Middle Cretaceous back-arc formation and arc evolution along the Asian margin: the Shyok Suture Zone in northern Ladakh (NW Himalaya)

The Shyok Suture Zone of the Ladakh palaeovolcanic are is made of Cretaceou s volcanosedimentary formations intruded by granodioritic plutons. Field ob servations show a tectonic stacking of lithe-units from different volcanic are and back-are environments. In the western part (Skardu area), the Shyok Suture Zone can be subdivided into two groups: (1) The Northern Group, which consists of olistolith basal tic blocks and tuffs. The basalts are LREE depleted with a LILE enrichment and a slight Nb depletion suggesting a back-are basin origin. (2) The South ern Group, which consists predominantly of andesites that show LREE enrichm ent, a flat HREE pattern, strong Nb-Ta depletion, and LILE enrichment. The volcanic rocks of the Southern Group appear to have island-are tholeiite (I AT) to calc-alkaline affinities. In the eastern part of the suture zone (Nubra-Shyok area), Albian to Cenoma nian age silicoclastic sediments predominate. These sediments correspond to a large detrital platform built on the northern part of the Ladakh Are. At the top, these sediments interlayer with basaltic to andesitic lavas. Thes e lavas appear to be more differentiated and calc-alkaline in nature than t he Skardu Southern Group lavas, but show similar volcanic are affinities. N o evidence of a back-are basin was found in this area. Our data from these two areas shows a northwest-southeast evolution, from b ack-are to are formations in northern Ladakh. Opening of this back-are basi n occurred on the northwestern side of the Ladakh Are. This back-are was pr ogressively closing eastward. The are itself was more mature in the east, r esembling the southern Tibetan continental are, and was characterised by mo re continental sedimentation. Subsequent Himalayan tectonometamorphic evolution, closure of the back-are basin and deformation along the Shyok Suture, reflects an early asymmetrica l geometry along the Asian margin and Kohistan-Ladakh Are. (C) 2000 Elsevie r Science B.V. All rights reserved.