Preliminary geochemical data of Mississippian volcanic rocks from the southern Puna (NW Argentina): evidence of extension after Chilenia terrane collision?

MARTINA, F.; ASTINI, R.A.

Heidelberg

Congreso; ? XXII Colloquium on Latin America Earth Sciences; 2011

Recent works have recognized in NW Argentina a major magmatic event of Mississippian age (Martina and Astini 2009; Martina et al., 2011), previously only known through indirect evidences (Willner et al. 2008). It consist of ~2000 m thick non metamorphosed bimodal volcanic and volcaniclastic rocks resting on a heterogeneous metamorphic basement and covered by thick (>1000 m) Late Paleozoic siliciclastic successions. Petrographic and geochemical data from the southern Puna (27¨¬01¡¯54¡±S-67¨¬04¡¯16¡±W) suggest the presence of rhyolites, trachytes, trachyandesites and picritic basalts with silica contents ranging from 44.3% to 78.8%. In the K2O versus SiO2 diagram the felsic volcanics plot in the K-rich field and the basalt in the medium-K. The alumina saturation index is >1 in the rhyolites and <1 in the trachytes, while the agpaitic index is <1 in all samples. The basalt has high Fe2O3 and TiO2 contents and low Mg number, typical of evolved magmas. The low concentration of Ni and Cr may indicate fractionation of olivine and clinopyroxene, respectively. Lithophile elements (LILE) content is high in all samples excepting Ba which is low. Rare-earth elements (REE) patterns display light REE enrichment (La/YbN = 2.74-10.34) and relatively flat medium to heavy REE profiles (Dy/YbN = 0.95-1.40), with strongly negative Eu anomalies (Eu/Eu* = 0.79-0.08) in the most felsic samples. The basalt profile is similar to those of the trachytes but with a slightly lower total REE concentration. Another difference with the felsic rocks is its positive Eu anomaly (Eu/Eu* = 1.38) which characterize the early crystallizing assemblages. Although the results are limited and not conclusive in order to define the source of the volcanism, the different SiO2 content between the basalt (44%) and the rhyolites and trachytes (60-79%) suggests they are not cogenetic. Basalts show high LILE, LREE, Nb and TiO2 and low Ba contents, typical of OIB-type magmas. In contrast, the felsic rocks evidence crustal melting processes due to its high incompatible elements content and prominent negative Eu anomaly suggesting feldspar in the source. Moreover, the low Zr/Nb (6.20-14.74) and Nb/Th (0.97-2.61) ratios are common of continental crust. This is consistent with the reported high initial 87Sr/86Sr ratios (Martina et al. 2011). The pronounced negative Ba anomaly is typical of rift settings. This interpretation is in agreement with the tectonic discrimination diagrams where samples plot in the intraplate granites field. An extensional environment could also explain the juvenile ¥åNd(t) values of the rhyolites (Martina et al. 2011) and it suggests mixing of crustal and mantle melts. A similar explanation was proposed for coetaneous A-type granites in the Andean foreland region (Dahlquist et al. 2010). Widespread crustal melting can be explained through underplating processes. In this sense, the OIB-like basalt may represent the primitive component. This magmatism occurred immediately to the east of the suggested Chilenia terrane collision (~390 Ma) and thus it could represent post-collisional extension.