关键词: 印支地块, Kontum 地块, 前寒武纪地壳, 岩石成因, 构造背景

Abstract: The Kontum Massif in the central Vietnam is one of the most important Precambrian basement exposures in the Indochina Block. This Massif consists mainly of different periods and different grade of metamorphic rocks. This study presents their petrologic, geochemical, zircon U-Pb and Hf isotopic data, in order to reveal the components of the basement rocks and their formation and tectonic evolution. Petrography and geochemical results indicate that the basement is mainly composed of metasedimentary rocks with a few meta-igneous rocks. Zircon U-Pb dating results suggest that the sedimentary rocks were formed in late Paleoproterozoic to late Neoproterozoic, and can be divided into five stages. First stage of sedimentary rocks were formed in an island-arc setting, second stage of sedimentary rocks were deposited in passive margin setting, third stage of sedimentation evironment is an active continental to continental arc setting, fourth stage is a transition from active continental margin to passive continental margin setting, and fifth stage of sedimentary basin is in an active continental margin setting. The protoliths of two metamorphic igneous rocks are tholeiite and S-type granite, and formed at 1424 Ma and 1485 Ma, respectively. Tholeiite is enriched in Rb, Ba and Sr and depleted in Nb, Ta, Zr and Ti. These features are consistent with the geochemical characteristics of typical island arc basalts. Most zircon εHf(t) values are positive with a mean value of + 5.66, except two grains, indicating that its parental magma originated from the depleted lithospheric mantle.Geochemical characteristics suggest that this tholeiite was formed in the island arc setting, in accordance with the deposition environment of coeval sedimentaryrocks. S-type granite has high SiO2, ALK and A/CNK (>1.1), and medium REE content and medium Eu negative anomaly. The magmatic zircon εHf(t) values varies from +5.97 to +12.11, suggesting that the clasts of the meta-sedimentary rocks in the source mainly came from the juvenile crust. Similarities of the formation time and geochemical features of Mesoproterozoic igneous rocks suggest that the Kontum Massif probably was connected with Hainan Island in Mesoproterozoic, and they were on the edge of the Columbia supercontinent during the disintegration of the supercontinent based on their tectonic setting.

Key words: Indochina Block, Kontum Massif, Precambrian crust component, petrogenesis, tectonic setting evolution