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

Abstract: Blocks of highly foliated amphibolite are locally embedded within a serpentinite mélange 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+Pl (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 13–15kbar and 750–875°C. Partial replacement of pyrope-rich (up to 35 mole%) garnet by Al-tschermakite (Al2O3 up to 21 wt%) reflects a high pressure (≈18kbar, 600°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. [Copyright &y& Elsevier]