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Three main styles of early Precambrian metamorphic evolution in northern Sino-Korea Platform and their geodynamics



Three main styles of early Precambrian metamorphic evolution in northern Sino-Korea Platform and their geodynamics



Gaoxiao Dizhi Xuebao = Geological Journal of China Universities 4(1): 1-10



The northern portion of Sino-Korea Craton consists of four independent blocks and each includes several metamorphic terrains or belts. Three styles of early Precambrian metamorphic evolution and their PTt-D paths have been distinguished in these terrains. The first style was found in Jning-Huaian Archaean high-grade terrain of Inner Mongolia. This district is characterized by widespread Al-rich para-gneiss (Khondalite suite) and some hypersthene plagioclase gneisses of granulite facies. They were subjected to five episodes of tectonic deformation and six stages of metamorphism from 2.6 Ga to 1.9 Ga. The mineral paragenesis and transformation of Al-rich gneisses in the whole process of metamorphism show a trend of Ky-Sil+Grt+Bt+Kfs-->Crd+ or -Opx-->And. The metamorphic PTt-D path of this terrain is clockwise with T = 800 degrees -850 degrees C, P = 0.90 GPa-1.00 GPa in its peak stage and a typical isothermal decompression (ITD) process in post-peak stage. It reflects double thickening of crust by geodynamic process such as A-subduction and tectonic stacking in a collisional belt, followed by tectonic thinning and uplifting in the later stage. The second style has been established in eastern Hebei-western Liaoning Archaean high-grade terrain which is mainly composed of late Archaean grey gneiss (TTG intrusions) and other granitic rocks with Subordinate volcano-sedimentary supracrustal sequences and some layered basic intrusions. The mineral evolution of basic rocks in granulite-facies belt is characterized by assemblage of Opx+Cpx+Hb+Grt+pl in its peak metamorphism with T = 800 degrees -850 degrees C, P = 0.70-0.80 GPa, and extensive development of Grt corona around Opx or Cpx during its post-peak stage. All data give rise to a counter clockwise PTt-D path with a nearly isobaric cooling process at its post-peak stage. It indicates the tectonic environment of vertical crustal thickening by magmatic accretion. The third style of PTt-d path was constructed in eastern Liaoning-southern Jilin early Proterozoic metamorphic belt. It was a rifted ensialic depression within the Jiao-Liao Massif during the middle stage of early Proterozoic with the formation of sedimentary sequences of shallow sea and continental shelf. by the end of early Proterozoic, they were subjected to three deformational episodes and five stages of metamorphism with the development progradal zones and a typical thermal anticline. Its axial area is characterized by high amphibolite-facies metamorphism. The mineralogic evolution in its Al-rich rocks is And-->Sil+Grt+Crd-->Ky. Its PTt-D loop is also counter clockwise, but it is distinguished from the second style by rapid raising pressure in its early stage and succeeded by a nearly isobaric heating process and lower pressure (0.50-0.60 GPa) in peak stage. Such PTt-D path may be genetically related to the closing process of a rifted depression within a rather stable continental massif.

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