Composition and metamorphism of the root of the southern Sulu orogen

The root of the continental orogenic belt in the southern Sulu area consists of five rock types with different chemical compositions.Their representative rocks are garnet peridotite,eclogite,gneiss,quartzite and marble.Their protoliths are old granitic intrusive,supracrustal and mafic-ultramafic intrusive rocks,which have all been subjected to ultrahigh-pressure(UHP )metamorphism.The granitic orthogneiss-es constitute the main part of the UHP belt in the southern Sulu area;they form a granitic and trond-hjemitic rock association,which is different from the typical TTG magmatic series.The metavolcanic rocks in the supracrustal rocks show the geochemical characteristics of bimodal volcanic rocks,indicating a tec-tonic continental rift environment.Except the mantle-derived garnet peridotites,all of the rocks have vari-able,lowerδ 18 O values than those of normal metamorphic rocks.This indicates that the protoliths of UHP rocks had undergone various degrees of fluid-rock interaction in the shallow level of the crust,and their meter-scale heterogeneity was preserved during the formation and exhumation of the UHP metamorphic rocks.Recent temperature-pressure(pT)calculations suggest that the pT conditions of garnet peridotites were probably1100～1200℃and6.5～8.0GPa with a geothermal gradient of than5℃/km or less,i.e.in the forbidden-zone of metamorphism in the Earth.