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Field Relations and Chemistry of Sapphirine

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Field Relations and Chemistry of Sapphirine Canadian Mineralogist Yol. 22, pp. 417421 (1984) FIELDRELATIONS AND CHEMISTRYOF SAPPHIRINE.BEARING ROCKSFROM THE R'@RNESUNDAREA. FISKENESSET,WESTERN GREENLAND HOWARD R. WILLIAMS Dewrtnent of Geologicol Sciences,Brock University, St. Catharines, Ontario L2S 3Al ABSTRACT (1910),has sincebeen found worldwide in hig[-grade met?morphic rocks of diverse composition and his- Two contrasting sapphirine-bearing assemblagesare tory (Hiegins et al. 1979).This paper dessribestlree described: a higbly maguesianone, developed sporadical- more sapphirine-bearing localities some 50 kn east ly at the upper contact of the FisketrEssetComplex in as- of the tlpe locality. Herd et al. (1969) and Herd sociation with layeredultramafic rocks, and the other, more (1973) reinvestigated the Fiskenaessetlocality, iron-rich, in the amphibolite bordering the Complex. Miner- describing it in detail; the sapphirine was found to al assemblagesdescribed (1) are oftwo kincls: iron-poor: be associatedwitl rocks marginal to the Fiskenas- enstatite - phlogopite - olivine - pargasite - - sapphirine (Windley spinel, gedrite - sapphirine - spinel, and enstatite - gedrite set Complex et al. 1973). - phlogopite - sapphirine - spi4el; (2) iron-rich: magnesio- The tlree occurrencesdescribed in this paper have hornblende - sapphirine - spinel - anorthite. Only the iron- similar field relations to the classicFiskenasset lo- poor sapphirine-bearing assemblagesand their associates cality Gig. l). The local geologyis dominatedby a are characterizedby the presenceof accessoryapatite, ru- high-eradequartzofeldspathic gneiss terrane contain- tile and zircon, which may indicate a possible sedimentary ing intercalated layers of amphibolite and outlying origin for at least some of the sapphirine-bearing materi- parts of the Fiskenessetgemplex. Petrographic evi- al. The iron-rich assemblageis found within rocks consi- dence indicates that these rocks failed to enter dered to be entirely of igneous origin. eranulite-facies conditions during late Archean (Williams Keywords: sapphirine, west Greenland, chemistry, petrol- met?morphism 1973). ogy, Fiskenesset Complex. Two types of sapphirine-bearing lithology are described,with contrasting mineralogy and minsl{ chemistry. Both fit into the classification systempro- Souuarnr posed by Herd et al. (L969). On d6crit deux assemblagese sapphirine du complexe PETRocRAPHY AND MINERALOGY oF SAPPHIRN.IE. de Fiskenesset (Groilnland occidental). Le premier, trts BEARING LITHOLOGIES AND ASSOCIATED ROCKS magndsien, apparalt par ci, par li au contact supdrieur, associ€aux roches ultramafiques stratifides. L'autre, plus Iron-poor assemblages riche en fer, setrouve d4ns les amphibolites en bordure du Two examplesof an iron-poor sapphirine-bearing complexe. On note les assemblagessuivants: (l) dans les between the roches i faible teneur etr fer: enstatite - phlogopite - oli- assemblage occur at the contact vine - pargasite - sapphirine - spinelle, gedrite - sapphi- Fiskenesset Complex and amphibolite (Fig. lb). rine - spinelle,et enstatite- gedrite - phlogopite - sapphirine Four distinct parageneseshave beenrecopized (Ia- - spinelle; (2) dans les roches riches en fer, magn6sio- ble 1). Where present, sapphirine oscurs as rims hornblende - sapphirine - spinelle - anorthite. Seuls les around spinel blasts up to l0 mm acrossin a matrix assemblagesd sapphirine pauvres en fer, et les assembla- containingrelics of olivine and enstatitewithin ged- gesqui leur sont associds,contiennent apatite, rutile et zir- rite and phlogopite. The gedrite and enstatiteare g6a ggmms min6raux accessoires,ce qui fait penser d une straw-colored in hand specimen and predominate origine s6dimentaire pour partie au moins une des roches over oliyine, pargasite and phlogopite, which are ir- d sapphirine. L'assemblage riche en fer, par contre, se and spinel trouve dans desroches dont I'origine est considdrdecomme regularly distributed. Where sapphirine purement ign6e. are present, amphibole and mica commonly radiate from them. Clraduit par la R€daction) In thin section, amphibole and sapphirine are colorless; the spinel is palest blue and phlogopite is Mots-clds: sapphirine, Groihland oocidental, chimisme, pale brown. Grains of amphibole and nica clearly p6trologie, complexe de Fiskenesset. replace olivine and enstatite. However, spinel may be intergrown symplectitically with gedrite, and is commonly rimmed by both gedrite and phlogopite. INTRoDUcTIoN Spinel and olivine are found as large blasts with in- Sapphirine, first described from the Fiskenasset clusions of rutile, zircon and apatite. Anorthitic district of western Greenland in 1809 by Giesecke plagioclaseis rarely found but occurs enclosing eu- 4t7 THE CANADIAN MINERALOGIST Ftc. l. (a) Map showing sapphirine localities; inset left indicates position of (a) within Greenland. Positions of Figure lb and lc are indicated. Stipple: amphibolite and anorthosite in the Bjdrnound area only. O) Ceological map of tle two iron-poor sapphirineoccurrences. Insets indicate detailedrelationships. (c) Geologicalmap of relatively iron-rich sapphirine occurrence. SAPPHIRINE.BEARING ROCKS FROM WESTERN GREENLAND 4t9 'I. hedral gedrite needlespoikiloblastically. In general, TABLE MINERALASSEI4BLAGES AND HERD CLASSIFICATION SAPPHIRINE-BEARINGAND ASsOCIATED ROCKS these four lithologies show abundant textural evi- OF IRON-POOR dence for rnineral disequilibrium, with particularly Sample Paraqenesls Herd Class complicated textural relationships. 106619 Enstatl te-ph] ogopite-o l i vl ne-pargaslte- AlBn The four lithologies are developedoverlying and sapphirine-sp I nel -zl rcontanorthlte within magnetite- and pleonaste-rich dunites and 'l I nel -ruti e-apatl te C peridotites, which occur as well-layered, discontilu- 106622 Gedrite-sapphi ri ne-sp 106625 Gedrlte-phlogoplte-anorthite No sapph. ous lensesalong the upper anorthositic contact. In- Type 3 terlayered with these ultramafic units are thin lay- 106626 Enstati te-gedrite-ph l ogopi te-sapphirine- AlC ers of marble. High chromium and niqkel splnel -zl rcon-ruti l e concentrations in the ultramafic rocks suggestanl ig- Sanples are ldentified by thelr GreenlandGeological survey neous origin. Locally derived pegmatites of div$rse (G.G.U.)number. Material is housedat the Unlversity of 'granitic' compositions are comm.only associaited Exeter, U.K. with the zoneof contact betweenthe ulhamafic rd,cks and the amphibolite. They range in compositiion from true granite through syenite to subsilicic robks Anorthite-rich plagioclase,magnetite and corundum containing only albite and corundum (plumasife). occur as accessories,in bands parallel to the folia- tion. Corundu'n and magnetite appear to have Iron-rich assembloge formed from the breakdown of spinel, whereaszoi- site formed from the degradation of anorthite. The An exampleof an iron-rich sapphirine-bearingas- generaltexture of amphibole, spinel, sapphirine and (GGU semblage 10479) is found near the nortlbrn anorthite is one of mineral equilibrium, with simple parag(ne- end of Bjdrnesund (Fig. lc), and has the grain-boundaries and triple-point contacts. sis magncio-hornglende - sapphirine- spinel - a4or- thte a magnetite t corundum t zoisite QIerdt[pe B). The sapphirine is contained within a sacchafoi- MINERAL CHEMISTRY OF THE SEPPHNTNS-SEARTNIC dal hornblende-rich rock found withiu fine-grained LITHOLOGIES amphibolites lying structurally above the Fiskenas- set Complex. Composed chiefly of slightly foliated The sapphirine-bearinglithologies and associated pale green granoblastic magnesio-hornblende, the rocks have been analyzed (Table 2). Except for rock contains medium blue, pleochroic sapphirine 106619,there is little evidenceof compositional zon- and olive green aluminous spinel as elongate grains ing; therefore, the compositionsare consideredto up to 1 mm asross.Sapphirine commonly includes represent the typical chemistry of each phase. The spinel and, in some instances, perhaps replaces it. composition of olivine, enstatite and phlogopite in TABLE2. COI4POSITIONOF SAPPHIRINE AND ASSOCIATED MINERALS n6a2z1 I 066261 I 066191 rc47gg2 Spr # Spl Opx Ged Spr Spl Opx Ged Phl Hbl Opx 0l Spr SPI Hbl si02 tr. 56.1 46,4 14.0 56.4 5l.5 40.3 50.7 58.6 42.7 13.3 tr' 45.7 Ti02 tr. 0.02 0.1 0.3 0.04 tr. 0.03 - 0.6 0.77 0.07 0.01 0.02 0.02 0.2 Al^0^ 66.I 68.3 3,47 17.4 62,5 68.5 3.01 16.4 17.2 9.09 2.11 tr. 63.5 65.0 15.? 18.8 7.16 Fe0 J.l t 0.I 6.54 5.5 ?.4 8.1 6.31 5.27 2.13 1.62 3.82 4.M 5.91 Mn0 0.03 0.03 u.uo 0.03 0.lI 0.06 0.06 0.09 0.12 .16.0 Mso t 9.4 21,9 34.8 25.7 20.4 23.6 35.0 26.8 24.5 21.3 37.3 53.6 17.6 15.5 Ca0 o_ou 0.63 tr. 0.03 0.46 tr. 12.9 0.0s 0.03 0.01 0.02 IL6 Na20 : 1.84 tr. - u.v4 l.1 l tr. tr. l.l4 Kr0 u.uJ 0.02 - 7.8 - 0'16 fnO 0.02 tr. tr. 0.16 0.28 0.05 "'2-3 0.12 0.02 0.02 .l01.0 Tota l 101.8loo.4 97.7 99.3 100.0100.7 100.4 93.4 97.4 101.9 100.8 100.4 99.6 97,1 Mg/(l,tg+Fe)0.92 0.78 0.90 0.89 0.94 0.84 0.91 0.90 0.9s 0.96 0.95 0.96 0.84 0.60 0.80 l. SeeTable I for iron-poor assemblages.2. Iron-rich assenblage..# Analyst,..J.C..Bevan..Abbreviations: ollvlne' Spr sapphlrine, Spl splnel, Opx orthopyroxene,Ged gedrite, Phl -phlogopJte'Hbl hornblende'ul ti.. trirte. foiat'jron'calculaLed as Feb-. Prob6 condilions ts t<v, 2b nA, torrection program"MAGlc" adapted by R.N. t^lilson. 420 TI{E CANADIAN MINERALOGIST the iron-poor lithologies is extremely magnesian,in tribution coefficient Kp for both is gimilu, 6d contrast to the adjacent ultramafis rocks, which are identical to that determined by Lal et al. (1978). charasterized by the assemblageforsterite - calcite The sapphirine compositions, plotted on an - magFetite - pleonaste. Al2O3-SiO2-(MgO+FeO) diagram, lie within the The distinction betweenthe two sapphirine-bearing linear zone (Fig. 3) defined by most previous data groups ofrocks basedon host lithologies and mineral lcl, Hissins et sl. (L979)1. assemblagesis confirmed by the Mg,/Fe ratio of the component minerals.
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