Postglacial Eruptive History Established by Mapping and Tephra Stratigraphy Provides Perspectives on Magmatic System beneath Laguna del Maule, Chile

Abstract:

Mapping and tephra studies have established the eruptive history of Laguna del Maule (LdM), including 78 postglacial events (50-77.6% SiO₂) that erupted from 57 vents around the ~100-km² LdM basin on the Chile-Argentina border. Most (65) of the young events were silicic, making the LdM volcanic field a unique focus of postglacial rhyolitic eruptions. Spatial and temporal distribution of the eruptive vents reflect the magmatic system that supports the volcanic field. Correlations of distal tephra deposits in Argentina to vents in Chile and radiocarbon dating show that many of the silicic vents cluster into a few multi-vent complexes that each erupted intermittently for decades or millennia between ~15 ka and 2 ka. The most voluminous postglacial eruption (~20 km³: the Plinian rhyolite of Laguna del Maule, unit rdm) is inferred to have erupted at a site covered by the subsequent lake ~15 ka, producing aphyric, high-silica rhyolite (76-77.6% SiO₂) pyroclastic flows and fallout that impacted both sides of the border. Rhyolite and rhyodacite eruptions that followed issued from vents that surround the lake, nearly all within 10 km of the rdm vent site. The most long-lived center is the Barrancas complex--with 2 edifices that together include as many as 17 vents, which erupted episodically between ~14.5 and ~3.5 ka, ~16 km SE of rdm. All of the rhyolites are crystal-poor, some are truly aphyric, but most rhyodacites include 5-25% phenocrysts (pl, hb, bt). In and near the LdM basin, an additional 13 mafic vents (50-61% SiO₂) and two dacites (63%; 66% SiO₂) erupted during the same time interval, most in a sector west to southwest, 6–10 km from the lake center. Notably, one scoria ring along the SE lakeshore, and two scoria rings on the NW shore are only 4 and 5.5 km, respectively, from the rdm vent site. Mafic components are conspicuous in the rhyodacite eruptions as enclaves and mixed pumices, but are rarely seen in the rhyolites. One exception is rdm itself, which has mafic enclaves in the rhyolite pumice and cognate cauliflower-shaped mafic clasts (53-61% SiO₂) that were comagmatic liquids. The eruption sequence shows that rhyolite, dacite, and mafic magma erupted throughout the entire postglacial interval, sometimes concurrently, with no systematic trends in either vent location or composition through time.