High-pressure granulites in the Fuping Complex of the central North China Craton: Metamorphic P-T-t evolution and tectonic implications

Mafic granulites in the Fuping Complex occur as lenses or boudins within high-grade TTG (Trondhjemite-Tonalite-Granodiorite) gneisses. Petrographic observations reveal four generations of mineral assemblage in the granulites: an inclusion assemblage of hornblende + plagioclase + ihnenite + quartz within garnet core; an inferred peak assemblage composed of garnet hornblende + plagioclase + clinopyroxene + menite + quartz; a decompression assemblage characterized by symplectites of clinopyroxene orthopyroxene + plagioclase, coronae of plagioclase +/- clinopyroxene +/- hornblende around embayed garnet porphyroblasts or a two-pyroxene association; and a late amphibolite-facies retrogressive assemblage. Two representative samples were used for pseudosection modeling in NCFMASHTO model system to determine their metamorphic evolution. The results show that these granulites experienced a high-pressure stage of metamorphism with peak P-T conditions of 12-13 kbar and 760-800 degrees C (P-max) and a post -peak history under P-T conditions of similar to 9.0 kbar and 805-835 degrees C (T-max), indicating a nearly isothermal decompression process (ITD) with a slight heating. Metamorphic evolution from the P-max to the T-max is predicted to be dominated by garnet breakdown through continuous metamorphic reactions of garnet + quartz +/- diopside = hornblende + plagioclase + liquid and garnet + quartz + homblende = plagioclase + diopside + liquid + orthopyroxene. Further metamorphic evolution after the T-max is dominated by cooling, suggesting that high-pressure (HP) granulites may also exist in the Fuping Complex. Metamorphic zircons in the Fuping HP mafic granulites have left inclined REE patterns, Ti contents of 1.68-6.88 ppm and crystallization temperatures of 602-712 degrees C. SIMS zircon U-Pb dating on these zircons yields Pb-207/Pb-206 ages of 1891 +/- 14 Ma and 1849 +/- 6 Ma, interpreted to represent the cooling stage of metamorphism. The P-T-t evolution of the Fuping HP mafic granulites records well the protracted Paleoproterozoic orogenic event occurred in the central North China Craton.