| 1. |
- Batki, Aniko, et al.
(författare)
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Petrogenetic significance of ocellar camptonite dykes in the Ditrau Alkaline Massif, Romania
- 2014
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 200, s. 181-196
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Tidskriftsartikel (refereegranskat)abstract
- Camptonite dykes intrude the rift-related Mesozoic igneous body of the Ditrau Alkaline Massif, Eastern Carpathians, Romania. We present and discuss mineral chemical data, major and trace elements, and the Nd isotopic compositions of the dykes in order to define their nature and origin. The dykes are classified as the clinopyroxene-bearing (camptonite-I) and clinopyroxene-free (camptonite-II) varieties. Camptonite-I consists of aluminian-ferroan diopside phenocrysts accompanied by kaersutite, subordinate Ti-rich annite, albite to oligoclase and abundant calcite-albite ocelli. Camptonite-II comprises K-rich hastingsite to magnesiohastingsite, Ti-rich annite, albite to andesine, abundant accessory titanite and apatite, and silicate ocelli filled mainly with plagioclase (An(4-34)). Age-corrected Nd-143/Nd-144 ratios vary from 0.51258 to 0.51269. The high epsilon(Nd) values of +4.0 to +6.1 which are consistent with intra-plate composition, together with light rare earth element (LREE), large ion lithophile element (LIE) and high field strength element (HFSE) enrichment in the camptonites is ascribed to the formation of small melt batches of a metasomatised sub-lithospheric mantle source. The presence of an asthenospheric 'high mu' ocean island basalt (HIMU-OIB)-type mantle component in the source region has also been revealed. A 1-4% degree of partial melting of an enriched garnet Iherzolite mantle source containing pargasitic amphibole followed by fractionation is inferred to have been involved in the generation of the camptonites. They are deduced to be parental melts to the Ditrau Alkaline Massif.
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| 2. |
- Bejgarn, Therese, et al.
(författare)
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Palaeoproterozoic porphyry Cu-Au, intrusion-hosted Au and ultramafic Cu-Ni deposits in the Fennoscandian Shield: Temporal constraints using U-Pb geochronology
- 2013
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 174, s. 236-254
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Tidskriftsartikel (refereegranskat)abstract
- The Skellefte district, northern Sweden, is known for the occurrence of 1.89 Ga Palaeoproterozoic volcanogenic massive sulphide (VMS) deposits. The deposits are hosted by the older part of a volcanosedimentary succession, which was intruded at 1.88-1.86 Ga by multiple phases of the syn-volcanic, early orogenic Jorn intrusive complex (JIC). The oldest phase of the JIC hosts different styles of mineralisation, among them porphyry Cu-Mo-Au, intrusion-related Au, and mafic-hosted Fe and Cu-Ni deposits. To discriminate between the different intrusive and ore related events, U-Pb ages of zircons have been obtained for nine intrusive phases and from Na-Ca alteration spatially related to mineralisation, while U-Pb ages of baddeleyite (ZrO2) have been used to constrain intrusive ages of three mineralised and barren mafic-ultramafic intrusive rocks. The two main JIC intrusive phases of a granodioritic-tonalitic composition in the southern study area intruded at 1887 +/- 3 Ma and 1886 +/- 3 Ma, respectively, and were succeeded by the intrusion of layered mafic-ultramafic intrusive rocks in the northern and southern study area at 1879 +/- 1 Ma and 1884 +/- 2 Ma, respectively. Emplacement of porphyry dykes took place at ca. 1877 Ma in the southern, western and northern JIC. The dykes are spatially and temporally associated with formation of porphyry style mineralisation, alteration and Au-mineralisation, as inferred from 1879 +/- 5 Ma zircons in adjacent Na-Ca alteration zones. High SiO2 and Al2O3 contents together with high Sr/Y ratios, mingling structures, mafic xenoliths and hornblende phenocrysts in the porphyry dykes suggest that the magma originated from hydrated partial melts, possibly from the base of the crust at a mature stage of subduction. Local extension resulted in intrusion of mafic-ultramafic rocks around 1.88 Ga prior to and after, the porphyry dykes and associated mineralisation, approximately 10 Ma after the formation of the spatially related 1.89 Ga VMS deposits in the Skellefte district. This 1.88 Ga event correlates with other 1.88 Ga mafic-ultramafic units widespread around the world, and could possibly be interpreted as a large scale response to supercontinent formation. (C) 2012 Elsevier B.V. All rights reserved.
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| 3. |
- Broska, Igor, et al.
(författare)
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Devonian/Mississippian I-type granitoids in the Western Carpathians : A subduction-related hybrid magmatism
- 2013
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 162, s. 27-36
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Tidskriftsartikel (refereegranskat)abstract
- SIMS zircon U-Pb dating of I-type granitoids from four Variscan crystalline basement outcrops in the Western Carpathians (Tribec, Nizke Tatry, Cierna Hora and Slovenske Rudohorie Mts.) implies that they originated between 367 and 353 Ma. Therefore, they belong to the earliest Variscan I-type granitoids. The oldest Devonian age at ca. 367-364 Ma is obtained from an enclave-bearing tonalite and associated dikes of the Tribe Mts. Several granodiorites from the Nizke Tatry, Vepor and Cierna Hora Mts. (353-357 Ma) show younger, Early Carboniferous ages. These data require a re-assessment of older models that explained the genesis of I-type granitoids in the Western Carpathians based on the assumption of younger, Middle to Upper Carboniferous ages. The I-type granite massifs of the Western Carpathians most probably originated in an arc-related environment within the Galatian superterrane, an assemblage of Gondwana derived fragments. The early age of I-type magmatism in the Western Carpathians marks the beginning of a north-dipping subduction of the Paleotethys ocean under Ibero-Ligerian and intra-Alpine terranes. We suggest a term "Proto-Tatricum" for that part of the Galatian superterrane where Devonian/Mississippian I-type granitoids were emplaced. Now the granitoids are incorporated as a part of crystalline basement into the Alpine Tatric and Veporic units within present West-Carpathian mountain chain.
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| 4. |
- Jastrzebski, Miroslaw, et al.
(författare)
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Constraints on the Devonian-Carboniferous closure of the Rheic Ocean from a multi-method geochronology study of the Staré Město Belt in the Sudetes (Poland and the Czech Republic)
- 2013
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 170-171, s. 54-72
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Tidskriftsartikel (refereegranskat)abstract
- This paper attempts to clarify the complex nature of how and when the Rheic Ocean closed in what is now Central Europe and, with respect to the various terranes that were involved, offer a regional chronology for the associated structural, metamorphic and igneous processes that accompanied and followed this closure. The Variscan orogen in Europe originated from the multiple collisions of Gondwana-derived terranes (the Armorican Terrane Assemblage) with Laurussia: however, many important structural details on the timing of these collision-related events remain obscure. In the Sudetes, the Stare Mesto Belt represents a WNW-dipping part of the Rheic suture that developed from the continental collision of the eastern terranes of the Armorican Terrane Assemblage (now in the Bohemian Massif) with the Brunovistulian Terrane (a part of Laurussia/Old Red Continent). In this study, the results of monazite Th-U-total Pb, garnet Lu-Hf and zircon U-Pb geochronology were integrated into a newly established D1-D3 tectonometamorphic sequence. A Th-U-total Pb age of similar to 368 Ma from a monazite that grew concurrently with D2 metasedimentary garnet, as well as Lu-Hf ages of similar to 361 Ma and similar to 355 Ma obtained from D2 metasedimentaly garnets, implies that the regional contractional deformation and progressive metamorphism of D2 took place mainly during the Famennian (Late Devonian) and extended into the Visean (Middle Mississippian of the Early Carboniferous). The ion probe U-Pb zircon ages of similar to 355 Ma and similar to 359 Ma obtained from leucocratic neosomes in migmatized amphibolites confirmed a lag in the peak temperature that followed crustal thickening during D1-D2. Metamorphic monazites dated at similar to 340 Ma provide a time for the subsequent D3 dextral transpressional regime. The closure of the Stare Mesto Belt segment of the Rheic Ocean probably resulted from a head-on hard collision between the westerly subducting Brunovistulian promontory of Laurussia and the eastern members of the Armorican Terrane Assemblage. Thus, the Rheic Ocean closed during the Late Devonian at similar to 370-360 Ma and preceded the collision of the Armorican Terrane Assemblage with East Avalonia at the western margin of what is now the Bohemian Massif. Following ocean closure, the Rheic slab may have broken off, resulting in the suture zone becoming dominated by lateral "tectonic escape" movements of the colliding terranes at similar to 340-330 Ma (Visean). Syntectonic D3 intrusions of granodiorite/tonalite magma acted as a hot lubricant and stitched the suture zone together.
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| 5. |
- Mattsson, Hannes B., et al.
(författare)
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Petrogenesis of the melilititic and nephelinitic rock suites in the Lake Natron-Engaruka monogenetic volcanic field, northern Tanzania
- 2013
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 179, s. 175-192
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Tidskriftsartikel (refereegranskat)abstract
- The Lake Natron-Engaruka Monogenetic Volcanic Field (LNE-MVF) in northern Tanzania consists of more than 150 vents of Upper Pleistocene to Holocene age that are scattered over an area of 2500 km(2). Here we describe the petrological characteristics of these eruptions in detail and link the magma chemistry to eruptive behavior when the magmas reach the surface. Erupted magmas are predominantly of melilititic or nephelinitic compositions (70 and 25%, respectively), together with minor amounts of basanites (5%). The melilititic magmas form by small degrees (1-2%) of partial melting of a metasomatized upper-mantle source (containing 1-4% gamet together with both amphibole and phlogopite). The melilitites ascend very rapidly through the lithosphere prior to eruption minimizing the effect of fractional crystallization and/or crustal contamination. These eruptions also frequently carry relatively large amounts of mantle debris to the surface which is also reflected in their bulk-rock compositions. The nephelinitic rock suite, on the other hand, forms by larger degrees of melting (2-4%) at higher levels of the sub-continental lithosphere containing less garnet (<< 1%). The scarcity of mantle debris in the nephelinitic eruption deposits, combined with the more evolved magma chemistry, indicates ponding in crustal reservoirs en-route to the surface. For many of the nephelinitic magmas this ponding resulted in fractional crystallization of predominantly olivine, which is also one of the principal phenocryst phases in these rocks. However, these periods of residence in the crust must have been short as none of the investigated rocks show any clear evidence of being affected by crustal contamination. Within the LNE-MVF a rough correlation between magma chemistry and resulting volcanic landforms is recognized. Large maar volcanoes and tuff cones/rings are predominantly of melilititic composition, whereas the nephelinites typically form scoria cones. This is attributed to the fact that melilititic magmas can hold more CO2 dissolved in the liquid compared to nephelinites, in combination with a rapid ascent from the upper mantle to the surface for the melilitites (<1-2 days). We interpret the violent exsolution of CO2 (in response to rapid decompression) to be responsible for the higher explosivity of the melilititic eruptions compared to the nephelinitic magmas within the LNE-MVF. (C) 2013 Elsevier B.V. All rights reserved.
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| 6. |
- Merle, Renaud E., 1976-, et al.
(författare)
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40Ar/39Ar ages and Sr–Nd–Pb–Os geochemistry of CAMP tholeiites from Western Maranhão basin (NE Brazil)
- 2011
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Ingår i: Lithos. - 0024-4937 .- 1872-6143. ; 122:3, s. 137-151
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Tidskriftsartikel (refereegranskat)abstract
- The Central Atlantic Magmatic Province (CAMP), emplaced at the Triassic–Jurassic (T–J) boundary (~200Ma), is among the largest igneous provinces on Earth. The Maranhão basin in NE Brazil is located around 700km inland and 2000km from the site of the earliest Pangea disruption. The CAMP tholeiites occur only in the western part of the basin and have been described as low and high-Ti. Here we document the occurrence of two sub-groups among the high-Ti tholeiites in the Western Maranhão basin. The major and trace elements and the Sr–Nd–Pb isotopic ratios define three chemical groups corresponding to the low-Ti (TiO2<1.3wt.%), high-Ti (TiO2~2.0wt.%) and evolved high-Ti (TiO2>3wt.%) western Maranhão basin tholeiites (WMBT). The new 40Ar/39Ar plateau ages obtained on plagioclase separates for high-Ti (199.7±2.4Ma) and evolved high-Ti WMBT (197.2±0.5Ma and 198.2±0.6Ma) are indistinguishable and identical to those of previously analyzed low-Ti WMBT (198.5±0.8Ma) and to the mean 40Ar/39Ar age of the CAMP (199±2.4Ma). We also present the first Re–Os isotopic data for CAMP basalts. The low and high-Ti samples display mantle-like initial (187Os/188Os)i ranging from 0.1267 to 0.1299, while the evolved high-Ti samples are more radiogenic ((187Os/188Os)i up to 0.184) We propose that the high-Ti WMBT were derived from the sub-lithospheric asthenosphere, and contaminated during ascent by interaction with the subcontinental lithospheric mantle (SCLM). The evolved high-Ti WMBT were derived from the same asthenospheric source but experienced crustal contamination. The chemical characteristics of the low-Ti group can be explained by partial melting of the most fertile portions of the SCLM metasomatized during paleo-subduction. Alternatively, the low-Ti WMBT could be derived from the sub-lithospheric asthenosphere but the resulting melts may have undergone contamination by the SCLM. The occurrences of high-Ti basalts are apparently not restricted to the area of initial continental disruption which may bring into question previous interpretations such as those relating high-Ti CAMP magmatism to the initiation of Atlantic ridge spreading or as the expression of a deep mantle plume. We propose that the CAMP magmatism in the Maranhão basin may be attributed to local hotter mantle conditions due to the combined effects of edge-driven convection and large-scale mantle warming under the Pangea supercontinent. The involvement of a mantle-plume with asthenosphere-like isotopic characteristics cannot be ruled out either as one of the main source components of the WMBT or as a heat supplier.
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| 7. |
- Naeraa, Tomas, et al.
(författare)
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A lower crustal mafic source for the ca. 2550 Ma Qorqut Granite Complex in southern West Greenland
- 2014
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 192, s. 291-304
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Tidskriftsartikel (refereegranskat)abstract
- The late Neoarchaean Qorqut Granite Complex is the youngest large igneous intrusion in the Nuuk region in southern West Greenland, where basement is primarily of Eoarchaean and Mesoarchaean age with a tonalite-trondhjemite-granodiorite (TTG) composition. The Qorqut granite is generally undeformed and it intruded during a prolonged period, starting at ca. 2730 Ma, characterised by crustal reworking, possibly related to syn- or post accretion tectonics or continental collision. We present major and trace element whole rock chemistry and combined U/Pb, Hf and O isotope data from zircon. We obtained a mean zircon U/Pb age of 2547 +/- 4 Ma (MSWD = 0.63). Initial sHf values range from - 12 to -18 requiring a long residence time and a rather homogeneous source. Sample averaged zircon delta O-18 values range from 6.1 +/- 0.2%. to 6.5 +/- 0.3/0.7%o best interpreted with a source region of mainly unweathered mantle derived igneous rocks. Compared to the regional TTG basement, the QGC is characterised by low CaO and Na2O and high K2O, LREE and Rb contents, and a stronger fractionated REE pattern with a negative Eu anomaly. We show that the homogeneous Hf isotope signature of the granite together with its low epsilon value and its pristine oxygen isotope composition are best explained with an Eoarchaean mafic source with a Lu-176/Hf-176 around 0.015-0.019. Trace element modelling confirms that a mafic source in REE and with an eclogitic residue and with plagioclase as a fractionating phase would generate appropriate melt compositions. Modelling requires residual rutile in the source which constrain the pressures to > ca. 13-18 kbar. Zirconium saturation temperatures suggest magma temperatures in the range 750-850 degrees C. The obtained P-T conditions suggest a lower crustal source region in a thickened crustal unit consistent with a post or late continental collisional setting. (C) 2014 Elsevier B.V. All rights reserved.
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| 8. |
- Petersson, Jesper, et al.
(författare)
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Albitization and quartz dissolution in Paleoproterozoic metagranite, central Sweden - Implications for the disposal of spent nuclear fuel in a deep geological repository
- 2012
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 148, s. 10-26
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Tidskriftsartikel (refereegranskat)abstract
- Hydrothermal alteration resulting in albitization and quartz dissolution has been identified in Paleoproterozoic metagranites down to - 1000 m elevation at Forsmark, Sweden. The alteration features were discovered during investigations to locate a site for the disposal of spent nuclear fuel in a deep geological repository. In general, albitization occurs extensively, but it is also observed locally adjacent to minor intrusive bodies of amphibolite. The altered rocks show a marked decrease in K-feldspar and an increase in quartz relative to the unaltered equivalents, resulting in an epitonalitic composition. Plagioclase is metamorphic in character and generally richer in albite than in the unaltered rocks. It is inferred that albitization was triggered by the input of basic or intermediate melts into the crust during igneous activity close to the peak of regional metamorphism at 1.87-1.86 Ga. The mineralogy of the epitonalites gives rise to an increased thermal conductivity and, thereby, a positive influence for the design and safety of a deep geological repository for spent nuclear fuel. However, the increased frequency of low conductive amphibolite in the albitized volumes, consistent with the proposed mechanism for alteration, gives a negative influence. In sharp contrast to the albitization, a majority of the occurrences of quartz dissolution, which resulted in the formation of episyenite, are located along fracture zones. Quartz dissolution took place between or after 1.8-1.7 Ga, when the bedrock was able to respond to deformation in a brittle manner. Most of the vugs left after the removal of quartz are, to a variable extent, refilled by hydrothermal assemblages, including quartz, albite, K-feldspar, hematite, chlorite and calcite. The geometry and spatial distribution of episyenite argue against an extreme fluid/rock ratio and it is inferred that the fluids had at least a moderate salinity with a temperature in excess of 300 degrees C. The dissolution process was promoted by the generation of secondary permeability localized in columnar or pipe-like volumes. The close spatial connection to fracture zones provides a basis to avoid bedrock affected by this type of alteration and, thereby, reduce the negative mechanical and hydrogeological aspects for a deep geological repository. (C) 2012 Elsevier B.V. All rights reserved.
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| 9. |
- Petersson, Jesper, et al.
(författare)
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Imprints of multiple fluid regimes on episyenites in the Bohus granite, Sweden
- 2014
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 196, s. 99-114
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Tidskriftsartikel (refereegranskat)abstract
- A number of episyenite occurrences within the Neoproterozoic Bohus granite, southwestern Sweden, were investigated in order to elucidate their genesis and past associated fluid regimes. Fluid inclusion microthermometry and O-H isotope systematics integrated with information from detailed mineralogy reveal a paragenetic succession within individual episyenites, including: (1) albitization of plagioclase, accompanied by dissolution of quartz, with a vuggy feldspathic reservoir rock as a result; (2) resumed infilling and replacement of igneous Fe-Ti-phases by various phyllosilicates (muscovite, chlorite and/or illite) +/- hematite assemblages and quartz, due to rift-related reactivation and (3) sporadic argillitic alteration and precipitation of fracture calcite. Although sparse owing to later overprinting, there are a few remnants that provide arguments for the involvement of meteoric water, shifted to a more O-18-rich composition, during the initial stage of dissolution and albitization. The main vug-filling phase is proposed to be associated with the ingress of surface-derived fluids with negative delta O-18 values, during the regional thermal anomaly accompanying the development of the nearby Permo-Carboniferous Oslo Rift at similar to 250 Ma The fluids involved during this event show consistently a marked, but variable O-18-depletion and resemble the modern-day shield brines, with modal salinities of 11-24 wt.% NaCl eq., low NaCl/(NaCl + CaCl2) weight ratios and no discernible CO2. Even though no P-T path can be defined for this phase, the fluid pressure can be restricted to 03-0.8 kbar and temperature to 160-280 degrees C, assuming a sedimentary overburden of similar to 3 km. An overall variability in the delta O-18 and delta D data can mainly be ascribed to (1) precipitation from fluids affected by isotopic exchange with the host rock and/or sedimentary overburden under highly variable water/rock ratios, and to some extent with (2) varying degrees of partial re-equilibration during subsequent hydrothermal interaction at lower temperatures, as emphasized by the presence of fluid inclusions of suspected secondary origin in quartz.
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| 10. |
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