Shifts in the source of the Paraná basalts through time

Magmatism occurred intermittently in the Paraná region of South America between 240-50 Ma, with the peak of magmatic activity between 138-126 Ma, comprising the Paraná continental flood basalts (CFB). Detailed chemical subdivision of the Paraná basalts into six units allowed stratigraphic correlations to be made, implying a northward migration of magmatism following the rifting of the South Atlantic. However, ⁴⁰Ar-³⁹Ar ages do not uphold the chronostratigraphic nature of such magmatic units, instead they indicate a southeastwards migration. Major elements (extrapolated to 8 wt.% MgO) are used to investigate changes in magma type with time, and are interpreted in terms of parameters such as pressure, depth of melting and the nature of the source region (fertile/refractory and water content). The oldest Paraná unit originated by melting at the greatest depths (90-120 km), shifting to shallow levels (30-60 km) for the younger units. Accompanying this decrease in pressure, the source material undergoing partial melting became progressively more refractory in major elements and also more hydrous. This hydrous metasomatism occurred after major element depletion of the lithosphere at shallow levels. The data is consistent with conductive heating of a stratified source region, inferred to lie within the lithospheric mantle, which is relatively fertile but anhydrous at depth, and also appears to be younger in age than the relatively major element refractory, hydrous lithosphere at shallower depths. Thus the interaction of asthenosphere and lithosphere can be determined within an integrated age, tectonic and thermal framework.