CONICET | Buscador de Institutos y Recursos Humanos

The data presented here are a summary of petrographic investigations, geochemistry and geo-chronological data from the main lithological units of the Sierras La Huerta and Valle Fértil, in the Province of San Juan, NW-Argentina, which form part of the Western Sierras Pampeanas. The Sierras de Valle Fértil and La Huerta are linked to a subduction related magmatic arc that experienced granulite facies peak conditions at 860°C and 6 - 7 kbar at Late Cambrian and Early Ordovician. The magmatic suite consists of meta-granodiorites, meta-granites, meta-tonalites and meta-gabbros-gabbronorites that are incorporated into a metamorphic sequence composed of garnet-cordierite-sillimanite-biotite gneisses, migmatites, amphibolites and impure marbles. Applied geo-thermobarometry on the gneissic and gabbroic lithologies results in peak-temperatures of 800 - 850°C at 6 - 7 kbar. The lack of retrograde evidence, such as hydration reactions, in the gneiss and marble units, implies that continuous updip fluid release may have led to almost fluid absent high temperature paragenesis, where only biotite is representing a H2O bearing phase in the pelitic host. Melting textures replacing biotite are commonly found associated with sillimanite. All the investigated magmatic bodies experienced slow cooling at still intermediate crustal levels that led to metamorphic recrystallization. Corona reaction textures composed of orthopyroxene pargasitic amphibole - clinopyroxene and spinel occur at the olivine - plagioclase interface in gabbroic rocks. Magnetite - orthopyroxene symplectites consume olivine in gabbroic rocks. This is evidence for increasing oxygen fugacity quite above the QFM - buffer indicated by the reaction 6(Mg,Fe)2SiO4 + O2 = 2Fe3O4 + 6MgSiO3. An isobaric cooling trajectory is documented from the related marbles units, where coronal grandite reaction rims are developed around scapolite and at the immediate scapolite - wollastonite interface. The isobaric cooling trajectories are consistent with an advective heat input originated from the related magmatic bodies (e.g. tonalites and gabbros) that is responsible for a "regional contact-metamorphic" high temperature, intermediate pressure event. Magmatic zircon ages of the meta-tonalites yield concordant Cambrian-Ordovician ages of 479.7 ± 3.2 Ma, which can be considered to be the age of peak-temperature metamorphism. Hence subduction related magmatism was still active at the beginning of the Ordovician.

enviar mensaje