An early Pennsylvanian thermal anomaly at the forearc of SW-Gondwana: P-T constraints and origin of the Caleta Loa Metamorphic Complex, Coastal Cordillera of N-Chile (~ 21°30' lat. S)
Abstract
This study presents a comprehensive characterization of the earliest Late Carboniferous Caleta Loa Metamorphic Complex (CLMC), located in the Coastal Cordillera of northern Chile (~21°30′S). Through integrated petrographic, geochemical, thermodynamic, and geochronological analyses, the research aims to constrain the pressure-temperature (P-T) conditions of its migmatite formation, refine the protolith identity, and assess the broader geodynamic context of the complex.
Thermodynamic modelling using P-T pseudosections and mineral chemistry reveals that the CLMC migmatites formed under upper amphibolite facies conditions, with peak estimates around 3.3–3.4 kbar and 650 °C. These results are consistent with a high geothermal gradient and suggest a crustal depth ca. 12.5 km during peak metamorphism. U-Pb detrital zircon age patterns indicate a protolith sedimentation age no younger than ~350 Ma, supporting a correlation with the nearby El Toco Formation, a low-grade metasedimentary unit part of 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 at the SW-Gondwana forearc during the early Pennsylvanian (ca. 318-316 Ma), likely associated with the so-called ‘Toco Orogeny’. The current juxtaposition of the CLMC and the El Toco Formation across a major fault suggests significant uplift and exhumation prior to the Late Triassic.
On a regional scale, the CLMC represents a hitherto isolated early Pennsylvanian thermal anomaly within the forearc of SW-Gondwana, potentially linked to contemporaneous inland magmatic activity. Its formation supports a scenario of renewed subduction and/or the end of a magmatic lull phase along the proto-Pacific Gondwana margin.
Thermodynamic modelling using P-T pseudosections and mineral chemistry reveals that the CLMC migmatites formed under upper amphibolite facies conditions, with peak estimates around 3.3–3.4 kbar and 650 °C. These results are consistent with a high geothermal gradient and suggest a crustal depth ca. 12.5 km during peak metamorphism. U-Pb detrital zircon age patterns indicate a protolith sedimentation age no younger than ~350 Ma, supporting a correlation with the nearby El Toco Formation, a low-grade metasedimentary unit part of 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 at the SW-Gondwana forearc during the early Pennsylvanian (ca. 318-316 Ma), likely associated with the so-called ‘Toco Orogeny’. The current juxtaposition of the CLMC and the El Toco Formation across a major fault suggests significant uplift and exhumation prior to the Late Triassic.
On a regional scale, the CLMC represents a hitherto isolated early Pennsylvanian thermal anomaly within the forearc of SW-Gondwana, potentially linked to contemporaneous inland magmatic activity. Its formation supports a scenario of renewed subduction and/or the end of a magmatic lull phase along the proto-Pacific Gondwana margin.
Keywords
migmatites, SW-Gondwana, Carboniferous, P-T pseudosections
PDF
SUPP MAT1
This work is licensed under a Creative Commons Attribution 4.0 International License.
