The Carboniferous Baralacha La basaltic dykes (Upper Lahul, Ladakh) : remnants of an early rifting...

Bull. Soc. géol. Fr., 2005, no 6

The Carboniferous Baralacha La basaltic dykes (Upper Lahul, Ladakh) :remnants of an early rifting event along the Indian northern plate

FRANÇOIS CHAUVET1, HENRIETTE LAPIERRE1, DELPHINE BOSCH2, ALAIN DEMANT3, FRANÇOIS BUSSY4,JEAN-CLAUDE VANNAY4, GEORGES H. MASCLE1, PIERRE BRUNET5, JOSEPH COTTEN6 and FRANCINE KELLER1

Keywords. – Within-plate volcanism, Geochemistry, Crustal contamination, Indian northern margin, Ladakh.

Abstract. – The Lower Carboniferous Baralacha La basaltic dykes were emplaced along transtensional faults. The ba-salts exhibit tholeiitic and alkaline affinities. The tholeiites are TiO2-poor, moderately enriched in light rare earth(LREE), and display Nb and Ta negative and Th positive anomalies. The alkali basalts, compared to the tholeiites, havehigher TiO2, rare earth and highly incompatible trace element contents and greater LREE enrichments. The Nd and Pbisotope compositions of the Baralacha La basalts suggest that they derive from the partial melting of an enriched OIBmantle source, characterized by a HIMU component, and contaminated by the lower continental crust. The Baralacha Ladyke swarm represent the remnants of an early rifting event on the northern Indian passive margin.

Les filons basaltiques carbonifères de Baralacha La (Haut Lahul, Ladakh) : témoins d’unephase précoce de rifting de la plaque nord-indienne

Mots clés. – Volcanisme intra-plaque, Géochimie, Contamination crustale, Marge nord-indienne, Ladakh.

Résumé. – Les dykes de Baralacha La, d’âge carbonifère inférieur, sont mis en place à la faveur de failles transtensives,fréquemment syn-sédimentaires. Ces filons présentent des affinités tholéiitiques et alcalines. Les tholéiites, pauvres enTiO2, se caractérisent par un enrichissement modéré en terres rares ainsi que des anomalies négatives en Nb et Ta et po-sitives en Th. Les laves alcalines, comparées aux tholéiites, ont des concentrations plus élevées en TiO2, en terres rareset en éléments fortement incompatibles ; leur enrichissement en terres rares légères est nettement plus marqué. Lescompositions isotopiques en Nd et Pb de ces basaltes suggèrent qu’ils dérivent d’une source mantellique enrichie, carac-térisée par un composant de type HIMU, contaminée par la croûte continentale inférieure. Ainsi, l’essaim filonien deBaralacha La représente le témoin d’une phase précoce de rifting qui affecte la bordure nord de la plaque indienne.

INTRODUCTION

A major change in the geographical distribution of thelithospheric plates occurred during the late Paleozoic[Nikishin et al., 2002]. This change corresponds to the tran-sitional period between the development of Pangea by ac-cretion of lithospheric fragments, island arcs linked to theclosure of oceanic basins and its break-up during Perm-ian-Triassic times [Stampfli et al., 2001; Nikishin et al.,2002]. The Neotethyan opening took place also duringPermian-Triassic times, leading to the quick northward driftof the Cimmerian blocks [Sëngor, 1984], detached from thenorthern margin of the Indian-Arabian plate. TheNeotethyan opening began during the late Carbonifer-ous-early Permian, east of Australia and propagated

towards the west, along the Indian and Arabian plates dur-ing the middle to late Permian [Stampfli et al., 2001].

This geodynamic reorganization of oceanic and conti-nental realms was accompanied by the emplacement oflarge igneous provinces (LIPS), which is attributed to animportant thermal activity and a reorganization of the man-tle convecting system [Nikishin et al., 2002]. Among thebest documented Permian LIPS, one finds: (i) the Siberian[Arndt et al., 1998], Emeishian-Song Da [Xu et al., 2001]and Panjal Traps [Gupta et al., 1983], (ii) the magmatism ofthe Hawasina nappes in Oman [Béchennec et al., 1991;Maury et al., 2003; Lapierre et al., 2004]. According toAbbott and Isley [2002], the Paleozoic LIPS developed dur-ing the Devonian (377-368 Ma) and Permian (254-252 Ma)

Bull. Soc. géol. Fr., 2005, t. 176, no 6, pp. 499-511

1. Lab. Géodynamique des Chaînes Alpines, UMR-CNRS 5025, Univ. Joseph Fourier, Géosciences, B.P. 53, 38041 Grenoble cedex, France2. Lab. Tectonophysique, UMR-CNRS 5568, Institut des Sciences de la Terre, de l’Eau et de l’Espace de Montpellier, Univ. Montpellier 2, Place Eugènebataillon, 34095 Montpellier cedex 05, France.3. Lab. Pétrologie magmatique, Faculté des Sciences de St Jérôme, case courrier 441, 13397 Marseille cedex 20.4. Institute of Mineralogy and Geochemistry, BFSH-2, University of Lausanne, CH-1015 Lausanne.5. Lab. Mécanismes de Transferts en Géologie, UMR-CNRS 5563, Unité mixte de recherche, Centre National de la Recherche Scientifique, UniversitéPaul Sabatier, 38 rue des 36 ponts, 31400 Toulouse, France.6. Lab. Domaines océaniques, UMR-CNRS 6538, Univ. Bretagne Occidentale, Place Nicolas Copernic, 29280 Plouzané, FranceManuscrit déposé le 14 janvier 2004; accepté après révision le 6 avril 2005.

times and the duration of their emplacement did not exceed12 Ma (12 ± 3 Ma). Tectono-magmatic activity began to de-velop along the Gondwana eastern margin in the MiddleCarboniferous [Stampfli et al., 2001] likely leading to thebreak-up of the Cimmerian blocks. On the northern Indianplate, a Carboniferous mafic volcanism was emplaced alongextensional, locally synsedimentary faults. The latter repre-sents evidence of the first extension occurring on the futureIndian northern margin. Very few geochemical and espe-cially isotopic data on the Carboniferous magmatism devel-oped on the Gondwana margins are available in theliterature. The only study published on the Baralacha La ba-saltic dykes is based on major and few trace elements[Vannay and Spring, 1993]. That is why we have developednew geochemical (major, trace element, Nd, Sr and Pb iso-topes) investigations on the Baralacha La basalts and theirclinopyroxenes.

GEOLOGIC SETTING

The Himalayan Ranges result from the collision of the Indianand Asian plates [Gansser, 1964]. The initial convergence ledto the development of an Andean-type margin during the Cre-taceous and lower Paleocene as the result of the subduction ofan oceanic realm (the Neotethys) beneath southern Asia (Ti-bet). In contrast, the Indian northern margin remained passive[Bassoullet et al., 1980; Baud et al., 1982]. After the lowerEocene, the Indian northern margin was shortened in a seriesof tectonic slices, which are nowadays caught between the Up-per Himalayan crystalline domain to the south and theIndus-Zangbo suture to the north [Frank et al., 1977]. Thesedismembered units of the Indian northern passive margin arelargely exposed in the Zanskar, Spiti and Upper Lahul areas ofnorthwestern Himalaya (fig. 1) [Srikantia and Barghava, 1982;Baud et al., 1982; Fuchs, 1982].

The “pre-rift units” of the Indian northern margin con-sist of platform-type detritic sediments, the age of whichranges from the late Precambrian up to the Lower to MiddlePermian [Bagati, 1990; Spring, 1993; Vannay, 1993]. The“syn-rift” units (Upper Permian to Middle Triassic) arecomposed of platform-type limestones and flyschs[Lamayuru Unit, Bassoullet et al., 1980; Cannat andMascle, 1990] deposited on the shelf and slope, respec-tively. The “post-rift” units (Upper Triassic to LowerEocene) consist of platform-type limestones (Trias-sic-Dogger, Upper Cretaceous-Paleocene) or detritic sedi-ments deposited on the shelf (Upper Cretaceous) whilepelagic sediments deposited on the slope [Garzanti et al.,1987]. All these sedimentary units have been metamor-phosed during the collision [Colchen et al., 1986; Steck etal., 1993; Steck, 2003].

Four igneous events are present within the “pre-riftunits” [Spring et al., 1993; Vannay and Spring, 1993], i. e.(i) Cambrian-Ordovician granites [Le Fort et al., 1986; Gi-rard and Bussy, 1999], (ii) Carboniferous Baralacha La ba-saltic dykes [Vannay, 1993] (figs. 2 and 3), (iii) earlyPermian alkaline granite and microgabbro intrusions[Spring et al., 1993], (iv) Middle Permian Panjal Traps[Bagati, 1990; Gaetani et al., 1990].

The Baralacha La basaltic dykes were emplaced alongtranstensional, locally synsedimentary faults, that never af-fect formations younger than the Lower Carboniferous

(fig. 3) [Vannay, 1993]. A few exposures of conformablebasaltic flows occur within the evaporites of the lowermostCarboniferous (Lipak Formation). The late Carboniferous islocally absent and the Paleozoic ends with Permian detriticsediments that overlie unconformably the older units.

In order to better constrain the genesis of the BaralachaLa basalts, new investigations were undertaken based on thetrace element of eleven rocks (table II) sampled by one ofus (Vannay). Their location is shown in figure 2.Clinopyroxenes of four samples (V148, V300, V326 andB50) were analyzed for major and trace element chemistry(table I). Nine among the best preserved samples were ana-lysed for isotope compositions (Nd, Sr, Pb, table II).

Before Spring and Vannay [1993], the Baralacha La bas-alts were considered as the feeder dykes of the Panjal Traps[Fuchs, 1982; Gaetani et al., 1990], which are related to thePermian-Triassic Neotethyan opening [Honneger et al., 1982;Stampfli et al., 1991]. These authors have clearly shown onthe basis of stratigraphic and some geochemical data, that theBaralacha La basalts represent remnants of an igneous activitycompletely different from that of the Panjal Traps.

PETROGRAPHY AND MINERALOGY OF THEBARALACHA LA BASALTIC DYKES

Textures of the dykes

Among the Carboniferous dykes, two textures have beenobserved: aphyric basalt with intergranular texture and por-phyritic basalt.

The intergranular basalts (V300, V326, V324, fig. 2,tabl. I and II) consist of millimeter sized-plagioclase lathsreplaced by albite, subhedral clinopyroxene and interstitialaggregates of chlorite, actinolite and epidote. Some samples(V324) include also millimeter-sized olivine pseudomorphs,now replaced by chlorite.

The porphyritic basalts differ from the aphyric ones bythe presence of centimeter-sized clinopyroxene (B50,V148) or plagioclase (V164) phenocrysts. The intersertalgroundmass is formed of tiny plagioclase microlites, nowreplaced by albite, anhedral oxides, chlorite, albite, epidoteand calcite. Locally, clinopyroxene microphenocrysts mayoccur in the groundmass (B50). Quartz – (V148) or chlorite+ pistachite-filled vesicles may occur.

Clinopyroxene compositions

Major elements

Major elements compositions of clinopyroxenes were deter-mined using a Cameca SX-100 electron microprobe fittedwith five spectrometers at the « Service communMicrosonde » (Université de Montpellier). The standard pro-cedures are 20 kV and 10 nA with an electron beam of 1 µmwidth and integrated counting times of 20-30 s. Syntheticand natural minerals were used as standards. Alkalis weredetermined first to minimize Na loss during measurements.A computer correction program was used to calculate the ele-ment concentrations. The accuracy of major element contentsis better than ± 5% of the total values.

Clinopyroxenes of the two aphyric basalts (V300,V326) are light-pink colored and plot in the diopside fieldor at the boundary between the diopside and augite fields

500 CHAUVET F. et al.


(fig. 4a) (Wo40-47, En33-36, Fs19-22). In contrast, colorlessclinopyroxenes of the porphyritic basalts (B50, V148) showaugite composition (Wo32-43, En44-49, Fs11-20) (fig. 4a). Inthe Ti versus Ca + Na diagram [Leterrier et al., 1982,fig.4b], the Ti-rich diopsides (3.4 < TiO2 < 1.6) of theaphyric basalts (V300, V326) cluster in the alkali field,while augites of the porphyritic basalts, characterized bylow Ti contents (TiO2 = 0.6) for a wide range of Ca + Nalevels plot in the tholeiitic field.

Figure 4c shows the behavior of clinopyroxene compati-ble elements within the same sample and/or within a single

grain. In the aphyric basalts (V300, V326), clinopyroxene isCr-poor and its MgO and CaO contents remain rather con-stant while TiO2 and FeO increase significantly from coreto rim and within the same sample. In the porphyritic ones(B50, V148), clinopyroxenes are less FeO-rich but are char-acterized by a significant FeO and TiO2 enrichment andCaO and Cr2O3 depletion within the same sample. Such anevolution is likely to reflect a differentiation process.Clinopyroxenes from sample B50 are relatively primitiveand enriched in Cr2O3, CaO and MgO. The very low Cr2O3contents of clinopyroxene in V148 indicates the differenti-ated nature of the lava.

THE CARBONIFEROUS BARALACHA LA BASALTIC DYKES (UPPER LAHUL, LADAKH) 501


FIG. 1 – General map of the NW Himalaya afterGansser [1981], Sinha [1981], Franck et al.[1987], Stutz [1988] and Vannay [1993]. A showsthe studied area the geological map of which isillustrated in figure 2. Tectonic units and thrusts:MBT: main boundary thrust; MCT: main centralthrust; HHC: crystalline of higher Himalaya; TH:Tethyan Himalaya; IYS: Indus-Yarlung suture;HKS : syntaxis Hazara-Kashmir.FIG. 1. – Carte générale de l’Himalaya du NWétablie d’après Gansser [1981], Sinha [1981],Franck et al. [1987], Stutz [1988] et Vannay[1993]. A désigne la région étudiée dont la cartegéologique est présentée sur la figure 2. Unitéstectoniques et chevauchements : MBT : mainboundary thrust ; MCT : main central thrust ;HHC : Cristallin du Haut Himalaya ; TH : Hima-laya téthysien ; IYS : suture de l’Indus-Yarlung ;HKS : Hazara-Kashmir syntaxis.

Trace elements

Trace-elements contents determinations on minerals wereobtained by laser-ablation ICP-MS mass spectrometry usinga 193 nm an Ar-F 193 nm Lambda Physics© Excimer lasercoupled with a Perkin-Elmer 6100DRC ICPMS at theInstitut of Mineralogy and Geochemistry of the Universityof Lausanne. NIST610 and 612 glasses were used as exter-nal standards, Ca and Si as internal standards aftermicroprobe measurements on the pit sites. Ablation pit sizevaried from 40 to 60 mm. BCR2 basaltic glass was regu-larly used as a monitor to check for reproductibility and ac-curacy of the system. Results were always within ± 10% ofthe certified values.

The C1 chondrite-normalized [Sun and McDonough,1989, fig. 5] rare earth elements (REE) patterns of theBaralacha La clinopyroxenes are light (L)REE depletedwith respect to the heavy (H)REE [(La/Yb)N ranging be-tween 0.3 and 0.7]. The tholeiitic clinopyroxenes differfrom the alkalic ones by their lower REE contents (less than10 times the chondritic abundance), the lack of HREE de-pletion with respect to the middle (M)REE, and more orless marked Eu negative anomalies (Eu/Eu* = 0.9).

The primitive mantle-normalized [Sun and McDonough,1989, fig. 5] multi-element plots of the Baralacha Laclinopyroxenes show common Nb, Ta, Zr and Ti negativeanomalies. The behavior of Rb, Ba and U is not consistentwith the clinopyroxene affinity and can be attributed to themobility of these lithophile elements (large ion lithophile

elements, LILE) during alteration or weathering processes.The Rb mobility is shown by sample B50 clinopyroxene. In-deed, as this rock sample has the most primitive composi-tion, clinopyroxenes in it should have the lowest Rbcontent. On the contrary clinopyroxenes in B50 have thehighest Rb content. Therefore, this is consistent with aremobilization of Rb after crystallization. The alkaliclinopyroxenes show a more or less marked Pb negativeanomaly while the tholeiitic clinopyroxenes have a smallpositive Pb anomaly. The Sr positive or negative anomaliescould be related to alteration processes. However, the pres-ence of both Pb and Sr negative anomalies in the tholeiiticclinopyroxenes, could reflect early plagioclase removal.



Alkalic Alcalic Tholeiitic TholeiiticHTi HTi LTi LTiV300 V326 V148 B50

SiO 2 (pds %) 50,04 46,49 49,39 48,29 51,81 51,11 50,94 51,30Al 2O3 2,58 5,10 2,83 3,43 2,73 2,31 3,47 2,48FeO 12,73 11,85 12,08 10,87 8,89 12,21 6,79 9,31

MnO 0,30 0,26 0,28 0,22 0,24 0,33 0,21 0,30MgO 11,49 11,01 11,91 11,28 16,22 17,02 16,10 16,57CaO 21,14 20,95 21,21 21,74 19,18 15,85 20,72 18,55Na2O 0,66 0,65 0,57 0,52 0,26 0,17 0,31 0,23K2O 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00TiO 2 1,60 3,09 1,96 3,45 0,60 0,70 0,47 0,62Cr2O3 0,01 0,02 0,00 0,02 0,24 0,12 0,72 0,60Total 100,54 99,41 100,22 99,84 100,17 99,83 99,72 99,95

V300 V326 V148 B50Rb (ppm) 0,8 1,2 1,1 4,9

Ba 1,3 87,5 1,4 40,3Th 0,1 0,2 0,1 0,0U 0,0 0,3 0,0 0,0

Nb 0,4 0,3 0,4 0,1Ta 0,0 0,1 0,0 0,0La 6,4 5,4 1,1 0,5Ce 21,6 17,9 3,7 1,9Pb 0,1 1,3 0,5 0,7Sr 30,8 76,0 13,5 68,6Nd 25,2 21,7 5,1 2,2Sm 9,4 8,5 1,8 1,0Zr 127,0 150,0 17,3 8,3Hf 5,7 6,9 0,7 0,4Eu 2,8 2,6 0,6 0,3Ti 9954,0 14073,6 3702,0 1982,0Gd 11,5 11,0 2,6 1,4Tb 2,1 2,0 0,5 0,3Dy 13,3 12,5 3,2 2,1Y 64,0 61,7 17,8 11,8Er 7,0 6,5 1,8 1,3Yb 6,3 6,1 1,9 1,3Lu 1,0 1,0 0,3 0,2

TABLE I. – Geochemical compositions (major and trace elements) ofthe Baralacha La clinopyroxenesTABL. I. – Compositions chimiques (majeurs et traces) des clinopyroxè-nes des basaltes de Baralacha La.

Alcalic Alcalic Alcalic AlcalicHTi HTi HTi HTi

N° échantillon V164 V300 V324 V326Sample Number

SiO 2 % 45.1a 46,62b 43,50 45,21 45,20 47,21 44,80 46,67TiO 2 3,50 3,61 3,35 3,48 2,35 2,45 2,36 2,46Al 2O3 14,80 15,30 14,85 15,43 15,50 16,19 15,70 16,35Fe2O3 16,15 16,69 16,35 16,99 14,90 15,56 14,70 15,31MnO 0,24 0,25 0,22 0,23 0,20 0,21 0,19 0,20MgO 3,78 3,91 6,25 6,50 6,30 6,58 6,25 6,51CaO 8,08 8,35 7,55 7,85 7,10 7,42 7,90 8,23Na2O 3,34 3,45 3,00 3,12 3,25 3,39 3,12 3,25K2O 0,89 0,92 0,58 0,60 0,59 0,62 0,66 0,69P2O5 0,80 0,80 0,56 0,58 0,35 0,37 0,32 0,33LOI 2,71 3,28 3,78 3,77

Total 54,29 99,49 99,52 99,77

Sc (ppm) 33* 33,0 28,5 28,0TiV 220,0 293,0 244,0 247,0Ni 27,0 86,0 88,0 95,0

Co 55,0 60,96c 65,0 49,3 60,0 56,4 54,0 51,6Cr 13,0 76,0 94,0 91,0Cs 1,5 1,3 2,5Rb 29,5 31,8 18,5 28,3 11,7 11,7 15,8 16,0Ba 375,0 398,0 410,0 364,1 580,0 531,9 415,0 402,5Th 3,4 4,4 1,7 1,7 1,8 2,1 1,9 2,1U 0,9 0,4 0,4 0,4

Nb 31,5 38,7 17,9 11,9 13,0 14,6 12,4 14,7Ta 2,4 0,7 0,9 0,8Pb 5,1 2,8 3,7 4,0Sr 335,0 397,3 346,6 495,0 491,3 584,0 596,6Zr 225,0 347,0 106,0 97,1 138,0 132,0 135,0 136,1Hf 7,8 2,4 3,3 3,3Y 57,5 70,6 35,5 32,7 36,0 37,1 34,5 37,5

Zr/Hf 44,7 44,7 40,9 40,5 41,7Nb/Ta 16,0 16,0 17,3 16,5 18,6

La (ppm) 34,0 37,3 19,4 13,4 15,3 15,9 15,8 15,6Ce 79,0 82,4 44,0 28,1 34,0 34,4 33,0 33,1Pr 10,9 3,8 4,4 4,4Nd 46,0 48,6 27,5 16,4 21,0 19,9 20,0 19,4Sm 10,4 11,8 6,7 4,5 5,5 5,5 5,6 5,2Eu 3,5 3,9 2,4 1,6 1,5 1,9 2,0 1,9Gd 12,1 12,5 7,3 5,1 6,1 6,2 6,1 6,0Tb 2,0 0,9 1,0 1,0Dy 10,7 11,4 6,7 5,3 6,4 6,3 6,2 6,2Ho 2,4 1,2 1,3 1,3Er 5,5 6,5 3,2 3,3 3,3 3,8 3,2 3,7

Tm nd 1,6 1,9 1,5Yb 4,4 5,4 2,8 2,7 3,0 3,1 3,0 3,1Lu 0,8 0,4 0,5 0,5

La/Nb 1,0 1,1 1,1 1,1(La/Yb)N 5,0 4,5 3,7 3,7

Mg# 0,2 0,3 0,3 0,387Sr/86Sr 0,713335 ± 10 0,711425 ± 9 0,712375 ± 8

87Rb/86Sr 0,2316 0,2362 0,0778

(87Sr/86Sr)i 0,712181 0,710248 0,711987143Nd/ 144Nd 0,512501 ± 5 0,512656 ± 6 0,512650 ± 7147Sm/ 144Nd 0,1460 0,1660 0,1623

(143Nd/ 144Nd)i 0,512165 0,512276 0,512278( Nd)i -0,40 1,70 1,80

(206Pb/204Pb) 18,848 18,496 18,489

(207Pb/204Pb) 15,737 15,709 15,709

(208Pb/204Pb) 39,541 39,037 39,089

(206Pb/204Pb)i 17,886 18,047 18,176

(207Pb/204Pb)i 15,656 15,685 15,692

(208Pb/204Pb)i 38,331 38,338 38,490

aAnalyses performed by ICP-optical spectroscopy at the Université de Bretagne occidentale in Brest.bMajor elements recalculated to 100% on an anhydrous basis. Analyses performed by ICP-MS at the Université

Joseph Fourier

c

TABLE IIa. – Major, trace element and isotopic (Nd, Sr and Pb) com-positions of the Baralacha La basaltic dykes.TABL. IIa. – Compositions en éléments majeurs, traces et isotopes (Nd,Sr, Pb) des basaltes de Baralacha La.

In summary, clinopyroxenes from the Baralacha Lashow tholeiitic and alkaline affinities and Nb and Ta nega-tive anomalies with respect to La and Th, respectively.

GEOCHEMISTRY OF THE BARALACHA LABASALTIC DYKES

The whole rock major, trace and isotope compositions of se-lected samples of the Baralacha La basalts are reported ontable IIa, b, c.

Analytical procedures

Major, compatible and incompatible trace elements weredetermined (table II) by ICP-optical spectroscopy at theUniversité de Bretagne occidentale in Brest, using the pro-cedure of Cotten et al. [1995]. Trace elements including U,Pb and the whole set of REE (table II) were analyzed byICP-MS at the Université Joseph Fourier in Grenoble, after

acid dissolution of 100 mg sample, using the procedure ofBarrat et al. [1996]. Limits of detection for REE andY = 0.03 ppm, U, Pb and Th = 0.5 ppm, Hf andNb = 0.1 ppm, Ta = 0.03 ppm and Zr = 0.04 ppm. Standardsused for the analyses were JB2, WSE, BIR-1 and JR1. Ana-lytical errors are 1-3% for major elements and less than 3%for trace elements. All the samples were pulverised in anagate mill. Ta contents in some samples (V192) were notaccurately measured.

Sr (static acquisition) and Nd (dynamic acquisition) iso-tope ratios (tabl. II) were determined on 9 samples at theLaboratoire de Géochimie Isotopique de l’Université PaulSabatier de Toulouse on a Finnigan MAT261 multicollectormass spectrometer using the analytical procedure of Lapierreet al. [1997]. Results on standards yielded 143Nd/144Nd =0.511850 ± 0.000017 (2� external reproductibility) on 12standards analysed. Results on NBS 987 Sr standard yielded87Sr/86Sr = 0.710250 ± major 0.000030 (2� external



Alcalic Alcalic Alcalic TholeiiticHTi HTi HTi LTi

N° échantillon V327 V333b S7 V192Sample Number

SiO2% 44,80 46,62 43,00 45,86 43,90 45,30 45,60 47,42TiO2 3,64 3,78 3,57 3,80 3,32 3,43 1,33 1,38Al2O3 14,80 15,40 14,20 15,14 15,00 15,48 17,10 17,78Fe2O3 16,00 16,65 16,05 17,12 16,35 16,87 13,00 13,52MnO 0,23 0,24 0,23 0,24 0,22 0,23 0,17 0,18MgO 3,80 3,95 5,40 5,76 6,35 6,55 7,12 7,40CaO 8,05 8,37 7,00 7,46 7,55 7,79 8,85 9,20Na2O 3,04 3,16 2,70 2,87 3,02 3,12 2,55 2,65K2O 1,02 1,06 0,93 0,99 0,60 0,62 0,25 0,26P2O5 0,70 0,70 0,68 0,72 0,59 0,61 0,19 0,20LOI 3,17 5,44 2,96 3,43

Total 99,25 99,20 99,86 99,59

Sc (ppm) 31,0 35,0 33,0 27,0TiV 220,0 306,0 300,0 200,0Ni 25,0 79,0 90,0 127,0Co 47,0 49,4 55,0 49,1 65,0 61,6 68,0 65,8Cr 7,0 70,0 80,0 47,0Cs 1,9Rb 28,5 26,6 28,9 19,0 17,7 8,0 7,9Ba 825,0 769,9 266,4 415,0 392,0 390,0 364,3Th 3,3 4,1 1,9 2,1 1,7 2,0 1,2 1,4U 0,7 0,4 0,3 0,2

Nb 27,5 29,2 23,0 22,7 18,0 19,3 7,0 6,9Ta 1,7 1,4 1,2 ndPb 6,9 2,0 4,7 2,5Sr 405,0 409,0 221,6 296,0 300,0 405,0 393,8Zr 205,0 325,6 154,0 193,5 136,0 118,8 90,0 91,0Hf 7,2 4,6 2,8 2,3Y 52,5 56,9 43,0 39,8 35,5 38,4 28,5 28,4

Zr/Hf 45,1 42,2 43,0 39,1Nb/Ta 17,7 16,5 15,7 nd

La (ppm) 30,5 29,1 24,5 21,5 19,0 19,2 11,3 10,8Ce 67,0 62,4 54,0 50,6 44,0 42,2 23,0 22,1Pr 8,3 6,8 5,8 2,8Nd 40,5 37,0 34,0 29,6 27,0 26,0 13,0 12,5Sm 9,5 9,3 7,9 7,2 6,5 6,7 3,6 3,2Eu 3,1 3,1 2,2 2,6 2,4 2,4 1,3 1,3Gd 10,7 10,0 8,6 7,7 7,1 7,0 4,5 4,2Tb 1,6 1,2 1,1 0,7Dy 9,7 9,2 7,9 7,0 6,6 6,4 4,8 4,4Ho 2,0 1,4 1,3 1,0Er 5,2 5,2 4,1 3,9 3,2 3,6 2,7 2,9

Tm 1,7Yb 4,0 4,6 3,4 3,5 2,9 2,7 2,7 2,7Lu 0,7 0,5 0,4 0,4

La/Nb 1,0 1,0 1,0 1,6(La/Yb)N 4,6 3,6 5,2 2,8

Mg# 0,2 0,3 0,3 0,487Sr/ 86Sr 0,711165 ± 9 0,723900 ± 8 0,711635 ± 887Rb/ 86Sr 0,1882 0,3777 0,0582

( 87Sr/ 86Sr)i 0,710228 0,722020 0,711345143Nd/ 144Nd 0,512544 ± 6 0,512647 ± 4 0,512569 ± 5147Sm/ 144Nd 0,1517 0,1475 0,1566

(143Nd/ 144Nd)i 0,512196 0,512309 0,5122100,20 2,40 0,45

(206Pb/204Pb) 18,735 19,053 18,252(207Pb/204Pb) 15,735 15,748 15,693(208Pb/204Pb) 39,414 39,823 38,861(206Pb/204Pb)i 18,354 18,326 17,955(207Pb/204Pb)i 15,714 15,708 15,677(208Pb/204Pb)i 38,733 38,594 38,241

Tholeiitic Tholeiitic TholeiiticLTi LTi LTi

N° échantillon V148 B29 B50Sample Number

SiO 2 % 49,70 51,75 45,65 47,60 48,10 50,22TiO 2 1,71 1,78 1,28 1,33 0,89 0,93Al 2O3 13,75 14,32 17,00 17,72 15,40 16,08Fe2O3 13,75 14,32 12,75 13,29 10,00 10,44MnO 0,22 0,23 0,16 0,17 0,17 0,18MgO 5,74 5,98 7,00 7,30 9,00 9,40CaO 8,45 8,80 8,65 9,02 7,55 7,88Na2O 2,40 2,50 2,95 3,08 3,15 3,29K2O 0,08 0,08 0,27 0,28 1,38 1,44P2O5 0,23 0,24 0,20 0,21 0,14 0,15LOI 3,57 3,84 3,73

Total 99,60 99,75 99,51

Sc (ppm) 45,0 27,0 34,0TiV 390,0 192,0 25,0Ni 62,0 118,0 170,0Co 65,0 60,1 65,0 56,9 58,0 55,9Cr 115,0 47,0 ndCs 1,1 2,1 3,1Rb 4,4 3,3 10,7 8,9 32,9Ba 32,0 30,2 195,0 182,4 430,8Th 1,0 1,5 1,4 1,4 2,4U 0,3 0,2 0,4

Nb 10,8 11,5 6,6 6,6 6,4Ta 1,1 0,6 0,5Pb 2,5 2,3 3,4Sr 227,0 225,6 245,0 234,5 489,3Zr 100,0 103,0 88,0 86,1 80,4Hf 2,7 2,4 2,2Y 29,0 30,5 28,0 27,4 29,7

Zr/Hf 38,6 36,7 37,0Nb/Ta 10,8 11,0 11,9

La (ppm) 11,5 11,1 10,8 10,6 10,0Ce 26,0 24,9 22,5 21,8 20,4Pr 3,4 2,8 2,7Nd 16,0 14,6 12,5 12,6 11,2Sm 4,3 4,1 3,7 3,4 3,2Eu 1,5 1,4 1,2 1,3 1,0Gd 4,9 4,5 4,2 4,0 4,0Tb 0,8 0,7 0,7Dy 5,1 5,0 4,6 4,6 4,8Ho 1,1 1,0 1,1Er 2,8 3,2 2,8 2,9 3,1

Tm 1,4 1,5 1,8Yb 2,6 2,9 2,8 2,7 2,9Lu 0,5 0,4 0,4

La/Nb 1,0 1,6 1,6(La/Yb)N 2,8 2,9 2,5

Mg# 0,3 0,4 0,587Sr/86Sr 0,714232 ± 9 0,709331 ± 7 0,715513 ± 8

87Rb/86Sr 0,0419 0,1101 0,1945(87Sr/86Sr)i 0,714023 0,708782 0,714544

143Nd/ 144Nd 0,512695 ± 7 0,512575 ± 6 0,512506 ± 5147Sm/ 144Nd 0,1693 0,1634 0,1703

(143Nd/ 144Nd)i 0,512307 0,512201 0,512116( Nd)i 2,34 0,30 -1,40

(206Pb/204Pb) 19,014 18,287 18,787(207Pb/204Pb) 15,776 15,685 15,747(208Pb/204Pb) 39,680 38,875 39,368(206Pb/204Pb)i 18,536 17,996 18,382(207Pb/204Pb)i 15,750 15,669 15,725(208Pb/204Pb)i 38,963 38,181 38,546

TABLE IIb-c. – Major, trace element and isotopic (Nd, Sr and Pb) compositions of the Baralacha La basaltic dykes.TABL. IIb-c. – Compositions en éléments majeurs, traces et isotopes (Nd, Sr, Pb) des basaltes de Baralacha La.

reproductibility) on 11 standard determinations. 87Sr/86Srand 143Nd/144Nd were normalized for mass fractionation rel-ative to 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219 re-spectively. εNdi were calculated with the present day(143Nd/144Nd)CHUR = 0.512638 and (147Sm/144Nd)CHUR =0.1967. Prior to Nd and Sr separation and acid digestion,two leaching steps in 2.5N HCL during 20 minutes at 100oCwere made.

For lead separation, 9 powdered samples were weightedto obtain approximatively 100 to 200 ng of lead. A leachingstep with 6N HCl during 30 minutes at 65oC was made be-fore acid digestion. Samples were then dissolved during36-48 hours on a hotplate with a mixing of tridistilled Hf:HNO3 concentrated acids. After evaporation to dryness,1 ml of HNO3 was added to the residue and kept at about90oC for 12-24 h. After complete evaporation, 0.5 ml of 8 NHBr was added to the sample which was kept at 70oC for2-3 h before complete evaporation. The chemical separationof lead was done using 50 µl of anion exchange resin(AG1X8, 200-400 mesh) and samples were loaded andwashed in 0.5 N HBr. Lead was then eluted in 6 N HCl. Pb

blanks were less than 40 pg and are negligible for the pres-ent analyses.

Lead isotope analyses were made on a VG modelPlasma 54 magnetic sector-inductively coupled plasma-mass spectrometer (MC-ICP-MS) at the Ecole NormaleSupérieure de Lyon. Lead isotope compositions were mea-sured using the Tl normalization method described byWhite et al. [2000]. For Pb isotope analysis, samples werebracketed between NBS 981 standards and calculated withrespect to the value reported for this standard by Todt et al.[1996]. This technique yields internal precision of ca.50 ppm (2σ and an external reproductiblity of ca. 150 ppm[2σ Blichert-Toft et al., 2003] for 206Pb/204Pb ratios deter-mined on 20 NBS 981 standards.

Major element geochemistry

The loss of ignition (LOI) of the Baralacha La dykes rangesbetween 3 and 6.5% (tabl. II). Considering the alteration, thelow grade metamorphism and weathering processes that haveaffected the Carboniferous dykes, which prevent us to use



FIG. 2. – Detailed geological map of the SE Zanskar, NWSpiti et Haut Lahul [after Vannay and Spring, 1993] showingthe location of the samples analyzed in this work.FIG. 2. – Carte géologique détaillée du SE Zanskar, NW Spitiet Haut Lahul [d’après Vannay et Spring, 1993] montrant lalocalisation des échantillons analysés du Baralacha La.

mobile elements (K2O, Na2O, SiO2, CaO) to characterize themagmatic affinities, we have used the TiO2 contents and theREE patterns of the whole rocks to distinguish the alkali andtholeiitic basalts. The tholeiites (V192, V148, B29, B50) arecharacterized by low TiO2 contents (LTi, TiO2 < 2%) andslightly LREE enriched patterns while the alkali basalts(V164, V300, V324, V326, V327, S7) have higher TiO2 con-tents (HTi, TiO2 > 2%), and greater enrichments in LREE.Moreover, the tholeiites have higher MgO and lower P2O5contents than the alkali basalts.

Trace element geochemistry

The chondrite-normalized [Sun and McDonough, 1989, fig.6b] REE patterns of the Baralacha La tholeiites are slightlyenriched in LREE [(La/Yb)N < 2.9] and show Eu positive ornegative anomalies. The alkali basalts differ from the

tholeiites by higher REE contents, greater LREE enrich-ments [3.5 (La/Yb)N < 5.2] and no Eu anomalies.

Because of LILE mobility during alteration and weath-ering processes, these elements are not shown in the primi-tive mantle-normalized [Sun and McDonough, 1989, fig. 6]multi-element plots of the Baralacha La basalts. Thetholeiites are characterized by Nb and Ta (La/Nb ~ 1.5 withthe exception of V148) negative and Th positive anomalies,LREE enrichments and rather flat patterns. These featuresare those of continental tholeiites. The alkali basalts, com-pared to the tholeiites, exhibit greater enrichments in in-compatible elements, and negative anomalies in U. Somesamples (S7, V300) show Zr and Hf negative anomalies;others (V300, V324, V326) weak Nb and Ta negative anom-alies (1.06 < La/Nb < 1.13).



FIG. 3. – Panorama (a) and lithostratigra-phic columns [after Vannay, 1993], sho-wing the geometric layout of the LowerCarboniferous syn-sedimentary faults andBaralacha La basaltic dykes.FIG. 3. – Panorama (a) et colonnes lithostra-tigraphiques (b) de la succession paléo-zoïque [d’après Vannay, 1993] illustrant ladisposition géométrique des failles synsé-dimentaires du Carbonifère inférieur etdes filons basaltiques du Baralacha La.

Isotopic geochemistry

The complete isotopic data set has been corrected for in situdecay assuming an age of 350 Ma, based on stratigraphicconstraints (refer to the stratigraphic column, fig. 3b). Themeasured and initial ratios are shown in table II.

All the Baralacha La basalts have high initial 87Sr/86Srratios that do not reflect the composition of the sources ofthe mafic melts. These high 87Sr/86Sr are due to alteration,low grade metamorphism or weathering processes that haveaffected these rocks.

The εNdi values of the Baralacha La basalts range be-tween + 2.3 and – 1.4. Such values suggest that the compo-sition of the source (or sources) of the Baralacha Labasalts resembles that of the Bulk Silica Earth (BSE) andreflects to some extent crustal contamination or assimila-tion. In order to characterize the importance of this crustalcontamination, we have plotted εNdi versus La/Nb andNb/Th (fig. 7a). Three out of four tholeiites (V148, V192,B29) and three alkali basalts (V300, V326, V33B) out offive show a negative correlation in the εNdi versus La/Nbplot. In the eNdi–Nb/Th correlation diagram, only thetholeiites exhibit a negative correlation; the samples withthe lowest (B50) and highest (V148) εNdi values have thehighest and lowest Nb/Th ratios, respectively. To deter-mine if the crustal contamination of the Baralacha La

basalts is related to assimilation, fractional, crystallizationprocesses (AFC), we have plotted these rocks in anεNdi-Zr (ppm) diagram (fig. 7a), Zr being considered hereas fractionation index. In this diagram, solely the tholeiitesdisplay a correlation and surprisingly, the rock (B50) withthe lowest Zr content (and the highest MgO) has the lowestεNdi value.

In Pb-Pb correlation diagrams (fig. 7b), the BaralachaLa basalts, whatever their affinity, plot above the NorthHemisphere Reference Line [NHRL, Zindler and Hart,1986] and are aligned between either BSE and high µ(238U/204Pb ratio, HIMU) fields or BSE and enriched man-tle Type I (EMI) fields. Samples B29 (tholeiite) and V300(alkali), characterized by the highest La/Nb ratios, plotnear the EMI field. Inversely, samples V148 (tholeiite) andV333B (alkali) that have the highest eNd values and thelowest La/Nb ratios plot towards the HIMU field. How-ever, the two alkali basalts (V164, V327) that are not lo-cated on the εNdi-La/Nb negative correlation trend(fig. 7a) plot at both ends of the trends observed in thePb-Pb correlation diagrams. Sample V164 with one of thelowest eNdi values (-0.43, tabl. II) plots near EMI. SampleB50 which has the lowest εNd value (-1.4, tabl. II) plotsnear EMII.

In the εNdi versus (206Pb/204Pb)i diagram (fig. 7c), allthe basalts, with the exception of samples B50 and V327,



FIG. 4. – (a) Nomenclature of the Baralacha La clinopyroxenes after Morimoto et al. [1988]. (b) Magmatic affinities of the Baralacha La clinopyroxenesafter Leterrier et al. [1982]. (c) Major element distribution of the Baralacha La clinopyroxenes during differentiation process.FIG. 4. – (a) Nomenclature des clinopyroxènes des basaltes de Baralacha La d’après Morimoto et al. [1988]. (b) Affinités magmatiques des clinopyroxènesdes basaltes de Baralacha La d’après Leterrier et al. [1982]. (c) Comportement des éléments des clinopyroxènes de Baralacha La au cours de la différen-ciation.

display a positive correlation between HIMU and EMIfields. Located at the two ends of this trend, B29(tholeiite) and V164 (alkali) are close to the BSE and EMIfields, respectively, while V148 (tholeiite) and V333B (al-kali) plot towards the HIMU field. B50 and V327 that dif-fer by higher (206Pb/204Pb)i plot towards the EMII field.

DISCUSSION

Involvement of the continental crust in the genesis ofthe Baralacha La basalts

Whatever the affinity of the Baralacha La basalts, their Nband Ta negative anomalies and Nd and Pb isotopic ratiosshow that these volcanic rocks derived from the melting ofan enriched mantle source (Oceanic Island Basalt, OIB),contaminated to some extent by continental crust.

On the basis of the εNdi-Zr diagram (fig. 7a), we mayassume that the variations of the εNdi of the Baralacha Labasalts towards 0 and negative values, at least for thetholeiites, are not related to AFC processes.

The correlation shown by the Baralacha La tholeiiticdykes in the εNdi-Nb/Th diagram (fig. 7a) suggests that theyhave been more or less contaminated by the upper crust be-cause their lowest Nb/Th (B29, B50) ratios approach that ofthe upper crust [2.38, Weaver and Tarney, 1984]. The Nb/Thratio of the lower crust [11.90, Taylor and McLennan, 1995]is significantly higher than that of the lower crust. Accordingto the εNdi-Nb/Th diagram, sample B50 appears to be themost contaminated by the upper crust. This is in agreement

with its Pb and Nd ratios because this sample plots near theEMII field in the Pb-Pb and εNdi-(206Pb/204Pb)i diagrams.This assumption does not fit with sample V148 because, evenif it plots near the EMII and BSE fields in the Pb-Pb dia-grams, (i) its Nb/Th ratio is greater than that of the uppercrust, and (ii) it plots near the HIMU field in the εNdi and(206Pb/204Pb)i diagram (fig. 7c). Thus, we can assume thatsamples V148 and B50 derive from the melting of at leasttwo components, an enriched source containing some HIMUcomponent and the upper continental crust (EMII). The iso-topic composition of sample V148 could represent that of theenriched mantle source containing some HIMU component.For samples B29 and V192, it seems more difficult to charac-terize the different sources that contributed to their genesisbecause, they plot in the middle of the BSE-EMI, BSE-EMIItrends in the Pb correlation diagrams. However, the lowercrust (EMI) could be involved in the genesis of samples B29and V192 because they plot towards the EMI field in theεNdi-(206Pb/204Pb)i diagram.

No correlation is observed in the εNdi-Nb/Th diagram(fig. 7a) for the Baralacha La alkali dykes. For more or lesssimilar εNdi values (+ 1.7), the Nb/Th ratio ranges between5.5 and 6.9. Moreover, sample V164 with the lowest εNdivalue (– 0.45) has one of the greatest Nb/Th ratios (8.86).This suggests that the Nb/Th variations are more linked todifferences in the degree of mantle partial melting than to theextent of crustal contamination of the mantle source. The up-per crust does not seem to be the main crustal contaminant ofthe alkali dykes because their Nb/Th are higher than that ofthe upper crust, and closer to that of the lower crust.



FIG. 5. – (C1) Chondrite-normalized rare earth element patterns and primi-tive mantle-normalized multi-element plots [Sun and McDonough, 1989]of the Baralacha La clinopyroxenes.FIG. 5. – Spectres des terres rares normalisés aux chondrites (C1) et dia-grammes multi-éléments normalisés au manteau primitif [d’après Sun etMcDonough, 1989] des clinopyroxènes des basaltes de Baralacha La.

FIG. 6. – Chondrite-normalized rare earth element patterns and primitivemantle-normalized [Sun and McDonough, 1989] multi-element plots of theBaralacha La basalts.FIG. 6. – Spectres des terres rares et multi-éléments normalisés aux chon-drites et au manteau primitif [d’après Sun and McDonough, 1989] des ba-saltes de Baralacha La.

Moreover, the alkali basalts, in the (208Pb/204Pb)i and εNdiversus (206Pb/204Pb)i diagrams (figs. 7b and c), are locatedalong the BSE-EMI trend (with the exception of V327).

Origin of the geochemical differences betweentholeiitic and alkalic basalts

On the basis of correlations between P2O5 (% wt) and Zr(ppm) and the total alkali versus silica diagram, Vannay andSpring [1993] already pointed out the tholeiitic and alkalineaffinities of the Baralacha La dykes. They also suggestedthat the Baralacha La basalts were cogenetic and that crys-tal fractionation was a major process during their genesis.According to these authors, the most enriched basalts in in-compatible elements derived from the most depleted onesby crystal fractionation.

We do not agree that tholeiitic and alkalic basalts arecogenetic and linked by crystal fractionation process. Wethink, on the basis of elemental and isotopic chemistry, thatthe differences in incompatible trace element contents be-tween the tholeiitic and alkalic Baralacha La basalts arelinked to variations of the mantle partial melting ratios anddegrees of enrichment of the sources.

Indeed, no correlation trends are observed in La (ppm)versus (La/Yb)N plot (not presented here) [Caroff et al.,1997]. If the tholeiites and alkali basalts were linked bycrystal fractionation, the (La/Yb)N ratio should remain con-stant while La increases. This is not the case.

Samples V327, V192, and B29 have similar εNdi values(ranging between + 0.2 and 0.45, tabl. II). However, the al-kali sample V327 differs from the two others by higher con-tents in LREE and Th (tabl. II). Similarly, samples V333Band V148 with similar εNdi values (+ 2) differ by incompa-tible trace element contents. The alkali sample V333B,compared to tholeiitic V148 sample, has a higher (La/Yb)Nratio (3.4) and Th content (2.1 ppm, tabl. II). In both cases,

the differences in incompatible trace element contentsbetween tholeiitic and alkaline rocks can be attributed tovariations of the mantle partial melting degrees because thesources of both rocks have similar Nd isotopic composi-tions. Inversely, some tholeiitic basalts (V148, B29, B50,table II) have similar (La/Yb)N ratios (ranging between 2.5and 2.8) but differ by their εNdi, ranging between + 2.3 and– 1.4 (table. II). In this case, the differences in the incom-patible trace element contents of the three tholeiites can beattributed to differences in composition of the sources(mantle versus continental crust) involved in their genesis.

Geodynamic implications

The Baralacha La dykes never intrude rocks younger thanearly Carboniferous and are associated with contemporane-ous transtensional faults, which are locally synsedimentary.These tectonic structures evidence the presence of an earlyCarboniferous extensional tectonic event in the Tethyan Hi-malayan Ranges [Vannay and Spring, 1993]. The earlyPermian granite and alkalic microgabbro of Yunam wereemplaced during another likely extensional event of thenorthern Indian margin. In the Middle Permian, the Panjalcontinental flood tholeiites were emplaced during a majorrifting event that led to the thinning and break-up of thenorthern Indian margin; the Neotethyan opening and thenorthward drift of the Cimmerian blocks (Lhassa, Iran).

Thus, three rifting related events associated with exten-sion and magmatism characterize the evolution of the north-ern Indian margin. The early Carboniferous event predatesby about 70 Ma the Permian Neotethyan opening and theemplacement of the widespread volcanism known from theHimalayan Ranges up to Oman and through Iran. Takinginto account the important time gap between the emplace-ment of the Baralacha La basaltic dykes and the PanjalTraps, it seems difficult to consider the Baralacha La ba-saltic dykes as an early event of the Neotethyan opening.



FIG.7a. – Isotope compositions of the Baralacha Labasaltic dykes. (a) εNdi versus La/Nb, Nb/Th and Zr(ppm) correlation diagrams.HIMU = enriched mantle component characterizedby a high µ (238U/204Pb) ratio. EMI = enrichedmantle component type I. EMII = enriched mantlecomponent type II. BSE = bulk silica earth.FIG.7a. – Compositions isotopiques des filons basal-tiques de Baralacha La. (a) Diagrammes de corréla-tion eNdi en fonction de La/Nb, de Nb/Th et de Zr(ppm). a

The two tectono-magmatic events (early Carboniferous andearly Permian) are most probably related to the thinning andfaulting of the northern Indian plate prior to the emplace-ment of the Panjal Traps during the Middle Permian.

CONCLUSION

Tholeiitic and alkalic basalts deriving from an enrichedmantle source, characterized by an HIMU type component,and contaminated by the lower continental crust (and insome samples by the upper crust) have been emplaced in theearly Carboniferous on the northern Indian plate. These bas-alts are related to a transtensional synsedimentary fault sys-tem. This early Carboniferous magmatism was followedsome 70 Ma after by the eruption of the Mid to UpperPermian Panjal Traps which are associated to the rifting ofthe Indian northern passive margin and the Neotethyanopening. The mid to late Permian Panjal Traps can be corre-lated with the mid-Permian tholeiitic and alkaline volcanicsof the Oman Ranges [Maury et al., 2003] considered to

represent the igneous remnants of the Arabian margin rift-ing and Neotethyan opening. This widespread Permianwithin-plate volcanism is probably related to the melting ofa large plume, named the “Tethyan” plume [Lapierre et al.,2004]. Taking into account the important temporal gap be-tween the emplacement of the Baralacha La dykes and thevoluminous flows of the Panjal Traps, it is difficult to con-sider the early Carboniferous tectono-magmatic activity asbeing related to the Neotethyan opening. However, we as-sume that the Baralacha La dykes represent: (i) the rem-nants of a faulting event recorded on the future Indiannorthern margin and (ii) the upwelling of an enrichedasthenospheric mantle during the early Carboniferous butnot the initial Permian Tethyan plume.

Acknowledgements. – This work was supported by the Institut National desSciences de l’Univers, programme « Intérieur de la Terre ». Many thanksto P. Telouk and A. Agranier and the Service commun National duMC-ICPMS of the Ecole Nationale Supérieure de Lyon for their help du-ring the Pb isotope measurements.



FIG.7b-c. – Isotope compositions of the BaralachaLa basaltic dykes. (b) (208Pb/204Pb)i and(207Pb/204Pb)i versus (206Pb/204Pb)i correlation dia-grams (c) εNdi versus (206Pb/204Pb)i correlation dia-gram.HIMU = enriched mantle component characterizedby a high µ (238U/204Pb) ratio. EMI = enrichedmantle component type I. EMII = enriched mantlecomponent type II. BSE = bulk silica earth.FIG.7b-c. – Compositions isotopiques des filons ba-saltiques de Baralacha La. (b) Diagrammes de cor-rélation (208Pb/204Pb)i et (207Pb/204Pb)i en fonctionde (206Pb/204Pb)i. (c) Diagramme de corrélationeNdi en fonction de (206Pb/204Pb)i.

b

c

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The Carboniferous Baralacha La basaltic dykes (Upper Lahul, Ladakh) : remnants of an early rifting...

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