Petrogenesis of the Late Mesozoic Guposhan composite plutons from the Nanling range, South China: Implications for W-SN mineralization

Abstract

Voluminous granite plutons were emplaced in the Late Mesozoic in the Nanling Range, South China, accompanied by large-scale polymetallic deposits. Most plutons are composite, composed of major-phase granites and highly differentiated late-stage granites. Previous studies have proposed fractionation of both granite types and polymetallic deposits from a common magma. We choose to study the ages, chemistry and Nd-Hf isotopes of the Guposhan complex from the Range, and propose a different model for the origin of the granite and relationship with associated polymetallic mineralization in the Nanling Range. The major-phase granites of the Guposhan Complex contain medium-coarse grained biotite granite and hornblendebearing biotite granite with mafic microenclaves (MMEs), emplaced at 161 ± 1 Ma.On the contrary, the late-stage granite is fine-grained, with apparent zircon ages of 154 ± 3 Ma. The major-phase granites show large variation in chemical compositions and in whole-rock Nd and zircon Hf isotope ratios, with εNd(t)=-6.0 to -1.9 and εHf(t) = -4.5 to 1.5, respectively. We suggest that their origin is attributable to a magma mixing process between felsic magma derived from melting of Proterozoic basement rocks and mantle-derived mafic magma. The mafic magma could be represented by the parental magma of the MMEs within the plutons, which have low SiO2, high MgO, and high εNd(t) values (up to 1.8). The late-stage granite shows εNd(t) and εHf(t) values similar to the major-phase granites, but are chemically more evolved (SiO2=75-80%), more reduced and water-deficient, and, in particular, are fluorine-bearing. Numerical geochemical modeling and the petrological data indicate that the late-stage granite was produced during a new thermal event, rather than being produced due to fractional crystallization of the earlier major-phase granite. The polymetallic deposits are genetically linked with the late-stage granite. We suggest that a break-off of the flat-subducted Pacific slab beneath the Cathaysia block in the late Mesozoic resulted in the upwelling of asthenosphere and formation of basaltic magma that underplated and triggered partial melting of the basement rocks, forming felsic melts, and followed by mixing of the mafic and felsic magmas in an extensional regime.