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1 Introduction Cambridge University Press 978-0-521-86810-5 - Metamorphic Rocks: A Classification and Glossary of Terms Edited by D. Fettes and J. Desmons Excerpt More information 1 Introduction Inzwischen herrscht in den mineralogischen that they should be based on non-genetic criteria Schriften eine erstaunliche Verwirrung in der and that these criteria should be measurable in the Bestimmung dieser Gesteinsarten, und in dem field or under the microscope. Nomenclature Gebrauche der Benennungen. based on criteria such as rock chemistry or meta- Werner, 1786 morphic grade was obviously unsuitable and a systematic scheme was devised based on com- (Meanwhile there prevails in mineralogical pound names with structural root terms and literature an astonishing confusion in the mineral qualifiers (see Schmid et al., Section 2.1). determination of rock types and in the This scheme allows a systematic name to be nomenclature.) given to any rock. However, it was accepted that there were many well-established specific names, This position described by Werner still has a cer- such as marble, amphibolite and eclogite,that tain resonance today. Despite many excellent would have to remain and could potentially be works proposing classification schemes for meta- used as alternative names. In addition many meta- morphic rocks, much of the terminology remains morphic rocks can be named by reference to their ill-defined and ambiguous. Practice differs across protolith. Thus, metamorphic rocks may poten- the international geological community and even tially have up to three acceptable names, that is a such core words as ‘gneiss’ and ‘schist’ vary in their systematic name, a specific name and a protolith- usage and meaning. To some extent this position based name, for example carbonate granofels – reflects the fact that metamorphic rocks, and here marble – metalimestone or hornblende-plagioclase we include metasomatic and diagenetic rocks, schist – amphibolite – metabasalt. The nomencla- represent a vast range of lithologies and processes, ture scheme has to take account of this situation. overlapping with the nomenclature of sedimentary This book is accordingly structured in two rocks on the one hand and with that of igneous main parts, the first dealing with the classifica- rocks on the other. It also involves a range of tion and nomenclature scheme, the second con- processes from sedimentation and burial through sisting of a glossary containing comprehensive deep-seated orogenesis to planetary collision. definitions of all terms related to metamorphic Against this background the aim of the terminology. The first part contains 12 sections: Subcommission on the Systematics of Meta- the first deals with the basis of the systematic morphic Rocks (SCMR) is to present a systematic scheme and how it should be used, the remainder nomenclature and to present an agreed definition deal with various specialist areas and discuss, as of all related terms. The SCMR took its remit to appropriate, the evolution of the terminology in cover all metamorphic rocks including quenched these areas or the rationale that lies behind rocks not normally considered as igneous rocks, approved definitions. The second part is the glos- such as fulgurite, tektite and pseudotachylite. It sary, which consists of around 1100 entries. It also covered structural terms including fault struc- gives comprehensive definitions, etymology and tures and the systematics of impactites. The prime source references, and categorizes the terms objective was to provide a scheme for naming and into three classes, namely: recommended use, describing metamorphic rocks; no attempt has restricted use and obsolete or unnecessary. been made to cover the terminology relating to All the terms were fully researched and the the detail or theory behind metamorphic processes, source and significant references individually mineral chemistry, graphical presentations, etc. checked. In addition, all the principal terms It was agreed that rock names should, as far as were considered by the Study Groups. In some possible, be applicable at the hand-specimen scale, cases this involved intense debate and wide Metamorphic Rocks: A Classification and Glossary of Terms. Recommendations of the International Union of Geological Sciences, eds. Douglas Fettes and Jacqueline Desmons. Published by Cambridge University Press. # Cambridge University Press 2007. © Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-86810-5 - Metamorphic Rocks: A Classification and Glossary of Terms Edited by D. Fettes and J. Desmons Excerpt More information 2 Introduction consultation with the international community accompanying guidelines and a table outlining before a consensus was reached. It is hoped the use of qualifiers. therefore that it will be accepted that these defini- 2. The specialist sections (2.2–2.12) should be con- tions are designed for international use and may sulted for extra information. A list of approved differ in detail from established usage in some terms (recommended and restricted) arranged areas. by specialist subject is given in section 3.3 of the glossary, and this may be used to find out the 1.1 Guidelines on how to use the book terminology related to a specialist subject. 3. The glossary should be used for the definition, 1. The systematic scheme for naming a rock is source references etc. of all terms. It contains sum- given in section 2.1. The basis of the scheme is mary lists of all approved terms arranged alpha- given in subsections 2.1.1 to 2.1.7. The proce- betically (section 3.2) and by subject (section 3.3). dure for naming a rock is given in subsections 4. All references cited in the book are in a con- 2.1.8 onwards. This includes a flow chart with solidated list at the end. © Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-86810-5 - Metamorphic Rocks: A Classification and Glossary of Terms Edited by D. Fettes and J. Desmons Excerpt More information 2 Classification and nomenclature scheme 2.1 How to name a metamorphic rock rock’ enables any earth scientist to assign a name to a metamorphic rock, following this scheme. R OLF S CHMID,DOUGLAS F ETTES, The section further gives guidelines on the appro- B EN H ARTE,ELEUTHERIA D AVIS and priate use of these systematic names and on the J ACQUELINE D ESMONS use of possible alternatives based on the protolith and other specific names. 2.1.1 Introduction PRINCIPLES OF NOMENCLATURE The usage of some common terms in metamor- A nomenclature scheme consists of defined terms phic petrology has developed differently in dif- and the rules governing their use. In erecting a ferent countries and a range of specialized rock nomenclature scheme the SCMR was guided by names have been applied locally. The SCMR aims the following underlying principles. to provide systematic schemes for terminology and rock definitions that are widely acceptable (a) The scheme must provide a consistent set of and suitable for international use. This first names to cover the spectrum of rock types section explains the basic classification scheme and their characteristics without any termi- for common metamorphic rocks proposed by the nology gaps. SCMR, and lays out the general principles which (b) The scheme must ensure that all users can were used by the SCMR when defining terms for apply the same criteria to give any rock or its metamorphic rocks, their features, conditions of characteristic features the same name. These formation and processes. Subsequent sections names should be understood uniquely and discuss and present more detailed terminology without ambiguity. for particular metamorphic rock groups and processes. In any system of nomenclature a number of The SCMR recognizes the very wide usage characteristic features or parameters are used to of some rock names (for example, amphibolite, divide rocks into groups or sets, and the criteria marble, hornfels) and the existence of many for such divisions or subdivisions are fundamen- name sets related to specific types of metamorph- tal to the terminology. The SCMR decided (see ism (for example, high P/T rocks, migmatites, Schmid & Sassi, 1986) that the above principles impactites). These names and name sets clearly would only be fulfilled if the criteria for any must be retained but they have not developed on specific division/subdivision were defined using the basis of systematic classification. Another set only one type of characteristic feature. For of metamorphic rock names, which are com- example, the criterion for a specific division/ monly formed by combining mineral names subdivision might be a particular feature of with structural terms (for example, quartz-mica mineral content or structure, but it should not be schist, plagioclase-pyroxene granofels) is capable both mineralogical and structural. In a series of of being used in a systematic way. The SCMR divisions/subdivisions in a classification scheme, recommends that such compound names are structure and mineral content may be applied at systematically applied using three root names different stages, but they should not be applied (schist, gneiss and granofels), which are defined simultaneously. solely by structural criteria. Such systematic At a given stage of division/subdivision a set of names are considered particularly appropriate rock groups may be recognized in a classification when specific names are unknown or uncertain. scheme, and these will be given group names A flow chart on ‘How to name a metamorphic (or root names in the case of major divisions). Metamorphic Rocks: A Classification and Glossary of Terms. Recommendations of the International Union of Geological Sciences, eds. Douglas Fettes and Jacqueline Desmons. Published by Cambridge University Press. # Cambridge University Press 2007. © Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-86810-5 - Metamorphic Rocks: A Classification and Glossary of Terms Edited by D. Fettes and J. Desmons Excerpt More information 4 Classification and nomenclature scheme Such names form a fundamental element of the that they may be formed from any pre-existing classification.
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