GEOCHEMISTRY OF SEDIMENTARY ROCKS FROM MÉLANGE AND FLYSCH UNITS SOUTH OF THE YARLUNG ZANGBO SUTURE ZONE, SOUTHERN TIBET

Abstract

The Mesozoic Yamdrock mélange and the Triassic flysch lie immediately south of the Yarlung Zangbo Suture Zone (YZSZ) ophiolitic belt, in southern Tibet. The whole-rock chemical signature of sandstones (mostly greywackes), red shales and black shales of the Yamdrock mélange and flysch units do not present significant variations between the different rock types and geological units, and is concordant with a continental passive margin source. Despite fairly important chemical weathering, the geochemical signature was not affected by significant sedimentary recycling nor significant heavy-mineral accumulation. REE patterns show an enrichment in LREE typical of shales and indicate that the Neo-Tethyan turbidites mostly derive from a felsic old upper continental crust source. Slight enrichments in Ti and Sc indicate a mafic contribution that could originate from mafic blocks of enriched intraplate geochemical affinity found in the sedimentary units. Sedimentary rocks from both units were thus derived from the old Indian continental crust, including granitic and intraplate mafic blocks, but were deposited in different tectonic settings in the Neo-Tethys ocean. The homogeneity of the YZSZ data, which cover the whole Neo-Tethyan basin, and correlation with Himalayan data support the formation of all Himalayan lithotectonic units along a continuous passive margin of Northern India. © 2004 Elsevier Ltd. All rights reserved.