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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
This is also derived from basic rocks such as dolerite, but at a rather higher grade of metamorphism and contains essentially hornblende and quartz (Fig. 7.2b). The term amphibolite is used as a group name for metamorphic rocks in which hornblende, quartz, and plagioclase are the main minerals. Hornblende-schist is one member of this group. British examples of hornblendeschist are found in the Dalradian rocks of the Scottish Highlands (Fig. 7.6), as in the Loch Tay area, Perthshire; and in the Lewisian near Loch Maree, Ross-shire; and in England at Start Point, Devon, and Landewednack, S. Cornwall.
Metamorphic Rocks
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
Figure 10.18 shows an example of a garnet amphibolite. Amphibolites form by metamorphism and recrystallization of basalt or other mafic igneous rocks. All amphibolites contain mostly hornblende and plagioclase, but other minerals including garnet and minor quartz are typically present. In Figure 10.18, the garnet porphyroblasts, 1–2 cm wide, are large and conspicuous; sometimes when garnet is present, it is less easily seen. Some amphibolites are strongly foliated but most are not.
Metamorphism
Published in Aurèle Parriaux, Geology, 2018
As metamorphism intensifies, the rock loses its carbon as CO2 is released. Feldspars and long prisms of calcic amphiboles crystallize. Amphibolite is composed of a fine alternation of amphibole layers and feldspar layers. It generally has a schistose structure.
Lithostratigraphy of Paleozoic metasediments in southern Fiordland, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2023
Richard Jongens, Ian M. Turnbull, Andrew H. Allibone
The metasediments of south-central Fiordland are generally more foliated than those in southwest Fiordland. This is particularly so for pelitic rocks, which always show some degree of schistosity. Gneissic rocks are common in the Lake Hauroko area (Ladley 1999). Foliation is axial planar to rare tight or isoclinal folds and subparallel to bedding. The foliation pre-dates intrusion of the undeformed c. 364 Ma Merrie Granite (Allibone et al. 2009a), which hornfelses adjacent Russet Formation psammites. Foliation and bedding are subsequently folded by open northeast or southwest gently plunging folds without associated cleavage development. Metamorphism appears similar to Cameron Group with assemblages of sillimanite ± K-feldspar ± muscovite ± cordierite in pelitic lithologies, diopside in calc-silicate rocks, and migmatisation in pelites (Ireland and Gibson 1998; Powell 2006; this study). The pelitic metamorphic mineralogy suggests a high temperature-low pressure regional upper amphibolite facies metamorphism. However, geothermobarometry in rare garnet-bearing pelite of the Russet Formation indicates pressures just into the kyanite stability field (Scott et al. 2009) although kyanite has not been observed in south-central Fiordland. Kyanite only appears north of the southernmost splay of the Spey-Mica Burn Fault System (Ireland and Gibson 1998; Powell 2006). Metamorphism is thought to have taken place around c.350–370 Ma, based on zircon rims (Scott et al. 2009) and monazites (Ireland and Gibson 1998), and the lack of metamorphic textures in the cross-cutting Merrie Granite (Allibone et al. 2009a).
Two belts of HTLP sub-regional metamorphism in the New England Orogen, eastern Australia: occurrence and characteristics exemplified by the Wongwibinda Metamorphic Complex
Published in Australian Journal of Earth Sciences, 2020
K. Jessop, N. R. Daczko, S. Piazolo
Grade increases to the north against the Shacks Mylonite Zone (Figure 19). The northern half of the area contains typical accretionary complex rock types of psammitic and pelitic metasedimentary rocks mixed with meta-basalts, meta-cherts and minor calc-silicate lenses that have been metamorphosed to migmatite gneiss, amphibolite and quartzite. The gneiss contains segregations, veins and irregular intrusions of S-type granitic rocks formed by partial melting of the metasedimentary rocks (Morand, 1993). Large grains of muscovite coexist with quartz and K-feldspar in the gneisses and have been interpreted as part of a prograde assemblage (Leitch et al., 1993). Psammitic metasedimentary rocks contain quartz, biotite, plagioclase, muscovite, and sillimanite. In addition, pelitic sedimentary rocks contain K-feldspar and almandine-garnet (Leitch et al., 1993; Morand, 1993). Cordierite has not been recorded, possibly because of the quartz-rich nature of the protolith Shoalwater Formation sedimentary rocks. Amphibolites are predominantly hornblende and plagioclase with subordinate titanite, clinopyroxene and almandine garnet.
The origin of mafic–ultramafic rocks and felsic plutons along the Clarke River suture zone: implications for porphyry exploration in the northern Tasmanides
Published in Australian Journal of Earth Sciences, 2023
A. Edgar, I. Sanislav, P. Dirks
Samples of amphibolite were collected from outcrops and subcrops proximal to the ultramafic lithologies. The amphibolite is dark green to black in colour, strongly lineated, medium to coarse-grained, and it contains 60–90 vol% amphibole, 10–30 vol% plagioclase and accessory opaque phases. The amphibole species consists predominantly of magnesian hornblende to ferromagnesian hornblende (online data).