Tectonic implications of Early Cretaceous low-Mg adakitic rocks generated by partial melting of thickened lower continental crust at the southern margin of the central North China Craton

Abstract

This paper reports new whole-rock geochemical, Sr–Nd–Pb isotopic, and zircon U–Pb and Hf isotopic data for Early Cretaceous intrusive rocks in the Sanmenxia–Houma area of central China, and uses these data to constrain the petrogenesis of low-Mg adakitic rocks (LMAR) and the spatial extent of the influence of the deeply subducted Yangtze slab during the Triassic evolution of this region. New zircon laser-ablation inductivity coupled plasma mass spectrometry (LA-ICP-MS) U–Pb data indicate that the early- and late-stage southern Quli, Qiligou, and Gaomiao porphyritic quartz diorites, the Canfang granodiorite, and the northern Wangmao porphyritic quartz monzodiorite were emplaced during the Early Cretaceous (~130Ma) and the late Early Cretaceous (~116Ma). These rocks are characterized by high Na2O/K2O, Sr/Y, and (La/Yb)n ratios as well as high Sr concentrations, low Mg# [molar 100×Mg/(Mg+Fe2+tot)] values, and low heavy rare earth element and Y concentrations, all of which indicate an LMAR affinity. The samples have relatively high initial 87Sr/86Sr ratios (0.7054–0.7095), and low εNd(t) (−11.90 to −22.20) and εHf(t) (−16.7 to −32.7) values, indicative of a lower continental crust origin. The presence of Neoproterozoic (754–542Ma) and inherited Late Triassic (220Ma) metamorphic zircons within the late Early Cretaceous LMAR and the relatively high 206Pb/204Pb ratios of these rocks suggest that they formed from primary magmas derived from partial melting of Yangtze Craton (YC) basement material that had undergone ultrahigh-pressure metamorphism. In contrast, the presence of Paleoproterozoic and Archean inherited zircons within early Early Cretaceous LMAR in this area and the relatively low 206Pb/204Pb ratios of these rocks are indicative of derivation from primary magmas generated by partial melting of the thickened lower continental crust of the North China Craton (NCC). These rocks may have formed in an extensional environment associated with the upwelling of asthenospheric mantle material. The presence of YC basement material within the NCC in the Sanmenxia–Houma area suggests that the deeply subducted Yangtze slab influenced an area of ~100km in lateral extent within the southern margin of the central NCC during the Triassic.