Juxtaposition of Variably Depleted Ultramafic Rocks across a Mantle Mylonite Zone: Ingalls Ophiolite, Washington Cascades

The Jurassic Ingalls ophiolite contains a wide array of variably depleted ultramafic rocks that were deformed in a steep mantle shear zone inferred to represent the deeper levels of an oceanic fracture zone in a supra-subduction-zone setting. This E-W-striking shear zone, which extends for 30 km and is 2-km-wide, separates harzburgite and dunite on the south from lherzolite and cpx-rich harzburgite on the north. Modes, chrome spinel chemistry, and REE abundances imply that the harzburgitic southern unit is the residue of a transitional MORB-IAT magma, whereas the lherzolitic northern unit is a residue of MORB magma. The lherzolite contains local pods of strongly deformed, amphibolite-facies metagabbro and veinlets of plagioclase and clinopyroxene that postdate high-T deformation of olivine; textures and a weak Eu anomaly suggest impregnation by infiltrating basaltic melt. The shear zone largely reworks the lherzolitic unit, and shear-zone mylonites are less depleted than either of the adjacent units. The mylonites are marked by commonly dramatic grain-size reduction. Most are lherzolite, but there are also scattered domains of hornblende peridotite and plagioclase peridotite. In these mylonites, opx and cpx porphyroclasts are set in a very fine-grained mosaic of olivine, cpx, plagioclase, and opx. Some of the plagioclase peridotite may represent strongly deformed impregnated lherzolite. The mylonites display moderately to steeply dipping foliation and gently to moderately plunging lineation. Local kinematic indicators (asymmetric porphyroclasts of pyroxene and spinel) mostly indicate dextral shear. Deformation predated dikes of diabase and gabbro (MORB and transitional MORB-IAT affinities) that display magmatic foliation. Localized high-T deformation marked by recrystallization and grain-size reduction does affect some m- to 10s of m-thick bodies of cumulate wehrlites and clinopyroxenites, whereas others are only weakly recrystallized. Overall, the ultramafites preserve a complex history of depletion, impregnation and refertilization, and deformation in and adjacent to a major mantle shear zone.