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Melilitolites: a New Scheme of Classification 897

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Melilitolites: a New Scheme of Classification 897 895 The Canadtan M inerak> gist Vol. 36, pp. 895-903(1998) MELILITOLITES:A NEW SCHEME OF CLASSIFICATION ELIZABETH ANN DUNWORTH' .aTNoKEITH BELL Ottawa - Carleton Geoscience Cente, Depamnent of Earth Sciences, Carleton University, Onawq Ontario KIS 586 Arsrnecr A new scheme of classification is proposed for metilite-bearing plutonic rocks with more than l07o melilite and less than 507o pimary carbonate phases,based on the absolute modal abundancesof the dominant minerals. Melilitolite is retained as a "general" root-name for all such rocks. Previous definitions of terms used in the classification of these rocks, including afrikandite' kugdite, okaite, turjaite and uncompahgrite, are modified in order to provide "specific" root-names for melilirolites. A new rock-name, "ultramelilitolite", is proposed as a specific root-nams f6l samples with more than 657o melilite. The use of multiple root-na.mesand the problems associatedwith the use of mineral modifiers in rock nomenclature are discussed. Keyword:; melilitolite, afrikandite, kugdite, okaite, tudaite, uncompahgrite, "ultramelilitolite", modifier, root-name. Sonauenn, Nous pr6sentons ici un nouveau sch6ma de classification visant les roches plutoniques contenant plu s de 70Vode mdlilite et moins de 507o de phasescarbonat6es primaires, et fond6 sur Ia teneur des min6raux dominants en termes absolus. Nous re- tenons le terme "m6lilitolite" comme racine du nom de telles roches. Les d6finitions utilis6es ant6rieurement dans le classification de ces roches, par exemple, afrikandite, kugdite, okarte, tu{ar'te et uncompahgrite, sont ici modifi6es afin d'en arriver d des noms de base spdcifiques dans ce sch6ma.Un nouveau nom, "ultram6lilitolite", est propos6 pour les 6chantillons contenant plus de 65Vode m6lilite. Nous discutons I'utilisation de noms de base multiples et les problbmes associ6str I'utili- sation de noms de mindraux collme qualificatifs dans la nomenclature de ces roches. (Traduit par la R6daction) Mots-cl6s; m6li1itolite, afrikandite, kugdite, okar'te,turjarte, uncompahgrite, "ultram6lilitolite", qualificatifs, noms de base. IxrnooucrroN rocks. Included in this scheme was a section on melilite-bearing intrusive rocks, based on a classifica- Althoughmelilite-bearing igneous rocks are compara- tion scheme first proposed by Egorov (1969). This tively rareon a worldwidescale, these unusual rocks play scheme uses (and in some casesredefines) nomenclature an importantrole in assessingthe petrogeneticsignifi- taken from the intemational literatwe. Although the clas- canceof mantlederivd SiOlpoor magmas.The current sification schemeby MikhasTovet aL (1995) is one of the IUGS classificationscheme for melilite-bearingintrusive most realistic attempts to classify melilite-bearing intru- rocks proposedby Streckeisen(1979) nd Le Maitre sive rocks thus far, it has two drawbacks: (i) it strays too (1989),and recently aftumed by Wootleyet al. (7996), far from the original definitions of the terms used in the definesboth innusive and extusive melilite-bearingrocks nomenclature, and (ii) it does not follow current IUGS solelyon thebasis of threeminelal components:melilite, guidelines on rock nomenclature. In a detailed classi olivineand pyroxene. Unforfimately, ttrese minerals do not fication of undersaturatedalkaline rocks, Mitchell adequatelyreflect the mineral assemblagespresent in (1996) included a section on extrusive and intrusive worldwide,plulonic, melilite-bearingrocks. melilite-bearing rocks which provided a compromise A classificationscheme called the Petrographic between the schemes of Woolley et al. (1996) and Codewas formulated by Mikhailov et aI. (1995)in an Mikhailov et al. (1995), but did not sufficiently reflect attemptto classifyall magmaticand metamorphic natural mineral assemblages.The present paper sum- I Presentaddress: Mineralogisk-Geologisk Museum, San'gate 1, OsloN-0562, Norway. E- maiJ address: [email protected] 896 THE CANADIAN MINERALOGIST marizes previous work on the description and classifi- Olivine-bearingmelilite rocksfrom the KugdaMassif cation of worldwide intrusive melilite-bearing rocks, within the Maimecha-Kotui Province in Siberia were and provides a new, comprehensivescheme of classifi- termed"kugdite' by Egorov(1969). Most melilite-bearing cation based on their modal mineralogy, which respects rocksfrom the otherMaimecha-Kotui massifs have been both old terminology and modern guidelines regarding classifiedas kugdite,okaite, turjaite anduncompahgrite, the nomenclature of igneous rocks. usingthe scheme of Egorov(1969). The Maimecha-Konri provinceincludes the Guli massif,the largestalkaline complexin theworld (1500km'z), which alsocontains Swnraenv oF PREYIoUsWoRK melilitoliteand kugdite (Yegorov 1989). Kovdorite, also describedas an olivine turjaite, was found in the Early afiempts to name intusive melilite-bearing rocks KovdorMassifbyAatlctnd(1945). Detailed sutdies of the were made in the filsthalf of the twentieth cenurry.l,arsen Kovdorrocks were subsequently carried out by Kupletsky & Hunter (1914)used ttre terrn'tncompahgrite'to define (1948),who devisedan earlythree-component scheme of a melilitelyroxene rock from hon Hill, Colorado. l.arsen classificationfor the samples.Kukharenko et al. (1965) (1942) subsequentlydescribed an averageuncompahgnte describedmelilite-bearing samples from Kovdor,includ- from Iron Hill as containing 68Vo melilite, 15% ing kovdoritesand tudaites,4s goltaining significant clinopyroxen e, | IVo magnetite o 3 Vo perov skite, 2Vo quantitiesof olivine (<l9%o)and monticellite (<20Vo), mica, LVoapatite and minor calcite. However, the moda.l aswell as nepheline-,clinopyroxene- and mica-bearing abundances of uncompahgrites at Iron Hill vary widely. varieties.Atzamastsev (1994) described the dominant Melilite can make up ftom 50 tD 100% of the total, and mineralogyof the turjaitesfrom Kovdor as melilite nepheline and garnet also can be present (Larsen 1942). (25-5 OVo), nepheline (l 5-3 07o), mica (20-25 Vo) ar^d Further detailed penological sndies of the uncompahgttes clinopyroxene(5-25Vo). An unusualmelilitolite sill at and relaredrocks atlron Hill werecanied outbyTi:mple & Pian de Celle, Italy, was describedby Stoppaet aL Grogan(1965) and Nash(1972). (1997).It contains-l2%o calciteand257o leucite, and Intrusive melilite-bearing rocks ils d$undsnl in Russia, wasgiven the name"calcite leucite melilitolite". particularly in the Kola Peninsulaand the Maimecha-Kotui hovince in Siberia. Ramsay (1921) and Kranck (1928) 'turjaite" defined the term to describe the abundant ClassrlceuoN or MsLLnoI-[Es nepheline-bearingmelilitolites ftom the Tlriy Peninsula in the Kola Peninsula-The type-locality turjaitss from Ttriy Rationale contain melilite (20-65Vo), nepheline or cancrinite orbottr (lG3 5Ea),mca Q-j 57o),m agnetite (!307o), perovskite (4Vo),with minor apatileand gamet and rare calcite and Many melilite-bearing intrusive rocks are dominated clinopyroxene (IGanck 1928, Bell et al. 1996). The lerm by the presence of one (or two) other mineralso apart "okaite" was defined by Stansfield (1923) for the haiilne- from melilite. Table I demonstrates the three principal (di- bearing equivalent of turjaite, described from the Oka melilite-bearing mineral assemblages: (A) silicates vided into mafic and felsic groups), oxides, and (C) Complex in western Quebec. Samples of okaite and re- @) lated melilite-bearing rocks from Oka tlpically contain carbonates. Most melilite-bearing rocks fall into cat- melilite (30-907o), nepheline (<40Vo),haiiyne (<40Vo), egory A, but categories B and C are represented in clinopyroxene (<IIVo), with minor apatite,calcite, mag- severalintrusions, and any proposedscheme of nomen- clature needs to take these assemblages into netite, mica and perovskite @by 1973, Treiman & Essene 1985,Gold et al.1986). consideration. Afrikandite, from the Aftikanda complex in the Kola Peninsul4 was firstdefinedby Chirvilskii et al. (194O)as an intrusive rock containing clinopyroxene, magnedte, TABLB 1. MINERAL ASSEMBLAGES IN MBLILITOLIIES AND DEFINMON OF SPECIFIC R@T-NAMES melilite, mic4 olivine and perovskite. A later definition of aftikandite by S/rensen (1974) as a melilile - perovskite crcup Domindt minemlogy Definingmineml (>10%) Spcific @t'nme - magnetite rock was used by Nielsen (1980) to classiff melil'rc (>657o) "ulmelilholite" pyrcxene uncomPalErite ring-dyke rocks from the Gardiner intrusion in Greenland olivine kugdite which also contain mica and apatite in substantial quanti- nepheline ruqdrc ties. Other rocks from Gardiner were classified as turjaite hauyne okaiE and uncompahgrite. The Rangwa complex in Kenya was B oxide FrcvskiE afrikddhe described by Le Bas (1977) as containing turjaite (40- C @bonde (prinary) [carbowe > 50Va carboMite] 59Vo melulite, l-j%o nepheltne and cancrinite,2-57o cdbone<5070 e.S.calcite-nelilitolite* perovskite, \s clinopyroxene, 3-l5Vo 5457o micU l-ISVo Ifatrysqle @d#. llm tbe ldlo of 'mtl4 @ oftht d€dniag mhsal4 th' ffi magnetiteatd3-luVo apatite),with rarer uncompahgnte ah';d;,ffig di!@l EiI p@vids the spaiflc @ts''9@ Ii4odif,@ Esy bs usedb d@td re thm 10/o of poniolar Ei@d!' @d @ lided h ordc of iwing abudne 5f% (<1,7Voclnopyroxene and 4Vo obvne), and afrikandite ' No ep*ift ffi--@s is Fovided frr @pl6 ffiining ffi tbe ICp/o@d ls rhe (157operovskite and l6Vo magnetite). priE6y @rbond6 pb!s. MELILITOLITES: A NEW SCHEME OF CLASSIFICATION 897 The schemeproposed in the presentpaper is based i) follow thecurrent IUGS guidelineson thenaming of on the modalmineralogy of a worldwide
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