Geochemistry, geochronology and Sr–Nd–Hf isotopes of two Mesozoic granitoids in the Xiaoqinling gold district: Implication for large-scale lithospheric thinning in the North China Craton

Abstract

The Mesozoic granitoids at the southern margin of the North China Craton are of considerable geologic interest, because of their geodynamic context and likely relationship to Mesozoic lithospheric delamination, but also because they are associated with one of the most important gold belts in China. However, their detailed age relations and petrogeneses have not been described by systematic and comprehensive geochemical studies. Here we report a systematical zircon U–Pb dating, petrogeochemistry, and Sr–Nd–Hf isotopic data on two of these granitoids (namely the Wenyu and the Niangniangshan plutons) in the Xiaoqinling gold district, Henan Province, North China. Both plutons are metaluminous to weakly peraluminous, and can be classified as I-type granites with similar zircon U–Pb ages of 131±1Ma and 134±1Ma. They are enriched in Si, K, Ba, and Sr, while depleted in Nb, Ta, P, Ti, Mg, Ni, V and Y, and both have Archaean-type tonalite–trondhjemite–granodiorite (TTG) and adakitic affinity, e.g. enrichment in LREE with high Sr/Y and La/Yb ratios and without Eu anomalies. Both the plutons show relatively high initial 87Sr/86Sr ratios (0.7076–0.7089 for the Wenyu pluton and 0.7074–0.7086 for the Niangniangshan pluton), negative εNd(t) values (−15.0 to −10.1 for the Wenyu pluton and −18.5 to −12.2 for the Niangniangshan pluton), and negative εHf(t) values (−21.9 to −10.4 for zircons from the Wenyu pluton and −30.8 to −20.8 for zircons from the Niangniangshan pluton). All the isotopic data indicate old crustal sources for the magma. Furthermore, low concentrations of MgO, Ni, and V also indicate little or no mantle materials in the source. The local Neoarchaean to Palaeoproterozoic basement rocks are the most likely sources, but their isotopic compositions do not cover the total isotopic range of the plutons, hence there maybe other sources involved. The occurrence of 1.78Ga inherited zircon grains on the depleted mantle Hf isotopic evolution line indicates 1.78Ga underplating mafic rocks to be a possible source. Detailed petrologic and geochemical data indicate that the two plutons derived from partial melting of Neoarchaean to Palaeoproterozoic metamorphic rocks and possibly with minor amounts of 1.78Ga mafic lower-crustal rocks at a depth of at least 50km, with eclogite, garnet amphibolite or amphibolite residues. During the magma crystallization stage, amphibole, apatite and titanite fractionation caused the observed variations of the element compositions. Meanwhile, the existence of >50-km-depth crust indicates that large-scale lithospheric thinning in this part of the south margin of North China Craton did not take place prior to the emplacement of these granitoids 130Myr ago. However, a fuller understanding of the geological significance of the widespread occurrence of Mesozoic granitic magmatism in the south margin of North China Craton will require similarly detailed studies of other granitoids in the region. In this way it should be possible to constrain their source compositions and the nature of the heat source(s) that led to this lithospheric thinning event.