Comment to “The Devonian-Carboniferous boundary in the Graz Paleozoic (Eastern Alps, Austria) and its global significance” by Kaiser and Hubmann

The Devonian-Carboniferous boundary in the Graz Paleozoic (Eastern Alps, Austria) and its global significance

Reply to the Comment to “The Devonian-Carboniferous boundary in the Graz Paleozoic (Eastern Alps, Austria) and its global significance” by Ana-Voica Bojar and Franz Neubauer

In the east-central part of the Eastern Alps, three major deformation events can be distinguished within the Koralm Complex and adjacent units (Plankogel Complex, Gleinalm Complex, Seckau Crystalline Complex, Paleozoic of Graz). A first deformation event D1 is characterized by the formation of a penetrative foliation and an E-W stretching lineation. Remnants of deformational micro-structures indicate a top-to-the-west sense of shear during this deformation event. Most of the D1-related fabrics were overprinted by subsequent metamorphism. This metamorphic event did affect the presumed tectonic boundary between the Koralm Complex and the Gleinalm Complex below. Particularly, D2 is related to the Plattengneis shear zone, which formed in the uppermost structural sections of the Koralm Complex, characterized by a N-S-oriented stretching lineation. Eclogites in the footwall have been affected by this deformation event, too. This deformation event is associated with pure shear in the central parts of the Koralm Complex, probably with top-to-the-south displacement in the southern parts, and top-to-the-north displacement in the northern parts. Deformation within the Plattengneis and the eclogites below occurred along the decompressional path, indicated by decreasing minimum pressures within the eclogites, and by northward and southward decreasing pressures and temperatures. The Plattengneis shear zone continuously passes into a low-angle normal fault in the northeastern part of the Koralm, forming the contact between the Koralm Complex and the Paleozoic of Graz. Thus, the Plattengneis shear zone primarily formed as an extensional structure and triggered exhumation of the eclogites. D3-related structures are restricted to distinct low-angle normal shear zones along the northern and southern margins of the Koralm Complex, with top-to-the-N/NE and top-to-the-S/SE displacement, respectively. These are related to the juxtaposition of exhumed high-pressure rocks of the Koralm Complex, and medium- to low-grade metamorphic units above. According to this evolution, the Cretaceous collisional process (Eo-Alpine cycle), which formed the present Austroalpine Nappe Complex, may be subdivided into two distinct phases: The first phase is the (ES)E-ward subduction and closure of the Hallstatt-Meliata Basin, resulting in the assembly of the Upper Austroalpine Nappe Complex. After closing of the Hallstatt-Meliata oceanic basin during the Late Jurassic, the Cretaceous orogeny in the Eastern Alps encompasses the collision between (south)easternmost parts of the Austroalpine continental crust and a continental fragment to the east. The second phase involves the southward underplating of the southern Apulian continental margin and resulted in the imbrication of the Middle Austroalpine basement complex. These units were additionally affected by pronounced metamorphim, increasing from greenschist-facies conditions in the northern parts to amphibolite- and eclogite-facies conditions in its southern-most parts. Continuous underplating was accompanied by extension in the internal parts of the orogen, resulting in the formation of an extensional detachment in the lower crust (Plattengneis shear zone), and exhumation of high-pressure metamorphic rocks during the Late Cretaceous. Coeval extension in the upper plate resulted in the formation of the Gosau sedimentary basins. Toward the north, the Plattengneis shear zone continuously climbed toward shallower crustal levels, and passed into a foreland-directed thrust. This thrust is conjectured to have affected the Upper Austroalpine Nappe Complex as well as the formation of distinct out-of-sequence thrusts.

In the Graz Paleozoic in the Eastern Alps (Austria), stratiform Pb-Zn-Ag-Ba deposits have been mined in the past. They are classified as sedimentary-exhalative (SEDEX) deposits and are hosted by polyphase deformed greenschist facies metasediments and volcanics of the Schönberg Formation (Schöckel nappe; Drauzug-Gurktal nappe system), which is upper Silurian to Lower Devonian in age. The stratiform ore horizons are dominated by galena, sphalerite, barite, pyrite, pyrrhotite, and magnetite and are accompanied by chalcopyrite, arsenopyrite, fahlore, pyrargyrite, tetradymite, cobaltite, ullmannite, breithauptite, electrum, and others. The project MRI_SEDEXPOT investigates these deposits to evaluate their exploration potential by focusing on hydrothermal alteration zones occurring in the footwall and hanging wall of the ore. Such alteration zones are globally used as exploration tools not only for SEDEX deposits. The investigated alteration zones show distinct mineralogical and geochemical characteristics. They contain Fe rich carbonates and chlorite, K‑ and Ba-feldspars, Ba bearing white mica, fluorapatite, REE and HFSE minerals. Disseminated sulphides are widespread, and albitization is typical below sulphide rich horizons. Geochemical profiles correlate well with the observed mineralogy. Ongoing investigations of these alteration zones should lead into defining proximity indicators to the ore and evaluation of the exploration potential of the stratiform Pb-Zn-Ba-Ag deposits of the Graz Paleozoic.

<p>In low-grade metamorphic units, precise thermobarometric and geochronologic data are often ambiguous or entirely lacking, thus complicating the temporal interpretation of metamorphism and hampering the identification of complex polymetamorphic histories. We present new P-T-t-D data from samples collected in two Austroalpine nappes exposed in the Eastern Alps, Austria: the structurally upper greenschist-facies Schöckel Nappe (“Graz Paleozoic,” Drauzug-Gurktal Nappe System) and the structurally lower amphibolite-facies Waxenegg Nappe (Koralpe-Wölz Nappe System). Although polymetamorphism was previously inferred from garnet zonation indicating multiphase growth in the Waxenegg Nappe, the timing of metamorphism is poorly resolved and only limited geochronology exists in the Schöckel Nappe.</p><p>Detailed petrographic investigations revealed that the chloritoid-bearing phyllite and micaschist of the Schöckel Nappe contain allanite that occasionally show partial replacement by small (<10 µm) monazite and thorite. Large (up to 500 µm) monazite exhibiting distinct core-rim chemical zoning were observed in the garnet-bearing micaschist of the Waxenegg Nappe. Careful documentation of the microstructural phase relations, thermodynamic modeling in the MnCNKFMASHT system, Raman spectroscopy of carbonaceous matter and in-situ LA-ICPMS U-(Th)-Pb dating of the accessory phases allow us to reconstruct a first metamorphic imprint at ~560°C and 4 kbar in the Waxenegg Nappe at c. 270 Ma (Permian event). Overprinting occurred at ~540°C and 8-10 kbar at c. 90 Ma (Eo-Alpine event). In the Schöckel Nappe, peak metamorphic conditions of ~470°C and 3-4 kbar existed during the Permian event at c. 260 Ma and the Eo-Alpine event in the upper part of the nappe did not exceed lower to middle greenschist-facies conditions.</p><p>Our results provide unequivocal evidence for Permian metamorphism in the Schöckel Nappe, which was hitherto unknown in this part of the Austroalpine Unit. Moreover, it demonstrates that the main metamorphic signature in this unit occurred during the Permian event and that the Eo-Alpine overprint is relatively lower grade than previously proposed. Combined with the data from the Waxenegg Nappe, there is an obvious marked increase in the Eo-Alpine peak conditions of ~130°C and 5 kbar across the nappe contact with higher grade in the footwall compared to the hanging wall. This is consistent with the existence of a major normal fault between the Drauzug-Gurktal Nappe System and the Koralpe-Wölz Nappe System in the easternmost part of the Austroalpine Unit, as already identified in its central and western parts. Modern thermobarometric analytical approaches coupled with high spatial resolution geochronology on accessory minerals is allowing a more thorough assessment of the subtle metamorphic histories recorded in the fundamentally important low-grade units of orogens.</p>

Precise thermobarometric and geochronologic data are crucial to correctly interpret the timing of metamorphism and identify complex polymetamorphic histories. We present new P-T-t-D data from samples collected in two Austroalpine nappes exposed in the Eastern Alps, Austria: the structurally upper greenschist-facies Schöckel Nappe (“Graz Paleozoic,” Drauzug-Gurktal Nappe System) and the structurally lower amphibolite-facies Waxenegg Nappe (Koralpe-Wölz Nappe System). In the latter, polymetamorphism was previously inferred. However, the timing of metamorphism is poorly resolved and only limited geochronology exists in the Schöckel Nappe.Detailed petrographic investigations of chloritoid-bearing phyllite and micaschist samples collected at two localities at the base and in a higher structural level of the Schöckel Nappe revealed complex phase relations of REE-minerals, involving multiple REE-epidote generations that may be associated with monazite, xenotime, apatite and zircon. In garnet-bearing micaschist of the Waxenegg Nappe, we observed large (up to 500 µm) monazite exhibiting distinct core-rim chemical zoning. From careful documentation of the microstructural phase relations, thermodynamic modeling, Raman spectroscopy of carbonaceous matter and in-situ LA-ICPMS U-(Th)-Pb dating of REE-epidote and monazite we show that rocks in all three localities were affected by LP metamorphism (0.3 – 0.4 GPa) during the Permian event (250 – 282 Ma) with peak temperatures decreasing from 560°C in the lower to 475°C in the upper nappe. During the Eo-Alpine event, overprinting at c. 90 Ma occurred under conditions of ~550°C and 1.0 – 1.1 GPa in the Waxenegg Nappe. At the base of the Schöckel Nappe, peak metamorphism at ~450 – 470°C and 0.4 – 0.7 GPa and cooling below 300°C likely took place before 110 Ma. Towards higher structural levels, only limited Eo-Alpine overprinting at low P-T conditions (<400°C, 0.3 – 0.5 GPa) is evident, thus the observed mineral assemblage reflects mostly Permian metamorphism.Our results demonstrate that the main metamorphic signature in the Schöckel Nappe can be resolved as the Permian event and that the Eo-Alpine overprint is relatively lower grade than previously proposed. We observe a marked increase in Eo-Alpine peak conditions (~80 – 100°C, 0.3 – 0.5 GPa) across the nappe contact with higher grade rocks in the footwall compared to the hanging wall. The metamorphic pattern is consistent with the existence of a major normal fault between the Drauzug-Gurktal Nappe and Koralpe-Wölz Nappe systems in the easternmost part of the Austroalpine Unit, as already identified in its central and western parts. Finally, our study highlights that coupling modern thermobarometric analytical approaches with high spatial resolution geochronology on accessory minerals is critical to improve our understanding of the fundamentally important low-grade units of orogens.

Evolution of veins and sub-economic ore at Strassegg, Paleozoic of Graz, Eastern Alps, Austria: evidence for local fluid transport during metamorphism

The taxonomy of Zeapora Penecke, 1894 has a long but inconsistent taxonomic history. Originally identified as cyclostomate bryozoan, it was later assigned to the trepostomate bryozoans, amphiporid stromatoporoids, thamnoporid tabulate corals and dasycladacean algae before it was finally classified as Halimedaceae or Udoteaceae. Based on newly collected material from the type locality in the Graz Paleozoic (Eastern Alps, Austria) the taxonomic description of the type species Z. gracilis is emended and the udoteacean genus Litanaella Schuysky & Schirschova, 1987 is placed in synonymy with Zeapora. Following this opinion, Zeapora remains no longer endemic in the alpine Noric Terrane but shows a peculiar circum‐tropical distribution during the Early to Middle Devonian similar to modern Halimeda.

Geochemical exploration in a mountainous area by statistical modeling of polypopulational data distributions

Young uplift in the non-glaciated parts of the Eastern Alps

The Paleozoic of Graz is an isolated nappe complex of about 1,500 km2 size and belongs to the Austroalpine units of the eastern European Alps. Despite more than 500 publications on stratigraphy, paleontology and local structure, many aspects of the internal geometry of this complex as a whole remained unclear. In this contribution, we present integrated geological profiles through the entire nappe complex. Based on these profiles, we present (1) a simplified lithological subdivision into 13 rock associations, (2) a modified tectonostratigraphy where we consider only two major tectonic units: an upper and a lower nappe system and in which we abandon the traditionally used facies nappe concept, and (3) a modified paleogeography for the whole complex. Finally, we discuss whether the internal deformation of the Paleozoic of Graz is of Variscan or Eo-Alpine age and which of the published models best explain the tectonic evolution of the Paleozoic of Graz.

Effects of lithology and bulk chemistry on phyllosilicate reaction progress in the low-T metamorphic Graz Paleozoic, Eastern Alps, Austria

Thermal evolution of an extensional detachment as constrained by organic metamorphic data and thermal modeling: Graz Paleozoic Nappe Complex (Eastern Alps)

Late Cretaceous structures within the eastern Graz Paleozoic Nappe Complex define an extruding wedge with north-eastward directed thrusting in eastern portions and strike-slip shear along the margins. Stacking structures are overprinted by south-westward directed extension with low-grade metamorphic rocks in the hangingwall and high-grade basement rocks in the footwall. Pressure–temperature and structural data are obtained from successively opening quartz veins that record various stages of progressive deformation and metamorphism. Fluid inclusion data and related structures show that during extension isothermal decompression from ca. 550°C and 8 kbar down to ca. 450°C and 2 kbar was related to exhumation of rocks from deep crustal levels. The data point to a high geothermal gradient and explain condensed paleo-isotherms due to ductile normal faulting in the eastern areas of the Graz Paleozoic Nappe Complex. The investigated Late Cretaceous structural elements suggest that the Graz Paleozoic Nappe Complex decoupled from the surrounding basement units and operated as a large-scale extension–extrusion corridor that evolved prior to Miocene extrusion tectonics in the Eastern Alps.