Petrogenesis and tectonic evolution of Yarlung Tsangpo ophiolites, south Tibet

• Main Author: Chan, Heung Ngai
• Published: University of Oxford 2008
• Subjects: 552
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.491339

Ophiolite complexes emplaced onto the Indian passive margin sequence in southwest Tibet represent the largest thrust sheet of the Neo-Tethyan oceanic crust and mantle that is preserved along the Yarlung Tsangpo Suture Zone (YTSZ). Field observations, petrological, geochemical and geochronological studies have revealed the supra-subduction zone (SSZ) type ophiolitic rocks formed in two different time frames, c. 127-124 Ma and c. ?4 Ma. The Early Cretaceous suite comprises voluminous mantle rocks, with subordinate mafic and ultramafic intrusions, while plutonic rocks are exposed locally. A shear zone complex probably representing a transform fault zone is also present. Geochemical analysis shows that the crustal rocks evolved from MORB-like to IAT to boninitic magmatism. The Late Cretaceous suite is represented by limited exposures of basaltic lavas, which have MORB-like geochemical compositions. Petrographic and geochemical evidence indicates that the majority of the mantle rocks are residues after extraction of MORB-type magma, which subsequently reacted with boninitic melts in a SSZ. Sub-ophiolite melange zones contain diverse rock types set in a serpentinte or mudstone matrix. Amongst a variety of lithologies, mid Jurassic and mid Cretaceous radiolarian cherts are exposed. Alkaline seamount volcanic rocks of inferred mid Cretaceous age were also found interbedded with cherts or overlain by limestones. Ophiolitic tholeiitic rocks were also included in the melange zones, two of which have 4°Ar_39Ar whole rock ages of c. 86 and 106 Ma. Evidence from the ophiolites and associated melange zones suggests that an intra-oceanic subduction zone initiated in the Early Cretaceous in this part of Neo-Tethyan Ocean. This SSZ system continued at least for c. 40 Ma, from the Early Cretaceous to Late Cretaceous.