Mineralogy and Geological Setting of Allanite-(Ce)-Pegmatites 341

NORWEGIAN JOURNAL OF GEOLOGY Mineralogy and geological setting of allanite-(Ce)-pegmatites 341

Mineralogy and geological setting of allanite-(Ce)- pegmatites in western Hurrungane, Jotun Nappe Complex, Norway: an EMP and ID-TIMS study

Simon Spürgin, Rune S. Selbekk and Anders Mattias Lundmark

Spürgin, S., Selbekk, R. & Lundmark, M. 2009: Mineralogy and geological setting of allanite-(Ce)- pegmatites in western Hurrungane, Jotun Nappe Complex, Norway: an EMP and ID-TIMS study. Norwegian Journal of Geology, vol. 89, pp 341-356, Trondheim 2009, ISSN 029-196X.

The 950 ± 1 Ma Berdalsbandet Pegmatite Swarm (BPS) in Hurrungane, in the Jotun Nappe Complex, SW Norway, consists of ≤2 m wide subparallel granitic dykes emplaced in a sinistral non-coaxial shear zone, constraining the age of local Sveconorwegian deformation. The abyssal class dyke swarm is interpreted to be related to local Sveconorwegian anatexis; mineralogy and REE characteristics indicate low degree partial melting. The pegmatites are symmetrically zoned and typically consist of an outer zone of feldspar + allanite-(Ce) + biotite, a transitional zone of feldspar + quartz + schorl + Fe-Ti-oxides, an inner zone of quartz + schorl + garnet + beryl and a core of hydrothermal quartz. A metasomatic rim of biotite is locally developed along contacts to the amphibolite wallrock. Secondary zeolites are suggested to relate to late- to post-Caledonian alteration. The dykes contain various rare-earth minerals, predominantly allanite-(Ce). Its composition, determined by electron microprobe (EMP) and 2+ expressed by representative analyses from three samples, is: (Ca0.86REE0.80Mn0.09Th0.01Na0.03)Σ1.79(Al1.79Fe 0.99Mg0.16Ti0.10)Σ3.04(Si3.17P0.01)O12(OH), (Ca1 2+ 3+ 2+ 3+ 0.30 .12REE0.72Mn0.03Th0.01Na0.02)Σ1.90(Al1.73Fe 0.66Fe 0.33Mg0.26Ti0.03)Σ3.01Si3.07O12(OH) and (Ca1.10REE0.74Mn0.02Th0.01Na0.01)Σ1.88(Al1.75Fe 0.69Fe 0.26Mg Ti0.04) 3+ 3+ 2+ 2+ 3+ 3+ 2+ 2+ 3+ Σ3.04Si3.08O12(OH). The ratio of Fe /(Fe +Fe ) is variable. Dominant substitution mechanisms are Ca + M = REE + M and 3Ca = 2REE . Isotope dilution thermal ionisation mass spectrometry (ID-TIMS) documents excess 206Pb reflecting high Th/U ratios, showing that the allanite largely retains its magmatic Pb isotopic signature despite the regional Caledonian upper greenschist- to lower amphibolite facies metamorphism. Local compositional heterogeneities in the allanite crystals are suggested to reflect Caledonian annealing of metamict domains accompanied by minor Pb-loss.

Simon Spürgin, Mineralogical-Geochemical Institute, Albertstr. 23b, 79104 Freiburg, Germany, Rune S. Selbekk, Natural History Museum, Geology, University of Oslo, Box 1172, Blindern, 0318 Oslo, Norway, Anders Mattias Lundmark, Department of Geosciences, University of Oslo, N-0316 Oslo, Norway

Introduction This study focuses on the 950 ± 1 Ma (Lundmark et al. 2007), allanite-(Ce)-bearing Berdalsbandet Pegmatite During Sveconorwegian collisional tectonics, igneous Swarm (BPS) in the western Hurrungane region, part rocks of late Paleoproterozoic to Mesoproterozoic age, of the Upper Jotun Nappe (Fig. 1). It presents the first now exposed in the Upper Jotun Nappe, SW Norway, detailed mineralogical description of the BPS as well as were metamorphosed and deformed under granulite- mineral data from its amphibolite host rock together to amphibolite facies conditions (Fig. 1). This led to at with a description of the local geological setting. The least two episodes of anatexis and the emplacement of BPS displays a mineral spectrum dominated by quartz, several generations of granitic pegmatites (Lundmark alkali feldspar, plagioclase and allanite-(Ce). Allanite 2+ et al. 2007). During Caledonian thrusting of the (Aln), CaREEAl2Fe (SiO4)3OH, is of particular interest Upper Jotun Nappe, renewed magmatism led to the since it is one of the main repositories for light rare emplacement of 427 ± 1 Ma, synkinematic granitic earth elements (LREE) in the crust, and can be used in dykes, the Årdal dyke complex (Lundmark & Corfu U-Th-Pb geochronology (e.g. Oberli et al. 2004, Gieré 2007). The different generations of granitic pegmatites & Sorensen 2004). To investigate the response of the and dykes offer a reliable way of constraining the relative, BPS allanites to late-Sveconorwegian and Caledonian and sometimes absolute ages of different phases of metamorphic events, and their potential for U-Pb deformation and metamorphism in this polydeformed geochronology, isotope dilution thermal ionisation mass and polymetamorphic area. Detailed studies of mineral spectrometry (ID-TIMS) analyses were performed on textures and compositions may allow conclusions parts of two grains. The mineral chemistry of allanite on the specific conditions leading to generation and was determined by electron microprobe (EMP) to emplacement of the dykes at well constrained times to be complement the limited published compositional data drawn. record on pegmatitic allanite. The results are used to 342 S. Spürgin et al. NORWEGIAN JOURNAL OF GEOLOGY

Fig. 1. Simplified geological map of Hurrungane, showing the NW margin of the Jotun Nappe Complex and the location of the study area (modified from Lutro & Tveten 1996 and Koestler 1983). Inset: Tectonostratigraphy of southern Norway (simplified from Roberts & Gee 1985). WGR = Western Gneiss Region. Black square in inset marks map section. NORWEGIAN JOURNAL OF GEOLOGY Mineralogy and geological setting of allanite-(Ce)-pegmatites 343 discuss and constrain the origin and metamorphic mainly of syenitic to monzonitic Gothian igneous rocks evolution of the pegmatites and the Hurrungane region. crystallised between 1694 ± 20 Ma (Schärer 1980) and ca. 1600 Ma (Bryhni & Sturt 1985), and intruded by gabbros at 1252 ± 28 Ma (Schärer 1980). Amphibolite facies Geological background metamorphism was dated by Schärer (1980) to 909 ± 16 Ma. The Lower Jotun Nappe is separated by mylonitic Southern Norway (Fig. 1) is traditionally divided into shear zones from the overlying, variably retrograded three main geological units. (I) The autochthonous and granulite facies rocks of the Upper Jotun Nappe, where parautochthonous Precambrian crystalline basement 1660 ± 2 to 1634 ± 5 Ma felsic gneisses and 1253 ± 15 (Baltic Shield and Western Gneiss Region, WGR). (II) Ma monzonite and gabbro conform to a similar pattern The overlying nappes of the Caledonian orogen, subdi- of protolith ages (Lundmark et al. 2007). The central vided into the Lower Allochthon (LA), Middle Alloch- and south-western parts of the Upper Jotun Nappe thon (MA), Upper Allochthon (UA) and Uppermost are dominated by an anorthosite-gabbro-troctolite Allochthon (UMA), the latter absent in southern Norway suite. A 965 ± 4 Ma U-Pb zircon age was interpreted (Roberts & Gee 1985). (III) Post-Caledonian formations as its protolith age (Lundmark and Corfu 2007a). The comprising Devonian molasse sediments, preserved in minimum age of one event of Sveconorwegian granulite basins on the west coast of Norway and at Røragen, and facies metamorphism in the Upper Jotun Nappe was the Permocarboniferous Oslo Graben with related sedi- constrained by 954 ± 3 Ma granitic anatexite in felsic ments and igneous rocks. The study area is located in the country rock, whereas a second high-grade event, also Jotun Nappe Complex (Bryhni & Sturt 1985), which associated with local anatexis, was dated to 934 ± 3 Ma makes up the largest part of the MA in central southern (Lundmark et al. 2007). Norway. The Upper Jotun Nappe is transected by numerous dykes, and several types have been mapped and The Jotun Nappe Complex described in the north-western Hurrungane region (Koestler 1989), where the study area is located. Granitic The medium- to high-grade Jotun Nappe Complex pegmatites are common, and different generations derives from Caledonian imbrication and thrusting of have been dated to 950 ± 1 Ma (the BPS), 942 ± 3 Ma the Baltoscandian margin (e.g., Lundmark et al. 2007; and 927 ± 3 Ma, respectively (Lundmark et al. 2007); Fig. 1). Caledonian strain is largely concentrated in the they exhibit preferred orientations on the outcrop/ basal thrust zone of the nappe complex, the Turtagrø local scale, but the presence and possible significance Zone, and in the phyllites of the Fortun/Vang nappes of a regional-scale geometric pattern is so far uncertain. (Lower Allochthon) that underlie the Jotun Nappe The ages disprove a previous suggestion that the Complex (Milnes & Koestler 1985; Lutro & Tveten pegmatites were cogenetic with the granulite host rocks 1996). Caledonian top-to-southeast thrusting and late- and intruded during a late phase of their solidification orogenic, top-to-northwest extension have been dated under high grade metamorphic conditions (Koestler at ca. 410 Ma and 400 Ma, respectively, in the phyllites 1983). Also present are 1-10 m wide metadolerites of (40Ar-39Ar cooling ages, Fossen & Dunlap 1998). unknown age, which locally contain primary ophitic Caledonian metamorphism in the Jotun Nappe Complex plagioclase-clinopyroxene-orthopyroxene assemblages reached upper greenschist- to lower amphibolite facies and metamorphic garnet (e.g., Battey & McRitchie 1973; conditions (Schärer 1980; Koestler 1983; Bryhni & Koestler 1983). The youngest dykes belong to the 427 ± Andréasson 1985; Milnes et al. 1997), and Caledonian 1 Ma synkinematic Årdal dyke complex (Lundmark & metamorphic overprinting is extensive in the Turtagrø Corfu 2007). Zone, which comprises tonalitic to granitic mylonites and blastomylonites, metasediments and slices of crystalline lithologies (Milnes & Koestler 1985). In Geological setting of the study area contrast, nappe-internal Caledonian deformation above the basal thrust zone is typically limited to distinct shear The study area is located in the Upper Jotun Nappe on zones (Bryhni & Sturt 1985), except for areas of intense the western flank of the Hurrungane mountains (Fig. Caledonian magmatism and hydration, associated with 2). The dominant lithologies are felsic to mafic two- locally extensive deformation (Lundmark & Corfu pyroxene granulites, mesocratic gabbroids, granitic 2008a). gneisses and subordinate garnet amphibolite and spinel- bearing two-pyroxene peridotites. A N-S striking shear The Jotun Nappe Complex can be subdivided into two zone, defined by a wide zone of mylonitic gneisses, main units distinguished by their different metamorphic divides the area into an eastern and a western part, histories, which were juxtaposed during the Silurian and encloses a lens of garnet amphibolite. The eastern south-eastwards translation over the WGR (Lundmark part is dominated by variably deformed pyroxene & Corfu 2008b). The medium grade Lower Jotun Nappe granulites. Growth of hornblende blasts at the expense is exposed along the margins of the complex. It consists of orthopyroxene and clinopyroxene testifies to local 344 S. Spürgin et al. NORWEGIAN JOURNAL OF GEOLOGY

Fig. 2. Major lithological units at Ber- dalsbandet, Hurrungane.

retrogression under amphibolite facies conditions. clinopyroxene in the presence of H2O has produced Two different types of corona structures are observed hornblende. In the pyroxene peridotites, olivine grains in the garnet amphibolite lens and in related spinel- are surrounded by orthopyroxene against clinopyroxene. bearing pyroxene peridotites, respectively. In the garnet The metamorphic assemblages are compatible with a amphibolite, cores of rounded plagioclase are rimmed common metamorphic history for these rock types. West by garnet against clinopyroxene. Reaction of garnet with of the N-S striking shear zone, gabbroic and granitic NORWEGIAN JOURNAL OF GEOLOGY Mineralogy and geological setting of allanite-(Ce)-pegmatites 345

Table 1. List of minerals found Table 1 in the Berdalsbandet pegma- Alkali feldspar a) ++ p Heulandite-(Ca), -(Sr) c) - s Pyrite -- s tites and their relative abun- Allanite-(Ce) ++ p Ilmenite ++ p Quartz ++ p dance. Apatite-(F) + p Magnetite ++ Rutile - p Biotite ++ p Malachite - s Stilbite - s Beryl -- p Monazite-(Ce) - p Thorite -- p Chabazite - s Muscovite + p Titanite + p Chalcopyrite - s Parisite-(Ce) -- s Tourmaline e) ++ p Epidote + s Plagioclase d) ++ p Xenotime-(Y) + p Fluorite -- p Polycrase-(Y) -- p Zircon + p Garnet b) + p Prehnite - s relative abundance: ++ = very frequent, + = frequent, - = rare, - - = very rare p = primary, s = secondary a) Orthoclase and microcline; b) Spessartine; c) analyses in Nordrum et al. (2005); d) Oligoclase; e) Schörl

gneisses dominate. Caledonian upper greenschist-lower in the strike direction. The BPS forms a vein-like, amphibolite facies metamorphism (see below) overprints close-meshed network of steeply dipping dykes with and obscures mineral assemblages representative of a predominant strike direction of 300-320°. The total earlier metamorphic cycles. However, a minimum volume of the BPS is estimated to ≤5x105 m3, assuming estimate of pre-Caledonian p-T conditions from relict ≤50 dykes of tabular shape (100 x 100 x 1 m). A small plagioclase + clinopyroxene + garnet assemblages points number of pegmatites belonging to the BPS is also to an event reaching at least upper amphibolite facies found in gabbros to the west of the garnet amphibolite grade. Evidence of partial melting is found in rocks both lens. The mylonitic shear zones that enclose the garnet inside (garnet amphibolite lens) and west (gabbroic and amphibolite body cross-cut all other structures in the granitic gneiss) of the shear zone. field area.

The N-S striking shear zone displays the highest strain immediately west of the garnet amphibolite lens (Fig. 2), where it exhibits a sharp contact to the garnet amphibolite. The assemblage epidote + green hornblende + chlorite indicates upper greenschist- to amphibolite facies conditions, typical of the Caledonian in the Jotun Nappe Complex, suggesting either a Caledonian age of the shear zone, or Caledonian reactivation and/or overprinting of an older shear zone.

The Berdalsbandet Pegmatite Swarm

The mineralised pegmatites that make up the BPS occur in an area of approximately 1 km2, and are mainly concentrated in a garnet amphibolite lens enclosed by steeply dipping, N-S striking mylonitic shear zones (Fig. 2). The medium-grained, olivine-normative amphibolite mainly contains faintly zoned to unzoned, tschermakitic to Mg-hastingsitic hornblende, equal amounts of plagioclase (An≥88) and garnet (Prp44Alm55Sps01) as well as clinopyroxene (En38Fs15Wo47) >> orthopyroxene and ore (representative analyses in Table 2). Reactions between the BPS granitic melt and the amphibolite can locally be seen as cm-thick metasomatic biotite rims developed in the country rock along the contacts with the pegmatites (Fig. 3). The mean width of the felsic Fig. 3. Symmetrically zoned, ca. 0.5 m wide pegmatite in garnet pegmatites is ca. 50 cm, the maximum width ca. 2 m, amphibolite. Fsp: feldspar-rich zone, Qtz: quartz core. The biotite and they can typically be traced for some tens of metres rim (Bt) towards the wall rock is covered with moss (black). 346 S. Spürgin et al. NORWEGIAN JOURNAL OF GEOLOGY

Table 2. Representative EMP analy- Table 2 ses of major minerals from the garnet hornblende plagioclase clinopyroxene garnet amphibolite hosting the BPS. Oxygen wt% am4 am7 pl1 pl5 cpx1 cpx5 grt1 equivalents for formula calculation

Ts MgHs An92 An88 En41Wo48 En38Wo47 Prp44Alm55 are indicated (O).

SiO2 43.19 42.13 44.54 45.89 51.30 50.45 39.63

TiO2 0.97 1.12 -- 0.03 0.19 0.23 0.03

Al2O3 13.51 14.30 35.68 34.90 3.63 4.78 22.33

Cr2O3 0.06 0.03 ------0.02 0.03 FeOt 12.49 12.89 0.20 0.17 6.59 8.82 22.05 MnO 0.02 0.03 -- -- 0.03 0.21 0.47 MgO 13.61 13.31 -- -- 14.52 12.63 9.89 CaO 12.20 11.84 18.66 17.74 23.26 22.19 6.68

Na2O 1.66 1.92 0.85 1.37 0.44 0.58 --

K2O 0.83 0.86 0.02 0.03 ------Total 98.55 98.44 99.97 100.13 99.99 99.90 101.13 O 12.5 12.5 8 8 6 6 12 Si 6.30 6.17 2.06 2.11 1.90 1.89 2.98 Ti 0.11 0.12 0.01 0.01 Al 2.32 2.47 1.94 1.89 0.16 0.21 1.98 Cr 0.01 Fe2+ 1.52 1.58 0.01 0.01 0.20 0.28 1.39 Mn 0.01 0.03 Mg 2.96 2.91 0.80 0.70 1.11 Ca 1.91 1.86 0.92 0.87 0.92 0.89 0.54 Na 0.47 0.55 0.08 0.12 0.03 0.04 K 0.15 0.16 Sum 15.74 15.82 5.01 5.01 4.03 4.02 8.03 -- = below detection limit. Ts = tschermakite, MgHs = magnesiohastingsite

Mineral content of the BPS in the rock and as crusts covering the primary minerals described above. The secondary minerals include Major minerals are quartz, perthitic orthoclase/ epidote, prehnite, chabazite, stilbite, heulandite-(Ca), microcline and oligoclase, minor constituents are heulandite-(Sr) (both analysed by Nordrum et al. 2005) allanite-(Ce), biotite and Fe-Ti-oxides. The pegmatites and the Cu-minerals malachite, chalcopyrite along also contain muscovite, zircon, tourmaline (schorl), with unidentified mixtures of various phases, probably garnet (spessartine-dominated), thorite, phosphates and cuprite/tenorite. other accessory minerals, along with the first reported occurrence of beryl in the Jotun Nappe Complex. Other REE phases beside allanite-(Ce) are xenotime-(Y), Analytical methods monazite-(Ce), polycrase-(Y) and parisite-(Ce), the latter occurring as an alteration product of allanite (Table EMP 1). However, the modal proportions of the minerals are highly variable since the pegmatites typically are Mineral analyses were performed on a CAMECA SX100 made up of largely barren quartzofeldspathic parts electron microprobe at the Geochemical-Mineralogical with interspersed mineralised parts hosting most of the Institute, University of Freiburg, Germany. The EMP accessories. is equipped with five WD spectrometers and one ED detector. Natural and synthetic standards were used for Some of the pegmatites are symmetrically zoned along calibration, and the data were corrected for background the main axis (Fig. 3). They show a progression in the using a mean atomic number procedure. Operating mineralogy (Fig. 4) from feldspar + allanite + biotite at conditions for all measured elements, except REE, the margin to feldspar + quartz + tourmaline (schorl) + were 20 nA and 15 kV, counting time 20 s for Kα-lines Fe-Ti-oxide and quartz + tourmaline + garnet + beryl and 30 s for Th Mα-line. Sodium was analysed first in the interior parts. Graphic intergrowth of quartz and to minimise loss of the element. The REE (including feldspar occurs locally in the larger dykes. The centres of Y) Lα-lines (Lβ for Sm) were calibrated on phosphate zoned dykes consist of hydrothermal quartz, locally with standards and counted for 50 s with 100 nA and 25 kV. a core of rose quartz. Zones are not continuous for more All elements detected in the ED spectra were analysed than a few metres, and their boundaries are undulating. quantitatively. In total 21 elements were measured, Secondary, typically euhedral minerals occur in fissures including 10 REEs. NORWEGIAN JOURNAL OF GEOLOGY Mineralogy and geological setting of allanite-(Ce)-pegmatites 347

Fig. 4. Growth relations between some pegmatite minerals, as deduced from textures.

XRD Allanite-(Ce) in the BPS Crystalline materials (e.g., allanite and other pegmatite 2+ minerals) were identified using a BRUKER axs D8 Allanite, CaREEAl2Fe (SiO4)3OH, is the most abundant Advance X-ray powder diffractometer (XRD) equipped rare-earth element (REE)-bearing mineral of the epidote with a ceramic X-ray tube (2.2 kW) and a Cu-Kα- group (Armbruster et al. 2006). It is a common accessory anode, combined with DIFFRACplus v5.0 software for in many types of evolved igneous rocks, as well as in a evaluation of the data. variety of metamorphic rocks of both igneous and sedimentary origin. Magmatic allanite shows a strong ID-TIMS preference for light rare-earth elements (LREE) over heavy rare-earth elements (HREE), while the REE Uranium-lead ID-TIMS analyses were performed on distribution in metamorphic allanite is highly variable fragments mechanically broken off from larger crystals. and ultimately depends on the specific REE activity in the The fragments were air abraded (Krogh 1982), spiked precursor rock (Gieré & Sorensen 2004 and references with a 202Pb/205Pb/235U tracer and dissolved in Savillex therein). Due to the presence of minor amounts of vials on a hot-plate. The U and Pb from each fragment radioactive elements, typically Th and U, (partial) were purified using one-stage HCl or HCl - HBr, or metamictisation of allanite is a common phenomenon. two stage HBr and HNO3 ion exchange separation (see This results in an increase of unit cell volume and below), loaded together or separately on Re filaments cracking of neighbouring minerals, and diminished and analysed on a MAT262 mass-spectrometer at Oslo sharpness of diffraction patterns (Ueda 1957). University. The data were corrected using fractionation factors determined with the 202Pb/205Pb spike. Blank The Ce-dominated variety of allanite, allanite-(Ce), is correction was 2 pg Pb and 0.1 pg U. The results were locally the major constituent of the pegmatites in the BPS plotted and regressed using the Isoplot software (v. 3.00) after quartz and feldspars. In some mineralised pockets of Ludwig (2003). The decay constants are those of Jaffey it makes up >50 vol% of the rock, and its frequency in et al. (1971). For other details of the analytical procedure less-mineralised, non-barren areas of the rock is gauged see Lundmark and Corfu (2007) and references therein. to approximately 1 vol%. The allanite-(Ce) typically Uncertainties in the isotopic ratios and the ages are given occurs as euhedral tabular or thin-tabular crystals of 1 and plotted at 2σ. to 3 cm, but can be up to 15 x 8 x 3 cm large. Usually the allanite-(Ce) is associated with biotite, which grows 348 S. Spürgin et al. NORWEGIAN JOURNAL OF GEOLOGY as flakes on the crystal faces or in the interstices between allanite grains. Other REE-rich minerals typically found Table 3 with the allanite are parisite-(Ce), monazite-(Ce), RB 1 RB 6 RB 15 polycrase-(Y), thorite and zircon. Common fracturing (wt%) min. max. min. max. min. max. of the surrounding minerals suggests an advanced state SiO2 31.20 32.31 31.17 33.76 31.02 32.83 of metamictisation, but the samples are all optically TiO2 0.90 1.29 0.32 0.62 0.51 0.69 anisotropic and XRD peaks are detectable (though ThO2 0.40 0.62 0.40 0.70 0.36 1.07 diffuse), which would not be expected from completely Al2O3 14.20 15.10 15.64 16.93 14.91 16.35 non-crystalline materials (Ueda 1957). Y2O3 0.03 0.06 0.06 0.17 0.04 0.10 La2O3 6.03 6.40 4.91 6.69 6.62 6.95 Mineral chemistry of allanite-(Ce) Ce2O3 11.58 12.47 9.02 12.01 11.61 14.11 Pr2O3 1.05 1.21 0.83 1.07 0.96 1.15 Analytical results from EMP measurements of allanite Nd2O3 1.42 1.76 0.83 1.52 1.05 1.24 were recalculated on the basis of 8 cations per formula Sm2O3 0.21 0.29 0.16 0.27 0.17 0.32 unit (apfu). Iron was measured as FeOtot, and the Fe2+/ Eu2O3 -- 0.13 0.06 0.12 0.07 0.13 Fe3+ ratio was calculated by balancing 25 negative Gd2O3 0.06 0.09 0.07 0.15 0.06 0.11 charges. All other components were assumed to occur in Dy2O3 ------0.02 one oxidation state only. Yb2O3 ------0.02 -- -- FeOt 11.34 13.45 10.13 13.37 9.66 12.90 The monoclinic structure of allanite can absorb many MnO 0.29 1.05 0.25 0.53 0.23 0.32 elements by different substitution mechanisms and MgO 0.99 1.83 1.04 2.09 1.08 2.19 allocates them to IXA(1), XIA(2), VIM(1-3) and IVT positions. CaO 7.99 11.32 10.47 14.13 7.20 11.08 Following the suggestions in the recent compilations of Na2O 0.05 0.18 0.03 0.11 0.06 0.12 Gieré & Sorensen (2004) and Ercit (2002) the various ions K2O -- 0.07 -- 0.03 -- 0.09 in the allanite are distributed in the following way: P2O5 -- 0.46 -- 0.02 -- 0.05 Total 91.57 96.24 90.20 97.31 91.06 96.96 • The A(1) position is fully occupied by Ca2+ -- = below detection limit. • A(2) contains a broad assortment of larger ions: all Table 3. Maximum and minimum values (wt%) for allanite-(Ce) REE3+, remaining Ca2+ (if present), Th4+, Mn2+, analyses from samples RB 1, RB 6 and RB 15. Na+ and K+ • M(1) is filled with Al3+ (excess from M(2)), Ti4+ and Mg2+ • M(2) contains Al3+ Analytical totals of EMP measurements are relatively • M(3) is the position of ferrous and ferric iron low (< 97.2 wt%). Since no other significant components • The T position holds Si4+, P5+; excess Al3+, which have been found in XRF or in EMP spectra, this deficit can be located in T, was not found in this study. may reflect additional incorporation of OH- groups

or H2O molecules into the semi-metamict structure The composition of allanite-(Ce) from the Berdalsbandet (Poitrasson 2002). pegmatites (Table 3 and 4) meets the criteria for belonging to the allanite subgroup of the epidote group Representative analyses from the allanite samples provide

(Armbruster et al. 2006). The EMP measurements also an average calculated formula composition of (Ca1.11RE 2+ 3+ reveal internal compositional variations distributed in E0.72Mn0.03Th0.01Na0.01)Σ=1.89(Al1.76Fe 0.54Fe 0.43Mg0.24Ti0.05) a patchwork-like pattern within the grains, also visible Σ=3.02Si3.08O12(OH). As the A(1) site only contains Ca, the by microscopical investigation due to different optical dominant cations on A(2) are the REEs and Ca which fill properties of these domains (Fig. 5a). approximately 70-90 % of A(2) space (Fig. 6a). Mn and

Fig. 5. Textures of pegmatite minerals, optical and backscatter electron (BSE) images. (a) Allanite-(Ce) crystal (Aln) with patchy domains (brown and reddish-brown) and biotite (Bt) inclusions. Image width 3.7 mm. (b) Fractured zircon (Zrn) in pegmatite, healed by large, pegmatitic crystals of titanite (Ttn) and ilmenite (Ilm). NORWEGIAN JOURNAL OF GEOLOGY Mineralogy and geological setting of allanite-(Ce)-pegmatites 349 ------→ 0.05 0.74 0.31 0.09 1.00 2.19 0.75 0.01 3.07 8.00 0.26 1.00 0.02 1.87 3.06 3.07 0.65 0.61 0.05 6.86 1.01 1.21 0.23 0.12 0.67 0.08 0.03 0.28 0.30 0.01 0.07 28 10.80 11.82 32.52 15.72 12.04 96.28 ------0.04 0.73 0.30 0.09 1.00 2.16 0.76 0.01 3.06 8.00 0.32 1.00 0.03 1.87 3.06 3.06 0.60 0.62 0.07 6.87 1.03 1.10 0.20 0.12 0.69 0.05 0.06 0.27 0.28 0.01 0.07 27 10.78 11.83 32.54 15.88 12.12 96.42 ------0.04 0.74 0.30 0.10 1.00 2.14 0.75 0.01 3.08 8.00 0.24 1.00 0.02 1.88 3.04 3.08 0.60 0.57 0.07 6.87 0.96 1.14 0.21 0.12 0.69 0.08 0.26 0.28 0.01 0.06 26 10.83 11.86 32.57 15.66 12.00 95.99 ------0.04 0.73 0.21 0.12 1.00 1.47 0.80 0.02 3.04 8.00 0.23 1.00 0.02 1.90 3.07 3.04 0.51 0.65 0.06 6.62 1.06 1.07 0.22 0.13 0.70 0.12 0.32 0.31 0.01 0.09 25 11.05 11.73 32.22 16.16 12.90 96.31 ------0.04 0.76 0.30 0.09 1.00 2.00 0.75 0.01 3.09 8.00 0.23 1.00 0.02 1.89 3.02 3.09 0.54 0.38 0.04 6.81 1.03 1.05 0.17 0.08 0.04 0.72 0.06 0.21 0.23 0.01 0.07 24 10.21 11.61 31.02 14.91 11.11 91.34 ------0.04 0.74 0.30 0.11 1.00 2.10 0.76 0.01 3.08 8.00 0.25 1.00 0.02 1.89 3.03 3.08 0.56 0.36 0.04 6.95 1.03 1.15 0.20 0.12 0.06 0.03 0.68 0.06 0.25 0.27 0.01 23 10.97 11.83 32.57 15.78 11.75 95.83 15 ------RB 0.04 0.73 0.26 0.11 1.00 1.84 0.79 0.01 3.07 8.00 0.24 1.00 0.02 1.88 3.05 3.07 0.60 0.55 0.07 6.87 1.03 1.14 0.23 0.10 0.07 0.05 0.73 0.06 0.23 0.24 0.01 22 11.08 11.86 32.83 16.19 12.16 96.96 -- -- 0.05 0.92 0.17 7.20 0.80 1.08 0.88 0.01 0.01 3.26 8.00 0.32 1.00 0.03 1.80 2.94 3.26 9.66 0.69 1.07 0.10 6.94 1.15 1.24 0.32 0.13 0.11 0.05 0.84 0.06 0.09 0.00 0.03 21 14.11 31.37 15.38 91.06 -- -- 0.01 0.04 0.73 0.30 0.11 1.00 2.13 0.75 0.01 3.07 8.00 0.28 1.00 0.02 1.88 3.04 3.07 0.02 0.62 0.56 0.09 6.83 1.00 1.16 0.24 0.08 0.09 0.02 0.67 0.08 0.28 0.30 20 11.00 11.84 32.69 15.85 12.12 96.69 ------0.01 0.04 0.74 0.26 0.10 1.00 1.84 0.81 0.02 3.08 8.00 0.31 1.00 0.02 1.89 3.03 3.08 0.57 0.54 0.08 6.91 1.06 1.17 0.23 0.10 0.11 0.02 0.72 0.09 0.20 0.22 19 10.89 11.90 32.78 16.35 11.72 96.67 ------0.01 0.04 0.74 0.30 0.11 1.00 2.15 0.74 0.01 3.05 8.00 0.26 1.00 0.02 1.89 3.05 3.05 0.58 0.55 0.07 6.82 1.00 1.13 0.25 0.09 0.11 0.65 0.06 0.32 0.33 18 10.99 11.88 32.35 15.66 12.24 96.18 ------← 0.01 0.05 0.74 0.30 0.11 1.00 2.16 0.74 0.01 3.09 8.00 0.28 1.00 0.02 1.89 3.02 3.09 0.04 0.64 0.47 0.07 6.87 1.09 1.14 0.23 0.07 0.06 0.69 0.07 0.24 0.25 17 10.93 11.88 32.72 15.67 11.72 96.13 ------→ 0.01 0.04 0.74 0.28 0.12 1.00 1.98 0.73 0.02 3.08 8.00 0.37 1.00 0.03 1.92 3.01 3.08 0.60 0.61 0.06 6.62 1.02 1.29 0.17 0.12 0.07 0.67 0.11 0.29 0.30 11.13 12.01 32.88 15.64 12.24 16-B 96.92 -- -- 0.01 0.02 0.54 0.14 0.37 1.00 1.04 0.81 0.01 3.07 8.00 0.41 1.00 0.03 1.96 2.95 3.07 9.02 0.02 0.35 0.40 0.09 5.00 0.86 1.02 0.16 0.10 0.09 0.53 0.46 0.05 0.02 0.45 14.13 33.76 16.93 12.97 96.39 15-D ------0.01 0.02 0.74 0.22 0.13 1.00 1.50 0.86 0.01 3.13 8.00 0.30 1.00 0.03 1.92 2.95 3.13 0.02 0.32 0.57 0.09 6.13 1.01 1.19 0.22 0.08 0.09 0.77 0.09 0.05 0.08 10.47 11.16 31.17 15.70 10.13 90.20 14-D ------0.01 0.03 0.72 0.26 0.12 1.00 1.88 0.73 0.02 3.07 8.00 0.41 1.00 0.03 1.90 3.01 3.07 0.45 0.65 0.07 6.61 1.05 1.17 0.21 0.08 0.08 0.66 0.33 0.11 0.33 11.22 11.81 32.93 15.75 12.69 13-B 97.17 ------0.01 0.04 0.67 0.21 0.17 1.00 1.49 0.75 0.01 3.13 8.00 0.36 1.00 0.03 1.89 2.99 3.13 0.50 0.58 0.11 5.86 0.95 1.28 0.22 0.09 0.09 0.76 0.23 0.04 0.23 11.76 11.02 33.59 15.90 12.63 12-B 96.46 ------RB6 0.01 0.04 0.72 0.26 0.14 1.00 1.84 0.71 0.01 3.08 8.00 0.53 1.00 0.04 1.92 2.99 3.08 0.02 0.56 0.48 0.17 6.26 1.03 1.52 0.26 0.06 0.15 0.67 0.32 0.07 0.31 11.36 11.44 32.93 15.67 12.48 11-B 96.84 -- -- 0.01 0.04 0.65 0.22 0.21 1.00 1.63 0.76 0.01 3.07 8.00 0.47 1.00 0.04 1.92 3.00 3.07 0.50 0.59 0.11 5.91 0.94 1.17 0.24 0.07 0.08 0.02 0.61 0.38 0.07 0.03 0.37 12.28 10.82 33.32 16.27 12.80 10-B 97.31 ------0.01 0.02 0.56 0.16 0.38 1.00 1.16 0.83 0.01 2.97 8.00 0.32 1.00 0.03 1.99 3.05 2.97 9.07 0.33 0.46 0.08 4.91 0.83 0.86 0.21 0.08 0.10 0.42 0.60 0.05 0.62 9-D 13.66 31.54 16.53 13.19 93.38 ------0.01 0.02 0.57 0.16 0.34 1.00 1.16 0.82 0.01 2.99 8.00 0.25 1.00 0.02 1.95 3.05 2.99 9.32 0.35 0.50 0.07 5.06 0.83 0.83 0.19 0.08 0.09 0.48 0.54 0.03 0.57 8-D 13.33 31.76 16.44 13.37 93.65 ------← 0.01 0.04 0.74 0.29 0.10 1.00 2.09 0.73 0.02 3.06 8.00 0.33 1.00 0.03 1.90 3.04 3.06 0.62 0.70 0.06 6.69 1.07 1.19 0.27 0.09 0.10 0.66 0.33 0.10 0.32 7-B 10.90 32.55 15.64 11.92 12.51 96.84 ------→ 0.01 0.09 0.78 0.15 8.19 0.01 0.04 0.89 0.99 0.80 0.04 3.15 8.00 1.05 1.00 0.09 1.82 3.00 3.19 1.22 0.62 0.04 6.03 1.05 1.42 0.21 0.07 0.46 0.96 0.00 0.18 0.07 6-R 31.20 15.10 12.33 11.34 91.57 ------0.01 0.10 0.80 0.16 7.99 0.01 0.86 1.06 0.79 0.03 3.17 8.00 1.02 1.00 0.09 1.79 3.04 3.18 1.29 0.53 0.05 6.12 1.08 1.59 0.28 0.09 0.09 0.99 0.00 0.16 5-R 31.60 15.08 12.45 11.78 92.29 ------0.01 0.07 0.75 0.16 0.26 1.00 1.83 0.61 0.01 3.03 8.00 0.32 1.00 0.03 1.96 3.02 3.03 0.92 0.46 0.03 6.40 1.09 1.66 0.27 0.13 0.08 0.63 0.42 0.07 0.45 4-C 11.32 31.51 14.20 11.58 13.41 95.27 RB1 ------0.01 0.06 0.74 0.13 0.22 1.00 1.57 0.66 0.01 3.07 8.00 0.29 1.00 0.02 1.91 3.01 3.07 0.90 0.40 0.03 6.34 1.12 1.76 0.24 0.14 0.06 0.74 0.30 0.05 0.02 0.33 3-C 11.09 32.31 14.82 11.66 13.45 96.24 -- -- ← 0.01 0.08 0.81 0.16 8.68 0.01 0.92 1.09 0.75 0.03 3.19 8.00 0.77 1.00 0.06 1.83 2.98 3.20 1.04 0.52 0.06 6.28 1.21 1.75 0.29 0.08 0.08 0.08 0.99 0.00 0.14 0.02 2-R 32.13 14.95 12.47 11.91 93.54 P 2+ 3+ K Si Ti Al Al Ca Ca Th Na Mg Mn Fe Fe REE 3 3 3 3 3 3 3 3 3 3 2 t 3 2 5 O O O O 2 O O O O 2 O 2 2 O 2 O 2 2 2 2 2 O 2 ox 2 O O wt% 2 2

A M T 2 Σ Na A(2) CaO Σ Σ Fe MgO A(1) M(1) M(3) T Sum Pr FeO TiO Y MnO ThO Al La Ce Nd Sm Eu Gd Total K P Yb SiO M(2) -R= rim, -C -B = =center, bright domain, -D = dark domain, -- = below detection limit. Dy Table 4 Table Table 4. Electron microprobe analyses and formula calculations of allanite-(Ce). of calculations formula and analyses microprobe Electron 4. Table 350 S. Spürgin et al. NORWEGIAN JOURNAL OF GEOLOGY

Fig. 6. Composition of allanite-(Ce) from Berdalsbandet. (a) Variation in A(2) site. A(1) is exclusively occupied by Ca. (b) Variation in M(1) plus M(3) sites. M(2) is exclusively occupied by Al. Some epidote-group end-members are indicated.

Th are present to a lesser degree. Data points cluster on the REE-rich end of the Ca-REE connecting line of the A(2)-site diagram, which is expected in pure allanite. Fig. 6b shows the variability on the M(1)- and M(3)-sites, whereas M(2) is fully occupied by Al (not plotted in the diagram). A large spread in distribution is found between Fe2+ and trivalent cations, whereas incorporation of the Mg-component (dollaseite) is homogeneous with an average of ca. 10-15 %. Again, most data points are close to the allanite end-member composition with equal amounts of Fe2+ and Al3+ in M(3) and M(1), respectively.

The most significant compositional difference between 3+ the allanite-(Ce) samples is found in the Feox = Fe / 3+ 2+ (Fe +Fe ) value. Feox is elevated in sample RB 6 (0.36) compared to RB 1 (0.14) and RB 15 (0.25). In general, RB 6 shows the widest compositional range of all analysed allanite-(Ce) samples. RB 6 data points are nearly in the same relative order in Fig. 6a and b: measurements resulting in high Ca values on A(2) also show high Al+Fe3+, whereas REE-rich ones yield high Fe2+. Fig. 7. C1-normalised REE pattern of allanite-(Ce) from the Berdals- bandet pegmatites. C1 data from Evensen et al. 1978. REE distribution in BPS allanite-(Ce) The structure of allanite is strongly selective for LREE. Geochronology The Berdalsbandet allanite-(Ce) has a total REE-content between 0.56 and 0.92 apfu (average 0.72 apfu). Ce (average 0.40 apfu) is dominant, followed by La (0.22 The allanites are texturally part of the pegmatite apfu) and the other LREE. The HREE are commonly mineral assemblage, and their crystallisation age is below the detection limit of the EMP. C1-chondrite thus constrained by a 950 ± 1 Ma zircon ID-TIMS age normalised REE patterns in Fig. 7 (C1 data from Evensen (Lundmark et al. 2007). To test the resilience of the et al. 1978) show a distinct enrichment of LREE by allanites to resetting, and potentially to assign an age 105 with a rapid decrease with higher atomic number to the annealing of metamict domains revealed by BSE (sample/C1 ≈ 103). This is comparable to data reported imaging (see below), eight analyses were obtained from from other igneous allanite (Gromet & Silver 1983; fragments of two cm-sized allanite crystals, RB15 and Peterson & MacFarlane 1993; Wood & Ricketts 2000; SS97 (Table 5). The analyses display a generally low Gieré & Sorensen 2004). analytical precision that mainly reflects the uncertainty NORWEGIAN JOURNAL OF GEOLOGY Mineralogy and geological setting of allanite-(Ce)-pegmatites 351

Fig. 8. Substitution mechanism in analysed allanite-(Ce). End- member compositions of allanite, dissakisite (Mg-equivalent of Aln) and hypothetical oxyalla- 3+ nite, CaREEAl2Fe (SiO4)3O, are labeled.

introduced by the correction for initial Pb, the result of the precursor (-s) to the pegmatites. Use of a more of both a relatively low radiogenic Pb content due to U primitive initial Pb correction corresponding to a concentrations of only 18 to 38 ppm, and a high initial Depleted Mantle composition (Neymark 1990) shifts Pb content of 6 to 16 ppm (Table 5). Additionally, the data more firmly onto Concordia, but also towards incomplete purification using one-stage HCl (5, 8; Table ages that are distinctly older than the 950 ± 1 Ma age 5), or one stage HCl-HBr (1; Table 5) separation led to of the pegmatite. Given the large, pegmatitic size of the weak ion beams. The remaining analyses were obtained allanite crystals it seems unlikely that the data reflect using two-stage HBr and HNO3 separation, and the inheritance. Also, no reverse discordance indicative purified U and Pb were loaded on separate Re filaments of a more primitive initial Pb has been observed in which improved the precision of the measurements. titanite data from other samples in the Upper Jotun Correction for initial Pb using the Stacey and Kramers Nappe (Lundmark et al. 2007; Lundmark and Corfu (1975) two-stage Pb evolution model yields reversely 2008a). Therefore, a more likely reason for the reverse discordant analyses (but still overlapping the Concordia discordance is the presence of excess 206Pb, an effect curve). Initial common Pb with lower 206Pb/204Pb and commonly observed in minerals with high Th/U 207Pb/204Pb ratios than predicted by the Stacey and ratios such as monazite and allanite (Schärer 1984). Kramers (1975) model could result from long-standing Incorporation of excess 230Th, an intermediate daughter depletion of U following high-grade metamorphism nuclide in the 238U decay series, during magmatic

Table 5

Nr (grain) Weighta Ua Th/Ub Pbca,c 206Pbd 207Pbe 2 sigma 206 Pbe 2 sigma rhof 207Pbe 2 sigma 206Pbe 2 sigma 207 Pbe 2 sigma Disc 204Pb 235U 238U 206Pb 238U 235U [ug] [ppm] [ppm] [abs] [abs] [abs] [Ma] [abs] [Ma] [abs] [%] 1 (SS97) 80 31.2 138.8 15.53 37.2 1.54 0.19 0.1643 0.0031 0.06 0.0682 0.0083 981 17 948 72 -13 2 (SS97) 13 25.9 168.6 7.70 49.5 1.45 0.11 0.1603 0.0017 0.00 0.0655 0.0049 958 10 908 44 -23 3 (SS97) 15 38.0 148.9 12.28 46.7 1.46 0.13 0.1583 0.0019 0.11 0.0668 0.0059 948 11 913 53 -15 4 (SS97) 29 34.7 163.0 11.22 46.8 1.45 0.11 0.1583 0.0018 0.00 0.0663 0.0053 947 10 909 47 -17 5 (RB15) 57 18.1 255.8 5.81 46.8 1.39 0.55 0.1572 0.0095 0.74 0.064 0.023 941 53 887 210 -27 6 (SS97) 17 33.2 161.3 8.62 53.1 1.421 0.093 0.1549 0.0015 0.00 0.0665 0.0044 928.6 8.8 898 39 -14 7 (SS97) 24 31.4 163.7 7.26 57.1 1.407 0.083 0.1532 0.0013 0.00 0.0666 0.0040 918.7 7.8 892 35 -12 8 (SS97) 2 37.4 169.5 11.92 44.2 1.36 0.12 0.1523 0.0019 0.02 0.0649 0.0057 914 11 873 50 -20 a) weight and concentrations are known to about 10% for samples larger than 5 ug and not better than 50% for smaller fractions b) Th/U model ratio inferred from 208/206 ratio and age of sample c) Pbc = initial common Pb in sample (corrected for spike, fractionation and blank) d) raw data corrected for fractionation e) corrected for fractionation, spike, blank and initial common Pb; error calculated by propagating the main sources of uncertainty f) the low error correlations reflect the large proportion of common Pb 352 S. Spürgin et al. NORWEGIAN JOURNAL OF GEOLOGY crystallisation leads to formation of 206Pb that is not Ca2+ + Fe3+ → REE3+ + Fe2+ (1) related to the 238U content of the mineral, resulting in too high 206Pb/238U ratios and hence reverse discordance. Ca2+ + Al3+ → REE3+ + Fe2+ (2) The data may be corrected if the Th/U ratio of the host rock is known, and assuming that it mirrors the Th/U To maintain charge balance in natural allanite, Ca, Al and ratio of the melt from which the allanites grew (Schärer, Fe3+ may be replaced by REE3+ and by various divalent 1984). In our case, excess 206Pb is supported by the very M- and A-site species. The universal substitution high Th/U ratios of 139-256 (Table 1). Unfortunately it is is presented in Peterson & MacFarlane (1993) and difficult to correct the data for initial disequilibrium due Armbruster et al. (2006): to the mineralogical inhomogeneity of the pegmatite. Even so, the presence of excess 206Pb testifies to a Pb A2Ca2+ + M3M3+ → A2(REE)3+ + M3M2+ (3) isotopic system that has undergone but very minor post- magmatic resetting (e.g., von Blanckenburg 1992). with Al3+ and Fe3+ for M3+ and Fe2+, Mg2+ and Mn2+ for M2+. Substitution (3) is prominent in the analysed Excess 206Pb does not affect the 207Pb/235U ages. Corrected allanite-(Ce). The linear relationship for the BPS allanite with crustal Pb (Stacey and Kramers 1975) they range is apparent in Fig. 9. from 948 to 873 Ma, overlapping with or slightly younger than the 950 ± 1 Ma zircon age. This is most Another mechanism of charge balancing, described by easily explained by minor post-magmatic Pb loss, which Peterson & MacFarlane (1993) and shown in Fig. 10, fits with observations supporting recrystallisation of is restricted to the main A-site cations and leaves one partly metamict allanite (see below). The analytical position unfilled: uncertainties in the data do not allow us to completely resolve the contributions from the initial Pb composition, 3Ca2+ → 2REE3+ (4) excess 206Pb, and subsequent Pb loss. Nevertheless, the data firmly support the first order conclusion that the Allanite-(Ce) and bulk rock REE in the BPS allanite fragments largely retain magmatic Pb isotopic signatures, with only marginal resetting of the isotopic Allanite-(Ce) is the major REE-bearing phase in the systems during subsequent events. BPS, dominating the LREE spectrum of the rock. Monazite-(Ce), the second most abundant LREE-rich mineral, is far less common than allanite-(Ce). It shows Discussion C1-normalised LREE patterns (Cresbron 1989; Möller 1989) that generally follow the same trend as in allanite- Exchange mechanisms in allanite-(Ce) (Ce). The LREE concentrations in major minerals in the pegmatites beside allanite-(Ce) (quartz, K-feldspar, Ideal allanite is connected to epidote and to clinoziosite plagioclase) are negligible (Fig. 11) (Rollinson 1993). The by REE-incorporating coupled substitutions (1) and (2), HREE spectrum is likely controlled by other minerals respectively (Gieré & Sorensen 2004): than allanite-(Ce), such as zircon (Brooks et al. 1981)

Fig. 9. Charge-balanced substitution of 3 Ca2+ by 2 REE3+ in allanite-(Ce). NORWEGIAN JOURNAL OF GEOLOGY Mineralogy and geological setting of allanite-(Ce)-pegmatites 353 and xenotime-(Y). Therefore, it can be assumed that the magmatic fluid flow, known to cause alteration along LREE (La-Sm) content of allanite-(Ce) represents a good grain-internal cracks (Petrík et al. 1995). This typically approximation of the bulk rock LREE content. The LREE produces sectors with elevated Feox, while Feox in original content for mineralised areas in the pegmatites can then igneous allanite is buffered by biotite and remains close to be estimated by the linear function 0.4 (Petrík et al. 1995). However, the analysed samples are hardly oxidised, in fact, they are mildly reduced compared

LREEN,rock = LREEN,Aln XAln,rock (5) to the biotite buffer (Table 3). Only a few domains in RB 6 show increased Feox-values of 0.57 and 0.60 (Table 4). Also, with LREEN,rock representing the C1-normalised LREE there is typically no direct correlation between cracks content in the bulk rock, LREEN,Aln the normalised in the grains and compositional domains; in fact, most LREE content in analysed allanite-(Ce), and XAln,rock the compositional domains are independent of the density and modal abundance of allanite-(Ce) in the rock, in this orientation of cracks (Fig. 5a). Alternatively, allanite may case estimated to 1% in mineralised parts (given the re-equilibrate and its components may be redistributed precision of this estimate vol% is assumed equal to wt%). into domains of more stable composition during post- It is evident that the shape of a C1-normalised curve is magmatic p-T-changes. Minor recrystallisation of identical to that of allanite-(Ce) (Fig. 7) diluted to 1%. metamict allanite is reported to start at temperatures as Compared to the chondritic value, and depending on the low as 200-300°C, with major annealing occurring in chosen constraints, the mineralised pegmatite batches two stages at 500-600°C (greenschist-amphibolite facies are enriched in La by a factor of 2000 and in Sm by 100. transition) and at 700-800°C (amphibolite-granulite facies transition), reflecting different annealing mechanisms Recrystallisation of allanite-(Ce) in the BPS (Janeczek & Eby 1993; Paulmann et al. 2000; Paulmann & Bismayer 2001). Magmatic zoning in allanite is typically characterised by straight compositional boundaries parallel to the The annealing of metamict domains observed as crystal faces, and oscillatory zoning, whereas irregular zoning in BSE and the minor Pb-loss indicated by the zoning with different brightness in backscatter electron ID-TIMS data in this study are unlikely to reflect the (BSE) imaging is indicative of secondary alteration (e.g., late-Sveconorwegian magmatic / metamorphic events Petrík et al. 1995; Poitrasson 2002; Hoshino et al. 2006). reported by Lundmark et al. (2007), since the allanite The patchwork-like, irregular compositional variations would not have had time to accumulate significant observed in this study are suggestive of late- or post- radiation damage at this time. Therefore, we attribute the observed textures to Caledonian annealing, at which time the Hurrungane rocks were exposed to upper greenschist-lower amphibolite facies conditions. The fact that the allanite fragments have largely retained their magmatic Pb isotopic signatures through Caledonian metamorphism when the allanites were already more than 500 m. y. old is remarkable, given the locally strong metamorphic overprint affecting the Upper Jotun Nappe (Lundmark et al. 2007; Lundmark and Corfu 2008a) and the tendency of allanite to become metamict (e.g., Poitrasson 2002 and references therein).

The origin and emplacement of the BPS

Plutonic activity in the Hurrungane area at ca. 1660 to 1630 Ma is distinctly older than the 950 ± 1 Ma pegmatites (Lundmark et al. 2007), indicating that large-scale plutonism is not the source of the evolved, REE-enriched melt manifested in the pegmatites. The presence of 954 ± 3 Ma syntectonic anatexite in the Hurrungane area and ca. 1600 Ma strongly discordant inhereted zircons in the BPS led Lundmark et al. (2007) to suggest that the pegmatites are related to partial melting in the country rock. When classified according to the scheme of Černý & Ercit (2005) and Černý (1990, 1992), the Berdalsbandet pegmatites belong to the LREE- Fig. 10. Partition coefficients (D) for REE between rhyolitic melt and some of the minerals occurring in the Berdalsbandet pegmati- enriched abyssal class (AB-LREE). These are known tes. Data from Rollinson (1993). LREE partition chiefly in allanite, to form under upper amphibolite- to granulite-facies HREE in zircon. conditions in anatectic terranes and typically contain 354 S. Spürgin et al. NORWEGIAN JOURNAL OF GEOLOGY minerals such as allanite, monazite and thorite. The high side of the amphibolite lens suggest a limited amount REE content of the pegmatites (modelled from allanite- of Caledonian displacement along this part of the N-S (Ce)) indicates either a low degree of partial melting, stretching mylonitic shear zone, despite the pervasive concentrating incompatible elements from a large Caledonian overprint. volume of country rock into a small volume of melt, or fractionation processes in the anatectic melt batches; the Although zeolites may exist as primary phases in small volumes of anatexite observed in the country rock pegmatites (Gottardi 1989, London 2008), the p-T in the extended field area supports the first alternative. sensitive zeolites in the BPS cannot be older than the Caledonian greenschist-facies event. Their growth is The generally subparallel pegmatites (Figure 12b) likely related to late- to post-Caledonian retrogression are oblique to the shear zones surrounding the from greenschist- to zeolite facies conditions. The copper garnet amphibolite lens (Figure 12a). The geometry mineralisation is clearly a subsolidus event, but its age is consistent with emplacement of the dykes normal remains uncertain. to the minimum compressive stress (σ3) in an active sinistral non-coaxial shear zone (Figure 12c). This is further supported by the concentration of the dykes Conclusions in the sheared, lens-shaped garnet amphibolite. Furthermore, shear deformation during solidification Four stages of mineral growth are distinguished in the of the dykes is indicated by fractured pegmatitic Berdalsbandet pegmatites: (1) an initial pegmatitic minerals (e.g., zircon, beryl) that were healed by later stage with mineral growth from the melt and formation pegmatitic phases (titanite, quartz) (Fig. 5b), and by of a metasomatic biotite rim by interaction of the dynamic high-T recrystallisation of quartz and biotite pegmatitic melt with the wallrock; (2) a hydrothermal near deformation zones. Therefore, we conclude stage, leading to the formation of massive quartz cores that the BPS is syntectonic, and that the age of the during the final dyke solidification; (3) a stage of allanite- pegmatites dates Sveconorwegian movements along (Ce) recrystallisation (4) a subsolidus, late- or post- the shear zones surrounding the garnet amphibolite Caledonian stage, where local zeolitisation of the dykes lens. It follows that the Caledonian deformation took place. The age of a secondary copper mineralisation observed in the mylonites constitutes reactivation of remains uncertain. Sveconorwegian structures. Allanite-(Ce) constitutes an important part of the BPS Field observations of a small number of BPS type mineralogy. Its composition, expressed by representative pegmatites in medium-grained gabbros on the western analyses of grains from three samples, is: (Ca0.86R

Fig. 11. a) The oblique orientation of sub-parallel, syntectonic Berdalsban- det pegmatites to the north-trending garnet amphibolite lens and surrounding shear zones indicates that synmag- matic sinistral shear controls the geometry of the pegmatites. b) Stereoplot of representative measurements of the orientation of twenty dm-wide pegmatites, arrows mark inferred mini-

mum compressive stress (σ3). c) The dykes are accommodated normal to the

minimum compressive stress (σ3) in the shear zone. The model illustrates a possible mechanism for the formation of en echelon dykes in an active shear zone (modified from Hoek 1991). NORWEGIAN JOURNAL OF GEOLOGY Mineralogy and geological setting of allanite-(Ce)-pegmatites 355

2+ EE0.80Mn0.09Th0.01Na0.03)Σ1.79(Al1.79Fe 0.99Mg0.16Ti0.10) References Σ3.04(Si3.17P0.01)O12(OH), (Ca1.12REE0.72Mn0.03Th0.01Na0.02) 2+ 3+ Armbruster, T., Bonazzi, P., Akasaka, M., Bermanec, V., Chopin, C., Σ1.90(Al1.73Fe 0.66Fe 0.33Mg0.26Ti0.03)Σ3.01Si3.07O12(OH) and 2+ 3+ Gieré, R., Heuss-Assbichler, S., Liebscher, A., Menchetti, S., Pan, Y. (Ca1.10REE0.74Mn0.02Th0.01Na0.01)Σ1.88(Al1.75Fe 0.69Fe 0.26M 0.04 & Pasero, M. 2006: Recommended nomenclature of epidote-group g0.30Ti )Σ3.04Si3.08O12(OH). The dominant substitution minerals. European Journal of Mineralogy 18, 551-567. 2+ 3+ 3+ 2+ 2+ mechanisms are Ca + M = REE + M and 3Ca Battey, M.H. & McRitchie, W.D. 1973: A geological traverse across the = 2REE3+. A pattern of distinct LREE enrichment pyroxene-granulites of Jotunheimen in the Norwegian Caledo- is comparable to data reported from other igneous nides. Norwegian Journal of Geology Supplement 53(3), 237–265. allanites. Additionally, a series of accessory minerals are Brooks, C. K., Henderson, P. & Rønsbo, J. G. 1981: Rare-earth parti- identified that contain High Field Strength Elements. tioning between allanite and glass in the obsidian of Sandy Braes, Northern Ireland. Mineralogical Magazine 44, 157-160.

207 235 Bryhni, I. & Andréasson, P. G. 1985: Metamorphism in the Scandina- A range of Pb/ U ages from 948 to 873 Ma and the vian Caledonides. In Gee, D. G. & Sturt, B. A. (eds.): The Caledo- 206 presence of excess Pb show that allanites in this nide Orogen – Scandinavia and related areas, 763-781, John Wiley study have largely retained their magmatic Pb isotopic & Sons, London. signatures despite the locally strong Caledonian upper Bryhni, I. & Sturt, B. A. 1985: Caledonides of southwestern Norway. In greenschist-lower amphibolite facies metamorphic Gee, D. G. & Sturt, B. A. (eds.): The Caledonide Orogen – Scandina- overprint in the field area. A patchwork-like distribution via and related areas, 89-107, John Wiley and Sons, London. Černý, P. 1990: Distribution, affiliation and derivation of rare-element of compositional variations in the allanite-(Ce) grains granitic pegmatites in the Canadian Shield. Geologische Rundschau is thought to reflect Caledonian annealing of metamict 79, 183-226. allanite, associated with minor Pb loss. Černý, P. 1992: Geochemical and petrogenetic features of mineraliza- tion in rare-element granitic pegmatites in the light of current rese- The BPS belongs to the abyssal class of pegmatites arch. Applied Geochemistry 7, 393-416. (AB-LREE), supporting a genetic link to coeval, granitic Černý, P. & Ercit, T. S. 2005: The classification of granitic pegmatites anatexite present in the field area. A strong enrichment revisited. The Canadian Mineralogist 43, 2005-2026. Cresbron, F. P. 1989: Mineralogy of the rare-earth elements. In Möller, in LREE content of the BPS, inferred from allanite- P., Černý, P. & Saupé, F. (eds.): Lanthanides, tantalum and niobium. (Ce) compositions and relative abundance in the BPS, Proceedings of a workshop in Berlin, 1989. Special Publication of indicates a low degree of partial melting consistent with the Society for Geology applied to Mineral Deposits 7, 3-26. field observations. Ercit, T. S. 2002: The mess that is “allanite”. The Canadian Mineralogist 40, 1411-1419. The geometric relationship of the BPS to its amphibolite Evensen, N. M., Hamilton, P. J. & O’Nions, R. K. 1978: Rare-earth host rock and to the surrounding shear zones, along abundances in chondritic meteorites. Geochimica et Cosmochimica Acta 42, 1199-1212. with detailed observations of minerals in the BPS, lead Fossen, H. & Dunlap, W. J. 1998: Timing and kinematics of Caledo- us to conclude that the BPS was emplaced in an active nian thrusting and extension collapse, southern Norway; evidence sinistral, non-coaxial shear zone, and that the pegmatites from 40Ar/39Ar thermochronology. Journal of Structural Geology 20, date Sveconorwegian shearing to 950 ± 1 Ma. 765-781. Gieré, R. & Sorensen, S. S. 2004: Allanite and other REE-rich Epi- It is further proposed that a strong Caledonian overprint dote-group minerals. In Liebscher, A. & Franz, G. (eds.): Epidotes. in the area surrounding the amphibolite lens hosting Reviews in Mineralogy and Geochemistry 56, 431-493. Gottardi, G. 1989: The genesis of zeolites. European Journal of Minera- the BPS constititutes reactivation of Sveconorwegian logy 1, 479-487. shear zones, though field observations point to a limited Gromet, L. P. & Silver, L. T. 1983: Rare earth element distributions amount of displacement despite the locally pervasive among minerals in a granodiorite and their petrogenetic implica- metamorphic overprint. tions. Geochimica et Cosmochimica Acta 47, 925-939. Hoek, J. D. 1991: A classification of dyke-fracture geometry with examples from Precambrian dyke swarms in the Vestfold Hills, Antarctica. Geologische Rundschau 80, 233-248. Acknowledgements Hoshino, M., Kimata, M., Shimizu, M., Nishida, N., & Fujiwara, T. The authors are grateful to the staff of the Mineralogical-Geochemical 2006: Allanite-(Ce) in granitic rocks from Japan: Genetic implica- Institute in Freiburg, especially H. Müller-Sigmund for her help with tions of patterns of REE and Mn enrichment. The Canadian Mine- microprobe analysis and R. Gieré for his advice concerning aspects of ralogist 44, 45-62. allanite mineralogy. Discussions with R. Maurer in the field and later Jaffey, A. H. K., Flynn, F., Glendenin, L. E., Bentley, W. C. & Essling, A. in the office were essential for correct interpretation of the geology of M. 1971: Precision measurment of half-lives and specific activities 235 238 Berdalsbandet. The considerations of R. Grapes strengthened the signi- of U and U. Physical Review C, Nuclear Physics 4, 1889-1906. Janeczek, J. & Eby, R. K. 1993: Annealing of radiation damage in alla- ficance of the presented data. We thank T. Garmo for sharing his know- nite and gadolinite. 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