Low-Grade Metamorphic Conditions and Isotopic Age Constraints of the La Horqueta Pre-Carboniferous Sequence, Argentinian San Rafael Block

Abstract

The San Rafael block is a pre-Andean geological entity situated in central-western Mendoza Province, Argentina. It is part of the Cuyania composite terrane. In this terrane it is important to consider the units exposed at the ‘pre-Carboniferous’ outcrops because they record geological events before the accretion to Gondwana. One of which is the siliciclastic La Horqueta Formation of an uncertain Lower-Middle Paleozoic sedimentary age. It is characterized by asymmetric, open to similar folds, with southeast vergence. KI values obtained from the La Horqueta Formation in its type area vary from 0.24 to 0.33 Δ°2θ, indicating very low-grade (high anchizonal) to low-grade (epizonal) metamorphic conditions, that increase slightly from south to north. The white mica b-parameter measured (9.016 ± 0.007 Å) suggest an intermediate pressure regime. Whole rock Rb-Sr isochronic ages were obtained on metapelites from the key outcrops in the La Horqueta area (379 ± 15 Ma, MSWD: 1.4) and in the Los Gateados area (371 ± 62 Ma, MSWD: 3.7), indicating that metamorphism and deformation occurred during the Devonian Chanic Orogenic phase, probably related to Chilenia terrane collision. U-Pb LA-MC-ICPMS detrital zircon ages patterns suggest that the La Horqueta Formation received a dominant sedimentary input from Mesoproterozoic sources, minor contributions from cratonic environments of Paleoproterozoic and Neoarchean ages, and finally a younger input from Pampean and Famatinian orogenic belts. U-Pb detrital zircon ages indicate a maximum sedimentation age close to the Silurian-Devonian limit for the La Horqueta Formation.