Metamorphic aragonite in the prehnite-pumpellyite facies, northwest Washington

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Abstract
Metamorphic aragonite is widely distributed in the San Juan Islands and the adjacent mainland of Washington State in rocks of the lower grade portion of the prehnite-pumpellyite facies. It occurs both in large, recrystallized limestone bodies and as a vein mineral in associated metasedimentary and metavolcanic rocks. The metamorphic facies and stratigraphic evidence indicate formation of the aragonite at low or moderate pressures. It is concluded that this aragonite is either metastable or that the experimental data reflect metastable conditions. These relations raise fundamental questions as to: (1) the reliability of metamorphic aragonite as an index of high pressure, and (2) the validity of the extreme pressures and especially of the unique geological history assigned to certain rocks of the glaucophane-schist facies on the basis of the occurrence of aragonite.

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