RENDJCONT! DELLA SOCIETA ITALlANA DJ MINERAl-OGJA E PETROLOGIA, 1987. Vol. 42, pp. 9H02

Preliminary data on the High Himalayan Crystallines along the - Traverse (South-Eastern , )

UGO POGNANTE Dipartimento di Sdenze della Terra, Via Valperga Caluw 37, 10125 Torina

GIUSEPPE GENOVESE Viale Roma 50, 10078 Venaria Reale, Torino

BRUNO LoMBARDO Centm di Studio sui Problemi cIcll'OrogellO delle Alpi Occidentali, C.N.R., Via kcademia delle Scierue 5, lOU3 Torino

PIERGIORGIO ROSSEITI Dipartimento di Georisorse e Territorio, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino

ABSTRACT. _ Preliminary study of the High Himalayan di rocre crostali. 11 HHC e stato teltonicamente sovra· CrystaHines (HHCj in SE Zanskar has shown the ex­ scorso da metasedimenti di basso grado (meugrovacche istence of an early high temperature event of hypothetical dello ) e dalle sequence merasedimentarie di Late Proterowic age associated with migmatization and gudo molto basso della Zona Tibetana. to production of leucocratic granitoids; this event is Parole chiave: mctamorfismo, migmatiti, Icucograniti, followed by a lower pressure high/medium temperature sovrascorrimenti, Himalaya. metamorphism of Himalayan age. During Miocene times the HHC have been intruded by the Gumbutanjon leucogranite which, like other Himalayan leucogranites, derives by partial melting of crustal rocks and post-dates the main Himalayan metamorphism. The HHC are over­ Introduction thTUSt by a sequence of low grade metasediments (Shingo La metagraywackes) and are in tectonic contact with the This work deals with a preliminary study very low grade metasediments of the Tibetan Zone. carried out during summer 1985 in south­ Key words: metamorphism, migmathe, leucogranite, eastern Zanskar (Fig. 1) on the High thrust, Himalaya. Himalayan Crystallines (HHC; FRANK et al., 1977; SRIKANTIA et aI., 1980; HONEGGER et RIASSUNTO. - Lo studio preliminare del Cristallino del­ al., 1982; SEARLE and FRYER, 1986) near the l'Alto Himalaya (HHCj nel settore sud-orientale dello boundary with the Late Precambrian­ Zanskar (India nord-occidentalej ha messo in evidenza Paleocene sedimentary nappes of the Tibetan un evenlo metamotfico di alca temperatura di ipotetica Zone (BAUD et al., 1984; GAETANI et al., 1985) eta tardo ProtCTozoica associato a processi di migmatiz· zazione con produzione di granitoidi leucocrntici. Un sue­ (Fig. 2). In particular the work describes the cessivo evento metamorfico di eta himalayana ha pro· geological setting and the metamorphic evolu­ dolto una ricristallizzazione in condizioni di tempera· tion of the HHC along the traverse Padum tura mediofalta e ptessioni piu basse associata ad inten· (Zanskar) - Darcha (Lahul). The areas studied si fenomeni deformativi e traspositivl. Durante il Mio­ are, from north-west to south-east, the Hap­ cene il BHC estato intruso dalleucogranito del Gum­ buranjon, che esuccessivo ai principali eventi metamor­ tal valley, the Temasa valley and the Shingo fid himalayani e deriva da processi di fusione parziale La - Gumburanjon area (Fig. 2). 96 U. J'OGNANTE. G. GENQVESE. B. LOMI\AROO. ]> ROSSETTI

± 50 Ma; FRANK et al., 1977; MEHTA, 1977), [SJ~[Zl[] while has not been wholly successful in the leucocratic onhogneisses of the Zanskar 456 region. The laner have been ascribed to the pre-Himalayan cycle (550 ± 50 Ma) by HONEGGER et al. (1982) and to the Himalayan o cycle (Miocene) by SEARLE and FRYER (t986). As to the metamorphism it has been describ­ 7 ed by HONEGGER et al. (1982) which defined a series of isograds of Himalayan age (cool­ ing ages for biotite: 11-13 Ma). Our data demonstrate that the history is more complicated than suggested by the previous authors; notably two generations of leucocratic granitoids ofdifferent ages occur in Zanskar and, in addition to the Himalayan mineral zones described by HONEGGER et al. (1982), the metamorphic crystallization has • a composite history with relics ofahypothetical pre-HimakJyan event overprinted by the Tertiary ~A HimakJyan recrystallization.

~~.~~.(BET Geologic and metamorphic settings Qt} INDIA\..." The geologic and tectonic settings of south· eastern Zanskar are shown in Figures 2 and 3, while the main mineral compositions are Fig. 1. - Tectonic sketch map of western HimaJaya. summarized in Table 1. Legend. 1, Tibetan block; 2: Trans.Himalllyan balOliths; 3: Ophiolites and related sedimems; 4: Sediments of the Tibetan Zone; 5: High Himalayan Crystallincs; 6: Lesser Haptal valley Himalaya; 7: Molasse.typc sedimenls (Siwaliks). met: main central thrust; mbt: main boundary thrust. Inset The Haptal valley is located south of Sani, indicates location of the studied area shown in detail a small hamlet NW of Padum. In this valley, in Fig. 2. the tectonic boundary between the HHC and the overlying late Precambrian-Paleozoic Pughtal nappe of the Tibetan Zone (BAUD et al., 1984) crops out just SW of Sani. This Previous studies on the HHC in the boundary is defined by a mylonitic belt, a few Zanskar.Lahul region tcns of metres thick, which dips 30-400 north. In the last decade peuographical studies eastward (Figs. 2, 3). In the Haptal valley the of the HHC in this sector of the Himalayan rocks of the PughtaJ nappe consist of very low belt were mainly concentrated in north­ grade metagraywackes (Phe Formation; NAN· westernmore (HONEGGER et al., 1982) and in DA and $INGH, 1977) with a pervasive folia­ sourh-easternmore areas (FRANK et al., 1977). tion defined by fine grained white mica and These studies have demonstrated that the chlorite. Mica and chlorite form films that HHC mainly consist of high/medium grade anastomose around pre-metamorphic clasts of metamorphics and of granitoids more or less quartz, plagioclase and K-feldspar. In the transformed into orthogneisses. Reliable mylonitic belt similar white mica-chlorite­ radiometric dating of these onhogneisses has bearing assemblages also occur in the HHC been only obtained on some Lahul stocks (550 rocks. PRELIMINARY DATA ON TilE lIIGll IlIMAI.AYAN CRYSTAlUNES ALONG THE PADUM·IlI\RCllA ETC. 97

The HHC include a sequence of dark mon assemblages include quartz, plagioclase, gneisses associated with dykes a few biotite, white mica, garnet ± K-feldspar por­ decimetres to rm=tres thick and with kilometric phyroclasts ± rare c1inopyroxene. About 3 intrusive bodies; both dykes and bodies con­ kilometres from the contact with the overly­ sist of leucocratic orthogneisses rich in garnet ing Pughtal nappe, the dark gneisses also in­ and in tourmaline. These rock-types are per­ clude sillimanite. vasively deformed and are refolded with the Near the sillimanite-bearing dark gneisses,

lmIo Elb oh • j l ~- d • .. e 0 m i... f v 0 I' • 0 o 9 -~ P

Fig. 2. - Geologic (I) and tectonic (bl maps of SE Zanskar along the Padum·Dan::ha geotrl\·ersc. Legend. a: mcutSC'dimc:nu of the Tiixtan naJlP6; b: Shingo u dark melagra)"l'ack". High Himalaran (riSlanines; c: dark ~neisses and micaschisu: d: kucoeratic onhogneisscs; c: posl-mc:tamorphic Gumburanjon kurogranite; f: Kade (hu onhogncissc:s. - ~Ier:amorphic as:scmbIages (for abbreviations Stt Tallk: 1).8' wm-dtl; h: am :E. gt porphyroblasu in mctasomatic vciru; i: 8t·bi in felsic rocks; I: gt.bi.p1.sill :l:: kr in fc:lsic rocks; m, gl-bi.pl·st in fc:lsic rocks; n: gl-cpx.qlz.hbl.pl.bi in mane rocks; 0: hbl.pl.bi in mafic rocks; p: main thrusts and tectonic contacts. I and 11 an' locations of the cross·sections shown in Fig. }. development of a foliation marked by brown rare lenses of mafk rocks are closely associated biotite and white mica. The dark gneisses and with leucocratic gneisses and pegmatoids rich the leucocratic orthogneisses are crosscut by in K-feldspar, plagioclase, biotite and white rare mafic dykes a few metres thick which mica. The mafic rocks show an early high have been also deformed during the metamor­ temperature assemblage with c1inopyroxene, phic recrystallization. They still contain relics garnet and quartz. The garnet has been later of magmatic minerals including pyroxene and corroded with the formation of a composite plagioclase phenocrysts; the magmatic corona consisting of zoned Ca-rich plagioclase minerals are largely altered to a metamorphic ± hornblende ± biotite ± opaques; assemblage consisting of hornblende, plagioclase in the coronas is usually crowded recrystallized plagioclase and pyroxene, garnet by smaIJ quartz inclusions. The garnet altera­ coronas around plagioclase, biotite and tion is coeval with the transformation of sphene. In the dark gneisses the most com- c1inopyroxene into hornblende. These 98 U, l'OGNANTE. G. GENOVESE. B. LOMBARDO. 1'. ROSSETIl

TABLE 1 dark gneisses an early assemblage including Main minerals ofthe rock-types occurring along kyanite, garnet, biotite, plagioclase ± rutile the Padum-Darcha traverse ± K-feldspar is replaced by an assemblage (5E Zanskar - NW Lahul, India) with muscovite, biotite, plagioclase, prismatic/acicular silIimanite, sphene. Among "",.-tn>o. .,..,." "'-'" the dark gneisses have been observed abun­ ~. dant rock-types rich in large K-feldspar por­ (0"1-1., H''l-,..dol·to "'l""" "n...... ""'..~ ..~ phyroclasts. Locally the latter rocks are strong­ $IIU'OO " ...,...... """

radiometric ages in the Gophu La and Nupt­ se granites of the eastern HimaJaya (VIlli. and LoMBARDO, 1986). The Shingo La metagraywackes are rich in clam of quartz and feldspar and show a mark­ ed foliation defined by fine grained white mica ssw ± biotite. The most common metamorphic NNE assemblage includes white mica, brown/pale brown biotite partly altered to chlorite, garnet ± dinozoisite. Between Gumburanjon and Shingo La they are characterized by cen· timetric porphyroclasrs of biotite ± garnet. The porphyroclasts aN: characterized by an in­ ternal foliation defined by quartz and opa­ ques; in many samples they are rotated and Fig. J. - Geologic cross-sections through lh~ High strained during a deformation even't which 11 ima!nyan Crystalljn~s and the ov~r1yjng metas~dimcn· also produced a more or less pervasive crenula· tary nnpp~s. For legend and cross·set:tion orientations tion cleavage marked by white mica and fine see Fig. 2. grained biotite. Moreover the Shingo La rocks are locally crosscut by centimetric metasomatic veins with amphibole ± garnet; metagraywackes are separated from the rocks these veins probably record a metasomatic of the Pughtal nappe by a normal fault. event which may be related to the intrusion The HHC metamorphics include a se­ of the Gumburanjon leucogranite. Although quence of biotite - garnet ± staurolite ± the Shingo La metagraywackes seem to in­ sillimanite ± kyanite ± clinopyroxene­ clude deformed and transposed leucocratic or­ bearing dark gneisses, transposed dykes of thogneisses, the contact with the other HHC leucocratic onhogneisses and minor rocks probably represents a tectonic thrust; metabasics (amphibolites). that is suggested by the existence of mylonites The unmetamorphosed leucogranitic body and by the apparent metamorphic jump from shows a mineralogical composition similar to the underlying medium/high grade metamor­ the leucocratic orthogneisses, but is more phics (stauroHte-garnet·kyanite-sillimanite heterogeneous and crosscuts the leucocratic bearing assemblages) to the overlying low orrhogneisses. The Gumburanjon leucogranite grade metagraywackes (biotite-muscovite ± contains white mica, garnet ± biotite and is dinozoisite ± garnet-bearing assemblages). locally very rich in tourmaline; it shows sud­ South of Shingo La, in the Kade Chu valley den and marked grain size variations from (I:igs. 2, 3), the Shingo La metasedimenrs pegmatoid 'to medium-grained varieties. display a sharp contact with the other HHC Xenoliths ranging in size from a few rocks, here represented by a huge body of or­ decimetres to sllvera! metres and consisting thogneisses which are less leucocratic than of dark gneisses with transposed dykes of those described above. leucocratic orthogneisses rich in garnet, are The Kade Chu orthogneisses consist of abundant in the Gumburanjon leucogranite. quartz, K-feldspar, plagioclase ± biotite and Rb-Sr mineral ages (FERRARA, LoMBAROO and show a foliation defined by fine grained white TONARlNI, in prep.) suggest that, like most mica ± biotite. Rb/Sr whole-rock investiga­ Himalayan leucogranites, the Gumburanjon tions (FERRARA et al., in progress) on these body was emplaced in the HHC during orthogneisses point to ages around 600 Ma, Miocene times. The age difference between in agreement with previous data on similar muscovite and biotite, about 2 Ma, suggests HHC orthogneisses of the Rohtang-La area, a cooling rate around 100° CfMa. Comparable some 50 km to the SSE, which yielded a Rb­ cooling rates have been computed from Sr isochron age of 601 ± 9 Ma (MEHTA, 100 u. fUGNANTF.. G GENOVESE, H LO~llIAR[)(). J' ROSSETTI

1977). The Kade Chu orthogneisses thus ap­ deformations and large scale transpositions in pear to belong to the plutonic cycle spanning all the HHC rocks, except the Gumburanjon the Late Proterozoic-Cambrian Ixluooary (550 leucogranite. In the ~tabasics it produced ± 50 Ma) which is incrt=asingly documented a meclium/high temperature crystallization throughout the (for a summary see with transformation of the older cpx-gt-qtz BoRTOLAMI et al., 1983). The 550 ± 50 Ma assemblage to a hocnblende-plagioclase­ cycle is dx= youngest of the thrtt granite cycles hiotite-ilmenite assemblage. Less obvious is recognized so far in the Himalayas from the microstructural evolution of the dark radiometric evidences, the older cycles occur­ gneisses; however, they locally show an early ring at 1200·1300 Ma and 1800-1900 Ma assemblage including kyanite, garnet, biotite respectively (BHANOT et al., 1979). ± K-feldspar ± rutile probably related to event I, transformed to a later assemblage rich in muscovite, sillimanite, sphene related to Discussion and conclusions event 2a. With respect to event 1, a decrease in pressure and in temperature is suggested The preliminary results of our study on the for event 2a by the previous features and by High Himalayan Crystallines in south-eastern the crystallization of staurolite ± garnet in Zanskar can be summarised as follows: some gnelsses. 1) Two main events have been recognized A Tertiary Himalayan age is implied for in the HHC. An early event 1 produced event 2a by the avaiJable radiometric ages migmatization in dark gneisses and formation from the studied area (FERRARA et al., in prep.) of leucocratic gcanitoids (transformed to and from adjacent sectors (HONEGGER et al., leucocratk onhogneisses during event 2a). 1982; SEARLE and FRYER, 1986). The lauer intrude as dykes or concentrate for· Event 2a is followed by the growth of ming km-sized bodies. In some mafic rocks biotite ± amphibole (event 2b) as a thermal associated with orthogneisses and with effect of the Gumhuranjon leucogranite. The migmatites event 1 produced dinopyroxene­ very low temperature syn-tectonic crystalliza­ garnet-quanz-bearing assemblages. tion (event 2e) occurring at the contact with A hypothetical pre·Himalayan (possibly the overlying nappes of the Tibetan Zone pro­ Late Proterozoic) age is suggested for event bably post-dates the intrusion of the Gum­ 1 by: a) the pervasive syn-tet:tonic medium buranjon stock; indeed, though no obvious temperature recrystallization of some evidence of deformation has been observed migmatites and of the associated leucocratic in the Gumburanjon leucogranite, some granitoids to leucocratic orthogneisses during minerals crystallized during event 2b sustained event 2a; b) the possible correlation of these a post-crystalline deformation. In that view leucocratic granitoids with granitoids dated the tectomc contact with the Tibetan nappes around 600 my in the Kade Chu valley; Rb­ is younger than intrusion of the Gumburan· Sr dating of the leucocratic granitoids in NE ion stock. Zanskar (HONEGGER et al., 1982) has not been successful possibly in consequence of the lack 2) North of Shingo La the HHC metamor­ of isotopic homogeneization; c) the existence phics (dark gneisses and leucocratic or· of xenoliths rich in leucocratic orthogneisses thogneisses) are intruded by the Gumburan­ within the Gumburanjon leucogranite which jon leucogranite lxx:Iy which clearly post-dates post-dates the main Himalayan metamor­ event 2a. The leucogranite lxx:Iy shows mark. phism; d) the rare porphyritic mafic dykes ed heterogeneity defined by grain size varia­ which intrude the leucocratic granitoids, but tions; in some host rocks it produced the are later a£fet:ted by event 2a; though the growth of biotite and of amphibole ± garnet significance of these dykes is still unclear, they in veins (event 2b). These evidences and the support a major time-gap between events 1 geochronological works in progress suggest and 2. that the Gumburanjon leucogranite is corn· A later event 2a is associated with pervasive parable to the Miocene leucogranites of the I'RELlMINARY DATA ON TilE HJGH HJMALAYAN CRYSTALUNES ,.,!.ONC TilE PAnUM.DARCJlA ETC 101

High Himalayas (e.g. LE FORT, 1973, 1981; REFERENCES DIETRlCH and GANSSER, 1981; FERRARA et al., 1983; SEARU: and FRYER, 1986) which an: in­ BAUD A., GAETANI M., G.uZANTI E., FOIS E., NICORA A., TINTORI A. (1984) . G~Jog;aI o/nnwtiOtfS itf terpreted as anatectic melts produced from JOulhusUrn z."Jktu find mJj«ml Wul tlTt" (110" high temper-ature metamorphics. Rb/Sr age Ihwt-stnn HimaJ.y.). &Iogae GC'Oi. Hdv., 77/1, differences on muscovites and biotites of the 171-197. Gumburanjon leucogranite (FERRARA et al., BltA.'«1TV.B., &IANOAiJ A.K., SL'«iH V.P., KANw..A.K. (I979) . G~hrotfolctfrtll tllld vo/cgicfli JluJin Otf a 0 in progress) point to cooling rates around 100 fJ'tlniU 01 Higbn HimtlJ.y., NortMrzsJ 01 Mtllfi1urlltf, elM•. l-limtlchGl.Prudnh.jour. GC'oI. Soc. India, 20, 90·94. Consequently two generatiom ofleucocratic 8ofI.TOLAMI G., LoMBARDO B., PoUJ\o"O R. (1983) . TIx grani/aids of different ages occur in the HHC UIlni~ oll~ uppn Imj4 KhoitJ (E~ Tt'f,jon), nUlmI N~/. In: cGnlnites of Himalayas, Karakorum and of SE Zanskar. Hindu Kush., FA Shams cd., Ins!. ofGeology, Pun­ jab Univ('rsity, Lahore, Pakistan, 2H·270. 3) As already inferred by BAUOet al. (1984) DIETRICH V., GANSSER A. (1981) - T~ kUCOgfllnius 01 and by GAETANI et al. (I985), our data sug­ I~ Bhuttl" Himtllaya (crustill anattxis vesus manll~ gest that a major tectonic boundary exists bet­ melling). Schweiz. Mineral. Petrogr. Mitt., 61, ween the HHC rocks and the sedimentary 177-202. FERRARA G., LoMIIARDO S., TONARINI S. (1983) - Rb/S, nappes of the Tibetan Zone. The petrographic geochronology 0/ granill'S and gneisses from lhr MOllnt evidences also indicate a metamorphic jump Everesl ~ion, Nrpa/ Himtl/aya. GceL Rondschau. 72, from the high/medium grade assemblages 119-136. of the HHC metamorphites to the very FRAXK W., TItONI M., PuRTSCliEU..£R F. (1977) ­ low grade assemblages of the Tibetan ~logy and ~trogfllphy 0/ Kulu· S. Lahul aTt'a. Coil. Int. CNRS, 268, 147·172. metasedimenrs. A recent work on this boun­ GAETANI M., GARZANTI E., jADOUL F. (198') . Main dary (HERREN, 1987) suggests it is an impor­ Stnli:luf/l/ rkmmts ofwska', NW Himalttytl (lndi4). tant normal fault. The interpretation of the RC'nd. Soc. Geo!. h., 8, 3·8. Shingo La metagraywackes and of their rela­ HEllEN E. (1987) . T« Z.nskllr shuT ~on~: Northrtul­ wutht«st nttDIuon unthin llH Hig/xT Himtll4ytIJ tionships with the HHC metamorphics is less (Luitlkh, /ndU). Geology, in peas. obvious. 1bey may either represent a .sequence HOSE.GCER K., DIETRICII V., FRA.>;"IC W., GANSSU A., of metasediments which shared, in part, a T11&., M., TROl\.111.1$DORFF V. (I982) - Mllfl'U2tWn Ilnd common Himalayan history with the HHC, m~14morphism in llH LadIlkh Himll!tJy1lJ (llH Indus­ TJlUltpo JIlt"" zotfr). Earlh and Planel. Sci. lc-uC'l"$, or they may derive from sediments of the 60, 2'3-292. Tibetan Zone (Le. similar to the rocks of the LE. FollY P. (1973) - Ln kuCOffllni~ik I'Himtll4yll, SUT Phe formation, Pughtal nappe) which, l'txmrpk du uanitr du MlltutSlu (Nipal unfTtl/J. Bull. however, sustained a metamorphic crystalliza­ Soc. Gee!. Fr., 7, "'·'61. LE FORT P. (1981) - Mllnllslu kucogfllnitr: 11 collision tion at higher temperature with respect to the sig,nawrr of IIN Hima/aya. jour. Geoph)'$. Res., 86, other Tibetan metasediments. In the former 10545-10'68. hypothesis, the existence of mylonitic belts ME-IITA P. K. (1977) - Rb/S, Grochronology a/the Kulll· and of a metamorphic jump between the Mandi belt: ilS implicationJ fa, the Himaftlyan Trc· Shingo La metasediments and the other HHC togrnrsis. Geo!. Rundschau, 66, 156·175. NANflA M.M., SlNCIl M.P. (1977) . Stratigraphy al/d metamorphics may record an early thrust dur­ scdimenlllliofl 0/thr Zamkar arra, lAdIlkh and ad;oin­ ing the Himalayan history. ing parts of lhe Lahaul rrgion of Himtlchtl/ Pradesh. Himalayan Geology, 6, 367·388. SEARI£ M.P., FRYER BJ. (1986)· Gllmet, tollmtlllin~afld AcknowkdgmffltJ. - 1be work has !xn('fited from musroviu-brIlring kueoUllnit~, gnd~ and m;flIlatit~ discussions and revi('ws by E. CAU..£GARI, M. GAETANI, ofllH I-lighn Himaillyas from uns1u" KIIlu, Lahoul E. GAKZAlm and two anonimous rcf=. 1be work was tllfd K4Jhmir. In: cCollision Tectonics., Coward & sUPPOrled by M.P.I. (40% M. GAETANI and U. RiC'S cds., 19, 18'·201. PoGNANTE, 1984). In particular Ihe aUlhon Ihank M. SRlKANTIA S.V., GANE$AN T.M., RAo R.N., SL>;"IlA P.I-I., GAETANI for introducifl8 them 10 Ihe spcdfk problC'ms TtRttY B. (1980) - G«Hoo ofllH~all"ll. Ladalth of Zanshr g=Iogy. Himllillya. Himalayan Geology, 8, 1009-1033. VIlL\ l.M., LoMBARDO B. (1986) - Onnvllzioni ~JM/ ,.ffrnJJmnmto In f.TIl11iti himllJ.y.ni. MANUSCJ.IPT ACCEPn:D jANU.uy 20, 1987 RC'IKl. S.I.M.P., 41/2, 410.