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

• Published in: Geology (Boulder) 1989-03, Vol.17 (3), p.273-276
• Main Authors: Brouxel, Marc, Lapierre, Henriette, Zimmermann, Jean-Louis
• Format: Article
• Language: English
• ISSN: 0091-7613
• DOI: 10.1130/0091-7613(1989)0172.3.CO;2

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 TiO2, high incompatible trace-element contents, and light rare earth element enrichment). These dikes, of Late Jurassic age (149 plus/minus 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 epsilonSr (between -9.7 and -12.5) and epsilonNd (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 epsilonNd suggest, moreover, that no old continental crust underlies the Klamath Mountains.