Petrogenesis of Permo-Triassic intrusive rocks in Northern Liaoning Province, NE China: implications for the closure of the eastern Paleo-Asian Ocean

摘要

The Changchun-Yanji belt recorded widespread Permo-Triassic magmatism, but their origins remain unclear, inhibiting a comprehensive understanding of the magmatic response to the final closure of the Paleo-Asian ocean in the eastern Central Asian Orogenic Belt (CAOB). Here, we present new geochronological, geochemical, and Hf isotopic data for the Permo-Triassic plutons from Northern Liaoning province, NE China. Combined the published ages with our new data, the Permo-Triassic magmatism in the eastern CAOB can be divided into five episodes: early Permian (293-274 Ma), middle-late Permian (270-257 Ma), latest late Permian-Middle Triassic (255-242 Ma), Late Triassic (240-215 Ma), and latest Late Triassic (209-200 Ma). The middle Permian and Late Triassic mafic plutons (i.e. ~266 Ma Mengjiagou gabbro-diorite, ~240 Ma Jiancaicun gabbro and ~224 Ma Shudetun gabbro-diorite) contain relatively high TFe2O3, MgO, Cr and Ni contents with positive epsilon(Hf)(t) values (+1.2 to +7.2), suggesting a depleted mantle origin. These mafic rocks together with the coeval granitoids make up typical bimodal associations, suggesting that they were formed under an extensional environment. The conclusions are also supported by occurrence of A-type granites during 270-257 Ma and 240-215 Ma. By contrast, the granitoids of 255-242 Ma in the eastern CAOB, including the Jianshanzi (~251 Ma) and Daganhe (~242 Ma) monzogranites, show typical geochemical features of adakitic granites, with high Sr/Y ratios and negative epsilon(Hf)(t) values (-8.6 to - 22.0), suggesting that the magmas were generated through partial melting of thickened ancient lower crust. Combined with previous studies, a four-stage tectonic evolution scenario was proposed: (1) active continental margin stage during 293-274 Ma; (2) continuing subduction resulted in the initiation collision, moderate crustal thickening, and slab break-off during 270-257 Ma; (3) final closure of the Paleo-Asian Ocean associated with continued crustal thickening occurred during 255-242 Ma; (4) lithospheric delamination in a post-collisional extensional environment occurred during 240-215 Ma.