The role of regional tectonics and magma flow coupling versus magmatic processes in generating contrasting magmatic fabrics within the Land’s End Granite, Cornwall.

Abstract

The Lower Permian Land’s End Granite intruded Upper Devonian metasedimentary and metavolcanic rocks of the Mylor Slate Formation that had been previously deformed and regionally metamorphosed during Variscan convergence. Structural studies of the host rocks have been used to infer that granite generation and emplacement occurred in response to regional D3 NNW-SSE extension of moderately thickened crust. Detailed field mapping along the northwestern margin of the granite reveals complex magmatic fabrics defined by K-feldspar and biotite. Close to the pluton margins, a gently NW or SE dipping magmatic foliation is defined by the preferred orientation of K-feldspar and biotite. Further from the pluton margin, the foliation dips moderately NW or SE, or occurs in steeply dipping NW-SE trending zones with very strong fabrics. Magmatic foliations tend towards parallelism with the margins of stoped blocks over a distance of one to several metres, but there is no evidence that the blocks deform a previously formed magmatic fabric. The Land’s End Granite exhibits low degrees of anisotropy of magnetic susceptibility (AMS) and biotite is the carrier. The AMS foliation generally dips gently to the NW or SE and contains two, near-orthogonal lineations that trend ENEWSW and N-S. The variations in magnetic lineation orientation correlate with the intensity of the macroscopic feldspar fabric. In zones where the feldspar fabric is strongly developed, the AMS has a NW plunging lineation, whereas in zones where the feldspar fabric is weak, the AMS has a NE plunging lineation. There is close correspondence between the orientation of the AMS fabrics and D3 structures within the Mylor Slate Formation. However, it is possible that there was only partial coupling between the granite magma and extensional deformation of the host rock. Stoped blocks and host rock irregular contacts exert a significant control on fabric orientation.