Dark margins at igneous contacts: A Critical Study with Special Reference to those in Jersey, C.I.
Abstract Megascopically dark zones may occur in rocks at the contact with granite intrusions. The zones vary in width and nature. Several explanations of their origin have been advanced, particularly for those in basic igneous rocks. Chilling of basic magma against acid magma and reaction between basic rocks and acid magma are the most common of these and the criteria by which they may be distinguished are discussed with particular reference to dark margins occurring in Jersey. Both processes are known to occur, but the tectonic setting of a particular granite mass in large measure determines which is dominant in a particular area. Chilled margins are thus found to be more common in the high-level volcanic association, whereas reaction is dominant at deeper plutonic levels, but can occur wherever there is a compositional difference between the rocks involved.