PETROLOGY AND TECTONICS OF THE CENTRAL CRYSTALLINES BETWEEN BHAGIRATHI AND YAMUNA VALLEYS, GARHWAL HIMALAYA

Abstract

The area under investigation forms a part of the Central Himalaya in Uttarkashi District along the Bhagirathi and Yamuna Valleys, i'he rocks exposed in the area belong to three distinct groups s. the Garhwal Group, the Bhatwari Group and the Suki kroup. The latter two have been considered as parts of the Central Crystallines. The Garhwal Group is represented by the Gamri Quartzite lying below the Main Central Thrust (MCT) and the Banas and Janki Chatti Formations, which are technically involved slices of this Group with the Central Crystallines (migmatites and metasediments) within the Central Himalaya. The Banas Formation (ypta $50m) is comprised of thin to enormously thick horizons of quartzite and quartz-sericite schist and bear a very strorg similarity with the Gamri Quartzite in terms of lithology and haavy mineral assemblage. The Banas Formation is overlain by calc-schist of the Janki Chatti Formation with a gradational contact. Immediately overlying the MCT, the Bhatwari Group reveals atleast one stage of deformation and metamorphism more than the rocks being referred to as the 3uki Group. The Bhatwari Group is, thus, the oldest group in the area, as evidenced by petrographic and structural data and the Rb/Sr isochro^ ages from the adjoining Bhillangana Valley (Chaili gneiss- 2.15b . my), The Bhatwari Group is separated from the Suki Group by the intervening tectonic slice of the low grade metamorphics of the Garhwal Group; the northern contact of which is marked by the Joti Bhandali Thrust (JBT). 11 The main rock typesc of the Bhatwari Group are chlorite schist, phyllonite, augen gneiss and schist, quartz-mica schist, garnetiferous mica schist, amphibolite and migmatite. The group is characterized by atlleast four phases of deformation, three phases of metamorphism and two phases of migmatization. The rocks belong mainly to the garnet grade and have undergone retrogressive metamorphism near the tectonic margins. The intensity of migmatization is maximum towards south and decreases gradually further northwards. The rocks are generally dipping due NE to NNE. The Bhatwari Group is also characterized by the presence of several bands of amphibolite which represent the original basic sills. Various standard plots, indices and ratios were used to study the geochemistry of these amphibolites. Their parental nature was deciphered to be tholeiitic basalt resembling Karoo dolerite and occupy the position of middle differentiate. From the trac§ element studies, it is evident that the original rock was continental tholeiitic dolerite which could have crystallized at a depth of 40 to 50 km and that was far off from the primitive magma source. The rocks overlying the JBT and extending upto Jhala in the Bhagirathi Valley and upto Yamnotri in the Yamuna Valley belong to the Suki Group. The 3uki Group is charact erized by an increasing grade of metamorphism from garnet grade near the JBT to sillimanite grade towards the top, thus representing an inverted metamorphic sequence. The degree of migmatization in the Suki Group similarly increases towards north.

Chemical data from the migmatite and the country: rock of the Bhatwari Group and the Suki Group reveal that composition of anatectic melts producing these migmatites and formed during different periods of earth history was not significantly different, although ->£ . _ . isochemical nature of trans formations of country rocks into migmatite are decipherable. The normative mineral plots (Q-Ab-Or and An-Ab-Or) for migmatite fall near the cotectic lines when compared with experimental diagrams and reveal that the migmatite of both groups have been formed due to anatexis. The anatectic origin of migmatite is further supported from trace element data. Large variability in concentration with the intensity of migmatization indicates an external source of migmatizing fluids. Co/Ni and Sb/Bi ratios of these rocks also suggest a hydro thermal activity in the area. Several traces of sulphide mineralization have been loc ated in the Bhatwari Group. The mineralization is related to quartz and quartz-calcite veins. The main mineralization is pyrite + pyrrhotite + sphalerite + chalcopyrite + galena. A higher Co/Ni ratio in pyrite from Janki Chatti (average 1.09) and Banas (average 1.19) and low Sb/Bi ratio (average 0.46) in galena from Pindki indicates a relatively high temperature hydrothermal environment. Fluid inclusion studies of quartz from quartz sulphide veins reveal a minimum temperature of formation as 180-320°C. On the basis of geological, petrological, structural, geochemical and radiometric data a tectonic model for the

geological evolution of the Central Himalaya has been postulated in the present work in which deformation of a PrecambriBn micro continent has been Dost<jL&-tec\ during the Himalayan Orogeny.