RELATIONSHIP BETWEEN METAMORPHISM AND STRUCTURE IN THE SKIDDAW GROUP,ENGLISH LAKE DISTRICT

Regional variation of white mica (illite) crystallinity in the Skiddaw Group is set against the structural interpretation of Hughes, Cooper & Stone (1993, this issue) in which early- or pre-Ludlow deformation a nd slaty cleavage development (S1) were succeeded by southward thrusti ng and an associated development of S2 and S3 crenulation cleavages, p ossibly during early Devonian times. Kubler index (KI) values are plot ted in relation to geological structure for a major part of the Skidda w Group, and cross-sections constructed. The pattern is interpreted in terms of three processes: (1) diagenetic to low anchizonal burial met amorphism under a relatively high geothermal gradient during the late- arc stage (early to mid-Silurian) which preceded the orogenic phase an d formation of S1; (2) upper anchizonal to epizonal metamorphism due t o tectonic thickening and slaty cleavage development during the early to pre-Ludlow orogenic phase following closure of Iapetus; (3) late-te ctonic uplift of already metamorphosed rocks by southward-directed mov ement on the Loweswater, Gasgale Gill and Causey Pike Thrusts possibly during early Devonian time. An analogous interpretation is made for t he Skiddaw area, though with the addition of major post-S1 contact met amorphism.