Decompression textures in garnet–cordierite gneiss from Kosavankovilpatti, Southern India: Constraints from reaction textures and phase equilibria modelling
Abstract: The Kosavankovilpatti area consists mainly of granulite facies rocks and is part of the Madurai block in the Southern Granulite Terrane, India. The garnet–cordierite gneiss collected at this location exhibits well-preserved metamorphic reaction textures and is used to infer the metamorphic history of the study area. Garnet resorption at the expense of orthopyroxene–cordierite and spinel–cordierite symplectites characterize the retrograde stages of metamorphism. Phase relationships for bulk rock composition in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2 (MnNCKFMASHT) system, as well as the application of multi equilibrium calculations for the peak metamorphic assemblages, yield maximum temperature (T) and pressure (P) conditions of 900 °C and 9 kbar, respectively. These estimated results are lower compared to those reported for the sapphirine-bearing granulites from the Madurai block that characterize UHT metamorphism. The post-peak P–T path, constructed for garnet-cordierite gneiss based on observed microstructural relationships and thermobarometry results, is characterized by a decompressive P–T segment ending at 810 °C and 5.5 kbar. The geochronological results of SHRIMP U–Pb give an age of 521 ± 30 Ma for the decompression metamorphic overprint that was superimposed on the protolith formation of the garnet–cordierite gneiss at 2561 ± 72 Ma. The proposed P–T path implies that the granulites in the Kosavankovilpatti section probably resulted from denudation of the thickened continental crust. In this way, the present study allows us to understand how this part of the lower continental crust could have evolved during prolonged heating without actually attaining UHT conditions.
Paleozoic xenoliths in Eocene plutons: The evidence for the destruction of pre-Jurassic crystalline basement beneath Adjara–Trialeti belt, Lesser Caucasus
Abstract: In Georgia the Paleogene Adjara–Trialeti riftogenic belt (length 350 km, width 50–2 km) is dominantly composed of trachytic and trachytic–andesitic pyroclastic deposits, though plutonic rocks also play an important role in the structure. In this article, we report new data on the (LA-ICP-MS) U–Pb zircon geochronology and petrochemistry of the plutons, their xenoliths and restite from this belt. The results indicate that the magmatism in the basin began in the Early Eocene (~50 Ma) associated with the formation of pyroclastic rocks. The mafic intrusions (~46–44 Ma) led to the assimilation and contamination of sialic crust and formation of monzo-syenite melts emplaced at ~43–42 Ma. The Eocene monzo-syenite plutons contain xenoliths of Paleozoic granites (312±7 to 474±5 Ma) and tholeiitic basalts that contain inherited zircon grains ranging in age from Neo-Proterozoic (747±33 Ma, 632±29 Ma) to Cambrian (515±9 Ma). Paleozoic granite xenoliths show complete mineralogical and age similarity to the Adjara–Trialeti belt adjacent pre-Jurassic massif granites. Inherited zircon grains most likely are captured by magmas during ascent that cuts through the Gondwana-derived old continental crust. Obtained results and regional geological analysis demonstrate that the riftogenic basin of the Adjara–Trialeti belt developed on the pre-Jurassic crystalline basement, from Late Cretaceous to Eocene into a back-arc extensional regime to post-collision geodynamic setting.
New biostratigraphic interpretation of the Middle Miocene (Badenian) transgression in the southern margin of the Pannonian Basin (Hrvaćani, northern Bosnia, Central Paratethys), based on the fossil assemblages
Abstract: This paper focuses mainly on the representatives of calcareous nannofossils and invertebrates from the Hrvaćani section of the Middle Miocene (Badenian) (Prnjavor Basin, southern margin of Pannonian Basin), and it defines the time of Badenian transgression in this area. Rich resources of fossil mollusk species, isolated parts of coral colonies, and other invertebrate remains were found in the investigated section. Moreover, turtle remains from marine sediments at the southern margin of Central Paratethys were recorded for the first time. Calcareous nannofossil biostratigraphy has established an NN5 Zone by the presence of Sphenolithus heteromorphus and the absence of Helicosphaera ampliaperta. The foraminifera associations from the analyzed sandstones belong to the Lower Badenian, namely the older zones of Ammonia viennensis and Elphidium crispum (the equivalent of Lagenidae Zone of Vienna Basin). The specimen of a turtle is currently attributed to order Testudines (suborder Pleurodira). From a stratigraphic and paleoecological point of view, all the fossil assemblages showed similar patterns for the subject of study, reflecting favorable conditions for developing a subtropical fauna in the open shallow marine environment. Consequently, by combining nannofossils, foraminifers, corals, malacological, and herpetological data, we can conclude that the early Middle Miocene (Badenian) in Hrvaćani (Bosnia) corresponds to the Lagenidae Zone (NN5 Zone) and was characterized by a relatively warm climate.
IR spectroscopy as a fast method of determining carbonate content in the Sarmatian–Badenian sandstone reservoirs: A case study from the Carpathian Foredeep (Poland)
Abstract: The work describes and compares the methods for determination of the carbonate content in fifteen samples of Sarmatian–Badenian sandstones, which are reservoir rocks for natural gas in the central part of the Carpathian Foredeep in Poland. Typical sandstone samples were taken from a depth of 1600–2000 m.b.s. (metres below the surface) from drilling cores from well W-1 located about 4 km south of Sędziszów Małopolski, for the development and testing of a new technique for determining the carbonate content. Sandstones are composed of mainly quartz (up to 82 %), then carbonates (calcite and dolomite), feldspar, micas, rarely glauconite and pyrite. The carbonate content of reservoirs is very important for making decisions regarding to intensification of gas production by acid treatment of a near-wellbore region of sandstone. The carbonate content in the tested samples was determined using Scheibler’s volumetric method, petrographic modal data and ATR FT-IR spectroscopy (Attenuated Total Reflectance Fourier Transformation-Infrared Spectroscopy). According to the volumetric and petrographic analyses, the average carbonate content in the tested sandstones was 14.57 and 20.50 vol. %. The similar value 14.51 % (standard deviation in comparison to other methodologies is 0.59–7.75 %) has been obtained by simple and fast ATR FT-IR spectroscopy based on integration of the band coming from out-of-plane bending vibration of the CO32− group (ν2) about 870 cm−1. A comparison of obtained data showed that the ATR FT-IR procedure has the potential to be routinely used for determination of the carbonate content in sandstone samples.
The 40Ar–39Ar dating and geochemistry of the Carpathian C1 obsidians (Zemplín, Slovakia)
Abstract: In situ 40Ar–39Ar UV laser ablation dating of the Carpathian C1 obsidians from the Slovakian part of the Zemplín – Tokaj area yielded new 40Ar–39Ar obsidian glass ages that fall in a narrow time interval of 12.07 ± 0.37 to 11.44 ± 0.39 Ma. This indicate that most of the Zemplín obsidian findings come from one short-time monogenic volcano, forming part of a long-lasting volcanism over the 15–10 Ma period. Chemical compositions of the Carpathian C1 obsidians clearly indicate common similarities between all examined localities (Brehov, Cejkov, Hraň, and Viničky). Geochemically, these obsidians belong to the silica-rich, peraluminous, high-potassium, calc-alkaline rhyolite series volcanic rocks of ferroan character. They were derived by multi-stage magmatic processes, from mixed mantle and crustal sources, and generated during subduction in a volcanic arc tectonic setting. The primary basaltic magma formed from the melting of the lower crustal source at the mantle/crust boundary. Subsequent formation of melt reservoirs in the middle and upper crust, accompanied by secondary melting of the surrounding rocks with continual addition of ascending melt, and repeated processes of assimilation and fractionation produced rhyolitic rocks with obsidians in the Zemplínske vrchy Mts.