Phenocryst

About: Phenocryst is a research topic. Over the lifetime, 4132 publications have been published within this topic receiving 158441 citations.

High-Ca, low-alkali carbonatite volcanism at Fort Portal, Uganda

Abstract: A Quaternary volcanic field at Fort Portal, SW Uganda, contains approximately 50 vents that erupted only carbonatite. The vents are marked by monogenetic tuff cones defining two ENE-trending belts. Lava from a fissure at the west end of the northern belt formed a flow 0.3 km2 in area and 1–5 m thick. The lava is vesicular throughout with a scoriaceous top, and probably formed by agglutination of spatter from lava fountains. “Phenocrysts” are olivine, clinopyroxene, phlogopite, and titanomagnetite enclosing blebs of pyrrhotite. Rims of monticellite, gehlenite, and reinhardbraunsite surround olivine, clinopyroxene and phlogopite, and magnetite is rimmed by spinel. The reaction relations suggest that these “phenocryst” phases are actually xenocrysts, perhaps from a source similar to that which supplied phlogopite clinopyroxenite xenoliths in the Katwe-Kikorongo volcanic field 75 km SW of Fort Portal. The groundmass of fresh carbonatite lava consists of tabular calcite, spurrite, periclase, hydroxylapatite, perovskite, spinel, pyrrhotite, and barite. The lava was readily altered; where meteoric water had access, spurrite and periclase are lacking, and some calcite is recrystallized. Vesicles in lava and rare dike rocks are partly filled with calcite, followed by jennite and thaumasite. Pyroclastic deposits cover 142 km2 and are far more voluminous than lava. Carbonatite ejecta were identical to lava in primary mineralogy, but are much more contaminated by crustal rock fragments and xenocrysts. At Fort Portal, eruption of a CaO-MgO-CO2-SiO2-P2O5-SO2-H2O-F liquid was unaccompanied by that of a more silica-rich or alkali-rich liquid. Alkali-rich carbonatite lavas and pyroclastic deposits have been documented elsewhere in East Africa, and calcite-rich volcanic carbonatites have been attributed to replacement of magmatic alkali carbonates by calcite. However, the alkali-poor volcanic carbonatites at Fort Portal were not formed by leaching of alkalis in meteoric water; tabular calcite is not pseudomorphous after alkali carbonates such as nyerereite. The Fort Portal magma was low in alkalis at the time of eruption.

Mineral and chemical variations within an ash-flow sheet from Aso caldera, Southwestern Japan

Abstract: Although products of individual volcanic eruptions, especially voluminous ash-flow eruptions, have been considered among the best available samples of natural magmas, detailed petrographic and chemical study indicates that bulk compositions of unaltered Pleistocene ash-flow tuffs from Aso caldera, Japan, deviate significantly from original magmatic compositions. The last major ash-flow sheet from Aso caldera is as much as 150 meters thick and shows a general vertical compositional change from phenocryst-poor rhyodacite upward into phenocryst-rich trachyandesite; this change apparently reflects in inverse order a compositionally zoned magma chamber in which more silicic magma overlay more mafic magma. Details of these magmatic variations were obscured, however, by: (1) mixing of compositionally distinct batches of magma during upwelling in the vent, as indicated by layering and other heterogeneities within single pumice lumps; (2) mixing of particulate fragments—pumice lumps, ash, and phenocrysts—of varied compositions during emplacement, with the result that separate pumice lenses from a single small outcrop may have a compositional range nearly as great as the bulk-rook variation of the entire sheet; (3) density sorting of phenocrysts and ash during eruption and emplacement, resulting in systematic modal variations with distance from the caldera; (4) addition of xenocrysts, resulting in significant contamination and modification of proportions of crystals in the tuffs; and (5) ground-water leaching of glassy fractions during hydration after cooling. Similar complexities characterize ash-flow tuffs under study in southwestern Nevada and in the San Juan Mountains, Colorado, and probably are widespread in other ash-flow fields as well. Caution and careful planning are required in study of the magmatic chemistry and phenocryst mineralogy of these rocks.

Two contrasting styles of interaction between basic magmas and continental crust in the British Tertiary Volcanic Province

Abstract: Some of the magmas that were extruded to form the 59 Ma, early, plateau-forming, basalt-dominated floods in the British Tertiary Volcanic Province mixed with small amounts of sialic melt during their uprise through the continental crust, while others underwent concomitant fractional crystallization and crustal assimilation. Magma batches of the first group originated in the uppermost asthenosphere and mostly ponded at the Moho, where they crystallized fractionally to form a basalt-benmoreite suite. The major element compositions of members of this series show that they underwent negligible further fractional crystallization between the Moho and the surface. Nevertheless, they paused long enough during their uprise to dissolve their high-pressure phenocrysts and, in many cases, to mix with small amounts of acid melt from lower crustal, granulite-facies, Archaean Lewisian leucogneisses. Basalts and tholeiitic andesites of the second group outcrop locally at the base of the lava piles in Mull and Skye. Their major element compositions show that they equilibrated within the upper third of the crust, under conditions approximating to anhydrous 1-atm cotectic equilibria. Most of these lavas are in SW Mull, where Proterozoic Moine metasediments form the uppermost crust. The major and trace element and isotopic compositions of the SW Mull basal lavas show that they assimilated substantial amounts of Moine metasediments progressively as they fractionated within the upper crust. Pb isotope data reveal that, before their final upper crust evolution, some of the SW Mull basal lavas had already mixed with small amounts of lower crust acid melts.