Detrital graphite particles in the Cryogenian Nantuo Formation of South China: Implications for sedimentary provenance and tectonic history

Abstract

The tectonic history of the Yangtze block of South China prior to the Ediacaran Period has been a matter of debate. In particular, the tectonic history of the Shennongjia terrane in the central-northern margin of the Yangtze block is poorly understood, and the tectonic relationship between the Shennongjia terrane and the Huangling massif (exposed in the Huangling anticline) is unclear. Previous researchers have reported U-Pb dates of detrital zircons and Nd isotopic compositions of clastic rocks from Cryogenian successions in the Shennongjia terrane to identify ancient crustal remnants and the provenance of clastic sediments, and to constrain the age and crustal evolution of the Yangtze block. Here, we describe abundant graphite discs from silty mudstone of the Cryogenian Nantuo Formation in the Shennongjia area of the Yangtze block. These graphite discs are opaque, ∼23–171 μm in size, and highly reflective. Most are abraded or rounded, although some show hexagonal crystalline morphology. Their Raman spectra are distinct from those of indigenous carbonaceous material (CM) in the sedimentary matrix from the Nantuo Formation at the same locality. Raman CM geothermometric analysis indicates that graphite discs experienced a maximum metamorphic temperature of nearly 600 °C, which is inconsistent with the relatively low metamorphic grade of the host rock. These observations indicate that the graphite discs are of detrital origin, most likely derived from high-grade metamorphic rocks in the basement of the Yangtze block (e.g., the Archean-Paleoproterozoic Kongling terrain in the core of the Huangling anticline), consistent with the hypothesis that the Shennongjia region was a part of Yangtze block by the end of the Cryogenian Period.