| 1. |
- Janak, M., et al.
(författare)
-
Eclogite-hosting metapelites from the Pohorje Mountains (Eastern Alps): P-T evolution, zircon geochronology and tectonic implications
- 2009
-
Ingår i: European Journal of Mineralogy. - : Schweizerbart. - 0935-1221.
-
Tidskriftsartikel (refereegranskat)abstract
- Abstract: Phase-equilibrium modelling, geothermobarometry, ion-microprobe dating and mineral chemistry of zircon have been used to constrain the P–T–t evolution of metapelitic kyanite-bearing gneisses from the ultrahigh-pressure (UHP) metamorphic terrane of the Pohorje Mountains in the Eastern Alps. These eclogite-hosting rocks are part of the continental basement of the Austroalpine nappes. Based on calculated phase diagrams in the system Na2O-CaO-K2O-FeO-MgO-MnO-Al2O3-SiO2-H2O (NCKFMMnASH) and conventional geothermobarometry, the garnet-phengite-kyanite-quartz assemblages of gneisses record metamorphic conditions of 2.2–2.7 GPa at 700–800 C. These are considered as minima because of the potential for a diffusion-related modification and reequilibration of the garnet and phengite during early stages of decompression. It is therefore most likely that the gneisses experienced the same peak UHP metamorphism at 3 GPa as associated kyanite eclogites. Decompression and cooling to 0.5 GPa and 550 C led to the consumption of garnet and phengite, and the development of matrix consisting of biotite, plagioclase, K-feldspar sillimanite and staurolite. Textures and phase diagrams suggest a low extent of partial melting during decompression. Cathodoluminescence images as well as zircon chemistry reveal cores encompassed by two types of metamorphic zircon rims. Ion probe U-Pb dating of three zircon cores yielded Permian (286 10, 258 7 Ma) and Triassic (238 7 Ma) concordia ages. The zircon rims are Cretaceous with a mean concordia age of 92.0 0.5 Ma and some cores gave a similar age. The Cretaceous zircons all exhibit very low Th/U ratio (,0.02) typical of metamorphic origin. In these zircons, nearly flat HREE patterns, (Lu/Gd)N ¼ 1–4, and only small negative Eu anomalies indicate formation in the presence of garnet and absence of plagioclase, which is corroborated by occurrence of Mg- and Ca-rich garnet inclusions. Therefore, these zircons are interpreted to record the Cretaceous HP/UHP metamorphism. The 92.0 0.5 Ma age obtained in this study agrees with that (93–91 Ma) determined earlier in the Pohorje eclogites from U/Pb zircon, Sm-Nd and Lu-Hf garnet-whole-rock dating. This implies that the eclogites and their country rocks were subducted and exhumed together as a coherent piece of continental crust. There is no evidence for a me´lange-like assemblage of rocks, which followed different P–T–t paths, or several subduction and exhumation cycles as proposed for some other UHP metamorphic terranes.
|
|
| 2. |
- Holtstam, Dan, 1963-, et al.
(författare)
-
Ferri-taramite, a new member of the amphibole supergroup, from the Jakobsberg Mn-Fe deposit, Varmland, Sweden
- 2022
-
Ingår i: European Journal of Mineralogy. - : Copernicus GmbH. - 0935-1221 .- 1617-4011. ; 34:5, s. 451-462
-
Tidskriftsartikel (refereegranskat)abstract
- Ferri-taramite (IMA CNMNC 2021-046), ideally Na-A(B) (CaNa)(C) (Mg3Fe23+)(Si6Al2)O-22(W) (OH)(2), occurs in skarn from the Jakobsberg manganese mine, Varmland, Sweden. Associated minerals are celsian, phlogopite, aegirine-augite, andradite, hancockite, melanotekite, microcline (var. hyalophane), calcite, baryte, prehnite, macedonite and oxyplumboromeite. Conditions of formation, close to peak metamorphism (at circa 650 degrees C and 0.4 GPa), include silica undersaturation, a slightly peralkaline character and relatively high oxygen fugacities. Ferri-taramite forms poikiloblastic crystals up to 5 mm and is dark brownish black with a yellowish grey streak. The amphibole is brittle with an uneven to splintery fracture. Cleavage parallel to {110} is good. Hardness (Mohs) is similar to 6, and D-calc = 3.227(5) g cm(-3). Holotype ferri-taramite has the experimental unit formula (A)(Na0.79K0.16Pb0.01)(Sigma 0.96)(B) (Ca1.26Na0.72Mn0.022+)(Sigma 2)(C )(Mg2.66Mn0.582+ Fe0.162+Zn0.02Fe1.263+ Al0.26Ti0.06)(Sigma)(T)(5.00) (Al1.86Si6.14)Sigma 8O(22)(W) (OH)(2), based on chemical analyses (EDS, laser-ablation ICP-MS) and spectroscopic (Mossbauer, infrared) and single-crystal X-ray diffraction data. The mineral is optically biaxial (-), with alpha = 1.670(5), beta = 1.680(5) and gamma = 1.685(5) in white light and 2 V-meas = 70(10)degrees and 2 V-calc = 70.2 degrees. Ferri-taramite is distinctly pleochroic in transmitted light, with X pale yellow, Y dark brown, Z yellowish brown and absorption Y> Z> X. The eight strongest reflections in the X-ray powder pattern (d values (in angstrom), I-rel, hkl) are 8.44, 60, 110; 3.392, 25, 131; 3.281, 39, 240; 3.140, 100, 310; 2.816, 45, 330; 2.7104, 38, 151; 1.3654, 26, 461; and 1.4451, 33, (6) over bar 61. Refined unit-cell parameters from single-crystal diffraction data are a = 9.89596(13), b = 18.015(2), c = 5.32164(7) angstrom, beta = 105.003(13)degrees and V = 916.38(2) angstrom(3) for Z = 2. Refinement of the crystal structure yielded R = 2.26 % for 2722 reflections with I-0 >2 sigma (I). The Mn2+ and Fe2+ ions show preference for the M1 and M3 octahedrally coordinated sites, whereas Fe3+ is strongly ordered at M2. The A-group cations, K and Na, are split over two subsites, A (m) and A(2), respectively.
|
|
| 3. |
- Karlsson, Andreas, 1986-, et al.
(författare)
-
Adding complexity to the garnet supergroup: monteneveite, Ca3Sb5+2(Fe3+2Fe2+)O12, a new mineral from the Monteneve mine, Bolzano Province, Italy
- 2020
-
Ingår i: European Journal of Mineralogy. - : Copernicus GmbH. - 0935-1221 .- 1617-4011. ; 32:1, s. 77-87
-
Tidskriftsartikel (refereegranskat)abstract
- Monteneveite, ideally Ca3Sb5+2(Fe3+2Fe2+)O12, is a new member of the garnet supergroup (IMA 2018-060). The mineral was discovered in a small specimen belonging to the Swedish Museum of Natural History coming from the now abandoned Monteneve Pb-Zn mine in Passiria Valley, Bolzano Province, Alto Adige (South Tyrol), Italy. The specimen consists of mainly magnetite, sphalerite, tetrahedrite-(Fe) and oxycalcioroméite. Monteneveite occurs as black, subhedral crystals with adamantine lustre. They are equidimensional and up to 400 μm in size, with a subconchoidal fracture. Monteneveite is opaque, grey in reflected light, and isotropic under crossed polars. Measured reflectance values (%) at the four COM wavelengths are 12.6 (470 nm), 12.0 (546 nm), 11.6 (589 nm) and 11.4 (650 nm). The Vickers hardness (VHN100/ is 1141 kgmm-2, corresponding to H D 6:5-7, and the calculated density is 4.72(1) g cm-3. A mean of 10 electron microprobe analyses gave (wt %) CaO 23.67, FeO 3.75, Fe2O3 29.54, Sb2O5 39.81, SnO2 2.22, ZnO 2.29, MgO 0.15, MnO 0.03 and CoO 0.03. The crystal chemical formula calculated on the basis of a total of eight cations and 12 anions, and taking into account the available structural and spectroscopic data, is (Ca2:97Mg0:03)6D3:00 (Sb5+1.73Sn4+0.10Fe3+0.17)6D2.00(Fe3+2.43Fe2+0.37Zn0:20)6D3:00O12. The most significant chemical variations encountered in the sample are related to a substitution of the type Y Sn4CCZFe3CY Sb5CCZFe2C. Mössbauer data obtained at RT and 77K indicate the presence of tetrahedrally coordinated Fe2C. Raman spectroscopy demonstrates that there is no measurable hydrogarnet component in monteneveite. The six strongest Bragg peaks in the powder X-ray diffraction pattern are [d (Å), I (%), (hkl)]: 4.45, 100, (220); 3.147, 60, (400); 2.814, 40, (420); 2.571, 80, (422); 1.993, 40, (620); 1.683, 60, (642). Monteneveite is cubic, space group Ia-3d, with a D 12:6093(2) Å, V D 2004:8(1)Å3, and Z D 8. The crystal structure was refined up to R1 D 0:0197 for 305 reflections with Fo 4.Fo/ and 19 parameters. Monteneveite is related to the other Ca-, Sb- and Fe-bearing, nominally Si-free members of the bitikleite group, but it differs in that it is the only known garnet species with mixed trivalent and divalent cations (2 V 1) at the tetrahedral Z site. Textural and mineralogical evidence suggests that monteneveite formed during peak metamorphism (at ca. 600 C) during partial breakdown of tetrahedrite-(Fe) by reactions with carbonate, under relatively oxidizing conditions. The mineral is named after the type locality, the Monteneve (Schneeberg) mine. © Author(s) 2020.
|
|
| 4. |
- Konrad-Schmolke, Matthias, 1970, et al.
(författare)
-
Compositional re-equilibration of garnet: the importance of sub-grain boundaries
- 2007
-
Ingår i: European journal of mineralogy. - 0935-1221 .- 1617-4011. ; 19:4, s. 431-438
-
Tidskriftsartikel (refereegranskat)abstract
- Garnets from meta-granitoid high pressure rocks (Sesia Zone, Western Alps) show complex internal sub-grain textures in electron forescatter images. All investigated garnets consist of a large number of sub-grains with different shapes and sizes. Some garnets exhibit a sub-texture with very fine-grained (<20µm) sub-grains in their cores overgrown by palisade-like sub-grains in the rims. Sub-grain boundaries in these garnets have enabled diffusive element exchange between the garnet core and the surrounding matrix. Compositional mapping reveals zonation patterns of Mg that indicate modification of the garnet composition during prograde metamorphism. The extent of diffusional re-equilibration is dependent on sub-grain size and element diffusivities. Our samples show that XMg is strongly influenced by diffusion along the sub-grain boundaries, whereas apparently slow diffusing elements, such as Ca, Ti and Y preserve their original concentric zonation pattern. This differential re-equilibration leads to very complex chemical zonation that cannot be easily interpreted in terms of simple prograde growth zonation or of normally-applied spherical diffusion models. The observation that almost all garnets in the investigated samples exhibit a sub-grain pattern suggests this might be a common feature in high pressure/low temperature rocks.
|
|
| 5. |
- Mohammad, Y O, 7602, et al.
(författare)
-
Mg-staurolite and kyanite inclusions in metabasic garnet amphibolite from the Swedish Eastern Segment: evidence for a Mesoproterozoic subduction event.
- 2011
-
Ingår i: European Journal of Mineralogy. - 0935-1221. ; 23:4, s. 609-631
-
Tidskriftsartikel (refereegranskat)abstract
- We present evidence that the 1674 Ma Bora°s Mafic Intrusion of the Swedish Eastern Segment experienced high-pressure metamorphism related to a Sveconorwegian subduction-exhumation cycle. Mg-rich staurolite is found as inclusions in garnet in metaluminous amphibolites. The inclusion assemblages include staurolite (XMg 0.34–0.40), kyanite, euhedral anorthite, clinozoisite and quartz. The thermodynamic packages winTWQ and Theriak-Domino were used to investigate the P-T conditions of the matrix and inclusion mineral parageneses. The bulk composition of the rock does not have a stability field for staurolite-bearing parageneses. In our samples minerals of an eclogite-facies paragenesis became isolated from the whole rock in the first stage as inclusions in garnet. High Zn levels in the staurolite (0.6–1 wt% ZnO) show that it must have formed as either chloritoid or staurolite, both of which concentrate Zn. Euhedral anorthite inclusions have trace-element compositions including high Sr and insignificant Eu anomalies, which support their interpretation as pseudomorphs after lawsonite in plagioclase-out conditions. Rutile lamellae in the garnet are also indicative of a high-pressure origin. Calculated phase diagrams show that the most likely original paragenesis was garnet þ clinopyroxene þ Mg-rich chloritoid þ lawsonite þ kyanite þ quartz, which has a stability field for the whole-rock composition at 600 °C and 2.23–2.45 GPa. These conditions correspond to depths greater than 75 km, thus the Bora°s Mafic Intrusion was situated in the mantle at that time, implying subduction of the crustal block in which it was situated. The minerals now observed in the inclusions and in the rock matrix formed under amphibolite-facies conditions at lower pressures of 0.6–0.9 GPa and slightly increased temperatures around 650 °C, reflecting rapid exhumation from the mantle. Sm-Nd dating of garnet gives 957.1 ° 9.4 Ma, consistent with less precise Lu-Hf data, and represents either garnet growth during subduction or resetting during exhumation. Our investigations of staurolite in amphibolites documented in the literature show that staurolite cannot form in equilibrium with amphibolitefacies parageneses in normal metabasic rocks, which always have metaluminous compositions. A two-stage process is required in which a peraluminous assemblage with kyanite and possibly chloritoid first forms, due to plagioclase-out reactions in eclogite-facies conditions. Staurolite forms in the second stage during exhumation as pressure decreases, in domains which are not in contact with the common amphibolite-facies assemblage, for example by hydration reactions involving kyanite and garnet or by breakdown of chloritoid at higher temperatures. The pressures estimated for garnet growth and the development of inclusions correspond to minimum depths of 75–83 km (for basaltic or granitic overburden) and the Bora°s Mafic Intrusion is an integral part of the Eastern Segment in which retrograde eclogite metabasic bodies occur within orthogneisses in at least four other localities. This implies that a major part of the Eastern Segment experienced a high-pressure metamorphic event and the entire block of continental crust was involved in a subduction-exhumation cycle during the Sveconorwegian orogeny.
|
|
| 6. |
|
|
