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Magmatism and Geodynamics in the Archaean
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
Summarizing the results of studies on this rock series, Barker (1979) emphasizes that trondhjemites predominate only in the Archaean grey gneiss complexes. The main mineralogical association of grey gneisses is oligoclase + quartz + biotite (K-feldspar and hornblende are minor phases). Archaean grey gneiss complexes have a distinctive geochemistry that does not recur in later formations (Kratz, 1978, 1981; Martin; 1994). In comparison with the younger tonalite– trondhj emite complexes, they are characterized by increased aluminium, magnesium, sodium, nickel, chromium, strontium and REE content and by decreased potassium, rubidium, radiogenic lead and uranium content (Table 6.1). Both mineralogical and chemical compositions indicate that TTG belong to the I-type granitoids of Chappel and White (1974).
Minerals
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
Depending on the composition, we give plagioclase a variety name: albite, oligoclase, andesine, labradorite, bytownite, or anorthite (Fig. 3.20). Adding some confusion, the name albite is used in two ways. It refers to both the end-member composition (NaAlSi3O8) and to any plagioclase that contains 90%–100% NaAlSi3O8. Similarly, the name anorthite refers to the end-member composition CaAl2Si2O8 and to plagioclase that contains 90%–100% CaAl2Si2O8.
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
Orthoclase or other alkali feldspar usually forms over half the rock, with smaller amount of plagioclase (oligoclase); the dark minerals may be biotite, hornblende, or a pyroxene; and apatite, sphene, zircon and opaque iron oxides are accessories. A little quartz may be present, filling interstices between the other minerals. The texture is coarse-grained, sometimes porphyritic. Hand specimens are usually pale coloured.
Pyrometamorphosed Otago Schist xenoliths cause minor contamination of Dunedin Volcanic Group basanite
Published in New Zealand Journal of Geology and Geophysics, 2020
Emma J. Scanlan, James M. Scott, Petrus J. le Roux
The mafic segregations have more variable compositions compared to the felsic segregations. Their mineral assemblages comprise combinations of plagioclase, cordierite, olivine, spinel, ilmenite and glass (Figure 5D). Plagioclase is present as euhedral laths, prismatic grains or anhedral grains that vary from 10–50 µm (Figure 5E,F). Feldspar compositions vary from anorthite to oligoclase and anorthoclase, with most grains either being calcic (Ab15-25An70-80Or5) or sodic (Ab50-60An40-50Or2-15) plagioclase (Figure 6; Table 1). Normal zoning and replacement features of Ca-rich plagioclase are present (Figure 5E,F). The purple colouration noted in some hand samples (Figure 2B) is made up of fine 10–20 µm prismatic grains of cordierite within glass (Figure 7A). Cordierite grains contain 7–9 wt% FeO and 7.5–8.5 wt% MgO and have a 38–40 mol% sekaninaite (Fe2Al3(AlSi5O18)) component (Table 1). Olivine occurs mainly in the form of very fine (1–5 µm) symplectic intergrowths with spinel (Figure 7B). Stunning cruciform olivine grains, ∼4 µm in size, are common (Figure 7C). The symplectic olivine grains have a composition of Fo45-53Fa47-65Tp1, whereas the cruciform grains have higher iron contents (Fo31Fa68Tp1; Table 1). The intergrown spinel is hercynite and has 22–40 wt% FeO and 7–14 wt% MgO, with V2O3 reaching up to 1.6 wt%. One very large symplectite (∼150 µm) contains anorthite, spinel, olivine, and ilmenite surrounded by pigeonite (Figure 7D). Ilmenite occurs in a lobate morphology and is surrounded by glass. Ilmenite often contains minor MgO (∼2 wt%) and up to 2.6 wt% V2O3. An acicular mineral is present within all samples but remains unidentified due to the fine grainsize; this mineral often has a dendritic morphology and may be mullite (Figure 7E). Glass in the mafic section is trachyandesite to trachydacite (Figure 3) and the totals slightly lower than 100% indicate that it is hydrous (Table 3).