A complete ophiolitic succession crops out along the easternmost exposure of the eastern Klamath Mountains (Trinity ultramafic sheet, ultrabasic-basic plutonic rocks, sheeted-dike complex, and basaltic pillow lavas). The ophiolitic succession is characterized by small amounts of iron enrichment, low titanium and incompatible trace-element contents, and depleted LREE patterns. The strong island-are geochemical affinities of the Trinity basatts and the εNd values of the ophiolitic succession ranging between 6.6 and 7.5 suggest that the Trinity ophiolite is the remnant of a back-arc basin located in the vicinity of an island arc. Important Sr-isotopic variations (εSr range between -17.3 and 23.6), contrasting with the rather constant εNd values, suggest that sea-water interaction occurred. The calculated water/rock ratios, ranging between 0 and 10, show that the hydrothermal activity was maximal in the high-level gabbros, just below the sheeted-dike complex.