Petrogenesis of Cretaceous granites in the southern Sulu Orogen: Implications for the flare‐up of post‐collisional magmatism

Abstract

To well understand the dynamic mechanism of Cretaceous magmatism flare‐up in the southern Sulu Orogen (SSO), the ages and petrogenesis of Qingyun monzogranites and the other coeval granitic plutons in the SSO are investigated herein through an integrated study of petrology, geochemistry, geochronology, and Sr–Nd–Pb–Hf isotopes. The studied granites are characterized by high Si, weak peraluminous, poor Fe and Mg, rich alkaline and high K. They are also characterized by enriched light rare earth element (LREE) and depleted heavy rare earth element (HREE), with the strong fractionation between LREE and HREE [(La/Yb)N = 19.71–45.78]. Besides, the rocks are depleted in HFSEs (i.e., Ta, Nb, Ti, and P) and relatively enriched in Pb and LILEs (i.e., Rb, Th, and K), with moderate negative Eu anomalies (δEu = 0.62–0.72). The lithogeochemical characteristics indicate that they are highly differentiated I‐type granite. Zircon U–Pb dating shows the age of monzogranite is 126.7 ± 1.0 Ma, which formed by magmatism in the Early Cretaceous. In addition, all the samples have low (206Pb/204Pb)i, (207Pb/204Pb)i, and (208Pb/204Pb)i (15.927–16.173, 15.356–15.383, and 37.084–37.412, respectively), similar to coeval felsic and mafic rocks from the Sulu Orogen and the North China Craton (NCC). Moreover, they display negative εHf(t) values (−21.53 to −34.43) with TDM2 model ages (2.55–3.35 Ga). Both the εHf(t) and TDM2 model ages indicate that they are derived from partial melting of the NCC Neoarchean continental crust. Therefore, it is believed that the rocks are primarily derived from partial melting of lower crust in granulite facies beneath the moderate pressure (0.8–1.3 GPa), corresponding to a depth of 35–45 km, within an extensional tectonic environment. The mineral fractional crystallization could occur in the diagenetic process. The delamination of lithosphere and the underplating of upwelling asthenospheric magma, which are reinforced by the subduction of the Palaeo‐Pacific Plate, could be the main factors to lead to partial melting of lower crust.