Pasto Ventura Quaternary basic monogenetic volcanic cluster, (Southern Puna, Argentina): volcanologic, morphological features, and eruptive history.

BAEZ WALTER; VIRAMONTE JOSE G.

Foz do Iguazu

Congreso; Meeting of the Americas 2010; 2010

AGU

The Pasto Ventura basic monogenetic volcanic cluster covers an area of ~ 700 km2 in the southeastern - Southern Puna in Catamarca province, Northewestern Argentina on the eastern side of the CAGH (Central Andes Gravity High ;Viramonte et al., this meeting). It is characterized by over 30 monogenetic vents, such as cinder cones, valley-Lava flows, mini-shield volcanoes, ring tuff and maars, overlying mainly the Upper-Precambrian-Lower-Paleozoic puna basement. The volcanic structures are small in volume and mafic in composition. The cinder cones are 60m to 100m high, diameter of the craters from 120m to 270m, which in many cases are U-shaped. Associated valley lava flow extends up to 3.59 Kmfrom the vent. The small shield volcanoes occupy areas from 2.76 km2 to 0.35 km2 and heights up to 30m. The low and flat profile of these features is considered to indicate that they were produced by the piling up of relatively thin lava flows ( 2-4 m) erupted in relatively fluid conditions. The lava flow expanded radially from a central vent and in most cases, they were probably formed by a single eruption that may have lasted for up to several days or weeks. Effusion rates were probably relatively very low, consistent with lava flow lengths of a few tens or hundreds of metres. All mini-shields in PV have a single central summit crater. They are generated by magmatic activity from a central vent. In some cases, where there is a significant slope, they form an elongated lava flow and reach distances around 2.2 Km. In other cases depending to its reology, they reach a few hundreds of meters. Proximal pyroclastic fall deposits are usually associated with them. The phreatomagmatic structures (ring tuff and maar) show 0.63 km to 0.59 km in diameter, 20 to 10 diameter/deep ratio and their related deposits are base surge ones. Many volcanic structures in PV are associated to extensional faults N60°E related to the regional trasntensional tectonic and produced by sinistral transcurret movement on the main Carachipampa-Culampaja NNWSSE fracture zone. In some cases, the vents are on the trace of the fault, having been cut again by later movementts. In other cases, the alignement of vents and cones and their relation to a main dyke are evident. Hidrovolcanic structures such as maares and tuff rings, do not appear as related to any structure or position inside the region. It is posible to differentiate three groups of rocks from their petrographic and geochemical features. First, basaltic trachy-andesites belonging to high-K calcalkaline series, with porphyric textures, Ol Prx and Pl phenocrysts within a pilotaxic to intersertal matrix, built by Pl microlitic and glass. The second group is formed by basaltic-andesites belonging to calcalkaline series, with textures and mineralogy similar to the first group but a smaller percentage of microliths but a larger percentage ofglass. The last group is the least abundant. It consists of andesite belonging to high-K calcalkaline series, presenting porphyric textures with phenocrysts of Pl y Prx within a glass-rich matrix. The general age of the monogenetic magmatism is < 2 Ma [Rise et al. 2008] and it is possible to establish a sequence of eruptive events using the degree of erosion and stratigraphic relationships. The oldest activity is represented by the small shield volcanoes, which display the highest degrees of erosion. Recent activity is represented by cinder cones and lava flows which are better preserved. Phreatomagmatic activity may be associated with a period of higher humidity during the Upper Pleistocene.2 in the southeastern - Southern Puna in Catamarca province, Northewestern Argentina on the eastern side of the CAGH (Central Andes Gravity High ;Viramonte et al., this meeting). It is characterized by over 30 monogenetic vents, such as cinder cones, valley-Lava flows, mini-shield volcanoes, ring tuff and maars, overlying mainly the Upper-Precambrian-Lower-Paleozoic puna basement. The volcanic structures are small in volume and mafic in composition. The cinder cones are 60m to 100m high, diameter of the craters from 120m to 270m, which in many cases are U-shaped. Associated valley lava flow extends up to 3.59 Kmfrom the vent. The small shield volcanoes occupy areas from 2.76 km2 to 0.35 km2 and heights up to 30m. The low and flat profile of these features is considered to indicate that they were produced by the piling up of relatively thin lava flows ( 2-4 m) erupted in relatively fluid conditions. The lava flow expanded radially from a central vent and in most cases, they were probably formed by a single eruption that may have lasted for up to several days or weeks. Effusion rates were probably relatively very low, consistent with lava flow lengths of a few tens or hundreds of metres. All mini-shields in PV have a single central summit crater. They are generated by magmatic activity from a central vent. In some cases, where there is a significant slope, they form an elongated lava flow and reach distances around 2.2 Km. In other cases depending to its reology, they reach a few hundreds of meters. Proximal pyroclastic fall deposits are usually associated with them. The phreatomagmatic structures (ring tuff and maar) show 0.63 km to 0.59 km in diameter, 20 to 10 diameter/deep ratio and their related deposits are base surge ones. Many volcanic structures in PV are associated to extensional faults N60°E related to the regional trasntensional tectonic and produced by sinistral transcurret movement on the main Carachipampa-Culampaja NNWSSE fracture zone. In some cases, the vents are on the trace of the fault, having been cut again by later movementts. In other cases, the alignement of vents and cones and their relation to a main dyke are evident. Hidrovolcanic structures such as maares and tuff rings, do not appear as related to any structure or position inside the region. It is posible to differentiate three groups of rocks from their petrographic and geochemical features. First, basaltic trachy-andesites belonging to high-K calcalkaline series, with porphyric textures, Ol Prx and Pl phenocrysts within a pilotaxic to intersertal matrix, built by Pl microlitic and glass. The second group is formed by basaltic-andesites belonging to calcalkaline series, with textures and mineralogy similar to the first group but a smaller percentage of microliths but a larger percentage ofglass. The last group is the least abundant. It consists of andesite belonging to high-K calcalkaline series, presenting porphyric textures with phenocrysts of Pl y Prx within a glass-rich matrix. The general age of the monogenetic magmatism is < 2 Ma [Rise et al. 2008] and it is possible to establish a sequence of eruptive events using the degree of erosion and stratigraphic relationships. The oldest activity is represented by the small shield volcanoes, which display the highest degrees of erosion. Recent activity is represented by cinder cones and lava flows which are better preserved. Phreatomagmatic activity may be associated with a period of higher humidity during the Upper Pleistocene.