The Yarlung-Zangbo Suture Zone, a major geological structure in Tibet, is well known as the locus of tectonic emplacement of the Tethyan ophiolites. Current models propose that most of the East Tethyan oceanic lithosphere was subducted within a single subduction zone, active during the Middle or Late Cretaceous, which was completed during the Paleogene collision between India and Asia. The Early Cretaceous sedimentary Giabulin Formation in southern Tibet, includes conglomeratic members that contain ultramafic and mafic plutonic pebbles, as well as radiolarian chert clasts, that record the erosion of oceanic lithosphere involved in a subduction event which occurred earlier than previously believed. Geochemical analyses, mineral chemistry, stratigraphic chronology, and sedimentary analysis, including source provenance, suggest that the pebbly conglomerate was formed through erosion of an unknown ophiolitic source that was geochemically distinguishable from the Xigaze ophiolites within the Yarlung-Zangbo Suture Zone, southern Tibet. We infer the existence of an older ophiolitic source, termed the Yarlung-Zangbo paleo-ophiolite, that was dismembered and eroded during an earlier subduction stage not taken into account in current models. (C) 2000 Elsevier Science Ltd. All rights reserved.
