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Nb spikes. They have variable e t (Nd) values (−0•5 to 12•1), regions) is characterized by Early Proterozoic (2•0 Ga) calcwhich mainly reflect derivation from a depleted source mantle. alkaline and Middle Proterozoic (1•6 Ga) tholeiitic dyke swarms High-TiO 2 tholeiitic dykes have more enriched IE-PM patterns intruding the crystalline basement involved in the Transamazonian and are characterized by e t (Nd) values (−1•4 to −7•5 ) typical Orogeny (2•2-1•9 Ga). The calc-alkaline dykes have andesitic of an enriched source mantle. Chemical and isotopic data and and rhyolitic compositions and trend east-west, whereas the melting modelling indicate that both calc-alkaline and tholeiitic tholeiitic dykes mainly trend N30°W and are represented by dykes originated by different melting degrees of a heterogeneous basalts with low (0•9-1•7 wt %) and high TiO 2 (up to source mantle, the variable IE enrichment of which may have 3•7 wt %). The calc-alkaline dykes have primitive mantle occurred in Late Archaean to Early Proterozoic times. The emplacement of the calc-alkaline dykes is associated with the (PM)-normalized trace element patterns enriched inRb, Ba, K, La, Ce and Nd, and significant negative Nb and Ti transtensional stage of the Transamazonian Orogeny, whereas the tholeiitic dykes reflect extensional tectonics succeeding the anomalies. These dykes are characterized by e t (Nd) values of −3 to −4, similar to those of the EMI mantle component. Transamazonian event. The calc-alkaline and tholeiitic dykes are similar in emplacement age and characteristics to meta-Low-TiO 2 tholeiitic dykes have low incompatible-element (IE) contents and PM-IE patterns with slightly positive or negative morphosed granites and volcanic rocks outcropping within the

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