A systematic mineralogical, REE and Sr-Nd isotopic study of the eastern Klamath Mountains pre- to Early Devonian volcanism (Copley tholeiites, Lovers Leap volcanic rocks, Trinity ophiolitic basalts) show that all of the rocks belong to a low-K, island-arc, tholelitic suite, with the exception of the Late Ordovician Lovers Leap Butte calc-alkalic volcanic rocks. All of the latter have low ϵNd values (ϵNd(T)= 3.9), suggesting that they suffered crustal contamination.

The Copley and Lovers Leap low-K tholeiites exhibit variable LREE enrichment or depletion and variable ϵNd values (ϵNd(T)range mostly between 5 and 8), suggesting that variable partial melting degree and/or contamination by sediments occur(s). The isotopic features of the Copley high-Mg andesites (boninitic affinities, ϵNd(T) range between 6.2 and 7.9) show that they were derived from the same source as the low-K tholeiites. The Trinity ophiolitic basalts, along with the LREE-depleted Copley and Lovers Leap tholeiites, show similar geochemical and isotopic features (ϵNd(T) = 7.3 and 7.4), suggesting that they belong to the same island-arc-back-arc basin system.

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