Disequilibrium melting of recycled phosphate-rich crust in Yunkai Massif, South China

Abstract

We conduct a comprehensive study including bulk-rock and mineral geochemical analyses, Rhyolite-MELTS and phase equilibrium modeling on Caledonian (422–436 Ma) granitic plutons from the Yangchun region of Yunkai Massif, South China, aiming to estimate the melting P-T-H2O conditions and decipher the mechanism for crustal anatexis. These rocks are typical S-type granites with presence of muscovite and garnet, resembling those plutons in the European Variscan Belt. The Rhyolite-MELTS modeling yields a crystallization sequence of quartz, plagioclase, K-feldspar, biotite and muscovite. Apatite shows homogeneous texture and occurs as columnar euhedral to subhedral crystal hosted in plagioclase and mica. Further Nd isotope analyses on apatite yield highly nonradiogenic Nd compositions with an εNd(t) from −14.3 to −8.2 and a TDM2Nd range of 1.83–2.33 Ga, which are obviously less radiogenic than the bulk-rock εNd(t) (−8.4 ∼ −7.1) and TDM2Nd (1.75–1.85 Ga). The zircon crystals span a wide εHf(t) range from −13.0 to +0.8 and TDM2Hf range of 1.36–2.22 Ga. The remarkable Nd isotopic difference between the bulk granite and apatite, systematic bulk-rock εNd(t) variations against SiO2, P2O5 and Sm/Nd, and the large εHf(t) variation of zircon in these S-type granites are likely attributed to disequilibrium melting of Proterozoic heterogeneous crust, during which residual garnet and apatite had been variably entrained in the anatectic melt. Further phase equilibrium modeling indicates that the melting conditions of Yangchun S-type granites include metasedimentary protoliths, ∼5.0 wt% water, P = 6–7 kbar, T = 700–800 °C, with a geothermal gradient of 35–40 °C/km. The estimated residual apatite and garnet are 0.07–0.10 wt% and 3.8–9.0 wt% in the protoliths, respectively. The combined results therefore suggest a relatively hot crust across the Yunkai Massif, probably due to mantle upwelling and crustal extension during early to middle Silurian.