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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).
Stratigraphy of the Agnew-Wiluna Greenstone Belt: review, synopsis and implications for the late Mesoarchean to Neoarchean geological evolution of the Yilgarn Craton
Published in Australian Journal of Earth Sciences, 2022
Q. Masurel, N. Thébaud, J. Sapkota, M. C. De Paoli, M. Drummond, R. H. Smithies
The Maria Mine Formation of the Mt White Group overlies the Vivien Formation and Halfway Well Basalt (Figure 7), with unresolved contact relationships (Beardsmore, 2017). The Maria Mine Formation is composed of a dominant basal conglomerate unit, which is conformably overlain by a sequence of siltstones and argillites (Beardsmore, 2017; Liu et al., 1995). The basal conglomerate is polymict and includes rounded to well-rounded clasts that range in size from pebbles to cobbles, locally to boulders (Beardsmore, 2017; Hayman et al., 2020). Clast types include coarse-grained mosaic equigranular granite, feldspar–quartz-phyric, sub-volcanic, granite–granodiorite, feldspar-phyric dacite, fine-grained aphyric basalt, medium-grained amphibole-phyric dolerite, coarse-grained gabbro, granule- to pebble-sized sandstone, siltstone and mudstone (Fullard, 2013). The large size and mixed provenance of the clasts imply a local talus provenance, related to uplift and erosion of nearby felsic igneous centres and underlying greenstones (Boggs, 1987). Geochemical comparison of the mafic clasts within the Maria Mine conglomerate with underlying stratigraphy suggest that these clasts are derived from the Halfway Well Basalt, Turret Dolerite, White Hope Basalt and Redeemer Basalt (Hayman et al., 2020). By implication, the Maria Mine conglomerate must be stratigraphically younger than the Halfway Well Basalt and volcano-sedimentary rocks of the Vivien Formation, with an inferred unconformable basal contact.
Character and tectonic setting of plutonic rocks in the Gällivare area, northern Norrbotten, Sweden
Published in GFF, 2019
Zmar Sarlus, Olof Martinsson, Tobias E. Bauer, Christina Wanhainen, Joel B. H. Andersson, Roger Nordin
he Vassaravaara complex forms an irregular-shaped body surrounded by granite. The central part of the complex is composed of a coarse-grained layered gabbro, which is composed of olivine, pyroxene, plagioclase, amphibole and abundant magnetite, and exhibits an equigranular to hiatal texture (Fig. 2F). Plagioclase is the dominant mineral (50%) and it is characterized by its subhedral shape forming an intergranular texture with olivine (7%) and pyroxene (20%), the latter two occurring as anhedral grains between plagioclase crystals. Pyroxene crystals give the rock a brownish tint and they are locally altered to amphibole. Both ortho- and clinopyroxene are present. Olivine is partly altered to pyroxene, amphibole, biotite and chlorite. Plagioclase is partly replaced by sericite while anhedral amphibole (~ 10%) is altered to biotite (~ 8%) and chlorite (~ 2%). Oxides (~ 3%) are dominated by magnetite. The outer part of the complex is finer-grained, with similar mineral composition as the central part, but with a lower olivine content. Locally, as shown by one sample (no. 35 in Fig. 1), the outer part of the intrusion may be weakly deformed (Fig. 2G), while the central part appears undeformed. A northeast-southwest striking high-strain zone cuts through the western part of the complex (Fig. 1).
The Lord Howe Volcanic Complex, Australia: its geochemistry and origins
Published in New Zealand Journal of Geology and Geophysics, 2022
Megan L. Williams, Brian G. Jones
Sample DR134, from the stratigraphically lowest outcrop of the Boat Harbour Breccia on the western side of LHI, is weakly porphyritic with sparse subhedral plagioclase phenocrysts up to 1 mm in length. The very fine-grained groundmass consists of plagioclase, glass and pyroxene, and the texture does not appear to be trachytic. The unit is thus consistent with alkali basalt, however it is petrographically distinct from all other flows, having an almost equigranular texture with very weak, if any, flow fabric. It is possible, therefore, that the sample originates from a large clast within the Boat Harbour Breccia rather than from a primary flow.