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Metamorphosed pillow basalt from the actively spreading basin west of the Marianas Island Arc contains albite or oligoclase of intermediate to low structural state. A feldspathic vein occupying late fractures in one metabasalt was found to contain highly disordered plagioclase ranging in composition from An3 to An31. From the absence of thermal metamorphic effects in the chlorite-montmorilloniteactinolitic amphibole-epidote-sodic plagioclase assemblage of the host rock we infer formation of the vein at low temperature.

Individual feldspar grains consist of lamellae or irregular domains (compositionally distinct volumes, not the crystallographers' domains) of varied compositions. Electron microprobe analyses of these domains yield a bimodal distribution of compositions. The large sizes of the domains and the inferred low temperature of formation of the vein suggest to us that the domains may have formed as primary metastable growth features. The bimodality and other features of the compositional distribution suggest that the composition adjusted, but incompletely so, in accord with the peristerite solvus.

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