Exhumation of an Archean Granulite Terrane by Paleoproterozoic Orogenesis: Evidence from the North China Craton

Granulites represent high-grade metamorphic rocks of the deep continental crust. The metamorphism and exhumation of granulites from Archean terranes provide insights into the crustal evolution of Archean cratons and shed light on the formation and reactivation of cratons. We present petrology, U-Pb geochronology (zircon, rutile, and titanite), and pressure-temperature (P-T) paths for metadiabase dikes in an Archean granulite terrane of the North China Craton. Garnet (Grt) coronae in the metadiabase dikes are developed between plagioclase (Pl) and clinopyroxene (Cpx) via the reaction Pl(igneous) + Cpx -> Pl(1) + Grt +/- quartz. The reaction proceeds inward within the plagioclase, progressively consuming Pl(igneous) (X-An = 0.65-0.69) and leaving Ca-poor Pl(1) (X-An = 0.46-0.53) as the residue. Geothermobarometry and P-T pseudosections suggest peak conditions for garnet formation at similar to 800 degrees C and 10-13 kbar. During retrograde metamorphism, Grt broke down to Ca-rich Pl(2) (X-An = 0.73-0.74), and ilmenite replaced rutile. Geothermobarometry and Zr-in-titanite temperatures constrain the P-T conditions of retrograde metamorphism at 700-750 degrees C and 4.5-7.5 kbar. Zircon and titanite U-Pb geochronology shows that the protolith of the metadiabase dike was formed at 2.4 Ga and underwent granulite-facies metamorphism at 1.86 Ga. The intrusion of mafic dikes into the Archean granulite terrane indicates that the Archean basement was also heated and buried in the Paleoproterozoic. The metadiabase dikes and the hosting Archean basement underwent Paleoproterozoic granulite-facies metamorphism at a depth of similar to 40 km, followed by near-isothermal decompression and subsequent near-isobaric cooling (cooling rate of 1-3 degrees C Myr(-1)) at depths of 15-25 km. Crustal shortening and thickening may have been caused by the underplating of the Khondalite series beneath the Archean basement during the amalgamation of supercontinent Columbia. The Paleoproterozoic orogeny induced a second generation of metamorphism of the Archean basement along the margin of the craton and drove the exhumation of the Archean granulite terranes to the middle crust (similar to 15 km).