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Journal Article

Neoproterozoic Mafic-Ultramafic Intrusions from the Fanjingshan Region, South China: Implications for Subduction-Related Magmatism in the Jiangnan Fold Belt

Wei Wang, Jun-Hong Zhao, Mei-Fu Zhou, Sheng-Hong Yang and Fu-Kun Chen
The Journal of Geology
Vol. 122, No. 4 (July 2014), pp. 455-473
DOI: 10.1086/676596
Stable URL: http://www.jstor.org/stable/10.1086/676596
Page Count: 19

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Topics: Olivine, Gabbro, Magma, Melting, Chemical composition, Rocks, Isotopes, Mantle, Geochemistry, Tectonics
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Neoproterozoic Mafic-Ultramafic Intrusions from the Fanjingshan Region, South China: Implications for Subduction-Related Magmatism in the Jiangnan Fold Belt
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Abstract

AbstractThe Jiangnan Fold Belt was formed through the collision of the Yangtze and Cathaysia Blocks during the Neoproterozoic. The ca. 820 Ma mafic-ultramafic rocks from the Fanjingshan region in the western Jiangnan Fold Belt, South China, are composed mainly of olivine pyroxenite, clinopyroxenite, and gabbros with minor wehrlite. Olivine pyroxenites have low and constant K2O (<1 wt%) and Na2O (<0.17 wt%) and a narrow range of εNd(t) (−3.2 to −1.6) and 207Pb/204Pb (15.65–15.89), suggesting insignificant crustal contamination. They have high Os (0.182–1.70 ppb), low Re/Os (0.29–2.24) and γOs (−1.9 to +20.3), indicating their origination from a heterogeneous mantle source. By contrast, two gabbros have γOs values ranging from 179 to 243, which may have resulted from later addition of Re and minor crustal contamination. Olivine pyroxenites and calculated parental magmas show similar primitive mantle–normalized trace element patterns with variable depletion of high field strength elements (e.g., Nb, Ta Zr, Hf, and Ti) and enrichment of large-ion lithophiles (e.g., Th, U, Rb, and Pb). Their Sr-Nd-Pb isotopic compositions are also similar to those of enriched mantle II–type mantle. These features are consistent with magma derived from a mantle wedge that was previously metasomatized by slab-derived fluids and melts. The mafic-ultramafic rocks from Fanjingshan have bulk rock and mineral compositions similar to those of Alaskan-type intrusions, suggesting that they were formed in a subduction-related environment just before amalgamation of the Yangtze and Cathaysia Blocks.

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