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

Abstract

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 + garnel + 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 symplectiles on garnet. The THERMOCALC program yielded peak (M 2) 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 (M 3) were estimated at ~6-5-8.0 kbar and 750-830°C, and hornblende + plagioclase symplectile (M 4) at ~4.5-6.0 kbar and 680-790°C. 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 collison between two continental blocks, which resulted in the final assembly of the North China Craton at ~ 1.8 Ga.