Daniel Demaiffe

Redistribution of rare earth elements, thorium, and uranium over accessory minerals in the course of amphibolite to granulite facies metamorphism: The role of apatite and monazite in orthogneisses from southwestern Norway

The amphibolite to granulite facies transition has been studied in a high-K calc-alkaline, hornblende-biotite, K-feldspar megacrystic augen gneiss series from the Rogaland-Vest-Agder (Rog-VA) sector of the Sveconorwegian province (southwest Norway). Hornblende begins to break down mainly to clinopyroxene at the Cpx-in isograd and biotite mainly to orthopyroxene at the Opx-in isograd (abbreviations for minerals following Kretz, 1983). The magmatic accessory mineral association comprises titanite and allanite, which begin to break down before the Cpx-in isograd. Titanite is preserved as relict inclusions in other minerals at higher grade. Monazite and thorite formed in the breakdown of allanite. Monazite abundance reaches a maximum between the Cpx-in and Opx-in isograds. The middle and heavy rare earth element (M-HREEs, except Eu) content of apatite steeply increases with increasing metamorphic grade and is directly correlated to the decrease of the modal abundances of titanite, hornblende, and biotite. The U contents of apatite are low and do not increase with metamorphic grade. The light rare earth elements (LREEs) and Th content of apatite are not correlated to the breakdown of allanite around the Cpx-in isograd but increase around the Opx-in isograd. A simplified equation is proposed for monazite crystallisation in the vicinity of the Cpx-in isograd that accounts for the allanite, titanite, and hornblende breakdowns and the M-HREE substitution in apatite: 3 (M-HREE)2O3in hornblende and litanite+3(LREE)2in allaniteO3+2Ca5(PO4)3apatite(F,OH)+6SiO2quartz⇔6(LREE)PmonaziteO4+2Ca2(M-HREE)3(SiO4)3(F,OH)lessingite in apatite+6CaOin plagioclase. At the Opx-in isograd, the increase of LREEs and Th content of apatite results either from the breakdown of some monazite: 3(LREE)PO4monazite+3SiO2quartz+4CaOin plagioclase+(F2,H2O)fluid⇔Ca5(PO4)3(F,OH)+apatiteCa2(LREE)3(SiO4)3(F,OH)less ingite in apatite. or the breakdown of the remaining allanite: 3(LREE)2O3in allanite+6SiO2quartz+4CaOin plagioclase+(F2,H2O)fluid⇔2Ca2(LREE)3(SiO4)3(F,OH)lessingite in apatite. The release of fluorine from the breakdown of biotite at the Opx-in isograd may increase apatite stability relatively to monazite in the granulite facies. when compared to amphibolite facies, the granulite facies augen gneisses do not show any U or Th depletion. This means that changes in the accessory mineral associations and the simultaneous breakdown of hydrous minerals at the amphibolite-granulite facies transition do not inevitably result in depleted granulite facies rocks but rather to an isochemical element redistribution. The coexistence of small amounts of metamorphic monazite and of relict inclusions of titanite in upper amphibolite facies augen gneisses suggests that (high-K) calc-alkaline orthogneisses are a suitable material to date (with the U-Pb method) prograde path of amphibolite facies regional metamorphism on monazite and the cooling path on titanite.

The 616 Ma Old Egersund Basaltic Dike Swarm, Sw Norway, and Late Neoproterozoic Opening of the Iapetus Ocean

The Egersund dike swarm of SW Norway is made up of 11 basaltic dikes trending ESE‐WNW. Two groups are defined: porphyritic dikes with bytownite phenocrysts of tholeiitic affinity and aphyric dikes of tholeiitic to alkaline affinity. Baddeleyite in the most alkaline dike gives a U‐Pb intrusive age of 616 ± 3 Ma. The swarm is parallel to the Neoproterozoic southwestern (present‐day orientation) Tornquist margin of Baltica. It is coeval with the Long Range swarm of Labrador which is parallel to the southeastern proto‐Appalachian margin of Laurentia. Both swarms are related to rifting that resulted in the opening of Iapetus ocean. Chronocorrelation of Neoproterozoic rift‐related magmatic suites in western Baltica and eastern Laurentia suggests diachronic opening of Iapetan oceanic basins at ca. 610 Ma along the northwestern Baltoscandian margin of Baltica and at ca. 550 Ma along the proto‐Appalachian margin of Laurentia and Tornquist margin of Baltica.

Nitrogen and carbon isotopic composition in the diamonds of Mbuji Mayi (Zaïre)

The concentration and isotopic composition of nitrogen, measured in large diamonds (gram size) from the Mbuji Mayi kimberlite district (Zaire) show a large range of variation (100<N<2100 ppm, −11.2<δ15N< +6.0). The15N-depleted nitrogen is associated with the higher nitrogen concentrations. The large diamonds are individually rather homogeneous in13C (range ofδ13C < 0.9‰) while variations occur within small octahedral diamonds from the same district (range up to 5.8‰). The total range ofδ13C variation is about the same for the large diamonds (−10.5 <δ13C < −5.5), the small octahedral diamonds (−10 <δ13C < −4.6), and the carbonates from local kimberlites (−11.8 < δ13C < −5.5). The diamond carbon isotopic data could indicate a complex story of crystallization within a deep-seated system fractionating its carbon isotopes. The nitrogen results indicate that nitrogen in diamonds is, on the average, markedly depleted in15N (weighted average −5.15‰) relative to atmosphere, sediments and upper mantle.

Sr and O isotope constraints on source and crustal contamination in the high-K calc-alkaline and shoshonitic neogene volcanic rocks of SE Spain

The Neogene volcanic province of SE Spain NVPS is characterized by calc-alkaline CA , high-K calc-alkaline KCA , . . . shoshonitic SH , ultrapotassic UP , and alkaline basaltic AB volcanic series. All these series, except the AB, have high LILErLREE, LILErHFSE and BrBe ratios and high but variable Sr, Pb and O isotope compositions. The KCA and SH lavas contain metapelitic xenoliths whose mineralogical and chemical composition are typical of anatectic restites. The geochemical characteristics of CA, KCA, SH and UP series suggest that they originated from the lithospheric mantle, previously contaminated by fluids derived from pelagic sediments. Additionally, the presence of restite xenoliths in the KCA and SH lavas indicates some sort of interaction between the mantle-derived magmas and the continental crust. Trace element and isotope modeling for the KCA and SH lavas and the restites, point towards the existence of two mixing stages. During the first stage, the lithospheric mantle was contaminated by 1-5% of fluids derived from pelagic sediments, which produced

U-Pb and Lu-Hf isotopes in baddeleyite and zircon megacrysts from the Mbuji-Mayi kimberlite: constraints on the subcontinental mantle

Megacrysts of baddeleyite (ZrO2) and zircon (ZrSiO4) from the diamond-bearing Mbuji-Mayi kimberlite were analyzed to determine their age, origin, and mantle source characteristics. Two of the five baddeleyites studied show 90° twins on the 0.05–0.2 mm scale suggesting formation at pressures ≥ 4.5 GPa, while zircon reveals a mosaic-like structure, indicative of a sudden pressure release. The 45 UPb analyses define an array that intercepts the concordia curve at 69.8 ± 0.5 (2σ) and 2528 ± 452 Ma. Initial epsilon-Hf values for an age of 70 Ma are +8.4 and +8.1 for zircon and +5.1, +6.0, +6.2, +6.5, +10.2 for baddeleyite. Average ZrHf ratios in both minerals are almost a factor two higher than those of primitive mantle, MORB or continental crust. Baddeleyites have uranium concentrations that are exceptionally high for mantle-derived grains (780–2050 ppm). Taken together, the data indicate that zircon and baddeleyite crystallized coevally at 70 Ma in mantle reservoirs that experienced different time-integrated LILE-fractionation histories. Some limited secondary enrichment of strongly depleted mantle may have contributed to the distinct range of Hf signatures. Small amounts (< 5%) of 2.5 Ga old radiogenic Pb detected in both minerals are probably inherited from pre-existing crystals in the mantle, and high ZrHf ratios most likely reflect very small degrees of lherzolite melting, possibly in association with mantle metasomatism by carbonate-rich fluids. Formation of the zircon and baddeleyite megacrysts can be explained either by pre-kimberlite crystallization from different magmas or subsolidus reaction during subduction of differentiated material such as oceanic crust (+ sediments?). This latter interpretation would be in agreement with the fact that inclusions in Mbuji-Mayi diamonds, and nodules in the kimberlite are dominantly eclogitic in nature. To produce the kimberlite, and to concentrate the megacrysts in a single pipe, subsequent melting of intermingled mantle domains seems the most plausible mechanism. Moreover, formation and residence times of the evolving kimberlite magmas must have been long enough to allow extraction, re-crystallization and beginning resorption of zircon and baddeleyite.

UPb and RbSr geochronology of the eastern part of the south Rogaland igneous complex, southern Norway

Abstract The successive intrusive events in the eastern part of the south Rogaland anorthosito-mangeritic complex have been dated by the UPb method applied to zircon, sphene, monazite, uraninite and accessorily by the RbSr whole rock isochron method. The whole magmatic activity in that part of the complex takes place in a short time interval: between 955 m.y., the intrusion age of the Bjerkrem-Sogndal layered norites, and 910 m.y., the emplacement age of a pergmatite in the Lyngdal granodiorite. The UPb geochronological data are consistent with field and petrological data on origin and mutual relations of the intrusives. The RbSr data give incoherent results probably due to open system behaviour in some cases.

TRACE ELEMENTS AND ANORTHOSITE GENESIS

Abstract Terrestrial massif anorthosites have gained new interest for the understanding of the deep zones of the crust and for the reconstitution of its history in Proterozoic time. The purpose of this paper is to show how trace elements can enlighten two controversial questions in the problem of anorthosites, namely the nature of the parental magma and the process which gives rise to related acidic rocks. Data obtained on rocks and minerals coming from the Rogaland anorthositic province, South Norway, are presented together with those available in the literature. The Sr and Ba in plagioclase and their relationship with Ca and K, K and Rb in rocks and plagioclases, rare earth elements (REE) in cumulate minerals and in various liquids, 87Sr/86Sr initial ratios on rocks and minerals as well as a few data on transition elements and on 18O/16O ratios are discussed in the different sections. Quantitative modelling of the behaviour of trace elements is realized mainly by graphical methods in the Bjerkrem-Sogndal layered lopolith and in the Hidra body, both andesine-type massifs. The major conclusions are as follows: (1) The parental magma of the andesine-type massifs has a jotunitic (hypersthene-monzodioritic) composition characterized by variable K/Rb ratios (from 480 to 1700), by the absence of an Eu anomaly, by variable REE contents (from 50 to 220 for La chondrite-normalized content) and by La/Yb ratios almost constant (from 6 to 12), as well as by high Ti and Fe contents, by transition elements indicating calc-alkaline affinities and by 87Sr/86Sr initial ratios similar to those of rocks derived from the upper mantle or the deep crust. (2) A jotunitic composition appears not to be compatible with the parental magma of the labradorite-type massif anorthosite. The relationship between andesine anorthosite and labradorite anorthosite cannot be described simply in terms of fractional crystallization. (3) Fractional crystallization can explain the succession of rocks from andesine anorthosite to leuconorite, to norite and finally to acidic rocks. In some cases however, namely the Bjerkrem-Sogndal lopolith in Rogaland, contamination by supracrustal material must be invoked and seems to have superimposed its effects on those of fractional crystallization. (4) Deformation, granulation and recrystallization do not appear to fractionate both the major and trace elements of the plagioclase, except Ti which is lowered.

Petrology of the bimodal Cenozoic volcanism of the Kapsiki plateau (northernmost Cameroon, Central Africa)

The Kapsiki Plateau is the northernmost volcanic zone of the Cameroon Line. The volcanism (27‐35 Ma) is of alkaline type and has a typical bimodal lava series diversity with basalts and hawaiite as mafic lavas and phonolites, trachytes, and rhyolites as felsic lavas. Mg-rich olivine phenocrysts occur only in basalts. The hawaiite contains andesine, olivine and Ca-rich pyroxene phenocrysts, and sanidine and quartz xenocrysts. The phonolites contain alkali‐feldspar and Na-rich clinopyroxene phenocrysts. Two types of trachytes occur: peralkaline trachytes, with Ti-, Na- and F-rich aegirine augite, richterite, arfvedsonite phenocrysts and non-peralkaline trachytes, with an aenigmatite-type undetermined mineral, Ti-rich biotite and zircon phenocrysts. Similarly, two types of rhyolites occur: peralkaline with quartz and arfvedsonite phenocrysts and non-peralkaline with quartz and biotite phenocrysts. Differentiation indices (Thornton and Tuttle, 1960) of the lavas range from 22 to 97 with a large gap in the range 34‐82. Some basalts are primitive (530 ppm Ni, 1100 ppm Cr). In basaltic lavas, phonolites and non-peralkaline trachytes and rhyolites, Zr and Nb covary with approximately constant ratios O3:1 , Zr=Nb , 5:3U: However, peralkaline trachytes and rhyolites have high concentrations in Zr (up to 2180 ppm) and Nb (up to 780 ppm), with correlative higher Zr/Nb ratios (6.3‐8.3). Some rhyolites have abnormal REE patterns (with kinks), depleted in light-REE, probably resulting from stability of Na‐REE‐F complexes under hydrothermal conditions. Despite a large gap between basaltic and felsic lavas, major- and trace-element distributions indicate co-magmatism for both the basaltic and felsic lavas. The differentiation of the lava series is dominated by crystal fractionation, the role of fluids in rhyolite genesis being of minor effect, as evidenced by constant values of Y/Ho and Zr/Hf throughout the series. The Kapsiki Plateau basalts are similar in their chemical and isotopic data character to other basalts from both the continental and oceanic sectors of the Cameroon Line. The continental crust appears to have no significant role in their genesis. The hawaiite mineralogical and geochemical characteristics are consistent with an origin by mixing of basaltic and felsic (phonolitic) magmas. The Kapsiki Plateau basaltic magmas may have originated from an infra-asthenospheric reservoir similarly to other basaltic magmas generated throughout the Cameroon Line. The Sr-isotope variations observed in trachytes and rhyolites point to some contamination of the magmas by crustal materials, while the Nd isotopic composition is only slightly affected. q 2000 Elsevier Science B.V. All rights reserved.

Derivation of the 1.0-0.9 Ga ferro-potassic A-type granitoids of southern Norway by extreme differentiation from basic magmas

Major and trace elements, Sr and Nd isotopic data as well as mineral compositions are presented for a selection of the 1.0–0.9 ferro-potassic A-type granitoids (Bessefjellet, Rustfjellet, Verhuskjerringi, Valle, Holum, Svofjell, Handeland-Tveit, Aseral, Lyngdal gabbronorites) that occur close to the Mandal-Ustaoset Line (MUL) of southern Norway. These hornblende biotite granitoids (HBG) define an extensive differentiation trend ranging from gabbronorites (50 wt.% SiO 2) to granites (77 wt.% SiO2). This trend is interpreted as resulting from extreme fractional crystallization of several basaltic magma batches with similar major and trace elements compositions. At 930 Ma, the HBG suite displays a narrower range in ISr (0.7027–0.7056) than in eNd(t) (+1.97 down to −4.90) suggesting some assimilation of a Rb-depleted lower crust (AFC process) or/and source variability. An age of 929 ± 47 Ma is given by a Rb-Sr isochron on the Holum granite (Sri = 0.7046 ± 0.0006, MSWD = 1.7). Geothermobarometers indicate a low pressure of emplacement (1.3–2.7 kbar) and an oxygen fugacity close to NNO. High liquidus temperatures are given by the apatite saturation thermometer (1005–1054 ◦ C) and are in agreement with results from other studies. The basaltic parent magmas of the HBG suite are partial melts of an hydrous mafic, potassic source lying either in the lithospheric upper mantle or in the mafic lower crust derived from it. This contrasts with the 930 Ma anorthosite–mangerite–charnockite suite (AMC suite) of the Rogaland Province for which a depleted lower crustal anhydrous gabbronoritic source has been indicated. The present data imply the penecontemporaneous melting of two contrasting sources in southern Norway. The source duality could result from an increasing degree of metamorphism (amphibolite to granulite) from East to West, an horizontal stratification of the lower crust or from the stratification of the lithosphere (melting of the lower crust or upper mantle). It may also indicate that the AMC and HBG suites formed in two distinct crustal segments. The linear alignment of the HBG suite along the Mandal-Ustaoset shear zone suggests that a linear uprise of the asthenosphere, following a lithospheric delamination under this structure, could be the vector of the mantle heat.

Rare earth element geochemistry and strontium isotopic composition of a massif-type anorthositic-charnockitic body: the Hidra Massif (Rogaland, SW Norway)

Abstract The Hidra Massif (Rogaland Complex, SW Norway) mainly consists of plagioclase cumulates (anorthosites and leuconorites), which grade progressively into a fine-grained (200 μm). locally porphyritic, jotunitic rock towards the contact with the granulite facies gneisses. The massif is cross-cut by thin (10 cm up to 1 m) charnockitic dykes. The petrographical and geochemical evolution of the Hidra Massif can be explained by fractional crystallization of a jotunitic parental magma. Major and trace element constraints indicate that mafic phases are underabundant in the exposed levels of the massif, most likely as a result of plagioclase flotation in the early stages of solidification. Partitioning into the cumulate minerals (mainly plagioclase and orthopyroxene) governs the trace element contents of the leuconoritic adcumulates. However, the trace element geochemistry of the apparently early formed anorthositic orthocumulates largely depends upon the amount of a trapped intercumulus liquid. On the basis of trace element abundances (high REE, Rb, Th, U; negative Eu anomalies) the silicic charnockitic dykes can be considered as the residual liquid of the anorthositic fractionation trend. The higher initial 87 Sr 86 Sr ratios (0.7086 ± 0.0006 vs 0.7055 ± 0.0004 for the plagioclase cumulates and jotunites) point to contamination of the charnockitic liquids by surrounding gneissic material.