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Thin Section from SP101
Thin-section under crossed-nicols (5 mm in diameter).
Monzonite Porphyry. This rock is holocrystalline, porphyritic-aphanitic, with fine- to medium-grained subhedral phenocrysts in a grey-brown groundmass. The groundmass comprises 30% of the rock and is composed of plagioclase, opaques and fine- grained mica. All crystals are subhedral except the opaque minerals (<1 mm) which are subhedral to anhedral. Fluorite (<.1 mm), carbonate (<.1 mm), and epidote (<.1 mm) are present as anhedral masses. The following are present as phenocryst phases: plagioclase and potassium feldspar (1-5 mm) occur with the former moderately and the latter only slightly altered to fine-grained phyllosilicates. Potassium feldspar exhibits oscillatory zonation; titanaugite (1 mm) exhibits hourglass zonation; hornblende (.1 mm); analcite (.5 - 1 mm); apatite (<1 mm).