Mineralogy And-Rare Earth Geochemistry of Apatite And
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American Mineralogist, Volume 60, pages 607-620, 1975 Mineralogy_and-RareEarth Geochemistryof Apatite and Xenotimefrom the GloserheiaGranite Pegmatite,Froland, SouthernNorway Rrmen Aur-r Mineralogisk-Geologisk Museum, Sarsgt. I, Oslo 5, Norway Abstract The Gloserheia granite pegmatite, situated in the Precambrian Kongsberg-Bamble forma- tion, contains eight different zones (core, intermediate zones I-V, wall zone, and border zone). Apatite occurs in the core and in zones I and llI. Xenotime has been found included in apatite from all three zones, and in other parageneseswithin these zones. The apatite close to xenotime inclusions is depleted in REE. Zoningof xenotime is characterizedby an enrrchment of the light REE relative to the heavy REE, from the center towards the rim of the cryslal. Xenotime crystals included in apatite from the outer zones show lessrelative enrichment in the light REE than do thosein the inner zonesof the pegmatite.No suchtrend in REE fractiona- tion is shown by xenotimes from other parageneses.The rare earth minerals included in apatiteare consideredto have formed by exsolution,possibly accompanied by metasomatism. Fractionation in the apatite-xenotimesystem was causedby processesbearing a simple rela- tion to the crystallization of the zones where theseminerals occur. ln contrast, the REE frac- tionation in the other xenotimesprobably has a more complex relation to the sequenceof zones in the pegmatite. Introduction termediate zone I, where it is found as roughly equidimensionaleuhedral-to-subhedral The Gloserheia granite pegmatite is situated in the crystalsup to a sizeof one meter. Commonly well Precambrian Kongsberg-Bambleformation, in Fro- developedforms include a prism, pyramid (occasionally land commune approximately 9 km N of Arendal two sets),and pinacoid, (8"43',36'E, 59.32'20' N). basal and crystals are sometimesdoubly terminated.The color is dominantly medium to The pegmatite has been briefly described by dark green, but sometimesgrayish to reddish. Andersen (1926, l93l) and the geology of the Arendal-Gloserheia ar ea by Bugge (1943). According In the core, apatite is much lesscommon, medium to green to Bugge the rocks are of both granulite and dark in color, and generally smaller and bet- ter amphibolite facies. Radiometric dating of rocks in developedthan in intermediate zone I. The crystal the Arendal area has yielded agesfrom 850 to 1300 forms are the same. It is closely associatedwith m.y.; ages for the pegmatitesare from 875 to 1030 microcline perthite and sporadically with lM m.y. (Broch, 1963). muscovite. The pegmatite is V-shaped and exhibits in general a Apatite occurs in intermediate zone III as infre- well developed and regular zoning. The following quent yellowish-greensubhedral to anhedral crystals zones (Cameron et al, 1949) can be distinguished: from a few mm to several cm in length. Very rarely, core, intermediatezones I-V, wall zone, and border colorless euhedral crystals, several mm in size, are zone. Table I shows the mineral composition of the found embeddedin calcite(all later discussionrefers, different zones. however, to the first mentioned type). This study concerns apatite and xenotime from In intermedi ate zone I, veins of colorless apatite- different zones in the pegmatite, with special some with calcite, small amounts of hematite, and emphasison rare earth geochemistryand the genesis more rarely mm-sized xenotime crystals-cut the of REE-bearing inclusionsin apatite. earlier apatite crystals, these veins presumably representinga late stageformation. It is not known if Apatite these "veins" extend outside the apatite. Apatite is among the most abundant minor All apatite from the Gloserheia pegmatite contains minerals in the pegmatite(Table l), especiallyin in- variable amounts of inclusions.Dissolution of 6.088 607 R, AMLI Tlsr-e l Mineral Compositionof the Different Zones in the revealed euhedral-to-anhedraland short-to-long GloserheiaPegmatite prismaticxenotimes (Fig. l, I-IV). Whenviewed with Core Lt. tut. ht' Int. ht. YaU 3older microscope(X 65), xenotimeappears zoae I ?one II zone III zore Iv zone v a binocular or light yellow. '-rn o2-1n ro@ lo ctr 5-8 n O 1-1.5n ro cD colorless il Studieswith the universalstage show that all long Q1Etz lAUMU Ulcrocline u AAMA ?fagloclase uuMAil u prismatic xenotimesare oriented with their c axis Biotlte AAAl L/r ^/r r(?) 1(?) r(?) r(?) (as is true of long 1[ T parallel to the apatite c axis T T T T I T prismatic quartz crystals). Some inclusions Apatite T T T Euenlte T (plagioclase?) haveaxes of elongationparallel to the Calclte T ?Fite T T SaFetlte T T apatitec axisbut showinclined extinction. Randomly Rutile T T T T orientedequidimensional inclusions (both quartzand Zircon alsooccur. The long prismaticxenotimes T T xenotime) Chlorlte occur throughout the crystalsbut tend to be con- Ka6olrte centrated along planes or lines (Fig. l, Vl). Urulnite frorite T Equidimensionalinclusions (mostly xenotime)also Beryl T not always Heoatite T tend to definesuch surfaces,which are [: {aln nlneral planar. Long prismatic and equidimensionalinclu- A: Accessoly Meral T: Tlace ninerd sions frequently occur together in highly varying amounts in such distributions(Fig. 1, VI). The kg of apatite from intermediatezone I in 6N HCI apatite shows a weakly undulating extinction adja- yieldedl.l5 wt percentinsoluble inclusions identified cent to these planes/lines.The featuresdescribed as xenotime,quartz, plagioclase (?), monazite, rutile, above are observedin apatitesin all zones' pyrite, and geothite.Their habit variesfrom euhedral Xenotime also occursas euhedrallong prismatic to anhedraland from short prismatic to extremely brown crystals,up to one cm long, on the facesof long-prismatic(Fig. l, I). Examinationof apatitethin apatitesfrom the coreand intermediatezone I. These sectionswith electronmicroprobe (semi-quantitative) havetheir c axesparallel or subparallelto the apatite and optical microscoperevealed, as additionalinclu- prism facebut are otherwiserandomly orientated. In sions,calcite (?) in apatitefrom the coreand from in- intermediate zone I, xenotime is also found as termediatezones I and III, and thorite in apatitefrom euhedrallong prismaticmm-sized brown crystalsin intermediatezone III. calcite"veins" cutting apatite. Similar inclusionsin apatiteshave been reported In the same zone, colorlesseuhedral mm-sized by severalinvestigators. Taborszky (1962) has found xenotimesoccur rarely bs the latestmineral formed zircon,hornblende, and micaoriented mostly parallel (growing on quartz, muscovite,and rutile) in the to the apatitec axis in the granitic to gabbroicrocks replacementof microcline. of the Odenwald province, West Germany. Xenotime has been found in only a few samples McKeown and Klemic (1957)observed monazite, from intermediatezone III. Two different associa- bastnaesite,and hematite in apatite from the tions are recognized;xenotime-euxenite-tourmaline- magnetitedeposits at Mineville,New York; Pigorini rutile-muscovite and xenotime-euxenite-calcite- and Veniale (1968) found monazite, xenotime, muscovite. Muscovitization of the plagioclaseis plagioclase(?), and biotite (the long prismaticinclu- much more pronouncedin the latter paragenesis.The sions oriented parallel to the apatite c axis) in the xenotimefrom both paragenesesoccurs as up to ca 2 granitic to granodioritic rocks of the Val Sessera X 6 mm euhedralcrystals with brown color. province,Italy. Microprobeanalysis of anhedralinclusions (up to - Fracturesin the Gloserheiaapatites tend to besub- 100pm across)in the xenotimefrom the tourmaline parallel to prism or base in accordancewith the associationshowed Ca, Si, and Ti as the main ele- reported parting for apatite (Tr6ger, 1959).Long- ments. The inclusionsare thereforeassumed to be prismaticcrystals crossing a fracture in the apatite sphene. showedno lateraldisplacement even on the pm scale. The colorlessxenotimes have <.rp' = 1.713+ 0.004 andthe yellow xenotimes <.rtq6 ) 1.713 + 0'004. Xenotime Monazite Xenotime is the most abundantmineral found in the apatitefrom all zones.Examination by scanning Monazite has been found only as inclusionsin electronmicroscope and polarizingmicroscope has apatite. It has been observedin matrix only 1n RARE EARTH GEOCHEMISTRY OF APATITE AND XENOTIME 609 FIc. I'Inclusionsinapatite.I: Scanningelectronmicroscope(SEM)photo(X400)showinggeneraltypesofinclusions.II:SEMphoto(x 2000) showing a short prismatic xenotime. III and lV: SEM photo ( X 3500; X 3000) showing xenotimes with staircase-liketerminations. V: Photo (x 500) showing needlesofxenotimes oriented parallel to apatite c axis. VI: Photo (x 55) showing part ofa curved face or line defined by parallel-oriented xenotimes. material from the quartz core, where it occurs as Analytical Methods colorlessanhedral grains (up to -100 pm) in contact with xenotime.It is alsopresent, in amounts(l per- The analyseshave beencarried out using an AnL- cent,in the materialdissolved out of the apatitefrom rvx microprobe with a 52.5" takeoff angle, housed intermediatezoneI. Hereit occursas anhedral yellow in the Central Institute for Industrial Research,Oslo. grains up to t mm across, which are easily The analytical methods and the empirical correction distinguishedfrom the xenotimesby their yellow factors used in this study were those described by fluorescenceunder short wave ultraviolet light (not Amli and Griffin (1975). filteredthrough cobalt-glass).The identificationwas The error bars in the figures (except Fig. 2) corre- confirmedby X-ray powderpattern,