长江中下游铜陵地区宝山岩体地球化学研究

摘要

The petrogenesis of adakitic intrusions in the Lower Yangtze River Belt has been hotly debated for the past decade.The Baoshan pluton is located in the southwest of the Tongling district.Here we conducted a systematic mineralogical,petrological and geochemical study on the Baoshan rocks and mafic microgranular enclaves to understand their petrogenesis.The results indicate:(1) the Baoshan pluton is composed of several kinds of magmatic rocks,e.g.,pyroxene diorite to granodiorite;(2) the mafic rocks are usually enrolled by felsic magma,with or without distinctive boundaries;(3) U-Pb dating on zircons by LA-ICPMS suggests that granodiorite and pyroxene diorite enclaves were emplaced at (143.8±3.2) Ma (2σ,MSWD=2.3) and (142.9±2.9) Ma (2σ,MSWD=2.2),respectively;(4) the Baoshan pluton is similar to adakite in geochemical affinity,e.g.,a wide-range of SiO2 (SiO2 52.88％-63.66％),high Al2O3 (averaging 16.03％),high Sr/Y (33.4-102,average 72.9) and Sr/La ratios,low Y and Yb,low (La/Yb)N (14.6-26.6,averaging 21.4) and K2O/Na2O (averaging 0.39) ratios;(5) the field contact relationship between mafic and felsic rocks and the geochemical characteristics of the Baoshan rocks indicate they were derived from magma mixing.The two end members of the Baoshan pluton are high-silica and low-silica end-members.High-silica end-member was formed by partial melting of subducted oceanic crust,followed by fractional crystallization of magnetite in varying degrees while upwelling.The other is the lithospheric mantle,which has high Ni,Cr,V and Co contents.The Baoshan rocks may indicate that the magmatism and mineralization in Tongling,even the LYRB,were formed in a subduction system during the Mesozoic time.%长江中下游地区与铜金矿床有关的埃达克岩成因仍然存在分歧.对铜陵宝山地区的侵入岩和其中的包体进行了系统的矿物学、岩石学和元素地球化学研究.结果表明:(1)宝山岩体岩性变化较大,从辉石闪长岩到花岗闪长岩均有;(2)暗色基性岩常常被淡色酸性的花岗闪长岩所包裹,部分界限呈过渡状态;(3)LA-ICPMS锆石U-Pb定年结果表明,宝山花岗闪长岩和其中的辉石闪长岩包体分别形成于(143.8±3.2) Ma(2σ,MSWD=2.3)和(142.9±2.9) Ma (2σ,MSWD=2.2),属于早白垩世;(4)宝山岩体地球化学特征表明其主体具有典型的埃达克岩特征,如较大SiO2含量变化范围(52.88％～63.66％),高Al2O3含量(平均值为16.03％)、高Sr/Y(33.4～102,平均为72.9)和Sr/La比值,低Y和Yb含量、低(La/Yb)N(14.6～26.6,平均值为21.4)和K2O/Na2O(平均值为0.39)比值.(5)野外岩石接触关系和岩石地球化学特征表明,宝山埃达克质岩可能为岩浆混合成因,混合的两个端元分别为高硅和低硅端元.高硅端元为俯冲洋壳熔融形成的熔体并发生不同程度磁铁矿的结晶分异,而低硅端元为高Ni、Cr、V、Co的岩石圈地幔熔体.铜陵宝山岩体的地质与地球化学研究结果表明,铜陵地区中生代埃达克质岩和大规模铜金成矿作用可能形成于俯冲背景之下.