Reconstruction of the Early Ordovician Famatinian arc through thermobarometry in lower and middle crustal exposures, Sierra de Valle Fértil, Argentina

Abstract

The crustal structure of the Famatinian paleoarc is reconstructed by determining the metamorphic crystallization P–T conditions from metasedimentary rocks at various structural levels in the Valle Fértil section. The bulk section exhibits a 15-km-thick arc crustal section. Thermobarometry shows that nested tonalitic and granodioritic plutons constructed the arc crust at depths < 20 km. Dioritic and tonalitic bodies dominated between 20 and 25 km. The deepest exposed paleodepths are formed by cumulate and non-cumulate gabbroic rocks with interlayered quartz diorites that crystallized below 25 km. The boundary between the crust and the lithospheric mantle is not observed and would have been underneath a depth of 27 km. P–T estimates throughout the section reveals high geothermal gradients ranging from 25 °C/km to 35 °C/km. The thermal regime inferred for middle crustal levels is observed in active and ancient magmatic arcs. Thermophysical models predicting the array of retrieved P–T estimates require heat advection conducted by mafic magmas which either emplaced in the lower crust and/or intruded into middle crustal levels. Calculated seismic wave velocities of plutonic rocks dominating progressively deeper paleodepths are used to deduce the internal architecture of the Valle Fértil section. This result indicates that the Famatinian arc had a middle crustal structure very similar to that of the ancient Talkeetna arc; however a rapid increase of seismic wave velocities from ~ 6.3 km/s to > 6.6 km/s is located at deeper depths in the Famatinian arc than in Talkeetna arc. The thickness of a crustal layer dominated by plutonic rocks with low seismic wave velocities (< 6.2 km/s) is 10 km thinner than the crustal layer with similar physical properties in the Sierra Nevada batholith. A putative model for the whole Famatinian arc suggests a total crustal thickness between 30 and 35 km with three distinct layers.