Campi Flegrei is a densely populated active volcanic field. Two major explosive volcanic events have led to the formation of nested calderas. Detailed stratigraphy of the volcanic rocks outcropping in part of this area contributes toward a better understanding and definition of the volcanic hazard. Our research activity focuses on the southwestern sector of Campi Flegrei including Procida Island. This area is particularly suitable for stratigraphic reconstruction due to the thick pyroclastic sequences exposed on the coastal cliffs. These sequences include several paleosol horizons and substantially represent the products of all the volcanic activity of Campi Flegrei. The onset of volcanic activity in this area is represented by products related to the activity of scattered vents of unconstrained age. From 74 to 55 ka, they were mantled by products erupted in the nearby island of Ischia. Circa 39 ka, the Campanian Ignim-brite eruption occurred in the Campi Flegrei area, producing a large caldera. A thick succession of welded pyroclasts, lithic breccias, and associated ash- and pumice-flow deposits was emplaced in the proximal area. The local activity resumed at 19–17 ka with the formation of monogenetic volcanoes. A phreatoplinian eruption (Neapolitan Yellow Tuff) occurred at 15 ka, which produced a second, nested caldera. Stratified yellow tuff volcanoes, ranging in age between 9 and 5 ka, developed along the Neapolitan Yellow Tuff caldera boundary. Minor tephra layers testify to the final explosive activity vented in this area.

ABSTRACT The Mogollon-Datil volcanic field is a 40–24 Ma cluster of calderas that formed during ignimbrite flare-up eruptions in southern New Mexico associated with sub-duction, and possible delamination, of the Farallon plate beneath the North American plate. This study uses magmatic zircon sampled from four ignimbrites from a nested caldera system and an additional ignimbrite located outside of the nested system to compare the processes and timing of magma accumulation in southern New Mexico. These ignimbrites include: the Whitewater Tuff, the Cooney Canyon Tuff, the Davis Canyon Tuff, and the Shelley Peak Tuff from the Mogollon Mountains and the Bell Top 4 Tuff from the Uvas volcanic field. The ignimbrites range from crystal-poor, high-silica rhyolite to crystal-rich, low-silica rhyolite. We compare previous 40 Ar/ 39 Ar sanidine eruption ages to new U-Pb crystallization ages and trace-element compositions of zircon. Weighted mean zircon ages define two magmatic groups. Group one includes the Bell Top Tuff (34.5 ± 0.5 Ma), the Cooney Canyon Tuff (34.8 ± 0.8 Ma), and the Whitewater Creek Tuff (36.2 ± 0.4 Ma). The second group includes the Davis Canyon Tuff (28.7 ± 0.5 Ma) and the Shelley Peak Tuff (29.6 ± 0.5 Ma). Weighted mean zircon ages are within published 40 Ar/ 39 Ar ages, with the exception of the Shelley Peak Tuff, which is ~1 m.y. older. Hafnium contents and Th/U and Yb/Gd ratios suggest the dominant mechanism that produced eruptible melt was rejuvenation or remobilization of a crystal mush accompanied by minimal partial melting of the continental crust.

Many source areas for voluminous ash flows have histories of repeated catastrophic eruption and caldera-forming collapse within a few million years or less. Examples range in complexity from simple calderas related to eruption of a single cooling unit, through sequential collapses of the same caldera related to successive eruptions of separate cooling units, and nested calderas related to eruption of successively smaller cooling units between successively longer time intervals, to complexes of overlapping calderas related to eruption of several cooling units from overlapping source areas within a large volcanic field. Some caldera complexes are related to collapse of adjacent, simultaneously active ring-fracture zones by immediately successive ash-flow eruptions to form composite sheets. Despite this wide range in complexity, a basically similar primary sequence of events in each area reflects the formation of a large volume of magma, at least part of which rose to form shallow epizonal magma chambers. In some instances, only a single high-level chamber formed and produced ash-flow eruption and collapse one or more times. In others, several high-level chambers formed either simultaneously or in succession; where several chambers were active simultaneously, major ash-flow eruption and collapse at one chamber may have triggered events in an adjacent one. Variations in the pattern of ash-flow–caldera relations reflect variations in relative volumes of magma in the chambers, of tectonic controls on intrusions and the timing of eruptions, and of the continuity of magma generation beneath a volcanic region. These diverse patterns represent variations on R. L. Smith’s concept that voluminous ash-flow eruptions and related caldera formation are surficial expressions of the degassing, but not the total emptying, of large magma chambers at high levels in the Earth’s crust. Study of these systems offers unique insights into the physical and chemical processes and evolution of silicic magmas.