L101 - granite

L101 hand specimen. Note the coarsely crystalline groundmass and large alkali feldspar.

L101 thin section, plane-polarised light. Note freshness of alkali feldspar.

L01 thin section, cross-polarised light. Note biotite alteration to chlorite.

Specimen Age and Location

Caledonian, Devonian

Shap Fell, Cumbria

Hand Specimen

Coarse-grained, porphyritic texture.
Phenocrysts of pink feldspar, 10-30 mm long, tabular, display clear cleavage planes.
Groundmass of coarse-grained white feldspar, pink feldspar, quartz and biotite.
Quartz grains are anhedral and display their characteristic conchoidal fracture and vitreous lustre.

Thin-section

Alkali feldspar (orthoclase)
- Pink phenocrysts up to ~10 mm long, relatively fresh (unaltered).

Groundmass
- Feldspar (both plagioclase and alkali) grains 1-4mm long, mainly subhedral, though some are quite lath shaped. Partially altered. The plagioclase grains display their characteristic lamellar twinning.
- Biotite grains (0.5-2 mm) with subhedral to anhedral form. Grains have ragged edges and are partially altered to chlorite, mainly along cleavage planes.
- Quartz grains (typically 2-5 mm) contain lots of tiny inclusions and display undulose extinction.

Accessory minerals
- Sphene
- Apatite
- Opaque mineral

Rock History

Porphyritic texture means this rock underwent two stages of cooling; phenocrysts crystallised first followed by the groundmass. The groundmass is coarse-grained, and must have crystallised slowly as part of an intrusion.

Plagioclase and alkali feldspar both occur in this rock, indicating that it formed at high pressures.

Granitic composition (essential quartz and feldspar) indicates that it was produced in a continental setting. Late-stage alteration.

Advanced Notes

This rock is classified as an adamellite because it contains roughly equal proportions of alkali feldspar and plagioclase feldspar, as well as biotite and quartz.

Rock Name

granite
adamellite