Abstract. We investigated rare earth element (REE) minerals in low- to medium-grade metapelites sampled in two nappes of the Austroalpine Unit (Eastern Alps, Austria). Combining microstructural and chemical characterization of the main and REE minerals with thermodynamic forward modeling, Raman spectroscopy on carbonaceous material (RSCM) thermometry and in situ U–Th–Pb dating reveal a polymetamorphic evolution of all samples. In the hanging wall nappe, allanite and REE epidote formed during Permian metamorphism (275–261 Ma, 475–520 °C, 0.3–0.4 GPa). In one sample, Cretaceous (ca. 109 Ma) REE epidote formed at ∼440 °C and 0.4–0.8 GPa at the expense of Permian monazite clusters. In the footwall nappe, large, chemically zoned monazite porphyroblasts record both Permian (283–256 Ma, 560 °C, 0.4 GPa) and Cretaceous (ca. 87 Ma, 550 °C, 1.0–1.1 GPa) metamorphism. Polymetamorphism produced a wide range of complex REE-mineral-phase relationships and microstructures. Despite the complexity, we found that bulk rock Ca, Al and Na contents are the main factor controlling REE mineral stability; variations thereof explain differences in the REE mineral assemblages of samples with identical pressure and temperature (P–T) paths. Therefore, REE minerals are also excellent geochronometers to resolve the metamorphic evolution of low- to medium-grade rocks in complex tectonic settings. The recognition that the main metamorphic signature in the hanging wall is Permian implies a marked P–T difference of ∼250 °C and at least 0.5 GPa, requiring a major normal fault between the two nappes which accommodated the exhumation of the footwall in the Cretaceous. Due to striking similarities in setting and timing, we put this low-angle detachment in context with other Late Cretaceous low-angle detachments from the Austroalpine domain. Together, they form an extensive crustal structure that we tentatively term the “Austroalpine Detachment System”.
<p>This contribution reports LA-ICP-MS zircon ages and Rb-Sr biotite ages from the Troiseck-Floning Nappe, forming the northeasternmost extension of the Silvretta-Seckau Nappe System in the Eastern Alps. The Troiseck-Floning Nappe comprises a basement formed by the Troiseck Complex and a Permo-Triassic cover sequence. The basement consists of paragneiss with intercalations of micaschist, amphibolite and different types of orthogneiss, which was affected by a Variscan (Late Devonian) amphibolite facies metamorphic overprint. The cover sequence includes Permian clastic metasediments and metavolcanics, as well as Triassic quartzite, rauhwacke, calcitic marble and dolomite. During the Eoalpine (Cretaceous) tectonothermal event the nappe experienced deformation at lower greenschist facies conditions.</p> <p>Detrital zircon grains from paragneiss are in the range of 530-590 Ma, indicating an Ediacarian to earliest Cambrian source and a Cambrian to Ordovizian deposition age of the protolith. Late Cambrian to Ordovician crystallization ages from leucogranitic intrusions represent the earliest magmatic event of the Troiseck Complex. The amphibolite bodies derived from basalt with a calc-alkaline to island arc tholeiitic signature.</p> <p>Leucocratic orthogneiss with K-feldspar porphyroclasts and a calc-alkaline granitic composition plots in the field of volcanic arc granite. The youngest zircon grains indicate a Late Devonian crystallization. Two pegmatite gneisses with a calc-alkaline composition are early Mississippian in age.</p> <p>Mylonitic orthogneiss with a pronounced stretching lineation appears as irregularly shaped layers. It is leucocratic, very fine grained and contains scattered feldspar porphyroclasts with a round shape and a diameter of about 1 mm. Its chemical composition is granitic/rhyolitic with an alkali-calcic signature. In classification diagrams it plots in the field of syn-collision granite. Zircon ages of about 270 Ma indicate a Permian crystallization. Similar rocks interpreted as Permian rhyolitic metavolcanics appear in the cover sequence. They share a similar chemical composition and crystallization age of 270 Ma. Associated intermediate metavolcanics developed from calc-alkaline basaltic andesite.</p> <p>According to Rb-Sr biotite ages cooling of the Troiseck-Floning Nappe below c. 300&#176;C occurred at about 85 Ma in the west and 75 Ma in the east.</p> <p>In summary, the Troiseck Complex developed from Cambrian to Ordovizian clastic metasediments and granitic and basaltic magmatic rocks emplaced in the same time range. During the Late Devonian, it was affected by the Variscan collisional event, causing deformation at amphibolite facies conditions and intrusion of calc-alkaline granites. In early Mississippian time pegmatite dikes intruded, maybe induced by decompression and exhumation. The deposition of clastic sediments and (sub)volcanic rocks (rhyolite and basaltic andesite) constrains a surface position of the Troiseck Complex during the Permian. Based on regional considerations an extensional environment is assumed. In Triassic times carbonate platform sediments were deposited. During the Eo-Alpine collision in the Cretaceous the unit was part of the tectonic lower plate and subducted to shallow crustal levels, indicated by a lower greenschist facies metamorphic overprint. The Troiseck-Floning Nappe was formed and exhumed since about 85 Ma. Rb-Sr as well as apatite fission track data from the literature indicate tilting with more pronounced exhumation and erosion in the eastern part during Miocene lateral extrusion of the Eastern Alps.</p>
The Bavarian Unit in the Moldanubian Superunit exposes a distinct segment of the central European Variscan orogen, which is characterized by strong, late Variscan, low pressure, high temperature (LP–HT) metamorphism. The predominant lithology of the Bavarian Unit is a paragneiss-derived migmatite with abundant cordierite and K-feldspar. Rare paragneiss varieties with large garnets from the Lichtenberg complex near Linz (Upper Austria) record detailed information regarding the regional P–T–t evolution. The large garnet porphyroblasts of this exceptional rock preserve complex three-phase growth zoning indicative of a polymetamorphic history. Garnet cores with uniformly elevated grossular contents are discontinuously mantled by lower grossular garnet, and this calcium-poor central garnet zone is further overgrown by a garnet rim zone with elevated grossular content. Garnet zones also display different mineral inclusions, i.e. sillimanite, plagioclase and spinel in the core, and staurolite, biotite, plagioclase, sillimanite and muscovite in the mantle. Cordierite, sillimanite, K-feldspar and spinel in the matrix are equilibrated with the garnet rim. Geothermobarometric calculations coupled with thermodynamic modelling constrain the P–T peak of the first prograde metamorphism at 0·85–1·10 GPa and 720–780 °C (garnet core). Subsequently there was a stage of decompression and cooling during which the first garnet generation became partly resorbed. A second prograde metamorphic stage resulted in a new growth phase of garnet. This started at 0·45–0·60 GPa and 580–630 °C (garnet mantle) and progressed to granulite facies peak conditions of 0·55–0·65 GPa and 830–900 °C (garnet rim). Th–U–total Pb dating of monazite inclusions in the garnet cores indicate a Visean age for the first medium pressure, medium temperature (MP–MT) metamorphic event (340 ± 7 Ma), relating it to the Variscan collision stage. Dating of matrix monazite yields a Pennsylvanian age (312 ± 5 Ma) for the LP–HT overprint, consistent with existing geochronological data from the Bavarian Unit. Our study documents that deeply buried Variscan collisional crust was exhumed to mid-crustal levels in the Visean, before being near-isobarically heated to LP–HT granulite facies conditions in the Pennsylvanian (Bavarian event of the Variscan orogeny). This P–T–t evolution implies a significant external heat influx into mid-crustal levels at the end of the Variscan orogeny.
Current models for Miocene backarc extension of the Aegean region generally suggest that stretching was accommodated mainly by NE-dipping low-angle normal faults with N to NE sense of shear. A crustal-scale low-angle normal fault system trending over a length of more than 200 km forms the North Cycladic detachment system, which records a NE-directed normal shear sense separating the Cycladic Blueschist unit in the footwall from the Upper Cycladic unit in the hanging wall. Based on new structural field data, we propose the existence of another large-scale low-angle normal fault system, the West Cycladic detachment system, which is exposed on Kea, Kythnos, and Serifos, strikes over a length of at least 100 km, and has a possible extension to the SE, where the existence of a South Cycladic detachment system has been recently postulated. The West Cycladic detachment system shares many similarities with the North Cycladic detachment system, with the notable exception that the structure dips toward the SW with top-to-the-SSW kinematics. New 40Ar/39Ar and U-Th/He thermochronological data suggest that the West Cycladic detachment system accommodated extension throughout the Miocene. Since both the North and the West Cycladic detachment systems were active until the late Miocene but exhibit opposing shear sense, we propose that a large part of the stretching of the Aegean crust was accommodated by these two bivergent crustal-scale detachment systems.
<p>Based on new structural field data and Raman micro-spectroscopy on carbonaceous material a major detachment juxtaposing Drauzug-Gurktal Nappe System (DGN) against the transgressive Permo-Mesozoic cover sequence of the &#214;tztal-Bundschuh Nappe System (BN, Stangalm Mesozoic s. str.) in the area SE of Flattnitz (Carinthia, Austria). An Eo-alpine top-SE kinematic has been identified.</p><p>The hanging wall unit comprise lithologies of the DGN phyllites, conglomerates and graphite schists (Stolzalpe nappe), which have experienced only low grade greenschist deformation. Raman constrains 350&#176;C&#177;40&#176;C.</p><p>The footwall unit consists of dolomitic ultra-mylonites, calcitic marble mylonites, meta-conglomerates and quarzites (Stangalm Mesozoic and Kuster nappe), which have experienced at least four main deformation phases. The oldest structures (D1) corresponding to Eo-Alpine nappe stacking are overprinted by (D2) isoclinal recumbent folds with E-W oriented shallow dipping fold axis and an axial plane schistosity, dipping shallowly to WSW. Ductile to brittle-ductile top to the E shearing (D3) is indicated by ESE-trending stretching lineation, C-type shear bands, stylolites, crystal- and shape preferred orientations of mineral grains. Late brittle deformation (D4) is recorded in steep joint sets with dip-directions to NW. Raman constrains 480&#176;C&#177;40&#176;C.</p><p>The detachment zone comprises a complicate zone of high strain including units from DGN folded together within the Stangalm Mesozoic, which have experienced the same deformation as the BN.</p>
The affiliation of the Ennstal Phyllite Zone (EPZ) to either the micaschist units of the Koralpe-Wölz nappe system (KW-NS) to its south or to nappes of the “Greywacke Zone” to its north and east is still debated. Due to similarities with phyllites of the “Greywacke Zone” in the north and phyllonitic micaschists in the south, no clear lithological boundary between these units is observable. Petrographic observations suggest a continuous eoalpine metamorphic gradient with no metamorphic gap between the KW-NS and the EPZ. In order to clear this debate and further constrain the tectonic and temporal evolution of these units, we present new LA-MC-ICP-MS U/Pb age dating results for metapelite samples from the EPZ as well as for the adjacent units of the KW-NS.Two samples (EA09 and SP02) from the central EPZ and one sample (SP62) from the northernmost part of the Wölz-Complex of the KW-NS were selected for detrital zircon age dating. The distribution of approximately 150 dates per sample reveals major peaks at the Ediacaran-Cryogenian boundary (624 – 646 Ma), a smaller peak at the Neoproterozoic-Mesoproterozoic boundary (~1000 Ma) followed by a hiatus and a smaller peak in the mid-Paleoproterozoic (~2000 Ma). All samples show similar mid-Paleoproterozoic and Neoproterozoic-Mesoproterozoic peaks. Sample SP62 contains one grain of Cambrian age (523 Ma) and one grain of mid-Ordovician age (460 Ma) whereas the youngest zircons from the EPZ samples yield Ediacaran ages of 629 Ma and 625 Ma. The lack of zircons of Ordovician age in samples EA09 and SP02 indicate an affiliation of the EPZ with the basal units of the “Greywacke Zone”.We also dated metamorphic allanite and REE-bearing epidote rims which are interpreted to form at low pressure and temperature conditions in metapelites. Allanites from the EPZ yield metamorphic ages of 105 ± 3.5 Ma in the northern part of the unit and 279 ± 6 Ma in the southern part. Allanite cores from two micaschist samples from the northern and central Wölz-Complex yield ages of 316 ± 21 Ma and 286 ± 11 Ma. Their respective epidote rims yield eoalpine ages of 98 ± 2 Ma and 96 ± 2 Ma. One micaschist sample from the Rappold-Complex yields ages of 326 ± 9 Ma for the allanite cores and 101 ± 1 Ma for the epidote rims. These ages are interpreted as prograde crystallization of allanite and epidote and give us petrochronological information about three distinct metamorphic events: Variscan, Permian and Eoalpine. By gathering three distinct eoalpine ages within the EPZ and the KW-NS, we can further constrain the metamorphic evolution of the eoalpine lower plate.
