The Josephine ophiolite, northwestern California

The 157-m.y.-old Josephine ophiolite is a complete ophiolite consisting of the following units: harzburgite tectonite, cumulate ultramafics, cumulate gabbro interlayered with cumulate ultramafics, cumulate gabbro, high-level gabbro and diorite, a 1.5-km-thick sheeted dike complex, and 0.4 km of pillow lava and breccia. The ophiolite is conformably overlain by the “Galice Formation,” consisting of a thin, siliceous pelagic sequence overlain by a thick sequence of turbidites having both volcanic arc and continental-margin sources. The ophiolite and overlying “Galice Formation” were intruded by numerous 150- to 151-m.y.-old calc-alkaline dikes and sills followed shortly by deformation and regional metamorphism under conditions of prehnite-pumpellyite to lower-greenschist facies. The Josephine Peridotite makes up the base of the ophiolite and covers >800 km2. It is an alpine-type peridotite that represents upper-mantle material and the residue of partial melting. Ultramafic rocks overlying the Josephine Peridotite are olivine + spinel ± clinopyroxene cumulates that grade upward into clinopyroxene and orthopyroxene ± olivine ± plagioclase cumulates, and finally to clinopyroxene + plagioclase cumulates. High-level gabbros and diorites are characterized by highly variable textures, complex intrusive relationships, strongly zoned plagioclase, and abundant mafic dikes; plagiogranites occur at this level as dikes, small intrusions, and screens between mafic dikes, but they are rare. The sheeted dike complex consists of mafic dikes averaging 0.6 m in thickness. Chilled margins are locally well preserved, and many of the dikes have been split and intruded by younger dikes, resulting in “half” and “marginless” dikes. The dike complex is composed of 100% sheeted dikes near the center but contains gabbro screens near the base and pillow lava and breccia screens in its upper parts. The dikes are inclined at ∼ 40° to bedding after correcting for folding and are inferred to have been rotated shortly after formation. The extrusive sequence consists predominantly of pillow lavas but contains abundant pillow breccias and some massive lavas. Some of the breccias formed as talus deposits, whereas others are graded and matrix-supported, indicating transport and redeposition, possibly in an area of rugged submarine topography. The dikes and lavas have geochemical affinities to island-arc tholeiites but are in some aspects transitional to mid-ocean–ridge basalts. The dikes and lavas are cogenetic with the underlying plutonic sequence, as indicated by similarity in crystallization sequence, contradictory intrusive relationships between sheeted dikes and high-level plutonic rocks, and lack of diabase dikes in the lowermost cumulates and harzburgite. The upper portions of the ophiolite were hydrothermally metamorphosed at the spreading center, resulting in downward changes in mineralogy, texture, and δ18O; in addition, metalliferous sediments and massive sulfides that occur overlying and within the pillow lavas probably were formed by precipitation from thermal springs. A geothermal gradient of 250–350 °C/km is inferred from mineral assemblages. On the basis primarily of its regional setting, the ophiolite is inferred to have formed in a back-arc basin; in particular, it is within a thrust sheet sandwiched between a coeval magmatic arc complex and a slightly older arc complex (remnant arc). This interpretation is consistent with abundant volcanic-arc detritus in graywackes overlying the ophiolite. The island-arc magmatic affinities of the lavas and dikes may be the result of formation of the ophiolite during the earliest stages of back-arc spreading.