Clockwise P–T–t paths with considerable heating processes during decompression from high-pressure mafic granulites in the Wuhe Complex, southeastern North China Craton

Abstract

The Wuhe Complex lies in the southern section of the Jiao–Liao–Ji mobile/orogenic belt (JLMB/JLJB), critical for understanding the Proterozoic tectonic evolution of the southeastern North China Craton (NCC). Detailed microstructural observations, mineral chemistry, phase equilibria modeling, and zircon and titanite chronological dating were conducted on the mafic granulites from the Wuhe Complex. Five stages of mineral assemblages were identified: the prograde stage (Pl1 + Cpx1 + Amp1 + Ilm1 + Ap1 ± Qz1 ± Kfs1 ± Ttn1 ± Py1), the pressure–peak stage (the core of matrix plagioclase and Grt2a + Cpx2a + Rt2a + Qz2a + Liq2a ± Amp2a ± Kfs2a ± Ttn2a), the temperature–peak stage (the rim of garnet porphyroblast and Pl2b + Cpx2b + Amp2b + Liq2b ± Ilm2b ± Qz2b ± Rt2b), the retrograde stage (Pl3 + Cpx3 + Opx3 + Amp3 + Ilm3 + Ap3 ± Grt3 ± Qz3 ± Scp3), and the cooling stage (Mag4 ± Chl4 ± Py4 ± Ccp4). Traditional thermobarometry calculations and phase equilibria modeling illustrated that clockwise metamorphic pressure–temperature (P–T) paths were retrieved, passing from 9.4 to 13.4 kbar/701 ℃–781 ℃ (prograde stage) through 16.3–17.2 kbar/807 ℃–846 ℃ (pressure–peak stage), and through 10.2–10.8 kbar/895 ℃–912 ℃ (temperature–peak stage) to 5.6–9.1 kbar/803 ℃–883 ℃ (retrograde stage). Hence, the high-pressure (HP) granulites show clockwise P–T paths with heating processes (∼80 ℃) from the lower crust (pressure–peak stage) to the middle-lower crust (temperature–peak stage). Therefore, HP mafic granulites might be buried to the depth of ∼ 57 km, with a slow exhumation rate. The metamorphic zircon and titanite grains from HP mafic granulites yield U–Pb ages of 1991–1758 Ma and 1832–1810 Ma, respectively. The 1832–1810 Ma might represent the retrograde time when HP mafic granulites cooled to 820 ℃–800 ℃. Therefore, the HP mafic granulites of the Wuhe Complex record the Late Paleoproterozoic subduction/collision orogenic event of the southeastern NCC; therefore, the JLMB/JLJB is a prolonged orogenic belt.