Abstract
The Las Lozas volcanic sequence, which crops out at northwestern border of the Famatina belt-southeastern Puna, NW Argentina, is constituted mainly by rhyolites and a lesser volume of basalts and trachytes, and volcanoclastic deposits. These rocks, previously considered of Early Paleozoic age, are now assigned to the Lower Pennsylvanian (320 Ma U-Pb age). They represent a bimodal volcanic succession that plot in the subalkaline/tholeiitic (rhyolites), alkaline basalts (basalts) and alkaline (trachytes) fields on the total alkali-silica diagram. The basalts display features comparable to transitional MORB and within-plate tholeiites, with contributions from a mantle source affected by crustal contamination. The acid members also show geochemical affinities to within-plate magmas, and their composition suggest a derivation from continental crustal material with mantle source interaction or a juvenile essentially mantle derived crust. The 320 Ma age from the Las Lozas volcanic succession as well as the 342 and 348 Ma U-Pb ages, from rocks in the nearby Cazadero Grande section, to the south, and the U-Pb ages from Sierra Pampeanas granites (332-357 Ma) highlight the importance achieved by Carboniferous magmatic activity in that region, framed between 320 and 350 Ma. Low strontium initial ratios from the Las Lozas (0.70479-0.70164) indicate a predominant contribution by a juvenile component, while the ratios in the nearby Cazadero Grande (0.71433-0.71233) and Sierras Pampeanas granites (0.717 to 0.7124) point to an input by enriched sources with restricted contribution of a mantle component. Nd isotopes from a basalt from the Las Lozas section (εNd(320) with +3.11, TDM with 774.6 Ma) indicates a possible asthenospheric source, with evidence for some mixture between juvenile and reworked crustal material. In contrast Sr-Nd isotopes in a rhyolite from Cazadero Grande (εNd(t) of -2.91 and -0.3, TDM of 1.09 and 1.1 Ga) and of Sierra Pampeanas granites (εNd(t) of -0.6, TDM of 1.19 and 1.1 Ga) suggest a crustal source with minor juvenile input. Taking in account the age difference in the region between Mississippian and the Lower Pennsylvanian magmatism, this would indicate a change in the magma source consistent with a more pronounced extensional tectonic regime for the Lower Pennsylvanian. This assumption need to be considered together with observations along a transect at 27º30’S, where the Carboniferous Eastern Sierras Pampeanas granitic rocks show, regardless of age, a greater contribution of juvenile material of mantle character to the west. Among these rocks, with a major juvenile component, are those of the Las Lozas succession as well the Cerro Gloria Granite, the eastern manifestations of the Carboniferous Eastern Sierras Pampeanas magmatism. From the foregoing emerges that the contribution of juvenile material could be continuously varied in the region through the Carboniferous, due to varying lithospheric extension. An example of this arise from the presented data of the Lozas succession, which indicate that this rocks resulted from reworking of supra-crustal material with input of juvenile magmas, linked to the change in the lithospheric extension
Highlights
The present study deals with the volcanicvolcanoclastic section that outcrops along the Las Lozas Valley at the northwestern of the Famatina System-southern Puna (Fig. 1), to the north of the Mississippian rhyolites reported in Cazadero Grande by Martina et al (2011) and to the west of the coeval granites of the Sierras Pampeanas (Dahlquist et al, 2006, 2013; Grosse et al, 2009)
On the total alkali (Na2O+K2O)-silica diagram (Le Bas et al, 1986), the felsic members fall into the rhyolite field and the subalkaline-tholeiitic series, the basic rocks plot within the alkaline basalt field and the intermediate types are alkaline trachyte and trahyandacites (Fig. 5a)
The geochemical characteristics of the basalts are comparable to those of transitional MORB rocks and within-plate tholeiitic basalts from attenuated continental margins, where their mantle source is affected by crustal contamination
Summary
The present study deals with the volcanicvolcanoclastic section that outcrops along the Las Lozas Valley at the northwestern of the Famatina System-southern Puna (Fig. 1), to the north of the Mississippian rhyolites reported in Cazadero Grande by Martina et al (2011) and to the west of the coeval granites of the Sierras Pampeanas (Dahlquist et al, 2006, 2013; Grosse et al, 2009). Lower Pennsylvanian and Mississippian successions are new records of the Carboniferous magmatism in the region, locate west of the small and scattered A-type granites distributed along more than 1,000 km in the Sierras Pampeanas (Dahlquist et al, 2010). The geodynamic setting of the Carboniferous magmatism in the region remains controversial It has been considered product of crustal reheating during final phase of Famatinian orogeny (e.g., Grissom et al, 1998; Llambias et al, 1998; Höckenreiner et al, 2003; Grosse et al, 2009) or part of the Achalian orogen (e.g., Sims et al.,1998; Siesgesmund et al, 2004; Dahlquist et al, 2006, López de Lucchi et al, 2004). Dahlquist et al, 2010 proposed that those Carboniferous granites represent a new tectonothermal event, considering their field and geochemical characteristics, indicative of an extensional within-plate setting and noncompressive tectonic regime and not syn-kinematic emplacement as has been invoked for Devonian granite of Sierras de Cordoba or San Luis (Sims et al.,1998; Siesgesmund et al, 2004)
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