The association of Hawaiian-Emperor volcanism with a large-scale central Pacific anisotropy anomaly at ~150 km depth can be explained by tapping of shallow melt sources in a perisphere/LLAMA (layer of lateral advection of mass and anisotropy) model. The origin of the anisotropy anomaly can be traced to the formation of a phlogopite-garnet-pyroxenite assemblage in the perisphere beneath an island arc on the Stikine terrane of the North American Cordillera in the Carboniferous. The pyroxenites were formed when subduction-related melts invaded the mantle wedge at ~150–200 km depth. The enriched region inherited the thermal profile of the mantle wedge, along with a solar-like noble gas isotopic composition from earlier fluxing of hydrothermal fluids between interplanetary dust particle–bearing deep-sea sediments and ultramafic layers of the oceanic crust prior to subduction. After termination of subduction, the enriched perisphere was displaced to the northeast beneath the Farallon plate, and then to the northwest beneath the Izanagi and Pacific plates, eventually becoming distorted into the shape of the present-day central Pacific anisotropy anomaly. During the thermal equilibration time, estimated at ~170 m.y., the phlogopite-garnet-pyroxenite assemblage followed a horizontal trajectory in pressure-temperature ( P-T ) space. As the P-T path crossed the solidi for volatile-bearing pyroxenite compositions, diabatic partial melting generated carbonatitic to alkaline melts which began to ascend and metasomatize shallower levels of the perisphere, carrying with them the geochemical signature of the original pyroxenites. The present central Pacific anisotropy anomaly is the current manifestation of the metasomatized domain. The latter was tapped from the Late Cretaceous to the present, by propagating fractures induced by large-scale plate reorganizations in the northwest of the Pacific Basin, to produce the Hawaiian-Emperor volcanic chain.

ABSTRACT Under fast-moving oceanic plates, the asthenosphere seismic B″ region becomes isolated from the convecting mantle by plate drag and acts as an advecting layer, which can serve as a long-lived source for intraplate volcanism. Geochemical enrichment of B″ can occur via infiltration by melts generated from the breakdown of serpentinite at ~200 km depth in subducting slabs. Ocean-island chains arise when melts generated within metasomatized B″ by shear melting and localized convection are released along lithospheric fractures controlled by the stress field of the plate. Intersection of metasomatized B″ with ocean-ridge systems produces oceanic plateaus. A strong anisotropy anomaly (V SH /V SV >1) at depths of ~150 km in the Pacific asthenosphere marks a metasomatized B″ domain that originated in the western paleo-Pacific basin in the Carboniferous, and that is now associated with Hawaiian volcanism. Metasomatized B″ can be trapped beneath orogenic belts during continental aggregation and tapped by edge-driven convection upon rifting to produce the correlation between intraplate volcanism and the fabric of sutures in opening ocean basins such as the Atlantic Ocean basin.

The Ilopango Caldera, located 10 km east of San Salvador, has erupted voluminous silicic pyroclastics four times in the last 57,000 years. The present caldera has a quasi-rectangular shape and is filled by Lake Ilopango. This paper provides a detailed description of a segment of the intracaldera stratigraphy at Ilopango caldera, with emphasis on the San Agustín Block Unit. Physical volcanology, petrology, and geochemistry establish the depositional environment and eruptive conditions of the intracaldera sequence and help to model the emplacement of the San Agustín Block Unit. The intracaldera stratigraphy comprises a sequence of pyroclastic density currents, unconformably overlain by lacustrine sediments and conformably overlain by the San Agustín Block Unit. A new radiocarbon age on wood near the top of the Lacustrine Unit indicates that a lake was present ≥43,670 years ago. The intracaldera sequence displays abundant evidence of emplacement in a subaqueous environment. The San Agustín Block Unit comprises a basal Fine Ash facies and an overlying Pumice Breccia facies. The basal Fine Ash facies is a hydromagmatic layer containing pumiceous and blocky angular glass shards, aggregates of fine ash and phenocryst fragments, and phenocrysts with a fine ash coating. The overlying Pumice Breccia facies is composed of pumice clasts up to three meters in length. The pumice clasts display a series of jointing textures indicative of hot emplacement and rapid cooling. These two facies suggest an initial subaqueous explosive eruption in which a vesiculated silicic melt fragmented upon contact with the water. When the magma had degassed sufficiently, the eruption style evolved to subaqueous dome growth that spalled quenched pumice clasts from a moderately vesiculated carapace. La Caldera de Ilopango se encuentra a 10 km al este de San Salvador. Ilopango ha tenido cuatro grandes erupciones de piroclastos ricos en sílice durante los últimos 57,000 años. La caldera actualmente tiene forma rectangular y el Lago de Ilopango se encuentra dentro de ella. Este artículo proporciona una descripción detallada de un segmento de la estratigrafía en el interior de la caldera de Ilopango con énfasis en una unidad que se llama “Unidad de Bloque San Agustín.” La volcanología física, petrología y geoquímica describen el medio ambiente deposicional y las condiciones eruptivas de la secuencia en el interior de la caldera y ayudan a modelar el emplazamiento de la “Unidad de Bloque San Agustín.” La secuencia en el interior de la caldera es constituida de flujos piroclásticos discordantes sobre ella yacen sedimentos lacustres y sobre estos últimos yace de manera concordante la “Unidad de Bloque San Agustín.” Una datación reciente con el método de radiocarbón de madera encontrada cerca del techo de la secuencia lacustre indica que un lago ya existia hace ≥43,670 años. La secuencia en el interior de la caldera muestra evidencia de un emplazamiento en un medio ambiente subacuático. La “Unidad de Bloque San Agustín” constituye una facie basal de ceniza fina y una facie superpuesta de pómez brecha. La facie basal de ceniza fina es un estrato hidromagmático conteniendo pedazos de vidrio, algunos en forma de bloque y otros vesiculares con textura de pómez. Contiene además un agregado de ceniza fina con fragmentos fenocristales y fenocristales con una recubierta de ceniza fina. La pómez brecha está compuesta de pómez con una elongación a trece metros. Los clastos pómez muestran diaclasas radiales, diaclasas concéntricas y diaclasas perpendiculares a la superficie que indican una deposición caliente y un enfriamiento rápido. Las dos facies sugieren una erupcíon inicialemente explosiva donde un magma sílice vesiculado es fragmentado cuando se pone en contacto con el agua. Cuando el magma está lo suficientemente desgasificado, la erupcíon desarrolla un domo subacuático des-boronando clastos templados de pómez proveniendo de una caparazón vesiculada.