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At least 235 records · Page 13Linked to original sources
USGS1997

The geology and petrology of Mauna Kea Volcano, Hawaii — A study of postshield volcanism

Mauna Kea Volcano, on the Island of Hawaii, is capped by lavas of alkalic and transitional basalt (Hamakua Volcanics) erupted between approximately 250-200 and 70-65 ka and hawaiite, mugearite, and benmoreite (Laupahoehoe Volcanics) erupted between approximately 65 and 4 ka. These lavas, which form the entire subaerial surface of the volcano, issued from numerous scattered vents and are intercalated on the upper slopes with glacial deposits. The lavas record diminishing magma-supply rate and degree of partial melting from the shield stage through the postshield stage. Much of the compositional variation apparently reflects fractionation of basaltic magma in reservoirs within and beneath the volcano.

Hawaii↗
USGS2008

Chemistry, mineralogy, and petrology of amphibole in Mount St. Helens 2004-2006 dacite

Textural, compositional, and mineralogical data are reported and interpreted for a large population of clinoamphibole phenocrysts in 22 samples from the seven successive dacite spines erupted at Mount St. Helens between October 2004 and January 2006. Despite the uniformity in bulk composition of magma erupted since 2004, there is striking textural and compositional diversity among amphibole phenocrysts and crystal fragments that have grown from, partly dissolved in, or been accidentally incorporated in the new dacite. This study demonstrates that magma erupted throughout the current dome-building episode is the end product of small-scale, thorough mixing of multiple generations of crystal-laden magma. The mixed amphibole population provides important clues to magma conditions within the dacite magma reservoir prior to ascent and, to some extent, the dynamics of mixing and ascent. The predominant amphibole in new dome rock ranges from moderate- to high-alumina tschermakite and magnesiohastingsite compositions. As substantiated by major- and trace-element geochemistry and barometry calculations, this compositional range of crystals, along with plagioclase, orthopyroxene, and iron-titanium oxide, is likely to have precipitated from dacite magma over a range of pressures and temperatures consistent with experimentally determined phase relations (~900&deg;C to ~800&deg;C between 100 MPa and ~350-400 MPa or ~4-km and 13.5-15-km depth). Along with traceelement characteristics, textural and compositional data help to distinguish some low-alumina magnesiohornblende crystals as xenocrysts. The diverse range in composition of amphibole in all samples of 2004-6 dacite, and the complex zonation observed in many phenocrysts, suggests a well-mixed source magma with components that are subjected to repeated heating and (or) pressurization within this pressure-temperature window. Amphibole textural and compositional diversity suggest dynamic conditions in the upper-reservoir zone, which has been tapped steadily during ~2 years of continuous and monotonous eruption. This well-mixed crystal mush is likely to have been subjected to repeated injection of hotter magma into cooler crystal-laden magma while simultaneously assimilating earlier generations of dacitic roof material and surrounding gabbroic rock. Decompression-related reaction rims around subhedral, rounded, resorbed, and fragmented amphibole phenocrysts, regardless of composition, indicate that this mixed-crystal assemblage was being broken, abraded, and dissolved in the magma as a result of mechanical mixing before and during early stages of ascent from conduit roots extending into a mushy cupola of the shallow reservoir. In the earliest lava samples (October 2004), amphiboles with <3-&mu;m rims associated with a glassier matrix than later samples suggest a slightly faster ascent rate consistent with the relatively high eruptive flux of the earliest phases of dome extrusion. Reaction rim widths of ~5 &mu;m on amphibole in all subsequently extruded lava result from a steady influx and upward transport of magma from 3.5-2.5-km to ~1-km depth at rates of ~600 to ~1,200 m/day, through a conduit less than 10 m in radius. Slower ascent rates inferred from volumetric-flux and matrixcrystallization parameters are explained by a widening of the conduit to greater than 60 m radius within 1 km of the surface.

Washington↗
USGS2021

Petrology and geochronology of 1.48 to 1.45 Ga igneous rocks in the St. Francois Mountains terrane, southeast Missouri

The igneous geology of the St. Francois Mountains terrane in southeast Missouri is dominated by the products of 1.48 to 1.45 billion year old volcanic and plutonic magmatism but also includes volumetrically minor, compositionally bimodal contributions added during plutonism between 1.34 and 1.27 billion years ago. The 1.48 to 1.45 billion year old igneous rocks in the St. Francois Mountains terrane are bimodally distributed between volumetrically dominant felsic rocks and volumetrically minor rocks with mafic to intermediate compositions. All of these rocks are ferroan, which like most of their trace element abundances, suggests a genesis associated with farfield intraplate extensional tectonism and decompression-related magmatism. The diversity of compositions among 1.48 to 1.45 billion year old igneous rocks in the St. Francois Mountains terrane probably reflects mixtures of mantle-derived mafic inputs and low-degree partial melting of more evolved crustal protoliths. Newly determined ages define essentially continuous magmatism during the 30-million-year period between 1.48 and 1.45 billion years ago. The products of this magmatism are essentially coeval, whether intrusive or extrusive or having mafic, intermediate, or felsic compositions. In addition, the iron oxide-apatite (for example, Pea Ridge) and likely the iron oxide-copper gold (Boss) deposits in the St. Francois Mountains terrane have ages coincident with this magmatic episode. Spatial and temporal relations between 1.48 to 1.45 billion year old igneous rocks in the St. Francois Mountains terrane and the mineral deposits they host suggest the associated magmatic and mineralization processes are also genetically related. Geochemical, petrographic, geochronologic, and terrane-wide physical characteristics of the 1.48 to 1.45 billion year old igneous rocks in the St. Francois Mountains terrane are consistent with an origin involving extension well inboard from the margin of the Laurentian craton, associated mantle upwelling, lower crustal melting in response to mantle-derived thermal inputs, and mixing of mantle- and juvenile lower crustal-derived melts. Significant major and trace element compositional dispersion characteristics of these rocks likely reflect midcrustal magma reservoir fractionation of their principal rock-forming minerals. The resultant magmas constitute a series of variably hybridized reservoirs, emplaced at upper levels in the crust, that form a series of plutonic and associated eruptive products.

Missouri↗
USGS1957

Geology of Saipan, Mariana Islands; Part 2, Petrology and soils

The rocks that comprise the volcanic formations of Saipan are of two principal types: dacites, which are characteristically glassy, and andesites, which are comparatively crystalline. The dacites consist primarily of silicic glass, oligoclase, and silica minerals (quartz, tridymite, cristobalite, chalcedony, and opal). Minor constituents in these rocks are green hornblende, biotite, magnetite, and hematite. The andesites are composed principally of labradorite, hypersthene, augite, and subcalcrc augite. Minor but also characteristic constituents of the andesites are quartz, tridymite, cristobalite, anorthoclase, and accessory magnetite, ilmenite, rutile, and apatite. Nine varieties of dacite and andesite are recognized on the basis of chemical composition, mineralogy, and texture. These are dacite, dacite vitrophyre, dacite perlite, hornblende-bearing dacite porphyry, augite-hypersthene andesite, quartz-bearing augite-hypersthene andesite, quartz-bearing augite-hypersthene andesite porphyry, augite andesite, and hypersthene andesite.

Professional Paper↗