江南造山带黟县复式岩体的成因研究

黟县岩体位于江南造山带北东段，是该区侵入新元古代基底中面积最大的复式岩体。本文对黟县岩体开展岩浆岩锆石U-Pb年代学和全岩地球化学分析和研究。该岩体可划分为两个侵入体，岩石类型分别为闪长岩、花岗闪长岩。锆石U-Pb定年结果表明，闪长岩和花岗闪长岩的LA-ICP-MS锆石U-Pb年龄分别为812±19 Ma和150.6±3.3 Ma，即上述岩石分别形成于晋宁期和燕山期。花岗闪长岩的地球化学特征表现为钾质、钙碱性、过铝质岩石，具有类似埃达克岩特征：高SiO₂（>65%），高Sr/Y（>21.44）和（La/Yb）_N（>11.57）比值，高Al₂O₃（<16%），低MgO（<2.20%），低Yb（<1.36×10^-6）和Y（<15.11×10^-6），但贫钠（K₂O/Na₂O>0.5%）。闪长岩的地球化学特征表现为钾质、高钾钙碱性过铝质岩石，具有较高的Al₂O₃（>16%），MgO（> 2.39%）和高TiO₂（> 0.41%）等含量，高Sr/Y（> 15.49）比值，低Yb（< 1.93×10^-6）和Y（<15.5×10^-6）含量，具有相似埃达克岩特征。两者的稀土元素配分图显示LREE富集，HREE亏损，强烈亏损P、Nb，具有微弱的Eu负异常。两者均为Ⅰ型花岗岩。两期岩浆源区主要源自古老下地壳岩石的部分熔融，晋宁期的闪长岩伴有地幔成分的参与。晋宁期的闪长岩可能形成于火山弧构造环境中，显示华夏地块向扬子地块俯冲汇聚过程；而燕山早期的花岗闪长岩形成于由挤压至拉张转换构造环境中，可能与古太平洋斜向俯冲作用有关。

Genesis of the Yixian complex pluton in the Jiangnan orogenic belt

The Yixian pluton is located in South Anhui district, northeastern Jiangnan orogenic belt, which is the largest complex pluton of the Neoproterozoic basement in the district. In this study, the authors systematically analyzed and studied the zircon U-Pb geochronology and geochemistry of Yixian complex pluton. It can subdivided into diorite and granodiorite from lithologically. The LA-ICP-MS U-Pb dating results of the diorite and granodiorite intrusions are 856.2±21.2 Ma~765.2±19.3 Ma and 165.2±6.8 Ma~144.4±1.5 Ma respectively. That is to say, they were formed Yanshanian and Jinning stage, respectively. The geochemistry of the granodiorite shows that they are potassium, calc-alkaline and peraluminous rocks and have similar characteristics of adakites:high SiO2(>65%), High Sr/Y(>21.44) and(La/Yb)N(>11.57) ratios, low Al2O3(<16%), Low MgO(<2.20%), Low Yb(<1.36×10-6) and Y(< 15.11×10-6) values, But poor Na(K2O/Na2O> 0.5%), etc. However, the diorite geochemistry is characterized by potassium and high K and calc-alkaline peraluminous rocks:high Al2O3(>16%), high MgO(>2.39%) and TiO2(>0.41%) contents, high Sr/Y(>15.49) ratios, low Yb(<1.93×10-6) and Y(<15.5×10-6) values, and show similar characteristics of adakites, too. The REE partition maps of the two granites show that LREE is enriched, HREE is depleted, P and Nb are strongly depleted, with weak negative Eu anomaly. Those indicate that they are Ⅰ-type granitoids. The two magma sources are mainly derived from the partial melting of the ancient lower crust metamorphic rocks, and the Jinning epoch diorites are accompanied by mantle material components. The Jinning diorite may have formed in the volcanic arc tectonic setting, it may showed the subduction and convergence of the Cathaysian Block to Yangtze Block, while the Early Yanshanian ranodiorite may have formed in the tectonic setting from compression to tension, which may be closely related to the subduction of the paleo-Pacific Plate.