Petrogenesis of late Mesozoic granitoids from the southern segment of the Tan–Lu Fault, eastern China: implications for the tectonic affinity of the Zhangbaling Uplift

Abstract

ABSTRACT Late Mesozoic magmatic rocks of the Zhangbaling Uplift (ZBU) in eastern China, which comprises the northern Zhangbaling area and the southern Feidong area, can provide insights into the deep crustal processes and tectonic affinity of the massif. These rocks are dominated by granitoids with formation ages of 132–120 Ma and a peak at 129 Ma. The Shankouling dyke (120.4 ± 1.1 Ma) and the Zhuyuanzhang quartz monzodiorite (129.0 ± 2.2 Ma) crop out sporadically in the Feidong area. These rocks show adakitic signatures of high Al2O3 and Sr contents, high Sr/Y, (La/Yb)N, and (Dy/Yb)N ratios, and low Y and YbN contents. They have enriched (87Sr/86Sr)i ratios, negative εNd(t) and εHf(t) values, and unradiogenic Pb isotopic compositions that resemble those of coeval high Sr/Y granitoids in the Dabie Orogen. These ZBU rocks can be divided into two groups: low-Mg granitoids with more enriched Sr–Nd isotopic signatures and low Mg# values (<50), and high-Mg granitoids with weakly enriched Sr–Nd isotopic signatures and high Mg# values (mostly >50). Combining their geochemical features with previous results of experimental melts, we conclude that the low-Mg granitoids were derived from fluid-absent partial melting of in situ thickened lower continental crust, resulted from Triassic continental subduction and collision in this area, whereas the high-Mg granitoids were derived from delaminated eclogitic lower crust followed by interaction with mantle peridotites during magma ascent. Isotopic compositions show that these granitoids could have been derived from ancient crust of the Yangtze Block via re-melting of Neoarchean and Paleoproterozoic lower continental crust. The great depth of the Tan–Lu Fault Zone enabled delamination of the thickened eclogitic mountain root beneath the ZBU, which resulted in the formation and emplacement of the high-Mg rocks, followed by uplift of the Zhangbaling massif associated with rollback of the subducting Paleo-Pacific (Izanagi) Plate.