Petrogenesis of the enclave-bearing granitoids from the Chinese Altai: implications for the differentiation of mantle-derived magmas and formation of calc-alkaline plutons in subduction zones
- 1Department of Earth Sciences, The University of Hong Kong, Hong Kong, China
- 2Department of Geology, Northwest University, Xi’an, China (710069)
- 3Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences, Guangzhou, China ( 510301)
The chemical differentiation of mantle-derived magmas in subduction zones during the generation, transport, and emplacement has always been a concern, which is closely related to the petrogenesis of calc-alkaline granitoids. A systematic study of petrography, mineralogy, and geochemistry is conducted on typical arc granitoids and associated mafic microgranular enclaves (MME) from the Chinese Altai, Central Asian Orogenic Belt. Magma hybridization modeling using major and trace element compositions suggests that the parental magma of granitoids is a mixture of a mafic and a felsic endmember. The sharp decrease of plagioclase An values from cores to rims (e.g., from ca. 80 to 40) implies polybaric crystallization of water-saturated magmas accompanied by degassing. Petrographic evidence and plagioclase in situ Sr isotopic compositions ((87Sr/86Sr)i = 0.7053–0.7071) show the involvement of isotopically different magmas during the mineral crystallization. The positive zircon εHf(t) values of MME (+2.3 to +5.4) and granitoids (+0.6 to +4.6) further show that the mafic melts are mantle-derived, while felsic melts should originate from juvenile lower crust with the slightly more evolved isotopic composition. An evolution scenario of the mantle-derived mafic magma and formation of enclave-bearing calc-alkaline plutons in arc settings is demonstrated: Hydrous mantle melts rose to the deep crustal-mantle boundary, where they effectively mixed with juvenile lower crustal melts to form the hybrid parental magma of the granitoids. In the high crustal-level chambers, decompression-dominated crystallization, mingling, and limited mixing of mafic magma blobs and enclosing granitic melts ultimately determined the rock texture, mineral composition, and enclave morphology. This work was financially supported by Hong Kong RGC GRF (17302317), National Key R&D Program of China (2017YFC0601205), NSFC Projects (41730213, 42072264, 41902229, and 41972237).
How to cite: Cui, X., Sun, M., Zhao, G., Zhang, Y., Yao, J., and Wong, J.: Petrogenesis of the enclave-bearing granitoids from the Chinese Altai: implications for the differentiation of mantle-derived magmas and formation of calc-alkaline plutons in subduction zones, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-177, https://doi.org/10.5194/egusphere-egu23-177, 2023.
