Late Llandovery Bentonites from Gotland, Sweden, As Chemostratigraphic Markers

Journal of the Geological Society, London, Vol. 151, 1994, pp. 741-746, 6 figs. 2 tables. Printed in Northern Ireland

Late Llandovery bentonites from Gotland, Sweden, as chemostratigraphic markers

R. A. BATCHELOR' & L. JEPPSSON2 'Department of Geology, School of Geography & Geology, University of St Andrews, North Haugh, St Andrews, Fife KY16 9ST, UK 2Department of Historical Geology & Paleontology, Solvegatan 13, S-223 62 Lund, Sweden

Abstract: Geochemicalanalysis of twodistinctive bentonites and their constituent apatite crystals from the Late Llandovery of Gotland, has provided unique chemical signatures for each bentonite. These data have allowed the beds to be correlated across the island. The chemical composition of constituent apatite crystals indicates that apatite crystallized in a transitional alkaline environment. Chemical and mineralogical evidence suggests that the earlier Lusklint Bentonite originated from a potassic magma generated in a waning subduction zone, while the later Ireviken Bentonite was not consanguinous with its predecessor, but was derived from a medium-K calc-alkaline magma produced in a continental margin environment. The volcanoes which gave rise to these bentonites probably lay in theTornquist Sea, during its latestages of closure,somewhere between southern Poland and Denmark.These chemically distinct bentonites could act as sensitive chemostratigraphic tools for correlation of Late Llandovery horizons elsewhere in Northern Europe.

Rocks of Late Llandovery age are exposed along a narrow the Ireviken Bentoniteat Locality C,and 0.06f0.03m strip on theNW coast of Gotland (Fig. 1) and are composed below at Locality A. No fauna1 changes connected with the mostly of mark and limestones. Soft clay bands which are bentonitehave been found on Gotland using thecurrent laterally persistent occur within the sequence and have been sampling interval of a few centimetres. described as bentonites by Thorslund (1948) and Spjeldnaes In view of their occurrence at an important stratigraphic (1959). Five bentonite samplesfrom the Lower Visby horizon, these bentonite beds and their constituent apatite Formation (Hede 1960) were sampled from the two major crystals were analysed in order to confirm their correlation bentonite beds. These beds form importantmarker horizons across the island, to classify their parental magmas and to close to or at the Llandovery-Wenlock boundary. reach a conclusion abouttheir petrogenesis and tectonic The Lower Visby Formation, with its distinctive marine setting.Bulk analysis was carried out by XRF using the fauna is characterized by an assemblage of conodonts, standard fused bead and pressedpowder pellet technique representing the uppermost Pterospathoclus amorphog- (Table 1). The bentonites contain euhedral apatite crystals nathoides Zone, and the rugose coral Palaeocyclus porpita (50-2OOpm long) and thesewere separatedand analysed which becameextinct atthe top of the Lower Visby for their rare earth element (REE) content by inductively- Formation (Hede 1921; Aldridge et al. 1993). These fauna coupled plasma-mass spectrometry (ICP-MS). Four apatite equate at least partly with the M. spirab Biozone. As these sampleswere analysed by electronmicroprobe for major beds were laid down in >70 m of water (Grey et al. 1974), and minor elements (Table 2). volcanic ash would have settled down below wave base and remained relatively undisturbed. The two major bentonites are exposed in this formation and can be readily located, Correlation even when covered by scree, by plants, commonly Tussilago It is clear from Fig. h,& b that the two Lusklint Bentonite and Equisetum, which preferentially grow in the water- samples (SW59, 61) canbe correlated onthe basis of retentive clay bands.These twohorizons were sampled similarities in selected ratios of Nb, Y, Zr, Si, Ti and Al. along strike, the lower one, Lusklint Bentonite (c.50 mm They are unique in containing euhedral apatite (squat form) thick),as sample SW59 from Locality A (Ireviken 3) and and biotite crystals, but no zircon. The three samples from SW61 from Locality C (Lusklint 1); the upper one - Ireviken the Ireviken Bentonite (SW@, 83, 63) show a small spread Bentonite (c. 100 mm thick), as sample SW60 from Locality incomposition though they still form a coherentgroup. A, SW83 from Locality B (Stenkyrkehuk 1) and SW63 from Each sample contains abundant apatite (lath form), biotite Locality C (Fig. 1). The locality names in parentheses are and zircon crystals and is rich in kaolinite. Relative based onthe compilation of Laufeld (1974) andlater variations in bulk rock geochemical indicators Zr X lOoO/Ti additions registered at Allekvia Geological Field Station on and Ce X 1O/Y for the two pairs of bentonites (Fig. 3) show Gotland.The distancebetween localities A and C is a complementary pattern.The chemical composition of 11.6 km. A third, thinner clay band occurs at Locality C apatite crystals also supportsthe correlationbetween the between these two and is namedthe Storbrut Bentonite two pairs of bentonites, particularly with respect to F, Cl, Sr (c. 25mm thick). The Ireviken Bentonite was deposited (Fig. 4) and chondrite-normalized REE patterns (Fig. 5). shortly after Datum 2 of the Ireviken Event (Jeppsson in Onthe basis of biostratigraphicalcontrol, chemical press), while the base of the Lusklint Bentonite forms the similarities and mineralogy, the Lusklint and Ireviken reference level to which sampling levels arereferred. Bentonites can be correlated along strike in Gotland. These Detailed collecting has established Datum 2 at 0.14 m below parameters could be used for correlatingbentonites at 741

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Wenlock & Ludlow Formations

Samplelocality

VlSBY

Locality A Locality B Locality C Lower Visby Formation

lrevlken Bentonite 2.46m (Starbrut Bentank)

Fig. 1. Sketch map of northern Gotland ------1 level O.OOm Lusklint Benlonite and sample locations. Simplified litholo- Sea level -5.1 m Sea level -5.9m Sea level -8.5m gical logs are superimposed.

Table 1. Whole-rock chemical analysis of Lower Visby benronites

Lusklint Bentonite Ireviken Bentonite

S W 59 SW61 SW60 SW63 SW83 SW63 SW60 SW61 SW59 Sample locality Ireviken 3 Lusklint 1 Ireviken 3 Lusklint 1 Stenkyrkehuk 1 Illite/smectite % 85 60 60 60

(wt %) SiO, 46.17 45.36 49.23 45.60 47.91 TiO, 0.90 0.91 0.56 0.57 0.66 AI203 22.28 22.92 23.41 18.37 22.41 Fe,O$2.19 3.25 5.75 3.45 2.45 MnO 0.02 0.02 0.030.01 0.03 M@ 2.29 3.16 2.28 3.63 3.26 CaO 3.50 6.33 2.49 7.93 4.06 Na,O 0.23 0.57 0.43 0.50 0.04 K20 4.39 3.65 4.29 3.96 3.60 p2os 0.06 0.04 0.18 0.13 0.19 L01 10.20 14.40 12.00 14.00 12.40 Total 95.79 98.68 98.30 98.19 97.73

PPm Nb 28 26 19 18 18 522 490Zr 522 242 233 222 49 45 Y 49 17 19 17 Sr 215 178 50 110 95 Rb 81 66 111 115 109 Th 46 40 31 27 31 12 4 43 Pb 124 27 26 20 1 9 17 Ga 19 18 18 18 Zn 92 369 23 44 41 32 20 16 20 Ni 32 24 22 87 88 Ce 87 44 57 47 4 9 6 sc 0 9 5 4 93 110 V 93 50 34 56 Ba 328 230 186 288 1379 La 14 22 32 22 26 1 8.6 18.8 12.9 12.3 12.9 Zr/Nb18.8 18.6 12.7

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Table 2. Chemical analyses of apatite crystals extracted from Lusklintand Ireviken bentonites

Lusklint Bentonite Ireviken Bentonite Chondrite

SW59SW61 error f 1SDSW60 error f 1SDSW63 errorf 1SDSW83 error f 1SD Sample Locality Ireviken 3 Lusklint 1 Ireviken 3 Lusklint 1 Stenkyrkehuk 1 PPm La 2360 2091 18 1364 15 1457 15 1198 23 0.2446 ce 5749 5101 46 3533 44 3779 31 3145 16 0.6379 Pr 761 658 15 457 3 478 4 409 4 0.09637 Nd 3171 2690 56 1850 36 1951 45 1648 20 0.4738 Sm 543 456 17 314 22 321 16 284 11 0.154 Eu 120.0 105.0 2.0 42.6 3.0 44.1 2.6 37.5 3.1 0.05802 Gd 438 357 17 273 7 283 9 242 7 0.2043 Tb 41.6 36.8 1.3 31.2 2.1 29.7 0.4 26.4 0.6 0.03745 DY 191 169 10 165 7 168 3 142 6 0.2541 Ho 24.8 20.7 2.5 25.2 0.6 24.9 0.5 21.6 0.8 0.0567 Er 52.5 47.2 4.5 60.8 4.5 67.1 4.1 55.5 2.3 0.166 Tm 6.0 4.3 0.8 8.1 0.2 7.6 0.8 5.8 0.6 0.02561 Yb 26.7 24.2 3.4 37.8 4.2 42.1 2.6 36.2 2.5 0.1651 Lu 3.5 2.9 1.9 6.1 1.1 6.0 0.3 4.2 0.5 0.02539

ZREE 1.35% 1.18% 0.80% 0.87% 0.73% Mn 733 654 40 1473 19 1514 30 1263 15 Sr 5553 6115 106 846 10 948 4 745 3 Th 203 130 6 240 3 211 4 212 3 U 13.4 12.6 2.2 9.1 0.9 10.1 0.5 8.4 1.0 Y 648 557 13 617 9 676 11 577 7 Eu/Eu* 0.73 0.77 0.43 0.44 0.43 Ce/Y 8.9 9.1 5.7 5.6 5.4

wt % SiO, 0.76 0.56 0.04 0.00 0.00 0.00 0.00 Fe0 0.09 0.11 0.01 0.19 ’ 0.02 0.19 0.01 MnO 0.10 0.07 0.01 0.21 0.00 0.21 0.02 0.08 0.13 0.01 0.12 0.01 0.13 0.00 2: 52.1 51.8 0.3 52.8 0.3 52.5 0.6 SrO 0.50 1.26 0.04 0.16 0.06 0.13 0.04 Na,O 0.16 0.18 0.05 0.26 0.06 0.26 0.02 403 0.42 0.26 0.10 0.12 0.07 0.14 0.18 %O3 0.97 0.57 0.12 0.37 0.06 0.37 0.14 p205 38.3 38.7 0.3 40.0 0.2 40.2 0.2 so3 0.63 0.63 0.06 0.40 0.08 0.36 0.19 F 3.0 2.97 0.39 1.95 0.56 1.68 0.17 Cl 0.28 0.27 0.01 1.10 0.09 1.14 0.06 CI/F 0.09 0.09 0.56 0.68 Chondrite data taken from Evensen er al. (1978)

similar horizons elsewhere (Batchelor & Clarkson 1993), a The two Lusklint Bentonite samples have a unique conceptemphasized by Bergstrom et al. (1992). The chemical signature.They have the highest absolute Lusklint Bentonite hasbeen identified, using biostrat- abundance of Sr and high-field strength elements (HFSE) igraphic control, in the Viki core from Saaermaa, Estonia such as Zr, Y, Th and Nb but contain no zircon crystals, (Jeppsson & Mannik 1993). only euhedral biotite and apatite. Apatite crystals contain >l% XREE (Table 2), the richest of all the suite of analysed apatitesfrom Gotland bentonites(Batchelor, Classification and petrogenesis unpublished data). The apatite REE pattern also shows a Traditionally, the Winchester & Floyd (1977) volcanic rocks LREE enrichment and a weak negative chondrite- discriminationdiagram has been applied to bentonites on normalized (negative) Eu anomaly (Eu/Eu* = 0.75). Ro- the assumption that Zr, Ti, Nb and Y are immobile under eder et al. (1987) reported that the Ce/Y value in apatite conditions of devitrification of volcanic glass to clay. Since reflects the alkalinity of the host melt. Values greater than doubt has been cast onthe immobility of Tiand other 7.7 indicate an origin from highly alkaline environments. ‘immobile’ elementsunder hydrothermalcondition in Apatite from the Lusklint bentonite has a value of 9.0. If sedimentaryrocks (Hole et al. 1992), a classification of this value represents an alkaline host melt, it could explain bentonites based on atomic proportions of selected REE in the absence of zircon in the bulk sample. The absence of apatite (Fleischer & Altschuler 1986) is used here. There is zircon combined with a weak negative Eu anomaly and the some overlap between fields but values for at. (La + Ce + presence of apatite crystals implies the source was not an Pr) > 70% indicate a strongly alkaline origin, whereas values oversaturated peralkaline melt (Macdonald 1987). Assuming ~65%do not. Apatite from both bentonite beds has values that the negative Eu anomaly in the apatite reflects the Eu of 66% and plotsin the transitionalzone between the budget in the parental melt (Roeder et al. 1987) then the intermediate and alkaline fields (Fig. 6). weak negative Eu anomaly suggests that either plagioclase

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"."" 1 I Locallty A LUSKLINT lrevlken 0.04 - BENrONrrESW61 SW60 Bentonlte

0 SW63 0.03 - Luskllnt IREVIKEN SW83 SW59 Bentonlte BENTONrrE SW60 I

H Ce'l OIY 0.01o'02* W Zr*l OOO/Ti 1 2 3 Si02/A1203 Locallty C

. .-

lrevlken SW63 Bentonite 1.o IREVIKEN SW63 BENTONlTE Luskllnt Bentonlte

.::l 0 20 40 60 80 100 0.7 Fig. 3. Bar graph showingthe complementary variation in Zr X loOO/Ti and Ce X 1O/Y for the Lusklint and Ireviken 0.6 1 sw59 LUSKLINT Bentonites. m sw61 BENrONiTE b

10 11 12 11 10 14 13 15 negative Eu anomaly appear. The rocks of Stromboli range Zr/Y from calc-alkaline andesites through shoshonites, latites to potassic rocks. The shoshonites and latites from Stromboli Fig. 2. Bivariate diagrams for Lower Visby bentonites are enrichedin REE, K, Rb,Sr and Ba, theycontain (a) Ti0,/AI,0,-Si0,/AI,03. (b) Nb/Y-Zr/Y. apatiteand biotiteas phenocryst phases but no zircon (Francalanci, pers. comm.). The spectrum of magmas are thought to havebeen generated in a magma chamber feldspar was not a major fractionating phase or the melt had undergoing refilled tapped fractionatingand assimilation a high oxygen fugacity. The combination of high processes although a heterogeneous mantle source enriched concentrations of HFSEand REE in the bulk sample in incompatible elements hasnot been ruled out indicates thatthe Lusklint Bentonite source magma from (Francalanci et al. 1989). The chemical similarities between which the apatite crystallized had either fractionated to a the Lusklint Bentonite and Stromboli lavas suggest that the high degreeor had tapped a source rich in incompatible elements. High Sr levels and a weak negative Eu anomaly in the apatite indicatelittle or no plagioclase removal which argues against crystal fractionationand supports an origin from HFSE- and REE-enriched material. The relatively low CI/Fratio (0.09) in apatite indicates growth in a wet magma, since Cl preferentiallypartitions into the hydrous phase (Candela 1986). Recent work on subduction-related calc-alkaline vulcan- ism on Stromboli, Southern Italy(Francalanci 1989) has shown that it is feasible to have no negative Eu anomaly in meltsfractionating both plagioclase feldspar and mafic minerals. This situation can arise, not due to a high oxygen m I I I fugacity, but because of the differences in DplagIEu and DCPX/Eu. Plagioclase favours Eu whereas clinopyroxene does not. The presence or absence of a negative Eu anomaly will thereforedepend crucially onthe relativeproportions of ,l 1 10 100001000001000 100 these two minerals crystallizing out of the melt. Only when Fig. 4. Bar graph showing element variations in apatite extracted plagioclase becomes the dominant crystallizing phase will a from the Lusklint and Ireviken Bentonites.

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mafic magma. The Ce/Y value of 5.6 for apatite means its Luskllnt Bentonlte source is less alkaline than that for the Lusklint Bentonite. SW59 These two bentonites have a vertical separation of 2.8 m, an --t SW6 1 interval which representsabout 60 OOO years (Jeppsson lrevlken Bentonlte 1987). If these bentonites originated from the samevolcano, SW60 then the change in bulk chemical composition, from potassic -0- SW63 --t SW83 to medium-K calc-alkaline, could have been achieved within this timeaccording to Francalanci (1989). However the difference in Zr/Nb(Table 1) suggests they are not co-magmatic. The bulk rock and apatite chemical evidence

La Ce Pr Nd Srn Eu Gd Tb Ey Ho Er Trn yb Lu indicates thatthe Ireviken Bentonite originatedfrom a medium-K calc-alkaline magmasource which was not Fig. 5. Chondrite-normalized rare earth elements in apatite consanguinous with themore potassic magma which extracted from the Lusklint and Ireviken Bentonites. Chondrite produced the Lusklint Bentonite. data taken from Evensen er al. (1978). Source and tectonic setting bentonitecould have been derived from magmas with potassic affinities enriched in lithophile elements. On the basis of bentonite thicknesses and a.consideration of regional volcanicity, Laufeld & Jeppsson (1976) surmized The bulkcomposition of thelater Ireviken Bentonite that a magma source lay either to the south of 'Gotland, shows a reduction in Zr, Y, Nb, and TiO, relative to the around the Barrandian area of Czechoslovakia, or to the Lusklint Bentonite. Key traceelement ratios (Zr/Y 12.5, southwest aroundthe Dingle Peninsula in Ireland. These La/Y Sc/Ni 0.4) resemble those orogenic andesites 1.3, of relative geographical positions are confirmed by palaeoge- of Andean type(Bailey 1981). The general trace element ographicalreconstructions forthe Silurian (Scotese & assemblage is similar to that of dacites rhyodacites from / McKerrow 1992). By the early Silurian, Baltica was closing the medium-K calc-alkaline series compiled by Mann in onLaurentia, while East Avalonia was moving (1983). Apatite composition differs markedly from that of northwards towards Baltica. Arc volcanism was still active in the Lusklint Bentonite.In particular, it has a strong Late Llandoverytimes, preserved as tuffs in theTonalee negative Eu anomaly, lower LREE enrichment (Fig. 5), Sr Formation of Galway, Ireland, but is believed to have levels are six timeslower and CI/F (0.62) is seven times ceased by the Early Wenlock, as oceanic subduction gave higher (Fig. 4). Such a change in compositions could be way to continental collision (Williams et al. 1992). The explained by plagioclase fractionation andapatite crystal- Tornquist Sea had closed by mid-Silurian times (Trench & lization in a drier magma which allowed C1 to partition into Torsvik 1992) andthe Northern Iapetus Ocean had also the melt. A relative fall in bulk concentrations of Y and closed by 420Ma,the end of the Wenlock (Soper et al. Ti02 would beconsistent with removal of pyroxene and 1992). Fe-Ti oxide, while the reduction in Zr and Nb could The maximum length of euhedralapatite crystals indicate an influx of HFSE-poor mafic magma butthis is extractedfrom the IrevikenBentonite is 0.2mm. This inconsistent with the drop in TiO,. Neither can the relative represents a minimum size for volcanic ash particles if the fall in apatiteLREE be explained by fractionation crystals are assumed to have been transported in a glassy processes. It is therefore unlikely thatthe Ireviken host particle. According to Walker (1971), a particle 0.2 mm Bentonite originated from a fractionated Lusklint Bentonite long could be transported from 2 to 500 km depending on parent magma or a hybrid formed by mixing the latter with the force of eruption. Clearly, particle shape and density, and prevailing winds will influence the dispersal of tephra. 2 The nearest active arc in Late Llandovery times was in the .13 closing Tornquist Sea, which was situated in what is now 012 southern Poland and southern Denmark. Silurian volcanic rockshave been described fromthe Klodzko Terrane in Poland where local marbles contain Ludlow fossils (Oliver et 1993). Denmark has no known Silurian volcanic rocks U al. but bentonites do occur in the Llandovery of Bornholm, t I : :9 m' SlLlClC I, 400 km SW of Gotland (Bjerreskov 1975). The Tornquist 4 ALKALINE Suture Zone at Klodzko lay at least 800 km SSW of Gotland and is the nearest hitherto known source of Silurian 1. 14. I5.j : . IFmRMEDlATE : volcanism. The continuation of this Zone to theNW takes it slightly closer to Gotland (approximately 500 km), but a thick cover of youngersediments may obscure any 0 volcanoes which generated the Gotland bentonites. In view 40 5030 40 60 70 80 90 of the distances involved, it is assumed that the ash which La+Ce+Pr % deposited the Ireviken Bentonite must have originated from Fig. 6. Rock classification based on apatite REE content after a powerful Plinian eruption. Fleischer & Altschuler (1986). 1, Granite pegmatite; 2, granite; 3, In contrast, the Silurian volcanic activity in SW Ireland phosphorite; 4, granodiorite; 5, gabbro; 6, kimberlite; 7, syenite; 8, lay about 2000 km away. These subalkaline volcanics have carbonatite; 9, alkali ultramafic; 10, iron ores; 11, ultramafic; 12, Zr/Nb values of 22.4-60 (Sloan & Bennett 1990) compared alkalic; 13, alkalic pegmatite; 14, gneiss/migmatite. with an average value of 13 for the Ireviken Bentonite. On

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the basis of distance and Zr/Nb values, the Irish source is fractionalcrystallisation, magmamixing, crustal contaminationand discounted. source heterogeneity. Bulletin Volcanologique, 51, 355-378. Based on maximum grain sizes and palaeogeographical GREY,J., LAUFELD,S. & BOUCOT, A.J.1974. Silurian triletespores and spore tetradsfrom Gotland: their implications for land plant evolution. considerations, it is likely that the Lower Visby Formation Science, 185,260-263. bentonites originated from the Tornquist Zone. The use of HEDE, J.E. 1921. GottlandsSilurstratigrafi. Sveriges Geologiska apatite chemistry provides sensitive fingerprints for correlat- Undersokning Avhandlingar och uppsatser,Qo5, 1-100. ing bentonites (Samson et al. 1988) and offers clues to the - 1960.The Silurian of Gotland. In: REGNELL,G. & HEDE,J.E. The LowerPalaeozoic of Scania. InternationalGeological Congress XXI nature of parental magmas. An alkaline tointermediate Session, Norden Guidebook, Sweden. composition is indicated forthe magmas which generated HOLE,M.J., TREWIN,N.H. & STILL,J. 1992.Mobility of the high field the Lower Visby bentonites. The potassic nature of the strength, rare earth elements and yttrium during late diagenesis.Journal Lusklint Bentonite indicatessourcea from a waning of the Geological Society, London, 149, 689-692. subduction zone which incorporated a significant proportion JEPPSSON,L. 1987.Lithological and conodont distributional evidence for episodes of anomalousoceanic conditions during the Siurian. In: of crustal material, as subduction gave way to continental ALDRIDGE,R.J. (ed.) Palaeobiology of Conodonts, Ellis Horwood Ltd, collision. A similarsituation exists today at Stromboli, 129-145. where the volcano lies above 18 km of continental crust and -, The anatomy of the mid-Early Silurian Ireviken Event. In: BRETT, C. is erupting shoshonitic lavas (Francalanci et al. 1989). The (ed.) PaleontologicalEvent Horizons. Ecological and Evolutionary Implications. Columbia Press. In press. younger Ireviken Bentonite was derived from an independ- -& MANNIK,P. 1993. High resolution correlations between Gotland and ent medium-K calc-alkaline magma generatedain Estonia near the base of the Wenlock. Terra Nova, 5, 348-355. continental margin environment. These two bentonites, with LAUFELD,S. 1974. Reference localities for palaeontology and geology in the their distinctive chemical and mineralogical composition, Silurian of Gotland. Sveriges Geologiska Undersokning,C705, 1-172. - & JEPPSSON,L. 1976. Silicification and bentonites in the Silurian of form unique chemostratigraphiccorrelation tools for Late Gotland. Geologiska Foreningens i Stockholm Forhandlingar,98,31-44. Llandovery sequences in Northern Europe. MACDONALD,R. 1987. Quaternaryperalkaline silicic rocks and caldera volcanoes of Kenya. In: FITTON,J.G. & UFTON, B.G.J. (eds) Alkaline Igneous Rocks. GeologicalSociety, London, Special Publications, Fieldwork was funded by the Carnegie Trust and analytical work by 30, 313-333. The Leverhulme Trust (R.A.B.). L.J. is financed by the Swedish MANN, A.C. 1983. Traceelement geochemistry of High-Alumina NaturalScience Research Council. Fieldwork was basedat the Basalt-Dacite-Rhyodacite Lavas of the MainVolcanic Series of formerAllekvia Geological Field Station, Endre, Gotland. The Santorini Volcano, Greece. Contributions to Mineralogy and Petrology, manuscript was improved by referees C. Elders and M. Hole. 84,43-57. OLIVER,G.J.H., CORFU,F. & KROGH,T.E. 1993.U-Pb ages from SW Poland:evidence for a Caledoniansuture zone betweenBaltica and References Gondwana. Journal of the Geological Society, London, E&, 355-369. ROEDER,P.L., MAcAR~uR,D., MA, X-P.,PALMER, G.R. & MARIANO, ALDRIDGE,R.J., JEPPSSON, L.& DORNING,K.J. 1993. Early Silurian oceanic A.N. 1987. Cathodoluminescenceand microprobe study of rare-earth episodes and events. Journal of theGeological Society, London, 150, elements in apatite. American Mineralogist, 72, 801-811. 501-513. SAMSON,S.D., KYLE, P.R. & ALEXANDER,E.C. 1988. Correlation of North BAILEY,J.C. 1981. Geochemical criteria for a refined tectonic discrimination AmericanOrdovician bentonites byusing apatitechemistry. Geology, of orogenic andesites. Chemical Geology, 32, 139-154. 16,444-447 BATCHELOR,R.A. & CLARKSON,E.N.K. 1993.Geochemistry of a Silurian SCOTESE, C.R. & MCKERROW,W.S. (eds) 1992. Palaeozoic Palaeogeography metabentoniteand associated apatite from the North Esk inlier, and Biogeography. Geological Society, London, Memoira 12. Pentland Hills. Scorrish Journal of Geology, 29, 123-130. SLOAN,R.J. & BENNETT,M.C. 1990. Geochemicalcharacter of Silurian BERGSTROM, S.M., HUFF,W.O., KOLATA,D.R. & KAUO, D. 1992. Silurian volcanismin SW Ireland. Journal of theGeological Society, London, K-bentonites in the lapetus Region: a preliminary event-stratigraphic and 147,1051-1060. tectonomagmaticassessment. Geologiska Foreningens i Srockholm SOPER, N.J. STRACHAN, R.A. HOLDSWORTW,GAYER, R.E. R.A. & GREILING, Forhandlingar, 114, 327-334. R.O. 1992. Sinistral transpression and the Silurian closure of Iapetus. BJERRESKOV,M.1975. Llandoverian and Wenlockian graptolites from Journal of the Geological Society, London, 149,871-880. Bornholm. Fossils and Strata, 8, 1-109. SPJELDNAES, N. 1959.Silurian bentonites from Gotland, Sweden. Geologiska CANDELA,P.A. 1986. Towards a thermodynamic model for the halogens in Foreningens i Stockholm Forhandlingar,81, 582-587. magmaticsystems: an application to melt-vapour-apatiteequilibria. THORSLUND,P. 1948. Silurisk bentonit frHn Gotland. Geologiska Foreningens Chemical Geology, 57, 289-301. i Stockholm Forhandlingar, 70,245-246. EVENSEN,N.M., HAMILTON,P.J. & O’NIONS,R.K. 1978. Rare-earth TRENCH,A. & TORSVIK,T.H. 1992.The closure of IapetusOcean and abundances in chondrite meteorites. Geochimica et Cosmochimica Acta, Tornquist Sea:new palaeomagnetic constraints.Journal of the Geological 42, 1199-1212. Society, London, 149,867-870. FLEISCHER,M. & ALTSCHULER,Z.S. 1986.The lanthanides and yttrium in WALKER,G.P.L. 1971. Grain-size characteristics of pyroclasticdeposits. minerals of the apatite groupan analysis of the available data. News Journal of Geology, 79,696-714. Jahrbuch fur Mineralogie Monatschefte, 10, 467-480. WILLIAMS,D.M., O’CONNOR, P.D. & MENUGE,J. 1992.Silurian turbidite FRANCALANCI,L. 1989. Traceelement partition coefficients for minerals in provenance and the closureof Iapetus. Journal of rhe Geological Society, shoshonitic and calc-alkaline rocks from Stromboli Island (Aeolian Arc). London, 149,349-357. Neues Jahrbuchfur Mineralogie Abhandlungen,160, 229-241. WINCHESTER,J.A. & FLOYD,P.A. 1977.Geochemical discrimination of -, MANEITI,P. & PECCERILLO, A. 1989. Volcanological and magmatologi- differentseries and their differentiation products using immobile calevolution of Strombolivolcano (Aeolian Islands): the roles of elements. Chemical Geology, U), 325-343.

Received 2 August 1993; revised typescript accepted 18 January 1994

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