Geochemical Recognition of a Captured Back-Arc Basin Metabasaltic Complex, Southwestern Oregon

Abstract

An extensive fault-bounded amphibolite terrane of Late Jurassic ($145 \pm 2 Ma$) metamorphic age occurring in the northeastern Klamath Mountains of southern Oregon has been recognized as the remnants of an ancient back-arc basin. In spite of thorough metamorphic recrystallization under amphibolite-facies conditions, the amphibolite locally displays relict igneous textures which suggest that the protoliths included basaltic dikes or sills, shallow diabase intrusions, and gabbros. The major- and minor-element chemistry of the amphibolite indicates that some alteration of original compositions has occurred. Nevertheless, certain elements (e.g. Zr, Ti, Y, Hf, P, Th, Ta, and the REE) were probably immobile and retain their original igneous abundances. Chondrite-normalized REE patterns are similar to those of mid-ocean ridge basalt (MORB): concave-downward and flat or slightly LREE-depleted, with abundances about six to 30 times chondritic values. Patterns are slightly LREE-enriched and HREE-depleted relative t...