Petrogenesis of Early–Middle Jurassic intrusive rocks in northern Liaoning and central Jilin provinces, northeast China: Implications for the extent of spatial–temporal overprinting of the Mongol–Okhotsk and Paleo-Pacific tectonic regimes

Abstract

The Mesozoic tectonic evolution of NE China was controlled mainly by the Mongol–Okhotsk and Paleo-Pacific tectonic regimes. However, the extent of the spatial and temporal overprinting of these two regimes is poorly understood. Here, we report new zircon LA-ICP-MS U–Pb dating and geochemical analyses of Jurassic intrusive rocks in northern Liaoning and central Jilin provinces, northeast China, to discuss their petrogenesis and outline the extent of spatial and temporal overprinting of these two tectonic regimes. Dating results indicate that Jurassic magmatism occurred in two stages during the Early (ca. 175Ma) and Middle Jurassic (170–163Ma). These rocks represent two-stage typical bimodal igneous rock associations composed mainly of olivine gabbro, gabbro, and gneissic granitoids. The Early and Middle Jurassic gabbros have low rare earth element (REE) abundances, positive Eu anomalies, depletion in high field strength elements (HFSEs), and positive εHf(t) values (+4.0 to +10.3, except for one value of −17.8), suggesting that the primary magma was derived from partial melting of depleted lithospheric mantle metasomatized by subducted-slab-derived fluids. The Early Jurassic monzogranite exhibit high REE abundances (195–201ppm), weak negative Eu anomalies (δEu=0.63–0.64), and negative εHf(t) values (−11.9 to −8.2), suggesting a primary magma that was derived from partial melting of lower continental crust of the NCC. The Middle Jurassic granodiorites are enriched in light REEs (LREEs) and large ion lithophile elements (LILEs), and are depleted in heavy REEs (HREEs) and HFSEs, as well as high Sr/Y (29–132) and (La/Yb)N (15–44) ratios. In addition, the Middle Jurassic granitoids near or within the NCC exhibit negative εHf(t) values (−18.9 to +0.2), whereas those within the Xing'an–Mongolia Orogenic Belt (XMOB) have generally positive εHf(t) values (−0.6 to +6.4), suggesting their origin from partial melting of thickened ancient NCC and newly accreted continental crust of the XMOB, respectively.The above findings, combined with regional geologic data, suggest that the Early Jurassic bimodal igneous rocks formed in an extensional tectonic environment related to subduction of the Paleo-Pacific plate beneath the Eurasian continent. The Middle Jurassic bimodal igneous rocks possibly formed in an extensional environment related to the collapse of a thickened region of the lower crust after closure of the Mongol–Okhotsk Ocean.