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

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, M-1a to M-1c. M-1a is represented by fine-grained orthopyroxene + clinopyroxene + plagioclase +/- quartz, which is surrounded by the M-1b garnet + quartz symplectite, which itself is mantled by the M-1c plagiociasc + 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 800-850degreesC (M-1a) followed by isobaric cooling to 700-750degreesC (M-1b) and pressure-decreasing cooling to 630-700degreesC (M-1c). The second event also includes three mineral assemblages, M-2a to M-2c. M-2a represents growths of garnet porphyroblasts and matrix orthopyroxene + plagioclase + clinopyroxene + quartz; M-2b consists of orthopyroxene + plagioclase +/- clinopyroxene symplectites or coronas; and M-2c 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 820-850degreesC (M-2a), followed by near-isothermal decompression (M-2b) of 7.0-7.6 kbar and 760-810degreesC and cooling (M-2c) to 690-760degreesC. 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 We-tern Blocks that resulted in the final amalgamation of the North China Craton.