Late Neoarchean and Paleoproterozoic tectonothermal and metamorphic evolution recorded in high-pressure granulites: Evidence from the Xuanhua Complex, northern North China Craton

Archean cratons usually underwent reworked processes during the late Paleoproterozoic orogeny, which may result in multiple tectonothermal records preserved in some high-grade metamorphic rocks; thus, these rocks provide a key opportunity on understanding the tectonic styles during different periods. In this study, we report new petrological, mineral chemical and geochronological data of a suite of newly discovered mafic and felsic metamorphic rocks in the Xuanhua Complex from the northern North China Craton (NCC), revealing the complex metamorphic thermal history and the transition of tectonic regime from the late Neoarchean to the late Paleoproterozoic. The mafic granulite preserves the prograde quartz and clinopyroxene inclusions within garnets (M-1), and the peak stage (M-2) is recorded by garnet, clinopyroxene, hornblende, plagioclase, quartz and ilmenite. The first retrograde (M(3)a) and final cooling stages (M(3)b) are defined by the appearance of orthopyroxene, and the amphibolite-facies overprinting, respectively. The felsic granulite records the similar peak mineral assemblage (M-2), but has higher volumes of feldspar and quartz than the mafic granulite. The retrograde stage (M3) is represented by amphibolite-facies assemblage (hornblende +/- biotite +/- clinopyroxene) and fine-grained intergrowth of hornblende + plagioclase around garnet. Pseudosection modeling and thermobarometry results suggest that both high-pressure (HP) mafic and felsic granulites record clockwise P-T paths, including isothermal decompression and cooling from the peak stage (M-2: 10.4-10.8 kbar/830-860 degrees C and 9.6-11.2 kbar/850-930 degrees C, respectively) to the retrograde stages (M(3)a and M(3)b: 7.5-8.0 kbar/840-870 degrees C and 7.8-8.2 kbar/650-700 degrees C, respectively; M-3: 7.2-8.6 kbar/670-800 degrees C). Zircon U-Pb dating shows that the magmatic precursors of these rocks were emplaced at c. 2.5-2.47 Ga. The metamorphic zircons reveal two groups metamorphic ages of c. 2.51-2.45 Ga and c. 1.90-1.85 Ga. The c. 2.51-2.45 Ga ages indicate a late Neoarchean tectonothermal event, and the c. 1.95-1.85 Ga ages are interpreted to represent the Paleoproterozoic HP granulite-facies metamorphism overprinting. The highest positive epsilon(Hf)(t) value of 7.64 and 2.8-2.6 Ga Hf model ages of c. 2.5 Ga magmatic zircons indicate that the c. 2.5 Ga mantle-derived mafic magma and the c. 2.5 Ga partial melting of 2.8-2.6 Ga basaltic juvenile crust generated the protoliths of mafic and felsic granulites, respectively, revealing 2.8-2.6 Ga and 2.5 Ga crustal growth and c. 2.5 Ga crustal reworking in the northern NCC. To sum up, we consider that the late Neoarchean magmatism and coeval metamorphism recorded in our samples are probably resulted from the amalgamation of micro-blocks. The Paleoproterozoic clockwise P-T-t paths with near-isothermal decompression segments and low geothermal gradients (c. 21-26 degrees C/km) of HP mafic and felsic granulites indicate a rapid post-collisional exhumation process after the significantly crustal thickening, and this long-term orogeny along the TNCO (c. 1.95-1.85 Ga; > 100 Myr) is related to the incorporation of the NCC into the Columbia supercontinent.