Partial melting of subducted continental crust: Geochemical evidence from synexhumation granite in the Sulu orogen

The partial melting of crustal rocks is a fundamental mechanism for the emplacement of continental granites. However, difficulty can be encountered in building a direct petrogenetic link between a granite and its source region. Partial melting of deeply subducted continental crust is increasingly being recognized in collisional orogens, but it is unclear whether such a process can produce a considerable volume of granitic melts leading to pluton emplacement. A possible solution to these two problems is to find a direct source-product relationship in granite petrogenesis. Here, we present a combined study of zircon U-Pb ages, Lu-Hf and O isotopes, and whole-rock major and trace elements and Sr-Nd isotopes for a newly found granitic intrusion in the Sulu orogen, China. The results provide geochemical evidence for the partial melting of deeply subducted continental crust during collisional orogenesis. Secondary ion mass spectrometry (SIMS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb dating yielded Late Triassic ages of 216 +/- 2 Ma to 218 +/- 3 Ma, postdating Middle Triassic ages for the ultrahigh-pressure (UHP) metamorphism of country rocks, corresponding to synexhumation magmatism during continental collision. Relict zircon cores have concordant Neoproterozoic and Middle Triassic U-Pb ages, respectively, consistent with protolith and metamorphic ages for the UHP meta-igneous rocks in the Dabie-Sulu orogenic belt. The granite zircons exhibit low delta O-18 values of 1.10%-4.32%, comparable to those of the UHP meta-igneous rocks. The granite is high-K calc-alkaline to shoshonitic and shows trace-element distribution patterns with enrichment in large ion lithophile elements and light rare earth elements but depletion in high field strength elements, consistent with arc-type igneous rocks. Granites have high (Sr-87/Sr-86)i ratios of 0.7478-0.9709 and negative epsilon(Nd) (t) values of -12.9 to -7.8, with two-stage Nd model ages of 1.93-2.28 Ga for whole rock. Zircon Lu-Hf isotope analyses also yield negative epsilon(Hf) (t) values of -17.8 to -5.9 with two-stage Hf model ages of 1.62-2.37 Ga. These whole-rock and zircon geochemical characteristics are generally comparable with those of the UHP meta-igneous rocks, indicating a genetic link between them. In particular, the low delta O-18 values and the zircon relicts of Neoproterozoic and Triassic U-Pb ages are characteristic features of the subducted continental crust of the South China block. Therefore, the granite was derived from decompressional melting of the subducted upper continental crust during exhumation. The granitic melts produced in this stage may serve as metasomatic agents for crust-mantle interaction in the continental subduction channel. This provides a direct candidate for the crustal component in the mantle source of postcollisional mafic igneous rocks.