Geochemical characterization of the El Cortijo Formation metacherts: New evidence of an oceanic rock sequence in the Palaeoproterozoic basement of the Tandilia Belt, Argentine, Río de la Plata Craton.

LAJOINIE, M.F.; LANFRANCHINI, M.E.; CINGOLANI, C.A.; SALVIOLI, M.A.; DE LA CAL, H.G.; ETCHEVERRY, R.O.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2021

0895-9811

Archaean-Palaeoproterozoic cherts are rare and contain valuable information to understand the chemical evolution of oceans in the past, given that during most of this period an abiological deposition process has been interpreted for their formation, in which silica precipitated directly from seawater. In the paleoproterozoic basement of the Tandilia Belt (Argentina), which is a portion of the Río de la Plata Craton (RPC), cherts have been mentioned as part of the El Cortijo Formation (ECF). They include low-grade metamorphic rocks, such as metacherts, metawackes and metabasites. In order to provide a conclusive evidence of the occurrence of marine rocks, detailed geochemical analysis and petrographic and field studies were performed. They allowed two groups of metacherts to be identified: gray-whitish (GWM) and green-bluish metacherts (GBM). The GWM present nodular and lens morphologies and are mainly composed of micro and mesoquartz crystals with evidence of recrystallization processes. The GBM show a layered structure that consists of micro and meso quartz crystals recrystallized in fine bands, interspersed with bands composed of epidote, zoisite, chlorite and sericite. The multielement diagram (normalized to post-Archaean Australian shale) shows positive anomalies of La, Eu and Y and a slight enrichment of the heavy rare earth elements (HREE) with respect to the light rare earth elements (LREE), similar to seawater signatures, for the GWM. Eu anomalies between 1.2 and 1.4, a flat distribution of LREE and depleted contents of HREE are shown for the GBM. These characteristics indicate that the GWM derived from C-Cherts (chemical cherts) sensu-stricto, while the GBM protolith is compatible with silicification of volcanic rocks, or S-Cherts (silicified cherts), also considering the high lithophile element contents. Furthermore, the Ge/Si atomic ratios, from 1.48 x 10-6 and 4.79 x 10 -6 mol/Kg, and Y/Ho ratios lower than 43.36 (for both metachert types) indicate an influence of hydrothermal silica source. The cherts identified in the ECF have revealed the occurrence of a stratigraphic complex of oceanic plate immersed in a thickened continental crust that was developed by tectonic accretion, on the southern margin of the RPC.