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Magmatism of Phanerozoic Fold Belts
Published in O.A. Bogatikov, R.F. Fursenko, G.V. Lazareva, E.A. Miloradovskaya, A. Ya, R.E. Sorkina, Magmatism and Geodynamics Terrestrial Magmatism Throughout the Earth’s History, 2020
V.V. Yarmolyuk, V.I. Kovalenko
The second, transitional, stage is characterized by the presence of diverse andesite assemblages dominated by rocks of the calc-alkaline (basalt–andesite– dacite–rhyolite) and potassic subalkaline (shoshonite–latite–alkali rhyolite) series. The diverse gabbro–granite assemblages are their intrusive equivalents. Less common and of local occurrence are the bimodal basalt–rhyolite assemblages and their intrusive equivalents: the gabbro–plagiogranite assemblages (Borsuk et al., 1987). Unusual dunite-clinopyroxenite-gabbro complexes, similar to those of the Platinonosny (platinum-bearing) Belt of the Urals and the massifs of Alaska, were recorded locally (Irvine, 1974; Laz’ko, 1988b). And, finally, the third, continental stage (often classed as orogenic) includes a wide range of igneous rocks. The most common are andesite–dacite–rhyolite, dacite–rhyolite and their intrusive equivalents: the diverse granodiorite–granite series. Apart from this, high-titanium basic rocks of normal, moderate and high alkalinity, usually of the K–Na series, are also common; the acid rocks contain trachytes and alkaline rhyolites.
Volcanoes and Their Products
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Although sometimes absent, when present, phenocrysts help distinguish different kinds of volcanic rocks. Figure 7.47 shows the common phenocrysts in rocks ranging from felsic to ultramafic. These phenocryst minerals are the same minerals that may be present as microscopic crystals in the groundmass. From felsic to ultramafic, the volcanic rocks are rhyolite, dacite, andesite, basalt, and komatiite. Quartz and K-feldspar are generally restricted to relatively felsic rocks, olivine and pyroxene to relatively mafic rocks, and the other minerals to intermediate rocks. Plagioclase is a solid-solution mineral in volcanic rocks of many sorts and varies from being Na-rich for felsic rocks to being Ca-rich for mafic and ultramafic rocks. Figure 7.47 is only an approximation, because the minerals present as phenocrysts depend on many different things.
Igneous Rocks
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
Varieties Hornblende-andesite, augite-andesite, enstatite-andesite, biotite-andesite, and quartz-andesite (= dacite). The pyroxene-bearing varieties are abundant and may grade into basalts. Andesites that have been altered by hot mineralizing waters of volcanic origin (see Fig. 5.28a), with the production of secondary minerals, are called propylites.
Correlation between the Warepan/Otapirian and the Norian/Rhaetian stage boundary: implications of a global negative δ13Corg perturbation
Published in New Zealand Journal of Geology and Geophysics, 2022
The Arawi Shellbeds and Ngutunui Formation are a succession of volcaniclastic sedimentary rocks dominated by thin sandstones, siltstones and shales, with minor but conspicuous conglomerates, tuffs and shellbeds (Figure 3). Limestones are not present, although the shellbeds approach coquina limestone composition in places within the Arawi Shellbeds (Grant-Mackie 1985), and there are no radiolarian cherts. Compositionally, the volcanic lithologies are broadly andesitic but range from basaltic andesite to dacite. The tuffs vary in vitric, crystal and lithic composition. In every respect, the sedimentary rocks in the Kiritehere section are typical of Murihiku Supergroup. They have been weakly metamorphosed to zeolite facies grade with conspicuous zeolite veining (laumontite, stilbite) and zeolite ‘cements’ (laumontite, heulandite, analcime). There are minor faults in places and also some ‘slumps’ (Grant-Mackie and Lowry 1964) but in general the stratigraphy is more or less ‘layer cake’, easy to recognise in the field, and the named formations and groups can be traced for many tens of kilometres. However, the sedimentary sequence as a whole has been folded, and a number of anticlines and synclines are recognised and named within a broader Kawhia Regional Syncline (e.g. Kear 1960; Edbrooke 2005). In the Kiritehere section, the sequence is dipping and younging to the east, and is part of a large homoclinal structure that may be interpreted as the west limb of a syncline (e.g. Kear 1960; Kear and Schofield 1964; Edbrooke 2005).