Chronology, Petrogenesis And Tectonic Implications Of The Cumulate Gabbro In Tahe Area, Northern Great Xing'an Range

In this paper, the characteristics of petrology and mineralogy, the geochemical data of the major, trace and rare earth elements, the results of zircon U-Pb dating and the composition of Hf isotopes in zircon are reported for the Tahe cumulate gabbro exposed in the northern Great Xing'an Range. Basen on these data, the emplacement age and the petrogenesis of the pluton, protolith and tectonic implications are discussed in detail. Based on eletron microprobing analysis (EMPA), olivine of the Tahe gabbro is chrysolite, with high Fo77, clinopyroxene belongs to diopside, and plagioclase contains high An85, it should be bytownite. Elements analytical results show that the Tahe gabbro, characterized by low SiO₂ (42.5⁴7.4 %), TiO₂ (<0.5%), P₂O₅ (<0.1%) and high Al₂O₃ (15.9³2.9%), CaO (11.7¹7.1%), are enriched in large ion lithophile elements (LILE) such as K, Sr, Ba, et al., and depleted in high field strength elements (HFSE) such as Zr, Hf, Ti, P, and so on, showing the similar geochemical characteristics to the gabbros formed in the active continental margin or island arc. Zircon laser ablation (LA)-ICP-MS U-Pb analyses indicated that the Tahe gabbro was emplaced at 333±8Ma with negative εHf(t) values of -2^-5. Therefore, it is concluded that the Tahe cumulate gabbroic magma was derived from partial melting of the pyroxenite in the enriched lithospheric mantle or lower crust which was metasomatized by fluid released from the subducted slab. Hence, the occurrence of the Tahe gabbro indicates that a subduction process was conducted beneath this area at ³30Ma, which was considered to be related to the assembly of the Songnen and Xing'an blocks.