CONICET | Buscador de Institutos y Recursos Humanos

Alkaline rocks ranging from olivine basalts, basaltic trachyandesites, phonolites, trachytes to alkali rhyolites intrude through and erupt over the crystalline basement of the eastern Sierra de Valle Fertil, Western Sierras Pampeanas, Argentina. The La Aguadita Trachyte (nov. nom.), located at 30º46´24.26´´S and 67º26´16.02´´W, consists of a 15-20 m thick light grey microporphyritic to aphanitic and thinly layered lava flow that overlies the basement covering an area of 120 m x 200 m. It outcrops along a main NNW-trending normal fault. Euhedral K-feldspar is the major constituent (80% volume) present both as microphenocrysts (1mm) as in the orthophyric groundmass (0.06-0.3 mm). Strongly pleochroic zoned Na-amphibole (riebeckite-arfvedsonite?) is present in spongy growths, amongst the feldspars. It is often rimmed with aenigmatite. Interstitial patches of nepheline and partially devitrified glass are also part of the groundmass. Average chemical analyses indicate high SiO2 (67.25 wt%) and alkalis (7.27 wt% Na2O and 4.42 wt% K2O); with very low MgO (0.07 wt%) and TiO2 (0.01 wt%) contents, whereas Al2O3 is high (15.82 wt%) and exceeds the (Na2O+K2O+CaO) content. Trachytes have chondrite-normalized steep LREE and flat HREE patterns, showing strong negative Eu anomaly. Geological constraints and geochemistry of trachytes are consistent with a within-plate setting suggesting that these peralkaline rocks were formed in a continental rift setting. The presence of these magmas expands the already known extent of Early Triassic alkaline volcanism in the Western Sierras Pampeanas and indicates a regional volcanic event related to uplift in an extensional tectonic environment.

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