Neoarchean metamorphic evolution and geochronology of the Miyun metamorphic complex, North China Craton

Mafic and felsic granulite from the Miyun metamorphic complex, northeast Beijing, North China Craton, occur as either interlayers or lenses within the Late Archean TTG gneiss. In the mafic granulite, the metamorphic peak assemblages (M1) are represented by porphyroblast minerals plus matrix minerals (orthopyroxene + clinopyroxene + plagioclase + quartz +/- K-feldspar +/- hornblende +/- biotite) as well as accessory minerals of zircon + apatite + ilmenite +/- magnetite, the post-peak assemblages (M2) are characterized by red-eye socket symplectites (garnet + clinopyroxene + plagioclase + quartz) rimming the orthopyroxene or clinopyroxene porphyroblast as well as accessory minerals of rutile, and the final retrograde assemblages (M3) are marked by the hornblende-biotite intergrowth. As for the felsic granulite, the peak assemblages (M1) are represented by the porphyroblast and matrix minerals (garnet + orthopyroxene + biotite + plagioclase + quartz +/- K-feldspar) as well as accessory minerals in the matrix (zircon + monazite + apatite + pyrite), the post-peak assemblages (M2) are characterized by red-eye socket symplectites (garnet + plagioclase + quartz) rimming the orthopyroxene porphyroblast, and the final retrograde assemblages (M3) are marked by the biotite-calcite intergrowth. Geothermobarometric computation suggests that the M1, M2 and M3 assemblages of the mafic granulite were formed under P-T conditions of similar to 800 to 860 degrees C/similar to 6 to 10 kbar, similar to 620 to 700 degrees C/11 to 12 kbar and similar to 590 to 660 degrees C/4 to 6 kbar, respectively, suggesting that these granulites record anticlockwise P-T paths including final nearly isothermal decompression (ITD) segments, indicative of orogenic process. For the felsic granulite, metamorphic P-T conditions of the M1 and M3 assemblages were estimated to be similar to 680 to 750 degrees C/6-8 kbar and similar to 670 to 680 degrees C/5 kbar, respectively, also including ITD process. The widely penetrative gneissosity of the granulite and these P-T paths, suggest a collisional event of cold nappes overthrusting on hot ones or hot nappes being buried/subducted into cold ones and finally experienced fast tectonic exhumation. High -precision SIMS and LA-ICP-MS U-Pb dating of metamorphic zircon of the granulite suggest that the metamorphism occurred at similar to 2.5 Ga. These data confirm that the Miyun metamorphic complex experienced a Neoarchean collisional event within the Eastern Block of the North China Craton.