Abstract
A trace-element investigation has been conducted on Ordovician to Devonian volcanics of the eastern Klamath Mountains spatially associated to the Siluro-Ordovician Trinity peridotite and gabbros. All the studied volcanics (low-K tholeiites and calc-alkaline volcanics) present strong island-arc geochemical features (low TiO 2 contents, negative Nb, Zr and Ti anomalies, mild LREE enrichment and depletion). Some volcanics from Lovers Leap and Copley exhibit strong immature island-arc geochemical affinities (very depleted LREE patterns and very low Nb contents), suggesting that they were formed by high degrees of partial melting (20%) of a residual lithosphere. Diminution of the partial melting degree and addition of sediments to the source will explain the occurrence of less depleted or slightly LREE-enriched volcanics. The Grey Rocks basaltic pillow-lavas thrust onto the Trinity peridotitic sheet probably represent the extrusive member of the ophiolitic sequence. Their strong island-arc geochemical affinities suggest that the ophiolite was formed during the first stages of a back-arc basin development. Differentiation took place in the arc by melting of the deeper basaltic layers, leading to the formation of abundant rhyolitic and trondhjemitic magmatism representing the first stages of continental accretion.
Published Version
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