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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 photo in Figure 10.30b shows a hand sample of eclogite from Norway. Eclogites, which are rocks that contain conspicuous red garnet and a green pyroxene called omphacite, have a red-green Christmas-tree appearance. These rocks form when basalt and other mafic rocks reach eclogite facies conditions. The eclogite samples we find in outcrops today originated at great depth within Earth, and the minerals within them are unstable at Earth’s surface. Consequently, many eclogites show signs of retrograde metamorphism and have converted partially to become blueschists. The blueschist seen in Figure 10.30a may have formed this way—from rock that was, at one time, entirely eclogite.
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
Eclogite (page 218) is the name given to a rock of basaltic composition but where its mineralogy is very different from basalt in that it consists chiefly of a garnet and a clinopyroxene containing sodium and aluminium in significant amounts. It is known that this mineral assemblage is stable only under high pressure conditions but over a range of temperatures.
Geotourism and geoparks for sustainable rural development and poverty alleviation: Huanggang Dabieshan UNESCO Global Geopark, China
Published in Australian Journal of Earth Sciences, 2022
The eclogite, which is mainly exposed in the Zhujiahe and Taojiashan villages, occurs as bands, lenses and blocks up to 3 m thick (Figure 2c) and is strongly retrogressed to (garnet) amphibolite (Groppo et al., 2015; Shi & Wang, 2004). Owing to sparse outcrops, a direct contact between the eclogite and orthogneiss host rock is rarely visible. Eclogite is considered to be the most important indicator of continental subduction, collision and subsequent exhumation processes and provides significant insights for understanding the geodynamics of orogens (Song et al., 2003). Zircon in the eclogite has a U–Pb age of 240–220 Ma, suggesting that these rocks were formed in the Triassic by the collision of the North and South China blocks (Groppo et al., 2015; Liu et al., 2007, 2011a, 2011b; Wang et al., 2012; Zhao et al., 2008). The discovery of coesite and micro-diamonds in the eclogites of the DBGG has made this area an important site for studying UHP metamorphism and documenting continental subduction to mantle depths (Okay et al., 1989; X. M. Wang et al., 1989; Shutong et al., 1992).
Zircon U–Pb-Hf isotope data in eclogite and metagabbro from southern Sweden reveal a common long-lived evolution and enriched source
Published in GFF, 2020
Andreas Petersson, Lorraine Tual
Direct dating of the protolith of the largest known eclogite body in south-western Sweden yields an age of 1683 ± 17 Ma. A secondary age cluster at 1459 ± 44 Ma is coeval with a 1431 ± 26 Ma generation of metagabbro in the near vicinity. Both events predate the 1.39–1.37 Ga Torpa-Tjärnesjö granite that today envelope the eclogite nappe. Both the age of the eclogite, and the enriched zircon Hf isotope signature of the rock advocates an evolution coupled to the surrounding gneisses within the Eastern Segment and argues against an exotic origin of the eclogite nappe. The eclogite and what is today its neighbouring metagabbro are slightly more enriched than their surrounding gneisses, suggesting an enriched ultra-mafic source in the mid to lower crust, or within the subcontinental lithospheric mantle below the Fennoscandian Shield, also sourced by the oldest granitic rocks within the Transscandinavian Igneous Belt.
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
Two small blocks of eclogite (2 and 4 kg) were reported from an 800 m × 3.5 km serpentinite mass near Gleneden homestead, 15 km southeast of Nundle (Allan & Leitch, 1992) (Figure 7a, c). The eclogite contained crystals of reddish-brown almandine garnet (Alm 44–50%; Grs 30–33%; Prp 12–22%; Sps 3–7%) and colourless to pale green sodic-augite–omphacite clinopyroxene, embedded in a groundmass of retrograde actinolite after clinopyroxene and chlorite after garnet. Accessory minerals include prograde ilmenite and rutile with retrograde titanite, epidote and albite veinlets. From compositional data, the eclogite was inferred to have formed from an ocean-floor (MORB) basalt (Allan & Leitch, 1992).