As the largest exposure of ophiolitic rock in North America, the Trinity terrane constitutes an important part of the ophiolite record worldwide. It has been interpreted variously as a fragment of a mid-ocean ridge crust, a mantle diapir, the roots of a volcanic arc, and a backarc basin. A recent compilation of existing field, compositional, and geochronologic data from the Trinity terrane and the adjacent Eastern Klamath, Central Metamorphic, and Yreka terranes has led to the hypothesis that the Trinity terrane is a polygenetic supra-subduction zone (SSZ) ophiolite that formed in a trench-proximal extensional environment. New major and trace element geochemical data for mafic crustal rocks of the Trinity terrane are reported here and used to test the SSZ ophiolite hypothesis. Gabbro and mafic dike samples are depleted in high field strength elements relative to normal mid-oceanic-ridge basalt (MORB) and have high Th/Nb and Pb/Ce ratios, negative Ce anomalies, and positive Eu anomalies. These data are consistent with partial melting of a depleted, residual, MORB-type, mantle source enriched by subduction-derived fluids that included a sedimentary input. A model is presented for the development of the Trinity as a proto-arc formed during the inception of a single, east-dipping, middle Paleozoic subduction zone, analogous to the Eocene proto-arc of the Izu-Bonin-Mariana intraoceanic arc system.

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