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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.

The low-grade metasedimentary rocks of Medvednica Mt. (Northwest Croatia) are associated with ortho-greenschists (metabasalts). The Lower to Upper Triassic age of the protolith of the low-grade metamorphic rocks belonging to the Stari Potok, Adolfovac, Tusti Breg, Bliznec, Mrzljak, and Sljeme lithostratigraphic units is determined on the basis of conodonts and field relationships. The metamorphic age of the Triassic metasedimentary rocks from the Risnjak lithostratigraphic units is assumed to be Jurassic. Petrographic and geochemical analyses of ortho-greenschists point to a tholeiite basic magmatic protolith rock having T-MORB (transitional midocean-ridge basalt) characteristics. The ortho-greenschists of the Sljeme lithostratigraphic unit underwent retrograde metamorphism and show metamorphic zoning from the epidote–amphibolite facies to the lower parts of the greenschist facies. However, the greenschists of the Tusti Breg and Bliznec units are characterized by prograde metamorphism ranging from lower to higher greenschist facies. This metamorphism is associated with Jurassic subduction (165–150 Ma) and closure of the Western Tethys. Retrograde metamorphism took place during the collision of the Adriatic and Eurasian plates in the Lower Cretaceous (125–110 Ma).

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.

This contribution reports new findings on petrogenesis and possible geotectonic setting of rare subvolcanic Triassic(?) dolerite cropping out in External Dinarides with a goal to contribute to the debate on geodynamic evolution of the continental margin of the Adriatic-Dinaridic platform(s) at the onset of the Mesozoic. Petrography of studied rocks revealed the crystallization order: clinopyroxene → plagioclase → alkali-feldspar ± amphibole ± Fe-Ti oxides. The geochemistry of calc-alkaline dolerite suggested magma generation from a spinel-garnet bearing peridotite mantle source (60sp:40grt). Two mechanisms explaining the origin of dolerite are here suggested. The first involves a low-grade partial melting of the altered lithospheric mantle and the subduction-modified oceanic lithosphere accompanied with subducted/recycled sediments. The second proposes a melting of the subduction- altered heterogeneous lithospheric (subcontinental) mantle contaminated and/or metasomatized during an earlier Hercynian subduction in the Late Paleozoic. Accordingly, two geodynamic concepts have been suggested: (i) Middle Triassic north-vergent active subduction of the Paleotethys beneath the Andean-type active Laurassian continental margin coupled by the formation of a pericontinental volcanic arc and subsequent extension and back-arc rifting, within the Adria Plate, which led to the formation of locally aborted rift systems and subsidence of the platform and (ii) Middle Triassic extension, unaffected by contemporaneous active subduction, of the High Karst nappe unit of the Greater Adria as a result of an ephemeral syn-rift volcanic stage which featured the formation of the Dinaridic aborted rift system followed by the platform subsidence and emergence of local sedimentary basins.

This work aims to unveil the origin, geodynamic significance, and diagenetic history of pyroclastites and associated chert documented within the Upper Anisian volcanosedimentary succession of the Ivanščica Mt. in Central Europe. An abundance of pyroclastic material points to polyphase volcanic activity and deep‐water sedimentation along the rim of an oceanic realm. Radiolarian‐based dating revealed Illyrian age of chert intercalated with pyroclastites. The latter are largely vitro‐crystalloclastic and were emplaced as airborne tuff. The crystalloclasts of sanidine, plagioclase, altered pyroxene, and amphibole are principal tuff constituents merged in the altered glassy matrix consisted of palagonite, clay minerals, and calcite. Variations in alteration assemblages reflect an extensive in situ eogenesis in an open hydrologic system that gradually evolved toward a restricted fluid percolation environment. Mineralogy, chemistry, and occurrence of andesitic tuff clearly denote an explosive volcanic activity formed at the top of suprasubduction zone. Such scenario presumes a complex genesis outlined in following steps: (a) partial melting and dehydration of down‐going Palaeotethyan slab which gave rise to the subduction‐related magmatism {; ; LILE, Th, and LREE [(La/Lu)cn = 6.51−9.42] enrichment ; negative anomalies of Nb‐Ta, P, and Ti [e.g., (Nb/La)n = 0.31−0.44]}; ; ; (b) magma contamination via interaction with continental crust during magma uplift along tectonically weakened zones of upper crust [positive Pb spikes, negative εNd (−4.18 to −4.44), and 147Sm/144Nd ≤ 0.113175]. This is in favour of geodynamic evolution that hypothesizes the existence of an active, ensialic, and mature volcanic arc developed along the southern active continental margins of Euramerica (Laurussia) during Late Anisian subduction of Palaeotethyan lithosphere.

Ultramafic cumulate rocks represent the rarest allochthonous fragments of the Mesozoic oceanic lithosphere observed today in the Upper Jurassic to Lower Cretaceous mélange of Mt. Kalnik, located in the SW part of the Zagorje‒ Mid-Transdanubian Zone (ZMTDZ). Poikilitic heteroadcumulate ultramafic rocks of Mt. Kalnik are represented by amphibole lherzolites/ harzburgite sand plagioclase lherzolites. Both were formed by in-situ processes within a magma chamber following the general crystallization sequence of: Al-chromite → Mg-rich olivine → enstatite ± augite→ Ca-amphibole (pargasite ± edenite ± magnesiohornblende) → Ca-plagioclase (An82.6-87.4). Cumulate minerals are spinel and olivine as well as orthopyroxene and clinopyroxene which areusually enclosed in intercumulate phases such as amphibole and/or plagioclase that render aninterstitial mesostasis. Rocks’ textural characteristics, mineral crystallization order and their phase chemistry are all suggestive of low-pressure sub-solidus crystallization in an open system. The low Ti content in augite and scant HFSE abundances suggest the studied rocks may have formed from a depleted mantle source. In addition to the medium to high degree of partial melting of the source, the parental process that gave rise to the Mt. Kalnik ultramafic cumulates also included a low degree of fractional crystallization. The segregation of oxidized Al-chromite and oikocrysts of pargasite and edenite in an early crystallization stage illustrates the formation of a cumulate sequence from volatile-rich magmas. These magmas usually have a high oxidation potential and are exclusively found in intra-oceanic subduction zones, predominantly in island arcs. The overall whole-rock geochemistry [e.g. (Nb/La)n = 0.25- 0.34 ; (Ti/Gd)n = 0.49-0.89 ; (Th/Nb)n = 5.29- 8.63 ; (La/Lu)cn = 0.57-0.68] together with a record of Ca-rich plagioclase (up toAn87.4) and low Ti clinopyroxene (≤0.54 wt%) corroborate the supra-subduction tholeiitic nature of the magma source. Ultramafic cumulates from the ophiolitic mélange of Mts. Kalnik and Medvednica show common genetic features and geotectonic provenance. Comparison with analogous ultramafic lithotypes of the north- eastern segment of the ZMTDZ (the Szarvaskö Complex, Hungary), the ultramafic cumulates of Mts. Kalnik and Medvednica portray some subtle differences that may indicate their distinctive geotectonic provenance. Mts. Kalnik and Medvednica ultramafic cumulates represent the vestiges of a single Upper Jurassic intra- oceanic arc system formed in the western branch of the Meliata-Maliak segment of the Neotethyan oceanic realm.

The Dinaridic segment of Neotethys was affected by a widespread shortening and related subduction-accretion-obduction processes that commenced in the middle Jurassic. In the Dinarides, the Krivaja-Konjuh Ophiolite Complex (KKOC) stands as the largest ophiolite complex with a well-exposed metamorphic sole which is the key to understanding the dynamics of intraoceanic subduction initiation in this part of Neotethys. In this contribution we present Sm-Nd geochronology on a granulite facies amphibolite from the KKOC, as well as a detailed petrological description. A five-point isochrone age calculated from clinopyroxene, plagioclase, garnet, amphibole and whole rock is 162 ± 14 Ma (MSWD = 6.2), whereas garnet and whole rock yield 160 ± 7 Ma. Ages calculated from all data points except clinopyroxene are 162 ± 5 Ma (MSWD = 1.09). Petrographic investigations suggest that these ages date granulite facies metamorphic conditions (i.e. peak metamorphism of Grt-Cpx amphibolite) rather than post-peak exhumation or obduction processes. Phase textural relationships are in line with previous research, which indicated a peak metamorphism equilibration pressure and temperature of ~1 GPa and ~800 °C, respectively. Granulite facies conditions are elucidated for an igneous precursor, which underwent a multi-stage metamorphism that gave rise to recrystallization of igneous clinopyroxene and plagioclase, epitaxial growth of amphibole, and garnet blastosis. Taking into account the age of gabbronorite from the youngest segment of the KKOC oceanic crust (Taorcian to Bathonian) and ages of radiolarian assemblages from the KKOC mélange (Bajocian to Bathonian), it may be inferred that within ~25 Ma the Dinaridic segment of Neotethys evolved rapidly from active ridge spreading through a stage of intraoceanic subduction and arc magmatism toward sub-ophioliteexhumation and further obduction along the Adria passive margin at the end of the Jurassic era.

Geochemistry, petrology and tectonomagmatic significance of basaltic rocks from the ophiolite mélange at the NW External-Internal Dinarides junction (Croatia)At the NW inflexion of the Sava-Vardar Suture Zone ophiolite mélanges, known as the Kalnik Unit, form the surface of the slopes of several Pannonian inselbergs in the SW Zagorje-Mid-Transdanubian Zone. The Mt Samoborska Gora ophiolite mélange, thought to be a part of the Kalnik Unit, forms a separate sector obducted directly onto Dinaric Triassic carbonate sediments. Basaltic rocks, the only magmatic rocks incorporated in the mélange, include Middle-Triassic (Illyrian-Fassanian) alkali within-plate basalts and Middle Jurassic (uppermost Bathonian-Lower Callovian) tholeiitic basalts. The latter sporadically constitute composite olistoliths, and are geochemically divided into N-MORB-like (high-Ti basalts) and transitional MORB/IAT (medium-Ti basalts). These geochemically different rocks suggest crystallization at various tectonomagmatic settings, which is also indicated by the rock paragenesis and host clinopyroxene compositions. Alkali basalts reflect melts derived from an OIB-type enriched mantle source [Ti/V= 62.2-82.4; (La/Lu)cn= 6.4-12.8] with Nd-Sr isotope signatures close resembling the Bulk Earth [εNd(T=235 Ma)= + 1.6 to + 2.5]. They are recognized as preophiolite continental rift basin volcanic rocks that closely predate the opening of the Repno oceanic domain (ROD) of the Meliata-Maliac ocean system. The high-Ti and medium-Ti basalts from composite blocks derived from a similar depleted mantle source (εNd(T=165 Ma) = + 6.01 vs. + 6.35) succesively metasomatized by expulsion of fluids from a subducting slab leading to a more pronounced subduction signature in the latter [Ti/V=31.6-44.8 and (Nb/La)n=0.67-0.90 vs. Ti/V=21.5-33.9 and (Nb/La)n=0.32-0.49]. These composite blocks indicate crust formation in an extensional basin spreading over the still active subducting ridge. The majority of high-Ti basalts may represent the fragments of older crust formed at a spreading ridge and incorporated in the mélange of the accretionary wedge formed in the proto-arc-fore-arc region. The Mt Samoborska Gora ophiolite mélange represents the trailing edge of the Kalnik Unit as a discrete sector that records the shortest stage of tectonomagmatic evolution related to intraoceanic subduction in the ROD.

Mafic intrusive rocks are subordinately represented fragments of the oceanic crust in the ophiolite mélange exposedat Mts. Kalnik and Ivanščica located in the NW Dinaric-Vardar ophiolite zone. This ophiolite mélange occurs in thenorthern area of the Kalnik Unit and represents the SW surface boundary of the Zagorje-Mid-Transdanubian ShearZone. The mélange, except for mafic intrusive rocks, consists of a chaotic mixture of various extrusive rocks formedin different tectonic settings of the Repno Oceanic Domain (ROD). The ROD was the segment of Neo-Tethys thatconnects the Meliata-Maliak and Dinaric-Vardar oceanic systems. Previous study of mafic extrusive sequences suggestedan 80 Ma period of tectono-magmatic evolution of the ROD from intra-continental rifting during the Anisian,to the formation of proto-arc crust during the Callovian-Oxfordian. The domain exposes ophiolitic rocks in four mélangeareas. Isotropic gabbroic rocks that are abundant in two northern areas (Mts. Kalnik and Ivanščica), can be discriminatedinto three distinct geochemical groups: (A) N-MORB-type gabbro [(Th/Nb)n = 0.99–1.10; (Nb/La)n =0.95–0.99], (B) IAT-type amphibole gabbro with clear supra-subduction characteristics [(Th/Nb)n = 6.04–8.16; (Nb/La)n = 0.32–0.42] and (C) BABB-type amphibole-bearing gabbro [(Th/Nb)n = 2.88–4.02; (Nb/La)n = 0.58–0.69].Representative gabbro samples of each geochemical group were dated by the Ar-Ar and/or the K-Ar dating method.The Early Jurassic N-MORB-type gabbros (geochemical group A), ~185 Ma old, signifies a peculiar stage of Palaeo-Tethyan slab break-off. The Late Jurassic IAT-type gabbros (geochemical group B), ~147 Ma old, is the vestige of anascent intra-oceanic arc, whilst the Early Cretaceous BABB-type gabbros (geochemical group C), ~100 Ma old,provides evidence of magmatism in the back-arc marginal basin. The analyzed gabbroic rocks enable refinement andcompletion of the geodynamic evolution of the ROD, from the opening of an ensialic back-arc basin during the Ladinianand a continuous spreading event until the Bajocian. Intra-oceanic convergence was initiated in the Bathonian,with the formation of a nascent island-arc during the Tithonian, leading to formation of a Cretaceous ensimatic backarcmarginal basin. There are many lines of evidence that correlate the geodynamic evolution of the ROD with theAlbanide-Hellenide Neo-Tethyan oceanic segment.

Uzorci djelomično ili potpuno izmijenjenih bazaltnih ekstruzivnih stijena iz ofiolitnog melanža Medvednice analizirani su primjenom optičke mikroskopije i rendgenske difrakcije u prahu. Kemijski sastav minerala u različitim paragenezama određen je pomoću elektronske mikrosonde, a strukture analiziranih mikrodomena su dokumentirane slikama unatrag raspršenih elektrona. Glavni alteracijski procesi uključuju albitizaciju, kloritizaciju, epidotizaciju, prehnitizaciju, pumpellyitizaciju, zeolitizaciju i titanitizaciju. Sporedni procesi su limonitizacija, kalcitizacija, zoisitizacija, sideritizacija, hematitizacija, manganizacija, piritizacija i silicifikacija koji nisu detaljno razmatrani. Albitizacija plagioklasa je dominantni alteracijski proces čije je postupno napredovanje jasno vidljivo na slikama unatrag raspršenih elektrona. Komponente izlužene alteracijom plagioklasa i klinopiroksena, moguće i uz komponente izlužene iz sekundarnih minerala prve generacije, omogućuju stvaranje klorita, epidota, pumpellyita i kalcita. U izvorno vitričnim uzorcima dio klorita i pumpellyita je nastao devitrifikacijom vulkanskog stakla. Alteracijom plagioklasa također nastaju i prehnit i laumontit. Obilato zastupljene nakupine leukoksena u svim uzorcima stijena predstavljaju guste agregate nanokristalnog titanita. Titan za stvaranje leukoksena potječe iz ilmenita odnosno titanom bogatog magnetita. Parageneze s epidotom bilježe komparativno najviši stupanj hidrotermalne alteracije koji odgovara srednjem odjeljku facijesa zelenih škriljavaca., The samples of partly or completely altered basaltic extrusive rocks from Mt. Medvednica ophiolite mélange were analysed by the optical microscopy and by X-ray powder diffraction (XRD). The chemical composition of minerals from different paragenetic assembleges were determinated by electron probe microanalysis (EPMA) and the measured microdomains were documented by back-scattered electrons (BSE) imagery. The most widespread alteration processes that affected analysed rocks are albitization, chloritization, epidotization, prehnitization, pumpellyitization, zeolitization and titanitization. Subordinate processes that include formation of limonite, calcite, zoisite, siderite, hematite, Mn-oxydes, pyrite and SiO2-phases were not constrained in this paper. Albitization is fairly dominant alteration process in all rock and its progradation throughout a plagioclase grain is clearly documented by BSE microphotographs and microprobe chemical analysis. Prehnite and laumontite are sporadic alteration products of plagioclases. The chemical components released during the alterations of plagioclases and/or clinopyroxenes along with components from circulating and percolating heated see water and fluids enable formation of chlorites, epidote and pumpellyite. Chlorites and pumpellyite formed also by devitrification of volcanic glass in vitric rim of pillow lavas. Small patches of leucoxene, that is abundant in all samples, represent dense aggregates of titanite nanocrystals replacing ilmenite and/or Ti-magnetite. Epidote-bearing paragenesis in the analyzed rocks record comparatively the highest alterations grade that correspond to the middle grade of greenschist facies metamorphism typical of upper sequence of a see floor crust.