Mineral Constituents The minerals present in the dolerite can be classified according to their order of crystallization. The dolerite displays OIB-like trace element features and contains moderately radiogenic Sr-Nd-Pb-Hf and light Ca isotopic compositions. Acrobat 4.05 Import Plug-in for Windows The most important minerals in the gabbron are plagioclase and pyroxene. 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+()����� 2010-05-11T21:28:49+10:00 application/pdf Like other mafic rocks, it is relatively low in silica content. Dunite is an ultramafic plutonic rock that is composed almost exclusively of olivine. %PDF-1.6 %���� 78 16 Diabase formation took place millions of years ago. 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. • Dolerites also often include olivine or quartz and can be alkali basalts, olivine tholeiites or quartz tholeiites. 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. Description of constituent minerals 37 Mineral composition 38 . 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). Gabbro is greenish and dark. The mineral composition of the dolerites was determined by X-ray diffraction. All five of the analysed dykes in- 0000006590 00000 n 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. Some geologists … 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. 2000-07-20T23:26:09+11:00 0000003995 00000 n 0000001248 00000 n During this process, the ferromagnesian minerals weather chemically, releasing iron and magnesium which recrystallise to form iron-rich montmonllomte In the fine-clay fraction. It is compositionally equivalent to gabbro and basalt but texturally between them. 2010-05-11T21:28:49+10:00 A significant decrease. Diabase (dolerite) is a dark-colored igneous rock. 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). 0000005311 00000 n It is the primary component of the sedimentary rock known as dolostone and the metamorphic rock known as dolomitic marble. 0000002683 00000 n 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 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). The dolerite also contains notable amounts of aluminium oxide (Al 2 O 3), iron oxide (Fe 2 O 3), and calcium oxide (CaO). Limestone that contains some dolomite is known as dolomitic limestone. 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. noun a dark basic intrusive igneous rock consisting of plagioclase feldspar and a pyroxene, such as augite; often emplaced in dykes any dark igneous rock whose composition cannot be determined with … The rocks in the earth’s crust continuously undergo changes in their composition which leads to formation of other rocks. 1. m��@( �*�^;k/%l��. Dolerite is a rock of basaltic chemical composition abundant in Ca-rich minerals, which releases Ca and Mg in solution during chemical weathering. Diabase Formation. 4c). Adobe Acrobat 9.2 Paper Capture Plug-in 0000000926 00000 n Dolomite, type of limestone, the carbonate fraction of which is dominated by the mineral dolomite, calcium magnesium carbonate [CaMg (CO 3) 2 ]. 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 0000029943 00000 n It occurs mostly in shallow intrusions (dikes and sills) of basaltic composition. If there is more sodium in the plagioclase, then the rock type is called diorite. 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. Most of the remainder is made up of pyroxene minerals (usually augite). <<41E226E8319E8D4B90D7AA0969CA8EC0>]>> 0000001130 00000 n of calcium and an increase of magnesium in the augite was observed for the micropegmatitic dolerite group (Table 2). startxref 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. Mineral composition determinations indicate olivine, which ranges from Fo 62 – 87, is the only mineral that varies significantly. 3d). Minor amounts of hornblende, olivine, magnetite, and quartz can be found in some diabase rocks. 0000002020 00000 n dolerite {Initial Core Descriptions) is slight, since the chemical composition of this glass (microprobe data, Melson, this volume) resembles more closely that of the underlying aphyric basalts than that of the dolerite. 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