关键词: 暗色微粒包体, 地球化学, 岩石成因, 岩浆混合作用, 沟里地区, 东昆仑造山带

Abstract:

The authors report petrographical and whole-rock geochemical data of the host diorite and mafic microgranular enclaves(MMEs) within the Asiha diorite pluton at Gouli area in eastern Qinghai East Kunlun，and ascertain their petrogenesis and tectonic setting. The Asiha diorite pluton contains abundant MMEs which are mainly amphibole gabbro. The MMEs display igneous-texture and some of them are characterized by plastic flow deformation. The MMEs have plagioclase, amphibole and quartz xenocrysts and acicular apatite, which belongs to  petrographical characteristics of magma mixing. Authors systematically studied petrological and geochemical characteristics of the host rocks and the MMEs, and conclude that both are meta-aluminous, high-K calc-alkaline series-shoshonitic series rocks. The selected major oxides of the MMEs and the host rocks show a linear correlation in Harker diagrams. In addition, Al₂O₃/K₂O-CaO/K₂O and SiO₂/CaO-K₂O/CaO identical oxide ratio denominator covariant diagram of the MMEs and their host rocks also show a linear correlation, which indicate their composition change is related to the magma mixing and mingling. Trace elements data show that they are both enriched in large ion lithophile elements (Rb, K) and depleted in high field strength elements(Nb, Ta,  P and Ti). The MMEs are characterized by low SiO₂ contents (SiO₂=50.70%-53.88%), high MgO, Fe₂O₃ and CaO contents, as well as a higher Mg^# value (Mg^#=0.52-0.59), which indicates that the primary magma was derived from the partial melting of the mantle wedge. The Rb/Sr ratio of the host rocks varies from 0.22 to 0.27, which is close to but less than average values of the earth crust. The Nb/Ta ratio of the host rocks varies from 14.5 to 15.2, which is between the average values of the crust and mantle, suggesting that the magma of the host rocks is mainly from crust and experienced magma mixing. Combined with regional tectonic evolution and discrimination of tectonic setting, we suggest that the Asiha diorite pluton formed in the Andean-type active continental margin. In Early Triassic, the A’nyemaqen ocean subducted northward, the subduction slab dehydrate led to fluid metasomatism, inducing partial melting of mantle wedge to form the mafic magma. The mafic magma underplated the overlying lower crust, resulting in its partial melting and then mixed with the crustal magma to form the Asiha diorite pluton, while the MMEs are the residually basic part of magma mixing.

Key words: mafic microgranular enclaves, geochemistry, petrogenesis, magmatic mixing, Gouli area, Eastern Kunlun orogenic belt