Eight hornblende andesite breccia dikes intrude Tertiary andesite mudflows and lava of the Sierra Nevada near Blairsden, California. Similar dikes occur elsewhere in the vicinity and farther south. The dikes consist of blocks of andesite in a fine matrix rich in a clay of the montmorillonite group. The dikes are essentially the same age as their wall rocks, have the same primary mineralogy and textures, and therefore represent the same magma. They differ from the wall rocks in the kind and extent of alteration.

It is concluded that the dikes represent andesite magma injected into cold wet wall rocks and that the brecciation was due to expulsion of the volatiles on chilling and crystallization. Brecciation occurred before completion of intrusion, and breccia was erupted to the surface. The alteration occurred during and immediately after brecciation, under nonoxidizing conditions, and was due to the attack of the escaping volatiles. This is thought to have taken place at low temperature followed immediately by condensation of the vapors producing a mud matrix from the comminuted andesite and clay derived from it.

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