SHIFTS IN THE SOURCE OF THE PARANA BASALTS THROUGH TIME

Magmatism occurred intermittently in the Parana region of South Americ a between 240-50 Ma, with the peak of magmatic activity between 138-12 6 Ma, comprising the Parana continental flood basalts (CFB). Detailed chemical subdivision of the Parana basalts into six units allowed stra tigraphic correlations to be made, implying a northward migration of m agmatism following the rifting of the South Atlantic. However, Ar-40-A r-39 ages do not uphold the chronostratigraphic nature of such magmati c units, instead they indicate a southeastwards migration. Major eleme nts (extrapolated to 8 wt.% MgO) are used to investigate changes in ma gma type with time, and are interpreted in terms of parameters such as pressure, depth of melting and the nature of the source region (ferti le/refractory and water content). The oldest Parana unit originated by melting at the greatest depths (90-120 km), shifting to shallow level s (30-60 km) Ibr the younger units. Accompanying this decrease in pres sure, the source material undergoing partial melting became progressiv ely more refractory in major elements and also more hydrous. This hydr ous metasomatism occurred after major element depletion of the lithosp here at shallow levels. The data is consistent with conductive heating of a stratified source region, inferred to lie within the lithospheri c mantle, which is relatively fertile but anhydrous at depth, and also appears to be younger in age than the relatively major element refrac tory, hydrous lithosphere at shallower depths. Thus the interaction of asthenosphere and lithosphere can be determined within an integrated age, tectonic and thermal framework.