GEOLOGY AND GEOCHEMISTRY OF SILICEOUS ROCKS IN THE EOARCHEAN NUVVUAGITTUQ SUPRACRUSTAL BELT: ESTIMATION OF THEIR ORIGIN AND EVIDENCE FOR LIFE

Stable carbon isotope records in metasedimentary rocks showed that life has existed in the early Archean. Therefore, it may become next subjects of interest what kind of ecosystem existed and whether life existed prior to Archean. However, they are still ambiguous because little information other than isotope records is available from the early Archean and Hadean rocks.

Nuvvuagittuq Supracrustal Belt (NSB) in northern Quebec consists of volcano-sedimentary sequences, cut by the >3.75 Ga orthogneiss. The age of NSB is still controversial but possibly 4.4 Ga (O’Neil et al., 2012); thus, only NSB has the pre-Archean information of life. The belt contains a ~100 m thick laterally continuous siliceous layer. The layer has been suggested to be chemical sedimentary rock based on the geological occurrence and sulfur mass independent fractionation. However, more comprehensive research is needed to better constrain their protoliths and depositional environment.

They mainly contain variable amount of quartz, cummingtonite, garnet, biotite, magnetite, and graphite, and have layering structure consistent over µm- to m-scale, suggesting its sedimentary protolith. Their rare earth element patterns also support their chemical sedimentary origin precipitated in the ocean.

The graphite occurs in association with the layering structure, suggesting its syn-depositional origin. The crystallization temperature of most graphite grains was estimated as >650°C by laser Raman spectroscopy, consistent with the maximum metamorphic temperature of NSB. Therefore, the graphite did not originate from fluid after peak metamorphism.

Whole-rock organic carbon isotope ratios (δ¹³C­_org) range from -10 to -27‰ and total organic carbon (TOC) contents are up to ~1%. The variation of δ¹³C­_org values between TOC contents can be explained by mixing of three carbon sources with δ¹³C­_org values of (1) -10‰, (2) -19‰, and (3) -27‰. The isotopically light carbon sources of (2) and (3) suggest their biological origin from different species.

Our results show the graphite in the siliceous rocks is syn-depositional biological origin, and suggest diverse biosphere have existed in the ocean of >3.75 Ga or possibly in 4.4 Ga Earth.