Complex mid‐crustal flow within a growing granite–migmatite dome: An example from the Variscan belt illustrated by the anisotropy of magnetic susceptibility and fabric modelling

Abstract

AbstractThis study describes anisotropy of magnetic susceptibility (AMS) of migmatites associated with crustally derived granites of the Pelhřimov core complex, Bohemian Massif. In combination with numerical modelling, we use this case example to discuss some of the general complexities related to interpreting flow patterns of anatectic lower and middle crust from magnetic fabric during growth of metamorphic domes. Magnetic lineations, reflecting tiling or zone axis of magnetically oblate grains, commonly tend to develop along the principal stretching direction. Our fabric modelling, however, shows that a lineation may also develop at a high angle to the overall flow (tectonic transport) direction as a relict of a pre‐existing fabric or due to a component of core complex axis‐parallel stretching and may be maintained for a significant period of time. Furthermore, we demonstrate that the finite magnetic fabric orientation is largely independent on the initial migmatite fabric intensity (formed before dome growth) but is rather a function of (a) shear strain, causing magnetic foliation and lineation to rotate towards the shear plane, and (b) magnitude of dome growth, causing their external rotation. Similarly, the degree of anisotropy P is also a function of accumulated shear strain and increases over time. These findings suggest that low fabric intensities may point to rapidly cooling, less sheared domains that are more likely to preserve early formed fabrics. In dome limbs undergoing protracted simple shear, low and high degrees of oblateness (as expressed by the shape parameter T) may discriminate between domains with weak and strong initial fabrics and may thus be used to map out domains with different strain histories and perhaps also with different melt proportions and rheological behaviour within metamorphic domes.