A sequence of Jurassic rocks on Fidalgo Island, Washington, is interpreted to be ophiolite. The order of rock types, from the base upward, is serpentinite, layered gabbro, a dike complex made up mostly of plagiogranite, volcanic rocks that are dominantly keratophyre, coarse breccia with clasts of keratophyre and plagiogranite, pelagic argillite, and siltstone-sandstone turbidites. The plagiogranites and keratophyres have identical chemical compositions and are mutually gradational in field setting and textures, all of which suggests that they are cogenetic. These rocks are distinguished from calc-alkalic rock types by their very low content of K2O (where SiO2 = 70%, K2O = 0.2% to 0.7%). Metasomatic alteration of the rocks appears to be insignificant, judging from (1) well-preserved primary igneous textures, (2) well-preserved primary intrusive and extrusive contacts, and (3) uniformity of chemical composition across igneous units.

An oceanic origin of the ophiolite is suggested by the capping of pelagic sediments. Their fine grain size, abundance of radiolaria, and enrichment in Mn and other metals are virtually identical to those of modern Pacific pelagic sediments and unlike that of arc or epicontinental sediments. This interpretation conflicts with the apparent paucity of plagiogranite and keratophyre on the present-day sea floor.

Field relations and chemical trends indicate that the plagiogranite-keratophyre magma is not the product of fractionation of the same melt that crystallized layered gabbro. High water content of the plagiogranite-keratophyre magma is indicated by hydrothermal alteration of the gabbro near plagiogranite intrusions and the occurrence of hornblende instead of pyroxene in mafic varieties. We suggest that this water is from the sea and that the anomalously low K2O content of these magmas is due to exchange with sea water.

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