Titanite-hornblende alteration recording volatile exsolution: an example from the Famatinian Arc, Argentina

ORTIZ JOYA, GUILLERMO; WEINBERG, ROBERTO F.; DE ABREU BARBOSA ARAUJO, LUCAS EDUARDO; SOLA, ALFONSO; SUZAÑO, NÉSTOR O.; BECCHIO, R. A.

Baveno

Simposio; 10th Hutton Symposium on Granites and Related Rocks; 2023

10th Hutton Symposium on Granites and Related Rocks

The concentration of magmatic volatile phases such as H2O in subduction zone magmas has been estimated in 5 wt. % or more. Despite being a minor component in magma, water especially when carrying F, Cl, S, etc., may play an important role in magmatic processes. For example, it is responsible for increasing magma buoyancy and decreasing its viscosity, it influences phase equilibria and even promotes magma hybridization and mush rejuvenation. Although the final fate of most of the volatile content is exsolution, it is possible to track the evidence of degassing of a pluton through the crystallization of new phases or modification of pre-existing phases, as has been demonstrated by the presence of “aqueducts” of hydrated mineral phases in granitic plutons. Here we present a summary of recent findings in the Archibarca hybrid intrusion of the Ordovician Famatinian Arc in North-western Argentina, where there is an intense alteration of mafic and felsic magmas recorded by the presence of Hbl- and Ttn-rich alteration zones (±Bt ±Ep). The distribution and the structures formed by this assemblage indicate that the alteration is a product of degassing and transfer of volatiles between mafic and felsic members.The Archibarca batholith is a mingled intrusion whose felsic member displays typical calc-alkaline features, but its mafic endmember has an alkaline affinity (4.4-5.4 wt. % K2O at 51-54 wt. % SiO2), and anomalous content of Ba (800-1100 ppm), P2O5 (0.6-0.7 wt. %) and TiO2 (1.8-1.9 wt. %). The contacts between felsic and mafic members are usually irregular and have flame-shaped dark rims (1-10 cm thickness) composed of Hbl+Ttn+Bt+Ep (Graphic 1). This assemblage is also found as patches in hybrid diorites in contact with granitic pegmatites, as mafic rims surrounding Qz-ocelli in hybrid diorites, and even in irregular dikes of metric scale that cut the hybrid rocks (Graphic 2). The observed titanite crystals record in their texture and trace element composition the breakdown of Bt and growth of Hbl, in turn, Hbl phenocrysts are replaced by an assemblage Act+Ep+Ttn.The nature of these alteration zones rich in Hbl (±Act)+Ttn+Bt+Ep suggests H2O-rich and high oxidation conditions related to the exsolution of water-rich fluids in shallow magmatic environments, as supported by thermodynamic calculations done in GeoPS. These findings are part of several structures recording how fluids can modify the textural, mineralogical, and chemical nature of shallow-level plutons.