Garnetiferous ultramafic inclusions in minette from the Navajo volcanic field
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Abstract:
Ultramafic xenoliths at The Thumb, a small minette neck near Shiprock, New Mexico, are divisible into three overlapping textural/compositional groups: coarse garnet peridotites with minerals relatively low in Fe and Ti and high in Cr, sheared garnet peridotites with minerals relatively high in Fe, Ti, and Cr, and megacrystalline rocks (medium- to ultracoarse- grained intergrowths of clinopyroxene, orthopyroxene, olivine, garnet, phlogopite, and ilmenite) with minerals very rich in Fe and Ti and low in Cr. The megacrystalline rocks are compositionally similar to the "discrete nodule" suite of Lesotho kimberlites, but are fragments of polycrystalline aggregates rather than phenocryst-like single crystals. Pyroxene compositions indicate that the inclusions equilibrated at temperatures ranging from 930–1230°C within a narrow vertical interval at around 130 km depth. The extreme textural and compositional heterogeneity of this restricted source terrane and the correlation between xenolith textures and mineral compositions are explained by the following model: The megacrystalline rocks are interpreted as precipitates from liquids introduced into an older, previously depleted terrane consisting of the coarse garnet peridotites. The Ti-Fe-rich sheared peridotites are thought to be the products of deformation and Ti-Fe-metasomatism of the coarse peridotite country rocks in tectonically disrupted aureoles surrounding the intrusive megacrystalline regions. The suggested metasomatizing medium is a residual liquid intermediate in composition between clinopyroxene and ilmenite. The model for The Thumb's inclusion suite may also be extended to explain the texturally and compositionally analogous suite of xenoliths described by Boyd and Nixon (1974) from Lesotho kimberlites. The Lesotho sheared garnet peridotites are viewed as the products of extensive metasomatism of previously less enriched mantle in response to intrusion of magma from which the discrete nodule association crystallized.Keywords:
Phlogopite
Ultramafic rock
Peridotite
Pyroxene
Ilmenite
Xenolith
Xenolith
Phlogopite
Lapilli
Diatreme
Country rock
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Ilmenite
Phenocryst
Carbonatite
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Phlogopite
Country rock
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Ilmenite
Phenocryst
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Diatreme
Phlogopite
Xenolith
Breccia
Phenocryst
Ultramafic rock
Mineral redox buffer
Diopside
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Phlogopite
Xenolith
Lapilli
Diatreme
Country rock
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Phlogopite
Xenolith
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Phlogopite
Xenolith
Phenocryst
Diatreme
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Ilmenite populations (megacrysts and macrocrysts) from 26 kimberlites in North America have been characterized by electron microprobe analysis to assist in the understanding of the origin and significance of ilmenite in kimberlites worldwide. Most belong to the Cr-poor megacryst suite. Geochemical trends in Cr-poor-suite ilmenites are consistent with a mantle fractional crystallization origin, with ilmenite forming only a minor proportion of the crystallizing assemblage. Coprecipitating magnesite is inferred to be an important host for Mg, with its crystallization causing Mg depletion in coexisting ilmenite. Decrepitation of magnesite megacrysts during kimberlite ascent may have enriched kimberlite hosts in Mg, contributing to the Mg increase characteristic of ilmenite rims. Ilmenite rims commonly have lower hematite contents than do cores, suggesting that the oxidation state of the kimberlite, and thus its potential for diamond resorption, can be overestimated if core compositions alone are considered. No evidence has been found to support the hypothesis that oxidized ilmenite populations correlate with increased potential for diamond resorption in a given kimberlite.
Ilmenite
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