Structure of the Vulcan Peak alpine-type peridotite, southwestern Oregon

Loney, Robert Ahlberg; Himmelberg, Glen R.

doi:10.1130/0016-7606(1976)87<259:sotvpa>2.0.co;2

Cited by 29 publications

(24 citation statements)

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“…These characteristics are similar to those of samples dredged from forearc and backarc settings [Dick and Bullen, 1984;Bonatti and Michael, 1989], and different from the composition of samples dredged from mid-ocean ridges Dick, 1989]. The Josephine peridotite is cut by a variety of magmatic rocks, ranging from an older suite of olivine websterites, to a younger suite of discordant dunite and wehrlite bodies, with related dunite, pyroxenite and gabbroic dikes [Dick, 1976[Dick, , 1977aHimmelburg and Loney, 1973;Loney and Himmelburg, 1976;Harding, 1988]. In addition, late, finegrained basaltic dikes cut the peridotite and the basal thrust underlying it.…”

Section: Introductionmentioning

confidence: 71%

Focused melt flow and localized deformation in the upper mantle: Juxtaposition of replacive dunite and ductile shear zones in the Josephine peridotite, SW Oregon

Kelemen¹,

Dick²

1995

J. Geophys. Res.

205

131

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Results of a field study demonstrate that melt flow in the shallow mantle was focused along actively deforming ductile shear zones in the Josephine peridotite in SW Oregon. Intergranular flow of ascending liquids dissolved pyroxene and precipitated olivine, forming zones of dunite replacing harzburgite. Syntectonic formation of dunite was focused along near‐vertical shear zones, with both vertical and dextral displacement, in the shallow mantle at less than 30 km depth and temperatures between 950 and 1100°C. The migrating magmas which formed dunite and related pyroxenite included high‐Mg, calc‐alkaline andesites characteristic of subduction‐related magmatic arcs. Explanations for focusing of melt flow along shear zones include the presence of a pressure gradient between strong and weak material during active deformation (Stevenson, 1989) and rapid infiltration of liquid when grain boundaries are aligned in mantle tectonites (Waff and Faul, 1992). Focused flow of melt may, in turn, lead to localized deformation and the development of shear zones. It is likely that melt‐lubricated shear zones provide an important locus for both melt extraction and strain in the shallow mantle beneath spreading ridges and in subduction‐related magmatic arcs.

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Section: Introductionmentioning

confidence: 71%

Focused melt flow and localized deformation in the upper mantle: Juxtaposition of replacive dunite and ductile shear zones in the Josephine peridotite, SW Oregon

Kelemen¹,

Dick²

1995

J. Geophys. Res.

205

131

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“…As illustrated in Figure 1, Fresno Bench is dominated by layered peridotites of varying pyroxene content. The temperature of deformation during shear zone formation was ∼1000°C, based on syntectonic magmatism [Kelemen and Dick, 1995] and geothermometry using orthopyroxene/clinopyroxene pairs [Loney and Himmelberg, 1976;Harding, 1988].…”

Section: Sample Collectionmentioning

confidence: 99%

Quantifying the effect of pyroxene on deformation of peridotite in a natural shear zone

Hansen

Warren

2015

JGR Solid Earth

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The influence of pyroxene on upper mantle viscosity remains unclear but may occur either through its difference in strength relative to olivine or through its effect on olivine microstructure. To determine the role of pyroxene in moderating peridotite viscosity, we analyzed microstructures in paired harzburgites and dunites from a natural shear zone exposed in the Josephine Peridotite. Olivine crystallographic textures evolve similarly in harzburgites and dunites with increasing strain, indicating the operation of similar deformation mechanisms in olivine. The mean olivine grain sizes are~1.5 times larger in dunites than in harzburgites, whereas the mean olivine subgrain sizes are a factor of~1.2 smaller in dunites than in harzburgites. The average stresses in olivine, estimated with a subgrain-size piezometer, are inversely correlated with pyroxene volume fraction and directly correlated with the mean olivine grain size. The calculated ratio of pyroxene viscosity to olivine viscosity for each harzburgite/dunite pair varies from 1.2 to 3.3. Notably, our data indicate that olivine viscosity is non-Newtonian with a finite grain-size sensitivity. We suggest that at the conditions of the shear zone, pyroxene is more viscous than olivine but that the increase in aggregate strength with larger pyroxene fractions is offset by correspondingly smaller olivine grain sizes. Thus, major rheological weakening associated with pyroxene content may only be possible if (1) temperatures are higher than those characterizing this shear zone, increasing the viscosity contrast between olivine and pyroxene or (2) stresses are high enough that grain-size reduction promotes a transition to a deformation mechanism with extreme grain-size sensitivity, such as diffusion creep.

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“…The Josephine ophiolite is found in the Klamath Mountains of southwest Oregon and northern California [ Himmelberg and Loney , 1973; Loney and Himmelberg , 1976; Dick , 1977a, 1977b; Harper , 1980, 1984; Kelemen and Dick , 1995] and is dated at 157 Ma (U/Pb age of zircon [ Saleeby et al , 1982]). The mantle section of the Josephine ophiolite is dominated by harzburgite containing pyroxene rich bands and minor amounts of dunite [ Dick , 1977b].…”

Section: Field Descriptionsmentioning

confidence: 99%

Significance of the concentration gradients associated with dunite bodies in the Josephine and Trinity ophiolites

Morgan

Liang

Kelemen

2008

Geochem Geophys Geosyst

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[1] Detailed transects were sampled across dunite bodies in the Josephine and Trinity ophiolites. The major peridotite lithologies sampled in the Josephine transect are a sequence of dunite and harzburgite and in the Trinity transect a sequence of dunite, harzburgite, lherzolite, and plagioclase lherzolite (DHL-PL). Major, minor, and selected trace element abundances in olivine, orthopyroxene, clinopyroxene, and spinel were measured. The composition profile from the Josephine transect has revealed a concentration gradient near the dunite-harzburgite contact. The composition profile from the Trinity transect has revealed several concentration gradients: two within the dunite, one in the harzburgite, and at least two in the plagioclase lherzolite. The composition profiles record complex histories of melt transport, melt-rock reaction, and subsequent subsolidus reequilibration. Analyses of closure distance suggest that compositional variation trends for a majority of major and minor elements in olivine, clinopyroxene, orthopyroxene, and spinel reported in this study were magmatic in origin. Subsolidus reequilibration may reduce the range or magnitude of variations for the 2+ cations such as Fe and Mg in olivine and spinel and significantly redistribute Ca and Li in coexisting minerals. Numerical simulations exploring the coupling of diffusion and advection in a porous matrix were used to explain compositional variations across the peridotite sequences. Melt flow from the host harzburgite into the dunite produces composition gradients near the dunite-harzburgite contacts similar to those from the Josephine transect. In contrast, melt flow from the dunite into the surrounding peridotite lithologies can produce concentration gradients similar to those observed in the Trinity transect. At least two chemically distinct episodes of melt flow within the same dunite channel system are proposed. Results from this study show that concentration gradients developed around the dunite-harzburgite and DHL-PL sequences can be used to infer part of the melt flow history of the dunite channel systems in the mantle. Results from this study and those from other ophiolite studies also demonstrate that concentration profiles in dunite and the surrounding peridotite lithologies are highly variable even among differing peridotite sequences within the same ophiolite, suggesting that the composition of instantaneous melt flowing through individual dunite channels is quite variable and the mantle source regions are heterogeneous.

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Structure of the Vulcan Peak alpine-type peridotite, southwestern Oregon

Cited by 29 publications

References 0 publications

Focused melt flow and localized deformation in the upper mantle: Juxtaposition of replacive dunite and ductile shear zones in the Josephine peridotite, SW Oregon

Focused melt flow and localized deformation in the upper mantle: Juxtaposition of replacive dunite and ductile shear zones in the Josephine peridotite, SW Oregon

Quantifying the effect of pyroxene on deformation of peridotite in a natural shear zone

Significance of the concentration gradients associated with dunite bodies in the Josephine and Trinity ophiolites

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