Genesis of a synmagmatic charnockite associated with the Weinsberg granite, southern Bohemian Batholith, northern Austria
Abstract: The Weinsberg granite, a coarse-grained biotite granite with abundant K-feldspar megacrystals, is the volumetrically dominant and most characteristic granite type of the late-Variscan Moldanubian Batholith in the Moldanubian zone of the Bohemian Massif. In the western batholith area, a local orthopyroxene-bearing variant (charnockite) of the Weinsberg granite has been identified and given the name of the Sarleinsbach quartz-monzodiorite in previous studies. Whole rock analysis of the charnockite and the relatively mafic Weinsberg granite in the immediate neighborhood show no significant geochemical differences with respect to either the major or trace elements. The mineralogy and petrology of the charnockite and surrounding granite are the same except for the presence of orthopyroxene ± clinopyroxene in the charnockite. In addition, the charnockite is characterized by the presence of dark grey, glassy orthoclase megacrysts with only some partial conversion to microcline, whereas in the granite the K-feldspar megacrysts consist of white microcline. The Fe–Mg silicates in the charnockite (orthopyroxene, clinopyroxene, amphibole, and biotite) are relatively Fe-rich (XFe = 0.6–0.7) whereas the plagioclase is more albitic (XAb = 0.6) than anorthitic. Fluid inclusions from the granite and associated charnockite are investigated and the results compared. The basic conclusion is that the magma responsible for the granite was dominated by an H2O-rich fluid with a CaCl2 component. The magma responsible for the charnockite was dominated by a CO2-rich fluid with a minor NaCl component, which lowered the H2O activity sufficiently below 1 such that orthopyroxene ± clinopyroxene was the stable Fe–Mg silicate phase during crystallization as opposed to the biotite in the granite. Taking into account that CO2-rich and H2O-rich fluids are immiscible in the presence of NaCl and CaCl2 over the P–T range of the overall crust, the implication is that in granitoid melts, if CO2 is present, there will be regions dominated by CO2 and regions dominated by H2O. The extent of either region will be determined by the overall CO2/H2O ratio in the melt. In the CO2-dominated regions, the H2O activity could be sufficiently lowered such that orthopyroxene is the stable Fe–Mg silicate phase during crystallization, though this will also be dependent on the Fe/Mg ratio of these phases and the overall whole rock chemistry of the melt. In addition to incipient solid state charnockitization, commonly seen in the Archean terranes of southern India and elsewhere, this suggests that a certain subset of granites and granitoids worldwide should have patches and/or limited areas of charnockite if the amount of CO2 present in the original magma goes above a certain fraction.
Geochemistry and tectonic significance of metamorphosed mafic ophiolitic rocks in the upper high-grade basement unit of the eastern Rhodope Massif (Bulgaria–Greece)
Abstract: Metamorphosed mafic ophiolitic rocks in the metamorphic section of the eastern Rhodope Massif in Bulgaria and Greece are important for understanding the oceanic lithosphere fragments, which have been involved in Alpine tectonic–metamorphic processes. Petrography and mineral compositions of the meta-mafic rocks (mostly gabbro–basalt to minor diorite–andesite) point to main amphibolite-facies overprint, which strongly obliterated the primary textures, and the original igneous grain-sizes are partly preserved only of the plagioclase. The meta-mafic rocks are classified as low-K and low- to high-Ti tholeiitic affinity igneous protoliths of basaltic to andesitic compositions, in which high-Ti and low-Ti groups are identified on the basis of Ti concentrations. They also differ with respect to trace element and REE characteristics. A complex chemistry of high-Ti group indicates an origin primarily from MORB mantle source, subsequently modified by subduction-zone derived LILE- and REE-enriched melts and contribution of HFSE-enriched component that produce the oceanic island tholeiites. The low-Ti group displays IAT affinity, with clearly defined subduction-related component demonstrated by LILE enrichment, HFSE and HREE depletion relative to N-MORB and negative Nb and Ti anomalies, all indicative for an island arc petrogenesis. A single dunite sample studied also displays geochemical characteristics of the low-Ti group meta-mafic rocks. Geochemical diversity of the meta-basic rocks with MORB, transitional MORB/IAT and IAT affinities hints their supra-subduction zone (SSZ) origin in an island arc/back-arc system, with identifiable arc-related and rifting/sea-floor spreading magmatic products represented by the low-Ti and high-Ti groups, respectively. At present, the available Middle-Late Paleozoic/Early Triassic radiometric ages of the meta-mafic rocks protoliths predominate over the Early Paleozoic ages, which suggests that the development of the inferred arc/back-arc system relates mostly to the ocean-floor magmatic evolution of the Paleotethyan realm.
Biostratigraphy of calcareous nannofossils and palaeoenvironments in the Lower Miocene of the Albanian–Thessalian Basin (Albania)
Abstract: The Albanian–Thessalian Basin, which is located in the vicinity of Mount Morava, includes Eocene to Middle Miocene sediments with a poorly-known, calcareous nannofossil content. This study is focused on the investigation of calcareous nannofossil assemblages from the Lower Miocene sediments outcropping in the area, resulting in the correlation to the global nannofossils zones/subzones NN2–NN3/CN1c–CN2, as well as to the zonation from the Mediterranean area, where the MNN2b and MNN3a biozones have been identified. An early-middle Burdigalian age for the studied outcrops is supported by primary and secondary index species (Helicosphaera ampliaperta, Helicosphaera mediterranea, Sphenolithus belemnos, Sphenolithus disbelemnos, Sphenolithus pseudoheteromorphus). Quantitative analyses were performed on calcareous nannofossils and statistics were applied to all of the counted samples. Based on the statistical analysis, short-time fluctuations in palaeoenvironmental parameters, such as temperature, salinity and eutrophic regime, are documented through the changes in the calcareous nannofossil assemblages and their abundance patterns. The depositional palaeoenvironment indicates changes in basin water depth, with influence of cold upwelling currents, terrigenous nutrient influx, and increased palaeoenvironmental perturbations over short time intervals.
Evidence of Early Sarmatian volcanism in the Hrvatsko Zagorje Basin, Croatia: Mineralogical, geochemical and biostratigraphic approaches
Abstract: A bentonite clay layer is documented in the Sutla-II column in the Hrvatsko Zagorje Basin, which is a part of the south-western marginal belt of the Pannonian Basin System. The origin of the clay is attributed to the alteration of felsic to intermediate volcanic ash, which had been deposited between horizontally-laminated marls in a marine environment. Provenance analysis indicates that the marls were sourced from mixed, dominantly-felsic source rocks. Smectite present in the marls is therefore not solely of terrigenous origin and may also be related to volcanic ash weathering. Based on the fossil content, an inference has been made suggesting Early Sarmatian age of the sediment hosting the bentonite clay intercalation. The sedimentological and palaeontological data are in favour of the sedimentation at an inner shelf area marked by unstable palaeoenvironmental conditions. The upper part of the Sutla-II column was deposited in the high-energy environment consisting of impure biocalcarenite and biocalcrudite coupled with fossiliferous litharenite, which all mark an intensive redeposition of older rocks and fossiliferous formations. The bentonite clay likely originated from distant tephra sourced from volcanic eruptions, presumably located in the north-eastern part of the Carpathian–Pannonian Region during the post-rift stage of the back-arc Pannonian Basin System development.
Detrital zircon geochronology of Pliocene deltaic sediments in the Marmara region (Turkey): Implication for sedimentary provenance and morphotectonic evolution
Abstract: The İstanbul Pliocene deposits consist of an alternation of sand, clay, and coal in the northern part of İstanbul that characterizes a delta plain deposit on the southern coastal line of the Black Sea. The Pliocene sediments, which are located conformably on the fluvial sediments consisting of coarse clastics, are about 80 meters thick and outcrop as isolated patches in Şile in the east of the İstanbul Strait (Bosphorus) and Kısırkaya in the west. The U/Pb detrital zircon ages obtained from the sands of Kısırkaya and Şile region showed that the Pliocene deposits contain Proterozoic (2396 ± 72–542.4 ± 7.9 Ma), Paleozoic (540 ± 12–258.9 ± 5.2 Ma), Mesozoic (248.8 ± 4.4–71.8 ± 1.2 Ma), and Cenozoic (63 ± 1.8–22.18 ± 0.95 Ma) zircons derived from a piedmont plateau. Presence of the youngest Oligocene–early Miocene zircons (22.18 ± 0.95–31.1 ± 1.2 Ma) reveals that the source of this succession may be the Northwest Anatolia and/or northern Aegean region where magmatic rocks of the same age crop out. In addition to the zircon data in the sandy deposits, trace element geochemistry also shows that the drainage basin of the Pliocene rivers transporting clastics to the basin is located in the southwestern region of İstanbul and flowed into the Black Sea before the formation of the Marmara Sea. These rivers would have been blocked in the early Quaternary by the Marmara Sea depression, which was formed by extensional faults, the product of an approximately N–S extensional tectonic regime in the region. This tectonic regime caused the rapid uplifting of the İstanbul region and the Istranca Mountains in the north of the Marmara, and the eroded flattened areas called the Bursa–Balıkesir plateau in the south, in the form of horsts. Subsequently, before the North Anatolian fault reached the region, it formed deformation structures under the effect of dextral shear in a wide zone in the Marmara region. This tectonic regime was ended when the North Anatolian fault reached and cut the Marmara Sea region in the Latest Quaternary.