REVISITING THE PETROLOGY AND AGE OF ACCRETIONARY COMPLEX ROCKS AT BANDON, OREGON

Block-in-matrix accretionary complex rocks at the inboard margin of Siletzia in SW Oregon have been historically correlated with the Franciscan accretionary complex on the basis of age and structural position. Unlike the Franciscan, however, the rocks in SW Oregon have received little scrutiny since the 1970s. Here, we present new petrological and geochronological data from accretionary complex rocks exposed on the beach at Bandon, Oregon to provide a first step toward testing their correlation with Franciscan rocks.

Accretionary complex rocks at Bandon consist of oceanic-affinity blocks set into a weakly carbonate-cemented greywacke matrix. Blocks include: coarsely-crystalline garnet-epidote blueschist, finely-crystalline epidote blueschist, chlorite-muscovite schist, greenstone, metabasalt pillows, and ribbon chert. The coarse garnet-epidote blueschists contain banded glaucophane-rich and epidote-rich domains. The interiors of garnets in both domains display Mn-Fe-Mg zoning patterns that indicate a single prograde phase of garnet growth. The euhedral rims of the garnets are enriched in Mg relative to the rest of the garnet, which suggests heating at the end of metamorphism. Euhedral glaucophane is in equilibrium with the garnet rims. Phengite from the coarse blueschists has been dated at ca. 148 Ma (K-Ar).

The greywacke matrix is pervasively fractured, but lacks a penetrative cleavage and locally retains bedding. Detrital minerals are coarse and include quartz, plagioclase, twinned K-feldspar, and biotite, which suggest input from a plutonic source terrain. Detrital zircon U/Pb age spectra from the greywacke (n=318) displays three primary age populations at 90-110, 160, and 185 Ma, which we infer are locally sourced from Jura-Cretaceous arc terranes presently in northern California/SW Oregon and western Idaho.

Our preliminary data suggest correlation with the Franciscan complex should be reconsidered because: 1) the highest grade blocks at Bandon record a relatively simple prograde metamorphic path, rather than the complex multiphase P-T paths common of Franciscan blocks, 2) unlike most Franciscan locales, the greywacke at Bandon is unmetamorphosed, and 3) the Bandon greywacke contains an abundance of ca. 185 Ma detrital zircons, which have not been found in most Franciscan greywackes.