Metamorphic evolution of garnet amphibolites from the Fuping Complex: Implications for the Paleoproterozoic subduction-collision history of the Trans-North China Orogen

The Trans-North China Orogen (TNCO) is a major Palaeoproterozoic collisional orogen in the North China Craton and has experienced complex tectonic evolution during the assembly of the Eastern and Western Blocks. The vestiges of subducted oceanic crust preserved in this orogen can be used to gain insights into the subduction-collision process associated with the Palaeoproterozoic orogeny. In this study, we investigate garnet amphibolites from the Wanzi Group in the Fuping Com-plex along the TNCO through integrated petrography, mineral chemistry, phase equilib-rium modelling and zircon U-Pb geochronology. Three stages of metamorphic mineral assemblages are identified as follows: (i) the prograde assemblage is preserved as pla-gioclase, amphibole, biotite and quartz inclusions in garnet porphyroblasts; (ii) the peak mineral assemblage includes coarse-grained amphibole, garnet, plagioclase, ilmenite, biotite and quartz; and (iii) the retrograde assemblage is mainly represented by the corona of amphibole, plagioclase and biotite in the matrix. Petrological features and phase equilibrium modelling define a clockwise pressure-temperature path involving a prograde stage of 7.2-9.6 kbar and <670 degrees C, peak conditions of 10.0-10.2 kbar and 690-810 degrees C, and a retrograde stage with near-isothermal decompression to similar to 7.4 kbar and cooling. Zircon U-Pb dating shows weighted mean ages of 1848 +/- 22 Ma and 1836 +/- 31 Ma, representing the growth of zircon in the retrograde stage during the uplift and cooling process. In combination with available data from the TNCO, the post-peak decompression and cooling process is inferred to be long-lived during 1.90-1.76 Ga, following collision along the TNCO that occurred during 1.97-1.90 Ga, ultimately leading to the final assembly of the North China Craton.