江西广丰铜山中酸性岩体锆石U-Pb年代学、地球化学特征及成因研究

Geochronology,Geochemistry and Petrogenesis of the Tongshan Intermediate-Felsic Rocks in Guangfeng County,Jiangxi Province

广丰铜山中酸性岩体侵位于华夏地块和扬子地块的结合部位——钦杭结合带东段。岩性为花岗闪长斑岩和石英闪长玢岩,与之伴生有中型矽卡岩型铜矿床。花岗闪长斑岩的锆石环带结构明显,Th/U值较大（0.40-1.07）,为典型的岩浆锆石,LA-ICP-MS锆石U-Pb成岩年龄为171.0±1.6 Ma（n=15,MSWD=0.16）。主微量元素特征表明该套中酸性侵入岩属于高钾过铝—偏铝质I型花岗岩类,轻重稀土分异明显（LaN/YbN=10.53-38.37）,无明显Eu异常（δEu平均值1.05）,富集大离子亲石元素（Rb、Ba、Th、U、K）,亏损Nb、Ta、Ti,低Y（〈10.4×10^-6）、Yb（〈0.93×10^-6）,高Sr/Y比值（〉26）,具有弧型岩石和埃达克质岩石的双重地球化学特征。综合研究表明,铜山矿区的中酸性岩石形成压力大于1.5GPa,熔体起源深度大于50km,源区为富含金红石的角闪榴辉岩,形成于古太平洋板块斜向俯冲的挤压后松弛阶段,由新元古代洋壳俯冲形成的增厚下地壳部分熔融形成。其埃达克质特性使其易于萃取、聚集铜金等成矿物质,因此具有扩大找矿的潜力。

The Tongshan intermediate-felsic rocks emplaced in conjunction area between Yangtze block and Cathaysian block,i.e.in the eastern part of the Qinhang conjunction areas.Tongshan intermediate-felsic rocks consist of granite diorite porphyry and quartz diorite porphyrite,accompanied with medium-sized skarn copper deposits.The zircons from granite diorite porphyry show distinct zonation structure with high Th/U ratios（0.40-1.07）,suggesting that zircons are typical magmatic in origin.LA-ICP-MS zircon U-Pb dating yields an age of 171.0±1.6 Ma（n=15,MSWD=0.16）.Major element and trace element characteristics indicate that the intermediate-acid intrusive rocks are high K peraluminous-aluminous I-type granitoid,with strong LREE/HREE fractionation（LaN/YbN=10.53-38.37）and no significant negative Eu anomalies（δEu average of 1.05）.Enrichment of large ion lithophile elements（Rb,Ba,Th,U and K）and depletion of Nb,Ta and Ti,low contents of Y（〈10.4×10^-6）and Yb（〈0.93×10^-6）,high Sr/Y ratio（26）,suggest the rocks are of double geochemical characteristics of arc volcanic rock and adakite.Comprehensive study shows that the intermediate-felsic rocks formed at the pressure greater than 1.5GPa,magmatic source depth was more than 50 km,and the source region composition was rutile-rich amphibole eclogite facice rocks,which resulted from partial melting of thickening lower crust with relaxation environment after extension caused by the oblique subduction tectonic of the Paleo-Pacific Plate.The ore-forming materials were easily extracted and accumulated to form Cu-Au deposits from the magmatic melts with adakitic characteristics. Therefore,the Tongshan area has the potential of mineralization and is worth broadening exploration.