Anatomy of the Xáltipan ignimbrite at Los Humeros Volcanic Complex; the largest eruption of the Trans-Mexican Volcanic Belt

Abstract

The Xáltipan ignimbrite (ca. 290 km3 DRE) represents the major explosive event from Los Humeros Volcanic Complex (LHVC) that can be ranked as the largest caldera-forming eruption of the Trans-Mexican Volcanic Belt (TMVB). This eruption marked the onset of an explosive period of LHVC producing the collapse of the main Los Humeros caldera at 164 ± 4.2 ka. This paper provides a detailed characterization of the Xáltipan ignimbrite anatomy and eruptive chronology that records a multi-phase, single eruption. This eruption resulted in more complex architecture and lithofacies variations of the ignimbrite than previously reported. To support these findings, we present two representative composite Xáltipan stratigraphic columns based on two regional reference sections, together with a welding lithofacies map, digital mapping, energy line projections, geologic sections and high-resolution petrophysical characterization (micro-CT, He-pycnometry). Additionally, we have reevaluated the magnitude (DRE volume), and the total erupted mass of the deposit by taking into account critical primary and post-emplacement processes such as lithic concentration, welding grade, and bulk loss due to erosion. The internal architecture of the Xáltipan ignimbrite is composed of two flow units and two pumice fallout deposits (a basal one and an intercalated layer), which formed as a result of a voluminous and sequential eruption that can be resumed in four eruptive phases. The initial stage of the eruption produced a basal pumice fallout (BPF) (phase 1) derived from a buoyant plume that eventually collapsed, generating pyroclastic density current (PDC) deposits with intra-ignimbrite pumice fallout beds. This event was followed by the climactic, caldera-forming, boiling over-type eruptive phase (phase 2) that generated voluminous PDCs of the lower flow unit (LFU). As the eruption waned, a brief eruptive hiatus occurred, immediately followed by a second explosive event in the western rim of the caldera with a boiling-over type eruptive column depositing a discrete pumice fallout layer (IPF) (phase 3), which later collapsed depositing less voluminous PDC (UFU) in the western flank of the Los Humeros caldera (phase 4). The anatomy of the deposits shows that the Xáltipan ignimbrite was emplaced in different behaviors: radially in the proximal parts and channelized and confined in the distal parts, showing significant welding variations that range from non-to-highly welded conditions (welding ranks I-VI) controlled mainly by the paleotopography. The size calculations indicate that the deposits have an aerial extent of ca. 4600 km2, an ejected bulk volume of ca. 330 km3 with a DRE volume of ca. 290 km3, and an ejected mass of 1.1 × 1012 kg, which points out to a paroxysmal eruption (VEI 6-7).