Geology, petrology and mineralogy of the Inti Huasi Granite, southern Sierra de Comechingones, Córdoba

Abstract

The Inti Huasi granite at the southern end of the Sierra de Comechingones is one of three examples of post-Famatinian, alkali-rich, weakly-peraluminous monzogranitic magmatism of the Sierra de Córdoba. According to geophysical data the Inti Huasi granite is largely covered (> 90%) by Quaternary sediments. The sedimentary cover, however, makes it impossible to identify and map satisfactorily all the possible sub-intrusions units which represent individual magmatic pulses that fed the Inti Huasi granite. Nevertheless, rock diversity seems to reflect the effect of high-level differentiation processes. The most common rock type is a biotite ± muscovite bearing monzogranite. Monzogranites are characteristically felsic (SiO₂ > 73 %, and FeO* + MgO + TiO₂ < 2,4), weakly peraluminous (Al₂O₃/(CaO + Na₂O + K₂O = 0,98 - 1,11), and contain ilmenite, sphene, apatite and zircon as accessory minerals. Although the compositional range of the monzogranites is small, it defines a differentiation trend that is essentially controlled by biotite and plagioclase fractionation. Leucogranitic dykes, which intruded monzogranites at a late- to post-magmatic stage and were the feeders of layered aplites, have compositional features typical of liquids formed after significant fractionation. By contrast, muscovite-rich monzogranites occupying a high-stratigraphical position resulted from syn- to late-crystallisation interaction between normal monzogranites and fluids. Genetic links among granitic rocks and biotite-plagioclase rich enclaves are shown by the composition of minerals and the trend of major elements, indicating that enclaves are small cumulates of early crystallised phases trapped by the granitic magma as was proved for the large enclaves of the Achala batholith. Petrological evidence suggests that the granitic magma of Inti Huasi was generated by partial melting of intermediate non-peraluminous rocks residing in intra-continental crust.