Olivine-sanidine trachybasalt from the Sierra Nevada, California^*

Near Huntington Lake in the central Sierra Nevada are three peaks formed of Cenozoic olivine-sanidine trachybasalt. Intrusive masses form Black Point and Red Mountain; Chinese Peak is capped by flows. The Black Point mass, largest of the exposures, is half a mile in diameter. Surrounding rocks are Mesozoic granites.

Most of the trachybasalts consist of granular mosaics of sanidine crystals, which are densely crowded with microlites and minute granules of plagioclase (An∼₅₀), augite, and magnetite, and contain abundant phenocrysts and microphenocrysts of olivine and augite. Sanidine content of holocrystalline specimens ranges from 15 to 30 per cent, olivine ranges from 5 to 25 per cent, and augite from 20 to 30 per cent. The rocks contain many vuggy ellipsoidal masses of augite and sanidine and ellipsoidal aggregates of augite. Many of the larger augite crystals are poikilitic and enclose numerous anhedra of sanidine.

The average chemical composition of 3 analyzed specimens of intrusive trachybasalt and one of the extrusive type is SiO₂ = 54 per cent, A1₂CO₃ = 14, Fe₂O₃ + FeO = 9, MgO = 7, CaO =7, Na₂O = 3, K₂O = 3, and TiO₂ = l.

Magmatic differentiation is inadequate to explain the composition of the rocks. Assimilation of granitic rock is shown by relationships of xenoliths and xenocrysts, and the origin of the trachybasalts seems best explained as the result of assimilation of large quantities of wall rock in a basaltic magma.