IS A TRIASSIC SUBDUCTION INITIATION EVENT RECORDED IN THE FORT JONES TERRANE, KLAMATH MOUNTAINS, CALIFORNIA?
Several possibilities exist for origin of the amphibolite. Two adjacent terranes, Devonian Forest Mountain subterrane and the Trinity Ophiolite, contain amphibolitic gabbros that may have been recycled into the Late Triassic FJT subduction zone. Alternately, the hornblende may have formed during the inception of subduction, as newly subducted material came into contact with the hot hanging wall of the subduction zone.
In order to determine the origin of the amphibolite-facies metamorphism we studied these rocks with petrographic, geochemical, and radiometric methods. Major and trace element geochemistry show slight similarities between the FJT and Trinity Ophiolite in immobile elements. However, pressure conditions determined with the Al- and Ti-in-hornblende thermobarometer suggest that the amphibolitic gabbro in the Forest Mountain subterrane and the Trinity Ophiolite formed at higher pressures than the FJT metabasalt.
New Ar/Ar dating of the amphibole separates from the FJT yield an age of ~240 Ma for the metamorphic event. As this age is similar to the Late Triassic (~220 Ma) white mica dates that have been reported for the blueschist-facies event, we suggest that the amphibolite may have formed during the hot, early phases of the subduction zone. As the subduction zone continued, these rocks were cooled into blueschist-facies conditions, creating the apparent counterclockwise P-T-t path recorded in the amphiboles.