Polyphase structural deformation of low- to medium-grade metamorphic rocks of the Liaohe Group in the Jiao-Liao-Ji Orogenic Belt, North China Craton: Correlations with tectonic evolution

Abstract

The Paleoproterozoic Jiao-Liao-Ji Orogenic Belt in the eastern North China Craton provides evidence of the evolution from the early rifting and ocean opening to the final collision of the Longgang and Nangrim blocks. However, it is still open to debate, and thus controversial, as to whether the Jiao-Liao-Ji Orogenic Belt formed by closure of a rift without subduction or from an ocean with subduction. Based on detailed field mapping and structural analysis on a micro/meso-scale of the Liaohe Group in the center of the Orogenic Belt, we define at least four phases of deformation and metamorphism. S0 is bedding formed during deposition of the lower Liaohe sediments in the Liaohe back-arc basin. During D1 deformation an early greenschist facies event (M1) was associated with a bedding-parallel S1 metamorphic foliation, penetrative axial planar foliation, micro- to meso-scale folds (F1), and an L1 lineation, all formed during accretion, probably in a trench-subduction setting. An M2 metamorphism took place during medium-pressure Barrovian-type amphibolite facies conditions and associated D2 deformation that gave rise to major thrusts, open-isoclinal folds and a well-developed crenulation cleavage during exhumation of high-pressure and high-temperature rocks and collision of the Longgang and Nangrim blocks. M3 metamorphism at sillimanite (M31) and cordierite (M32) grade was associated with D3 deformation that produced regional gentle re-folds related to post-collisional exhumation of the Orogenic Belt. An M4 retrograde metamorphism and D4 deformation gave rise to a weak foliation that can be correlated with late Cretaceous extension that affected the whole eastern North China Craton. We present new isotopic ages (LA-ICPMS U-Pb zircon) of a granitic vein (2162Ma), a pegmatite (1840Ma), and a very late granite (114Ma) that are integrated with relevant published data. Accordingly, the formation age of deformation and metamorphism was set up, and at last, a modified tectonic model was presented.