METAMORPHIC EVOLUTION OF THE HIGHER HIMALAYAN CRYSTALUNE, ZANSKAR, JAMMU AND KASHMIR

Abstract

The Himalaya is a composite mountain be1t of the Trans- Himalayan batholith in north, followed successively in the south by the Indus-Tsangpo Suture Zone (ITSZ), Tethyan Sedimentary Zone (TSZ), Higher Himalayan Crystalline (HHC), Lesser Himalayan Proterozoic Foreland and Sub-Himalayan Tertiary belt with variable geological characters. However, metamorphic rocks occur mainly in the Higher Himalaya as a continuous belt and discontinuously in the Lesser Himalaya. The Higher Himalayan Crystalline (HHC) belt is deformed and metamorphosed due to intracontinental collision and crustal shortening in Cenozoic period. Detailed work on metamorphism and deformation has been carried along two sections of the HHC in Jammu and Kashmir, NWHimalaya. These include i) Chenab-Bhot Nala cross section from Thatri, Chhatru, Kishtwar, Atholi, Bhujaz and Mulung Tokpo, and ii) Suru-Doda Valleys section along the northern margin of the HHC from Sankoo, Panikhar, Parkachik, Ringdom, Pensila and Padam. Thick pile of Salkhala Group metamorphics of the Higher Himalayan Crystalline is thrust over the Lesser Himalayan Proterozoic Foreland of the Kishtwar Window along the Main Central Thrust (MCT). Towards southwest and west, it constitutes a part of Kashmir Nappe, which is overlain by the fossil iferous Paleozoic-Mesozoic succession of Kashmir-Bhadarwah Basin. On the other hand, the Zanskar Shear Zone (ZSZ) marks the northeastern margin of the HHC with the Tethyan Sedimentary Zone along the Suru - Doda Valleys. Chenab-Bhot Nala sections through the Lesser and Higher Himalayan tectonic units reveal a polyphase Barrovian - type regional metamorphism. Low grade rocks viz, metavolcanics, carbonaceous phyllite, mica schist and quartzite with granitic intrusives are exposed in the Lesser Himalayan Foreland . The HHC comprises mainly different varieties of mica schist, sillimanite gneiss, mica gneiss, calc-silicate, amphibolite and granitoids. In HHC, metamorphism increases from garnet grade at its base along the Main Central Thrust (MCT) to sillimanite-K-feldspar grade towards higher topographic and structural levels. The peak of metamorphism is accompanied by extensive anatexis, migmatisation and formation of leucogranite. Further NE towards Tethyan Sedimentary Zone, garnet to sillimanite-muscovite grades are truncated by the Zanskar Shear Zone along the Suru - Doda Valleys. Detailed petrographic work only on pelites reveal different mineral assemblages in metamorphic grades (a) garnet: garnetbiotite- muscovite-quartz±K-feldspar±plagioclase±chlorite, (b) staurolite-kyanite; garnet-biotite-muscovite-quartz±staurolitekyanite± K-feldspar±Plagioclase, (c) sillimanite-muscovite: silli manite /kyanite-garnet-biotite-muscovite-quartz-K-feldspar-plagioclase, and (d) sillimanite-K-feldspar: sillimanite-garnetbiotite- quartz-K-feldspar-plagioclase+cordierite. Final and textural relations indicate that mica, garnet, staurolite, kyanite, sillimanite etc. are grown syntectonically to most pervasive D2 deformation. This deformation produces a most pervasive axial planar foliation (S2), NE plunging reclined folds (F2) and a consistant co-axial lineation (L2). The growth of these minerals with respect to D2 deformation is indicated by helicitic growth with concordant internal Si fabric to external Se during main metamorphism M2 . Garnet has grown in at least two distinguishable stages of metamorphism M2 which is documented by inclusion-rich, syntectonic (G-I) and inclusion-free post-tectonic (G-II) garnet. In a few samples, garnet has a pre-tectonic core of metamorphism Ma. which is almost devoid of inclusions (G-0), syntectonic inner rim with prominant spiral inclusions like G-I and inclusionfree outer rim of G-II. In higher grades, garnet shows dissolution and corroded margins that is more prevalent in migmatite zone. Staurolite is found only in a few samples with kyanite and mica. Kyanite predominently occurs as blades and is restricted only to staurolite-kyanite grade. Sillimanite occurs as prisms and fibrolite. Sillimanite is found in association with feldspar, garnet, quartz and cordierite in higher grades and also as polymorphic transformation of kyanite in sillimanite-muscovite grade rocks. Fibrolite is commonly observed as epitaxial growth in biotite, feldspar and sometimes even in garnet. Cordierite is of very limited occurrence and is seen only in migmatite.

Retrogression is less in the area of investigation, excepting in Doda Valley and close to tectonic boundaries. Chloritisation of garnet, biotite and staurolite and sericitisation of feldspar are common in the rocks of this area. Mineralogical and textural studies suggest the growth of garnet, staurolite, kyanite, sillimanite etc. in different grades due to several continuous and discontinuous metamorphic reactions. Fe/Fe+Mg of coexisting garnet and biotite increases with metamorphic grade, except in garnet grade, which shows more Fe enrichment than staurolite-kyanite grade. Muscovite is poorer in paragonite component, while approaching from lower to higher metamorphic grades. Plagioclase is more albitic with anorthite reaching a maximum of 35% in a few samples and revealing no variation in anorthite content with respect to metamorphic grade. Staurolite shows Mg rich core and Fe rich rim showing evidence of zoning. Ilmenite is almost pure but sometimes, reveal Fe depletion. Garnet from all the metamorphic grades reveal the presence of zoning. Growth zoning is very common in the garnet to staurolite-kyanite grade rocks and is inferred to be because of prograde metamorphism. However, sillimanite-muscovite and sillimanite-k-feldspar grade rocks reveal reverse zoning with Mg rich and Mn poor core and Fe/Fe+Mg and Mn rich rim. The later phenomenon is due to thermal relaxation and volume diffusion after peak metamorphism. iv AFM topology of coexisting mineral phases indicate equilibrium condition. However, in the low to medium grade rocks crossing of tie lines are observed implying the non-ideal conditions of the participating phases. P-T condition of metamorphism are determined using suitable geothermobarometers for the relevant mineral assemblages. The garnet-biotite exchange thermometer is used to obtain temperature, applying various calibrations. Pressure is estimated with garnet-Al2SiOs-quartz-plagioclase and for garnet-muscovitebiotite- plagioclase assemblages using different calibrations. Error estimation for temperature and pressure is calculated following standard methods. The inferences made from the P-T data along the Chenab - Bhot Nala section and Suru-Doda Valleys are summarised below: 1) In topmost portion of the Himalayan footwall near the MCT, rims of garnet-I show temperature of 500-560°C and pressure of 7.00-9.60 kbar for garnet grade. The rocks occurring above MCT at the base of HHC on the SW side also show more or less the same P-T condition. 2) The HHC rocks, occurring within 1 km and above the MCT on the NE side of the Kishtwar Window in staurolite-kyanite grade, record the core and rim temperatures of 500-570cC and 550-650°C and rim pressure of 7.50-9.00 kbar. P-T values of subidioblastic garnet-II are exactly similar to the rim data obtained from garnet-I. 3) Further moving up NE in higher structural level, garnet- I of sillimanite-muscovite grade gives core temperature of 700- 740°C and rim P-T of 610-650°C and 6.00-8.50 kbar. Sillimanite-Kfeldspar grade rocks give core temperature and pressure of about 780"C and 9.00-11.00 kbar. However, the rim P-T show 600-665°C and 4.50-6.50 kbar. A few samples show substantial reduction in core and rim P-T from the above mentioned values, but these values are noteworthy in the zone of migmatite. 4) While moving further NE towards ZSZ in sillimanitemuscovite grade, garnet rim and subidioblastic garnet-II grains give temperature and pressure of about 675°C and 7.75 kbar, whereas staurolite-kyanite grade rocks show 600°C and 6.00 kbar from rims of garnet-I. 5) In the Suru Valley, along the main cross-section from Panikhar to Ringdom, the staurolite-kyanite grade rocks record core P-T of 450°C and 5.25 kbar, while rim gives 560-650°C and 6.75-9.75 kbar. P-T data, obtained staurolite-kyanite grade, suggest that these rocks have been developed during prograde metamorphic condition. The normal growth zoning and increasing temperature towards garnet rim support their heating during late stage of metamorphism M2. Presence of kyanite, rarity of staurolite and increasing Fe/Fe+Mg ratio towards garnet rim in some samples is interpreted to have been formed in near isobaric and increased temperature conditions. About 10-18 km from the MCT towards higher structural level, garnet-I core temperature is increased

by about 170°C with pressure remaining constant from that of staurolite-kyanite grade. On the other hand, the rim temperature remains more or less constant, but pressure is reduced by 2.00- 3.00 kbar from staurolite-kyanite grade. Still up in the topographic level and up to about 23 km in sillimanite-K-feldspar grade, garnet-I core records an increase of about 40-80°C from sillimanite-muscovite grade with a slight increase in pressure. For the same grade, P-T condition for garnet. rim or subidioblastic grains is reduced to about 150°C and 2.50-3.50 kbar from core. The rim pressure, however, is reduced by another about 1.5-2.00 kbar than the sillimanite-muscovite grade without much change in rim temperature. The only supporting evidence for prograde metamorphism in the higher grades is from garnet, which shows reverse zoning with high Mg/Fe ratio and lower Mn at the core. The temperature and pressure reduction is evidenced by epitaxial growth of fibrolite in biotite, and an increase in Fe/Fe+Mg ratio and Mn at garnet rim. At the highest topographic level between 23 and 40 km from window, the rocks are extensively migmatised in the sillimanite-K-feldspar grade. Garnet is partially reequilibrated and show grain-size reduction, and therefore, does not give peak metamorphic P-T condition. Detailed study on texture, petrography and P-T data establishes the presence of inverted metamorphism in HHC of Jammu and Kashmir like in Garhwal, Kumaon and Nepal-Bhutan-Sikkim Himalaya. Many models have been proposed to expalin inverted metamorphism in the Himalaya. The data obtained in this study have been compared with all the models of inverted metamorphism to understand their applicability for this region. The P-T values indicate that metamorphism has been initiated at depth because of subsidence of the Indian Plate during the onset of collision with Eurasian Plate. Further, collision has caused intense ductile shearing during main deformation and metamorphism at depth of about 25-35 km. Southward ductile shearing has probably displaced higher grade rocks over lower grade and caused inverted metamorphism. This seems to have resulted in the cooling of the higher grade and heating of the lower grade rocks giving an apparent isothermal uplift. Also, the pressure release during fast uplift of the higher structural levels in comparision to the lower level or basal part of the HHC, probably resulted in the thermal relaxation and equilibration in the whole of HHC. Therefore, it is possible that the inverted metamorphism observed in this part of the HHC may be due to ductile shearing and uplift after an initial subsidence.