Pleistocene to holocene growth of a large upper crustal rhyolitic magma reservoir beneath the active laguna del Maule Volcanic Field, Central Chile

Abstract

The rear-arc Laguna del Maule volcanic field (LdM) in the Andean Southern Volcanic Zone, 36°S, is among the most active latest Pleistocene-Holocene rhyolitic centers globally and has been inflating at a rate of > 20cm a-1 since 2007. At least 50 eruptions during the last 26 kyr allow for a thorough interrogation of changes in the physical and chemical state of this large, 20km diameter, silicic system. Trace element concentrations and Sr, Pb and Th isotope ratios, that the mafic precursors to the LdM rhyolites result from mixing between partial melts of garnet-bearing mantle and crust in Th-excess and partial melts of garnet-free crust in U-excess. The 238U/230Th ratios of the LdM lavas are decoupled from the slab fluid signature, similar to several recently studied frontal arc volcanic centers in the Southern Volcanic Zone. A narrow range of radiogenic isotope compositions and increasing isotopic homogeneity with differentiation indicate that silicic magma is generated by magma hybridization and crystallization in the upper crust with limited involvement of older, radiogenic material. New 40Ar/39Ar and 36Cl ages reveal a wide footprint of silicic volcanism during the early post-glacial (25-19 ka) and Holocene (c. 8-2 ka) periods, but focused within a single eruptive center during the interim period. Subtle temporal variations in trace element compositions and two-oxide temperatures indicate that these eruptions, issued from vents distributed within a similar area, tapped at least two physically discrete rhyolite reservoirs. This compositional distinction favors punctuated extraction and ephemeral storage of the erupted magma batches. Frequent mafic recharge incubates this long-lived, growing shallow silicic magma reservoir above the granite eutectic, which favors magma interactions over rejuvenation of near- to sub-solidus silicic cumulates. A long-term rate of mass addition-extrapolated from surface deformation accumulated over the past decade-is comparable with those that have produced moderate- to largevolume caldera-forming eruptions elsewhere.