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

CAFFE, PABLO J.; MARO, GUADALUPE; PRESTA, JUAN; FLORES, PATROCINIO; PERALTA, YÉSICA

Auckland

Conferencia; 4th International Maar Conference - A Multidisciplinary Congress on Monogenetic Volcanism; 2012

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 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 (