Abstract
Shoshonitic volcanic centres are scarce within the back-arc mafic volcanism of the Puna Plateau in the Central Volcanic Zone of the Andes. San Jerónimo and Negro de Chorrillos volcanoes are two well preserved shoshonitic monogenetic edifices that developed mainly Strombolian activity and lava flows, which extended more than 9 and 4 km from the vent, producing a volume DRE (Dense Rock Equivalent) of 0.091 and 0.020 km3, respectively. The new unspiked K–Ar (groundmass) ages of San Jerónimo (144 ± 3 ka) and Negro de Chorrillos (51 ± 2 ka) eruptions presented in this work are both younger than those previously published. Petrographic and compositional features allow distinguishing between potassic and ultrapotassic suites in volcanic products of both eruptions. Phlogopite occurrence and low glass content characterize the ultrapotassic suite. Based on the crustal thickness of this part of the Puna and the experimental constraint, these magmas last equilibrated at Moho depths at around 50 km (the highest depth of spinel lherzolite stability field) for temperatures of 1100–1200 °C. Once in the crust, the K-rich magma underwent a minor interaction with a trachytic-trachydacitic melt, which is evidenced petrographically and geochemically, e.g., mismatch of mineral assemblages (co-occurrence of olivine and quartz), disequilibrium crystal textures (dusty zones in plagioclase, embayments in sanidine), and a wide range of compositional variation in the glasses. Ultrapotassic (i.e., phlogopite-bearing) and potassic lavas in the same eruption are interpreted as pulses of the same magma but with a longer mixing time with the trachytic-trachydacitic melt for the resulting ultrapotassic rocks within an eruptive time frame of hours or days. The singular shoshonitic character of these magmas could be related to their origin along an old terrane boundary. Afterwards, as occurs for other mafic magmas of the Puna, major NW-SE fault systems would favour their ascent to the surface.
Highlights
Up to 10 cm thick ash-fall deposits identified in fluvial terraces were associated with the San Jeronimo (SJ) eruption (Kay et al, 2008), and pyroclastic flows and ash fallout were related to the Negro de Chorrillos (NdC) eruption (Seggiaro et al, 2016)
San Jeronimo and Negro de Chorrillos shoshonitic volcanoes are small monogenetic edifices mainly built by effusive activity, resulting from a Strombolian-type eruption with a Volcanic Explosivity Index (VEI) of 2 (Newhall and Self, 1982)
Volcanic products of potassic and ultrapotassic compositions occur in both eruptions
Summary
The Central Volcanic Zone of the Andes (CVZ; e.g., Thorpe, 1984; Stern, 2004) extends between 14◦ S in Peru and 27◦ S in Chile and Argentina (Fig. 1) and records a complex evolution of a volcanic arc and back-arc through hundreds of volcanoes, from constructional (poly genetic and monogenetic) to large collapse calderas (Grosse et al, 2017; Guzman et al, 2014; Kay and Coira, 2009; Maro et al, 2016b, 2017; Naranjo et al, 2018; Petrinovic et al, 2017; Tilling, 2009). This plateau consists of two morphotectonic units, the Altiplano (~14–23◦ S) and Puna (~23–27◦ S) In this geotectonic setting, the magmatism is driven by the geodynamic mechanisms of plateau uplift and the processes by which the continental crust overriding plate has been shortened and thickened, whereas the mantle lithosphere that originally underlay it has been thinned or entirely removed (Beck et al, 2015; Garzione et al, 2017; Kay and Coira, 2009). This thickness is around 50 km depth in the studied area (~24–25◦ S) (Beck et al, 2015; McGlashan et al, 2008; Yuan et al, 2002)
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