A source-depleted Early Jurassic granitic pluton from South China: Implication to the Mesozoic juvenile accretion of the South China crust

Abstract

Source-depleted granites were rarely reported in South China. Hereby we identified such a granitic pluton, the Tiandong pluton, at Northeastern Guangdong province in Southeastern (SE) China. Whole-rock Sr-Nd and zircon Hf isotopes of the Tiandong granites both revealed obviously depleted source signatures, with initial isotopic values of initial 87Sr/86Sr=0.7032–0.7040, εNd(t)=1.1–1.5, and εHf(t)=6–13, respectively. Zircon U-Pb dating implied the granite was intruded in Early Jurassic (188Ma). The dominant minerals of the Tiandong granite consist of K-feldspar, plagioclase, quartz and biotite, with accessory mineral assemblage of apatite+zircon+magnetite. Based on the mineralogy and the depleted isotopic signature, the granites chemically show I-type affinity such as low Zr+Nb+Ce+Y (131.6 to 212.2), 104×Ga/Al (2.12–2.27), A/CNK values <1.1 (0.97–1.03), corundum molecule <1 (0–0.55) and extremely low P2O5 contents (0.05wt%). The one-stage and two-stage depleted mantle Nd model ages (TDM=0.89 to 0.84Ga, T2DM=0.88 to 0.85Ga) are consistent. TDM(Hf) values of 0.31–0.63Ga are also indistinguishable from T2DM(Hf) values of 0.35–0.75Ga. The Nd and Hf isotopic compositions confirm that the Tiandong granites are juvenile crustal accretion but decoupled Nd-Hf isotopic systems. The juvenile crust is likely to originate from a mixed source of the primary asthenospheric mantle and the subordinate EMII. Combined with early studies of adjacent rocks, we propose that the early Jurassic (~200–175Ma) magmatism as evidenced by the Tiandong granites might be driven by upwelling of asthenosphere and subsequent underplating of mafic melts in an intra-plate extensional setting as a response to far-field stress during early stage subduction of the paleo-pacific plate.