The metamorphic series of the Pohorje Mountains represents a part of the Eastern Alpine realm that was subjected to ultrahigh-pressure conditions during the Cretaceous Eo-Alpine orogenic cycle. The ...Slovenska Bistrica Ultramafic Complex located in the south-eastern Pohorje Mountains is an 8km wide serpentinite body that contains lenses of garnet-bearing ultramafites and eclogites. It is embedded in and part of a mixed continental unit of metapelitic gneisses, orthogneisses, and eclogites. We present Lu–Hf garnet chronometry coupled with geochemical and petrological data from three samples: one garnet lherzolite, one eclogite from within the ultramafic complex, and one eclogite from the surrounding mixed unit. All obtained ages are identical within error, i.e. 96.6±1.2Ma and 94.8±5.1Ma, respectively, for the two eclogites and 91.6±4.1Ma for the garnet lherzolite. Garnet of all samples shows homogeneous concentrations of major bivalent elements due to high temperature re-equilibration. It does, however, preserve growth-related zoning with respect to Lu in all three samples implying that Lu–Hf ages still record garnet growth. The coincidence of ages suggests that the ultramafic complex and the surrounding continental mixed unit share the same subduction history, i.e. the complex was part of the subducting plate during and after the garnet growth stages.
•Lu–Hf garnet geochronology is applied to eclogites and garnet bearing ultramafites.•The ages are coupled to phases of garnet growth via the characterization of Lu zoning.•The ages indicate a coherent character of the Pohorje Nappe in the Eastern Alps.
The present contribution reports new whole-rock major and trace element data, REE, and Sr–Nd isotopic ratios for the early Miocene Pohorje igneous complex (PIC) in Slovenia (Eastern Alps), which, ...along with published data, are used to investigate its genesis and evolution. The complex comprises three main rock-groups. The largest one is made of granodiorite and minor tonalite (GTD) enclosing mafic microgranular enclaves (MME), and a small Q-dioritic body. The second group prevailingly consists of dacite stocks and dykes, and porphyritic microgranodiorite (DAMG). Andesitic dykes (AD), intruding metamorphic rocks and less often the GDT, constitute the third group. Aplites and pegmatites intrude both the Pohorje igneous complex and the country rocks.
Evidence of interaction between magmas has been observed only in the GDT rocks. In the southeastern part of the complex the GDT rocks are less evolved and emplaced at greater depth (ca. 16.5–17.6 km) than the rocks cropping out in the northwestern part (ca. 12.4–13.8 km). DAMG rocks can be divided geochemically into two groups, high-Ga DAMG and low-Ga DAMG, having similar chemical composition with the most evolved GDT, and the most evolved andesitic dykes, respectively. The andesitic dykes can be divided into three groups that differ in LILE contents from each other up to three orders of magnitude.
A Mixing plus Fractional Crystallization (MFC) process between the least evolved MME and the most evolved DAMG rocks is considered responsible for the formation of the two diversely evolved GDT. Two of the three AD groups (AD1, AD3) originated in the mantle, whereas the third one (AD2) is considered as the result of an AFC process between the less enriched andesite (AD1) and continental crust (gneiss). The same process is considered responsible for the low-Ga DAMG genesis.
The most mafic magma is presumably the result of melting of mantle wedge differently metasomatized and enriched in LILE due to fluids, sediment melts, and bulk sediments. The felsic end-member magma of the MFC process originated from partial melting of intermediate-lower crust having intermediate to basaltic composition. Geodynamic sequence for the formation of the PIC comprehend mantle metasomatism during Alpine subduction, thickening of crust during the collision that followed subduction, and production of mantle melts in response to delamination caused by opening of the Pannonian basin and asthenospheric upwelling. Crustal derived melts resulted by underplating of mantle-derived magmas.
•Miocene Pohorje igneous complex comprises plutonic, volcanic and sub volcanic rocks.•Melting of mantle metasomatized by fluids, sediments, and sediment melts.•Delamination caused by opening of the Pannonian basin and asthenosphere upwelling.•Crust-derived melts generated through underplating of mantle-derived magmas.•Complex evolutionary process like fractionation, magma mixing and crustal assimilation.
Phase relations among the mineral assemblages of UHP kyanite eclogite were investigated in the Pohorje Mountains of the Eastern Alps. Ultrahigh-pressure metamorphism resulted from intracontinental ...subduction during the Cretaceous (ca. 92Ma). Kyanite-bearing eclogites are associated with meta-ultramafic rocks including UHP garnet peridotites and are embedded in metapelitic gneisses and micaschists. The kyanite eclogites contain a peak metamorphic assemblage of garnet, omphacite, kyanite and phengite. Pyrope-rich garnet is unzoned and almost free of inclusions. The non-stoichiometric supersilicic omphacite contains up to 5mol% of Ca-Eskola molecule. Breakdown of omphacite during decompression resulted in exsolution of oriented rods of silica. Phengite contains up to 3.5 Si a.p.f.u. Polycrystalline quartz inclusions in peak-pressure minerals – garnet, omphacite and kyanite – are surrounded by radial fractures diagnostic of the former presence of coesite. Peak-pressure minerals are replaced by symplectites of diopside+plagioclase+amphibole after omphacite, plagioclase+biotite after phengite and sapphirine+corundum+spinel+anorthite after kyanite. Sapphirine has composition close to (Mg, Fe)12.4 Al38.9 Si4.5 O80 in average, which is amongst the most aluminous yet reported. Peak metamorphic conditions were constrained from calculated phase equilibria in the NKCFMASH system with the fixed bulk-rock composition, and conventional geothermobarometry. This approach led to consistent results, the calculated peak P–T conditions of 3.0–3.7GPa and 710–940°C, in the stability field of coesite and the same range as metamorphic conditions recorded by the associated garnet peridotites. This implies that eclogites and their host rocks were subducted to depths of about 100km. The relatively high temperature at peak pressure, compared to UHP rocks of Tertiary age in the Western Alps where mostly oceanic crust was subducted, probably resulted from radiogenic heat production by subducting continental crust, in the intra-continental setting of the Cretaceous subduction zone in the Eastern Alps.
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► Significant enlargement of UHPM terrane in Pohorje, Eastern Alps. ► UHPM (3.5–3.7GPa; 800–940°C) due to Cretaceous intracontinental subduction. ► Contrasting types of subduction and UHPM in the Alps: oceanic vs. continental.
The formation and deformation history of back-arc basins play a critical role in understanding the tectonics of plate interactions. Furthermore, opening of extensional back-arc basins during the ...overall convergence between Africa and Europe is a fundamental process in the overall tectonic evolution of the Mediterranean and adjacent areas. In this frame, Miocene tectonic evolution of the western Pannonian Basin of Central Europe and its connection to inherited Cretaceous structures of the Eastern Alpine nappes are presented.
Revision of published and addition of new structural and thermochronological data, as well as seismic profiles from the western Pannonian Basin is complemented by high-resolution thermo-mechanical numerical modeling in order to propose a new physically consistent tectono-sedimentary model for the basin evolution. The onset of extension is dated as ~25–23 Ma, and higher rates are inferred between 19 and 15 Ma at the south-western part of the area (Pohorje, Kozjak Domes, Murska Sobota Ridge, and Mura-Zala Basin). Rift initiation involved the exhumation of the middle part of the Austroalpine nappe pile along low-angle detachment faults and mylonite zones. The Miocene low-angle shear zones could reactivate major Cretaceous thrust boundaries, the exhumation channel of ultra-high-pressure rocks of the Pohorje Dome, or Late Cretaceous extensional structures. Miocene extension was associated with granodiorite and dacite intrusions between 18.64 and 15 Ma. The Pohorje pluton intruded at variable depth from ~4 to 16–18 km and experienced ductile stretching, westward tilting, and asymmetric exhumation of its eastern side. Terrestrial early Miocene (Ottnangian to Karpatian, 19–17.25 Ma) syn-rift depositional environment in supradetachment basins evolved to near-shore and bathyal one by the middle Miocene (Badenian, 15.97–12.8 Ma). Deformation subsequently migrated eastwards to the western part of the Transdanubian Range (Keszthely Hills) and to newly formed grabens. In this formerly emerged terrestrial area active faulting started at 15–14.5 Ma and continued through the late Miocene almost continuously up to ~8 Ma but basically terminated in the Mura-Zala Basin by ~15 Ma (early Badenian). These observations suggest a ~200 km shift of active faulting, basin formation, and related syn-tectonic sedimentation from the SW (Pohorje and Mura-Zala Basin) toward the Pannonian Basin center. Building on the above described observational and modeling data makes the Pannonian Basin an ideal natural laboratory for understanding the coupling between deep Earth and surface processes.
•Variable extensional structures characterize the western Pannonian Basin.•Exhumation of deeper crustal rocks along detachments occurred in the Miocene.•Depocenters and faulting shifted from basin margin toward basin center.•Thermomechanical modelling predicts depocenter migration within 12 Ma.•Miocene extensional structures reactivated and were bounded to inherited weakness zones.
Significantly different peak pressure–temperature (P–T) conditions (18–26 kbar and 630–760°C versus 29–37 kbar and 750–940°C) have previously been published for eclogite and related metabasites from ...the south‐eastern flank of the Pohorje Mountains in Slovenia. These rocks can show a bimodal distribution of chromium in the rock‐forming minerals, particularly garnet, the role of which in their metamorphic evolution is unclear. Therefore, we studied an eclogite and a related rock with clinopyroxene containing only 17 mol% jadeite + acmite (sample 18Ca35a). KαCr intensity maps of garnet particularly in sample 18Ca35a show a sharp irregular boundary between the core (Gt1) and the mantle (Gt2). Gt1 of millimetre‐sized garnet in this rock is nearly Cr‐free and unzoned, whereas Gt2 is of different composition (0.22 wt.% Cr2O3) and slightly zoned. Nearly Cr‐free amphibole, (clino)zoisite, kyanite and staurolite inclusions are present in Gt1. The matrix consists of garnet and Cr‐bearing clinopyroxene, (clino)zoisite and amphibole. Thermodynamic modelling suggests peak P–T conditions of 22.5 ± 2 kbar at 710 ± 25°C (Gt1) and 23 ± 2 kbar at 700 ± 25°C (Gt2) in both samples. We interpret these findings to suggest that olivine‐ and hornblende‐bearing gabbros with some chromite experienced early metamorphism in the eclogite facies, when Gt1 formed. The rock was subsequently exhumed and cooled leading to significant garnet corrosion. A second stage of metamorphism, recognized by mappable Cr contents in garnet, led to the growth of Gt2 and other Cr‐bearing minerals at the expense of chromite relics, which survived stage I. The peak P–T conditions of stage II are compatible with those previously derived by same authors and support the view that probably no ultrahigh‐pressure eclogite exists in the Pohorje Mountains. We relate the two metamorphic events to the Cretaceous and Palaeogene high‐pressure events recently reported from micaschists of the Pohorje Mountains.
The Austroalpine nappe stack of the Pohorje Mountains (Mts.) in northeastern Slovenia comprises a suite of eclogite facies metamorphic rocks that were partially assigned to Eo‐Alpine ...ultrahigh‐pressure metamorphism (UHPM). We selected a micaschist, which was previously related to this metamorphism, for a detailed study including the chemical zonation of garnet and potassic white mica, the identification of mineral inclusion assemblages, pseudosection modelling with PERPLE_X, and monazite in‐situ dating with the electron microprobe. Polymetamorphism was revealed by (at least) two generations of garnet and phengite and four populations of monazite yielding ages of 283.6 ± 6.1 (2σ), 94.1 ± 3.7, 47.9 ± 10.8 and 26.2 ± 2.8 Ma. The Permian monazite population is characterized by relatively high Y contents (~1.15 wt% Y) and low La/Gd mass ratios (8.7) indicating its formation before the growth of porphyroblastic garnet. The Eo‐Alpine population, however, grew synchronously with garnet based on low Y contents (~0.05 wt%) and high La/Gd ratios (21.4). The older Tertiary population (48 Ma) shows also high Y contents (1.1 wt%) and low La/Gd ratios (10.6) whereas the younger Tertiary population is characterized by low Y contents. The Permian P–T conditions of 7.5–10 kbar at 600–650°C were obtained using the inclusion assemblage of staurolite+rutile+biotite in porphyroblastic garnet. High pressure (HP) but no UHPM was reconstructed for both Eo‐Alpine coarse phengite (Si = 3.22 per formula unit = pfu) and small Tertiary garnet+fine‐grained phengite (Si = 3.27 pfu) at peak pressures ~16 kbar and 18.5–23 kbar respectively. Maximum temperatures close to 650°C were likely reached during the Eo‐Alpine HP event, whereas those of the Tertiary HP event were probably ~580°C. These HP metamorphic events suggest that the Pohorje Mts. experienced both an Eo‐Alpine and a Tertiary subduction–exhumation history, the latter of which was mainly reported for underlying Penninic nappes so far.
In order to constrain tectonic models for the nature of the Eoalpine high pressure belt at the eastern end of the Alps, we investigate the formation pressure of metamorphic rocks along a profile ...between the Koralpe and the well-known UHP rocks of the southern Pohorje mountains. Rocks from three different regions are considered: (i) the rocks of the southernmost Koralpe to the north, (ii) the rocks of the Plankogel Unit between the Plankogel detachment and the Drava valley and (iii) the rocks between the Possruck range and the southern Pohorje mountains. In the Koralpe, pelitic rocks record a formation pressure around 15 – 18 kbar, as reported in the literature. For the Plankogel Unit, we derive pressures between 7.1 ± 1.95 kbar and 11.5 ± 3.42 kbar at 650 °C and recognize only a single Eoalpine metamorphic event. For the high grade rocks of the Pohorje mountains, we derive peak metamorphic pressures (explored with the garnet-muscovite-kyanite-quartz assemblage) that rise from 16.2 ± 3.45 kbar (at 700°C) in the north, to 23.9 ± 2.49 kbar (at 700 °C) in the south. There, we also recognize a later lower pressure event that is derived from pressure calculations with the full equilibrium assemblage. This lower pressure event yields similar conditions around 10 ± 2 kbar at 650 °C for the entire north-south transect within the Pohorje mountains. Peak metamorphic conditions in the Koralpe and Pohorje regions are matched by a continuous field gradient of about 1.3 kbar per 10 kilometers distance corresponding to a depth increase of about 0.5 km per kilometers distance assuming lithostatic conditions. We suggest that this supports that the two units may be interpreted together in terms of a 45° dipping subducting plate. Above this subducting plate, it is inferred that a slab was extracted that was located between the Plankogel Unit and the high pressure rocks, causing a first exhumation stage that is associated with buoyant upwards tilting of the subducted slab to mid crustal levels. Within this model, the Plankogel Unit was located in the hanging wall of the extracted slab and the Plankogel detachment forms the suture of the extracted slab. Exhumation from mid crustal levels to the surface during a 2
stage occurred due to erosion and normal faulting. This normal faulting is responsible for some 10 km of upward displacement of the Pohorje mountains relative to the Koralpe and ultimately for the current distribution of lithologies on a map scale.
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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