越南中部Kontum地块前寒武纪地壳组成和构造演化

Precambrian Crust Components and Its Tectonic Evolution of the Kontum Massif,Central Vietnam

越南中部的Kontum地块是印支地块中前寒武纪变质岩最重要的出露地区之一。该地块由不同时代不同变质程度的岩石组成。该研究通过对该地区主要变质岩的岩石学、地球化学和锆石U-Pb年代学分析,探讨了Kontum地块的物质组成以及其形成的构造背景。岩相学和化学成分分析结果表明,这些基底变质岩主要由变质沉积岩和少量的变质火成岩组成。变质沉积岩的原岩主要是硬砂岩和页岩。锆石U-Pb定年结果显示,这些沉积岩形成于古元古代晚期到新元古代晚期,并大致可以划分为五期沉积盆地的形成和演化,分别对应岛弧环境、被动大陆边缘环境、活动大陆边缘或大陆弧环境、活动大陆边缘-被动大陆边缘过渡环境和活动大陆边缘的构造背景。二个变质火成岩的原岩分别是拉斑玄武岩和S型花岗岩,它们分别形成于1424 Ma和1485 Ma。拉斑玄武岩显示富集Rb、Ba、Sr等大离子亲石元素,亏损Nb、Ta、Zr、Ti等高场强元素,符合典型岛弧玄武质岩石的地球化学特征。其锆石的εHf(t)值主要为正值(-1.68^+14.2),表明其岩浆起源于亏损的岩石圈地幔。地球化学特征表明其形成于岛弧环境,与第三期沉积沉积作用环境相符。S型花岗岩具有较高的SiO2、ALK和A/CNK(>1.1),以及中等的稀土含量和中等的Eu负异常。其原岩岩浆锆石的εHf(t)值变化于+5.97^+12.1,表明源区沉积岩的碎屑主要来自新生地壳。对比显示Kontum地块与海南岛在中元古代时期很可能曾经相连,在Columbia超大陆裂解时期处于超大陆的边缘。

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 zirconεHf(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 zirconεHf(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.