Radiometric dating of the Tertiary volcanics in Lower Silesia, Poland. VI. K-Ar palaeomagnetic data from basaltic rocks of the West Sudety Mountains and their northern foreland
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This article presents the sixth and final contribution in a series of papers focused mainly on the K-Ar dating of the Oligocene and Neogene (Miocene and Pliocene) intraplate basaltic volcanics of the Lower Silesia, SW Poland. The present paper includes 22 new K-Ar dates from the West Sudety Mountains and their northern foreland. The K-Ar dates range from 30.7 to 22.2 Ma. The data are supplied with geological description of the sampled outcrops, petrographic, geochemical and palaeomagnetic data of the analysed samples. Palaeomagnetic investigation confirmed the existence of two important volcanic episodes distinguished already in 1997: the reversed polarity Odra Event (mean age 28.2±1.2 Ma), and the normal polarity Gracze Event (mean age 26.28±1.8 Ma).Keywords:
Radiometric dating
K–Ar dating
Neogene
Outcrop
Absolute dating
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Abstract Twenty-one new 40 Ar/ 39 Ar step-heating experiments on mineral separates from intrusive and extrusive Carboniferous and Permian igneous rocks in the Midland Valley of Scotland yielded 17 concordant experiments with a relative age precision better than 1% (2σ). These ages resolve inconsistencies between existing K-Ar dates on the same samples and their stratigraphical constraints correlated to recently published timescales. The precise 40 Ar/ 39 Ar dates are stratigraphically constrained to stage level and can contribute to Carboniferous timescale tie points at the Tournaisian-Visean boundary, within the Visean and at the Carboniferous-Permian boundary. Situated in the extending Variscan foreland, two distinct phases of extension-related transitional-alkaline volcanism have been resolved in the Dinantian: the Garleton Hills Volcanic Formation in the eastern Midland Valley near the Tournaisian-Visean boundary, 342.1 ± 1.3 and 342.4 ± 1.1 Ma; and the Clyde Plateau Volcanic Formation in the western Midland Valley during the mid-Visean, 335 ± 2–329.2 ± 1.4 Ma. Alkaline basic sills near Edinburgh, previously thought to be Namurian, appear to be coeval with the Clyde Plateau Volcanic Formation at 331.8 ± 1.3–329.3 ± 1.5 Ma. The new ages allow correlation between these short-lived Dinantian magmatic pulses and extensional and magmatic phases in the Northumberland-Solway and Tweed basins to the south. After late Westphalian, end-Variscan, compression and a regionally important tholeiitic intrusive phase at c. 301–295 Ma, alkaline magmatism related to post-Variscan extension occurred in the central and western Midland Valley during the latest Carboniferous or Permian from 298.3 ± 1.3 to 292.1 ± 1.1 Ma. This correlates well with post-Varsican extension and magmatism observed across the NW European foreland from 300 to 280 Ma.
Geochronology
K–Ar dating
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The main aim of this study is to define precisely the age of the alkaline basalts occurring within the lowermost part of Carboniferous succession in the northeastern part of LublinBasin(drill core Parczew IG-7 and IG-9; SE Poland). The new, whole-rocks 40 Ar/ 39 Ar data constrain the age of volcanic activity and emplacement of alkaline basalts to the Late Tournaisian (348 ± 0.8 Ma) with possible prolongation to the Middle Visean (338.5 ± 0.7 Ma). The younger age is however of evidently poorer quality than the older one. The new data caused to correlate the volcanic processes occurring within theLublinBasin with alkaline intrusions drilled inNE Poland inside the Paleozoic cover of the East European Platform. They also correspond to the volcanoclastic levels defined in different parts of the Trans European Suture Zone inPoland.
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On the basis of experiments presented in Part I of this series, most of the published thermobarometers relevant to four-phase peridotites are tested here for their ability to reproduce experimental conditions. They were rejected if any systematic discrepancy in either pressure or temperature was discernible. This test cautions against the use of all published versions of thermometers basad on the compositions of coexisting ortho- and clinopyroxenes and the use of existing barometers based on the Al content of orthopyroxene axxisting with garnet. Therefore, we formulated new versions of the two-pyroxene thermometer and the Al-in-opx barometer: with KD*=(1-Ca*)cpx/(1-Ca*)opx;Ca*=CaM2/(1-NaM2) and XFePx=Fe/Fe+Mg);TBKN is in degress Kelvin and P is in kilobars. Our new barometer is of the form (C1–C3) and site occupancies are given in the text. Temperatures may also be calculated from the Ca content of opx alone: This thermometer can be applied both to the CMS and the natural system experiments, which may indicate that Fe and Na have counter-balancing effects on the Ca content of opx. The partitioning of Na between opx and cpx can also serve as a useful thermometer, and was calibrated from natural rock data: where T is in degrees Kelvin, P is in kilobars, and DNa=Naopx/Nacpx. The following three published thermobarometers based on further exchange reactions are capable of reprducing experimental conditions: exchange of Ca between olivine and clinopyroxene as a barometer (PKB), exchange of Fe and Mg between garnet and clinopyroxene as a thermometer (TKrogh), exchange of Fe and Mg between garnet and olivine as a thermometer (TO'Neiii). Our tests also show that the most accurate pressure and temperature estimates arc obtained from the following combinations of thermometers and barometers: TBKN+PBKN, TBKN+PKB, TKrogh+PBKN, TO'Ne$$$ll+PBKN.
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Felsic
Paleogene
Continental Margin
Continental arc
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Dacite
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An Early Miocene acidic tuff that accumulated in a small tectonic graben at the south-eastern margin of the Bohemian Massif in 40 39 north-eastern Austria was dated by means of Ar/ Ar dating. Feldspar crystals from the tuff have an inverse isochron age of 17.23 ± 0.18 Ma, which is interpreted to reflect its crystallisation age. The tuff shows a reversed palaeomagnetic polarity and can be correlard ted with chron C5Cr of the late Burdigalian (early Karpatian) and with the lowstand systems tract (LST) of the Bur 4 global 3 order sea level cycle. The tephra originates from acid (rhyodacitic to dacitic) calc-alkaline arc volcanism. Our study demonstrates that the investigated volcaniclastics are significantly different from other Burdigalian (Eggenburgian, Ottnangian) and Langhian (early Badenian) tephra from the area with regard to their volcanic zircon and Rare Earth element (REE) composition. The volcanic source of the Straning tuffs might be traced back to the western Inner Carpathian volcanic arc. The tuffs are most likely genetically related to the Middle Rhyolite Tuff (late Burdigalian, Karpatian) of northern Hungary and southern Slovakia.
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Radiometric dating
K–Ar dating
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We present fifteen new K/Ar ages on amphibole phenocrysts of Neogene volcaniclastic rocks from the Styrian Basin, Austria. The westernmost sub-basin of the Pannonian Basin is the Styrian Basin, which hosts a large number of phreatomagmatic tuff occurrences beside Middle Miocene shield volcanoes and Pliocene effusive alkaline volcanic rocks. The investigated tuffs contain the well-known mantle xenoliths and frequent amphibole and pyroxene phenocrysts. The new K/Ar ages indicate that the Late Miocene phreatomagmatic volcanism started in Pontian (7.51 Ma) and ended in Romanian (2.73 Ma) times. The complete interval of the youngest volcanism in the Styrian Basin covers 5.8 Ma, similar to the volcanism of the western part of the Pannonian Basin and the Nógrád/Novohrad area (West Carpathians). The dated volcanic occurrences display NNW–SSE oriented arrays which are approximately parallel to the Auersbach basinal high zone. The new data indicate that the Late Miocene to Pliocene volcanism of the Styrian Basin is synchronous with the onset of a regional compressional event and eastward tilting of the Styrian basin fill.
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Neogene
Amphibole
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