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Cadomian Lower-Crustal Contributions to Variscan Granite Petrogenesis (South Bohemian Pluton, Austria): Constraints from Zircon Typology and Geochronology, Whole-Rock, and Feldspar Pb-Sr Isotope Systematics |
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| Authors: | KLOTZLI U S; KOLLER F; SCHARBERT S; HOCK V |
| Institution: | 1LABORATORY FOR GEOCHRONOLOGY, INSTITUTE OF GEOLOGY, UNIVERSITY OF VIENNA, GEOZENTRUM, ALTHANSTRASSE 14, A-1090 VIENNA, AUSTRIA 2INSTITUTE OF PETROLOGY, UNIVERSITY OF VIENNA, GEOZENTRUM, ALTHANSTRASSE 14, A-1090 VIENNA, AUSTRIA 3GEOLOGICAL SURVEY OF AUSTRIA, RASUMOFSKYGASSE 23, A-1030 VIENNA, AUSTRIA 4INSTITUTE OF GEOLOGY AND PALAEONTOLOGY, UNIVERSITY OF SALZBURG, HELLBRUNNERSTRASSE 34, A-5020 SALZBURG, AUSTRIA |
| Abstract: | A hybrid pyroxene-bearing Weinsberg type granitoid of the SouthBohemian batholith (Austria) consists of two independent mineralassemblages that were formed during two different magmatic events.The older, inherited assemblage forms unevenly distributed millimetre-sizedmulti-grain patches of quartz + mesoperthitic alkali feldspar+ andesine/bytownite + clinopyroxene (XMg = 0·50–0·54)+ orthopyroxene (XMg = 0·35–0·42) ±ilmenite ± accessories. It is interpreted to representremnants of a mangeritic igneous rock with a superimposed granulite-faciesre-equilibration texture characterized by unzoned pyroxenesand plagioclase. The enclosing younger assemblage with alkalifeldspar + oligoclase/andesine + quartz + biotite ± accessoriescrystallized from a biotite-bearing granitic melt with feldsparsexhibiting typical magmatic zoning. Coexisting with the inheritedassemblage are zircons with a characteristic typology (S23 toD, mean J4). Zircons belonging to the granitic assemblage, onthe other hand, show a distinctly different typology (L2 toS5, mean L4) or are anhedral. A Cambrian age of formation andsubsequent re-equilibration of the inherited assemblage is inferredfrom a mean U/Pb and 207Pb/206Pb evaporation age of 523 ±5 Ma for the J4 zircons. Granitic L4 zircons show a mean 207Pb/206Pbevaporation age of 355 ± 9 Ma, interpreted as the ageof zircon growth during a Carboniferous partial melting eventin the lower crust. Granite emplacement at 345 ± 5 Mais inferred from U/Pb analysis of the anhedral zircon population.The comparably low radiogenic common Pb isotope compositionof megacrystic alkali feldspars suggests that at least somedomains of these crystals are inherited from the older, pyroxene-bearingmineral assemblage. Rb/Sr whole-rock dating is thus severelyjeopardized by the presence of the inherited alkali feldsparcrystals, leading to widely scattering data points and errorchronages of no geological significance. KEY WORDS: Austria; Bohemian Massif; geochronology; granites; Pb–Sr isotopes |
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