Rock Classification Scheme (BGS).pdf - The Water, Sanitation and ...

Field S1 Rocks with the root name trachybasalt maybe named more precisely according to therelative amounts of Na 2 O and K 2 O. If Na 2 Ominus 2 is greater than K 2 O the rock is consideredto be sodic and is called hawaiite; ifNa 2 O minus 2 is less than K 2 O the rock isconsidered to be potassic and is calledpotassic-trachybasalt.Field S2 Using the same criteria as for Field S1,rocks with the root name basaltic-trachyandesitemay be named more preciselyas mugearite (sodic), and shoshonite(potassic), respectively.Field S3 Using the same criteria as for Field S1,rocks with the root name trachyandesitemay be named more precisely as benmoreite(sodic), and latite (potassic), respectively.Fields U1, F Field U1 contains rocks with the root namesbasanite and tephrite, while Field Fcontains foidite, of which the two mainvarieties are nephelinite and leucitite. Theboundary between these fields is dashed onthe TAS diagram, as nephelinites and leucititescan fall in both. Additional parametersare required to distinguish these rocks moreeffectively (see Le Maitre et al., 1989).6 ‘EXOTIC’ CRYSTALLINE IGNEOUS ROCKSSeven groups of crystalline igneous rocks — the Carbonatites,Melilitic rocks, Kalsilitic rocks, Kimberlites,Lamproites, Leucitic rocks and Lamprophyres — are classifiedas mineralogically and chemically ‘exotic’ (Figures1 and 2). Rocks in these groups have sufficiently unusualmineral and chemical compositions that they are not classifiedusing the modal (QAPF and ultramafic diagrams) orchemical (e.g. TAS) methods by which ‘normal’ crystallineigneous rocks are classified. Although theycomprise only a tiny proportion of all crystalline igneousrocks, the unusual characteristics and distinctive fieldsettings of the exotic igneous rocks means they haveattracted a large amount of attention from geologists. As aresult, there exists a large and confused nomenclature anda variety of classification schemes. The IUGS has recentlypublished new recommendations for the classification oflamprophyres, lamproites, kimberlites and the kalsilite-,melilite- and leucite-bearing rocks following an extensivereview (Woolley et al., 1996). New ‘characterisations’ ofthe rock types are given in mineralogical and/or geochemicalterms, and a revised sequence for the systematic classificationof the rocks is provided which integrates with theexisting IUGS system of Le Maitre et al. (1989). Therevised scheme is presented as the best compromise thatwas achievable, however it still suffers from somewhatconfusing terminology and a lack of clear definitions.The exotic alkaline rocks have never been classified satisfactorily,and this remains so despite recent advances,particularly in kimberlite and lamproite nomenclature. Therecent publication by the IUGS has tightened up aspects ofthe nomenclature, definitions and classification of theserocks, however a detailed petrographical and chemicalexamination is still required before most of these rocks canbe classified satisfactorily, and in many cases a knowledgeof the field setting provides a crucial piece of evidence.Any classification of a rock as a lamprophyre, kimberliteor lamproite using this system should be considered provisionaluntil further investigations using the specialised literatureas a key to correct classification have been undertaken.For no other groups of igneous rocks is the petrogeneticcomponent of classification more important than forlamprophyres, lamproites and kimberlites.In this section, definitions or characterisations for eachof the main ‘exotic’ rock types are presented, as they aregiven by the IUGS (Woolley et al., 1996). If the precisemineralogical and/or chemical nature of the rocks is notknown, no attempt should be made to classify them or toassign a root name unless unequivocal evidence isprovided by either the field setting or by a characteristicmineralogical assemblage that is identifiable in handspecimen. In the absence of either of these, a more generalfield name should be used, such as biotite-phyric-exoticmafite. Only when sufficient mineralogical and/orchemical evidence has been obtained to assign a rockunequivocally to one of the main ‘exotic’ rock types, asthey are defined below, should a more specific name suchas biotite-phyric lamprophyre or an approved root namesuch as spessartite be assigned.6.1 CarbonatitesCarbonatites are defined as igneous rocks that containmore than 50% modal primary carbonate (Streckeisen,1978; 1979). They are known to occur in extrusive,shallow and deep-seated environments.6.1.1 CLASSIFICATION OF CARBONATITESThe hierarchical classification scheme for carbonatites isshown in Figure 25. In mineralogical terms the followingtypes of carbonatite can be distinguished:calcite-carbonatite more than 90% of the carbonate iscalcite.dolomite-carbonatite more than 90% of the carbonate isdolomite.ferroan-carbonatite the main carbonate mineral isferroan dolomite, ankerite orsiderite. These can be distinguishedfrom other carbonates bystain tests.natrocarbonatite composed essentially of sodium,potassium and calcium carbonateminerals.6.1.2 USE OF QUALIFIER TERMS IN NAMING CARBONATITESThe root names for carbonatites have no grain size connotations,so it is important that grain size qualifier terms (seeSection 3.2 Grain size definitions and 7.2.1 Qualifiers toindicate grain size) are used in the rock name, for examplefine-grained dolomite-carbonatite. Mineral qualifiersshould be used to indicate the presence of minor componentsof carbonate minerals that are not implied by the rootname, for example calcite-bearing-fine-grained dolomitecarbonatite.Mineral qualifiers should be used for noncarbonatecomponents, for example apatite-bearingcalcite-carbonatite. Mineral qualifiers should also be usedwhere non-carbonatitic igneous rocks contain significantproportions (but less than 50%) of primary carbonate, forexample carbonate-bearing nephelinite (> 10% modalprimary carbonate) or calcite-rich nephelinite (20 to 50%modal primary calcite). Colour index qualifiers (seeSection 7.3 Qualifiers based on colour) should not be usedwhen naming carbonatitic rocks as all primary carbonate15