BGS classification of lithodemic units : proposals for classifying units of intrusive rock

Abstract

This report sets out a framework and terminology for classifying and naming lithodemic units composed entirely or predominantly of intrusive rock. It has been written with the geology of the UK in mind, but the principles and definitions recommended here should be applicable globally. The report anticipates a second phase of work, in which a BGS lithodemic classification for onshore intrusions in the UK will be constructed using the recommendations set out here. It is expected that this classification will in future be extended to include other classes of lithodemic unit (highly deformed, highly metamorphosed and ‘mixed’), and lithodemic units in offshore parts of the UK. A hierarchical framework of six formal ranks is proposed. Recommendations are made for classifying lithodemic units within the hierarchy and for constructing formal names for lithodemic units. A comprehensive glossary contains definitions for ‘approved’ lithodemic terms and for other terms associated with lithodemic classification. Some amendments to the existing form for making entries to the BGS Lexicon of Named Rock Units are suggested in order to make it compatible with some of the special characteristics of lithodemic units. It is proposed that a Lithodemic Framework Committee be established to approve new names for lithodemic units and to oversee management and development of the classification. Examples showing how the proposed classification and nomenclature will work are set out in an Appendix, generally in the form of simplified geological maps accompanied by a ‘classification’ of the illustrated lithodemic units in a hierarchical chain. In anticipation of a second phase of work that will produce a full lithodemic classification, it is proposed that intrusive lithodemic units associated with each of the four major Phanerozoic tectonothermal events to have affected the UK (‘Caledonian’, ‘Variscan’, ‘Carboniferous and Permian’, and ‘Atlantean’) be grouped at the highest formal rank proposed here to create four ‘supersuites’; a brief summary is provided of the salient aspects of each of these four events