A field, chemical, and stable isotope study of subseafloor metamorphism of the Josephine ophiolite, California‐Oregon

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Abstract
The Josephine ophiolite is a large, complete ophiolite generated in a Late Jurassic back arc basin along western North America. The ophiolite underwent subseafloor metamorphism under a steep thermal gradient as indicated by downward changes in mineralogy and δ18O values. Characteristic mineral zonation includes mica±orthoclase and hematite in the lower extrusives, greenschist‐facies assemblages in the upper sheeted dikes, and amphibolite‐facies assemblages in the lower sheeted dikes and high‐level gabbro. Cumulate gabbros are only incipiently altered but are depleted in 18O indicating high‐temperature alteration with fluids that evolved at low water(W)/rock(R) ratios. A second, off‐axis circulation system is recorded by metalliferous sediments that occur 8–21 m above the ophiolite. Alteration in the upper sheeted dike complex and especially the extrusive sequence is heterogeneous at outcrop scale as indicated by large variations in mineralogy, chemistry, and δ18O values within individual pillows and dikes. The heterogenity is due to (1) variations in W/R ratio and starting material during alteration by downwelling seawater (e.g., pillow cores versus glassy rims) and (2) localized discharge mineralization (mostly epidosites) superimposed on the background recharge alteration. Epidosites occur largely as dike‐parallel stringers, irregular replacement of pillow and massive lava, and replacement of interpillow and pillow‐breccia matrices. The chemistry of most extrusives and sheeted dikes is characterized by loss of Ca and gain in Na and Mg, consistent with alteration by downwelling fluids. In addition, the sheeted dikes are depleted in K2O, and the lower sheeted dikes and high‐level gabbros are depleted in Zn and Cu. The δ18O and δD values for most samples from the extrusive sequence indicate alteration by seawater at 50° rotation of the entire crustal sequence at the spreading axis, suggests that the ophiolite formed at a slow spreading center where magma chambers were episodic.