Contrasting P-T-t paths from a Paleoproterozoic metamorphic orogen: Petrology, phase equilibria, zircon and monazite geochronology of metapelites from the Jiao-Liao-Ji belt, North China Craton

摘要

The Paleoproterozoic metapelites within the major collisional orogenic belts of the North China Craton are generally characterized by clockwise metamorphic P-T-t paths. Here we report the case where an isobaric cooling followed by a final decompression is defined from high grade metapelites in the Jingshan Group at the southern part of the Jiao-Liao-Ji belt. The earlier prograde trajectory is constrained to be considerable heating with slight pressure increase to the peak metamorphism of 7.9-9.2 kbar at 763-770 degrees C by integrated methods of petrology, geothermobarometry and P-T-X pseudosection, suggesting an anticlockwise P-T path. Zircon and monazite geochronology combined with previous data suggests that the prograde metamorphism probably commenced at 1.90-1.85 Ga, followed by a slow isobaric cooling at mid-crustal level during 1.85-1.80 Ga and a final exhumation to upper crustal level presumable at 1.60-1.70 Ga. The defined P-T-t path seems to contrast with previously reported clockwise P-T-t paths. In the light of available geological evidence and recent numerical modelling, these contrasting P-T-t paths are reconciled by a tentative geodynamic model in which orogenic collapse and hot exhumation of the orogenic root during 1.90-1.85 Ga resulted in isothermal decompression of the granulites with clockwise P-T paths and triggered the prograde metamorphism for those regions displaying anticlockwise P-T paths. The pressure increase in the prograde trajectory of anticlockwise P-T path may be related to the sinking of upper-middle crustal materials to compensate for the contemporaneous ascend of the hot orogenic root. Subsequently, rocks of these contrasting P-T paths experienced a common slow isobaric cooling during 1.85-1.80 Ga. The 1.95-1.90 Ga metamorphic ages that are common in the Paleoproterozoic khondalitic rocks elsewhere in the NCC are absent in our samples can be ascribed to the high-temperature metamorphism related to this orogenic collapse and subseq