东秦岭金堆城大型斑岩钼矿床LA-ICP-MS锆石U-Pb定年及成矿动力学背景

U-Pb(LA-ICP-MS) Zircon Dating for the Large Jinduicheng Porphyry Mo Deposit in the East Qinling,China,and Its Metallogenetic Geodynamical Setting

对出露东秦岭金堆城大型斑岩钼矿床北部的老牛山黑云二长花岗岩和金堆城含矿斑岩利用单颗粒锆石激光探针LA-ICP-MS定年技术进行了U-Pb精确定年和岩石地球化学对比研究。LA-ICPMS锆石U-Pb测年得到老牛山黑云二长花岗岩和金堆城含矿斑岩锆石加权平均年龄值分别为146.35±0.55Ma和140.95±0.45Ma,与前人多次获得的金堆城钼矿床的辉钼矿Re-Os模式年龄141±4～127±7Ma下限值一致,显示成矿与成岩同时或略滞后于岩体,成岩成矿发生于侏罗纪-白垩纪的同一成岩成矿系统中,与发生于140Ma左右的中国东部构造体制大转换晚期的伸展地球动力学事件相吻合,岩体的形成和钼矿化的发生处于侏罗纪-白垩纪挤压-伸展转变期,对应的地球动力学背景为华北板块与扬子板块碰撞造山后的陆内造山和伸展过程。两岩体常量元素含量和特征地球化学参数变化范围基本一致,均属硅过饱和的钙碱系列岩类。两个岩体大离子亲石元素LILE相对富集,高场强元素HFSE贫化,LREE富集,轻重稀土分馏较明显,铕异常不明显或无铕异常,微量元素和稀土元素分布与板内花岗岩和后碰撞花岗岩相似。矿床地球化学资料证明,矿床成岩成矿物质来自深部,为下地壳和上地幔混合源。因此,金堆城矿床的成矿发生于大陆会聚作用向伸展拉张的转折时期,由于岩石圈减薄,软流圈物质上涌,导致强烈的壳-幔相互作用,加厚的下地壳物质发生熔融形成富含Mo元素的花岗质岩浆,当岩浆沿构造薄弱带上升到地壳浅部,岩浆冷凝分异出的成矿流体充填和交代围岩而成。

A precise U-Pb dating based on single particle zircon laser probe dating technology （LA-ICPMS）, and comparative study of petrological and geochemical characteristics have been applied to the Jinduicheng orebearing porphyry and Laoniushan biotite-monzonitic granite north of the large Jinduicheng porphyritic molybdenum deposit in the Eastern Qinling. The weighted average ages by LA-ICPMS zircon U-Pb dating of the Laoniushan biotite-monzogranite and the Jinduicheng ore-bearing porphyry are 146.35± 0.55 Ma and 140.95±0.45 Ma respectively, which are consistent with the lower limit of molybdenite Re-Os model ages by previous researchers. This indicates that the mineralization took place almost at the same with or slightly later than that of the diagenesis, with metallogenesis and diagenesis occurring in the same system in Jurassic-Cretaceous, which was coupled with the extensional geodynamical event during the later stage of tectonic system transformation occurring in East China at about 140 Ma. The formation of massif and mineralization of molybdenum occurred at the transitional period of compression-extension between the Jurassic and the Cretaceous, which corresponded to geodynamical setting of intracontinental orogeny and extension after the collision of the North China Block and the Yangtze Block. The two plutons have the similar variation On major elements and geochemical parameters, both of them belong to cal-alkaline series rock with silica super-saturation. Geochemically, the two plutons are enriched relatively in LILE, depleted in high field strength element （HFSE）, with LREE especially enriched, obvious differentiation between LREE and HREE, and or no distinct negative Eu anomaly. The distribution of trace elements and rare elements are similar to that of intraplate granite and post-collision granite. Geochemical data indicate that the metallogenetic substance was deep earth in origin, and was mixing of the lower crust and upper mantle. Thus, the metallogenesis of the Jinduicheng ore deposit betided at the transformation period from continental convergency to extension as a result of thinning of lithosphere and uprising of ore-forming substance from asthenosphere, which jointly caused interaction between the crust and the mantle, sped melting of the thickened lower crustal material, and then resulted in Mo-rich grantitic magma. When the magma moved along structures developed within the upper crust, ore-bearing fluid derived from condensation fraction of the magma filled the structures and replaced country rock, thus forming the deposits.