Low-grade metamorphism of Cambro-Ordovician successions in the Famatina belt, Southern-Central Andes: Burial-inversion history linked to the evolution of the proto-Andean Gondwana margin

Abstract

The metamorphic P-T conditions of low-grade units from the Famatina belt, Central Andes of Ar¬gentina, were estimated through petrography, X-ray diffraction, and electron microscopy. For the Middle-Upper Cambrian Negro Peinado Formation a tectono-metamorphic event associated with intense intrafoliar folding, with estimated temperatures between 290 and 400°C (KICIS: 0.16-0.27Δº2θ, biotite blastesis and compositional homo¬geneity in dioctahedral micas) and intermediate pressure conditions (white mica b parameter: 9.010Å-9.035Å), was recognized. The Achavil Formation (Middle-Upper Cambrian) presents a main metamorphic event associated with temperatures between 200 and 290°C (KICIS: 0.26-0.41Δº2θ) and intermediate- to low-pressure conditions (white mica b parameter values: 8.972Å-9.017Å). Some illitic substitution in dioctahedral micas also indicates lower metamorphic grade than the Negro Peinado Formation. For Upper Cambrian to Middle Ordovician sequences a burial metamorphic pattern, with a progressive decrease in metamorphic grade from Volcancito Formation to Cerro Morado Group (ca. 490-465 Ma; KICIS: 0.31-0.69Δº2θ) and absence of tendency changes linked to strati¬graphic discontinuities was proposed. Mica and chlorite are the main phyllosilicates in the oldest units, while Ilt/Sme (R3) mixed-layer is almost the only one in the youngest. White mica b parameter indicates intermediate- to low-pressure conditions for all these sequences. This burial metamorphic pattern presents a marked break as the youngest Ordovician unit (La Aguadita Formation, after ca. 452 Ma) records higher metamorphic conditions (IKCIS: 0.28-0.19Δº2θ) than units from the Ordovician arc, with estimated temperatures between 270 and 330ºC and intermediate-pressure conditions. Our results indicate that basin contraction and inversion processes related to the Ordovician Ocloyic Orogeny involved at least two well-discriminated and not superposed metamorphic episodes in this region.