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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
The kind of rock that forms under blueschist or eclogite facies conditions depends on the protolith. Pelitic rocks metamorphosed at high pressure contain kyanite and garnet, often with chloritoid, zoisite, an Na-rich amphibole, and phengite (a mica similar to muscovite but containing magnesium). Granites may contain the same minerals that pelites contain but in different proportions. Marbles may contain aragonite, a high-pressure polymorph of calcite, although aragonite tends to turn into calcite when high-pressure rocks are brought to the surface. The rock type called blueschist forms when basalt and other mafic compositions reach blueschist facies conditions. Blueschists contain glaucophane, a distinctive inky blue amphibole (but despite their name do not display schistosity). Figure 10.30a shows a blueschist, partially changed to a green eclogite, from an outcrop on the beach just north of Jenner, California, that contains blue glaucophane, green pyroxene, and red garnet. Glaucophane, an Na-rich amphibole, forms in blueschists because at high pressure, the Na-component of plagioclase reacts to make amphibole. Similarly, at high pressure, the Ca-component of plagioclase is unstable and reacts to form a pyroxene. So, high-pressure rocks commonly contain glaucophane and pyroxenes that are rich in Na and Al and other nonstandard components. The pyroxenes, such as the green ones seen in Figure 10.30a, are called omphacite. Many blueschists also contain lawsonite, a high-pressure mineral with composition equivalent to hydrated anorthite, and epidote.
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
The blueschist facies is defined in basic rocks by the coexistence of glaucophane (blue to lilac amphibole) and lawsonite [CaAl2(Si2O7)(OH)2.H2O] (stable at high pressure and low temperature) or epidote at higher temperatures. Garnet, quartz, muscovite and jadeitic pyroxene are often present. Schistose rocks with these parageneses, which are blue in hand specimen, are often referred to as blueschists.
Metamorphism in the New England Orogen, eastern Australia: a review
Published in Australian Journal of Earth Sciences, 2020
K. Jessop, N. R. Daczko, S. Piazolo
Blocks of blueschist of 1–10 m diameter contain amphiboles (glaucophane, barroisite, crossite), phengite, pumpellyite, chlorite, quartz, albite, titanite, epidote and calcite. Rare sodic pyroxene and hornblende are also recorded (Fukui et al., 1995). Phillips et al. (2015) described lawsonite-blueschists from both Glenrock and Pigna Barney and noted white mica, glaucophane, lawsonite, epidote, titanite in a groundmass of albite and quartz. Some samples contained relict garnet of a similar composition to those found in the Attunga eclogite. Phillips et al. (2015) reasoned that at least some of the blueschist was formed during retrograde metamorphism of eclogite.