Geochemistry and Genesis of the S-Type,Cordierite-Andalusite-Bearing Capillitas Batholith,Argentina

The Ordovician Famatinian-age magmatic cordierite-andalusite-bearing Capillitas batholith, in the Pampean Ranges of northwestern Argentina, encompasses a coarsely porphyritic to equigranular two-mica monzogranite with equigranular, fine-grained, late leucogranitic muscovite-rich facies. This batholith exhibits sharp, discordant contacts with low-pressure biotite-andalusite-cordierite schists of the La Cébila Formation, locally developing biotite-cordierite-muscovite-bearing contact hornfels aureoles.

The two-mica monzogranite contains cordierite, andalusite, and sillimanite, although cordierite and andalusite are more abundant in the leucogranitic equigranular facies. These minerals are not homogeneously distributed and the three minerals are found together only locally. The presence of biotite-rich xenoliths with a high amount of anhydrous aluminum silicates (andalusite, sillimanite) and cordierite, exhibiting textures similar to those of the host monzogranites, suggest that, at least in part, they have been incorporated into the magma and reequilibrated. The pressure during emplacement was probably 4 kbar under near-solidus temperature, thus preserving the anhydrous aluminum-silicate stability under high H₂O activity.

The major-element data indicate a peraluminous calc-alkalic trend with compositional gaps. They attest to the existence of two distinct magma pulses. The chondrite-normalized REE patterns and multi-element spidergrams point to a probable origin by crustal (metasediment?) anatexis for both pulses. Peraluminous granitic magma cannot be a primary melt of metaluminous quartz-amphibolite, since there is a great geochemical homogeneity of all the granitoids. Igneous xenoliths are absent and the isotopic compositions of the granitoids correspond to those of metasedimentary sources. Both major and trace elements point to a collisional tectonic environment of an inner-continental magmatic arc.