Metamorphic history and geodynamic significance of high-grade metabasites from the ophiolitic melange beneath the Yarlung Zangbo ophiolites, Xigaze area, Tibet

Blocks of highly foliated amphibolite are locally embedded within a serpentinite melange underlying the Yarlung Zangbo ophiolites in the Xigaze area of southern Tibet. The ophiolites are remnants of an Early Cretaceous back-arc basin within the Permo-Cretaceous Tethys Ocean, which are exposed along in the Yarlung Zangbo Suture Zone (YZSZ). These amphibolites are interpreted as fragments of a dismembered dynamothermal sole. Three types of amphibolite are present: (1) common amphibolite with assemblages of Hbl + Pl +/- Ep +/- Ap +/- Ttn, (2) clinopyroxene-bearing amphibolite with Hbl +/- Pl +/- Cpx +/- Ep +/- Ttn +/- Qtz +/- Ap and (3) garnet-clinopyroxene-bearing amphibolite characterized by the assemblages Hbl + Cpx + Grt + Pl +/- Rt and Grt + Hbl + PI (corona assemblage). In all three types, plagioclase is pseudomorphed by late albite-prehnite. Retrograde cataclastic veins containing assemblages of Prh + Ab + Ep +/- Chl are also present. P-T estimates indicate that the amphibolites reached peak metamorphic conditions of 13-15 kbar and 750-875 degrees C. Partial replacement of pyrope-rich (up to 35 mole%) garnet by Al-tschermakite (Al2O3 up to 21 wt%) reflects a high pressure (18 kbar, 600 degrees C) metamorphic event followed by rapid exhumation. Soon after exhumation, the amphibolites were intruded by very fine-grained diabase dykes that were then hydrothermally altered. The field relationships and metamorphic history of the amphibolites indicate formation during inception of subduction within a back-arc basin prior to obduction of the ophiolites onto the Indian passive margin. (C) 2007 Elsevier Ltd. All rights reserved.