RENDJCONT! DELLA SOCIETA ITALlANA DJ MINERAl-OGJA E PETROLOGIA, 1987. Vol. 42, pp. 9H02 Preliminary data on the High Himalayan Crystallines along the Padum-Darcha Traverse (South-Eastern Zanskar, India) 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 Shingo La) 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 " ...,......"""<IS ("'1-1" )-.t''''"!JIt",•.!;~_ ," ..... ly refolded and grade to migmatites which in turn grade to homogeneous leucocratic ,,,.•,.<1_.,....-', ....... granitoids. These rocks have later undergone -miCOOltM '" ; LEL.<:OO:II/oTIC synmetamorphic deformation and the j OOT'O'''USSES ,tt-pi '''-0'_00." , ~,- leucocratic granitoids are