Garnet perspectives on the metamorphic history and tectonic significance of Paleoproterozoic high-pressure mafic granulites from the northern Hengshan, North China Craton

Reconstructing the pressure-temperature (P-T) evolution of early Precambrian high-pressure granulites and eclogites is critical for decoding the tectono-metamorphic evolution of the early Earth. However, debates continue regarding the P-T history of these rocks because of various modifications by peak and retrograde rehydration metamorphism. As a significant rock-forming mineral in high-grade metamorphic rocks, garnet not only grows with increasing pressure or temperature and develops variably zoned patterns along given P-T loops, but its properties are stable subsquent to its formation when compared to other minerals in the matrix. Consequently, garnet is excellent for monitoring the thermo-metamorphic history of its host rock during orogeny. Selected garnet grains from the Paleoproterozoic high-pressure mafic granulites in the northern Hengshan area of the North China Craton, which exhibit variable microstructural and chemical zoning, present an excellent example for extracting metamorphic evolution of the host rocks. Microstructural zoning displays a zonal distribution of frozen inclusion-type minerals in garnet. Chemically, the garnet shows pronounced compositional zoning, with bell-shaped XSps, initially flat and then increasing/decreasing XPyr and Fe#, initially increasing and then decreasing XGrs from core to mantle, and resorbed rims. Both zoning patterns document multiple generations of radial growth history. The progressive evolution of the host rocks can be further divided into early prograde (M1–1), late prograde (M1–2), near peak (M2–1), and peak (M2–2) stages. The resorption textures indicate the succeeding decompression (M3) and cooling (M4) stages.