Jean-Robert Kienast

Ultrahigh-Pressure (60 Kbar) Magnesite-Bearing Garnet Peridotites from Northeastern Jiangsu, China

The very high P/T metamorphic belt in Chinas Shandong and fiangsu provinces, which has drawn much attention because of its numerous coesite-bearing eclogites, also contains little-studied massifs of garnet peridotites. Three samples, a chromite-bearing garnet peridotite (DZ01), a spinel-free garnet peridotite (DZ02) and a garnet websterite (DG01) from northeastern Jiangsu are studied here. Olivine compositions in peridotites are restricted to

Geochronology and metamorphic P-T-X evolution of the Eburnean granulite-facies metapelites of Tidjenouine (Central Hoggar, Algeria): witness of the LATEA metacratonic evolution

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Pressure‐Temperature‐Fluid Evolution in Eburnean Metabasites and Metapelites from Tamanrasset (Hoggar, Algeria)

. The spinel phase in DZ01 is mostly Fe-rich chromite, with increasing magnetite toward its rim. Garnets are pyropic; their rims are relatively enriched in almandine. Orthopyroxene is extremely poor in Al, reflecting the very high P conditions. Clinopyroxene is Cr-rich, Al-poor diopside. Magnesite and phlogopite in DZ01 are believed to have formed in the mantle but somewhat later than the other primary phases. Magnesite, with very low Sr, Ba, and K contents, displays breakdown reactions producing dolomite, calcite, and periclase. Titanoclinohumite and pentlandite have been observed. Amphibole and chlorite occur as secondary phases. Geothermobarometry yields extremely high P of equilibration for these rocks (65-45 kbar). The P-T paths followed by the two garnet peridotites were considerably lower in T than the normal continental geotherm. Evidence against partial melting during uplift of the peridotites is clear. Metasomatism and oxidation of the mantle source by a

Magmatic Underplating, Metamorphic Evolution and Ductile Shearing in A Mesozoic Lower Crustal‐Upper Mantle Unit (Tinaquillo, Venezuela) of the Caribbean Belt

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First report of ultrahigh-temperature sapphirine + spinel + quartz and orthopyroxene + spinel + quartz parageneses discovered in Al–Mg granulites from the Khanfous area (In Ouzzal metacraton, Hoggar, Algeria)

-bearing fluid are demonstrated. Oxygen fugacity in DZ01 evolves from +0.45 to + 1.37 log units relative to the FMQ buffer, with decreasing P and T. These results suggest that the garnet peridotites were derived from a mantle wedge overlying a subducted slab.

Integrating geologic and satellite radar data for mapping dome-and-basin patterns in the In Ouzzal Terrane, Western Hoggar, Algeria

Abstract Central Hoggar, within the Tuareg shield to the east of the West African craton, is known for its complexity owing to the interplay of the Eburnean and Pan-African orogenies. The Tidjenouine area in the Laouni terrane belongs to the LATEA metacraton and displays spectacular examples of granulite-facies migmatitic metapelites. Here, we present a detailed petrological study coupled with in situ U–Pb zircon dating by laser-ablation inductively coupled plasma mass spectrometry (ICP-MS) that allows us to constrain the relative role of the Eburnean and Pan-African orogenies and hence to constrain how the LATEA Eburnean microcontinent has been partly destabilized during the Pan-African orogeny; that is, its metacratonic evolution. These metapelites have recorded different metamorphic stages. A clockwise P–T evolution is demonstrated on the basis of textural relationships, modelling in KFMASH and FMASH systems and thermobarometry. The prograde evolution implies several melting reactions involving the breakdown of biotite and gedrite. Peak metamorphic P–T conditions of 860±50 °C and 7–8 kbar (M1) were followed by a decrease of pressure to 4.3±1 kbar and of temperature to around 700 °C, associated with the development of migmatites (M2). After cooling, a third thermal phase at c. 650 °C and 3–4 kbar (M3) occurred. U–Pb zircon laser ablation ICP-MS analysis allows us to date the protolith of the migmatites at 2151±8 Ma, the granulite-facies and migmatitic metamorphisms (M1–M2) at 2062±39 Ma and the medium-grade metamorphic assemblage (M3) at 614±11 Ma. This last event is coeval with the emplacement of large Pan-African granitic batholiths. These data show that the main metamorphic events are Eburnean in age. The Pan-African orogeny, in contrast, is associated mainly with medium-grade metamorphism but also mega-shear zones and granitic batholiths, characterized by a high temperature gradient. This can be considered as typical of a metacratonic evolution.

Prograde and retrograde evolution of eclogite from Adrar Izzilatène (Egéré-Aleksod terrane, Hoggar, Algeria) determined from chemical zoning and pseudosections, with geodynamic implications

Tamanrasset (Central Hoggar, Algeria) is part of the 2000 Ma Eburnean terrane in which spectacular examples of granulite‐grade metapelites and metabasites occur. Reactional textures in metabasites, supplemented by geothermobarometry, fluid inclusions, and mineral equilibria indicate a complex metamorphic history, characterized by a strong decompression during postpeak uplift. Peak metamorphic pressure‐temperature (PT) conditions of 800°C and 10 kbar, respectively, were followed by rapidly decreasing pressure (to 700°C and 6 kbar) at nearly constant temperature, during which time the rocks acquired their present mineral assemblages and textures (garnet pyroxenites). This isothermal decompression path was followed by nearly isobaric cooling. Throughout this history, water activity remained consistently low ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Mineral equilibrium modelling and calculated chemical potential relations of reaction textures in the ultrahigh-temperature In Ouzzal terrane (In Hihaou area, Western Hoggar, Algeria)

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