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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
A facies series is a sequence of different facies that a rock experiences while being metamorphosed from low grade to higher grade; Figure 10.29 shows several examples. A Barrovian facies series follows a normal geothermal gradient created during crustal thickening related to mountain building. Typical metamorphism progresses from the zeolite facies to the greenschist, amphibolite, and granulite facies. Other rocks may follow a Buchan facies series (low pressure). Buchan metamorphism occurs in places where Earth’s crust is rifting, allowing magmas to move upward while carrying a great deal of heat, or in other places where high upward heat flow causes metamorphism at elevated temperature without a great increase in pressure. Barrovian and Buchan are the most common facies series, but as shown in Figure 10.29, some rocks may follow a high pressure facies series or a contact metamorphism facies series. In either case, the minerals that form will be different from those formed by Barrovian or Buchan metamorphism.
Correlation between the Warepan/Otapirian and the Norian/Rhaetian stage boundary: implications of a global negative δ13Corg perturbation
Published in New Zealand Journal of Geology and Geophysics, 2022
The Arawi Shellbeds and Ngutunui Formation are a succession of volcaniclastic sedimentary rocks dominated by thin sandstones, siltstones and shales, with minor but conspicuous conglomerates, tuffs and shellbeds (Figure 3). Limestones are not present, although the shellbeds approach coquina limestone composition in places within the Arawi Shellbeds (Grant-Mackie 1985), and there are no radiolarian cherts. Compositionally, the volcanic lithologies are broadly andesitic but range from basaltic andesite to dacite. The tuffs vary in vitric, crystal and lithic composition. In every respect, the sedimentary rocks in the Kiritehere section are typical of Murihiku Supergroup. They have been weakly metamorphosed to zeolite facies grade with conspicuous zeolite veining (laumontite, stilbite) and zeolite ‘cements’ (laumontite, heulandite, analcime). There are minor faults in places and also some ‘slumps’ (Grant-Mackie and Lowry 1964) but in general the stratigraphy is more or less ‘layer cake’, easy to recognise in the field, and the named formations and groups can be traced for many tens of kilometres. However, the sedimentary sequence as a whole has been folded, and a number of anticlines and synclines are recognised and named within a broader Kawhia Regional Syncline (e.g. Kear 1960; Edbrooke 2005). In the Kiritehere section, the sequence is dipping and younging to the east, and is part of a large homoclinal structure that may be interpreted as the west limb of a syncline (e.g. Kear 1960; Kear and Schofield 1964; Edbrooke 2005).