Larvik Plutonic Complex Research Articles

Multi-stage evolution of the monzonitic Larvik Plutonic Complex (Oslo Rift, Norway) and its implications for the formation of the Kodal Fe-Ti-P(−REE) deposit

The Larvik Plutonic Complex is a monzonitic complex that was emplaced early during the formation of the Permian Oslo Rift (southern Norway). It extends across its entire width (around 50 km), and is made up of a majority of larvikite (augite monzonite) and lardalite (nepheline monzogabbro to nepheline syenite) distributed in ten successive intrusive units that partially intersect each other. The Larvik Plutonic Complex also hosts occurrences of singular Fe-Ti-P-rich rocks of magmatic origin, including the Kodal deposit. These are mainly composed of titanomagnetite, ilmenite, titanaugite and apatite, the latter also featuring rare earth elements (REE) enrichment. Here, we aim to unravel the conditions that allowed the Kodal deposit to form, and determine why other mineralized bodies are not seen elsewhere in the Larvik Plutonic Complex. We compared the petrography and elemental and isotope (Sr, Nd and Hf) geochemistry of both the Kodal mineralization and the neighboring larvikite, in order to provide evidence of their petrogenetic relationship. Both lithologies share the same isotopic ratios (87Sr/86Sr(i): 0.7035–0.7043; εNd(i): +2.37 − +3.48; εHf(i): +3.39 − +9.16), which would suggest a single homogeneous source. The very low dispersion of our isotopic data also suggests that crustal contamination levels in the area were low to negligible. Normalized trace diagrams of larvikite in several localities of the Larvik Plutonic Complex also show enrichments in most incompatible elements, which becomes more prevalent towards the Kodal deposit. Elements like Sr and Eu(2+) follow an opposite trend, because of their compatibility in plagioclase. We therefore infer that the region around the Kodal deposit hosts more fractionated larvikite due to the previous crystallization of successive plagioclase cumulates. We deduce that the Kodal lobe corresponds to a more evolved intrusion, which is no part of Pluton V per se, as considered in the literature until now, but instead derives at least from a monzonitic magma at the origin of the plutons V to VIII). This also implies that the formation of Fe-Ti-P mineralization at Kodal was most likely a consequence of enrichment of the residual melt in alkaline elements and incompatible elements. These conditions support an early hypothesis of formation by silicate-liquid immiscibility, along with petrographic evidence of disequilibrium at the mineralogical scale. However, further analyses would be required to test the hypotheses of silicate-liquid immiscibility against an accumulation of the Fe-Ti-P mineralization from an evolved intermediate magma.

Syenites

Syenites are felsic plutonic igneous rocks characterized by the lack of quartz (<5 percent) or feldspathoids (<10 percent), although quartz syenites have 5–20 percent quartz. Typically they consist of feldspars, predominantly alkali feldspar with minor ferromagnesian minerals, usually pyroxene, amphibole or biotite. Syenites are overwhelmingly rocks of extensional environments and generally occur in areas of continental rifting, such as the African Rift Valley, and also on oceanic islands, such as the Canary archipelago. The extrusive equivalent of syenite is trachyte. As examples, we will look at two plutonic complexes containing syenites: the little‐known Kangerlussuaq intrusion in East Greenland and the more familiar Larvik Plutonic Complex in the Oslo region of Norway. Syenites occur in association with both oversaturated (quartz syenites and granites) and foid‐bearing rocks (typically nepheline syenites). Agpaitic syenites are highly evolved types with titano‐zircono‐silicates such as eudialyte. Syenites find use as dimension stones and countertops.

Chemical controls on niobium and zirconium mobility inferred from dissolution experiments on wöhlerite in alkaline silica-undersaturated melts

The solubility of wöhlerite, ideally Na2Ca4ZrNb(Si2O7)2O3F, in peralkaline SiO2-undersaturated melts (Na2O–CaO–Al2O3–SiO2 ± F ± H2O) was studied at the following temperature (T) and pressure (P) conditions: 750 °C, 200 MPa, 850 °C, 100 MPa, and 1000 °C, 200 MPa. Wöhlerite appeared to be stable up to high temperature, with increasing solubility, which is consistent with field observations of early wöhlerite crystallization in pegmatites in the Larvik Plutonic Complex. A lovozerite-group mineral (combeite–zirsinalite–lovozerite) was frequently observed to crystallize in the experiments at 750 °C, and, at very high Na/Al ratios, also at 850 °C. Fluorcalciopyrochlore formed at 1000 °C in experiments where water was added. The concentrations of Zr and Nb in the quenched glasses were generally very high and found to increase strongly with temperature. Moreover, they also increase with addition of water and with increasing peralkalinity. The effect of CaF2 addition was insignificant. Based on comparison with the limited information in the available literature, the solubilities of Ca-bearing Zr and Nb minerals (wöhlerite, lovozerite-group minerals, fluorcalciopyrochlore) in peralkaline silica-undersaturated melts may be lower than those of nominally Ca-free Zr and Nb minerals (zircon, baddeleyite, wadeite, columbite-(Mn)). The experimental data do not support the existence of significant ZrF or NbF complexation. The high concentrations of Nb and Zr in the reacted peralkaline melts can be explained by strongly enhanced formation of alkali-Nb/Zr-silicate complexes compared to that at peraluminous conditions. These complexes are the underlying chemical reason for characteristic high-field-strength-element (HFSE) enrichment in agpaitic and hyperagpaitic rocks.

Timing and Petrogenesis of the Permo-Carboniferous Larvik Plutonic Complex, Oslo Rift, Norway: New Insights from U–Pb, Lu-Hf, and O Isotopes in Zircon

Abstract The Permo-Carboniferous Oslo Rift is a narrow, aborted continental rift with comparatively little extension but voluminous magmatism, developed at a Proterozoic lithospheric discontinuity. On the southern flank of the onshore part of the rift, the multiple intrusive phase Larvik plutonic complex (LPC) encompasses almost the width of the rift with a conspicuous over- to undersaturated assemblage of monzonite (larvikite, tønsbergite) and nepheline syenite (lardalite, foyaite). We present new single-crystal U–Pb–, Lu–Hf–, and O–in–zircon isotope data for the intrusive centers of the complex. U–Pb ages of slightly over- to undersaturated monzonites show a shift in igneous activity from 299 Ma on the eastern edge of the complex to 289 Ma in the west. The western part of the complex is built of two consanguineous magmatic systems with a northward shift in the locus of magmatism, from 296 to 289 Ma (undersaturated monzonite) and 296 to ~288 Ma (nepheline syenite). Moreover, an early oversaturated monzonite was emplaced in the eastern part of the complex at ~302 Ma. The Hf and O isotope composition of the monzonites and nepheline syenites shows little variation with zircon εHf (295 Ma) values of +5.5 to +8 and δ18Ο values of +4.79‰ to +5.49‰. No change in isotope values is observed with varying alkalinity and the Hf and O isotope compositions are interpreted as mantle source values. The source of the precursors of the monzonitic and nepheline syenitic magmas was probably a mildly depleted, sublithospheric peridotite in the lithosphere-asthenosphere boundary zone (at 3–4 GPa). Rhyolite–MELTS modeling implies polybaric fractionation of weakly alkaline basaltic melts from the source having led to an oversaturated/saturated liquid line of descent at ~0.5 GPa and an undersaturated one at ~1 GPa. Magmatism had an imperative role in the evolution of the rift by localizing strain and weakening the lithosphere along the discontinuity. A fractionating crustal melt column was tapped at varying depths in the course of advancing rupturing with no significant mixing of the over- and undersaturated melts, and magma batches were emplaced as sequential nested plutons in response to differential opening of the rift. Among continental saturated-undersaturated alkaline complexes, the LPC may be unique in the emplacement of successive magma batches across a continental rift in the direction of opening, tapping of increasingly deeper parts of the melt column with advancing rifting, and intrusion of two contemporaneous, contrasting magmatic lineages at the end of igneous activity. The marked lithospheric step at the rift locus in the Precambrian basement of southeastern Norway was probably the driving force for the inception and evolution of the Larvik magmatic system.

Zirconolite from Larvik Plutonic Complex, Norway, its relationship to stefanweissite and nöggerathite, and contribution to the improvement of zirconolite end-member systematics

Abstract The very first description of zirconolite, originally called polymignyte, discovered in alkaline pegmatites within the Larvik Plutonic Complex (LPC), Norway, was published almost 200 yr ago. We studied zirconolite from three occurrences located in this region using modern techniques—X-ray powder diffraction (XRPD) upon thermal annealing of initially radiation-damaged mineral, electron probe microanalysis, and Mössbauer spectroscopy. The initial XRPD pattern lacked any sharp diffraction maxima, as the accumulated radiation dose exceeded a critical value of ca. 1016 α-decays/mg. Annealing at 400–800 °C induced recrystallization to a transitional, cubic phase interpreted to have a disordered, defect fluorite structure [space group Fm3m; unit-cell parameters: a = 5.1047(4) Å, V = 133.02(2) Å3], with its XRPD pattern being very similar to that of cubic ZrO2. Rietveld analysis of the XRPD pattern obtained after a phase transition at 900 °C shows a mixture of -3O [wt. fraction of ca. 60%, space group Cmca, unit-cell parameters: a = 7.2664(8) Å, b = 14.1877(15) Å, c = 10.1472(12) Å, V = 1046.1(2) Å3], and -3T [wt. fraction of ca. 40%, space group P3121, unit-cell parameters: a = 7.2766(6) Å, c = 17.0627(15) Å, V = 752.42(11) Å3] zirconolite polytypes. However, the crystal habits of zirconolite from the LPC show a distinct orthorhombic symmetry. Although their chemical compositions are far from the ideal zirconolite composition and a large number of elements are involved in high concentrations [up to ca. 17 wt% REE2O3, ≤7 wt% ACTO2, ≤18 wt% Me25+O5, ≤9 wt% Me2+O + Me23+O3, Fe3+/(Fe3++Fe2+) ≈ 0.2, where ACT = Th + U, Me5+ = Nb ± Ta, Me2+ = Fe2+ ± Mg, Me3+ ≈ Fe3+], the compositional variability is relatively limited. To quantitatively describe the two distinct compositional trends observed, we introduced a concept called “edgemembers,” so that mixing is approximated to occur between two terminal compositions situated at two edges of the zirconolite composition space. These marginal compositions were determined from observed compositional trends, i.e., heterovalent substitution of Me5+ for Ti in octahedral sites and ACT enrichment associated with increasing Ti/Me5+ ratio. This approach provides general substitution vectors for both, Håkestad-type mode (ACT + 3 Ti + Me3+ = Ca + 3 Me5+ + Me2+), and Stålaker-type mode (0.7 ACT + 0.5 REE + 0.9 Ti + 0.7 Me3+ + 0.05 Zr = 1.2 Ca + 1.3 Me5+ + 0.3 Me2+ + 0.05 Mn). In terms of chemical composition, the studied zirconolite corresponds to recently approved zirconolite-related minerals stefanweissite (for Ca > REE) and nöggerathite (for REE > Ca). Based on a careful analysis of zirconolite composition space, we show that our observed Håkestad-type compositional trend, as well as a high number of published zirconolite compositions worldwide (with Me2+ + Me3+ sum of ca. 1 atom per 14 O), can be well approximated by a modified end-member set comprising Ca2Zr2Me25+TiMe2+O14, REE2Zr2Ti3Me2+O14, CaACTZr2Ti3Me2+O14, CaREEZr2Ti3Me3+O14, and Ca2Zr2Ti2Me5+Me3+O14 without a need to involve the ideal zirconolite formula Ca2Zr2Ti4O14. The redefined composition space constrained by end-members from this set, together with ideal zirconolite, may be representative of the vast majority of more than 450 published zirconolite compositions worldwide with Me2+ + Me3+ totaling ≤1 atom per 14 O. The equation XMe3+ = 2 – 2XREE* – 3XMe5+* provides an independent calculation of iron oxidation state or XMe3+ = Me3+/(Me2+ + Me3+) for this remarkable group of zirconolites, where XREE* is derived from XREE = REE/(REE + Ca) and XMe5+* from XMe5+ = Me5+/(Me5+ + Ti). Understanding the oxidation state of iron in zirconolite may be helpful to characterize redox conditions during its crystallization.

Boron isotope composition of coexisting tourmaline and hambergite in alkaline and granitic pegmatites

The boron isotopic composition of tourmaline and hambergite (Be2BO3[OH,F]) from peraluminous (n = 12), peralkaline (n = 1), and peralkaline nepheline syenite (n = 16) pegmatites has been measured by secondary ion mass spectrometry, for which a new hambergite reference material was developed. The focus of this study is on nepheline syenite pegmatites from the Larvik Plutonic Complex (Norway) and one peralkaline pegmatite related to the nearby Eikeren-Skrim Complex (Norway), where we investigate the source of boron as being from magmatic vs. external fluids. Tourmaline-hambergite mineral pairs were also analysed from peraluminous pegmatite localities (Russia, Tajikistan, and Pakistan) to test for systematic B-isotope fractionation between these two minerals.Tourmaline and hambergite from peraluminous granitic pegmatites have light boron ratios (δ11B = −12.9to −1.0‰) associated with S-type granites, whereas peralkaline granitic and nepheline syenite pegmatites have boron ratios (δ11B = −1.7 to 11.8‰), which we interpret is a result of heavy‑boron enrichment from external fluids. Our data show that hambergite tracks isotope variations of its geochemical setting and could therefore be used as a proxy mineral in place of tourmaline when geochemical stability favours hambergite.The results suggest a slight but consistent partitioning of B-isotopes between tourmaline and hambergite, with Δ11B = δ11Btourmaline−δ11Bhambergite in the range of approximately −3‰ to −5‰. Boron is in trigonal coordination with oxygen in both of these mineral phases as verified by NMR. Single crystal XRD analyses of tourmaline and hambergite reveal consistent longer <B-O > distances of tourmaline relative to hambergite. We attribute the boron isotopic fractionation to the longer <B-O > bond-lengths in tourmaline compared with hambergite.

Pegmatites of the Larvik Plutonic Complex, Oslo Rift, Norway: field relations and characterisation

Pegmatites of the Larvik Plutonic Complex, Oslo Rift, Norway: field relations and characterisation

Variation in Major and Trace Elements of Primary WÖhlerite As an Indicator of the Origin of Pegmatites in the Larvik Plutonic Complex, Norway

Variation in Major and Trace Elements of Primary WÖhlerite As an Indicator of the Origin of Pegmatites in the Larvik Plutonic Complex, Norway

Heyerdahlite, Na3Mn7Ti2(Si4O12)2O2(OH)4F(H2O)2, a new mineral of the astrophyllite supergroup from the Larvik Plutonic complex, Norway: Description and crystal structure

ABSTRACTHeyerdahlite, ideally Na3Mn7Ti2(Si4O12)2O2(OH)4F(H2O)2, is a new astrophyllite-supergroup mineral from the Larvik Plutonic complex, Norway. Heyerdahlite was found in a nepheline-syenite pegmatite with albite, aegirine, hastingsite/magnesio-hastingsite, kupletskite, lorenzenite and pyrophanite. Heyerdahlite is colourless to pale brown, transparent and has a vitreous lustre. Mohs hardness is 3, Dcalc. = 3.245 g/cm3. Heyerdahlite is biaxial (+), α = 1.694(2), β = 1.710(5), γ = 1.730(5), 2Vmeas. = 80(4)° and 2Vcalc. = 84.5°. Cleavage is perfect parallel to {001}. The empirical formula based on 32.18 (O + F) apfu is (Na1.18K0.68Rb0.12Cs0.01Pb0.01)Σ2Na1.00(Mn6.29Zn0.23Mg0.07Zr0.04${\rm Fe}_{{\rm 0}{\rm. 03}}^{{\rm 2 +}} $Ca0.01Na0.34)Σ7.01(Ti1.78Nb0.17 Mg0.03Zr0.02)Σ2(Si8.03O24)O2[(OH)3.92F0.08]Σ4F1.00[(H2O)1.18□0.82]Σ2, for Z = 1. Heyerdahlite is triclinic, space group P$\bar 1$, a = 5.392(2), b = 11.968(4), c = 11.868(4) Å, α = 112.743(8), β = 94.816(7), γ = 103.037(8)° and V = 675.6(7) Å3. The crystal structure was refined to R1 = 4.44% for 3577 unique (Fo &gt; 4σF) reflections. In the crystal structure of heyerdahlite, there are four [4]T sites occupied by Si. The [6]D site is occupied mainly by Ti. The T4O12 astrophyllite ribbons composed of SiO4 tetrahedra and D octahedra constitute the H (Heteropolyhedral) sheet. In the O (Octahedral) sheet, there are four Mn-dominant [6]M(1–4) sites. Two H sheets and one central O sheet form the HOH block, and adjacent HOH blocks link via a common ${\rm X}_{\rm D}^{\rm P} $ anion of the two D octahedra. In the I (Intermediate) block between HOH blocks, there are two interstitial cation sites, A and B, and a WA site, partly occupied by H2O. The A site splits into two sites, [12]A(1) and [6]A(2), partly occupied by K and Na, respectively, with A(1)–A(2) = 0.754 Å. The aggregate content of the A site is ideally Na2 apfu. The [10]B site is occupied by Na. The WA site is ideally occupied by (H2O)2 pfu. The mineral is named after the Norwegian explorer Thor Heyerdahl (1914–2002), who was born in Larvik.

Hydroxylgugiaite: A New Beryllium Silicate Mineral From the Larvik Plutonic Complex, Southern Norway and the Ilímaussaq Alkaline Complex, South Greenland; The First Member of the Melilite Group To Incorporate A Hydrogen Atom

Abstract Hydroxylgugiaite, ideally (Ca 3 □ 1 ) Σ4 (Si 3.5 Be 2.5 ) Σ6 O 11 (OH) 3 , is a new mineral species from two localities in the Larvik plutonic complex in Porsgrunn, Telemark, Norway, and one locality in Ilimaussaq, Greenland. Hydroxylgugiaite crystals occur as squat dipyramids {111} (30 × 50 μm) or as elongate tetragonal prisms. The crystals are translucent, white to pale grey in color, with a white streak and vitreous luster. It is brittle, with no apparent cleavage. Hydroxylgugiaite is uniaxial positive with ω = 1.622 ± 0.002 and ϵ = 1.632 ± 0.002. There is no pleochroism and birefringence is low. The average of eight analyses of a single grain of type material (oxide wt.%) gave Na 2 O 2.04, CaO 32.90, FeO 0.22, MnO 0.74, BeO 13.47 (LA-ICP-MS), Al 2 O 3 0.74, SiO 2 44.06, F 1.74, H 2 O (assuming 3 OH + F) 4.93, Total (–0.73 O = F) 100.10. Potassium, strontium, and magnesium were measured but not detected. The calculated density is 2.79 g cm –3 . The empirical formula on the basis of 14 anions including 3 OH – + F – is: (Ca 2.76 Na 0.31 Mn 0.05 Fe 0.01 ) Σ3.13 (Si 3.45 Be 2.53 Al 0.07 ) Σ6.05 O 11 [(OH) 2.57 F 0.43 ] Σ3 . The formula from crystal-structure analysis of the Saga specimen is: (Ca 3.02 □ 0.98 ) Σ4 (Si 1.79 Be 0.21 ) Σ2 (Be 2.29 Si 1.71 ) Σ4 O 11 (OH) 3 . Combined structural and chemical data gives the following formula for the Nakkaalaaq specimen: (Ca 2.88 □ 0.98 Na 0.12 Mn 0.02 ) Σ4 (Si 1.80 Be 0.17 Al 0.03 ) Σ2 (Be 2.32 Si 1.68 ) Σ4 O 11 [(OH) 2.70 F 0.30 ] Σ3 ; with simplified formula (Ca,□) 4 (Si,Be) 2 (Be,Si) 4 O 11 (OH) 3 . The crystal structure of hydroxylgugiaite is tetragonal in acentric space group P 2 1 / m , with a 7.4151(2), b 7.4151, c 4.9652(1) A, V 272.9(1) A 3 , and Z = 1. It has been refined to an R index of 0.028 on the basis of 342 observed reflections and a correction for the {110} twin law. It is an H-bearing member of the melilite group. The structure has two distinct layers. The one crystallographically distinct Ca site with eight-fold coordination is a square antiprism polyhedron. The Ca polyhedra are in a layer with the H atoms. A second layer consists of corner-sharing Si/Be atoms in tetrahedral coordination with O. One H atom is bonded to an apical O atom that is not shared by two tetrahedra. This H atom is present only when there is a Ca -site vacancy. The other H atom is loosely bonded to the same O atom but at a different site. The IR spectrum supports this H-bonding scheme. Additional hydroxylgugiaite data is given for the other localities.

Ti-RICH FLUORO-RICHTERITE FROM KARIÅSEN (NORWAY): THE OXO-COMPONENT AND THE USE OF Ti4+AS A PROXY

The crystal-chemical characterization of an amphibole with an unusual composition, A(Na0.76K0.24)B(Ca1.42Na0.56Mn2+0.02)C(Mg2.64Fe2+1.95Mn2+0.07Mg2.64Zn0.01Fe3+0.01Ti4+0.32)T(Si7.18Al0.82)O22W[(OH)0.58O0.27F1.15], found in pegmatitic veins at Kariasen, Larvik Plutonic Complex, Norway, provides an excellent example of the detection and estimation of the oxo component in amphibole. The use of Ti as a proxy for the oxo component is discussed and a procedure to derive accurate Ti partitioning from the results of structure refinement is described. Because the presence and amount of oxo component in amphiboles are important in order to determine values of ƒO2 and ƒH2O, especially in igneous and magmatic systems, this procedure should be applied any time the compositional data or the petrological context indicate the presence of significant Ti, or suggest that the oxo component may be a relevant issue.

MAFIC MINERALS OF THE ALKALINE PEGMATITES IN THE LARVIK PLUTONIC COMPLEX, OSLO RIFT, SOUTHERN NORWAY

The major and trace element geochemistry of mafic silicate minerals from alkaline pegmatites within the 10 ring sections of the Larvik plutonic complex, Oslo rift, southern Norway, have been studied to shed light on the complex evolutionary history of the pegmatites. Pyroxene compositions plot within the aegirine-diopside-hedenbergite ternary diagram, trending from the 50-50% Di-Hd tie line towards the 100% Ae endmember. Primary magmatic diopside and hedenbergite with minor aegirine occur primarily within miaskitic to low agpaitic pegmatites, with compositions ranging from Ae 3 Di 45 Hd 51 to Ae 89 Di 5 Hd 6 , whereas aegirine and aegirine-augite are the primary magmatic clinopyroxenes in agpaitic pegmatites, with compositions ranging from Ae 31 Di 27 Hd 42 to Ae 90 Di 6 Hd 4 . Secondary clinopyroxene, after amphibole or as a hydrothermal phase within fractures and vugs, is dominantly aegirine, with compositions ranging from Ae 59 Di 16 Hd 25 to Ae 99 Di 1 Hd 0 . Primary amphibole compositions are dominantly calcic (edenite, ferro-edenite, hastingsite, magnesio-hastingsite, pargasite, and ferro-pargasite), with less common late-stage sodic-calcic amphiboles (katophorite, magnesio-katophorite, ferro-richterite, and taramite); sodic amphiboles (ferro-ferri-nyboite, ferri-nyboite) are only observed in agpaitic pegmatites. All early pegmatite mafic minerals in low agpaitic pegmatites have negative Eu anomalies (Eu/Eu* = 0.16–0.26), positive Ce anomalies, and low Nb/Ta N and Y/Ho N ratios. Sodic mafic silicates in highly agpaitic pegmatites are HREE-enriched, with Ce/Yb N = 0.22–0.58 compared to Ce/Yb N > 1 for all other primary mafic phases, are strongly depleted in MREE, and have only slightly negative Eu anomalies (Eu/Eu* = 0.66–0.75). On the basis of major and trace element geochemistry, it can be concluded that miaskitic and alkaline pegmatites in Ring Sections 1–8 within the Larvik complex have been derived from the same parental melt, whereas pegmatites in Ring Sections 9 and 10 have a distinct geochemical signature, suggesting a separate source.

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.

Full analysis of feldspar texture and crystal structure by combining X-ray and electron techniques

Feldspar crystals typically show a range of exsolution and polysynthetic twinning textures that can present problems for their full characterization, but at the same time give important information about their genesis. We present an integrated procedure for the micro-texture analysis, twin law identification plus crystal structure refinement of all components in a feldspar intergrowth. This procedure was applied to perthitic intergrowths in feldspars from two different pegmatites in the Larvik plutonic complex in the southern part of the Oslo region, Norway. It revealed that the two starting high-temperature (HT) feldspars had similar global chemical compositions but underwent significantly different cooling histories, with cooling times probably differing by over an order of magnitude. Powder X-ray diffraction with Rietveld refinement was used for a preliminary identification of the mineral components and concluding quantitative phase analysis. Electron microprobe analysis was used to bracket the chemical compositions of the constituents. Electron backscatter diffraction was used to reveal the texture of the samples, twin laws and spatial distribution and crystallographic orientation of the crystal domains. Single-grain X-ray diffraction recorded by an area detector was applied for a simultaneous integration of reflection intensities for all crystallographic domains with different orientations and severe diffraction overlaps. The crystal structures were refined using the program JANA2006 that allows a simultaneous calculation for structurally different components. Combined results of various methods helped improve accuracy and resolve ambiguities that arise from the application of a single technique. The approach is widely applicable to the study of mineral intergrowths and bridges an existing gap in the routinely accessible data on the structural characteristics of rock constituents.

The mineralogy and crystal chemistry of alkaline pegmatites in the Larvik Plutonic Complex, Oslo rift valley, Norway. Part 1. Magmatic and secondary zircon: implications for petrogenesis from trace-element geochemistry

AbstractA detailed electron microprobe (EMP) and laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) study of zircon from six types of miaskitic and agpaitic alkaline pegmatite from the Larvik Plutonic Complex, Oslo rift valley, Norway, was undertaken to shed light on the pegmatite petrogenesis. Detailed rare earth element (REE) analyses indicate important differences between the zircon from each type of pegmatite. Primary zircon from miaskitic Stavern-, Tvedalen- and Stålaker-type pegmatites has a mean ΣREE = 704 ppm, is depleted in LREE and has a significant positive Ce anomaly (Ce/Ce* = 44–67) and negative Eu anomaly (Eu/Eu* = 0.15–0.18). Secondary Tvedalen-type zircon is REE-enriched (ΣREE = 5035 ppm), with a flatter REE pattern, Ce/Ce* = 0.97 and a Eu anomaly similar to primary Tvedalen-type zircon (Eu/Eu* = 0.21). Secondary zircon from agpaitic Langesundsfjord-type pegmatites display a distinctive flat REE pattern characterized by overall REE enrichment (ΣREE = 967), Ce/Ce* = 1.92, and a minor negative Eu anomaly (Eu/Eu* = 0.37). Zircon from agpaitic Bratthagen-type pegmatites occurs as both altered primary and secondary phases and is strongly enriched in REE relative to other zircon (ΣREE = 4178 and 8388, respectively). Primary Bratthagen-type zircon has a similar REE pattern to miaskitic zircon, with a steeper HREE profile and smaller Ce and Eu anomalies (Eu/Eu* = 0.73; Ce/Ce* = 6.22). Secondary Bratthagen-type zircon is strongly enriched in LREE compared to primary zircon, does not display a positive Ce anomaly and has Eu/Eu* = 0.56. The altered primary and secondary Bratthagen-type zircons have elevated Th/UN ratios, suggesting a different melt source for Bratthagen-type agpaitic pegmatites. Zircon from external pegmatites has trace-element signatures similar to Stavern-, Tvedalen- and Staålaker-type primary zircon with Ce/Ce* = 214 and Nb/Ta and Th/U ratios that are similar to those of secondary Langesundsfjord- and Bratthagen-type zircon. It is suggested that the parental melt of the external pegmatites is the same as the miaskitic pegmatites, but that it has undergone alteration by hydrothermal fluids derived from the host basalt, or by post-magmatic F-rich fluids which mobilize Nb and Th. On the basis of texture, morphology and geochemistry, two populations of zircon can be recognized: (1) primary zircon from miaskitic pegmatites; and (2) secondary zircon from post-magmatic, hydrothermal assemblages. The U–Th–Pb isotope analyses indicate that the secondary and altered zircon are depleted in 238U, and enriched in LREE. Interaction of a post-magmatic hydrothermal fluid with an externally derived meteoric fluid is suggested to have influenced the REE signatures, and in particular the Eu and Ce anomalies of the late-stage zircons.

Petrology of Nepheline Syenite Pegmatites in the Oslo Rift, Norway: Zirconium Silicate Mineral Assemblages as Indicators of Alkalinity and Volatile Fugacity in Mildly Agpaitic Magma

Agpaitic nepheline syenites contain complex zirconium silicate minerals as primary magmatic phases; agpaitic liquidus mineral assemblages are controlled by compositional parameters such as alkalinity and concentrations of water and halogens in the magma. The Larvik Plutonic Complex in the late Palaeozoic Oslo Rift, Norway consists of hypersolvus monzonite (larvikite) and different varieties of nepheline syenite. The nepheline syenite members have miaskitic mineralogy (i.e. zircon stable), except for a suite of late, mildly agpaitic pegmatites, which contain zirconium silicate minerals of the wohlerite, rosenbuschite and eudialyte groups, in some cases together with zircon. These minerals form part of the liquidus mineral assemblage of the nepheline syenite magma together with fluorite and the rock-forming minerals alkali feldspar, aegirine(–augite), alkali amphibole, biotite, nepheline and sodalite. Low-variance mineral assemblages with two or three zirconium silicate minerals, fluorite and the major rock-forming minerals can be modelled in terms of phase equilibria in a multi-component system involving the sodium disilicate component of the melt and the volatile components water, fluorine and chlorine. A semiquantitative isothermal and isobaric petrogenetic grid in log activity space constructed from the observed mineral assemblages and the compositions of the coexisting minerals indicate that there are at least three trends of evolution leading from a miaskitic to an agpaitic crystallization regime in peralkaline nepheline syenites: (1) the increasing alkalis trend, typically leading to eudialyte-dominated liquidus assemblages (and possibly further to hyperagpaitic residual liquids); (2) the increasing water trend, leading to catapleiite crystallization; (3) the increasing fluorine trend, giving transitionally agpaitic liquidus assemblages with wohlerite, lavenite and hiortdahlite, with or without zircon, but eventually with magmatic fluorite; this type of evolution is applicable to weakly agpaitic systems such as the Oslo Rift pegmatites. The evolution of such magmas will most probably terminate at liquidus boundaries also involving eudialyte and/or rosenbuschite, and they are unlikely to evolve to more strongly agpaitic compositions.