Crystalline basement from Laguna Amarga metamorphic complex in the high Andes of western Catamarca, Argentina (27° 15’ - 27° 40’ south): Petrology, structure and geodynamic implications

The Laguna Amarga Metamorphic Complex is a major stratigraphic component of the southern Central Andes. The occurrence of crystalline basement pieces extensively exposed in the western Andes of Catamarca (Argentina), has been known for decades, nevertheless, a lack of detailed field-based and geochronological survey has precluded an accurate correlation of the Laguna Amarga Complex with other basement exposures in the Central Andes. The Laguna Amarga Metamorphic Complex outcrops in several fault-bounded blocks spread over about 360 km2, always underlying Neopaleozoic strata and Neogene volcanic strata from the Central Volcanic Zone. The northern part of the complex consists of amphibolite-facies metamorphic rocks derived from sedimentary and igneous precursors grouped in an association named Cazadero Grande. The original stratigraphic succession was interbedded siliciclastic sediments, carbonaceous, and mafic rocks. The southern segment of the complex is dominated by granulite-facies migmatites and ortho-amphibolites grouped in an association called Los Aparejos. In the Cazadero Grande Association, four deformational events are recognized. The first three phases gave rise to an imbricate S-verging over-thrusting system and prevailing tectonic transport to the south in response to progressive non-coaxial strains. The mineral assemblages linked to the metamorphic climax equilibrated coeval with the early stages of fabric-forming deformation. The posthumous fourth phase of deformation produces gentle folds with kilometer-scale wavelengths whose axial planes (non-penetrative) have an NNE strike, and are as related to Andean deformation. Three incremental deformation events coeval with the metamorphic climax are registered in the Los Aparejos Association that indicate a kinematics transport to the east. First U–Pb zircon ages are reported for the Laguna Amarga Metamorphic Complex. Age distributions are polymodal, but the dominant populations make well-defined clusters. The most prominent concord age peak is at 389 Ma, revealing the age of the metamorphic climax. Another noteworthy aspect is the presence of dominant Mesoproterozoic concord age peaks (1119-1082 Ma), interpreted as inheritance ages of their protolith. A point to highlight is the absence of ages reflecting the source with Brasiliano-Pan-African orogenic ages that dominates detrital provenance from the West Gondwana. The Mesoproterozoic protolith ages allow the correlation of this metamorphic complex with nearby basement detached from Laurentian realms. The analysis of the ages shows that the metamorphic peak and the generalized deformation that affected the entire basement of the complex developed in the Middle Devonian (Eifelian), this being a very little known event for the Paleozoic evolution of western Gondwana. The work discusses how a better understanding of the crystalline basement of the high Andes impacts the geodynamic history of the proto-Andean Gondwana margin. It also provides new information on the geodynamic configuration of the Lower to Middle Paleozoic and its link with the processes of accretion-collision of terranes.