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

Abstract

Both medium- and high-pressure pelitic granulites were found in the Jiaobei massif in the southern segment of the Jiao-Liao-Ji Belt, which is a Paleoproterozoic tectonic belt in the Eastern Block of the North China Craton. Studies on mineral assemblages and metamorphic reaction textures reveal that the medium-pressure pelitic granulites from the Jiaobei massif underwent four metamorphic stages: (1) the prograde stage (M1) represented by mineral inclusions within the core of garnet grains; (2) the peak stage (M2) indicated by the formation of sillimanite+K-feldspar and the growth of inclusion-free garnet mantle, (3) the decompression stage (M3) characterized by the cordierite+sillimanite symplectite, and (4) the decompression-cooling stage (M4) forming cordierite coronas. An integrated study of mineral reaction textures, mineral chemistry and pseudosection modelling in the NCFKMASHTO system with the THERMOCALC technique constrains the P–T conditions of 9.4–10kbar and 870–900°C for the M2 stage, 6.3–6.6kbar and 840–900°C for the M3 stage, and 4–5.2kbar and 815–830°C for the M4 stage. The P–T conditions of the early prograde (M1) assemblage cannot be estimated due to reworking at later metamorphic stages (M2–M4). The mineral assemblages of the M2–M4 stages and their P–T conditions define a clockwise P–T path involving isothermal decompression (ITD) and subsequent decompression-cooling for the medium-pressure pelitic granulites, which are consistent with a continent–continent collision environment. This suggests that the Jiao-Liao-Ji Belt was not formed simply by the closure of a Paleoproterozoic rift basin as conventionally considered, but the tectonic evolution of its southern segment must have been involved in subduction and/or continent–continent collisional processes. Based on this study and previous data, we propose that the Jiao-Liao-Ji Belt represented a Paleoproterozoic rifting- and collision-belt along which the Langgang and Langrim Blocks amalgamated to form the Eastern Block of the North China Craton.