Petrology of the Miocene Igneous Rocks in the Altar Region, Main Cordillera of San Juan, Argentina. A Geodynamic model within the context of the Flat Slab Segment

MAYDAGÁNA L., MARTA FRANCHINIA M., CHIARADIA M., PONS M.J., IMPICCINI A., TOOHEYE J., REYE R

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2011 vol. 32 p. 30 - 48

0895-9811

The Altar porphyry Cu-Au-Mo deposit (31º 29´ S, 70º 28´ W) is located in the Andean Main Cordillera of the San Juan Province (Argentina), in the southern portion of the flat-slab segment (28-33ºS), 25 km north of the world-class porphyry Cu-Au-Mo deposits of Los Pelambres and El Pachón. Igneous rocks in the area have been grouped into the Early Miocene Lower Volcanic Complex -composed of intercalations of lava flows and thin volcaniclastic units that grade upwards to a thick massive tuff- and the Middle-Late Miocene Upper Subvolcanic Suite that consists of a series of porphyritic stocks and dikes and magmatic and hydrothermal breccias. The Lower Volcanic Complex represents an Early Miocene arc in which mantle-derived magmas have evolved at low pressures through plagioclase- and pyroxene-dominated fractional crystallization and assimilation of crustal rocks (AFC). A significant transition in magmatic trace element chemistry occurred with the emplacement of the Upper Subvolcanic Suite Middle- Late Miocene arc. Upper Subvolcanic Suite magmas require a garnet-bearing residual mineral assemblage that indicates higher pressure conditions in the melt generation or equilibration site. The relatively uniform radiogenic isotope compositions suggests a homogeneously mixed crust-mantle contribution in the source region of the Upper Subvolcanic Suite magmas, a process that can be typical of magmatic evolution in MASH-type zones within the lower crust. The geodynamic setting in the Altar region changed from steep subduction during the eruption of the Lower Volcanic Complex to shallow subduction during the emplacement of the Upper Subvolcanic Suite. Considering the reconstructions of the Juan Fernández Ridge track, it is suggested that such an increase in compression and crustal thickness and the intervening change in the geochemical signatures in Altar at ~10 Ma coincided with the arrival of the Juan Fernández Ridge to this area. The abrupt change in the REE signatures from the Early Miocene Lower Volcanic Complex to the Middle-Late Miocene Upper Subvolcanic Suite is due to the magmatic gap between ~20 and ~10 Ma, that is probably related to high rates of crustal shortening. The delay of ~2 Ma observed between the change in the REE signatures in El Indio and Altar could be related to the southward migration of the subducting Juan Fernández Ridge during Middle Miocene times. In the El Indio and Altar regions the arrival of the Juan Fernández Ridge may have played an important role enhancing the shallowing of the slab and favouring the generation of the geochemically peculiar Middle-Late Miocene magmas.