The Slovenska Bistrica ultramafic complex (SBUC; Eastern Alps, Slovenia) occupies the south-easternmost part of the Pohorje Mountains, which represent an exhumed piece of continental crust subducted ...during the Cretaceous Eo-Alpine orogeny. The SBUC is composed of serpentinised harzburgites with local occurrences of garnet lherzolite, and is the only known occurrence of ultramafic rocks within the high- to ultrahigh-pressure nappe system apart from a few small dismembered pieces in the near vicinity. The harzburgites are highly depleted following melting within the spinel stability field, as exemplified by high whole-rock MgO contents (41.5–44.3 wt.%), low Al
2O
3 (0.7–1.2 wt.%), low Lu
N (0.1–0.7), and high Cr# of Cr-spinel (ca. 0.5). Fluid-immobile incompatible trace elements (Ti, Sc, V, Zr, HREE, Th) correlate well with MgO, consistent with a melt depletion trend. Other incompatible elements (Ba, Sr, LREE) show little correlation and are probably modified by the serpentinisation process or later metamorphic overprint. However, comparable LREE enrichment of all samples and absence of negative Nb and Th anomalies suggests that this piece of mantle was already metasomatised by melts or fluids before serpentinisation.
Garnet lherzolite in the SBUC recorded an UHP stage (4 GPa, 900 °C) not visible in the harzburgites. Because of the evidence of an earlier lower pressure stage within the spinel stability field, the SBUC represents a piece of subducted mantle. The protolith of the harzburgites is probably oceanic mantle, considering the high degree of melt depletion yet the lack of a subduction-zone signature. It therefore most likely represents a part of previously subducted Meliata oceanic mantle, which was part of a deeper section of the hanging wall along which subduction of the continental crust that is now exposed in Pohorje took place. Alternatively, it may represent mantle depleted and metasomatised in a continental rift zone, which was later incorporated in the hanging wall of the subduction zone and subsequently dragged down to UHP conditions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The Miocene deformation history of magmatic and host metamorphic rocks and surrounding sediments was reconstructed by measuring meso- and microscale structures and anisotropy of magnetic ...susceptibility (AMS) data in order to constrain the structural evolution of the Pohorje pluton during the onset of lithospheric extension at the Eastern Alps–Pannonian Basin transition. Principal AMS axes, lineation and foliation are very similar to mesoscopic lineation and foliation data from the main intrusive body and from some dykes. Although contribution from syn-magmatic texture is possible, these structures were formed during the cooling of the pluton and associated subvolcanic dykes just shortly after the 18.64 Ma pluton intrusion. Dykes emplaced during progressively younger episodes reflect decreasing amount of ductile strain, while firstly mesoscopic foliation and lineation, and then the tectonic AMS signal gradually disappears. In the structurally highest N–S trending dacite dykes, the AMS fabric only reflects the magmatic flow. The Miocene sediments underwent the same, NE–SW to E–W extension as the magmatic and host metamorphic rocks as indicated by both AMS and fault-slip data. All these events occurred prior to ~ 15 Ma, i.e., during the main syn-rift extension of the Pannonian Basin and during the fastest exhumation of the Tauern and Rechnitz windows, both demonstrating considerable extension of diverse crustal segments of the Alpine nappe pile. After a counterclockwise rotation around ~ 15 Ma, the maximum stress axis changed to a SE–NW orientation, but it was only registered by brittle faulting. During this time, the overprinting of a syn-rift extensional AMS texture was not possible in the cooled or cemented magmatic, metamorphic and sedimentary rocks.
We investigate landscape evolution in a region of the Alps that has escaped glacial erosion during periodic glaciations of the last million years. The research is thus suited to investigate ...landscaping processes on a longer time scale at the eastern end of the Alps. Morphometric analysis reveals the presence of incised relict landscapes in several regions. In the Koralpe range topographic analysis is interpreted in terms of the relict landscape being present on both sides of the eastward tilted Koralpe block. This suggests that the relict landscape is younger than the tilting of the range, which is inferred to have taken place between 18 and 16Ma. In the Pohorje region, a relict landscape is developed across the contacts of a 19Ma pluton. We use apatite (U–Th)/He thermochronology to constrain the possible age of the Koralpe and Pohorje relict landscapes. The results indicate that the Pohorje massif had cooled below 70°C by about 15Ma suggesting that the relict landscape must be younger — consistent with the interpretation of the Koralpe range. These results suggest that many relict landscapes of the eastern Alps may have formed after 15Ma in a period of tectonic quiescence and erosion. However, in both ranges channel profile projections show that about 387±105m uplift and incision occurred subsequently. This incision is likely to have occurred during the last 6–5Ma in response to the uplift of the whole region. It testifies to a renewed and ongoing uplift event that is earlier than the glaciation periods but might easily be confused with impact of glacial erosion elsewhere in the eastern Alps.
•Morphologically, Koralpe and Pohorje ranges constitute incised relict landscapes.•Channel profile projection indicates an incision amount of c. 400m.•New apatite (U–Th)/He ages suggest that the relict landscape is <15Ma.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Fairy tales are fascinating fantasies, but they also gave people a better understanding of serious issues such as illness, injury, and even death. The results of the analysis of a large corpus of a ...total of 1512 published fairy tales, legends, and other documentary prose about Pohorje, most of which date back to the 19th and the first half of 20th centuries, and whose origins were in a time when diseases were ubiquitous and poorly understood, show that the theme of health and disease rarely appear in these sources, however, when they do mention it, they refer specifically to the living conditions of people in the Pohorje Mountains, epidemics, social diseases, mental illnesses, congenital deformities, and ideas about the relationship between disease, the human body, and supernatural forces. One of the reasons for such explanations is the widespread absence of doctors in this hilly area.
The first evidence for ultrahigh‐pressure (UHP) metamorphism in the Eastern Alps is reported from kyanite eclogites of the Pohorje Mountains in Slovenia. Polycrystalline quartz inclusions surrounded ...by radial fractures in garnet, omphacite, and kyanite are interpreted to be pseudomorphs after coesite. Abundant quartz rods and needles in omphacite indicate an exsolution from a preexisting supersilicic clinopyroxene that contained a Ca‐Eskola component. Geothermobarometry on the mineral assemblage garnet + omphacite + kyanite + phengite + quartz/or coesite yields peak pressure and temperature conditions of 3.0–3.1 GPa and 760°–825°C, well within the stability field of coesite, thus supporting the microtextural evidence for UHP metamorphism. This records the highest‐pressure conditions of Eo‐Alpine metamorphism during the Cretaceous orogeny in the Alps, implying a very deep subduction of the continental crust to at least 90–100 km depths. The new data are evidence for a regional southeastward increase of peak pressures in the Lower Central Austroalpine, indicating a south‐ to eastward dip of the subduction zone. Subduction was intracontinental; northwestern parts of the Austroalpine (Lower Central Austroalpine) were subducted under southeastern parts (Upper Central Austroalpine). The subduction zone formed in the Early Cretaceous in the northwestern foreland of the Meliata suture after Late Jurassic closure of the Meliata Ocean and the resulting collision, by a forward subduction shift to a Permian rift.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The granitic pegmatite dike intruded the Cretaceous UHP rocks at Visole, near Slovenska Bistrica, in the Pohorje Mountains (Slovenia). The rock consists mainly of K-feldspar, albite and quartz, ...subordinate muscovite and biotite, while the accessory minerals include spessartine-almandine, zircon, ferrocolumbite, fluorapatite, monazite- (Ce), uraninite, and magnetite. Compositions of garnet (Sps
Alm
Grs + And
Prp
), metamict zircon with 3.5 to 7.8 wt. % HfO
atom. 100Hf/(Hf + Zr) = 3.3-7.7 and ferrocolumbite atom. Mn/(Mn + Fe) = 0.27-0.43, Ta/(Ta + Nb) = 0.03-0.46 indicate a relatively low to medium degree of magmatic fractionation, characteristic of the muscovite - rare-element class or beryl-columbite subtype of the rare-element class pegmatites. Monazite-(Ce) reveals elevated Th and U contents (≤11 wt. % ThO2, ≤5 wt. % UO2). The monazite-garnet geothermometer shows a possible precipitation temperature of ~495 ± 30 °C at P~4 to 5 kbar. Chemical U-Th-Pb dating of the monazite yielded a Miocene age (17.2 ± 1.8 Ma), whereas uraninite gave a younger (~14 Ma) age. These ages are comtemporaneous with the main crystallization and emplacement of the Pohorje pluton and adjacent volcanic rocks (20 to 15 Ma), providing the first documented evidence of Neogene granitic pegmatites in the Eastern Alps. Consequently, the Visole pegmatite belongs to the youngest rare-element granitic pegmatite populations in Europe, together with the Paleogene pegmatite occurrences along the Periadriatic (Insubric) Fault System in the Alps and in the Rhodope Massif, as well as the Late Miocene to Pliocene pegmatites in the Tuscany magmatic province (mainly on the Island of Elba).
In steady-state orogens, topographic gradients are expected to increase with elevation whereas the European Alps feature a transition from increasing to decreasing slopes. This peculiar pattern has ...been interpreted to reflect either the critical slope stability angle or a premature fluvial landscape but is also consistent with the glacial buzz-saw hypothesis. To disentangle the contributions of each of these principles we split the Alps into contiguous domains of structural units and analyze their slope–elevation distributions emphasizing glaciated and non-glaciated realms. In comparable structural units within the extent of the last glacial maximum (LGM) the transition from increasing to decreasing slopes is located at the equilibrium line altitude (ELA) of the LGM and we interpret this to be evidence for the impact of glacial erosion. Decay rates of glacial landforms towards steady-state slopes depend on lithological properties leading to a landscape characterized by different transient states. Beyond the LGM limits the slope–elevation distributions show local maxima as well, but these are located at varying altitudes implying a tectonic driver. This observation and data from surrounding basins suggests that at least parts of the European Alps experienced a pre-Pleistocene pulse of tectonic uplift. The resulting presence of premature low-gradient terrain above the ELA during the global cooling in Plio–Pleistocene times would have heavily influenced the onset and the extent of an alpine ice cap.
•The transient topography of the Alps is caused by tectonics and glaciation.•Local maxima in slope occur at the LGM ELA: glacial buzz-saw.•Local maxima in slope occur at various altitudes: prematurity.•The persistence of transient landforms depends on lithology: slope stability.•Pre-Pleistocene prematurity may have influenced onset and extent of glaciations.
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New evidence for ultrahigh‐pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet‐bearing ultramafic rocks from the Pohorje Mountains in Slovenia. The garnet peridotites are closely ...associated with UHP kyanite eclogites. These rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. On the basis of petrochemical data, garnet peridotites could have been derived from depleted mantle rocks that were subsequently metasomatized by melts and/or fluids either in the plagioclase‐peridotite or the spinel‐peridotite field. At least four stages of recrystallization have been identified in the garnet peridotites based on an analysis of reaction textures and mineral compositions. Stage I was most probably a spinel peridotite stage, as inferred from the presence of chromian spinel and aluminous pyroxenes. Stage II is a UHPM stage defined by the assemblage garnet + olivine + low‐Al orthopyroxene + clinopyroxene + Cr‐spinel. Garnet formed as exsolutions from clinopyroxene, coronas around Cr‐spinel, and porphyroblasts. Stage III is a decompression stage, manifested by the formation of kelyphitic rims of high‐Al orthopyroxene, aluminous spinel, diopside and pargasitic hornblende replacing garnet. Stage IV is represented by the formation of tremolitic amphibole, chlorite, serpentine and talc. Geothermobarometric calculations using (i) garnet‐olivine and garnet‐orthopyroxene Fe‐Mg exchange thermometers and (ii) the Al‐in‐orthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at conditions of around 900 °C and 4 GPa. These results suggest that garnet peridotites in the Pohorje Mountains experienced UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of continental crust, which incorporated mantle peridotites from the upper plate, in an intracontinental subduction zone. Sinking of the overlying mantle and lower crustal wedge into the asthenosphere (slab extraction) caused the main stage of unroofing of the UHP rocks during the Upper Cretaceous. Final exhumation was achieved by Miocene extensional core complex formation.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Field relations and deformation structures in the southeastern part of the Pohorje Mountains constrain the tectonic evolution of the Austroalpine high-pressure/ultrahigh pressure (HP/UHP) terrane. ...The Slovenska Bistrica Ultramafic Complex (SBUC) forms a large (ca. 8 × 1 km size) body of serpentinized harzburgite and dunite including minor garnet peridotite and is associated with partly amphibolitized eclogite bodies. The SBUC occurs in the core of an isoclinal, recumbent, northward closing antiform and is mantled by metasedimentary rocks, mostly gneisses and a few marbles, including isolated eclogite/amphibolite lenses. Before this folding, the SBUC formed the deepest part of the exposed terrane. We interpret the SBUC to be derived from near-MOHO, uppermost mantle which was intruded by gabbros in the subsurface of a Permian rift zone. During Cretaceous intracontinental subduction, the SBUC was most likely part of the footwall plate which experienced HP to UHP metamorphism and was folded during exhumation. In the Miocene, the Pohorje Pluton intruded and, subsequently, the metamorphic rocks together with the pluton were deformed probably due to east-west extension and contemporaneous north-south shortening, thus forming an antiformal metamorphic core complex.
The investigation of eclogites and a precursor gabbro from the Austroalpine basement domains Koralpe, Saualpe and Pohorje shows that these mafic rocks are similar to oceanic gabbros that were derived ...from a depleted mantle source. The chemical variations of the eclogites are related to differences in the magmatic history of the precursor rocks and to seawater alteration. The trace element composition of the rocks has not changed significantly during the gabbro to eclogite transformation because trace elements are redistributed among the newly formed high-pressure major and accessory minerals. As other recent studies indicate, incompatible trace elements are predominantly hosted in zoisite/clinozoisite (Sr, Pb, U, Th, LREE), apatite (Sr, Pb, REE), phengite (Cs, Rb, Ba), garnet (Y, HREE, Sc), rutile (Ti, Nb, Ta) and zircon (Zr, Hf) at eclogite-facies conditions. Omphacite hosts most of the Li in addition to some Sr and major amounts of Sc and V
. This argues against significant liberation of LILE and LREE during subduction-related dehydration or fluid infiltration of these mafic rocks.
The trace element characteristics of accessory minerals in eclogites help to reconstruct the
P–
T–
t evolution of a subduction complex: U–Pb zircon ages will date the high-pressure event if U–Th characteristics and REE analyses constrain zircon growth to being metamorphic and essentially synchronous with the growth of garnet. Recent studies document that variations in Zr content of rutile grown in the presence of zircon and quartz are mainly attributable to differences in temperature. Zr-in-rutile thermometry of Koralpe, Saualpe and Pohorje eclogites yields temperatures of 700–730 °C (according to Zack et al. Zack, T., Moraes, R., Kronz, A., 2004. Temperature dependence of Zr in rutile: an empirical calibration of a rutile thermometer. Contrib. Mineral. Petrol. 148, 471–488) or between 630–650 °C (according to Watson et al. Watson, E.B., Wark, D.A., Thomas, J.B., 2006. Crystallization thermometers for zircon and rutile. Contrib. Mineral. Petrol. 151, 413–433). No systematic variation in rutile temperatures was observed for matrix rutile and rutile included in garnet, omphacite or kyanite, suggesting that these temperatures represent peak metamorphic conditions and that this part of the Austroalpine basement behaved as a coherent block during subduction.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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