越南石溪铁矿区二长花岗岩地球化学特征、锆石U-Pb年龄和Hf同位素组成及其成矿意义

Geochemical Characteristic,Zircon U-Pb Age and Hf Isotopic Compositions of Monzogranites from the Thack Khe Iron Deposit,Vietnam

位于印支陆块东北部长山成矿带内的越南石溪铁矿是东南亚地区最大的矽卡岩型富磁铁矿。本文对矿区与铁矿成矿有关的黑云母二长花岗岩进行了锆石U-Pb年代学、地球化学和Hf同位素研究。地球化学分析结果显示,岩体富硅(SiO2为69.82%~71.8%)、贫钙镁(TFeO/MgO比值为4.07~4.76)、弱过铝质(A/CNK为1.01~1.05),明显富集Rb、Th、Pb、Zr,亏损Ba、Sr-P和Eu-Ti,属于铝质A型花岗岩。锆石U-Pb定年结果显示,矿区Phia Bioc杂岩体中黑云母二长花岗岩侵位于257.8±2.2 Ma,属于晚二叠世。锆石的εHf(t)值为-2.6^-6.4,对应的二阶段Hf模式年龄tDM2为1296~1505 Ma。岩石地球化学特征及Hf同位素组成表明,石溪铁矿区Phia Bioc杂岩体中黑云母二长花岗岩主要形成于古老下地壳岩浆岩的部分熔融,应该形成于特提斯马江洋向长山带下俯冲时期的弧后伸展构造背景。

The Thack Khe iron deposit, located in the Truong Son metallogenic belt of the northeast of the Indochina Block, is the largest skarn-related magnetite deposit in Southeast Asia. In this study, we performed zircon U-Pb geochronology, geochemistry and zircon Lu-Hf isotope analyses on biotite monzogranite related with the metallogenesis of the Thack Khe iron deposit. The geochemical results show that the biotite monzogranite rock is enriched in Si （SiO2 =69.82% to 71.8%） and depleted in Ca and A1, with TFeO/MgO=4.07 to 4.76 and slightly peraluminous （A/CNK=1. 01 to 1.05）. Enrichment of Rb, Th, Pb, Zr and depletion of Ba, Sr-P, Eu-Ti indicate the rock belongs to the aluminium A-type granite. The zircon U-Pb dating indicates that the biotite monzogranite of Phia Bioc complex from the Thaek Khe iron deposit emplaced at 257.8±2.2 Ma, i.e. the Late Permian. The initial Hf isotope ratios （εHf（t） = --2.6 to --6.4） responds to tDM2 of 1296 Ma to 1505 Ma. The geochemical characteristic and Hf isotope suggest that the biotite monzogranite from the Phia Bioc complex in the Thack Khe iron deposit formed by the partial melting of the lower crustal igneous rock in the tectonic setting of a back-arc extension related to subduetion of the Tethyan Majiang Ocean towards the Truong Son metallogenie zone.