Laguna Amarga Metamorphic Complex (27° 15' - 27° 40' South), Catamarca Argentina: geochronology and geodynamic implications

OTAMENDI J E; CRISTOFOLINI E A; MOROSINI A F

Congreso; XVI Congreso Geologico Chileno; 2023

The crystalline basement ofLaguna Amarga Metamorphic Complex (LAMC) occurs as a mosaic of fault-boundedblocks in the Cenozoic eruptive volcanic field of the Andes of Catamarca. Thesebasement blocks are located between the Cazadero Grande River and the ValleAncho depression, around 150 km from Fiambalá, Catamarca. The outcrops havebeen mapped based on the correlation of the rock types and structural evidenceas a new stratigraphic nomenclature (LAMC), and subdivide in Cazadero Grandeand Los Aparejos associations. The Cazadero Grande Association is characterizedby amphibolite-facies metasedimentary rocks and ortho-amphibolites, grouped inthree lithological units: 1) Aguada de Tambillo Unit, composed of metamorphosedsiliciclastic rocks, including gneisses, quartzites, and cherts; 2) PuestoVerde Unit, constituted by marbles and calcsilicate gneisses; and 3) ElQuemadito Unit, dominated by ortho-amphibolites. The Los Aparejos metamorphicassociation is divided into two granulite-facies lithostratigraphic units: 1)Lagunas Frías Unit, consisting of metapelitic and metapsammitic migmatites withsubordinate marbles; and 2) Pirca Negra Unit, formed by meta-mafic rocks. The combination of apetrological-structural analysis, and detrital-metamorphic zircon ages isrelevant to the stratigraphic correlation of the LAMC at a regional scale. Inorder to know the age of the metamorphic peak and the ages of inheritanceassociated with their provenance areas, zircons from five metasedimentary rocksof siliciclastic protoliths (paragneiss, metagreywacke, metaquartzite,leucosome and schollen of metatexite,) were dated by U-Pb isotopes (LA-MC-ICPMS).Zircons occur in all samples as long prismatic to slightly elongated crystalswith straight or rounded edges that are 50-200 μm long and 25-70 μmthick. In cathodoluminescence images, the grains display oscillatory orsector-zoned, dark or bright cores surrounded by rims up to 30 μm thick,often truncated by homogeneous or oscillatory dark rims. In all samples, theU-Pb zircon age distributions show that the dominant populations makewell-defined clusters. In three samples the analyses on homogeneous zircon rimsgive a concordant age peak at 389 Ma. These younger peaks record a metamorphicevent (the metamorphic cycle happened under amphibolite to granulite facies atmean P-T condition) and the evolution to the nappe system that affected theentire LAMC during Middle Devonian. Analysis of zircons core in all samples isconcordant with a statistically strong constrained age range between 1000 and1370 Ma (concord age peaks 1119-1082-1071-1065-1031 Ma). The LateMesoproterozoic ages (1100-1000 Ma) are interpreted as the inheritance ages oftheir sedimentary protolith. The absence of typical zircon age of Gondwanalandmasses (650-600 Ma; Late Neoproterozoic Basiliano-Pan-African orogenicages), makes impossible a Gondwana provenance for the detritus that formed theLAMC protoliths. Consequently, the origin of the detritus that constructed themetamorphic sequence is consistent with a Laurentian affinity source arealinked with the southern Appalachian Grenvillian and the Granite-RhyoliteIgneous Province. Also, the lack of paleo-Proterozoic ages demonstrates thatthe LAMC is disconnected from the Arequipa-Antofalla terrane and the MARAblock. Based on the prevalence of Mesoproterozoic (Grenvillian) detrital agesand the analogous petrological (P-T-t) and structural style features, weconsider that the LAMC shows similarities in the geological evolution to theexotic Cuyania composite terrane transferred from Laurentia to the westernGondwana.The LAMC study has keyimplications for understating the evolution of the Paleozoic basement betweenthe Famatinian arc and the Andes Cordillera. A narrow area separates theMesoproterozoic blocks from the Ordovician arc exposed in the Sierra de LasPlanchadas. The region between the Chaschuil River and Los Aparejos valleys mayrepresent a geodynamic boundary zone among the autochthonous Famatinian arc andthe allochthonous Cuyania terrane and is considered as the northwardcontinuation of the Valle Fértil lineament. In line with this interpretation,the northern boundary of the allochthonous terrane would extend beyond 28°South latitude.