浙江新路盆地九华山火山-侵入杂岩年代学、地球化学特征及其地质意义

Geochronology and Geochemistry of Jiuhuashan Granitic Volcanic-Intrusive Complex Rock in Xinlu Basin of Quzhou City, Zhejiang Province, and Their Geological Significance

为探究新路火山岩盆地产于不同围岩的铀矿床成因机制,查明构造地质背景与铀成矿的关系,以浙江衢州九华山花岗质火山-侵入杂岩为对象,开展年代学和地球化学研究.SHRIMP锆石U-Pb年龄结果显示,花岗岩与火山岩可能为同期岩浆活动的产物,加权平均年龄为～134.9-～137.8 Ma.地球化学分析结果显示,九华山花岗岩和火山岩均具有高铝(11.96%~14.59%),高全碱(7.35%~9.30%),Ga/Al×10 000>2.6,Zr+Nb+Ce+Y>350,FeO*/MgO>10等特征,指示花岗质火山-侵入杂岩为典型的铝质A型花岗岩;而岩石尤其是SiO_2含量更高的岩石,表现出更强烈的Eu亏损,Ba、Ta、Nb、Ti负异常,Zr、P含量低等特征,指示岩石形成过程中经历了不同程度地结晶分异作用.同位素分析显示九华山火山-侵入杂岩体具有εNd(t)值变化较大且部分为正(2.26^-6.16),相对年轻和分散的Nd同位素模式年龄等,暗示岩浆源区可能发生了新老物质不同比例的混合.这表明本区花岗岩与火山岩具有相似的地球化学特征,为拉张背景下经历了壳幔组分混合和不同程度分离结晶作用的同期岩浆活动的产物.综合工作区已知铀成矿年龄与成矿特征认为,早白垩世NW向拉张构造体制和软流圈地幔的上涌为本区早期铀矿化提供了有利的构造环境、物质条件和良好的热源.从一个侧面证实本区主干铀矿床虽然含矿围岩具多样性,但从成因上来讲均为热液型铀矿床,均为同一构造背景下形成的产物.

To understand the genesis of uranium deposits in different ore hosting rocks and discuss the relationship between deposits and tectonic geological setting of Xinlu volcanic basin in Quzhou City, Zhejiang Province, we have focused on Jiuhuashan granitic volcanic-intrusive complex by zircon U-Pb geochronology,major and trace element analysis, and Sr-Nd isotopic analysis. SHRIMP U-Pb dating of zircon grains from the volcanic and intrusive rocks yield ages of 134.9~137.8 Ma, and both of them have similar geochemical characteristic with high Al（11.96%~14.59%）,Na2O＋K2O（7.35%~9.30%）, Zr＋Nb＋Ce＋Y（350）, FeO＊/MgO（10） and Ga/Al ratio（2.6×10^（-4））, indicating that they are products of a synchronous magmatism with a typical characteristic of aluminous A-type granite. While those more acidic rocks have lower Zr and P contents, more enrichment in some LILEs,and more depletion in Ba、Ta、Nb、Ti and Eu, which announces a varying degree fractional crystallization in magma source. The isotope geochemical characteristics display young and dispersed Nd model age, high εNd（t） values in the range of -6.16 to 2.26, suggesting an input of mantle materials. We conclude the volcanic and intrusive rock formed in the same magmatic event which occurred at a tectonic setting of extension and upwelling of asthenosphere in Xinlu basin in early Cretaceous, and it provides a favourite ore-controlling structure, material source and heat for uranium mineralization. This also means those uranium deposits in different ore hosting rocks have similar genetic mechanism.