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No abstract available
No abstract available
Comprehensive geochemical investigations of rnetabasites yielded constraints for a correlation of, or discrimination between the different tectonic units within the northeast Bavarian crystalline basement. The Münchberg nappe pile consists of at least five large tectonic units which exhibit differences in lithology, in part also in metamorphie grade and in metamorphie history. The metabasites in each of these nappes show their own, significant geochemical characteristics. The lowermost tectonic unit, the Bavarian lithofacies, includes the anchimetamorphie Ordovician Randschieferserie which contains alkaline basalts. In their geochemistry, they are sirnilar to the metabasites of the Fichtelgebirge crystalline complex in the autochthonous Saxo-thuringian. The next higher tectonic unit of the Münchberg nappe pile, the Prasinit-Phyllit-Serie contains metabasites which can be derived from subalkaline basalts with a clear calc-alkaline tendency. There is a striking geochemical resemblance to the metabasites of the Erbendorf Greenschist Zone (EGZ) underscorinq the similar lithology of both allochthonous units which appear to be in a similar tectonic position. The Randamphibolit-Serie higher up in the Münchberg nappe pile consists of metabasites with tholeiitic characteristics and a pronounced differentiation trend. The next higher tectonic unit, the Liegendserie of the Münchberg gneiss cornplex s. str., contains metagabbros to metagabbronorites with a high-Al basaltic composition. The amphibolites and banded hornblende gneisses of the overlying Hangendserie are of subalkaline basaltic character with calc-alkaline affinity. The Zone Erbendorf-Vohenstrauss (ZEV) is currently regarded as an allochthonous unit equivalent to the higher crystalline nappes of the Münchberg pile. However, the geochemical character of the metabasites do not encourage such a correlation. Neither the schistose and striped amphibolites nor the flaseramphibolites of the ZEV with their N-KORB and E-MORB character respectively, find convincing counterparts in the crystalline nappes of the Münchberg pile. However, an interestingly close resemblance exists between the schistose and striped amphibolites in the ZEV, on the one hand, and in the autochthonous Zone Tirschenreuth- Mähring (ZTM) and the adjacent Moldanubian sensu strictu, on the other. Owing to the absence of age criteria, our results cannot be used, so far, to reconstruct the paleogeographical position of the individual tectonic units, based on the geochemical characteristics of their respective metabasites.
No abstract available
No abstract available
Die Kupfer-Zink-Lagerstätte von Kupferberg-Neufang im Bereich der Münchberger Deckenstruktur besteht aus einem etwa 4 km langen Zug einzelner stratiformer Vererzungslinsen, der in NW-SE-Richtung vom anchimetamorphen Paläozoikum "Bayerischer" Faziesentwicklung in die Prasinit-Phyllit-Serie hinüberzieht. Bisherige lagerstättenkundliche Untersuchimgen interpretierten die gesamte Lagerstätte als einheitliche, submarin-exhalativ entstandene stratiforme Bildung in Zusammenhang mit ordovizischem basischem Vulkanismus, der in den Rahmengesteinen der Lagerstätte durch die Diabase der anchimetamorphen Randschieferserie bzw. die Prasinite der Prasinit-Phyllit-Serie dokumentiert ist. Diese Vorstellung steht jedoch in deutlichem Widerspruch zu den Ergebnissen neuerer geochemischer und biostratigraphischer Untersuchungen, die für die Diabase und Prasinite eine völlig unterschiedliche Zusammensetzung ergaben und die zeigten, daß die Prasinit-Phyllit-Serie schon im Oberproterozoikum, die erzführenden Schichten im anchimetamorphen Paläozoikum aber erst im Unterdevon sedimentiert wurden. Für eine Klärung der Situation sind weitere Untersuchungen im Bereich der Lagerstätte erforderlich.
The Ringgold Knoll pegmatite, a late-stage member of the Granite Harbour Intrusives, crosscuts high-grade Wilson gneisses of the Oates Coast, which forms the westernmost part of the Wilson Terrane at the Pacific end of the Cambro-Ordovician Ross orogenic belt in West Antarctica. The pegmatite mineral assemblage consists of K-feldspar, plagioclase, quartz, garnet (almandinespessartine-pyrope), dark tourmaline (schorl-dravite), muscovite, apatite, monazite, zircon, blue AI-rich tourmaline and dumortierite in order of decreasing abundances. Major, minor and rare earth elements are reported for the greater part of the mineral assemblage. The time of pegmatite emplacement is constrained by Rb-Sr and Sm-Nd isochron ages of 492 ± 8 (2a) Ma and 500 ± 40 (2a) Ma, respectively. High initial 87Sr/86Sr of 0.7315 ± 0.0003 and low E Nd,t of -8.7 ± 1.2 strongly support an origin of the magma from highly evolved crustal source rocks. K-Ar and Ar-Ar model ages of about 470 to 475 Ma for igneous muscovite indicate that the pegmatite together with its wall rocks spent a prolonged period at elevated temperatures before final cooling below about 350 °C. The muscovite dates may give an estimate for the time of exhumation of the Oates Coast crystalline basement along two major late Ross orogenic detachment zones within the Wilson Terrane i.e. the Wilson and the Exiles thrusts (c.f. FLÖTTMANN and KLEINSCHMIDT, 1991).
K-Ar dating on hornblendes and micas from the Tepla Domazlice zone revealed a pattern of dates which significantly deviates from the mid-Carboniferous to early Permian one that is found in the adjacent low-pressure metamorphic Moldanubian and Saxothuringian. Especially for the Marianske Lazne metabasic complex, confirming early Czech determinations, the dates resemble the early Devonian pattern determined for the Munchberg Gneiss Massif and the Erbendorf-Vohenstrau zone of northeastern Bavaria. This supports the idea that all three units are remnants of a huge' complex which suffered a metamorphic overprint under medium-pressure conditions, probably in the early Devonian. Strong rejuvenation is found in the southern part of the Tepla-Domazlice zone by which micas and even two hornblendes were reset to mid-Carboniferous ages. According to the geological setting, part of the apparently preDevonian dates may be explained by inherited argon from earlier metamorphic and magmatic events, e.g. the high-pressure metamorphism documented in eciogitic relics. However, excess argon, caused by the mid-Carboniferous overprint cannot be excluded.
no abstract available
no abstract available
No abstract available
K-Ar dating on hornblendes and micas from the TepläDomazlice zone revealed a pattern of dates which significantly deviates from the mid-Carboniferous to early Permian one that is found in the adjacent low-pressure metamorphic Moldanubian and Saxothuringian. Especially for the Mariänske Läzne metabasic complex, confirming early Czech determinations, the dates resemble the early Devonian pattern determined for the Münchberg Gneiss Massif and the Erbendorf-Vohenstrauß zone of northeastern Bavaria. This supports the idea that all three units are remnants of a huge complex which suffered a metamorphic overprint under medium-pressure conditions, probably in the early Devonian. Streng rejuvenation is found in the southern part of the Teplä-Domailice zone by which micas and even two hornblendes were reset to mid-Carboniferous ages. According to the geological setting, part of the apparently preDevonian dates may be explained by inherited argon from earlier metamorphic and magmatic events, e.g. the high-pressure metamorphism documented in eclogitic relics. However, excess argon, caused by the mid-Carboniferous overprint cannot be excluded.