Definitions and images to illustrate geological terms, links to images and website articles


paraconformitypegmatite dikeperidotitephacolithsphaneritic texturephenocrystphyllitic textureplunge, plunge direction (fold) ▪ pluton, plutonicporphyritic textureporphyry


peridotite xenolith in a typical olivine-rich peridotite cut by a centimeter-thick layer of greenish-black pyroxenite, San Carlos, southwestern United States. Peridotite is an ultramafic, ultrabasic (less than 45% silica), dense, plutonic igneous rock comprising mostly olivine and pyroxene. Most of the Earth's upper mantle (asthenosphere) is composed of peridotite that originated during the accretion and differentiation of the Earth, or that has differentiated, by precipitation of olivine ± pyroxenes, from basaltic or ultramafic magmas in turn derived from partial melting of the upper mantle peridotites. Deeper in the crust, olivine is replaced by a high pressure polymorphs, so peridotites do not occur at depths greater than 400 km.

Peridotite emplaced in the continental crust is typically found in obducted ophiolite complexes, as xenoliths in basalts and kimberlite pipes, and as orogenic peridotite massifs and alpine peridotites. Olivine is unstable at shallow depths and reacts rapidly with water, so that much surface peridotite has been altered to serpentinite by a process in which the pyroxenes and olivines are converted to green serpentine.

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Phacoliths are concordant plutonic bodies that lie parallel to the bedding plane or foliation of folded country rock. They occur along the crests of anticlines or the troughs of synclines in folded sedimentary strata.

Rarely, the body of a phacolith may extend as a sill from the crest of an anticline through the trough of an adjacent syncline, such that it has an S shape in cross section. The hinge of folds in intensely folded terrains are areas of reduced pressure, so are potential sites for magma migration and emplacement.

[links: images: close-up: autolith of biotite-rich, enclave bearing, porphyritic granite in another more phenocrystic granite, Wolf Mountain Intrusion (Phacolith), Proterozoic Llano Uplift, central Texas]

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plagioclase feldspar phenocryst, Lambert Dome, YosemiteA phenocryst is a conspicuous, large crystal embedded in a finer-grained matrix of smaller crystals in a porphyritic igneous rock.

(left - click to enlarge image - a plagioclase feldspar phenocryst, Lambert Dome, Yosemite, courtesy Daniel Mayer)

Porphyrys are formed by a two-stage cooling of rising magma. First, deep crustal magma cools slowly, allowing formation of large phenocrysts (diameter 2 mm or more). Second, the magma cools rapidly at shallower depths having been injected upward or extruded by a volcano, allowing for formation of small crystals in the groundmass.

[images : phenocrysts of feldspar in matrix of quartz, feldspar and mica; large feldspathic phenocrysts in granite; large zoned plagioclase phenocryst; andesites with phenocrysts; formation: Death Valley vitrophyre, a phenocryst-bearing obsidian; thin-section feldspar and clinopyroxene phenocrysts in Mt. Fuji basalt; thin-section clinopyroxene phenocrysts, ppl; thin-section augite phenocryst, same augite phenocryst in cross-polarized light; resorbed quartz phenocryst, in PPL; thin-section anhedral amphibole phenocryst; igneous rocks in thin section]

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map of plutonic rock of North AmericaPluton is a general term for any large igneous intrusion of crystallized magma. Rocks that emplaced in large intrusive structures are termed "plutonic", whereas those that form in small intrusions are called "hypabyssal".

Plutonic igneous rocks cooled and crystallized from intrusive or irruptive magma within the Earth's crust. Plutonic rocks exhibit a fine-grained aphanitic texture if the magma cooled close to the surface in volcanic necks or feeder pipes, with grain size increasing through to coarse-grained, phaneritic textures for magmas that cooled very slowly at great depth in very large magma chambers. (image - click to enlarge - plutonic rock of North America)

Plutonic rock types include granite, diorite, gabbro, and rhyolite. Plutonic structures include huge solidified magma chambers called batholiths, laccoliths, stocks, bysmaliths, and lopoliths, while smaller hypabyssal structures include diapirs, dikes and dikelets, ring dykes, sills, volcanic necks and plugs, and cone sheets. Less commonly encountered plutonic structures include: phacoliths, cactoliths, ductoliths, harpoliths, sphenoliths, akmoliths, and ethmoliths. Chonoliths are intrusive igneous structure with shapes so irregular that they do not fall into the normal categories. Any intrusive structure may later become exposed at the surface by erosion.

(Veins are finite volumes within rocks, which are filled with crystals of minerals precipitated from hot (aqueous) fluids.)

intrusive emplacement / structures: ▪ batholithboudinbysmalithconcordantconformablediapirdiatremedikediscordantenclaveshypabassallaccolithslopolithsplutonsillsstratavolcanovein

structures reflecting flow/crystallization within magma chambers: ▪ cumulatesenclavesschlierenxenoliths

magmatic processes: ▪ assimiliationexchange of volatilesfractional crystallizationigneous rocksigneous structuresmagmamagmatic differentiationmagmatic mixingophiolite complexestectonics extrusive: ◘ lavavolcanoesvulcanism

igneous rocks: ◘ basaltcumulatesdunitesenclavesfelsic (sial) ◘ gabbrogranitegranodioritegroundmasskeratophyreslavamafic (sima) ◘ magmaophiolite complexespegmatitesperidotiteperidotiteporphyryschlierentonalitesxenocrystxenoliths

Maps of North American rock types : rock types - metamorphic, plutonic, sedimentary, volcanic; tapestry of time and terrain, terrain.

[images : dike, sill, diapir, laccolith, batholith]

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Porphyry is classically a reddish-brown to purple igneous rock containing large phenocrysts of minerals such as feldpar or quartz embedded in a fine-grained matrix or groundmass of feldspar. More generally, the term porphyry encompasses any rock with a texture of phenocrysts embedded in a finer textured matrix (such as granite) or with visible crystals in an aphanitic matrix (such as basalts, aphanites or phanerites).

Porphyrys are formed by a two-stage cooling of rising magma. First, deep crustal magma cools slowly, allowing formation of large phenocrysts (diameter 2 mm or more). Second, the magma cools rapidly at shallower depths having been injected upward or extruded by a volcano, allowing for formation of small crystals in the groundmass. Differential cooling permits separation of dissolved metals into distinct zones, creating rich, localised metal ore deposits (gold, copper, molybdenum, lead, tin, zinc and tungsten).

[images - roll-over link for preview (where available); large images (well worth a visit) show only as a corner on preview : hand-specimen diabase porphyry : hand-specimen rhyolite porphyry : hand-specimen andesite porphyry, 2 : porphyry : hand specimen of "ore" from the Coed y Brenin porphyry-copper deposit : Wolf porphyry : red porphyry : red Imperial porphyry Egypt : 4th C porphyry head : xenolith-bearing hybrid porphyry : porphyrite dike cutting undeformed post-tectonic granite : closeup of hybrid porphyry texture, 2, microphenocryst of titaniferous augite rimmed with brown sodic ferrohornblende : outcrop of hybrid porphyry from pipe cluster : gabbro xenolith in hybrid porphyry : Guanella Pass porphyry : Porphyry Basin, 2, 3, 4, 5, 6, g : porphyry Trentino : porphyry quarry : porphyry copper mine : gold-copper porphyry at Quispe : thin-section plagioclase, augite, oxides porphyry, 2, 3, Skaergaard intrusion photomicrographs]

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