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

摘要

Archaean-Palaeoproterozoic cherts are rare and contain valuable information to understand the chemical evo-lution 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 Palaeoproterozoic basement of the Tandilia Belt (Argentina), Rio de la Plata Craton (RPC), cherts have been mentioned as part of the El Cortijo Formation, which includes low-grade metamorphic rocks, such as metacherts, metawackes and metabasites. In order to provide conclusive evidence about the origin of this unit, detailed geochemical analysis and petrographic and field studies of the metacherts 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 mesoquartz 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) to 4.79 x 10(-6), and Y/Ho ratios lower than 43.36 (for both chert types) indicate an influence of hydrothermal silica source. The study of the El Cortijo Formation cherts has revealed conclusive evidence about the origin of these marine rocks allowing a reinterpretation of the whole formation as an Oceanic Plate Stra-tigraphy complex in the Tandilia Belt basement, southern margin of the RPC.