The Carran-Los Venados volcanic field and its relationship with coeval and nearby polygenetic volcanism in an intra-arc setting

Abstract

Understanding the relationship between monogenetic and polygenetic volcanism has been a long-standing goal in volcanology, especially in cases where these two styles of volcanism are coeval and geographically adjacent. We studied the Carran-Los Venados (CLV) volcanic field and made comparisons with published data on CLV's polygenetic neighbor Puyehue-Cordon Caulle (PCC) in the Southern Andean arc, using quantitative tools and recent numerical simulations of magma reservoir formation. CLV is a basaltic to basaltic andesitic volcanic field composed of 65 post-glacial scoria cones and maars and a 1-km-high Pleistocene stratovolcano, whereas PCC is a basaltic to rhyolitic composite volcano. Our results point to three main differences between CLV and PCC: (1) the CLV magmas differentiate at low-crustal reservoirs, followed by rapid ascent to the surface, while the PCC magmas stagnate and differentiate in lower- and upper-crustal reservoirs; (2) CLV is elongated in the NE direction while FCC is elongated in the NW direction. Under the current stress field (N60 degrees sigma(Hmax)), these two volcanic alignments correspond, respectively, to local extensional and compressive deformation zones within the arc; and (3), the post-glacial CLV magma flux was estimated to be 3.1 +/- 1.0 km(3)/ky, which is similar to the average magma flux estimated for KC; however, the FCC magma flux is estimated at approximately twice this value during peak eruptive periods (5.5 +/- 1.1 km(3)/ky). Based on numerical simulations, CLV is in a limit situation to create and sustain a mush-type upper-crustal reservoir containing highly crystalline magma, which is however not eruptible. The FCC volcanic system would have been able to create a stable reservoir containing eruptible silicic magma during periods of peak magma flux. We postulate that monogenetic volcanism occurs at CLV due to both low magma flux and an extensional/transtensional regime that favors rapid magma rise without storage and differentiation in stable upper-crustal reservoirs. However, the CLV system seems to be at an inflection point, and could become polygenetic if magma flux increases. For PCC, high magma flux during some periods together with compressive deformation would have led to the construction of one or several stable upper-crustal magma reservoirs, with subsequent silicic volcanism and construction of central conduits for magma extrusion, resulting in polygenetic volcanism with evolved compositions. In this model, monogenetic volcanic systems can become polygenetic despite extensional regime if magma flux increases sufficiently to create and sustain a stable upper-crustal magma reservoir.