Cordilleran Section - 112th Annual Meeting - 2016

Paper No. 8-11
Presentation Time: 8:30 AM-5:30 PM


SAMPLE, Bret D. and SHIMABUKURO, David H., Department of Geology, California State University, Sacramento, CA 95819,

The Western Paleozoic and Triassic belt of the Klamath Mountains contains accreted units recording the Triassic and Jurassic history of plate convergence along the Cordilleran margin. The easternmost and structurally highest of these units, the Fort Jones terrane (FJT), or alternately, the Stuart Fork terrane, is a Late Triassic phyllitic-quartzite mélange that contains metabasalt and metasedimentary blocks that have undergone blueschist-facies metamorphism. The metabasalt block-in-matrix fabric of the FJT is well exposed along Soap Creek Ridge, approximately 6 miles southwest of Yreka. These metabasalts consist of zoned amphiboles which have hornblende cores and Na-amphibole rims. While the Na-amphibole can be linked to Late Triassic subduction, the origin of the early hornblende—indicative of an amphibolite-facies metamorphic event—is unclear.

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.