The metamorphic history of the concealed Caledonides of eastern England and their foreland

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Abstract
White mica (illite) crystallinity data, derived mostly from borehole samples, have been used to generate a contoured metamorphic map of the concealed Caledonide fold belt of eastern England and the foreland formed by the Midlands Microcraton. The northern subcrop of the fold belt is characterized by epizonal phyllites and quartzites of possible Cambrian age, whereas anchizonal grades characterize Silurian to Lower Devonian strata of the Anglian Basin in the southern subcrop of the fold belt. Regional metamorphism in the Anglian Basin resulted from deep burial and Acadian deformation beneath a possible overburden of 7 km, assuming a metamorphic field gradient of 36 °C km-1. Late Proterozoic volcaniclastic rocks forming the basement of the microcraton show anchizonal to epizonal grades that probably developed during late Avalonian metamorphism. Cambrian to Tremadoc strata, showing late diagenetic alteration, rest on the basement with varying degrees of metamorphic discordance. During early Palaeozoic times, much of the microcraton was a region of slow subsidence with overburden thicknesses of 3.3–5.5 km. However, concealed Tremadoc strata in the northeast of the microcraton reach anchizonal grades and may have been buried to depths of 7 km beneath an overburden of uncertain age.