Characteristics and pathways of primitive magma supply near Laguna del Maule

Abstract:

Magmatism in the Andean sector (35.5-36.5°S) of Laguna del Maule (LdM) is predominantly silicic in composition. Some less-evolved magmas have erupted over the last few 100 kyr. Truly primitive magmas are rare due to the thick crust (>45 km) and extensive crustal storage of silicic magmas (Hildreth et al. 2010). However, a few centers with magma compositions of >7 wt.% provide important constraints on the magma supply that sustains and creates the voluminous silicic magmatism associated with LdM and other silcic centers in the area. With the exception of Manantial Pelado (60 km NW of LdM) all predominantly mafic centers are cinder cones distributed along regional NNW-trending lineaments, often associated with conjugate ENE-trending structures (Salas et al. 2016). E.g. in the case of the La Resolana Craters and Los Hornitos cinder cones, in the vicinity of the Descabezado Grande volcanic field, these mafic vents coincide with the regional contact between the Miocene La Invernada pluton and the Eocene-Early Miocene volcaniclastic Abanico Formation.

Preliminary results compiling compositional diversity among these primitive magmas show that some liquid lines of descent follow a tholeiitic trend, with Fe enrichment during fractionation from 8 wt.% to 4 wt.% MgO (Zimmer et al. 2010). Tholeiitic indices > 1 (e.g., Manantial Pelado ~1.18) are common, suggesting relatively water poor conditions (mostly < 3 wt% H2O) for this part of the arc. In particular the glacially dissected Manantial Pelado is comprised almost exclusively of olivine-clinopyroxene bearing basaltic andesites. The primitive nature in these lavas manifests itself through the presence of Cr-spinel inclusions (Cr# 0.4-0.6) in olivines with Mg# 85. The most primitive magmas in the entire sector are from the Los Hornitos cinder cones (>12 wt.% MgO) that have olivine in equilibrium with the mantle (Mg# 92), thus providing unique access to source magmas for the silicic systems of this part of the Andes. Eruptions of these magmas suggests that pathways for primitive magmas remain open, while mixing and contamination with more evolved magma is limited despite the extensive distribution of evolved compositions in the subsurface.

Refs: Salas et al., 2016, IJES, 1-15; Zimmer et al. 2010, JPet, 2411-44; Hildreth, et al. (2010), SERNAGEOMIN-Boletin 63, 142.