GEODYNAMIC EVOLUTION OF THE SHYOK SUTURE ZONE, NW HIMALAYAS

Abstract

In the established framework of continent-continent collision between the Indian and the Eurasian plates two suture zones are usually recognized, viz., Indus-Tsangpo Suture Zone (ITSZ) and Shyok Suture Zone (SSZ). The present study aims at enhancing our knowledge regarding lateral, temporal, petrological and geochemical variations of the rock units of SSZ and its evolutionary history. Petrogenetic modelling using major and trace elements as well as isotopic compositions of different rock units in the SSZ and their relations are utilized to reconstruct the geodynamic evolution of the SSZ. Geochemical study involves major, trace and REE analyses of selected samples from each litho-tectonic unit. The data from different litho-tectonic units of SSZ have been analyzed separately using various discrimination diagrams and indicate subduction zone magmatism and arc affinity of the rock samples. Major and trace element abundances of the samples from different litho-tectonic units, viz., Hundar, Khardung, Shyok, Tsoltak, Nubra and Karakoram, demonstrate magmatic trends expected for a suite of rocks varying from mafic to felsic composition. This is indicative of fractionation of olivine, pyroxene, feldspars and Fe-Ti oxide phases during the evolution oftheir magmas. AI2O3, Fe203, CaO, MgO, Ti02and P205 show negative correlation with Si02. Scattering in Na20 and K20 variation diagrams may be due to post-crystallization processes or fluid activity in the area. In chondrite normalized rare earth element (REE) pattern of samples from different litho-tectonic units show light REEs enrichment and fractionation of middle and heavy REEs. Multi-element spidergrams show enrichment of large ion lithophile elements and depletion of high field strength elements with negative Nb, P, Ti, Sr and Eu anomalies. Shyok volcanics exhibit slightly less enrichment compared to other units. Two component mixing curve between an old crustal component and a depleted mantle component explains the isotopic characteristics of the SSZ rocks. Most of the samples from SSZ plot along this mixing hyperbola. Isotopic study of the rocks from SSZ indicates source multiplicity, either by mixing of crustal material due to subduction during melting or during the transport of magma to the surface. Sediment involvement at the source region and influx of aqueous fluid also played a major role in modifying the geochemical characteristics of the rock units. Isotopic and geochemical characteristics suggest that Khardung volcanics and Hundar intrusives are formed by melting of some components of the Kohistan-Ladakh island arc by recycling of the arc. The evidence from geochemical and isotopic study indicate that an active continental margin (Andean-type subduction) was present between the Kohistan and Karakoram during Cretaceous time. Shyok volcanics may be the basement on which the more calc-alkaline arc system developed. The more matured arc and back-arc basins are well preserved in the northern part (Pakistan area). These are translated alpng.the Karakoram Fault (KF) towards, Lhasa in Tibet. Therefore, the rock units exposed/ in the study/ate* probablyRepresent fore arc part and the accretionary complex ofan Andean-type arc setting.