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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 in the New England Orogen, eastern Australia: a review
Published in Australian Journal of Earth Sciences, 2020
K. Jessop, N. R. Daczko, S. Piazolo
East–west-trending zones of low-grade metamorphism have been mapped in the Nambucca Block, part of the ‘Barnard’ Basin of Leitch (1988). Four zones are outlined (Figure 6), ranging from prehnite–pumpellyite (sub-greenschist) to the sub-biotite part of the greenschist facies. Highest metamorphic grades occur in the central-eastern part of the block where P–T conditions of 4–7 kbar and 360–370 °C were estimated (Leitch, 1975). However, although these zones hold in general, degrees of deformation and associated metamorphic effects vary locally (Liu, Offler, & Brime, 1993; Offler & Brime, 1994). Regionally, burial metamorphism reached prehnite–pumpellyite facies (P: 1–3 kbar, T: ∼250–300 °C) with a geothermal gradient of ∼45 °C/km, indicative of the thermal perturbation of the rift phase (Liu et al., 1993). However, unlike other Permian basins in the NEO, a higher-grade overprint is associated with later multi-deformational events in the Nambucca Block. The McGraths Hump Basalt on the northern boundary of the block (Figure 6, zone 2) experienced deformation and metamorphism to greenschist facies; the Petroi Metabasalt (Figure 6, zone 3) shows evidence of hydrothermal high-greenschist to amphibolite-facies metamorphism (P: ∼1.5–2.5 kbar, T: 450–500 °C, geothermal gradient ∼80 °C/km); while metamorphism of a metadolerite (Figure 6, zone 4) is consistent with the lower greenschist facies of zone 4 (Liu et al., 1993).
Reconnaissance composition of river sand from northern South Island, New Zealand: a modern analogue for southern Taranaki Basin
Published in New Zealand Journal of Geology and Geophysics, 2020
Linda M. Doran, Kathleen M. Marsaglia, Greg H. Browne
Other differences are related to the percentages of quartz and types of lithic fragments. Of the two Caples Terrane samples, Sample NZ-11-45 exhibits higher proportions of monocrystalline quartz, quartz-biotite tectonite fragments, pumpellyite-bearing metasiltstones in the finer sand fractions, and quartz-chlorite tectonite fragments, along with a decrease in the proportion of pumpellyite-bearing metamudstone fragments. This is consistent with the observations of Turnbull (1980) who identified pumpellyite-actinolite and prehnite-pumpellyite metamorphic facies in the Caples terrane in northern Southland, New Zealand. Sample NZ-13-25 provides the clearest example of fragments of low-grade, prehnite-pumpellyite-facies metamorphic rocks. It has the most pumpellyite, the largest number of unmetamorphosed argillite and siltstone fragments, the lowest number of quartz-mica tectonite fragments, and almost no feldspar except for highly altered plagioclase grains in the very fine-sand fraction (Supplementary Tables 3 and 4, https://doi.org/10.1080/00288306.2019.1618881). Many individual clasts in metasiltstone fragments appear to have been replaced by pumpellyite and sericite. On occasion, the colourful birefringence of epidote is visible through the pumpellyite. Relict microlitic textures define some metavolcanic clasts.
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
Jongens et al. (2009) describe a number of mafic dikes and small stocks of microdiorite in the vicinity of Mt. Pember, c. 70 km northwest of Christchurch (Figure 1A). These bodies cut low-grade sandstones of the Rakaia Terrane, which have locally given Late Triassic fossil ages. The intrusive rocks are generally massive and unfoliated. However, petrographic studies indicate that they have experienced prehnite–pumpellyite facies metamorphism along with the enclosing clastic rocks. Geochemical analyses of two samples of microdiorite (included in Figure 4) indicate intermediate compositions and within-plate or back-arc basin signatures (Jongens et al. 2009), with some influence from subduction-related processes (possibly through crustal contamination of the magmas). An 40Ar–39Ar crystallization age of 185.6 ± 3.3 Ma was obtained for hornblende from one of the diorite bodies (Jongens et al. 2009). This age is only slightly younger than the U–Pb zircon ages that we determined for the North Rough Ridge felsic metavolcanic rocks. Although the intrusions in the Mt. Pember area have not experienced the same high degree of strain as the felsic metavolcanic rocks, they show the same relative age with respect to regional deformation and metamorphism, and we speculate that the diorites (and associated mafic dikes) at Mt. Pember, the felsic metavolcanic rocks and at least some of the greenschists may be broadly comagmatic and result from the same general tectonic processes.