藏南马扎拉矿区英安岩年代学与地球化学特征及其地质意义
Geochemical characteristics, chronology and geological significance of the dacite in the Mazhala deposit, southern Tibet
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摘要: 马扎拉金（锑）矿位于西藏北喜马拉雅金锑铅锌多金属成矿带，是扎西康整装勘查区发现的最早并最具代表性的金矿。对马扎拉矿区广泛出露的英安岩进行了锆石U-Pb年代学和岩石地球化学研究。全岩主量和微量元素地球化学特征显示，英安岩具有富硅（SiO2=66.86%~71.84%）、低碱（Na2O+K2O=3.79%~4.60%）、低Mg（MgO=0.50%~0.94%）、强过铝质（A/CNK=1.55~2.23）的特征，强烈富集Rb、U等大离子亲石元素（LILEs），相对亏损Ta、Ti、Zr等高场强元素（HFSEs）；稀土元素总量较高（ΣREE=283.74×10-6~380.04×10-6），轻稀土元素富集，且轻、重稀土元素分馏明显（LREE/HREE=10.57~15.22），中等负Eu异常（δEu=0.39~0.48），无明显Ce异常。英安岩LA-ICP-MS锆石U-Pb测年获得206Pb/238U年龄加权平均值为138.2±1.6Ma，表明马扎拉矿区英安岩形成于早白垩世。结合区域地质资料，初步认为马扎拉矿区英安岩形成于被动大陆边缘构造环境，可能是Comei-Bunury大火成岩省形成早期的产物。Abstract: Located in Northern Himalaya metallogenic belt, the Mazhala deposit is the most representative gold deposit in the Zhaxikang integrated exploration area. In this study,the authors carried out comprehensive researches including petrochemistry and zircon U-Pb dating analysis of the dacite in the Mazhala deposit,Comei County,southern Tibet. Chemical analyses show that the dacite is characterized by high Si (SiO2=66.86%~71.84%),low alkali (Na2O+K2O=3.79%~4.60%),low Mg (MgO=0.50%~0.94%), strong peraluminous nature (A/CNK=1.55~2.23),enrichment of LILE such as Rb,U and depletion of HFSE such as Ta, Ti and Zr. The rocks also exhibit high total REE content (ΣREE=295.71×10-6~343.82×10-6),enrichment of LREE and obvious differentiation (LREE/HREE=10.5~15.22),with moderately negative Eu anomalies (δEu=0.39~0.48) and no cerium anomaly. LA-ICP-MS zircon UPb chronology indicates that the Mazhala dacite was emplaced at 138.2±1.6Ma, being the product of the Early Cretaceous magmatic activity. According to regional geology and related data, it is considered that the dacite was formed on the Himalayan passive continental margin and may be a part of Comei-Bunbury large igneous province.
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