Origin of Early Cretaceous Guandian adakitic pluton in central eastern China: partial melting of delaminated lower continental crust triggered by ridge subduction

Early Cretaceous adakite or adakitic plutons are widely distributed in central eastern China, e.g. lower Yangtze river belt (LYRB), the south Tan-Lu fault (STLF), and the Dabie orogen. Their genesis, however, remains controversial. In this contribution, we present detailed geochemical and geochronological study on the Guandian pluton in central Anhui Province, eastern China, which has been formerly regarded as a part of the north belt in the LYRB and lately classified in the STLF. Namely, it is located near the boundary between ridge subduction related slab melting and partial melting of lower continental crust (LCC). The Guandian pluton consists of quartz monzonite and is metaluminous and high-K calc-alkaline according to the chemical composition. The samples show high SiO2 (59.15-62.32%), Al2O3 (14.51-15.39%), Sr (892-1184ppm), Sr/Y (56.74-86.32), and low Y (12.65-18.05ppm), similar to typical geochemical features of adakite. The Guandian adakitic rocks also exhibit high K2O (2.88-3.86%), MgO (3.89-5.24%), and Mg# (55-60), negative anomalies of high field strength elements (e.g. Nb, Ta, and Ti), and positive anomalies of Ba, Pb, and Sr. LA-ICP-MS zircon U-Pb dating yielded a weighted average age of 129.2 +/- 0.7Ma. Calculations of zircon Ce4+/Ce3+ (6.97-145) and (Eu/Eu*)(N) (0.23-0.42) on the basis of in situ zircon trace element analysis indicate that the magma had a lower oxygen fugacity relative to the ore-bearing adakites in the LYRB and Dexing, which is consistent with the fact of ore-barren in the research area. In combination with previous research, we propose that Guandian adakitic pluton was formed by partial melting of delaminated LCC triggered by Early Cretaceous ridge subduction of the Pacific and Izanagi plates. During ridge subduction, physical erosion destructed the thickened LCC and resulted in delamination, while thermal erosion facilitated partial melting of the delaminated LCC.