3.4 Mylor Slate Formation (Northern Cornwall)

Both Gurnard's Head samples (GUR001 and 2) share textural similarities with GpI-7 and GpIa-3; however, the exposure sections contain plentiful acicular plagioclase feldspar to 0.5mm, partly altered to granular amphibole, unlike the two axe sub-groups. Both samples also contain heavily altered pyroxenes, replaced by bright (xpl) fibrous amphibole. Plentiful, fine-grained, biotite is present in both slides. Although there are some petrographic similarities between Gurnard's Head thin-sections and the two sub-groups indicated, the match is less than conclusive and would need additional thin-sections to support the relationship.

Three thin-sections (ZEN001, 1A, 1B) from a single sample from the Zennor Point exposure within the Mylor Slate Formation are all laminated, very fine grained with mica (biotite and muscovite), quartz and probably albite in the groundmass. This, and the total lack of opaques, makes them unlike any axe thin-section examined in this work.

A further three thin-sections (ZEN002, 2A, 2B), also from a single Zennor Point greenstone, all show evidence that these heavily altered mafic rocks have been extensively epidotised. Fine, fan-shaped agglomerates of acicular tremolite-actinolite are present, along with finely disseminated opaque grains and heavily altered laths of plagioclase feldspar to 1mm long. The presence of biotite in one thin-section (ZEN002) indicates that this rock was metamorphosed on the edge of the Land's End granite contact aureole. The overall texture has not been seen in any axe thin-section, the acicular to lath-shaped feldspars being distinctive.

Hornfelsic Kenidjack sample (KEN001) has abundant granular biotite, very fine acicular to bladed amphibole and possible poikiloblasts of staurolite and larger andalusite. Mats of fibrous, first/low second order birefringent amphibole are intermixed with low-relief patches of albite/quartz. A further two samples (KEN002 and 003) are similar, but with larger amphibolite grains. All three thin-sections show schistosity and higher temperature metamorphic minerals, and are unlike any axe thin-section, except for KEN001, which shares similarities with GpIII-3, but with plentiful granular biotite not seen in the axe thin-section.


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Last updated: Wed Jul 29 2009