L162 - porphyritic basalt

Hand Specimen

Black, porphyritic rock comprising black clinopyroxene, grey feldspar and greenish olivine phenocrysts, in a fine-grained groundmass.

Plagioclase feldspar 
- Phenocrysts 1-4mm long, with a euhedral, tabular form. Randomly orientated. Characteristic lamellar twinning. Some are zoned. Inclusions of magnetite, clinopyroxene and other minerals. Extensive alteration along abundant cracks.

Clinopyroxene (augite) 
- Euhedral phenocrysts, 1-2 mm across. Moderate relief, inclined extinction and first order pink to second order orange interference colours. Pale pink in thin-section. Some are zoned.

Olivine 
- Very few phenocrysts are fresh enough to be easily identifiable. These are colourless in plane polarised light, have moderate relief, and up to second order pink interference colours. No cleavage.

- 1st phase of alteration along cracks: The majority of the phenocrysts have been completely altered to greenish serpentine.

- 2nd phase of alteration: orange brown mineral – probably the amphibole cummingtonite. Characteristic curving cracks and the prismatic form of olivine are still visible.

Groundmass 
- Fine-grained. Plagioclase feldspar needles, clinopyroxene, some olivine (mostly altered), and opaques, probably magnetite. Occasional calcite grain.

Two stage cooling 
1. Phenocrysts grow slowly in a magma chamber. 
2. Fine-grained groundmass grew rapidly → extrusive. 
Hydrothermal alteration, possibly as part of the volcanic pile.

Cummingtonite is an amphibole. It is colourless to pale green in thin-section, has moderate relief and two cleavages which intersect at 56°.

porphyritic basalt