Decoding geochemicai and gamma-spectrometric signatures from lavas and sand injectites at the base of the Parana volcanic province, Novo Hamburgo, Brazil

摘要

Two fundamental issues in the understanding of processes active at the onset of volcanism in the Parana volcanic province of South America are the description of individual lava flows and the relationships between the lavas and the underlying loose erg sand. In the Novo Hamburgo region, Brazil, four Cretaceous lavas cover the erg and are well exposed in large quarries. One basalt flow (51.52 wt.% SiO_2) and three basaltic andesites (52.28-55.74 wt.% SiO_2) of the low-Ti Gramado chemical type are present. Outstanding, partly silicified sand injectites are also exposed, including 100 m-long (1-2 m thick) extrudites and one sill of similar dimensions (up to 5 m thick) as well as their feeding dikes and smaller sills. The feeder dikes display funnel shapes near the lower contact of the sand extrudites. Gamma-ray emission rates of the four flows are 79, 128, 95, and 97 counts per second from the first to the fourth flow. A vertical basalt dike (47.10 wt.% SiO_2) cuts the Incopel flow in the Sultepa quarry and has 54 counts per second. This evidence indicates the intrusion of fluidized sand from the underlying erg turned into the Guarani aquifer, a prime example for the dynamic interaction of hot water-saturated erg sand with an overlying basalt seal. The Parana volcanic province is unique among large continental flood basalt provinces because of the presence of a heated aquifer underneath the extrusive pile and the geologic evidence of explosion of hot water and its vapour carrying sand into the overlying, cool lavas.