Extrusive Carbonatite from the Ouaternary Rockeskvll
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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 N + 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 N 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,