The tourmaline-bearing granite pluton of Bodmin (Cornwall, UK): magnetic fabric study and regional inference

Abstract

The magnetic fabrics and microstructures of the Variscan granite pluton of Bodmin (Cornwall) have been studied. Its low susceptibility magnitude, consistent with its lack of magnetite, comes from biotite and tourmaline. As the magneto-crystalline behaviour of tourmaline is ‘inverse’ compared with that of the phyllosilicates, the magnetic fabric was remeasured after heating the specimens to enhance the biotite magnetic signal by the growth of mimetic magnetite. The roughly similar magnetic fabrics before and after heating demonstrate that the effect of tourmaline is not important as long as the tourmaline content does not exceed that of the phyllosilicates. Regionally, this study reveals a well-defined NNW trend of the magnetic lineations with opposite plunges on each side of a separation line crossing the pluton centre. This pattern, very similar to that documented in the nearby Carnmenellis pluton, is ascribed to unroofing during emplacement. It agrees with the late to post-magmatic NW stretch that characterizes the Devonian to Carboniferous formations located to the north of this region, the thrust-and-fold structure of which is attributed to southward compression. We conclude that syntectonic emplacement may have characterized the whole Cornubian batholith at the very end of the Variscan orogeny in Cornwall.