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 (M₁) 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 (M₂) consists of clinopyroxene + garnet + sodic plagioclase + quartz ± hornblende in the high-pressure granulites and orthopyroxene + clinopyroxene + garnel + plagioclase + quartz in the medium-pressure granulites. Peak metamorphism was followed by near-isothermal decompression (M₃), which resulted in the development of orthopyroxene + clinopyroxene + plagioclase symplectites and coronas surrounding embayed garnet grains, and decompression-cooling (M₄), represented by hornblende + plagioclase symplectiles on garnet. The THERMOCALC program yielded peak (M₂) P-T conditions of 13.4-15.5 kbar and 770-840°C for the high-pressure granulites and 9-11 kbar and 820-870°C 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 (M₃) were estimated at ~6-5-8.0 kbar and 750-830°C, and hornblende + plagioclase symplectile (M₄) at ~4.5-6.0 kbar and 680-790°C. The P-T conditions of the early prograde assemblage (M₁) 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 collison between two continental blocks, which resulted in the final assembly of the North China Craton at ~ 1.8 Ga.