藏南扎西康矿区流纹岩的岩石地球化学、锆石U-Pb测年和Hf同位素组成

Petrochemical Features,Zircon U-Pb Dating and Hf Isotopic Composition of the Rhyolite in Zhaxikang Deposit,Southern Xizang(Tibet)

本文对藏南扎西康铅锌锑银矿区流纹岩进行详细的岩石地球化学、锆石U-Pb年代学和Hf同位素组成研究。详细的地球化学特征显示,扎西康流纹岩具有富硅(SiO2=73.37%~77.08%)、贫碱(Na2O+K2O=3.48%~3.56%),贫Mg(MgO=0.36%~0.49%),贫Ca(CaO=0.50%~0.66%),强过铝质(A/CNK=2.16~2.62),富集Rb、Th、U、Pb等大离子亲石元素及Hf、Nd等高场强元素,并明显亏损Sr、Ti等元素。稀土总量较高(∑REE=295.71×10-6~343.82×10-6),轻稀土富集,且轻重稀土分异明显(LREE/HREE=10.00~12.62),弱负Eu异常(δEu=0.57~0.88),无明显Ce异常。采用LA-MC-ICP-MS对流纹岩锆石U-Pb年龄进行测定,岩体成岩年龄为135.33±0.62Ma。锆石εHf(t)值较低,主要集中在-9.5^-24.2,显示其物源为壳源。扎西康矿区出露的流纹岩形成于早白垩世,可能是Comei—Bunury大火成岩省的一部分。

In this study, we carried out a comprehensive research including petrochemistry, zircon U-Pb dating and Hf isotopic composition analysis on the rhyolite in the Zhaxikang deposit, Lhiinzg County, South Xizang （Tibet）. Chemical analyses show the rhyolite has high SiO2 （SiO2 = 73.37% - 77.08% ）, low alkaline （Na20 ＋ K20=3.48% -3. 56%）, low Mg and Ca （MgO =0. 36% -0.49%,CAO =0. 500/o -0.66%）, strong peraluminous （A/CNK =2.16 - 2.62）, enriched in LILEs such as Rb, Th, U, Pb and HFSEs such as Hf and Nb, and depleted in Sr and Ti. The rocks also display high total REE contents （ ∑REE =295.71 × 10 -6 - 343.82 × 10-6）, enrich LREE and differentiate obviously （LREE/HREE = 10.00 -12.62）, with weakly negative Eu anomalies （ δEu = 0.57 - 0.88 ） , without cerium anomalies. LA-MC-ICP-MS zircon U-Pb chronology indicated that the Zhaxikang rhyolite emplaced at 135.33 ± 0.62Ma. Zircon Hf isotope analyses show relatively homogeneous Hf isotopic compositions with enf（t） values from -9.5 to -24.2, which indicate that the rhyolite are likely generated from crust. The rhyolite is the product of the Early Cretaceous magmatic activity and may be the part of Comei-- Bunbury large igneous province.