WHAT DO PLUTONS TELL US ABOUT TERRANE ACCRETION PROCESSES IN THE KLAMATH MOUNTIANS?

The Klamath Mountains province is an archetype of an accretionary orogens and was the birthplace of the terrane concept. Traditional interpretations of accretion suggested that growth of the orogen was by successive addition of terranes via underthrusting at the base of the orogen. This model assumes that exposed terranes represent the entire crustal section, and that the complete accretionary history can be read from the rock record. These assumptions now require testing. Recent U-Pb (zircon) dating by laser-ablation ICP-MS combined with Nd, Sr, and oxygen isotope analysis show that plutons emplaced during specific times also have specific isotopic and zircon inheritance characteristics. For example, post-Nevadan plutons in the 150-144 Ma range have low initial 87Sr/86Sr and high εNd. They also contain a distinctive array of zircons derived from flysch of the Jurassic Galice Formation, through with the magmas passed. Slightly younger tonalitic plutons (142-136 Ma) have similar isotope values but have and distinctive ~150 Ma zircon inheritance. These ~150 Ma zircons lack any known source terrane; they apparently image otherwise “hidden” basement rocks. In contrast, pre-Nevadan plutons (170-156 Ma) have higher initial 87Sr/86Sr and δ¹⁸O and lower εNd than post-Nevadan plutons and have characteristic 185 Ma inherited zircons. The lack of 185 Ma inheritance in post-Nevadan plutons suggests that the source for these zircons was excised during Nevadan thrusting. We conclude that in accretionary orogens, plutonic rocks can “remotely sense” changes in the composition, age, and distribution of “hidden terranes” through time. Integration of such data with structural studies will enhance 4-D reconstruction of accretionary orogens and may ultimately permit a much clearer picture of how accretion and terrane modification by magmatic processes contribute to crustal growth.