鲁西中生代高镁辉石闪长岩中超镁铁质捕虏体成因:氧化态和铂族元素证据

Origin of Ultramafic Xenoliths in High-Mg Pyroxene-Diorites from Western Shandong Province:Evidence from Oxidation State(fO_2) and Platinum Group Elements

鲁西莱芜晚中生代高镁闪长岩中含丰富的超镁铁质捕虏体,主要为纯橄岩(>80%),少量方辉橄榄岩(<5%)和橄榄辉石岩(<15%)。测定了代表性捕虏体的亲铁、亲铜元素含量,根据其矿物化学成分计算了捕虏体的氧化-还原状态。方辉橄榄岩遭受了不同程度富Si熔体的交代,全岩富含Cr、Co、Ni和IPGE,高IPGE/PPGE值,是地幔深度部分熔融的残留。纯橄岩由铬铁矿和高Mg橄榄石组成,全岩富含Cr、Co、Ni,贫IPGE,低IPGE/PPGE值,可能为一种镁铁质熔体的堆晶岩。纯橄岩中橄榄石成分表现出一定的变化范围,局部镁橄榄石(Fo)成分高达94,可能是堆晶中橄榄石与足够的铬铁矿反应的结果,原始熔体可能为玻镁安山质岩浆。方辉橄榄岩和纯橄岩都显示高fO2值,FMQ+1.4～+2.4,与研究区早古生代相对还原的陆下岩石圈地幔(fO2低于C-CO2缓冲反应)形成鲜明对比。数据表明,中生代扬子大陆和华北克拉通碰撞之前,特提斯大洋板块和扬子大陆边缘相继俯冲到华北克拉通东缘之下,导致当时岩石圈地幔的fO2陡升。

Ultramafic xenoliths are abundant in Late Mesozoic high-Mg pyroxene-diorites at Laiwu,western Shandong Province;they are composed of dominant dunite（80%） with minor harzburgite and olivine-pyroxenite.We determined the abundance of siderophile elements（PGEs,Cr,Co,and Ni） and chalcophile elements（Cu and S） of representative xenolith samples,and calculated oxidation states based on mineral chemistry.The harzburgite shows evidence of variable metasomatism by a Si-rich melt,and contains high concentrations of Cr,Co,Ni and IPGE with high ratios of IPGE/PPGE in bulk rock composition,most likely representing the residue of mantle after high degree of partial melting.Dunite,composed of chromite and high-Mg olivine,contains high Cr,Co and Ni,but low IPGE with low ratios of IPGE/PPGE in bulk rock composition,suggesting it is likely the cumulate of a mafic melt.Some olivine grains in dunite show variable,locally high Mg（up to Fo=94）.We attribute these high values to the interaction of the olivine with abundant chromite in the cumulate,and that the parental melt was likely boninitic.Both harzburgite and dunite indicate high log fO2 values,ranging from FMQ＋1.4 to ＋2.4.The values contrast with fO2 below that of the graphite-CO2 buffer for xenoliths in Early Paleozoic diamondiferous kimberlite pipes in the area.The data indicate a sharp increase in fO2 during Mesozoic time,likely caused by subduction of the Tethyan oceanic plate before collision with the Yangtze continent below the eastern margin of the North China craton.