Sample:
Fine-grained ‘High Cheviot’ porphyritic, granophyric alkali feldspar granite (3A.1a)
Location: Woolhope Crag NT922221
Prepared sample in reflected light
Thin section in plane polarised light (PPL)
Thin section with crossed polarising filters (XP)
Observations
Colour
Red/grey at surface, red-brown in section with distinct cream coloured plagioclase and black biotite phenocrysts
Alteration
Heavily altered feldspars showing brown colouration and sericite.
Chlorite and tourmaline present. There is a particularly striking area of chlorite with biotite showing radial flame texture in XP. This is adjacent to a large quartz crystal along with tourmaline and pyroxene.
Texture
Fine-grained (average length of matrix crystals 0,3mm,) porphyritic (av. phenocryst length 1.8mm, max. length 2.3mm) .
Most crystal boundaries are consertal with micrographic intergrowth between alkali feldspar and quartz. Continuous zoning and also discontinuous sector zoning appears in some plagioclase crystals.
Mineralogy
Mafic content: 7%
Pyroxenes
Very little pyroxene, <1%
Biotite
Mostly biotite appearing as ragged flakes up to 1mm in length with iron ore at rims and in cleavages and with no inclusions.
Alteration
Tourmaline crystals prismatic up to 0.6mm in length
Opaques
Felsic content: 93%
Plagioclase
Very little,1% of feldspars, in the form of Oligoclase (11% Anorthite). Mostly poorly formed but some euhedral phenocrysts up to 1.6mm in length.
Orthoclase
Mostly in anhedral crystals up to 0.7mm in length. Some crystals show simple twinning, some are heavily clouded.
Granophyric texture appears associated with altered orthoclase crystals.
Quartz
Up to 0.7mm crystals, subordinate to feldspars.
Accessory minerals:
One apatite inclusion in biotite.
Interpretation
The micrographic texture may be due to sudden chilling or as a result of quartz replacement of feldspar.
The area containing chlorite, biotite, pyroxene and tourmaline may represent pyroxene and/or biotite that has been subjected to hydrothermal alteration.