The Development of a Restless Rhyolite Magma Chamber at Laguna del Maule, Chile

The Laguna del Maule (LdM) volcanic field is a site of rapid crustal deformation at rates in excess of 200 mm/yr since 2007. The uplift is centered in the 16 km diameter LdM lake basin, which is ringed by 21 rhyolite domes and coulees erupted since the last glacial retreat. The lack of previously common andesite and dacite eruptions since 19 ka and coherent major and trace element variation throughout post-glacial time suggests the presence of a large silicic magma body beneath the LdM basin. Assimilation-fractional crystallization modeling predicts the rhyolites evolved at 5 km depth by 73% fractionation of a basaltic parent and modest assimilation of granodiorite accounting for up to 20% of the highest silica rhyolite. AFC processes dominate the evolution from basalt, however the differentiation of the silicic magma is complicated by liquid extraction from crystal mush, remelting of cumulate by intruding basalt, and trace element diffusion. Two-oxide thermometry indicates a relatively hot, oxidized system with eruptive temperatures ranging from 760 - 850° C and fO2 at QFM+2. Pilot ion microprobe 238U-230Th dating of zircon rims suggests the shallow LdM magma system was assembled over a period of 100-200 kyr. 40Ar/39Ar geochronology and field relationships reveal the post-glacial silicic volcanism occurred in two phases. Phase 1 began approximately coincident with deglaciation at 25 ka with the eruption of the rhyolite East of Presa Laguna del Maule. Over the next 6 ky, 6 small rhyodacite domes, a larger rhyodacite flow, and 4 andesite flows erupted in the NW basin and two silicic domes 12 km to the SE. Phase 1 culminates with the eruption of the Espejos rhyolite near the N shore of the lake at 19 ka. The locus of volcanism then migrates SE and phase 2 begins at ~10 ka with the eruption of the Cari Launa rhyolite and the early flows of the Barrancas complex. This period is more voluminous, erupting 4.8 km3 compared to 1.7 km3 during phase 1. Phase 2 produced lower silica rhyolite (72-74%) than the majority erupted during phase 1 (75-76%) but a smaller range of compositions overall as andesite and rhyodacite eruptions become rare and peripheral. The two phases are also distinguished by a small, but consistent, shift in REE contents. Phase 1 is marked by lower REE contents, but higher Ce/Sm ratios. The chemical trends are temporally, rather than spatially, correlated reflecting the evolution of an integrated magma body rather than local vagaries in magmatic process. Early eruptions in both phases 1 and 2 are characterized by elevated two-oxide temperatures, the presence of trace pyrrhotite, and Ta contents 2-3 times greater than subsequent eruptions, an enrichment of similar magnitude to that observed in the early Bishop Tuff. The intrusion of basalt to the base of the magma chamber could provide a source of heat and volatiles catalyzing the crystallization of Fe-sulfide and roofward diffusion of Ta. Such events have been followed by periods of heightened volcanic activity and produced an increasing rate of silicic magma generation. If the current unrest is indicative of basaltic intrusion, it could foreshadow continuing silicic volcanism at LdM, potentially leading to a catastrophic caldera forming eruption.