AGE CONSTRAINT AND DEGREE OF METAMORPHISM OF THE TURTLEBACK COMPLEX ON ORCAS ISLAND, SAN JUAN ISLANDS, WA

The Turtleback Complex located on the western limb of Orcas Island, WA has been structurally altered, intruded by bodies of igneous material, and highly weathered. The tectonic setting of the San Juan Islands has been relatively stable since Late Jurassic (Brandon et al., 1988). Small areas of Turtleback have been age dated using U-Pb radiometric zircon age dating (Brown 2010), however more samples need to be dated in order to further constrain the age of metamorphism. These U-Pb radiometric zircon age dates have given an approximate age of 417 Ma (Brown 2010). The major metamorphic even would have occurred some 200 million years before the islands were in its current position. Petrographic microscope mineral identification confirms rock types and level of metamorphic degree. Many of the granitic boulders examined throughout the complex were out of outcrop and appeared to be glacial deposits. The last apparent glacial event of the Pacific Northwest was Holocene in age (Ohkushi 2003). Geochemical analysis of the complex indicates that Turtleback remains a structurally and geochemically complex geologic unit.