Low grade metamorphism in the Lower Ordovician Skiddaw Group of the Lake District, England

SUMMARY A white-mica crystallinity (illite crystallinity) survey of the Lower Ordovician Skiddaw Group of the Lake District has revealed a complex pattern in which Kubler Index values range from <0.21 Δ°2θ (low epizone) to >0.42 Δ°2θ (diagenetic), large areas of the outcrop having anchizonal values (0.21–0.42 Δ°2θ). The <2 μ m mineralogy of the higher grade rocks is dominated by 2M 1 muscovite and chlorite, and interlayered paragonite-muscovite is common; lower grade rocks contain 1Md illite, chlorite and interlayered illite-smectite, in some cases accompanied by pyrophyllite and, rarely, kaolinite. The pattern of variation can be related to the metamorphic history of the area in which anchizonal burial metamorphism during the Silurian was superseded by Lower Devonian (Acadian) upper anchizonal to epizonal regional metamorphism in areas in which slaty cleavage is relatively well developed. The contrast between these latter areas and those retaining the burial signature was heightened by late Devonian uplift and faulting. Sub-volcanic metamorphism during Caradocian volcanism is also considered likely to have attained anchizonal grades. Numerous additional variations within the pattern are related to interference with the 10Å muscovite diffraction peaks by interlayered paragonite and smectite, induration by minor intrusions and movement on Caledonian and post-Carboniferous faults.