The El Cortijo Formation: Ocean Plate Stratigraphy in the Tandilia Belt Paleoproterozoic basement (Argentina): Insights into subduction-accretion processes in the southern sector of the Río de la Plata Craton

Abstract

An Ocean Plate Stratigraphy (OPS) sequence is a record of the oceanic crust geological evolution which allows the identification of paleo-oceans preserved in suture zones of orogenic belts. In the Paleoproterozoic basement of the Tandilia Belt (Argentina), Río de la Plata Craton, the El Cortijo Formation (ECF) crops out in a large suture zone with an E-W orientation and vertical to subvertical inclination. It includes a low-grade metavolcano-sedimentary sequence constituted by metacherts, metabasites, metacarbonate, metawacke, and phyllite. In order to provide genetic interpretations of the ECF, based on the OPS model, petrographic, geochemical, and geochronological studies of some of these rocks were performed. Mineral assemblages identified in metabasites, metawacke, and phyllite indicate a transition from prehnite-pumpellyite to greenschist facies conditions. Metabasites, classified as metavolcanite of intermediate to basic composition, chlorite schist and tremolite-actinolite schist, are interbedded between metacherts and metacarbonate bands. Geochemical analyses revealed particular chemical fingerprints which indicate that the protoliths of these metabasites would have been part of a volcanic arc rock association consistent with adakites, Nb-enriched basalts, high-Mg andesites, and “normal” tholeiitic to calc-alkaline basalts and andesites. The metawacke and the phyllite seem to derive from wackes and semi-pelites, respectively, whose main source is consistent with igneous-metamorphic rocks of acidic to intermediate composition linked to a continental arc system. The first LA-ICP-MS U–Pb data obtained for detrital zircons of the phyllite from the ECF yielded a weighted average age of 2138 ± 9 Ma and a youngest individual 207Pb/206Pb spot age of 2047 ± 22 Ma, which locates the maximum sedimentation age in the early Orosirian, during the maximum tectono-magmatic activity of the Tandilia Belt. According to Lu–Hf data, phyllite zircons derived from crustal sources and yielded Archean Hf model ages. Moreover, εHf(t) data of this rock exhibit a more subchondritic signal than the corresponding for some plutons of the Tandilia Belt, which could be indicating that zircons were not derived from plutons but are instead representative of the host rocks of such plutons. In this sense, the information indicates that the top of the sequence, constituted by the metawacke and the phyllite, represents the arrival of the oceanic plate to the trench zone. The study of the ECF has revealed conclusive evidence to postulate that the whole unit corresponds to the upper part of an OPS sequence of convergent type linked to intra-oceanic arc and continental fore-arc trench tectonic environments. The presence of this oceanic sequence, now preserved in a subduction-accretion complex, allows the Tandilia Belt to be classified as a Paleoproterozoic Pacific-type orogenic belt.