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Geochemistry and REE Minerals of Nepheline Syenites from The American Mineralogist, Volume 70, pages 1087-1 100, 1985 Geochemistryand REE mineralsof nephelinesyenites from the Motzfeldt Centre,South Greenland A. P. JoNrsr Diuisionof Geologicaland Planetary Sciences CalifurniaInstitute of Technology,Pasadena, California 91125 .rNp L. M. LensrN GrsnlandsGeologiske U ndersogelse @sterVoldgade 10, DK-1350,Copenhagen, Denmark Abctract Whole-rock analysesincluding the REE are given for nineteensamples from four of the six overlappingnepheline syenite intrusions which constitutethe Proterozoic(- 1310ma) Motz- feldt Centre. Larvikite margins of one unit (SM5) are apparently related to the nepheline syenites by early fractionation of Ol + Pl followed by fractionation of Ol + Pl + Cpx t Amph at current levels of exposure.Positive Eu-anomaliesin the larvikites might be 875r/865r explained by large-scaleincorporation of earlier Pl-rich cumulates.Low initial ratios imply a mantle source,and the most basic magma was a late alkali gabbro giant dike with plagioclasemegacrysts. REE- and incompatibleelement-rich peralkaline lujavrites with large negativeEu-anomalies are late stageproducts of extremefractionation. Electron probe analysesof REE in eudialyteconfirm this mineral as the major host to Zr and REE in the lujavrites. Other REE-bearingminerals in the nephelinesyenites are Ca-Na{e-Ti-Zr-Nb- silicatesof the rinkite-mssandrite and l6venite series,with up to 22 wt.% RE-oxides;some may be related to metasomaticfluids which had geochemicalsignatures similar to the lujav- rites. Introduction sphene.Electron probe analysesincluding the REE in sev- discussed. The Motzfeldt Centre is the older of four major centers eral of theseminerals are reportedand becausea of igneousactivity which make up the Igaliko Complexof The Motdeldt Centre is especiallyinteresting was developed. nephelinesyenites (Emeleus and Harry, 1970).It is situated late stage peralkaline magma, lujavrite, bulk of silica- at the eastern end of the Proterozoic alkaline igneous This provides a link between the and the un- Gardar Province of southern Greenland (Emeleus and undersaturatedsyenite plutons in the Gardar (Ferguson,1964, Upton, 1976).The Motzfeldt Centre has a determined usual peralkalineIlimaussaq Intrusion that Rb/Sr (whole-rock)age of 1310+31 ma and an initial 1970;Bailey et al., 1981;Sorenson, 1969). We suggest 8?5r/865rratio near 0.702 (Blaxland et al., 1978).The incorporation of earlier feldspar-richcumulates in addition played im- MotzfeldtCentre covers an areaof approximately350 km2 to extensivecrystal fractionation could have an and is well exposedas a steeply dissectedand glaciated portant role in the overall petrogenesisof the syenites. 2000 m plateau. The mineralogy of the mafic silicatesto- getherwith a briefoutlineofthe geologyhas been given by Geology Jones(1984). The Motzfeldt Centre was formed by a seriesof overlap- In this paper we present new whole-rock geochemical ping intrusions emplacedby a combination of block stop- data for major and trace elementsincluding the rare earth ing and 5-10 km radius ring fractures.The country rock is elements(REE). In particular the REE are contained in a mostly gneiss,known as Julianehib granite (Allart, 1973) variety of minerals,some of which show evidenceof meta- and a sequenceof supracrustalrocks including metasedi- somatism. These minerals are principally eudialyte and ments, agglomerates and lavas collectively called the alkali-zircono-silicates of the rinkite-mssandrite and Eriksfiord Formation (Emeleusand Upton, 1976).Meta- livenite series,and to a lesserextent, apatite, zircon and somatizedand alteredrelics of the supracrustalrocks form large founderedraftlike xenoliths within the syenitesand measure up to several kilometers in length and a few I Current address, School of Geological Sciences,Kingston hundred meters in thickness.Adjacent lavas in the supra- Polytechnic, Penrhyn Road, Kingston Upon Thames, England crustal volcanic rocks, including the altered xenoliths, KTl 2EE. range from alkali basalts to phonolites and are possibly 0003-004x/85/l l 12-1087$02.00 1087 JONESAND LARSEN:NEPHELINE SYENI?ES FROM THE MOTZFELDT CENTRE Fig. l. Geology of the Motzfeldt Centre and location of samples used for whole-rock analysesin this study. Map after Jones (1984) with revised eastward extension to the major E-W fault after Tukiainen et al (1984). AGGD is alkali gabbro giant dike; EM : East Motzfeldt : syenites; NM North Motzfeldt syenites. Ornamentation, solid : supracrustal rocks. NQ : North Q6roq Centre (Chambers, 1976),SQ : South Q6roq Centre (Stephenson,1976). related to the Motzfeldt Centre itself (Jones, 1980). The structurally high levels. Pegmatites are sometimes associ- center is cut by a major transcurrent E-W fault (The ated with contacts between xenoliths and host syenites. Flink's Dal Fault, Emeleus and Harry, 1970) which was Several pegmatites, microsyenite sills and altered limonite- shown to have a left lateral displacementof severalkilome- rich shear zones in the center are enriched in radioactive ters (Jones, 1980) measured as 6 km to the east of the minerals (Tukiainen et al., 1984). inland lake, Moztfeldt So (Tukiainen et al., 1984).The Geo- The simplified geological map in Figure 1 shows the logical Survey of Greenland is currently engaged in de- location of analyzed whole-rock samples (XRF) selected for tailed investigations of several syenites and of radioactive additional REE determination by Instrumental Neutron mineralization in and around the center. There are six Activation Analysis (INAA). The XRF analyses were ob- major syenite units, prefixed "SM" and numbered I to 6 in tained by Jones (1980) from which the following summary order of intrusion as indicated on the summary geological is largely extracted. The ranges of zoned mafic minerals, map in Figure 1. Future refinements to the geological especially Fe/(Fe + Mg) and Na/(Na + Ca) in pyroxene, boundaries of the syenite units are unlikely to affect the are related to the calculated normative differentiation index major conclusions of this paper, but might provide answers (D.L) of the host rock. The D.L was calculated after Thorn- to some of the outstanding problems. A unit of late-stage ton and Tuttle (1960) with the addition of normative lujavrites (SM6) and a sub-unit of cancrinite-rich roof-zone acmite (NaFe3Si2O6), which is important in the more frac- syenites (HY) were recognized subsequent to the work of tionated syenites.Whole rock P and Ti decreasewith in- Emeleus and Harry (1970) by Jones (1980). Unequivocal creasing D.I. and increasing Fe/Mg, consistent with p€tro- time relationships between units SM5 and SM6 have not graphic textures which indicate early crystallization ofapa. been established, since they are separated by the Flink's tite and Ti-magnetite. Many of the syenites show cumulus Dal Fault. The center is cut by a regional swarm of ENE- textures in thin section and classic mineral banding, al- trending trachytic dikes and a prominent alkali gabbro though rare, does occur in the center (Emeleus and Harry, giant dike (AGGD) intruded in similar orientation. A few 1970; Jones, 1980). Mineral compositions show normal near-vertical carbonatitic and ultrabasic breccia pipes up zoning and combined trends which are the expectod conse- to 200 m across also cut the center, but are not shown on quences of crystallization from increasingly fractionated oy- Figure 1. The minor intrusions remain an attractive goal enite magmas. In addition to continuous zonation, the for future study and it is not known how they relate to the mafic minerals also show discontinuous zonation of the syenites.Thin discontinuous fluorite- and calcite-rich veins type pyroxene-amphibole-pyroxene (Jones, 1984). Bulk are found sporadically throughout the center and may pro- feldspar probe analyses are difficult to obtain because of vide clues to late stage metasomatic processesseen in some perthitic textures and show a wide range in Ca-poor com- JONESAND LARSEN:NEPHELINE SYENITES FROM THE MOTZFELDT CENTRE 1089 dike relatedto the marginsof unit SM5 (SM5{')contains well defined plagioclasecores with irregular outlines and lamellar twinning abruptly terminatedby mantlesof crypt- operthitic alkali feldspar.Three reconnaissancemicroprobe analysesof the plagioclaseshow a range in composition from Ab.rAnrrOr, to AbroAnrrOr,n and are plotted in the feldspar ternary in Figure 2. Similar, though slightly lesscalcic plagioclasecores (<An2o) occur in chilled facies of augitesyenite from Ilimaussaq(Larsen, 1981). The bulk composition of the larvikite ring dike provides the best estimateof a basic magma availableat Motzfeldt that was directly associatedwith the syenites(Table 2). However,the most basicrocks in the center,apart from the little studied carbonateplugs, are representedby the late AGGD (Fig. l). The AGGD carries abundant plagioclasemegacrysts (-Anruor.) and is similar to a Ti-rich alkali basalt in Ab 0r composition,with about 47 wt.VoSiO, and l4%o nephel- ine (Ne)in the CIPW norm at Fe3lFe2:0.1 (Jones,1980). Fig. 2. Feldspar compositions (Ca Na-K). Symbols: solid tdangles : plagioclase cores in larvikites of SM5; open triangles : rims and normal feldspar from larvikitic syenites of SM5; open Analytical procedure circles : feldspar from nepheline syenites of SM5; hexagon X-ray onalyses(XRF) (AGGD) : plagioclase megacryst from alkali gabbro giant dike. fluorescence Ruled areas are loci of feldspar (including plagioclase relics) from Originalrock samples,typically 0.5 to 2.0kg in sizedepending chilled facies of augite
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