A hidden alkaline and carbonatite province of early carboniferous age in northeast Poland: Zircon U-Pb and pyrrhotite Re-Os geochronology

Extensive geophysical investigations in NE Poland in the 1950s and 1960s led to the discovery of an alkaline and carbonatite magmatic province buried under thick (600-800 m) Meso-Cenozoic cover north of the Trans-European Suture Zone, or Tornquist Line. Drilling focused on geophysical anomalies identified three intrusions in the Paleoproterozoic metasedimentary and metavolcanic rocks of the Mazowsze Domain: the Pisz gabbro-syenite massif, the Ełk syenite massif, and the small, differentiated Tajno body consisting of clinopyroxenite cumulates and syenites crosscut by carbonatite veins. Emplacement ages for these intrusions have been obtained by (1) zircon U-Pb geochronology on a gabbro from Pisz, a syenite from Ełk, and an albitite from Tajno and (2) a Re-Os model age for pyrrhotite from a Tajno carbonatite. The ages measured by both methods fall in the narrow range 354-345 Ma (Early Carboniferous: Tournaisian). This is slightly younger than the Late Devonian (380-360 Ma) Kola Peninsula alkaline and carbonatite province (20 intrusions) of NW Russia and Karelia but is of comparable age to the first manifestations of the long-lasting (~100 m.yr.) Carboniferous to Permian magmatic event (360-250 Ma) manifest in northern Europe (from the British Isles to southern Scandinavia, the North Sea, and northern Germany) in the foreland of the Variscan orogeny (in the so-called West European Carboniferous Basin) and the East European Craton

Contrasting styles of (U)HP rock exhumation along the Cenozoic Adria-Europe plate boundary (Western Alps, Calabria, Corsica)

Since the first discovery of ultrahigh pressure (UHP) rocks 30 years ago in the Western Alps, the mechanisms for exhumation of (U)HP terranes worldwide are still debated. In the western Mediterranean, the presently accepted model of synconvergent exhumation (e.g., the channel-flow model) is in conflict with parts of the geologic record. We synthesize regional geologic data and present alternative exhumation mechanisms that consider the role of divergence within subduction zones. These mechanisms, i.e., (i) the motion of the upper plate away from the trench and (ii) the rollback of the lower plate, are discussed in detail with particular reference to the Cenozoic Adria-Europe plate boundary, and along three different transects (Western Alps, Calabria-Sardinia, and Corsica-Northern Apennines). In the Western Alps, (U)HP rocks were exhumed from the greatest depth at the rear of the accretionary wedge during motion of the upper plate away from the trench. Exhumation was extremely fast, and associated with very low geothermal gradients. In Calabria, HP rocks were exhumed from shallower depths and at lower rates during rollback of the Adriatic plate, with repeated exhumation pulses progressively younging toward the foreland. Both mechanisms were active to create boundary divergence along the Corsica-Northern Apennines transect, where European southeastward subduction was progressively replaced along strike by Adriatic northwestward subduction. The tectonic scenario depicted for the Western Alps trench during Eocene exhumation of (U)HP rocks correlates well with present-day eastern Papua New Guinea, which is presented as a modern analog of the Paleogene Adria-Europe plate boundary

Minéralisations associées aux complexes mafiques-ultramafiques en domaine océanique et continental

Non disponible / Not availableCe mémoire de thèse comporte deux parties: l'une sur les ophiolites et l'autre sur les minéralisations de divers complexes basiques-ultrabasiques, dont les ophiolites, les komatiites et les sills archéens et protérozoïques. Les travaux sur les ophiolites ont abouti à deux résultats majeurs : 1) la reconstitution du puzzle ophiolitique corse ; 2) la diversité des ophiolites avec une classification en deux grands types, dont le magmatisme de l'un s'apparente à celui des rides médio-océaniques, tandis que le magmatisme de l'autre ressemble à celui des arcs immatures. Des études sur la minéralisation des corps basiques-ultrabasiques, il ressort que la nature des magmas parentaux est importante pour juger de leur potentialité métallogénique (Cr, Ni, éléments du groupe du platine EGP). Le caractère saturé à sursaturé des magmas basiques a été noté dans les ophiolites porteuses de gisements de chrome, dans les lopolites stratifiés enrichis en Cr, EGP. Des magmas de type boninitiques seraient à l'origine de certains de ces complexes. En conclusion de la thèse est présentée une classification des complexes basiques-ultrabasiques basées sur l'ordre de cristallisation des minéraux qui permet de discuter de l'origine des gisements des EGP par rapport à celle des autres minéralisations (chromite, concentration sulfurées)

Zincochromite from the Guaniamo river diamondiferous placers, Venezuela: evidence of its metasomatic origin

International audienceZincochromite associated with chromite and magnesiochromite was found in heavy-mineral concentrates from the Quebrada Grande placer deposit in the Guaniamo River diamondiferous district, Venezuela. It occurs in subhedral to anhedral grains from 0.1 to 0.7 mm in size. Reflectance measurements show an increase of reflectance values with FeFe2O4 content in spinel solid solution. The Vickers microhardness (load 100 g) is 1367(84) kg/mm(2). Zincochromite containing 60 mol.% ZnCr2O4 has a unit cell with a value of a equal to 8.271(3) angstrom. As revealed by EPMA, zincochromite grains are markedly zoned, with the rim and microfissures enriched in Zn. Furthermore, the chromite crystals partly replaced by zincochromite indicate high-temperature metasomatic processes involving a Zn- and Mn-bearing fluid phase. All chromian spinel compositions observed underwent, more or less intensely, this replacement. The chemical formula of the zincochromite richest in Zn from Guaniamo can be written: (Zn6.499Fe2+ Mn-0.613(2+) (0.511MgNi0.022)-Ni-0.434)(Sigma 8.079)(Cr(9.639A)l(4.742)Fe(3+) 1.262Ti0.213V (3+)(0.040))Sigma O-15.921(32). A large extent of Mn-for-Fe2+ substitution has been observed. In some grains, the number of Mn atoms per formula unit exceeds Fe2+. Statistically, taking into account the Fe3+ concentration, zincochromite was formed at higher oxygen fugacities than its chromite and magnesiochromite precursors. EPMA traverses of zoned zincochromite and Zn- bearing magnesiochromite crystals indicate the substitution mechanisms 3Zn -> 2(Fe2+ + Mn) + Mg and 4Zn -> (Fe2+ + Mn) + 3Mg, besides the Zn -> (Fe2+ + Mn) and Zn -> Mg replacements. Chromite and magnesiochromite contain low, but consistent Zn concentrations, which demonstrate an early Zn enrichment of the magmatic system. Their chemical compositions are compatible with chromian spinels occurring in diamondiferous kimberlites and lamproites. Considering the presence of Zn- bearing chromites in several kimberlite-bearing areas on a worldwide scale, the occurrence of zincochromite at Guaniamo points to its use as a mineral indicator of kimberlites

The Los Archipelago nepheline syenite ring-structure: A magmatic marker of the evolution of the central and equatorial atlantic

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Pétrologie, géochimie et genèse des gisements de tsavorite associés aux gneiss et roches calco-silicatées graphiteux de Lemshuku et Namalulu, Tanzanie

La tsavorite, grossulaire vert à V-Cr-Mn, est contenue dans des gneiss et roches calco-silicatées graphiteux, souvent associés à des marbres dolomitiques, et appartenant à la ceinture métamorphique néoprotérozoïque mozambicaine. La tsavorite se trouve soit dans des nodules ou des veines de quartz (gisements primaires), soit dans des placers (gisements secondaires). L'étude minéralogique des tsavorites propose un nouveau protocole de certification de leur origine géographique, à partir du rapport V/Cr, de la teneur en Mn et du [delta]18 O. L'étude des gisements de Lemshuku et Namalulu en Tanzanie montre que le métamorphisme des protolithes sédimentaires riches en matière organique et évaporites s'est effectué à P = 7,0 +- 0,4 kbar et T = 677 +- 14C, à 634 +- 22 Ma (datation U-Th-Pb sur monazite). Le bâti métamorphique s'est refroidi vers 500 Ma (datation 40Ar-39Ar sur muscovite). Deux stades de métasomatose sont reliés à la formation de la tsavorite : (i) une métasomatose de diffusion formant les nodules à P = 5,0-7,4 kbar et T = 580-691C; (ii) une métasomatose calcique d'infiltration contemporaine de la formation des veines de quartz à P = 3,6-4,9 kbar et T = 505-587C. Ces dernières sont datées in situ par la méthode Sm-Nd à 606 +- 36 Ma. Les évaporites continentales, déposées dans une sabkha de côte marine avec des sédiments silico-calcaires, sont transformées en tsavorite dans le cas des nodules, alors que les sels fondus sont associés à la formation des veines de quartz. Les minéralisations sont contrôlées par la lithostratigraphie et la tectoniqueTsavorite, a (V, Cr, Mn)-bearing green grossular, is hosted by graphitic gneisses or calc-silicates, often asssociated with dolomitic marbles, and belonging to the Metamorphic Neoproterozoic Mozambique Belt. Tsavorite is found either as nodules or in quartz veins (primary deposits), or in placers (secondary deposits). The mineralogical study of tsavorites suggests a new protocol to certificate their geographical origin, based on the V/Cr ratio, Mn content and delta18O. The study of the Lemshuku and Namalulu deposits in Tanzania has shown that the metamorphism of organic matter-rich and evaporites-rich sedimentary protoliths occurred at P = 7.0 +- 0.4 kbar and T = 677 +- 14C, at 634 +- 22 Ma (U-Th-Pb dating on monazite). The metamorphic series cooled down at around 500 Ma (40Ar-39Ar dating on muscovite). Two metasomatic stages are linked to the formation of tsavorite : (i) diffusion metasomatism forming nodules at P = 5.0-7.4 kbar and T = 580-691C; (ii) calcitic infiltration metasomatism forming quartz veins at P = 3.6-4.9 kbar and T = 505-587C. These last have been dated in situ with Sm-Nd dating at 606 +- 36 Ma. Continental evaporites, deposited in a coastal marine sabkha with (Si, Ca)-bearing sediments, transformed into tsavorite in the case of the nodules, while the molten salts are associated with the formation of the quartz veins. The mineralisations are controlled by lithostratigraphy and structureMETZ-SCD (574632105) / SudocNANCY1-Bib. numérique (543959902) / SudocNANCY2-Bibliotheque electronique (543959901) / SudocNANCY-INPL-Bib. électronique (545479901) / SudocSudocFranceF

Origin of micro-layering in a deep magma chamber: evidence from two ultramafic-mafic layered xenoliths from Puy Beaunit (French Massif Central)

The origin of magmatic layering is still hotly debated. To try to shed some light on this problem, two ultramafic-mafic layered xenoliths from Puy Beaunit (French Massif Central) were investigated in detail. The nodules belong to a stratiform intrusion emplaced in the deep crust during the Permian (257 ± 6 Ma; Féḿnias, O. Coussaert, N. Bingen, B. Whitehouse, M. Mercier, J.-C. Demaiffe, D. 2003. A Permian underplating event in late- to post-orogenic tectonic setting. Evidence from the mafic-ultramafic layered xenoliths from Beaunit (French Massif Central). Chem. Geol. 199 293-315). The 3 to 5 cm thick nodules have, in common, a central orthopyroxenite layer; the succession of layers is, respectively, norite-orthopyroxenite-norite (PBN 00-01) and norite-orthopyroxenite-harzburgite (PBN 00-03). The variations of both major (by electron microprobe) and trace, essentially the RE, elements (by LA-ICP-MS) were measured in major mineral phases (orthopyroxene, clinopyroxene, plagioclase, spinel) along cross-section perpendicular to the layering. Strong grain size, chemical and textural variations occur along these sections: they can be continuous or discontinuous, symmetrical or asymmetrical. Such complex variations cannot be solely related to a single magmatic history (fractional crystallisation, mineral sorting). Other processes such as element enrichment by residual liquid channelling along layer boundaries and/or sub-solidus recrystallisation and element redistribution must be invoked. It appears, in particular, that element distribution in the central orthopyroxenite layer could result from the injection of micro-sills of orthopyroxene-rich liquid between previously consolidated layers. © 2005 Elsevier B.V. All rights reserved.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Emplacement of the Meugueur- Meugueur cone-sheet (Niger, W.Africa),one of the world's largest igneous ring-structures

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