Basalt magmatism from Alagoas Stage (Campos Basin)

Abstract

Magmatism associated with the pre-evaporitic stage of the Campos Basin was studied through the boring cores of four boreholes. They are important in the tectonic and stratigraphic evolution context of the basin, and this study aimed to characterize these rocks based on petrography, lithochemistry, mineral chemistry and Sr-Nd isotopic composition. The application of conventional study methodologies was difficult due to the degree of alteration of the rocks, evidenced by mineralogical changes (replacement of minerals by clay minerals, palagonitization of volcanic glass, sericitization of plagioclase and presence of vesicles), high loss on ignition (LOI) and identification of sandy sediments enclosed by volcanic rock. In order to minimize the alteration effects, methodologies were used that allowed evaluating the degree of alteration of the samples and chemical element mobility in the whole rock. The results establish that the volcanic rocks are basalts derived from magmas with subalkaline composition, tholeiitic to transitional, low TiO₂ and high magnesium. The rare earth elements (REE) and the distribution pattern of trace elements are in conformity with the standard tholeiitic basalts of low TiO₂ of the Serra Geral Formation and are different from the Ocean Atlantic enriched-mid ocean ridge basalt (E-MORB) and ocean island basalts (OIB), although some elements have similar behavior. The dominant pyroxene is augite, which chemically confirms that the basalts belong to the subalkaline series and the transitional series between the calc-alkaline and tholeiitic series. Isotopically, the studied samples plot out of the mantle array field and follow the behavior of the Campos Basin basalts. The collected parameters establish the correlation of the volcanic rocks of Alagoas Stage in Campos Basin with the intracontinental basalts of Paraná Basin.