Geology, geochemistry, geochronology, and economic potential of Neogene volcanic rocks in the Laguna Pedernal and Salar de Aguas Calientes segments of the Archibarca lineament, northwest Argentina
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This study presents new geochemical, geochronological, isotopic, and mineralogical data, combined with new geological mapping for a 2400 km2 area of Neogene volcanic rocks in northwestern Argentina near the border with Chile, between 25°10′S and 25°45′S. The area covers the zone of intersection between the main axis of the Cordillera Occidental and a set of NW–SE-trending structures that form part of the transverse Archibarca lineament. This lineament has localized major ore deposits in Chile (e.g., the late Eocene La Escondida porphyry Cu deposit) and large volcanic centers such as the active Llullaillaco and Lastarría volcanoes on the border between Chile and Argentina, and the Neogene Archibarca, Antofalla, and Cerro Galán volcanoes in Argentina. Neogene volcanic rocks in the Laguna Pedernal and Salar de Aguas Calientes areas are mostly high-K calc-alkaline in composition, and range from basaltic andesites, through andesites and dacites, to rhyolites. Magmatic temperatures and oxidation states, estimated from mineral compositions, range from ~ 1000 °C and ∆FMQ ≈ 1.0–1.5 in andesites, to ~ 850 °C and ∆FMQ ≈ 1.5–2.0 in dacites and rhyolites.The oldest rocks consist of early–middle Miocene andesite–dacite plagioclase–pyroxene-phyric lava flows and ignimbrites, with 40Ar/39Ar ages ranging from 17.14 ± 0.10 Ma to 11.76 ± 0.27 Ma. Their major and trace element compositions are typical of the Andean Central Volcanic Zone, and show strong crustal contamination trends for highly incompatible elements such as Cs, Rb, Th, and U. These rocks are geochemically grouped as sub-suite 1. This widespread intermediate composition volcanism was followed in the middle–late Miocene by a period of more focused rhyodacitic flow–dome complex formation. These felsic rocks are characterized by less extreme enrichments in highly incompatible elements, and increasing depletion of heavy rare earth elements. These rocks are geochemically grouped as sub-suite 2. The youngest rocks in this sub-suite show the highest La/Yb ratios, and are characterized by abundant hornblende phenocrysts (not commonly seen in other rocks from the area). In the Pliocene–Pleistocene, there was a return to more typical andesite–dacite volcanism, with geochemical characteristics similar to the early–middle Miocene lavas, and are also grouped in sub-suite 1. Finally, extensional tectonics in the Quaternary led to localized outpouring of mafic (basaltic andesitic to andesitic) monogenetic lava flows and cones. One particularly large flow, the Vega Aguas Calientes lava flow, covers approximately 90 km2, and samples form two groupings, with affinities similar to the least-evolved samples from sub-suites 1 and 2 (sub-groups BA1 and BA2, respectively).Nd and Sr isotopic compositions indicate moderate to strong crustal contamination, especially in more felsic rocks, and extend from 87Sr/86Sr (0.706) and εNd (− 2.4), values typical of Central Volcanic Zone rocks, to more evolved compositions (0.709 and − 6.8, respectively) typical of large-volume ignimbrites of the Altiplano–Puna Volcanic Complex and Cerro Galán. The latter compositions are thought to be derived by extensive interaction between mantle-derived arc magmas and Paleozoic granitoid rocks that form much of the crustal column in this region. The distinctive mineralogy and geochemistry of the sub-suite 2 middle–late Miocene rhyodacitic flow–dome complexes indicate that these magmas had higher water content than both the earlier and later sub-suite 1 andesites–dacites. They were erupted during a period of tectonic quiescence following the Quechua orogenic phase, and geophysical evidence suggests that they were proximally derived from a large upper crustal magma chamber which partially collapsed to form a trap-door caldera. Strong fumarolic alteration associated with the youngest of these felsic volcanoes, Cerro Abra Grande, suggests the potential for the existence of epithermal-type mineralization within the volcanic edifice, or porphyry-type mineralization at depth.
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