3D model of the El Hornito pluton, Sierras Pampeanas of Argentina: Evidence of hybridization in the re-used feeder channels for emplacement in the ductile-brittle level

Abstract

A comprehensive structural, petrological, and geophysical study of the 385 ± 2 Ma Devonian El Hornito Pluton (EHP) in the Sierra Grande de San Luis, Argentina, is presented. We establish that the EHP results from the intrusion of three main magma batches. The three-dimensional shape of the pluton was modeled using stochastic litho-constrained gravity data inversion and was complemented with a field geology survey. Results suggest a shallow-rooted, irregularly shaped pluton with a mean depth of 1.5 km, 16 km long and 4 km deep with a volume of 217 km³. Two areas of negative residual Bouguer anomaly reveal the existence of two root zones interpreted to indicate feeder channels. One of the feeders does not have a surface expression, but the other coincides with the central part of the pluton. A hot mafic magma intruded into and rejuvenated a crystallizing granite mush and the thermal perturbations triggered local convection with the development of an ascending magma plume developing mixing/mingling structures. Pluton emplacement was controlled by a shear zone at the boundary between two major metamorphic units. Emplacement is evidenced by the location of the two root zones along the interpreted contacts of the metamorphic zones, and the shear zone along this basement anisotropy provided the space into which magma batches were emplaced. Internal and external structures indicate that space for pluton emplacement was generated by a combination of tectonism with ballooning and stoping assisted by floor depression, which were the most effective processes during the final emplacement of the magma. This was possible due to the contrasting rheological responses of the host rocks.