T12

We have studied eclogite, garnet clinopyroxenite, and garnet-bearing micaschist and gneiss from the southeastern flank of the Pohorje Mountains (Mts.) in order to better understand the pressure-temperature (P-T)-time evolution of these rocks. Geochronology was performed by in-situ analyses of monazite in different textural positions with an electron microprobe and a laser-ablation inductively coupled plasma mass-spectrometer. P-T trajectories were obtained by thermodynamic modelling considering strongly the chemical zoning of garnet and mica and the mineral inclusions in these phases. In addition, we calculated the influence on intracrystalline cation diffusion on garnet zoning also to gain time constraints.

Two high-pressure (HP) events were proved for metamorphic rocks of the Pohorje Mts. These events occurred at temperatures between 570-650 °C for micaschist and 670-740 °C for eclogite + garnet clinopyroxenite in Late Cretaceous and Eocene times. In addition, we found that a micaschist sample taken close to the Pohorje pluton was partially overprinted in the Miocene (18.9±0.2 Ma) by this intrusion at depths of 30-32 km. Thus, the subsequent uplift of the Pohorje pluton and its surrounding occurred at a mean rate of 1.6-1.7 mm/a. The studied metamorphic rocks were also significantly exhumed probably soon after the Eo-Alpine event that had led to peak pressures up to about 2.3 GPa. This exhumation was accompanied by cooling. Another burial process followed during which Eo-Alpine rocks were significantly overprinted at peak pressures up to 2.4 GPa in the Eocene. For example, two generations of potassic white mica (phengite) formed in micaschist. The Eo-Alpine one was relatively coarse grained, whereas the Eocene generation replaced this coarse-grained phengite by newly grown small flakes. No indications for ultrahigh-pressure metamorphism were found.

We interpret our findings, including previous results on rocks of our study area in the Pohorje Mts., in a geodynamic context as follows: A first collision of continental (micro)plates occurred in the Late Cretaceous after a branch of the Neotethys Ocean was closed. The subduction of the corresponding oceanic plate including sediments on top led to eclogite (+ HP garnet clinopyroxenite) and HP micaschist which were exhumed during the continent-continent collision in an exhumation channel. About 45 Ma after this Eo-Alpine collisional event, another part of the Neotethys Ocean was closed followed by a second collision of continental (micro)plates. This process led to clearly overthickened crust and deep burial of rocks residing in the Eo-Alpine exhumation channel. Exhumation of the studied metamorphic rock units, probably mainly caused by surface erosion, followed this Eocene collisional event. A particular event in the Miocene is characterized by intrusions of large volumes of acidic magma. These intrusions formed the Pohorje pluton, which produced discernable contact metamorphism, for instance in micaschist, close to its margin.

How to cite: Massonne, H.-J. and Li, B.: Pressure-temperature-time evolution of Austroalpine metamorphic rocks from the southeastern Pohorje Mountains, 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-7, https://doi.org/10.5194/egusphere-alpshop2022-7, 2022.

Skarnoid calc-silicate xenoliths composed of anorthite, clinopyroxene and Mg-Al spinel were discovered in an alkali basalt quarry located in the Belinsky vrch lava flow, near Fiľakovo (Southern Slovakia). Randomly oriented tschermakite pseudomorphs are replaced by olivine, spinel, and plagioclase. The relict amphibole within the pseudomorphs is characterized by high ^VIAl (1.95 to 2.1 apfu), and very low occupancy of the A-site (<0.1 apfu), which are a diagnostic feature of high-pressure metamorphic rocks. Pyroxene compositions plot along continuous mixing line extending from nearly pure diopside-augite towards a Ca(Fe³⁺Al)AlSiO₆ endmember with an equal proportion of ^VIAl³⁺ and Fe³⁺. Forsterite (Fo_72–83) and Fe³⁺-rich ilmenite crystallized from the melt, leaving behind the residual calcic carbonate with minor MgO (1–3 wt%). Euhedral aragonite and apatite embedded in the fine-grained calcite or aragonite groundmass indicate slow crystallization of residual carbonatite around the calcite-aragonite stability boundary. Olivine-ilmenite thermometry (Andersen & Lindsley, 1981) yielded temperatures between 770 and 860 °C. Pressures of 1.8–2.1 GPa were estimated by intersection of the olivine-ilmenite thermometer with the calcite-aragonite stability boundary calculated for a CO₂ saturated environment using Perple_X (Connolly, 1990). Tschermakite touching interstitial plagioclase was suitable for the application of the barometer of (Molina et al., 2021), which yielded 781±13 °C and 2.05±0.03 GPa consistent with the olivine-ilmenite-calcite-aragonite thermobarometry. The estimated PT conditions fall well inside the garnet stability field, although no garnet has been observed in the mineral assemblage. However, the presence of esseneite and kushiroite with melilite inclusions suggest high CO₂ partial pressure, low SiO₂ activity and strongly oxidizing conditions, in which the high Al, Fe pyroxenes are formed at the expense of the garnet (Ohashi & Hariya, 1975). The protolith is still ambiguous, and two options have been considered. The relict tschermakite in spinel-plagioclase-forsterite pseudomorphs suggests a metamorphosed calc-silicate marble originating from a sedimentary protolith. High Cr contents in spinel and pyroxene, abundant Cu-sulfides, and high CaO contents, 0.3–1.0 wt% CaO, in forsterite, suggest a magmatic protolith, similar to layered gabbro-anorthosite complexes modified by interaction with calcic carbonatite melt.

References:

Andersen, D., & Lindsley, D. (1981). A valid Margules formulation for an asymmetric ternary solution: revision of the olivine-ilmenite thermometer, with applications. Geochimica et Cosmochimica Acta, 45(6), 847–853.

Connolly, J. (1990). Multivariable phase diagrams; an algorithm based on generalized thermodynamics. American Journal of Science, 290(6), 666–718.

Molina, J., Cambeses, A., Moreno, J., Morales, I., Montero, P., & Bea, F. (2021). A reassessment of the amphibole-plagioclase NaSi-CaAl exchange thermometer with applications to igneous and high-grade metamorphic rocks. American Mineralogist, 106(5), 782–800.

Ohashi, H., & Hariya, Y. (1975). Phase relation of CaFeAlSiO6 pyroxene at high pressures and temperatures. The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists, 70(3), 93–95.

How to cite: Reato, L., Huraiová, M., Konečný, P., and Hurai, V.: Formation of esseneite and kushiroite in calc-silicate skarnoid xenoliths from Southern Slovakia, 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-11, https://doi.org/10.5194/egusphere-alpshop2022-11, 2022.

The Alps, as part of the Alpine-Mediterranean Mountain chain, are one of the classical localities for orogenic studies, where the Mesozoic-Cenozoic tectonic evolution is well known. Many classical models have been proposed to explain the tectonic evolution from Mesozoic rifting and breakup to Late Mesozoic-Cenozoic subduction, plate collision and exhumation. However, the pre-Mesozoic tectonic evolution of the pre-Alpine basement remains poorly known because of the lack of sufficient age data due to complex polyphase deformation and multiple metamorphic overprints. New data from mainly amphibolite-facies pre-Alpine basement of the Austroalpine mega-unit indicates that this basement is composed of a heterogeneous series of continental units, island arcs, ophiolites, subduction mélanges, accretionary wedges, and seamounts affected by different metamorphic grades. This study (Chang et al., 2021) presents new results of LA-ICP-MS U-Pb zircon dating and MC-ICP-MS Lu-Hf isotopic tracing of zircons from three key areas of Austroalpine basement, including the: i) Wechsel Gneiss and Waldbach Complexes, and Wechsel Phyllite Unit, (ii) Saualpe-Koralpe-Pohorje, and (iii) Schladming areas. We determine the Wechsel Gneiss Complex to be a continental magmatic arc formed during 500–560 Ma in the proximity to a continental block with a ‘memory’ of Late Archean to Early Proterozoic continental crust. The Wechsel Gneiss Complex has Hf model ages of 2.1 to 2.2 Ga and 2.5 to 2.8 Ga that indicate a close relationship to northern Gondwana, with depleted mantle Hf model ages as old as 3.5 Ga. The Wechsel Phyllite Unit structurally overlying the Wechsel Gneiss Complex has partly different sources, including juvenile crust formed at ca. 530 Ma. In contrast, the Waldbach Complex constantly added new crustal material during 490–470 Ma period and bears considerably more positive εHf(t) values than the underlying Wechsel Gneiss Complex and gives relatively young, depleted mantle model ages of 700 to 500 Ma. The Waldbach Complex is, therefore, interpreted to be part of a magmatic arc that formed during closure of the Prototethys and was metamorphosed during Variscan orogenic events at ca. 350–330 Ma. The Schladming-Seckau and Wechsel Complexes represent a Cambro-Ordovician magmatic arc system formed by Prototethys subduction processes with the associated Late Neoproterozoic to Early Ordovician ophiolitic Speik complex having formed in its back-arc basin or as Prototethyan lithosphere. The Plankogel Complex and structurally overlying micaschist and amphibolite units represent accreted ocean, ocean island, and continent-derived materials, interpreted to be an accretionary complex formed during the Permo-Triassic closure of the Paleotethys. Many granites with Permian ages (e.g., porphyric granite called Grobgneiss and other granite gneisses and associated pegmatites) were likely formed in an extensional environment that culminated in the opening of the Middle-Late Triassic Meliata oceanic rift. These granites formed by partial remelting of crust with mainly Middle Proterozoic Hf model ages. Taken all these data together, we find that the Austroalpine basement is heterogeneously composed and includes complexes of different ages, different tectonic evolutionary histories and different remolten sources representing different locations before final accretion. The composite of pre-Alpine complexes in the Austroalpine mega-unit likely assembled not earlier than Late Permian or Early Triassic.

How to cite: Chang, R., Neubauer, F., Genser, J., Liu, Y., Yuan, S., Guan, Q., and Huang, Q.: Hf isotopic constraints for Austroalpine basement evolution of Eastern Alps: review and new data, 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-29, https://doi.org/10.5194/egusphere-alpshop2022-29, 2022.