An Early Pennsylvanian thermal anomaly in the forearc of SW Gondwana: the origin of the Caleta Loa Metamorphic Complex, northern Chile

Abstract

This study presents a comprehensive characterization of the Lower Pennsylvanian Caleta Loa Metamorphic Complex (CLMC), located in the Coastal Cordillera of northern Chile at around 21°30′ S. Through integrated petrographic, geochemical, thermodynamic, and geochronological analyses, the research aims to constrain the pressure-temperature (P-T) conditions of migmatite formation, refine the protolith identity, and assess the regional geodynamic context of the complex. Thermodynamic modelling using P-T pseudosections and mineral composition reveals that the CLMC migmatites formed under upper amphibolite facies conditions, with peak estimates of approximately 3.3-3.4 kbar and 650 °C. These results are consistent with a high geothermal gradient and suggest a crustal depth of ~13 km during peak metamorphism. U-Pb detrital zircon age patterns indicate a protolith sedimentation age no older than ca. 350 Ma (Early Mississippian), consistent with a genetic correlation with the nearby El Toco Formation, a low-grade metasedimentary unit in the forearc of the SW Gondwana margin. Structural data show evidence of compressional deformation, including top-to-the-west shear indicators and westward verging folds, which are interpreted as syn-anatectic features. These observations, combined with the metamorphic and geochronological data, point to a tectono-thermal event in the SW Gondwana forearc during the Early Pennsylvanian (ca. 318-316 Ma), associated with the so-called ‘Toco Orogeny’ and potentially linked to contemporaneous inland magmatic activity. The existence of this event supports a scenario of renewed subduction and/or the end of a magmatic lull along the proto-Pacific Gondwana margin. The current juxtaposition of the CLMC and the El Toco Formation on either side of a major fault suggests significant uplift and exhumation of the former prior to the Late Triassic.