L110 - porphyritic rhyolite

L110 hand specimen. Note the bimodal crystal size between groundmass and phenocrysts, and the difference in habit of phenocryst quartz (rounded) and alkali feldspar (more angular).

L110 thin section, plane polarised light. Both a rounded quartz phenocryst and the finely crystalline groundmass are observed.

L110 thin section, cross-polarised light.

Ordovician
Armboth Dyke
Thirlmere, Cumbria

Hand Specimen

Pink-brown, porphyritic rock with a very fine-grained groundmass.
Abundant pink macrocrysts (5-10 mm).
Phenocrysts of feldspar (2-4 mm) and quartz (2-4 mm).
Accessory black mineral.

Thin-section

Quartz
- Rounded phenocrysts (1-4 mm) with embayments/cavities and tiny inclusions.

Feldspar
- Phenocrysts ~2mm, subhedral. Mainly alkali feldspar, some plagioclase. Alteration to white mica.

Garnet
- Very rare, euhedral phenocrysts (0.5-1 mm). Isotropic, high relief, colourless to pale pink.

Biotite
- Very rare fragments with chlorite alteration.

Groundmass
- Microcrystalline quartz and feldspar, with some muscovite and biotite.

Calcite
- Some of the feldspars phenocrysts and most of the garnet phenocrysts have been replaced by calcite. These are the pink macrocrysts visible in hand specimen.

Rock History

Porphyritic texture indicates two-stage cooling. The phenocrysts grew slowly in a magma chamber; the fine-grained groundmass formed during rapid cooling at the Earth’s surface.

Late stage hydrothermal alteration to white mica and carbonate.

Rock Name

porphyritic rhyolite
quartz porphyry