Depth of origin of late middle Jurassic garnet andesite, southern Klamath Mountains, California

Abstract

A distinctive set of andesitic dikes crops out in the Klamath Mountains province south of ̃41°15'. These dikes are characterized by phenocrysts of Al-rich amphibole, Ca-rich almandine-pyrope garnet, plagioclase, and, commonly, quartz, set in a finegrained groundmass of plagioclase, quartz, alkali feldspar, and amphibole. Conspicuous microphenocryst/accessory phases are allanite/epidote, zircon, and apatite. A variety of thermobarometric methods plus comparison with published experimental data indicates the phenocryst assemblage was stable at pressure >7 kb and at temperatures between 850 °C and the solidus. The best estimate of pressure of phenocryst equilibration is 8-9 kb and ̃800 °C, which corresponds to depths of 25-30 km. Emplacement pressure was probably 3-4 kb, and the lack of low-pressure equilibration features suggests that the dike magmas rose and cooled quickly. Rare earth element (REE) patterns for zircon are distinct from regional zircon compositions (samples in Klamath River sand) but 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. A U-Pb (zircon) age of 160.5 ± 1.9 Ma was determined for one of the dike samples. This age is coeval with emplacement of the voluminous Wooley Creek plutonic suite in the central and northern parts of the province (north of latitude 41°15'). It is not clear why sparse dikes with high-pressure assemblages crop out in the southern Klamath Mountains province and voluminous coeval plutons are present in the central and northern parts. This distribution may reflect distinct tectonic regimes in the north (extension) versus the south (contraction), differences in melt productivity, or differences in the composition of deep crustal rocks.