Metamorphic P–T path and implications of high-pressure pelitic granulites from the Jiaobei massif in the Jiao-Liao-Ji Belt, North China Craton

Abstract

High-pressure pelitic granulites have recently been found from the Jiaobei massif in the southern segment of the Jiao-Liao-Ji Belt (JLJB), which separates the Eastern Block of the North China Craton into the Longgang and Langrim Blocks. Petrological evidence from these high-pressure pelitic granulites indicates three distinct metamorphic stages (M1–M3). The pre-peak (M1) stage is represented by a mineral assemblage of garnet (core)+inclusion-type minerals within the garnet (biotite+kyanite+muscovite+plagioclase+quartz+ilmenite). The peak (M2) stage is marked by a mineral assemblage of garnet (mantle)+K-feldspar+kyanite+plagioclase+biotite+rutile+ilmenite+quartz. The post-peak (M3) stage is featured by a mineral assemblage of garnet (rim)+sillimanite+plagioclase+biotite+ilmenite+quartz. Using the THERMOCALAC technique, the NCKFMASHTO system has been applied to construct pseudosections for a representative high-pressure pelitic granulite sample. P–T conditions for the M1, M2 and M3 stages are constrained at 9.3–10.7kbar/645–670°C, 14.8–16.2kbar/860–890°C, and 6.3–8.5kbar/710–740°C, respectively. The mineral assemblages and P–T conditions of the high-pressure pelitic granulites define a clockwise P–T path involving decompression and cooling following the peak high-pressure granulite facies metamorphism. This suggests that the high-pressure pelitic granulites experienced the initial crustal thickening (M1 and M2), followed by exhumation and cooling (M3), which implies that the southern segment of the JLJB must have been involved in subduction- or collision-related tectonic processes. Therefore, the JLJB may represent another Paleoproterozoic collisional belt along which the Longgang and Langrim Blocks amalgamated to form the Eastern Block.