Ultramafic rocks of a fracture-zone ophiolite, North Cascades, Washington

Abstract

The Ingalls Complex was deformed in a Late Jurassic oceanic fracture zone. An unusually diverse group of ultramafic tectonites comprise three units in this ophiolite. Unit 1 consists mostly of poorly to moderately foliated harzburgite and dunite characterized by porphyroclastic textures. Irregular-shaped and tabular dunite bodies probably represent intrusive bodies or residues of partial melting. Voluminous Unit 2 consists mostly of poorly to strongly foliated Iherzolite and clinopyroxene-bearing harzburgite, plagioclase peridotite is present locally. Olivine and enstatite generally define equigranular mosaics or weakly porphyroclastic textures. Clinopyroxene, however, in some samples displays only weak deformation, compositional zoning, simple (growth?) twins and interstitial, commonly poikilitic texture. Clinopyroxene and plagioclase in these samples probably formed from a melt after recrystallization of olivine and enstatite, indicating that these Iherzolites are impregnated peridotites. Other Iherzolites and clinopyroxene-bearing harzburgites may represent weakly depleted mantle. Pods of metagabbroic gneiss within Unit 2 probably are small intrusions that were deformed as they cooled. Unit 3 represents a major high-temperature ( 700 °≥ 900 ° C ) shear zone that separates Units 1 and 2, and consists of strongly foliated, commonly mylonitic Iherzolites and hornblende peridotites. The latter are the most strongly foliated ultramafites, and olivine in them records stresses as high as 275 MPa. The abundance of hornblende implies a genetic relationship between mylonitization and the hydration and metasomatism necessary to form such rocks from Iherzolites. Mineral chemistry and geothermometry are typical of mantle tectonites in many ophiolites and oceanic fracture zones. There is a particularly strong similarity between the spinels in the Ingalls Complex and the spinels from the Owen and Vema fracture zones. Hornblende in Unit 3 ranges from edenite to edenitic hornblende. Calculated temperatures from CPX-OPX pairs in Unit 2 range from 950 ° to 1000 ° C. These data are best explained by horizontal and vertical displacements in oceanic fracture zones which may juxtapose lithosphère formed at different structural levels and with different petrologic histories. Unit 1 represents a block of upper mantle which probably experienced convective upwelling and partial melting beneath a spreading ridge near its intersection with a fracture zone. During convective upwelling at least parts of the upper mantle represented by Unit 2 were impregnated by a melt. The impregnated Iherzolite may be accounted for by the edge effect at a fracture zone or reduced melt production at a fracture zone due to ribbon spreading. The mylonite zone of Unit 3 reworks Iherzolites and harzburgites which were originally part of the block represented by Unit 2 and probably records deformation in the active segment of the fracture zone. These observations document the complexities that may occur in ultramafic rocks within oceanic fracture zones. They also suggest that impregnation, metasomatism, and high stresses are important in these zones.