Description: The combination of new 40 Ar/ 39 Ar and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U/Pb zircon ages with published geochemistry of the volcanic and plutonic rocks of the Organ caldera complex (New Mexico) provides a framework for understanding the origin of these silicic magmas and the time scales of caldera magmatism. The Organ caldera complex erupted three ignimbrites: the 36.45 ± 0.08 Ma Cueva Tuff, the 36.23 ± 0.14 Ma Achenback Park Tuff, and the 36.03 ± 0.16 Ma Squaw Mountain Tuff. The ignimbrite sequence is zoned from a crystal-poor, high-SiO 2 rhyolite at the base to a crystal-rich, low-SiO 2 rhyolite at the top. The ignimbrite sequence is intruded by the zoned Organ Needle pluton, which has previously been interpreted to be the nonerupted silicic cap and less-differentiated residual crystal mush of the caldera-forming magma chamber. The geochronology of the Organ Needle pluton indicates that these silicic magmas were generated via shallow-crustal in situ differentiation. U/Pb zircon and many 40 Ar/ 39 Ar biotite ages of the different phases of the Organ Needle pluton are temporally indistinguishable from the Squaw Mountain Tuff eruption age, indicating that this pluton was emplaced and rapidly cooled during or shortly after the youngest caldera eruption. New ages also suggest that Organ caldera magmatism was characterized by protracted emplacement of magmas following caldera collapse. Volcanism continued after the caldera eruptions until at least 35.7 Ma. Three silicic postcaldera plutons were emplaced between 36.0 and 34.3 Ma. Multiple diffusion domain thermal modeling of plutonic K-feldspar suggests reheating events, possibly related to postcaldera magmatism, at 34 Ma, 32–30 Ma, and as young as 26 Ma. Geochronology, geochemistry, and field-based observations of the Organ Needle pluton and caldera-forming ignimbrites support the hypothesis that some plutonic rocks are the nonerupted, geochemically complementary residues of large-volume silicic eruptions.

Description: Among large ignimbrites, the Bonanza Tuff and its source caldera in the Southern Rocky Mountain volcanic field display diverse depositional and structural features that provide special insights concerning eruptive processes and caldera development. In contrast to the nested loci for successive ignimbrite eruptions at many large multicyclic calderas elsewhere, Bonanza caldera is an areally isolated structure that formed in response to a single ignimbrite eruption. The adjacent Marshall caldera, the nonresurgent lava-filled source for the 33.9-Ma Thorn Ranch Tuff, is the immediate precursor for Bonanza, but projected structural boundaries of two calderas are largely or entirely separate even though the western topographic rim of Bonanza impinges on the older caldera. Bonanza, source of a compositionally complex regional ignimbrite sheet erupted at 33.12 ± 0.03 Ma, is a much larger caldera system than previously recognized. It is a subequant structure ~20 km in diameter that subsided at least 3.5 km during explosive eruption of ~1000 km 3 of magma, then resurgently domed its floor a similar distance vertically. Among its features: (1) varied exposure levels of an intact caldera due to rugged present-day topography—from Paleozoic and Precambrian basement rocks that are intruded by resurgent plutons, upward through precaldera volcanic floor, to a single thickly ponded intracaldera ignimbrite (Bonanza Tuff), interleaved landslide breccia, and overlying postcollapse lavas; (2) large compositional gradients in the Bonanza ignimbrite (silicic andesite to rhyolite ignimbrite; 60%–76% SiO 2 ); (3) multiple alternations of mafic and silicic zones within a single ignimbrite, rather than simple upward gradation to more mafic compositions; (4) compositional contrasts between outflow sectors of the ignimbrite (mainly crystal-poor rhyolite to east, crystal-rich dacite to west); (5) similarly large compositional diversity among postcollapse caldera-fill lavas and resurgent intrusions; (6) brief time span for the entire caldera cycle (33.12 to ca. 33.03 Ma); (7) an exceptionally steep-sided resurgent dome, with dips of 40°–50° on west and 70°–80° on northeast flanks. Some near-original caldera morphology has been erosionally exhumed and remains defined by present-day landforms (western topographic rim, resurgent core, and ring-fault valley), while tilting and deep erosion provide three-dimensional exposures of intracaldera fill, floor, and resurgent structures. The absence of Plinian-fall deposits beneath proximal ignimbrites at Bonanza and other calderas in the region is interpreted as evidence for early initiation of pyroclastic flows, rather than lack of a high eruption column. Although the absence of a Plinian deposit beneath some ignimbrites elsewhere has been interpreted to indicate that abrupt rapid foundering of the magma-body roof initiated the eruption, initial caldera collapse began at Bonanza only after several hundred kilometers of rhyolitic tuff had erupted, as indicated by the minor volume of this composition in the basal intracaldera ignimbrite. Caldera-filling ignimbrite has been largely stripped from the southern and eastern flank of the Bonanza dome, exposing large areas of caldera-floor as a structurally coherent domed plate, bounded by ring faults with locations that are geometrically closely constrained even though largely concealed beneath valley alluvium. The structurally coherent floor at Bonanza contrasts with fault-disrupted floors at some well-exposed multicyclic calderas where successive ignimbrite eruptions caused recurrent subsidence. Floor rocks at Bonanza are intensely brecciated within ~100 m inboard of ring faults, probably due to compression and crushing of the subsiding floor in proximity to steep inward-dipping faults. Upper levels of the floor are locally penetrated by dike-like crack fills of intracaldera ignimbrite, interpreted as dilatant fracture fills rather than ignimbrite vents. The resurgence geometry at Bonanza has implications for intracaldera-ignimbrite volume; this parameter may have been overestimated at some young calderas elsewhere, with bearing on outflow-intracaldera ratios and times of initial caldera collapse. Such features at Bonanza provide insights for interpreting calderas universally, with respect to processes of caldera collapse and resurgence, inception of subsidence in relation to progression of the ignimbrite eruption, complications with characterizing structural versus topographic margins of calderas, contrasts between intra- versus extracaldera ignimbrite, and limitations in assessing volumes of large caldera-forming eruptions. Bonanza provides a rare site where intact caldera margins and floor are exhumed and exposed, providing valuable perspectives for understanding younger similar calderas in some of the world’s most active and dangerous silicic provinces.