endstream endobj 79 0 obj <> endobj 80 0 obj <> endobj 81 0 obj <>/Font<>/ProcSet[/PDF/Text/ImageB/ImageC/ImageI]>> endobj 82 0 obj <>stream xref On the basis of texture and mineral proportions, the dolerite is divided into four groups: (1) picritic, (2) olivine-rich, (3) normal, (4) and micropegmatitic dolerites. Diabase is the preferred name in North America, while dolerite is the preferred name in the rest of English-speaking world, where sometimes the name diabase is applied to altered dolerites and basalts. Mineral compositions were determined with CAMECA S×100. Diorite is defined as follows (p.198): a group of plutonis rocks intermediate in composition between acidic and basic rocks, characteristically 4b).Furthermore, on the Nb*2–Zr/4–Y diagram (Meschede, 1986) the dolerite dykes plot in the fields of volcanic-arc basalts and N-MORB lavas (Fig. The wide range of modal composition is illu-Mcp 0, 0 o o 00.'!. The nearly identical Sr-Pb isotopes between plagioclase and bulk rock indicate a minor role of crustal assimilation during magmatic evolution. As originally applied by Brongniart in 1807, the term corresponded to what is now recognized as diorite. Dolerite is composed of two essential and several accessory minerals. It is a medium grained dark and heavy rock, harder than granite. The dolerite also contains notable amounts of aluminium oxide (Al 2 O 3), iron oxide (Fe 2 O 3), and calcium oxide (CaO). dolerite (diabase, microgabbro) A dark-coloured, medium-grained igneous rock which contains plagioclase feldspar of labradorite composition and pyroxene of augite or titanaugite composition as essential minerals, and magnetite, titano-magnetite, or ilmenite as accessory minerals. It occurs mostly in shallow intrusions (dikes and sills) of basaltic composition. Diabase formation took place millions of years ago. All five of the analysed dykes in- Etymol: Greek doleros, 'deceitful,' in reference to the fine-grained character of the rock that makes it difficult to identify megascopically. Synonym: A synonym of Dolerite In the United States, an intrusive rock whose main components are labradorite and pyroxene and that is characterized by ophitic texture. Description of constituent minerals 37 Mineral composition 38 . <<41E226E8319E8D4B90D7AA0969CA8EC0>]>> Dolerite is a dark igneous rock. 2000-07-20T23:26:09+11:00 It is compositionally equivalent to gabbro and basalt but texturally between them. Minor amounts of hornblende, olivine, magnetite, and quartz can be found in some diabase rocks. 78 16 area are either type-1 or type-2 dolerite with type-3 and type-4 dolerite occurring only as small sills of limited lateral extent. If there is more sodium in the plagioclase, then the rock type is called diorite. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine grained to aphanitic chilled margins which may contain tachylite(dark mafic glass). 0000002020 00000 n uuid:8f9efbd6-2214-4636-a733-f10b85f1dd66 The accessory minerals are quartz, orthoclase, chlorite and magnetite. Limestone that contains some dolomite is known as dolomitic limestone. Two dykes in the study area are of type-1 dolerite, whereas the other three dykes sampled do not correspond in composition to any of the dolerite types represented by sills. Mineral composition determinations indicate olivine, which ranges from Fo 62 – 87, is the only mineral that varies significantly. 0000003995 00000 n The Amata Dolerite has a medium grey to greenish grey colour on freshly broken surfaces and typically consists of interlocking white plagioclase crystals and dark coloured mafic minerals (Fig. Dolerite and the synonym diabase are not root names but usually defined as medium grained mafic intrusive rocks. %PDF-1.6 %���� uuid:6965ee2a-ca18-4027-9cb3-535df64e55ad 0000000016 00000 n The results are given in Table 2. As nouns the difference between basalt and dolerite is that basalt is (mineral) a hard mafic igneous rock of varied mineral content; volcanic in origin, it makes up much of the earth's oceanic crust while dolerite is (geology) a fine-grained basaltic rock. The dolerite is typically massive and samples with ophitic or subophitic texture display a conspicuous spotty pattern on polished surfaces (Fig. 0000000926 00000 n 0000005311 00000 n Diabase or Dolerite Diabase or dolerite is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diorite, gabbro, anorthosite — the three root names in this field are separated according to the colour index and the average composition of their plagioclase – anortho… Introduction: The intrusive equivalent of gabbro as it has the same mineral and chemical composition.Dolerite is found in either sills, sheet-like compositions that intrude horizontally, and dykes, which are vertical or inclined features. The dolerite displays OIB-like trace element features and contains moderately radiogenic Sr-Nd-Pb-Hf and light Ca isotopic compositions. It is a medium grained igneous rock, and can be dark grey or black with greenish shades in colour. It can include microgabbro, basalt, andesite, microdiorite and other rock types. The most important minerals in the gabbron are plagioclase and pyroxene. Dolomite is a common rock-forming mineral. Dolerite is a rock of basaltic chemical composition abundant in Ca-rich minerals, which releases Ca and Mg in solution during chemical weathering. 1. 0000006679 00000 n Introduction . 8�ژ��bu,��}������ˏ��w�ߋ�?�i�W�mp&�gR�������2�2�V��l. During this process, the ferromagnesian minerals weather chemically, releasing iron and magnesium which recrystallise to form iron-rich montmonllomte In the fine-clay fraction. startxref The term dolerite, synonymous with diabase and microgabbro, is used to describe an igneous hypabyssal rock of dark color composed of plagioclase (labradorite in composition) and clinopyroxene (normally augite or titanoaugite), with opaques as the main accessory minerals (magnetite, titanomagnetite or ilmenite). Diabase is a common rock type. of calcium and an increase of magnesium in the augite was observed for the micropegmatitic dolerite group (Table 2). 78 0 obj <> endobj Mineral composition. 93 0 obj <>stream 3c). 0000001130 00000 n It is the primary component of the sedimentary rock known as dolostone and the metamorphic rock known as dolomitic marble. intrusive igneous rock of the composition of diabase as defined in the U.S. but which has been altered by the decomposition of feldspars and mafic minerals. 0000004633 00000 n Mineral composition has been determinedattheEPMALab,CPL,CHQ,Geolog- ical Survey of India, Kolkata. 0000002683 00000 n These larger occurrences of dolerite mostly contain pyroxene as the dominant ferromagnesian mineral and are largely the rock called "diabase" by petrographers, while cases where hornblende is dominant are mostly confined to dikes and smaller intrusions, especially in the older rocks. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine grained to aphanitic chilled margins which may contain tachylite (dark mafic glass). Discussion 44 References 45 . � _*� Quartz, orthoclase and chlorite may comprise Gabbro is greenish and dark. 3d). Diabase typically has a mineral composition that is dominated by the plagioclase feldspar known as labradorite (approximately 40% to 70% of the rock). However, on the logarithmic Zr/TiO 2 *10 −4 vs. Nd/Y diagram (Winchester and Floyd, 1977) the dolerite dykes have an apparent sub-alkaline basalt composition (Fig. The rocks in the earth’s crust continuously undergo changes in their composition which leads to formation of other rocks. 0 Diabase or dolerite or microgabbro is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. One of the most common rocks in the assemblage is known as Preseli Spotted Dolerite—a chemically altered igneous rock containing spots or clusters of secondary minerals replacing plagioclase feldspar. 2010-05-11T21:28:49+10:00 0000001248 00000 n application/pdf Adobe Acrobat 9.2 Paper Capture Plug-in 0000006590 00000 n 2. It is a small amount of olivine and amphibole. 0000000616 00000 n The researchers wondered about the origins of the dolerite bluestones that Thomas had identified, and took a second look at the mineral composition of the rocks. 0000003340 00000 n %%EOF Acrobat 4.05 Import Plug-in for Windows The mineral composition of the dolerites was determined by X-ray diffraction. 0000005941 00000 n 4c). 1 0 obj <> endobj 66 0 obj <>stream Most of the remainder is made up of pyroxene minerals (usually augite). H��Wے����INi�s�T������Tl�^&y�S.���H��]o�>=�[�n���ؘ��>}��o�"� ����+����cq����������W.ԇ]}�o����t�����[�G�5��w.-�)��1���2�㡸��~.>����K�7C����]{���>|�tW�+��;|WƊզ�Ū+()����� Like other mafic rocks, it is relatively low in silica content. Diabase, also called Dolerite, fine- to medium-grained, dark gray to black intrusive igneous rock.It is extremely hard and tough and is commonly quarried for crushed stone, under the name of trap. x�b```e``�����Pm� �� �@Q� ���٤���Yj��'3X�N���F�\���V�@�R�x���!hG1�,�j&;&&3&U&_&Y&����� The essential minerals are plagioclase feldspar and pyroxene, which together constitute between about 60% and 80% of the total rock composition. phyric dolerite, small lenses of coarse-grained pegrna-toid, and bands of granophyre with a composition roughly equivalent to that of a granite (62 to 67% Si02). • A dolerite is the medium-grained equivalent of a basalt - a basic rock dominated by plagioclase and pyroxene. It is a mafic rock, containing magnesium and iron, often in form of olivine. • Dolerites also often include olivine or quartz and can be alkali basalts, olivine tholeiites or quartz tholeiites. 2010-05-11T21:28:49+10:00 A significant decrease. Most common mafic minerals in ultramafic rocks are definitely pyroxenes and olivine (if hornblende is present it is added to pyroxenes). Abstract Chemical weathering taking place in a soil formed on till derived solely from dolerite was investigated. They can contain a wide range of … Some geologists … Diabase (dolerite) is a dark-colored igneous rock. There are a variety of Diabase Uses and the Diabase Reserves are found in many countries around the world. 00 \ Plag FIG. %PDF-1.4 %���� H�|U�j�@��s��;�H���^Cݤ�C{ Analyses were per- formed with a 15 KV accelerating voltage, 15× 10−10A beam current, 5 μm spot size and 50 s live time for each element. 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