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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
Mafic rocks metamorphosed along a Barrovian or Buchan path generally appear similar. Unlike metamorphosed pelites, common mafic metamorphic rocks, excluding very high-pressure blueschists and eclogites, contain no distinctive minerals indicative of higher or lower pressure. Regardless of PT path, metamorphism of mafic rocks begins with the formation of zeolites. Rocks that form within the zeolite or prehnite-pumpellyite facies generally appear as altered basalt or other mafic rocks that contain distinctive zeolite minerals, or other minerals such as prehnite and pumpellyite. Greenschist facies rocks are green and commonly somewhat schistose due to the presence of chlorite, epidote, and actinolite. Amphibolites, which form within the amphibolite facies, are distinctive black (hornblende) and white (plagioclase) rocks, generally showing only minor or no foliation, that are dominated by plagioclase and hornblende. And granulites, which sometimes look like very high-grade amphibolites, are defined by the presence of orthopyroxene or of garnet with quartz.
Metamorphism
Published in Aurèle Parriaux, Geology, 2018
In addition, metamorphic conditions are determined by the assemblage of minerals present in rocks after transformation; these assemblages define different facies. For example, the “greenschist” facies is characterized by the presence of chlorite, albite, epidote, and actinolite.
Metamorphism
Published in Aurèle Parriaux, Geology, 2018
In addition, metamorphic conditions are determined by the assemblage of minerals present in rocks after transformation; these assemblages define different facies. For example, the “greenschist” facies is characterized by the presence of chlorite, albite, epidote, and actinolite.
Interpreting geology from geophysics in poly-deformed and mineralised terranes; the Otago Schist and the Hyde-Macraes Shear Zone
Published in New Zealand Journal of Geology and Geophysics, 2019
Casey C. Blundell, Robin Armit, Laurent Ailleres, Steven Micklethwaite, Adam Martin, Peter Betts
The study area contains a variety of geophysical responses that correspond with subtle differences in lithology and structure. The geophysical expression of the schist has been enhanced by a strong schistose foliation, regional-scale polyphase deformation, and metamorphism that reaches upper greenschist facies (Wood 1963; Means 1966; Craw 1985; Craw and MacKenzie 2016). Tightly folded meta-mafic horizons with magnetic susceptibilities >20–100 × 10−1 SI correspond in aeromagnetic data with anomalous highly magnetic features up to ∼70 nT (Figures 5 and 6). The geophysical data reveals multiple fold generations that were later overprinted by brittle faulting (Figure 7). These observations are consistent with those made in previous field analyses (e.g. Means, 1966; Wood, 1978; Martin et al. 2016), and the use of filtered geophysical data here permits regional correlation between such local structural analyses that would otherwise be enigmatic.
Early Jurassic felsic and associated mafic meta-igneous rocks in Otago Schist, Central Otago, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2018
James K. Mortensen, J. Anthony Coote, David Craw, Douglas J. MacKenzie
Greenschist layers are mineralogically variable internally, on the scale of centimetres to metres, but are dominated by albite, chlorite, actinolite and epidote in varying proportions (Craw 1984; Bierlein and Craw 2009). In this study, we describe an unusual felsic rock type in Central Otago previously unreported in the literature, that is spatially associated with a greenschist layer. This felsic rock is interpreted to have a volcanic protolith (see later discussion); however, its genetic relationship to the adjacent greenschist unit is uncertain. The felsic metavolcanic rock has been metamorphosed along with the greenschist and the adjacent quartzofeldspathic metasedimentary schist. The field relationships and original textures, although obscured by deformation and metamorphism, suggest that the felsic volcanic rock may have had a tuffaceous protolith. If correct, this felsic unit provides an opportunity to further unravel the geological history of the enclosing schist, by providing new information on the compositional range and tectonic affinities of the metavolcanic layers in the Torlesse Terrane, as described herein. Further, we report results of U–Pb dating of zircons separated from the felsic metavolcanic unit, which are interpreted to be of primary magmatic origin. Hence, we report here the first crystallization age for formation of rocks within the schistose Torlesse Terrane of Central Otago, which we interpret to be a depositional age. We then evaluate these results in the context of the geological evolution of the Torlesse Terrane.
Geochemistry and origin of a Mesozoic ophiolite: the Pounamu Ultramafics, Westland, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2018
Alan F Cooper, Richard C Price, Anthony Reay
Most of the serpentinite lenses in the Whitcombe area are symmetrically flanked by zones of felted greenschist up to 70 m wide. The greenschist is composed of the assemblage chlorite–actinolite–epidote, with only accessory amounts of quartz and plagioclase. The contacts between meta-serpentinite and felted greenschists are marked by a 4–5 m thick sequence of essentially mono- or bi-mineralic zones characterised by antigorite–magnesite, talc–magnesite, talc, tremolite and chlorite. These ‘blackwall’ zones have been interpreted as the result of metasomatic diffusion between the contrasted serpentinite and felted greenschist compositions during garnet zone metamorphism (Cooper 1976; Koons 1981; Cooper and Reay 1983; Ireland et al. 1984).