Origin of layered variations in crystallinity of graphitic carbons in Yunshan graphite deposit, Heilongjiang, China

• A spatial distribution pattern with a poor crystallinity in the graphite center and nearly perfect crystallinity at the edge is revealed. • Variably crystalline graphite in the Yunshan graphite deposit is of syngenetic and biological origin. • A three-stage evolutionary model is proposed for variably crystalline graphite. Yunshan graphite deposit (YGD) is the largest crystalline graphite deposit in China, which has undergone thermal metamorphism during the Meso-Neoproterozoic era. The mechanism by which metamorphism induces the formation of graphite can be determined by variations in crystallinity of graphitic carbons, yet this is still unknown for the YGD. This study aims to characterize graphite-quartz schists from the YGD for potential formation mechanism of graphite therein by an integrated approach, including petrographic, mineralogical, geochemical, and spectroscopic analyses. The geochemical and mineralogical compositions show that the graphite-quartz schists were likely derived from initial shale or claystone and subsequently subjected to greenschist to amphibolite facies metamorphism. Micro-Raman spectroscopy reveals that graphite in the graphite-quartz schists has I-D1/G ratios ranging from 0.02 to 1.88, exhibiting significant layered variations in crystallinity, with poor crystallinity in the center and nearly perfect crystallinity at the edge. The peak temperatures (357 ℃ to 637 ℃; avg. 475 ℃) for graphite crystallization calculated using Raman spectra align with the metamorphic temperatures experienced by the host rocks, pointing to a syngenetic origin. The δ 13 C gra values range from −24.0‰ to −24.3‰, indicating that the graphitic carbons in the YGD are of biological origin. In comparison, the δ 13 C carb values of carbonate minerals range from −7.5‰ to −12.7‰, which is significantly 13 C-depleted and lower than marine carbonate. This suggests that carbonate in the YGD could form from graphite oxidation during retrograde metamorphism, as revealed by the difference in δ 13 C values between graphitic and inorganic carbons that records a retrograde temperature from 276 ℃ to 365 ℃. Based on these new data, this study elucidates a synergistic control mechanism of temperature and H 2 fugacity on layered variations in crystallinity of graphitic carbons, and further proposes a three-stage evolutionary model involving deposition, metamorphism and retrograde metamorphism for biological carbon in graphite from the YGD.