We present the main petrographic and geochemical features of the Lower to Middle Eocene turbiditic sandstones from the northwestern portion of the External Dinaride flysch basin. Sampled areas cover SW Slovenia (Vipava and Brkini Basins) and the Istrian Peninsula (Trieste-Koper and Pazin Basins). Framework constituents of the lithic arenites reveal low-grade metamorphic, acidic plutonic, and to a lesser extent, mafic volcanic and ultrabasic sediment sources, with evidence for a small degree of sediment recycling as well. Among the processes that commonly influence sediment compositions, weathering in the source and sorting were probably negligible, but carbonate contribution of detrital or intrabasinal origin diluted the siliciclastic portions to various degrees. Main and trace element compositional data agree well with petrography and clearly indicate the predominance of felsic, crustal source lithologies. Exposed mafic-ultramafic source units were volumetrically less important.  From the Early Paleogene, extensive sediment mixing occurred in front of the Dinaride orogenic thrust wedge, with the components derived from different Dinaride units of felsic crystalline basement, platform carbonates and ophiolite. In the Eocene, a likely source of the mafic-ultramafic detritus was the Jurassic ophiolitic mélange in the NE Dinarides.

Abstract

The foliated low-grade metamorphic rocks of the Triassic Bagolyhegy Metarhyolite Formation, mainly of pyroclastic origin, host post-metamorphic quartz-albite veins containing abundant tourmaline and occasionally rutile/ilmenite. The study of the Ti-oxide-mineralized veins with SEM-EDX revealed an unusual mineral assemblage comprising fine-grained Nb–Ta-bearing oxides (columbite-tantalite series, fluorcalciomicrolite and other Nb–Ti–Y–Fe-REE-oxide minerals) intergrown with Nb-rich polymorphs of TiO₂ (anatase, rutile), ilmenite and zircon enriched with hafnium. This high field strength elements (HFSE)-bearing paragenesis is unexpected in this lithology, and was not described from any formation in the Paleozoic-Mesozoic rock suite of the Bükk Mountains (NE Hungary) before. The host metavolcanics are significantly depleted in all HFSE compared to the typical concentrations in felsic volcanics and the mineralized quartz-albite veins have even lower Ti–Nb–Ta concentration than the host rock, so the mineralization does not mean any enrichment. From proximal outcrops of the Triassic Szentistvánhegy Metavolcanics, potassic metasomatized lenses with albite-quartz vein fillings containing rutile/ilmenite are known. We studied them for comparison, but they only contain REE mineralization (allanite-monazite-xenotime); the Nb–Ta-content of Ti-oxide minerals is undetectably low. LA-ICP-MS measurements for U–Pb dating of Hf-rich zircon of the Nb–Ta-rich mineral assemblage gave 71.5 ± 5.9 Ma as lower intercept age while dating of allanite of the REE mineralized quartz-albite veins gave 113 ± 11 Ma as lower intercept age. The REE-bearing vein fillings formed during a separate mineralization phase in the Early Cretaceous, while the Nb–Ta mineralization was formed by post-metamorphic alkaline fluids in the Late Cretaceous., controlled by fault zones and fractures.

The 1,200-m-deep Budaörs-1 borehole provided important data for our understanding of the stratigraphy and tectonic setting of the southern part of the Buda Hills. Although previous reports contained valid observations and interpretations, a number of open questions remained. The importance of this borehole and the unsolved problems motivated us to revisit the archived core. The new studies confirmed the existing stratigraphic assignment for the upper dolomite unit (Budaörs Dolomite Formation) as the dasycladalean alga flora proved its late Anisian to Ladinian age assignment. An andesite dike was intersected within the Budaörs Dolomite. U–Pb age determination performed on zircon crystals revealed a Carnian age (~233 Ma), and settled the long-lasting dispute on the age of this dike, proving the existence of a Carnian volcanic activity in this area after the deposition of the Budaörs Dolomite. Palynostratigraphic studies provided evidence for a late Carnian to early Norian age of the upper part of the lower unit (Mátyáshegy Formation). This result verified an earlier assumption and reinforced the significance of the tectonic contact between the upper unit (Budaörs Formation) and the lower unit (Mátyáshegy Formation). Based on structural observations and construction of cross sections, two alternative models are presented for the structural style and kinematics of the contact zone between the Budaörs and Mátyáshegy Formations. Model A suggests a Cretaceous age for the juxtaposition, along an E–W striking sinistral transpressional fault. In contrast, model B postulates dextral transpression and an Eocene age for the deformation. The latter one is better supported by the scattered dip data; however, both scenarios are considered in this paper as possible models.

Abstract

The timing of Triassic magmatism of the Ditrău Alkaline Massif (Eastern Carpathians, Romania) is important for constraining the tectonic framework and emplacement context of this igneous suite during the closure of Paleotethys and coeval continental rifting, as well as formation of back-arc basins.

Our latest geochronological data refine the previously reported ages ranging between 237.4 ± 9.1 and 81.3 ± 3.1 Ma. New K/Ar and U–Pb age data combined with all recently (post-1990) published ages indicate a relatively short magmatic span (between 238.6 ± 8.9 Ma and 225.3 ± 2.7 Ma; adding that the most relevant U–Pb ages scatter around ∼230 Ma) of the Ditrău Alkaline Massif. The age data complemented by corresponding palinspastic reconstructions shed light on the paleogeographic environment wherein the investigated igneous suite was formed.

The magmatism of the Ditrău Alkaline Massif could be associated with an intra-plate, rift-related extensional tectonic setting at the southwestern margin of the East European Craton during the Middle–Late Triassic (Ladinian–Norian) period.