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

摘要

The 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 K_20 (<1 wt%) and Na_2_O (<0.17 wt%) and a narrow range of _ε_(>Nd(t)) (-3.2 to -1.6) and _(207)Pb/_(204)Pb (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.