Oxygen isotopic composition of lower cretaceous tholeiites and precambrian basement rocks from the paraná basin (Brazil) : The role of water-rock interaction

Abstract

107 rock samples from the Paraná plateau, southern Brazil (86 tholeiites and 21 various rock-types from the Precambrian crystalline basement) were measured for their oxygen isotopic composition. Both the crustally uncontaminated tholeiites from the Northern Paraná Province and the crustally contaminated basalts from the Southern Paraná Province exhibit quite positive δ¹⁸O-values ranging from +6.0 to +11.5% and from +7.0 to +12.8%, respectively. These values are considerably higher than those expected for rocks deriving from mantle partial fusion (+5.7±0.3%). A simple process of crustal contamination and fractional crystallization cannot explain the results obtained as the oxygen isotopic composition of the samples from the crystalline basement ( +4.8 to +12.7%, mean value +8.8%) cannot explain a δ¹⁸O increase of the basalt magma higher than ∼ 2%o. According to various considerations, we conclude that the positive oxygen isotope values exhibited by the Paraná basalts are the result of secondary post-eruptive hydrothermal exchange processes between rock and ¹⁸O-enriched water. The ¹⁸O-enriched water was substantially formed by isotopic exchange between normal (meteoric) groundwater and either the thick sedimentary sequence underlying the volcanic sequence in the Paraná basin or the crystalline basement rocks.