Abstract

The Josephine ophiolite and its sedimentary cover (Galice Formation) of the western Jurassic belt, Klamath Mountains, northern California, are the expression of a Late Jurassic rift basin which developed between an active Late Jurassic arc to the west and a remnant Middle Jurassic are to the east. The Devils Elbow ophiolite remnant (DEO) and its sedimentary and volcanic cover represent the southern continuation of the Josephine ophiolite and overlying Galice Formation. The DEO consists of a dike complex, volcanic sequence, and ophiolitic breccia. Overlying strata consist of terrigenously derived turbidites and andesitic to dacitic volcaniclastic rocks. The following relations suggest formation of the DEO in a marginal basin setting, in accordance with the setting inferred for the Josephine ophiolite basin: (1) dikes and lavas are characterized by an abundance of clinopyroxene phenocrysts, by crystallization of clinopyroxene before plagioclase, and by trace-element geochemistry which is transitional between IAT and MORB; (2) overlying sedimentary rocks appear to have been derived from erosion of older Klamath Mountain terranes and were deposited into the DEO basin shortly after and/or during its formation; (3) the texture and composition of the volcanic deposits interlayered with sedimentary strata indicate the presence of a nearby active volcanic arc source. The period of rifting which culminated in the formation of the Josephine ophiolite/DEO basin began by at least 164 Ma, the age of the DEO, but is unlikely to have begun much earlier, as the region was experiencing a major period of compression just prior to this time. Rifting began at least 5 m.y. prior to cessation of magmatism in the rifted Middle Jurassic arc. The overlap in rifting and arc magmatism can be explained if spreading in the Josephine ophiolite/DEO basin was asymmetric relative to the axis of Middle Jurassic are magmatism and/or if transform motion approximately parallel to the Middle Jurassic are was important in the spreading geometry of the basin.

The western boundary fault of the western Jurassic belt, and Klamath Mountain province, is the South Fork fault (previously referred to as part of the Coast Range thrust). In the southern western Jurassic belt, a serpentinite matrix méelange (SMM) occurs along this fault. Blocks in the SMM consist largely of greenstones with the trace-element geochemistry of MORB. Other block types include gabbroic to dioritic intrusive rocks, diabase, harzburgite, amphibolite, chert, sandstone, and shale. Generally similar serpentinite matrix mélanges have been described from other locations along the South Fork fault. This spatial association suggests that the South Fork fault provided a pathway for the bouyant rise of serpentinite or serpentinite matrix mélange from a source at depth. Obvious sources for the SMM include parts of the Franciscan complex, the Josephine ophiolite/DEO, or possibly the Coast Range ophiolite. The geochemistry and relative abundances of different lithologies in the SMM, however, indicate that none of these could represent the source. In contrast, the SMM is nearly identical in block content and block geochemistry to serpentinite matrix mélange of the Rattlesnake Creek terrane which structurally overlies the western Jurassic belt to the east. This suggests that a previously unrecognized part of the Rattlesnake Creek terrane may underlie the Josephine ophiolite/DEO at depth. As the Rattlesnake Creek terrane formed the western part of the basement for the rifted Middle Jurassic arc, the part of the Rattlesnake Creek terrane which underlies the Josephine ophiolite/DEO may represent a fragment of crust which was rifted oceanward with the opening of the Josephine ophiolite/DEO basin and then subsequently thrust beneath the basin during the Late Jurassic Nevadan Orogeny.

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