Geology, petrology and geochronology of sierra Valle Fertil - La Huerta batholith: Implications for the construction of a middle-crust magmatic-arc section

Abstract

The Valle Fertil-La Huerta batholith is a differentially tilted, compositionally stacked, calc-alkaline plutonic sequence in the central Famatinian arc of Argentina. It consists of two major lithologic units: these are (1) an intermediate unit largely dominated by amphibole- and biotite-bearing tonalites but encompassing from gabbro to leucogranite; and (2) an overlying silicic unit that shows the same petrological diversity as the intermediate unit, but granodiorite and monzogranite are its prevailing lithologies. The silicic unit is separated into four subunits using lithological changes at regional mapping scale. All the boundaries are gradational among lithologic units and subunits. The internal fabric along the entire batholith is either magmatic or submagmatic and it was developed during the arc formation. The original magmatic foliation is north-south striking and steeply-dipping. The magmatic foliation crosses over and transposes the gradational contacts among lithologic units. Petrological architecture, structural geology, geobarometric estimates and geochronology show that the entire batholith is a comagmatic system exposing more than 13 km of a middle arc crust. Crystallization-fractionation models computed using whole rock and mineral chemistry account for the mineralogy of plutonic rocks and coincide with the results of experimental petrology. Mass-balanced modeling predicts that the unexposed cumulate roots of the Famatinian arc should have been about 1.3 times larger than the intermediate and silicic plutonic batholith. However, the mass of calculated cumulate would diminish significantly if a portion of the host metasedimentary material were consumed in constructing the batholith. The batholith grew progressing upward from the lowest levels and acquired a stratified compositional organization. The driving mechanism was continuous influxes of mantle-derived mafic magmas that made up the lower crust, caused crustal-level melting and magma mixing, and formed intermediate - silicic rocks at increasingly shallower depths.