Paleoproterozoic medium to high pressure metamorphism in the Wanzi supracrustal association, Trans-North China Orogen: New insights from the gedrite-bearing gneiss, gedrite-free gneiss, and amphibolite

Abstract

The Wanzi supracrustal association locates in the Fuping Complex, middle Trans-North China Orogen, and it was generally considered before as a lithological unit that underwent medium pressure metamorphism. The newly discovered gedrite-bearing gneiss, gedrite-free gneiss, and amphibolite of the Wanzi supracrustal association show abundant mineral assemblages with complex reaction textures, which are critical clues to retrieve the metamorphic evolution of this area. Gedrite-bearing/free gneiss samples mainly consist of garnet porphyroblast plus matrix minerals including biotite, kyanite, sillimanite, staurolite, ±gedrite, plagioclase, quartz, as well as accessory minerals such as ilmenite, magnetite, apatite, monazite, and zircon. Amphibolite samples are generally composed of garnet porphyroblast plus matrix minerals amphibole, ±biotite, plagioclase, quartz, as well as accessory minerals including ilmenite, magnetite, and zircon. Two to four generations of metamorphic mineral assemblages formed at the prograde (M1), peak (M2), and retrograde (M3-M4) stages have been identified from these samples, recording clockwise P-T paths including sequential near-isothermal decompression (ITD) and near-isobaric cooling (IBC) segments, indicative of convergence tectonic settings. The peak P-T conditions of these samples are different, ranging from 8.2 to 13 kbar and 695 to 822 °C. SIMS/SHRIMP U-Pb dating of metamorphic zircon yields 207Pb/206Pb age of 1838–1821 Ma, representing the metamorphic event. Combined with the previous data, the Wanzi supracrustal association is inferred to have undergone middle to high pressure metamorphism during 1.84–1.82 Ga, which was involved in the subduction and/or collision event between the Western and Eastern Blocks and led to the final assembly of the North China Craton.