Petrography of the Quartz Tholeiites from the Misiones Province, Argentina

Abstract

<p>The Paraná basalts of the Misiones Province in Argentina are remarkably similar with respect to their mineralogy and major element chemistry. This is quite remarkable when considering the vast geographic area these samples cover (a base map with sample locations is provided in the back flap). The areal extent of the Misiones Province is approximately 3000km2 and sample localities separated by 300km may display a variation in their silica contents of only 1.5%. From a viewpoint considering the areal extent of some flood basalts, this is not at all uncommon but is what should be expected.</p> <p>The basalts sampled from the Misiones Province are all typical quartz tholeiites with hypersthene appearing as rare phenocrysts but more commonly is a constituent of the glassy groundmass. The essential mineral in all these samples is a calcic plagioclase which accounts for approximately 40% of most samples. One inconsistency of these samples is the relative abundance and nature of the volcanic glassy matrix which is generally controlled by the rate of change in temperature as the magma is brought to the surface where it subsequently crystallizes. This volcanic glass: crystalline phase ratio, being a near surface phenomenon requires a parameter(s) that may account for variations of tholeiitic equilibration at shallow depths. A feasible conclusion to satisfy this observation is that the magma has issued out of numerous fissures, with each fissure providing unique physical parameters that will effect the abundance of late stage quench products such as volcanic glass.</p> <p>To support this hypothesis is the abundance of dikes in the Misiones Province that could serve as fissures for the expulsion of the magma.</p> <p>Therefore, it seems probable that these quartz tholeiites have all been derived from a common source rock and their mineralogical and chemical differences are attributed to their unique near surface environments in which they equilibrated.</p>