Extrusive Carbonatite from the Ouaternary Rockeskvll

389

The Canadian Mineralo gist Vol. 34, pp. 389-401 (1996)

EXTRUSIVECARBONATITE FROMTHE OUATERNARY ROCKESKVLL COMPLEX, WESTEIFEL, GERMANV

TEAL R. RILEYI. D. KEN BAILEY ATO FELICITY E. LLOYD

Depafimentof Geology,University of Bristol, Wills Memorial Buil.ding,Queens Road, Bristol BS8 lRJ, U.K,

Aasrnasr

Spheroidalcarbonate lapilli arepresent in unconsolidatedash from the upperunit of a diatremein the QuatemaryWest Eifel volcanic province of Germany.The cm-sizedlapilli are carbonatiteautoliths with a groundmassmineralogy dorninated (>507o) by polycrystalline quenchedcarbonate. They are typically nucleatedon fragmentalmaterial and are concentricallylayered around the central nuclei. The carbonatitic groundmassincludes clinopyroxene (diopside), melilite, apatite, vishnevite, magnetite,schorlomite and tracesanidine, representing the preservedcomposition of the eruptedmelt. The compositionof the calcite (SrO -2Vo, BaQ *0.5Vo and,LaYb 125-21,5)im.Flies a magmaticorigin, and the equilibrium assemblageof minerals indicatesa high temperatureof eruption. The presenceof melilite suggestsa geneticlink with alkali-mafrc, melilite-beming nephelinitic maemaerupted as lava and pyroclasticrocks near the diatreme.Sanidine and melilite are widely consideredto be incompatible;however, their coexistencein the carbonatiticautotths and the presenceof Oq6_s5feldspar in syenitexenoliths and their phonolite hosts, as well as megacrystserupted from the diafteme,indicate that this compositionhas a broad petro- logical stability and may form a major reservoirofpotassium at low aH2Oconditions in the uppermantle.

Keywords:carbonatite, extrusive, autoliths, nephelinite, melilite, sanidine,Eifel, Germany.

Sovrnaans

Des lapilli sph6roldauxde carbonatefont partie de cendresnon litbifi6es de funitd supdrieured'un diatrbmeform6 par explosion dans la province volcanique de lEifel occidental, d'6ge quatemaire,en Allemagne. Ces lapilli, centim6triques, sont desautolithes de carbonatitedont la parcest surtout(>507o) faite de carbonatetrempd polycristallin. Cetle pdte,en couches concentriques,emobe un noyau de matdriau fragmentE.l,a pite carbonatitiquecontient clinopyroxdne(diopside), m6lilite, apatite,vishn6vite, magu6tite, schorlomite et une tracede sanidine,et prdserveraitla compositiondu -agna lors de l'6ruption. l,a composition de la calcite (-27a de SrO, -0.57o de BaO, La./Yb entre 125 et 215) implique une origine masrnatique, et I'assemblagede min6raux en 6quilibre supposeune temp6rature6lev6e d'6ruption. D'aprds la pr6sencede m6lilite, nous soupgonnonsun lien g6n6tiqueavec une suite mafiqueet alcaline,et en particulier avecun magna ndpheliniquei m6lilite, mis en place sousforme de lave et de produitspyroclastiques prbs du diatrbme.On considbreg6n6ralement I'association de sanidine et de rn6lilite comme exemplede paire incompatible;toutefois, elles semblentcoexister dans les autolithescaxbonatitiques. Ce faig et la pr6sencede feldspathde compositionOq6_65 dans des x6nolithes sy6nitiques et leursh6tes phonolitiques, de m6me que sousforme de m6gacristauxdans les tufs de la diatrdme,montrent que cette compositionpossbde un champ 6tendude stabilit6 pdtrologique,et pourrait bien former un r6servoirmajeur de potassiumsous conditions de faible activit6 de HrO dans le mantrgau. (Traduit par la R6daction)

Mots-cl6s:carbonatite, extrusion, autolithes, n6ph6linite, m6li1ite, sanidine, Eifel, Allemagne.

Ixrnotucnon and accompaniedby rapid discharge of magma. Therefore,preserved effrrsive carbonatites(especially Studies of carbonatites and related rocks have lapilli, ash eruptionsand melt droplets)have probably shownthat extrusivecarbonatites axe not aswidespread undergonelittle interactionwith lithologies other than as their intrusive counte{parts, and yet they are their sourceregion. The low viscosity of carbonatitic probably closer 10 a true melt composition.Extrusive melts (Treiman & Schedl 1983) also encourage carbonatiternagmatism will be potentially explosive efftcient separationfrom sourcerocks and rapid rate of eruption. Carbonatite magmatism from the Rockeskyll I hesent address: British Antaxctic Survey, High Cross, complex (West Eifel, Germany) erupted autoliths Madingley Road,Cambridge CB3 0ET, U.K. judged to be compositionally close to the original 390 TTIE CANADIAN MINERALOGIST melt. Their proximity to alkaline mafic rocks andsanidine fragments in the tuffs andfields to the east allows further constraints to be placed on magma of Kyllerkopf. However, he made no mention of any genesis. genetic links with the alkali-mafic deposits of Kyllerkopf, described by Haardt in the same year GeoI-ocrcAt-SE"rm{c (Ler4). A far clearervolcanological picture is now possible The QuaternaryWest Eifel volcanic field coversan at Rockeskyllowing to extensivequarrying, a common area of approximately600 km2 and was active from feature of the West Eifel landscape.Quarrying has 0.6 to 0.01Ma. The field comprisessome 250 eruptive exposed numerous horizons of maar deposits and centers(Biichel 1993)dominated by alkali-mafic lavas revealed the limits of the crater walls. Detailed and pyroclastic rocks. Within the West Eifel field, mapping,coupled with existing magneticdata (Mertes tbree phonolitic centershave been identified @iichel 1982), has enabled the identification of several 1993), one of which is located in the Rockeskyll eruptive centers(I, tr and Itr) within the Rockeskyll complex,northeast of Gerolstein(Frg. 1). In contrast, complex (Fig. 2). The quarry exposuresare shown in the neighboring volcanic field of the East Eifel is Figure 3, which hiehlights the two principal centers, dominated by more felsic rock-types, attributed by Kyllerkopf hill (centerIII), dominatedby alkali-mafic Schminckeet al. (1983) to a differing tectonicregime; lavas and pyroclasticrocks, and the AufDickel center they consideredthe East Eifel to be characterizedby (II1 toward the northeast. Carbonatite volcanism is the development of a subsurfacernagma chamber, restricted to Auf Dickel and forms the only known absentin the West Eifel. carbonatitein the West Eifel. At Auf Dickel, 10- to The occurrence of phonolitic rock-types at 15-mcuttings expose a partly erodeddiatreme that cuts Rockeskyll was noted by Hopmann (1914), who through the upper ash and some lower, bedded,but commented on the similarity of the center to the reworked pyroclastic material. A diatreme origin is volcanism of the Laacher See in the East Eifel. suggestedby the eruptedproducts, fragmental carbon- Parallels were drawn after finding syenite,phonolite atite and syenite xenoliths, and by carbonatitic

+ trryrED I Rhine River ol ADENAUtr l a Rockeskvll volcanic a r,O O KOBLENZ complex tr , wMAYEN EOCHEIFEL

s isrid? ****g|t-t dt9llE a Mosel River

COCHBMtr Germany WFfTEIItsL. :-tr' a . trMANTDERSCHEIDt" ?____:io* BAD

Ftc. l. Distribution of Quaternaryvolcanic centersfrom the East and West Eifel fields (adaptedfrom Merfes & Schmincke 1983). EXTRUSIVE CARBONATTTE.EIFEL. GERMAI{Y 39'.|

\ Walsdorf

Auf Dickel diatreme anomaly

Legend anomalY -400- _ contours (nT)

positive @m anomaly centre 030 EI m -600 'Gerolstein

Ftc. 2. Magneticanomaly map of the volcanic complexesnorth and west of Rockeskyllvillage (adaptedfrom Mertes 1982).

autoliths, which are commotr in diatreme eruDtions. and the presenceof slump structureswithin the unit This interpretationis reinforced by existing magnetic may indicate water-driven slumping of primary data of the region (IVlertes1982), clearly showing a depositsback into a depressioncreated by the diatreme. positive magnetichigh of 1140nT nearthe site of the However, such feafures are also consideredto be a presentexposures (Fig. 2). Biichel (1978)noted that a primary characteristic of tuffisite pipes (e.9., positive magretic anomaly exists over many maar Schwabianfield; Cloos 1941), and are a result of craters and volcanic edifices, and has been attributed fluidized emplacement. prorimity '1ava The of the vent site to a plug" remaining in the fossilized conduit. is indicatedby the abundanceof eruptedxenotths and The upper ash unit of the diatreme,which hosts the pyroclastic material, which decreasessharply away eruptedxenolitls and autoliths, is devoid of bedding, from Auf Dickel. 392 THE CANADIAN MINERAIIGIST

ov-4\. N 'i( Auf Dickel center II 'n " carbonatite-svenite diatrem6

\1 craterwallmargins

outerwall\:. facies \ quarry 1 f,( exPosures volcanic a cenrct triangulation A polnt quarry I track >. dip of l8 pymclasts I to Dohm-

I qate(war Kyllerkopf o^center III tonockeskvtlj$$ \,,/ffi"' -'-- 554.6m ventfacies- / *,*rhu,,pondedlava / spine / \,b"b-I \q_ gs11'mnaralkali- E' / Fto. 3. Quarry exposuresand mafic lava volcaniccomplexes of the \ _ 500m area north and west of Rockeskyll.

T?mAur Drcxrr, DIArnsluE:Enuprsr PRoDUcrs pyroxene. The extrusive carbonatite is erupted as lapilli, which are spheroidalto ellipsoidal; these are The compositionof magmaserupted throughout the termel autoliths Gig. ) sincethey areinterpreted to be lYest Eifel is dominantlyalkaline and mafic. However, cognatewith the host,carbonate-rich ash. The autoliths the eruptedproducts at Auf Dickel are distinctive in are carbonatitic" with more than 507o calcium their felsic and carbonatiticcharacter. The tuffisite ash carbonateconstifuting the groundmass.They are that forms the upper unit of tle diatremeis host to a erupted in large numbers, but do not form any wide range of alkaline and carbonatitic rock-types; preservedhorizons of lapilli. fragmental xenolitls of syenite, mafic syenite, Pelletal lapilli, including nucleated autolittrs, are phonolite and biotite-+alcite carbonatiteare commoq corlmon in diatremeeruptions, and are reportedfrom :rsare megacrysts of water-clearsanidine and diopsidic the Urach andDeckentuffpipes of the Hegau,southern EXTRUSIVE CARBONATITE, EIFEL, GERMANY 393

Germany (Keller er al. 1990). Bailey (1989) also PslRocHEtvtrsrRyoF TTIEAtrroL[TIs reportedmagmatic lapilli from the diatremebreccias of Rufunsa Qambia), and they me widely recognized Over one hundred autoliths have been recovered from kimberlitic diatremes(Ferguson et al. 1973). from the Auf Dickel tuffisite ash and from the soil above the quarry face. They are concenfated at the ANALYnCALPnocsu,:RFs locality marked by the positive magnetic anomaly (centerII) in Figure 2 and are distributed sparselyin Electon-microprobeanalyses were performedon a nearbyfields. Many of the collectedsamples have been JEOL JXA-8600 Superprobewith Link analytical slicedto investigatetheir internal structure,to ascertain AN10/85s atalyzer and LEMAS automation at the whetherthey are nucleate4 what is the compositionof Department of Geology, University of Bristol. the nucleus,and to check for the presenceor ab$ence Analysis was by wavelength-dispersionmethods, with of concentric 2qning. Thin sections of twenty-five beam conditions 15 kV, 15 nA (5 nA for carbonate) randomly chosen autoliths have been prepared for and a diameterof 5 pm (15 tim for carbonate).Data mineralogical and electron-microprobe analysis were reducedby ZAF correctionprocedwes. (EMPA); their mineral composition,nucleus composi- The cmbonatefraction was leached using 0.5 M tion andmorphology are listed in Table l. The autoliths acetic acid and was analyzed foi the rare-earth fall into two textural categories,although chemically elements(ftEE) and other trace elements(Ba, Rb, Tb, they are very simil21.Distinction is madeon the basis Nb, TA Sr, Z, IIf, Y) on separateruns, with the same of grain size, with one group characterizedby coarse solution. Analyses were performed using a VG crystallinecalcite and melilite, whereasthe othergtoup PlasmaQuadICP-MS insfiument at the Univenity of is very fine-grained, the phasesonly discernible by Bristol. The solutions for REE analysiswere prepared EMPA. The coarsely crystalline autoliths are best with Re and Ru as internal standards.Blank solutions observedunder cathodoluminesence, where the ragged and intemational rock standardsalso were nm as a calcite laths are easily idenhfied as the dominant control on instrument behavior. For ftace-element luminescingphase (Fig. 5). The back-scatteredelecton analysis, we used In as an internal standard.Each image @ig. 6) higblights the groundmassmineralogy solution was analyzedthree times during the samerun, of the finer-grained autoliths, with phenocrysts of with a sample-uptakeperiod of 3 minutesfollowed by melilite, apatite, schorlomite and calcite. Only two a 2O-secondwash period in 57o HNO3 + orthophos- of tle samples sectroned exhibit coarser-gnined phoric acid. T\e REE concentrationswere nonnalized minerals, and both are cored by zoned fragmentsof to the chondritic values of Nakamura(L974), and the clinopyroxene and dominated by ragged laths of remaining face elementsto the chondrite values of calcite, characteristicof quenchedcarbonate (Keller Thompsonet al. (L984). 1989). Microprobe analysesof the calcite (Iable 2)

1cm

Flc. 4. Carbonatitic,nucleated autolith (SQlapl) from the Auf Dickel diatreme. 394 THECANADIAN MINERALOGIST

TABLE 1. ST'MMARYOFAIIIOLITITMINERALOCY, MORPHOI.OGYANDNUCIEUS COMPOSMON

ADI yes amph cc, vh, ap,mt, mel,cpx zo 23 Arr2 yes cpx cc, vh, cpx, ap,mt, schl,idd, mel 26 26 AD3 no cc, mel,cpx, ap,mt, schl zl 20 ADT yes cpx+ap cc, cpx,mel, san,ap, mt, schl,vh 31 a1 AD5 yes anph+aprcpx cc, cpx, mel, san,ap, mt, schl, vh l6 l8 AD6 cc, ap,cpx, vh, mt ZU 2A AD7 yes cc, mel,ap, cpx, vh, mt, scbl 20 t7 AD8 no "l* cc,cpx, ap, idd, vh, mt 18 1a AD9 yes aprcpx+vhfragment cc, mel,vh, cpx, ap,mt t, 20 ADIO yes maiic-richfragment cc,ap, cpx, mt, vh, idd 18 ADIl yes cc, mel,cpx, ap, schl, mt l6 L2 AD12 no "l* cc, mel, cpx, ap, idd, vh, mt, schl I'T 16 AD13 yes amph+cpx cc, mel, ap,cpx, vh, idd, mt l6 l5 AD14 yes cpx cc, mel, ap,vh, cpx,mt, schl,idd 16 ll ADI5 yes rl* cc, mel, cpx,ap, vh, mt idd 15 l4 AD16 no cc, mt cpx, ap, mel, vh L7 l6 ADIT yes cpx+aPfml cc, ap, cpx, vh, mt idd, mel z5 t7 AD18 yes cpx+mt cc, cpx, ap, mel, yh, mt idd, schl ZL t9 AD19 yes lithic fragrnent cc, cpx, mel, ap, vh, ml, idd 26 22 AD20 no cc,cpx, ap,mel, vh, mt 43 34 RKK77 yes lithic fragment cc, ap, cpx, mt vh, idd 40 30 sQtapl yes cpx cc, mel, cpx,ap, vh, schl,mt 16 IJ SQlap2 yes vh cc, cpx, san,idd, ap,vh, mt, mel, schl 16 SQlap3 no cc, cpx, ap, idd, mt, vh, mel l'l 15

cc, calcite;vq vishnevitqcp& clinopyroxenoischl, scrholomile; e, aptitst mt, magnedls;san, sanidinei mel, nelilito; mph, ka€rsutite;id4 iddingsite.

Flc. 5. Cathodoluminescencephotomicrograph of autolith SQlapl; luminescingphases are calcite, cc (laths) and apatite,ap (euhedra).Field of view is 4.5 x 1.5mm. EXTRUSWE CARBONATITE. EIFEL. GERMANY 395

FIc. 6. Back-scatteredelectron image of autolith (AD5) groundmass.Melilite (me), apatite(ap), schorlomite(gt), calcite (cc) and vishnevite(vh). Scalebar: 100 pm.

reveal high levels of Sr (1.77o SrO) and Ba nucleus Clable 3). Mass-balancecalculations using (0.4VoBaO),indicative of primary magmaticcarbonate modal proportionsand microprobedata have allowed (Ngwenya& Bailey 1990). the determinationof their bulk chemistry; tlese are Modal analyseshave been determinedfor the two listed, with that of other extrusive carbonatites,in coarser-grainedsamples, AD11 andSQlapl, excluding Table4. the xenocrystic clinopyroxene, vishnevite and the The finer-grained samples are characterizedby mm-sized phenocrystsand xenocrystsof vishnevite, clinopyroxene,melilite and apatitein a groundmassof the same composition.They occur as subhedraland TABI.E2. REPRESENTATIVERFSUL$OFELECTRONMICROPROBE euhedralcrystals and commonlyform coresto smaller ANALYSES OFIIINERALPHASES FROMARANGE OF AUTOLTIIS lapilli included within the autolith groundmass.The si02 46.94 0.O7 2.28 36.56 47.94 43.22 28.10 2.51 15.05 7.51 AlPj 5.78 bd bd 24.48 9.43 8.29 3.26 2.67 TABLB 3. MODAL ANAI,YSES OFCOARSELY SYSTALIjNE Ct2q0.04BmME@ E m CARBONATTUC AUIOLIn'SI FeO" 10.49 O.17 0.39 1.39 4.89 5.91 18.05 75.99 Ii,tl0 0.49 0.18 bd m 0.29 0.38 0.43 1.63 MsO 10.68 0.07 bd 0.18 4.53 5.24 1.63 cs 23.95 51.20 55.31 8.93 29.U 31.95 32.72 m phas SQlapl ADlI N&O 0.57 0.05 0.37 10.14 3J4 4.65 o.24 m KzO m 0.22 m 4.31 0.38 0.25 0.05 m TiO'Bmmmmtra bd 0.26 cafsitg 56.E% 53.r % P2O5 D 0.12 37.05 m 0.21 0.20 u E

TABLE4. COMPOSITIONSOF EXTRUSIVE CARBONATTTES (ALL DrcEPT TABI3 5. GBMICAL @MPO$IION OF SANIDINB FROM AIJF DICKEL RffiS ROCKESKYLLDATA TAKEN FROM BAILEY I93)

Si02 65.03 65.05 64.29 66.43 AlzQ 18.28 19.26 t9.01 16.06 Dd 0.39 13 t3.3 15.8 0.45 1.45 t6.2 ::9trq 0.11 r. 1.74 1.16 0.75 0.03 l.0l 0,52 t\4q bd bd bd 0.41 Al2Ol 0.09 0.o7 3.03 7.38 5.45 0.15 t35 3.91 FeO' o.20 0.20 0.15 2.25 Fao3 0.2811 o34ll 7.93 od !d 0.98{ 8.3 3.69 CaO 0.05 0.03 0.07 0.22 RO nd !d 4.44 6.111 5.49, 0d 3.12 l.3l Na2O 3.r7 2.63 2,63 0.49 li,ld 0.04 o.22 0.4 0.2 0.07 0.41 0.46 0.07 Kro 13.36 12.49 12.60 13.93 Mgo u.Jl 0.2t 8.55 0.9 l.9l 0.36 BaO 1.l6 o.77 1.02 0.18 co 13.9 54 36 39.1 38.9 52.6 40.5 38.1 Total 10i.65 rco.92 100.86 100.07 NazO 32.2 0-l o;t3 2.a L92 0.04 o.23 0.05 An 0.23 0.15 0.35 0.62 K2o 8.21 0.05 0.2 1.3 0.22 0.5 0.14 0.5 Ab 25.94 23.86 23.55 t4.O2 &o5 0.9 1.82 3.32 2.5s 2.34 r.s6 7 0.6 71.21 73.79 73.67 Hp E. nd 3.45 0.1 0.2 l.16 r.2 3.12 78.03 34.1 41.6 14.8 2!.3 22.2 39.8 24-6 24-l CJ 1.92 1.41 1.85 7.64* 9a 2,93 nd 0.08 0,2 0.13 0.54 nd !d cl 4.2r nd nd dd nd !d Bd !d s03 2.18 !d nd 1.6 0.9 nd !d nd co@podd@lw6t€dhsL%dds, Syelols: bd:belrydlaecdorF€Or:totetb@repGtrd SrO 1J3 o.4 0.63 r.r4 1.25 0.67 nd nd 6 FeO,Ar @sddb (CsAl2SlzoJ,Ah elbde(NaAlslor, or 6eel@ (KAlsqoJ, cc BaO 1.04 0,2 0.r5 0.4 0.46 0.r5 Dd ad (BaAl"St?Oil.rtrFoSLOo Tohl lu2.9l 99.6 98.45 100.14$98.01$ 99.4 100.23 [email protected] €lda @&'Er@b6.

C@lcit'@ !@qrd h *% odd4 Syolols tr& @ doL e eq Foroil: bEl !@ repdtd I FqO!, FeOl el kortD6td @ FaO, S: ,@-!dl@ et-r,fu tu Eidd @fys @d el@. eriw060 dtb

larger grains of clinopyroxene (>1 mm diameter) and Soulard(1992) have demonstratedexperimentally are pale brown to green and commonly are zoned, that in olivine-free melts, melilite and plagioclasecan whereasthe smaller (<1 mm diameter) fragmentsof exist in equilibrium. The apparentincompatibility of clinopyroxene are typically green-cored.The fine- alkali feldspar and melilite also has been illustrated grainedgroundmass has been analyzedby EMPA and (Yoder 1985),although an examplehas beenreported has revealed a mineral assemblageconsistent v/ith in a melilite nephelinite in Djebel Targou, Morocco tle coarsersamples (Table 2): calcite (AD5), clino- (Velde& Rachdi1988). Another possible example has pyroxene (ADl), melilite (SQlap1, AD14), apatite beennoted by Stoppa& Cundari(1995). (AD13), vishnevite (SQlap3), magnetite (AD4) and Microlaths (<100 pm) of sanidinecoexisting with euhedraof the titanium-rich garnetschorlomite (AD5). melilite have been noted in two of the finer-grained Thesephases constitute an equilibrium assemblageand carbonatitic autoliths (AD4, AD5). The sanidine is are consideredto represent the composition of an chemically similar to the water-clearsanidine erupted eruptedmelt. as megacrystsand presentwithin the syenitexenoliths The two textural types also coexist within the same (Iable 5), with the exceptionof the FeO content,which sample. Finer-grained autoliths cornmonly have an is significantly higher in the sanidineof the autolith. outer concenfricrim of coarsercrystalline carbonatite, Bailey & Schairer(1966) atributed the high-Fe nature similar to tle texturesobserved in samplesADll of the alkali feldspars to their higher-temperature and SQlapl. These textures may indicate auto[th genesis,where the structureis expanded,and Fe canbe nucleation and accretioo occurring at more than one accommodated.This observation is congruent with level in the diatreme.If autolith growth takes place the likely high+emperature source regions of the within the diatremeneck itself, it is possiblethat the carbonatiticautoliths and the lower-temperaturesource finer-grained autoliths nucleate at higher levels in of the syenitexenoliths, eruptedfrom the Auf Dickel the diatreme column and are quenched prior to diatreme. Velde & Rachdi (1988) attributed the recirculationwithin a fluidization cell. However,there coexistenceof sanidineand melilite to the anomalously appffirs to be no systematicmineralogical variation in high Sr in the nephelinitic melt. This proposal was the carbonatitebetween coarser and finer lithologies, basedon the high Sr contentof the melilite (l2%oSfr), and analyses of groundmasspyroxene from the which was judged to be the only phase able to core to rim have not revealed anv marked chemical crystallize from a Sr-rich, silica-undersaturatedmelt, changes. since the coexistingnosean can only accommodateup to27o SrO1'the P2O5content of the melt wastoo low to MaLn rrE ANDSANIDn{E: allow sufficient apatiteto crystall2e and accommodate AN INCOIVPATBI.E ASSBMBLAGE? the excessSr. However, tlis explanationfor the occrurenceof K-feldspar and melilite by Velde & The absence of plagioclase with melilite in Rachdi (1988) is now regarded as not entirely magmatic rocks has been noted by Yoder (1973), satisfactory(D. Velde,pers. comm., 1993). although their coexistencehas been reported from The petrogeneticscenario at Rockeskyll has few hybrid rocks and in metamorphicrocks. Yoder (1973) parallels with the nephelinite of Morocco. Although EXTRUSIVE CARBONATITE. EIFEL" GERMANY

the Sr content of the carbonatiteis high, it partitions CensoNar[E Cns drsrRvAND strongly into the calcite and the apatite, whereasthe TTDCALCFICATION HYPONNSTS melilite rarely containsmore than LVoSrO (Table 2). The occurrenceof sanidine in the autoliths of Auf Three autolith samples were selected for trace- Dickel as <100 pm laths has only been identified element analysis, to determine the abundancesof following detailedEMPA. Conditionsfavorable to the large-ion lithophile elements (ULE) nd high-field- crystallization of sanidineare high activities of K, Si strength elements(HFSE in the carbonatefraction. and low activity of H2O,whereas conditions favorable The samplesADl, AD2 and AD4 are ellipsoidal and to melilite includehigh Na in the melt and an alkalinily 2.5 to 3 cm in diarneter.Only AD4 is nucleatedon a ratio [(Na+K/Al] greaterthan 0.8. Early crystallization core, hence the powderedsamples are dominatedby of melilite, nesemFaniedby calcite, will have driven groundmassmineralogy and as such are closer in the melt toward a Si and K-rich composition; composition to an erupted melt. To eliminate the providing the aH2O is low, following the crystal- xenocrystic material from analysis and to i$olate ltzatron of vishnevite, then conditions may favor the the carbonatefraction, the sampleswere acid-leached formation of sanidine. in 0.5 M acetic acid for 90 minutes at 200'C. The Sanidineis presentin many of the alkaline rocks at leachatesrevealed carbonate levels of 58Vo.65Voand Rockeskyll and Auf Dickel. Its occurrencein syenite, 68Von samplesAD4, ADl and AD2, respectively, phonolite, carbonatiteand as water-clearmegacrysts similar to the calcite modesof the crystalline autoliths indicates that the composition Orre*, has a broad (AD11, SQlapl; Table2). stability. Potassicfeldspar of Oqe_s5composition has The results(Frg. 7) for the threesamples are plotted been reported from other alkaline-carbonatite with the averagecarbonatite of Nelson et al. (L988). provinces: Rufunsa,Zanbia (Liyungu L992),I-aasher The high levels of the LILE are characteristic of See, East Eifel (Taylor et al. L967), Polino, Italy calciocarbonatites.whereas the low levels of the (Stoppa & Lupini 1993) and northeastemScotland HFSE are the result of the resistanceto leaching of (Aspen et al. 1990). Sanidine (Or$_ss) from a the schorlomitegarnet and magnetite,as theseare the coesite-grospyditexenolith (Sharp er aI. 1992) and major depository minerals for the heavy rare-earth from diamond(Novgorodov et al. 1,990)testifies to its elements(HREE), Zr, Ti, Nb, Ta and IIf. The I/FSE stabi[ty at higherpressures and confirms it may forrn a levelsin the averagecarbonatite ofNelson e/ al. (1988) major potassic phase at conditions where hydrous areindicative ofthe presenceofpyrochlore, garnetand phasesare unstable (Bailey 1987). masnetitein the carbonatite.

-----+- ADl ----.- AD2 lmm ---o- ADzl ---l- Nelsonet al. (1988) AverageCa$onatite a

F 100

a lll xl

Ba Rb ThNb Ta La Ce Sr NdSm Z Hf Tt Tb Y Tm

FIc. 7. Variation diagramfor the acid-solublefraction of Auf Dickel autoliths compared to the averagecarbonatite of Nelson er aL (1988). 398

T\e ULE-eniched signatureand the Sr, Ba levelsin (1973)also remarked upon the importanceof Ca, and the calcite unequivocally classify the carbonate as specifically carbonate,in the crystallizationof melilite, magmatic, making it distinct from altered alkali and commented on its association with limestone, carbonatites(Hay & O'Neil 1983). The carbonatite marble and carbonatite.The case for a genetic link volcano of Oldoinyo Lengai in northern Tanzania between melilite- and carbonate-bearingmagmas is erupts alkali carbonatitelavas @awson 1962). T\e strongand hasbeen proposed for the eruptivesuites at alkali-carbonateminerals nyerereite and gregoryiteare Kerimasi, Tanzanta(Mariano & Roeder1983) and the water-solubleand hygroscopic, and therefore highly Kaiserstuhlin Germany(Keller 1984).The Kaiserstuhl susceptibleto removalunder surface conditions, where alkaline complex consists of olivine nephelinite, they are often assumedto be replacedby calcite. This melilitite, tephrite, phonolite and carbonatite,and as hasprompted suggestions that many calciocarbonatites such presents parallels to the Quaternary volcanic are replacementsof a primary alkali carbonatite.The rocks of Rockeskyll.Keller (1984)proposed a close calcite laths within the Rockeskyll autoliths are poly- geneticlink berweencarbonatite and calcite - melilite crystalline, with ragged, feathery margins, a texture - haiiyne nephelinite("bergalite") at Kaiserstuhl.This that might be comparedwith pseudomorphsof alkali observationwas supportedby Schleicheret al. (L990), carbonate(Hay & O'Neil 1983). The replacement who showedthe Sr-isotopesystematics of the melilite- hypothesisis dependentupon percolatingsolutions of bearingmelts andthe carbonatitesto be consistentwith meteoricwater crystallizing calcite at low temperature, a genetic link, with '"bergalite" forming the inter- which would involve the rejection of Sr (Shearman& mediate composition. Isotopic evidence from t}le Shirmohammadi 1969), Ba and other incompatible Rockeskyll complex also is compatiblewith a genetic elements.However, the high levels of theseelements in link betweenthe carbonatiticrocks of Auf Dickel and the Rockeskyll carbonatesseem to rule out a low- the melilite nepheliniteof Kyllerkopf hill. Carbonatitic temperaturecalcification of alkali carbonatite.Also, autolithshave a 87Srf6Srvalue of 0.7(X21t 0.00014, the pristine nature of the melilite indicates minimal and the capping melilite nephelinite lava from alteration of the Auf Dickel autoliths, whereas the Kyllerkopf hill has a 87srf6sr value of 0.7M51 x. abundanceof apatite (Table 3) indicates a fa1 highsl 0.00012(Riley 1994). solubility of F than is possiblein alkali carbonatemelts Gittins (1989) commented on the problems (Gittins& Jago1991). In summary,the polycrystalline associatedwith the origin of carbonatiticmagmas and natureof the calcite may be a primary quench-related rejected ideas involving crystal fractionation from a featureor a result ofinversion from a high-temperature carbonatedalkali-mafic magma on the grounds that to a low-temperatureform. CO, activity is too low to allow abundantcarbonate to crystallize. He highlighted the problems of trace- OPJGn{s oF CARBoNATnE AND element budgeting between carbonatitic and rrn Axer-r-Merrc CoNNncnoN nepheliniticmagmas. The evidencesupporting primary carbonatemelts is now growingin support(e.9., Bailey Bailey (1993)listed known occlurencesof extrusive 1989). Dalton & Wood (1993) have demonstrated carbonatites(Iable 4) and reportedtheir field associa- experimentally that calcium carbonatemelts can be tions with silicaterock-types. Although all carbonatites produced by reacting near-solidus carbonate melts are not associatedwith silicate rock-types, the from mantle peridotite with depletedmantle wallrock. occurrence of carbonatite in association with Primary melts from mantle lherzolite (>25 kbar) have nephelinite,melilitite and phonotte is widespread(t e Cal(Ca+ Mg) valuesof 0.724.74. Reactionof these Bas 1977), and the role of CO2 in the genesisof all melts with depleted mantle assemblages(lherzolite, suchmagma-types is paramount@aney D74). harzburgite) at lower pressure drives the melt The scenario at Rockeskyll is the familiar compositionto aCal(Ca + Mg + Fe + Na) value of up associationof carbonatitic,nephelinitic and phonolitic to 0.95,providing the wallrock (lherzolite,harzburgite) magma-types.Spatial links between these lithotypes is converted to wehrlite (olivine + clinopyroxene). are clear, with carbonate and felsic magmatism Theseresults confirm that Ca-Mg carbonatemelts may restricted to the diatreme.and alkali-mafic lavas and erupt directly from the mantle and that calcium pylroclasticunits to the neighboringeruptive centerof carbonatemelts arethe productof reactionwith mantle Kyllerkopf hill. H6wsysr, the mineral assemblages peridotite. imply geneticlhks amongcarbonatite, the alkali-mafic Wehrlite compositionsare enrichedl,lnthe LILE rel- rocks and the felsic rocks. ative to depletedmantle assemblages.Therefore, they The presenceof melilite in the carbonatiticautoliths provide a possible parent for mantle-derived and the capping lavas of Kyllerkopf hill indicates nephelinitic and melilite-bearing melts without the possible links between source regions. Experimental problem of requiring an ulna-low degree of partial work by Lloyd (1985) on olivine melilitite indicated melting of a lherzolitic composition in order to that Ca (and,by inference,other cations)as well as Na concentratethe required incompatible elements.The and K promote the crystallization of melilite. Yoder problem with low-degreepartial melting is apparent EXTRUSIVE CARBONATITE, EIFEL, GERMANIY 399 when the volume of alkali-mafic magmaerupted in the retains its magmatic signature,with LalYb ratios of West Bifel is considered.Within a time span of only 125-2L5and Sr and Ba contentsin the calcite of.L.7Vo 0.5 Ma, 1.7 km3 (Mertes & Schmincke1983) of SrO and 0.47oBaO, respectively.The pristine natureof dominantlynephelinitic, leucititic and melilite-bearing the melilite seemsto rule out any interactionbefween magmas were erupted; therefore, melt production the autolithsand meteoricwater. would haveto be far higherthan the minutefractions of 3. Genetic links between the carbonatemagma and melt proposedto concentratethe ULE from a depleted melilite nepheliniteof a neighboringcenter are inferred mantle source. This petrogenetic problem was from Sr-isotopechemistry and the melilite connection. approachedby Frey et al. (L978), who useda pyrolite Partial melting of peridotite and wallrock reaction are starting composition with the ULE at 6-9 times favored to explain the origin of carbonatitic magma chondriticvalues and infened fractions of.4-:7Vopartral and provide the sourcerocks for the associatedalkali- melt to generate olivine melilitite and olivine mafic magmas. nephelinite. Wehdite and trace-element-enriched4. The petrologically incompatible assemblageof lherzolite from the West Eifel are characterizedby melilite and sanidineis reportedfrom the groundmass LILE contents at 1G-30 times chondritic values ofthe carbonatiteautoliths. The sanidinecomposition (Fenwick1991). Hence even a largerdegree ofpartial is close to that of the feldspar eruptedas megacrysts melting can be envisagedthan that proposedby Frey and rvithin the syenites and phonolites from the et al. (1978), thus providing a suitable model for diatreme pipe (Orzo+s).Sanidine from other carbon- alkali-mafic magmaproduction in the West Eifel. atite - 2lkaline provinces (Rufunsa,Zambia; Laacher Recent attempts to explain events of mantle See, Germany; northeasternScotland) and diamond- enrichmenthave implicated carbonate melts to produce bearing eruptive rocks testily to its broad pefrological the observedmineralogical and geochemicallchanges. stability, it may form a major reservoirof potassiumin Green & Wallace (1988), Thibault et al. (1992), the upper mantle. Fenwick(1991), Yaxley et al. (1991)andlonov et al. (1993)all looked to mantle-derivedcarbonate melt and AcrNowmocrprslrrs alkaline silicate melt as a viable mechanismfor producing enrichment n the ULE and converting Martin Palmer (BristoD is thanked for analysis of depletedmantle assemblages to wehrlite andultimately the samples for strontium isotopes, Wayne Taylor olivine clinopyroxenite.The appearanceof REE-rich (University College London) for cathodoluminesence, phases(monazite, apatite) and, in somecases, remnant Steve Lane (Bristol) for assistancewith the electron pockets of carbonate(Ionov er aI. 1993) in mantle microprobe, and Tony Kemp @ristol) for analytical xenoliths all point to interaction of depleted expertisewith the ICP-MS. Georg Biichel (Mainz) is mantlewith a melt with a signaturethat is carbonatitic. thankedfor accessto aerial photogmphsofthe region The implications are that interaction with carbonate and magneticdata of the areasurrounding Rockeskyll. melt and also an alkali-rich component acts as a The Alumni Foundation, University of Bristol is precursorto the genesisof alkali-mafic magm4 a link thanked for a significant conhibution to the cost of that is upheld by field observationsin the West Eifel. attending the GAC-MAC meeting in Waterloo in The dominantly alkali-mafic eruptive rocks are 1994. This text has benefitted from the constuctive accompaniedby suites of lherzolite, harzburgite, reviews of Dan Barker, Robert Martin and an wehrlite and olivine clinopyroxenite xenoliths with anonymousreferee. The researchwas supportsdby the trace-elementsignatures indicative of interaction Natural Environment ResearchCouncil (lK), Grant with carbonatemelt (Fenwick 1991" T\ibault et al. GT4I9LIGSI9. 1992). RsFRE{cFs CoNcrustoNs AspsN, P., UrroN B.G.J. & DtcrN, A.P. (1990): l. Extrusive carbonatites,erupted as autoliths from Anorthoclase,sanidine and associaGdmegacrysts in high-pressure an explosivediaheme pipe, representthe only known Scottishalkali basalts; syeniticdebris from Eur.J. Mineral.2, 503-517. carbonatite of the West Eifel. The autoliths have uppermantle sources. typically nucleatedon crystal or lithic fragmentsand BAtrEy,D.K. (1974):Nephelinites and ijolites.1z The arecharacterized by groundmassmineralogy of calcite, AlkatineRocks (H. Sgrensen,ed.). Wiley-Interscience, melilite, diopside, vishnevite, apatite, schorlomite, london,U.K. (53-66). magnetite and trace sanidine. The groundmass - constitutes an equilibrium assemblage and is (1987): Mantle metasomatism perspectiveand Rocks(J.G. Fitton & B.G.J. consideredto representthe compositionof the erupted prospect./n Alkaline Igneous Upton,eds.). Geol. Soc., Spec. Publ.30, 1-13. melt. 2. The calcificationhypothesis is not deemedvalid for (1989): Carbonatemelt from the mantle in the the autolith carbonates. The carbonate chemistrv volcanoesof south-easthmbia. Nature 33E, 415417. 400 TI]E CANADIAN MINERALOGIST

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