Cool kaolins in Finland

Palaeogeography, Palaeoclimatology, Palaeoecology 392 (2013) 454–462

Contents lists available at ScienceDirect

Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo

Cool kaolins in Finland H. Albert Gilg a,⁎, Adrian M. Hall b, Karin Ebert c, Anthony E. Fallick d a

Lehrstuhl für Ingenieurgeologie, Technische Universität München, Arcisstr. 21, 80333 Munich, Germany School of Geography and GeoSciences, University of St Andrews, Irvine Building, North Street, St Andrews, KY16 9AL Fife, Scotland, UK c Department of Physical Geography and Quaternary Geology, Stockholm University, 10691 Stockholm, Sweden d Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QF, Scotland, UK b

a r t i c l e

i n f o

Article history: Received 5 June 2013 Received in revised form 26 September 2013 Accepted 30 September 2013 Available online 9 October 2013 Keywords: Kaolin Palaeotemperatures Finland Weathering Fennoscandian Shield

a b s t r a c t We use D/H and 18O/16O ratios to explore the age of kaolins on the Fennoscandian Shield. Sub-Cretaceous kaolins in southern Scandinavia have isotopic compositions indicative of weathering under warm mean annual temperatures (MATs) of N15°C. Deep kaolins on the shield surface in Finland previously also have been regarded as products of humid tropical weathering of Mesoproterozoic to Eocene age. New oxygen and hydrogen isotope ratios indicate, however, weathering by cool groundwater under MATs of 13–15°C. Isotope ratios are also not consistent with deep (N1km) burial by cover rocks, indicating that a very old age for the weathering is unlikely. Palaeotemperatures are below Cretaceous MATs, yet substantially above Plio-Pleistocene MATs. Comparisons with palaeotemperatures in N Europe and around the Arctic Ocean indicate that the Finnish kaolins developed on the shield surface in the Palaeogene or, alternatively, Miocene. Deep weathering was selectively developed in highly fractured shield rocks and took place in response to latest Cretaceous and Palaeogene uplift and after stripping of Palaeozoic cover rocks. The cool kaolins in Finland indicate that previous routine attributions of kaolinitic weathering products in the geological record to humid tropical environments should be closely scrutinised. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The D/H and 18O/16O ratios of clay minerals formed during rock weathering reflect the isotopic composition of meteoric waters and temperature (Lawrence and Taylor, 1972; Sheppard and Gilg, 1996; Savin and Hsieh, 1998). Moreover, the original isotopic signatures may be retained in kaolinite clays over timescales of 108years (Lawrence and Rashkes Meaux, 1993; Savin and Hsieh, 1998; Gilg, 2000), unless the rock suffered a more severe diagenetic overprint (Bird and Chivas, 1988; Longstaffe and Ayalon, 1990). As meteoric waters have variable isotopic compositions related to climatic and geographic situations (Rozanski et al., 1993), the isotope composition of unaltered kaolins has been used to detect regional palaeoclimatic changes related to surface uplift (Chamberlain et al., 1999), latitudinal changes (Bird and Chivas, 1989) or global temperature changes (Mora and Pratt, 2001; Feng and Yapp, 2009). In some cases, the isotope data can provide valuable indirect evidence for the age of weathering (Bird and Chivas, 1989; Gilg, 2000). Deep weathering profiles with high kaolinite contents are developed on Proterozoic Shield rocks beneath Pleistocene glacial deposits in a few locations in Fennoscandia (Figs. 1 and 2). The timing of kaolinisation at these sites is poorly constrained except in southern Fennoscandia where overlying Jurassic–Cretaceous cover rocks give minimum ages. As Fennoscandia showed significant palaeolatitudinal changes due to continental drift during the Proterozoic and Phanerozoic (Fig. 3), stable ⁎ Corresponding author. Tel.: +49 89 28925855; fax: +49 89 28925852. E-mail address: [email protected] (H.A. Gilg). 0031-0182/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2013.09.030

hydrogen and oxygen isotope studies on kaolinites have the potential to yield useful clues to temperatures of kaolinisation and the isotope composition of palaeometeoric waters, and to the timing of weathering. This paper examines D/H and 18O/16O ratios for kaolins from sites on the Fennoscandian Shield in order to estimate groundwater palaeotemperatures during weathering and the likely ages of phases of deep kaolinisation. 2. Samples and methods We examine the mineral and H–O isotopic composition of two groups of residual kaolins between 55° and 67°N (Figs. 1 and 2). Group 1 comprises two kaolin deposits between 55° and 56°N on ~1.46Ga granitic rocks of the Blekinge–Bornholm Provinces (Zariņš and Johansson, 2009) that occur beneath Early to Late Cretaceous sedimentary rocks in SW Fennoscandia: Ivö in Skåne, southern Sweden (Lidmar-Bergström, 1999; Naqvi, 2013), and Rønne on Bornholm Island, Denmark (Almeborg et al., 1969; Bondam and Störr, 1988). Both deposits were mined during the 20th century and were the largest kaolin producers in their countries. We selected representative white and friable samples from surface outcrops. In contrast, Group 2 kaolins are prospects of unexposed residual occurrences on Palaeoproterozoic Svecofennian rocks (~1.9Ga) in Finland with a potential for economic use in paper and ceramic industries (Pekkala and Yevzerov, 1990; Lintinen and Al-Ani, 2005). We investigated drill core samples from three areas (Figs. 2 and 3A). In SE Finland, kaolins of the Virtasalmi district (61°N) (Fig. 2A) have developed on Svecofennian supracrustal rocks (gneisses, schists and amphibolites) at the Litmanen

H.A. Gilg et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 392 (2013) 454–462

455

Fig 1. Kaolins and other indicators of erosion history on the Fennoscandian Shield. Locations: 1 Rønne, 2 Ivö, 3 Virtasalmi, 4 Siurunmaa, 5 Vittajänkä, 6 Sokli; 7 Lappajärvi, 8 Karikkoselkä, and 9 Iso-Naakkima.

deposit and syn-tectonic plutonic rocks (metatonalites) at the Eteläkylä deposit (Sarapää, 1996). The kaolins are covered by 10 to 30m thick Pleistocene glacial deposits (Fig. 3). The lenticular kaolin bodies are 0.5 to 2km in length and 50 to 500m wide and attain thicknesses of ~20– 40m but reach depths of up to 100m (Sarapää, 1996). A zone of less intensely weathered rock typically surrounds and underlies the kaolins (Al-Ani et al., 2006). The surrounding shield plains lie at ~100ma.s.l. The shield terrain has a pronounced NW–SE grain due to partial excavation by glacial erosion of weathered fracture zones to create linear, lakefilled rock basins (Fig. 3A). In NE Finland (67.5°N), the Vittajänkä deposit in Salla (Lintinen and Al-Ani, 2005; Al-Ani and Sarapää, 2008), and the Siurunmaa prospect near Sodankylä (Rask and Lintinen, 2001) formed on Palaeoproterozoic metasediments of the Central Lapland Greenstone Belt. These greenstones show extensive albitisation, carbonatisation and sericitisation (Eilu, 1994). Overlying glacial, glacifluvial and glacilacustrine deposits are 10–20m thick (Fig. 3B). At Siurunmaa, the base of the overlying glacilacustrine deposits contains reworked Eocene marine diatoms (Tynni, 1982; Hall and Ebert, 2013). The kaolins in N Finland occur beneath poorly-drained, lithologically- and fracture-controlled plains and depressions at ~200ma.s.l. that are set between hill groups (Fig. 2B and C). Glacial erosion in this area has been very limited due to the repeated development of covers of cold-based ice during the Pleistocene (Hirvas, 1991). In the shield landscape classification scheme of Söderman (1985), the kaolin occurrences in N and S Finland

are regarded as Miocene in age and fall within the zones of Pliocene etchplains. For each drill core, we took samples from the top and bottom of the white kaolinised zone. We preferred white samples to avoid 18Odepleted Fe oxides and friable samples to avoid admixture of non-clay minerals during crushing. The samples were gently disaggregated and size fractions separated by settling in deionised water. For most samples we investigated the b2μm fraction enriched in kaolinite, in some samples additionally the 2–6μm fraction. X-ray diffraction analyses were run on powder and oriented mounts (air dried, ethylene glycolated and heated to 550°C) using a Phillips PW 1800 with CuKα radiation. The Rietveld programme BGMN (Bergmann et al., 1998) was used for mineral quantification. Stable isotope measurements were performed at SUERC. The extraction of hydrogen for isotopic analysis follows the principle of Bigeleisen et al. (1952). After degassing at 120°C in a vacuum to remove adsorbed moisture, water was extracted from the clays by vacuum heating in a Pt crucible with an induction furnace to ~1500°C. The water was converted to hydrogen gas by reduction over hot chromium. The yields were determined manometrically. Hydrogen isotopic composition was measured on a VG 602 mass spectrometer calibrated using VSMOW and GISP. NBS 30 biotite gives δD=−65‰ by this method. Oxygen was extracted from the clays using the ClF3 modification to the BrF5 method (Clayton and Mayeda, 1963) and converted to CO2 by

456

H.A. Gilg et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 392 (2013) 454–462

V-SMOW. The precision and accuracy of the isotope measurements are estimated at ±0.2‰ for oxygen and ±2‰ for hydrogen at one sigma. 3. Mineral compositions and stable isotope data

Fig. 2. Geomorphological context of kaolins in S and N Finland. Kaolin locations from Al-Ani et al. (2006) and Lintinen (2006). A. Virtasalmi district B. Sodankylä district C. Salla district.

reaction with hot platinised graphite. Isotopic measurements were conducted on a Micromass SIRA II mass spectrometer calibrated with the carbonate standards NBS-18 and NBS-19 and NBS 28 quartz gives δ18O=9.6‰. All delta (δ) values are reported in per mil (‰) relative to

The results of X-ray diffraction analysis (Table 1) show that the clay separates consist of more than 90% kaolinite with minor contents of illite (mostly 1M, at Vittajänkä 2M1 polytype), quartz and K-feldspar and traces (b1%) of anatase, crandallite and calcite. Halloysite has been reported in some samples of the Bornholm (Bondam and Störr, 1988) and Litmanen, Virtasalmi (Al-Ani and Sarapää, 2008) kaolins but it was not present in our samples. The two investigated 2–6μm fractions contain significantly more muscovite/illite (10 to 15%) than the b2μm fractions (b6%) of the same sample. The Hinckley Index (HI) of the kaolinites (Table 1) varies between deposits with low values for the Siurunmaa prospect (0.4 to 0.5) and moderately high values for the other deposits (0.8 to 1.3) but generally do not change much within a profile. This suggests that the deep weathering profiles are mature. An exception is the Eteläkylä, Virtasalmi deposit, where the crandallite-bearing sample from the upper part of the profile has much lower HI value (0.8) than the deep sample (1.3). The kaolin samples from Finland show unusually low δD (−126 to −105‰) and δ18O values (13.8 to 16.1‰) with generally lower values in the more northern deposits of Vittajänkä and Siurunmaa (Table 1). There is no systematic variation in isotope values with respect to depth in the alteration profiles and to the Hinckley Index. The illite/mica-rich 2–6μm fractions have identical oxygen and very similar hydrogen isotope values compared to the b2μm fractions indicating no need for the correction of isotope values considering the ancillary minerals in the b2μm fractions. Significantly higher oxygen (18.7 to 19.9‰) and hydrogen isotope values (−83 to −82‰) are reported for the two samples from southern Fennoscandia (Ivö and Rønne). The relatively low kaolinite content of the Rønne sample (~88% kaolinite) requires some correction of the oxygen isotope value for the quartz, feldspar, muscovite contaminant. Assuming values of 5 to 10‰ for the residual granite-derived minerals, a corrected δ18O value of 19.9 to 20.6‰ is calculated for the kaolinite. The isotope ratios of the kaolinites are plotted in δ18O–δD diagram in Fig. 5. The relationship between the δD and δ18O values of kaolinite formed in equilibrium with waters on the Global Meteoric Water Line (GMWL) is predicted by the equation of Savin and Epstein (1970). The temperature-dependent kaolinite–water fractionation factors of Sheppard and Gilg (1996) are used to calculate kaolinite isotherms in a plot of δD against δ18O for temperatures at 10 and 20°C. Additionally, the isotope compositions of present-day local meteoric waters (Burgman et al., 1987; Kortelainen and Karhu, 2004) and of corresponding kaolinites in equilibrium at mean annual air temperature for each site are indicated by triangles. For comparison, we also show previously published isotope data for Mesozoic and Cenozoic kaolins from Georgia, U.S.A. (Savin and Epstein, 1970; Hassanipak and Eslinger, 1985; Lawrence and Rashkes Meaux, 1993), western Europe (Portugal, Spain, France, England) (Sheppard, 1977; Bobos and Gomes, 1998; Boulvais et al., 2000; Gilg, 2003; Fernández-Caliani et al., 2010) and central Europe (Germany, Czech Republic, Austria, Poland) (Savin and Epstein, 1970; Gilg and Frei, 1997; Gilg, 2000, 2003). We note that the isotope compositions of weighted mean annual precipitation at low-latitude (b40°) and low-altitude (b500m) IAEA stations (Fig. 5) correspond to the isotope composition in equilibrium with almost all large kaolin deposits in Europe, U.S.A. and elsewhere (Bird and Chivas, 1989; Chivas and Bird, 1995). At these stations the average mean annual temperature (MAT) is 23±5°C and average rainfall is 1328±844mm/a; both conditions being favourable for kaolinisation (Thiry, 2000). The new hydrogen and oxygen values for the Fennoscandian kaolins (Table 1) do not support a hydrothermal origin for these clays but are

H.A. Gilg et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 392 (2013) 454–462

clearly indicative of a weathering origin (Fig. 3). All samples plot far to the right of the supergene–hypogene line that broadly discriminates between clays of hydrothermal versus weathering origin (Sheppard and Gilg, 1996)(Fig. 5). The clay mineral composition and decreasing alteration with depth in these kaolins are also consistent with a supergene origin (Sarapää, 1996; Lintinen and Al-Ani, 2005). Furthermore, the δD and δ18O values, set against present geothermal gradients in Finland of 8–15K/km (Kallio et al., 2011), preclude significant post-weathering geothermal heating or burial by N1km of cover rocks. Group 1 kaolin isotope data indicate MATs of 10 to 20°C, in contrast to modern MATs in southern Sweden of 6–9°C and palaeometeoric waters that are isotopically much higher than present-day groundwater (Fig. 5). The isotope composition of palaeometeoric water in equilibrium with the South Scandinavian kaolins is, however, lower than those for waters in equilibrium with Cretaceous and Palaeogene Georgia kaolins or the western European kaolins (N Portugal, NW Spain, Cornwall, Brittany) that formed at palaeolatitudes of about 35°N and 35 to 40°N, respectively in a paratropical, near coastal environment (Scotese, 1997). The Group 1 kaolins plot with kaolins from central Europe which formed in warmer, humid climates in the Cretaceous, Palaeogene and, in part, Miocene (Gilg, 2000; Störr, 2006). The Cretaceous of NW Europe was probably amongst the warmest climate periods of the Phanerozoic (Hallam, 1985) and Jurassic and Cretaceous kaolinitic and bauxitic weathering is recorded widely across Europe south of Pleistocene glacial limits (Migoń and Lidmar-Bergström, 2001; Ahlberg et al., 2003). The Group 2 samples from Finland are quite distinct, however, with very low δD and δ18O values that are incompatible with a formation at both low latitude (b40°) and low elevation (b500ma.s.l.) as recorded in most kaolin deposits (Fig. 5; see also Savin and Hsieh, 1998). The low δ18O values of the kaolinite exclude preferential D-exchange after formation at a low latitude and altitude. The absence of alunite suggests that kaolinisation is not related to supergene acid-sulphate alteration of

457

sulphide-rich protoliths (Thiry, 2000). The isotope data, however, indicate that Group 2 Finnish kaolins belong to a small sub-set of ancient deep weathering kaolins with low δ18O and δD, including Jurassic–Early Cretaceous kaolins in Australia (Bird and Chivas, 1988 and 1993), Late Cretaceous and Oligocene sedimentary kaolins from Japan (Mizota and Longstaffe, 1996) and the Miocene high elevation kaolinitic clays from Idaho (Lawrence and Rashkes Meaux, 1993), that have been attributed to intensive weathering operating under cool climates. 4. Age of the cool kaolins The Virtasalmi kaolins in southern Finland lie b10km from the Iso-Naakkima impact structure (Fig. 2A). The former crater holds Neoproterozoic (1000–650Ma) mudstones that rest on a thin kaolinitic weathering horizon later lithified to claystone (Elo et al., 1993). The impact age is uncertain but impact probably occurred at ~1200Ma at a time when Baltica was at~34°S (Pesonen et al., 1996) (Fig. 4). The unlithified Litmanen kaolin has yielded three Mesoproterozoic K–Ar dates of 1151±22 to 1189±18Ma for 1M illite-bearing size fractions (Sarapää, 1996). These K–Ar dates may reflect inheritance of trace amounts of degraded mica from the parent rock that partly lost 40Ar, as observed in most other illite-bearing kaolinitic saprolites (Gilg and Frei, 1997; Menegatti et al., 1999). Deep kaolinisation is unlikely before the evolution of land biota after ~500Ma led to the build up of organic acids in soils and a rise in oxygen levels in the Earth's atmosphere (Kennedy et al., 2006). Phases of Neoproterozoic or Palaeozoic deep weathering on the exposed Fennoscandian Shield are recorded by kaolins, kaolinitic sandstones and bauxites that occur beneath or in proximity to cover rocks found around the shield margin in southern Finland, Estonia and western Russia (Elo et al., 1993; Mordberg and Nesterova, 1996; Puura et al., 1997) (Fig. 1). The preservation of this material for most of the

Fig. 3. A. Schematic geological sections of investigated kaolin deposits with sample locations (stars). B. Cross sections of kaolin occurrences. Modified from Lohva and Lehtimäki (2005) and Lintinen and Al-Ani (2005).

458

H.A. Gilg et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 392 (2013) 454–462

B Vittajänkä, N Finland. Section A.

Vittajänkä, N Finland. Section B. 50

0

m

50 Phyllite

Quartzite and sericite schist

Basic volcanite

Litmanen, S Finland

Mica gneiss

Quartz feldspar gneiss

Amphibolite

Kaolin

Glacial deposits Fig. 3(continued).

last 500Myr is due to burial by Palaeozoic cover rocks. These cover rocks formerly extended over and beyond remnants of the sub-Cambrian peneplain (Fig. 1). The Vendian or older kaolinitic weathering profiles, up to 100m thick, found beneath Cambrian sedimentary rocks in Estonia, are weakly lithified and have been subject to low temperature diagenesis beneath b2km of Palaeozoic cover rocks (Kirsimäe et al., 1999; Kirsimäe and Jørgensen, 2000). The δD and δ18O values in the Group 2 kaolins indicate, however, very limited, if any, heating after weathering and so indicate burial by b1km cover rocks. Unless the kaolins were able to persist under conditions of virtually zero erosion under thin cover rocks for the last 500Myr, the Group 2 kaolins have developed after removal of Proterozoic and Early Palaeozoic cover rocks. Fennoscandia remained at sub-tropical and tropical latitudes from the Palaeozoic until the Cenozoic (Tikkanen, 2002) (Fig. 4). Permian uplift led to the stripping of cover rocks in south-west Fennoscandia

and to exposure of the shield surface, including the Group 1 sites, with subsequent deep weathering in the Mesozoic (Lidmar-Bergström, 1995). In southern Finland, however, the preservation of Proterozoic impact craters (Abels et al., 2002), together with some of the oldest Apatite Fission Track ages that have been documented anywhere on Earth (Hendriks et al., 2007) and the absence of traces of former Mesozoic rocks, indicate that large areas of the shield remained sealed from weathering and erosion through the Mesozoic beneath Neoproterozoic and Palaeozoic cover. In N Finland, the exposure history is different. Sites of deep kaolinisation are remote from remaining Proterozoic cover rocks. Exposure of the shield probably occurred first during uplift and km scale erosion that accompanied and followed carbonatite intrusion in NE Finland and adjacent parts of Russia at ~360Ma (Amelin and Zaitsev, 2002; Lee et al., 2006)(Fig. 1). Moreover, Mesozoic to Palaeogene sedimentary rocks are absent from

H.A. Gilg et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 392 (2013) 454–462

459

Table 1 Location and mineralogy of kaolin samples. Clay mineralogy expressed as percentages. K kaolinite, Q quartz, I illite (1M and 2M1), Cr crandallite, C calcite and Kf K-feldspar. Sample no.

Deposit

Protolith

Depth (m)

Drill hole code

Size

Mineralogy

HI

δD (‰)

δ18O (‰)

Finland South FIN1 FIN2 FIN3 FIN4 FIN4

Virtasalmi — Eteläkylä Virtasalmi — Eteläkylä Virtasalmi — Litmanen Virtasalmi — Litmanen Virtasalmi — Litmanen

Metatonalite Metatonalite Quartz–plagioclase-gneiss Quartz–plagioclase-gneiss Quartz–plagioclase-gneiss

22.6 76.3 31.7 51.0 51.0

3231/91/R697 3231/91/R697 3231/-88/R525 3231/-88/R525 3231/-88/R525

b2μm b2μm b2μm b2μm 2–6μm

K(97) I-1M(2) Cr(1) K(100) K(94) l-1M(5) Q, Kf, Cr (b1) K(97) I-1M(3) Q (b1) K(90) I-1M(10)

0.8 1.3 1.0 0.8 0.9

−123 −110 −105 −118 −108

15.5 16.1 15.4 14.5 14.5

Finland North FIN5 FIN6 FIN7 FIN7 FIN8

Siurunmaa Siurunmaa Vittajänkä Vittajänkä Vittajänkä

Arkosic quartzite Arkosic quartzite Quartz–muscovite schist Quartz–muscovite schist Quartz–muscovite schist

16.9 38.4 23.3 23.3 26.7

R19/3713/-79 R19/3713/-79 4621/01/R337 4621/01/R337 4621/01/R337

b2μm b2μm b2μm 2–6μm b2μm

K(90) I-1M(4) Q(4) Kf (1.5) Cr(b1) K(92) I-1M(3) Q(2) Kf (2) Cr(1) K(93) I-2M1(5) Q(2) K(79) I-2M1(15) Q(6) K(92) I-2M1(6) Q(2)

0.4 0.5 1.0 0.8 1.1

−125 −123 −123 −126 −112

14.2 13.8 14.2 14.0 14.4

Sweden (Skane) IVO1 b2 Ivö

Granite

Surface sample

b2μm

K(97) l(2) Q(1)

0.6

−82

19.9

Denmark (Bornholm) BO1 b20 Rønne

Granodiorite

Surface sample

b20μm

K(88) Q(4) Kf(4) l-2M1(4)

0.8

−83

18.7

N Finland due to non-deposition or to erosion in response to a later phase of uplift that commenced in the Late Cretaceous (Hendriks and Andriessen, 2002; Ebert et al., 2012; Hall and Ebert, 2013). The survival of Mesozoic weathering on the exposed shield surface would require unrealistically low erosion rates. The Group 2 Finnish kaolins are unlikely to relate to Mesozoic weathering as the shield surface was sealed from weathering in the S and any Mesozoic kaolins in the N would have been stripped by Cenozoic erosion. A post-Mesozoic age for the Group 2 kaolins is also consistent with an isotopic signature that is distinct from that of the Group 1 Mesozoic humid tropical weathering residuals in southern Sweden and northern Denmark. Although the Fennoscandian Shield drifted to between 50° and 65°N in the Cretaceous, very warm climates were maintained, with MATs reaching ~17°C even within the Arctic (Weijers et al., 2007), until temperatures peaked in the brief (~0.1Ma) period of the Palaeocene– Eocene Thermal Maximum (PETM) at ~55Ma (Sluijs et al., 2006). Towards the Eocene–Oligocene boundary (~34Ma), there was a step

change towards the modern ice-house climate (Moran et al., 2006). At the Palaeogene–Neogene transition (~23Ma), large polar ice caps built up, primarily in Antarctica, and cool to warm temperate conditions were established across Fennoscandia and around the Arctic (Lavrushin and Alekseev, 2005). MATs reached 11–13°C in Iceland and 15.5–20°C in Denmark (Larsson et al., 2011) for a period of ~12Myr during the Late Oligocene and Miocene. Cooling at ~10Ma led to the first appearance of sea ice and the establishment of boreal to sub-arctic biomes around the Arctic Ocean (Lavrushin and Alekseev, 2005). By the Early Pliocene (4– 5Ma), MATs had fallen to −1°C in the high Arctic (Csank et al., 2011). The onset of glaciation on the Fennoscandian Shield occurred in the Late Pliocene (~2.8Ma) (Flesche Kleiven et al., 2002). Modern MATs are ~1°C in N Finland. Isotope data indicate formation of Group 2 kaolins at MATs of ~13°C (Fig. 3). Taking into account modern latitudinal thermal gradients of 5– 7°C between NW Europe and sub-arctic Finland, MATs were high enough for the formation of the Group 2 kaolins in Finland in only two phases of

Fig. 4. Palaeolatitude changes of Fennoscandia since the Mesoproterozoic. Modified from Tikkanen (2002).

460

H.A. Gilg et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 392 (2013) 454–462

5. Conclusions The formation of kaolinitic weathering profiles is often attributed to the warm and humid environments of the tropics and sub-tropics (Scotese, 1997; Tabor and Montañez, 2005). Moreover, high detrital kaolin contents in sedimentary sequences are widely regarded as indicators of intense weathering of source terrains under humid tropical conditions (Thiry, 2000; John et al., 2012). Yet the deep kaolins in Finland formed at MATs of 13–15°C and are isotopically distinct from Mesozoic kaolins formed in the rest of Europe under tropical environments. The Finnish kaolins relate instead to humid temperate climate weathering in the Palaeogene or Miocene at high latitudes. Rainfall and the permeability of the parent rock, rather than temperature, should be regarded as the most critical factors in the development of thick kaolinite deposits. Attribution of deep kaolins to humid tropical conditions should be questioned in circumstances where other climate proxies indicate cool climates. Fig. 5. Stable H–O isotope composition of Fennoscandian kaolins and of kaolins from Georgia (U.S.A.), western and central Europe (Savin and Epstein, 1970; Sheppard, 1977; Hassanipak and Eslinger, 1985; Lawrence and Rashkes Meaux, 1993; Gilg and Frei, 1997; Bobos and Gomes, 1998; Boulvais et al., 2000; Gilg, 2000; Gilg, 2003; Fernández-Caliani et al., 2010). The isotope composition of mean annual precipitation at low latitude (b40°) and altitude (b500ma.s.l.) I.A.E.A. stations (Rozanski et al., 1993) and the supergene–hypogene line after Sheppard and Gilg (1996) are shown for comparison. The isotope composition of recent meteoric water (Burgman et al., 1987; Kortelainen and Karhu, 2004) and hypothetical kaolin compositions in equilibrium with the waters at present-day MAT from the Fennoscandian sampling sites are shown as triangles.

the Cenozoic: the Palaeocene–Eocene and the Late Oligocene to Middle Miocene. An added complication however is that during the Palaeogene Arctic meteoric waters may have been depleted in D and 18O. Values from wood cellulose for Eocene meteoric water on Axel Heiberg Island are −15.1‰ (Jahren and Sternberg, 2002), although other proxies indicate lower δ18O values of ~−20‰(Fricke and Wing, 2004; Richter et al., 2008). It remains to be demonstrated, however, that isotopic depletion at the PETM persisted for long enough to be recorded in thick kaolinitic weathering profiles (John et al., 2012). Without independent dating, available evidence does not allow a clear choice between an Eocene or Miocene age for kaolinisation. The conditions promoting kaolin formation in Finland under cool climates are clearer. The formation of thick kaolins is promoted by high temperatures, high rainfall, permeable substrates, susceptible mineralogy and lithology, high water tables and long timescales (Thomas, 1995). Climates were humid throughout the Cenozoic around the Arctic Ocean and in N Europe (Larsson et al., 2011; Eberle and Greenwood, 2012). The widespread development of kaolins and bauxites during the Eocene and Miocene periods further south under warmer temperatures is an additional sign of abundant rainfall over mid-latitude Europe (Migoń and Lidmar-Bergström, 2001). Deep kaolins in shield bedrock in Finland are localised along fracture zones (Laajoki, 1975). Kaolins in N Finland are also preferentially developed in greenstone rocks that were rendered susceptible to later subaerial weathering by hydrothermal carbonate alteration (Eilu, 1994). Cenozoic uplift of Finland and the long term fall in sea level from the PETM onwards would also have lowered regional water tables, allowing deep penetration of weathering. The 24 and 12 Myr duration of the Palaeogene and Miocene warm periods, respectively, were probably sufficient for deep kaolinisation in Finland because equivalent depths of kaolinisation are recorded for these intervals further south in Europe (Migoń and Lidmar-Bergström, 2001). Hence only the first factor promoting advanced kaolinisation, high temperature, is not met in Finland. The kaolins of Finland can be seen as belonging to the growing number of kaolin and bauxite occurrences recognised in the geological record where intense rock alteration by deep weathering has been accomplished under cool climates (Bird and Chivas, 1988; Bird and Chivas, 1993; Taylor et al., 1992; Chivas and Bird, 1995; Mizota and Longstaffe, 1996; Cravero et al., 2010).

Acknowledgements Olli Sarapää of the Geological Survey of Finland, Bart Hendriks and David Bridgwater are thanked for providing kaolin samples. The paper benefitted from review by Piotr Migoń and Allan Chivas. Field work in Finland by AMH was supported by the Carnegie Trust for the Universities of Scotland. SUERC is financially supported by funds derived from NERC and Scottish Universities.

References Abels, A., Plado, J., Pesonen, L.J., Lehtinen, M., 2002. The impact cratering record of Fennoscandia. In: Plado, J., Pesonen, L.J. (Eds.), Impacts in Precambrian Shields. Springer Verlag, pp. 1–58. Ahlberg, A., Olsson, I., Simkevicius, P., 2003. Triassic–Jurassic weathering and clay mineral dispersal in basement areas and sedimentary basins of southern Sweden. Sediment. Geol. 161, 15–29. Al-Ani, T., Sarapää, O., 2008. Clay and clay mineralogy. Geological Survey of Finland, Report M19/3232/2008/41, Espoo, p. 94. Al-Ani, T., Sarapää, O., Lehtinen, M.J., 2006. Mineralogical and chemical study of some kaolin samples from the Kahdeksaisiensuo and Hyväjärvi occurrences, Virtasalmi, SE Finland. Geological Survey of Finland, Report M19/3232/2006/1/822010, Espoo, p. 19. Almeborg, J., Bondam, J., Heller, E., 1969. Kaolin deposits of Denmark. Proc. XXIll Int. Geol. Congr. 15, 75–84. Amelin, Y., Zaitsev, A.N., 2002. Precise geochronology of phoscorites and carbonatites: — the critical role of U-series disequilibrium in age interpretations. Geochim. Cosmochim. Acta 66, 2399–2419. Bergmann, J., Friedel, P., Kleeberg, R., 1998. BGMN — a new fundamental parameters based Rietveld program for laboratory X-ray sources, it's (sic) use in quantitative analysis and structure investigations. Commission of Powder Diffraction, International Union of Crystallography CPD Newsletters. 20, 5–8. Bigeleisen, J., Perlman, M.L., Prosser, H.C., 1952. Conversion of hydrogenic materials to hydrogen for isotopic analyses. Anal. Chem. 24, 1356–1357. Bird, M.I., Chivas, A.R., 1988. Stable-isotope evidence for low-temperature kaolinitic weathering and post-formational hydrogen-isotope exchange in Permian kaolinites. Chem. Geol. 72, 249–265. Bird, M.I., Chivas, A.R., 1989. Stable isotope geochronology of the Australian regolith. Geochim. Cosmochim. Acta 53, 3239–3256. Bird, M.I., Chivas, A.R., 1993. Geomorphic and palaeoclimatic implications of an oxygenisotope chronology for Australian deeply weathered profiles. Aust. J. Earth Sci. 40, 345–358. Bobos, I., Gomes, C., 1998. Greisen and post-greisen alteration in the São Vicente de Pereira kaolinite deposit, Portugal. Can. Mineral. 36, 1615–1624. Bondam, J., Störr, M.T., 1988. Transition in chemical and mineralogical composition around unweathered relicts of granodiorite in the kaolin deposit on Bornholm, Denmark. Bull. Geol. Soc. Den. 37, 117–122. Boulvais, P., Vallet, J.M., Estouelle-Choux, J., Fourcade, S., Martineau, F., 2000. Origin of kaolinization in Brittany (NW France) with deposits over granite: stable isotopes (O, H) constraints. Chem. Geol. 168, 211–223. Burgman, J.O., Calles, B., Westman, F., 1987. Conclusions from a ten year study of oxygen18 in precipitation and runoff in Sweden. Isotope Techniques in Water Resource, Development, IAEA-SM-299/107. 579–590. Chamberlain, C.P., Poage, M.A., Craw, D., Reynolds, R.C., 1999. Topographic development of the Southern Alps recorded by the isotopic composition of authigenic clay minerals, South Island, New Zealand. Chem. Geol. 155, 279–294. Chivas, A.R., Bird, M.I., 1995. Palaeoclimate from Gondwanaland clays. In: Churchman, G.J., Fitzpatrick, R.W., Eggleton, R.A. (Eds.), Clays: Controlling the Environment. Proc. 10th

H.A. Gilg et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 392 (2013) 454–462 Int. Clay Conf., Adelaide, Australia, 1993. CSIRO Publishing, Melbourne, Australia, pp. 333–338. Clayton, R.N., Mayeda, T.K., 1963. The use of bromine pentafluorine in the extraction of oxygen from oxides and silicates for isotopic analysis. Geochim. Cosmochim. Acta 27, 43–52. Cravero, F., Marfil, S.A., Maiza, P.J., 2010. Statistical analysis of geochemical data: a tool for discriminating between kaolin deposits of hypogene and supergene origin, Patagonia, Argentina. Clay Miner. 45, 183–196. Csank, A.Z., Tripati, A.K., Patterson, W.P., Eagle, R.A., Rybczynski, N., Ballantyne, A.P., Eiler, J.M., 2011. Estimates of Arctic land surface temperatures during the Early Pliocene from two novel proxies. Earth Planet. Sci. Lett. 304, 291–299. Eberle, J.J., Greenwood, D.R., 2012. Life at the top of the greenhouse Eocene world — a review of the Eocene flora and vertebrate fauna from Canada's High Arctic. Geol. Soc. Am. Bull. 124, 3–23. Ebert, K., Hall, A.M., Hättestrand, C., 2012. Pre-glacial landforms on a glaciated shield: the inselberg plains of northern Sweden. Nor. J. Geol. 92, 1–17. Eilu, P., 1994. Hydrothermal alteration in volcano-sedimentary rocks in the Central Lapland greenstone belt, Finland. Bull. Geol. Soc. Finl. 374, 1–146. Elo, S., Kuivasaari, T., Lehtinen, M., Sarapää, O., Uutela, A., 1993. Iso-Naakkima, a circular structure filled with Neoproterozoic sediments, Pieksämäki, south-eastern Finland. Bull. Geol. Soc. Finl. 65, 3–30. Feng, W., Yapp, C.J., 2009. 18O/16O and D/H ratios of pedogenic kaolinite in a North American Cenomanian laterite: paleoclimatic implications. Geochim. Cosmochim. Acta 73, 6249–6263. Fernández-Caliani, J.C., Galán, E., Aparicio, P., Miras, A., Márquez, M.G., 2010. Origin and geochemical evolution of the Nuevo Montecastelo kaolin deposit (Galicia, NW Spain). Appl. Clay Sci. 49, 91–97. Flesche Kleiven, H., Jansen, E., Fronval, T., Smith, T., 2002. Intensification of Northern Hemisphere glaciations in the circum Atlantic region (3.5–2.4Ma) — ice-rafted detritus evidence. Palaeogeogr. Palaeoclimatol. Palaeoecol. 184, 213–223. Fricke, H.C., Wing, S.L., 2004. Oxygen isotope and paleobotanical estimates of temperature and δ18O-latitude gradients over North America during the Early Eocene. Am. J. Sci. 304, 612–635. Gilg, H.A., 2000. D–H evidence for the timing of kaolinization in northeast Bavaria, Germany. Chem. Geol. 170, 5–18. Gilg, H.A., 2003. Ein Beitrag zur Isotopengeochemie von Tonmineralen und Tonen. Habilitationschrift. Fakultät Chemie, Technische Universität München. Gilg, H.A., Frei, R., 1997. Isotope dating of residual kaolin deposits in Europe (Tirschenreuth, Germany and St. Yrieix, France). 30th International Geological Congress. VSP International Science Publisher, Beijing. Hall, A.M., Ebert, K., 2013. Cenozoic microfossils in northern Finland: local reworking or distant wind transport? Palaeogeogr. Palaeoclimatol. Palaeoecol. 388, 1–14. Hallam, A., 1985. A review of Mesozoic climates. J. Geol. Soc. 142, 433–445. Hassanipak, A.A., Eslinger, E., 1985. Mineralogy, crystallinity, O18/O16, and D/H of Georgia kaolins. Clay Clay Miner. 33, 99–106. Hendriks, B.W., Andriessen, P.A., 2002. Pattern and timing of the post-Caledonian denudation of northern Scandinavia constrained by apatite fission-track thermochronology. Geol. Soc. Lond. Spec. Publ. 196, 117–137. Hendriks, B., Andriessen, P., Huigen, Y., Leighton, C., Redfield, T., Murrell, G., Gallagher, K., Nielsen, S.B., 2007. A fission track data compilation for Fennoscandia. Nor. J. Geol. 87, 143–155. Hirvas, H., 1991. Pleistocene stratigraphy of Finnish Lapland. Geol. Surv. Finland Bull. 354, 1–123. Jahren, A.H., Sternberg, L.S.L., 2002. Eocene meridional weather patterns reflected in the oxygen Isotopes of Arctic fossil wood. GSA Today 12, 4–9. John, C.M., Banerjee, N.R., Longstaffe, F.J., Sica, C., Law, K.R., Zachos, James C., 2012. Clay assemblage and oxygen isotopic constraints on the weathering response to the Paleocene–Eocene thermal maximum, east coast of North America. Geology 40, 591–594. Kallio, J., Leppäharju, N., Martinkauppi, I., Nousiainen, M., 2011. Geoenergy research and its utilization in Finland. Geol. Surv. Finland Spec. Pap. 49, 179–185. Kennedy, M., Droser, M., Mayer, L.M., Pevear, D., Mrofka, D., 2006. Late Precambrian oxygenation; inception of the clay mineral factory. Science 311, 1446–1449. Kirsimäe, K., Jørgensen, P., 2000. Mineralogical and Rb–Sr isotope studies of lowtemperature diagenesis of Lower Cambrian clays of the Baltic paleobasin of North Estonia. Clay Clay Miner. 48, 95–105. Kirsimäe, K., Jørgensen, P., Kalm, V., 1999. Low-temperature diagenetic illite–smectite in Lower Cambrian clays in North Estonia. Clay Miner. 34, 151–163. Kortelainen, N.M., Karhu, J.A., 2004. Regional and seasonal trends in the oxygen and hydrogen isotope ratios of Finnish groundwaters: a key for mean annual precipitation. J. Hydrol. 285, 143–157. Laajoki, K., 1975. On the stratigraphic position of kaolin in Vayrylankyla, South Puolanka area, Finland. Bull. Geol. Soc. Finl. 47, 83–91. Larsson, L.M., Dybkjær, K., Rasmussen, E.S., Piasecki, S., Utescher, T., Vajda, V., 2011. Miocene climate evolution of northern Europe: a palynological investigation from Denmark. Palaeogeogr. Palaeoclimatol. Palaeoecol. 309, 161–175. Lavrushin, Y.A., Alekseev, M.N., 2005. The Arctic regions. In: Velichko, A.A., Wright, H.E. (Eds.), Cenozoic Climatic and Environmental Changes in Russia. Geological Society of America Special Publications, 382, pp. 13–30. Lawrence, J.R., Rashkes Meaux, J., 1993. The stable isotopic composition of ancient kaolinites of North America. In: Swart, P.K., Lohmann, K.C., McKenzie, J., Savin, S. (Eds.), Climate Change in Continental Isotopic Records. American Geophysical Union, Washington DC, pp. 249–261.

461

Lawrence, J.R., Taylor, H.P., 1972. Hydrogen and oxygen isotope systematics in weathering profiles. Geochim. Cosmochim. Acta 36, 1377–1393. Lee, M.J., Lee, J.I., Garcia, D., Moutte, J., Williams, C.T., Wall, F., Kim, Y., 2006. Pyrochlore chemistry from the Sokli phoscorite–carbonatite complex, Finland: implications for the genesis of phoscorite and carbonatite association. Geochem. J. 40, 1–13. Lidmar-Bergström, K., 1995. Relief and saprolites through time on the Baltic Shield. Geomorphology 12, 45–61. Lidmar-Bergström, 1999. Uplift histories revealed by landforms of the Scandinavian domes. In: Smith, B.J., Whalley, W.B., Warke, P.A. (Eds.), Uplift, Erosion and Stability: Perspectives on Long-Term Landscape Development. Special Publication of the Geological Society, London, 162, pp. 85–91. Lintinen, P., 2006. Kaoliinitutkimukset Sodankylän Kelujärven ympäristössä 2003–2004. Geological Survey of Finland, Report M19/3713,3714,3731,3732/2006/1/82, Espoo, p. 7. Lintinen, P., Al-Ani, T., 2005. The Vittajänkä kaolin deposit, Salla, Finnish Lapland. Geol. Surv. Finland Spec. Pap. 38, 41–47. Lohva, J., Lehtimäki, J., 2005. Geophysical investigation of kaolin and ilmenite deposits in Finland. Geol. Surv. Finland Spec. Pap. 39, 147–154. Longstaffe, F.J., Ayalon, A., 1990. Hydrogen-isotope geochemistry of diagenetic clay minerals from Cretaceous sandstones, Alberta, Canada: evidence for exchange. Appl. Geochem. 5, 657–668. Menegatti, A.P., Früh-Green, G.L., Stille, P., 1999. Removal of organic matter by disodium peroxodisulphate: effects on mineral structure, chemical composition and physicochemical properties of some clay minerals. Clay Miner. 34, 247–257. Migoń, P., Lidmar-Bergström, K., 2001. Weathering mantles and their significance for geomorphological evolution of central and northern Europe since the Mesozoic. Earth-Sci. Rev. 56, 285–324. Mizota, C., Longstaffe, F., 1996. Origin of Cretaceous and Oligocene kaolinites from the Iwaizumi clay deposit, Iwate, northwestern Japan. Clay Clay Miner. 44, 408–416. Mora, G., Pratt, L.M., 2001. Isotopic evidence for cooler and drier conditions in the tropical Andes during the last glacial stage. Geology 29, 519–522. Moran, K., Backman, J., Brinkhuis, H., Clemens, S.C., Cronin, T., Dickens, G.R., Eynaud, F., Gattacceca, J., Jakobsson, M., Jordan, R.W., Kaminski, M., King, J., Koc, N., Krylov, A., Martinez, N., Matthiessen, J., McInroy, D., Moore, T.C., Onodera, J., O'Regan, M., Pälike, H., Rea, B., Rio, D., Sakamoto, T., Smith, D.C., Stein, R., St John, K., Suto, I., Suzuki, N., Takahashi, K., Watanabe, M., Yamamoto, M., Farrell, J., Frank, M., Kubik, P., Jokat, W., Kristoffersen, Y., 2006. The Cenozoic palaeoenvironment of the Arctic Ocean. Nature 441, 601–605. Mordberg, L.E., Nesterova, E.N., 1996. Palaeozoic bauxite deposits of North Onega basin, Russia: evidence as to genesis. Transactions of the Institution of Mining and Metallurgy. Section B. Appl. Earth Sci. 105, 200–205. Naqvi, S.A.A.-E., 2013. Weathering of Precambrian Basement and Formation of Sedimentary Particles in Scania. Department of Geosciences, Oslo 108 (Masters thesis). Pekkala, Y., Yevzerov, V., 1990. Geology and prospects for exploitation of the kaolin deposits in the Eastern Fennoscandian Shield. In: Niini, H.H. (Ed.), Comprehensive Assessment of Nonmetalliferous Deposits: Proceedings of the Finnish–Soviet Symposium. Helsinki University of Technology, Helsinki, Finland, pp. 15–24. Pesonen, L.J., Jarvela, J., Sarapää, O., Pietarinen, H., 1996. The Iso-Naakkima meteorite impact structure: physical properties and paleomagnetism of a drill core. Meteorit. Planet. Sci. Suppl. 31, 105–106. Puura, V., Vaher, R., Miidel, A., 1997. Tectonics. In: Raukas, A., Teedumae, A. (Eds.), Geology and Mineral Resources of Estonia. Estonian Academy Publishers, Tallinn, pp. 163–180. Rask, M., Lintinen, P., 2001. Kaoliinitutkimukset Sodankylän Siurunmaalla vuosina 1978– 1988. Geological Survey of Finland, ReportM19/3713/2001/1/82, Espoo, p. 13. Richter, S., Johnson, A., Dranoff, M., LePage, B., Williams, C., 2008. Oxygen isotope ratios in fossil wood cellulose: isotopic composition of Eocene-to Holocene-aged cellulose. Geochim. Cosmochim. Acta 72, 2744–2753. Rozanski, K., Araguás-Araguás, L., Gonfiantini, G., 1993. Isotopic patterns in modern global precipitation. In: Swart, P., Lohmann, K.C., McKenzie, J., Savin, S.M. (Eds.), Climate Change in Continental Isotopic Records. , 78. American Geophysical Union, Washington D.C., pp. 1–36. Sarapää, O., 1996. Proterozoic primary kaolin deposits at Virtasalmi, southeastern Finland. Geol. Surv. Finland Spec. Publ. 18b, 1–152. Savin, S.M., Epstein, S., 1970. The oxygen and hydrogen isotope geochemistry of clay minerals. Geochim. Cosmochim. Acta 34, 25–42. Savin, S.M., Hsieh, J.C.C., 1998. The hydrogen and oxygen isotope geochemistry of pedogenic clay minerals: principles and theoretical background. Geoderma 82, 227–253. Scotese, C., 1997. Paleomap Project. http://www.scotese.com/. Sheppard, S.M.F., 1977. The Cornubian batholith of SW England: D/H and 18O/16O studies of kaolinite and other alteration minerals. J. Geol. Soc. Lond. 133, 573–591. Sheppard, S.M.F., Gilg, H.A., 1996. Stable isotope geochemistry of clay minerals. Clay Miner. 31, 1–24. Sluijs, A., Schouten, S., Pagani, M., Woltering, M., Brinkhuis, H., Damsté, J.S.S., Dickens, G.R., Huber, M., Reichart, G.-J., Stein, R., Matthiessen, J., Lourens, L.J., Pedentchouk, N., Backman, J., Moran, K., 2006. Subtropical Arctic Ocean temperatures during the Palaeocene/Eocene thermal maximum. Nature 441, 610–613. Söderman, G., 1985. Planation and weathering in eastern Fennoscandia. Fennia 163, 347–352. Störr, M., 2006. Zur Geologie und Genese der Kaoline des Böhmischen Massivs. Z. Geol. Wiss. 6, 389–421. Tabor, N.J., Montañez, I.P., 2005. Oxygen and hydrogen isotope compositions of Permian pedogenic phyllosilicates: development of modern surface domain arrays

462

H.A. Gilg et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 392 (2013) 454–462

and implications for paleotemperature reconstructions. Palaeogeogr. Palaeoclimatol. Palaeoecol. 223, 127–146. Taylor, G., Eggleton, R., Holzhauer, C., Maconachie, L., Gordon, M., Brown, M., McQueen, K., 1992. Cool climate lateritic and bauxitic weathering. J. Geol. 100, 669–677. Thiry, M., 2000. Palaeoclimatic interpretation of clay minerals in marine deposits: an outlook from the continental origin. Earth-Sci. Rev. 49, 201–221. Thomas, M.F., 1995. Models for landform development on passive margins. Some implications for relief development in glaciated areas. Geomorphology 12, 3–15. Tikkanen, M., 2002. The changing landforms of Finland. Fennia 180, 21–30.

Tynni, R., 1982. The reflection of geological evolution in Tertiary and interglacial diatoms and silicoflagellates in Finnish Lapland. Geol. Surv. Finland Bull. 320, 1–64. Weijers, J.W.H., Schouten, S., Sluijs, A., Brinkhuis, H., Sinninghe Damsté, J.S., 2007. Warm arctic continents during the Palaeocene–Eocene thermal maximum. Earth Planet. Sci. Lett. 261, 230–238. Zariņš, K., Johansson, Å., 2009. U–Pb geochronology of gneisses and granitoids from the Danish island of Bornholm: new evidence for 1.47–1.45Ga magmatism at the southwestern margin of the East European Craton. Int. J. Earth Sci. 98, 1561–1580.