THE METAMORPHIC HISTORY OF THE CONCEALED CALEDONIDES OF EASTERN ENGLAND AND THEIR FORELAND

White mica (illite) crystallinity data, derived mostly from borehole s amples, have been used to generate a contoured metamorphic map of the concealed Caledonide fold belt of eastern England and the foreland for med by the Midlands Microcraton. The northern subcrop of the fold belt is characterized by epizonal phyllites and quartzites of possible Cam brian age, whereas anchizonal grades characterize Silurian to Lower De vonian strata of the Anglian Basin in the southern subcrop-of the fold belt. Regional metamorphism in the Anglian Basin resulted from deep b urial and Acadian deformation beneath a possible overburden of 7 km, a ssuming a metamorphic field gradient of 36-degrees-C km-1. Late Proter ozoic volcaniclastic rocks forming the basement of the microcraton sho w anchizonal to epizonal grades that probably developed during late Av alonian metamorphism. Cambrian to Tremadoc strata, showing late diagen etic alteration, rest on the basement with varying degrees of metamorp hic discordance. During early Palaeozoic times, much of the microcrato n was a region of slow subsidence with overburden thicknesses of 3.3-5 .5 km. However, concealed Tremadoc strata in the northeast of the micr ocraton reach anchizonal grades and may have been buried to depths of 7 km beneath an overburden of uncertain age.