cretaceous Deccan Cood basalt and related alka-line complexes and (ii) the Proterozoic silicicMalani magmatism.The alkaline magmatism related to the Deccan

episode is manifested at the Sarnu–Dandali, theMer–Mundwara and the Tavidar volcanic com-plexes and encompasses a very wide spectrum(Udas 1974; Narayan Das et al. 1978; Srivastava1989; Chandrasekaran and Chawade 1990; Chan-drasekaran et al. 1990; Shastry and Kumar 1996;Simonetti et al. 1998; Bhushan and Chan-drasekaran 2002; Bhushan and Kumar 2013;Bhushan 2015; Vijayan et al. 2016), ranging fromthe silica-undersaturated phonolite, carbonatite,ijolite, nephelinite to the saturated alkali basalt,alkali gabbro, lamprophyre, gabbro, etc., anddated to be of 65–80 My (Basu et al. 1993; Pandeet al. 2017). No major structural fabric or any kindof deformation is imprinted over the regional litho-packages, except for the regional dyke emplace-ment and fractures of various trends, as discussedby Vijayan et al. (2016). However, alkaline mag-matism sensu stricto is restricted to theSarnu–Dandali and the Mer–Mundwara com-plexes, since the Tavidar complex (Sen et al. 2012)is characterized by minor sub-alkaline rocks.

(a) Sarnu–Dandali alkaline complex: Thenorthern-most Deccan-linked alkaline complex islocated towards the eastern part of the Barmerbasin (Bgure 1D; 40 km east of Barmer town; Udas1974; Narayan Das et al. 1978; Chawade and


Chandrasekaran 1985). The Malani igneous com-plex acts as the basement and the area is mostlycovered by the Thar Desert. Lithological associa-tion is diverse, ranging from the phonolite, car-bonatite, ijolite, nephelinite to saturated alkalibasalt, alkali gabbro, lamprophyre, and gabbro.India’s only carbonatite-hosted rare earth elementdeposit is located near Kamthai area of theSarnu–Dandali Complex (Bhushan and Kumar2013; Bhushan 2015). Based on the 40Ar–39Argeochronology, 88.9–86.8 My (for syenites,nephelinite, phonolite and rhyolite) and 66.3 ± 0.4My (melanephelinite) polychronous emplacementages have been reported for the Sarnu–Dandalialkaline complex (Sheth et al. 2017). The ages ofaround ca. 90 Ma for some of the magmatic pulsesfrom this complex led Sheth et al. (2017) to relatethem to the tectono-magmatic events coincidingwith the India–Madagascar breakup. Our recentstudy shows that the carbonatites of this complexare characterized by their leucocratic and carbon-ate-rich content (Bgure 2F). At places, alteration ofRare Earth Elements (REE) bearing phases leavesa leaching imprint on carbonatite (Bgure 2L).Mela-nephelinites in the Sarnu area are hard andcompact in nature and present as bouldary, dis-continuous outcrops and rich in maBc minerals andare of melanocratic nature (Bgure 2H). Spheroidalweathering of mela-nephelinites was pervasive.Phonolites of the same area are characterized bythe phenocrysts of maBc minerals. The rocks areBne to medium-grained, light greenish to red colourin Beld, hard and compact in nature (Bgure 2A).Phonolites are present as multiple intrusions andare hosted by alkali syenite. Picrobasalt is alsopresent in this area which is Bne-grained, greenish-red to black in colour, slightly weathered and atplaces have amygdaloidal structures. Silica andcalcite are the main secondary crystallized miner-als in amygdules (figure 2I).

(b) Mer–Mundwara alkaline complex: TheMer–Mundwara complex is located *130 kmsouth-west to the Sarnu–Dandali alkaline complex,and is an intrusive within the Erinpura Graniteand occupies an area of about 15 km2 (Bgure 1H).Three individual plutons, viz., the Mer (largest anda ring intrusion); the Toa pluton (partial ring), andthe Musala pluton (laccolithic plug), which collec-tively host a museum of varied rock types com-prising gabbros, essexites, pyroxenites, ijolites,basanites, lamprophyres, nepheline syenites andcarbonatites (Coulson 1933; Bose and Das Gupta

1973; Vishwanathan 1977; Subrahmaniam andRao 1977; Chakraborti and Bose 1978; Chakraborti1979; Subrahmaniam and Leelanandam 1989; Sri-vastava 1989). The age spectrum is quite wide-spread and can be categorized into three broadgroups: (i) 56 ± 8 My (apatite Bssion track ontheralite; Subrahmanyam et al. 1972); (ii)*68.5–64 My (40Ar/39Ar age mineral separates(primary biotite and amphibole) and whole-rocksamples; Basu et al. 1993; Rathore et al. 1996)and (iii) 80–84 My (40Ar–39Ar ages biotites of maBcrocks and of 102–110 My whole rocks of nephelinesyenite; Pande et al. 2017). A polychronousemplacement history has also been inferred for theMer–Mundwara alkaline complex (Pande et al.2017). Carbonatites of the complex are character-ized by their prominent brecciated nature(Bgure 2B).

2.7 Tethyan Suture zone

The locations of ultramaBc lamprophyre sills ofthis domain are located immediately towards thesouth of Tethyan Suture zone, elucidated by anophiolitic belt (Ahmed and McCormick 1990; Nabiet al. 2002) northeast of Quetta, Pakistan. The sillsintrude the Late Triassic to Early Jurassic WulgaiFromation (Shrinab Formation) and show steep([70�) dipping and have ENE–WSW trend. Mostof the sills are doleritic in nature and are similar;however, the sills from the west of Spangar peakare petrographically very different, having macro-crysts of olivine, phlogopite and andraditic garnetwith nepheline and Cr-spinel. The groundmasscomprises perovskite, olivine, phlogopite, apatite,monticellite, magnetite, chlorite, serpentine andpectolite (Ahmed and McCormick 1990).40Ar–39Ar plateau ages reveal the sills to representa significant pre-Deccan intrusion event(69.7 ± 0.2 My) linked to the advent of the Reu-nion plume at the base of the Indian lithosphere(Kerr et al. 2010).

3. Petrography and mineralogy

3.1 Alkali basalts

Location and mode of occurrences of alkali basaltsalong with important references are provided intable 1. Alkali basaltic lavas of the DLIP arecharacterized by the presence of inequigranular


Figure 3. Photomiccrographs and Back Scattered Electron (BSE) images of some alkaline rocks from the Deccan LIP: (A) Twogenerations of olivine in Behradih orangeite (in PPL; DEB-3A/90/1); (B) Calcite dominant medium grained carbonatite fromSarnu–Dandali complex (in XPL; AA/KT/1.1); (C) Pyroxene phenocryst in porphyritic phonolite from the Sarnu–Dandalicomplex (in PPL; AA/DD/10.1); (D) Pyroxene and olivine phenocryst in amela-nephelinite from the Sarnu–Dandali complex(in XPL; AA/SN/1.4); (E) Oscillatory zoned clinopyroxene (with inclusions of apatite and spinel) in the feldspathic groundmassof lamprophyre from Dongargaon area (in PPL; ND2/3); (F) Amphibole and clinopyroxene as dominant phenocryst phases inthe feldspathic groundmass from the Panwad lamprophyre (in PPL; NP1/2); (G)Macrocryst-megacryst of pyroxene ([7 mm) inlamprophyre from the Seraphalia area (in PPL; SPL/SS/2); (H) Biotite, amphibole and zircon in the phonolite of theSarnu–Dandali complex (in PPL; AA/DD/11.2); (I) Brown amphiboles as dominant phenocrysts in the feldspathic groundmassof lamprophyre from the Phenai Mata alkaline complex (in PPL; PML/SS/1); (J) BSE image showing slightly zoned pyroxenephenocryst from the Mundwara lamprophyre (MMU2-1-7); (K) Exsolution intergrowth texture of magnetite and ilmenite fromthe Mundwara lamprophyre (MRGL-SS-2-3); (L) Lozenge shaped titanite and alkali- as well as plagioclase-feldspar in thegroundmass of Chhotaudepur lamprophyre (CUL/SS/2); (M) Intergrowth texture of spinel and clinopyroxene in the HeranRiver lamprophyre (HRL/2); (N) Dendritic and quenched texture of feldspar in the Heran River lamprophyre (HRL/1) and (O)Ocellus shaped spinel in Seraphalia lamprophyre (SPL/SS/1). Abbreviations. Ol: olivine; Cal: calcite; Pyx: pyroxene; Cpx:clinopyroxene; Amp: amphibole; Bt: biotite; Zrn: zircon; Feld: feldspar; Ilm: ilmenite; Mg: magnetite; Ttn: titanite and Spl:spinel.


and porphyritic texture with the phenocrysts ofolivine, clinopyroxene, plagioclase and opaqueoxides (mostly magnetite). The groundmass com-prises microlites of plagioclase, Bne-grained olivine,clinopyroxene, plagioclase, magnetite, titanomag-netite, ilmenite, calcite, chalcopyrite and glass.Rare incidences of biotite and nepheline (bothmodal and normative) have also been reported(Talusani 2001; Karmalkar et al. 2005). Overall,the phenocrysts of pyroxene dominate over those ofthe olivine. Olivine phenocrysts-dominated mildlyalkaline basalts are conventionally termed as oli-vine basalts (Greenough et al. 1998). At manyplaces, resorbed and skeletal nature of olivine hasbeen reported and inferred to be the products ofrapid crystallization due to an inCux of hottermagma pulses (Kshirsagar et al. 2011). Variousmantle xenoliths and xenocrysts have also beenreported from the alkali basalts (Karmalkar et al.2005; Ray et al. 2016 and reference therein). Oli-vine from the xenocrysts/xenoliths (Fo88–90) in thealkali basalts is more Mg-rich, compared to thosefrom the phenocryst olivine (Fo70–75) (Paul et al.2008). To differentiate the primary olivines fromthose occurring as xenocrysts in alkali basalts, wehave plotted their CaO contents against Mg#,which reveal that a majority of the olivines are ofprimary nature and only a few are mantle orxenocrystic in nature (Bgure 4A) (Thompson andGibson 2000). Pyroxenes of alkali basalts haveundersaturated T sites and majority have lowoxygen fugacity indicating reducing conditions ofcrystallization (Bgure 4B and C). Spinels of alkalibasalts follow Fe–Ti fractionation trend, whichindicates the evolution of spinel composition dur-ing fractional crystallization of olivine and pyrox-ene with or without feldspar (Bgure not shown;Barnes and Roeder 2001). Alkali basalt spinelsdisplay titano-magnetite trend (magmatic trend 2)similar to that of the lamprophyre (Bgure 4D;Mitchell 1995).

3.2 Lamprophyres and orangeites

The Deccan lamprophyres are characterized by Bneto medium grained, inequigranular, porphyritic,and panidiomorphic textures (Dessai and Viegas2010; Chalapathi Rao et al. 2012; Pandey et al.2018a; Sahoo et al. 2020; Bgure 3F and I). Pheno-crysts comprise of pyroxene, mica, amphibole andolivine whereas the groundmass contains feldspar,feldspathoid and rarely carbonates at places. Other

observed textures in lamprophyres range frompoikilitic, cumuloporphyric, glomeroporphyritic,atoll, sieve and intergrowth. Feldspars in the lam-prophyres are restricted to the groundmass phaseonly, owing to their hydrous and volatile-richnature which suppresses the former growth asphenocrysts (Rock 1991). All varieties of feldsparhave been described from the lamprophyresreCecting extensive solid solution. The presence ofalkali, as well as plagioclase feldspar, can also beseen in the BSE images (Bgure 3L). BSE imagingalso reveals the presence of radiating dendriticquenched texture in the groundmass feldspar fromone of the (Heran River section) lamprophyres,which is indicative of rapid undercooling andslower diffusion rate than growth (Bgure 3N). Twomodels can account for the genesis of dendritictexture: Brstly, crystal tendrils grow to cross thedepleted diffusion zone for supply of necessaryelements and secondly, the dendritic shape isattained to overcome the local heat (undercooling)accompanied by crystallization (Winter 2010).Xenocrystic (crustal) feldspars have been noticedin one of lamprophyres from Jitnagar area, Chho-taudepur sub-province (Bgure 3H). In some Deccanlamprophyres (from the Seraphalia area), macro-crysts ([7 mm in size) and megacrysts of olivineand pyroxene are also observed (Bgure 3G).Megacrysts of biotite are also reported from thePhenai Mata lamprophyres (Sethna 1989).Macrocrysts of rare chrome diopside were studiedfrom the Mundwara complex (Sharma et al. 2018).Groundmass also includes microphenocrysts ofhydrous maBc silicates and other maBc minerals.Accessory minerals include apatite, titanite, spinel,pyrite, chalcopyrite, epidote, perovskite and car-bonate minerals. Apatite and titanite, which aremostly restricted as accessory phases, are alsorarely present as microphenocrysts (*1 mm insize) in some of the lamprophyres (Pandey et al.2019; Sahoo et al. 2020). Although the majority ofDeccan-related lamprophyres are fresh and areporphyritic in nature, at places, groundmass feld-spar alteration to sericite is seen. Likewise, poorlydeveloped porphyritic texture has also beenreported from the Mount Girnar lamprophyres(Bose 1973; Paul et al. 1977). A close association ofthe lamprophyre and syenite form composite dykesat the Mount Girnar (Bose 1971; Sethna 1989;Sahoo et al. 2020). Zoning in mineral phases fromthe lamprophyres has been attributed to disequi-librium conditions of pressure and temperatureduring crystallization. The presence of oscillatory


zoned phenocrysts has been reported widely(Bgure 3E). Cumulus intergrowth of spinel withclinopyroxene and exolution of magnetite withilmenite can also be seen (Bgure 3K and M). Thepresence of intergrown minerals with poikilitcinclusions of each other along with zoning and sievetexture (Pandey et al. 2019) provides an evidenceof magma mixing in the genesis of these lampro-phyres. BSE imaging also reveals the presence ofrare, rounded to sub-rounded spinel ocelli. Multiplemodes of spinel occurrence in the Deccan lampro-phyres is known as, viz., (i) discrete grains in thegroundmass, (ii) early crystallized poikilitic inclu-sions, (iii) intergrowth crystals with differentminerals and (iv) a rounded to sub-roundedanomalous ocellus (Bgure 3O). Carbonate ocelli arecommon in the alkaline lamprophyres and havebeen interpreted as immiscible carbonates in sili-cate melts. The Deccan lamprophyres from Ser-aphalia area comprise carbonate ocelli. Titanitesare present as needles to lozenge in shape(Bgure 3L). Modal classiBcation of these rocks isbased on the predominance of phenocryst phasesand nature of the groundmass (Rock 1991). Diversevariety of lamprophyres have been documentedfrom the DLIP, viz., monchiquite, camptonite,kersantite, vogesite, spessartite, minette, sannaite,alnoite and damtjernite (see Dessai and Viegas2010; Pandey et al. 2018a; Sahoo et al. 2020).Earlier, the Palanpur dyke from the Kutch areawas recognized as anorogenic lamproite dyke(Karmalkar et al. 2014) and subsequently wasshown to be of damtjernite variety (Pandey et al.2018a). The Behradih orangeites are characteri-zed by the presence of inequigranular texture,macrocryst grains and two generations of olivinewhich are characteristic textures of kimberlites(Bgure 3A). Macrocrysts and microcrysts of olivinefrom Behradih orangeites (Fo84–91) have overlap-ping compositions with those from the EasternDharwar craton kimberlites (Fo83–90) (ChalapathiRao et al. 2011).

3.3 Carbonatites

The mineralogy of the carbonatites is diverse andcommon minerals reported include calcite, alkalipyroxene, phlogopite-biotite, amphibole, apatite,pyrochlore, magnetite, monazite, perovskite, allan-ite, zircon, feldspar, olivine and melanite garnet(Viladkar 2010). The REE and Nb–Ta bearingminerals include bastnaesite, cerianite, chevkinite,fersmite and columbite (Krishnamurthy 2019 and

references therein). S€ovites of the Amba Dongar arecharacterized by Bne to coarse porphyritic grainedtexture. Coarse-grained s€ovites have fused crystalmargins and larger phenocrysts of calcite rimmed bythe iron oxides. Strongly zoned pyrochlores of theAmba Dongar carbonatite are characterized by widevariations of Nb, Ta, Ce and Ti (Viladkar 2010).The Amba Dongar alvikites display tabular texturewhile ankerites are richer in iron content (Viladkar1981). Fine-grained, dolomite-dominated, granularbeforsite of this area have different mineralogy fromalvikites (Sukheswala and Avasia 1972). Dolomitesand calcites in carbonatites of the Panwad–Siri-wasan domain are anhendral rhomb-shaped andoccur in the form of veins in the associated cherts(Khandelwal et al. 1997). Both alvikite as well ass€ovite type of carbonatites, have been reported fromthe Sarnu–Dandali complex (Bhushan and Kumar2013; Bhushan 2015). On the basis of older s€ovitexenoliths in the younger plugs of s€ovite, a multi-phase emplacement for carbonatites has been sug-gested (Doroshkevich et al. 2009). The Siriwasancarbonatites are characterized by the presence ofankerite (up to 10 vol%), zoned amphibole, aegirineaugite and lithium mica (Viladkar and Gittins2016). Calcite-dominated medium-grained carbon-atite from the Sarnu–Dandali can be noticed inBgure 3(B).

3.4 Nepheline syenites

The Deccan nepheline syenites are medium- tocoarse-grained, leucocratic to mesocratic, andhypidiomorphic to granular in nature. Essentialmineralogy includes pyroxene, alkali feldspar,nepheline, hornblende, biotite, and sodalite,whereas accessory phases include titanite, apatite,olivine, opaques and carbonates. Alkali syenites ofthe Mundwara alkaline complex have been dividedinto nepheline sodalite syenite, barkevikite-nephe-line microsyenite and leucosyenite by Bose (1973),whereas Chakraborti (1979) recognized aegerineaugite syenite and brown hornblende syenite only.Imprint of mixing and perthitic growth in feldsparas well as compositional zoning in alkali pyroxeneswas also highlighted in these early works. Thepresence of sodic augite and extreme alterationshas been reported for the Mount Girnar nephelinesyenites (Paul et al. 1977). Petrography of theMount Girnar nepheline syenite and kaolinizednature of feldspar was discussed by Mathur et al.(1926). Smaller dykes from the Mount Girnar,


Phenai Mata, Jawhar and north of Bombay exhibitporphyrtitc and trachytic texture, in which pla-gioclase and maBcs form the phenocryst phase(Sethna 1989). Some of the feldspathoidal syenitesfrom these areas have up to 70% of alkali feldspar,whereas nepheline, sodalite, cancrinite and analcitecomprise 20–30 vol% of the modal mineralogy. Thepresence of barkevikite and aegerine augite varietyof pyroxenes has also been described from thePhenaimata and the Mount Girnar samples. Thepresence of garnet and apatite has been reportedfrom the Panwad–Saidivasan area (Khandelwalet al. 1997). Recently, nepheline syenite occurrencehas been reported from the Kutch area which wastentatively linked to the Pre-Deccan Mesozoicplutonic events (Thakkar et al. 2021). Mineralchemistry plots of pyroxene for nepheline syenitesalso indicate the predominance of low oxygenfugacity and undersaturation in T sites (Bgure 4Band C).

3.5 Miscellaneous alkaline rocks (trachytes,ijolites, tinguaites, etc.)

This category includes a vast variety of silicaundersaturated to oversaturated alkaline rocks.It is impractical to provide individual descriptionof all the miscellaneous alkaline rocks; so only theirmost representative aspects are discussed here.Both melanephelinites as well as phonolites, dis-play a porphyritic texture in a majority of thesamples. Phenocrysts in the melanephelinites areconstituted by clinopyroxene, olivine, biotite(crystals as well as pseudomorphs), amphibole andspinel (Bgure 3C and D), whereas the groundmasscomprises of nepheline, Cuorphlogopite, biotite andFe–Ti oxides. Lozenge-shaped zircons in associa-tion with amphibole and biotite have also beennoticed in the phonolites of Sarnu–Dandali area(Bgure 3H). Alkali to sub-alkali picritic basaltsfrom the Murud Janjira are porphyritic to aphyric.

Figure 4. (A) Mg# vs. CaO variation plot (Thompson and Gibson 2000) for the olivines from alkali basalts, lamprophyres andmiscellaneous alkaline rocks showing presence of primary and mantle-derived olivines for lamprophyre and alkali basalt whereasonly primary olivines are present in the miscellaneous alkaline rocks. (B) Al(VI)+2Ti+Cr vs. Na+Al(IV) (Schweitzer et al. 1979)plot for pyroxenes from different alkaline rocks. Except for the nepheline syenite, all other alkaline rocks display variable oxygenfugacity. (C) Al (a.p.f.u.) vs. Si (a.p.f.u.) plot (Stoppa et al. 2014) for pyroxenes from the Deccan alkaline rocks indicatingsaturation of T sites for majority of samples (data sources for A, B and C: Viladkar and Avasia 1992; Simonetti et al. 1998; Rayet al. 2006; Dessai et al. 2008, 2010; Pandey et al. 2019 and references therein). (D) Fe2+/Fe2++Mg vs. Ti/(Ti+Cr+Al) plot forspinel (after Mitchell 1995) indicates a strong T2 trend rather than a T1 (kimberlitic trend) (data sources: Cucciniello et al. 2015;Pandey et al. 2019; Dongre et al. 2021 and references therein).


Olivines are the sole phenocrysts, whereas thegroundmass comprises pyroxene as well as glass.A wide range of variations in mineral chemistrywas reported from the olivine (Fo84–76), clinopy-roxene (Ca38–40Mg45–43Fe17–14) and feldspar(Or0.5–2Ab37–29An61–68) (Dessai et al. 2008).Melanephelinites from the Sarnu–Dandali havestrongly zoned clinopyroxene andmoderately zonedolivine and spinel (Chatterjee 2021). Zonedclinopyroxenes have oscillatory trends of decreasingMgO andNa2O and increasing Al2O3 andTiO2 fromtheir core to the rim. Cr–Al rich spinel core and Baand Ti coupled substitution in biotite were alsohighlighted by the previous workers. Clinopyroxenethermobarometry of the miscellaneous alkalinerocks have provided varying pressure (1.2–22.3kbar) and temperature (1039–1323�C) conditions(op. cit.). Phonolitic nephelinites of the AmbaDongar are associatedwith carbonatites and displaya porphyritic texture, in which the phenocrysts arecomposed of nepheline, melanite, aegirine augiteand barkevikite, whereas feldspar, feldspathoid andglassy matter are the groundmass constituents(Sethna 1989). Olivine-free nephelinite and phono-lite of the Amba Dongar and Sirivasan areas arequite similar in nature with minor differences incalcitic-rich groundmass (Viladkar and Avasia1992). Pyrochlore and titanite have also beenreported in these rocks from the Amba Dongar.Tinguite and phonolite of the Panwad–Kawant areaare characterized by microporphyritic nature withsome rarely occurring macrocrysts of pseudo-leucite(Viladkar 2010). The phonolites occurring in thisarea were further divided in anlacime-bearing maBcand felsic varieties (Viladkar and Avasia 1994).Pyroxenes of these miscellaneous rocks have widevariations in their chemistry. The Amba Dongarand Sirivasan alkaline rocks have acmite enrich-ment with increasing MgO, whereas those from thePanwad have hedenbergite component (Viladkarand Avasia 1992). Titan-augite and nepheline-bearing ijolites of the Panwad area are devoid offeldspar. Ijolites comprise accessory minerals likeanalcite, apatite and larger titanite (Sukheswalaand Avasia 1972). Interestingly, all the olivinespresent in these rocks are of primary (liquidus)nature rather than those of mantle (xenocrystic)origin (Bgure 4A). Fenitized nephelinites from theAmba Dongar display imprints of intense K-meta-somatism in the form of replacement of nephelineand pyroxene by hydromuscovite and phlogopite,respectively (Viladkar 2015). Determination of Tsite plot based on pyroxene mineral chemistry data

indicates prominent undersaturation of the T siteand is similar to those reported from the alkalinebasalts (Bgure 4C). Oxygen fugacity plots based onpyroxene cations for the miscellaneous alkalinerocks display a wide scatter in the data (Bgure 4B).Spinels of the miscellaneous alkaline rocks also fol-low the T2 trend (Bgure 4C).

4. Whole-rock geochemistry

Available geochemical data (major oxides, traceelements, and radiogenic Sr–Nd isotope andstable C and O isotope compositions) of differentalkaline rocks from the DLIP, compiled in thisstudy, are discussed in detail in this section. Basedon the availability of precise quality data in goodquantity, all the data are grouped into Bve majorrock groups, viz., alkali basalt, lamprophyre, car-bonatite, nepheline syenite and miscellaneousalkaline rocks (which include trachytes, ijolites,tinguaites, etc.). For geochemical interpretations,we have also selected the datasets containing com-plete geochemical analysis in terms of major oxides,trace elements and rare earth elements (REE). Foralkali basalts, we have selected 53 geochemicaldatasets, for lamprophyres 103, for carbonatites137, for nepheline syenite 26, and for the miscella-neous alkaline rocks 220. Only the representativegeochemical plots for each class have been presentedas Bgures; however, for interpretation many otherplots were also considered. Fractionation index foreach class is judicially selected on the basis of vari-ation and concentration of concerned element/ox-ide. MgO is chosen as fractionation index for alkalibasalt, lamprophyre and nepheline syenite, whereasfor carbonatite and miscellaneous alkaline rocksCaO and SiO2 have been selected.Highest variation in the SiO2 content is dis-

played by the carbonatites (ranging from 0.36 to52.09 wt%). On the contrary, least variation isshown by the alkali basalts (42.07–51.84 wt%).Lamprophyres also have a wide range of silicacontent (varying from 34.49 to 51.54 wt%), andalong with carbonatites, display their silicaundersaturated (\45 wt% SiO2) nature for most ofthe compiled data. Total alkali vs. silica (TAS)diagram (Supplementary Bgure S1A) for alkalibasalts demonstrates a weak negative correlationwith a narrow range of compositional Belds, viz.,basalt, trachy basalt and basanite. Interestingly,the maximum variation for the total alkalis is dis-played by the lamprophyres (0.32–13.04 wt%),


whereas the minimum and narrowest range ofvariation is shown by the alkali basalts (1.6–5.9wt%). Highest content of the bulk alkalis amongstthe Deccan alkaline rocks is shown by the alkalipyroxenites (up to 19.6 wt%) from the miscella-neous alkaline rock category. K2O/Na2O values forlamprophyres and nepheline syenite consistentlyfall in the range of 0.5–2.0, which emphasize theirshoshonitic nature. Alkali basalts have K2O/Na2Ovalues\0.5, highlighting their calc-alkaline aDnity,whereas carbonatite and other alkaline rocks displayerratic variations. High K and calc-alkaline toshoshonitic nature of the lamprophyres is furtherconBrmed with the help of Co vs. Th (Hastie et al.2007) and Zr/Y vs. Th/Yb (Ross and Bedard 2009)plots (Supplementary Bgures S1B and C, respec-tively). Negative anomalies of Nb–Ta–Ti are widelyregarded as signatures of orogenic alkaline magma-tism. Ubiquitous absence of the Nb–Ta–Ti negativeanomalies exclude their orogenic nature and alsosigniBes lack of substantial crustal contamination.Negative correlation of SiO2, Al2O3 and Na2O

with regard to MgO has been observed in the alkalibasalts, which suggest involvement of differentia-tion during magmatic evolution. CaO, Cr and Niare positively correlated with MgO, whereas ironshows slight scatter, indicating pyroxene and oli-vine fractionation (Bgure 6A–D). Total alkali vs.silica (TAS) diagram (Supplementary Bgure S1A)for alkali basalts demonstrate a weak positivecorrelation with a wide range of compositionalBelds, viz., picrobasalt, basalt, basaltic andesite,trachy basalt, phono tephrite and tephrite basan-ite. Alkali basalt data show scatter in the subal-kaline/tholeiitic Belds in the TAS plot.Incompatibility of Zr is more in comparison to Y inthe mantle phases and Zr/Y value tends to behigher when degree of melting is small. Zr/Y valuein the Deccan alkali basalts is fairly high (up to13.94) along with FeOt content and indicates roleof lower degree of melting in the genesis. Both Zrand Y are negatively correlated with MgO andFeO, and highlight the involvement of partialmelting process in melt generation (Supplementaryonline Bgure S2A–D). The chondrite-normalizedREE patterns and primitive mantle-normalizedmulti-element spider plots for the alkali basalts arepresented in Bgure 6(E). The REE patterns ofalkali basalts are uniform, parallel to sub-paralleland exhibits moderate to strong REE fractionation((La/Yb)N = 1 to 37). Significant LREE enrich-ment in comparison to HREE is also evident fromREE plot (Bgure 6F). A conspicuous positive Pb

anomaly is noticeable in the multi-element spider-gram (Bgure 6E). Pb is a very mobile element andits negative anomaly indicates loss by leaching,whereas its positive spike suggests either crustalinput or hydrothermal overprint. The absence ofany negative or positive anomaly of Eu negates therole of plagioclase in the fractionation and accu-mulation process. Positive Nb and Ta anomaly forsome samples is also apparent. Positive Ba, Th andNb anomalies forming hump patterns have beeninterpreted as derived from melting of plume/asthenosphere (Pearce et al. 1990).In the lamprophyres too, similar negative trends

of silica, alumina and alkalis are present, but dis-play relatively better correlation (Bgure 7A and B).Positive correlation of TiO2, FeOt and CaO withMgO is prominent in lamprophyres, whereas it isfound lacking in the alkali basalts. Elevated K2Ocontents in a majority of lamprophyres reCect theirhigher modal phlogopite. TiO2 content in all thelamprophyres is [1 wt%, which suggests theiranorogenic nature. Geochemical classiBcationdemonstrates the presence of all three lampro-phyres, viz., alkaline, calc-alkaline and ultramaBcfrom Deccan LIP (Dessai and Viegas 2010; Pandeyet al. 2018a, 2019; Sahoo et al. 2020). The vari-ation in lamprophyre geochemistry is very wellreCected in their primitive as well as evolvednature. Lamprophyres from the Mundwara, thePhenaimata, the Murud Janjira, the Saurashtraand a few lamprophyres from the Chhotaudepurexhibit an evolved nature, whereas those fromthe Kutch and the Tethyan UML, and a fewother lamprophyres (Panwad and Moradungri)from Chhotaudepur sub-province show primitivenature. Lamprophyres are believed to have beenderived from partial melting of metasomaticallyenriched lithosphere (Foley 1992) which is verywell reCected in elevated concentration ofincompatible trace elements, viz., Nb (18–293ppm), Ta (0.5–21 ppm), Zr (103–823 ppm) andHf (1.7–15.5 ppm).REE plots for the Deccan lamprophyres are

shown in Bgure 7E and F. A steep, sub-parallel andfractionated pattern of REE is evident and advo-cates a significant LREE enrichment over theHREE ((La/Yb)N = 5 to 136). In the multi-element spidergram, negative troughs at P, Pb, Tiand Zr can be observed. Negative troughs of P, Tiand Zr can be attributed to residual mineralogyinvolving apatite, titanite and zirconium-bearingphases respectively in the source region. Minorperturbations in Pb concentrations might be due


to the mobility and leaching eAect. The absence oftroughs at Zr–Hf, Nb–Ta and Ti also attests thenon-orogenic nature for Deccan lamprophyres. It issignificant that although many of Deccan lampro-phyres display shoshonitic characters and calc-alkaline varieties have also been identiBed, none ofthem display typical subduction signatures like theMesoproterozoic Dharwar craton lamprophyresfrom southern India (Pandey et al. 2017a). Nega-tive anomalies at Zr–Hf and Ti and increased Zr/Hf with silica content in a few Chhotaudepurlamprophyres have been explained by theclinopyroxene fractionation (Pandey et al. 2019).In the compiled REE plots for such a large dataset,it is difBcult to appreciate minor variations.Although a prominent Eu negative anomaly is notdistinguishable, it has been reported for the MountGirnar and the Saurashtra lamprophyres (Naushadet al. 2019; Sahoo et al. 2020). Eu anomalies are notvery common in lamprophyres and apart fromplagioclase fractionation they also at times reCectweathering and alteration. Majority of Deccanlamprophyres are fresh and lack any imprints ofalterations unlike the Precambrian lamprophyresfrom the Dharwar craton, southern India (Pandeyet al. 2018a, b, c). The Behradih orangeites (GroupII kimberlites) have been clubbed here with thelamprophyres, because of their overall closenesscompared to the other rock types. The Behradihorangeites display essentially potassic and ultra-maBc nature and share indistinguishable geo-chemical traits of the orangeites from the Kaapvaalcraton, southern Africa. Highly fractionated REEpattern of these rocks have been attributed to verysmall degree (\1%) of partial melting of a phlo-gopite–garnet lherzolite source (Chalapathi Raoet al. 2011).A negative correlation is observed between

SiO2, Al2O3, alkalis, TiO2 and FeOt vs. CaOcontent for the majority of carbonatites(Bgure 8A–D). CaO is chosen as a fractionationindex in variation diagram for carbonatitesinstead of SiO2, MgO and FeOt contents due tothe latter’s inconsistent values, and also for manysamples, they are as low as 0.5 wt%. MgO showsan overall scatter with respect to CaO, but ingeneral, depicts a negative trend. Negative trendsof iron, silica, magnesium and potassium withCaO suggest fractionation of maBc minerals, suchas clinopyroxene, amphibole and mica. Positivecorrelation of CaO and P2O5 is interpreted asapatite fractionation during magmatic evolution.Deccan-related carbonatites display an overall

scatter in the CaO vs. P2O5 plot (SupplementaryBgure 2E) which likely indicates the presence ofcumulus crystals of apatite rather than eAects offractionation. Compared to the Sarnu–Dandaliand Mer–Mundwara carbonatites, the other vari-eties are more silicic. Higher silica content ([20wt%) in a few Deccan carbonatites suggests theirsilicio-carbonatitic nature. High barium, silicaand moderate calcium content are characteristicof the benstonite carbonatites. Deccan carbon-atites show varying barium content (0.01–11.4wt%), but for the majority of them the concen-tration is not enough to classify them as ben-stonites. The compositional variation is alsoreCected in the primitive mantle normalized spi-dergram, amongst which the negative anomaly isprominent for P and Ti (Bgure 8E), perhaps bedue to apatite and titanite fractionation respec-tively. Negative troughs and depletion of K, Zr, Hfand Ti are characteristic of the global carbon-atites (Woolley and Kempe 1989), which are alsodiscernable in the Deccan carbonatites. Chondritenormalized rare earth element concentration ofDeccan carbonatites show (i) very high REEconcentration and (ii) LREE enrichment overHREE with a gentle slope without any Euanomalies (Bgure 8F). Sr and Ba enrichmentlevels are also quite high for the carbonatites ingeneral.All the compiled nepheline-bearing syenites from

DLIP are characterized by comparatively highersilica (49.8–61.8 wt%) content in comparison to theother alkaline rocks and highlights their silica-sat-urated nature. Interestingly a majority of nephe-line syenites display peraluminous (Al2O3[Na2O + K2O + CaO) nature, which is very wellreCected from their higher alumina content(15.4–24.5 wt%). Although K2O/Na2O ratio andplot indicate their shoshonitic nature, very highcontent of sodium in many of the nepheline syen-ites is distinctive. A positive trend can be noticedfor TiO2, FeOt, CaO and P2O5 with regard to theMgO, whereas a negative trend is prominent forK2O, SiO2 and Al2O3 (Bgure 9A–D). The lack ofgood quality and complete trace element and REEdatasets in sufBcient amounts preclude furtherdiscussion in this regard.Expectedly, the bulk-rock geochemical traits for

the miscellaneous alkaline rocks are highly variablewith a broad spectrum of rocks from silica under-saturated to oversaturated. Variation in the datais also reCected from the scatter of data in thegeochemical plots (Bgure 10A–D). A positive


correlation for Al2O3, Na2O and K2O with regardSiO2 is observed, whereas FeOt, CaO, MgO andTiO2 are negatively correlated. Positive correlationfor the alkalis, along with alumina, with increasingsilica proportions, suggests involvement of plagio-clase fractionation. Negative correlation of mag-nesium, iron and calcium with increasing silicacontent indicates fractionation of maBc mineralssuch as clinopyroxene and amphibole. Negativecorrelation of titanium with silica might beattributed to titanite fractionation. Co, Sc, V andSr are negatively correlated with silica while Rb, Yand Zr are positively correlated. Chondrite nor-malized REE contents of the miscellanous alkalinerocks show distinct LREE enrichment over HREE.Negative Eu anomaly in some samples is evidentand indicative of plagioclase fractionation(Bgure 10E). Negative P and Ti anomalies can beobserved in multi-element spidergrams for amajority of the miscellaneous alkaline rocks,indicative of residual apatite and titanite in thesource. Positive spikes of Pb and Ta are also pre-sent. Ta and Ti are not correlated with the differ-entiation index, which suggests their abundance,representing the source-related character.

5. Radiogenic isotopes

Radiogenic Sr–Nd isotope compositions are suf-Bciently available only for the carbonatite, lam-prophyres and miscellaneous alkaline rocks andvery few for the alkali basalts. Pre-Deccan(Kutch), synchronous with the Deccan (AmbaDongar and Mount Girnar) and post-Deccan(intruding the Deccan Traps: Chhotaudepur sub-province) alkaline rock ages are available fromthe DLIP (table 1). The Deccan alkaline rocksshow intrusive relationship with the DeccanTraps, undated syenites, Godhra Granite andeven Precambrian basement. However, in thisstudy, we have considered all of them to be coe-val and taken, 65 My age to correct their initialNd and Sr-isotopic ratios since (i) majority of thedated alkaline rocks have coeval ages (65 ± 1My), and (ii) alkaline rocks which have clearintrusive relationship with Deccan age tholeiiticbasalts, have yielded precise age of 65 My(Parisio et al. 2016). The variations in the87Sr/86Sri vs. 143Nd/144Ndi for Deccan alkalibasalts are presented in Bgure 11(A). The aver-age compositions of various mantle reservoirs,such as the depleted MORB mantle (DMM),

HIMU (high l; l = 238U/204Pb), EMI (enrichedmantle I), EMII (enriched mantle II) and OIB(Oceanic Island Basalt), are also marked in theplot (Zindler and Hart 1986). A wide range ofisotopic variability is evident, but majority of thebasalts have OIB-like signature. Some of thealkali basalts samples show aDnity towards theIndian Ocean type MORB-like signatures.The eNdinitial and

87Sr/86Sri plot for lampro-phyres, carbonatites and miscellaneous alkalinerocks from different alkaline complexes and fromspatially distinct domains reveal a clear trendparallel to the mantle array (Bgure 11B). Suchclear trend is indicative of the involvement ofmixing of two distinct mantle sources in thegenesis of Deccan alkaline rocks. eNdinitial valuesfor the Deccan lamprophyres range from –10.53to +4.76 with a majority of the negative values,indicating the varying role of enriched to deple-ted mantle sources in their genesis. On the otherhand, the 65 My aged Bastar organgeites havevery similar isotopic signature and are indistin-guishable from those of the Deccan lamprophyressuggesting an overall similarity in their mantlesource (Bgure 11B). Depleted mantle model ages(TDM) of the lamprophyres, carbonatites, alkalibasalts and miscellaneous alkaline rocks rangefrom 313 to 1804 My with majority of themhaving Meso- to Neoproterozoic model ages.Strikingly, all the carbonatites have negativeeNdinitial values indicating consistent enrichedmantle sources.

6. Stable isotopes

Since the carbonatites are rare, exotic, mantle-derived, and carbonate-bearing igneous rocks, thesource of the carbon remains puzzling and con-tributes to our understanding of the nature ofmantlecarbon and the long-term global carbon cycle. Theextremely low viscosity and short residence timequalify the carbonatites as tracers to back track thesource characteristics (Pyle et al. 1991; Dobson et al.1996). In the bivariate plot of d18O and d13C, theDeccan carbonatites display a mixed response(Sarkar and Bhattacharya 1992; Srivastava andTaylor 1996; Ray and Ramesh 1999; Viladkar andSchidlowski 2000). As a whole, the spread of d13Cdata of theDeccan carbonatites ranges from –8.62 to+2.1 (Bgure 12B). On the other hand, the d18O forthe carbonatites showawide range from+6 to+29.5


(Bgure 12B). In the d18O–d13C bivariate plot(Bgure 12A), the spread of d13C data of the AmbaDongar carbonatites is quite large with respect tothe otherDeccan carbonatites andmostly plot at thetransition between the primary carbonatites andsilica-assimilated carbonatites (Bgure 12A). On theother hand, the Sarnu–Dandali carbonatites mostlyplot in the altered or silicate assimilated Beld(Bgure 12A). Fractional crystallization of Cuid-richcarbonate melts is considered to be responsible forvariation in d18O and d13C values in the Deccan-carbonatites, particularly in the Sarnu–Dandali andtheAmbaDongar (Ray andRamesh 1999; Ray et al.2000). Various degrees of low-temperature Cuid-rock alteration, Cuid-related CO2 bearing mag-matic, hydrothermal or metasomatic secondaryalteration process, and sub-solidus groundwaterinteraction models are held responsible for thisscatter (Simonetti and Bell 1995; Ray et al. 2000).Imprints of recycled carbon for a restricted batch ofcarbonatitemelt at AmbaDongar were also invoked(Ray et al. 2000). Another study envisages thatenrichment in the carbonatitemantle source regionsoccurred as a consequence of metasomatism by Cu-ids derived from recycled oceanic crust throughsubduction (Ray and Ramesh 2006).The Sarnu–Dandali carbonatites alongwith those

from the Mer–Mundwara are closer to the primarycarbonatite Beld. The Mer–Mundwara carbonatitesalso share a similarity with the primary carbonatiteof Oldoinyo Lengai, Tanzania, Africa. Unlike thecarbonatites from the Amba Dongar, the sourcecarbon for the Sarnu–Dandali and Mer–Mundwaraappears to be of primordial nature and lessinvolvement of recycled crustal material in theirgenesis (Bgure 12A).

7. Geophysical studies and paleomagnetism

Sub-surface conBguration of the DLIP has beenexplained by numerous geophysical techniqueswith an emphasis on the structure, crustal inho-mogeneities, low-velocity zones and earthquakestudies (Mohan and Ravi Kumar 2004; Mad-husudhan Rao et al. 2013; Rajaram et al. 2016).Attempts have been made to understand the sub-trappean topography and structures utilizing var-ious techniques, such as deep-sounding proBles(Kaila et al. 1981) and also from the magnetotel-luric soundings (Gokarn et al. 1992). Receiverfunction analysis using teleseismic data near

Mumbai in the western Deccan volcanic provinceshows the presence of thick shield-like crust, whichappears to be inCuenced by the Deccan volcanism(Mohan and Ravi Kumar 2004). Gravity andmagnetic studies have found circular gravity highsand magnetic anomalies, which coincide with bothknown as well as concealed volcanic plugs associ-ated with Deccan volcanism (Chandrasekhar et al.2002). Magnetotelluric proBles have also broughtout that the traps lie directly over high resistivebasement with thin inter-trappean sediments(Harinarayana et al. 2007). A number of detailedgeophysical investigations contributed to under-standing the seismicity of the region, localizedcrust-mantle structure below the Kutch rift zoneand identiBcation of several subsurface lineaments(Mandal 2016; Rajaram et al. 2016). For example,Mandal (2011) delineated a thin lithosphere–asthenosphere boundary (LAB) beneath the Kutchsub-province based on teleseismic receiver functionstudies. His work also inferred a modern-dayaverage lithospheric thickness of 75.9 ± 5.9 kmfrom this domain. Interpretation of aeromagneticdata alongside gravity provided information ontrap thickness, depth to the bottom of the mag-netic crust and concealed structures (Rajaramet al. 2009).The study by Chandrasekhar et al. (2002)

highlighted the geomagnetic pole position close tothe early phase of Deccan eruption from thepaleomagnetic study of volcanic plugs and Dec-can Traps of Saurashtra. Palaeomagnetic studywith a particular emphasis on the Deccan episodelinked alkaline rocks of the Mundwara andSarnu–Dandali alkaline complexes of westernRajasthan corresponds paleopole position at 42�Nand 274�E with a palaeo-latitude of 24.5�S, sim-ilar to the Deccan pole position (Lakshmi Nar-asimhan et al. 2019). The characteristic remanentmagnetization explained that these alkalineintrusions were within the magnetic Chron C30N,coeval with the onset of massive Deccan volcan-ism (op. cit.). Combined gravity and magneticmodelling studies of Pavagadh and Phenai Mataigneous complexes (Singh et al. 2014) reveal thepresence of a dense maBc body along the strike ofNarmada Tapti Tectonic zone representingmagma accumulation in the deeper crust.Majority of Deccan alkaline rocks have intrudedin various rift and fault systems present in sub-surface to surface and some can be traced up tothe upper mantle.


8. Xenoliths

The evolution and nature of the continental litho-sphere is studied through xenoliths and xenocrystsentrained in the magmas. Various crustal, as well asmantle xenoliths and xenocrysts, have beendescribed fromDeccan alkaline rocks (Bgure 5A andB; Dessai et al. 2004). The Kutch is a significantdomain in terms of xenolith populations. Peridotitexenoliths in nephelinites from the Kutch were Brstreported by De (1981). A detailed account of spinellherzolites from the Kutch was provided byMukherjee and Biswas (1988). On the basis ofmineral chemistry data of xenoliths occurring in thealkali basalt plugs, both xenocrystic and phe-nocrystic nature of olivine was inferred from theKutch (Krishnamurthy et al. 1988, 1999). Highlyfractured and anomalous shaped olivine xenocrysts,occurring as disaggregate components of spinellherzolite- and dunite-nodules, have been alsoreported by Kshirsagar et al. (2011). The xenolithson which significant information is available arethose from the Bhujia, Dhrubia, Sayala Devi, Din-odhar Dongar, and Lodai in Kutch regions (Kar-malkar et al. 2009; Dessai et al. 2021). Thesefragments are hosted by the nephelinite/basaniteplugs and sill-like intrusives associated with thealkaline rocks. The mantle xenoliths are mostly ofspinel lherzolites, wehrlites and inBltrated by melts(Chattopadhyay et al. 2017). Micro-textural stud-ies of clinopyroxene, spinel and orthopyroxeneestablished a genetic link between spongy andcoronal texture. The Cower structure aggregation ofclinopyroxenes is interpreted to be a result ofmelt–xenocryst interactions (Chattopadhaya et al.2017).Four different varieties of crustal and mantle

xenoliths have been reported from the MurudJanjira lamprophyres, viz., pyroxene maBc gran-ulites, felsic granulites, pyroxenites/websteritesand spinel lherzolites (Dessai et al. 2008; Dessai andViegas 2010). The Amba Dongar carbonatites havealso entrained the xenocrysts of quartz, orthoclase,microcline and plagioclase representing the Baghbeds and basement metamorphics (Sukheswala andAvasia 1972). The Heran River and Dongargaonlamprophyres from Chhotaudepur sub-provincehave high concentration of xenoliths. Crustalxenoliths entrained in these lamprophyres aremostly feldspathic in nature (Bgure 5A). Petro-graphic studies have also attested the presence ofxenocryst plagioclase feldspar having corrodedmargins (Bgure 5D). Mantle xenoliths in the

Dongargaon lamprophyres are in 1–6 mm size rangeand in the form of rounded to sub-rounded shape(Bgure 5B). In majority of these xenoliths, presenceof olivine and at places kelyphitic reaction rim canbe appreciated (Bgure 5E). The presence of cryptic-carbonate metasomatism and involvement of fertilemantle was inferred on the basis of trace elementmineral data of ultramaBc xenoliths and glasscompositions (Karmalkar et al. 2000; Karmalkarand Sarma 2003). Rare mantle xenoliths providingdirect evidence of modal mantle metasomatismwere reported by Pandey et al. (2017c; Bgure 5E andF). Based on the detailed mineral chemistry ofmetasomatically grown minerals like phlogopiteand apatite (Bgure 5H), it has been inferred that theunderlying mantle in this domain is highly hetero-geneous and some of the readily fusible metasoma-tized portions have survived the wholesale meltingevent during the Deccan Trap event. Subsequentstudies have also demonstrated the petrographicevidence for the reactivation of the Narmada–Sonfault system based on xenoliths of earlier lampro-phyric event from the Dongargaon lamprophyre(Pandey et al. 2018b).Macrocrysts/megacrysts of olivine and pyroxenes

are also observed in many Deccan lamprophyres,which are different from the megacrysts from theDharwar craton (southern India) lamprophyres.Many of the macrocrysts in the Mundwara lampro-phyres have been found tobe antecrysts based on thedisequilibrium textures and elemental zoning,rather than early crystallized cognate minerals(Bgure 5C and G) (Sharma et al. 2021a).Alkaline rocks associated with the DLIP possibly

originated from complex magma chamber processesin a magmatic plumbing system (Sharma et al.2021b). It is important to note that megacrysts ofsouthern Indian lamprophyres are early crystallizedand of cognate origin, as they are devoid of the role ofsuch complex evolution system (Pandey et al.2017a, b). Corroded macrocrysts of olivine in theMundwara alkaline complex are characteristicallyenveloped by brown phlogopite, signifying theirparticipation in re-equilibrium chemical reaction(Bgure 5C; Sharma et al. 2021a). These macrocrystsare[2 mm in size and complexly zoned. P–T esti-mates on the chrome diopside megacrysts from the

Mundwara lamprophyre have yielded *100 km

depth of derivation (Sharma et al. 2018). P–T esti-

mates from different mantle xenoliths of the Kutch

and the Dongargaon area vary from 790 to 1060�Cand 1 to 2.7 GPa, respectively. P–T estimates of


various granulitic crustal xenoliths display a wide

temperature variation ranging from 750 to 870�Cand pressure ranging from 0.9 to 1.4 GPa. Pres-

sure–temperature variation of different representa-

tive xenoliths in concordance with the geothermalgradient has been plotted and presented in Bgure 13.Karmalkar et al. (2009) have shown the range ofequilibration temperatures between 500� and900�C,with a variation of pressure between 6 and 11 kbarand corresponding depth range of 20–35 km for thexenoliths from the Deccan.

9. Economic aspects

Carbonatites as a group are endowed with REE,Sr, Ba, Nb, Y, F and are host to many world-classdeposits of rare earth carbonates, pyrochlore,apatite, Cuorite and others (Simandl and Paradis2018). Though the carbonatites occur in consider-able proportion in Amba Dongar, only the Kam-thai carbonatites of the Sarnu–Dandali alkalinecomplex has been proved to be the world-classREE deposit in India (Bhushan and Kumar 2013;

Figure 5. (A) Crustal xenolith from the Heran River lamprophyres. Xenoliths are feldspathic in composition and sub-angular tosub-rounded in shape (HRL/2); (B) Rounded to sub-rounded mantle xenoliths from the Dongargaon lamprophyres (ND2/1);(C) Olivine macrocryst ([7 mm) mantled by phlogopite from the Phenai Mata lamprophyre (PML/SS/3); (D) Corrodedplagioclase xenocryst in the feldspathic groundmass from Jitnagar lamprophyre (in PPL; JTL/SS/2); (E) Olivine, clinopyroxeneand phlogopite bearing mantle xenolith from the Dongargaon lamprophyre. Kelyphytic reaction rim can be noticed in andaround xenoliths (in XPL; ND2/1); (F) BSE image of the same xenolith showing the presence of spinel at places. (G) BSE imageof olivine macrocryst from the Phenai Mata lamprophyre. Phlogopite mantling around olivine and the presence of spinels can benoticed (PML/SS/3); (H) X-ray elemental map showing metasomatically grown phlogopite (Ph) presence marked by potassiumcontent in the Dongargaon lamprophyre (ND2/1). Abbreviations are the same as in Bgure 3.


Bhushan 2015). The maximum value of totalLREE obtained in this deposit is 17.31%, with amean of 3.33% covering an area of 19,475 m2 and a4.91 mT of ores. Carbocernaite, britholite, allaniteand monazite contribute to the REE mineraliza-tion in the Sarnu–Dandali complex. In the Kam-thai area, the Malani rhyolites occur as basementrock towards the south, west and eastern sides,whereas the nepheline syenite intrudes the rhyolitein the north (Bhushan 2015). The Amba Dongarcarbonatite complex is credited as a world-classCuorite deposit (Subrahmaniam and Parimoo 1963;Palmer and William-Jones 1996; Viladkar et al.2005; Viladkar 2015) with 11.6 million tons of orewith an average of 30% CaF2. REE mineralizationof the complex is controlled by hydrothermal Cuidand sequestered mainly by Cuorite, apatite, Co-rencite, strontianite, etc. (Krishnamurthy et al.2000; Doroshkevich et al. 2009).In recent times, the mineral exploration

activity in the DLIP is being carried out toevaluate the REE and rare metals’ potentiality(Nagabhushnam et al. 2018; Singh 2020).Though continental Cood basalts hold immensepotential for hosting the copper-sulphide depos-its, lack of sulphide saturation can potentiallyinhibit this premise (Laxman and Kumar 2018).Anomalous contents of gold, platinum and pal-ladium from the picrite basalts in Saurashtrahave been reported (Banerjee et al. 2000).Shoshonitic rocks, which are rich in LILE andhalogen content, have the potential to be acarrier for gold, copper, as well as other basemetals (M€uller and Groves 1993). Many world-class gold deposits like Cripple Creek (USA)Emperor (Fiji), Ladolam and Porgera (PapuaNew Guinea) are associated with the shoshoni-tic, potassic, calc-alkaline to alkaline rocks(M€uller 2002 and reference therein). A widespectrum of shoshonitic alkaline rocks from theDeccan, thus holds a promise for future explo-ration programs. Incidences of Fe–Ni–platinumgroup of element (PGE) mineralization in thecumulate gabbros and intrusive dykes of lam-prophyre, picrobasalt, basalt of the Phenai MataIgneous complex have been reported (VijayaKumar and Randive 2021). On the other hand,Crocket and Paul (2008) have evaluated thePGE concentrations in the alkaline and maBcrocks of Kutch, but economically significantvalues have not been obtained. The Behradihorangeite is known to contain high diamondcontent (Newlay and Pashine 1993).

10. Discussion

10.1 Nature of source: Contrasting parentalmagmas

Initially, West (1958) proposed the tholeiitic nat-ure of parental magma as source for the genesis ofall different varieties of magmatic activities fromthe DLIP. Subba Rao (1971) suggested that thefollowing processes act either independently or incombination, for the genesis of Deccan alkalinerocks: (i) differentiation of primary alkali olivine-basalt magma; (ii) fractional crystallization of analkaline magma derived under special tectonicconditions; (iii) dissociation of feldspars in thepresence of fugitive constituents; (iv) substitutionof alkalis by the volatiles; and (v) desilication ofthe magma by limestone syntexis. Subsequently,the genesis of the alkaline rocks from the Deccanwas addressed by several workers; e.g., Bose(1973, 1980), Udas (1971), Sukheswala and Avasia(1972), Paul et al. (1977), Chakraborti (1979,1984), Viladkar (1998), Sukheswala (1982), Sethna(1989), Gwalani et al. (1993), Rock et al. (1994),Srivastava (1994, 1997), Rathore et al. (1996),Khandelwal et al. (1997), Sheth and Chan-drasekharam (1997) and Ray and Ramesh (1999).Although these proposals are not sufBcient toaccount for the genesis of vast variety of the Dec-can alkaline rocks, they nevertheless laid substan-tial foundation for in-depth studies. The plumeactivity in combination with crustal rifts was heldresponsible for the alkaline-carbonatite magma-tism (Bose 1980). Based on the 3He/4He ratios ofmineral separates and Rb–Sr in whole-rock samplesfrom alkaline–maBc complexes, the role of a lowermantle-derived plume in the genesis of these alka-line complexes was inferred (Basu et al. 1993).Subsequently, alkaline magmas of the DLIP wereproposed to be the manifestations of an earlysmall-scale melting of plume-modiBed lithospherewith ascendant lithospheric geochemical inputs(Simonetti et al. 1998).A variety of parental magmas have been inferred

for different Deccan alkaline rocks. A commonsource for the contemporaneous potassic alkalineas well as tholeiitic magmas from the Rajpipla areawas suggested (Mahoney et al. 1985). Phonolites ofthe Panwad–Kawant area are considered to havebeen originated from nephelinite magma owingto their higher percentage of Ba and Sr (Viladkarand Avasia 1994). A common primitive parentalmagma was inferred for the picrobasalt as well as


the lamprophyres from the Panwad and theChhaktalao lamprophyres on the basis of Sr iso-topic studies (Randive et al. 2005). Based on meltinclusion investigations of olivine and pyroxenephenocrysts from the Chhaktalao lamprophyres,Hari et al. (2000) inferred their origin from a lowerdepth, possibly from the spinel lherzolite source.Chondrite-normalized REE patterns reveal theabsence of Eu anomaly in samples of the Deccanalkaline rocks, negate the role of plagioclase dur-ing the fractionation of their original melt(Bgures 6–9). HREE concentrations as well as REEpatterns highlight a garnet or spinel source for themajority of the Deccan alkaline rocks.CContrasting magmatic sources ranging from

the OIB to MORB mantle have also beenobtained from the nitrogen and noble gas isotopic

investigations from alkali basalts of the Kutch(Mohapatra and Murty 2002). Similarly, mixedand intermediate OIB and E-MORB geochemicalsignatures were also noticed from alkali olivineand nepheline bearing gabbros from the PatchamIsland (Maitra 2003). In the alkaline complexes,such as the Amba Dongar and the Sarnu–Dan-dali, carbonatites are associated with other sili-cate rocks. These carbonatites are petrologicallyindistinct and derived from dissimilar mantlesources. The Amba Dongar carbonatites areconsidered to be the products of fractional crys-tallization of carbonate melt, whereas alkali sili-cate rocks were shown to be derived by the sameprocess, but from a parental silicate melt (Rayand Shukla 2004). On the contrary, the role ofijolite magma by partial melting of a mantle

Figure 6. Bi-variant plots for the Deccan alkali basalts: (A)MgO (wt%) vs. Al2O3 (wt%), (B) MgO (wt%) vs. Na2O (wt%), (C)MgO (wt%) vs. Ni (ppm), (D) MgO (wt%) vs. Cr (ppm), (E) Primitive mantle normalized multi-element spidergram for theDeccan alkali basalts. Prominent anomalies of Ba, Ta and Pb are evident in many of the samples (Sun and McDonough 1989).(F) Chondrite normalized REE pattern depicting a moderately high LREE/HREE fractionation (data sources: Talusani 2001;Paul et al. 2008; Sheth et al. 2013 and references therein).


source was invoked by Shastry and Kumar(1996) for the Sarnu–Dandali alkaline complex,whereas primary magmatic melt for the genera-tion of carbonatites was proposed by Bhushanand Kumar (2013) and Bhushan (2015) for thesame complex. Alkali basalt as the parentalmagma source was inferred for the lamprophyresof the Mundwara alkaline complex (Sharma et al.2021a). Varieties of lherzolite (garnet-bearing, Ti-rich, etc.) containing minor proportions of vola-tiles have been favoured for the genesis of themajority of alkaline rocks, whereas pyroxenite/hornblendite was also speculated as a source rockfor a few of the alkaline rocks from the Bhuj area

(Chatterjee 2021). The role of mixed mantlesources is reCected in trace element ratio plots,which indicate enriched as well as HIMU typeOIB mantle source for the genesis of the Deccanalkaline rocks (Bgure 14B and C). Similar HIMU-type depleted mantle source has been inferred forChhotaudepur sub-province and they are thoughtto have been generated from a previously car-bonatite metasomatized mantle (Pandey et al.2019). Radiogenic isotopic plots of the Deccanalkaline rocks and stable isotope studies of car-bonatites also support the participation of con-trasting parental magma sources (Bgures 11and 12).

Figure 7. Bi-variant plot of the Deccan lamprophyres (A) MgO (wt%) vs. Al2O3 (wt%), (B) MgO (wt%) vs. Na2O (wt%), (C)MgO (wt%) vs. Cr (ppm), (D) MgO (wt%) vs. Ni (ppm) and (E) Primitive mantle normalized multi-element spidergram for theDeccan lamprophyres. Conspicuous anomalies of Rb, Pb, P and Ti are noticeable in many of the samples (Sun and McDonough1989). (F) Chondrite normalized REE pattern depicting a strong LREE/HREE fractionation with higher content of HREE (datasources: Pandey et al. 2019; Sharma et al. 2021a; Dongre et al. 2021 and references therein).


10.2 Contaminated mantle source vs. crustalcontamination

Diverse alkaline rocks originate from the meltingof sources involving either the lithosphere orasthenosphere or both, and reCect their sourcesignatures in their geochemistry. Ascending mag-matic pulses also significantly incorporate materialfrom the varied mantle, as well as crustal domainswhich can inCuence the overall geochemical signa-tures along with the petrogenetic interpretations.The presence of such crustal, as well as mantlecomponents, has been discussed in the precedingsection on xenoliths. Numerous studies have alsoevaluated the nature and extent of such

contamination on the primary melt composition ofthe alkaline rocks from the Deccan.Helium isotopic signatures of olivine and

pyroxene from the Phenai Mata igneous complexhave revealed the role of crustal contamination bylate pulses of the Reunion mantle plume (Basuet al. 1993). Calcium isotopic studies on thealkaline rocks of the Phenai Mata alkaline rocksalong with the Amba Dongar carbonatites haverevealed the involvement of 20% recycled car-bonate-bearing the mantle plume source (Banerjeeand Chakrabarti 2019). Recent studies of noblegas, systematic on the carbonatites from the AmbaDongar and the Siriwasan areas have negated themodels which involved the contribution of a

Figure 8. Bivariant plots of the Deccan carbonatites (A) CaO (wt%) vs. SiO2 (wt%), (B) CaO (wt%) vs. K2O (wt%), (C) Sr(ppm) vs. Ba (ppm), (D) Sr (ppm) vs. Ce (ppm) and (E) Primitive mantle normalized multi-element spidergram for the Deccancarbonatites. Conspicuous anomalies of Nb, Pb, Zr, Ti and Y are noticeable in many of the samples (Sun and McDonough 1989).(F) Chondrite normalized REE pattern depicting a strong LREE/HREE fractionation with lesser HREE content (data sources:Srivastava 1997; Viladkar and Gittins 2016; Chandra et al. 2017 and references therein).


lithospheric mantle source enriched in recycledcomponents (Hopp and Viladkar 2018). Clumpedisotopes were used to assess the diagenetic alter-ations in the post-magmatic evolution of the car-bonatites from the Siriwasan and the AmbaDongar (Fosu et al. 2020). Although variousgranulitic/crustal xenoliths have been reportedfrom the Murud–Janjira alkaline rocks, reverseAFC (assimilation and fractional crystallization)modelling of basanites negated any serious inCu-ence of their contamination with the primaryalkaline magma (Chatterjee 2021). Contaminationof the source itself can be accounted only bysubduction and associated Cuid activities. Con-crete evidence in favour of ancient subductionevents from the Deccan alkaline rocks is scarce.Pande et al. (2017) and Mohapatra and Murty(2002) provided some geochemical evidence forrecycled crustal components and involvement ofancient subduction events. Subduction is animportant process that not only deBnes the crustaland mantle evolution but also contributes to theextensive mantle heterogeneity. Therefore, somedegree of recycled crustal component in the globalmantle is ubiquitous (Hutchison et al. 2019). The

arguments in favour of ancient subduction fromthe Deccan alkaline rocks come from very limiteddomains, which can be also explained alternativelyby other processes. For example, Pande et al.(2017) have invoked subduction based on hydrousparental magma, which can also be accountedfrom metasomatism or enrichment by an earlyincubating mantle plume. Similarly, a very limiteddataset has been utilized by Mohapatra and Murty(2002) and multiproxy geochemical, as well asisotopic evidences on more samples from differentdomains, are necessary to ascertain the same.Besides the studies undertaken by previous

workers, we have also attempted to evaluate therole of crustal contamination in the Deccan alka-line rocks. Except for the miscellaneous alkalinerocks, other groups clearly demonstrate a goodcorrelation of MgO content with regard to theircompatible element concentrations (Bgures 6C andD, 7C and D). Higher abundance of transitionelements (average values of Ni: 125, 227, 14, 16 and87 ppm; Cr: 248, 391, 10, 65 and 365 ppm in alkalibasalt, lamprophyres, carbonatites, nephelinesyenites and miscellaneous alkaline rocks, respec-tively) in all the alkaline rocks also support their

Figure 9. Bivariant plots of the Deccan nepheline syenites (A) MgO (wt%) vs. Al2O3 (wt%), (B) MgO (wt%) vs. CaO (wt%),(C) MgO (wt%) vs. TiO2 (wt%) and (D) MgO (wt%) vs. P2O5 (wt%) (data sources: Bose 1971, 1973; Gwalani et al. 1993 andreferences therein).


derivation from the mantle or its constituent pri-mary components. Concentration of crustallyinCuenced trace elements Sr (average values inppm: 470, 1088, 5485, 1202, and 1081 respectivelyin aforesaid rock types) and likewise, that of Zr(213, 319, 500, 26 and 406 ppm) and Nb (27, 127,581, 123 and 118 ppm) are much higher in com-parison to their contents in the continental crust(Sr = 360 ppm; Zr = 130 ppm and Nb = 17 ppm;Rudnick and Gao 2003). This clearly demonstratesthat a majority of Deccan alkaline rocks were nei-ther derived from the crust nor they contain any

significant component of it. Conspicuous positivePb spike in some of the alkali basalts and miscel-laneous alkaline rocks may suggest some crustalinputs. Lu/Yb and Th/La ratios are also critical inassessing the inCuence of crustal inputs and areconsidered as indices of crustal contamination.Alkali basalts exhibit distinctive low ratios of Lu/Yb (0.10–0.17) and Th/La (0.10–0.31) indicatingnegligible crustal inputs, whereas the miscellaneousalkaline rocks have moderately low values(0.08–0.46 and 0.03–0.81) suggesting limited crus-tal incorporation during their ascent.

Figure 10. Bivariant plots of miscellaneous alkaline rocks from the Deccan: (A) SiO2 (wt%) vs. MgO (wt%), (B) SiO2 (wt%) vs.CaO (wt%), (C) SiO2 (wt%) vs. Ni (ppm), (D) SiO2 (wt%) vs. Rb (ppm), and (E) Primitive mantle normalized multi-elementspidergram (normalized after Sun and McDonough 1989) for miscellaneous alkaline rocks. Prominent anomalies of Ce, Pb, P andTi are noticeable in many of the samples. (F) Chondrite normalized REE pattern depicting LREE/HREE fractionation withmoderate content of HREE (data sources: Viladkar and Avasia 1994; Cucciniello et al. 2015; Vijayan et al. 2016 and referencestherein).


10.3 Evolution of the source and mantlemetasomatism

Enrichment of incompatible elements in the sub-continental lithospheric mantle (SCLM) in thealkaline rocks is accounted by two broad metaso-matic processes, viz., subduction-generated Cuidsand mantle plume and associated processes. As faras the role of any ancient subduction in the genesisof the Deccan alkaline rocks is concerned, concreteevidences are lacking but extensive studies ofxenoliths have clearly demonstrated the geochem-ical as well as petrographic evidences of mantlemetasomatism. Substantial enrichment of Th, Nband LREE in the spinel lherzolite xenoliths from

alkaline rocks of Kutch area evidence crypticmetasomatism beneath the Deccan LIP (Kar-malkar and Rege 2002). Direct evidence for modalmetasomatism from the formation of metasomati-cally introduced minerals like phlogopite and apa-tite (O’Reilly and GriDn 2000) in the xenolithsfrom Dongargaon lamprophyres have also beendocumented (Pandey et al. 2017c). The presence ofthese two phases in the source regions was alsosuspected on the basis of geochemical studies ofalkaline rocks from the Kutch area (Karmalkaret al. 2005). Whereas low degree partial melting ofa metasomatized mantle is known to generatealkaline rocks, the role of low degrees of meltingand re-solidiBcation as a causative factor of meta-somatism was also brought out from the TethyanUML (Kerr et al. 2010). Interestingly, the TethyanUML (69.7 ± 0.2 Ma) represents one of the earliestmelts of the R�eunion plume and the metasomatismcommenced at the leading edge of early plumehead. McKenzie (1989) has proposed that in duecourse of plume–lithosphere interactions, the Brstmelts were produced from the stretching of sufB-ciently thick metasomatic layer and not from thethermal boundary layer. Many of the Deccan-syn-chronous and post-Deccan alkaline rocks alsodepict their origin from metasomatized mantle(Pandey et al. 2019). Extensive signatures ofcryptic metasomatism from xenoliths of alkalinerocks in Kutch and adjoining areas have beenattributed to the imprints of the rising Reunionplume (Karmalkar et al. 2000).Sen et al. (2009) have suggested the involvement

of a carbonatitic melt as an active agent of car-bonatite metasomatism which was generated dur-ing decompression melting of asthenosphere. Theearliest carbonatitic metasomatic melts were sug-gested to have generated the CO2-rich lherzolitemantle beneath Kutch area. The presence of an oldand enriched lithospheric mantle beneath Deccanwas also inferred on the basis of the presence ofmelilite and potassic richterite in sodic alkalinemagmas of the Murud Janjira area (Melluso et al.2021). Considering the subduction as a metasom-atizing factor, it has been already pointed out(above) that involvement of some degrees of enri-ched mantle, possibly by ancient subductingmaterial, is omnipresent in many of the alkalinerocks, especially the carbonatites (Ray andRamesh 2006). However, there exists an uncer-tainty with regard to the time frame of metaso-matic enrichment: Whether the metasomatism waspreceded by eruption of the Deccan Traps or an old

Figure 11. (A) 87Sr/86Sri vs.143Nd/144Ndi isotopic variation

plot for the Deccan alkali basalts; (B) 87Sr/86Sri vs. eNdiisotopic variation plot displaying a prominent trend parallel tothe mantle array for the Deccan lamprophyres and miscella-neous alkaline rocks. An enriched-mantle source for thecarbonatites and majority of lamprophyres can be noticed.Different geochemical reservoirs are abbreviated as EM-I(enriched mantle I), DM (depleted mantle), MORB (mid-ocean ridge basalt), OIB (ocean-island basalt), PREMA(prevalent mantle), HIMU (high l or U/Pb) and BSE (bulk-silicate earth) are from Zindler and Hart (1986) and Hofmann(2007) (data sources: Kerr et al. 2010; Pandey et al. 2019;Banerjee and Chakrabarti 2019 and references therein).


and enriched lithospheric mantle already existed?On the basis of the isotopic studies of contempo-raneous tholeiitic and K-rich alkaline rocks, it wassuggested that mantle enrichment events precededthe magmatism (Mahoney et al. 1985). Therefore,the information pertaining to the age, evolution aswell as the causative factor of metasomatizedmantle source regions in the Deccan remains anopen question.

10.4 Complex magma chamber systems

Based on the seismological studies, the presence ofmagmatic underplating beneath the central andwestern portions of Narmada Son valley was sug-gested (Reddy et al. 1999). Other geophysicalmethods have also evinced the existence of mag-matic ponding and accumulations at crustal andsub-crustal levels from the geophysical study of thealkaline igneous complexes of the DLIP (Singhet al. 2014). Of late, it has been established thatsuch magmatic accumulations at different sub-crustal levels are actually the manifestation of a

magmatic plumbing system which are integralconstituents of LIPs and play a pivotal role in thegenesis of maBc and ultramaBc dyke system (Ernst2014). ‘‘A magma plumbing system, therefore,consists of interconnected magma conduits andreservoirs, which store magma as it evolves into acrystal mush, ultimately fed from a zone of partialmelting’’ (Magee et al. 2018). Apart from the geo-physical evidence, direct petrological evidencessuch as complex zoning, sieve textures andnumerous cumulous intergrowth textures also evi-dence the existence of a complex magma chambersystem (Rock et al. 1994; Pandey et al. 2019).Importantly, Rock et al. (1994) have discussed thecomplexly zoned clinopyroxene phenocrysts fromdifferent alkaline rocks and alkaline complexesfrom the Deccan LIP and invoked the role of thefollowing processes, viz., xenoliths assimilations,oxidation, kinetic eAects, co-precipitation of otherphases, polybaric differentiation and magma mix-ing either solitary or in combination. Amongst allthe processes, magma mixing is a prominent can-didate to account for the various disequilibrium

Figure 12. Stable isotope of the Deccan carbonatites: (A) Bivariant plot of d18O (SMOW) vs. d13C (PDB) and (B) d13C andd18O. (Data sources: Sarkar and Bhattacharya 1992; Srivastava and Taylor 1996; Ray and Ramesh 1999 and references therein.)


textures. Complexly zoned xenoliths of MurudJanjira area were also explained by mixing ofprimitive and fractionated pulses at crustal levelalong with polybaric crystallization during ascent(Dessai et al. 1990).Sharma et al. (2021a, b) have not only identiBed

the disequilibrium textural clues from the Mund-wara and the Phenai Mata igneous complexes, butalso highlighted the role and imprints of complexmagma chamber systems in the LIPs on the genesisof the alkaline rocks. Antecryst bearing Mer lam-prophyres from the Mundwara alkaline complexare shown to have been formed from early segre-gation of Cr-pyroxene, Cr-spinel and olivine fromthe magma in crustal magma chambers. Theremaining magma after segregation has undergoneprogressive differentiation and produced an alto-gether different magma source which has generatedthe Musala lamprophyres within the same alkalinecomplex. Similarly, on the basis of detailed mineralchemistry of olivine antecrysts and complexlyzoned minerals, two distinct magmas – generatedthrough evolution in mushy shallow level crustalmagmatic chambers – have been identiBed for thetwo camptonite lamprophyres from the PhenaiMata igneous complex. Recurrent magmatic eventsin response to reactivation of rifts have also been

recently established from the Deccan alkalinecomplexes. Polychronous magmatic ages from theMundwara alkaline complex, lamprophyric xeno-liths in Dongargaon lamprophyre and contrastingvarieties of carbonatite in different pulses from theSarnu–Dandali area demonstrate the presence ofputative pulses of alkaline magmatism (Pandeet al. 2017; Sheth et al. 2017; Pandey et al. 2018b).In case of the Mundwara and the Sarnu–Dandalicomplexes, the recurrent pulses have an age gap of15–45 My and for the Dongargaon, time con-straints are yet to be established. In light of theabove, it is imperative to re-investigate the petro-genetic characters associated with the recurrentmagmatic events. Many of the obtained poly-chronous ages are from mineral separates and inview of the reported occurrences of antecrysts orrecycled crystal cargos from the Mundwara as wellas the Phenai Mata igneous complexes, these agesshould be re-assessed. Contrasting mantle sourceinferred for the olivine phenocrysts and ultramaBcxenoliths from alkaline rocks of same domain in theDLIP (Mohapatra and Murty 2002) assumesimportance in this context.

10.5 Geodynamic implications

Interaction and inCuence of the mantle plume onthe SCLM is a complex process and is dependenton many factors such as: (i) motion of the plate and(ii) lithospheric heterogeneities which inCuencesthe lithospheric dynamics. From the earlier dis-cussions of xenoliths, it is quite evident thatlithospheric mantle beneath the DLIP is consider-ably heterogeneous in nature and so is its litho-spheric thickness. Sharma et al. (2018) haveinferred a minimum thickness of *100 km for thepre-Deccan lithosphere from P–T studies of chromediopside megacrysts from the Mundwara alkalinecomplex. Earlier isotopic studies have also sug-gested an upper mantle origin for Mundwaraalkaline complex and extreme differentiation hadgenerated the whole suite of the different rocktypes (Rathore et al. 1996). Similar litho-sphere–asthenosphere boundary (LAB) depthshave been obtained from the studies of REEinverse modelling in synchronous to post-Deccanlamprophyres from Chhotaudepur sub-province(Pandey et al. 2019). The Mount Girnar lampro-phyres which have been dated as 65 ± 0.9 My, areshown to be derived from spinel-garnet transitionzone and a minimum LAB thickness of 85 km from

Figure 13. P–T estimates from the conventional geothermo-barometry of Kutch xenoliths. Mineral chemistry of orthopy-roxene, clinopyroxene, spinel and olivine has been used in thisestimation. The plagioclase–spinel and spinel–garnet phasetransitions, anhydrous and hydrous peridotite solidus andgeotherms (40, 60 and 80 mW/m2) are after Jagoutz and Behn(2013) and references therein. Data sources: Kutch (Chat-topadhaya et al. 2022 and references therein); granulite/crustal xenoliths (Dessai and Vaselli 1999; Dessai et al. 2004and references therein); Dongargaon xenoliths (Pandey et al.2017a, b, c) and Mundwara Cr-diopside (Sharma et al. 2018).


western domain (Sahoo et al. 2020). On the otherhand, the presence of diamonds in Behradih oran-geites constrains the LAB thickness at 65 My to be[140 km beneath the Bastar craton (Lehmannet al. 2010). Various alkaline rocks can originatefrom varying depths and the generated melts canhave different geochemical signatures depending onthe involved degrees of melting and interaction ofthe melt fractions with the continental lithosphereduring their ascent. Even within the same alkalinecomplex, involvement of different depth sourceshas been discussed. For example, on the basis ofd44/40Ca and K/Rb values, Banerjee and Chakra-barti (2019) inferred the derivation of the AmbaDongar carbonatites from the deeper parts of theReunion mantle plume in comparison to the asso-ciated alkaline rocks.From the isotopic evidence from the carbonatites

and associated alkaline rocks, Simonetti et al.(1998) have concluded the mixing and involvementof three distinct mantle components viz., theReunion plume, the continental lithosphere andthe asthenosphere. Carbonatites and associatedalkaline rocks can also attain an isotopic variabilityfrom the parental carbonate-silicate magma itselfwithout the need to invoke heterogeneous source(Chandra et al. 2019). Although the whole-rockgeochemistry as well as the isotopic systematics ofa majority of the Deccan-related alkaline rocks,clearly demonstrates a dominating plume source,Pande et al. (2017) postulated the role of eitherJurassic subduction under Gondwanaland or Pre-cambrian subduction events from their studies onthe Mundwara alkaline complex. Unusual isotopicsignature of recycled material was also noticed inthe inclusions of alkali basalt of the Kutch and theReunion Island and was related to global enrich-ment of LAB by the Cuids derived from subductedslab (Mohapatra and Murty 2002). Interestingly,majority of discrete lamprophyre dykes of Chho-taudepur sub-province are shoshonitic in naturewhile some belong to the calc-alkaline variety,which are generally known to be associated with asubduction-related tectonic setting. Some of theselamprophyres exhibit a geochemical or isotopicsignature of subduction modiBed lithosphere(Pandey et al. 2019). Pyroxene mineral chemistrydata of the Deccan alkaline rocks indicate mixedoxygen fugacity values on Al(VI) + 2Ti + Cr vs. Na+ Al(IV) plot (Bgure 4B), indicating the presence ofboth orogenic as well as anorogenic signatures.Similarly, mixed orogenic and anorogenic expres-sions have also been obtained on Ca (a.p.f.u.) vs. Ti

(a.p.f.u.) pyroxene plot (not shown). To ascertainthe exact geodynamic setting of the alkaline rocks,we have implied more robust geochemical traceelement ratio plots, which distinctly exhibit ananorogenic geodynamic setting and plume sourcefor all the alkaline rocks from the Deccan(Bgure 14). However, involvement of mixed anddifferent mantle sources is also apparent in thesegeochemical discrimination diagrams.

10.6 Future directions of research

Major gaps are listed here to serve as the futuredirections of research:

(a) Many of the alkaline rocks of the DLIP areassociated with other alkaline as well as non-alkaline igneous rocks. The genetic relation-ships of these rocks are still not wellestablished.

(b) There is a substantial paucity of good qualitydata – especially the mineral chemistry andisotope geochemistry – for the alkaline rockswith the exception of lamprophyres and fewalkali basalts. Trace element mineral dataare almost entirely lacking and futureresearch should address this aspect bydeploying state-of-the-art instruments suchas LA-ICPMS. Mineral-based trace elementand radiogenic data also act as tool forfuture REE exploration programs and alsohelp improvising the existing models withbetter resolution. Stable isotopic studies arelimited to carbonatites only. Stable isotopedata from mineral separates can enhance ourcontrol in the characterization of source,deciphering inputs from crustal sources/crustal contamination and in constrainingthe exact nature of evolution of the alkalinerocks and complexes. Nepheline syenites arethe least explored rocks, even though theyhave the potential to host REE-bearingmineral phases.

(c) The presence of complex magma chamberplumbing system has been recently broughtout and their significance in the genesis ofalkaline rocks and igneous complex is not wellunderstood. Many of the Deccan alkaline rocksassociated with the igneous complex systemsstill remains unexplored on these lines. Eventhe discrete alkaline rock occurrences seem toposses the signature texture of plumbing sys-tem. Re-evaluation of the alkaline rock


distribution in light of the evolving plumbingsystem models needs to be taken up.

(d) There is a scarcity of high-quality radiomet-ric ages for the Deccan alkaline rocks. Someof the pre-Deccan ages range from 102 to 110My (Mundwara nepheline syenite; Pandeet al. 2017) to 124 My (Palanpur UML;Karmalkar et al. 2014). Polychronous mag-matic activities in the form of putative pulseshave also been observed from the Mundwara,the Sarnu–Dandali and the Dongargaondomains. Precise age determinations play asignificant role in constraining the petroge-netic models.

(e) Alkaline rocks are known to originate fromdeeper depths and are relatively least con-taminated than the Cood basalts. Re-assess-ment of the source contamination by anearlier ancient subduction event(s) by multi-proxy radiogenic as well as non-traditionalstable isotopic studies of alkaline rocks and

their mineral separates can shed further lighton these aspects.

(f) Both cryptic as well as modal metasomatismhas been reported from various xenolithsentrained in the alkaline rocks. Carbonatiticmetasomatized enriched lithosphere mantle hasbeen invoked to account for the generation ofvarious alkaline rocks from the DLIP. But theexact nature, timing and causative factors havenot been well established. Precise dating of themetasomatic minerals in the mantle xenolithscan clarify this aspect.

11. Conclusions

Salient aspects of this review on the alkaline rocksfrom the DLIP are summarized as under:

• Seven alkaline rock provinces have been delin-eated based on the spatial and temporal rela-tionships. Two modes of occurrences of the

Figure 14. Tectonic discrimination diagrams: (A) Nb9100/Zr vs. Th9100/Zr (Wilson and Biachini 1999) demonstrating ananorogenic setting for all the Deccan alkaline rocks; (B) Zr/Y vs. Nb/Y plot depicting a plume source for all the alkaline rocks.Majority of the alkali basalt and lamprophyres share the similar Beld as the OIB. Mixed source is displayed by some alkalinerocks which overlap with the EM1, EM2 and HIMU sources. Various Belds have been marked after Sun and McDonough (1989).(C) Nb/Th vs. Zr/Nb plot (Condie 2003) showing lack of significant inCuence of subduction on these rocks. Indications of mixedsources can be noticed for alkali basalt and miscellaneous alkaline rocks OIB, N-MORB and ARC Belds have been marked afterSun and McDonough (1989). (D) TiO2/Al2O3 vs. Zr/Al2O3 plot (M€uller and Groves 2000) indicates that majority of alkalinerocks fall within plate Beld. (WIP: Within plate; CAP; continental arc; PAP; Post-collisional arc; IOP: Initial arc; LOP: Lateoceanic arc; Data sources remain the same as in Bgures 6–10).


alkaline rocks, viz., (i) as alkaline complexes(e.g., Amba Dongar, Phenai Mata) or (ii) asdiscrete bodies (Palanpur, Behradih, etc.) arehighlighted. Temporally, all of them representthe earliest-, synchronous, post-Deccan and evenlast phase of igneous activity in the DLIP.

• The maximum mineralogical diversity is dis-played by the carbonatites, lamprophyres andmiscellaneous alkaline rocks in that order. Abroad spectrum of textures is displayed by theserocks. Fine to coarse-grained, calcite-dolomitecarbonatites are present including various vari-eties of sovite, ferrocarbonatite, ankerite andalvikites. Similarly, the diversity in lamprophyresis also represented by the alkaline, calc-alkalineand ultramaBc-varieties.

• Olivines present in the lamprophyres and alkalibasalts belong to both primary as well as mantle-derived (xenocryst) categories, whereas those inthe miscellaneous alkaline rocks are strictly pri-mary innature.Pyroxenesdisplayoverlappingandmixed signature of oxygen fugacity and saturationof T-structural site for a majority of data, with theexception of a few alkali basalts and miscellaneousalkaline rocks which are undersaturated in nature.Spinels display titano-magnetite trend (magmatictrend 2) and follow Fe–Ti fractionation path.

• Major, trace and REE geochemical data display aconsiderable diversity of varying compositions fordifferent alkaline rocks. Highest alkali variationsare exhibited by the lamprophyres, whereas theleast by alkali basalts. Shoshonitic aDnities arealso displayed by some lamprophyres and nephe-line syenites. With the exception of the miscella-neous alkaline rocks, others lack any substantialevidence of crustal contamination. The role ofpyroxene and olivine fractionation is prevalent inalkali basalts and lamprophyres, whereas plagio-clase fractionation is present only in miscella-neous alkaline rocks. Significant LREEenrichment over HREE and moderate to strongREE fractionation are hallmarks of the Deccanalkaline rocks. Primitive mantle normalized mul-ti-element spidergrams depict apatite, titaniteand zirconium-bearing phases to constitute theresidual source mineralogy.

• Radiogenic isotopes reveal a dominant OIBsignature from the alkaline rocks of differentsub-provinces with the exception of MORB-likesignature displayed by few alkali basalts.Depleted to enriched mantle sources wereinvolved in the generation of the lamprophyres,

whereas distinct enriched mantle sources areshown by carbonatites. Depleted mantle modelages (TDM) for a majority of alkaline rocks rangefrom Meso- to Neoproterozoic. The Bastar oran-geites are isotopically indistinguishable from theDeccan lamprophyres. Stable isotopic studies ofAmba Dongar carbonatites reveal transitionalnature with enrichment signatures imparted bythe recycled oceanic crust. Stable isotope geo-chemistry also reveals the eAect of fractionalcrystallization of Cuid-rich carbonate melts andprimordial nature of the Sarnu–Dandali and theMer–Mundwara carbonatites.

• Geophysical investigations in and around alka-line rocks and complexes highlight the presenceof magma accumulations, lineaments and thickshield-like crust. A vast variety of entrainedmantle and crustal xenoliths/xenocrysts includedunite, spinel lherzolite, websterite, pyroxenite,granulite, feldspar, olivine, quartz, orthoclase,microcline, etc. Xenoliths preserve imprints ofcryptic as well modal mantle metasomatism.P–T estimates have depicted a fairly wide rangeof equilibrium temperature of 500–1200�C andpressure 0.9–4.4 GPa (20–100 km correspondingdepths). LREE and Cuorite from the carbon-atites and diamonds from the Bastar orangeitesconstitute notable economic resources.

• Small degree of melting of contrasting mantlesources under the inCuence of mantle plume hasbeen inferred in the genesis of vast variety ofalkaline rocks. Carbonatite metasomatism is wide-spread and existence of complex magma chamberprocesses and long-lived plumbing systems playedan important role in the magmatic evolution.

• A varying lithospheric thickness of 85 to [140km from the western to central sub-provincereCects the interaction of the mantle plume withthe heterogeneous lithosphere.

Acknowledgements

It is a great pleasure to contribute to this specialissue in memory of Prof Gautam Sen with whom oneof us (NVCR) was closely associated for severalyears. We tried to accommodate as many relevantreferences as possible in this review, and anyexclusion (given the magnitude of the literatureavailable) is inadvertent, unintentional and con-strained by the page limit and we solicit an under-standing and pardon by the aggrieved author, if


any. We are thankful to the Head, Department ofGeology, BHU, Varanasi for his support. NVCRthanks DST-SERB, New Delhi for granting researchprojects on the alkaline rocks (No. SR/S4/ES-554/2011 and IR/S4/ESF-18/2011 dated 12.11.2013)and Institute of Eminence Project (IoE). IoE andUGC are also acknowledged by RP for seed grantand start-up grant projects. MKS thanks CSIR-NewDelhi for awarding an SRF (NET). CSIR is alsoacknowledged for awarding JRF to DT. We alsothank Samrendra Sahoo, B Lehmann, AbhinaySharma and Prashant Dhote for their help andassistance during our study on the Deccan-relatedalkaline rocks. Insightful reviews by four anonymousjournal reviewers and editorial comments by Jyoti-sankar Ray are gratefully acknowledged.

Author statement

Rohit Pandey: Data compilation, interpretationand manuscript drafting; N V Chalapathi Rao:Conceptualization, overall supervision, and Bnal-ization of the manuscript; Mahendra K Singh: Datacompilation, graphic preparation and manuscriptdrafting; Debojit Talukdar: Compilation, inter-pretation and manuscript drafting.

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Corresponding editor: : JYOTISANKAR RAY


Alkaline rocks from the Deccan Large Igneous Province

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