Experimental constraints on the pre-eruptive conditions of the Caxias do Sul dacite: Implications for high temperature silicic volcanism of the Paraná Magmatic Province, Brazil

The Paraná-Etendeka Magmatic Province is the second largest continental large igneous province (LIP) known. It is characterised by the occurrence of an unusually high proportion (∼3%) of silica-rich magmatic products (dacitic to rhyolitic in composition). Here we provide the first experimental constraints on the pre-eruptive crystallisation conditions of the Caxias do Sul dacite (CSd), the oldest silicic end member of the Palmas-type volcanic sequence of the Paraná Magmatic Province. We use crystallisation experiments to define the phase assemblage of the CSd and compare our experimental results with observations from natural samples to constrain pre-eruptive conditions. We find that the crystallisation conditions are consistent with temperatures between 975 and 1000 °C, pressures of 150–350 MPa and a H2O content of ∼2 wt%. Quantitative textural analysis of natural products provides an estimated crystal content of 15–29 vol%, which likely crystallised within the storage region prior to magma ascent. The experimentally estimated high temperature and low H2O content suggest that low undercooling (ΔT) was maintained within the conduit, accompanied by the sole crystallisation of syn-eruptive microlites, generating minimal rheological change during magma ascent. Compared with other dacitic compositions, the CSd represents an atypical, high temperature and H2O-poor end member and possessed a comparatively low viscosity.