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 actas 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 bentonite have been found on Gotland using the current 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 separated and 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/151/5/741/4889869/gsjgs.151.5.0741.pdf by guest on 26 September 2021 l I 0- 10 - Lower Visby Beds 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/151/5/741/4889869/gsjgs.151.5.0741.pdf by guest on 26 September 2021 SILURIANBENTONITE CHEMOSTRATIGRAPHY 743 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.