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This contribution provides insights into ocean-floor hydrothermal metamorphism of the fast-evolving Dinaridic Neotethys. Mineralogical, geochemical and Sr isotope data collected from altered ophiolites and non-ophiolite basalts/andesites and tuffs of the active continental margin are consistent with hydrothermal alteration trajectories that reflect the host-rock composition. This suggests that hydrothermal fluxes were restricted within a simple closed seawater-fed system. Based on the initial isotopic ratios of Sr, two fluid–rock interaction trends are established: (a) low-to-medium degrees of metasomatism in pre-Middle Jurassic anorogenic ophiolites that progressively abated, and (b) increased intensities of metasomatism in post-Middle Jurassic orogenic ophiolites. This agrees with chlorite thermometry and Ca-Al-(Fe)-silicate phase chemistry. The metamorphic assemblages belong to the zeolite, prehnite-pumpellyite, prehnite-actinolite and greenschist facies. The facies is reliant on the temperature of hydrothermal systems and their fluid chemistry. Rare earth element (REE) phase geochemistry shows (a) variable fluid–rock ratios in chlorite and pumpellyite dependent on fluid temperatures, (b) prominent Eu and Ce anomalies that reflect the fluid oxidation state, (c) light REE/heavy REE mobilization attributed to prevalent ligand complexation, and (d) multi-phase fluid percolation across reaction zones of heterogeneous permeability. This study proposes initiation of simple hydrothermal system(s) at or near a spreading centre(s) in the infancy of the Dinaridic Neotethys. Such a system became more complex during Middle Jurassic and Early Cretaceous time with reactive hydrothermal fluids passing the recharge area and reaching the hot reaction zone. An abrupt obliteration of the established high-temperature regime ensued, following the final closure of the Neotethys.

This study provides new insights into the distribution of rare earth elements (REE) in a range of chlorite from hydrothermally altered mafic rocks from the Dinarides. Low-medium grade metamorphic conditions to include zeolite, prehnite-pumpellyite, and greenschist facies assemblages reveal unique geochemical signatures of chlorite, thus reflecting the interaction between hydrothermal fluids and host rock lithologies. A diverse set of chlorite have been identified and may comply with characteristic rare earth element signatures that can be categorized and defined by specific geochemical and textural parameters. Fe-rich chlorite may contain significant enrichments in REE in contrast to Mg- rich members, particularly with the LREE. Additionally, numerous textural varieties such as amygdales and pseudomorphic chlorite have been identified and may also reflect their own distinct trace element signatures. This research will (a) use stable isotope geothermometric methods to constrain the formation conditions of a variety of chlorite, (b) make estimates on redox state using the enrichment/depletion of Ce and Eu, and (c) make inferences concerning the sources of REE which provide the initial rare earth element budget of the fluids, all to provide further support for the rare earth elemental codependency on the conditions in which they themselves and the fluid have been subjected to.

Ivanščica Mt. is an inselberg in the transitional area where S-verging Southern Alpine structures overprint NNW-verging Dinaric structures (van Gelder et al. 2015, and references therein). It is composed of Triassic to Cretaceous shallow to deep-marine sedimentary succession of the Adriatic continental passive margin (Šimunić et al. 1976), overthrust by ophiolitic mélange (Babić et al. 2002). Early Cretaceous siliciclastic flysch-type deposits continuously overlaying pelagic limestones clearly indicates distal position of these successions on the continental margin. However, so far complete Jurassic succession on Mt. Ivanščica was never found, causing different interpretations of its Mesozoic history. Babić (1974) assumed the existence of Jurassic pelagic succession on top of the Late Triassic shallow-water carbonates, describing up to few meters of Early to Middle Jurassic condensed pelagic limestone overlain by Late Jurassic radiolarian cherts and pelagic limestones. Contrary, Šimunić et al. (1976) assumed shallow-marine conditions throughout the Early Jurassic followed by emersion until latest Jurassic. Our study revealed for the first time complete shallow-water to pelagic Jurassic succession on Mt. Ivanščica. Southern slopes of Ivanščica Mt. are mostly built of ophiolitic mélange, except for prominent hills built of the Late Triassic Dachstein limestone, which were so far interpreted as klippes (Šimunić et al. 1976), or olistoliths (Babić & Zupanič 1978). Our new data indicate continuous succession on top of Dachstein limestones composed of shallow-marine carbonates, represented by intraclastic-peloidal packstones to grainstones, gradually transitioning to wackestones with pelagic influence. The onset of pelagic sedimentation took place around the end of the Early Jurassic when thin bedded marls, shales, marly limestones with intercalated fine-grained calciturbidites were deposited. Higher in the succession Callovian to possibly early Tithonian radiolarian cherts are overlaid by calcarenites, late Tithonian to Early Cretaceous pelagic limestones and Early Cretaceous flysch-type deposits. Discovery of the Jurassic pelagic sediments allows for a new interpretation of structural relations on the Ivanščica Mt. In our opinion occurrence of Dachstein limestone, previously interpreted as klippes or olistoliths represent an imbricate fan. Because the Southern Alps thrust front in this area was interpreted according to these mapped klippes and nappe contact, our findings may also have an impact on the redefining of the easternmost Southern Alps thrust front. Babić, Lj. (1974). Jurassic−Cretaceous sequence of Mt. Ivanščica (North Croatia). Bull. Sci. Cons. Acad. Yugoslav. (A), 19/7−8, 180−181. Zagreb. Babić, Lj. & Zupanič, J. (1978). Mlađi mezozoik Ivanščice. In Babić, Lj. & Jelaska, V. (ured.): Fieldtrip Guidebook 3. Skupa sedimentologa Jugoslavije, 11–23. Hrvatsko Geološko Društvo, Zagreb. Babić, Lj., Hochuli P. A.. & Zupanič, J. (2002). The Jurassic ophiolitic mélange in the NE Dinarides: dating, internal structure and geotectonic implications. Eclogae Geologicae Helvetiae 95, 263–75. Šimunić, A., Pikija, M., Šimunič, Al., Šikić, K. & Milanović, M. (1976): Stratigrafsko - tektonski odnosi centralnog i istočnog dijela Ivanščice. 8. Jugosl.geol.kongres, Bled, 1974., 2, 3003-313, Ljubljana. van Gelder, I., Matenco, L., Willingshofer, E., Tomljenović, B., & Andriessen, P., Ducea, M., Beniest, A. & Gruić, A. (2015). The tectonic evolution of a critical segment of the Dinarides-Alps connection: Kinematic and geochronological inferences from the Medvednica Mountains, NE Croatia. Tectonics. 34. n/a-n/a. 10.1002/2015TC003937.