Mineralogical and geochemical constraints on contribution of magma mixing and fractional crystallization to high-Mg adakite-like diorites in eastern Dabie orogen, East China

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• 作者：Chao Zhang^a ; ^b ; ^{c.zhang@mineralogie.uni-hannover.de} ; Changqian Ma^a ; ^{cqma@cug.edu.cn} ; Francois Holtz^b ; Jü ; rgen Koepke^b ; Paul Eric Wolff^b ; Jasper Berndt^c
• 关键词：Magma mixing ; Fractional crystallization ; High-Mg adakite ; Dabie orogen ; Tan-Lu fault ; MLYR belt
• 刊名：Lithos
• 出版年：2013
• 出版时间：July, 2013
• 年：2013
• 卷：172-173
• 期：Complete
• 页码：118-138
• 全文大小：2067 K

文摘

The Liujiawa pluton which is located near the eastern boundary of the Dabie orogen is composed of multiple lithologic units including mainly gabbronorites, diorites, granodiorites and hornblende gabbros. Gabbronorites and hornblende gabbros occur as enclaves in dioritic hosts which show gradual contact with granodiorites. Zircon U-Pb dating indicates that gabbronorites and diorites formed coevally at ~ 128 Ma, but they have distinct zircon Hf isotopes with ¦ÅHf(t) of ? 26 to ? 23 (gabbronorite) and of ? 32 to ? 27 (diorite) respectively. Petrographic observations and rock-forming mineral compositions clearly show mixing between mafic and felsic magma end-members, which might have formed the homogeneous whole-rock Sr-Nd isotopes with ¦ÅNd(t) of ? 17 to ? 25 and initial ⁸⁷Sr/⁸⁶Sr of 0.707 to 0.709. As revealed by zircon Hf isotopes, F concentrations in amphibole and biotite and thermodynamic modeling of crystallization, the gabbronorites represent enriched lithospheric mantle-derived magmas which evolved by fractional crystallization of orthopyroxene, clinopyroxene, magnetite and/or amphibole, whereas the granodiorites may be derived from the Dabie Archean basement. Mineralogical and geochemical data as well as major and trace element modeling show that the origin of diorites, previously interpreted as high-Mg adakites, can be explained by magma mixing between the crust-derived granodioritic magmas and the differentiation products of mantle-derived gabbronoritic magmas. As a result, the high-Mg adakite-like geochemistry of the diorites is a consequence of magma differentiation at a crustal depth, involving fractional crystallization and magma mixing, rather than an intrinsic feature of primitive melts. The mantle upwelling in the adjacent central Middle-Lower Yangtze River metallogenic (MLYR) belt during Late Jurassic-Early Cretaceous belt might have acted as a precursor and triggered the partial melting of lithospheric mantle beneath the eastern Dabie orogen and the further melting of orogenic basement, consistent with the model of Zhang et al. (2010) suggesting a magmatic link between the MLYR belt and the southeastern Dabie orogen.