POSSIBLE COUNTERCLOCKWISE P-T-T PATH FROM THE FORT JONES TERRANE NEAR YREKA, CALIFORNIA
The underlying FJT, usually considered to be an extension of the Stuart Fork Terrane to the south, consists of a diverse assemblage of phyllite, quartzite, chert, and metabasalt, in both coherent and mélange tectonic textures. These rocks were affected by a widespread blueschist-facies metamorphic event, presumably of Triassic age. Good exposures of the block-in-matrix fabric of the FJT exist in the Soap Creek Valley, just west of Yreka. Here, we individually describe blocks mapped by previous workers, finding lawsonite-blueschist and omphacite-lawsonite metabasalt, actinolite schist, and amphibolite. In addition, eclogite has been reported in the literature.
One metabasalt block has late glaucophane overprinting early hornblende, suggesting a blueschist-facies overprint on an amphibolite protolith. Furthermore, widespread late lawsonite overprints omphacite-rich rocks that likely co-existed with garnet. These observations, indicative of a higher-temperature event followed by a lower-temperature event, may indicate a counterclockwise pressure-temperature-time path, one which may record the early stages of subduction in the Klamath Mountains. If so, geochronology on these rocks may provide the best opportunity to date of initiation of this subduction zone. Alternately, the blueschist-over-amphibolite metamorphic relationship could be explained by the blocks having undergone two cycles of subduction.