Jurassic granitic magmatism in the lesser Xing’an-Zhangguangcai ranges of NE China: the Dong’an example

The ~100,000 km2 granitoid belt in the Lesser Xing'an-Zhangguangcai Ranges (LXZR) is an important component of the Phanerozoic granitic provinces (known as the 'granitoid ocean') in NE China. To reveal the mechanisms of Mesozoic magmatism and crustal growth in the LXZR, we present systematic geologic data, lithofacies information, zircon U-Pb ages, Hf isotopic, and element geochemical data for the Dong'an granites in the northern LXZR. The results show the following: (1) the Dong'an granites can be subdivided into three lithofacies, namely, medium- to coarse-grained alkali-feldspar granite, fine-grained alkali-feldspar granite, and granitic aplite, which were emplaced during the Early Jurassic (187–178 Ma); (2) the Dong'an granites are characterized by high SiO2 and total alkali contents and low Fe2O3t contents and Mg# values, and they are enriched in LILEs (e.g., Rb and K) and depleted in HFSEs (e.g., Nb and Ta); and (3) the zircons from these granites yield εHf(t) values of −2.4 to +5.7 and two-stage model ages of 772–1542 Ma. These results indicate that the Dong'an granites were emplaced during the Early Jurassic, are highly fractionated I-type (HFI) granites and were likely derived from partial melting of the thickened lower crust. Regionally, the Dong'an granites and the Early-Middle Jurassic intrusive rocks (201–163 Ma; 90,000 km2) in the LXZR show a trend of comagmatic evolution controlled by fractional crystallization, and they were successively emplaced during magma evolution. The HFI granites that are geochemically similar to the Dong'an granites formed in the later stage of magma evolution. Combined with regional geological observations, we conclude that the granitic rocks in the LXZR formed in an active continental margin setting related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent. This process may record an important episode of crustal accretion in the LXZR.