Abstract
Patchy zoning in plagioclase has been observed in gabbroic rocks in southwestern Oregon. The plagioclase consists of a partially resorbed calcic core, and sodic rims and patches surrounding and included in the core. The mechanism employed to explain this type of zoning is an increase in water pressure in the magma during the initial stages of crystallization, and is discussed in terms of the three-component system diopside (hornblende)-albite-anorthite rather than the two-component system albite-anorthite. Textural features seen in thin section support the proposed mechanism.
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