Distribution and significance of crystalline, perlitic and vesicular textures in the Ordovician Garth Tuff (Wales)
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- McArthur, A., Cas, R. & Orton, G. Bull Volcanol (1998) 60: 260. doi:10.1007/s004450050232
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Diverse spherulitic and granular crystalline fabrics, perlitic textures and fabrics related to the growth and migration of vesicles occur in the Garth Tuff, a largely welded Ordovician ignimbrite. Defining the distribution of such textures helps to constrain the ignimbrite's cooling and degassing history. Suites of spherulitic and perlitic textures closely reflect variation in cooling rates. Seven facies are defined based on the style and intensity of crystallisation: (1) a medium to coarsely crystalline, equigranular facies; (2) an intensely spherulitic facies; (3) a sparsely spherulitic facies; (4) a pectinate facies; (5) a microcrystalline to cryptocrystalline, equigranular facies; (6) a lithophysal facies; and (7) a transitional perlite–pectinate facies. Textural changes from facies 1 to 5 reflect progressively higher cooling rates. Facies 1 occurs in proximal settings in the ignimbrite's core. Facies 2 to 5 successively envelop facies 1, with facies 2 becoming the dominant fabric in the ignimbrite's core in medial settings. Facies 5 is typically developed in the originally glassy perlitic zones at the ignimbrite's welded margins. Crystallisation under hydrous conditions is reflected by second-boiling textures in the sporadically developed lithophysal facies. The seventh facies reflects a subtle interplay between cooling, hydration and crystallisation which locally prevented perlitic fracturing. The distribution of amygdales reflects patterns of volatile migration and entrapment. In the lower levels of the ignimbrite, amygdales occur in irregular concentrations or rare subvertical pipe-like structures. Pipe-like structures attest to fumarolic activity while the ignimbrite was in a rheomorphic state. Amygdales are widespread and evenly distributed in the upper levels of the ignimbrite. However, the top of the welding profile is characterised by a thin, poorly vesiculated, originally vitrophyric horizon that abruptly caps an intense concentration of amygdales. Ductile and brittle fabrics developed during the upward migration of gas. Microscopic drag folds occur around some amygdales. Jigsaw-fit to clast-rotated breccias originated through both late-stage pneumatic fracturing and autobrecciation. Vaporisation of water at the flow base provided a significant source of volatiles in addition to gas released during cooling and crystallisation. Secondary alteration has enhanced or modified some fabrics. Perlitic zones were susceptible to patchy chlorite–sericite–carbonate diagenetic alteration. Diagenesis and metamorphism have contributed to the infilling of vesicles.