Considerable heating during clockwise decompression of a long-lived Paleoproterozoic hot orogen: Evidence from the Xuanhua Complex, North China Craton

Abstract

Granulites comprise the root of collisional orogens, and decoding their evolution may provide key information on orogenic processes from collisional thickening to post-collision extensional thinning. However, the records of collisional orogeny in most cases have been obscured or even erased by later high-/ultrahigh-temperature (HT/UHT) overprinting. Here, we report new petrological, geochemical and geochronological data for granulite-facies rocks in the Xuanhua Complex of the northern Trans-North China Orogen (TNCO). We reveal the complex metamorphic features and tectonic evolution of a long-lived Paleoproterozoic hot orogen as well as their link with Phanerozoic orogens. The rocks are composed mainly of garnet + clinopyroxene + orthopyroxene ± amphibole + plagioclase + quartz + rutile + ilmenite with symplectitic intergrowths of clinopyroxene + orthopyroxene ± amphibole + plagioclase ± quartz. Pseudosection modeling and thermobarometric calculations suggest that the high-pressure (HP) mafic granulites record clockwise pressure–temperature (P-T) paths, from the prograde stage (M1) to the pressure-peak stage (M2; 13.5–15.0 kbar and 850–875 °C), followed by considerable heating during decompression to the temperature-peak stage (M3a; 8.0–9.6 kbar and 910–950 °C) and final cooling to the M3b stage. Magmatic zircon U-Pb dating reveals c. 2.1–2.0 Ga protolith ages for the granulites. Metamorphic zircon and titanite U-Pb results, and trace elements in zircons indicate that the age of c. 1.92 Ga is possibly close to the lower limit of HP granulite-facies pressure-peak, and c. 1.84–1.81 Ga ages are related to the timing of the final cooling stage. Hence, by comparison with existing geological data, the synthesized metamorphic P-T-t data of the Xuanhua granulites indicate two stages of tectonic evolution, including: (1) crustal thickening by continental collision between the Western and Eastern Blocks at c. 1.95–1.92 Ga, resulting in an early HP granulite-facies metamorphism at relatively low thermal gradients of 16–19 °C/km; and (2) the collision-thickened orogenic lithosphere was thinned controlled by continental extension and gravitational instability, which might provide additional heat in the interval c. 1.90–1.84 Ga, resulting in a later UHT overprinting at reduced pressures and higher gradients of 30–36 °C/km. In addition, the post-collisional extension accompanying exhumation decompression shows a long-lived timescale of > 80 Myr, corresponding roughly to a slow exhumation rate of < 0.3 mm/yr. To sum up, these features of long-lived tectono-thermal evolution (tectonic transition from crustal thickening to lithospheric thinning, and prolonged orogeny and exhumation), slow exhumation rates, and the occurrence of UHT overprinting, are becoming more commonly identified in Precambrian hot orogens like the TNCO.