Rune S. Selbekk

Generation of anorthositic magma by H2O-fluxed anatexis of silica-undersaturated gabbro: an example from the north Norwegian Caledonides

The Skattora migmatite complex in the north Norwegian Caledonides consists of migmatized slightly nepheline-normative metagabbros that are net-veined by numerous (up to 90%) anorthositic and leucodioritic dykes. The average chemical composition of 17 anorthosite dykes is (wt %) 58.4% SiO 2 , 0.2% TiO 2 , 23% Al 2 O 3 , 1.8% FeO t , 0.7% MgO, 6.3% CaO, 7.8% Na 2 O, 0.2% K 2 O. A migmatite leucosome and a dyke have been dated by the U/Pb method on titanite to 456±4 Ma. In low melt fraction areas minor leucosomes are orientated parallel to the foliation. More intense anatexis formed stromatic to schlieric migmatites. The leucosomes are commonly connected to dykes, suggesting that melt segregated and left its source. Dyke thicknesses range from a few centimetres up to several metres. In general, early dykes are parallel to the foliation in the host rock, while the later dykes cut the foliation. Plagioclase (An 20–50 ) is the dominant mineral (85–100%) in the dykes and the leucosome, but 0–15% amphibole is generally present. Field relations, geochemistry and preliminary melting-experiments strongly suggest that the anorthosites originated by H 2 O-fluxed anatexis of the gabbroic host rock.

Contrasting tonalite genesis in the lyngen magmatic complex, north norwegian caledonides

Abstract The Lyngen gabbro (LG), defining the major part of the Lyngen magmatic complex, is characterised by layered gabbros of N-MORB affinity (western suite) and layered gabbronorites, quartz-bearing gabbros and diorites/quartz-diorites of IAT (island-arc tholeiite) to boninitic affinity (eastern suite). The boundary between the eastern and western suites is generally defined by a large-scale ductile shear zone of suboceanic origin, the Rypdalen shear zone (RSZ). Tonalites occur within the RSZ and in the eastern suite of the LG. Variations in field occurrence and chemical composition of the tonalites suggest that they represent two petrologically different groups. Tonalite intrusion (the Vakkas pluton) up to 5 km2 large occur in the eastern suite of the LG, and are characterised by high Y contents (average 26 ppm) and high K 2 O Rb ratios (average 0.062) compared to tonalites on the RSZ. The Vakkas pluton has lightly concave REE (rare earth element) patterns with negative Eu-anomalies, and positive eND-values (+3.7 to +3.9). Geochemical modelling based on the REE and field evidence suggests that these tonalites may have formed by fractional crystallization from a boninitic parental magma. Tonalites related to the RSZ form irregular veins and dikes that net vein the shear zone. They are characterised by low Y contents (average 6 ppm), low K 2 O Rb ratios (average 0.025), and highly variable contents of Na2O, K2O, Sr and Ba, compared to the Vakkas pluton. Tonalites related to the RSZ show substantial variation in the content of the LREEs. They possess low abundances of the HREEs, and absence of, or slightly positive Eu-anomalies. The tonalites have highly variable eND-values (−0.6 to −9.4), probably resulting from enrichment of Nd from an external source. Geochemical modelling suggests that the LREE-rich tonalites formed by H2O-rich partial melting of differentiated products from the eastern suite of the LG. The presence of B in the fluid phase is suggested by the presence of tourmaline-bearing tonalite pegmatites. Thus, the anatectic tonalites of this group could have been formed by water-excess melting of a variety of gabbroic cumulates of the LG. In the LG, LREE-depleted tonalites (eND-values +5.1) also occur, and these are best explained in terms of partial melting of gabbroic cumulates from the transition zone between the eastern and the western suites of the LG.

Petrology of nepheline syenite pegmatites in the Oslo Rift, Norway: Zr and Ti mineral assemblages in miaskitic and agpaitic pegmatites in the Larvik Plutonic Complex

Abstract Agpaitic nepheline syenites have complex, Na-Ca-Zr-Ti minerals as the main hosts for zirconium and titanium, rather than zircon and titanite, which are characteristic for miaskitic rocks. The transition from a miaskitic to an agpaitic crystallization regime in silica-undersaturated magma has traditionally been related to increasing peralkalinity of the magma, but halogen and water contents are also important parameters. The Larvik Plutonic Complex (LPC) in the Permian Oslo Rift, Norway consists of intrusions of hypersolvus monzonite (larvikite), nepheline monzonite (lardalite) and nepheline syenite. Pegmatites ranging in composition from miaskitic syenite with or without nepheline to mildly agpaitic nepheline syenite are the latest products of magmatic differentiation in the complex. The pegmatites can be grouped in (at least) four distinct suites from their magmatic Ti and Zr silicate mineral assemblages. Semiquantitative petrogenetic grids for pegmatites in log aNa2SiO5 - log aH2O - log aHF space can be constructed using information on the composition and distribution of minerals in the pegmatites, including the Zr-rich minerals zircon, parakeldyshite, eudialyte, låvenite, wöhlerite, rosenbuschite, hiortdahlite and catapleiite, and the Ti-dominated minerals aenigmatite, zirconolite (polymignite), astrophyllite, lorenzenite, titanite, mosandrite and rinkite. The chemographic analysis indicates that although increasing peralkalinity of the residual magma (given by the activity of the Na2Si2O5 or Nds component) is an important driving force for the miaskitic to agpaitic transition, water, fluoride (HF) and chloride (HCl) activity controls the actual mineral assemblages forming during crystallization of the residual magmas. The most distinctive mineral in the miaskitic pegmatites is zirconolite. At low fluoride activity, parakeldyshite, lorenzenite and wöhlerite are stable in mildly agpaitic systems. High fluorine (or HF) activity favours minerals such as låvenite, hiortdahlite,rosenbuschite and rinkite, and elevated water activity mosandrite and catapleiite. Astrophyllite and aenigmatite are stable over large ranges of Nds activity, at intermediate and low water activities, respectively.

Formation of tonalite in island arcs by seawater-induced anatexis of mafic rocks; evidence from the Lyngen Magmatic Complex, North Norwegian Caledonides

Abstract The Lyngen Magmatic Complex (LMC) of North Norway, consists of a western suite of layered gabbros of normal-mid oceanic ridge basalt (N-MORB) affinity and an eastern suite of layered gabbronorites, quartz-bearing gabbros and diorites/quartz-diorites of IAT (island-arc tholeiitte) to boninitic affinity. The boundary between the suites is defined by a large-scale ductile shear zone, the Rypdalen shear zone (RSZ). In this shear zone anatectic tonalites were generated by partial melting of the gabbro in the presence of an H 2 O bearing fluid phase. Quartz from the tonalites contains early secondary and secondary liquid-dominated inclusions (88–99 wt.% H 2 O), with an average salinity of 18 wt.% (calculated as NaCl eq ). Combined gas and ion chromatography shows that the major ions in the fluid are Cl − , Ca 2+ , Na + with smaller amounts of K + , Mg 2+ , Sr 2+ , Br − and NO 3 − . The dominant non-H 2 O volatile species is N 2 (0.5–10%), and small amounts of CO 2 , CH 4 and other hydrocarbons are also present. The cation concentrations in the fluid are variable, due to element exchange during interaction of the fluids with the tonalites, amphibolites and metagabbros of the RSZ. The fluid contributed Na + and K + to the melt and gained Ca 2+ in exchange, explaining the variable Na + /Ca 2+ ratio of the fluid. The Br − and Cl − contents of the fluid inclusions plot on the same line as evaporating sea water, which strongly suggests a seawater origin for the fluid phase, and a seawater source fits well with other geochemical signatures and the tectonic setting of the LMC. It is suggested that seawater escaped from a subducting slab and was channelled along the Rypdalen shear zone. This caused anatexis of the gabbro, generating tonalitic melts at 0.5–0.9 GPa and 680–800°C.

Geological relations and U-Pb geochronology of Hyttsjö granites in the Långban-Nordmark area, western Bergslagen, Sweden

Abstract The Hyttsjö granites occur in the extensively mineralised Långban-Nordmark area in the westernmost part of the Bergslagen ore province. They have been classified as late Svecokarelian granites due to their homogeneous and generally isotropic appearance in addition to a WR Rb/Sr age. Moreover, they have been considered as possible candidates for supplying essential metals to epigenetic mineralisation in this classic ore district. Two Hyttsjö granites yield U-Pb zircon data ages of 1791±2 and 1793±3 Ma, respectively, which overlap with emplacement ages of the adjacent 1.80-1.78 Ga Filipstad suite belonging to the Transscandinavian Igneous Belt (TIB). Mafic rocks occur quite abundantly associated with the Filipstad-type granite (sensu lato) and various types of mafic enclaves as well as hybrid rocks are present, suggesting a co-magmatic origin. Such mafic intrusives are also exposed in the vicinity of most known Hyttsjö-type plutons. Not least our observations that the former exhibit back-veining by granitic melts suggest intimate causal and temporal relationships between granite formation and mafic TIB rocks. The Hyttsjö granites were probably produced through partial melting related to the intrusion of hot, mafic magmas in and along the border between the TIB and the Svecofennian supracrustal and subvolcanic rocks. Thus, all available observations and data suggest that the Hyttsjö granites are intimately related to and most probable a product of TIB magmatism. Also, they do not lend any support for the hypothesis that the formation of the Hyttsjö granites represent a temporally separate intrusive episode. The Hyttsjö granites are therefore unlikely to be discernibly responsible for specific mineralisation in this area.

Peterandresenite, Mn4Nb6O19·14H2O, a new mineral containing the Lindqvist ion from a syenite pegmatite of the Larvik Plutonic Complex, southern Norway

Peterandresenite (IMA2012-084), ideally Mn4Nb6O19·14H2O, is the first naturally occurring hexaniobate. It was found at the AS Granit larvikite quarry in Tvedalen, Larvik, Vestfold, Norway, by private collector Peter Andresen, after whom the mineral is named. The mineral was found on fracture surfaces and in tiny vugs in the centre of a miaskitic pegmatite dike. It occurs as equidimensional, transparent to translucent orange crystals up to 1 mm with a pale orange streak and a vitreous to resinous lustre. The Mohs hardness is 2–2.5 and the mineral is brittle with uneven fracture and no cleavage. D (calc.) = 3.05 g/cm3 and D (meas.) = 3.10(1) g/cm3. Peterandresenite is biaxial (−) and the refractive indices (white light) are: α = 1.760(5), β = 1.795(5) and γ = 1.800(5); 2V (meas.) = 43(2)° and 2V (calc.) = 40.7°. The mineral exhibits strong dispersion (r > v) and is pleochroic with X (colourless) < Z (pale orange) ≪ Y(medium orange). The optical orientation is: X ≈ c,y ≈ a*,Z = b. The empirical formula based on electron microprobe analyses is (Mn3.92Ca0.05Na0.03)Σ4.00(Nb5.71Mn.13Fe.12Si0.03)Σ5.99O18.57·14H2O. The five strongest reflections in the X-ray diffraction pattern [ d obs. in A ( I ) ( hkl )] are: 9.8977 (82) (001), 7.7104 (42) (110), 7.4689 (39) (20–1), 7.1026 (63) (11–1) and 2.9260 (100) (42–2). The mineral is monoclinic, C 2/ m , with a = 15.329(1), b = 9.4121(5), c = 11.2832(9) A, β = 118.650(4)°, V = 1428.6(2) A3 and Z = 2. Peterandresenite has a novel structure consisting of six edge-sharing Nb-octahedra forming a super octahedron known as a Lindqvist ion. One Mn2+ octahedron interconnects three Lindqvist ions to form a two-dimensional layer perpendicular to the c -axis. A second Mn2+ octahedron bridging with the Lindqvist ion protrudes into the adjacent layer along the c -axis and creates a three-dimensional structure via hydrogen bonds.

Deep-seated Carbonatite Intrusion and Metasomatism in the UHP Tromsø Nappe, Northern Scandinavian Caledonides—a Natural Example of Generation of Carbonatite from Carbonated Eclogite

Carbonatites (sensu stricto) are igneous rocks typically associated with continental rifts, being emplaced at relatively shallow crustal levels or as extrusive rocks. Some carbonatites are, however, related to subduction and lithospheric collision zones, but so far no carbonatite has been reported from ultrahigh-pressure (UHP) metamorphic terranes. In this study, we present detailed petrological and geochemical data on carbonatites from the Tromsø Nappe—a UHP metamorphic terrane in the Scandinavian Caledonides. Massive to weakly foliated silicate-rich carbonate rocks, comprising the high-P mineral assemblage of Mg–Fe-calcite 6 Fe-dolomiteþgarnetþomphacitic clinopyroxeneþphlogopiteþ apatiteþ rutileþ ilmenite, are inferred to be carbonatites. They show apparent intrusive relationships to eclogite, garnet pyroxenite, garnet–mica gneiss, foliated calc-silicate marble and massive marble. Large grains of omphacitic pyroxene and megacrysts (up to 5 cm across) of Cr-diopside in the carbonatite contain rods of phlogopite oriented parallel to the c-axis, the density of rods being highest in the central part of the megacrysts. Garnet contains numerous inclusions of all the other phases of the carbonatite, and, in places, composite polyphase inclusions. Zircon, monazite and allanite are common accessory phases. Locally, veins of silicate-poor carbonatite (up to 10 cm across) occur. Extensive fenitization by K-rich fluids, with enrichment in phlogopite along contacts between carbonatite and silicate country rocks, is common. Primitive mantle-normalized incompatible element patterns for the carbonatite document a strong enrichment of light rare earth elements, Ba and Rb, and negative anomalies in Th, Nb, Ta, Zr and Hf. The carbon and oxygen isotope compositions of the carbonatite are distinctly different from those of the spatially associated calc-silicate marble, but also from mantle-derived carbonatites elsewhere. Neodymium and Sr isotope data coupled with the trace element distribution indicate a similarity of the Tromsø carbonatite to orogenic (off-craton) carbonatites rather than to anorogenic (on-craton) ones. U–Pb dating of relatively U-rich prismatic, oscillatory-zoned zircon gives an age of 454 5 6 1 1 Ma. We suggest that VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 2403 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2017, Vol. 58, No. 12, 2403–2428 doi: 10.1093/petrology/egy016 Advance Access Publication Date: 27 February 2018

Staining of an Indoor Carrara Marble Floor at the Opera House Oslo Norway

Surface staining is common for several types of marble, including the Carrara marble. The indoor Carrara marble floor in the new Oslo Opera House suffered, shortly after laying, a yellow to brown discoloration. The stain occurs only on the surface of the marble tiles and cannot be seen on edges or fracture surfaces inside the marble. Chemical analyses of the stain shows mainly sulfur, potassium, sodium, and chlorine. These elements can be found in the organic parts of the marble, and are more easily leached compared to the dissolution of other minerals in the marble. The few pyrite grains observed are unaltered, and can by no means be related to the color changes. The main potassium and sodium source is the mortar over which the marble tiles were laid. During the drying process, the upward moisture has concentrated the dissolved S, Cl, and Br on the surface of the marble where a change in physical conditions caused oxidation of sulfide and precipitation of elementary sulfur, generating the yellow to brown discoloration. Our investigations show the surface stain to contain crystals of native sulfur (S) and aphthitalite (K,Na)3Na(SO4)2. The sulfur originates from the marble itself and possibly some from the mortar cement in dissolved state as sulfide ions. Elementary sulfur precipitated at low pressure, room temperature, and low relative humidity is unstable, and will oxidize to SO2 gas and disappear when the marble is dried. The aphthitalite crystallization is a reaction of the potassium, possibly also some sodium and sulfur, from the mortar and sulfur and sodium in the marble. Aphthitalite is stable at 1 atm pressure and partly dissolves in water. The surface stains can be removed by treatment with dilute H2O2, carefully avoiding the incipient disintegration of pyrite.