PETROLOGY AND GEOCHEMISTRY OF A MESOZOIC OCEANIC ARC COMPLEX: AN INVESTIGATION OF THE NORTH FORK TERRANE, SOUTHERN KLAMATH MOUNTAINS, CALIFORNIA

The central Klamath Mountains of northern California are composed of numerous tectonically disrupted oceanic + terrigenous units. These terranes lie outboard of—and are sutured against—continental margin complexes of Paleozoic-earliest Mesozoic age. These terranes offer an excellent opportunity to investigate the processes that contributed to the growth of the North American cordillera, especially during Mesozoic time. The North Fork terrane of the Western Paleozoic and Triassic belt (wTrPz) belt records a portion of the late Paleozoic to early Mesozoic history of this complex system. Previous field and geochemical investigations of the North Fork terrane in the Sawyers Bar area led to the identification of two distinct suites of volcanics, an alkalic unit (OIB) and a tholeiitic unit (IAT), that are interstratified with each other and a siliceous metasedimentary unit. Glomeroporphyritic clots of relict magmatic hornblende ± minor augite characterize North Fork metabasaltic and associated metagabbroic units in the Sawyers Bar area, whereas mafic igneous units of the Hayfork Summit area are mostly clinopyroxene-phyric. New mapping, petrology, and bulk-rock geochemistry in the Hayfork Summit area of the southern Klamath Mountains indicate the presence of OIB- and MORB-like suites that are both transitional to IAT. In contrast to the Sawyers Bar area, Hayfork Summit volcanics are not interfingered and the MORB-like volcanics are present exclusively (?) as blocks in a serpentinite matrix mélange unit. Volcanics with a strong arc signature have not been recognized in the Hayfork Summit area. These relationships have led the authors to suggest that the evolving basaltic arc may have formed further offshore in the south and more proximal to the continental margin in the north. The ophiolitic basement to this arc may be represented by the serpentinite matrix mélange present in the Hayfork Summit study area.