Abstract
The amorphous carbonaceous material (CM) in mafic igneous rocks is important for deciphering the relationships between igneous and hydrocarbon systems in Earth interior. Deciphering of CM in Precambrian rocks it is a challenge because of reworking by later superimposed events but could give insights into evolution of these systems in time. We report the results of combined field and petrological studies, Raman spectroscopy and carbon isotope analysis of well-preserved ca. 2.0 Ga pillow lavas in the Onega Basin, Karelian craton, Eastern Fennoscandian Shield. In these rocks CM is present in both pillow and inter-pillow areas as poorly ordered carbon. In the MORB-like basalts of the Zaonega Formation, CM occurs in post-magmatic veinlets formed via migration of hydrothermal fluids. In the OIB-type basalts of the Suissary Formation, CM is found in amygdales, inclusions in feldspar, and in devitrified volcanic glass indicating a CM deposition via a rapid cooling of C-rich magma. Despite morphological diversity carbon isotopic composition indicates a biogenic origin of CM in both the MORB- and OIB-like basalts. The most likely source was the Paleoproterozoic oilfield in the Zaonega Formation, commonly referred to as the Shunga event.
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