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...