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Manx Group Sedimentation |
1 Geology Department, University College, Belfield, Dublin 4, Ireland
2 Geological Survey of Ireland, Beggars Bush, Haddington Road, Dublin 4, Ireland
Spessartine-garnet quartzites (coticule) are now widely recognized as extremely useful marker horizons throughout the Appalachian-Caledonian Orogen. In many instances, they appear to be of early Ordovician age. They reflect a syn-sedimentary, volcanic exhalative origin and in many areas, e.g. the Leinster region in southeast Ireland, they are intimately associated with, inter alia, base metal mineralization and with tourmalinite. However, though the metamorphic coticule is common, the pre-metamorphic coticule precursor has proved difficult to recognize.
Manganiferous ironstone rocks of coticule aspect are a distinctive feature in parts of the Manx Group exposed along the west coast of the Isle of Man. Though they lack any trace of the characteristic manganese garnet, perhaps reflecting a relatively low metamorphic grade, they do bear a strong morphological resemblance in outcrop to the typical coticule lithology of the Ribband Group in southeast Ireland. In both places, tourmaline-rich rocks occur nearby. On the Isle of Man, a major tourmalinite occurrence is linked to an important shear zone.
In briefly reviewing the field occurrence, petrography and chemistry of the manganiferous ironstone beds, and by comparison with coticule elsewhere, the possibility that these ironstones are a coticule precursor or a coticule facies variant is introduced. That a lithology now termed coticule might be a significant component in the Manx Group (‘Slates’) was noted no less than 70 years ago. Any recognition of coticule rock would have an important bearing on the understanding of the stratigraphy, metamorphism and mineralization of the Manx Group.
