| ◊ albite ◊ amphiboles ◙ Archean rocks ◙ amphibolites ◙ andesites ◊ anorthite ◙ anorthosites ◙ aplites |
| ◊◊ alumino-silicates (zeolites) |
| alphabetic sections ◊ A ◊ B ◊ C ◊ D ◊ E ◊ F ◊ G ◊ H ◊ I ◊ J ◊ K ◊ L ◊ M ◊ N ◊ O ◊ P ◊ Q ◊ R ◊ S ◊ T ◊ U ◊ V ◊ W ◊ X ◊ Y ◊ Z ◊ elements of periodic table ◊◊◊ Mineral Index ◊◊◊ ◙◙ Rock Index ◙◙ |
mineral / chemical formula |
properties / significance / occurrence | |
albite, NaAlSi3O8 alkali feldspar endmember |
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Triclinic pinacoidal, tectosilicate alkali feldspar with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. |
| albite: | ||
| composition | Sodium aluminum silicate; "pivot" endpoint mineral of two different feldspar series | |
| color | usually white, can be shades of blue, yellow, orange, or brown | |
| lustre | vitreous; dull if weathered | |
| transparency | mostly translucent to opaque | |
| crystal system | triclinic; bar 1 | |
| crystal habits | blocky, tabular and platy crystals; almost universal crystal twinning, often as minute parallel striations on the crystal face; typical crystal has a nearly rectangular or square cross-section with slightly slanted dome and pinacoid terminations. | |
| cleavage | perfect in one and good in another direction forming nearly right angled prisms | |
| fracture | conchoidal | |
| hardness | 6 - 6.5 | |
| specific gravity | 2.61 (av.) | |
| streak | white | |
| other | index of refraction is 1.53; lamellar twinning may grooves on cystal surfaces that appear as striations | |
| associations | often occurs as fine parallel segregations alternating with pink microcline in perthite as a result of exolution on cooling; most often associated with the plagioclase series; quartz, tourmalines, and muscovite | |
| occurences | granitic and pegmatite masses and also in some hydrothermal vein deposits: Labrador, Canada; Scandinavian Peninsula | |
| indicators | occurence, crystal habit, twinning, striations, density and index of refraction | |
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mineral / chemical formula |
properties / significance / occurrence |
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amphiboles double-chain inosilicates [Si4nO11n]6n- |
image - click to enlarge -hornblendes and crystal.
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Double-chain monoclinic and orthorhombic inosilicates, with iron and magnesium. Similar to the single-chain inosilicate pyroxenes but with extra -OH group and oblique cleavage planes (~ 120 degrees), rather than ~ 90 degrees as in pyroxenes. Amphiboles exhibit stronger pleochroism than pyroxenes. Amphiboles are mineral components of igneous and metamorphic rocks. Igneous rocks such as granite, diorite, and andesite contain hornblende. Amphibole-containing metamorphic rocks include limestones altered by contact metamorphism (tremolite) and rocks formed by the alteration of other ferromagnesian minerals (hornblende). Pseudomorphs of amphibole after pyroxene are known as uralite. |
| hornblende: name given to the series of minerals | ||
| composition | iron, magnesium and aluminum ions can freely substitute | |
| color | dark green to black | |
| lustre | dull to vitreous | |
| transparency | translucent when thin, opaque crystals | |
| crystal system | monoclinic; 2/m | |
| crystal habits | short stocky prismatic crystals, long thin crystal forms, granular, massive, and occassionally acicular aggregates; hexagonal cross-section; termination appears as the two faces of a dome (actually two of the four faces of a prism) | |
| cleavage | imperfect in two directions at 56 and 124 degrees | |
| fracture | uneven | |
| hardness | 5-6 | |
| specific gravity | 2.9 - 3.4 | |
| streak | gray to brown | |
| other | translucent speciments are pleochroic, large crystals have an almost striated or grainy appearance | |
| associations | quartz, feldspars, augite, magnetite, micas, and many medium grade metamorphic minerals | |
| occurences | Bancroft, Ontario; Norway; Bohemia; Mt. Vesuvius, Italy and New York, USA | |
| indicators | crystal habit (especially cross-section), color and cleavage | |
| [images: crystals: hornblende, hornblende crystals are generally longer, thinner and shinier than augite and the mineral cross-sections are diamond-shaped, amphibole. | ||
The amphibolite facies signifies that Buchan, Barrovian, and or advanced Abukuma regional metamorphic temperature and pressure conditions operated at the time of formation of these prograde metamorphic rocks. Amphibolites generally form during the regional metamorphism of igneous rocks primarily composed of ferromagnesian minerals. Amphibolite facies result from higher temperature metamorphism of rocks of the greenschist facies, whereas higher temperatures coupled with greater pressures will result in the granulite facies. The protolith, or parent rock of an amphibolite may be:
◙ volcanic sediments
◙ mafic rocks
--- ◙ basalt ortho-amphibolite yield hornblende/actinolite ± albite ± biotite ± quartz ± accessories; and, frequently remnant greenschist facies assemblages including, notably, chlorites
--- ◙ high-magnesia basalts yield similar minerals to ortho-amphibolites, and may contain anthophyllite, a Mg-rich amphibole
--- ◙ ultramafic rocks yield tremolite, asbestiform amphibole, talc, pyroxene, wollastonite, prograde metamorphic olivine (rarely)
◙ impure marls (derived from CaCO3 or lime-rich muds)
◙ deposits containing dolomite ((CaMg(CO3)2)
◙ deposits containing siderite (FeCO3)
--- ◙ sedimentary para-amphibolites yield hornblende/actinolite ± albite ± biotite ± quartz ± garnets (calcite ± wollastonite)
--- ◙ pelites yield quartz, orthoclase ± albite ± biotite ± actinolite ± garnets ± staurolite ± sillimanite
Typically cumulate, holocrystalline plutonic igneous rocks comprising primarily hornblende amphibole are termed hornblendites. Porphyritic igneous rocks comprising >90% amphibole in a feldspar groundmass may be lamprophyres. Metamorphosed basalts generate ortho-amphibolites and other chemically appropriate lithologies yield para-amphibolites. Tremolites are metamorphic amphiboles typically derived from highly metamorphosed ultramafic rocks, so tremolite-talc schists are not generally considered as 'amphibolites'. Uralites are hydrothermally altered pyroxenites. |
| links: rock: amphibolite, 2, 3, 4, amphibolite, 2, 3; close-up: garnet amphibolite; garnet-amphibolite with prismatic, bluish aggregates which consist of spinel + plagioclase aggregates; formations: boudinaged-amphibolite (dark) layer within gneisses, Laxfordian complex, near Durness, Scotland (the gneiss has "flowed" into the spaces between the more competent amphibolite boudins (higher viscosity); Amphibolite boudins in gneisses, Amphibolite layers in gneisses, Amphibolite layers in gneisses 2, Migmatitic gneisses of the Nanga Parbat massif cross-cut by an amphibolite sheet, Amphibolite Blacktail Gulch, folded epidotized amphibolite, cavities created by dissolution of calcite in amphibolite, Union marble with amphibolite inclusion, coarse amphibolite gneiss, amphibolite gneiss with folds, close up of the folds, (Greenstone); thin-sections: Amphibolite, 2, amphibolite, b, wp, Amphibolite à Fantômes de Grenat, Amphibolite, Auvergne, France; webpages: amphibolite, Metamorphic lab: Norway Caledonian eclogites and related rocks; Antarctica: Metamorphic complexes |
alphabetic sections ◊ A ◊ B ◊ C ◊ D ◊ E ◊ F ◊ G ◊ H ◊ I ◊ J ◊ K ◊ L ◊ M ◊ N ◊ O ◊ P ◊ Q ◊ R ◊ S ◊ T ◊ U ◊ V ◊ W ◊ X ◊ Y ◊ Z
◊ elements of periodic table ◙◙ Rock Index: alphabetic ◙◙ Rock Index: Igneous Rocks ◙◙ Rock Index: Metamorphic Rocks ◙◙ Rock Index: Sedimentary Rocks ◙◙ ◊◊◊ Mineral Index ◊◊◊
Andesites and diorites commonly have a grayish to salt-and-pepper appearance. Lighter-colored andesites could be mistaken for rhyolite except that rhyolite commonly has pink potassium feldspar and always has quartz. Andesites are composed mainly of plagioclase (mostly andesine) plus pyroxenes (clinopyroxene and orthopyroxene) and/or hornblende. Biotite, quartz, magnetite, and sphene are common accessory minerals, while alkali feldspars may be present in minor amounts. At the lower end of the silica range, andesite lava may also contain olivine. Classification of porphyritic andesites reflects the mineral composition of the most abundant phenocrysts. Andesites form at accretionary (convergent) plate margins, where intermediate volcanic rocks originate via several processes: Andesite is considered the product of partial melts of the water-rich subducting oceanic crustal basalts or of the intervening wedge of lower crustal rocks above the subducting plate. Thick lava flows of andesitic magma commonly erupts from stratovolcanoes at temperatures of 900-1100° C. Eruptions often begin with an explosive phase as volatiles escape, so deposits of layers of pyroclasts are common on and around the volcano. The explosive phase of the eruption is then followed by a flow of fluid lava from the volcano, which then protects the underlying layer of pyroclasts from erosion. Volcanos that result from a series of such eruptions become layered (stratified), and so are referred to as strato- or composite volcanos. Being less viscous and hotter than rhyolitic lava, some lava flows reach several km in length. Andesite magma can also generate strong explosive eruptions to form pyroclastic flows and surges and enormous eruption columns. Andesite was the main rock type erupted during the great Krakatau eruption of 1883, and Mt. Mazama consisted of andesite and dacite lava flows before it exploded 7,700 years ago. Andesites are named for the stratovolcanoes of the Andean cordillera, though they occur in the cordillera (parallel mountain chains) of Central and North America (including the Cascades) and are common around the Pacific "ring of fire". Andesites are associated with the volcanoes: Montagne Pelée; the Soufriére of St. Vincent; Hekla in Iceland; Krakatoa; Bandai-san; Popocatepetl; Fuji; Ngauruhoe; Shasta; Hood; and Adams. Andesites sometimes also occur in dikes and small volcanic plugs. |
| link: images: hand-samples: andesite NZ, andesite porphyry; andesite, 2, 3, 4, 5, 6, 7; Andesite; andesite packed with plagioclase and hornblende phenocrysts, and close-up showing black phenocrysts (hornblende) and white crystals (plagioclase); Submarine Arc Volcanism and andesite lava (~10 cm) newly formed near Brimstone Pit, showing elemental sulfur infilling the vesicles in the lava; andesite (with a little tundra moss) from Kiziman volcano; basaltic andesite from the 1996 Karymsky eruption; porphyritic andesite, 2, 3; close-up: andesite; Andesite; andesite lava flow; secondary copper mineralization (green) in vugs within a vesicular andesite, found west of the Alous deposit; brecciated andesite altered to quartz and K-feldspar (adularia) stained yellow with sodium cobaltinitrate ), from basin margin fault zone at Rhynie (breccia); thin-section: olivine pyroxine andesite, 2, 3; andesite - toggle ppl and xp; andesite with crystals of plagioclase feldspar; andesite with phenocrysts of plagioclase and hornblende; formations: thick, short andesite flow on the flanks of the Colima Volcano, Mexico; columnar andesitic lava, Studen Kladenets, Rhodopes, Bulgaria; andesite flows exhibiting thick flow margins tens of meters high and well-developed lava levees (the transverse ridges on the central andesite flow are called ogives, and develop as folds perpendicular to the direction of flow, which are sometimes ramped upward along faults generated by brittle deformation of the flow interior), Lascar Volcano, Chile; lava flow breccias incorporating boulders of andesitic composition (15 feet from top to bottom), Conejo Volcanics; Andesite Ridge; andesite lava flow, Brokeoff Volcano, California; andesite breccia; andesite lava flow, Kupreanof and Zarembo Islands; andesitic lava- and tuff-layers exposed as the massive cliffs of the Galiuro escarpment; Mt. Baker; webpages: NOAA Ocean Explorer. |
mineral / chemical formula | properties / significance / occurrence | |
| anorthite | [image, 2, 3, 4, 5, 6, 7] | Anorthite is a calcium-rich, plagioclase feldspar end member, and is one of the more rare members of the plagioclase series |
| anorthite: less than 10% sodium and more than 90% calcium | ||
| composition | CaAl2 Si2 O8 calcium aluminum silicate | |
| color | white, gray, colorless, or pale shades of other colors | |
| lustre | vitreous; dull if weathered | |
| transparency | usually translucent to opaque, rarely transparent | |
| crystal system | triclinic; bar 1 | |
| crystal habits | blocky, or tabular crystals with a nearly rectangular or square cross-section with slightly slanted dome and pinacoid terminations; twinning is almost universal. | |
| cleavage | perfect in one and good in another direction forming nearly right angled prisms | |
| fracture | conchoidal | |
| hardness | 6 - 6.5 | |
| specific gravity | 2.76 (approx.) | |
| streak | white | |
| other | lamellar twinning can cause a grooved effect on cystal and cleavage surfaces that appear as striations. | |
| associations | biotite, augite, hornblende, and pyroxenes | |
| occurences | Anorthite is mostly found in contact metamorphic limestones and as a constituent in mafic igneous rocks. Lake Co, California; Franklin, New Jersey and Italy | |
| indicators | occurence, twinning, color and luster | |
 Anorthosites are typically pale gray, and are phaneritic, intrusive igneous rocks comprising predominantly plagioclase feldspars (90-100%) with minimal mafic constituents (less than 10%). The plagioclase feldspars exist as solid solutions between the albite and anorthite endmembers, and are classified according to their percentage of anorthite (An0-100). Modified anorthosites have less than 90% but more than 78% of plagioclase, for example gabbroic anorthosite, whereas anorthositic rocks have 78–65% of plagioclase, such as anorthositic gabbro. The mafic minerals in anorthosites are usually pyroxenes, ilmenite, magnetite, and olivine. However, the mafic minerals in Proterozoic anorthosite can also include clinopyroxene, orthopyroxene, or, more rarely, amphiboles. Other rare minerals include biotite, apatite, zircon, scapolite, and calcite. The moon has more anorthosite than does Earth, although large bodies of anorthosite were emplaced early in geological history when massif anorthosites were produced about 1–2 Ga. Valuable deposits of Fe and Ti ores are associated with anorthosites. Large emplacements are found in New England region of the US (Adirondack Fe-Ti deposits) and other large anorthosite masses are found in Zimbabwe and Canada. Archean and Proterozoic anorthosites differ in geological age, geochemistry, modes of occurrence, and, it is believed, genesis. Mode of genesis of anorthosites, which are comparatively dry, remains controversial because anorthositic magmas (with higher plagioclase feldspar than basalts) would not likely remain fluid at estimated ancient crustal temperatures. Anorthosites occur as: If the magma was more basaltic than anorthositic, the magma could have generated anorthosites by undergoing gravitative fractionation of mafic minerals into ultramafic cumulates, while the plagioclase crystals ascended and were emplaced as anorthosite plutons. High-alumina orthopyroxene megacrysts (HAOM) contain anomalously large amounts of aluminum, which is more soluble with increasing pressure, suggesting that the HAOM crystallized at depth, near the base of the earth's crust. |
| Archean anorthosites have equant megacrysts of plagioclase surrounded by a fine-grained mafic groundmass, and higher percentage composition of plagioclase feldspars than do Proterozoic anorthosites. Archean anorthosites are less common, and are found in a few anorthosite bodies, associated with ultramafic rocks, that were emplaced in the late Archean or early Phanerozoic Eons. |
Proterozoic anorthosites, or 'massifs', or 'massif-type anorthosites' have plagioclase feldspar typically ranges from An40 to An60 (andesine, labradorite). Massifs occur as large domes and are weakly layered. Proterozoic anorthosites can occur in association with iron-rich diorites, gabbro, norite, leucotroctolite, leuconorite, monzonite, and rapakivi granite and are termed 'Anorthosite suites' or 'Anorthosite-Mangerite-Charnockite complexes' or, in association with granites, AMCG or AMG. Unlike Archean anorthosites, Proterozoic anorthosites are not found in association with large volumes of ultramafic rocks. (diagram). During the Middle Proterozoic (1.4-1.0 Ga), orogenic belts such as the Grenville and Nairn Structural Provinces (Canada) and the Svecco-Norwegian (Scandinavia) were intruded by igneous complexes. These plutonic rocks are characterized by large coarse-grained bodies comprising mostly cumulate plagioclase feldspar and intercumulus orthopyroxene. These bodies are surrounded by orthopyroxene- and clinopyroxene-bearing rocks of variable quartz content (dioritic to adamellitic) that are termed jotunite, opdalite, mangerite, and farsundite (for Norwegian type localities). Isolated bodies of hyperthene-bearing alkali granites associated with high grade granulite facies metamorphic rocks are termed charnockite (for type rocks in southern India). Rocks of the Anorthosite suite are also often found associated with gneisses of the granulite metamorphic facies, such as the Morin Anorthosite (north of Montreal) and rocks of the Grenville Structural Province of Ontario. These anorthosite suite rocks often contain garnets and minor amounts of amphiboles and biotite. Some components of the Anorthosite suite include intrusive sheets of ilmenite and apatite. |
| links: images: formations: anorthositic replacement along the tops of the two left-most beds, Skaergaard pluton, southwestern Greenland, giant gabbroic anorthosite block with graded layering on eastern Kraemer Island, and "Christmas tree" anorthositic replacement features, 2, irregular anorthositic replacement rock, small replacement anorthosite sub-parallel to some layering, and having a mafic layer on the bottom, irregular gabbroic anorthosite rock, gabbroic anorthosite autolith thought to have broken off the partly solidified Skaergaard roof zone, layered and crossbedded rock with several bodies of replacement anorthosite above a large trough; beds lapping up on the margin of a large anorthositic autolith, foot of Pukugagryggen, mushroom-shaped anorthositic replacement bodies with more mafic margins in their lower parts and not at the caps, end of very large layered gabbro autolith, coast of Uttental Sund; small gabbroic troctolite autoliths in the giant gabbro block, angular autolith in the giant gabbroic block with base that looks like replacement anorthositic gabbro; graded layer with abundant autoliths, plagioclase porphyry dike, leucotroctolite autolith; contact-zone magmatic slump deposits of anorthosite breccia amid layers of pyroxenite, Greenland; modally graded "bed" of pyroxene cumulate in Neuburg Pyroxenite Member, showing sharp base and gradational top showing as thin dark layer in gabbro above anorthosites; Walker Peak anorthosites (plagioclase cumulates) at lowest-exposed parts of the Greenland intrusion; US: San Gabriel anorthosite-syenite body; Africa: Neoporterozoic anorthosite body within the Uluguru Mountains East African Orogen of central Tanzania; anorthosite and chromitite layering in the critical zone of the Bushveld layered mafic intrusion at the Dwars River locality (a famous example of rhythmic layering in the Bushveld); mottled anorthosite, a marker bed in the Bushveld complex; NA: roadcut through anorthosite, Soledad Canyon Rd.; Norway: Liåsen: bands of ilmenite in anorthosite and close-up of ilmenite bands; light coloured Ala-Penikka PGE Reef anorthosite layer in a darker coloured gabbronoritic cumulates; anorthosite and massive ilmenite layer in anorthosite, Kydlandsvatn, and leuconoritic blocks in anorthosite, Eigerøy, and anorthosite landscape, Åna-Sira, and anorthosite xenoliths in norite, 2, Norway; NA: anorthosite-dunite layers in ultramafic cumulates of the Bay of Islands ophiolite; contact of anorthosite with diabase at the base of Split Rock, in front of contact of diabase host rock with the anorthosite, Lake Superior; microbanding in anorthosite; hand-specimens: anorthosite; tightly folded anorthosite, Namaqualand; lunar anorthosite, 2, 3; close-up: anorthosite; garnet in anorthosite, Labradorite anorthosite, both Adirondacks; Piggstein (stretched orthopyroxene) megacrystals in anorthosite; thin-sections: fsu anorthosite, 2; anorthosite; anorthosite with albitic plagioclase; feldspar in anorthosite; maps: reference [86]Early Proterozoic gabbro-anorthosite, East Greenland Caledonian fold belt, North-East Greenland (76º N); webpages: Sybille Canyon, Wyoming; Lac St Jean anorthosite complex, Canada; History of the Emplacement and Deformation of Anorthosite Bodies in the Eastern Marcy Massif, Adirondacks Mountains, New York; Igneous Petrology Reference Series: |
(image at left - click to enlarge - Aplite dikes in Kinzigito, Morro Dois Irmãos) Unlike pegmatites, which are much coarser-grained, aplites occur in small, hypabassal bodies that rarely contain zones of different minerals. Pegmatites and aplites may occur in association in thin strata, or layers, within each other and are assumed to have formed simultaneously from similar magmas. A relatively rare gemstone composed of beryllium and aluminum oxide (chrysoberyl) can be found in aplites and pegmatites. Chrysoberyl varies in color depending upon which optical axis is being observed. Aplite dikes are commonly found in granitic bodies. The aplites formed from the ultimate residual melt after most of the crystallization of the granitoid was completed, so aplites are rich in quartz, alkali feldspars, and sometimes muscovite. |
| links: images: rocks/formations: paired shear zones as ductile boundary reactivation of an aplite dike (aplite dike and shear zones offset by late stage epidote-bearing fractures; aplite dike in dark granite (l); sheared aplite veins in a deformed granitoid, Ticino, Switzerland; aplite seam (pink) recuts a granite (gray) in which are darker enclaves (under finger); shear zone developed in granodiorite with xenoliths); convoluted, banded aplites in quartz, Tanco; late stage aplite dykes, South Lamma Granite, Kwai Chung Suite; Aplite dike cutting granodiorite of Summit Lake pluton; aplite dikes (Cathedral Peak Granodiorite , near Tenaya Lake in Yosemite National Park), 2, 3, 4; aplite dike; close-up: paired shear zones as ductile boundary reactivation of an aplite dike (aplite dike and shear zones offset by late stage epidote-bearing fractures; close-up of aplite vein in granite; aplite matrix with schorl inclusions; aplite dikes at sharp angles, 2 (Cathedral Peak Granodiorite, near Tenaya Lake in Yosemite National Park); Aplitos cortando gnaisse aluminosos, Costão do Leblon, Rio de Janeiro; aplite dykes showing compositional layering; hand-specimen: pegmatite-aplite zone within amazonite granite; thin-section: aplite; |
The Superior Upland of Wisconsin and Minnesota, dipping south from Lake Superior, forms the largest U.S. surface exposure of the Precambrian Canadian Shield (2.6-1.6 Ga). Much of the Earth's continental crust formed in the Archean, and small, highly deformed remnants of Archean structures persist as tectonically incorporated terranes within Archean crustal provinces. The active tectonics of the Archean produced numerous, mobile, relatively small protocontinental landmasses that began to coalesce in the late Archean. By the close of the Archean, about 2.5 Ga, a more tectonically stable supercontinent had assembled from accreted landmasses. Vaalbara is Earth's theorized first supercontinent, which, according to radiometric data, existed by 3.3 billion years ago (3.3 Ga) and possibly even as far back as 3.6 Ga. By about 2.7 Ga, Neoarchean sanukitoid cratons plus new continental crust accreted to form another of Earth's earliest supercontinents, Kenorland, which comprised the Laurentia, Baltica, Australia, and Kalahari cratons and persisted from about 2.7 Ga until about 2.48 to 2.10 Ga. About 70% of modern continents are derived from the single large late Archean landmass. Modern shields, far distant from the site of their formation, contain cratonic cores consisting of old basement rocks that have been relatively undisturbed since the Precambrian era. Cratons have a thick continental crust and deep roots that extend into the underlying, anomalously cold mantle mantle for depths up to 200 km (twice the approximately 100 km thickness of mature oceanic or noncratonic continental lithosphere). The thickness and coolness of the coalesced Archean supercontinent led to a reduction in volcanic and tectonic activity within and along the margins of the supercontinent by the start of the Proterozoic. Most geologists believe that crust forming mechanisms in the Hadean and early Archean differed from modern tectonic mechanisms. Plate tectonics appears to have been operative – and highly active – by 4-3.5 Ga, when rock assemblages typical of magmatic arcs, oceanic plateaus, oceanic islands, and accretionary prisms appear in the geologic record. By 3 Ga, cratons, passive margins, and continental rifts were widespread, but the geochemical differences between Archean and younger rocks suggest that Archean tectonic regimes differed from modern tectonic environments. The majority of Archean rocks still in existence are metamorphic descendants and crystalline cratonic remnants. These granitic-gneiss products of sedimentary or igneous protoliths suggest a high degree of lithospheric recycling. Vast lava flows later in the Archean erupted from undersea rift zones as pillow basalts, and later metamorphism altered these basalts to greenstones. Archean igneous rocks include unusual lavas, such as komatiite, intrusive igneous rocks such as great melt sheets, and voluminous plutonic masses of granite, granodiorite, diorite, ultramafic to mafic layered intrusions, anorthosites and monzonites known as sanukitoids. Other Archean igneous rocks with unusual mineralogy include lamprophyres and kimberlites. Gneisses, granulites and greenstone belts are common Archean rocks. Greenstone–granite belts represent the upper crust, granulite–gneiss belts formed in the mid-lower crust, and other Archean rocks formed in sedimentary basins, basic dikes, and layered complexes that were either deposited on or intruded into the greenstone–granite and granulite–gneiss belts. Pillow lavas occur within these belts, and patchy interbeds of conglomerates and chemical sediments formed later. The mechanisms of belt genesis remains uncertain, but their chemistry is consistent with partial melting of water-infused basalts under high temperature and pressure. Reef-forming stromatolite microfossils, which persist to this day in scattered stressed environments, provide evidence of life in the warm Archean oceans. Stromatolites were built by the photosynthetic Cyanobacteria that transformed the primordial atmosphere. The oldest known stromatolites are found in south Africa's 3.5 Ga Barberton greenstone and in western Australia's 3.5 Ga Pilbara greenstone. Other evidence of Archean life is found in fossils of cells or cellular tissue, and carbonaceous matter that is identifiable as a product of biologic activity based on its carbon isotopic composition. | |
Granulites are high grade regional metamorphic related to gneisses, and may represent residues of partial melting, or may result from rocks that failed to melt. Granulites are considered to be derived from plutons associated with volcanic arcs. Granites and gneisses are the dominant rock types within granulites. | |
Greenstones belts are found between granite and gneiss bodies in Archaean and Proterozoic cratons. Greenstone belts are interpreted as having formed in the back arc and/or forearc basins of ancient oceanic spreading centers adjacent to volcanic island arc terranes. The belts comprise volcanic and volcanosedimentary units. At the base are ultramafic volcanic rocks (komatiites), above which lie mafic, intermediate and felsic volcanic rocks, which are overlain by tuffaceous rocks and sediments. The basal sediments are mostly cherts, jaspers and banded iron-formations, whereas the uppermost sediments consist mostly of shales, sandstones, conglomerates, and quartzites, and eroded reworked volcanics that were deposited rapidly by turbidity currents and debris flows. The pattern of decreasing volcanics and increasing volcaniclastics reflects progressive phases during which catastrophic volcanism gradually abated, and erosional and depositional activity increased. The greenstones were simultaneously and later intruded by inflating plumes of low-density magma that emplaced huge granitic domes or batholiths, which were exhumed in the Proterozoic and later. Greenstone belts include massive volcanogenic sulfide deposits (MVS) resulting from the old mid-ocean ridge or volcanic arc sites, so are enriched in copper and lead sulfides. | |
Maps: Distribution of North American rocks in : ● Precambrian ● Archean • Early Archean • Middle Archean • Late Archean ● Proterozoic : map : simplified geology of modern continents: Minnesota : Greenland: geological map; geological maps of Greenland; main periods of crust formation and orogeny; diagram: Geological Column | |
| links: images: formations : Acasta gneiss ; Archean gneisses and amphibolites (upper right), contact with Skaergaard (map), and northwest contact between the Skaergaard pluton and the Archean gneisses (upper left), a broad synformal structure in the Archean rocks is cut by numerous Tertiary basaltic dikes; |
Associated science sites • Abiogenesis and Evolution • Algorithms of Evolution • Archea Eubacteria • Cancer • Cell Biology • Complex Systems • Cyanobacteria • Diagrams Tables • Endosymbiosis • Enzymes • Evo Devo • Evolution in Action • Fat • Geology • Glossary • Immunology • Life Chemistry • Medical Science • Mechanisms of Evolution • Molecule • Molecular Biology • Molecular Paths • Neurosciences • Organics • Origin of Life • Paleogeology • Pathways • Photosynthesis • Protein • Receptor • Rocks & Minerals • SET • Signaling • Sleep • Stem & Progenitor Cells • Stromatolites • Taxonomy Phylogeny • Tissue • Virus • And some philosophy/general interest sites • A-Deistic • Adeistic • Black Out • cosmic • Eine Kleine Nattermusing • eMusings • eVolition • Galaria • Godborygmi • Godspell Follies • Gray Matters • MetaThoughts • Mimble Wimble • Naturalism • BLogodaedaly • palimpsest • Salient • Science of Evolution • Sechuam • Sintheist • Tabula Flexuosa • The Scarlet A • Weltschauung •
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