The anisotropy of granites – First results of a systematic study of German felsic plutonites

Granites represent suitable crystalline host rocks for nuclear waste repositories because of their mechanical strength and apparent isotropy. However, all granites have a primary structural and petrophysical anisotropy that developed during the emplacement and crystallization of the melt. The primary anisotropy likely controls the orientation of post-magmatic structural features such as extensional fractures. This secondary anisotropy controls potential fluid pathways. Thus, a causal relationship between primary and secondary anisotropies could be an important constraint in terms of the site selection process. We present the first results of a systematic study of German felsic plutonites. We focus on samples of syn-variscan peraluminous granite plutons from two localities, namely the Fichtelgebirge and the Erzgebirge. These areas represent different tectonic settings during intrusion, i.e., compression and transtension, respectively. To estimate their primary anisotropy, we analyzed the crystallographic preferred orientation (CPO) of the rock-forming minerals. CPOs were measured using the neutron time-of-flight (ToF) texture diffractometer “SKAT” and electron backscatter diffraction (EBSD). Based on this data intrinsic bulk rock elastic properties are modeled. All granites show weak but distinct preferred orientations of the rock-forming minerals, which are coherent on a local scale. The quartz textures, for example, show similar CPOs, with point maxima of the positive rhombs combined with small-circle to crossed-girdle c-axis distributions. However, the orientation with respect to the geographic reference system strongly varies on a regional scale. We will discuss the CPOs regarding their tectonic setting and correlate the primary anisotropy with the post-magmatic fracture patterns of the particular granites.