LATE MIDDLE JURASSIC GARNET ANDESITE MAGMATISM IN THE SOUTHERN KLAMATH MOUNTAINS

A distinctive set of andesitic dikes crops out in the Klamath Mountain province south of approximately 41°15'. The dikes are characterized by phenocrysts of Al-rich amphibole, Ca-rich almandine-pyrope garnet, plagioclase, and quartz, set in a fine-grained groundmass of plagioclase, quartz, alkali feldspar, and amphibole. Conspicuous microphenocryst/accessory phases are allanite/epidote, zircon, and apatite. The phenocryst assemblage was at equilibrium in a P range of 800–900 MPa and T ~800°C, which corresponds to depths of 25 to 30 km. Emplacement pressure was probably 300–400 MPa, and the lack of low-P equilibration features suggests that the dike magmas rose and cooled quickly. Rare earth element (REE) patterns for zircon are consistent with crystallization in equilibrium with garnet. Moreover, the zircons have strongly depleted light REE patterns, which is consistent with co-precipitation of allanite/epidote. The bulk rock REE patterns are not indicative of an origin by partial melting of metabasites with residual garnet.

A U-Pb (zircon) age of 160.5 ± 1.9 Ma was determined for one of the dike samples, which places the garnet andesite dike magmatism as coeval with the voluminous Wooley Creek suite of plutons in the central and northern parts of the province (north of latitude 41°15'). Previous petrologic and geochemical studies showed that Wooley Creek suite magmatism involved significant lower crustal assimilation, mixing, and crustal melting, with suggestions that crustal thickness was greater than commonly assumed for an oceanic arc setting. The presence of quartz phenocrysts in the garnet andesite dikes attests to their crustal origin, and supports the idea that these high-P assemblages grew in a crust that had thickened during contractional deformation associated with ca. 170 Ma thrusting on the Wilson Point fault.