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19-775: The Mozambique Belt includes components created when the Mozambique Ocean opened and others created when the ocean later closed. The western portion of the Mozambique Belt is mostly composed of upper amphibolite -grade gneisses , reworked rocks from the Tanzania craton and Usagaran Belt, emplaced between 2,970 and 2,648 million years ago. The eastern portion has high-grade arc-derived rocks ranging in age from 841 to 632 million years ago. Both terranes also include Neoproterozoic metasedimentary rocks. The differences may indicate origins in two different basins, with

38-506: A certain amount of contact metamorphism by adjacent granitic masses. Metamorphosed basalt ( metabasalt ) creates ortho-amphibolite and other chemically appropriate lithologies create para-amphibolite . Although tremolite is a metamorphic amphibole, it is most commonly derived from highly metamorphosed ultramafic rocks , and thus tremolite-talc schist is not generally considered a variety of amphibolite. A holocrystalline plutonic igneous rock composed primarily of hornblende amphibole

57-436: A metamorphic amphibolite, it must be certain that the amphibole in the rock is a prograde metamorphic product, and not a retrograde metamorphic product. For instance, actinolite amphibole is a common product of retrograde metamorphism of metabasalt at (upper) greenschist facies conditions. Often, this will take on the crystal form and habit of the original protolith assemblage; actinolite pseudomorphically replacing pyroxene

76-538: A period of residence in the mid-lower crust followed by cooling between 640 and 620 million years ago. The rocks were exhumed over 500 million years ago. The deeper crustal levels were elevated by the closure of the Mozambique Ocean and the upper crustal arc remnants were removed by erosion. Amphibolite Amphibolite ( / æ m ˈ f ɪ b ə l aɪ t / ) is a metamorphic rock that contains amphibole , especially hornblende and actinolite , as well as plagioclase feldspar , but with little or no quartz . It

95-433: Is a plutonic rock consisting mainly of the amphibole hornblende. Hornblende -rich ultramafic rocks are rare and when hornblende is the dominant mineral phase they are classified as hornblendites with qualifiers such as garnet hornblendite identifying a second abundant contained mineral. Metamorphic rocks composed dominantly of amphiboles are referred to as amphibolites . This igneous rock -related article

114-507: Is an indication that the amphibolite may not represent a peak metamorphic grade in the amphibolite facies. Actinolite schist is often the result of hydrothermal alteration or metasomatism , and thus may not, necessarily, be a good indicator of metamorphic conditions when taken in isolation. Second, the microstructure and crystal size of the rock must be appropriate. Amphibolite facies conditions are experienced at temperatures in excess of 500 °C and pressures less than 1.2 GPa, well within

133-496: Is called a hornblendite , which is usually a crystal cumulate rock . Igneous rocks with greater than 90% amphiboles, which have a feldspar groundmass , may be lamprophyres . Metamorphic rocks composed primarily of amphibole , plagioclase , with subordinate epidote , zoisite , chlorite , quartz , titanite , and accessory leucoxene , ilmenite and magnetite which have a protolith of an igneous rock are known as ortho-amphibolite . Para-amphibolite will generally have

152-473: Is distinctive, the pyroxene altered to fuzzy, radially arranged actinolite pseudomorphically after pyroxene, and saussuritised plagioclase. The archaic term epidiorite is sometimes used, especially in Europe, to refer to a metamorphosed ortho-amphibolite with a protolith of diorite , gabbro or other mafic intrusive rock. In epidiorite the original clinopyroxene (most often augite ) has been replaced by

171-504: Is often preserved. Amphibolite facies equilibrium mineral assemblages of various protolith rock types consist of: Amphibolite facies is usually a product of Barrovian Facies Sequence or advanced Abukuma Facies Sequence metamorphic trajectories. Amphibolite facies is a result of continuing burial and thermal heating after greenschist facies is exceeded. Further burial and metamorphic compression (but little extra heat) will lead to eclogite facies metamorphism; with more advanced heating

190-599: Is recommended by the British Geological Survey when it is possible to determine the origin of the rock from its characteristics alone (and not from field relationships), particularly when the degree of metamorphism is low. Amphibolite as a rock defines a particular set of temperature and pressure conditions known as the amphibolite facies . However, caution must be applied here before embarking on metamorphic mapping based on amphibolite alone. First, for an ortho-amphibolite or amphibolite to be classed as

209-640: Is typically dark-colored and dense, with a weakly foliated or schistose (flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance. Amphibolite frequently forms by metamorphism of mafic igneous rocks, such as basalt . However, because metamorphism creates minerals entirely based upon the chemistry of the protolith , certain 'dirty marls ' and volcanic sediments may also metamorphose to an amphibolite assemblage. Deposits containing dolomite and siderite also readily yield amphibolite ( tremolite -schist, grunerite -schist, and others) especially where there has been

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228-626: The Kalahari craton from the combined Congo – Tanzania craton, and separated the Madagascar–India block from the Congo–Tanzania block. The ocean began closing between Madagascar–India and Congo–Tanzania between 700 and 580 million years ago, with closure between 600 and 500 million years ago. Exposed rocks in Kenya and Tanzania include rift sediments and passive margin metasediments from

247-434: The amphibolite appears to transgress apparent protolith bedding surfaces it is an ortho-amphibolite, as this suggests it was a dyke . Picking a sill and thin metamorphosed lava flows may be more troublesome. Thereafter, whole rock geochemistry will suitably identify ortho- from para-amphibolite. The word metabasalt was thus coined, largely to avoid the confusion between ortho-amphibolite and para-amphibolite. This term

266-522: The ductile deformation field. Gneissic texture may occur nearby, if not then mylonite zones, foliations and ductile behaviour, including stretching lineations may occur. While it is not impossible to have remnant protolith mineralogy, this is rare. More common is to find phenocrysts of pyroxene, olivine , plagioclase and even magmatic amphibole such as pargasite rhombohedra, pseudomorphed by hornblende amphibole. Original magmatic textures, especially crude magmatic layering in layered intrusions ,

285-478: The early Neoproterozoic continental margin. They also include older crustal material from the Tanzanian craton that has been reworked. At one time the belt was thought to have been caused by the collision of east and west Gondwana. According to a more recent theory, Gondwana formed over an extended period from accretion of independent terranes . The Mozambique Belt rocks formed from magmatic underplating and

304-514: The eastern basin rocks being thrust over those of the western basin during closure of the Mozambique Ocean between 585 and 550 million years ago. Many geologists consider that the Mozambique Ocean formed during the break-up of Rodinia between 800 and 700 million years ago, accompanied by volcanic activity. Magmatism from this period is found in East Africa and Madagascar, although not in the southern Mozambique Belt. The ocean probably separated

323-460: The fibrous amphibole uralite . Amphibolite was a favourite material for the production of adzes ( shoe-last-celts ) in the central European early Neolithic ( Linearbandkeramic and Rössen cultures). Amphibolite is a common dimension stone used in construction, paving, facing of buildings, especially because of its attractive textures, dark color, hardness and polishability and its ready availability. Hornblendite Hornblendite

342-439: The majority of rocks begin melting in excess of 650 to 700 °C in the presence of water. In dry rocks, however, additional heat (and burial) may result in granulite facies conditions. Uralite is a particular hydrothermally altered pyroxenite ; during autogenic hydrothermal circulation the primary mineralogy of pyroxene and plagioclase , etc. has altered to actinolite and saussurite ( albite + epidote ). The texture

361-453: The same equilibrium mineral assemblage as ortho-amphibolite, with more biotite, and may include more quartz, plagioclase, and depending on the protolith, more calcite / aragonite and wollastonite . Often the easiest way to determine the true nature of an amphibolite is to inspect its field relationships; especially whether it is interfingered with other metasedimentary rocks, especially greywacke and other poorly sorted sedimentary rocks. If

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