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Upper Jurassic mafic magmatic rocks of the eastern Klamath Mountains, northern California: remnant of a volcanic arc built on young continental crust

M. Brouxel·H. Lapierre·J.-L. Zimmermann
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Abstract

Diabasic and gabbroic dikes intruding the lower Paleozoic Trinity Ophiolite in the Lovers Leap section, Klamath Mountains, California, display strong calc-alkalic petrological and geochemical features (occurrence of primary amphiboles, zoned plagioclase phenocrysts and biotite, low TiO 2 , high incompatible trace-element contents, and light rare earth element enrichment). These dikes, of Late Jurassic age (149 ±6 Ma by K-Ar), are petrographically and geochemically similar to the contemporaneous calc-alkalic ultramafic-mafic magmatism well developed through the Klamath Mountains. They present negative Nb, Zr, and Ti anomalies typical of subduction-related magmatism and probably belong to a volcanic arc on an active continental margin. Their ϵ Sr (between -9.7 and -12.5) and ϵ Nd (between 5.6 and 6.3) values compare with some western U.S. Mesozoic granites. The Nd isotopic values, lower than those of mid-oceanic ridge basalts and intra-oceanic island arcs, suggest that these dikes, deriving from a depleted mantle source, have been slightly contaminated by continental material, probably subducted sediments. Values of ϵ Nd suggest, moreover, that no old continental crust underlies the Klamath Mountains.

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M. Brouxel, H. Lapierre, J.-L. Zimmermann. 1989. Upper Jurassic mafic magmatic rocks of the eastern Klamath Mountains, northern California: remnant of a volcanic arc built on young continental crust. https://doi.org/10.1130/0091-7613(1989)017%3C0273%3Aujmmro%3E2.3.co%3B2

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