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

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 9.4-10 kbar and 870-900 degrees C for the M2 stage, 6.3-6.6 kbar and 840-900 degrees C for the M3 stage, and 4-5.2 kbar and 815-830 degrees 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. (C) 2012 Elsevier B.V. All rights reserved.