New chemical analyses have been made of a suite of recently collected samples of pillow lava, breccia, and minor intrusive rocks from the Olympic Peninsula. The rocks have been altered through addition of at least water and carbon dioxide, but the recalculated analyses show that these rocks are tholeiitic basalt rather than spilite as previously reported. A petrogenetic model, involving partial melting of mantle peridotite to produce a magma that underwent fractional crystallization of olivine and equilibrated near atmospheric pressure, accounts for the observed chemical features of the basalt. The characteristics and distribution of both the volcanic and associated sedimentary rocks suggests possible origin in an island-arc environment.

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