Tectonophysics, 142 (1987) 261-289 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 261 Uitramafic rocks of a fracture-zone ophiolite, North Cascades, Washington ROBERT B. MILLER’ and DAVID W. MOGK * I Department of Geology, San Jose!State University, San Josh, CA 95192-0102 (U.S.A.) ’ Department of Earth Sciences, Montana State University, Bozeman, MT 59717 (U.S.A.) (Received May 23,1986; revised version accepted January 30,1987) zyxwvutsrqponmlkjihgfedcbaZYXW Abstract Miller, R.B. and Mogk, D.W., 1987. Uitramafic zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG ro c ks of a fracture-zone ophioiite, North Cascades, Was~ngton. Tectonophysics, 142: 261-289. The Ingails Complex was deformed in a Late Jurassic oceanic fracture zone. An unusually diverse group of uitramafic tectonites comprise three units in this ophioiite. Unit 1 consists mostly of poorly to moderately foliated harzburgite and dunite characterized by porphyrociastic 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 lherzolite and ciinopyroxene-bearing harzburgite; plagiociase peridotite is present locally. Oiivine and enstatite generally define equigranuiar mosaics or weakly porphyrociastic textures. Clinopyroxene, however, in some samples displays only weak deformation, compositional zoning, simple {growth?) twins and interstitial, commonly poikiiitic texture. Clinopyroxene and plagioclase in these samples probably formed from a melt after r~~st~lization of oiivine and enstatite, indicating that these lherzolites are imprecated peridotites. Other lherzolites and clinop~oxen~b~~ng harzburgites may represent weakly depleted mantle. Pods of metagabbroic gneiss within Unit 2 probably are small intrnsions that were deformed as they cooled. Unit 3 represents a major high-temperature (700 o 2 900 o C) shear zone that separates Units 1 and 2, and consists of strongly foliated, commonly myionitic lherzolites and hornblende peridotites. The latter are the most strongly foliated uitramafites, and olivine in them records stresses as high as 275 MPa. The abundance of hornblende implies a genetic relationship between myionitization and the hydration and metasomatism necessary to form such rocks from lherzoiites. Mineral chemistry and geothermometry are typical of mantle tectonites in many ophioiites 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 o to 1000 o C. These data are best explained by horizontal and vertical displacements in oceanic fracture zones which may juxtapose lithosphere 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 lherzoiite 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 lherzoiites 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. 0040-1951/87/.$03.50 0 1987 Elsevier Science Publishers B.V.