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Onset of seafloor spreading in the Iapetus Ocean at 608 Ma: precise age of the Sarek Dyke Swarm, northern Swedish Caledonides
Precambrian Research 110 (2001) 241– 254 www.elsevier.com/locate/precamres
Onset of seafloor spreading in the Iapetus Ocean at 608 Ma: precise age of the Sarek Dyke Swarm, northern Swedish Caledonides O.M. Svenningsen Lamont-Doherty Earth Obser6atory, 61 Route 9W, P.O. Box 1000, Palisades, NY 10964 -8000, USA
Abstract The Sarek Dyke Swarm (SDS) crops out in the Sarektja˚kka˚ Nappe (SN) of the Seve-Kalak Superterrane in the northern Swedish Caledonides. The SN has two main components: (1) a 4 – 5 km thick succession of rift-related sedimentary rocks, which is intruded by (2) a suite of tholeiitic dykes (the SDS) constituting 70 – 80% of the nappe. The nappe was deformed during Caledonian thrusting, but dykes and sedimentary rocks in the interior of the eastern parts of the SN are preserved in a pristine state. The tholeiitic dykes of the SDS commonly occur in sheeted dyke complexes, and up to 11 successive generations can be identified from crosscutting relations. The SN represents the fossil continent–ocean transition between the Baltic craton and the Iapetus Ocean, marking the initiation of seafloor spreading. Bubble-shaped pods and veinlets of diorite are present in the SDS sheeted dyke complexes. The pods are absent in the oldest dykes, but the younger a dyke, the more frequent the pods. The diorite pods are the equivalent of gabbro pegmatites, and both cogenetic and coeval with the dykes. The rapid successive emplacement of tholeiitic magma raised the ambient temperature in the dyke complex, so that crystallization in the youngest dykes mimicked similar processes in gabbro plutons. Six zircon fractions, from the diorite pods including two single grains, were analysed geochronologically by the U–Pb thermal ionization mass spectrometry method. The data yield a linear array of points that are 0.4–0.8% normally discordant, indicating a crystallization age of 608 9 1 Ma (207Pb/206Pb= 607.9 90.7 Ma, MSWD= 0.33). This age is inferred to date the onset of seafloor spreading in the Iapetus Ocean along the Baltoscandian margin. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Continent– ocean transition; Iapetus Ocean; Isotope age dating; Rodinia; Sarek Dyke Swarm; Scandinavian Caledonides
1. Introduction During the Late Proterozoic and Early Palaeozoic, the supercontinent Rodinia broke up E-mail address: [email protected] (O.M. Svenningsen).
in a process that was probably similar to the Mesozoic –Cenozoic break-up of Pangea (Torsvik et al. 1996; Dalziel, 1997). The Iapetus Ocean emerged by continental rifting, progressing to seafloor spreading in approximately the same position as the Atlantic Ocean formed later during the Early Tertiary (Svenningsen, 1996). The Iapetus
0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 ( 0 1 ) 0 0 1 8 9 - 9
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Ocean was obliterated during the subsequent Ordovician– Silurian Caledonian orogeny, but fragments of its oceanic crust and passive margins are scattered throughout Caledonian orogenic belts bordering the present North Atlantic Ocean. Continental break-up along the Baltoscandian margin of the Iapetus Ocean has been dated chiefly through age determinations of mafic intrusive rocks, mostly dykes. The allochthon of the Scandinavian Caledonides contains a large number of mafic intrusive complexes; mostly dense dyke swarms penetrating sedimentary sequences. The Seve-Kalak Superterrane (SKS) is characterized by this kind of rift-related mafic magmatic suite, which is inferred to have originated during and in connection with the rift episode leading to the formation of the Iapetus Ocean (Andre´ asson et al., 1998). The evidence for this connection suffers from the lack of precise dating of most of the dyke swarms. This is partly explained by the considerable technical problems facing anyone wishing to date mafic dykes (Black et al., 1991). Age control is fundamental in providing constraint on the reconstruction of the Baltoscandian margin, or indeed any dismembered passive margin found in an orogen. Dating, primarily from Norway, demonstrates that other dyke swarms in the Caledonian allochthon, disturbingly similar in appearance to the SKS swarms, may be of different age and tectonic setting (Andre´ asson, 1994; Stølen, 1994; Svenningsen, 1995; Andre´ asson et al., 1998). Age alone does not determine the tectonic setting of a dyke swarm, but without reliable ages, any reconstruction based on the occurrence of mafic dyke swarms will be uncertain, at best. In this paper, the crystallization age of one of the most prominent dyke swarms in the Caledonides, the Sarek Dyke Swarm, has been determined with high precision by U–Pb dating of zircons from comagmatic dioritic pods. The age presented here, together with a number of supplementary arguments, provides powerful evidence that the initiation of sea-floor spreading in the Iapetus Ocean occurred during a short period around 620–605 Ma.
2. Geological setting The Sarek Dyke Swarm (SDS) occurs in the Sarektja˚ kka˚ Nappe of the SKS (Andre´ asson et al., 1998) in the allochthon of the northern Swedish Caledonides (Fig. 1). The Caledonides can be subdivided into tectonostratigraphic units containing, from east to west: (A) Early Palaeozoic cover sediments; (B) overthrust Precambrian crystalline rocks originating from the Baltic Shield; (C) rocks representing the passive margin between Baltica and the Iapetus Ocean (the SKS); (D) terranes originating from Iapetus lithosphere; and (E) thrust sheets of Laurentian affinity (Stephens and Gee, 1989; Andre´ asson et al., 1998). The polymetamorphic SKS contains remains of the transition between the continental crust of Baltica and the oceanic crust of the Iapetus Ocean in various states of preservation. Conditions of peak metamorphism in the SKS range from eclogite facies to no visual signs of any penetrative Caledonian overprint. However, the metamorphism follows a pattern: Caledonian garnet amphibolite grade rocks normally characterize the margins of the various thrust sheets in the SKS, whereas their interiors preserve records of older histories. Thrust sheets bearing Late Cambrian– Early Ordovician eclogites (e.g. Mikka, Tsa¨ kkok Nappes) are thus juxtaposed with the Sarektja˚ kka˚ Nappe, displaying perfectly preserved Vendian dolerite dykes intruded into sediments with intact sedimentary structures (Andre´ asson, 1986; Svenningsen, 1993). Evidence of an early, extensional deformation event preceding, but connected to, the dyke intrusion episode abound (Svenningsen, 1994b, 1995). The local geology of the Caledonides in the Sarek area is summarized in Table 1.
2.1. The Sarek Dyke Swarm The Sarektja˚ kka˚ Nappe has two main components: a 4–5 km thick succession of rift-related sedimentary rocks (the Favoritkammen Group (FG); Svenningsen, 1994b), which is penetrated by a suite of tholeiitic dykes (the SDS) constituting 70–80% of the nappe (Fig. 2). The nappe was deformed during Caledonian thrusting, which seg-
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mented the nappe, rotated the segments, and transformed the dyke-sediment rocks in the marginal parts of the nappe into garnet amphibolite and schists. This deformation commonly left the
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interior of the nappe untouched by penetrative strain (cf. Svenningsen, 1994a, 1995). Particularly in the east, very large lenses (up to 30× 8× 1.5 km) of intact dyke swarm within host rock are
Fig. 1. Tectonostratigraphic map of the Scandinavian Caledonides. Place names referred to in the text are indicated, and the study area, the Sarek National Park, is marked by a grey square.
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Table 1 Tectonostratigraphy of the Sarek Mountains Unit
Lithology
Metamorphism
La˚ utak Nappe
Monotonous quartzwacke, eclogite
Eclogite facies
Terrane affinity:
Seve-Kalak Superterrane Sierkavagge Nappe Up to 80–85% amphibolite, garben schist, Pervasive garnet amphibolite grade Seve-Kalak mica schist, graphite-rich marble Superterrane Sarektja˚ kka˚ Nappe 70–80% mafic dykes intruded into quartzite, Garnet amphibolite grade mylonites along Seve-Kalak metapsammite, marble; garnet amphibolite marginal and in Western parts of nappe; Superterrane and schist in marginal parts of nappe no penetrative Caledonian deformation in central Eastern parts Mikka Nappe Quartzite with locally abundant calc–silicate Traces of eclogite facies in central parts Seve-Kalak rock; garnet amphibolite, sometimes of nappe; margins retrogressed to garnet Superterrane retro-eclogitic, mafic rocks locally dominate amphibolite grade mylonitic schists over the metasedimentary rocks Matua˚ lke Nappe Garnet amphibolite with calcite; widespread Garnet amphibolite grade Seve-Kalak calc–silicate alteration of the amphibolite Superterrane Skarja Nappe Feldspar-rich, strongly foliated garnet mica Upper greenschist to amphibolite grade Seve-Kalak schist, very rare boudins of amphibolite Superterrane (without garnet) Akkajaure Nappe Thrust sheets of plutonic Precambrian Greenschist grade; interior of thrust sheets TSMB Complex basement rocks, often with thin cover of often not visibly affected by Caledonian sedimentary rocks metamorphism Lower Allochthon Quartzite, black shale, conglomerate, Low grade Basement metavolcanic rocks Precambrian Gneiss and granite; porphyritic volcanic Generally no Caledonian overprinting Basement crystalline rock, quartzite basement The transition between cratonic rocks derived from the Baltic Shield and Iapetus Ocean is defined as the border between the shortened margin of continent Baltica (TSMB) and the Seve-Kalak Nappe Complex.
preserved. At the sample locality (Fig. 2), the dykes are generally parallel to the bounding thrusts of the approximately 1.5 km thick nappe, which is folded in a large, gentle WNW-trending synform (the A8 par Synform) with a wavelength of 5 –10 km (Svenningsen, 1993). The FG consists of a sedimentary succession at least 4 km thick. Its lowermost formations contain marble and calc– silicate rocks derived from evaporitic protoliths, and shallow-water sediments deposited in mixed continental and marine settings (Svenningsen, 1991). These rocks grade stratigraphically upward into thick monotonous quartzite and metapsammite. Brittle extensional deformation, partly syn-depositional, characterizes the entire FG (Svenningsen, 1995). The dykes are compositionally homogeneous and vary little in chemical composition. They are
MORB tholeiites with a somewhat ambiguous trace-element signature, which fit well with a passive margin setting (Andre´ asson et al., 1992; Svenningsen, 1994a, 1996). Gabbro and olivine leucogabbro have been encountered only as xenoliths, occurring in large numbers in certain dyke generations. Some dykes also show evidence of hydrothermal alteration, which occurred during the dyke emplacement period (Svenningsen, 1994a). A conspicuous feature is the bubble-shaped pods of diorite occurring in the younger dyke generations (cf. Fig. 2).
2.2. Dioritic pods The zircons used for dating were extracted from small pod-like bodies of diorite to quartz diorite in the Sarek dykes. These pods are examples of a
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petrological phenomenon in mafic dykes that has not received much attention in the literature. In some respects, they are similar to the gabbroic pegmatites found in layered gabbro intrusions (Larsen and Brooks, 1994), but they cannot be the exact equivalents for reasons discussed later.
2.2.1. Petrography of the diorite The diorite is normally medium grained and equigranular, commonly with an aplite-like texture. The plagioclase is commonly intergrown with scapolite, which can constitute as much as 25 modal percent of the rock, forming a granophyrelike texture. Quartz, dark green amphibole, twinned clinopyroxene and sulphides are the most common accessory minerals. In the pods, calcite, apatite, and zircon are conspicuous, but in most
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places subordinate. The pods are generally compositionally zoned, commonly with a 5– 15 mm wide rim that is richer in plagioclase than the internal parts.
2.2.2. Field characteristics of diorite pods The pods tend to occur in groups and have not been encountered in the single dykes intruding the sedimentary rocks (cf. Fig. 2). Furthermore, the pods become increasingly common in successively younger dykes. The pods generally have softly rounded, commonly bubble-shaped outlines, in many places narrowing into stems, in some places veins or dykelets, revealing that they are not xenoliths. In contrast to the pods, xenoliths are either roughly circular to elliptical or angular (Fig. 3). In a few places, pods grow into balloon shapes from centimeter-thin, dyke-parallel alteration zones (Fig. 3e). These diapir-like swells tend to occur towards the central parts of the dykes. Thin veins and fractures filled with the diorite– quartz diorite commonly emanate from the pods. These veins both cut and are crosscut by dolerite dyke contacts (Fig. 3c). The pods are everywhere surrounded by a halo of altered host dolerite, in which the original pyroxene is altered to dark green hornblende. This halo varies in thickness, generally between 0.5 and 5 cm. Nowhere do the pods crosscut contacts between dykes, but in some places they straddle the boundaries between different zones (e.g. aphyric and porphyritic) in a dyke, demonstrating that they obtained their final shape after the main body of the dyke solidified (Svenningsen, 1994a).
3. Previous geochronology Fig. 2. Detailed map of the SDS at the sampling locality (Favoritha¨ llen (‘the Favorite Outcrop’) north of the Ruopsokjekna Glacier in the A8 par Massif; described in detail in Svenningsen (1994a, 1995)). In the northern part of the outcrop, dykes cut the sedimentary rocks of the Favoritkammen Group. In the southern part, the dykes form a sheeted complex. Successively younger dykes are indicated by progressively darker shading (youngest is darkest). The dioritic pods are more common in the sheeted complex, and absent in the single dykes emplaced into sedimentary rocks. The effect of Caledonian deformation is minimal, expressed as thin (1 – 5 cm) shear zones.
Svenningsen (1994a) presented a Sm–Nd mineral-whole rock isochron age of 5739 74 Ma (Ndi = 0.51217, mNd = + 5.3), interpreted to represent the crystallization age of the dolerite. The poor precision results from insufficient spread between the end data points in the isochron, caused by differences between the Sm–Nd compositions of plagioclase and pyroxene being too small. This is the only previous dating of the Sarek dykes.
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Fig. 3. Field photographs of dioritic –quartz dioritic pods in the sheeted dyke complex from Favoritha¨ llen (Fig. 2), except F, which shows a diorite pod in the Tertiary Coast –Parallel Dyke Swarm in East Greenland. The scale bar is 10 cm in all photographs except E, where the pencil is 15 cm long. (a) Typical shape and size of the diorite pods. Softly rounded outlines, the irregular bubble shape, and the dark halo distinguish them from xenoliths. The pods tend to occur in groups, but not in any particular location relative to the hosting dyke (e.g. near margins, in centre). The only obvious localizing factor is where pods occur in connection with the thin skarn zones (E). (b) Small group of diorite pods (left) occurring in the aphyric and xenolith-free marginal zone of a xenolith-bearing dyke. Leucogabbro xenoliths in the central part of the dyke are abundant in the right part of the photograph. In spite of the obvious risk for confusion, the mode of occurrence — xenoliths in the central zone of a dyke and pods in scattered irregular groups — as well as the characteristic dark halo, which is absent around xenoliths, makes pods and xenoliths easy to differentiate from each other. (c) Diorite vein and pod cut by the chilled contact of a younger dolerite dyke, demonstrating the coeval emplacement age of the diorite and dolerite. Inset: the diorite pods shown with dotted pattern and the chilled margin as a hatched line, with the hatching towards the younger dyke. (d) Diorite dykelet cutting across dolerite. The dark halo is absent around dykes and veins. (e) Diapir-shaped pod, apparently ‘growing’ from a skarn zone parallel to the dyke margins (and the base of the photo). Thin diorite veins emanate from the pod and from the skarn zone (far left; thin vein is parallel with left side of photo). Similar veins both cut and are crosscut by dolerite dykes (cf. D). (f) Diorite pod in the Tertiary dykes at IC Jacobsen Fjord, East Greenland. The pod has similar dimensions and shape, and shares other features with the SDS pods, most notably the concentration of plagioclase along the rim of the pod and the dark alteration halo. Also, some pods seem to ‘grow’ along dyke-parallel zones.
However, there are numerous datings of mafic intrusive rocks in Scandinavia that have been attributed to represent the opening of the Iapetus Ocean. There is a substantial spread in these ages, and a correlation between ages obtained and method used can be discerned (Fig. 4).
4. Samples and analytical procedure The analytical work was carried out at the thermal ionization mass spectrometer laboratory at the Department of Earth, Atmospheric and Planetary Sciences at Massachusetts Institute of
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Fig. 4. Diagram showing results of different methods of dating of Iapetus rift-related mafic rocks in the Scandinavian Caledonides, grouped according to method used. Analytical precision ( 9 Ma) is indicated with the thinner black bar. Numbers at base of columns indicate cited publications according to the following: 1, Cadow (1993); 2, Mørk and Stabel (1990); 3, Svenningsen (1994a); 4, Zwaan and Van Roermund (1990); 5, Daly et al. (1991); 6, this study; 7, Bingen et al. (1998); 8, Krill (1983); and 9, Claesson and Roddick (1983).
Technology, Cambridge, MA. Zircons were separated from the diorite using standard crushing, Wilfley table, heavy liquids and magnetic methods. The zircons have two main morphological types: one group (A) as rod-like crystals with a large length/width ratio (10:1 or more), commonly containing inclusions and fractures, the second group (B) as unusual, almost rhombic shapes (Fig. 5). The zircons of the second group are commonly exceptionally clear, inclusion-free and without zoning. Only one fraction (2c 2, Fig. 5) of the needle-shaped zircons (group A) was analyzed; the rest belong to group B. All zircons were abraded in air and then washed in warm 30% HNO3. Samples were dissolved in Teflon microcapsules in standard Parr digestion bombs, and spiked before dissolution with a mixed 205Pb – 233U – 235U tracer, and then dissolved in concentrated HF at 220°C for 40– 60 hours. Pb and U were separated using HCl-based ion chromatography procedures, modified after Krogh (1973). Lead was analyzed with conventional thermal ionization mass spectrometry in either static mode using Faraday detectors with the Daly detector in
the axial position to measure 204Pb, or using a Daly detector in the ion-counting mode. Uranium was analyzed in the static, multicollector mode using Faraday collectors. Additional analytical details are included in Table 2.
Fig. 5. The two morphological zircon groups from the diorite pods at Favoritha¨ llen. (a) The needle or rod-shaped zircons of group A; and (b) the unusual crystal shapes and exceptionally clear grains of group B. The large crystal in B constituted the single-zircon fraction 0 c0 (see Table 2).
248
Sample Number fractiona of grains
0c 0 0 c1 1c 2 2 c1A 2 c1B 2 c2 a
1 16 20 1 6 25
Weight (mg)
27.5 20.6 17.8 86.3 12.5 25.4
Composition
Atomic ratios
U (p.p.m.)b
Pb (p.p.m.)b
Pb(c) (pg)c
206
433.37 792.41 787.68 121.86 600.37 528.48
40.65 75.93 77.00 11.52 58.28 53.25
20.5 23.8 44.0 25.3 28.3 43.0
3580.99 4228.68 1962.73 2567.68 1643.60 1923.30
204
Pb/
d
Pb
208
Ages (Ma) 206
Pb/
0.03270 0.05610 0.06600 0.03210 0.04860 0.09910
e
Pb
206
238
Pb/
0.09833 0.09855 0.09798 0.09849 0.09838 0.09811
f
U
g
%err
207
235
0.09 0.13 0.08 0.08 0.1 0.1
0.81515 0.18710 0.81194 0.81653 0.81519 0.81320
Pb/
f
U
g
%err
207
206
0.12 0.15 0.12 0.12 0.16 0.14
0.0601 0.0601 0.0601 0.0601 0.0601 0.0601
Pb/
f
Pb
%err
0.08 0.08 0.08 0.08 0.11 0.09
g
206
Pb/238U
207
Pb/235U
207
Pb/206Pb Err ( 9Ma)h
604.6 605.9 602.6 605.5 604.9 603.3
605.3 606.4 603.5 606.1 605.4 604.2
608.0 608.4 607.3 608.3 607.0 607.9
1.7 1.7 1.7 1.7 2.5 1.8
All fractions are air-abraded. Expressed as p.p.m. U and p.p.m. radiogenic Pb. Picograms of common Pb. d Measured ratio corrected for fractionation; Pb fractionation correction is 0.12 9 0.04% per atomic mass unit (a.m.u.) for multicollector analyses or 0.15 90.04% per a.m.u. for single collector analyses. e Radiogenic Pb. f Corrected for fractionation, spike, blank, and initial common Pb; U blank =1 pg9 50%, Pb blank = 3.5 pg 950%; initial common Pb composition calculated from Stacey and Kramers (1975) using the interpreted age of the sample. g Errors are reported in percent at the two-sigma confidence interval. h Two-sigma error in the 207Pb/206Pb age in million years. b c
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Table 2 U–Pb data for zircons from diorite pods in the SDS
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Fig. 6. U – Pb concordia diagram of zircon analyses from Table 2. Note that the concordia is enlarged due to the small scatter of data points. The points are near-concordant (0.4 –0.8% discordance). Additional analytical information is given in the text and in Table 2.
5. Results Six fractions, including two single-grain analyses, were analyzed and the results are listed in Table 1 and Fig. 6. The data form a linear array of points that are 0.4– 0.8% normally discordant. The fraction with the group A zircon is indistinguishable from the other fractions. Linear regression using the York 1 algorithm of Ludwig (1989) gives an upper intercept age of 609.1+ 30/ − 2.1 Ma and a lower intercept 1889 368 Ma (MSWD =0.18). The uncertainty in the upper intercept age is given by the intersection between the error envelope and the concordia, and the low angle between this and the best fit line results in the highly asymmetric errors. The large uncertainty in the lower intercept results from the tight clustering of data points very close to the concordia (Fig. 6). There is little reason to assign any geological significance to the lower intercept age. The overlap of the lower intercept with the origin allows the use of the
weighted mean of the 207Pb/206Pb age of 607.99 0.7 Ma (MSWD=0.33) instead of relying on the asymmetric error in the upper intercept age. Both the upper intercept age and the weighted mean of the 207Pb/206Pb ages are statistically valid and both yield the same age within uncertainty. It is therefore argued that the crystallization age of the zircons is 6089 1 Ma.
6. Discussion The interpretation of the 608 Ma age obtained from the SDS as representing the onset of seafloor spreading in the Iapetus Ocean requires a discussion of the following questions. 1. Does the age represent the intrusion age of the dolerite dykes? 2. Did the Sarek Dyke Swarm originate as a continent–ocean transition and, if so … 3. Can the SDS be connected to the opening of the Iapetus Ocean?
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6.1. Does the age represent the intrusion age of the dolerite? The dioritic pods in the Sarek dykes are in many respects similar to the gabbro pegmatites described from the Skaergaard intrusion in East Greenland (Larsen and Brooks, 1994). These gabbro pegmatites formed by the migration, growth, and trapping of intercumulus liquids from the main magma. However, this genetic model can hardly be used for the dioritic pods in the SDS, since they occur in dykes, which have a different crystallization history compared with gabbro plutons. Still, the increasing number of diorite pods in progressively younger dykes (Fig. 2) indicates a relationship between the pods and the thermal history of the dyke complex. The following model is proposed. The petrology of the contacts between dykes in the Sarek sheeted dyke complex indicate that the difference in temperature between intruding magma and the dolerite wallrock generally was small, in the order of at most 200°C (Huppert and Sparks, 1989; Svenningsen, 1994a). This in turn suggests that most of the SDS dykes were emplaced within a short time — not more than a few years between intrusions. Considering this emplacement history of the SDS, it is evident that the dyke emplacement is commonly localized to the centres of the evolving sheeted dyke complexes. Thermal softening from older, still warm intrusions may have helped. This is also indicated by the dominance of dykes intruded into the central part of older dykes; intrusions following the contacts between dykes have not been observed in the SDS, except locally. The rapid succession of dyke intrusions must have increased the heat content of the dyke complex, retarded the cooling, and thus also the crystallization rates, of the younger dykes. This resulted in a prolonged stage of late-magmatic processes for the later dykes — compared with ‘normal’, single dykes — mimicking those in layered mafic plutons (for example, Nicolas, 1992; McBirney, 1995; McBirney and Nicolas, 1997). The retarded late crystallization history of the younger dykes produced an interstitial liquid of dioritic to quartz dioritic composition, similar to late intercumulus liquids in gabbros. Na– Cl-rich
fluids emanating from the meta-evaporite wallrock apparently migrated deep into the dyke complex and contaminated the interstitial melt, as indicated by the high scapolite content in many pods and in the spots affected by hydrothermal alteration (Svenningsen, 1994a). The fluids from the metaevaporites may also have lowered the melting temperature of the interstitial melt, prolonging the crystallization further. The late-magmatic liquid migrated either along fractures, and is locally preserved as thin diorite veins, or penetrated along narrow zones of enhanced permeability, oriented parallel to the contacts of the hosting dyke. These migration channels are preserved as skarn zones, normally in lateral parts of the central zones of dykes (cf. Figs. 2 and 3e). The zircons are thus inferred to have crystallized from an interstitial magmatic liquid during the late stages of the formation of the dyke swarm. There is no indication, either from field, microscopic or analytical data, of any subsequent growth of zircon or Pb loss/gain, and the age obtained is thus interpreted to represent the age of the zircons and thus also the SDS. Regardless of whether this model is valid, field data demonstrate that the pods crystallized during the formation of the sheeted dykes in the SDS (Svenningsen, 1994a; see also Fig. 3). In addition, the unusual morphology of the zircons may be interpreted as a result of rapid cooling compared with zircons found in plutonic rocks (Vavra, 1993), which would support the model for pod formation proposed here. The 608 9 1 Ma age falls within error of the previously obtained Sm– Nd age of 5739 74 Ma, but the U–Pb age is considered a better indication of the true age of the dykes, because of the greater stability of the U–Pb system.
6.2. Did the SDS originate in a continent–ocean transition? Based on a number of arguments, Svenningsen (1994a,b, 1995) suggested that the emplacement of the SDS marked the onset of seafloor spreading in the Iapetus Ocean. The structure and composition of the SDS is remarkably similar to that of the
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coast-parallel dyke swarm (CPDS) of East Greenland, emplaced during Early Tertiary opening of the Atlantic Ocean (Nielsen, 1987). For the following reasons, a similar setting of a rift evolving into seafloor spreading is still considered the most probable origin of the Sarek dykes: All the key features typical of the SDS are present within the CPDS; for example, at Fladø Island, the dyke density is 70– 80% and most of the dykes are parallel; however, there is a gradual change in orientation with age. In East Greenland, this can be attributed to the evolution of the so-called Coastal Flexure, in which the crust bends in a large monocline towards the continent – ocean transition in response to crustal thinning (Nielsen, 1978; Myers, 1980; Klausen et al., 1996). This change in orientation of the dykes is not always gradual, and thinner dykes with deviating orientations (up to 60° both in SDS and the CPDS) of intermediate age are conspicuous, but subordinate, components of the dyke swarm. In both dyke swarms, brittle normal faulting was most active during the early stages of dyke emplacement. Faulting probably remained active throughout the intrusive period, but was obscured by the intense magmatic activity (Svenningsen, 1995; Karson et al., 1998). Fladø Island, the structural analogue of the SDS, is situated within a few kilometers of the first magnetic spreading anomaly in the Atlantic oceanic crust (Larsen, 1990). Initiation of seafloor spreading was probably very close in time, if not contemporaneous with the emplacement of the CPDS; most of the magmatic activity along the East Greenland volcanic margin occurred within a couple of million years (Tegner et al., 1998). In addition to the arguments put forward by Svenningsen (1994a,b, 1995), the variation in dyke density from 50 to 100% of dyke material (average around 80%), in the SDS, and the sharing of most of the structural features with the CPDS (and with slow-to-medium-spreading oceanic crust; Karson et al., 1998), are the basis for arguing that the most likely original tectonic setting was a continent –ocean transition, similar, but not identical to the East Greenland margin (the CPDS shows evidence of influence of a mantle plume, not seen in Sarek; see Svenningsen 1996). If the Iapetus
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Ocean opened later than around 580–610 Ma, the very widespread mafic magmatism, the deposition of thick sediments in rift basins and seafloorspreading processes along the 2000 km of the Scandinavian Caledonides must represent another ocean, for which there is no other independent evidence. The question to be asked is: Why would these processes start, stop, then begin again the second time, without leaving a trace in the geologic record?
6.3. Why the Iapetus Ocean? The age of the Late Precambrian rifting along the Baltoscandian margin is still indicated only by a few geographically scattered radiometric ages (cf. Fig. 4 and references). Judging from these, the rifting occurred at different times in various parts of the orogen; in the central Scandinavian Caledonides, ages from ca. 745 to ca. 545 Ma have been attributed to Iapetus rifting (Andre´ asson et al., 1998; Andre´ asson, 1994), whereas in the north, ages cluster between 610 and 570 Ma. If this age difference is real, the different dyke swarms may not represent the same event, as dense dyke swarms related to continental splitting are generally emplaced rapidly, within less than 10 million years. Even though magmatic activity can be very long-lived in rifts, voluminous intrusive activity related to one event over an extended (several tens of million years) period is, at best, rare. The Iapetus-rift age data from Scandinavia can be broken down to sets, depending on the method used; ages within the U–Pb set do not scatter over large time intervals, whereas other methods do (Fig. 4). The oldest ages have been obtained with methods that may not have been optimally suited for dating the age of intrusion of the dykes. The K–Ar/Ar –Ar age of 6659 10 Ma (Claesson and Roddick, 1983) and the 7459 37 Ma age based on results from Rb– Sr biotite (Krill, 1983), although technically very good, need critical re-evaluation of the significance of the data and/or testing with other dating methods. If they are found to date the intrusion of the Sa¨ rv dykes, these dykes probably do not represent the opening of the Iapetus. Recent U–Pb dating on baddeleyite of the Egersund dykes, connected to the Iapetus rifting
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in southern Norway, indicates an intrusion age of 61693 Ma (Bingen et al., 1998). This dyke swarm is not as dense as the SDS, and may have been emplaced during an earlier phase of rifting, whereas the slightly younger Sarek dykes were intruded during the peak of continental splitting. Similar Sm–Nd ages are also reported from Iapetus-rift related rocks in northern Norway: 5829 30 Ma from the Corrovare dykes (Zwaan and Van Roermund, 1990) and 6129 33 Ma from Seiland gabbros (Daly et al., 1991). The latter date is considered by Daly et al. (1991) to represent the age of the majority of the intrusive rocks of Seiland Igneous Province. As more high-quality age data are obtained, there is a tendency for ages connected to the Iapetus opening along the Baltoscandian margin to cluster around 620– 605 Ma. While there is commonly some discrepancy between Sm– Nd ages and U– Pb ages, for the SDS the Sm –Nd age agrees within error with the U –Pb ages. Minor resetting of the Sm– Nd system during later, presumably Caledonian influence may explain this discrepancy in some cases, as in Sarek. There is compelling evidence for a major rifting event, progressing into seafloor spreading along the Baltoscandian margin, and isotopic data indicate that this event occurred around 620– 605 Ma. Indications of subduction (eclogite facies metamorphism) along the same margin have been dated at ca. 500 Ma (Andre´ asson and Albrecht, 1995); this leaves 100 million years for the Iapetus Ocean to open and close, which is more than enough for a major ocean basin. However, the Iapetus appears to have been an ocean with a long and complex history. Onset of ocean floor spreading along other adjacent rifts in the Rodinia supercontinent occurred at different times. Along the southern arm of Iapetus (now represented in the North American Appalachian Range) rifting occurred in several episodes, 760– 700, 620–590, and 570– 550 Ma, the latter representing opening of the ‘southern’ arm of Iapetus (Cawood et al., 1999). However, the ocean facing the Baltoscandian margin was by definition, as originally proposed by Harland and Gayer (1972), the Iapetus Ocean. All evidence indicates that the SDS and contem-
poraneous mafic igneous rocks of the Scandinavian Caledonides represent the passive margin of Baltoscandia, and consequently that ocean should be the Iapetus Ocean.
7. Conclusions Widespread rifting and the inception of seafloor spreading in the Iapetus Ocean along the Baltoscandian margin occurred during the Vendian. The SDS represents an ancient, fragmented, obducted and overthrust continent–ocean transition, which retains a detailed record of the geological events during continental break-up. This extraordinary geological setting provides a unique possibility of dating the onset of seafloor spreading, providing the age of the dykes can be determined. The diorite pods in the dykes of the central parts of sheeted complexes represent a variety of late magmatic bodies, similar, but not identical to gabbro pegmatites. The diorite crystallized from an interstitial liquid, contaminated with fluids from the sedimentary host rock, during the late stages of solidification of the youngest dolerite dykes. The same kinds of pod exist in other sheeted complexes, e.g. the Tertiary CPDS of East Greenland, but have probably been overlooked or interpreted as xenoliths, since no description of similar phenomena has been found in the literature. Zircons extracted from the diorite pods thus gives the age of crystallization of the dykes, and the U– Pb data define a near-concordant age. The near-concordance and the absence of a lower intercept allow the use of the weighted mean of the 207Pb/206Pb ages, which is 607.99 0.7 Ma, indicating an age of 6089 1 Ma. This is the age of the diorite pods, the dyke swarm and thus the onset of seafloor spreading in the Iapetus Ocean.
Acknowledgements Sam Bowring and David Hawkins at MIT gave invaluable help with sample preparation, analytical work and insights into the U–Pb dating
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method. Ulf So¨ derlund and Gunilla Otterberg assisted with the fieldwork, and the Danish Lithosphere Centre in Copenhagen, Denmark gave the author an opportunity to study the Late Tertiary East Greenland volcanic margin. The work was supported financially by grants from the Swedish Institute, the Royal Physiographic Society in Lund, the Swedish Royal Academy of Science, and the Swedish Natural Science Research Council, which are gratefully acknowledged. This paper is a contribution to IGCP 440: Assembly and Breakup of Rodinia. References Andre´ asson, P.G., 1986. The Sarektja˚ kka˚ Nappe, Seve Terranes of the northern Swedish Caledonides. Geol. Fo¨ ren. Stockholm Fo¨ rha. 108, 263 –266. Andre´ asson, P.G., 1994. The Baltoscandian Margin in Neoproterozoic– Early Palaeozoic time. Some constraints on terrane derivation and accretion in the Arctic Scandinavian Caledonides. Tectonophysics 231 (1/3), 1 –32. Andre´ asson, P.-G., Albrecht, L., 1995. Derivation of 500 Ma eclogites from the passive margin of Baltica and a note on the tectonometamorphic heterogeneity of eclogite-bearing crust. Geol. Mag. 132 (6), 729 –738. Andre´ asson, P.G., Svenningsen, O., Johansson, I., Solyom, Z., Xiaodan, T., 1992. Mafic dyke swarms of the Baltica –Iapetus transition, Seve Nappe Complex of the Sarek Mts., Swedish Caledonides. Geol. Fo¨ ren. Stockholm Fo¨ rha. 114, 31– 45. Andre´ asson, P.-G., Svenningsen, O.M., Albrecht, L., 1998. Dawn of Phanerozoic orogeny in the North Atlantic Tract; evidence from the Seve-Kalak Superterrane, Scandinavian Caledonides. Geol. Fo¨ ren. Stockholm Fo¨ rha. 120 (2), 159 – 172. Bingen, B., Demaiffe, D., van Breemen, O., 1998. The 616 Ma old Egersund basaltic dike swarm, SW Norway, and Late Neoproterozoic opening of the Iapetus Ocean. J. Geol. 106, 565 – 574. Black, L.P., Kinny, P.D., Sheraton, J.W., 1991. The difficulties of dating mafic dykes: an Antarctic example. Contrib. Mineral. Petrol. 109, 183 –194. Cadow, R., 1993. Sm – Nd and Rb–Sr ages of hornblende clinopyroxenite and metagabbro from the Lillebukt Alkaline complex, Seiland Igneous Province. Norsk Geol. Tidsskr. Issn 73 (4), 243 –249. Cawood, P.A., McCausland, P.J.A., Dunning, G.R., 1999. Rift related magmatism associated with Rodinian breakup and opening Iapetus in the northeastern Appalachians. J. Conf. Abs. 4 (1), 121. Claesson, S., Roddick, J.C., 1983. 40Ar– 39Ar data on the age and metamorphism of the Ottfja¨ llet dolerites, Sa¨ rv Nappe, Swedish Caledonides. Lithos 16, 61 –73.
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Daly, J.S., Aitcheson, S.J., Cliff, R.A., Gayer, R.A., Rice, A.H.N., 1991. Geochronological evidence from discordant plutons for a late Proterozoic orogen in the Caledonides of Finnmark, northern Norway. J. Geol. Soc. London 148, 29 – 40. Dalziel, I.W.D., 1997. Neoproterozoic – Palaeozoic geography and tectonics: review, hypothesis, environmental speculation. Bull. Geol. Soc. Am. 109 (1), 16 – 42. Harland, W.B., Gayer, R.A., 1972. The Arctic Caledonides and earlier oceans. Geol. Mag. 109, 289 – 384. Huppert, H.E., Sparks, R.S.J., 1989. Chilled margins in igneous rocks. Earth Planet. Sci. Lett. 92, 397 – 405. Karson, J.A., Brooks, C.K., Storey, M., Pringle, M.S., 1998. Tertiary faulting and pseudotachylytes in the East Greenland volcanic rifted margin: Seismogenic faulting during magmatic construction. Geology 26 (1), 39 – 42. Klausen, M.B., Brooks, C.K., Nielsen, T.F.D., Karson, J.A., Svenningsen, O.M., 1996. Infrastructure of the coast — parallel dike swarm along the rifted continental margin of southeast Greenland. EOS Trans. Am. Geophys. Un. 77 (46), 827. Krill, A.G., 1983. Rb – Sr study of dolerite dikes and psammite from the Western Gneiss Region of Norway. Lithos 16, 85 –93. Krogh, T.E., 1973. A low contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determination. Geochim. Cosmochim. Acta 37, 488 – 494. Larsen, H.C., 1990. The East Greenland Shelf. In: Grantz, A., Johnson, L., Sweeney, J.F. (Eds.), The Arctic Ocean Region. The Geology of North America. Geological Society of America, Boulder, CO, pp. 185 – 210. Larsen, R.B., Brooks, C.K., 1994. Origin and evolution of gabbroic pegmatites in the Skaergaard Intrusion, East Greenland. J. Petrol. 35 (6), 1651 – 1679. Ludwig, K.R., 1989. PB.Dat: a computer program for processing raw Pb – U– Th isotope data. Open File Report 88-557. US Geological Survey. McBirney, A.R., 1995. Mechanisms of differentiation in the Skaergaard Intrusion. J. Geol. Soc. London 152, 421 – 435. McBirney, A.R., Nicolas, A., 1997. The Skaergaard layered series 2. Magmatic flow and dynamic layering. J. Petrol. 38 (5), 569 – 580. Mørk, M.B.E., Stabel, A., 1990. Cambrian Sm – Nd dates for an Ultramafic Intrusion and for High-Grade Metamorphism on the Oksfjord Peninsula, Finnmark, North Norway. Norsk Geol. Tidsskr. Issn 70 (4), 275 – 291. Myers, J.S., 1980. Structure of the coastal dyke swarm and associated plutonic intrusions of East Greenland. Earth Planet. Sci. Lett. 46, 407 – 418. Nicolas, A., 1992. Kinematics in magmatic rocks with special reference to gabbros. J. Petrol. 33, 891 – 915. Nielsen, T.F.D., 1978. The Tertiary dike swarms of the Kangerdlugssuaq area, East Greenland. Contrib. Mineral. Petrol. 67 (1), 63 – 78. Nielsen, T.F.D., 1987. Mafic dyke swarms in Greenland: a review. In: Halls, H.C., Fahrig, W.F. (Eds.), Mafic Dyke
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Swarms. Geological Association of Canada, Special Paper 34, pp. 349 – 360. Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planet. Sci. Lett. 26 (2), 207 – 221. Stephens, M.B., Gee, D.G., 1989. Terranes and polyphase accretionary history in the Scandinavian Caledonides. Geol. Soc. Am. Spec. Pap. 230, 17 –30. Stølen, L.-K., 1994. The rift-related mafic dyke complex of the Rohkunborri Nappe, Indre Troms, northern Norwegian Caledonides. Norsk Geol. Tidsskr. 74 (1), 35 – 47. Svenningsen, O., 1991. Tectonic setting and evaporitic origin of the magnesite-dolomite-calcsilicate Spika Formation, Sarektja˚ kka˚ Nappe, northern Swedish Caledonides. TERRA NOVA Abs. Suppl. 4, 30. Svenningsen, O.M., 1993. Tectonic evolution of the Sarektja˚ kka˚ Nappe, northern Swedish Caledonides. Doctoral Thesis, Lund University, 190 pp. Svenningsen, O.M., 1994a. The Baltica – Iapetus passive margin dyke complex in the Sarektja˚ kka˚ Nappe, northern Swedish Caledonides. Geol. J. 29 (4), 323 – 354. Svenningsen, O.M., 1994b. Tectonic significance of the metaevaporitic magnesite and scapolite deposits in the Seve Nappes, Sarek Mts., Swedish Caledonides. Tectonophysics 231 (1/3), 33 – 44.
Svenningsen, O.M., 1995. Extensional deformation along the Late Precambrian – Cambrian Baltoscandian passive margin: the Sarektja˚ kka˚ Nappe, Swedish Caledonides. Geol. Rundsch. 84, 649 – 664. Svenningsen, O.M., 1996. Passive margins — past and (almost) present, Geol. Fo¨ ren. Stockholm Fo¨ rha. 118 (Jubilee Issue), A41 – A42. Tegner, C., Duncan, R.A., Bernstein, S., Brooks, C.K., Bird, D.K., Storey, M., 1998. 40Ar– 39Ar geochronology of Tertiary mafic intrusions along the East Greenland rifted margin: Relation to flood basalts and the Iceland hotspot track. Earth Planet. Sci. Lett. 156 (1), 75 – 88. Torsvik, T.H., Smethurst, M., Meert, J., Van der Voo, R., McKerrow, W., Brasier, M., Sturt, B., Walderhaug, H., 1996. Continental break-up and collision in the Neoproterozoic and Palaeozoic — a tale of Baltica and Laurentia. Earth-Sci. Rev. 40 (3/4), 229 – 258. Vavra, G., 1993. A guide to quantitative morphology of accessory zircon. Chem. Geol. 110, 15 – 28. Zwaan, B.Z., Van Roermund, H.L.M., 1990. A rift-related mafic dyke swarm in the Corrovarre Nappe of the Caledonian Middle Allochthon, Troms, North Norway, and its tectonometamorphic evolution. Norges Geol. Under. Bull. 419, 25 – 44.
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Onset of seafloor spreading in the Iapetus Ocean at 608 Ma: precise age of the Sarek Dyke Swarm, northern Swedish Caledonides O.M. Svenningsen Lamont-Doherty Earth Obser6atory, 61 Route 9W, P.O. Box 1000, Palisades, NY 10964 -8000, USA
Abstract The Sarek Dyke Swarm (SDS) crops out in the Sarektja˚kka˚ Nappe (SN) of the Seve-Kalak Superterrane in the northern Swedish Caledonides. The SN has two main components: (1) a 4 – 5 km thick succession of rift-related sedimentary rocks, which is intruded by (2) a suite of tholeiitic dykes (the SDS) constituting 70 – 80% of the nappe. The nappe was deformed during Caledonian thrusting, but dykes and sedimentary rocks in the interior of the eastern parts of the SN are preserved in a pristine state. The tholeiitic dykes of the SDS commonly occur in sheeted dyke complexes, and up to 11 successive generations can be identified from crosscutting relations. The SN represents the fossil continent–ocean transition between the Baltic craton and the Iapetus Ocean, marking the initiation of seafloor spreading. Bubble-shaped pods and veinlets of diorite are present in the SDS sheeted dyke complexes. The pods are absent in the oldest dykes, but the younger a dyke, the more frequent the pods. The diorite pods are the equivalent of gabbro pegmatites, and both cogenetic and coeval with the dykes. The rapid successive emplacement of tholeiitic magma raised the ambient temperature in the dyke complex, so that crystallization in the youngest dykes mimicked similar processes in gabbro plutons. Six zircon fractions, from the diorite pods including two single grains, were analysed geochronologically by the U–Pb thermal ionization mass spectrometry method. The data yield a linear array of points that are 0.4–0.8% normally discordant, indicating a crystallization age of 608 9 1 Ma (207Pb/206Pb= 607.9 90.7 Ma, MSWD= 0.33). This age is inferred to date the onset of seafloor spreading in the Iapetus Ocean along the Baltoscandian margin. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Continent– ocean transition; Iapetus Ocean; Isotope age dating; Rodinia; Sarek Dyke Swarm; Scandinavian Caledonides
1. Introduction During the Late Proterozoic and Early Palaeozoic, the supercontinent Rodinia broke up E-mail address: [email protected] (O.M. Svenningsen).
in a process that was probably similar to the Mesozoic –Cenozoic break-up of Pangea (Torsvik et al. 1996; Dalziel, 1997). The Iapetus Ocean emerged by continental rifting, progressing to seafloor spreading in approximately the same position as the Atlantic Ocean formed later during the Early Tertiary (Svenningsen, 1996). The Iapetus
0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 ( 0 1 ) 0 0 1 8 9 - 9
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Ocean was obliterated during the subsequent Ordovician– Silurian Caledonian orogeny, but fragments of its oceanic crust and passive margins are scattered throughout Caledonian orogenic belts bordering the present North Atlantic Ocean. Continental break-up along the Baltoscandian margin of the Iapetus Ocean has been dated chiefly through age determinations of mafic intrusive rocks, mostly dykes. The allochthon of the Scandinavian Caledonides contains a large number of mafic intrusive complexes; mostly dense dyke swarms penetrating sedimentary sequences. The Seve-Kalak Superterrane (SKS) is characterized by this kind of rift-related mafic magmatic suite, which is inferred to have originated during and in connection with the rift episode leading to the formation of the Iapetus Ocean (Andre´ asson et al., 1998). The evidence for this connection suffers from the lack of precise dating of most of the dyke swarms. This is partly explained by the considerable technical problems facing anyone wishing to date mafic dykes (Black et al., 1991). Age control is fundamental in providing constraint on the reconstruction of the Baltoscandian margin, or indeed any dismembered passive margin found in an orogen. Dating, primarily from Norway, demonstrates that other dyke swarms in the Caledonian allochthon, disturbingly similar in appearance to the SKS swarms, may be of different age and tectonic setting (Andre´ asson, 1994; Stølen, 1994; Svenningsen, 1995; Andre´ asson et al., 1998). Age alone does not determine the tectonic setting of a dyke swarm, but without reliable ages, any reconstruction based on the occurrence of mafic dyke swarms will be uncertain, at best. In this paper, the crystallization age of one of the most prominent dyke swarms in the Caledonides, the Sarek Dyke Swarm, has been determined with high precision by U–Pb dating of zircons from comagmatic dioritic pods. The age presented here, together with a number of supplementary arguments, provides powerful evidence that the initiation of sea-floor spreading in the Iapetus Ocean occurred during a short period around 620–605 Ma.
2. Geological setting The Sarek Dyke Swarm (SDS) occurs in the Sarektja˚ kka˚ Nappe of the SKS (Andre´ asson et al., 1998) in the allochthon of the northern Swedish Caledonides (Fig. 1). The Caledonides can be subdivided into tectonostratigraphic units containing, from east to west: (A) Early Palaeozoic cover sediments; (B) overthrust Precambrian crystalline rocks originating from the Baltic Shield; (C) rocks representing the passive margin between Baltica and the Iapetus Ocean (the SKS); (D) terranes originating from Iapetus lithosphere; and (E) thrust sheets of Laurentian affinity (Stephens and Gee, 1989; Andre´ asson et al., 1998). The polymetamorphic SKS contains remains of the transition between the continental crust of Baltica and the oceanic crust of the Iapetus Ocean in various states of preservation. Conditions of peak metamorphism in the SKS range from eclogite facies to no visual signs of any penetrative Caledonian overprint. However, the metamorphism follows a pattern: Caledonian garnet amphibolite grade rocks normally characterize the margins of the various thrust sheets in the SKS, whereas their interiors preserve records of older histories. Thrust sheets bearing Late Cambrian– Early Ordovician eclogites (e.g. Mikka, Tsa¨ kkok Nappes) are thus juxtaposed with the Sarektja˚ kka˚ Nappe, displaying perfectly preserved Vendian dolerite dykes intruded into sediments with intact sedimentary structures (Andre´ asson, 1986; Svenningsen, 1993). Evidence of an early, extensional deformation event preceding, but connected to, the dyke intrusion episode abound (Svenningsen, 1994b, 1995). The local geology of the Caledonides in the Sarek area is summarized in Table 1.
2.1. The Sarek Dyke Swarm The Sarektja˚ kka˚ Nappe has two main components: a 4–5 km thick succession of rift-related sedimentary rocks (the Favoritkammen Group (FG); Svenningsen, 1994b), which is penetrated by a suite of tholeiitic dykes (the SDS) constituting 70–80% of the nappe (Fig. 2). The nappe was deformed during Caledonian thrusting, which seg-
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mented the nappe, rotated the segments, and transformed the dyke-sediment rocks in the marginal parts of the nappe into garnet amphibolite and schists. This deformation commonly left the
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interior of the nappe untouched by penetrative strain (cf. Svenningsen, 1994a, 1995). Particularly in the east, very large lenses (up to 30× 8× 1.5 km) of intact dyke swarm within host rock are
Fig. 1. Tectonostratigraphic map of the Scandinavian Caledonides. Place names referred to in the text are indicated, and the study area, the Sarek National Park, is marked by a grey square.
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Table 1 Tectonostratigraphy of the Sarek Mountains Unit
Lithology
Metamorphism
La˚ utak Nappe
Monotonous quartzwacke, eclogite
Eclogite facies
Terrane affinity:
Seve-Kalak Superterrane Sierkavagge Nappe Up to 80–85% amphibolite, garben schist, Pervasive garnet amphibolite grade Seve-Kalak mica schist, graphite-rich marble Superterrane Sarektja˚ kka˚ Nappe 70–80% mafic dykes intruded into quartzite, Garnet amphibolite grade mylonites along Seve-Kalak metapsammite, marble; garnet amphibolite marginal and in Western parts of nappe; Superterrane and schist in marginal parts of nappe no penetrative Caledonian deformation in central Eastern parts Mikka Nappe Quartzite with locally abundant calc–silicate Traces of eclogite facies in central parts Seve-Kalak rock; garnet amphibolite, sometimes of nappe; margins retrogressed to garnet Superterrane retro-eclogitic, mafic rocks locally dominate amphibolite grade mylonitic schists over the metasedimentary rocks Matua˚ lke Nappe Garnet amphibolite with calcite; widespread Garnet amphibolite grade Seve-Kalak calc–silicate alteration of the amphibolite Superterrane Skarja Nappe Feldspar-rich, strongly foliated garnet mica Upper greenschist to amphibolite grade Seve-Kalak schist, very rare boudins of amphibolite Superterrane (without garnet) Akkajaure Nappe Thrust sheets of plutonic Precambrian Greenschist grade; interior of thrust sheets TSMB Complex basement rocks, often with thin cover of often not visibly affected by Caledonian sedimentary rocks metamorphism Lower Allochthon Quartzite, black shale, conglomerate, Low grade Basement metavolcanic rocks Precambrian Gneiss and granite; porphyritic volcanic Generally no Caledonian overprinting Basement crystalline rock, quartzite basement The transition between cratonic rocks derived from the Baltic Shield and Iapetus Ocean is defined as the border between the shortened margin of continent Baltica (TSMB) and the Seve-Kalak Nappe Complex.
preserved. At the sample locality (Fig. 2), the dykes are generally parallel to the bounding thrusts of the approximately 1.5 km thick nappe, which is folded in a large, gentle WNW-trending synform (the A8 par Synform) with a wavelength of 5 –10 km (Svenningsen, 1993). The FG consists of a sedimentary succession at least 4 km thick. Its lowermost formations contain marble and calc– silicate rocks derived from evaporitic protoliths, and shallow-water sediments deposited in mixed continental and marine settings (Svenningsen, 1991). These rocks grade stratigraphically upward into thick monotonous quartzite and metapsammite. Brittle extensional deformation, partly syn-depositional, characterizes the entire FG (Svenningsen, 1995). The dykes are compositionally homogeneous and vary little in chemical composition. They are
MORB tholeiites with a somewhat ambiguous trace-element signature, which fit well with a passive margin setting (Andre´ asson et al., 1992; Svenningsen, 1994a, 1996). Gabbro and olivine leucogabbro have been encountered only as xenoliths, occurring in large numbers in certain dyke generations. Some dykes also show evidence of hydrothermal alteration, which occurred during the dyke emplacement period (Svenningsen, 1994a). A conspicuous feature is the bubble-shaped pods of diorite occurring in the younger dyke generations (cf. Fig. 2).
2.2. Dioritic pods The zircons used for dating were extracted from small pod-like bodies of diorite to quartz diorite in the Sarek dykes. These pods are examples of a
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petrological phenomenon in mafic dykes that has not received much attention in the literature. In some respects, they are similar to the gabbroic pegmatites found in layered gabbro intrusions (Larsen and Brooks, 1994), but they cannot be the exact equivalents for reasons discussed later.
2.2.1. Petrography of the diorite The diorite is normally medium grained and equigranular, commonly with an aplite-like texture. The plagioclase is commonly intergrown with scapolite, which can constitute as much as 25 modal percent of the rock, forming a granophyrelike texture. Quartz, dark green amphibole, twinned clinopyroxene and sulphides are the most common accessory minerals. In the pods, calcite, apatite, and zircon are conspicuous, but in most
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places subordinate. The pods are generally compositionally zoned, commonly with a 5– 15 mm wide rim that is richer in plagioclase than the internal parts.
2.2.2. Field characteristics of diorite pods The pods tend to occur in groups and have not been encountered in the single dykes intruding the sedimentary rocks (cf. Fig. 2). Furthermore, the pods become increasingly common in successively younger dykes. The pods generally have softly rounded, commonly bubble-shaped outlines, in many places narrowing into stems, in some places veins or dykelets, revealing that they are not xenoliths. In contrast to the pods, xenoliths are either roughly circular to elliptical or angular (Fig. 3). In a few places, pods grow into balloon shapes from centimeter-thin, dyke-parallel alteration zones (Fig. 3e). These diapir-like swells tend to occur towards the central parts of the dykes. Thin veins and fractures filled with the diorite– quartz diorite commonly emanate from the pods. These veins both cut and are crosscut by dolerite dyke contacts (Fig. 3c). The pods are everywhere surrounded by a halo of altered host dolerite, in which the original pyroxene is altered to dark green hornblende. This halo varies in thickness, generally between 0.5 and 5 cm. Nowhere do the pods crosscut contacts between dykes, but in some places they straddle the boundaries between different zones (e.g. aphyric and porphyritic) in a dyke, demonstrating that they obtained their final shape after the main body of the dyke solidified (Svenningsen, 1994a).
3. Previous geochronology Fig. 2. Detailed map of the SDS at the sampling locality (Favoritha¨ llen (‘the Favorite Outcrop’) north of the Ruopsokjekna Glacier in the A8 par Massif; described in detail in Svenningsen (1994a, 1995)). In the northern part of the outcrop, dykes cut the sedimentary rocks of the Favoritkammen Group. In the southern part, the dykes form a sheeted complex. Successively younger dykes are indicated by progressively darker shading (youngest is darkest). The dioritic pods are more common in the sheeted complex, and absent in the single dykes emplaced into sedimentary rocks. The effect of Caledonian deformation is minimal, expressed as thin (1 – 5 cm) shear zones.
Svenningsen (1994a) presented a Sm–Nd mineral-whole rock isochron age of 5739 74 Ma (Ndi = 0.51217, mNd = + 5.3), interpreted to represent the crystallization age of the dolerite. The poor precision results from insufficient spread between the end data points in the isochron, caused by differences between the Sm–Nd compositions of plagioclase and pyroxene being too small. This is the only previous dating of the Sarek dykes.
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Fig. 3. Field photographs of dioritic –quartz dioritic pods in the sheeted dyke complex from Favoritha¨ llen (Fig. 2), except F, which shows a diorite pod in the Tertiary Coast –Parallel Dyke Swarm in East Greenland. The scale bar is 10 cm in all photographs except E, where the pencil is 15 cm long. (a) Typical shape and size of the diorite pods. Softly rounded outlines, the irregular bubble shape, and the dark halo distinguish them from xenoliths. The pods tend to occur in groups, but not in any particular location relative to the hosting dyke (e.g. near margins, in centre). The only obvious localizing factor is where pods occur in connection with the thin skarn zones (E). (b) Small group of diorite pods (left) occurring in the aphyric and xenolith-free marginal zone of a xenolith-bearing dyke. Leucogabbro xenoliths in the central part of the dyke are abundant in the right part of the photograph. In spite of the obvious risk for confusion, the mode of occurrence — xenoliths in the central zone of a dyke and pods in scattered irregular groups — as well as the characteristic dark halo, which is absent around xenoliths, makes pods and xenoliths easy to differentiate from each other. (c) Diorite vein and pod cut by the chilled contact of a younger dolerite dyke, demonstrating the coeval emplacement age of the diorite and dolerite. Inset: the diorite pods shown with dotted pattern and the chilled margin as a hatched line, with the hatching towards the younger dyke. (d) Diorite dykelet cutting across dolerite. The dark halo is absent around dykes and veins. (e) Diapir-shaped pod, apparently ‘growing’ from a skarn zone parallel to the dyke margins (and the base of the photo). Thin diorite veins emanate from the pod and from the skarn zone (far left; thin vein is parallel with left side of photo). Similar veins both cut and are crosscut by dolerite dykes (cf. D). (f) Diorite pod in the Tertiary dykes at IC Jacobsen Fjord, East Greenland. The pod has similar dimensions and shape, and shares other features with the SDS pods, most notably the concentration of plagioclase along the rim of the pod and the dark alteration halo. Also, some pods seem to ‘grow’ along dyke-parallel zones.
However, there are numerous datings of mafic intrusive rocks in Scandinavia that have been attributed to represent the opening of the Iapetus Ocean. There is a substantial spread in these ages, and a correlation between ages obtained and method used can be discerned (Fig. 4).
4. Samples and analytical procedure The analytical work was carried out at the thermal ionization mass spectrometer laboratory at the Department of Earth, Atmospheric and Planetary Sciences at Massachusetts Institute of
O.M. S6enningsen / Precambrian Research 110 (2001) 241–254
247
Fig. 4. Diagram showing results of different methods of dating of Iapetus rift-related mafic rocks in the Scandinavian Caledonides, grouped according to method used. Analytical precision ( 9 Ma) is indicated with the thinner black bar. Numbers at base of columns indicate cited publications according to the following: 1, Cadow (1993); 2, Mørk and Stabel (1990); 3, Svenningsen (1994a); 4, Zwaan and Van Roermund (1990); 5, Daly et al. (1991); 6, this study; 7, Bingen et al. (1998); 8, Krill (1983); and 9, Claesson and Roddick (1983).
Technology, Cambridge, MA. Zircons were separated from the diorite using standard crushing, Wilfley table, heavy liquids and magnetic methods. The zircons have two main morphological types: one group (A) as rod-like crystals with a large length/width ratio (10:1 or more), commonly containing inclusions and fractures, the second group (B) as unusual, almost rhombic shapes (Fig. 5). The zircons of the second group are commonly exceptionally clear, inclusion-free and without zoning. Only one fraction (2c 2, Fig. 5) of the needle-shaped zircons (group A) was analyzed; the rest belong to group B. All zircons were abraded in air and then washed in warm 30% HNO3. Samples were dissolved in Teflon microcapsules in standard Parr digestion bombs, and spiked before dissolution with a mixed 205Pb – 233U – 235U tracer, and then dissolved in concentrated HF at 220°C for 40– 60 hours. Pb and U were separated using HCl-based ion chromatography procedures, modified after Krogh (1973). Lead was analyzed with conventional thermal ionization mass spectrometry in either static mode using Faraday detectors with the Daly detector in
the axial position to measure 204Pb, or using a Daly detector in the ion-counting mode. Uranium was analyzed in the static, multicollector mode using Faraday collectors. Additional analytical details are included in Table 2.
Fig. 5. The two morphological zircon groups from the diorite pods at Favoritha¨ llen. (a) The needle or rod-shaped zircons of group A; and (b) the unusual crystal shapes and exceptionally clear grains of group B. The large crystal in B constituted the single-zircon fraction 0 c0 (see Table 2).
248
Sample Number fractiona of grains
0c 0 0 c1 1c 2 2 c1A 2 c1B 2 c2 a
1 16 20 1 6 25
Weight (mg)
27.5 20.6 17.8 86.3 12.5 25.4
Composition
Atomic ratios
U (p.p.m.)b
Pb (p.p.m.)b
Pb(c) (pg)c
206
433.37 792.41 787.68 121.86 600.37 528.48
40.65 75.93 77.00 11.52 58.28 53.25
20.5 23.8 44.0 25.3 28.3 43.0
3580.99 4228.68 1962.73 2567.68 1643.60 1923.30
204
Pb/
d
Pb
208
Ages (Ma) 206
Pb/
0.03270 0.05610 0.06600 0.03210 0.04860 0.09910
e
Pb
206
238
Pb/
0.09833 0.09855 0.09798 0.09849 0.09838 0.09811
f
U
g
%err
207
235
0.09 0.13 0.08 0.08 0.1 0.1
0.81515 0.18710 0.81194 0.81653 0.81519 0.81320
Pb/
f
U
g
%err
207
206
0.12 0.15 0.12 0.12 0.16 0.14
0.0601 0.0601 0.0601 0.0601 0.0601 0.0601
Pb/
f
Pb
%err
0.08 0.08 0.08 0.08 0.11 0.09
g
206
Pb/238U
207
Pb/235U
207
Pb/206Pb Err ( 9Ma)h
604.6 605.9 602.6 605.5 604.9 603.3
605.3 606.4 603.5 606.1 605.4 604.2
608.0 608.4 607.3 608.3 607.0 607.9
1.7 1.7 1.7 1.7 2.5 1.8
All fractions are air-abraded. Expressed as p.p.m. U and p.p.m. radiogenic Pb. Picograms of common Pb. d Measured ratio corrected for fractionation; Pb fractionation correction is 0.12 9 0.04% per atomic mass unit (a.m.u.) for multicollector analyses or 0.15 90.04% per a.m.u. for single collector analyses. e Radiogenic Pb. f Corrected for fractionation, spike, blank, and initial common Pb; U blank =1 pg9 50%, Pb blank = 3.5 pg 950%; initial common Pb composition calculated from Stacey and Kramers (1975) using the interpreted age of the sample. g Errors are reported in percent at the two-sigma confidence interval. h Two-sigma error in the 207Pb/206Pb age in million years. b c
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Table 2 U–Pb data for zircons from diorite pods in the SDS
O.M. S6enningsen / Precambrian Research 110 (2001) 241–254
249
Fig. 6. U – Pb concordia diagram of zircon analyses from Table 2. Note that the concordia is enlarged due to the small scatter of data points. The points are near-concordant (0.4 –0.8% discordance). Additional analytical information is given in the text and in Table 2.
5. Results Six fractions, including two single-grain analyses, were analyzed and the results are listed in Table 1 and Fig. 6. The data form a linear array of points that are 0.4– 0.8% normally discordant. The fraction with the group A zircon is indistinguishable from the other fractions. Linear regression using the York 1 algorithm of Ludwig (1989) gives an upper intercept age of 609.1+ 30/ − 2.1 Ma and a lower intercept 1889 368 Ma (MSWD =0.18). The uncertainty in the upper intercept age is given by the intersection between the error envelope and the concordia, and the low angle between this and the best fit line results in the highly asymmetric errors. The large uncertainty in the lower intercept results from the tight clustering of data points very close to the concordia (Fig. 6). There is little reason to assign any geological significance to the lower intercept age. The overlap of the lower intercept with the origin allows the use of the
weighted mean of the 207Pb/206Pb age of 607.99 0.7 Ma (MSWD=0.33) instead of relying on the asymmetric error in the upper intercept age. Both the upper intercept age and the weighted mean of the 207Pb/206Pb ages are statistically valid and both yield the same age within uncertainty. It is therefore argued that the crystallization age of the zircons is 6089 1 Ma.
6. Discussion The interpretation of the 608 Ma age obtained from the SDS as representing the onset of seafloor spreading in the Iapetus Ocean requires a discussion of the following questions. 1. Does the age represent the intrusion age of the dolerite dykes? 2. Did the Sarek Dyke Swarm originate as a continent–ocean transition and, if so … 3. Can the SDS be connected to the opening of the Iapetus Ocean?
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6.1. Does the age represent the intrusion age of the dolerite? The dioritic pods in the Sarek dykes are in many respects similar to the gabbro pegmatites described from the Skaergaard intrusion in East Greenland (Larsen and Brooks, 1994). These gabbro pegmatites formed by the migration, growth, and trapping of intercumulus liquids from the main magma. However, this genetic model can hardly be used for the dioritic pods in the SDS, since they occur in dykes, which have a different crystallization history compared with gabbro plutons. Still, the increasing number of diorite pods in progressively younger dykes (Fig. 2) indicates a relationship between the pods and the thermal history of the dyke complex. The following model is proposed. The petrology of the contacts between dykes in the Sarek sheeted dyke complex indicate that the difference in temperature between intruding magma and the dolerite wallrock generally was small, in the order of at most 200°C (Huppert and Sparks, 1989; Svenningsen, 1994a). This in turn suggests that most of the SDS dykes were emplaced within a short time — not more than a few years between intrusions. Considering this emplacement history of the SDS, it is evident that the dyke emplacement is commonly localized to the centres of the evolving sheeted dyke complexes. Thermal softening from older, still warm intrusions may have helped. This is also indicated by the dominance of dykes intruded into the central part of older dykes; intrusions following the contacts between dykes have not been observed in the SDS, except locally. The rapid succession of dyke intrusions must have increased the heat content of the dyke complex, retarded the cooling, and thus also the crystallization rates, of the younger dykes. This resulted in a prolonged stage of late-magmatic processes for the later dykes — compared with ‘normal’, single dykes — mimicking those in layered mafic plutons (for example, Nicolas, 1992; McBirney, 1995; McBirney and Nicolas, 1997). The retarded late crystallization history of the younger dykes produced an interstitial liquid of dioritic to quartz dioritic composition, similar to late intercumulus liquids in gabbros. Na– Cl-rich
fluids emanating from the meta-evaporite wallrock apparently migrated deep into the dyke complex and contaminated the interstitial melt, as indicated by the high scapolite content in many pods and in the spots affected by hydrothermal alteration (Svenningsen, 1994a). The fluids from the metaevaporites may also have lowered the melting temperature of the interstitial melt, prolonging the crystallization further. The late-magmatic liquid migrated either along fractures, and is locally preserved as thin diorite veins, or penetrated along narrow zones of enhanced permeability, oriented parallel to the contacts of the hosting dyke. These migration channels are preserved as skarn zones, normally in lateral parts of the central zones of dykes (cf. Figs. 2 and 3e). The zircons are thus inferred to have crystallized from an interstitial magmatic liquid during the late stages of the formation of the dyke swarm. There is no indication, either from field, microscopic or analytical data, of any subsequent growth of zircon or Pb loss/gain, and the age obtained is thus interpreted to represent the age of the zircons and thus also the SDS. Regardless of whether this model is valid, field data demonstrate that the pods crystallized during the formation of the sheeted dykes in the SDS (Svenningsen, 1994a; see also Fig. 3). In addition, the unusual morphology of the zircons may be interpreted as a result of rapid cooling compared with zircons found in plutonic rocks (Vavra, 1993), which would support the model for pod formation proposed here. The 608 9 1 Ma age falls within error of the previously obtained Sm– Nd age of 5739 74 Ma, but the U–Pb age is considered a better indication of the true age of the dykes, because of the greater stability of the U–Pb system.
6.2. Did the SDS originate in a continent–ocean transition? Based on a number of arguments, Svenningsen (1994a,b, 1995) suggested that the emplacement of the SDS marked the onset of seafloor spreading in the Iapetus Ocean. The structure and composition of the SDS is remarkably similar to that of the
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coast-parallel dyke swarm (CPDS) of East Greenland, emplaced during Early Tertiary opening of the Atlantic Ocean (Nielsen, 1987). For the following reasons, a similar setting of a rift evolving into seafloor spreading is still considered the most probable origin of the Sarek dykes: All the key features typical of the SDS are present within the CPDS; for example, at Fladø Island, the dyke density is 70– 80% and most of the dykes are parallel; however, there is a gradual change in orientation with age. In East Greenland, this can be attributed to the evolution of the so-called Coastal Flexure, in which the crust bends in a large monocline towards the continent – ocean transition in response to crustal thinning (Nielsen, 1978; Myers, 1980; Klausen et al., 1996). This change in orientation of the dykes is not always gradual, and thinner dykes with deviating orientations (up to 60° both in SDS and the CPDS) of intermediate age are conspicuous, but subordinate, components of the dyke swarm. In both dyke swarms, brittle normal faulting was most active during the early stages of dyke emplacement. Faulting probably remained active throughout the intrusive period, but was obscured by the intense magmatic activity (Svenningsen, 1995; Karson et al., 1998). Fladø Island, the structural analogue of the SDS, is situated within a few kilometers of the first magnetic spreading anomaly in the Atlantic oceanic crust (Larsen, 1990). Initiation of seafloor spreading was probably very close in time, if not contemporaneous with the emplacement of the CPDS; most of the magmatic activity along the East Greenland volcanic margin occurred within a couple of million years (Tegner et al., 1998). In addition to the arguments put forward by Svenningsen (1994a,b, 1995), the variation in dyke density from 50 to 100% of dyke material (average around 80%), in the SDS, and the sharing of most of the structural features with the CPDS (and with slow-to-medium-spreading oceanic crust; Karson et al., 1998), are the basis for arguing that the most likely original tectonic setting was a continent –ocean transition, similar, but not identical to the East Greenland margin (the CPDS shows evidence of influence of a mantle plume, not seen in Sarek; see Svenningsen 1996). If the Iapetus
251
Ocean opened later than around 580–610 Ma, the very widespread mafic magmatism, the deposition of thick sediments in rift basins and seafloorspreading processes along the 2000 km of the Scandinavian Caledonides must represent another ocean, for which there is no other independent evidence. The question to be asked is: Why would these processes start, stop, then begin again the second time, without leaving a trace in the geologic record?
6.3. Why the Iapetus Ocean? The age of the Late Precambrian rifting along the Baltoscandian margin is still indicated only by a few geographically scattered radiometric ages (cf. Fig. 4 and references). Judging from these, the rifting occurred at different times in various parts of the orogen; in the central Scandinavian Caledonides, ages from ca. 745 to ca. 545 Ma have been attributed to Iapetus rifting (Andre´ asson et al., 1998; Andre´ asson, 1994), whereas in the north, ages cluster between 610 and 570 Ma. If this age difference is real, the different dyke swarms may not represent the same event, as dense dyke swarms related to continental splitting are generally emplaced rapidly, within less than 10 million years. Even though magmatic activity can be very long-lived in rifts, voluminous intrusive activity related to one event over an extended (several tens of million years) period is, at best, rare. The Iapetus-rift age data from Scandinavia can be broken down to sets, depending on the method used; ages within the U–Pb set do not scatter over large time intervals, whereas other methods do (Fig. 4). The oldest ages have been obtained with methods that may not have been optimally suited for dating the age of intrusion of the dykes. The K–Ar/Ar –Ar age of 6659 10 Ma (Claesson and Roddick, 1983) and the 7459 37 Ma age based on results from Rb– Sr biotite (Krill, 1983), although technically very good, need critical re-evaluation of the significance of the data and/or testing with other dating methods. If they are found to date the intrusion of the Sa¨ rv dykes, these dykes probably do not represent the opening of the Iapetus. Recent U–Pb dating on baddeleyite of the Egersund dykes, connected to the Iapetus rifting
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in southern Norway, indicates an intrusion age of 61693 Ma (Bingen et al., 1998). This dyke swarm is not as dense as the SDS, and may have been emplaced during an earlier phase of rifting, whereas the slightly younger Sarek dykes were intruded during the peak of continental splitting. Similar Sm–Nd ages are also reported from Iapetus-rift related rocks in northern Norway: 5829 30 Ma from the Corrovare dykes (Zwaan and Van Roermund, 1990) and 6129 33 Ma from Seiland gabbros (Daly et al., 1991). The latter date is considered by Daly et al. (1991) to represent the age of the majority of the intrusive rocks of Seiland Igneous Province. As more high-quality age data are obtained, there is a tendency for ages connected to the Iapetus opening along the Baltoscandian margin to cluster around 620– 605 Ma. While there is commonly some discrepancy between Sm– Nd ages and U– Pb ages, for the SDS the Sm –Nd age agrees within error with the U –Pb ages. Minor resetting of the Sm– Nd system during later, presumably Caledonian influence may explain this discrepancy in some cases, as in Sarek. There is compelling evidence for a major rifting event, progressing into seafloor spreading along the Baltoscandian margin, and isotopic data indicate that this event occurred around 620– 605 Ma. Indications of subduction (eclogite facies metamorphism) along the same margin have been dated at ca. 500 Ma (Andre´ asson and Albrecht, 1995); this leaves 100 million years for the Iapetus Ocean to open and close, which is more than enough for a major ocean basin. However, the Iapetus appears to have been an ocean with a long and complex history. Onset of ocean floor spreading along other adjacent rifts in the Rodinia supercontinent occurred at different times. Along the southern arm of Iapetus (now represented in the North American Appalachian Range) rifting occurred in several episodes, 760– 700, 620–590, and 570– 550 Ma, the latter representing opening of the ‘southern’ arm of Iapetus (Cawood et al., 1999). However, the ocean facing the Baltoscandian margin was by definition, as originally proposed by Harland and Gayer (1972), the Iapetus Ocean. All evidence indicates that the SDS and contem-
poraneous mafic igneous rocks of the Scandinavian Caledonides represent the passive margin of Baltoscandia, and consequently that ocean should be the Iapetus Ocean.
7. Conclusions Widespread rifting and the inception of seafloor spreading in the Iapetus Ocean along the Baltoscandian margin occurred during the Vendian. The SDS represents an ancient, fragmented, obducted and overthrust continent–ocean transition, which retains a detailed record of the geological events during continental break-up. This extraordinary geological setting provides a unique possibility of dating the onset of seafloor spreading, providing the age of the dykes can be determined. The diorite pods in the dykes of the central parts of sheeted complexes represent a variety of late magmatic bodies, similar, but not identical to gabbro pegmatites. The diorite crystallized from an interstitial liquid, contaminated with fluids from the sedimentary host rock, during the late stages of solidification of the youngest dolerite dykes. The same kinds of pod exist in other sheeted complexes, e.g. the Tertiary CPDS of East Greenland, but have probably been overlooked or interpreted as xenoliths, since no description of similar phenomena has been found in the literature. Zircons extracted from the diorite pods thus gives the age of crystallization of the dykes, and the U– Pb data define a near-concordant age. The near-concordance and the absence of a lower intercept allow the use of the weighted mean of the 207Pb/206Pb ages, which is 607.99 0.7 Ma, indicating an age of 6089 1 Ma. This is the age of the diorite pods, the dyke swarm and thus the onset of seafloor spreading in the Iapetus Ocean.
Acknowledgements Sam Bowring and David Hawkins at MIT gave invaluable help with sample preparation, analytical work and insights into the U–Pb dating
O.M. S6enningsen / Precambrian Research 110 (2001) 241–254
method. Ulf So¨ derlund and Gunilla Otterberg assisted with the fieldwork, and the Danish Lithosphere Centre in Copenhagen, Denmark gave the author an opportunity to study the Late Tertiary East Greenland volcanic margin. The work was supported financially by grants from the Swedish Institute, the Royal Physiographic Society in Lund, the Swedish Royal Academy of Science, and the Swedish Natural Science Research Council, which are gratefully acknowledged. This paper is a contribution to IGCP 440: Assembly and Breakup of Rodinia. References Andre´ asson, P.G., 1986. The Sarektja˚ kka˚ Nappe, Seve Terranes of the northern Swedish Caledonides. Geol. Fo¨ ren. Stockholm Fo¨ rha. 108, 263 –266. Andre´ asson, P.G., 1994. The Baltoscandian Margin in Neoproterozoic– Early Palaeozoic time. Some constraints on terrane derivation and accretion in the Arctic Scandinavian Caledonides. Tectonophysics 231 (1/3), 1 –32. Andre´ asson, P.-G., Albrecht, L., 1995. Derivation of 500 Ma eclogites from the passive margin of Baltica and a note on the tectonometamorphic heterogeneity of eclogite-bearing crust. Geol. Mag. 132 (6), 729 –738. Andre´ asson, P.G., Svenningsen, O., Johansson, I., Solyom, Z., Xiaodan, T., 1992. Mafic dyke swarms of the Baltica –Iapetus transition, Seve Nappe Complex of the Sarek Mts., Swedish Caledonides. Geol. Fo¨ ren. Stockholm Fo¨ rha. 114, 31– 45. Andre´ asson, P.-G., Svenningsen, O.M., Albrecht, L., 1998. Dawn of Phanerozoic orogeny in the North Atlantic Tract; evidence from the Seve-Kalak Superterrane, Scandinavian Caledonides. Geol. Fo¨ ren. Stockholm Fo¨ rha. 120 (2), 159 – 172. Bingen, B., Demaiffe, D., van Breemen, O., 1998. The 616 Ma old Egersund basaltic dike swarm, SW Norway, and Late Neoproterozoic opening of the Iapetus Ocean. J. Geol. 106, 565 – 574. Black, L.P., Kinny, P.D., Sheraton, J.W., 1991. The difficulties of dating mafic dykes: an Antarctic example. Contrib. Mineral. Petrol. 109, 183 –194. Cadow, R., 1993. Sm – Nd and Rb–Sr ages of hornblende clinopyroxenite and metagabbro from the Lillebukt Alkaline complex, Seiland Igneous Province. Norsk Geol. Tidsskr. Issn 73 (4), 243 –249. Cawood, P.A., McCausland, P.J.A., Dunning, G.R., 1999. Rift related magmatism associated with Rodinian breakup and opening Iapetus in the northeastern Appalachians. J. Conf. Abs. 4 (1), 121. Claesson, S., Roddick, J.C., 1983. 40Ar– 39Ar data on the age and metamorphism of the Ottfja¨ llet dolerites, Sa¨ rv Nappe, Swedish Caledonides. Lithos 16, 61 –73.
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