Geochemical and Sr-Nd isotopic constraints from the Kontum massif, central Vietnam on the crustal evolution of the Indochina block

The Kontum massif, central Vietnam, consists mainly of high-grade (amphibolite to granulite facies) metamorphic rocks and represents the largest basement exposure (core complex) of the Indochina block. To explore the crustal evolution of Indochina, Sr and Nd isotopic and geochemical data for various rock types from the massif are reported. The basement rocks show a wide range of present day ε_Nd values from -22 (gneiss) to +15 (amphibolite), yielding depleted-mantle model ages (T_DM) from 1.2 to 2.4 Ga along with an "exceptionally" old T_DM of 2.7 Ga for a granulite. These data indicate that crustal formation in the Indochina block took place principally during the Paleoproterozoic and Mesoproterozoic, and do not support the conventional notion that the Kontum core complex is composed of Archean rocks. Geochemical data indicate that the gneisses and schists have heterogeneous compositions characterized by a calc-alkaline nature, whereas most of the amphibolites are tholeiitic basalts with intraplate magmatic signatures. Therefore, the former may be interpreted as products from pre-existing Proterozoic crustal materials and the latter as resulting from the Paleozoic rifting event that disintegrated the Indochina block from Gondwanaland. During its accretion with other SE Asian continental blocks in Permo-Triassic time, the Indochina core complex was subjected to the Indosinian orogeny, characterized by a high-temperature, granulite facies metamorphism in the lower crust with associated charnockite magmatism and subsequent regional exhumation.