Early Proterozoic calc-alkaline and Middle Proterozoic tholeiitic dyke swarms from central-eastern Argentina: Petrology, geochemistry, Sr-Nd isotopesand tectonic implications

The Rio de La Plata craton in Argentina (Azul and Tandil regions) is charac terized by Early Proterozoic (2.0 Ga) calc-alkaline and Middle Proterozoic (1.6 Ga) tholeiitic dyke swarms intruding the crystalline basement involved in the Transamazonian Orogeny (2.2-1.9 Ga). The calc-alkaline dykes have a ndesitic and rhyolitic compositions and trend east-west, whereas the tholei itic dykes mainly trend N30 degreesW and are represented by basalts with lo w (0.9-1.7 wt %) and high TiO2 (up to 3.7 wt %). The calc-alkaline dykes ha ve primitive mantle (PM)-normalized trace element patterns enriched in Rb, Ba, K, La, Ce and Nd, and significant negative Nb and Ti anomalies. These d ykes are characterized by epsilon (t)(Nd) values of -3 to -4, similar to th ose of the EMI mantle component. Low-TiO2 tholeiitic dykes have low incompa tible-element (IE) contents and PM-IE patterns with slightly positive or ne gative Nb spikes. They have variable epsilon (t)(Nd) values (-0.5 to 12.1), which mainly reflect derivation from a depleted source mantle. High-TiO2 t holeiitic dykes have more enriched IE-PM patterns and are characterized by epsilon (t)(Nd) values (-1.4 to -7.5) typical of an enriched source mantle. Chemical and isotopic data and melting modelling indicate that both calc-a lkaline and tholeiitic dykes originated by different melting degrees of a h eterogeneous source mantle, the variable IE enrichment of which may have oc curred in Late Archaean to Early Proterozoic times. The emplacement of the calc-alkaline dykes is associated with the transtensional stage of the Tran samazonian Orogeny, whereas the tholeiitic dykes reflect extensional tecton ics succeeding the Transamazonian event. The calc-alkaline and tholeiitic d ykes are similar in emplacement age and characteristics to metamorphosed gr anites and volcanic rocks outcropping within the Namaqua fold belts of sout hwestern Africa (Richtersveld and Witberg-Aggeneys-Gamsberg provinces); thi s may indicate that the Rio de La Plata craton and southwestern Africa were contiguous in Early-Middle Proterozoic times.