OCEANIC VERSUS EMPLACEMENT AGE SERPENTINIZATION IN THE JOSEPHINE OPHIOLITE - IMPLICATIONS FOR THE NATURE OF THE MOHO AT INTERMEDIATE AND SLOW-SPREADING RIDGES

We present field, petrographic, and geochemical evidence for oceanic serpentinization in the 162 Ma Josephine ophiolite of NW California and SW Oregon. Undeformed and unrodingitized dikes that intruded into serpentinized shear zones provide time markers for serpentinization and deformation. The dikes intruding serpentinites are of two types: (1) Fe-Ti enriched dikes with normal mid-ocean ridge basalt (NMORB) magmatic affinity which are geochemically linked to the uppermost lavas and a Late Fe-Ti dike within the crustal sequence of the ophiolite; and (2) hornblende-bearing, calc-alkaline dikes intruded during ophiolite emplacement between 150 and 146 Ma. Based on crosscutting relationships between dikes and serpentinites, serpentinization of upper mantle peridotites took place at or near the ridge axis, during periods of amagmatic structural extension in the absence of a magma chamber. Lizardite-bearing serpentinites in the northern peridotite are constrained to be oceanic and indicate temperatures of <350 degrees C within the oceanic upper mantle. Oceanic, lizardite-bearing, serpentinized shear zones, from the ultramafic cumulate section to the basal sole, indicate that the entire Josephine peridotite may have been transected by serpentinized shear zones prior to ophiolite emplacement. The ultramafic cumulate sequence was completely serpentinized prior to ophiolite emplacement, and we suggest that the paleo-Moho in the Josephine ophiolite is a serpentinization boundary. The basal sole is interpreted to be a reactivated oceanic fault along which antigorite mylonites formed from a preexisting serpentinite during ophiolite emplacement. Oceanic serpentinization in the Josephine ophiolite took place beneath a 2-3 km crustal sequence and suggests that serpentinization may be an important feature at intermediate spreading rate mid-ocean ridges, as well as at slower spreading ridges.