Polymetamorphism of mafic granulites in the North China Craton: Textural and thermobarometric evidence and tectonic implications

Abstract

The basement of the North China Craton can be divided into the Archaean Eastern and Western Blocks, separated by major Palaeoproterozoic terrane boundaries that roughly correspond with the limits of a 100-300 km wide zone, named the Trans-North China Orogen. Some mafic granulites from the orogen and adjoining areas in the Eastern and Western Blocks preserve textural evidence for two granulite facies events involving contrasting P-T paths. The first event is characterized by three distinct mineral assemblages, M1a to M1c. M1a is represented by fine-grained orthopyroxene + clinopyroxene + plagioclase ± quartz, which is surrounded by the M1'b garnet + quartz symplectite, which itself is mantled by the M1c plagioclase + biotite symplectite. These assemblages and their P-T estimates define an anticlockwise P-T path, with peak metamorphism of 7.0-8.0 kbar and 800-850°C (M1a) followed by isobaric cooling to 700-750°C (M1b) and pressure-decreasing cooling to 630-700°C (M1c). The second event also includes three mineral assemblages, M2a to M2c. M2a represents growths of garnet porphyroblasts and matrix orthopyroxene + plagioclase + clinopyroxene + quartz; M2b consists of orthopyroxene + plagioclase ± clinopyroxene symplectites or coronas; and M2c is represented by plagioclase + hornblende symplectites. These assemblages and their P-T estimates define a clockwise P-T path, with peak metamorphism of 9.2-9.8 kbar and 820-850°C (M2a), followed by near-isothermal decompression (M2b) of 7.0-7.6 kbar and 760-810°C and cooling (M2c) to 690-760°C. The isobaric cooling, anticlockwise, P-T path of the first granulite facies event is similar to the P-T paths inferred for the c. 2.5 Ga metamorphosed mafic granulites from the Eastern and Western Blocks, whereas the near-isothermal decompression, clockwise, P-T path of the second granulite facies event is similar to the P-T paths inferred for the c. 1.8 Ga metamorphosed khondalite series in the Western Block and some mafic granulites in the Trans-North China Orogen. These relations suggest that the polymetamorphic granulites were derived from the reworking of the 2.5 Ga metamorphosed granulites during the 1.8 Ga collision between the Eastern and Western Blocks that resulted in the final amalgamation of the North China Craton.