Magmatic garnet-bearing mafic xenoliths (Puy Beaunit, French Massif Central): P-T path from crystallisation to exhumation

Berger, Julien; Féménias, Olivier; Coussaert, Nicolas; Demaiffe, Daniel

doi:10.1127/0935-1221/2005/0017-0687

Cited by 8 publications

(3 citation statements)

References 58 publications

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“…In the metagabbros where spinel is absent, there is no textural evidence for a reaction between pyroxenes and plagioclase to form garnet, as is the case in deep-seated granulitic xenoliths (Berger et al . 2005). In the Amalaoulaou metagabbros, garnet is indeed euhedral and coarse-grained, and occurs in leucocratic as well as in melanocratic layers.…”

Section: Discussionmentioning

confidence: 99%

Dehydration, melting and related garnet growth in the deep root of the Amalaoulaou Neoproterozoic magmatic arc (Gourma, NE Mali)

et al. 2008

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International audienceThe Amalaoulaou Neoproterozoic island-arc massif belongs to the Gourma belt in Mali. The metagabbros and pyroxenites forming the main body of this arc root show the pervasive development of garnet. In the pyroxenites, the latter has grown by reaction between pyroxene and spinel during isobaric cooling. By contrast, in the metagabbros, garnet textures and relations to felsic veins exclude an origin through solid-state reactions only. It is proposed that garnet has grown following dehydration and localized melting of amphibole-bearing gabbros at the base of the arc. The plagioclase-saturated melts represented by anorthositic veins in the metagabbros and by trondhjemites in the upper part of the massif provide evidence for melting in the deep arc crust, which locally generated high-density garnet clinopyroxene rutile residues. Garnet growth and melting began around 850 degrees C at 10 kbar and the tonalitic melts were most probably generated around 1050 degrees C at P >= 10 kbar. This HT granulitic imprint can be related to arc maturation, leading to a P-T increase in the deep arc root and dehydration and/or dehydration-melting of amphibole-bearing gabbros. Observation of such features in the root of this Neoproterozoic island arc has important consequences, as it provides a link to models concerning the early generation of continental crust

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Section: Discussionmentioning

confidence: 99%

Dehydration, melting and related garnet growth in the deep root of the Amalaoulaou Neoproterozoic magmatic arc (Gourma, NE Mali)

et al. 2008

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“…Helium isotope ratios were also detected in some pyroxenite xenoliths [45]. In particular, 3 He/ 4 He were measured in CO 2 -dominated fluid inclusions in clinopyroxene grains, indicating MORB-type values (e.g., Ra = 7.1-7.6, where Ra is the 3 He/ 4 He ratio of 1.38 × 10 −6 as measured in air). Accepting, for instance, the pyroxenite formation mechanism put forward in the present paper, it may be suggested that He (as well as CO 2 and other mantle-derived gases) trapped in these pyroxene grains derived from the degassing of the basalt magma intruded in the amphibolite and hence responsible for pyrometamorphism and amphibole breakdown.…”

Section: Further Discussionmentioning

confidence: 89%

“…Garnet-bearing pyroxenites from ultramafic massifs and xenolith suites the world over can be due to crystal accumulation from mafic magmas at mantle depths, together with variable amounts of trapped interstitial magma [1]. Garnet is either considered a primary igneous phase [2][3][4] or product of reaction between pyroxene and spinel, due to variations in pressure and/or temperature conditions in the system [5,6]. Some garnet pyroxenites, generally called "recycled" [7], are instead thought to have origin from high-pressure metamorphism of crustal igneous mafic rocks during subduction events.…”

Section: Introductionmentioning

confidence: 99%

Convective Instability in Intraplate Oceanic Mantle Caused by Amphibolite-Derived Garnet-Pyroxenites—A Xenolith Perspective (Hyblean Plateau, Sicily)

Scribano

Carbone

2020

Geosciences

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Geochemical characteristics of middle ocean ridge basalts (MORBs) testify partial melting of spinel-peridotite mixed with a few amounts of garnet-pyroxenite. The latter can be considered either autochthonous products of the crystallization of partial melts in the sub-oceanic mantle or allocthonous recycled crustal materials originated in subduction contexts. Here we suggest the “autocthnous recycled” origin for garnet-pyroxenites. Such a hypothesis derives from the study of garnet-bearing pyroxenite xenoliths from the Hyblean Plateau (Sicily). These consist of Al-diopside, pyralspite-series garnet, Al-spinel and Al-rich orthopyroxene. Trace element distribution resembles an enriched MORB but lower chromium. Major-element abundances closely fit in a tschermakitic-horneblende composition. Assuming that a high-Al amphibolite was formed by hydrothermal metasomatism of a troctolitic gabbro in a slow-spreading ridge segment, a transient temperature increasing induced dehydroxilization reaction in amphiboles, giving Al-spinel-pyroxenite and vapor as products. Garnet partially replaced spinel during an isobaric cooling stage. Density measurements at room conditions on representative samples gave values in the range 3290–3380 kg m−3. In general, a density contrast ≥300 kg m−3 can give rise to convective instability, provided a sufficient large size of the heavy masses and adequate rheological conditions of the system. Garnet-pyroxenite lumps can therefore sink in the underlying mantle, imparting the “garnet geochemical signature” to newly forming basaltic magma.

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Cumulating processes at the crust-mantle transition zone inferred from Permian mafic-ultramafic xenoliths (Puy Beaunit, France)

Berger

Féménias

Coussaert

et al. 2006

Contrib Mineral Petrol

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Magmatic garnet-bearing mafic xenoliths (Puy Beaunit, French Massif Central): P-T path from crystallisation to exhumation

Cited by 8 publications

References 58 publications

Dehydration, melting and related garnet growth in the deep root of the Amalaoulaou Neoproterozoic magmatic arc (Gourma, NE Mali)

Dehydration, melting and related garnet growth in the deep root of the Amalaoulaou Neoproterozoic magmatic arc (Gourma, NE Mali)

Convective Instability in Intraplate Oceanic Mantle Caused by Amphibolite-Derived Garnet-Pyroxenites—A Xenolith Perspective (Hyblean Plateau, Sicily)

Cumulating processes at the crust-mantle transition zone inferred from Permian mafic-ultramafic xenoliths (Puy Beaunit, France)

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