Les dunites en enclaves dans les basaltes alcalins des îles océaniques : approche pétrologique

Berger, E. T.; Vannier, Michel

doi:10.3406/bulmi.1984.7809

Cited by 27 publications

(17 citation statements)

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“…Residual dunite can be attributed to melting of orthopyroxene out of harzburgite or to reaction of orthopyroxene out of harzburgite accompanied by crystallization of olivine from ascending diapirs of picritic magma. These diapirs would be undersaturated in orthopyroxene after rising from the zone of melting [Irvine, 1967;Nicolas, 1972, 1977;Dick, 1977a, b;Quick, 1981;Pallister and Gregory, 1983;Berger and Vannier, 1984;Pallister, 1984]. There are no discriminating data to help us choose between the two possible origins.…”

Section: Discussionmentioning

confidence: 99%

The Kanuti ophiolite, Alaska

Loney¹,

Himmelberg²

1989

J. Geophys. Res.

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The Kanuti ophiolite is a mafic‐ultramafic thrust sheet of probable Jurassic age, formerly considered to be the upper part of the Yukon‐Koyukuk ophiolite belt (Angayucham terrane). It is here called the Kanuti ophiolite after the Kanuti River region on the southeastern flank of the Yukon‐Koyukuk Basin. The thrust sheet crops out discontinuously for a distance of more than 900 km along the northern and southeastern margins of the basin. It is probably correlative with similar ophiolite thrust sheets to the north in the western Brooks Range and to the south in the Ruby geanticline. Technically, the ophiolite is considered to be the Kanuti thrust panel of the Angayucham‐Totzitna terrane. The Kanuti consistently overlies another extensive thrust sheet, consisting mostly of pillow basalt and radiolarian chert of Devonian to Jurassic age (Narvak thrust panel). This sheet is thrust over a third sheet consisting of probable Devonian phyllite and metagraywacke, which is in turn thrust over older metamorphic rocks (Slate Creek thrust panel). The Kanuti ophiolite is a partial ophiolite that consists of a lower residual mantle suite and an upper magmatic suite, but dikes, extrusives, and sediments are absent. The residual mantle suite is composed of harzburgite and dunite with refractory mineral compositions. The harzburgite is attributed to partial melting and extraction of basaltic magma; residual dunite is attributed to partial melting or to reaction of orthopyroxene out of harzburgite in contact with ascending melt diapirs. The magmatic suite consists of layered ultramafic and gabbroic rocks, containing minerals having limited iron enrichment. The absence of large volumes of magmatic rocks intermediate in composition between cumulus ultramafics and evolved gabbros favors periodic introduction of magma, rather than closed system fractional crystallization. The ultramafic rocks of both the residual mantle and magmatic suites are tectonites, which have undergone high‐temperature deformation involving isoclinal folding on all scales and related syntectonic recrystallization. The olivine fabric is consistent with the glide system {0kl} [100], which has been produced experimentally at 800°–1190°C at 20 kbar. Olivine Z axes and subparallel isoclinal fold axes have consistent, northeast trends throughout the Kanuti region (>100 km NE‐SW) and may be close to the original upper mantle flow direction, despite later low‐angle thrust faulting. The order of crystallization in the cumulus ultramafic rocks of the magmatic suite is olivine, clinopyroxene, plagioclase, and orthopyroxene. The high Mg numbers of clinopyroxene (0.85–0.93) coexisting with olivine suggest that the cumulus ultramafic rocks crystallized at relatively high pressures (>10 kbar). The effects of parental magma composition cannot be evaluated, but the small difference in Mg numbers of coexisting olivine and clinopyroxene in the cumulus ultramafic rocks and in residual harzburgite suggests that regardless of absolute pressures, the pressure difference between the melt...

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Section: Discussionmentioning

confidence: 99%

The Kanuti ophiolite, Alaska

Loney¹,

Himmelberg²

1989

J. Geophys. Res.

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“…In addition, interaction of Si-undersaturated basalts with peridotites results in crystallization of olivine at the expenses of pyroxenes, forming dunites (Quick, 1982;Berger and Vannier, 1984;Kelemen et al, 1990;Morgan and Liang, 2003). Dunites are ubiquitous in shallow sections of…”

Section: Accepted Manuscriptmentioning

confidence: 99%

Heterogeneity and anisotropy in the lithospheric mantle

2015

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International audienceThe lithospheric mantle is intrinsically heterogeneous and anisotropic. These two properties govern the repartition of deformation, controlling intraplate strain localization and development of new plate boundaries. Geophysical and geological observations provide clues on the types, ranges, and characteristic length scales of heterogeneity and anisotropy in the lithospheric mantle. Seismic tomography points to variations in geothermal gradient and hence in rheological behavior at scales of hundreds of km. Seismic anisotropy data substantiate anisotropic physical properties consistent at scales of tens to hundreds of km. Receiver functions imply lateral and vertical heterogeneity at scales < 10 km, which might record gradients in composition or anisotropy. Observations on naturally deformed peridotites establish that compositional heterogeneity and Crystal Preferred Orientations (CPOs) are ubiquitous from the mm to the km scales. These data allow discussing the processes that produce/destroy heterogeneity and anisotropy and constraining the time scales over which they are active. This analysis highlights: (i) the role of deformation and reactive percolation of melts and fluids in producing compositional and structural heterogeneity and the feedbacks between these processes, (ii) the weak mechanical effect of mineralogical variations, and (iii) the low volumes of fine-grained microstructures and difficulty to preserve them. In contrast, olivine CPO and the resulting anisotropy of mechanical and thermal properties are only modified by deformation. Based on this analysis, we propose that strain localization at the plate scale is, at first order, controlled by large-scale variations in thermal structure and in CPO-induced anisotropy. In cold parts of the lithospheric mantle , grain size reduction may contribute to strain localization, but the low volume of fine-grained domains limits this effect

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“…Nevertheless, the relative light REE enrichment of these patterns also can be explained by progressive reequilibration processes occurring between melt and peridotite (Bodinier et al, 1991;Takazawa et al, 1992), resulting in the formation of harzburgite by clinopyroxene dissolution and orthopyroxene precipitation (Kelemen et al, 1992;Takazawa et al, 1992), and of dunite by pyroxene dissolution and olivine precipitation (Kelemen, 1990). The narrow compositional range of olivine (Fo90.4-01.5), from zones i to 4, and the lack of significant compositional variations between dunire lenses and enclosing lherzolite (Fo90.•-•1.1), is more consistent with reequilibration processes (Berger and Vannier, 1984;Bodinier, 1988) than with a refractory evolution which must produce a more magnesian olivine. The development of harzburgite and dunire should correspond to an increasing degree of melt percolation and melt-rock interactions.…”

Section: Origin Of Spinel Lherzolites Harzburgites and Dunitesmentioning

confidence: 99%

Chromitite and ultramafic rock compositional zoning through a paleotransform fault, Poum, New Caledonia

Leblanc

1995

Economic Geology

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The major mylonitic shear zone of the northern part of the ophiolitic peridotite sheet of New Caledonia has been described as a paleotransform fault (S•cher, 1981). Located along this structure, the Poum ultramarie massif shows a wide spectrum of peridotite compositions ranging from plagioclase lherzolite to dunite across a 2-kin-thick transition zone. The mylonitized plagioclase lherzolite contains abundant pyroxenite and mafic veins. The Poum ultramafic massif includes more than 30 chromitite orebodies. Their composition varies according to the nature of the host peridotitc: A1 rich in the plagioclase lherzolite zone (31-43% A12Oa and 25-36% Cr2Oa), Cr rich (39-60% Cr•Oa) in the spinel lherzolite and clinopyroxene harzburgite zones, and still richer in the harzburgite-dunite zone (49-64% Cr•2Oa). In peridotires, the accessory Cr spinel also displays a Cr enrichment from lherzolite to dunitc. Segregations of Cr-rich chromite (50-60% Cr•Oa) are present in orthopyroxenite veins displaying boninitic affinities. The Poum chromitites display low platinum-group element (PGE) contents and Pd/Ir ratios as do the chromitite segregations of the orthopyroxenite veins, suggesting a common parent magma or/and common fractionation processes. The compositional zoning of the Poum massif can be explained by the contrasting behavior of ultramarie rocks and upwelling marie melts, away from the axis of the transform fault, using the current ideas about the genesis of boninitic melts and the formation of dunitc and harzburgite by magma-rock interactions during melt percolation (Nicolas and Prinzhofer, 1983; Kelemen, 1990; Kelemen et al., 1992; Takazawa et al, 1992). In the lithospheric lherzolite zone that was cooled and mylonitized along the older wall of the transform fault, melts were channeled along fractures leaving M-rich chromitite pods. By contrast, pervasive percolation along the lithosphere-asthenosphere boundary allowed important melt-rock interactions resulting in the progressive formation of harzburgite and dunite and generating Mg-Cr-rich melts from which precipitated Cr-rich chromitites.

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Les dunites en enclaves dans les basaltes alcalins des îles océaniques : approche pétrologique

Cited by 27 publications

References 0 publications

The Kanuti ophiolite, Alaska

The Kanuti ophiolite, Alaska

Heterogeneity and anisotropy in the lithospheric mantle

Chromitite and ultramafic rock compositional zoning through a paleotransform fault, Poum, New Caledonia

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