Low grade metamorphism of the Manx Group, Isle of Man: a comparative study of white mica ‘crystallinity’ techniques

Abstract

Studies have been made in two laboratories on metapelites from the Manx Group, Isle of Man, showing the distribution of white mica ‘crystallinity’, using the Kubler index (Δ°2Θ) and the Weber index (Hb rel ). Two isocryst maps, which show broadly similar distributions of metamorphic grade, are illustrated and a correlation between the laboratories is presented. The mineralogy of the <2 m fraction in relation to ‘crystallinity’ has been determined. The distribution of metamorphic grade over central and eastern parts of the Isle of Man, as depicted by the Kubler index isocryst map, is related to subjacent granite plutons, but these contact metamorphic effects overprint an earlier, largely anchizonal, regional metamorphism. Localized, late, retrograde effects are related to low temperature hydrothermal mineralization and to cold re-working in a large fault zone. Differences between the maps are largely in the degrees of resolution which stem directly from the different sampling intervals employed. Comparisons between the maps suggest a sampling interval of 1-2 km is adequate in regions of simpler geology; but that an interval of <1 km may be necessary in more complex regions. Well resolved maps can be produced only when the sampling interval is linked to geology. The changes in the mineralogy of the <2 m fraction in relation to grade are closely similar to those described from Lower Palaeozoic metapelites from North Wales, although higher grades have been reached in the Manx Group with the development of biotite.

Kübler Illite “Crystallinity” Index of the Cretaceous Gyeongsang Basin, Korea: Implications for Basin Evolution

“…Strain energy can also be important in the aggradation of illite crystallites (Roberts and Merriman, 1985;Roberts et al, 1990Roberts et al, , 1996Merriman et al, 1990). In the Gyeongsang Basin, however, no prominent fold or thrust is found within the basin.…”

“…KI is especially useful in reconstructing structural and metamorphic history in very low-to low-grade pelitic rocks (e.g., Frey et al, 1980;Roberts and Merriman, 1985;Frey, 1988;Roberts et al, 1996;Warr et al, 1996;Lee and Ko, 1997). Also, the spatial variation in sediment thickness (e.g., Awan and Woodcock, 1991) and the existence of concealed plutons (e.g., Roberts et al, 1990) can be examined on the basis of KI variation. In this study, KI was used to reconstruct an early history of sedimentary basin development by detecting a major fault related to the initiation of basin formation.…”

In sight of the suture: the early Palaeozoic geological history of the Isle of Man

“…Roberts et al (1990) suggested that the high metamorphic grade along the central northeast-southwest spine of the island records the enhanced thermal gradient due to granite emplacement. However, Power & Barnes show that the two main granitic bodies have different time relations to the main deformation, with the Foxdale intrusion postdating D2, and the Dhoon intrusion pre-dating D2 and possibly overlapping D1.…”

“…Roberts et al 1990;Rushton 1993;Ford 1993). Invaluable though this would be, the difficulty and cost of such an exercise should not be underestimated.…”

“…The polyphase deformation history established by Simpson (1963) for the Manx Group is assumed to be predominantly late Caledonian in age, and generally due to the Silurian-Early Devonian impingement of Avalonia with the Laurentian continent (Soper et al 1987(Soper et al , 1992. Recent interest in the Manx Group was rekindled through biostratigraphic work by Molyneux (1979), metamorphic studies by Roberts et al (1990) and a field guidebook by Ford (1993). New research (e.g.…”