Collision zone metamorphism in Vietnam and adjacent South-eastern Asia: Proposition for Trans Vietnam Orogenic Belt

Recent geological and petrological investigations of the metamorphic rocks from Vietnam revealed the following new evidences to understand the tectonic evolution of Southeast Asia; 1) findings of ultrahigh-T(∼ 1000 °C) pelitic granulites, high- to ultrahigh-P (∼ 40 kbar) mafic metamorphic rocks and high-P/medium-T gneisses from the Kontum Massif, 2) eclogite and high-P granulite from the Song Ma suture zone, and 3) ultrahigh-T aluminous granulite rrom the Red River zone. These lines or evidence are strongly indicative of the highest-metamorphic conditions in each metamorphic terrane. Estimated P-T conditions and reaction textures from these rocks delineate a characteristic clockwise pressure-temperature-time (P-T-t) path for each other. Based on the combination of P-T paths from these complexes, two-stages of metamorphic field gradient are identified. An earlier MO-stage of high-P/T gradient is recognized, based on the peak-P conditions from the Kontum Massif and Song Ma suture zone. A later M1-stage of low-P/T gradient is also identified by linking the peak metamorphic conditions from the Kontum Massif, the Song Ma suture zone and the Red River zone. The former metamorphic field gradient could represent an early continental collision event and the latter would indicate a peak metamorphic stage caused by very high-T magmatic intrusion (asthenosphere upwelling) as a heat source of ultrahigh-T metamorphism. A simultaneous collision metamorphism throughout Vietnam should have taken place during the continental collision between Indochina and South China cratons, which led to the formation of Trans Vietnam Orogertic Belt.