High-pressure granulites (retrograded eclogites) from the Hengshan Complex, North China Craton: Petrology and tectonic implications

Both high- and medium-pressure granulites have been found as enclaves and boudins in tonalitic-trondhjemitic-granodioritic gneisses in the Hengshan Complex. Petrological evidence from these rocks indicates four distinct metamorphic assemblages. The early prograde assemblage (M-1) is preserved only in the high-pressure granulites and represented by quartz and rutile inclusions within the cores of garnet porphyroblasts, and omphacite pseudomorphs that are indicated by clinopyroxene + sodic plagioclase symplectic intergrowths. The peak assemblage (M-2) consists of clinopyroxene + garnet + sodic plagioclase + quartz +/- hornblende in the high-pressure granulites and orthopyroxene + clinopyroxene + garnet + plagioclase + quartz in the medium-pressure granulites. Peak metamorphism was followed by near-isothermal decompression (M-3), which resulted in the development of orthopyroxene + clinopyroxene + plagioclase symplectites and coronas surrounding embayed garnet grains, and decompression-cooling (M-4), represented by hornblende + plagioclase symplectites on garnet. The THERMOCALC program yielded peak (M-2) P-T conditions of 13.4-15.5 kbar and 770-840 degreesC for the high-pressure granulites and 9-11 kbar and 820-870 degreesC for the medium-pressure granulites, based on the core compositions of garnet, matrix pyroxene and plagioclase. The P-T conditions of pyroxene + plagioclase symplectite and corona (M-3) were estimated at similar to 6.5-8.0 kbar and 750-830 degreesC, and hornblende + plagioclase symplectite (M-4) at similar to 4.5-6.0 kbar and 680-790 degreesC. The P-T conditions of the early prograde assemblage (M-1) cannot be quantitatively estimated because of the absence of modal minerals. The combination of petrographic textures, mineral compositions, metamorphic reaction history, petrogenetic grids and thermobarometric data defines a near-isothermal decompressional clockwise P-T path for the Hengshan granulites, suggesting that the Hengshan Complex underwent initial crustal thickening, subsequent exhumation, and cooling and retrogression. This tectonothermal path is considered to record a major phase of collision between two continental blocks, which resulted in the final assembly of the North China Craton at similar to 1.8 Ga.