Long-lived (>100 m.y.) postcollisional exhumation and cooling in the Paleoproterozoic Trans-North China Orogen; evidence from phase equilibria modeling and monazite petrochronology of granulite-facies metapelites in the Fuping Complex

Long-lived collisional orogens that formed over tens to hundreds of millions of years are common in the geologic record. The Trans-North China orogen marks the collision between the Eastern and Western blocks of the North China craton, and it preserves metamorphic rocks with ages between 1.98 Ga and 1.80 Ga. These units allow detailed assessment of the time scale and duration of crustal thickening, exhumation, and cooling associated with a major Proterozoic orogeny. In this study, we present integrated petrography, mineral chemistry, phase equilibria modeling, and texturally controlled in situ mass spectrometry of monazite U-Th-Pb and trace-element analyses performed on a suite of orthopyroxene-bearing pelitic granulites and garnet-biotite gneisses from the Fuping Complex within the Trans-North China orogen. These rocks record clockwise pressure-temperature (P-T) paths involving granulite-facies peak conditions of 9.9-11.0 kbar and 850-880 degrees C for pelitic granulites, and 10.9-11.6 kbar and 860-880 degrees C for garnet-biotite gneisses, followed by postpeak decompression to approximately 8-9 kbar and later cooling, with final solidification of melt at <840 degrees C. Four monazite populations were identified in these samples. Group I grains are irregular and elongated and occur in contact with or embay garnet. They have high rare earth element (REE) and Y contents and metamorphic ages of 1.90-1.86 Ga, which correspond to the breakdown of garnet during postpeak decompression. Most monazite grains crystallized from melt are represented by groups II+III+IV and are associated with orthopyroxene, biotite, plagioclase, and quartz in the matrix. They have crystallization ages between 1.86 Ga and 1.76 Ga and relatively low REE and Y concentrations. These data imply a long-lived (>100 m.y.) postcollisional exhumation and cooling involving decompression from 10-12 kbar to approximately 9 kbar during 1.90-1.86 Ga, followed by retrograde cooling from 1.86 to 1.76 Ga under prolonged residence in the middle to lower crust. Initial collision and peak metamorphism occurred before 1.90 Ga, ultimately leading to the final cratonization of the North China craton and its incorporation into the Columbia supercontinent.