SILURIAN PLUTONISM IN THE TRINITY TERRANE (NEOPROTEROZOIC AND ORDOVICIAN), KLAMATH MOUNTAINS, CALIFORNIA, UNITED-STATES

New data indicate that the Trinity terrane of northern California is a polygenetic composite terrane rather than a single slice of oceanic l ithosphere. We suggest approximately one third of the Trinity terrane consists of Silurian intrusive rocks that represent the roots of a pre viously unrecognized Silurian magmatic are. Crosscutting relations and U-Pb zircon isotopic data document at least one early Paleozoic defor mation in the Trinity terrane of northern California. A ductile shear zone between Neoproterozoic metagabbro and Ordovician(?) harzburgite i s intruded by the Upper Silurian China Mountain pluton. This evidence indicates a major early Paleozoic shear zone formed in the eastern Kla math Mountains after the Middle Ordovician but prior to Late Silurian plutonism.