Serie correlación geológica
On-line version ISSN 1666-9479
TOSELLI, Alejandro J; ROSSI, Juana N; BASEI, Miguel A. S and LARROVERE, Mariano. Geochemical and isotopic constraints on petrogénesis of devonian-carboniferous Santa Cruz and Asha granites: Sierra de Velasco, Arentina. Ser. correl. geol. [online]. 2011, vol.27, n.2, pp. 77-98. ISSN 1666-9479.
The Santa Cruz and Asha granites crop out as north-south striking associated plutons in the northeast sector of the Sierra de Velasco. Even though both granites have been emplaced nearly synchronous, they are clearly different in their textural and geochemical characteristics. The Santa Cruz Granite is a medium-grained equigranular monzogranite, in which muscovite predominates over biotite, and the magnetic susceptibility is 0.10 - 0.14 x 10-3 IS units. The Porphyritic Asha Granite is a coarse-grained porphyritic monzogranite with biotite largely predominating over muscovite. The magnetic susceptibility is 2 to 16 x 10-3 IS units. Both granites present A/CNK > 1,1 and have Rb/Sr > 2. In a modal QAP, both granites are project on the granite field. A U-Pb monazite age was of 361+/- 4.1 Ma for the Porphyritic Asha Granite (GPA) and 354.3 +/-3.8 Ma for the Santa Cruz Granite (GSC). These data indicate that the intrusions took place during the Upper Devonian to the Lower Carboniferous. The Fe2O3, MgO, CaO and TiO2 values are significantly lower, and SiO2 , Al2O3 , K2O and P2O5 are higher than those in GPA. Trace elements (Sr, Ba, V, Sc and Σ REE) are higher than those in GSC, although both showed marked negative Eu anomaly. Both granites show low content of Sr (<200 ppm). The geochemical data of both granites plotted on different diagrams in comparison with experimental diagrams melts, suggesting a metagreywackes in transition to metabasites as dominated sources, corresponding to crustal provenance. On the tectonic setting diagrams the granites fit into a continental collision scheme. The Sm-Nd isotopic geochemistry yields eNdt of -6,01 for GSC and eNdt of -5,41 for GPA, and the Nd model ages of 1586 Ma for the GSC and 1538 Ma for the GPA, in the same range as the Famatinian Arc of the Central Sierras Pampeanas. The Sm/Nd fractionation index (ƒSm/Nd) and the Nd model ages of both granites are close to those of the Puncoviscana Formation. This suggests that the Puncoviscana Formation could be the source of generation of both granites, through a process of strike-slip.
Keywords : Peraluminous Granites; Devonian-Carboniferous; Petrogenesis; Crustal Source; Sierras Pampeanas.