Zircon geochronology and compositional record of late- to post-kinematic granitoids associated with the Bavarian Pfahl zone (Bavarian Forest)

Abstract

The northwest-striking Pfahl zone, Bavarian Forest, is a mylonitic shear zone that is associated with brittle-ductile deformation fabrics and a conspicuous hydrothermal quartz mineralization. Two granites from this shear zone yield U–Pb and Pb–Pb evaporation ages between 321–329 Ma and two granodiorites give concordant 238U–206Pb and 235U–207Pb ages of 325±3 Ma and 326±3 Ma, respectively. Zircon populations of the granitoids show sub-types clustering around S20, S10 (granite) and S22 to S24 (granodiorite) testifying different magma affinity. Compositional and isotopic characteristics indicate that the granites and granodiorites were coeval melts, but not differentiates of a single parent magma. The granodiorites were derived from a source with higher time-integrated 87Rb/86Sr and lower 147Sm/144Nd ratios than the granites. One granite body is transected by the shear zone but the main mass of the granite is largely undeformed. This finding suggests that granite intrusion predates the final stage of ductile deformation along the Pfahl shear zone.