Abstract

Late Paleozoic accretionary complexes crop out along the coast of south-central Chile (c. 34-41°S). They are composed of two units with contrasting styles of deformation and metamorphism; the Eastern Series (ES), formed by frontal accretion under lowgrade metamorphism, and the Western Series, formed by basal accretion under high P/T metamorphic gradients. The Lumaco and Capitán Pastene formations (c. 38°S) are local denominations for parts of the Eastern Series and crop out <15 km north of the Lanalhue Fault Zone (1). The Capitán Pastene Formation comprises deformed slates and pelitic schists, while the Lumaco Formation consists of poorly deformed slates and fine-grained metasandstones (2). The age of the protolith of the Lumaco Formation has been considered as Silurian based on the occurrence of trace fossils described in metasandstone beds. A garnetbearing pelitic schist of the Capitán Pastene Formation (sample FO2107) and a fine-grained metasandstone of the Lumaco Formation (sample FO2109) were collected and 70 zircon grains from each were dated by U-Pb SHRIMP for provenance analysis. The zircons from sample FO2107 are mainly elongate euhedral grains with pyramidal terminations together with subrounded grains. Importantly, some zircon grains have low U rims and embayments. Sample FO2109 contains prismatic zircons with central cavities and length parallel zoning; low U rims are less common than in FO2107. Both samples show similar provenance patterns with major age peaks at c. 340-345 Ma, subordinate age peaks at c. 330 and 355 Ma, and a less prominent Devonian component of c. 360-380 Ma. A similar, c. 330 Ma, maximum age of deposition for Lumaco and Capitán Pastene formations discards the previous interpretation of a Silurian age for the protolith. The near absence of older cratonic zircon dates and the abundance of euhedral igneous zircon grains implies derivation from relatively local sources. Notably, igneous units with 340-355 Ma crystallization ages in northern Patagonia are unknown. The ES is intruded by the Pennsylvanian Coastal Batholith generating a HT-LP metamorphic overprint, and near the study area c. 310 Ma U-Pb SHRIMP crystallization ages are reported (3). The ages obtained in the low U rims (sample FO2107) are scattered, but some grains yielded concordant Mississippian ages (~325-340 Ma) and are comparable to previously reported metamorphic ages in the basement of north-western Patagonia (4). It is suggested that the studied formations represent different portions of the same paleo-basin that was later deformed and metamorphosed to different extents during the late Carboniferous to Permian Gondwanide tectonic cycle. (1) Glodny et al. (2008). Journal of South American Earth Scinces 26: 397-411. (2) Tavera (1979). Unpublished report. (3) Deckart et al. (2014). Andean Geology 41: 49–82. (4) Renda et al. (2021). Journal of South American Earth Scinces 106: 103045. Financing: This research was funded by Consejo Nacional de Investigación Científica y Tecnológica (presently ANID) through Project Fondecyt N°1180457.