The polygenetic Ingalls ophiolite complex in the central Cascades, Washington, is one of several Middle to Late Jurassic ophiolites of the North American Cordillera. It consists primarily of mantle tectonites. High-temperature mylonitic peridotite, overprinted by serpentinite mélange (Navaho Divide fault zone), separates harzburgite and dunite in the south from lherzolite in the north. Crustal units of the ophiolite occur as steeply dipping, kilometer-scale fault blocks within the Navaho Divide fault zone. These units are the Iron Mountain, Esmeralda Peaks, and Ingalls sedimentary rocks. Volcanic rocks of the Iron Mountain unit have transitional within-plate–enriched mid-ocean-ridge basalt affinities, and a rhyolite yields a U-Pb zircon age of ca. 192 Ma. Minor sedimentary rocks include local oolitic limestones and cherts that contain Lower Jurassic (Pliensbachian) Radiolaria. This unit probably formed as a seamount within close proximity to a spreading ridge. The Esmeralda Peaks unit forms the crustal section of the ophiolite, and it consists of gabbro, diabase, basalt, lesser felsic volcanics, and minor sedimentary rocks. U-Pb zircon indicates that the age of this unit is ca. 161 Ma. The Esmeralda Peaks unit has transitional island-arc–mid-ocean ridge basalt and minor boninitic affinities. A preferred interpretation for this unit is that it formed initially by forearc rifting that evolved into back-arc spreading, and it was subsequently deformed by a fracture zone. The Iron Mountain unit is the rifted basement of the Esmeralda Peaks unit, indicating that the Ingalls ophiolite complex is polygenetic. Ingalls sedimentary rocks consist primarily of argillite with minor graywacke, conglomerate, chert, and ophiolite-derived breccias and olistoliths. Radiolaria from chert give lower Oxfordian ages. The Ingalls ophiolite complex is similar in age and geochemistry to the Josephine ophiolite and its related rift-edge facies and to the Coast Range ophiolite of California and Oregon. The Ingalls and Josephine ophiolites are polygenetic, while the Coast Range ophiolite is not, and sedimentary rocks (Galice Formation) that sit on the Josephine and its rift-edge facies have the same Radiolaria fauna as Ingalls sedimentary rocks. Therefore, we correlate the Ingalls ophiolite complex with the Josephine ophiolite of the Klamath Mountains. Taking known Cretaceous and younger strike-slip faulting into account, this correlation implies that the Josephine ophiolite either continued northward ~440 km—thus increasing the known length of the Josephine basin—or that the Ingalls ophiolite was translated northward ~440 km along the continental margin.

The Upper Jurassic Galice Formation of the Klamath Mountains, Oregon-California, overlies the ca. 162-Ma Josephine ophiolite and the slightly younger Rogue–Chetco volcano-plutonic arc complex. The Galice Formation that overlies the Josephine ophiolite consists of a siliceous hemipelagic sequence, which grades upward into a thick turbidite sequence. Bedded hemipelagic rocks and scarce sandstone, however, also occur at several localities within the Josephine ophiolite pillow basalts. Corrected paleoflow current data suggest that the Galice Formation was derived predominantly from the east and north. Detrital modes of sandstones from the Galice Formation indicate an arc source as well as a predominantly chert-argillite source with minor metamorphic rocks. A sandstone located ∼20 m below the top of the Josephine ophiolite has detrital modes and heavy mineral suites similar to the turbidite sandstones. Detrital Cr-spinel compositions from the turbidite and intra-pillow lava sandstones are also similar, indicating supra-subduction zone mantle peridotite and volcanic sources. Published detrital zircon data from a turbidite sandstone chiefly give a bimodal age distribution of 153 Ma and ca. 227 Ma but with a minor Proterozoic component. Whole-rock geochemistry from intra-pillow lava sedimentary rocks, the hemipelagic sequence, and the turbidites suggest a mixture between mafic and cratonic sources. It is suggested that the source area for the intra-pillow lava sedimentary rocks, hemipelagic sequence, and turbidites resulted from the mixing of arc and accreted terranes. These data indicate that the source areas for the Galice Formation were already established by ca. 162 Ma, probably during a Middle Jurassic orogeny that predated formation of the Josephine basin.

The Galice Formation is characterized by slaty cleavage, overturned tight-to-isoclinal folds having variable hingeline orientations, and a south–southeast-trending stretching lineation formed during the Nevadan orogeny. Calc-alkaline dikes and sills (151–146 Ma) that intruded the Galice Formation and its basement (Josephine ophiolite) are regionally metamorphosed, and some are deformed; however, some plutons of this age also overprint slaty cleavage, suggesting syntectonic intrusion. Amoeboid margins on some sills suggest intrusion began prior to lithification of the Galice Formation. Some dikes are intruded into pre-existing small thrust faults that predate the slaty cleavage. Dikes show a wide range of orientations, and poles to dikes are consistently oriented at a high angle to poles to extension veins and to the stretching lineation in the Galice Formation. Poles to dikes define two quadrants on an equal-area, lower-hemisphere projection separated by planes oriented at right angles. These planes are analogous to nodal planes of a fault-plane solution, and thus allow determination of P- and T- axes. Restoration of structures to their original (Nevadan) orientation results in the P - and T -axes, stretching lineations, poles to extension veins, poles to small syn-cleavage faults, and poles to cleavage all being essentially coplanar with the “movement plane” that strikes to the northwest and dips steeply. The “fault-plane solution” derived from dike orientations indicates northwest-southeast contraction, consistent with slip directions for most small faults having slickenfibers. A wide range of fold hingeline orientations and slip directions on small pre-cleavage faults, however, may record early west-directed shortening.

A newly discovered dike complex and serpentinite-matrix mélange may represent basement for the Upper Jurassic Galice Formation in the Elk outlier of the western Klamath terrane, which lies far to the west of the Klamath Mountains province. The fault-bounded dike complex consists of virtually 100% parallel dikes with scarce diorite screens and amphibole-rich hydrothermal metamorphic assemblages, all consistent with it being a sheeted dike complex of an ophiolite. Magmatic affinities of the dikes are transitional island-arc tholeiite (IAT) to mid-ocean ridge basalt (MORB) and boninitic, similar to the Josephine ophiolite that underlies the Galice Formation elsewhere. The mélange includes blocks of mafic volcanic (many pillowed) and hypabyssal rocks, igneous breccia, amphibolite, pyroxenite, peridotite, and metasandstone in a sheared serpentinite matrix. Volcanic rocks and amphibolites have MORB magmatic affinities, and some of the pillow lavas are unusual highly fractionated Fe-Ti basalts. Hypabyssal blocks have MORB, within-plate, and calc-alkaline affinities. The magmatic affinities of the mélange blocks are distinct from the dike complex, and thus the mélange and dike complex are unrelated petrogenetically. The mélange may belong to the underlying Pickett Peak terrane of the Franciscan Complex, or it may be part of the western Klamath terrane and correlative to Lower Mesozoic ophiolitic rocks found elsewhere in “rift-edge facies” of the Josephine ophiolite. The Elk outlier is far to the north of the part of the western Klamath terrane that contains the Josephine ophiolite. Thus, the presence of a remnant of the Josephine ophiolite in the Elk outlier would be consistent with Cretaceous dextral strike-slip displacement proposed by previous workers or with the Josephine extending farther north than previously recognized.

Field mapping and structural analysis indicate that the Snowcamp remnant of the Coast Range ophiolite, in the vicinity of Game Lake, southwest Oregon, was thrust to the northeast over mafic phyllonite and amphibolite correlated with the Chetco complex. The contact between the upper ophiolitic and lower mafic sections is marked by a ∼5- to 10-m-thick serpentinite unit and paralleled by mylonitic foliations in the lower mafic section. Syndeformational tonalites intrude the lower plate. The fabric-forming metamorphic mineral assemblages (amphibolite facies) in the mafic section suggest a temperature range of ∼550–600+ °C and medium pressure, whereas quantitative thermobarometry indicates 615–720 °C and 3.5–6 kb (11.5–20 km depth). A mylonitic greenschist-facies mineral assemblage in phyllonites below the thrust overprints the amphibolite-facies fabric, indicating that thrusting continued during cooling. The 40 Ar/ 39 Ar analyses indicate that thrusting and syntectonic intrusion occurred by at least 154.2 ± 2.0 and 149.1 ± 0.4 Ma, respectively. The character of this thrust strongly resembles that of the Madstone Cabin thrust that emplaced the Josephine ophiolite over the Rogue–Chetco arc complex. The proposed correlation between the two faults implies that either the Snowcamp remnant of the Coast Range ophiolite has undergone a similar emplacement history as the Josephine ophiolite or that the ophiolitic rocks near Game Lake may be an outlier of the Josephine ophiolite juxtaposed by postemplacement faulting against a Coast Range remnant around Snowcamp Mountain to the south.

A group of plutons were emplaced in the western Klamath Mountains province during the waning stages of the Late Jurassic Nevadan orogeny. Published U-Pb (zircon) ages indicate that the “western Klamath plutonic suite” was emplaced in the age range of 151–144 Ma. Crosscutting relationships, development of contact metamorphic aureoles, and the presence of distinctive inherited zircon populations indicate that the magmas intruded the footwall and hanging-wall rocks of the principal Nevadan thrust fault. The plutons are chiefly gabbroic to dioritic in composition, but commonly include ultramafic rocks and contain smaller volumes of tonalite and granodiorite. Hornblende is the most common mafic phase, except for some ultramafic rocks in which clinopyroxene ± olivine are locally distinctive, the two-pyroxene dioritic to monzodioritic rocks of the Buck Lake unit of the Bear Mountain pluton, and the most felsic rocks in which biotite is the most abundant mafic phase. Compositions of fine-grained mafic dikes suggest the presence of two principal parental, H 2 O-rich magmas: primitive basalt and evolved basalt/basaltic andesite. The former was parental to the ultramafic rocks of this suite. It was also parental to the basalt/basaltic andesite magmas by deep-seated fractional crystallization processes. The latter magmas were parental to the gabbroic and dioritic units. Many of the felsic rocks show evidence of origins by partial melting of metabasaltic crustal rocks, particularly their low heavy rare-earth element concentrations and high Sr/Y ratios. Mixing of crustal melts with primitive basaltic magmas was locally important (e.g., Pony Peak pluton). The mafic parental magmas show trace element features typical of an origin by partial melting of a subduction-modified mantle wedge. It is unclear whether subduction was coeval with western Klamath magmatism or whether the subduction signature developed as the result of Middle Jurassic subduction.

Abstract The Josephine Ophiolite is a large complete ophiolite flanked by arc complexes, including rifted arc facies, and overlain by volcanopelagic and volcaniclastic sedimentary rocks. The extrusive sequence and sheeted dyke complex record a wide range in magma types and degree of fractionation. The upper part of the extrusive sequence, as well as late dykes in the ophiolite, have mid-ocean ridge basalt (MORB) affinities and include unusual highly fractionated Fe-Ti basalts. The sheeted dyke complex and lower pillow lavas are dominated by transitional island-arc tholeiite (IAT) to MORB, but about 10% consist of low-Ti, high-Mg basalts and andesites. Whole-rock chemistry and Cr-spinel compositions indicate that the low-Ti rocks range from boninite (BON) to primitive arc basalt. The low-Ti samples have trace element characteristics indicating a greater subduction component than the IAT-MORB or MORB samples, as well as derivation from a wide range of sources ranging from depleted to enriched relative to an average N-MORB mantle source. Mixing of low-Ti and MORB magmas may have produced the IAT-MORB magma type that is most characteristic of the ophiolite. Podiform chromites and late magmatic features in the mantle peridotite, described by previous workers, appear to have been formed from the low-Ti magmas. Regional geological relationships and the presence of boninitic magmas suggest that arc rifting and initial sea-floor spreading to form the Josephine Ophiolite occurred in the forearc of a west-facing arc built on edge of the North American plate. Arc magmatism appears to have jumped westward, at which time the Josephine basin became situated in a back-arc setting, analogous to the inferred evolution of the modern Lau back-arc basin. Alternatively, the Josephine Ophiolite may have formed in a setting analogous to the north end of the Tonga Trench or the south end of the North Fiji basin, both sites of modern boninites, where a back-arc spreading centre has propagated across an arc into the forearc. Rift propagation during formation of the Josephine Ophiolite is consistent with the presence of highly fractionated Fe-Ti basalts.

Abstract The Ingalls Ophiolite Complex is a suprasubduction-zone ophiolite formed largely in a fracture-zone setting. Mantle tectonites are cut by a large, high-T shear zone overprinted by sheared serpentinite. Mafic complexes of ca. 161 Ma gabbro, sheeted dikes, and pillow lava occur as large blocks in the sheared serpentinite. An overlying Late Jurassic argillite unit contains minor chert, graywacke, and pebble conglomerate, along with lenses of ophiolite breccias. Detrital serpentinite forms some of these breccias, and mafic blocks in other breccias range up to hundreds of meters in diameter. Older basement is locally present in the ophiolite complex, includ ing (1) phyllite, metachert, and pillow basalt of the undated De Roux unit overlain by (2) Early Jurassic pillow lava, basalt breccia, and minor chert and oolitic limestone of the Iron Mountain unit (fossil seamount). This older basement indicates that at least part of the ophiolite is polygenetic. The presence of a high-T mantle shear zone and ophiolitic breccias containing clasts derived from the lower crust and upper mantle suggest formation of the ophiolite in a fracture zone setting. Late calc-alkaline dikes cut the various units, some of which are related to the middle Cretaceous Mount Stu art batholith, and others of which are probably Late Jurassic.

Abstract Volcanic-associated massive sulfide (VMS) deposits in ophiolites are generally considered to be ancient analogs to sulfide deposits forming today at 350°C hot springs, hot smokers, on mid-ocean ridges and in back-arc basins (Fornari and Embley, 1995; Ishibashi and Urabe, 1995). Most ophiolites appear to have formed in supra-subduction zone settings (back-arc, fore-arc, or nascent arc), a conclusion based largely on the common presence of lavas having arc-like magmatic affinities and, in a number of lavas, affinities to boninites (e.g., Pearce et al., 1984; Meffre and Crawford, 1996). Ophiolites provide an opportunity to observe the pathways of hot-smoker fluids, providing that alteration resulting from interaction with such fluids can be recognized. Epidosites (granoblastic epidote + quartz + chlorite + titanite ± magnetite) in sheeted dike complexes have been inferred to record the pathways of such fluids (Richardson et al., 1987; Shiffman and Smith, 1988; Schiffman et al., 1990; Harper et al., 1988; Nehlig et al., 1995). The intense Ca metasomatism, high-variance assemblages, and complete textural reconstitution to granoblastic textures of epidosites suggest interaction with large volumes of highly reacted seawater-derived fluids. Epidosites in the Josephine ophiolite appear to represent more diffuse discharge during periods when faulting was poorly developed (high magma supply). Large volumes of epidosites (tens of km 3 ) occur beneath VMS deposits in the Troodos Cyprus and Oman ophiolites (Richardson et al., 1987; Schiffman and Smith, 1988; Nehlig et al., 1995). More recently, fault-controlled discharge has been documented in the Josephine ophiolite as a distinct structural style that generally postdates epidosites (Alexander and Harper, 1992; Alexander et al., 1993). Some of the oceanic fault zones are characterized by mineralized breccias and thus appear to have been highly permeable pathways for hot smoker-like discharging fluids. VMS deposits in the Josephine ophiolite appear to have been formed by discharge and venting along fault zones (Kuhns and Baitis, 1987; Zierenberg et al., 1988).

Pumpellyite and prehnite are associated closely with epidosite in two well-exposed sections of the Josephine ophiolite and are interpreted to have formed during hydrothermal metamorphism beneath a spreading axis. In the upper 75 m of the extrusive sequence, epidosite grades upward into “pumpellyosite” (granoblastic pumpellyite + quartz + chlorite ± epidote rock) and, in interpillow hyaloclastite, into “prehnitite” (granoblastic prehnite + quartz + epidote ± chlorite rock). Probable hydrothermal pumpellyite also occurs in the lower hematitic pillow lavas as amygdules that contain pumpellyite + chlorite ± epidote ± chalcopyrite. The second occurrence of pumpellyosite and prehnitite is in the basal sheeted dike complex, where these minerals formed during the late stages of retrograde hydrothermal metamorphism. The bulk-rock composition of pumpellyosite and prehnitite shows extensive Ca metasomatism very similar to that of epidosite. Like epidosites, these rocks are inferred to have formed by interaction with large volumes of upwelling, highly reacted hydrothermal fluids, similar to those at modern high-temperature hot springs on mid-ocean ridges. The change in the upper 75 m of the pillow lavas from epidosite to pumpellyosite and prehnitite may reflect cooling of upwelling fluids to less than ~315 °C. The presence of interpillow prehnitite immediately below sediments overlying the ophiolite implies that these fluids leaked directly onto the sea floor. The Josephine pumpellyosites and prehnitites formed at temperatures between 200 and 315 °C, within the overlapping stability fields of epidote, prehnite, and pumpellyite. The influence of fluid composition on mineral equilibria is evaluated at 250 °C and 500 bar using an a Ca 2 + / a H + 2 versus a Fe 3 + / a H + 3 diagram modified from Rose and Bird (1987). The topology of the pumpellyite-epidote and pumpellyite-prehnite phase boundaries was derived using compositions of coexisting Ca-Al silicates in the Josephine samples. The pumpellyite and prehnite stability fields are generally at lower a Fe 3 + / a H + 3 than epidote, whereas the pumpellyite stability field is generally at lower a Ca 2 + / a H + 3 than prehnite.

The 162-Ma Josephine ophiolite was emplaced over an active mafic batholith (Chetco River complex) along the Madstone thrust in southwestern Oregon during the Nevadan orogeny, beginning at ∼155 Ma. Strongly deformed amphibolite and minor quartzite occur between the ophiolite and the batholith and are interpreted to make up a metamorphic sole formed during thrusting. Retrograde metamorphism is ubiquitous, and amphibolite has been locally converted to greenschist-facies mafic phyllonite adjacent to the Madstone thrust. Pegmatite dikes locally cut the amphibolite but are also penetratively deformed, indicating syntectonic intrusion. A geochronologic study (Harper and others, 1989) indicates cooling from ∼450°C at 153 Ma, intrusion of the pegmatite at 150 Ma, and cooling to ∼350°C at 146 Ma. Geobarometry, using amphibole composition and phengite content of muscovite, indicates relatively low P/T metamorphism. The lower contact of the amphibolite sole with the Chetco River complex, as described by previous workers, is intrusive and syntectonic with deformation of the amphibolite sole. In the hanging wall of the Madstone thrust, 20 to 40 m of high-T serpentinite mylonite occurs along the base of the Josephine Peridotite. The serpentinite apparently formed during ophiolite emplacement because it is structurally concordant with the underlying amphibolite and phyllonite. In addition, the serpentinite locally shows metasomatism, which probably resulted from interaction with fluids derived from the amphibolite sole. The amphibolite shows two generations of folds having fold hinges parallel to a NNE-stretching lineation. These structures, along with grain-size reduction and asymmetric fabrics, indicate that the amphibolites are mylonites formed by progressive simple shear. The lineations and sense-of-shear criteria for the amphibolite and serpentinite mylonite indicate thrusting of the Josephine ophiolite toward the north-northeast, over the Chetco River complex. Continued north-northeast thrusting during greenschist retrograde metamorphism is indicated by lineations and microstructures in phyllonites and a pegmatite dike. A minimum displacement of 12 km is inferred from the outcrop pattern of the Madstone thrust. The metamorphic sole and regional geologic setting of the Josephine ophiolite are distinct from other ophiolites. There is no inverted gradient, maximum temperatures were lower, syntectonic magmas were intruded into both the metamorphic sole and the ophiolite, and the ophiolite was thrust over an active magmatic arc rather than a continental margin. In addition, the ophiolite and overlying Galice Formation were thrust beneath the North American continent by >40 km along the roof thrust (Orleans fault) and regionally metamorphosed to low grade. Geochronologic and structural studies indicate that the basal Madstone thrust and the roof thrust were both active at 150 ± 1 Ma, but the thrusting direction along the roof thrust appears to have been west or northwest. The cause and tectonic significance of nearly orthogonal thrusting directions between the basal and roof thrusts of the ophiolite is enigmatic. One possibility is that thrusting occurred during sinistral oblique subduction, and the Josephine thrust sheet was effectively decoupled along the roof thrust due to high pore-fluid pressures in the Galice Formation.