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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
The rocks of all three formations underwent granulitic metamorphism at pressures of 10–12 kbar and temperatures, of 90Q°C. The age of premetamorphic rocks of the Daldynskaya Formation, from the data of E.V. Bibikova (Rosen et al., 1988), is evaluated as no younger than 3.32 Ga, and that of granulite metamorphism is taken as 2.7 ± 0.1 Ga ago. The Anabar Shield granulitic complex is crosscut by a number of thick, Early Proterozoic zones of retrograde metamorphism under amphibolite-facies conditions with an age of 1.97 ± 0.02 Ga. These are associated with formations similar in age to the extensive anorthosite-mangerite massifs, which were found to be 2.1 Ga old at a εNd(T) of –6.0 (Sukhanov, 1989).
Metamorphic Rocks
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
Igneous rocks which were much weathered before metamorphism may acquire calc-silicate minerals (e.g. Ca-garnet) derived from calcium and Al-silicates in the original rock, and hornblende from chlorite. Andesites and andesitic tuffs during contact metamorphism may develop many small flakes of brown mica and crystals of magnetite, as in the Shap Granite contact zone. Basic granulites (rocks of equigranular texture, p. 141) are formed from basic rocks such as gabbro by prolonged high-grade metamorphism, involving high temperature and complete recrystallization; pyroxenes and plagioclase are the main minerals. The term granulite is used broadly for rocks in which the main minerals are roughly equidimensional (see further under regional metamorphism).
Metamorphic rocks
Published in W.S. MacKenzie, A.E. Adams, K.H. Brodie, Rocks and Minerals in Thin Section, 2017
W.S. MacKenzie, A.E. Adams, K.H. Brodie
Most metamorphic rocks are designated by a term which denotes the texture preceded by the names of one or more of the constituent minerals in order of increasing abundance (e.g. garnet- sillimanite schist). These minerals may be indicators of the grade of metamorphism of the rock. The terms used to designate the texture are schist (182–185), gneiss (186–187) and granofels (180–181) (or hornfels (227–228) if the rock is known to be formed by contact metamorphism). Fine grained schists can be termed slates or phyllites. The term granulite has in the past been used both to denote a texture and also a facies – the highest temperature and pressure facies in regional metamorphism. In its original use to describe a texture it meant that the rock is massive and that the mineral grains show no preferred elongation and are generally of uniform size: the term recommended for a rock with such a texture is granofels (page 168). In fact the original granulite is a strongly deformed felsic rock now termed a mylonite! Other definitions require the presence of specific minerals, such as feldspar and quartz. In general, it is best to avoid using granulite as a rock type, and to use granulite facies to refer to rocks metamorphosed at high T and P conditions.
A geophysically constrained crustal element map of East Antarctica between Enderby Land and Princess Elizabeth Land
Published in Australian Journal of Earth Sciences, 2023
Y. Giri, P. G. Betts, M. Radhakrishna, M. A. McLean, T. K. Biswal, R. J. Armit
The Vestfold Hills and Rauer Group are located at the eastern end of the Prydz Bay coast further east of Larsemann Hills (Figure 6c). The Vestfold Hills Province is dominated by the Neoarchean (ca 2520 Ma) granodiorites, tonalitic orthogneisses and pyroxene granulites, and metavolcanic rocks and semi-pelitic rocks (Harley, 1993), which record magmatic accretion crust formation events between ca 2520 and 2480 Ma (Harley & Kelly, 2007). These rocks had undergone granulite facies metamorphosed at ca 2500 Ma. Grenvillian and Pan-African metamorphic ages are absent in this terrane (Clark et al., 2018; Hoek & Seitz, 1995; Mikhalsky et al., 2019). These rocks are intruded by extensive dyke arrays of Meso- to Neoproterozoic age (Clark et al., 2012; Harley & Kelly, 2007; Liu et al., 2021).
Crystallographic preferred orientation of quartz deformed at granulite conditions: the Kalinjala Shear Zone, Port Neill, South Australia
Published in Australian Journal of Earth Sciences, 2022
C. J. L. Wilson, N. J. R. Hunter, V. Luzin
The development of CPOs in quartz has been studied in the mylonitic orthogneiss from the Kalinjala Shear Zone using neutron diffraction and optical techniques. The processes that cause the localisation of strain in these lower crustal granulite facies metamorphic rocks include metamorphic phase changes, development of CPOs and grain boundary migration by dynamic recrystallisation. Quartz CPOs from the central section of the Kalinjala Shear Zone are dominated by prism-< a > slip, with concentrations parallel to the Y-strain axis. The CPO patterns are approximately symmetrical with respect to the finite strain axes. This serves as an independent kinematic indicator of a predominantly late flattening or a plane strain component superimposed on the initial constrictional dextral shear strain generally observed in the Kalinjala Shear Zone. The CPOs are consistent with an approximately east–west-directed compression late in the evolution of the Kalinjala Shear Zone. The results of this investigation also demonstrate the validity of applying semi-quantitative microstructural and crystal fabric methods to support field observations.
Raman-XPS spectroscopy, REE chemistry, and surface morphology of Fe-Ti oxide heavy mineral sands: a case study from Varkala-Kovalam coast, south-west India
Published in Applied Earth Science, 2021
R. G. Rejith, M. Sundararajan, A. Peer Mohamed, M. Satyanarayanan
The beach minerals as accessory phases in provenance rocks and the west-flowing rivers originate from Western Ghats act as the geomorphic control for the occurrence of heavy mineral-rich sands along the coast of Kerala. The occurrence of provenance rocks, climatic factors, coastal dynamics and processes, drainage patterns (west-flowing rivers), and coastal geomorphology are driving factors behind the formations of heavy mineral-rich sands. The beach sand transported and deposited over the sea undergoes relative sorting based on their variation in density by the strong action of waves and long currents. Thiruvananthapuram district is mainly occupied by two geological divisions, the Tertiary and Quaternary sediments along the western coastal fringe and the eastern Archean crystalline rocks. The Archean crystalline rocks are composed of khondalites, charnockites, and migmatite groups. The khondalites mainly occupy the southern part of Kerala (including the present study area). It is made of garnetiferous biotite-sillimanite gneiss and rarely calc-granulite and quartzite. The primary sources of heavy minerals are khondalites, charnockites, and also the outcrops of pegmatites and quartz veins seen in these rock types. The Tertiary sediments that linearly stretches along the coast, unconformably overlay the crystalline rocks, are one of the intermediate sources. The laterite weathered from charnockites consists of ilmenite and magnetite, whereas khondalite consists of sillimanite and graphite (Krishnan et al. 2001).