Metamorphic evolution and SIMS U–Pb geochronology of orthopyroxene‐bearing high‐Psemipelitic granulite in the Fuping area, middle Trans‐North China Orogen

Abstract

The Fuping area lies in the middle part of the Trans‐North China Orogen, which is a critical region for understanding the metamorphic–tectonic evolution of this orogenic belt in the Precambrian. The newly discovered orthopyroxene‐bearing semipelitic granulite in this area is a coherent stratigraphic unit, among which three generations of metamorphic mineral assemblages (peak, post‐peak and cooling retrograde) have been identified based on petrographic observation and mineral chemical analysis. The peak metamorphic mineral assemblage mainly consists of porphyroblastic garnet plus matrix minerals including biotite, plagioclase, orthopyroxene, quartz as well as accessory minerals including pyrite, monazite, zircon and apatite. Geothermobarometry calculations and phase equilibria modelling confirm that the representative samples record different P–T conditions and metamorphic stages due to retrogressive metamorphism, with the highest P–T conditions reaching high‐P granulite facies. The retrieved clockwise metamorphic P–T paths pass from 12.5 to 13.5 kbar/855–880℃ (peak stage) through 8.5–11 kbar/850–886℃ (post‐peak stage) and to a speculative cooling phase (retrograde stage), reflecting near isothermal decompression (ITD) followed by near isobaric cooling (IBC). This P–T path is interpreted to reflect a subduction and/or collision event followed by a rapid exhumation. Secondary ion mass spectrometry (SIMS) U–Pb dating on metamorphic monazite and zircon yielded metamorphic ages ranging from 1,825 to 1,815 Ma, possibly constraining a retrograde metamorphic age. Therefore, the semipelitic granulite in the Fuping area records the subduction/collision event between the Eastern and Western Blocks of the North China Craton during the Late Palaeoproterozoic.

Metamorphism and P-T Evolution of High-Pressure Granulites from the Fuping Complex, North China Craton

“…The results of the present study are similar to the above and show that the Fuping Complex underwent regional metamorphism ~1.85 Ga, which may be related to the collision between continental blocks in the NCC [34,47,75].…”

“…Metamorphic zircons in the Fuping HP mafic granulites show crystallization temperatures of 602-712 • C and SIMS zircon U-Pb ages of 1891 ± 14 Ma and 1849 ± 6 Ma, interpreted to represent the cooling stage. Liu et al [75] investigated the mafic granulites exposed in the TTG gneisses in the Daliushu area, delineating four metamorphic stages with the peak mineral assemblage of Gt + Cpx + Opx + Amp + Pl + Q + Zr + Ilm and obtaining the P-T conditions of 680~820 • C/7.2~11.7 kbar. The P-T paths are clockwise with near-isothermal decompression (ITD) and subsequent nearisobaric cooling (IBC) segments.…”

Metamorphic evolution of garnet amphibolites from the Fuping Complex: Implications for the Paleoproterozoic subduction–collision history of the Trans‐North China Orogen

“…A combination of P–T paths and the timing and duration of metamorphism are crucial in understanding the evolution of metamorphic terranes. In the last few decades, several studies on the metamorphic evolution of the TNCO have been carried out in the Hengshan Complex (Qian et al, 2015, 2019; Zhang et al, 2021), Wutai Complex (Qian et al, 2013; Qian & Wei, 2016), FPC (Liu, Zhang, et al, 2021; Tang et al, 2017; Zhao, Wilde, et al, 2002), Huai'an Complex (Liu et al, 2019; Wu et al, 2016), Taihua Complex (Lu et al, 2013, 2020) and Lüliang Complex (Guo et al, 2021; Lu et al, 2022; Zhao et al, 2017). Metamorphic evolution of high‐grade metamorphic rocks in the TNCO generally shows clockwise P–T paths, including heating and pressure increase from the prograde to the peak stages, followed by near‐isothermal decompression and cooling processes, which are thought to be the result of subduction–collision process and rapid exhumation (Brown, 1993; England & Thompson, 1984; Harley, 1989).…”

Diverse P‐T‐t Paths Reveal High‐Grade Metamorphosed Forearc Complexes in NW China

“…A bulk-rock X Fe 3+ = Fe 3+ /(Fe 3+ +Fe 2+ ) ratio was set for each sample (Table S1 in Supporting Information S1) based on the amount of Fe 3+ in the minerals (Data Set S1 in Supporting Information S1) using the recommended values of Palin et al (2016). Because rocks that underwent partial melting (i.e., HP mafic granulites D46 and D49 and metapelite DH53) generally record the retrograde process accompanied with cooling and crystallization of remaining melt, the contents of H 2 O in each sample involved in the thermodynamic modeling were adjusted (Table S1 in Supporting Information S1) using the T-M H2O diagram so that their final mineral assemblages (i.e., the observed M 3 assemblages in this study) were stable just above the solidus and reached equilibrium with the last remnant of melt (e.g., Diener et al, 2008;Liu et al, 2020). For rocks metamorphosed under sub-solidus P-T conditions (i.e., garnet amphibolites PT55 and DH54), the contents of H 2 O in each sample were also adjusted (Table S1 in Supporting Information S1) using the T-M H2O diagram so that the water content saturates the rocks and their peak mineral assemblages are consistent with practical observations.…”