COLLISION TECTONICS : METAMORPHIC AND GEOCHRONOLOGICAL CONSTRAINTS FROM PARTS OF HIMACHAL PRADESH, NW-HIMALAYA

Abstract

The Himalaya is located on the seismically-active northern margin of the Indian Plate, and provides an unique opportunity to investigate collisionrelated geodynamic processes of a young orogenic belt during the Cenozoic- Quaternary. Collision of the northerly-moving Indian Plate with the Eurasian Plate has considerably remobilised the Precambrian basement and cover sediments of the Indian Plate since 50 Ma. The basic objective of the present work is to provide better tectonomecanorphic and geochronological constraints on the collision tectonics of various metamorphic nappes along the Sutlej Valley and Chor Mountains in HimachalPradesh. Detailed geological work along the Sutlej valley and Chor Mountain has revealed the following major tectonic units: (a) The Lesser Himalayan Proterozoic sedimentary foreland of quartzite-volcanic association, now exposed in many windows. (b) The Bajura/Kulu Nappe of the Middle Proterozoic mylonitised augen gneiss, bounded by the Kulu Thrust at its base and the Main Central Thrust (MCT) / Jutogh Thrust (JT) at the top. (c) The Higher Himalayan Crystalline (HHC) belt in the north is thrust southwestward over the Proterozoic Lesser Himalayan foreland of the Kulu- Rampur Window and Bajura/Kulu Nappe along the folded MCT/JT. Atleast two Proterozoic concordant to discordant magmatic events have been identified in the HHC, which lacks any Tertiary leucogranite bodies in the basal part. (d) The Jutogh Nappe in parts of Chor Mountain and Luhri-Anni-Dalash region is separated from the Bajura/Kulu Nappe and less metamorphosed Chail Nappe by the Jutogh Thrust. The contact of the Chor granitoid, occurring at the top of the Jutogh metamorphics is characterised by intensely mylonitised porphyritic granite gneiss due to presence of the Chor thrust. Between Jakhri and Wangtu, the HHC belt has undergone four distinct deformational phases. The earliest isolated tightly appressed "flame type' F, fold hinges are developed on lithological/metamorphic banding SQ having axial plane foliation Sp which appears to be the earliest recognisable penetrative planar fabric. The most pervasive second deformational phase develops an intense planar fabric S2, which is axial planar to the N-NE/S-SW plunging F2 folds. Being the main foliation, it is a composite planar fabric with S- surface being consistently rotated SW due to C-shear fabric. It possesses a strong down-the-dip Nto NNE plunging stretching/mineral lineation L2, that is coaxial to the F2 isoclinal reclined folds. Asymmetric megacrysts, pressure fringes and rotational garnet etc. reveal a consistent southwestward verging broad ductile shear zone during the Himalayan Orogeny. The third deformational D3 phase is characterised by two distinct early D3 and late D3b phases. In the D3a phase, F3a folds are mainly isoclinal, inclined-type with gentle to moderately-dipping axial surfaces (S1o). F, folds plunge either N100° or N300° at low angles. In the later D3b phase of the same deformational event, coaxial asymmetric, SW-verging F3b folds are mostly crenulation-type, whose hinges form a prominent lineation. Earlier L2 lineation is folded around many F3b hinges. The axial plane foliation S3b of these folds in metapelite and granite gneiss strike N100° with very steep dips of about 60°-80° towards N or S. The effects of late deformational phase D^ are weak and sporadic during brittle-ductile and brittle regime. Thrust-faults, shear bands and quartz-filled tension gashes are developed during this late stage deformation. The deformational pattern of the HHC contrasts with the Jutogh Nappe in its frontal parts of the Simla Hills and Chor Mountains. In the frontal Jutogh Nappe, earliest Dcl deformation phase produces an E or W plunging tight to isoclinal, recumbent to moderately inclined folds on lithological layering of the sedimentary origin Sq and is of Himalayan in age. This corresponds to the D2 deformational phase of the HHC. The HHC and Jutogh metamorphic nappe have undergone a polyphase Barrovian-type metamorphism under upper greenschist to amphibolite facies condition. Two sections have been investigated for detailed metamorphic evolution along (a) the Nauhra-Shamra treverse in southern parts of the Chor Mountain within the Jutogh Nappe and (b) Jakhri-Wangtu section along the Sutlej valley in the lower parts of the HHC. Mineral assembleges reveal the presence of two metamorphic grades in the HHC: (i) garnet and staurolite grade transition zone and (ii) staurolite-kyanite grade, while garnet grade rocks are present in basal Jutogh Nappe of the Chor Mountain. These assemblages appear to grow during the main deformational event, predating the mylonitic textures of the nappe translation. Suitable assamblages were probed to evaluate the pressure and temperature conditions of metamorphism, by using the garnet-biotite (GB) thermometer and garnet-plagioclase-sillimanite/kyanite-quartz (GPAQ) and garnet-plagioclasebiotite- muscovite (GPBM) barometers. Along the Nauhra-Shamra section, syntectonic garnet core of MCj indicate a variation of about 130° C from 520° C to 650° C, while post-tectonic garnet rim reveals a cooling path of the metamorphic pile with temperature falling between 480° C and 550° C at 5.4 to 7.8 kbar from its base towards higher structural levels. This variation seems to be real rather than apparent, hence is indicative of the metamorphic inversion in the Jutogh Nappe. In addition the Sutlej valley section of the HHC, garnet records a temperature of about 520° C at the base along the MCT during the M2 metamorphism and attains a maximum of 650° C towards higher structural levels. The pressure could not be estimated due to lack of suitable assemblage in this section. On the basis of the temperature data, this section also provides evidences of possible inversion in HHC. Major and trace elemental analyses of 15 samples from important granitoid bodies reveal their peraluminous S-type character within the granite, quartz-monzonite and monzonitic field on quartz-orthoclase- plagioclase normative plot. The Rb vs Y + Nb discrimination plot indicate that these bodies are of Within Plate Granite (WPG). The Spidergram plotting of compatible and incompatible elements as well as REE normalised plots indicate similar chemical characters for porphyroclastic granite, the Wangtu granite and the Chor granite, like the Hunza granite of North Pakistan. However, these bodies are geochemically different from the Tertiary Gangotri (Garhwal) and Manaslu (Nepal) leucogranites. Four granitoid samples from the hanging wall of the MCT/JT in the HHC and Jutogh Nappe have been dated by U-Pb systematics on zircons using isotopic dilution technique. The U-Pb zircon ages are of about 2.0 Ga, 1.8 Ga and 0.9 Ga, indicating the presence of the Proterozoic elements in the NW-Himalaya. The Chor granitoid in frontal parts of the Jutogh Nappe presents a complex history in the deformed and undeformed components; the latter yields an upper intercept of primary crystallisation age of 910 ± 23 Ma (2o) from 7-point regression line, though better age of 912 i 6 Ma (2o) is estimated from 5-point regression line. Although the deformed Chor granitoid is not the best sample for this analysis, it also confirms the 900 Ma age for the granitoid. Basal parts of the HHC contain discordant aplitic granite bodies, whose U-Pb zircon age of 2068 +5Ma (2o) from 5-fraction regression line indicates an Early Proterozoic period of granite crystallisation. Another Early Proterozoic period of 1866 ± 10 Ma (2o) for crystallisation event is indicated from U-Pb zircon age from the Wangtu granitoid from a 5-fraction regression line. 87Sr/8"Sr ratios from all these samples ranges between 0.706 to 0.716 and indicate their protolith to be the Archean-Proterozoic continental crust of the Indian Plate. Deformational patterns in this part of the Himalayan metamorphic belt indicate that the Main Central Thrust and Jutogh Thrust sensu stricto were formed during or after late D3 deformational event, and transported the ductily deformed metamorphic pile. These thrusts post-date the main progressive metamorphic events. P-T estimation, textural relationship and localised inverted metamorphic isograds can be explained in terms of ductile shearing, which caused the S-C fabric and, in turn, the inverted metamorphism in the HHC. The HHC and Jutogh Nappe contain numerous Proterozoic granitoid bodies, emplaced during an early to syn-Dj deformational event, and are now deformed into gneisses due to the Himalayan Orogeny. Although various bodies were emplaced atleast during three Proterozoic magmatic events having identical crustal protoliths, their geographical distribution may possibly be controlled by involvement of distinct tectonostratigraphic units within the Proterozoic basement. It, therefore, appears that collision tectonics in parts of NW-Himalaya has caused, not only intense imbrication of remobilised Proterozoic basement in various metamorphic nappes, but also the Lesser Himalayan Foreland in window zone. Tectonometamorphic data from these nappes clearly reveal that main metamorphic evolution is synchronous with ductile shearing in a thick intracontinental ductile shear zone and predates the brittle-ductlile to brittle emplacement of metamorphic nappes along the Main Central Thrust and Jutogh Thrust.