Description and Petrogenesis of the Paraná Rzhyolites, Southern Brazil

The Paraná continental flood basalt province is a voluminous bimodal volcanic sequence, with <5% silicic rocks (‘rhyolites’) lying on top of the basalts, concentrated towards the South Atlantic margin. Petrographically, the rhyolites have an anhydrous mineralogy (plagioclase, pyroxene, Fe–Ti oxides), and. two distinct groups are defined on the basis of phenocryst abundance. The Palmas group rhyolites are almost aphyric (<5% phenocrysts), in contrast to the plagioclase-rith Chapecó group rhyolites (<25% phenocrysts). The plagioclase and clinopyroxene phenocrysts in the Palmas group rhyolites are rounded and poorly preserved, and are compositionally less evolved than those in the Chapecó group. Calculated eruption temperatures are unusually high for silicic magmas (950–1100°C), and lie within the range of temperatures for the associated flood basalts. Chemically, the Palmas and Chapecó group rhyolites are clearly distinguishable, with the most striking feature being the higher high field strength elements, notably Ti, in the Chapecó group. This mirrors the well-documented low- and high-Ti division of the Paraná basalts, and in addition there is a geographic correlation between the low- and high- Ti basalt and rhyolite provinces, with high-Ti volcanics predominating in the north of the Paraná Basin, and low-Ti in the south. The Chapecó group have Sr and Nd isotope ratios which overlap with those of the high-Ti basalts (87Sr/86Sr130 0⋅705–0⋅708), whereas the Palmas group exhibit a range towards high Sr isotope ratios (87Sr/86Sr130 0⋅714–0⋅727), continuing the trend of the low-Ti basalts to more radiogenic values. This suggests that assimilation of radiogenic material has occurred. Both rhyolite groups plot away from the isotopic fields for crustal basement types beneath the Paraná, thus an origin by simple crustal melting is discounted. Based on petrographic, chemical and isotopic data, petrogenetic models for the two rhyolite groups are developed, focusing on the clear genetic link between the Palmas rhyolites and the low-Ti basalts, and the Chapecó rhyolites and the high-Ti basalts. The Chapecó rhyolites are modelled as partial melts ( ∼30%) of underplated high-Ti basalts, rather than fractionates, primarily because of the time gap between eruption of the high-Ti basalts and Chapecó rhyolites. However, the Palmas rhyolites are almost coeval with the low-Ti basalts, and are modelled as the products of open-system fractional crystallization from these low-Ti basaltic magmas. In addition, this low-Ti suite shows a continuous trend from basalt to rhyolite in highly incompatible elements such as Zr and Hf consistent with a liquid line of descent, whereas the high-Ti magmas have a substantial gap in the concentration of these elements between the basalts and rhyolites. Experimental data support the derivation of both Paraná rhyolite groups from basaltic parents with moderately low water contents. Pressure calculations suggest shallower ponding for the Palmas magmas than for the Chapecó magma (<5 kbar vs 5–15 kbar), and the style of eruption inferred for the two groups is explosive (rheoignimbritic) for the Palmas group, and effusive (lava flows) for the Chapecó group.