Timing and evolution of Jurassic-Cretaceous granitoid magmatisms in the Mongol-Okhotsk belt and adjacent areas, NE Asia: Implications for transition from contractional crustal thickening to extensional thinning and geodynamic settings

摘要

The Mongol-Okhotsk belt and adjacent areas are key areas to study the relationship between the Okhotsk and Paleo-Pacific tectonic regimes and their superposition on the older Paleo-Asian regimes during late Mesozoic times. This paper summarizes the spatial-temporal evolution of Late Mesozoic (Jurassic-Cretaceous) granitoids and related intrusions in these areas, and interprets the magmatic evolution in terms of a transition from contractional crustal thickening to extensional thinning. According to 407 published zircon ages, these granitoids were mainly emplaced during the intervals 200-180 Ma, 180-165 Ma, 165-145 Ma, 145-135 Ma and 135-100 Ma. Jurassic granitoids (200-145 Ma) predominately occur in the Baikal-NE Mongolia (BNEM) and Great Xing'an Range. Early Cretaceous (145-100 Ma) granitoids are mainly occur in the Great Xing'an Range, and display a southward-younging migration. Significantly, Early Cretaceous granitoids also extend into the Trans-Baikal area across the Mongol-Okhotsk suture, far away from the Paleo-Pacific plate margin (in NE China); thus they were more plausibly related to post-orogenic collapse of the Mongol-Okhotsk orogen. From the Late Jurassic to Early Cretaceous, the granitoids evolved compositionally from calc-alkaline and high-K calc-alkaline, I-type, with some adakite-like features, to high-K calc-alkaline and shoshonitic, highly fractionated I-, transitional I-A or, A-type, characterized by a significant decrease in their Sr/Y ratios. This evolution coincided with a tectonic transition from contractional crustal thickening to extensional thinning. Combined with regional geology, we speculate that the Jurassic granitoids were likely derived from melting of the deep-seated, thickened lower continental crustal (LCC) sources, whereas the Cretaceous granitoids produced through crustal melting from an extensional thinning setting. Our results provide a case study demonstrating that the petrogen-esis of granitic magmatism was closely associated with crustal tectonics. Early Jurassic granitoids along the Okhotsk belt formed in a subduction/collision setting related to closure of the Mongol-Okhotsk Ocean, whereas Late Jurassic granitoids in the Great Xing'an Range and in the northern North China Craton may have formed in a syn- or post-collisional setting superposed by far-field affects of subduction of the Paleo-Pacific plate. Early Cretaceous granitoids in these areas formed in response to post-orogenic extensional collapse of the Mongol-Okhotsk belt, coupled with back-arc extension related to Paleo-Pacific plate subduction.