D

◊ diagenetic alteration ◙ diorites ◊

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dacite

Dacite is a felsic, aphanitic to porphyritic, light to dark gray to black, extrusive igneous rock of composition intermediate between that of andesite and rhyolite (63-68 % SiO2). Like andesite, dacite comprises mostly plagioclase feldspar with biotite, hornblende, and pyroxenes (augite and/or enstatite). Quartz is present within the groundmass or as rounded phenocrysts. Dacites are often found associated with andesites and trachytes. They form lava flows, dikes, and sometimes massive intrusions at the center of volcanoes. Despite containing less silica than rhyolite, dacite can be more viscous and equally as dangerous as rhyolite. Its expolosive potential results from the high crystal content of many dacites, within a relatively high-silica melt matrix. Dacite typically erupts at temperatures of 800-1000°C, and is a common rock type associated with enormous Plinian-style eruptions. Named for Pliny the Younger who described the 79 CE eruption of Vesuvius, Plinian eruptions eject large quantities of tephra and volatiles high into the stratosphere. Dacite was erupted from Mount St. Helens 1980-86, Mount Pinatubo in 1991, and Mount Unzen 1991-1996. Eruptions of relatively gas-poor dacite typically form thick rounded lava flow in the shape of a dome. Dacites were named for the Roman Province of Dacia (Carpathians), and occur in the many localities including the western US, the Andes, Greece, Iceland, Scotland, Spain, and New Zealand.

link: images: hand-specimens: dacite, dacite from Unzen, 2, 3, 4, 5; close-up: plagioclase phenocrysts in dacite porphyry; Newberry Springs dacite outcrop; thin-sections: dacite fsu; dacite from Mt. St. Helens eruption; dacite with zoned plagioclase, xp; dacite; dacite with zoned plagioclase phenocryts and rounded quartz grains; formations: dacite pinnacles, and shattered dacite, Gnarl Ridge, Mt. Hood Wilderness, OR; Wood's Mountain; Unzen Volcano, a dacite dome with an apron of pyroclastic flow deposits; breadcrusted dacite bomb on Kameni Island near Thera volcano, Greece; cracks on this breadcrust bomb developed when the brittle outer surface cooled and cracked due to expansion associated with the continued growth of gas bubbles in the interior (dacite breadcrust bomb (~15 cm across) that erupted from the Mt. St. Helens lava dome); spectacular columnar joints in glassy dacite lava, north side of Cone Glacier valley, west side of Veniaminof volcano; dacite columns; columnar jointing in dacite; Glacier Peak, in the north central Cascade Mountains, is comprised mainly of dacite lava flows that have erupted in the last 770,000 years; Santiaguito dacite dome complex, and cupola dome of Santiaguito with 1999-2004 dacite flow complete with levees, and beginnings of a Vulcanian eruption, 2, 3; Casita is a composite volcano with clustered dacite domes at its peak and deeply dissected morphology; Elden Mountain is a steep-sided exogenous dacite lava dome in the San Francisco Volcanic Field; dacite dome that rose about 30 meters on flank of Mt. St. Helens, and dome, and thermal image, followed by emission of gas and steam; explosive eruption (dacite and rhyolite magma) in Plinian-style eruption of Mt. St. Helens; Black Butte is a complex of four hornblende-dacite domes; Cretaceous mafic dike at right cuts the dated Jurassic (~148 Ma) dacite dike at center, Onion Valley mafic complex and Aberdeen mafic sill complex, felsic (dacitic) Independence dikes cut metavolcanic rocks in the northern Alabama Hills; three parallel dacite lava flows that form Negit island; Trollafell, a dacite dome above Leirufjordur; webpages: Juan de Fuca Ridge and the Blanco Transform Zone; dacite lava as talus on the slopes of submarine cone site; Mars: dacite flow (magenta) and the volcanic cone associated with it have a composition distinct from the basaltic lavas that comprise most of the caldera floor (blue)

image courtesy of USGS

diorites

Diorites are hard, intermediate, plutonic igneous rocks composed mainly of plagioclase feldspars (usually andesine), hornblende, and/or pyroxenes. Depending upon the mineral mix, diorites are colored salt-and-pepper, gray, bluish gray, to dark gray, and may have a greenish cast. Diorites that are deficient in dark minerals such as hornblende are light colored (leucodiorites). The presence of olivine and iron-rich augite grades diorites into ferrodiorites, which are transitional to gabbro. The presence of quartz grades the rock type into quartz-diorite or tonalite, and the presence of potassium feldspar orthoclase grades diorites into granodiorites. (image at right - click to enlarge -QAPF diagram) Plutonic dioritic intrusions form large batholiths. Diorites may be associated with intrusions of granite or gabbro. Dioritic magma results from the partial melting of a mafic rock above a subduction zone, so it is associated with volcanic arcs, and with cordilleran mountain building resulting from the convergence of tectonic plates at continental margins. Andesite is the extrusive volcanic equivalent of diorite. Diorite is a comparatively rare rock, and is found in Sondrio, Italy; Thuringia and Sassonia in Germany; Finland; Romania; central Sweden; Scotland; the Andes Mountains; the Isle of Guernsey; the Basin and Range province and Minnesota in the USA. Orbicular diorite is an extremely rare variety of diorite rock of unknown genesis, which contains many spherical orbs with concentric zones of light and dark color. Orbicular diorites are found on the island of Corsica, in South Africa, and North Carolina.

links: images: hand-specimens: diorite, 2, 3, 4, 5, 6, 7, 8, near equal mixture of light plagioclase and dark hornblende; Smithyford Kiln; Glenbuchat diorite, Limekilns wp; orbicular diorite, an extremely rare variety of the rock that contains many spherical lumps, or orbs, that exhibit concentric zones of light and dark color; Diorite, Diorites & Diorite Gneiss & Granodiorite, 2 & Tonalite & Quartz Diorite, stained-1, 2, & Quartz Monzodiorite, 2, 3; close-up: Quartz and albite tend to be pale-colored and without obvious cleavages. Quartz, however, is classy clear and usually grayish whereas albite is usually milky white. Biotite and amphibole are both black, but biotite has brilliantly shiny cleavage surfaces, whereas amphibole cleavages are rougher and dull: diorite, 2, 3, 4, 5, 6, nepheline diorite, 1 amphibole: 1 Na-plagioclase, minimal quartz; Dioritoid a.k.a. Diorite; orbicular diorite; Biotite diorite: plutonic; medium-grained with all crystals about the same size; formations: Proterozoic-age Pinto Diorite (white crystals are plagioclase feldspar and the dark green crystals are pyroxene), diorite with silicic dike, Fort Irwin, and South Fort Irwin; Glenbuchat diorite boulder, Stony Hill, Glenbuchat ; diorite intrusion, Lake Tiorati darker rock (left), roof pendant (xenolith) of felsic gneiss (right) with a thin layer of contact melt rock, some of the melt rock can be seen in small veins in the diorite, & Diorite intrusion, close-up with hammer on the diorite (contact has a 10 cim pink pegmatoidal melt vein (right of the hammer head) ; orbicular diorite (8.5 inches) from pipe pocket, Talkeetna Mountains; orbicular 'granite'; gold-mineralized quartz veins in diorite, Finland, 2; weathered; Decomposed Diorite, Nevada SR 207; Granite vein intruding the pyroxene diorite, Gabbro Point, contact between gabbro (left) and intrusive pyroxene diorite; Diorite Cliff Band, below Matahpi Peak, wp, prominent diorite tower just off the main ridge crest (Dunderberg's southwest ridge, Yosemite); jp; Kdi Diorite (Cretaceous): Diorite (clinopyroxine inclusions), quartz diorite, and tonalite, contains inclusions of very coarse-grained pyroxene gabbro; layered tonalite (light grey layers) - monzogranite (pinkish) - diorite (dark grey) gneiss(age of 2827+8/-7 Ma); El Capitan granite with dark diorite intrusion; thin-section: diorite, 2, diorite wp; diorite gneiss fsu, 2; orbicular diorite fsu; webpages: diorite, unforbidden-geology; related: Diorite statue of King Chephren (Khafre, close-up), Egypt 4th dynasty; Diorite statue, Gudea of Lagash Girsu, Louvre; a diorite weight from the Treasury of Persepolis.

dunite

Dunites are gray to olive green to 'dun', coarse-grained or phaneritic plutonic igneous rock of ultramafic composition. Comprising greater than 90% olivine, with minor pyroxenes and chromite, dunites are olivine-rich end-members of the peridotite group of mantle-derived rocks. Dunites are common components of the oceanic lithosphere, and are generally believed to arise from the effect of gravity and other forces on accumulations of dense olivine grains. Thus, dunites may represent upper mantle refractory residues from which basaltic magmas have been extracted. Dunites may also result from the accumulation of olivine crystals at the base of large basaltic or picritic magma chambers. Such cumulate dunites typically form thick layered mafic intrusions, and may be associated with cumulate layers of wehrlite, olivine pyroxenite, harzburgite, and even chromitite (a chromite cumulate). Dunite typically occurs in sills, but may appear in dikes and laccoliths. Dunites are rarely found as refractory-residue dunites, are typically found associated with layers of wehrlite, pyroxenite, and gabbro at the base of ophiolite complexes, in which slabs of mantle rock have been obducted onto continental crust during continental or island arc collisions (orogenic). Dunites are also found in alpine peridotite massifs that contain segments of sub-continental mantle exposed during collisional orogeny. Because the minerals that comprise dunite are less stable at the surface, dunites typically manifest retrograde metamorphism in near-surface environments, whereby dunite is altered to serpentinite and soapstone. Dunites are important commercial source of chromium, but may also contain copper, magnesium, and platinum ores.

links: images: hand-specimens: NZ, Pitt, 2, 3 ; close-ups: dunite-jp, 2; chromitite pod in dunite; formations: Dunite vein, Lizard; cumulate-textured dunite (Kluane Ranges Mafic-Ultramafic Suite (TrK), Tatamagouche Mafic-Ultramafic Complex); layered chromitite-dunite; orbicular chromitite in a dunite pipe from within the harzburgitic mantle, TMOC, Mine Grand-Père; dunite, West Ridge, South Twin Sister, Mt. Baker; weathered dunite on South Twin Sister; antinodular PGE-rich chromitite forming veins, pods and schlieren in a metasomatic dunite pipe [schlieren]; wispy pyroxene schlieren in pyroxene-dunite separating the massive metasomatic dunite pipe (Hall orebody) from surrounding layered websterite-peridotite; dunite, Quingling; dunites forment des poches décamétriques aux limites diffuses avec les lherzolites, Le Monte Maggiore; ultramafic rocks, the lowest unit of the Ophiolite Sequence (Franciscan Complex) consisting of intensely sheared serpentinite, hanzburgite, dunite and pyroxenite; Luobusha dunite, Tibet; chromite in dunite lenses that are hosted in harzburgite; anorthosite/dunite layers in ultramafic cumulates of the Bay of Islands ophiolite; 'restite' veins of dunite in harzburgite; dunite with a 10 cm pod of chromite, Trout River, Newfoundland; anorthosite-dunite-troctolite cumulates, Lewis Hills,, Newfoundland; gabbro sill in dunite, modal layering, Maqsad diapir; gabbro dyke in harzburgite mantle, dunite pods at base, Moho, Wadi Al Abyad; dunite mine, North Carolina; Cullain Complex (large laccolith) layers of huge pyroxene crystals interbedded with finer-grained dunite, 2; gabbro lens within dunite, Tuf (Maqsad massif); impregnated dunite (Wadi Bani Hina, Haylayn massif); banded harzburgite, layering (dunite, pyroxenite, harzburgite) resulting from high temperature asthenospheric flow; dunite; interlayering of brown dunite; dunite-spinel lherzolite contact in the Lanzo peridotite; dunitexenolith, vesicular basalt with dunite xenoliths; dunite (predominently olivine) fragment in an impact breccia (moon), (breccia); fold in dunite at Lien, Almklovdalen; Related: Serpentine; serpentinite and its alteration to asbestos, veins and nodules of talc; thin-sections: fsu, dunite, 2, partially serpentinized dunite, 2; dunite, olivine cumulate, partly serpentinised - XP, PPL; cumulate textures with olivine in dunite XPL, 2, 3, PPL; wp, webpages: Southern Quebec ophiolite belt; Geological Pictures: