Resumen
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 (eNd(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 (eNd(t) of -2.91 and -0.3, TDM of 1.09 and 1.1 Ga) and of Sierra Pampeanas granites (eNd(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.