Geochemistry, geochronology and metamorphism of high-pressure mafic granulites in the Huai'an Complex, North China Craton: Implications for the tectonic evolution of the Paleoproterozoic orogeny

Abstract

The Paleoproterozoic was a critical tectonic transition stage for the North China Craton (NCC), with several collisional orogenic belts identified. However, the tectonic boundaries, temporal-spatial evolution, and geodynamic mechanisms of these orogens remain controversial. In this contribution, we document a newly identified largest high-pressure (HP) mafic granulite outcrop at Xizhaojiayao in the Huai'an Complex, which is composed of metamorphosed supracrustal (volcano-sedimentary) sequences, including HP mafic granulites (metabasalts), marbles (metalimestones), quartzites (metasilicalites), and pelitic granulites (metapelites). They occur as tectonic slices within Neoarchean migmatized tonalitic gneisses. The Xizhaojiayao HP mafic granulites show different field occurrences and rock assemblages from the dike-type HP mafic granulites that are widely documented in the Huai'an and Hengshan Complexes. Microstructural studies, geothermobarometer calculations, and phase equilibrium modeling of the HP mafic granulites defined a clockwise pressure–temperature (P–T) path with prograde evolution from amphibolite-facies (M1, 5.2–5.5 kbar/538–668 °C) to HP granulite-facies (M2, 11.2–12.2 kbar/850–880 °C), followed by a significant near-isothermal decompression to medium-low-pressure granulite-facies (M3-1, 8.4–9.1 kbar/800–900 °C; M3-2, 4.9–5.6 kbar/779–805 °C). Zircon/titanite U–Pb dating constrains peak HP metamorphism, granulite-facies decompression, and amphibolite-facies cooling retrograde at 1902–1909 Ma, ∼1830 Ma, and < 1818 Ma, respectively. Whole-rock major and trace-element compositions, as well as positive εNd(t) (+2.0 to + 4.9) and zircon εHf(t) values (+2.9 to + 8.8) of these HP mafic granulites, indicate a mid-ocean ridge basalt geochemical affinity, whereas an island-arc signature is common for the dike-type HP mafic granulites. The Xizhaojiayao supracrustal sequences, which represent dismembered and metamorphosed upper oceanic crustal fragments, were subducted from the upper crust to depths of 35–40 km, followed by rapid exhumation to the middle-shallow crust and were tectonically involved in the melt-weakened tonalitic crust. These tectono-metamorphic scenarios illustrate the Paleoproterozoic orogenic evolution of a consecutive subduction-collision-exhumation process from 1.95 to 1.80 Ga, which led to the final assembly of the NCC.