New zircon data supporting models of short-lived igneous activity at 1 . 89 Ga in the western Skellefte District , central Fennoscandian Shield

New U-Th-Pb zircon data (SIMS) from three intrusive phases of the Palaeoproterozoic Viterliden intrusion in the western Skellefte District, central Fennoscandian Shield, dates igneous emplacement in a narrow time interval at about 1.89 Ga. A locally occurring quartz-plagioclase porphyritic tonalite, here dated at 1889 ± 3 Ma, is considered the youngest of the intrusive units, based on the new age data and field evidence. This supports an existing interpretation of its fault-controlled emplacement after intrusion of the dominating hornblende-tonalite units, in this study dated at 1892± 3 Ma. The Viterliden magmatism was synchronous with the oldest units of the J örn type early-orogenic intrusions in the eastern part of the district (1.89–1.88 Ga; cf. Gonz̀ales Rold́an, 2010). A U-Pb zircon age for a felsic metavolcanic rock from the hanging-wall to the Kristineberg VMS deposit, immediately south of the Viterliden intrusion, is constrained at 1883 ± 6 Ma in this study. It provides a minimum age for the Kristineberg ore deposit and suggests contemporaneous igneous/volcanic activity throughout the Skellefte District. Furthermore, it supports the view that the Skellefte Group defines a laterally continuous belt throughout this “ore district”. Tentative correlation of the 1889± 3 Ma quartz-plagioclase porphyritic tonalite with the Kristineberg “mine porphyry” suggests that these units are coeval at about 1.89 Ga. Based on the new age determinations, the Viterliden intrusion may equally well have intruded into or locally acted as a basement for the ore-hosting Skellefte Group volcanic rocks. Correspondence to: P. Skytẗa (pietari.skytta@ltu.se)


Introduction
The Skellefte District (Fig. 1) is one of the most important mining districts in northern Europe with numerous VMS deposits and a large potential for future discoveries (Carranza and Sadeghi, 2010).The Kristineberg mine is located in the western part of the district.It is the largest past and present VMS mine in the district, with a total production of 26.5 Mt ore, grading 1.05 Cu %, 3.56 Zn %, 0.24 Pb %, 1.31 g t −1 Au, 38 g t −1 Ag, and 25.6 % S, from the start of production in 1940 to the end of 2010.The deposit is hosted by felsic to intermediate metavolcanic rocks suggested to be part of the ore hosting, 1.89-1.88Ga Skellefte Group that occurs throughout the Skellefte District (cf.Allen et al., 1996;Kathol and Weihed, 2005).The ore-hosting metavolcanic rocks structurally overlie the composite, pre-tectonic ∼1.90 Ga Viterliden intrusion (Bergström et al., 1999;Skyttä et al., 2010), both occurring in the core of a regional-scale antiformal structure (Fig. 1; Skyttä et al., 2009).Previous geochronological work in the Skellefte District has been focused on the metavolcanic and granitoid rocks of the eastern and central parts (Billström and Weihed, 1996;Lundström and Antal, 2000;Weihed et al., 2002;Gonzàles Roldán, 2010).Apart from imprecise U-Pb zircon data from the composite Viterliden intrusion dated at 1907 ± 13 Ma by Bergström et al. (1999), age data from the western part of the Skellefte District is lacking and the time scale of volcanic activity is unknown.Correlation of the Kristineberg volcanic and intrusive units in the West with the ore-bearing Skellefte Group volcanic and early-orogenic, calc-alkaline, intrusive units in the other parts of the Skellefte District, requires control over the timing of magmatism across the district.Furthermore, temporal relationships between intrusive and volcanic events Published by Copernicus Publications on behalf of the European Geosciences Union.Weihed (2005) and Bergman Weihed (2001).
are needed to indirectly constrain the age of the VMS mineralization, and to better understand the crustal scale accretionary processes during the Svecokarelian orogeny.
This study presents new U-Th-Pb zircon data from four different igneous units in the Kristineberg area in the western part of the Skellefte District: three compositionally different intrusive units of the composite Viterliden intrusion and one volcanic unit within the stratigraphic hanging-wall to the Kristineberg deposit.Besides constraining the emplacement history of the composite intrusion, dating several intrusive units from a geographically small area aims at investigating the consistency of igneous zircon crystallization ages within different magmatic components of this plutonic complex.The latter is especially important because of the relatively old published age of the Viterliden intrusion with respect to the other early-orogenic intrusive units in the district (Wilson et al., 1987;Weihed and Schöberg, 1991;Lundström et al., 1997;Bergström et al., 1999;Weihed et al., 2002;Gonzàles Roldán, 2010).The 1907 ± 13 Ma igneous zircon crystallisation age of the Viterliden intrusion presented by Bergström et al. (1999) was, however, constrained from regression of highly discordant, large multi-grain zircon fractions.The intercept age is defined with a high MSWD of 7.6 and a high lower intercept at about 400 Ma, factors indicative of an unconformable data set and a possibly unreliable age (cf.Metzger and Krogstad, 1997).
Dating the metavolcanic rock aims at correlating the volcanic units in different parts of the Skellefte District, and at providing an estimate for the minimum age of the mineralization of the Kristineberg deposit.The obtained age data will be interpreted and discussed with respect to the evolution of igneous activity in the Skellefte District, with special emphasis on the timing of VMS mineralization at Kristineberg.

Geological and structural setting
The bedrock of the Skellefte district is composed of ∼1.90-1.86Ga Palaeoproterozoic Svecofennian supracrustal and associated intrusive rocks that were deformed and metamorphosed during the Svecokarelian orogeny at 1.87-1.80Ga (Weihed et al., 2002).The majority of models for the crustal evolution of the Skellefte district suggest that it is a remnant of a volcanic arc accreted towards the Karelian craton in the NE (Hietanen, 1975;Gaál, 1990;Weihed et al., 1992).However, an alternative interpretation (Rutland et al., 2001a, b;Skiöld and Rutland, 2006) suggests that the Skellefte district was deposited in a rift setting on the Bothnian Basin metasedimentary rocks (their Robertsfors Group).Other suggestions for the basement are 2.0-1.9Ga granitoids south of the Skellefte district (Billström and Weihed, 1996) and the Bothnian Basin rocks beneath a north-dipping crustal-scale reflector in the Kristineberg area (Malehmir et al., 2007).
The structural evolution of the Skellefte District is controlled by a complex fault pattern developed during early crustal extension, later followed by fault inversion (Allen et al., 1996;Bauer et al., 2011) during the main compressional deformation at around 1. 87-1.82 Ga (Bilström and Weihed, 1996;Rutland et al., 2001b;Weihed et al., 2002).This event reflects approximately N-S shortening and is characterized by reverse south-side-up faults and upright folds in the central part of the district (Bergman Weihed, 2001;Bauer et al., 2011), whereas a regional-scale west-plunging antiform was formed in the Kristineberg area.Another episode of crustal shortening took place at ∼1.80 Ga, and was characterized by approximately E-W bulk compression (Bergman Weihed, 2001;Weihed et al., 2002;Skyttä et al., 2010).Metamorphic peak conditions reached partial melting in the south-eastern part of the district, whereas sub-solidus PT-conditions at ∼3 kbars and ∼600 • C prevailed in the Kristineberg area at ∼1. 85-1.80 Ga (cf. Kathol and Weihed, 2005).
The predominantly felsic, volcanic Skellefte Group is attributed to a stage of extensional continental margin arc volcanism (Allen et al., 1996), and constitutes the lowermost supracrustal unit in the Skellefte District stratigraphy (Figs. 2  and 3).It is the main host to the VMS deposits, the majority of which are located within the upper part of the unit (Allen et al., 1996).Age of the Skellefte Group volcanism has been dated at ∼1.89-1.88Ga by U-Pb data on zircons from the central part of the Skellefte District (Welin, 1987;Billström and Weihed, 1996;Montelius, 2005).The dominantly metasedimentary Vargfors Group (1875 ± 4 Ma; Billström and Weihed, 1996) lies stratigraphically above the Skellefte Group rocks and is coeval with deposition of the sub-aerial Arvidsjaur Group occurring further to the north (Skiöld et al., 1993).The Bothnian Basin metasedimentary rocks occur to the south of the Skellefte district and range in age from >1.95 to 1.87 Ga (Lundqvist et al., 1998).
The Kristineberg hanging-wall rhyolite (Fig. 4c) is fine-grained, contains up to 0.2 mm large qtz-and plgphenocrysts in a fine-grained matrix of qtz, fsp and bt (totally <10 % mafic minerals), and has a well developed a SW-plunging lineation defined by elongate bt-aggregates.The hanging-wall rhyolite is associated with the stratigraphically lower of the two ore horizons in the Kristineberg area, i.e. the Kristineberg-Kimheden ore horizon (Figs. 2, 3).The stratigraphically higher, Hornträsk-Rävlidsmyran-Rävliden ore horizon is located in the upper part of the Skellefte Group volcanic rocks, which is the most common stratigraphic position for VMS deposits in the Skellefte District.Based on alteration patterns, the mine porphyry vs. the ore-hosting metavolcanic rock cross-cutting relationship, and geochemical modelling, Galley and Bailes (1999) suggested that the development of the lower of the Kristineberg ore horizons was associated with a pre-Viterliden subvolcanic intrusion at depth.In contrast, the upper ore horizon was considered coeval with the emplacement of the Viterliden intrusion into the volcanic pile (Galley and Bailes, 1999).

Sampling and analytical procedures
A total of seven samples were selected for geochronological analyses from the Kristineberg area.The samples were

A
Inverted normal fault taken both at the surface and from drill cores available in the Boliden Mineral AB drill core archives.They were milled into fine-grained powder using a swing-mill.Heavy mineral separates were obtained using a full size Wilfley water panning table.Magnetic mineral fractions were removed with a hand-magnet.Zircon grains selected for further analytic work were hand-picked using a stereomicroscope.Since all samples are rather poor in zircon, as much as about 10 kg of rock were processed for most samples.Of all seven samples, zircon was obtained from one metavolcanic and three meta-intrusive rocks, while no zircon was found from one hornblende-tonalite, one hanging-wall rhyolite, and one "mine porphyry" sample.Zircons selected for analytical work were mounted on double faced tape and embedded in transparent epoxy resin together with the 1065 Ma Geostandards zircon 91 500 (Wiedenbeck et al., 1995).The epoxy mount was polished to expose the central parts of the crystals and potentially older cores.

III
Back scattered electron (BSE) imaging was used for selection of the location of analytical spots and subsequent reexamination of spot sites after analysis.The overall analytical quality of the zircon populations was low, and most crystals contained cracks and inclusions.Areas large enough to host the spot without including these features were hard to find, and therefore several of the analytical spots also hit inclusions and cracks (cf.Description of spot location in Table 1 and representative zircon crystals shown in Fig. 5).A much larger number of analyses were obtained from the samples with a high abundance of cracks and inclusions to match problems with low analytical quality of analysed areas.The problem with low analytical quality of the grains was more accentuated for samples with a low yield of zircon in which the selection of grains was limited, e.g.sample IV, the Kristineberg hanging-wall rhyolite.
BSE imaging prior to analysis was done at the Luleå University of Technology in Luleå using a Phillips XL30 electron microscope with LaB6 filament.Post-analysis BSE imaging was done at the Evolutionary Biology Centre at Uppsala University using a Zeiss Supra 35-VP field emission SEM electron microscope, with a Robinson back scatter detector.Prior to U-Th-Pb analysis, the mount was coated with ca. 30 nm of gold.Secondary Ionisation Mass Spectrometry (SIMS) U-Th-Pb in situ analyses on zircon was carried out using a Cameca IMS 1280 high mass-resolution instrument, at the NORDSIM facility at the Swedish Museum of Natural History in Stockholm.The analytical and data reduction procedures closely followed Whitehouse et al. (1999) and Whitehouse and Kamber (2005).The instrument was operated using an O 2 − primary beam with a spot size less than 25 µm, high transmission secondary beam settings and a mass resolution (MRP) of ca.5400, sufficient to resolve Pb from molecular interferences in zircon.
All isotopic data are presented in Table 1.Age calculations were done using Isoplot/Ex (Ludwig, 2003).Ages are reported with 2 sigma errors, except for sample IV (60.1pmsk-09), which is reported with 95 %-confidence limits.In the figures and in the discussion, concordia ages are presented without decay constant errors.However, in the section below, age calculations are presented both with and without decay constant errors, for future reference.

Sample I: Viterliden hornblende-tonalite (47.1-pmsk-09)
Zircon was quite abundant in the sample that is dominated by up to 0.6 mm long, euhedral prismatic grains with sharp terminations.The crystals have approximate width/length ratios of 1:3, and are transparent to semi-transparent and colourless to weakly yellowish (light brownish) in colour.Most grains are cracked.The sample also contains a subordinate group of small (up to 150 µm), rounded and slightly brownish grains.
The analytical quality of the population is rather poor and cracks and inclusions (both dark and colourless) are frequent.The crystals are typically subhedral prisms, with rounded outer terminations.They are between 100-200 µm long with aspect ratios of about 1:2.Uncracked domains are clear to semi-transparent, often with a yellow/orange tint.
In BSE-images, the zircons generally show a weak, broadbanded oscillatory zoning, whereas some marginal domains appear more or less unzoned (Fig. 5, n3450-03ab).Some grains display intense oscillatory zoning in specific domains, which are typically cracked and appear more altered (Fig. 5, n3450-21a).Most grains are texturally non-complex.However, some grains contain texturally older core domains, surrounded and cut by texturally younger unzoned or weakly oscillatory zoned zircon (Fig. 5, n3450-05a, 12a).Secondary alteration features in the zircon population typically occur as BSE-dark thin alteration fronts preferentially located along the outer margins of the grains, broadly following bands in the zoning of the crystal.These domains are interpreted as poorly crystalline regions, tentatively indicating progressive metamictisation of the grain.
The low analytical quality of the zircon population made it difficult to locate areas devoid of cracks, inclusions, and BSE-dark altered domains large enough to host an analytical spot.As a consequence, 18 of the totally 32 analytical spots hit cracks, inclusions or straddled the crystal margin-epoxy interface.This resulted in a varying degree of discordance for these analyses (Table 1), which show a more or less complex pattern indicating both ancient and recent Pb-loss.The discordant data were excluded from the age calculation.The remaining 14 concordant analyses were located in domains showing a weak, broad-banded BSE-zoning, covering both internal and marginal parts of the crystals.Together the 14 concordant analyses define a concordia age (ignoring decay constant errors) of 1891 ± 3 Ma (Fig. 6b; MSWD conc.+equiv.= 1.4,probability = 0.082), identical to a weighted average 207 Pb/ 206 Pb age of the same analyses at 1892 ± 4 Ma (MSWD = 0.89, probability = 0.56).The concordia age including decay constant errors is 1890 ± 5 Ma (MSWD conc.+equiv.= 1.4,probability = 0.086).The 1891 ± 3 Ma concordia age is interpreted to directly date igneous crystallisation of the plagioclase porphyritic tonalite.

Sample III: quartz-plagioclase porphyritic tonalite (coarse "mine porphyry"; 29.1-pmsk-08)
The zircon population is dominated by prismatic, typically between 100-250 µm long and more or less euhedral crystals with length/width ratios of 2.5-3.They are semi-transparent light-brown to nearly colourless, with slight orange shades.Cracks and inclusions are common.In BSE-imaging, the crystals typically show an intense but somewhat blurred oscillatory zoning that in most grains continues throughout the grain all the way to the crystal edge (Fig. 5, n3449-02a, 03a).This is indicative of a single, non-complex growth stage.
A total of 27 analyses were made in 25 different crystals.The analytical result is affected by the abundance of cracks and inclusions in the analysed zircon population.18 out of 27 analyses are between 1-46 % discordant.This discordance is directly correlated to the location of the analytical spot hitting cracks or inclusions, or the crystal-epoxy interface (see location of analysed area in Table 1).U contents in un-cracked, inclusion-free domains are between 350-660 ppm.Discordant analyses, where the analytical spot has hit cracks and/or inclusions, may have up to 1000 ppm U and generally also significantly higher amounts of common Pb ( 206 Pb/ 204 Pb ratios often well below 10 000, Table 1).These analyses have been excluded from the age calculation.The remaining nine analyses are concordant and give a concordia age (ignoring the decay constant errors) of 1889 ± 3 Ma (Fig. 6c; MSWD conc.+equiv.= 1.4,probability = 0.13), identical to a weighted average 207 Pb/ 206 Pb age of the same analyses calculated at 1890 ± 4 Ma (MSWD = 1.1, probability = 0.33).The concordia age including decay constant errors is 1888 ± 6 Ma (MSWD conc.+equiv.= 1.4,probability = 0.14).The concordia age defined by the nine concordant analyses is interpreted to date igneous crystallisation of the quartzplagioclase porphyritic tonalite at 1889 ± 3 Ma.

Sample IV: Kristineberg hanging-wall rhyolite (60.1-pmsk-09)
Only about 50 zircons were retrieved from a 10 kg sample of the Kristineberg hanging-wall rhyolite, and all crystals were selected for analytical work.The crystals are rather small in size, typically short prismatic and more or less euhedral with aspect ratios between 1:2 and 1:3 and maximum lengths of about 150 µm.They are turbid to semi-transparent and cracks and inclusions are common in all crystals.In BSE-images the zircons are mostly texturally uniform, typically showing a weak broad-banded zoning or are unzoned, suggesting a non-complex single stage of zircon growth (Fig. 5, n3447-10a and b).A couple of grains have BSE-brighter inner domains with a narrow-banded oscillatory zonation surrounded by unzoned or weak broad-banded zoned zircon that dominates the sample (estimated >90 vol-% of the sample).17 analyses were obtained from the sampling.Some of the spots hit cracks and inclusions (Table 1), of which six analyses are discordant and discarded from age calculation.Eleven analysis are concordant (Fig. 6d, Table 1).The data set does not allow calculation of a Concordia age, but together these analysis define a common weighted average 207 Pb/ 206 Pb age of 1883 ± 7 Ma (MSWD = 2.4, probability = 0.01).One of the concordant analysis (n3447x-32a) hit a BSE-brighter oscillatory zoned core surrounded by unzoned BSE-darker zircon that dominates the sample (analysis marked in Fig. 6d).This texturally older domain also gives a slightly higher 207 Pb/ 206 Pb age (Table 1) and may potentially be a xenocrystic domain.If this analysis is excluded from age calculation, the remaining 10 concordant analyses define a weighted average 207 Pb/ 206 Pb age of 1883 ± 6 Ma (MSWD = 1.4,probability = 0.16).This age is interpreted to date igneous crystallisation of the Kristineberg Rhyolite at 1883 ± 6 Ma.

Discussion
The new ∼1.89Ga U-Pb ages for the three different phases of the composite Viterliden intrusion demonstrate that it was emplaced synchronously with the majority of the pre-to early-orogenic granitoids further east in the Skellefte District (Wilson et al., 1987;Weihed and Schöberg, 1991;Lundström et al., 1997;Weihed et al., 2002;Gonzàles Roldán, 2010).Exceptions are the ∼1.91 Ma Björkdal intrusion (Lundström and Antal, 2000) and the younger phases of the Jörn intrusive complex (Gonzàles Roldán, 2010).Of these two, the Björkdal intrusion has a complex history including several zircon-forming events (Lundström and Antal, 2000), and for this reason, it is not clear if it may be classified into the group of Jörn type intrusives.According to recent petrological investigations of the younger phases of the Jörn intrusive complex (GIII-IV; 1.87 Ga), it is not likely that they are cogenetic with the older 1.89-1.88Ga GI-phase (Gonzàles Roldán, 2010).The 1.89-1.88Ga age for the Kristineberg hangingwall rhyolite shows that not only intrusive, but also felsic volcanic activity was synchronous throughout the whole Skellefte District, culminating at around 1.89 Ga (cf.Welin, 1987;Billström and Weihed, 1996).This also supports the idea that the Skellefte District was originally a laterally (sub)continuous volcanic belt (Carranza and Sadeghi, 2010) that was later split and transposed by tectonic events (Skyttä et al., 2010).The new geochronology data presented here highlight the importance of the 1.89 Ga igneous activity in the build-up of the Skellefte District.The synchronous timing of the igneous events allows for geological correlations throughout the Skellefte District.
The relationship between the intrusive and volcanic units in the Skellefte District is important, since it affects interpretations on the mineralization processes.Because nearly all early-orogenic intrusions within the district (e.g.Karsträsk, Sikträsk, Rengård; Fig. 1) experienced high of tectonic often including development of shear zones along their margins (Bergman Weihed, 2001), it is not possible with the available data to constrain their stratigraphic position relative to the surrounding canic rocks.Not even from the now reasonably well-dated Kristineberg area is it possible to satisfactorily determine the temporal relationship between all the intrusive phases and the structurally overlying volcanic sequence.The regionally overlapping ages of the early-orogenic intrusives (Wilson et al., 1987;Weihed and Schönberg, 1991;Lundström et al., 1997;Weihed et al., 2002;Gonzàles Roldán, 2010) and the Skellefte Group metavolcanic rocks (Welin, 1987;Billström and Weihed, 1996) are in agreement with the idea that the Viterliden complex intruded into the succession of the Skellefte Group metavolcanic rocks (Galley and Bailes, 1999).This is also supported by the xenoliths of altered ore-hosting metavolcanic rocks in the "mine porphyry" at Kristineberg ( Årebäck et al., 2005).The new data presented here show that the volumetrically largest phases of the Viterliden intrusion probably are slightly older than the Kristineberg hanging-wall rhyolite.For this reason, it is possible that these parts of the intrusive complex acted as a basement for the overlying volcanic rocks.This interpretation agrees with Billström and Weihed (1996), who considered the Sm-Nd signature of the Skellefte Group metavolcanic rocks to indicate the presence of an intrusive basement, not much older than the metavolcanic rocks themselves.However, the new data does not exclude the possibility that the whole complex is intrusive into the volcanic sequence, since it only constrains the relationship between the intrusion and the hanging-wall rhyolite.The age of the volcanic footwall rocks still remains unknown.Furthermore, the interpretation, which considers part of the intrusive complex to be the basement to the volcanic sequence, would require some time for erosion and uplift between the crystallisation of the intrusion and the onset of volcanism.This may have been accomplished by normal faulting-related footwall uplift prior to, or in the early stages of extension-related volcanism.Regionally, pre-compressional uplift and unroofing of the Jörn GI-type granitoids are attributed to major dip-slip faulting along the intrusive-volcanic contact in the central part of the Skellefte District (Bauer, 2010).However, uplift-related granitoid clast-conglomerates, similar to those in the central Skellefte District area, are lacking in the Kristineberg area.
The "mine porphyry" is considered the youngest of the Viterliden intrusive phases (Galley and Bailes, 1999;Årebäck et al., 2005), but no geochronological evidence has been provided.The overlapping or perhaps slightly younger age of the qtz-plg-porphyritic tonalite presented in this paper (1889 ± 3 Ma), with respect to the other tonalite types (1892 ± 3 Ma; 1891 ± 3 Ma), may support this interpretation.Such a relative timing is also supported by Skyttä et al. (2010), who proposed that the qtz-plg-porphyritic tonalites were emplaced along fault zones truncating the hornblende-tonalites.However, since no zircon was obtained from the "mine porphyry proper" the possibility still remains that it is even younger than the 1889 ± 3 Ma age for its coarser equivalent.This would imply that successive pulses of magma were intruded along the fault zones, after the volumetrically largest phases of the intrusion.Considering the narrow time interval of the dated intrusive units and the lack of further evidence on the age of the "mine porphyry proper", we tentatively estimate that it was emplaced 1889 ± 3 m.y.ago.If correct, this sets a minimum age for the VMS deposition, which is in agreement with the more loosely-defined minimum age constrained by the hangingwall rhyolite.

Conclusions
The early-orogenic magmatism within the Kristineberg area occurred at ∼1.89 Ga when the Viterliden composite intrusion was emplaced during a period of ∼5 Ma.The intrusion was coeval with the emplacement of the earliest phases of the Jörn GI -type intrusions further east.The oldest phase of the Viterliden intrusion is a hornblende-tonalite (1892 ± 3 Ma) and the youngest is a quartz-plagioclase porphyritic tonalite ("mine porphyry"; 1889 ± 3 Ma).The emplacement of the latter was controlled by major faults.Volcanism in the Kristineberg area took place at 1.89-1.88Ga and was contemporaneous with volcanism in the other parts of the Skellefte District.Furthermore, it defines the minimum age for the Kristineberg VMS deposit at ≥1.88 Ga.However, if the tentative correlation between the dated quartz-plagioclase porphyritic tonalite and the "mine porphyry proper" is correct, the minimum age is constrained even further, at 1889 ± 3 Ma.The age relationships between the volcanic and intrusive units within the Kristineberg area are compatible with the Viterliden intrusion being both the local basement for, and/or intruding into, the ore-hosting Skellefte Group volcanic rocks.

Fig. 5 .
Fig. 5. BSE images for selected ion microprobe-dated zircons.Sites of analyses are indicated by analyze number, see also Table 1.Note the different scale in n3448-01ab.Qtz = quartz, plg = plagioclase.

Table 1 .
SIMS U-Th-Pb zircon data from the Viterliden meta-igneous rocks.Zircon ion microprobe data for the intrusive and volcanic rocks dated in this study.
Stacey and Kramers (1975)lation shown with normal letters; data in italics have been excluded from age calculation.bTh/Uratios calculated from 208 Pb/ 206 Pb ratios corrected for Pb com .c% of common 206 Pb in measured 206 Pb, estimated from 204 Pb assuming a present dayStacey and Kramers (1975)model.d Degree of discordance; positive numbers are reverse discordant.Blanks indicate that analysis is concordant within 2σ error.