| ◊ garnets ◊ geothermobarometry ◙ gneisses ◙ granodiorite ◙ granulites ◊ groundmass ◊ |
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 Gabbro is a coarse-grained, mafic, plutonic igneous rock that forms at spreading centers in rift zones and mid-ocean ridges (so underlies oceanic crust). Gabbros can form as massive uniform intrusions or as layered ultramafic intrusions formed by settling of pyroxene and plagioclase feldspar (pyroxene-plagioclase cumulate).
As an essential component of the oceanic crust, gabbros are found in many ophiolite complexes in the sheeted dike zone to massive gabbro zone (zones III and IV). Long belts of gabbroic intrusions are typical at proto-rift zones and around ancient rift zone margins, where they intrude into the rift flanks.
Gabbros are commercially important because they are often associated with Ni, Cr, and Pt ores, which occur almost exclusively in association with gabbroic or related ultramafic rocks. Gabbroic complexes are often associated with primary magnetite (Fe) and ilmenite (Ti) deposits. Banded, or layered, gabbroic complexes with well-developed monomineral or bimineral varieties are found in Montana, the Bushveld Complex in South Africa, and the Black Cuillin mountains of Skye (Ghreadaidh via Coire a Ghreadaidh) Inhomogeneous gabbro complexes that are not regularly layered are associated with the large, basinlike intrusion in Sudbury, Ontario, large diabase sills (tabular intrusions) in the Palisades, New Jersey, and many Karoo diabases (fine-grained gabbro) in South Africa.
A finer-grained rock with the same composition as gabbro is termed diabase. |
| images : formations: layered gabbro, North Cascades : Salem gabbro-diorite cut by a a composite dike with felsic margins and a central core of basaltic rock : White Mountain Magma Series; gabbro dike in Adirondacks, and chill zone of gabbro dike; Black Cuillin mtns., Skye: pegmatitic gabbro : oceanic crust exposed on Cyprus; Gabbro Dyke with some Visible Gold; dikes in gabbro, Lizard; hummocky terrain of gabbro-diabase, Hess Mine, Yuma; Bird's Eye Gabbro, with spotted bands of large hornblende crystals; hand-specimens: gabbro, 1, 2, 3, 4, 5, 6, l; gabbro that is roughly half pyroxene and half plagioclase, and close-up, and gabbro with lots of coarse, dark pyroxene crystals, and close-up, gabbro in which it is difficult to discern the minerals, and close-up (Pyroxene is typically duller and more blocky than hornblende, but most important are the cleavages intersecting at 90°); layered gabbro; mii; close-up: gabbro; gabbro, Lizard, 2; gabbro, Cuillin; polished surface; migmatitic gabbro, and garnet vein in gabbro, and veins crossing contacts; oceanic crust gabbro; thin sections: Oman Ophiolite gabbro; gabbro fsu, 2; thin section of olivine gabbro - pyroxene and olivine show bright colours, striped grey rectangular crystals are plagioclase feldspar : thin section of gabbro with plagioclase and hypersthene (orthopyroxene) : hypersthene gabbro : thin section with pyroxene and (striped) plagioclases : thin section orthopyroxenes crystals surrounded by alteration (uralite) : thin section : thin section with twinned plagioclases; gabbro, Mt. Megantic, Monteregian Hills; gabbro; gabbro, Hess Deep, plagioclase (gray tones) and clinopyroxene (brown to blue tones); pigeonite exsolution |
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mineral / chemical formula |
properties / significance / occurrence |
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garnets X3Y2(SiO4)3 |
images - click to enlarge - top, garnet crystals; second, andradite; third, espessartite; fourth, uvarovite; fifth, almandine in gneissic rock; bottom, garnet structure
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Minerals of the garnet group are nesosilicates in the isometric system that display either dodecahedral or trapezohedral crystal habits. Garnets are red, orange, yellow, green, blue, purple, brown, black, pink or colorless gemstones. There are six common varieties of garnets – pyrope, almandine, spessartite, grossular, uvarovite and andradite. Garnets are key indicator minerals used in geothermobarometric determination of the genesis of many igneous and metamorphic rocks. Garnets are relatively resistant to alteration, and diffusion of elements is relatively slow in garnet. Therefore, individual garnets commonly preserve compositional zonations that can be employed for interpretion of temperature-time histories of rock. When garnet grains lack compositional zonation, they are interpreted as having been homogenized by diffusion. Garnets are also employed to definine metamorphic facies of rocks. For example, eclogite has basaltic composition, but chiefly comprises garnet and omphacite. Pyrope-rich garnet is restricted to relatively high-pressure metamorphic rocks, such as those in the lower crust and in the Earth's mantle. Peridotite may contain plagioclase, or aluminum-rich spinel, or pyrope-rich garnet, and the presence of each of the three minerals defines a pressure-temperature range in which the mineral could equilibrate with olivine plus pyroxene: the three are listed in order of increasing pressure for stability of the peridotite mineral assemblage. Hence, garnet peridotite must have been formed at great depth in the earth. Xenoliths of garnet peridotite have been carried up from depths of 100 km and greater by kimberlite, and garnets from such disaggegated xenoliths are used as a kimberlite indicator minerals in diamond prospecting. At depths of about 300 to 400 km and greater, a pyroxene component is dissolved in garnet, by the substitution of (Mg,Fe) plus Si for 2Al in the octahedral (Y) site in the garnet structure, creating unusually silica-rich garnets that have solid solution towards majorite. Such silica-rich garnets have been identified as inclusions within diamonds. Garnets are found in mica schists (almandite); mantle derived rocks, such as peridotites and eclogites (pyrope is indicator mineral for high pressure metamorphism); granite pegmatite and grade metamorphic phyllites (spessartite ); deep-seated igneous rocks like syenite as well as serpentines, schists, and crystalline limestone (andradite); contact metamorphosed limestones with vesuvianite, diopside, wollastonite and wernerite (grossular); crystalline marbles and schists associated with chromite in peridotite, serpentinite, and kimberlites (uvarovite). |
| almandine: | ||
| composition | Fe3Al2(SiO4)3 iron aluminum silicate | |
| color | typically red to brown, sometimes with a tinge of purple, may appear black | |
| lustre | vitreous | |
| transparency | transparent to translucent | |
| crystal system | isometric; 4/m bar 3 2/m | |
| crystal habits | typical rhombic dodecahedron, also 24 sided trapezohedron, and combinations of these forms; massive occurrences also common; crystals are typically embedded and isolated, from other alamadine crystals, in metamorphic rocks | |
| cleavage | absent | |
| fracture | conchoidal | |
| hardness | 6.5 - 7.5 | |
| specific gravity | ~ 4.3 | |
| streak | white | |
| other | commonest garnet - usually the garnet found in garnet schists | |
| associations | micas, staurolite, quartz, and feldspars | |
| occurences | Wrangel Alaska; Germany; Norway and India | |
| indicators | crystal habit, color, density, and hardness | |
| pyrope: pure pyrope is unknown in nature and the various proportions are referred to as pyrope-almandine | ||
| composition | Mg3Al2(SiO4)3 magnesium aluminum silicate | |
| color | red to reddish purple, sometimes appears black | |
| lustre | vitreous | |
| transparency | transparent to translucent | |
| crystal system | isometric; 4/m bar 3 2/m | |
| crystal habits | typical rhombic dodecahedron, also 24 sided trapezohedron, and combinations of these forms; massive and granular occurrences are also common. | |
| cleavage | absent | |
| fracture | conchoidal | |
| hardness | 7 - 7.5 | |
| specific gravity | approximately 3.6 | |
| streak | white | |
| other | only garnet that is always a shade of red, although less common than most other garnets | |
| associations | olivine, serpentine, biotite, hornblende, augite, pyroxenes and diamond. | |
| occurences | ultramafic igneous rocks; metamorphism of ultramafic igneous rocks; magnesium rich rocks subjected to high grade metamorphism; Europe; Arizona and New Mexico, USA; South Africa; Australia | |
| indicators | crystal habit, color, hardness and enviroment | |
| links: images: grossular garnet, crystal, 2, 3, 4, pyrope, spessartine, tsavorite garnet, uvarovite; rocks: garnet-mica schist, 2; thin-section: garnet, 2, zonal distribution of quartz inclusions in this garnet porphyroblast, 2, wp; weblinks: garnets, mii, wp, wp2, garnet, thermobarometry; structure: garnet (UC), (crystals UC) | ||
| Geothermobarometry is employed to determine the pressures and temperatures operating during the formation of particular mineral assemblages during the genesis of igneous and metamorphic rocks. | ||
image at left - click to enlarge - stability fields of common pelitic minerals: Compare the mineral stability field chart above with chart of metamorphic facies & chart of trajectories of metamorphic facies (on different scale). Andalusite is a polymorph of the other aluminosilicate minerals kyanite and sillimanite. Cordierite or iolite is a magnesium iron aluminium cyclosilicate that is particularly common in hornfels produced by contact metamorphism of pelitic rocks. Determinations rely upon: | ||
Barrovian regional metamorphic zones are regions defined by reactions in which minerals appear or disappear, and these mineral zones can be mapped as isograds. The A(K)FM diagram is used to describe the phase relations of pelites by assuming that excess SiO2, H2O are present, and that a plagioclase (identity unimportant) is also stable . The diagram results in a tetrahedron where A=Al2O3, K=K2O, F=FeO, M=MgO | ||
Barrovian zone | mineral assemblage | |
| chlorite zone | chlorite + mus + qtz + H2O + relict minerals | |
| biotite zone | chlorite + biotite + mus + qtz + H2O | |
| garnet zone | chlorite + biotite + garnet + mus + qtz + H2O | |
| staurolite zone | staurolite + 2 AKFM phases + mus + qtz + H2O | |
| kyanite zone | kyanite + 2 AKFM phases + mus + qtz + H2O | |
| sillimanite zone | garnet + biotite + sillimanite + mus + qtz + H2O | |
| 2nd sillimanite zone, or sillimanite + orthoclase zone | sill + or + qtz + H2O + melt and no mus | |
Examples/ Links: | ||
(left - click to enlarge image : top, banded gneiss; center, close-up of augen gneiss showing characteristic elliptic or lenticular feldspaths (normally microcline); bottom kinked banding in gneiss) Gneisses typically occur in gneiss belts comprising large areas within the high-grade cores of regional metamorphic belts. The high temperatures and shear stresses of high grade metamorphism are probably due to deep tectonic burial and major regional compression, so gneissic terranes can form in areas of convergent plate tectonics. Gneissic rocks are coarsely foliated rocks with alternating light (quartz and feldspar) and dark (hornblende and biotite) bands. Micas are absent of present only in small amounts in gneissic rocks, but predominate in the often finer grained schists. Individual bands in gneisses are 1 mm to 1 cm in thickness and result from recrystalization of component minerals during subjection to formative high temperature and pressure (shear stress). Those rocks without obvious banding are termed leptites. Individual mineral grains are often flattened parallel to banding, and gneiss is defined by this texture although the term gneiss often indicates mineral composition of granitic type, dominated by quartz and feldspars. |
| Where not of granitic origin, gneisses are named for their parent rock such as diorite and amphibolite, or for the presence of minerals such as albite, biotite, biotite-plagioclase, chlorite, and garnet, hornblende-plagioclase. |
| links: images, roll-over for preview : biotite gneiss : pyroxene gneiss : Passagassawakeag gneiss 1 : Passagassawakeag gneiss 2 : Passagassawakeag gneiss 3 : augen gneiss with gneissic banding : augen gneiss close-up : late, coarse grained leucosomes in pelitic gneiss that contain large aggregates of cordierite (dark patches), and biotite-amphibole gneiss containing melt patches with euhedral orthopyroxene, both Antarctica; gneiss sample : alternating pink K-spar and black amphibole and white plagioclase layers : chloritoid gneiss 1 : chloritoid gneiss 2 : chloritoid gneiss 3 : migmatitic gneiss : transposed gneiss : diagram of banding in gneiss : gneiss boulders : Archean banded gneiss, Black River, Wi : formations: granite sheets and diatexites with schlieric stucture (lower part) are commonly interlayered with migmatites and gneisses, and disrupted mafic dikes in a leucocratic gneiss, both Antarctica. |
 Granite is typically a medium to coarse grained felsic, intrusive igneous rock (plutonic) that is usually pink to dark gray, sometimes black, depending on its chemistry and mineralogy. Granites are the commonest basement rocks of the continental crust, many dating from the Precambrian. In some granites, individual crystals are larger than the groundmass (porphyrys). Granites primarily comprises orthoclase and plagioclase feldspars, quartz, hornblende, muscovite and/or biotite micas, with minor accessory minerals such as magnetite, garnets, zircon and apatite. Rarely, a pyroxene is present. Very rarely, iron-rich olivine, fayalite, occurs.
Granites are classified according to the QAPF diagram for granitoids and phaneritic foidolites (plutonic rocks) that compares the percentages of quartz, alkali feldspar (orthoclase, sanidine, or microcline) and plagioclase feldspar. As a plutonic rock, granite is often exposed in weathered tors, dykes and as massive batholiths. |
| links: images: hand-specimens: light gray granite, Salinian, 2; pink-gray granite, Llano Uplift, Texas; pink-gray; pink-tan granite; pink granite, Pikes Peak, Colorado; orbicular granite formed by coronas of reaction rims of oriented hornblende formed around xenolithic inclusions, Granite Harbour, Antarctica; close-up: El Capitan Granite contains quartz (gray glassy looking grains), orthoclase feldspar (white grains), and biotite mica (black grains), Yosemite granite; a light gray granite; pink-gray, Gran Violet (Brazil); blue gray, Lavanda Blue (Brazil); pale gray-tan, Giallo Veneziano (Brazil); gray granite, Idaho batholith; Pikes Peak granite; Rapakivi granite; orbicular granite, Caldera, Chile; very dark gray, Azul Noce (Spain); formations: Big Rock, Yosemite, sheeted granite, Tioga Road, Yosemite ; granite; granite boulders; pink granite on Great Head Trail; schist over granite; gorge in pink granite; granite cascade; glacial scratches and grooves on exposed Milbank granite; granite island, Aus; webpages: Rock Gallery Drexel ; Mineral Gallery Drexel ; |
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Tonalite–trondhjemite–granodiorite (TTG) series are rock aggregates formed by melting of wet mafic crust at high pressures. Evidence indicates that "wet melting" of garnet-amphibolite and/or eclogite was an important element of early continent formation (cratonization). It is widely accepted that most Archaean granite–greenstones are dominated by TTG, although Late Archaean terranes, such as in the Yilgarn Craton, are dominated by K-rich granitoid rocks derived from remelting of older felsic TTG-dominated crust. According to this model, a much greater degree of crustal reworking has occurred in the Pilbara craton than is required by TTG-dominated crust. |
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| ◙ links: rocks: granodiorite, 2, magnified x 2.5, biotite granodiorite, hornblende granodiorite, hornblende-biotite granodiorite; field appearance: granodiorite, Grand Canyon, boulder, USGS, Layered granodiorite wp; thin-section: granodiorite, 2, magmatiche wp; webpages: Granodiorite definition at USGS; | |
Granulites are similar to gneisses, being composed of similar minerals. However, granulites are finer-grained, typically have less perfect foliation, contain more garnet, and have a different microscopic structure (small, rounded grains forming a closely-fitted mosaic). The component minerals of granulites depend upon the composition of the parent rock (protolith) and temperatures and pressures experienced during metamorphism. Common high-grade granulites contain plagioclase feldspar, pyroxenes (the coexistence of clino- and orthopyroxene in a metamorphosed basalt defines the granulite facies). Granulites may also contain accessory garnets, oxides, and amphiboles. |
| links: images: hand-specimens: 1, 2, 3, granulite-facies metasedimentary rock, 2; close-ups: 1, 2, 3, 4, 5, 6, 7, 8, mafic granulite, Kwinitsa quarry, B.C. & garnet bearing mafic granulite, postkinematic garnet in granulite gneiss, Lake Toxaway; formations: polished granulite, Bear Mt., NY, shear zone between granulite and amphibolite facies rocks, haloes, Winding Stair Gap granulite, western Blue Ridge Province, retrograded HP mafic granulite (gabbroic dyke) showing garnet rimmed by plagioclase, Shombach Cape Shear Zone, strongly sheared, granulite-facies mafic orthogneiss (dark, top and center), interlayered with lighter-coloured metasediments containing high-grade, coarse-grained partial melt composed of garnet (reddish), orthopyroxene (brown/black), quartz and feldspar; purplish grey orthogneiss retrogressed from granulite facies, with angular fragments of homogeneous amphibolite; thin-sections: 1, 2, 3, 4, 5, wp, bytownite plagioclase grains exhibiting polysynthetic twins, wp-jp; webpages: Granulite Gallery: |
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Greenschists or greenstones are schistose or foliated rocks tinted by the greenish chlorite phyllosilicates, actinolite, and epidote, and can be very dark when primary pyroxene does not revert to chlorite or actinolite.
Greenschist alteration typically affects oceanic basalts in the vicinity of mid-ocean ridges. The greenstone belts that are found between granite and gneiss bodies in Archaean and Proterozoic cratons are commonly altered to the greenschist facies, and host rocks for a variety of ore deposits in Australia, Namibia, and Canada. |
images: greenschist: hand-specimen: greenschist, polished, hand-sample containing actinolite, chlorite, epidote, and sphene; actinolite schist; formations: tightly folded quartz veins in greenschist, a basement rock of eastern Philippines; outcrop of greenschist; close-up: field appearance facies muscovite-chlorite schist, the dark spots indicate plagioclase porphyroblasts with characteristic graphite inclusion trails; Pebble Beach; formations: folded greenschist of the Kôszeg-Rechnitz window; sheeted dike partially altered to greenschist, a hallmark of magmatic extension; metabasic pod embedded in gneisses overprinted by amphibolite-greenschist facies assemblages; eye structure in sub-greenschist facies (fish-like greenstone blocks, C-S fabric in melange Kodiak Is. Alaska; lower greenschist facies shear zone cutting basement schists; assymmetric pod of schist; Tianshan greenschist rocks, 2; thin-section: subhedral crsytals of epidote XPL greenschist, PPL, and anhedral crsytals of epidote XPL, PPL, Siphnos, Greece; webpages: Metamorphism of the Kluane metamorphic assemblage, SW Yukon; Pebble Beach: The Schist (greenschist gallery) chlorite: chlorite, 2, chamosite, sphalerite, quartz cluster with chlorite inclusion; rocks: chlorite schist, 2, chlorite, 2, greenish hue due to chlorite, light colored minerals are serasite, a fine grained muscovite, along with K-feldspar, chlorites in diabase; formations: Série métapélitique de l'Agly, Schiste à chlorite; weblinks: chlorite group, chlorites, chlorite, petrographic; thin-section: chlorite, 2, chlorites, sem-Authigenic chlorite, Norphlet sandstone, sem, sem strzegomite, wp, wp2; structure: crosslinked chains, sheets |
