The origin and fate of organic carbon in graphite-manganese bearing rocks and implications for the Lomagundi-Jatuli Event.

Our study helps to unravel the complexity of the Lomagundi-Jatuli event, the largest and longest positive carbon isotope excursion ever recorded on the Earth's surface, by providing a unique view of Paleoproterozoic graphitic rocks from the Borborema province of Northeastern Brazil. Through detailed mineralogical, textural, chemical and isotopic analyses, we bring a new perspective that provide support to elevated primary productivity and large-scale organic carbon burial during the Lomagundi-Jatuli event. Graphite crystals with distinctive textural features occur in association with silicate and oxidised manganese ores, manganese quartzites, garnetites, and gneisses. The graphites were crystallised at temperatures up to 634 °C, consistent with amphibolite facies metamorphism, according to Raman thermometry. An average total carbon content of 2.1 wt%, with δ13C values ranging from - 15.0 to - 21.5‰, is indicated by whole-rock geochemistry and carbon isotopic composition, respectively. Based on these results, our study proposes that these graphitic rocks may represent remnants of organic matter, possibly derived from bacterial biomass associated with manganese-rich sediments, preserved under reducing environmental conditions in a redox-stratified marine setting. Biological mediation on the origin of silicates is suggested by the close relationship between reduced manganese silicates and graphite. These constraints indicate that Paleoproterozoic graphite-rich rocks represent an important but overlooked reservoir of organic carbon that was partially degassed during the metamorphism of organic-rich sequences. Overall, this research provides new insights for the enigmatic emergence of the Lomagundi-Jatuli event, highlighting the intricate interplay among organic carbon, manganese-rich rocks and Earth's evolutionary processes during this period.

Petrology of dioritic, tonalitic and trondhjemitic gneisses from Encantadas Complex, Santana da Boa Vista, southernmost Brazil: paleoproterozoic continental-arc magmatism

The Encantadas Complex is a unit composed of dioritic, tonalitic and trondhjemitic gneisses with minor hornblendite. This complex is intruded by granites of Neoproterozoic age. Major and trace element data indicate metaluminous to slightly peraluminous composition related to the medium-K calc-alkaline series. Compositional parameters are consistent with a common evolution from less differentiated magmas, probably through fractional crystallization. The orthogneisses show LaN/YbN ratios from 10 to 50, K2O/Na2O varying from 1.1 to 3.0, Y contents from 3 to 39 ppm, Yb from 0.3 to 3.7, and Lu with contents in the range 0.06 and 0.54 ppm. Such geochemical features are similar to those of Archaean tonalitic rocks and are usually described in rocks formed by partial melting of mafic rocks under high-pressure conditions leaving an eclogitic residue. The presence of diorites and hornblendites, associated to tonalitic and trondhjemitic gneisses suggests, on the other hand, that tonalitic magmas could be formed by hornblende-controlled fractionation of hydrous basaltic magmas. Tonalitic gneisses show U-PbSHRIMP zircon age of 2, 263 ± 6 Ma for igneous crystallization and 2, 045 ± 10 Ma for the metamorphism. The geochemical parameters, tectonic and geochronological features of Encantadas Complex are consistent with magmas derived from mantle metasomatized by subduction-related fluids in a continental-arc.

O Complexo Encantadas é constituído por gnaisses tonalíticos, trondhjemíticos e dioríticos com presença subordinada de hornblenditos. Os elementos maiores e traços indicam uma composição metaluminosa a fracamente peraluminosa relacionada às séries cálcico-alcalinas médio-K. Os parâmetros composicionais são consistentes com uma evolução associada provavelmente a cristalização fracionada a partir de magmas menos diferenciados. Os ortognaisses mostram razões LaN/YbN variando entre 10 até 50, K2O/ Na2O entre 1,1 e 3,0, com conteúdos de Y entre 3 e 39, Yb entre 0,3 e 3,7 e Lu entre 0,06 e 0,54. Estas feições são similares às descritas em tonalitos arqueanos, usualmente atribuídas à fusão parcial de seqüências máficas sob condições de alta pressão deixando um resíduoeclogítico. A presença de dioritos e hornblenditos, associados aos gnaisses tonalíticos e trondhjemíticos sugere, por outro lado, que os mesmos podem representar rochas com uma evolução comum, geradas provavelmente por fracionamento de hornblenda a partir de magmas básicos hidratados. Os tonalitos apresentam idade U-PbSHRIMP em zircão de 2,263±6Ma para a cristalização e 2,045±10Ma para o metamorfismo. Os parâmetros geoquímicos, tectônicos e geocronológicos são consistentes com magmas derivados de manto metassomatizado por fluídos relacionados com subducção oceânica em uma margem continental ativa.