Late Famatinian (440–410 Ma) overprint of Grenvillian metamorphism in Grt-St schists from the Sierra de Maz (Argentina): Phase equilibrium modelling, geochronology, and tectonic significance

Abstract

The analysis of major and trace elements in zoned minerals is useful for deciphering parts of the tectonothermal evolution of polymetamorphic tarrain. We applied this approach to the Maz Metasedimentary Series in Western Sierras Pampeanas of Argentina, where polymetamorphism resulted in the overprinting of a Grenvillian basement (the Maz Complex) during the pervasive Rinconada tectonic phase of the Famatinian orogeny. The older metamorphism (M₁) is assigned to the youngest Grenvillian metamorphic event recognized in this basement at c. 1035 Ma, whereas the Rinconada metamorphism (M₂) was Silurian to early Devonian, essentially between 440 and 410 Ma. The latter resulted from oceanward migration of the orogenic front relative to earlier late Cambrian to Ordovician (490–470 Ma) tectonic phases of the Famatinian orogeny. The M₁ and M₂ metamorphic events have been recognized in a staurolite-garnet schist from the Maz Metasedimentary Series. Most metamorphic minerals from this rock were formed during the M₂ event which was of the Barrovian type (±kyanite). Part of the metamorphic P–T evolution is recorded in the complex compositional zoning of garnet porphyroblasts. Three types of garnet were identified based on texture and chemistry, including trace elements (REEs). Phase equilibrium analysis, compositional isopleth, and multi-equilibrium thermobarometry were applied in order to establish the P–T history. M₁ is represented by preservation of Grt₁ ± Kfs ± Sil, with peak P–T condition of 790°C and 5.2 kbar, that is, granulite facies. This early metamorphic event was related to a deformational D₁ episode represented by a relict S₁ foliation. The latter is preserved as aligned inclusions in staurolite porphyroblasts and as relics of an older crenulated foliation in microlithons from the matrix. M₂ followed a clockwise P–T path with three mineral growth stages. The earliest occurred at ~585°C and ~8.7 kbar and is represented by Grt₂ ± St₁ ± Bt₁ + Qz. Grt₂ was partially coeval with growth of St₁, which was stable at ~625°C and 9.0 kbar. Grt₂ + St₁ are syn-kinematic to the main S₂ foliation (D₂ episode). Subsequently, decompression (D₃) started as St₂ (+ Bt₂ + Ms₁ + Qz + Pl) crystallized, and garnet was partially consumed at ~612–620°C and ~7.3–7.7 kbar. St₃ + Grt₃ crystallized at ~608°C and ~6.8 kbar at the end of D₃. Increasing P–T conditions during the earlier M₂ growth stage suggest burial of the Maz Metasedimentary Series, probably linked to tectonic thickening by underthrusting (tectonic phase D₂). Peak metamorphic conditions were attained during thrust stacking. The tectonic phase (D₂) was responsible for the main S₂ foliation, which was penetrative at all scales. The nearly isothermal decompression path is compatible with exhumation, probably resulting from extension along discrete shear zones (tectonic phase D₃) that produced a mylonitic foliation S₃.