Development of a synorogenic composite sill at deep structural levels of a magmatic arc (Odenwald, Germany). Part 2: Rheological inversion and mullion formation under bulk constriction

Abstract

We present structural, microfabric and strain data of a spessartite sill and its quartzmonzodiorite host that were affected by bulk constriction at T = ca. 660 °C. Constrictional deformation led to (1) subvertical prolate grain-shape fabrics, (2) increase of the sill's dip, (3) doubling of the sill's thickness, (4) mullions with cusps pointing into the host, and (5) boudinage of felsic veins cutting through the mafic sill. The sigmoidal shape of the foliation/lineation within the sill was caused by sill rotation, which led to shear stresses along the contacts with the host resulting in antithetic rotation of the grain-shape fabric that is subvertical in its central part and subparallel along the margins. The shape of the mullions, the boudinage of the felsic veins and the sigmoidal foliation/lineation indicate that the mafic sill was incompetent and the felsic material of host and veins was competent. This inversion in rheology can be explained by phase-boundary diffusion that was more effective in the fine grained mafic sill than in the coarse grained felsic host. Evidence for phase-boundary diffusion in form of strongly lobate phase boundaries is widespread and concerns all of the constituent phases present within the sill, host and felsic veins.