Incipient metamorphism in the Lower Palaeozoic marginal basin of Wales

Abstract

A diagcnctic through anchizone to epizone transition is demonstrated in pelitic rocks of thc Lower Palaeozoic marginal basin of Walcs by cxamination of variations in phyllosilicate mineralogy. illite crystallinity and h,, parameter of white micas. This transition represents a temperature range from -150°C to -400°C and the metamorphism is of a lowpressure facies series type. with a geothermal gradient of -4O"Ckm-'. Variations in grade can be correlated largely with the original basin and shelf form, suggesting a depth-related metamorphism. However, in areas closer to the site of Caledonian plate collision an increasingly syn-tectonic metamorphic event is apparent.Correlation of pelite data with metabasite assemblages is variable. the most consistent relationship being between epizone crystallinity values and epidote-actinolite (greenschist facies) assemblages. Diagenetic clay mineral assemblages are found associated with prehnitepumpellyite assemblages in metabasites and it is suggested that the latter represent nonbuffered, and therefore non-diagnostic, assemblages.

Prehnite-pumpellyite facies metamorphism in the Cenozoic Abanico Formation, Andes of central Chile (33°50'S): chemical and scale controls on mineral assemblages, reaction progress and the equilibrium state

“…5). These values are similar to those reported for pumpellyite-chlorite mineral pairs in rocks under prehnite-pumpellyite facies metamorphism in other low grade metamorphic terrains (e.g., Robinson and Bevins, 1986;Robinson, 1993, 2003). Although it has been established in other areas that the Abanico Formation reaches zeolite facies in its upper parts (Levi, 1970;Robinson et al, 2004), this would not be the case at the localities studied here considering the ubiquitous presence of epidote and the absence of zeolites in metamorphic mineral assemblages throughout the entire exposed series.…”

Occurrence of chloritoid-bearing metapelitic rocks and their significance in the metamorphism of the Silgará Formation at the Central Santander Massif

“…The PT conditions of chloritoid-bearing metapelites range from the sub-greenschist facies to the middle part of the amphibolite facies in the staurolite-kyanite zone. The occurrence of chloritoid in the lowest grade rocks are reported by Primmer (1985) in Upper Devonian epizone slates, and Robinson and Bevins (1986) close to the upper anchizone-epizone boundary. On the other hand, Spear (1993) reports the occurrence of the paragenesis chloritoid + pyrophyllite.…”

Prograde Transitions of Corrensite and Chlorite in Low-Grade Pelitic Rocks from the Gaspé Peninsula, Quebec

“…Such a sequence has been only rarely described in pelitic rocks (Frey 1978;Weaver et al 1984;Robinson and Bevins 1986), for which chlorite commonly has been inferred to form as a byproduct of the smectite-to-illite reaction or from alteration of kaolinite or berthierine (Hower et al 1976;Boles and Franks 1979;Ahn and Peacor 1985;Walker and Thompson 1990). Dioctahedral phyllosilicates such as illite or muscovite are usually present in amounts much greater than those of trioctahedral minerals in shales, commonly reflecting the presence of either dioctahedral smectite as an original detrital phase as in Gulf Coast sediments, or volcanic ash that was subsequently altered to form dioctahedral smectite.…”

“…The Gasp6 rocks thus represent a special type of pelitic sequence involving deposition of abundant andesitic volcanics derived from an island arc system. Pelites deposited in other backarc basins, such as the Lower Paleozoic rocks of Wales (Robinson and Bevins 1986) and Oslo Region (Bjrrlykke 1974) similarly contain abundant C/S and/or chlorite. Such rocks may be precursors to chlorite-rich rocks free of C/S by regional or contact metamorphism as is the case in the Gasp6 rocks.…”