Mineral Chemistry and Petrogenesis of the Puesto La Peña Undersaturated Alkaline Potassic Complex, Mendoza, Argentina

Abstract

The late Early Miocene Puesto La Pena alkaline potassic complex, Mendoza, Argentina, is a multiphase intrusion with a large variety of rock types (including pyroxenite, malignite, borolanite, syenite, mafic to felsic dikes, and volcanic necks). This study focuses on the mineralogy of the various lithologies and the mineral chemical data in order to define a petrogenetic model that explains the fractionating phases of the complex. Geothermobarometric data and characteristics of and variations in mineral composition are in agreement with a parental magma that began the crystallization process at around 35–40 km depth with initial temperatures of around 1350 °C. After cooling to about 900 °C, a fractional crystallization process was dominant in a crustal environment at moderate to shallow depths, the ascent of magma being related to crustal extension. Crustal contamination seems to have been limited. In the proposed model, pyroxenite is a cumulate phase of the original intrusion, being comagmatic with the malignite-syenite. The late-stage evolution of the complex involves evolution towards the calcite liquidus field, which explains the presence of clusters of calcite-dolomite crystals and carbonatitic dikelets associated with late ouachititic dikes. The mineral composition reflects an environment of crystallization at moderate to shallow depths, low pressure, low silica, and high temperature, which is consistent with the model for the complex, considered to represent the root zone of an alkaline caldera-style volcano emplaced in an extensional back-arc setting.