Two distinctive granite suites in the SW Bohemian Massif and their record of emplacement: constraints from geochemistry and zircon 207Pb/206Pb chronology

Abstract

Various types of gneisses, migmatites and granites dominate the outcrops of the Bavarian Forest, in the southwestern Moldanubian Sector of the Bohemian Massif, providing evidence for the geological characteristics of a crustal root zone. Fourteen granite intrusions from this area have been dated by the single-zircon Pb-evaporation technique. Major and trace elements, and Rb-Sr and Sm-Nd isotope compositions of these rocks have also been determined. The 207Pb/206Pb single-zircon ages indicate Carboniferous magma production in only a few million years. The intrusions NE of the Bavarian Pfahl Zone are dated between 328 and 321 Ma, whereas ages between 324 and 321 Ma are obtained in the area SW of this zone. The coincidence between these ages and regional peak-metamorphism in this area ('Bavarian Phase') indicates a connection between metamorphism and melt extraction. When the geochemical and isotopic compositions of the granites are examined, supplemented by data from previous geochemical studies, it becomes evident that the Bavarian Pfahl Zone, which runs diagonally across the Bavarian Forest, is a terrane boundary that separates two regions of peraluminous S-type plutons predominantly of crustal anatectic origin but with distinct compositional features. Plutons located in the SW and along the Pfahl Zone (Bavarian Terrane) define a high Ca-Sr-Y suite, whereas plutons from the NE and neighbouring crystalline units (Ostrong Terrane) constitute a low Ca-Sr-Y suite. Granites from the Bavarian Terrane have significantly more radiogenic initial Nd and less radiogenic initial Sr isotope ratios than granites from the Ostrong Terrane, indicative of different source materials. These findings support the idea that the difference in magma composition is linked to the nature of the underlying crust. © The Author 2008. Published by Oxford University Press. All rights reserved.