Alkaline Rocks. Alkaline rocks contain some of the world's largest deposits of REE in some of the most diverse mineral assemblages, although they account for only 0.5% of the world's igneous rocks. All rock classifications are based on two criteria, however, mineral content of the rock, and texture (grain size). If they erupt from volcanoes onto the surface as lava, they are called extrusive rocks.By contrast, Intrusive rocks are formed from magma that cools underground. Q - Af - Pf diagram for classification of felsic and intermediate plutonic rocks. Immobile trace elements and SiO 2 classification for igneous rocks, especially volcanics. 2007) Hastie G Co vs. Th This expanded classification is presented in Table 2, where the peralkaline granites are included along with other alkaline rocks. Extrusive rocks, because of their small crystals and glass, are less durable. Alkaline rocks contain higher Na and K that other igneous rocks, such that they contain feldspathoids, alkali pyroxenes or alkali amphiboles. The volcanic rocks consist of an early group of alkalic basalt lavas and a later group of siliceous ignimbrites. Sporadically out-cropping throughout the region, the Nama Group is a platform sequence that is partly complementary to the Gariep Group and lies unconformably on the rocks of the Namaqua Metamorphic Province. 3.4 Classification of Igneous Rocks As has already been described, igneous rocks are classified into four categories, based on either their chemistry or their mineral composition: felsic, intermediate, mafic, and ultramafic. Streckeisen, 1967. Igneous Rock Bodies. Igneous Rocks are classified is several ways, and methods of classification have evolved a lot over the past 100 years.Each classification is useful for a certain purpose and reflect a particular way of looking at igneous rocks. This data release includes new major and trace element geochemical data acquired by the U.S. Geological Survey (USGS) for igneous rocks in the Cripple Creek district in Colorado. The most important are Pocos de Caldas, Jacobina, Serido, and Figueira. Cripple Creek is among the largest epithermal districts in the world, with more than 800 metric tons (t) Au (>26.4 Moz). The plutonic rocks are also alkaline. These deposits are in alkaline igneous rocks, in sandstones, and in veins in many rock types. The diagram in Figure 3.16 can be used to help classify igneous rocks by … SiO 2 vs. K 2 O for volcanic rocks. Plus, they are, by definition, exposed to the elements of erosion immediately. SiO 2 vs. Na 2 O+K 2 O for volcanic rocks. Trachyte; Trachyte is a extrusive igneous rock that light coloured is composed mostly of alkali feldspar with minor amount of dark coloured minerals such as biotite, amphibole, or pyroxene. Winchester and Floyd, 1977. Here, both volcanic and intrusive igneous rocks are exposed. Co - Th (Hastie et al. Diagram of Miyashiro (1974) distinguishing between tholeiitic and calc-alkaline igneous rocks. The alkaline rocks fall into the alkalic (or rarely alkali–calcic) peralkaline category in that table. Another failed rift is the Oslo Graben of southern Norway, of late Paleozoic age. respectively, calc-alkaline igneous rocks and gneisses of the Richtersveld and Gordonia Subprovinces of the Namaqua Metamorphic Province. Igneous rocks are those that form via the process of melting and cooling. With the addition of the AI and FSSI indices, we can expand the classification of alkaline rocks. Rocks in the tholeiitic magma series are classified as subalkaline (they contain less sodium than some other basalts) and are distinguished from rocks in the calc-alkaline magma series by the redox state of the magma they crystallized from (tholeiitic magmas are reduced; calc-alkaline magmas are oxidized). LeBas et al., 1986. SiO2 - K2O (Peccerillo + Taylor 1976) PeceTaylor G SiO 2 vs. K 2 O binary Diagram proposed by Peccerillo & Taylor (1976) to distinguish various series of tholeiitic, calc-alkaline and shoshonitic rocks. Igneous rocks are common in the geologic record, but surprisingly, it is the intrusive rocks that are more common. If the intrusive rock cooled underground but near the surface, it is called subvolcanic or hypabyssal, and often has visible, but tiny mineral grains.