Neogene monogenetic volcanoes from the northern Puna of Argentina, Central Andean plateau.

CAFFE, P.J., MARO, G., PRESTA, J.F., FLORES, P.I., PERALTA, Y.

Auckland

Congreso; Fourth International Maar Conference; 2012

IAVCEI

Mafic (basaltic andesite to andesite) volcanic rocks are rare in the Neogene volcanic record of the northern Puna (Fig. 1), an area dominated by voluminous silicic (dacite to rhyolite) ignimbrites and lavas sourced from large calderas or composite volcanoes (Coira et al., 1993) erupted during the Miocene to Pleistocene. Most mafic volcanic rocks in the northern Puna were sourced from small scoria cones that are scattered across a 115 x 185 km area (Fig. 1) near the boundaries between Argentina, Chile and Bolivia (~70 km east of the current arc). Although scarce, understanding the eruption style of these volcanoes is relevant, because they behave in a similar way as basaltic volcanic centers that are the most common eruptive structures on Earth (Walker, 2000). Additionally, as the mafic rocks from Puna are coeval with the extensive dacitic ignimbrites, and the latter are considered as a mixture of 50:50 crustal/mantle magmas (Kay et al., 2010), the record of compositional variations in the mafic magmatism is also important for defining the mantle end-member in the dacitic Puna mix. Fourteen scoria cones and related mafic lava flows were studied to define facies architecture and compositional variations. These centers comprise either small isolated edifices (e.g., Pabellón) with no relation to other volcanoes, or may be related to large composite volcanoes (e.g., Tropapete), but most frequently form ammalgamated clusters of different size (15 km2 – 120 km2) and complexity. Main mafic volcanic fields are aligned in the NNE-SSW direction (Fig. 1), coincident with the orientation of the principal Andean thrusts. Transverse, NW-SE and E-W faults that usually act as transfer structures (Petrinovic et al., 2006) also seem to have participated in the eruptions. Almost all centers involve pyroclastic as well as lava flow units. Scoria cone remnants preserve their original shape, but are rather low in altitude (mean height ~70 m; mean height/basal diameter ~0.07), with external maximum slopes <16º. These parameters are consistent with moderate erosion due to arid climate, considering the Late Miocene-Early Pliocene ages of most edifices. Some cones (El Toro, Campo Negro, Bitiche) show horse-shoe morphologies due to partial collapse caused by rafting during lava outpours.  Petrographic and geochemical composition is very variable, ranging between crystal-poor (3-10%) rocks with skeletal microphenocrysts of olivine and/or pyroxene, to well-crystallized and crystal-rich (~20-30%) plagioclase-pyroxene ± amphibole ± olivine phyric andesites. Geochemical compositions range between calc-alkaline basaltic andesite to andesite, with a few centers (Rachaite, Barro Negro) transitionally trending to trachyandesite or shoshonite. Most of these rocks have SiO2 contents (54-63 %) and Mg# values (>45) typical of high-Mg andesites (Kelemen et al., 2004). The most mafic samples fall in the Mg# range 60-67, overlapping with values shown by the scarce (pre-Neogene) Cenozoic basalts erupted in the Central Andes.      Lavas have slabby or massive aspects, some of them showing meso- to macro-scale sheath–like flow folding in flow fronts (e.g., Cerro Morado, Jama, El Toro), or compressional ridges in the top (e.g., Campo Negro). Blocky lavas are scarce, but remnants of aa surfaces are conserved occasionally (e.g., Cerro Negro) on top of massive lavas. In the contrary, blocky lavas are ubiquitous only in the Quaternary Tuzgle center. Stacking of flows and presence of partly eroded or well preserved rafted pyroclastic deposits dragged during eruption are common features. Rare pseudofiamme in a few lavas (Cerro Morado, El Toro) suggest origins by clastogenesis (Cabrera and Caffe, 2009). Only in Patahuasi, intrusive andesite bodies develop fluidal peperite margins or dispersion of fragments as small (<1 m) rounded pillows with blocky peperite margins that indicate in-situ brecciation during injection of magma in wet silicic volcaniclastic sequences. Scoria cones exhibit typical facies of Strombolian edifices elsewhere (e.g., Vespermann and Schmincke, 2000). Recognized facies include: a) rare massive and cross bedded beds, interpreted as hydrovolcanic deposits formed during explosive  encountering of magma and ground-water; b) unwelded spindle-shaped bomb and scoria deposits that dip away from the vent, typical of the external wall facies of the cone; c) minor beds of  better sorted and finer material (ash or fine lapilli) interstratified with the coarser facies; d) interstratified beds of weakly welded scoria and moderately to strongly welded spatter in many rafts, and especially in layers that dip towards the interior of the edifice, interpreted as the internal wall and crater facies of the cone (e.g., El Toro, Cerro Morado, Jama); d) vertical to inclined lava dykes, as well as lava filling breaks in the cone, interpreted as the representant of the complex plumbing system that cut different parts of the cones. From cone facies and morphology, as well as compositional correlation between deposits of several cones and lava flows, it is possible to infer that pyroclastic and effusive eruptions were probably concurrent, as is confirmed by abundant rafts of welded (internal) to unwelded (external) cone deposits on the top of lava flows. Eruptions had a typical Strombolian style, with brief periods of fountaining and/or development of short-lived eruptive columns that alternated with the predominantly pulsatory Strombolian dynamics. Estimated discharge rates (to 10-20 m3/s) deduced from lava flow lengths (Walker, 1973) and the absence of interruptions in the volcanic activity evidenced by the lack of paleosoils or interstratification of other volcanic rocks are consistent with a short eruptive life-span, as observed in monogenetic fields elsewhere