Sample SP119 - Black Hills

Hornblendite Xenolith

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40Ar/39Ar age spectra for SP119

Thin-section description

(mafic enclave). Pyroxene Hornblendite. This is a holocrystalline, phaneritic, fine- to medium-grained, hypidiomorphic, granular rock. All crystals are subhedral except the opaque minerals (<1 mm) which are subhedral to anhedral. Around the borders of some of the opaques is an iron-oxide phase. Fluorite (<.1 mm) is present as anhedral masses after hornblende. The following are present as phenocryst phases: green hornblende (2-5 mm) is present with some crystals exhibiting well-developed 120 boundaries; sector zoned titanaugite (2-4 mm); olivine (<1 mm); plagioclase (<1 mm); apatite (<.1 mm) is present as inclusions in hornblende.