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Journal of Petrology Volume 42 Number 11 Pages 2109-2143 2001
© Oxford University Press 2001
Early Proterozoic Calc-Alkaline and Middle Proterozoic Tholeiitic Dyke Swarms from Central–Eastern Argentina: Petrology, Geochemistry, Sr–Nd Isotopes and Tectonic Implications
1DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITY OF TRIESTE, VIA E. WEISS, 8, 34127 TRIESTE, ITALY
2DIPARTIMENTO DI INGEGNERIA CHIMICA, DELL’AMBIENTE E DELLE MATERIE PRIME, UNIVERSITY OF TRIESTE, PIAZZALE EUROPA 1, 34100 TRIESTE, ITALY
3INSTITUTO DE GEOCIENCIAS, UNIVERSITY OF SÃO PAULO (USP), PO BOX 11348, 01051 SÃO PAULO, SP, BRAZIL
4DIPARTIMENTO DI MINERALOGIA E PETROLOGIA, UNIVERSITY OF PADOVA, CORSO GARIBALDI, 37, 35137 PADOVA, ITALY
5INSTITUTO DE RECURSOS MINERALES (FCNyM-UNLP), CALLE 47 N° 522, 1900 LA PLATA, ARGENTINA
6DEPARTAMENTO DE GEOCIENCIAS, UNIVERSITY OF LONDRINA, PO BOX 6001, 86051-990 LONDRINA, PR, BRAZIL
7LABORATORIO DE ENTRENAMIENTO MULTIDISCIPLINARIO PARA LA INVESTIGACION TECNOLOGICA (LEMIT-CIC and FCNyM-UNLP), PO BOX 128, 1900 LA PLATA, ARGENTINA
The Rio de La Plata craton in Argentina (Azul and Tandil regions) is characterized 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 andesitic and rhyolitic compositions and trend east–west, whereas the tholeiitic dykes mainly trend N30°W and are represented by basalts with low (0·9–1·7 wt %) and high TiO2 (up to 3·7 wt %). The calc-alkaline dykes have primitive mantle (PM)-normalized trace element patterns enriched in Rb, Ba, K, La, Ce and Nd, and significant negative Nb and Ti anomalies. These dykes are characterized by 
t(Nd) values of -3 to -4, similar to those of the EMI mantle component. Low-TiO2 tholeiitic dykes have low incompatible-element (IE) contents and PM-IE patterns with slightly positive or negative Nb spikes. They have variable t(Nd) values (-0·5 to 12·1), which mainly reflect derivation from a depleted source mantle. High-TiO2 tholeiitic dykes have more enriched IE-PM patterns and are characterized by 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-alkaline and tholeiitic dykes originated by different melting degrees of a heterogeneous source mantle, the variable IE enrichment of which may have occurred in Late Archaean to Early Proterozoic times. The emplacement of the calc-alkaline dykes is associated with the transtensional stage of the Transamazonian Orogeny, whereas the tholeiitic dykes reflect extensional tectonics succeeding the Transamazonian event. The calc-alkaline and tholeiitic dykes are similar in emplacement age and characteristics to metamorphosed granites and volcanic rocks outcropping within the Namaqua fold belts of southwestern Africa (Richtersveld and Witberg–Aggeneys–Gamsberg provinces); this may indicate that the Rio de La Plata craton and southwestern Africa were contiguous in Early–Middle Proterozoic times.
KEY WORDS: Argentina; geochemistry; petrology; Proterozoic dykes; Rio de La Plata craton
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