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Review of Basic Chemistry and Geology
Published in Arthur W. Hounslow, Water Quality Data, 2018
Granites are coarse-grained igneous rocks occurring at continental edges. They consist of quartz and potash feldspar with smaller amounts or sodic plagioclase and some ferromagnesian minerals (containing iron and magnesium such as mica [biotite and/or muscovite] and sometimes hornblende). They are plutonic, resulting from the melting of marine sediments as they descend into the mantle. They are lighter than the surrounding rock, and therefore rise and intrude it. Rhyolites are fine-grained volcanic equivalents of granite.
Nordic Europe
Published in Ian Sims, Alan Poole, Alkali-Aggregate Reaction in Concrete: A World Review, 2017
Jan Lindgård, B. Grelk, Børge J Wigum, J. Trägårdh, Karin Appelqvist, E. Holt, M. Ferreira, M. Leivo
Rhyolite is an acid volcanic rock with phenocrysts of quartz and alkali feldspar embedded in a glassy or cryptocrystalline groundmass. Rhyolite can have a high proportion of glass. It is the extrusive equivalent of granite. The alkali-reactive components can be the glass and cryptocrystalline quartz.
Early Devonian volcanic facies, central Lachlan Orogen, New South Wales: implications for tectonic and metallogenic models
Published in Australian Journal of Earth Sciences, 2022
Felsic coherent facies and associated autoclastic breccias within the Gurragong Group are collectively referred to as the Mount Ariah Rhyolite (Figure 2) and are characterised by porphyritic texture and similar phenocryst types and abundances. Phenocrysts include high modal quartz, lesser feldspar and biotite, and, commonly, trace amounts of primary garnet (Table S3). Additional phenocryst phases differ between the coherent units and may include cordierite, orthopyroxene or clinopyroxene. Accessory minerals (trace–2%) include fluorite, muscovite, zircon, and opaque minerals (predominantly non-magnetic). The groundmass in the felsic coherent units is generally a recrystallised mosaic of quartzo-feldspathic granophyric texture, locally perlitic and/or containing spherulites. Flow banding is common in the coherent facies, now defined by contrasting alteration products (e.g. bands of chlorite or iron oxide vs albite; or sericite + clay vs granophyric-textured recrystallised glass), probably after original variations in composition or spherulite contents that separated during laminar flow. Geochemical analyses indicate the coherent units are predominantly rhyolites (see geochemical data below). Irregular quartz veinlets containing fluorite uncommonly occur in the felsic units.
Resolving tectonic settings of ancient magmatic suites using structural, geochemical and isotopic constraints: the example of the St Peter Suite, southern Australia
Published in Australian Journal of Earth Sciences, 2020
A. J. Reid, M. J. Pawley, C. Wade, E. A. Jagodzinski, R. A. Dutch, R. Armstrong
Associated with the St Peter Suite are the Nuyts Volcanics and the St Francis Granite, which are exposed only on islands of the Nuyts Archipelago (Figure 2a). The Nuyts Volcanics have been dated at 1631 ± 3 Ma and 1627 ± 2 Ma (Cooper, Mortimer, Rosier, & Uppill, 1985; Rankin, Flint, & Fanning, 1990), indicating they are co-eval with the St Peter Suite. The Nuyts Volcanics comprise porphyritic rhyodacite and rhyolite that preserve quartz and feldspar phenocrysts in a fine-grained matrix, with local examples of flow banding and lapilli (Rankin et al., 1990). These porphyritic rocks are intruded by rhyodacite to rhyolite dykes and a distinctive leucogranite, the latter of which is termed the St Francis Granite. These porphyritic volcanics preserve a foliation defined by fine-grained mica growth that is steeply dipping and north–south oriented (Flint & Rankin, 1991; Rankin & Flint, 1989).
The You Yangs batholith in Southeastern Australia, the sources of its magmas and inferences for local crustal architecture
Published in Australian Journal of Earth Sciences, 2019
Cross-cutting, steeply dipping felsic dykes (aplites, pegmatites and porphyritic microgranites) range up to several metres wide (Baker, 1935), although most aplitic dykes are decimetric in width. A particularly long dyke, designated as D1 by Baker (1935), is orientated NNW–SSE and cuts through essentially the whole extent of the outcrop (nearly 5 km) in the You Yangs Regional Park (Figure 2). This dyke is a porphyritic rhyolite to dacite. It varies in phenocryst proportion and mineralogy along its length, with hornblende phenocrysts in some places and feldspar only in others, but most commonly with quartz and feldspar phenocrysts. The margins are chilled against the granite and, in some places, have millimetre-scale magmatic layering (Baker, 1935). This dyke is represented by samples YY21 and YY30. A few younger, mafic lamprophyre dykes of Cenozoic age were also mapped by Baker (1935) and are shown in Figure 2.