Sample SP11 - Granite Porphyry

SP11 (East Spanish Peak). Granite Porphyry. This rock is holocrystalline, porphyritic aphanitic, with fine- to medium-grained subhedral to anhedral phenocrysts in a grey-brown groundmass. The groundmass comprises over 45% of the rock and is composed fine-grained mica, quartz, plagioclase, and potassium feldspar. All phenocrysts are subhedral to euhedral except quartz (.5-3 mm) which is anhedral to subcircular and the opaque minerals (<.5 mm) which are subhedral to euhedral. The following are present as phenocryst phases: quartz is rounded and partially resorbed; plagioclase and potassium feldspar (both 1-4 mm) occur with moderate alteration to fine-grained phyllosilicates; biotite (.5-1 mm); apatite (.5 mm); brown hornblende (.5-1 mm). Anti-rapakivi texture is evidenced by potassium feldspar rimming grains of plagioclase exhibiting polysynthetic twinning. Orthoclase was separated for 40Ar/39Ar dating, but the age spectra produced was discordant.