DESCRIPTION AND PETROGENESIS OF THE PARANA RHYOLITES, SOUTHERN BRAZIL

The Parana 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 aphpric (<5% phenocrysts), in contrast to the plagioclase-rich Chapeco 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 Chapeco group. Calculated eruption temperatures are unusually high for silicic magmas (950-1100 degrees C), and lie within the range of temperatures for the associated flood basalts. Chemically, the Palmas and Chapeco' group rhyolites are clearly distinguishable, with the mast striking feature being the higher high field strength elements, notably Ti, in the Chapeco group. This mirrors the well-documented low- and high-Ti division of the Parana 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 Parana Basin, and low-Ti in the south. The Chapeco group have Sr and Nd isotope ratios which overlap with those of the high-Ti basalts (Sr-87/Sr-86(130) 0.705-0.708), whereas the Palmas group exhibit a range towards high Sr isotope ratios (Sr-87/Sr-86(130) 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 Parana 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 an the clear genetic link between the Palmas rhyolites and the low-Ti basalts, and the Chapeco' rhyolites and the high-Ti basalts. The Chapeco rhyolites are modelled as partial melts (similar to 30%) of underplated high-Ti basalts, rather than fractionates, primarily because of the time gap between eruption of the high-Ti basalts and Chapeco 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 Parana rhyolite groups from basaltic parents with moderately low water contents. Pressure calculations suggest shallower ponding for the Palmas magmas than for the Chapeco 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 Chapeco group.