Two dark phenocrysts in a dyke: augite or hornblende?
A 12 m thick, east-west trending basaltic andesite dyke intrudes gneiss. Phenocrysts from the core reach 4 mm; those from the chilled margin are 0.3 mm. In thin section the dark phenocrysts are euhedral, high relief, pale green-brown with weak pleochroism. Extinction is inclined at 40-44 degrees, 2V is about 55 degrees, and hand specimens show two cleavages near 87 degrees. Specific gravity is 3.3. Plagioclase phenocrysts are An 60 and unaltered; interstitial glass is fresh. A nearby andesite lava carries the same dark phenocrysts but they are 1 mm and partly chloritized, and its plagioclase is An 45. No orthopyroxene is present. The dyke margin is glassy and contains 5 percent vesicles, while the core is holocrystalline. A student labels the dark mineral as hornblende because it is dark and has cleavages.
1. Identify the dark phenocryst and justify the identification with at least four properties. Explain why hornblende is excluded.
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Identification: augite, a clinopyroxene.
- Extinction: inclined at 40-44 degrees. Hornblende typically shows extinction of about 10-30 degrees; augite is commonly 35-48 degrees.
- 2V: about 55 degrees. Augite is usually 50-60 degrees; hornblende is commonly 60-90 degrees and commonly negative.
- Pleochroism: weak, pale green-brown. Hornblende is usually strongly pleochroic from pale yellow-green to brown-green.
- Cleavage: two cleavages near 87 degrees. Hornblende has cleavages near 56 and 124 degrees.
- Relief and specific gravity: high relief and 3.3 are consistent with augite. Hornblende has lower relief and specific gravity near 3.1-3.3.
Why not hornblende? The weak pleochroism, the 87 degree cleavage and the low 2V are the decisive exclusions. The student is relying on colour and cleavage alone, which is exactly the trap. The association with An 60 plagioclase and fresh glass is also typical of a basalt-andesite suite, not a hornblende-bearing magma.
Caveat: 2V alone is not enough; the combination of extinction, cleavage and pleochroism is what makes the identification robust.
2. The same mineral is 0.3 mm at the margin and 4 mm in the core. Interpret the cooling history and decide whether the dyke is one intrusion or several pulses.
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This is a single cooling unit, not several unrelated intrusions.
- The margin is glassy, vesicular and fine-grained, with 0.3 mm phenocrysts. That is a chilled margin produced by rapid cooling against cold gneiss. The core is holocrystalline with 4 mm phenocrysts, indicating slower cooling and more time for crystal growth.
- A single intrusion can produce this textural gradient by conductive cooling from the walls inward. The absence of an internal chilled margin, chilled selvage or back-veining within the dyke argues against multiple pulses.
- The 5 percent vesicles at the margin indicate volatile exsolution at shallow depth. If the dyke were deeply buried, the vesicles would be rarer; this supports emplacement at shallow to moderate crustal levels.
Test: if there were multiple pulses, the core should contain a fine-grained internal contact or a screen of altered material. The student should log the dyke perpendicular to its length and check for such contacts before concluding.
Interpretation: the core is coarser because it cooled more slowly, not because it came from a different magma batch.
3. Could this dyke be the feeder to the nearby andesite lava? Give a reasoned judgement, not just yes or no.
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It is possible, but the present evidence is not diagnostic.
- Supporting evidence: the dyke and lava carry the same dark phenocryst, which suggests a common magmatic lineage. The dyke is shallow, vesicular at the margin and intrudes gneiss, so it could have reached the surface.
- Contradicting evidence: the lava phenocrysts are only 1 mm and partly chloritized, and its plagioclase is An 45, whereas the dyke has An 60 and fresh glass. A feeder should generally be more primitive than the lava only if fractionation occurred between the dyke and the surface; but An 60 to An 45 is a large jump for a single feeder.
- The chloritization in the lava could be deuteric or burial alteration, not proof of a different batch. The smaller phenocryst size could reflect degassing and rapid growth, or a separate magma pulse.
Judgement: the dyke is a plausible but unproven feeder. To confirm it, compare whole-rock major and trace elements, especially MgO, Cr and Ni, and check whether the dyke and lava share the same groundmass glass composition. A contact relation showing the dyke feeding the lava flow would be decisive.
Verdict: treat it as a possible feeder and test it; do not map it as a proven feeder on the basis of mineral appearance alone.