塔里木和中亚造山带西段二叠纪大火成岩省的两类地幔源区

A review on two types of mantle domains of the Permian large igneous province in Tarim and the western section of Central Asian orogenic belt

对塔里木和中亚造山带西段二叠纪玄武质岩石地质、年龄、元素地球化学、同位素组成的系统总结表明,二叠纪火成岩在分布面积、岩石类型(以玄武岩占绝对优势)、活动时间(以275Ma左右为峰期)等方面均与世界典型的大火成岩省一致,将其命名为巴楚大火成岩省(Bachu LIP)。元素和同位素地球化学特征表明,塔里木玄武岩来自长期富集的岩石圈地幔,来源深度为60～80km。塔里木基性岩墙和超镁铁-镁铁杂岩的原始岩浆可能来自软流圈地幔(OIB)部分熔融。中亚造山带西段的玄武岩、基性岩墙和超镁铁-镁铁杂岩主要来自被俯冲带熔体交代的强烈亏损的岩石圈地幔,其中部分地区可能有软流圈物质的加入,如东天山和阿勒泰南缘高Ti系列的玄武质岩石。根据元素和同位素地球化学资料,将巴楚大火成岩省分为2个地幔省(mantledomain),即塔里木省和中亚省。这2个不同地幔省的成矿系列也有显著的差异,塔里木省为钒-钛磁铁矿矿床,而中亚则以铜-镍-(铂族金属)硫化物矿床为主,成矿作用的差异和岩浆地幔源区的差异是完全对应的。综合地质、地球化学和成矿作用,认为巴楚大火成岩省的形成和二叠纪地幔柱密切相关。

A comprehensive analysis on the reported geological, geochronological and geochemical data of the Permian basaltic rocks in Tarim and the western section of the Central Asian orogenic belt （CAOB） indicates that the volume, petrography and their peaking activity at 275 Ma are correlated with those characteristics of other large igneous provinces （LIPs） in the world and we termed it as Bachu LIP. Elements and Sr-Nd isotopic compositions of the basalts in Tarim suggest that they may originate from partial melting of a long-term enriched hthospheric mantle at 60-80 km depth, while the mafic dykes and ultramafic-mafic intrusions in Tarim most possibly originated from asthospheric mantle （OIB-like）. On the other hand, the basaltic rocks from the CAOB predominantly originated from partial melting of a subduction-released fluid （and/or melts） metasomatized lithospheric mantle involved with variable OIB-like basaltic magma addition, e.g., the high-Ti series basaltic rocks in southern Altaid and Eastern Tianshan areas. Thus based on geochemical data of major and trace elements, two different mantle domains can be divided within Bachu LIP, i.e., a long-term enriched Tarim domain and a subduction-metasomatized depleted CAOB domain. These two mantle domains have different metalogenesis types, i.e., the metalogenesis in Tarim is dominated by V-Ti magnetite mineralization, while in CAOB, dominated by Cu-Ni- PGE sulphide and co-genesis of Cu-Ni-PGE sulphide and V-Ti magnetite mineralization. The metal]genetic difference between in Tarim and in the western section of the CAOB is concurrent with the two different mantle domains. By integrating geological, geochronlogical, metallgenetic and geochemical data, we conclude that the formation of Bachu LIP is closely linked with a Permian mantle plume, which we term it as Bachu plume.