POSSIBLE COUNTERCLOCKWISE P-T-T PATH FROM THE FORT JONES TERRANE NEAR YREKA, CALIFORNIA

The Klamath Mountains preserve a Devonian to late Jurassic accretionary complex emplaced beneath the Eastern Klamath Terrane. At the latitude of Yreka, California, the structurally highest lower-plate unit is the Central Metamorphic Belt, which is underlain by the Fort Jones Terrane (FJT). Although the amphibolite-facies Central Metamorphic Belt has been considered to be a Devonian metamorphic sole based on old K/Ar dates, recently published Ar/Ar hornblende to the south yield a Triassic age, leaving open the possibility that these rocks are not related to the early history of the subduction zone.

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.