Sr isotope systematics of K-feldspars in plutonic rocks revealed by the Rb–Sr microdrilling technique

Abstract

87Sr / 86Sr isotope variation was studied in subareas of K-feldspar megacrysts from Late-Palaeozoic granodiorite, diorite and syenite of the Bavarian Forest. Most samples were collected close to the Bavarian Pfahl shear zone and feldspars from these sites reveal remarkably constant initial 87Sr / 86Sr isotope ratios. These ratios are similar to those of the bulk groundmass or feldspar crystals from the groundmass. Individual feldspar grains however, define isochron ages that are considerably younger (by ∼15 to ∼30 Ma) than the crystallisation age of the host rocks as defined by U–Pb and Pb–Pb zircon geochronology. The Rb–Sr feldspar data probably reflect diffusion controlled isotopic homogenisation under hydrous conditions between 290–310 Ma. A pink feldspar crystal from a hydrothermally altered syenite reveals comparatively large degree of isotopic variation, indicating local disturbance of the Rb–Sr system caused by hydrothermal alteration during Permian–Triassic Pfahl quartz precipitation. A diorite from the Fürstenstein pluton displays some isotopic scatter and slight differences in the initial 87Sr / 86Sr ratios between the more radiogenic feldspar–megacryst and the less radiogenic bulk groundmass. This rock type was subsequently intruded by the Saldenburg granite and the more radiogenic 87Sr / 86Sr isotope signature of the feldspar relative to the groundmass is interpreted as being consistent with the feldspar having exchanged with the fluids derived from the granite. This study shows that K-feldspar crystals from slowly cooled plutonic rocks do not necessarily preserve their original Sr isotope composition. However, Sr isotope traverses through single crystals can give robust records of the mm-scale isotope system behaviour also in older plutonic rocks relevant for solving petrological or geochronological problems.