Differential stress control on the growth and orientation of flame perthite: A palaeostress-direction indicator

Abstract

Albite flames result from the replacement of K-feldspar by albite through alkali exchange. In flame perthites from deformed granites within the Grenville Front Tectonic Zone, replacement is driven by retrograde breakdown of plagioclase and hydration of K-feldspar. Within the strain fabric of the host rock, albite flames preferentially form subparallel to the inferred maximum compression direction and also develop at high-stress points such as grain-to-grain contacts. Flame tips are generally parallel to the ‘normal’ perthite crystallographic direction i.e. the Murchison plane, which is the orientation of the plane of minimum crystallographic lattice misfit between the albite and K-feldspar. We propose that the morphology of an albite flame is controlled by the coherent propagation of its tip into the K-feldspar crystal lattice, and that the orientation of the flames with respect to the rock fabric is controlled by the differential stress imposed during metamorphism. This type of replacement requires dry conditions in which there would be inefficient dissolution-reprecipitation along completely incoherent interfaces (normal replacement), and increased strength of feldspar. Under these conditions coherency is maintained to avoid a very large kinetic barrier. The differences in lattice misfit along the Murchison plane between K-feldspar and albite requires either high elastic strain energy (if the fit is coherent), or lattice dislocation energy (if the fit is semi-coherent). Such energy might be expected to inhibit the replacement growth of albite lamellae in a host feldspar. However, compression parallel to the Murchison plane reduces the lattice misfit along the compression direction. This results (1) in an increase in the Helmholtz free energy of the K-feldspar, and (2) in a reduction in the Helmholtz energy of the albite lamellae or of the semi-coherent interface. Thus, albite replacement allows a decrease in net free energy without the destruction of the alumino-silicate framework and is therefore favoured under conditions of high differential stress. Since flame growth is controlled by the imposed differential stress and not by strain in the host rock, flame perthite has potential as a palaeostress-direction indicator.