Graphitization in Paleoproterozoic granulites from the Taihua Complex, North China Craton: Constraints from laser Raman spectroscopy, carbon isotopes and geochronology

Abstract

The presence of graphite as a refractory form of solid carbon in high grade meta-sedimentary rocks serves as a valuable tool for identifying carbon sources and assessing the movement of carbon through the Earth's interior. In this study, we investigate the graphite formation processes from different lithologies (marbles, metapelites) from the Paleoproterozoic granulites in the southern margin of North China Craton. By utilizing a range of analytical techniques, including petrographic, mineral-chemical, laser Raman spectroscopic, carbon stable isotopic, and geochronological studies, we characterize the textural and isotopic variety of graphite and the timing of metamorphism associated with graphitization. The results suggest a progressive growth and graphitization of graphite during medium to high temperature metamorphic facies. The δ13C values of crystalline graphite in metapelites and marble range in −24.00‰ to −20.10‰, and − 22.50‰ to −21.60‰, respectively, both close to organic carbon averages. The calcite from the graphite-bearing marble shows δ13C values in the range of −2.90‰ to −2.80‰, which corresponds to marine carbonates. In contrast, graphite-bearing quartzo-feldspathic gneiss and diopside/augite-bearing marble show heavier isotopic values of −14.4‰ to −16.5‰, suggesting graphite precipitation from fluids of both organic and magmatic origin. The spectral and isotopic signatures of graphite in these rocks indicate a dominant biogenic signature with mixture of abiogenic carbon sources. Zircon UPb geochronology marks the timing of high-grade metamorphism at ca. 1.93 to 1.92 Ga, associated with graphite formation during subduction and accretion process associated with the final assembly of the North China Craton. We propose that crystalline graphite was formed during the continental collision-triggered medium to high temperature metamorphic event, consistent with a global graphite metallogenic event in the Paleoproterozoic.