Petrogenesis of the Late Mesozoic Qijinfeng Granite Complex in the Tongbai orogen: Geochronological, geochemical and Sr-Nd-Pb-Hf isotope evidence

Abstract

The Tongbai orogen is located in the northern Hubei and southern Henan provinces, together with the Qinling, Hong'an and Dabie orogens they constitute one of the most important orogenic belt separated the North China Block and Yangtze Block. Among these orogens, the Tongbai receives less attention and its magmatic and tectonic evolution is still unclear. In this paper, we present a detailed study on geochronology, geochemistry and Sr-Nd-Pb-Hf isotopes of the five plutons (Daxianduo, Jinziling, Sanguanduo, Sanhedian and Zhoulou) that composes the Qijianfeng Granite Complex in Suizao area, in order to better understanding the Late Mesozoic magmatic and tectonic evolution of the Tongbai orogen. The LA-ICP-MS UPb dating of zircon suggests an Early Cretaceous emplacement age (135–139 Ma) for the Qijianfeng Granite Complex. All the five plutons of the Qijianfeng Granite Complex are weakly peraluminous and belong to high-K calc-alkaline series and post-collisional I type granites. The chemical and isotopic compositions of the Qijianfeng Granite Complex show SiO2 contents of 69.84–74.36 wt%, Al2O3 of 13.85–15.62 wt%, MgO of 0.16–0.52 wt%, N2O/K2O of 0.99–1.73; 506–1205 ppm Sr, 3.4–12.9 ppm Y, 0.49–1.25 ppm Yb; Eu/Eu* values of 0.7–1.0, LREE/HREE values of 3.1–9.5; 87Sr/86Sr values of 0.7057–0.7080 and εNd(t) values of −16.59 to −6.18, which share similarities with the C-type (high-K) adakite. The low MgO, Ni and Cr contents further suggest that the Qijianfeng Granite Complex belongs to adakite derived from partial melting of thickened lower continental crust. Combined with the geochemical characteristics of other contemporaneous intrusions in the whole orogen, our study shows that the extensive Early Cretaceous adakitic magmatic activity is triggered by lower crustal thickened, mountain-basin coupling and crust-mantle interaction in the Tongbai orogeny.