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

Abstract

A hybrid pyroxene-bearing Weinsberg type granitoid of the South Bohemian batholith (Austria) consists of two independent mineral assemblages that were formed during two different magmatic events. The older, inherited assemblage forms unevenly distributed millimetre-sized multi-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 represent remnants of a mangeritic igneous rock with a superimposed granulite-facies re-equilibration texture characterized by unzoned pyroxenes and plagioclase. The enclosing younger assemblage with alkali feldspar + oligoclase/ andesine + quartz + biotile ± accessories crystallized from a biotite-bearing granitic melt with feldspars exhibiting typical magmatic zoning. Coexisting with the inherited assemblage are zircons with a characteristic typology (S23 to D, mean J4). Zircons belonging to the granitic assemblage, on the other hand, show a distinctly different typology (L2 to S5, mean L4) or are anhedral. A Cambrian age of formation and subsequent re-equilibration of the inherited assemblage is inferred from a mean U/Pb and 207Pb/206Pb evaporation age of 523 ± 5 Ma for the J4 zircons. Granitic L4 zircons show a mean 207Pb/206Pb evaporation age of 355 ± 9 Ma, interpreted as the age of zircons growth during a Carboniferous partial melting event in the lower crust. Granite emplacement at 345 ± 5 Ma is inferred from U/Pb analysis of the anhedral zircon population. The comparably low radiogenic common Pb isotope composition of megacrystic alkali feldspars suggests that at least some domains of these crystals are inherited from the older, pyroxene-bearing mineral assemblage. Rb/Sr whole-rock dating is thus severely jeopardized by the presence of the inherited alkali feldspar crystals, leading to widely scattering data points and errorchron ages of no geological significance.