Journal of Petrology Advance Access originally published online on July 22, 2004
Journal of Petrology 2004 45(9):1799-1819; doi:10.1093/petrology/egh034
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Journal of Petrology 45(9) © Oxford University Press 2004; all rights reserved
U–Pb Age, Setting and Tectonic Significance of the Anorthosite–Mangerite–Charnockite–Granite Suite, Lofoten–Vesterålen, Norway
DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF OSLO, PB 1047 BLINDERN, N-0316 OSLO, NORWAY
RECEIVED APRIL 28, 2003; ACCEPTED MARCH 26, 2004
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGPREVIOUS GEOCHRONOLOGYANALYTICAL APPROACHU-Pb RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Mangerites, charnockites, anorthosites, gabbros and granites occur within a high-grade metamorphic complex in the Lofoten–Vesterålen islands of northern Norway. U–Pb dating of zircon, titanite and monazite indicates a three-stage magmatic history beginning at 1870–1860 Ma with the emplacement of the Lødingen and Hopen plutons, followed by a dominant stage at 1800–1790 Ma that formed the bulk of the suite, and concluded by the emplacement of pegmatites, local rehydration and retrogression between 1790 and 1770 Ma. On the scale of the Baltic Shield the 1870–1860 Ma episode corresponds to contraction, amalgamation of arcs, and regional deformation. By contrast, the episode at 1800–1790 Ma was characterized by major shifts in plate convergence, by intraplate deformation, and by a diversity of magmatic associations including suites derived from the subcontinental mantle and widespread granitoid rocks extracted from the continental crust. The diversity of concurrent magmatic events across the Svecofennian orogen, and the temporal coincidence with collisional events in coeval orogenic belts, suggests that the genesis of the suite of magmatic rocks may have been related to tectonically driven mechanisms of magma generation.
KEY WORDS: anorthosite–mangerite–charnockite–granite; lithospheric processes; Lofoten–Vesterålen; Svecofennian orogen; U–Pb geochronology
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPREVIOUS GEOCHRONOLOGYANALYTICAL APPROACHU-Pb RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Anorthosite–mangerite–charnockite–granite (AMCG) magmatic suites are a typical, and nearly exclusive, feature of Proterozoic crust. There has been much debate concerning their genesis, the nature of the parental melts, the extent of crust and mantle involvement in the generation of the magmas, the tectonic setting, and the larger-scale mechanisms controlling these processes [see reviews by Ashwal (1993)
, Windley (1993), Emslie et al. (1994) and Bédard (2001), and references therein]. To improve our understanding of the origin of AMCG complexes it is useful to examine these questions within the broader temporal and spatial context of crustal evolution (e.g. Corrigan & Hanmer, 1997). The Baltic Shield provides a particularly interesting ground for this kind of discussion because it grew through intermittent episodes over a period of more than 1 billion years. This long evolution, which included two major orogenic cycles (Svecofennian and Sveconorwegian) separated by a variety of magmatic, metamorphic and deformational processes, contributed to the addition of large amounts of new crust to the original Archaean nucleus (e.g. Gaál & Gorbatschev, 1987; Åhäll et al., 2000). This study is focused on the AMCG suite exposed in the Lofoten–Vesterålen islands of northern Norway (Fig. 1). New U–Pb ages are reported for the dominant geological elements of the complex and their significance is evaluated in the context of crustal growth in the Svecofennian orogen in an attempt to provide further insights into the questions discussed above.
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GEOLOGICAL SETTING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPREVIOUS GEOCHRONOLOGYANALYTICAL APPROACHU-Pb RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The Lofoten–Vesterålen islands are underlain by Precambrian basement, which emerges from beneath the stack of allochthonous nappes occupying the central, NNE-trending axis of the Scandinavian Caledonides (Fig. 1). This basement high stretches to the north into the Western Troms area. The position is also similar to that of the Western Gneiss Region in central and southern Norway. The rocks in Lofoten–Vesterålen, however, display only subordinate evidence of the Caledonian deformation that intensely overprinted the southern regions (Griffin et al., 1978
; Tull et al., 1985; Tucker et al., 1990). The weak Caledonian overprint in Lofoten–Vesterålen has promoted discussions on the nature of the links between these rocks and the autochthonous basement east of the Caledonides. A parautochthonous to allochthonous setting of Lofoten–Vesterålen has been considered by some workers (Brueckner, 1971; Tull, 1977; Hodges et al., 1982) whereas others, on the basis of geological similarities and large-scale magnetic anomalies, have proposed that the Lofoten–Vesterålen basement is directly linked to the Baltic Shield below the nappes (Griffin et al., 1978; Olesen et al., 1997).
Archaean crust occupies large parts of the islands of Langøy and Hinnøy (Fig. 2). Small remnants may also be present at the southwestern tip of Austvågøy (Wade, 1985), whereas the Archaean age previously suggested for the gneisses on Moskenesøy (Green & Jorde, 1971) has not been supported by subsequent Nd isotopic studies (Jacobsen & Wasserburg, 1978; Wade, 1985). The Archaean rocks of Langøy consist mainly of high-grade gneisses that have been interpreted to represent supracrustal rocks of intermediate composition, migmatized in the Late Archaean, and then metamorphosed at granulite-facies conditions in the Proterozoic (Griffin et al., 1978). The appearance of orthopyroxene to the west of the amphibolite-facies domain on Hinnøy has been interpreted as a prograde metamorphic transition (Fig. 2; Heier, 1960; Griffin et al., 1978). On Hinnøy the Archaean domain is dominated mostly by tonalitic to granitic rocks with local greenstone belt remnants.
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A younger sequence of paragneisses is dominated by fine-grained gneisses, locally quartzitic, with subordinate graphite schist, iron formation and marble. Although they have been overprinted by the same high-grade metamorphism as the Archaean migmatites the paragneisses are of more homogeneous appearance and have been interpreted to represent a Palaeoproterozoic supracrustal succession, which has been thought to be of volcanogenic derivation based on geochemical compositions (Griffin et al., 1978
).
These early assemblages were intruded by plutons of the AMCG suite, which occupies about 50% of the islands (Malm & Ormaasen, 1978). The suite is dominated by mangerite and charnockite and their retrograded equivalents, with local but important occurrences of gabbro, anorthosite and granite. The mafic and felsic phases locally grade into, or mutually cross-cut, each other indicating that some of the intrusions may be broadly genetically related. Markl (2001) proposed a multistage evolution with a broadly basaltic parental melt undergoing polybaric crystallization and differentiation to form anorthosites as cumulates and ferrodiorites and jotunites as residual melts. The mangerites and charnockites are inferred to represent feldspar cumulates and residual liquids, respectively, derived from magmas broadly syenitic in composition, which may or may not be directly related to the ferrodiorites. Isotopic data show that Archaean crust played an important role in the genesis of these rocks, both as a source and as a contaminant (Wade, 1985; Markl & Höhndorf, 2003). The available Pb isotopic data define a linear trend, interpreted to represent mixing between Archaean lower-crustal components and late Palaeoproterozoic juvenile additions, whereby the mangerites and charnockites contain more Archaean Pb than the mafic rocks. Most anorthosites, ferrodiorites and the felsic rocks yield overlapping 
Nd values ranging from –3 to –9. Exceptions yielding more primitive Nd values between –1 and +1 are ferrodiorite to gabbro from the Eidsfjord Complex (Markl & Höhndorf, 2003) and mangerite from northern domains of Austvågøy and Flakstadøy (Wade, 1985). The distribution of Sr isotopic compositions broadly matches that obtained from Nd.
Charnockites and mangerites are widespread throughout the region, especially in the form of large plutons such as the Raftsund, Hopen, Sund–Ølkona and SW Lofoten intrusions (Griffin et al., 1974; Ormaasen, 1977; Malm & Ormaasen, 1978). Anorthosite occurs at three sites: on Moskenesøy, Flakstadøy and at Eidsfjord. Gabbro forms smaller plutons in all the islands; the largest one is the Eidet–Hovden pluton on Langøy (Heier, 1960; Romey, 1971; Markl, 1998; Markl et al., 1998; Markl & Frost, 1999).
There is one distinctive body of granite–syenite, the Torset pluton on Langøy (Heier, 1960; Brueckner, 1971). In addition, a widespread and diffuse network of feldspathic veins is present throughout the granulite-facies domain, especially on Langøy, and it is generally nearly impossible to distinguish this late veining from the original migmatitic structures described by Griffin et al. (1978). A feldspar porphyritic and generally gneissic variety of granite forms the Lødingen pluton in southeastern Hinnøy (Andresen & Tull, 1983), whereas the Tysfjord granite covers a large area on the mainland (Andresen & Tull, 1986). Local granitic pegmatites belong to distinct generations, one Palaeoproterozoic (this study), the other Caledonian (Corfu, 2004).
On Vestvågøy the basement is overlain by the Leknes group, a distinct assemblage of amphibolite-grade paragneisses and schists that is in tectonic contact with the Palaeoproterozoic basement and was multiply deformed and metamorphosed during the Caledonian orogeny (Tull, 1977; Corfu, 2004). Other probable expressions of the Caledonian orogeny are shear zones that locally preserve the record of a high-pressure event (Wade, 1985; Markl & Bucher, 1997; Kullerud et al., 2001).
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PREVIOUS GEOCHRONOLOGY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPREVIOUS GEOCHRONOLOGYANALYTICAL APPROACHU-Pb RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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An extensive programme of geochronological work was carried out over a period of some 30 years using mainly the Rb–Sr and Pb–Pb whole-rock isotopic methods, complemented in the final stages by Sm–Nd isotopic work (Heier & Compston, 1969
; Brueckner, 1971; Griffin et al., 1974, 1978; Taylor, 1974, 1975; Jacobsen & Wasserburg, 1978; Andresen & Tull, 1983; Wade, 1985). The first indications for the existence of Archaean crust were provided by the Rb–Sr data of Heier & Compston (1969). Subsequently, Taylor (1975) reported a Pb–Pb age of 3600 Ma for the gneisses in western Langøy, but this interpretation was challenged by Jacobsen & Wasserburg (1978), whose Sm–Nd results indicated an age not older than about 2700 Ma. Their reintepretation was then implicitly accepted by, and integrated into the review of Griffin et al. (1978). The timing of granulite-facies metamorphism was estimated at 1830 ± 35 Ma (Rb–Sr whole-rock; Griffin et al., 1978). The mangeritic suite yielded ages mainly in the range between 1800 and 1700 Ma. The Lødingen granite yielded Rb–Sr whole-rock dates between 1380 and 1640 Ma based on the work of different groups (Heier & Compston, 1969; Griffin et al., 1974, 1978; Taylor, 1974; Andresen & Tull, 1983; Wade, 1985).
Sveconorwegian Rb–Sr whole-rock ages were obtained from metasedimentary rocks of the Leknes Group on Vestvågøy and from similar rocks on the small island of Værøy south of Lofoten (Taylor, 1974; Griffin & Taylor, 1978). Sveconorwegian ages were also defined by Rb–Sr on biotite in Langøy (Heier & Compston, 1969). A set of K/Ar and Ar/Ar data for biotite in mangerite and paragneiss from Vestvågøy also included some Sveconorwegian ages but there was an overall variation from 1991 to 350 Ma [unpublished data by J. Sutter quoted by Tull (1977)]. The Sveconorwegian results were taken by Griffin et al. (1978) to indicate the time of a retrogressive event at amphibolite-facies conditions coeval with metamorphism and deformation of the Leknes Group. On the basis of these inferences, Markl & Bucher (1997) concluded that the eclogite-facies event observed locally in shear zones was probably related to this Mesoproterozoic activity. The previous isotopic work also yielded sporadic Ordovician to Carboniferous Rb–Sr and Ar–Ar mineral ages, which were difficult to interpret in detail, and in part reflected late- and post-Caledonian extensional events (Griffin et al., 1978; Hames & Andresen, 1996).
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ANALYTICAL APPROACH |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPREVIOUS GEOCHRONOLOGYANALYTICAL APPROACHU-Pb RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Samples of 1–5 kg were crushed in a jaw crusher and pulverized with a hammer mill. Mineral separation was carried out by a combination of Wilfley table, sieving at 250 µm, magnetic separation, and heavy liquids flotation. The grains to be analysed were selected by handpicking under a binocular microscope and abraded to remove external disturbed domains (Davis et al., 1982
; Krogh, 1982). The selected samples were subsequently washed using HNO3, H2O and acetone, weighed on a microbalance, and spiked with a mixed 205Pb/235U tracer. Dissolution of zircon was carried out with HF (+HNO3) in Teflon bombs (Krogh, 1973) at 184°C whereas the other minerals were dissolved in screw-top Savillex vials on a hot plate. Monazite and larger zircon fractions were processed through a single-stage HCl chemical separation as described by Krogh (1973), and zircon samples below 4–5 µg in weight were measured directly without undertaking any chemical purification, whereas the other minerals were treated with a two-stage HBr–HCl–HNO3 procedure as summarized by Corfu & Stone (1998).
The samples were loaded on zone-refined Re filaments with Si-gel and H3PO4 and measured on a MAT 262 mass spectrometer either on Faraday cups in static mode, or, for smaller samples and all 207Pb/204Pb ratios, by peak-jumping in an ion-counting secondary electron multiplier. The secondary electron multiplier data were corrected for a non-linear bias using an exponential equation whose parameters were adjusted based on concurrent measurements of the NBS982 Pb standard. In addition, all the data were corrected for 0·1%/a.m.u. fractionation using reproducibility factors of ± 0·05%/a.m.u. for Faraday data and ±0·1%/a.m.u. for secondary electron multiplier data. Bulk reproducibility for zircon was tested by measuring aliquots of samples split after dissolution but before chemistry, and also by analysing separate fragments of zircon standards 91500 (Wiedenbeck et al., 1995) and GJ (provided by W. L. Griffin). Results of seven analyses of 91500 yield an intercept age (essentially the mean 207Pb/206Pb age) of 1066·5 ± 0·7 Ma with an MSWD value of 0·7; this age is slightly higher than the 1065·4 ± 0·3 Ma reported by Wiedenbeck et al. (1995). Fifteen reproducibility analyses on split samples and standards, from this and other concurrent studies, yield average differences of 0·26% for the 207Pb/235U ratio, 0·24% for the 206Pb/238U ratio, and 0·05% for the 207Pb/206Pb ratio. In detail the overlaps range from excellent to poor, the three worst cases being data reported in this study: two of these duplicate analyses are reported for sample BAL-1 (Table 1) and another for sample 19. The latter shows perfectly matching 207Pb/206Pb ratios, but a divergence of 0·9% for the U–Pb ratios. The 207Pb/205Pb ratios measured for the two aliquots of this fraction actually show an even larger difference of about 1·6%, but they are counterbalanced by a difference of –0·8% between the two 238U/235U ratios. These differences are much larger than expected from the usual reproducibility of Pb measurements, especially as both datasets were obtained from very solid mass spectrometer runs. They are thus more likely to reflect an improper equilibration between spike and sample solution before aliquoting. Duplicate measurements of seven monazite pairs and one titanite pair yield in all cases data that overlap within the respective calculated analytical uncertainties, whereby the overlap of two monazite duplicates reported in this paper for sample 30 is again rather marginal.
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The zircon analyses were corrected for a Pb blank of 2 pg and 0·1 pg U, but in some instances it was evident that the actual blank must have been higher than that and allowance has been made for that during the calculation of the data. The titanite and the larger monazite fractions were corrected for 10 pg Pb and 0·3 pg U blank, using a blank composition of 206Pb/204Pb = 18·3 (±2%) and 207Pb/204Pb = 15·555 (±1%). The residual initial common Pb was subtracted using compositions calculated with the Stacey & Kramers (1975)
model for the age of the sample. Decay constants are those of Jaffey et al. (1971). The data were reduced with ROMAGE 5.1, a program originally written by T. E. Krogh and expanded by L. Heaman. Data plotting and age calculations were performed with the program Isoplot of Ludwig (1999). |
U–Pb RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPREVIOUS GEOCHRONOLOGYANALYTICAL APPROACHU-Pb RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Samples
This paper reports U–Pb data for 14 samples, which were selected to represent the more characteristic phases across the region. They include anorthosite, gabbro, ferrodiorite, mangerite, charnockite, monzonite, granite and pegmatite (Table 1, Fig. 2). The data are presented below, starting with the oldest set of plutons (Lødingen and Hopen), followed by the main group of intrusions whose ages vary in the 1800–1790 Ma range, and concluding with the youngest expressions of magmatism.
Lødingen pluton (Hinnøy)
The zircon crystals extracted from sample 34 of the Lødingen granite are generally euhedral and prismatic but contain local inclusions and some metamict domains that may represent older cores. Four fractions of various zircon types (1–4, Table 1; Fig. 3) yield 5–13% discordant analyses. Coexisting titanite shows a variation from red–brown grains with 110 ppm U and Th/U = 0·9 to very pale brown fragments with 30–40 ppm U and Th/U = 0·1. Two analyses of brown titanite (5–6) are about 3% discordant, whereas three fractions of pale brown titanite (7–9) plot close to the lower intercept of the array. A line defined by all the zircon and titanite data has an MSWD of 3·3 and yields an upper intercept age of 1873 ± 2 Ma, which is taken to indicate the time of magmatic crystallization of the pluton. The excess scatter, caused mainly by the poor collinearity of some of the zircon fractions, may reflect either subordinate amounts of inheritance or the effect of additional isotopic disturbances. The titanite analyses alone show a better linear correlation with an MSWD of 1·3, yielding about the same upper intercept age and a lower intercept age of 422 ± 5 Ma, which probably reflects resetting and/or new growth of titanite during the Caledonian event.
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.Hopen pluton (Austvågøy)
The two samples (22 and 21) collected for the U–Pb work represent the dominant charnockitic and mangeritic facies of the pluton. Interestingly, the two zircon populations are different in terms of external appearance and internal geometry. In the charnockite (sample 22) zircon occurs mainly as euhedral, prismatic crystals with well-developed internal oscillatory zoning (Fig. 4a–c). By contrast, the mangerite (sample 21) contains highly resorbed, irregularly shaped, anhedral to subhedral zircon grains with no, or locally faintly developed, oscillatory zoning (Fig. 4d–f), but showing resorption fronts invading the primary zircon from the margins of the grains. Despite these distinct morphological differences, the two samples yield overlapping U–Pb ages. Four zircon analyses (10–13) for the charnockite are 0·5–2·3% discordant, but collinear (MSWD = 0·2), and define an upper intercept age of 1860 ± 5 Ma (Fig. 3), whereas two mangerite zircon analyses (14–15) overlap on concordia defining a concordia age of 1864 ± 1 Ma (Fig. 3). The identical age of the two rock types is consistent with the geological evidence suggesting that they are part of a single, zoned but coherent intrusive body (Ormaasen, 1977
).
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Ballstad mangerite, SW Lofoten pluton (Vestvågøy)
The zircon population of this rock (sample BAL-1) resembles that of the Hopen mangerite, consisting entirely of anhedral grains with irregular but smooth external shapes, and generally occurring as broken fragments. The analyses (16–22; Fig. 5) are characterized by relatively low U contents and are all less than 1% discordant, defining a mean 207Pb/206Pb age of 1794 ± 1 Ma (MSWD = 1·9). A slightly better fit (MSWD = 1·7) to the data is achieved by projecting the data through a Caledonian lower intercept age, yielding an upper intercept age of 1795 ± 1 Ma, which is interpreted as the age of intrusion of the mangerite.
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Raftsund charnockite (Austvågøy)
The sample (23) provided abundant euhedral zircon crystals, ranging from colourless to dark pink in colour, and containing 200–500 ppm U. The analyses plot on or near concordia defining a line (MSWD = 1·3) with an upper intercept age of 1796 ± 2 Ma, which probably dates the magmatic crystallization of the body.
Sund–Ølkona mangerite (Flakstadøy)
The zircon population in sample 7 is composed of euhedral prisms that are commonly clear but locally contain metamict interiors. The analyses (29–35) are up to about 2% discordant, but are not collinear, defining instead a fan-shaped pattern indicative of a complex isotopic evolution (Fig. 5). Some of the analyses (33–35) were carried out on tips broken away from metamict cores to verify whether the scatter was caused by inheritance, but the tips tend to yield older 207Pb/206Pb ages than the whole grains, and do not support this hypothesis. The strongest deviation towards younger 207Pb/206Pb ages shows a correlation with U content, suggesting that the pattern reflects an early Pb loss event, possibly connected to alteration (e.g. Corfu & Ayres, 1984). The four analyses that seem to be less affected by this early disturbance (29, 33–35) define a line (MSWD = 1·3) with an upper intercept age of 1800 ± 2 Ma, which is taken as the best indication for the time of emplacement of the unit. The lower intercept of this line also indicates a Caledonian Pb loss event, whereas analyses 30–32 can be fitted to lines projecting towards Sveconorwegian lower intercept ages.
Pegmatitic pod in gabbro, Flakstadøy complex, Flakstadøy
The zircons in this pegmatitic domain of the gabbro (sample 4) are euhedral but generally fragmented. One of the analyses (36) is concordant and collinear (MSDW = 0) with two more discordant ones (37 and 38). The data define a line with an upper intercept age of 1793 ± 4 Ma and projecting towards a Caledonian lower intercept age. The fourth analysis (39) deviates significantly to the left of this line, pointing towards a Sveconorwegian lower intercept age.
Pegmatite in Napp gabbro, Flakstadøy complex, Flakstadøy
A 3–4 mm wide zircon crystal was recovered from a steeply dipping pegmatitic vein (sample 52) in the gabbro. Three abraded pieces of the crystal yield more or less concordant data (40–42) that define an age of 1789 ± 2 Ma (MSWD = 1·5).
Borge gabbro, Vestvågøy
A coarse-grained portion of this gabbro body (sample 19) yielded mainly fragments of large euhedral zircon blades and prisms. The data (44–47) indicate low U contents and define an array (MSWD = 1·5) with an upper intercept age of 1796 ± 1 Ma, which dates intrusion of the pluton.
Eidsfjord complex anorthosite and ferrodiorite (Langøy)
Sample 27, representing coarse anorthosite, contains mainly clear to pinkish zircon fragments and a few euhedral prisms. A concordant analysis was obtained from a fraction of two clear but U-rich fragments (48). This point is collinear with three discordant analyses (49, 51, 52; MSWD = 0·2), the line defining an upper intercept age of 1796 ± 2 Ma. The fifth analysis (50), obtained from five grains having a slightly dark and shady appearance, deviates somewhat to the left of the line.
The ferrodiorite (sample 28) zircon population contains grains that are subrounded and commonly broken. Euhedral grains seem to be absent. The analyses (53–57), carried out on various subrounded grains and fragments, yield data that are about 0·5–2% discordant and have 207Pb/206Pb ages varying between 1777 and 1747 Ma. There is no straightforward explanation for this pattern and there is no consistent relation between zircon characteristics and apparent age that could help in the interpretation. If the ferrodiorite is coeval with the anorthosite the data must indicate either secondary zircon recrystallization or a severe Palaeo- to Mesoproterozoic disturbance affecting the zircon systematics. Scoates & Chamberlain (1997) isolated a secondary generation of zircon in an anorthosite from Wyoming and interpreted it as having formed from Zr released during breakdown of ilmenite. Amelin et al. (1999) also found a secondary generation of zircon in a troctolite and suggested that its growth may have been promoted by the local intrusion of syenite. An analogous mechanism could possibly also have produced the subrounded zircon in the ferrodiorite, although at present we have no evidence for the existence of such younger intrusive phases in the area. The alternative interpretation is that the ferrodiorite dyke significantly post-dates the anorthosite and that the scatter reflects inheritance together with complex Pb loss. More work is needed to resolve this problem.
Eidsfjord monzonite (Langøy)
Although this sample (24) contains abundant and good quality zircon crystals, all the analyses are at least 1–1·5% discordant. Five of them (58–61, 63) define a line (MSWD = 1·1) with an upper intercept age of 1800 ± 3 Ma and pointing towards Caledonian Pb loss (Fig. 5). Two other analyses (62, 64) deviate from the line, suggesting that they were affected by an earlier isotopic disturbance.
Titanite occurs in this sample as yellow to almost colourless fragments, as anhedral to subhedral lentils, and locally as aggregates of polycrystalline grains (raspberry titanite). Even though it is difficult to clearly distinguish the different titanite types it was possible to separate a predominantly Proterozoic group of yellow titanites (analyses 65–67) with up to 28 ppm U, about 2·3 ppm initial Pb, and relatively high Th/U of 1·3–1·7 from a predominantly Palaeozoic group of very pale titanite (68–72) with 11–17 ppm U, relatively high initial Pb contents of 3–3·5 ppm, and Th/U as low as 0·14. The analysis of a fraction of raspberry titanites (69) containing only 3·3 ppm U is very imprecise and plots close to the younger group, but still has a substantial amount of Proterozoic Pb. The two groups of analyses plot on a common discordia line with intercepts at 1769 ± 11 Ma and 513 ± 39 Ma (Fig. 6). The lower intercept age is imprecise mainly as a result of the low 206Pb/204Pb ratios (44–24; Table 1) and, hence, large uncertainties of the analyses of the young group. The youngest analysis (72) yields a concordia age of 478 ± 9 Ma, which must be considered as an upper limit for the age of the Palaeozoic titanite component. The upper intercept of 1769 ± 11 Ma is significantly younger than the age of coexisting zircon of 1800 ± 3 Ma, and thus indicates metamorphic crystallization of the titanite, probably during the episode of post-magmatic retrogression of the dry mangeritic protolith.
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Torset granite (Langøy)
The granite (sample 30) is composed of a population of transparent, euhedral zircon crystals, which contain xenocrystic cores, as revealed by the 207Pb/206Pb ages of two analyses of short-prismatic crystals and fragments (73 and 74). Three analyses of tips and of one euhedral crystal (75–77) define a common age of 1789 ± 1 Ma. Another single crystal (78) yields a younger but discordant 207Pb/206Pb age of 1780 Ma, which overlaps that of the oldest monazite grain (79). The other monazite analyses indicate a high degree of isotopic heterogeneity: one grain (80) yields a reversely discordant analysis, a second (81) is concordant, but at a younger age of 1771 Ma, and finally three multigrain, unabraded fractions (82–86), including two duplicate pairs, yield more discordant data with 207Pb/206Pb ages between 1759 and 1751 Ma. This data pattern provides no simple unambiguous interpretation. The age of 1789 ± 1 Ma defined by the three overlapping zircon analyses is considered the most likely age of crystallization. This is a valid interpretation if the analysed grains in fractions 75–77 were free of inherited cores, a presumption supported by the reproducibility of the 1789 Ma age, and the expectation that a core component would probably be somewhat visible in this type of pinkish, transparent grains. The alternative is that these three fractions all had equal amounts of inheritance and that the granite actually intruded at around 1780 Ma, the apparent age of the youngest zircon and oldest monazite (78–79). This alternative is considered less likely because of the above arguments and also keeping in mind that some discordance is possible in this zircon grain (78) because of its relatively high content of U (235 ppm) and only moderate degree of abrasion. The preferred interpretation is that this grain underwent Pb loss, possibly at around 1200 Ma as suggested by the trajectory of a line projected through this point from 1789 Ma. Such an interpretation could also account for the position of the discordant monazite data points. The position of the other monazite analyses is difficult to interpret with great certainty. It is possible that monazite underwent multiple growth events between 1780 and 1770 Ma. Although these monazites are extremely rich in Th with respect to U (Th/U = 122–269), with the highest value occurring in the reversely discordant analysis, it appears unlikely that the latter can be related to excess initial 230Th (Schärer, 1984
). The inverse discordance may be related to some alteration (e.g. Corfu & Evins, 2002) or to incomplete dissolution.
Felsic pegmatite in Borge gabbro (Vestvågøy)
The zircons separated from this pegmatite (sample 20) occur mostly as blocky fragments of large euhedral crystals. Analyses of transparent pieces, ranging in colour from colourless to brown, reveal very high U contents of up to 4000 ppm and also substantial common Pb at around 0·6 ppm. The data points are between 4·7 and 11% discordant (Fig. 6). Three of them (87, 89, 90) define a line with an upper intercept age of 1773 ± 2 Ma and a Caledonian lower intercept age, but the fourth point (88) plots off the line. Coexisting titanite occurs mainly as pale yellow fragments with around 100 ppm U and 4·5 ppm initial Pb, and the results (91–93) plot 7–13% discordantly. The low 206Pb/204Pb ratio and large correction for initial Pb introduce a large uncertainty in these analyses. When they are corrected with Pb compositions calculated with the model of Stacey & Kramers (1975) the analyses plot well to the left of the zircon discordia line, but the use of the most primitive Pb composition reported by Markl & Höhndorf (2003) for AMCG rocks in Lofoten shifts the data points onto the zircon discordia line (Table 1, Fig. 6) yielding an MSWD of 0·5 for a common regression of titanite and zircon. Such a primitive Pb would imply that the pegmatite was formed by melting of Archaean lower-crustal rocks. A verification of this Pb composition using feldspar from the pegmatite, however, failed because the latter is highly altered and generally rich in inclusions (haematite and other minerals), and contains 27 ppm unsupported radiogenic Pb (together with 26 ppm common Pb and 0·0 ppm U). One single, dark brown titanite grain (94) contains a much higher U content of 1218 ppm and is substantially more discordant (34%) than yellow titanite. It is possible that both the high U content and the discordance are related to the presence of minute black inclusions in the grain, and that these are identical to brown–black, opaque grains present among the separates. Two analyses of such grains (95–96) reveal U contents of 10–18%; one of them is slightly reversely discordant at around 450–460 Ma, whereas the second is normally discordant. The two data points define a line with an intercept at 451 ± 2 Ma, suggesting that this U-rich phase grew or was reset during an event at this time. Interestingly, the zircon data yield an identical but less precise lower intercept age of 449 ± 21 Ma. The four titanite analyses (91–94) define lines with younger Caledonian intercept ages of 412 ± 12 or 395 ± 12 Ma (MSWD = 0·7 or 1·3), depending on the correction used [AMCG primitive Pb or Stacey & Kramers (1975)]. The titanite lower intercept ages are largely controlled by the discordant brown titanite analysis (94), itself possibly controlled by black and U-rich inclusions. Thus the titanite analysis may have been biased by the same post-Caledonian Pb loss seen in analysis 96 (it is in fact possible to calculate a line with a good fit through this analysis and the four titanite data points). The three yellow titanite analyses alone best fit lines trending towards Ordovician lower intercept ages.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPREVIOUS GEOCHRONOLOGYANALYTICAL APPROACHU-Pb RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Proterozoic evolution
The new U–Pb results show that the Lofoten AMCG suite was emplaced during two distinct events, the first one at 1870–1860 Ma followed by a second and dominant magmatic event at 1800–1790 Ma (Fig. 7). A concluding period, lasting some 20–30 Myr, was characterized by local hydration of the dry AMCG rocks, and by the infiltration of pegmatitic melts.
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The 1870–1860 Ma event
The early period of magmatism led to the emplacement of two distinct plutonic units: the Lødingen granite and the Hopen pluton. The Hopen pluton is a composite intrusion consisting of gabbro, mangerite and charnockite. Although it is compositionally similar to the other AMCG rocks in Lofoten–Vesterålen, its age of 1864 Ma, defined by zircon in the mangerite and supported by those in the charnockite, sets this unit apart from all the others that have been dated. The age shows instead that the Hopen pluton intruded just some 10 Myr after emplacement of the Lødingen granite.
The Lødingen pluton consists of coarse-grained, variously gneissic, pink granite with large hornblende aggregates, and locally encloses rafts and enclaves of mafic rocks. The unit had previously been dated four times using Rb–Sr on whole rocks, yielding dates of 1520 ± 35, 1380 ± 80, 1644 ± 36 and 1644 ± 89 Ma (Heier & Compston, 1969; Griffin et al., 1978; Andresen & Tull, 1983; Wade, 1985). These dates are all distinctly younger than the U–Pb age of 1873 ± 2 Ma reported here. The fact that the latter is defined by both zircon and titanite largely excludes a potential age bias caused by inherited components. It is more likely that the Caledonian overprint, which is recorded by partially reset and/or newly formed titanite, had a strong influence on the Rb–Sr system, creating geologically meaningless isochron ages. It has been stated by Griffin et al. (1978) and Wade (1985) that the Lødingen granite cuts the Raftsund mangerite, a relationship that would support the apparently younger ages of the former. However, whereas the two units are very distinct in appearance, their border zone, of several hundred metres width, is very complex, comprising a variety of intrusive phases whose affinity to one or the other body is not readily discerned. Hence, the field evidence in favour of a post-mangerite age of the Lødingen granite seems to have been speculative and probably invalid.
The 1800–1790 Ma event
Except for the Hopen pluton discussed above, the bulk of the AMCG suite in Lofoten–Vesterålen was emplaced during an interval of 10–15 Myr after about 1800 Ma (Fig. 7). There is no clear-cut trend between age and composition of the investigated phases. The oldest ages of 1800 Ma were obtained for the Sund–Ølkona mangerite and the Eidsfjord monzonite, whose protolith was probably mangeritic (Heier, 1960). Intermediate ages of 1796–1793 Ma are given by various gabbroic, anorthositic, mangeritic and charnockitic units including the very extensive Raftsund intrusion. The youngest dates were obtained from the Torset granite and a pegmatitic dyke in the Napp gabbro. The latter pegmatite represents either a differentiated portion of the host gabbro or a younger dyke.
Markl (2001) concluded that the AMCG suite in Lofoten–Vesterålen defines two separate fractionation trends. One trend is related to ferrodioritic and jotunitic rocks, as the residual liquids, and anorthosites as the plagioclase cumulates of a common gabbroic liquid. The other trend is related to charnockites and mangerites, the mangerites representing feldspar cumulates, and the charnockites the probable residual liquids, of an intermediate, possibly syenitic, liquid. Markl also suggested that there may be a direct genetic link between the two trends, possibly even a direct fractionation trend from gabbro to ferrodiorite to mangerite and charnockite. The age sequence observed in Lofoten, including the fact that the initial plutons are of felsic composition, shows that formation of the AMCG suite was a complex process with multiple magma batches sequentially differentiating, and probably undergoing mixing and crustal assimilation. The close spatial and temporal grouping of all these components confirms that they must have been broadly related to some common first-order process. The type of complexity is a common feature of AMCG complexes (e.g. Amelin et al., 1994; Emslie et al., 1994; Scoates & Frost, 1996).
Late- to post-magmatic stage
Emplacement of the dry AMCG suite was succeeded by a period of higher fluid activity, which led to local hydration of the mangerites. Titanite is interpreted to have grown in the Eidsfjord monzonite at 1769 ± 11 Ma as part of this process. Another product of this late activity is the 1773 ± 2 Ma granitic pegmatite cutting the Borge gabbro. Other effects of these post-magmatic processes are apparently also recorded by monazite in the Torset granite, although the exact interpretation of that set of data remains speculative.
Timing of high-grade metamorphism
Unravelling the metamorphic evolution of the rocks in Lofoten–Vesterålen is complicated by the composite mineralogical record left by several prograde and retrograde events. Griffin et al. (1978) summarized the then existing geobarometric evidence, which suggested emplacement of the AMCG suite and granulite-facies metamorphism at pressures of 9–12 kbar. Markl et al. (1998), however, revised this estimate to about 4 kbar, indicative of mid-crustal conditions. This promotes the question of whether the granulite-facies assemblages in the country rocks represent regional or just contact metamorphic effects. The amphibolite- to granulite-facies transition had been interpreted by Heier (1960) to be a gradual metamorphic transition, a relationship subsequently discussed by Olsen (1978). The preservation of the 1873 Ma titanite age in the Lødingen granite sample 34, which was collected only some 10 km away from the projection of the orthopyroxene isograd, argues against a gradual metamorphic transition because amphibolite-facies metamorphism affecting this region at c. 1800 Ma would probably have reset or at least overprinted the original titanite. This and other considerations suggest that the orthopyroxene isograd, which also matches the sharp boundary to the Lofoten magnetic anomaly, represents a Caledonian tectonic boundary (Corfu, 2004).
Additional complications on the regional metamorphic picture were introduced by lower-grade metamorphic overprints at 1780–1760 Ma, as described above, and during the Caledonian orogeny, which also included a high-pressure event (Olsen, 1978; Markl & Bucher, 1997; Corfu, 2004).
Regional correlation
The Palaeoproterozoic geology of the Baltic Shield presents a good framework within which one can attempt to interpret the genesis of the plutonic suites in Lofoten–Vesterålen. The general pattern of crustal growth (Fig. 7) can be subdivided broadly into: (1) an early period, between 1·95 and 1·88 Ga, of arc magmatism with volcanic and plutonic events commonly of tholeiitic to calc-alkalic affinity; (2) at around 1·88–1·86 Ga episodes of regional deformation with local deposition of turbidites and alluvial–fluvial sedimentary rocks and extensive magmatism of calc-alkalic to alkalic character reflecting compression and closure of marginal basins; (3) a protracted period from 1·86 Ga to at least 1·77–1·76 Ga interspersed by vigorous plutonic events, with episodes of local deformation and regional metamorphism (e.g. Huhma, 1986; Skjöld et al., 1988, 1993; Nironen, 1997; Bergman et al. 2001; Högdahl & Sjöström, 2001; Rämö et al., 2001; Corfu & Evins, 2002; Weihed et al., 2002). These events appear to have climaxed somewhat diachronously across the region.
Vestiges of the first period are scarce in Lofoten–Vesterålen. The protoliths of the Palaeoproterozoic paragneisses have been considered to represent volcanic and sedimentary rocks formed in a marginal or back-arc basin (Griffin et al., 1978). They were probably deposited during the 1·95–1·87 Ga period of arc development documented elsewhere in the Svecofennian belt, but absolute age constraints are missing in Lofoten–Vesterålen, and the interpretations offered in this respect seem rather speculative.
There is also very little information concerning the kinematic history that accompanied the second period at 1880–1860 Ma, regionally related to arc accretion, basin closure and collision. In terms of plutonism we now know that at least two units, the Lødingen granite and Hopen pluton, were formed during this event. They are temporally correlative with late- to post-tectonic intrusions composed of granite, syenite, monzogranite, monzonite and local gabbro and diorite that are widespread in the Baltic Shield east of the Caledonides (Skjöld et al., 1993; Rämö et al., 2001; Weihed et al., 2002; Rehnström & Corfu, 2004). Examples include the ‘perthite–monzonite suite’ in Sweden (e.g. Bergman et al., 2001), and the ‘post-kinematic plutons’ in central Finland, a bimodal suite of ferroan, alkalic–calcic to alkaline, peraluminous to metaluminous plutons (see Frost et al., 2001) interpreted as the product of partial melting of deep mafic crust underplated during the preceding orogenic phases (Rämö et al., 2001).
The subsequent period between 1860 and 1800 Ma was relatively quiescent across the Svecofennian orogen. Magmatism formed c. 1850–1840 Ma alkalic–calcic plutons in central Sweden, perhaps the earliest expression of TIB (Transscandinavian Igneous Belt) magmatism (Åhäll & Larson, 2000; Andersson & Wikström, 2001). Granulite-facies metamorphism, associated with the emplacement of ferroan, alkali–calcic suites of metaluminous monzodiorites and peraluminous granites, occurred at 1830–1810 Ma in southern Finland (Väisänen et al., 2000).
Volumetrically important and spatially widespread magmatic activity was characteristic of the period at about 1800–1780 Ma when granitoid rocks rich in feldspar and quartz invaded large tracts of crust across the shield. In northern Sweden Öhlander & Skjöld (1994) distinguished two main types of intrusives: one group of alkali-rich syenite to granite crystallized from relatively dry magmas, and another group of fluid-rich, minimum melt granites and pegmatites. These two groups of rocks are both ferroan, dominantly alkaline, and peraluminous to metaluminous, and were interpreted to reflect widespread melting of the crust promoted by heat from asthenospheric sources. In southern Finland and western Russia a coeval suite of post-collisional magnesian alkaline intrusions occurs along east-trending crustal lineaments, and has been inferred to be the product of melting of metasomatized lithospheric mantle (Eklund et al., 1998). Another characteristic rock association related to these 1800–1780 Ma events is the Revsund (or TIB1) plutonic suite of alkaline to calc-alkaline granite, monzonite and diorite, which occurs along a north-trending belt of 2000 km length oblique to the main Svecokarelian structures and crustal boundaries (Fig. 1; Åhäll & Larson, 2000; Weihed et al., 2002). Granitoid rocks in this age bracket are also common in the basement windows below and west of the Caledonides, both south (Skår, 2002) and north of Lofoten–Vesterålen (Ersfjord granite, Romer et al., 1992; Corfu et al., 2003). The period around 1800 Ma was also characterized by deformation along major NW- to north-trending crustal-scale shear systems, for example in central and northern Sweden (Högdahl et al., 2001; Weihed et al., 2002). Weihed et al. interpreted the kinematics on these faults and the association with plutonism to indicate a regime of east–west shortening within a Cordilleran-type convergent system. A similar setting has also been considered by Åhäll & Larson (2000) in discussing the genesis of the TIB1 suite.
Magmatic activity in the period between 1780 and 1750 Ma was locally important but less widespread on a regional scale. In northern Finland anatexis and granitic intrusions at 1780–1770 Ma were associated with compressional deformation (Corfu & Evins, 2002) whereas in the West Troms area north of Vesterålen (Fig. 1) the emplacement of granitic dykes at about 1770 Ma accompanied the last stages of shearing along major NW-trending shear zones (Corfu et al., 2003). An AMCG complex present in the Caledonian nappes east of Lofoten, but originally located somewhere to the west of it, yields ages between 1780 and 1760 Ma, and contains granitic dykes as young as 1730 Ma (Rehnström, 2003).
Indications for the genesis of the AMCG suite in Lofoten–Vesterålen
Anorthosites and related rocks are thought to have been derived from basic parental melts (tholeiitic basalt, high-Al basalt or jotunitic–gabbronoritic melts) undergoing polybaric fractionation and ending up as plagioclase cumulates and residual melts in the middle crust (e.g. Emslie et al., 1994). There is still a great deal of controversy regarding the derivation of the parental melts: either from the mantle, with variable contamination by crustal material (e.g. Emslie et al., 1994), or from lower-crustal sources (e.g. Duchesne et al., 1999; Schjellerup et al., 2000).
In Lofoten–Vesterålen the interpretation of the isotopic evidence is complicated by the fact that the potential crustal contaminants included both Archaean and juvenile Palaeoproterozoic crust, the latter isotopically similar to Palaeoproterozoic mantle. In general the mangerites and charnockites contain Pb that is less radiogenic than that in anorthositic and gabbroic rocks, reflecting a higher proportion of Archaean lower-crustal Pb in the former (Markl & Höhndorf, 2003). On the basis of the isotopic evidence, Wade (1985) estimated the proportion of Archaean crust in the mangerites to be over 50%. Some mangerites (e.g. some from the SW Lofoten pluton), however, yield very evolved radiogenic Pb compositions (as measured in feldspar and whole rock) together with Nd values close to zero (Wade, 1985), suggesting that juvenile Proterozoic lower crust also had some input in the magma generation process. It is thus evident from the isotopic data, including the coincidence of Nd isotopic compositions in basic and acidic rocks, that crust provided a large proportion of the material of the AMCG suite.
The production of AMCG rocks in Lofoten–Vesterålen at 1800–1790 Ma must also be viewed within the above-described regional context of extensive late Svekokarelian magmatism and crustal deformation in a general convergence setting. The various suites across the shield are dominantly felsic (to intermediate) in composition and they were derived largely from melting of continental crust (e.g. Öhlander & Skjöld, 1994), but there are also mafic intrusive bodies and the potassic magnesian granitoid suite in Finland, which probably formed by differentiation of mantle-derived melts (e.g. Eklund et al., 1998).
The widespread magmatic activity at 1800–1780 Ma coincides with, and may have been controlled by, a shift in the general pattern of plate convergence, notably the shift from the north to NE direction typical of the Svecokarelian period to the east to SE direction at around 1800 Ma (Romer et al., 1992; Åhäll & Larson, 2000; Weihed et al., 2002). The tectonic process presumably reflected plate interactions at a much larger scale, such as the collisional events recorded at that time in northern Labrador and Baffin Island of the Trans-Hudson orogen (St-Onge et al., 1999).
It is thus apparent that the production of the AMCG rocks in Lofoten–Vesterålen must have been related to the major tectonic upheaval, which affected the entire Svecofennian orogen during its final stages of consolidation. The AMCG suite may have been formed primarily by melting of a mantle upwelling, and subsequent widespread interaction of these melts with the crust below areas of continental extension. As an alternative, the orogenic context and the evidence for widespread crustal sources could point towards the ‘crustal tongue’ mechanism hypothesized by Duchesne et al. (1999). According to this model, monzonoritic parental melts are produced by extensive melting of gabbronoritic lower-crustal slivers thrust into the mantle along deep shear zones. A somewhat analogous concept of crustal thickening by underthrusting of the continental lithosphere under the arc has been advanced by Ducea (2001) to explain similar ‘bursts’ of magmatic activity in the California arc.
Caledonian effects
A detailed discussion of the Caledonian history is beyond the scope of this study and has been given elsewhere (Corfu, 2004). It is nevertheless appropriate to summarize the information obtained in different parts of Lofoten–Vesterålen. One important observation concerns the divergent ages obtained for titanite in different rocks. Pale titanite in the Lødingen granite indicates resetting or new growth at 422 ± 5 Ma during the Silurian and coincides crudely with the time of the main Scandian collisional events. By contrast, titanite in the Eidsfjord monzonite points to an age of 
478 ± 9 Ma. Because the titanite in this sample occurs as a mixture of Proterozoic and Palaeozoic generations that are not easily distinguished, the 478 Ma age could be overestimating the actual age of formation. Support for the occurrence of an Early Ordovician metamorphic event, however, is also provided by the lower intercept age of 513 ± 39 Ma defined by the titanite line. An alternative interpretation would be to postulate a more complex three-stage evolution with both Late Neoproterozoic to Cambrian and Silurian to Devonian events combining to yield the observed pattern. The U-rich phase in the pegmatite dyke yields an age of 451 ± 2 Ma. Because the behaviour of U-rich minerals can be unpredictable, this age must be treated with caution; however, it is interesting to note that it fits well into the age pattern provided by other work in Lofoten–Vesterålen (Corfu, 2004).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPREVIOUS GEOCHRONOLOGYANALYTICAL APPROACHU-Pb RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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U–Pb dating of the anorthosite–mangerite–charnockite–granite suite in Lofoten–Vesterålen indicates two distinct periods of magmatic activity: an early one at 1870–1860 Ma and a second and dominant one at 1800–1790 Ma. Subsequent activity persisting for some 30 Myr caused the partial hydration of the AMCG rocks, recorded by the local development of titanite in monzonite and the emplacement of pegmatites.
The 1870–1860 Ma phase of magmatism, marked by the emplacement of one mangeritic–charnockitic pluton and a granite body, was coeval with the main collisional and arc amalgamation event in the Svecofennian orogen. The second and dominant period of magmatism in Lofoten–Vesterålen was characterized by the emplacement of major charnockitic to mangeritic plutons together with gabbroic, anorthositic and granitic intrusions. On the scale of the orogen, the period at 1800–1780 Ma was distinguished by the emplacement of very widespread and compositionally variable plutonic associations of both mantle and crustal origin and also corresponded to a shift in convergence rates, possibly related to more distant continental collisional processes such as those in the Trans-Hudson orogen. It appears likely that the AMCG suite was formed as a consequence of these major tectonic events. Geometric and kinematic factors such as the position of the area at the margin of the Archaean craton and the changing plate kinematics may have been the most important variables that led to the development of this unique rock association in Lofoten–Vesterålen.
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ACKNOWLEDGEMENTS |
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Thanks are extended to K. Kullerud for help in sampling, H. Austrheim for providing one of the samples, and M. Holte and B. H. Eriksen for digitizing the maps. Constructive comments by reviewers J. S. Scoates, G. Markl, M. E. Bickford and editor B. R. Frost are gratefully acknowledged. This work was partly supported by NGU network grant 256600.
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FOOTNOTES |
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* Telephone: (+47) 22 85 66 80. Fax: (+47) 22 85 42 15. E-mail: fernando.corfu{at}geo.uio.no
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