Two-million-year eruptive history of Laguna del Maule volcanic field

Abstract

The Laguna del Maule (LdM) volcanic field, which surrounds the 54-km² lake of that name, covers ∼500 km² of mountainous glaciated terrain with Quaternary lavas and tuffs that extend 40 km westward from the Argentine frontier and 30 km north-south from the Río Campanario to Laguna Fea. Complementing recent investigations of postglacial volcanism and the ongoing geophysical unrest around the lake, we here review the longer eruptive history that spanned the entire Quaternary.

The distributed rear-arc LdM volcanic field is contiguous with the Tatara-San Pedro stratovolcano complex on the volcanic front of the Quaternary Andean arc. The LdM field has had only a few large edifices, but we identified at least 140 separate vents, from which >350 km³ of products have erupted since 1.5 Ma. Eruptive products of 14 (early and middle Pleistocene) stratocones and shields, and of ∼125 monogenetic cones, domes, and lava flows, were mapped on foot, studied petrographically, and chemically analyzed. More than 80 ⁴⁰Ar/³⁹Ar and K-Ar ages have been determined to calibrate the Pleistocene eruptive sequence. An extensive welded ignimbrite erupted at 1.5 Ma and was followed by another at ∼950 ka, producing a 12 x 8 km-wide caldera that underlies the north part of the lake basin and the ruggedly eroded highlands north of it. Outside the caldera, the southern two-thirds of the lake basin is a drainage network cut on Tertiary andesites and dacites.

A ring of ∼29 postglacial rhyolite and rhyodacite coulees and domes plus associated pyroclastic deposits that erupted from >30 separate vents (and together cover ∼100 km²) encircles the lake. The large number of postglacial silicic vents around the lake basin, several comagmatic multi-vent compositional arrays, and scarcity of mafic enclaves in the rhyolites are features that suggest growth of a latest Pleistocene to Holocene magma reservoir beneath the LdM Basin. The Barrancas center on the divide southeast of the Basin has an additional 21 lavas from 15 vents and represents a second independent postglacial rhyolitic reservoir. About 21 postglacial mafic and intermediate eruptive units accompany the rhyolites around the margins of the LdM Basin. Ongoing work by Fierstein et al. (this volume) has brought the total number of postglacial vents recognized to more than 73 and has determined ∼70 radiocarbon dates that bracket the abundant tephra deposits, thus providing a 17,000-year-long calibration of the postglacial eruptive sequence.

In addition to the many postglacial silicic units, glacially eroded silicic lavas yield ages of 3.7, 2.5, 2.4, 2.0, 1.6, and 1.35 Ma, and 924, 880, 712, 695, 680, 460, 335, 240, 203, 114, 97, 83, and 25 ka, providing evidence of a prolonged history of explosive silicic eruptions from vents scattered throughout the volcanic field. Production of widely distributed rhyolites throughout the long history of the volcanic field demonstrates intensive crustal processing as well as the enduring potential for explosive eruptions.

For the Quaternary LdM volcanic field, chemical analyses define an array continuous from 49% to 77.6% SiO₂, medium-K toward its mafic end (1.5% K₂O @ 55% SiO₂) but high-K at its silicic end (4.5% K₂O @ 75% SiO₂). Quaternary eruptive units include 5 basalts, ∼30 mafic andesites (52–57% SiO₂), 33 andesites, 11 dacites (63–68% SiO₂), 25 rhyodacites, ∼27 rhyolites (>72% SiO₂), and 6 ignimbrites (andesitic to rhyolitic). None of the basalts is primitive, and most of the mafic rocks display petrographic and/or chemical evidence for diverse crustal contributions (Hildreth et al., 2010).