Geochemistry Of The Baoshan Pluton In The Tongling Region Of The Lower Yangtze River Belt

The petrogenesis of adakitic intrusions in the Tongling district and Lower Yangtze River Belt have been hotly debated in past decades.Baoshan pluton is located on the southwest of Tongling district.Here we do a systematic mineralogy,petrology and geochemical study for the Baoshan rocks and mafic microgranular enclaves to understand their petrogenesis.The results indicate:1)Baoshan pluton is composed by several kinds of magmatic rocks,e.g.,pyroxene diorite to granodiorite;2)the mafic rocks are usually enrolled by felsic magmas,with or without distinctive boundaries;3)the calculated Ce⁴⁺/Ce³⁺（average 329）of zircon in BS-3 which represents host rocks is obviously higher than the calculated Ce⁴⁺/Ce³⁺（average 60）of zircon in BS-2which represents enclaves,indicating the host rocks have higher oxygen fugacity.the Ti-in-zircon temperature（average 679）in BS-3 is slightly lower than BS-2（average689）,but both of them are in the range of adakites from the Lower Yangtze River Belt,far below the range of adakites from the South Tan-Lu Fault,suggesting a water-rich source of Baoshan pluton;4)U-Pb dating on zircon by LA-ICP-MS suggests that granodiorite and the pyroxene diorite enclaves were emplaced at 143.8±3.2Ma（2σ,MSWD=2.3）and 142.9±2.9Ma（2σ,MSWD=2.2）,respectively;5)Baoshan pluton is geochemical affinity to adakite,e.g.,wide-range of SiO₂（SiO₂=52.88wt.%⁶3.66wt.%）,high Al₃O₂（average 16.03 wt.%）,high Sr/Y（33.4-102,average 72.9）and Sr/La ratios,low Y and Yb,low（La/Yb）_N（14.6²6.6,average 21.4）and K₂O/Na₂O（average0.39）ratios;6)The field contact relationship between mafic and felsic rocks and the geochemical characteristics of Baoshan rocks indicate they were originated by magma mixing.The two end members of 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 lithospheric mantle,which has high Ni,Cr,V and Co.7)The LA-ICP-MS in-situ and mapping results of zoned plagioclase of reveal the magma evolving process as follows:the melt of lithospheric mantle ejected into the magma chamber which was formed by the melt of subducted oceanic crust and high silica melt captured the plagioclase phenocryst from low silica melt,after the equilibrium of the thermodynamic environment of the xenocrysts and the host magma,the plagioclase phenocryst continued to grow.There are several reasons for why adakites are beneficial to large-scale magmatic mineralization:high oxygen fugacity,rich in water content and abundant metallogenic（Cu、Au）elements.On the condition of rich water and high oxygen fugacity,valence of sulfur is mainly+6,leading to the unsaturation of sulphide magma and more chalcophile elements were extracted from the mantle,thus the melt became the potential metallogenic magma which contained abundant chalcophile elements.Baoshan rocks may indicate that the magmatism and mineralization in Tongling even the LYRB were formed in a subduction system in Mesozoic.