PETROCHEMISTRY AND PETROGENESIS OF THE GANGOTRl GRANITE AND ASSOCIATED GRANITOIDS GARHWAL HIMALAYA

Abstract

The Himalaya is the world's highest mountain chain belonging to the Alpine-Himalayan System. Recent views suggest its origin in terms of collision between Indian plate and Tibetan plate which resulted in large scale anatexis and generation of granitic melt on both side of the suture. Two types of granites have been recorded within the metamorphic belts lying southwards in Garhwal Himalaya (Western part of Kumaun Himalaya of Gansser). One is a pre-Mesozoic biotite granite gneiss (BGG), which has tectonic contacts with the metamorphics. The other and the younger, is a tourmaline-muscovite granitoid (TMG) which occurs as intrusive in the metamorphic rocks and BGG. This work documents the petrography and geochemistry of the two types of granites and their environs exposed in the upper parts of Bhagirathi valley in the Garhwal Himalaya. The rock formations in the area have been classified into Harsil Formation and Martoli Formation Comprising the metasediments and lying tectonically below and at the top of two granites, the BGG and TMG. The Harsil formation comprises the quartzite, quartz-mica-garnet schist and quartz-biotite-kyanitesillimanite schist. The maximum metamorphic grade is near Jhala with the grade decreasing outward ("divergent type metamorphism"). Along Harsil fault a thin band of hypidiomorphic two-mica granite (remobilised granite) has intruded the metasediments resulting into a contact aureole of about 0.5 m in the adjacent quartz-mica schist of the Harsil formation. The contact between BGG and Martoli formation is also tectonic (Nilang-Martoli Thrust). The

biotite granite gneiss (BGG) extends upto Gaumukh and Nilang. The foliation direction varies from N 65°W-S 65°E to N 50°W-S 50°E although, at places, it shows N 10°E-S 10°W/N 25°E-S25°W and N25°E-S25°W/N 40°E-S40°W trend. The foliation within BGG shows a continuity with the regional structure as defined by the metasediments. The BGG contains remnants of biotite schist, quartz-muscovite schist and quartzite. It has developed into augen gneiss along transverse faults and shear zones. Petrographically discrenable types in BGG are : (1) quartz - rich granitoids, (2) granite, (3) granodiorite (4) tonalite and (5) quartz-diorite Tourmaline-muscovite granitoid (TMG) is a light-coloured granite with foliation defined by muscovite and occasionally strongly oriented tourmaline. It has intrusive relationship with BGG and occasionally also with metasediments. It is associated with pegmatites and tourmaline veins which intrude into BGG and Harsil formation for a considerable distance. There is no thermal effect along the contacts of TMG. Petrographically, TMG consists of granite, granodiorite and tonalite. Xenoliths in TMG comprise the elements of BGG and garnetiferous muscovite schist. In BGG, shear zones producing augen gneiss and the faults are conspicuously oriented in N or NE direction. The foliation in TMG is basically governed by the foliation trend of the contact formations. Biotite granite gneiss is light grey rock with predominantly inequigranular texture. Grains are subidioblastic to xenoblastic

with plagioclase megacryst. Three generations of plagioclase have been recorded. Potash feldspar is microcline or perthite. It occurs as crystal aggregates or as crystals replacing plagioclase. Granophyric intergrowth with quartz or myrmekitic intergrowth in adjoining or enclosed plagioclase is commonly observed. Biotite and muscovite are randomly distributed. In the shear zones, BGG is dynamically metamorphosed to protomylonite and augen gneiss and shows cataclastic textures. Tourmaline-muscovitegranitoidshow generally hypidiomorphic texture, although at times these may be porphyritic. Near the contacts with BGG, they have acquired gneissosity due to tourmaline bands and occasional muscovite. The BGG shows a large variation in chemistry. Following data summarises the value range in percentage with averages in bracket for 26 rock analyses : Si02 - 56.00 - 76.33 (72.20) Fe203 - 0.57 - 1.77 (1.27) MgO - 0.27 - 2.48 (1.05) Na20 - 0.84 - 7.75 (3.88) Ti02 - 0.12 - 1.07 (0.42) MnO - 0.00 - 0.21 (0.02) A12°3 ~ H'07 - 21.55 (13.34) FeO - 0.10 - 2.52 (0.93) CaO - 0.17 - 2.84 (1.48) K20 - 2.11 - 6.93 (4.48) P205 - 0.04 - 1.16 (0.35) Na20/Al203 vs. K20/A1203 ratios and AFM plots of BGG indicate igneous rock chemistry. Similarly mol. Al203/(Na20+K20+CaO) ratio is less than 1.1, the normative corundum is less than 1% and Na20/K20 ratio is more than 0.6 which also gives the rock an Itype chemistry. Trace element data also point towards the igneous

trend. On the basis of many diagrams, the mineral assemblage indicates crystallization at ternary minimum. After comparing the petrochemistry of BGG with that of the neighbouring sediments, it is vividly brought out that BGG is a product of granitization. The generation of granitizing fluids must be related to the original pre-Mesozoic regional metamorphism. The 'I'-type chemistry is probably a reflection of the involvement of crustal elements during anatexis, although the heavy influx of alkalies during first phase of Himalayan orogeny may also have contributed towards this chemistry. The alkali metasomatism is reflected in the form of large plagioclase megacrysts and late stage microcline which engulfs the earlier plagioclase at a number of places. The prsence of remnants of quartz-biotite-feldspar-garnet geniss, quartzite and quartz-muscovite schist also favour that BGG were originally granitized metasediments. Local scale remobilised two mica granites are developed along Harsil fault along with small thermal aureole around it. Tourmaline-muscovite granitoid, also referred to as leucogranite, is found in different parts of higher Himalaya. Leucogranites have been considered as anatectic product of metasediments or 'S'-type by several workers. TMG of Bhagirathi region shows 'I'-type chemistry with a sporadic 'S'-type trait. This shows that melting at least in this region is of crustal part of the basement. 'S'-type traits could be the reflection either of the contamination of magma with the metasediments at upper levels or melting of the latter contributing to the magma. The following data summarises the value range in percentage with average in brackets for 17 rock analyses of TMG; Si02 - 69.00 - 75.33 (71.95) Fe20 - 0.44 - 1.24 (0.79) MgO - 0.26 - 1.06 (0.52) Na20 - 3.79 - 6.31 (4.85) Ti02 - 0.01 - 0.24 (0.14) MnO - 0.00 - 0.04 (0.008). A1203 - 13.58 - 15.85 (14.70) FeO - 0.04 - 0.46 (0.26) CaO - 0.42 - 1.63 (1.07) K20 - 3.31 - 6.39 (4.76) P205 - 0.22 - 1.39 (0.53) In the case of TMG the mode of occurrence, I-type chemistry and texture proves it to be an intrusive. Rb/Sr data of these granite fall in more than 30 km depth (10Kb). It is difficult for any sediment to survive at this depth. I-type chemistry of granite in this area as against S-type reported by several workers assume, therefore, added significance to the petrogenetic model. It is envisaged that the BGG were metasediments which were granitized during the pre-Mesozoic period (Precambrian? Lower Paleozoic?). These granitized metasediments were involved in the first phase of Himalayan orogeny while subducting between Indian and Tibetan plates. During this phase remetamorphism and anatexis of the whole sequence took place giving rise to alkali-rich mobilizates which ultimately led to the development of megacrysts of plagioclase and K-feldspar. The addition of alkalies modified the original chemistry of the granitized metasediments so much so as to change the chemistry to I-type traits or else the original granitization was due to crustal melting. During subsequent phase of tectonism and anatexis, the generated magma gave rise to TMG which came up along weak planes

as intrusion. Depending upon whether the melting or anatexis took place of the crust or, of the BGG along with the metasediments depended the I-type or S-type chemistry of TMG. This naturally would depend upon the crustal thickness and the geothermal gradient. There is evidence of yet another phase of granite formation locally along major faults, where magmatic looking granites have intruded, causing thermal metamorphism of the contact rocks. The shear zone tectonics gave rise to protomylonitic augen gneiss, probably postdating all magmatic activities.