PALEOZOIC-MESOZOIC SUBDUCTION RELATED CRUSTAL GROWTH IN THE KLAMATH MOUNTAINS: A VIEW FROM THE EASTERN KLAMATH TERRANE

The Klamath province consists of a sequence of east-dipping lithotectonic terranes ranging from mid-Paleozoic in the east to Jura-Cretaceous in the west and provides a record of over 350 million years of subduction-related crustal growth. A record of the initiation of convergent margin tectonism and the nucleus of subsequent crustal growth is found in the Eastern Klamath terrane (Trinity, Redding and Yreka subterranes). Ophiolitic rocks of the Trinity-Redding subterranes record initial extension and magmatism between 431-398 Ma (Silurian-Devonian) and were generated by partial melting of a depleted, residual MORB mantle source enriched by subduction-derived fluids. Devonian melange and volcaniclastic turbidites (Yreka subterrane) deposited on Trinity basement indicates a trench-proximal position. The origin of the Eastern Klamath terrane is comparable to Eocene forearc of the IBM arc system. Unlike the IBM system, however, the Eastern Klamath system was initiated in proximity to a continental (Laurentian) margin as evidence by (1) Precambrian detrital zircon in Yreka strata; (2) inherited Precambrian zircon in Silurian-Devonian Trinity gabbro; and (3) a continental signature in Pb isotopes for Trinity-Redding magmatic rocks. Metabasite of the Central Metamorphic terrane (CMT) was thrust beneath the Trinity ophiolite in Devonian time. Metabasite trace element data is consistent with a MORB protolith, suggesting the CMT records accretion of oceanic lithosphere in an east-dipping subduction zone. Subsequent crustal growth processes are largely recorded west of the CMT. Permian basaltic rocks (Redding subterrane) and Early Cretaceous mafic plutonic rocks (Trinity-CMT), however, carry a subduction signature with trace elements indicating a progressively more enriched mantle source and may provide evidence of Late Paleozoic - Mesozoic replenishment of fertile mantle beneath the Eastern Klamath terrane.