Metamorphism and P–T evolution of high pressure granulite in Chicheng, northern part of the Paleoproterozoic Trans-North China Orogen

Abstract

It is widely accepted that the North China Craton (NCC) was finally formed by collision of the Eastern and the Western Blocks along the Trans-North China Orogen (TNCO) at approximately 1.85Ga. The continent–continent collision along the TNCO was partly defined by metamorphism of high pressure mafic granulites with clockwise P–T paths. However, the continental subduction process has not been well recorded by previously reported HP granulites due to lack of prograde metamorphism and their plutonic protoliths. In this paper, we report HP mafic granulites among a metamorphic volcanic-sedimentary sequence south to Chicheng, northern part of the TNCO, in which the prograde metamorphism was well preserved. The prograde metamorphic stage (M1) has been revealed by a distinct growth zone of garnet (Grt) porphyroblasts with an inclusion-rich core and inclusion-free mantle. The peak metamorphic mineral assemblage (M2) consists of Grt and clinopyroxene (Cpx) predominantly with minor plagioclase (Pl), Rutile (Ru), hornblende (Hb) and quartz (Q). The Pl+Hb symplectite or corona surrounding garnet and Cpx+Ilm (ilmenite) symplectite surrounding sphene provide evidence for the post-peak decompression (M3) stage. Ubiquitous Hb especially along cracks represents further retrogression (M4) within amphibolite facies. Phase equilibria modeling by the THERMOCALC program combined with thermobarometric calculations yielded a consecutive hairpin shaped clockwise P–T path, which indicate that the HP granulites experienced remarkable prograde metamorphism, during which the temperatures increased from 630°C to 740°C while the pressures increased from 8kbar to the barometric peak of 16kbar. After the barometric peak, the HP granulites subjected to decompression from 16kbar to 13kbar while the temperature continued to increased from 750°C to 790°C, suggesting a thermal relaxation. After the thermal peak, the HP granulites began their consecutive decompression with cooling from 790°C and 13kbar to 700°C and 7kbar through 740°C and 9kbar, avoiding access to the orthopyroxene (Opx) stability field as experienced by many other HP granulites in the TNCO. The hairpin-shaped P–T path of the Chicheng HP granulites suggests a continuous continental subduction and tectonic uplift, providing a better metamorphic constraint on the final assembly of the NCC along the TNCO.