Formation of mafic magmas through lower crustal AFC processes 鈥?An example from the Jinan gabbroic intrusion in the North China Block

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• 作者：Feng Guo ; Jiangtao Guo ; ChristinaYan Wang ; Weiming Fan ; Chaowen Li ; Liang Zhao ; Hongxia Li ; Jingyan Li
• 关键词：Lower crustal assimilation ; Sr&ndash ; Nd&ndash ; Hf&ndash ; O isotopes ; Jinan gabbroic intrusion ; Early Cretaceous ; North China Block
• 刊名：Lithos
• 出版年：1 October, 2013
• 年：2013
• 卷：179
• 期：Complete
• 页码：157-174
• 全文大小：2777 K

文摘

The Jinan gabbroic intrusion from the North China Block (NCB) experienced complex differentiation from a reduced and Si-undersaturated magma to an oxidized and Si-saturated melt, with a mineral assemblage of olivine + pyroxene + plagioclase + biotite + ilmenite. New dating of baddeleyite and zircon yield a consistent U-Pb age range from 130 to 134 Ma, confirming emplacement during the early Cretaceous. In situ oxygen analyses on olivine show relatively homogeneous O isotopic compositions, and the olivine separates give a limited 未¹⁸O range from 6.0 to 6.1鈥? higher than that of olivine from normal mantle peridotites. The zircons have even higher 未¹⁸O (6.9-7.3鈥? than olivine and clinopyroxene separates (未¹⁸O = 6.2-6.9鈥? and show clear O isotope disequilibrium with olivine/clinopyroxene. The 未¹⁸O of the primary magma for olivine and clinopyroxene crystallization was increased by 1.2-1.4鈥?to form the evolved melt for zircon crystallization. This 未¹⁸O variation points to assimilation or contamination by high-未¹⁸O crustal components during magmatic evolution. Systematic decreases in whole-rock 蔚_Nd(t) and 蔚_Hf(t) are accompanied by increases of 未¹⁸O, Ba/Nb, Ba/Th and Zr/Nb and decrease of Th/La, along with a change in oxygen fugacity of the magma. Modeling results suggest that the Jinan gabbroic intrusion experienced assimilation-fractional crystallization (AFC) with felsic material at lower crustal levels. Combined geochemical and Sr-Nd-Hf-O isotopic features suggest that the primary melts were derived from the NCB lithospheric mantle that had been further metasomatized by a small volume of carbonatitic melt, possibly derived from the subducted Yangtze crust.

A compilation of the published mineral O isotope data shows that most Mesozoic mafic intrusions in NCB exhibit O isotope disequilibrium between zircon and olivine/clinopyroxene, with higher 未¹⁸O of zircon than olivine and clinopyroxene, indicating their crystallization from ¹⁸O-richer differentiated melts due to crustal contamination or assimilation. Therefore, 未¹⁸O of zircon as a tracer of mafic magmas should be used with caution, particularly for primary Si-undersaturated magmas.