PETROLOGY AND STRUCTURE OF THE GRANITOIDS WITH IN NELLORE SCHIST BELT, RAPUR AREA, NELLORE DISTRICT,ANDHRA PRADESH.

Syed HamimJeelani1,Sumit K. Mitra2and M.Lingadevaru3

1. Dept. of Geology, School of Earth Sciences, Central University of Karnataka, Aland Road, Kadaganchi-585367, Karnataka, India.

2. Geological Survey of India, Southern region, Hyderabad

ABSTRACT

Rapur area forms southern part of the Nellore schist belt of Andhra Pradesh. The area under studylocated between latitude 14o 15’ 00” and 14o 11’ 30” and longitude 79o 32’ 55” and 79o 37’ 30”and coversan area about 30 Km2. The dominant rock types exposed in the area are Metabasalt, Quartz Mica Schist,Quartzite belonging to Nellore Schist Belt (NSB), undifferentiated granitoids of PGC II and youngeracidic and basic intrusives. These granitoids are sandwiched between NSB and Nallamalai Fold Belt (NFB)rocks, which are exposed along eastern and western side of the mapped area. The unclassified granitoids of the area has been mapped and classified on the basis of their modalpercentage, mineralogical and geochemical characteristics, which are grouped into two suites: 1.Tonalite-Granodiorite-Monzonite suites (TGM) 2. Monzonite-Syenite suite (MS).Rapur area predominated by TGM suite of rocks, which includes mainly Hornblende Biotite Gneiss andOrbicular Granite. Hornblende Biotite Gneiss are tonalitic to granodioritic in composition. Thegneissosity is defined by parallel alignment of mafic and felsic minerals in the Hornblende BiotiteGneiss. Gneissosity plane is parallel to the schistosity of metabasalt of NSB trending NW-SE directiondefining the regional foliation in the area. The orbicular granite, a variant of Hornblende BiotiteGneiss, contains oval to orb-shaped phenocryst ofplagioclase and k-feldspar set within fine grainedmatrix of quartz, feldspars, biotite and hornblende.The MS suites of rock consist of alkali feldspargranite and monzogranite, which are exposed nearMarlapudi and NE of Rapur. The alkali feldspargranite intruded along NS trending plane, whileMonzogranite intruded along NE trending axial planeof regional F3 fold.The area suffered a polyphasedeformation. The first phase of deformation producedisoclinal (F1) folds with the development ofpervassive schistosity (S1). These have been involvedin coaxial upright (F2) folding with the axial planestriking NW’ly, with the development of crenulationcleavage (S2) at places. The last phase ofdeformation produced (F3) folds with three sets of vertical axial planes striking N330, N75 and N285.

Key words: Nellore Schist Belt, Nallamalai Fold Belt, Tonalite-Granodiorite-Monzonite, Monzo Granite,polyphase deformation.

INTRODUCTION:

Nellore Schist belt (NSB), named after the district town Nellore, occurs over 140km long and 25 to 30 kmwide, N-S trending arcuate belt with the west facing convexity. The Nellore Schist Belt is situatedbetween North latitude 13 50’: 15 00’ and East longitude 79 30’: 80 00, covering parts of Chittore,Nellore, Prakasam, Guntur and Khammam districts. NSB extending in NNW-SSE direction, extends fromSirasanambedu of Nellore district in the south to Vinukonda of Guntur district in the north.

The curvilinear NSB is sandwiched between the Eastern Ghat Mobile Belt (EGMB) to the east and NallamalaiFold Belt (NFB) to its west. EGMB is thrusted over NSB while NSB is thrusted over NFB on its westernpart. The Archean Proterozoic rock of NSB occuring within the Eastern Dharwar Craton (EDC) arecharacterized by upper amphibolite facies rocks. NSB is composed of two groups namely Lower

Fig: 1a. Generalized Geological Map, showing

Gudur/Vinjamuru Group and Upper Udaigiri Group. The Lower Gudur Group comprises of Lower ChagnamFormation (metabasalts) and Upper Malakonda Formation (quartzite, quartz-mica-schist).

The metavolcano and metasedimentary lithological assemblage of the Nellore Schist Belt has a greenstone affinity but the metamorphic grade is highly variable from Lower greennschist to Upper amphibolote facies. The rocks of the belt are intruded by the Granitoids, Gabbro, Dolerite, etc. Numerous pegmatites of various dimensions, some of which are well known mica pegmatites, traverse rocks of NSB.

GEOLOGICAL BACKGROUND:

Oldham and King (1861) were the earliest workers who mapped the Nellore Schist Belt. Their observationswere recorded by W.King in his Memoir published in 1880. The term Schistose Series was introduced by themcovering the entire rock formation. The metavolcanics were referred to as ‘Kandra outburst’ and graniteas a younger intrusive into schistose and metavolcanics. B.C. Roy (1950) mapped entire Nellore schistbelt in 1:63,360 scale and placed it in Achaean rocks. . He has given brief account of the geology of thebelt and introduced the term Kandra volcanic to denote amphibolites, chlorite schist and epidotequartzite. The granitoids observed in the area are considered to be intrusive into the basic rocksKandra. Subramanyam(1963) carried out investigation of feldspar in NSB and opined that Kandra volacanicsand later granite and pegmatite intrusive correspond to the time gap between the close of the Dharwar(Achaean) and the deposition of Cuddapah –the Eparchean interval. Rao (1963), based on mapping of theUdaygiri taluka, considered the metadolerites and metabasalt as intrusive into older schistose formationalong the foliation planes and equated them with Kandra suite of volacanics. P. Subramanyam and P.R.Phillips (1965) considered the schist and quartzite as oldest unit which is intruded by the Kandravolcanic. Granite has folded this orogenic

movement and granite mass developed a gneissic structure. According to Murthy and Pantulu (1989) therocks of Kandra are the product of Island arc volcanism with an impression of ridge magmatism suggestingmarginal basin environment during the Proterozoic time. B.K. Nagaraja Rao, P.M.Katti and D.Roop Kumar(1989-90), introduced the term Kandra Igneous Complex (KIC). Ophiolite affinity was suggested to the KICbased on the field relation, petrography and geochemistry of the rocks. Leelanandan (1991) opined thatthe Kandra volcanics of the Nellore Schist Belt as possible ophiolite. Sehsa Sai (1989) from his studiesaround Ananthasagram area brought out a major NW-SE lineament. Kaila et.al (1979) indicated deep crustalfaults as a result of which the Nellore Schist Belt is juxtaposed against the Cuddapah basin rocks. DeepSeismic Sounding studies across the Cuddapah basin by NGRI (Kaila et. al. 1979); (Kaila and Tiwari 1983)suggested that the western part of the basin has

almost acted like a stable rigid block while, the NSB and Eastern Ghat were thrust up and now lie near and in juxtaposition with Cuddapah sediments. K.N. Srinivasan and D.Roop Kumar

(1991-92) reported undeformed nature of the orbicular granite and placed it as younger granite, suggestedits emplacement is post-kinematic in the schist belt.

Fig 2a Geological Map of Study the Area

Super Group Group Formation Lithology AgeAcid/FelsicIntrusive

Quartz Vein Mesoproterozoic

Mafic IntrusiveDolerite MesoproterozoicGabbro

PeninsularGneissic Complex

MSSuite

Monzo-Granite

Archean toPalaeoproterozoic

Alkali FeldsparGranite

TGMSuite

Orbiculargranite

HornblendeBiotite Gneiss

Dharwar SuperGroup/ NelloreSchist Belt

Gudur GroupMalakondaFormation

Quartzite Mica Schist/Quartzite

ArchaenChaganamFormation

Metabasalt

Table 1: Stratigraphy of the area based on the field observations is asfollows.

GEOLOGY OF THE AREA:

The NSB is divided into two domains on the basis of their structure and metamorphism named as Udayagiriand Vinjamuru domain. NSB rocks are classified into Older Gudur Group in the area, which is part ofVinjamuru domain, comprises of Lower Chagnam and Upper Malakonda Formations. Chagnam Formation mainlycomposed of metavolcanics i.e., metabasalt. These rocks occurring along the eastern boundary of the areaare exposed, west of Marlapudi, north of Virayapalem, west of Gilkapadu, and also occur as enclave withingranitoid rocks. Basement for these rocks has not been recognized in the area. Metabasalt are fine tocoarse grained, grayish green to black in colour, massive to schistose, consist of hornblende, actinoliteand plagioclase as essential minerals with biotite, epidote, muscovite, zoisite, ilmenite, sphene, pyriteas accessory minerals. These metabasalt, at places, show a strong compositional banding with thin laminaeof quartzofelspathic rich and mafic rich banding. Plagioclase, quartz forms the quartzofelspathic layerwhile hornblende, actinolite, biotite forms the mafic layer. The dominant schistosity trending NNW-SSEdirection with moderate to steep eastern dip. A number of quartz veins intruded along the schistosityplane and at places occurs across the schistosity plane.

Malakonda formation mainly composed of metasedimentary rock i.e., Quartzite and Quartz Mica Schist.Quartzites are exposed north of Virayapalem and NE of Bojjanapalle. They are thickly bedded, fine tocoarse grained and white to grey in colour. The compositional banding is defined by the thick quartzoserich layer and thin dark argillaceous layer. Gritty nature in the quartzite is generally seen at places.This shows NW-SE and NE-SW strike with varying dip as a result of regional folding. The limited exposureof Quartz Mica Schist (QMS) occurs NE of Bojjanapalle. These are grey, brown in colour, fine to mediumgrained, with alternate thin bands of quartzose rich and mica rich layer.

Apart from these, variants of granitoids occurring in two suits namely Tonalite Granodiorite MonzograniteSuite (TGM suite) and Monzogranite Syenogranite (MS suite) are considerably noticed.

TGM Suite:

Hornblende Biotite Granite gneiss and Orbicular Granite comprises PGC-II rocks and are exposed aroundRapur, Bojjanapalle, Marlapudi. The Hornblende Biotite Gneisses occupies the major part of the mappedarea. They are leucocratic, medium to coarse grained, equigranular to prorphyritic at places orbicularnature with plagioclase, K-feldspar and quartz as the major constituents with varying mafic content.Hornblende Biotite Gneiss shows well defined gneissosity having alternate mafic rich (Biotite,hornblende) bands and quartzofeldspathic rich bands. At places shearing within hornblende biotite gneiss,south of Telugu Ganga canal is noticed.

The orbicular granite is unusual rock; it gives dotted appearance due to inhomogeneous distribution oforbicules. Most of the orbicules are nearly spherical, elliptical to elongated, which ranges from 2 to 4cm in diameter. Present study (both field & laboratory), clearly indicates these are deformed granite anda variant of a Hornblende Biotite Gneiss, having gradational contact between them. They are mainlycomposed of plagioclase and K-feldspar along with biotite as mafic minerals, forming clear and darksingle ring around orbicule. The two types of textures described by previous workers for Orbicules, isas follows: 1. One having a core of mafic mineral surrounded by rims of plagioclase. This type oforbicular nature may be related to crystallothrasmatic orbicular nature. 2. Other variety is devoid ofany mafic core. These may be related esboitic orbicular nature. These suites also contain micro-granularmafic rich enclave (MME), more pronounced in the hornblende biotite gneiss, varying in abundance fromplace to place. They exhibit smooth, lobate to straight and some time cuspate margins. The MMEs range incomposition between diorite/quartz diorite, tonalite with higher mafic content than the host granitoids.

MS Suite:

Alkali Feldspar Granite: It is leucocratic (pink color), medium grained, composed of K-feldspar, quartz,biotite and plagioclase with decreasing modal percentage. It is intruded along NS plane extending up toCherlapalli village. It is mostly massive but at contact it shows schistose nature. Monzogranite: Therock type is grey colored, fine to medium grained, composed of quartz, K- feldspar, plagioclase andbiotite. It is massive and undeformed in nature, having intrusive contact with TGM suite, well exposed atGilakapadu quarry section. It is intruded along the NE trending axial planes of F3 fold, north of Rapur,which is the last phase of deformational event in the area, indicating late-post deformation intrusivenature of this granite.

Mafic intrusives like Gabbro, are intruded into the metabasalt and Granitoids at number of places,trending NS direction sharing its contact with metabasalt and granitoids. It is massive, melanocratic,coarse grained, mainly composed of pyroxenes, amphiboles, plagioclase with subordinate biotite. And alsoDolerite dykes also which are very small in size having dimension 5m X 1m, occurs as a small intrusion inthe area. It is fine grained, massive, melanocratic, mainly consist of pyroxenes, amphiboles, plagioclaseand biotite. Some sulphide specks also noticed in the form of dissemination. At most of the places, thinquartz veins are intruded parallel to the bedding plane, schistosity in the rocks of NSB and alonggneissosity in the Granitoids i.e., NNW-SSE direction.

PETROGRAPHY:

Metabasalt: The rock is medium to fine grained, massive to schistose, mainly composed of amphibole,plagioclase feldspars with minor amounts of quartzand biotite, chlorite, with accessories likeepidotes, sphene, apatite and opaque minerals. Theglameroporphyritic to intergranular texture is verycommon, while ophitic texture are also noticed atplaces (Fig.3.1). The amphiboles are mainly ofhornblende in composition, subhedral in shape andgreen in colour. Elongated, needle shaped grains ofactinolite which has very low extinction angle arealso present. Most of the plagioclase feldspars arealbitic in composition and are often sericitised. Atplaces porphyroclast of plagioclase grains could bedistinguished with fine grained quartz, plagioclaseand biotite matrix. Subhedral laths of plagioclaseare medium to fine grained shows polysynthetic twinning. The mafic amphibole layer alternate withquartzo-feldspatic layer, define the compositional banding of the rock at places. Chlorite, epidote andsericite form as a result of alteration due to deformation and metamorphism. Amphibole grains showsalteration to chlorite and plagioclase feldspars due to saussuritisation. The dark green colored biotiteis the dominant mafic mineral varying in proportion from 5% to 15%.

Granitoids (Peninsular Gneissic Complex): Granitoids are divided according to their mineral assemblages,their field and petrographical features. The granitic rocks in the area which was considered as part of

Peninsular Gneissic Complex-II (2.5 Ga) is classified into two suites of intrusive granitoids namely,Tonalite-Granodiorite-Monzogranite (TGM) and Monzogranite-Syenogranite (MS) / Alkali Feldspar Granitewith undeformed Monzogranite (Biotite Granite).

TGM is compositionally expanded suite from tonalite to monzogranite in composition.

Hornblende Biotite Granite Gneiss:

It is medium to coarse grained rock; mainly composed of sub-equal amounts of plagioclase + K-feldsparsalong with quartz as dominant mineral phase. The biotite and hornblende are present as mafic minerals.The plagioclase + K-feldspars occur as subhedral phenocryst which are set in fine grain matrix ofplagioclase + k-feldspar + quartz. The foliation is defined by the parallel alignment of hornblende andat places biotite, which swerves around plagioclase porphyroclast and microcline, at places (Fig 3.3).The Apatite + Epidote + Sphene + Zoisite occur as main accessory minerals. Plagioclase is subhedral inshape, shows polysynthetic twining and saussuritised at places. Alkali feldspars are less in proportioncompared to plagioclase. They are mostly microcline; anhedral shaped exhibiting extensive marginalgranulation. At a number of places perthite in K-feldspar are also noticed. Biotite occurs as inclusionin the hornblende grain, which are subhedral in shape. Quartz is medium to fine grained, equant shaped,showing recystallisation and sub-grains formation. Mafic minerals both hornblende and biotite arechloritised at places.

Orbicular Granite: Orbicular granite is the important variant of TGM suite, medium grained and leuco to mesocratic withdevelopment of orbicules / proto-orbicule / resorbed phenocryst of plagioclase and k-feldspar aggregates.Orbicular monzogranite, are medium to coarse grained characterized by orbs of plagioclase + K-feldspar.Orbs are nothing but resorbed phenocrysts, which are spherical to oval in shape (Fig 3.2). It is mainlycomposed of plagioclase + k-feldspar + quartz as felsic phase while hornblende + biotite as maficconstituent. The plagioclase shows polysynthetic twining and most of the places are saussuritised. K-feldspar comprises both orthoclase and microcline. K-feldspars are mostly perthite.

However flame perthite in monzogranite of TGM suite is dominant over normal perthite. The orb shows bothtextures which are described as esbiotic (concentric rims of biotite) and crystallothrausmatic(concentric rims of titanite). At places graphic to granophyric texture is very common indicationmagmatic origin of the rock. Most of the orbicules are deformed and fractured, which are filled withinterstitial quartz. The boundary of obrs is covered with recrystalised quartz gain.

Monzogranite Syenogranite (MS suite)

Alkali Feldspar Granite: It is fine to medium grained, poorly foliated; mainly composed of K- feldspars (mainlymicrocline with some orthoclase), quartz, plagioclase, hornblende, biotite in decreasing modalpercentages. The epidote, zoisite, apatite, sphene and monazite are present as accessory phases. Thephenocrysts of k-feldspars are subhedral in shape and embedded in the fine grained matrix of k-felspar +plagioclase + quartz. The microclines are mostly perthitic. Plagioclase is much less in proportioncompared to Alkali feldspars, randomly oriented in the ground mass. These alkali feldspars are mostlymicrocline/orthoclase of anhedral shape exhibiting extensive marginal granulation and also showsmyrmakitic and granophyric texture (Fig 3.4). Quartz is fine grained, equant shape showingrecystallisation and sub-grains formation. Polycrystalline quartz with irregular grain boundaries formedin response to grain boundary migration (GBM) recystallisation.

Monzogranite: It is light gray in colour, massive with a fine grained granitic texture / salt & peppertexture. These rocks are mainly composed of alkali feldspar, quartz with small amounts of plagioclase andmafic minerals such as biotite. Alkali feldspar is dominantly perthite and plagioclase is mostly albiteplus small abundances of oligoclase. Alkali feldspars are mostly microcline, subhedral to anhedral shape,shows cross-hatched twinning. Quartz-alkali feldspar show intergrowth and micrographic textures,indicating its magmatic origin. Mafic minerals consisting of biotite and minor muscovite, mainly formirregular crystals interstitial to alkali feldspar and quartz. Accessory mineral includes zircon,epidote, zoisite, apatite, sphene and monazite. The plagioclases are saussuritised and shows deformed

Fig 3: QAP diagram showing modal analysis of

polysynthetic twinning indicates plastic crystal deformation. Quartz is fine grained, equant shapedpresent mostly in matrix and shows undulatory extinction.

Quartzite:

It is fine to medium grained, composed of predominantly quartz with small amount of muscovite and opaque.The quartz grains are elongated to elliptical, sub-rounded shaped shows undulose extinction, due todeformation (Fig 3.6). The thick quartzose layer alternate with muscovite layer define bedding to therock.

Gabbro: It is medium to coarse grained, shows ophitic to subophitic texture, mainly composed ofhornblende, plagioclase, orthopyroxenes as essential minerals and quartz in subordinate amount with someaccessory minerals and opaques. The presence of hornblende indicating conversion of pyroxene, formed dueto retrograde metamorphism (Fig 3.5). Dolerite: It is medium grained rock, composed of plagioclase, alteredpyroxene (hornblende), epidote and opaques. Subhedral grains of plagioclase and hornblende occur ascumulus aggregates representing the grameroporphyritic to equigranular texture to the rocks. Pyroxenesare mostly altered to amphiboles. At many places shows subophitic texture where plagioclase laths arepartially enclosed within the pyroxene/hornblende grains.

GEOCHEMISTRY: The oxide and trace element values of TGM suites shows high SiO2 - 70.93 % to 48.2% withAvg. 59.565%, Al2O3 - 16.54% to12.7% with Avg. 14.62%, CaO -10.51% to 0.63% with Avg.5.57%, high Na2O -5.16% to 2.28% with Avg. 3.72%, K2O -7.06 to 1.4% with Avg. 4.23%, Fe2O3 - 11.83% to 1.28% with Avg.6.555% while low MnO - Avg.0.085%, MgO -Avg. 5%, TiO2- Avg.0.56%. They are rich in Ba (747.4ppm), Sr(349.15ppm), Zr (1808.9ppm), & low, Zn (87.285ppm), Rb (124.3ppm) in average. The REE pattern showstypical enriched LREE (235.7 ppm to 2.54 ppm & Avg. 119.12 ppm), with negative & positive Eu anomaly (Fig4.6 & 4.7) and flat to depleting HREE (0.16 ppm to 23.93 ppm & Avg. 12.045 ppm), while the Total REEranges (72.25 ppm to 671.23 ppm & Avg. 371.74 ppm), with the dominating LREE over HREE.

Fig 3.1 Metabasalt showing parallel alignment of prismatic

Fig 3.2 Orbicule of K-Feldspar in orbicular granite showing inclusion of

Fig 3.3 Gneiss showing augen structure

Fig 3.4 Alkali feldspar granite

Fig 3.5 Ophitic texture in Gabbro defined by the inclusion of

Fig 3.6 Quartzite with elongated or lenticular

The MS suite of rocks shows geochemical similarities like TGM suite of rock. They have silica (SiO2)values in Granite 74.29% (in average 65% in TTG and 67% in TGM) slightly moderate A12O3 (Avg. 11.84 %),lower Fe2O3 (Avg. 2.44 %), CaO (Avg. 1.16%) and MgO (Avg. 0.35%) with substantial increase in K2O (Avg.4.91%) (Table 3). They are rich in Ba (744.70ppm), Zr (317.48ppm) and having low Zn (35.55ppm) andoccasionally in total REE (Table. 4 and 5). The REE pattern shows typical increased LREE (348.79 ppm to386.32 ppm & Avg. 367.43 ppm), with negative europium anomaly and flat HREE (40.78 ppm to 64.31 ppm &Avg. 52.57 ppm), with the Total REE ranges (389.57 ppm to 450.63 ppm & Avg. 420 ppm). (Fig 4.6 & 4.7)

.

Fig 4.3: NormativeAb-An-Or diagramfor the Granitoidsof the study area(O’Connor 1965).

Fig 4.4: The binary diagram of Si02 vs K2O (Peccerillo and Taylor 1976) shows Shoshonitic affinity

Fig 4.1: AFM diagram for Granitoids showing tholeitic and calc alkaline affinity

Fig 4.2: TAS diagram shows alkaline distribution to thegranitoids which

Fig 4.5 A/NK vs A/ CNK plot (Frost, 2001)showing peralkaline and metaluminous characters of the granites.

In the TAS diagram, as defined by Cox et al. (1979) (Fig 4.2) and Middlemost (1997) (Fig not shown),indicated all samples plot in the tonalite to granite field, belonging to the silica saturated alkalineseries (Irvine and Bangar 1971) (Fig 4.1). It shows strong affinity to calc-alkaline series in AFMdiagram. Potassium rich nature of these Granitoids is indicated by samples plot in high K-, calc alkalinefiled in K2O Vs SiO2 diagram Fig (4.4). This character is also concordant with their moderate to highAl2O3 content of TGM & MS and supported by the noticeable abundance of potash feldspars. This behavior isdue to strong influence of K2O on the calculation of total alkali/R1 parameter. These different variantsof Granitoids nature also confirmed by samples plot in normatic Ab-An-Or diagram (O Connor, 1965) (Fig4.3), which mainly falls within granitic field. These are also indicated Al-saturated nature and displaya dominant metaluminous behavior with A/CNK (molar Al2O3/ (CaO+Na2O+ K2O; Clark, 1981) value ranging from0.7 to 0.9. This is further supported by the samples plot in alumina saturation diagram (A/CNK Vs A/NK;Sand, 1994) where clustering is closed to the dividing line between metaluminous to peraluminous field(Fig 4.5).

TECTONIC IMPLICATION:

In Ta+ Yb vs Rb and Yb vs Ta diagram (Pearce et al., 1984) shows concentrations based tectonicdiscrimination of Granitoids samples falls in the Within Plate Granite field (WPG) indicating allgranitoids are intruded in the craton (Fig 4.8)

Fig 4.8 Ta+ Yb vs Rb and Yb vs Ta diagram (Pearce et al., 1984) shows concentrations based tectonic discrimination of Granitoids samplesfalls in the Within Plate Granite field

On the geotectonic discrimination diagram based on multi-cationic R1-R2 factors as proposed by Batchelor+ Bowden, 1985, Granitoids samples plots in the late orogenic field. This suggests emplacement of magmaat the end of an orogenic cycle in continental rift zones. This is further substantiated by thehypothesis of Kusky (1993) envisaging that the interaction, ascent and emplacement of high k-granite aregenerally associated with late orogenic extensional regimes exhibiting syn - to post-tectonic signature.

Fig 4.6: Chondrite-normalized REE distribution pattern for All Granitic suites. Chondrite

Fig. 4.7: Primitive Mantle normalized REEdistribution pattern for All Granitic suites. The normalized values are

MantleFractionates

Pre-plateCollisionPost-

collisionUplift

Late-orogenic

AnorogenicSyn-collision

Post-orogenic

OrAb

An

Bt

Ph

En

Di

Fo

Hd

Ha

-1000 0 1000 2000 3000 4000

010

0020

0030

0040

00R 1 R 2BatchelorBowden1985

R1= 4Si - 11(Na + K) - 2(Fe + Ti)

R 2= 6Ca

+ 2Mg + Al

Fig 4.9 Geotectonic discrimination diagram based on multi-cationic R1-R2 factors as proposed by Batchelor + Bowden,1985.

Table 3: Major oxide (in wt %) of Granitoids from Rapur Area.

Lithology MB HBG HBG HBG HBG OG OG AFG AFG MG MGSiO2 48.2 65.86 68.32 63.68 67.64 67.66 69.25 70.41 69.6 67.88 70.93TiO2 0.99 0.56 0.44 0.32 0.51 0.41 0.35 0.13 0.27 0.41 0.51Al2O3 13.41 13.56 12.08 16.54 14.24 13.99 12.7 14.38 14.02 14.01 13.13Fe2O3 11.83 5.8 5.16 2.94 3.57 3.56 3.48 1.28 2.45 3.87 2.76MnO 0.15 0.08 0.09 0.02 0.04 0.03 0.04 0.02 0.04 0.04 0.03MgO 9.93 1.67 0.73 3.47 2.04 3.72 0.71 0.07 0.09 1.42 1.99CaO 10.51 1.98 1.5 4.22 2.71 0.63 1.73 1.7 1.02 1.43 3.32Na2O 2.28 3.41 4.24 5.16 6.05 4.21 3.81 4.56 4 4.11 3.07K2O 0.98 5.44 6 2.53 1.4 5.65 5.91 5.55 7.06 5.82 0.72P2O5 0.43 0.16 0.09 0.19 0.1 0.07 0.06 0.01 0.02 0.08 0.08Total 98.71 98.52 98.65 99.07 98.3 99.93 98.04 98.11 98.57 99.07 96.54

Table 4: Trace element (ppm) data of different suits of Rapur Granitoids

Lithology MB HBG HBG HBG HBG OG OG AFG AFG MG MG

Li 10.04 9.56 6.05 8.77 4.58 7.23 5.66 0.22 0.31 12.82 5.76

Be 1.21 3.22 3.63 2.54 4.23 4.75 5.86 3.48 6.53 4.4 4.08

Sc 45.77 16.12 10.73 8.02 8.99 6.05 6.34 1.41 1.62 8.32 9.29

V 343.3 17.82 15.07 70.08 22.79 13.43 13.78 8 4.66 17.68 27.02

Cr 441.2 147 162.3 204.8 219.3 148.4 175.7 173.1 200.7 171.8 196.2

Co 52.79 6.3 4.73 10.99 6.28 4.61 6.44 1.03 1.52 5.9 8.15

Ni 100.2 5.76 5.27 20.96 7.25 4.36 5.36 3.25 3.64 5.16 11.41

Cu 101.3 12.22 5.75 13.37 11.58 6.09 39.41 3.08 3.66 18.89 11.97

Zn 120.2 88.04 71.53 40.51 26.49 58.54 63.99 6.52 111.8 45.24 28.81

Ga 10.51 19.2 24.36 16.99 13.99 20.46 24.97 12.33 23.37 24.75 10.76

Ge 3.76 1.97 2.15 1.42 1.91 1.69 2.2 1.26 2.29 1.83 1.79

Rb 30.8 154.7 217.8 94.19 51.54 140.6 201.9 147.9 230.2 195.2 39.92

Sr 650.4 107.3 67.6 662.8 144.2 35.5 104.7 40.22 22.55 85.56 158

Zn 28.63 82.17 70.94 13.67 117.6 112.5 143 137.2 160.9 117.3 132.5

Zr 110.8 1047 1285 143.2 1748 1487 1652 782.1 3501 1164 1214

Nb 4.09 21.61 25.26 6.76 28.41 38.04 43.36 40.63 53.42 31.97 35.34

Pd 708.6 1444 1200 451.6 1833 1727 2181 2226 2471 1782 2140

Cd 131.1 305.4 322.3 80.16 317.5 323.6 321.6 257 645.2 264.1 299.5

In 78.7 133.6 111 35.07 167.2 103.1 210.1 104.9 229.4 103.2 113.1

Sn 1970 3573 3441 2410 6560 4543 5892 4932 4929 4719 5238

Cs 1.76 1.88 0.56 0.68 0.29 0.54 0.6 0.29 0.18 1.45 0.65

Ba 340.4 1011 1385 668.5 303.1 914.5 1150 168.5 867.4 1347 109.8

Hf 4.73 25.83 30.21 3.66 32.03 34.72 30.69 32.85 56.61 25.89 25.99

Ta 0.38 1.5 1.51 0.38 1.91 2.21 2.46 3.35 2.69 2.2 2.35

Re 4.58 11.14 10.44 1.96 14.1 13.41 18.03 19.45 19.24 14.84 17.9

Ir 6.78 34.41 40.69 5.63 43.25 45.66 42.31 42.72 80.45 36.57 36.49

Pt 38.11 187.5 214 28.86 233.4 241.7 209 234.5 425.5 174.7 188.8

Au 71.94 292.4 299.4 67.68 363.1 402.4 450 585.9 568 383.7 439.7

Pb 9.33 22.85 19.3 31.67 14.43 13.27 21.49 15.36 12.86 22.85 28.57

Bi 42.02 60.73 97.05 19.54 173.7 14.16 148.6 98.48 12.32 59.96 78.79

Table 5: REE data of different suits of Rapur Granitoids:

Lithology MB HBG HBG HBG HBG OG OG AFG AFG MG MG

La 11.02 48.58 75.91 52.84 91.52 100.3 157.4 81.18 142.8 53.6 82.26

Ce 22.37 88.43 119.6 84.02 145.1 145.2 235.7 138.4 222.1 96.9 138

Pr 3.7 12.08 16.66 10.38 19.61 21.83 32.85 19.15 30.61 14.54 19.12

Nd 19.23 54.69 69.34 40.29 80.18 89.79 129.7 79.47 124.6 65.24 80.72

Sm 5.26 13.45 14.36 7.02 18.19 19.36 26.07 18.78 26.9 16.66 19.35

Eu 1.43 2.54 1.99 1.51 1.91 1.95 2.23 0.31 2.24 2.1 2.09

Gd 3.44 9.1 9.02 3.93 12.21 12.67 16.5 12.46 17.61 11.16 13.07

Tb 0.73 1.98 1.77 0.54 2.58 2.65 3.24 2.85 3.62 2.52 2.89

Dy 5.09 14.18 11.91 2.78 17.56 18.29 21.18 21.46 23.93 18.05 20.28

Ho 1.1 3.1 2.63 0.53 4.04 4.16 4.91 4.87 5.64 4.16 4.67

Er 2.56 7.32 6.24 1.12 9.56 9.98 11.72 11.82 13.42 9.94 11.11

Tm 0.49 1.45 1.21 0.17 1.8 1.92 2.18 2.37 2.46 1.92 2.12

Yb 3.13 9.26 7.79 1.05 11.94 12.56 14.47 14.63 16.71 12.57 13.93

Th 2.05 17.29 32.79 30.38 71.21 45.3 50.02 61.79 39.38 41.65 56.39

U 0.6 5.43 5.08 2.14 22.67 6.67 13.19 10.37 9.2 14.75 15.89

In Table 3, Table 4, Table 5 the lithology is denoted as Hornblende Biotite Gneiss (HBG), AlkaliFeldspar Granite (AFG), Metabasalt (MB), Monzogranite (MG), Orbicular Granite (OG)

STRUCTUREStructures of small and intermediate scale, Bedding and compositional banding: Bedding/ stratification plane (So) planer structure in the quartzite and mica schist, defined by colour and compositional banding varying inthickness from few mm to few cm. In the quartzite bedding is defined by alternate thick quartzose rich layer and very thin dark argillaceous layer, while in the schist it is defined by light colour quartzose layer and dark colour micaceous to argillaceous layer.In the metabasalt, alternate dark colour amphibole rich layer and light colour quartzofelspathic layerdefined the compositional banding. In the biotite gneiss, alternate quartzofelspathic rich bands andmicaceous and amphibole rich bands defined the gneissosity.

Diastrophic structures:Three generations of folds have been deciphering in the scale of hand specimen and outcrop on the basisof relationship of one another.

F1 folds and related structures: F1 folds: The F1 folds are very tight to isoclinal and are characterised by high amplitude/wavelength ratio(fig 5.1). As a consequence of these folds, except for the hinge zone which are of insignificant aerialextent compare to the limbs, bedding and compositional banding are virtually parallel the axial plane ofthese folds. A large no. of pre and early kinematic quartz and pegmatite veins have been affected by F1folding. The veins which are intruded parallel to the axial plane so the forming pinch and swell andboudinage structures. Axial plane schistosity (S1): A penetrative schistosity, defined by parallel alignment of flaky andprismatic minerals and is parallel to the axial plane of F1 fold have developed at places, the envelopingsurface formed by the series of S1 schistosity in metabasalt, defines the contact relationship with theadjoining Granite Gneiss (fig 5.2).

Fig 5.1Rootless Isoclinal FoldsLinear structures associated with the F1 folding are, Axis of F1 fold, Boudinage structure, Lineation dueto intersection of bedding/compositional banding and schistocity, Mineral elongation.

F2 and related structures:F2 folds: The axial plane on F1 fold along with the schistosity has been involved in coaxial upright F2folding. The F2 fold are upright with the axial plane striking NNE to NE and axis plunging gently towardsNNE/NE or SSW/SW. S2 crenulation cleavages seen in the east of the area near Saidapuram. Crenulationscleavage (S2) has developed because of sharp crinking of S1 cleavage specially in metabasalt. At placesin the mica schist , crenulations cleavage (S2) has also developed. Linear structures associated with the F2 folds are Puckers and axis of F2 folds, at places intersectionof S2 and S1, also define the F2 linear structure.

Fig 5.2 Round hinged Isoclinal Fold Fig 5.3. F2 micro folds with development of S2

crenulation cleavages

F3 and related structures: In the thickly banded rocks there are usually round hinge upright warps whereas in the thinly layeredsuch as in mica schist and metabasalt, they occurred as kink and conjugate folds. The F3 folds havevertical axial plane with the three sets of strikes N330, N75 and N285 . At places sets of fractures havedeveloped parallel to the axial plane of F3 folds.

Determination of attitude of principle compression direction from axial planes of conjugate folds(N330and N75 ) indicate that maximum compression direction δ1 is horizontal in N15 E /S15 W direction,parallel to the dominant orientation of the S plane striking N15 . Therefore, compression parallel to thedominant NNW striking S plane must have been responsible for the formation of F3 fold.

Interrelation of structure: The three sets of folds vary not only in style but also in their orientation.Depending on the attitude axial plane of the fold the interference pattern between the sets has takendifferent shape.

a. Type-3 interference patterns due to superposition of F2 folds on F1 isoclines which has resultedin folding of F1 axial plane with the axis remaining constant have been observed at places.

b. Type-1 interference pattern due to superposition of upright F3 fold and upright F2 folds hasresulted in the dome and basin / eye fold structures (Fig 5.5).

Fig 5.4. Open wraps of F3 generation with the axial Fig 5.5 Type 1 interference pattern in metabasalt plane striking NE-SW

6. Summary and conclusion

The 30 Sq Km area has been mapped on 1:25,000 scale around Rapur Village in the southern part of Nelloreschist belt, which comes under the toposheet no. 57N/11, 57N/12 to establish the contact relationshipbetween the various litho-unit and also to study the structural set up of the area. Present study of thearea reveals that the rocks are dominantly different variants of Granitoids. The main dominant lithologyexposed around Rapur area is Nellore schist belt (NBS) on eastern side, Nallamalai Fold Belt (NFB) onwestern side and major Granitic gneiss, granitoids are exposed as sandwich in between the two. Thedifferent lithounits exposed in the area are metabasalt, quartz mica schist, and quartzite of NSB,various variants of Granitoids i.e., Hornblende biotite gneiss and orbicular granite which are part ofTGM suite and Alkali feldspar granite and Monzogranite of MS suite and mafic intrusives such as gabbro,dolerite. Mineralogically, these granitic suites mainly consist of Plagioclase, K-feldspar, quartz asfelsic mineral phases while biotite and hornblende as mafic mineral constituent. The apatite, monazite,zircon, epidote, zoisite occur as in accessory amount. As per the IUGS classification and according totheir mineral assemblages, their field relationship, petrographical characters, and their chemicalclassifications these granitoids are divided into following two types. 1) tonalite-granodiorite-monzogranite (TGM suite) and 2) monzogranite-syenogranite (MS) / Alkali Feldspar Granite (MS suite) withMonzogranite Biotite Granite (undeformed). Tonalite Granodiorite Monzogranite: (TGM suite), are manlycomposed of variants of Hornblende biotite granite and orbicular granite. Monzogranite Syenogranite (MSsuite), are mainly consist of Alkali feldspar Granite and Monzogranite.

All these Granitoids show remarkable chemical similarity, suggesting their formation from similar sourcein probably at different events, specially TGM & MS suite. The evolution has resulted in decrease ofcompatible trace elements, Ti, Zr, CaO, FeO, MgO and increase in the Sr and Ba concentrations (Table. No.3 & 4). The mineralogical heterogeneity is reflected in the major and trace elemental abundance.Predominantly these gneisses being tonalitic to Granitic in nature, with low-moderate Rb and high Srcontents (Table 4), show calc-alkaline affinity with high K2O percentage and metaluminous nature. Inprimitive-mantle normalized spidergrams (Fig. 7.7), these granites are enriched in some LILE (Large IonLithophile Elements) (Ba,Rb, Th, U, K) and HFSE (High Field Strength Elements) (Nd, Zr,), but stronglydepleted in Sr, P, Tb and to a lesser extent Eu mainly is observed in the spidergrams. These granitecharacteristically have enriched LREE, relatively flat to somewhat depleted HREE chondrite normalized REEpatterns with significant strong negative Eu anomalies. Negative Eu anomaly in granitoids of Rapur ispronounced (Fig.7.6), indicating variation in degree of plagioclase fractionation and REE partitioning.Average Σ REE contents of 123 ppm to 565 ppm have been recorded. Normalized REE plot reveal a generalLREE enrichment, strong negative Eu anomaly and flat HREE patterns (Fig.7.6 & 7.7) in these granitesindicating an enriched lower crustal source. In the tectonic discrimination diagram Y vs. Nb and Y+Nb vs.Rb, samples plot in Within- Plate -Granite (WPG) field. The overall geochemical data and the variation inimportant major and trace elements and their ratios suggest that the granite is of late -anorogenicnature. Three generations of folds have been marked, with the tight isoclinals F1 folds with inmetabasalt with axial plane parallel to schistosity, as observed near Chaganam(NSB), indicating thatthese folds were formed during first phase of deformational episode. This F1 folds are overprinted bylow plunging open upright F2 folds, with in schistose metabasalt, coaxially with development ofcrenulation cleavages/puckers, having axial plane striking NNW-SSE. The F3 folds occurs as open, uprightfold with three sets of vertical axial planes striking N330, N75 and N285.

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