T10

Numerous outcrops of ultramafic rocks consisting mostly of peridotites occur in the area of Banovina, in Croatia. These rocks were formed as parts of the former Earth's mantle and belong to the Central Dinaride Ophiolite Belt (CDOB), which is direct proof of the existence and closure of the Neothetys ocean in the northern part of the Balkan area. Previous studies have considered these peridotites as fertile, subcontinental parts of the mantle with complex chemistry. This research presents a more detailed petrographic and chemical analysis, focused solely on peridotites as dominant ophiolitic member, with the intention to sort between different types of Banovina peridotites and offer the model for their petrogenesis.   

Detailed field work, mapping and petrographic analyses have revealed that Banovina peridotites occur as two texturally, lithologicaly and mineralogicaly different types, that crop out in two geographically different belts, the northern (N-belt) and the southern (S-belt). The N-belt contains mostly serpentinite breccias and serpentinized, depleted and mostly porphyroclastic spinel lherzolites that occur in the form of mélange, while S-belt comprises larger masses of peridotites which consist predominantly of fertile spinel lherzolites with equigranular to porphyroclastic textures. Bulk rock analyses have shown that spinel lherzolites from the S-belt have lower Cr# and Mg# and higher content of Al₂O₃, CaO, Na₂O, TiO₂ and REE than spinel lherzolites from N-belt, and same relations, excluding the REE, can be seen in the chemistry of clinopyroxenes and orthopyroxenes. Spinels from the N-belt spinel lherzolites have a significantly higher Cr# (12,7 – 50,7) then those from the S-belt spinel lherzolites (7,7 – 10,8). Two types of dunites, which were found only within S-belt peridotites, have very different petrographic and chemical characteristics. Pyroxene rich dunite is characterized by a coarse-grained protogranular to porphyroclastic texture, high modal pyroxenes (up to 10 vol. %) and spinels enriched in MgO, Al₂O₃ and NiO. The second type of dunite has small-grained equigranular texture, contains amphibole (up to 1 vol. %), pyroxene (< 1 vol. %) and spinels enriched in Cr₂O₃ and FeO_T. Geochemical analysis of all peridotites indicate that the S-belt peridotites represent a subcontinental mantle which have been formed through the initial rifting phase during which they ascended to the upper crust. Peridotites from the S-belt are classified as orogenic peridotites. The geochemical characteristics of N-belt peridotites indicate their origin from a suboceanic mantle formed within mid ocean ridge environment and are classified as ophiolitic peridotites. Dunites show different geochemical characteristics and may have been formed by different geological processes. The diverse lithology of ultramafics in the limited space of the S-belt indicates very heterogeneous nature of the subcontinental mantle. As a part of the CDOBs, peridotites from Banovina indicate that the CDOB record three different phases of ocean evolution, the early phase of the initial rift and opening of the ocean (S-belt peridotites), later phase of the already developed ocean (N-belt peridotites) and also the phase of ocean closure which is evident from the mélange occurrences.

How to cite: Bilić, Š., Garašić, V., and Woodland, A. B.: Two types of peridotites in the area of Banovina, Croatia and their petrogenesis, 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-10, https://doi.org/10.5194/egusphere-alpshop2022-10, 2022.

The Middle Triassic period represents a very dynamic time in a broader Tethyan region. Tectonic movements related to disintegration of Pangea and opening of the Neotethys gave rise to the formation of volcanic and volcaniclastic deposits. In the NW Croatia, these rocks outcrop in approximately 60 km long intra-Pannonian Mountain chain (Mts. Kalnik, Ivanščica, Strahinjščica, Kuna Gora, Desinić Gora and Ravna Gora), representing the junction between the Southern Alps and Internal Dinarides. Among these mountains, Mt. Ivanščica represents a complex structure built of shallow-marine to pelagic successions originating off the passive continental margin of Adria microplate. These deposits are found in a tectonic contact with ophiolitic mélange containing remnants of Neotethys. Middle Triassic volcanic and volcaniclastic rocks from this area include basic/intermediate to acidic effusive and pyroclastic lithologies and are interfingered with marine sediments.

The specific area of Vudelja quarry, situated in the central part of the northern Ivanščica slopes, is composed mainly of mafic lithologies and their volcaniclastic derivates. Volcanic and volcaniclastic rocks of basaltic composition were studied in detail to distinguish different facies and their spatial distribution. Facies characteristics enabled the interpretation of their genesis. Three different facies were recognized: 1) effusive coherent facies; 2) pyroclastic flow facies, and 3) resedimented autoclastic facies. Effusive coherent facies is composed of hyaline basalts, with incorporated clasts of the same lithotype. This facies was formed by effusion of basaltic magma. Basaltic clasts were likely incorporated by the primary effusive flow during magma ascent, or following effusion while flowing over the fragmented basaltic material. Quenching fragmentation of a basaltic effusion in the marine environment is another possible process explaining the formation of basalt clasts. Pyroclastic flow facies is composed of plagioclase crystalloclasts, basaltic lithoclasts and scoria fragments, with the flow texture indicated by clast arrangement and glassy matrix with flow features. This facies occurs only locally and its geographical extent is limited thus indicating a small volume of the flow, characteristic for basaltic pyroclastic flows of scoria and ash type. Resedimented autoclastic facies is dominant at the outcropping quarry front and is composed of several lithotypes with various grain sizes and types of matrix. Clast dimensions vary from block to ash size. Some samples exhibit common sedimentary features such as horizontal lamination. The facies was formed by the autofragmention processes of hot basaltic magma in contact with sea water, and subsequent resedimentation of newly formed volcaniclastic particles. Due to an intense tectonic disruption, spatial organization of determined facies is not clear, though the alienation from the primary source can generally be followed from south to north.

Studied locality presents a portion of Middle Triassic volcanic and volcano-sedimentary formations well known from the Southern Alps, Transdanubian Range and the Dinarides. Intense volcanic activity, related to the rifting between the future Adria microplate and southern edge of Laurasia, fed the material to a complicated pattern of sedimentary environments formed along the future Adria passive margin. Extensional tectonics created deep faults within the continental crust which might have served as conduits for submarine basaltic extrusions.

How to cite: Smirčić, D., Vukovski, M., Slovenec, D., Kukoč, D., Belak, M., Grgasović, T., Šegvić, B., and Badurina, L.: Differentiation and genesis of the Middle Triassic mafic volcaniclastic facies in NW Croatia, a case study from Vudelja quarry, 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-43, https://doi.org/10.5194/egusphere-alpshop2022-43, 2022.

Alpine landscapes form in mountain belts that likely experienced tectonic uplift during plate’s convergence, and efficient erosion dominated by glacial carving and circle retreat. In southern Patagonia N-S oriented late Miocene plutonic complexes are exposed in deep incised valleys with summits topographically above the glacial equilibrium line altitude. Two of the most emblematic ones are the Fitz Roy (Chaltén, latitude 49°S) and the Torres del Paine (latitude 51°S) plutonic complexes, ~2 km higher than the mostly flat bottom valley that is partially covered by the Southern Patagonian Icefield. This continental region is located above an asthenospheric window that opens and migrates from the latitude 54 °S towards the latitude 46 °S since ~16 Ma, and experienced dynamic uplift during episodes of spreading ridge collision with the continental margin. Here we present a new dataset of combined low-temperature thermochronometers from the Chaltén and Torres del Paine plutonic complexes, and their thermal history inversion numerical modeling, to identify the geodynamic processes forcing on the exhumation of the mountain belt. These complexes are separated by 200 km along the strike of the belt, and share a pulse of rapid exhumation at ca. 6 Ma, likely showing that glaciation was regionally starting at this moment. After a period of quiescence, in Torres del Paine the exhumation rate is accelerated from ~2 Ma to the present, interpreted as a signal of the Pleistocene climatic transition creating incise valleys. Only in the Fitz Roy a pulse of rapid exhumation is present at ca. 10 Ma, approximately coincident with the time range in which the ridge was subducting beneath the continent at that latitude. This allows us to separate the climatic from the tectonic/mantle forcing to the exhumation in southern Patagonia, and represents the first in-situ observation of the passage of the asthenospheric window in the low-temperature thermochronometric record of the region.

How to cite: Paiva Muller, V. A., Sue, C., Valla, P., Sternai, P., Simon-Labric, T., Gautheron, C., Martinod, J., Ghiglione, M., Baumgartner, L., Hérman, F., Reiners, P., Grujic, D., Shuster, D., Braun, J., Husson, L., and Bernet, M.: Exhumation response to climate and tectonic forcing in the southern Patagonian Andes (Torres del Paine and Fitz Roy plutonic complexes), 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-4, https://doi.org/10.5194/egusphere-alpshop2022-4, 2022.