Timing of Variscan syn-collisional metamorphism constrained by Lu–Hf and Sm–Nd garnet petrochronology (The Tatra Mountains, Western Carpathians)
Abstract: The Variscan high-pressure (HP) eclogite and medium pressure (MP) metapelitic gneisses from the Tatra Mts. crystalline basement (Western Carpathians, Slovakia) were dated by means of the Lu–Hf and Sm–Nd methods. The Lu–Hf and Sm–Nd garnet dating of mafic eclogite and metapelitic gneisses provides a new set of precise ages on Variscan syn-collisional metamorphism in the Western Carpathians. The published P–T conditions document peak eclogite facies metamorphism at 15–17 kbar and 650–695 °C, whereas garnet–sillimanite gneisses reached their climax at 5–6 kbar and 650–700 °C. Lu–Hf bulk garnet dating gave the precise ages of 354.5 ± 1.2 Ma for eclogite, while the ages of 346.7 ± 1.0 Ma and 345.9 ± 1.0 Ma were determined for metapelites. The Sm–Nd dating of the same garnet fractions from metapelites gave the ages 344.9 ± 2.6 and 345.4 ± 9.3 Ma. Both the Sm–Nd and Lu–Hf ages are indistinguishable within their uncertainties, and so, when considering the major and trace element zonation in garnet, we interpret them as dating the time of prograde garnet crystallization. The data therefore indicates high-pressure, eclogite facies metamorphism in the Tournaisian, and medium-pressure metamorphism in Visean times. We interpret the eclogite facies metamorphism as a record of initial Variscan continental collision in the Early Carboniferous. Early metamorphism in the eclogite facies is a deeper expression of the subducting slab, and thus, garnet started to grow in eclogite ca. 10 Myr earlier than that in the sillimanite-bearing gneiss in a shallower level.
The composite zircon–xenotime–monazite–allanite assemblage in the leucogranite from the Low Tatra Pluton, Western Carpathians: Interplay of melts and fluids
Abstract: The highly peraluminous Zámostská Hoľa leucogranite (ZHG) forms a small, dyke-like body within the granodiorites of the Prašivá type in the composite, 353–351 myr Low Tatra Pluton (LTP), Central Western Carpathians, Slovakia. The ZHG fabric recorded syn-magmatic brecciation, multiple influxes of fluid-rich melt, and fluid-rock interaction. The accessory mineral assemblage includes zircon, xenotime-(Y), monazite-(Ce) and allanite-(Ce). The zircon in the ZHG shows very low Zr/Hf and Th/U; apparently, primary zircon is euhedral, oscillatory zoned and P, Al, Ca, Fe, U, Y, HREE-rich; however, most of the zircon grains are to variable extents altered by dissolution–reprecipitation process, and show depletion or redistribution of trace elements within the crystal. The xenotime-(Y) is spatially associated with zircon and varies from the euhedral, igneous-like to strongly irregular secondary grains formed from the zircon dissolution in the presence of P-bearing fluid. Some zircon and xenotime-(Y) grains are high in As. The monazite-(Ce) shows morphology typical for early-magmatic origin, and alteration is restricted to huttonite-enriched, U, Ca, Y-depleted rims and patches; however, the entire crystals are unusually high in F up to 0.8 wt %. The monazite-(Ce), followed by the trace element-rich zircon and euhedral xenotime-(Y), crystallized from peraluminous, F, U, Y + HREE-enriched melt and resemble ones from highly fractionated granites or pegmatites. The alteration of primary accessory minerals and formation of the secondary xenotime-(Y) is related to an influx of alkali-enriched, fluid-dominated magma. The reaction of REE in fluid with brecciated and altered plagioclase and micas led to the local formation of secondary allanite-(Ce). The magmatic age of the ZHG monazite is 345 ± 2.5 Ma and emphasizes its formation in the final stage of the prolonged evolution of the LTP; but whether the leucogranite represents residual differentiates of the magma of hosting Prašivá type, or independent magmatic pulses, remains an unresolved issue. The exceptional accessory phases assemblage records mineral–melt–fluid interactions in the late magmatic-hydrothermal transition, which is the critical ore-forming phase, and overall characteristics of the ZHG minerals shares attributes of those in the fertile intrusions and provides arguments for the close relationship between the composite magmatism and the ore petrogenesis in the LTP.
Campanian U–Pb ages of volcaniclastic deposits of the Haţeg Basin (Southern Carpathians): Implications for future intrabasinal lithostratigraphic correlations
Abstract: Here we report new LA-ICP-MS zircon U–Pb ages of the Upper Cretaceous volcaniclastic deposits of the Haƫeg Basin (Southern Carpathians, Romania). These deposits crop out around the area of Densuş and Răchitova localities in NW Haţeg and were previously included in the Maastrichtian continental Densuş-Ciula Formation. They are interpreted as eruptive sequences generated by repeated explosive volcanic eruptions, probably related to the Apuseni–Banat–Timok–Srednogorie (ABTS) magmatic activity, that manifested in Eastern Europe during the Late Cretaceous. The three samples selected for U–Pb age determination were collected from the Densuş volcaniclastic sequences; they are primary andesitic–dacitic lithic clasts resulted directly from volcanic eruptions. The new zircon U–Pb ages of 80.44 ± 0.14, 80.22 ± 0.25, and 81.88 ± 0.17 (Early to earliest-Late Campanian), as well as geochemical analyses of the three sampled volcaniclastics, overlap with previously reported data for Banatite rock samples from the Banat and Apuseni segments of the ABTS belt. Age and composition similarities strongly suggest an affinity between the Late Cretaceous Neotethyan subduction-initiated magmatism and the volcaniclastic deposits of the Haţeg Basin. The Densuş–Ciula Formation unconformably overlies the Răchitova Formation – the youngest marine beds in this area, divided into Upper and Lower members. Recently, the age of the Upper Member was biostratigraphically restricted to the Early to earliest-Late Campanian, about the same period of time during which, according to the new U–Pb ages, subaerial volcanism took place. Considering their Early to earliest-Late Campanian age, the volcaniclastic deposits may represent a different lithostratigraphic unit from the Maastrichtian Densuş–Ciula Formation, to which they were formerly associated with. On these grounds, we suggest that the association of the volcaniclastic products with this formation, along with their stratigraphic and tectonic relationships, is revised.
Paleomagnetic contribution to resolving the tectonic evolution of the Drina–Ivanjica Unit, Internal Dinarides
Abstract: The Adria-derived Drina–Ivanjica Unit of the Internal Dinarides represents a composite basement-cover thrust sheet passively carrying obducted Jurassic ophiolites and post-obduction Upper Cretaceous sedimentary rocks, penetrated by post-orogenic Miocene igneous rocks. This study presents the first paleomagnetic results from the Drina–Ivanjica Unit obtained at a total of 34 geographically distributed localities, representing Triassic–Jurassic carbonates, Upper Cretaceous carbonate and flysch successions, as well as Miocene igneous rocks. Paleomagnetic directions for the igneous rocks and the Upper Cretaceous flysch, which had clearly remagnetized during Miocene magmatism, point to a 30° clockwise (CW) rotation, which must have taken place after 20 Ma. This angle perfectly agrees with the earlier-published value for both the Western and Eastern Vardar zones (including the Jadar–Kopaonik thrust sheet, the Sava Zone, and the Cretaceous overstepping sequence of the Eastern Vardar ophiolitic unit), suggesting a coordinated rotation between them and the Drina–Ivanjica Unit. Results of the present study set a younger age constraint (20 Ma) for the commencement of the coordinated CW rotation instead of the previously suggested 23 Ma. The rotation may have connected to the extension induced by the slab rollback of the Carpathians, which was also responsible for the opening of the Pannonian Basin. The paleomagnetic directions for the Cenomanian–Turonian carbonates, which are situated far from the magmatic bodies, are interpreted in terms of a minor counter-clockwise (CCW) rotation taking place before the Miocene CW rotation. The Triassic–Jurassic carbonates of the Drina–Ivanjica Unit have post-tilting magnetizations, which were likely induced by the thermal effect of the obducted ophiolites. The overall mean paleomagnetic direction suggests a rotation of about 50° CCW between 150–20 Ma, which is in line with the well-documented post-150 Ma CCW rotation of the Adriatic microplate.
