Abstract

In the western foothills of the Sierra Nevadas near Auburn, California, two volcanic suites of probable Triassic-Jurassic age have been investigated geochemically. The older suite is a mélange that shows pervasive shearing and low-amphibolite metamorphism. This suite contains basalts with the mineralogical and chemical properties of MORB and was almost certainly formed along the spreading ridge of a major ocean basin or in a small ocean, perhaps a back-arc, basin. The younger suite is probably Jurassic and is less deformed and less metamorphosed than are the basalts of the older suite. Rocks in the younger suite are primarily basalts but include some andesites. Mineralogical and chemical properties indicate formation of the younger suite above a subduction zone, either along a thin continental margin or in an offshore island arc later accreted to the continental margin.

Relative to modern equivalents (MORB or subduction-zone basalts), the Sierran suites: (1) have approximately the same average abundances of most elements; (2) have a greater range of abundances, even for such comparatively immobile elements as Ni and Cr; (3) are enriched in K, Rb, Ba, and Sr, presumably because of posteruptive processes; and (4) have lower A12O3 and higher total Fe contents.

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