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 Paran a-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 Paran a 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 degrees C, pressures of 150 - 350 MPa and a H 2 O 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 H 2 O content suggest that low undercooling ( Delta T ) 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 H 2 O-poor end member and possessed a comparatively low viscosity.