Petrogenesis Of Granitic Dykes In Zhongposhanbei And Hongshishan Layered Intrusions In Beishan Area, Xinjiang, P.R. China

Numerous Permian granitic dykes are spatially and temporally associated with the Zhongposhanbei and Hongshishan mafic-ultramafic layered intrusions. There are four types of granitic dykes totally, including biotite granite, quartz diorite porphyrite, moyite and granite porphyry. The former two are distributed in the Pobei intrusion and the latter two in the Hongshishan intrusion.Both biotite granite and moyite belong to the shoshonite series, they are enriched in LREE and LILE, obviously depleted in Eu, P2O5 and TiO2, relatively depleted in Nb and K2O, and both of them are A-type granites. TheεNd and Isr values of the biotite granite and moyite range from -4.54 to -5.53, and 0.6999 to 0.7090 respectively, but Isr value of the biotite granite is lower than 0.7000, and thus has no geological meaning. All the samples are distributed between EMⅡand MORB field in the (207Pb/204Pb)i vs. (206Pb/204Pb)i diagram, hence they should be the products of partial melting of old crust heated by mantle derived magmas, and there might be some mantle derived materials involved in this process.Amphiboles in quartz diorite porphyrite are mainly brown amphiboles. These amphiboles have low Si/(Si+Ti+Al) values (lower than 0.765), indicating mantle origin. The geochemical characteristics of the quartz diorite porphyry are similar to the cumulus plagioclase bearing quartz diorite in the layered intrusion, and the quartz diorite should be M-type granite. Samples of quartz diorite porphyry, quartz diorite and mafic rocks compose a well fractionate trend on the Harker diagrams, which indicate that the quartz diorite porphyry and quartz diorite should be products formed at the end of the evolution of mafic magma. TheεNd values of the quartz diorite range from -2.67 to -2.61, indicating crustal contamination. But in the (207Pb/204Pb)i vs (206Pb/204Pb)i diagram, the samples lie near the BSE area, indicating insignificant involvement of crustal materials. This may be due to less effection of Pb isotopes than that of Nd and Sr isotopes in the process of crustal contamination.The granite porphyry belong to I-type granites evidenced by high SiO2 contents, low ratios (<1), low Ga/Al ratios and Zr+Nb+Ce+Y values and negative correlation between P2O5 and SiO2.TheεNd values of granite porphyry are 0.13～0.77, distributing near the MORB area, and composing a well fractionate trend in the Nd-Sr isotope diagram together with samples of olivine gabbro and moyite, which indicate that the granite porphyry may be generated by mixing of a mantle-derived magma and a felsic magma which originated from partial melting of ancient crust heated by underplating of mantle-derived magmas.In conclusion, the mantle-derived magma can not only provide the heat source for the granites, but also can provide materials for the granites to some extend, and thus I-M-A type granites can be formed in the same environment at the same time.