Zircon U-Pb geochronology of the Chimpa volcano (Central Andes, Puna plateau, NW Argentina): Inferences on the temporal evolution of the magmatic system

Abstract

We investigated the temporal evolution of the andesitic Chimpa volcano of the northern Puna plateau, Central Andes, situated at the geological boundary between the plateau and the Eastern Cordillera domains. The volcanic activity consisted in three constructive volcanic cycles (Basal, Cajon and Chimpa units) showing complex eruptive behaviors (ignimbrites, lava domes. block-and-ash flows, lava flows). We present new U-Pb analyses conducted on the zircon crystals from the Chimpa volcanic rocks. These analyses provide constraints on the magmatic/volcanic tempos and offer insights into the Th and U (and Th/U ratios) systematics of the analyzed zircon crystals. The results suggest a lifespan for volcanism ranging 7.5 to <7.0 Ma, in concomitance with the regional steady-state magmatic phase separating the first two pulses of ignimbritic flare-up in the Altiplano-Puna Volcanic Complex. Moreover, the analyzed zircon crystals exhibit Th/U ratios (0.11–0.34) and Th (33–8860 ppm) and U (52–4258 ppm) that indicate magmatic crystallization from poorly evolved melts at high temperatures. Some discrepancies exist between the calculated zircon concordia age for the third volcanic phase (Chimpa Unit, ca. 7.35 ± 0.071 Ma) and that of the second cycle (Cajon Unit, 6.98 ± 0.057 Ma). We interpret these differences as stemming from the presence of zircon antecrysts in the final eruptive melts. Indeed, a true pre-eruptive event of zircon crystallization (i.e., formation of autocrysts) could not be proven by the existing dataset.

There is a geochronological affinity with some nearby volcanic rocks from the Puna plateau and the Eastern Cordillera domains, particularly considering the rhyolitic products of the Ramadas Volcanic Center and the andesitic to dacitic Almagro volcanic rocks. This highlights the complex behavior of the local magma plumbing system beneath this particular area, resulting in the emission of geochemically variegate volcanic rocks at similar times. This scenario suggests that the composition of the erupting melts are affected by the rheological behavior of the mid-upper-crustal MASH reservoir (the Altiplano-Puna Magmatic Body), which may either facilitate or impede the ascent of poorly evolved magmas derived from the deep crust towards the surface. From this point of view, the relative location of the volcanic centers relatively to the position of the geophysical anomaly may exert an important influence on the petrogenetic paths of magmas.