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

Structurally-controlled hydrothermal alteration in the syntectonic Neoproterozoic Upper Ruvubu Alkaline Plutonic Complex (Burundi): Implications for REE and HFSE mobilities

International audienceThe Neoproterozoic Upper Ruvubu Alkaline Plutonic Complex (URAPC), Burundi, is located along the western branch of the East African Rift. It comprises oversaturated and undersaturated syenites and a shallow level carbonatite body (the Matongo carbonatite) that does not outcrop but has been sampled by drill-cores. The elliptic map contour of the URAPC points to a syntectonic emplacement. Large shear zones that were active during magmatic emplacement have accommodated a regional NE-SW shortening. Mineralization features of late-magmatic to hydrothermal origin are associated with the carbonatite, which, by itself, contains a dense network of calcitic veins. HFSE mineralization occurring as zircon and ilmenite megacrysts can be found in an area of intense and extensive K-fenitization, which lead to the transformation of the surrounding syenite into a dominant K-feldspar + biotite mineral assemblage (Inamvumvu area). Carbonatitic dykes (overprinted by a hydrothermal alteration) are present a few kilometers north of the Matongo carbonatite, within highly deformed zones in the syenite. These dykes occur along with Na-fenites (resulting from the transformation of the feldspathoidal syenite into an albite-dominant paragenesis) and are enriched in REE-minerals (monazite and ancylite-(Ce)). Many magmatic (pegmatoid) dykes and hydrothermal (quartz + hematite) veins also occur in shear zones in the URAPC. Most of them can be interpreted as tension gashes. The chondrite-normalized REE patterns of some carbonatite whole rock samples are highly disturbed, in relation to post-magmatic hydrothermal alteration. The HFSE and REE distribution in the minerals from the hydrothermal veins/dykes (calcitic veins within the carbonatite, carbonatite dykes overprinted by a hydrothermal alteration in deformed zones, and zircon and ilmenite megacrysts) attests for a complex behaviour of REE during alteration. Oxygen and carbon isotope compositions of the Matongo carbonatite and the carbonatitic dykes have a magmatic signature, with 7.2 < δ18O (vs. SMOW) < 8.5‰ and -4.7 < δ13C (vs. PDB) < -5.4‰ in agreement with the Sr isotopic composition. The oxygen isotope composition of zircon and ilmenite megacrysts (δ18OZr = 4 to 4.7‰, δ18OIlm = -4.3 to -1.5‰ respectively) also point to a magmato-hydrothermal origin of the forming fluids. Some samples of the Matongo carbonatite and the carbonatitic dykes, with high δ18O values (δ18O = 8.6 to 21.8‰), show evidence of a medium- to low-temperature hydrothermal alteration event by an aqueous fluid. Calcitic veins in the carbonatite record another alteration event, outlined by the co-variation of δ18O and δ13C values (δ18O = 16.3 to 24.7‰ and δ13C = -4.7 to 0.2‰), implying the involvement of a mixed H2O-CO2 fluid. As a whole, the circulation of fluids in the URAPC was initiated during magmatic emplacement and the geometry of this circulation was controlled by the syn-emplacement crustal scale shear zones. Element mobility, one expression of which being the mineralization features described here, follow the same scheme

Etude pétrologique de l'apophyse sud-est du massif de Bjerkrem-Sogndal (Norvège méridionale)

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Les kimberlites d'Afrique Centrale: pétrologie, géochimie et intérêt économique

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De l'origine des anorthosites: pétrologie, géochimie et géochimie isotopique des massifs anorthositiques d'Hidra et de Garsaknatt ( Rogaland - Norvège mériodionale)

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La tectonique des plaques: Une révolution dans les sciences de la terre

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Le magmatisme alcalin et carbonatitique: synthèse sur la province paléozoïque de Kola (Russie) et caractéristiques générales du massif protérozoïque de Matongo (Burundi)

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Rare earth element geochemistry and strontium isotopic composition of a massif-type anorthositic-charnockitic body: the Hidra Massif (Rogaland, SW Norway)

The Hidra Massif (Rogaland Complex, SW Norway) mainly consists of plagioclase cumulates (anorthosites and leuconorites), which grade progressively into a fine-grained (200 μm). locally porphyritic, jotunitic rock towards the contact with the granulite facies gneisses. The massif is cross-cut by thin (10 cm up to 1 m) charnockitic dykes. The petrographical and geochemical evolution of the Hidra Massif can be explained by fractional crystallization of a jotunitic parental magma. Major and trace element constraints indicate that mafic phases are underabundant in the exposed levels of the massif, most likely as a result of plagioclase flotation in the early stages of solidification. Partitioning into the cumulate minerals (mainly plagioclase and orthopyroxene) governs the trace element contents of the leuconoritic adcumulates. However, the trace element geochemistry of the apparently early formed anorthositic orthocumulates largely depends upon the amount of a trapped intercumulus liquid. On the basis of trace element abundances (high REE, Rb, Th, U; negative Eu anomalies) the silicic charnockitic dykes can be considered as the residual liquid of the anorthositic fractionation trend. The higher initial 87Sr86Sr ratios (0.7086 ± 0.0006 vs 0.7055 ± 0.0004 for the plagioclase cumulates and jotunites) point to contamination of the charnockitic liquids by surrounding gneissic material. © 1981.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Strontium-isotopic geochemistry of the Mbuji Mayi and Kundelungu kimberlites (Zaire, Central Africa)

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Pb isotope geochemistry of the Hidra anorthositic-charnockitic massif (Rogaland, S.W. Norway)

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