Selective thermal enhancement of magnetic fabrics from the Carnmenellis granite (British Cornwall)

The anisotropy of magnetic susceptibility (AMS) of magnetite-free granites having complex paramagnetic mineralogies can be thermally enhanced. In-air heating at 650 °C during 2 h helps growing new magnetites that are mimetic with respect to the biotite cleavages. As attested by the similar after-heating AMS and AAR fabrics, the signal of the silicates having an inverse fabric, like tourmaline and cordierite, becomes negligible when compared to the signal of magnetite. When applied to the granite body of Carnmenellis (British Cornwall), the after-heating magnetic lineation map favourably compares with the magmatic lineations early reported by Ghosh [J. Geol. Soc. London, XC (1934) 241]. We interpret this new structural map in terms of NE-trending late-magmatic tensional domains, among which a corridor is clearly defined at pluton center, and along which the magma flowed during its late stage of emplacement. Preferred percolation of boron-bearing fluids was responsible for tourmaline enrichment in these domains which, globally, are perpendicular to the overall NW-trending magma stretch recorded by the majority of lineations.