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.
Highlights
The Famatina belt, in the Central Andean Region of Argentina (Fig. 1), is part of an ancient subduction-collisional margin that formed along western Gondwana during the Early Palaeozoic (Vaughan and Pankhurst, 2008)
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 homogeneity in dioctahedral micas) and intermediate pressure conditions, was recognized
Petrologic, mineralogical (XRD), and textural analyses (Collo et al, 2005) of metasandstones and metapelites from the Loma de las Damas outcrop indicate that the rocks were affected by a regional low-grade metamorphicdeformational episode whose main blastesis was contemporary with the development of the S1 foliation
Summary
The Famatina belt, in the Central Andean Region of Argentina (Fig. 1), is part of an ancient subduction-collisional margin that formed along western Gondwana during the Early Palaeozoic (Vaughan and Pankhurst, 2008). Within this belt, Middle-Upper Cambrian to Middle-Upper Ordovician deposits constitute an almost continuous succession affected by low-grade metamorphism. Three major stages of sedimentation record the period preceding the initiation of subduction up to the uplift and basin inversion in the final stages of the Ocloyic
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