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Constraining the origin of the adakitehigh-Nb basalt (HNB) association in Baja California, Mexico, is critical to a better understanding of global arc magmatism. Currently the preferred explanation for the close spatial and temporal association of the two rock suites is through melting of the basaltic portion of the suhducted FaralIon-Cocos plate, thus providing support for the slab-melting origin of adakites elsewhere. Moreover, a tectono-magmatic model involving the production of both adakite and HNB from slab melts offers a comprehensive explanation for the origin of the atypical, arc-related, postsubduction magmatism in Baja California. This paper proposes alternative models for the origin of HNB and postsubduction magmatism in Baja California, wherein the unusual geologic setting of western Mexico and westward movement of North America permitted the influx of Pacific asthenosphere beneath the adjacent Gulf of California after the cessation of subduction. Unlike the previous tectono-magmatic model, the new models propose that the asthenosphere provided a direct source for postsubduction tholeiitic and rare alkali magmas that were erupted in Baja California as tholeiites and HNB, respectively. Fractional crystallization of some of the HNB magmas plus assimilation of tholeiitic materials produced Nb-enriched basalts (NEB). The influx of Pacific asthenosphere alter the cessation of subduction also provided thermal energy to melt the mafic lower Baja California crust, producing adakite rocks, and the preexisting metasomatized mantle wedge, producing bajaites and calc-alkaline magmas. Keywords: High-Nb basalts, adakite, subduction zone, bajaite, near-ridge seamount, Baja California.

Large explosive eruptions are generally rare, random events in the history of any particular volcano, volcanic area, or worldwide. In the Taupo Volcanic Zone, New Zealand, temporal clustering of eruptions occurs on a 15 smaller eruptions over a total ~100 k.y. period. After a precursor eruption from a nearby source (and a break of years to decades), these paired eruptions in turn generated a wet ash-fall deposit and a dry pumice-fall deposit; the Mamaku ignimbrite (>145 [km.sup.3] magma); a fine-grained vitric ash-fall deposit; then the Ohakuri ignimbrite (>100 [km.sup.3] magma). Rotorua and Ohakuri, spaced ~30 km apart, are the inferred collapse calderas associated with the Mamaku and Ohakuri ignimbrites, respectively. The early wet and dry fall deposits came from southerly sources, close to or within the subsequent Ohakuri caldera, while the fine-grained vitric ash is inferred to represent a co-ignimbrite ash from the Mamaku ignimbrite. At its southwest margin, the Mamaku ignimbrite overlies, but is also intercalated within and then overlain by, the pumice fall deposit, demonstrating that at least two widely spaced vents were active simultaneously for part of the eruption sequence. The post-Mamaku vitric ash-fall deposit underwent only trivial reworkiug prior to emplacement of the Ohakuri ignimbrite. This and other field evidence imply continuity, or time gaps of only days to months, in the whole paired sequence. Syneruptive volcanotectonic faulting may have permitted accumulation of >400 m of nonwelded Ohakuri ignimbrite through graben subsidence. Posteruptive faulting within years to decades of the eruption produced an ~300 m extra-caldera offset of the Mamaku ignimbrite and collateral subsidence of a >40 [km.sup.2] area immediately south of Rotorua caldera. Temporal linkages between ignimbrite eruptions and graben subsidence, the NNE-SSW alignment of associated faulting between the Rotorua and Ohakuri calderas, and the eruption-related subsidence indicate a tectonic control on volcanism associated with Taupo Volcanic Zone rifting processes. Statistical forecasts of the frequency of large-volume explosive events based on averages may be inaccurate because of tectonic triggering effects. Keywords: caldera collapse, collateral subsidence, ignimbrite eruption, Taupo Volcanic Zone, Mamaku ignimbrite, Ohakuri ignimbrite.

A detailed textural and chemical study of a section through the 121 Ma Ossipee ring dyke in New Hampshire provides a comprehensive record of magmatic and volcanic events during caldera formation. Phenocryst-rich quartz syenite is the main rock type; syenite, rhyolite, and basalt form small mingled dykes, xenoliths, and magmatic enclaves. Enclaves of fine-grained syenite have positive Eu anomalies, are more mafic, and are depleted in crystals relative to host quartz syenite. Anorthoclase phenocrysts in enclaves have compositions similar to the bulk enclave; high CaO contents in the crystals indicate magmatic temperatures up to 1050 [degrees]C. Rhyolite in the ring dyke is texturally and chemically equivalent to ignimbrite inside the caldera. Basalt occurs both as xenoliths and megabreccias from earlier volcanoes and as dykes that mixed and mingled with rhyolitic dykes during eruption. We suggest the following sequence of events: (1) Convection, crystal settling, and filtration of melt away from solidification fronts formed a zoned magma chamber containing voluminous rhyolite overlying crystal-rich quartz syenite and cumulate syenite. (2) Basalt partially melted the anorthoclase syenite cumulate at the base of the chamber to form a hot mobile syenitic liquid, now represented by enclaves in the ring dyke. (3) Continued intrusion of basalt replenished and rejuvenated the Ossipee chamber, driving large eruptions of rhyolitic ignimbrite that ponded in the caldera as the chamber roof collapsed. (4) Caldera collapse stirred the chamber, mingling and mixing rhyolitic, quartz syenitic, and syenitic magma. Vortices around subsiding blocks during caldera collapse caused the mixing and mingling. (5) Quartz syenite, rhyolite, and small volumes of basalt magma were intruded simultaneously into the ring dyke, causing the basalt and rhyolite to mingle explosively. The rejuvenation of the magma chamber and subsequent subsidence of the caldera produced a mixed and mingled crystal mush now preserved in the ring dyke. The ring dyke represents residual magma from a caldera-forming eruption. Keywords: caldera, ring dyke, rhyolite, syenite, rejuvenation, replenishment.

The Sudbury Igneous Complex of Ontario, Canada, is the remnant of a voluminous melt sheet produced in a few minutes by impact of a massive meteorite into continental crust 1.85 Ga ago. The transient cavity and melting zone reached the Moho and instantly (~2 min) relaxed to form a more familiar large, shallow crater holding a thick, superheated (~1700[degrees]C) melt sheet covered by ~2 km of breccia. There is little about the resulting bimodal igneous complex that resembles crystallization of well-known sheet-like bodies of similar composition. Yet, the norite and granophyre exhibit a remarkable similarity in isotopic and trace element compositions, suggesting an intimate common parentage from the surrounding crust. This petrogenetic enigma is explained here as a natural, unavoidable consequence of the impact process in the rapid formation of a superheated magmatic emulsion, which we take as the high-temperature equivalent of breccia. A wide spectrum of viscously discrete, interdispersed parcels of mafic and felsic liquids, reflecting the compositional heterogeneity of the target crustal materials, formed the emulsion. Within days to mouths, the emulsion components separated according to their relative densities into a bimodal norite-granophyre assemblage that formed the basic structure of the present Sudbury Igneous Complex. There is clear evidence of this emulsion in the earliest dikes (i.e., offsets), which likely give the earliest state of the nascent melt sheet. Immediately following emulsion separation, the strongly superheated bimodal melt sheet underwent vigorous thermal convection in each layer. These convective motions homogenized and rapidly cooled the magma to the liquidus temperatures, whereupon convection ceased. The pattern of convection was pinned in place by the embayment topography of the crater floor, which in turn played a pivotal role in directing sulfide deposition into the embayments. All further cooling was by conduction of heat through the upper and lower boundaries during which time solidification fronts were established and propagated inward from the upper and lower margins. There is clear evidence of solidification from the floor upward and the roof downward. Minimal differentiation and compositional modification took place throughout cooling and solidification. Nevertheless, during the solidification stage, granitic rock fragments on the crater floor and rafts of fallback breccia from the thick overlying Onaping Formation became unstable and entered the melt sheet, and the partially melted remnants collected at the interface between the norite and granophyre. Some interstitial melt from the norite also percolated upward, and, altogether, the blocks and melt produced the distinctive chemical and physical characteristics of the unusual Transition Zone. The Sudbury melt sheet is, in essence, a full-scale magmatic experiment. The conditions of formation, relative to any other large terrestrial magma, are 'precisely' known. Thus the clear lack of any significant modal layering, the overall homogeneous nature of each unit, and the lack of any significant chemical differentiation through crystal fractionation establish Sudbury as a valuable example of what does not happen under the initial conditions long assumed to prevail at the formation of most large magma chambers. Keywords: magma, viscous emulsion, Sudbury, melt sheet, impact melt, differentiation.