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

Abstract

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 °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 subsequent slow cooling. Therefore, our new data further supports that the JLJB is a long-lived collisional orogenic belt with post-orogenic extension at 1.90–1.85 Ga and subsequent slow cooling during 1.85–1.80 Ga. Hence, this study provides new metamorphic constraint on the post-orogenic evolution of the JLJB and insights into the mechanism of generating contrasting P-T-t paths in regional metamorphic orogens.