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Plutonic 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
For classifying ultramafic rocks, the IUGS uses the proportions of olivine, orthopyroxene, and clinopyroxene (Fig. 6.25). Peridotites, which all contain more than 50 wt% olivine, include harzburgite (orthopyroxene >> clinopyroxene), wehrlite (clinopyroxene >> orthopyroxene), and lherzolites (subequal amounts of orthopyroxene and clinopyroxene). Pyroxenites may be orthopyroxene-rich (orthopyroxenite), clinopyroxene-rich (clinopyroxenite), or in-between (websterite). The modifier olivine is put in front of a pyroxenite name if a rock contains more than 5% olivine. A different triangular diagram (not shown) is used for the rare ultramafic rocks that contain hornblende.
Composition and Miocene deformation of the lithospheric mantle adjacent to the Marlborough Fault System in North Canterbury
Published in New Zealand Journal of Geology and Geophysics, 2023
Sophie J. Bonnington, James M. Scott, Marshall C. Palmer, Nadine P. Cooper, Malcolm R. Reid, Claudine H. Stirling
Olivine within the peridotites is magnesium-rich but the sample suite covers a large range in Mg# ( = 100*Mg/Mg + Fe) (all mineral chemistry is in the Data Supplementary), with the majority below the typical primitive upper mantle peridotite composition (89.2; Ionov and Hofmann 2007) (Figure 5A). The lherzolite samples have the tightest olivine Mg# cluster (82.2–83.9), whereas the harzburgite xenoliths range from 86.6 to 91.2 and dunite xenoliths range from 82.6 to 92.3. The wehrlite sample had an average olivine Mg# of 84.5. Orthopyroxene is typically magnesium-rich with average end-member compositions of Wo2En87Fs11 and therefore is enstatite. Clinopyroxene has an average end-member diopside composition of Wo45En50Fs5. Six harzburgite samples contain Cr-spinel butthis phase is absent in all other samples (Table 1). However, even when present, the spinel is of small abundance (<1%). Spinel Cr# ( = 100*Cr/(Cr + Al)) ranges from 25.1 to 55.6 and the samples trend towards refractory compositions. Furthermore, the absence of spinel correlates with lower olivine Mg# (Figure 5B). Ilmenite is a minor component of the spinel-free peridotite samples (<1%), but only occurs in those with olivine Mg# < 89.6.
Petrogenesis of the Kalka, Ewarara and Gosse Pile layered intrusions, Musgrave Province, South Australia, and implications for magmatic sulfide prospectivity
Published in Australian Journal of Earth Sciences, 2023
W. D. Maier, B. Wade, Sarah-Jane Barnes, R. Dutch
The Giles intrusions show considerable variation in size (from a few km2 to >3000 km2), depth of emplacement (from sub-volcanic to possibly more than 30 km; Goode & Moore, 1975), stratigraphy and style of associated mineral deposits. Maier et al. (2014, 2015) grouped the intrusions by their predominant lithologies. Intrusions with important ultramafic segments of wehrlite, harzburgite, websterite and (olivine) orthopyroxenite include Wingellina Hills, Pirntirri Mulari, The Wart, Gosse Pile and Ewarara (Figure 1c). Intrusions that are predominantly leucogabbronoritic comprise Hinckley Range, Michael Hills, Latitude Hill, Murray Range, Morgan Range, Cavenagh Range and Saturn. The Blackstone, Jameson-Finlayson and Bell Rock ranges also belong to this group, but are now believed to be tectonically segmented portions of an originally single body, named Mantamaru by Maier et al. (2014, 2015). Some of the gabbronoritic intrusions contain pyroxenite layers (e.g. Woodroffe/Mt Caroline; Pirajno & Hoatson, 2012). Several contain troctolitic successions, namely Cavenagh, Morgan, Mantamaru and Kalka. Intrusions that contain both ultramafic and mafic segments are rare, comprising Pirntirri Mulari, Wingellina Hills and Morgan in Western Australia and Kalka and Ngunala in South Australia. In most intrusions, anorthosite is absent or forms thin, centimetric to decametric layers, but Ngunala and Kalka have anorthosites reaching thicknesses of several tens of metres, and one intrusion (Teizi) consists predominantly of anorthosite.
An updated catalogue of New Zealand’s mantle peridotite and serpentinite
Published in New Zealand Journal of Geology and Geophysics, 2020
Analysis of several hundred peridotite xenoliths in the Dunedin Volcanic Group shows them to mainly be spinel facies lherzolitic to harzburgitic with minor amounts of dunite and wehrlite (Figure 2) (Scott et al. 2014a, 2014b; Dalton et al. 2017). The clustering of olivine Mg# between 90 and 91 indicates that the sampled mantle lithosphere is moderately depleted (McCoy-West et al. 2013; Scott et al. 2014a, 2014b; Dalton et al. 2017), but not as depleted as the similar-aged exhumed peridotites in the Alpine Dyke Swarm. Rhenium-depletion Os model ages range from 1.78 Ga to 0.5 (McCoy-West et al. 2013) and these data, coupled with several radiogenic εHf analyses extending up to +175 (Scott et al. 2014b), and radiogenic 143Nd/144Nd (Scott et al. 2014b; McCoy-West et al. 2016; Dalton et al. 2017), indicate that the sampled lithospheric mantle contains some material significantly older than the overlying Mesozoic crust.