On the origin of microrhythmic layering in arfvedsonite lujavrite from the Ilímaussaq alkaline complex, South Greenland

Microrhythmic layering is locally developed in agpaitic arfvedsonite lujavrite from the Ilímaussaq alkaline complex, South Greenland. Three–15-cm-thick laminated dark layers alternate with 1–10-cm-thick, light-coloured granular urtitic layers. Dark layers are uniform (isomodal) but the urtitic layers are enriched in early nepheline and eudialyte in their lower parts and in late analcime and REE phosphate minerals in the upper parts. The layers are separated by sharp contacts; they are draped around rafts from the overlying roof zone and lack structures indicative of current processes or post-cumulus deformation. Compared with the background arfvedsonite lujavrite of the complex, the dark layers are richer in sodalite, microcline and arfvedsonite and poorer in analcime and eudialyte. They have higher K₂O, Cl, FeO and S but lower Na₂O, H₂O⁺, Zr and P contents, the opposite of the light-coloured layers. The complementary chemistry of the two types of layers oscillates about the composition of the background arfvedsonite lujavrite. Layers probably formed in a stagnant bottom layer of the lujavrite magma chamber. Each layer started as a liquid layer which exchanged components with the underlying crystallization front. On cooling, it crystallized primocrysts and exchanged components with the overlying magma which became a new, complementary liquid layer and, during further cooling and burial within the sequence of layers, it underwent largely closed-system interstitial crystallization. Exhaustion of Cl and a sharp decrease in a_NaCl relative to a_H2O terminated the crystallization of a sodalite-rich dark layer and initiated abundant crystallization of nepheline in the overlying liquid layer (urtitic layer). The layered sequence represents a local K₂O-, Cl-rich but Na₂O-, H₂O-poor facies of arfvedsonite lujavrite and may have formed by exchanging components with sodalite-bearing rafts from the roof zone.     

Sector zoned aegirine from the Ilímaussaq alkaline intrusion,South Greenland

Sector zoned aegirine crystals occur in the interstices of peralkaline nepheline syenites in Ilímaussaq. The crystals have grass-green [001] sectors enriched in Ca and Fe²⁺ (as CaFeSi₂O₆), Mn and Zr; pale green {010} sectors enriched in Al (as NaAlSi₂O₆); blue-green {110} sectors enriched in Ti (as NaTi_0.5Fe _0.5 ²⁺ Si₂O₆); and light green {100} sectors enriched in Fe₃₊ (as NaFe³⁺ Si₂O₆).The crystals grew in the liquid with a rate that did not exceed the diffusion rate of most elements in the liquid. However. Fe³⁺ seems to have had diffusion rates lower than the crystal growth rate, and this probably caused the development of some sectors enriched in acmite and others enriched in the hedenbergite component. For Al, Ti and Zr a crystal structural control is envisaged although a recent structure-based model for sector zoning fails to explain the efficient separation of these elements into different sectors.Three more occurrences of sector zoned aegirine are noted, all from peralkaline nepheline syenites. The phenomenon is probably more widespread than hitherto realised.Contribution to the mineralogy of Ilímaussaq no. 62     

Geochemistry of boron in the Ilímaussaq alkaline complex, South Greenland

The distribution of boron has been studied in rocks and minerals of the Ilímaussaq complex, South Greenland, using optical emission spectrometry. In the silica-undersaturated rocks of intrusive phases 1 and 3, average B contents increased from 5.6 ppm in augite syenite to a maximum of 8.9 ppm in sodalite-rich agpaitic nepheline syenite (naujaite roof cumulate) and then decreased to 5.4 ppm in the final lujavrites. Boron only behaved as an incompatible element during certain stages of the fractionation history. Starting at the naujaite stage, sodalite crystals (60–45 ppm B) were fractionated by flotation and were also trapped among the heavy cumulus phases of the bottom cumulates. This prevented the significant build-up of B in late derivatives as seen in other nepheline syenites. Nevertheless, in late pegmatites and veins associated with the agpaitic rocks, B was locally concentrated in certain Be minerals and metamict/reworked minerals. In the silica-oversaturated rocks of intrusive phase 2, average B contents increased from 8.6 ppm in quartz syenite to 13 ppm in alkali granite.     

Aenigmetites from the Ilímaussaq intrusion, south Greenland: Chemistry and petrological implications

Aenigmatite in the Ilímaussaq intrusion shows a variety of textural relations to the other mafic minerals and an unusual range in chemical compositions. The saturated and oversaturated rocks contain zoned aenigmatities with Ca, Al, Fe²⁺-rich cores coexisting with katophorite, and near-ideal Ti-aenigmatite rims coexisting with arfvedsonite and aegirine. The aenigmatite substitutions are outlined, and the varying chemistry discussed. A no-oxide field seems to have existed in the (log f_O2, T) space for the undersaturated magma, and an arfvedsonite-aenigmatite oxygen buffer equilibrium is suggested for the coexisting Fe³⁺-rich aenigmatite and katophorite. This buffer was later invalidated by the stabilisation of aegirine, whereby the Fe³⁺-aenigmatite component broke down, causing extensive recrystallisation into near-ideal Ti-aenigmatite.     

The origin and mode of emplacement of lujavrites in the Ilímaussaq alkaline complex, South Greenland

Lujavrites are rare meso- to melanocratic agpaitic nepheline syenites that are characterized by elevated contents of elements such as Li, Be, Zr, REE, Nb, Th and U. They are the most evolved members of the three large composite agpaitic complexes – Lovozero, Kola Peninsula, Russia; Pilansberg, South Africa; and Ilímaussaq, South Greenland – and are inferred to stem from the same deep fractionating magma sources that fed the earlier members of the complexes. The composition of the melts that evolved into lujavrites is, however, not well known. The agpaitic part of the Ilímaussaq complex is divided into a roof series, a floor series of cumulates and an intermediate series of lujavrites sandwiched between the two. In the traditional view, the lujavrites formed from residual melts left between the downward crystallizing roof series and the floor cumulates. New field observations and geochemical data suggest that the floor cumulates and the main mass of lujavrites constituted a separate intrusive phase which was emplaced into the already consolidated roof series rocks largely by piecemeal stoping. Studies of the contact facies of the floor cumulates indicate that the initial magma of the floor cumulate–lujavrite sequence was peralkaline nepheline syenitic with enhanced contents of Zr, Hf, HREE, Y, Nb, Ta, F, Ba and Sr. Subsequent crystallization in a closed system resulted in the formation of the floor cumulates and lujavrites. Chemical analyses of dykes within and outside the complex represent stages in the magmatic evolution of the agpaitic rocks.     

Fluid–rock interaction in autoliths of agpaitic nepheline syenites in the Ilímaussaq intrusion, South Greenland

Within the 1.16 Ga old Ilímaussaq intrusion, up to 700 m large autoliths occur in one stratigraphic unit of the layered floor series of agpaitic nepheline syenites (kakortokites). These autoliths consist of two different rock types: augite syenite and naujaite (agpaitic nepheline syenite). All three rock types show a number of alteration features related to the entrapment of the autoliths in the kakortokite magma caused by the interaction with a fluid phase.

In the kakortokites, the oxidation of primary arfvedsonite to aegirine and fluorite is restricted to the close proximity to the autoliths. Close to the surrounding kakortokite, the primary mafic phases of the augite syenites (augite, fayalite, Fe–Ti oxides) are completely replaced by arfvedsonite, aenigmatite, biotite, aegirine and fluorite. The decomposition of primary hastingsite to spectacular aegirine–augite–nepheline–aenigmatite symplectites can be observed up to several meters inside the autoliths. Additionally, fluorite formed at grain boundaries of primary nepheline. In the naujaite autoliths, primary arfvedsonite is replaced by aegirine–biotite intergrowths and abundant aenigmatite is occasionally replaced by Ti-rich aegirine and Fe–Ti oxides.

The mineral reactions in the autoliths are used to decipher details of the late to post-magmatic processes in a peralkaline syenitic intrusion. Mineral equilibria record an evolution governed by falling temperature (620 to ca. 500 °C) and increasing relative oxygen fugacity from FMQ + 1 to above FMQ + 4. Quantification of the observed mineral reactions reveals the infiltration of the autoliths with an oxidizing fluid phase rich in Na and F and minor addition of K. Volatiles (H and F) and in some cases also Fe, Ti and Ca (± Mg) released from primary autolith phases were mainly just relocated within the autoliths.     

Crust-mantle interaction in the evolution of the Ilímaussaq Complex, South Greenland: Nd isotopic studies

Sm-Nd isotopic compositions were determined for the peralkaline Ilímaussaq Complex of the Gardar Province of southern Greenland. The majority of the samples in the agpaitic and augite syenitic units have near chondritic initial _Nd(≈ 0), whereas a few samples trend towards _Nd values as low as − 6 at the time of intrusion (1143 Ma). This latter value, from a sample taken from the margin of the complex, lying on the evolutionary trend for Ketilidian country-rock granitoids, suggests that large-scale contamination took place only at the margins of the complex. The similarity of the Nd isotopic compositions of the augite syenite and agpaitic units suggests that their parental magmas were derived from the same reservoir. A comparison of the Nd with existing Sr and Hf isotopic data for the complex suggests an origin by combined assimilation fractionation processes. Assimilation-fractional crystallization modeling of the isotopic compositions indicates that the Ilímaussaq magmas could have formed through fractional crystallization of a basaltic melt while assimilating granitic crust. The model requires initially higher assimilation rates from basalt to augite syenite composition with subsequent decreasing assimilation rates from augite syenite to agpaitic compositions. Alkali granites, which formed after the intrusion of the augite syenites, have isotopic compositions intermediate between those of the augite syenites and the surrounding Ketilidian basement. This implies even greater amounts of assimilation and is interpreted as evidence for an origin through fractionation of a basaltic or augite syenite magma with concurrent assimilation of Ketilidian crust.     

Volatiles in a peralkaline system: Abiogenic hydrocarbons and F–Cl–Br systematics in the naujaite of the Ilímaussaq intrusion, South Greenland

Agpaitic rocks comprise most of the exposed part of the 1.16 Ga old, 8 × 17 km large and about 1700 m thick Ilímaussaq intrusion in South Greenland. Within these, more than 600 m thick sequence of sodalite-rich “naujaites” (mainly sodalite + arfvedsonite + alkali feldspar + nepheline + eudialyte + aenigmatite) are interpreted as a sodalite flotation cumulate. Sodalites show two to three different zones in cathodoluminescence (CL) and at least two zones in thin sections. The CL zones can be related to chemical differences detectable by electron microprobe, whereas relations with optical zonations are less obvious. Compositional trends in sodalite reflect trends in the evolution of volatile contents in the melt. The sodalite at Ilímaussaq is almost free of Ca and closely corresponds to the pure Na–Cl sodalite endmember with about 7 wt.% of Cl; S contents reach up to 0.9 wt.%. Cl/Br ratios range from 500 to 1700. Raman spectroscopy shows that S is present as [SO₄]²⁻ in sodalite, although sphalerite (ZnS) is a stable phase in naujaites. Peralkalinity and fO₂ conditions allow S²⁻ and [SO₄]²⁻ to be present contemporaneously.

The whole naujaite sequence is divided into two parts, an upper part with low, homogeneous S contents and Cl/Br ratios in the sodalite cores, and a lower part with strongly variable and higher S contents and with Cl/Br ratios, which are decreasing downwards. The details of the S content and the Cl/Br ratio evolution show that sodalite strongly influences the halogen contents of the melt by scavenging Cl and Br.

The naujaites were formed from a highly reduced, halogen-rich magma in equilibrium with magmatic methane at about 800 °C, which, upon ascent, cooling and fractionation, exsolved an aqueous fluid phase. Both fluids were trapped in separate inclusions indicating their immiscibility.

Micrometer-sized aegirine crystals and primary hydrocarbon-bearing inclusions are abundant in the crystal cores. The inclusions were trapped at pressures up to 4 kbar, although the emplacement pressure of the intrusion is about 1 kbar. This indicates growth of the sodalite during melt ascent and a very effective mechanism of trace element scavenging during sodalite growth. Sodalite rims are devoid of aegirine or primary hydrocarbon inclusions and probably reflect the emplacement stage.     

Variations in the content of uranium in eudialyte from the differentiated alkaline Ilímaussaq intrusion,south Greenland: (Contribution to the Mineralogy of Ilímaussaq,no. 36)

Uranium analyses by the fission-track method on eudialytes from the undersaturated rocks of the Ilímaussaq intrusion demonstrate that uranium enters eudialyte in isomorphous substitution. The content of uranium in the eudialytes varies with the crystallization of the magma in two ways.In the downwards-crystallizing roof rocks, eudialyte is interstitial and the content of uranium in eudialyte decreases with proceeding crystallization, whereas in the bottom rocks, formed by upwards accumulation of liquidus minerals including eudialyte, the uranium content in eudialyte increases with crystallization. The reason for the abnormal trend in the roof rocks is discussed and compared with similar trends elsewhere.     

The petrogenesis of the Kangerdlugssuaq alkaline intrusion,east Greenland

Evidence of layering and physical conditions of emplacement are discussed for this saucer-shaped, differentiated syenite mass. The intrusion is thought to derive from a magma of quartz trachytic composition,rather than from partial melting of the basement gneiss. The derivation of an undersaturated residual liquid, necessary to produce the pulaskite and the foyaite, is discussed, together with the mechanism whereby the ‘thermal barrier’ is crossed. Possible explanations considered are the depression of the thermal barrier through increased vapour pressure; the formation of iron-bearing feldspar; escape of silica and some potassium with volatile constituents; and crystal ? liquid equilibrium control.     

Subsolidus deuteric/hydrothermal alteration of eudialyte in lujavrite from the Pilansberg alkaline complex, South Africa

The most evolved rocks of the Pilansberg alkaline complex are aegirine lujavrites in which three varieties of eudialyte are recognized on the basis of textural relationships and composition. Manganoan eudialyte-I is a relict orthomagmatic phase occurring as poikilitic plates or as relict grains in pseudomorphed euhedral phenocrysts. Late eudialyte-II ranges in composition from manganoan eudialyte through kentbrooksite to taseqite-like varieties and is considered to be formed by cation exchange with eudialyte-I and alkaline fluids. Eudialyte-III is a hydrothermal phase replacing eudialyte-II, and has either taseqite-like (5–7.3 wt.% SrO, < 2.0 wt.% REE₂O₃) or kentbrooksite (< 1.5 wt.% SrO,  8.5 wt.% REE₂O₃) compositions. Three styles of replacement of eudialyte-I and -II are recognizable. Type 1 involves replacement by complex aggregates of zircon, fergusonite-(Ce), allanite-(Ce), britholite-(Ce), titanite, pyrochlore, albite and potassium feldspar, i.e. a “miaskitic” paragenesis. Type 2 alteration consists of complex aggregates dominated by deuteric Na–Zr-silicates (?catapleiite), stronalsite, strontium-apatite and lamprophyllite replacing eudialyte-I and -II and relicts of the “miaskitic paragenesis”, i.e. a highly sodic “agpaitic-to-hyperagpaitic” paragenesis. Type 3 replacement involves mantling of any residual eudialyte-II and zircon, and replacement of deuteric Na–Zr-silicates by eudialyte-III together with barytolamprophyllite as late hydrothermal phases. Further alteration and replacement resulted in the superposition of natrolite, britholite, pyrochlore, allanite and diverse Ba- and Mn-based minerals onto the types 2 and 3 assemblages, and ultimately to the deposition of allanite-(La), La-dominant REE carbonates and rarely a silica phase. All of the alteration styles are considered to have occurred in situ under subsolidus conditions (< 450 °C) by interaction of pre-existing eudialyte and other minerals with deuteric, sodium- and chlorine-bearing aqueous fluids. The evolution of the replacement products is from a miaskitic through an agpaitic to a hyperagpaitic paragenesis and ultimately back to a low agpaitic-to-miaskitic assemblage, reflecting changes in the a(Na⁺)/a(Cl⁻) ratio and alkalinity of the deuteric/hydrothermal fluids.     

The cumulus and post-cumulus evolution of chrome-spinels in ultrabasic layered intrusions: evidence from the Cuillin Igneous Complex,Isle of Skye,Scotland

Chrome-spinels from the layered Peridotilte Series of the unmetamorphosed, anorogenic 60 Ma Cuillin Igneous Complex, Isle of Skye, display a wide variety of compositions. Cumulus (within seams) chrome-spinels from the lowest exposed portion of the Peridotite Series exhibit features indicative of textural equilibrium, are rich in Al and Mg, and have low values of the ratio Cr/(Cr+Al). Cumulus chrome-spinels from higher up in the series are different from these: particularly, textural disequilibrium is evident, intercumulus plagioclase and olivine are present, and the chrome-spinels are rich in Cr, Fe and Ti, with high values of the ratio Cr/(Cr+Al). Intercumulus (dispersed) chrome-spinels tend towards anhedral forms and define enrichment trends towards Fe (both Fe²⁺ and Fe³⁺) with decreasing Mg, Cr and Al, and towards Al, with decreasing Fe²⁺ and Cr (and increasing Mg). Individual crystals are completely homogeneous and are devoid of reaction rims. The observed textural characteristics and compositional data of the chrome-spinels documented here suggest that the semi-quantitative peritectic reaction: aluminous chrome-spinel + meltplagioclase + olivine + chromian chrome-spinel, is responsible for the observed parageneses, and that both the environment of crystallization (eumulus or intercumulus) and the role of plagioclase ±olivine crystallization are critical parameters for this geochemical trend in spinels within upper crustal magmatic systems. The effects of pyroxene crystallization on the development of this geochemical trend are also considered. This investigation highlights the need to consider the role of post-cumulus mineral-melt reactions and their influences upon the final compositions of major oxide and silicate phases within layered intrusions.     

Age and origin of agpaitic magmatism at Ilímaussaq,south Greenland: RbSr study

A RbSr whole-rock isochron gives an age of 1168±21 m.y. for the agpaitic units of Ilímaussaq, showing that this complex belongs to the main phase of Gardar igneous activity in south Greenland and is not, as previously supposed, a significantly younger intrusion. Moreover, the agpaites must have intruded very soon after the earlier augite syenite phase of Ilímaussaq. The initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7096±0.0022 for the agpaites is in marked contrast to the low (～0.703) ratio obtained for the augite syenites and suggests that selective enrichment of ⁸⁷Sr occurred by preferential leaching of radiogenic strontium from unstable positions in Rb lattice sites in older crustal material by the highly reactive agpaitic magma.     

Independent ages of magmatic and hydrothermal activity in alkaline igneous rocks: The Motzfeldt Centre,Gardar Province,South Greenland

The Motzfeldt Centre is one of four major alkaline centres belonging to the Igaliko complex, part of the Mid-Proterozoic Gardar province of South Greenland. Motzfeldt comprises syenites and nepheline syenites displaying complex magmatic and subsolidus histories. Ta, Nb and REE-rich zones are associated with a pervasive hydrothermal alteration process in the North and East of the complex, part of a subdivision of the Centre called the Motzfeldt Sø Formation (MSF). The zircons from the MSF show textures that are both magmatic and altered, whereas the textures of pyrochlore have a predominantly subsolidus chemistry. We compare and contrast the radiogenic isotope geochemistry of the zircon and pyrochlore to obtain independent age estimates of the magmatic and hydrothermal episodes. Selected analyses of magmatic zircons are concordant with an age of 1,273 ± 6 Ma. Altered zircons are normally discordant with one intercept at 1,261 ± 28 Ma and the other at 349 ± 49 Ma. This younger age would appear on face value to date the hydrothermal event at Motzfeldt to ~900 Ma after the magmatism. However, the Pb–Pb isotope systematics from altered pyrochlore define an isochron age of 1,267 ± 6 Ma for isotopic closure of the pyrochlore following alteration during the subsolidus. There is no known magmatic event in Southern Greenland at ~350 Ma and discordant points have high common lead. We therefore infer that the younger intercept age is spurious. The overlap between the magmatic U–Pb zircon and the Pb–Pb pyrochlore age suggests that alteration of the Motzfeldt centre occurred shortly after magmatism.     

塔里木北缘波孜果尔碱性(花岗)岩铌-钽-锆-铷-稀土矿床钠铁闪石、霓石特征及意义

碱性造岩矿物能够记录碱性岩源区特征、岩浆演化以及晚期成矿的重要信息,是开展碱性岩成岩成矿研究的有效手段.波孜果尔碱性花岗岩型铌-钽-锆-铷-稀土矿床位于塔里木北缘-中亚南天山晚古生代造山带,是塔里木地块北缘铌成矿带中典型的碱性岩型矿床.本文通过对含矿岩体中的霓石和钠铁闪石开展矿物学研究,结合全岩成分揭示波孜果尔稀有-稀...     

The crystallisation history of the Igdlerfigssalik nepheline syenite intrusion,Greenland

During slow cooling of plutonic igneous rocks the initial high temperature minerals crystallised from the magma continue to re-equilibrate with each other to varying degrees with falling temperature. Thermodynamic studies of mineral equilibria are used to calculate crystallisation temperatures for the cumulus assemblage ol-cpx-mt-ne-fsp in the Igdlerfigssalik syenites and to calculate composition parameters for the original magmas. Cumulus crystallisation occurred in the range 900–980°C. Nepheline and alkali feldspar continued to equilibrate in some rocks down to 650°C, while macroscopic exsolution in alkali feldspar and titanomagnetite continued to temperatures below 600°C. Oxygen activities during the crystallisation of the cumulus minerals were below magnetite-wustite.     

Rare-earth distribution in the Tugtutôq younger giant dyke complex: evidence bearing on alkaline magma genesis in south Greenland

Rare-earth element data for 14 gabbro and syenogabbro samples from the Tugtutôq younger giant dyke complex (YGDC) and for three anorthosite inclusions lend support to other geochemical and experimental evidence that the anorthosites represent early fractionates that formed at depth from a magma similar to (but not identical with) the liquid which formed the giant dykes themselves. All samples are light REE enriched and absolute REE concentrations increase with increasing degree of differentiation in both chilled marginal samples and cumulates. Strong positive Eu anomalies in early cumulates decrease with advancing fractionation and there may even be negative Eu anomalies in the most differentiated cumulates. Lack of significant Eu anomalies in the chilled samples confirms that large quantities of anorthosite (large positive Eu anomalies) could not have formed directly from the YGDC magma. A comparison with REE distributions in the nearby, younger Ilimaussaq alkaline complex (from data reported by Gerasimovskiy & Balashov (1968) shows that the late fractionates of the YGDC and the early, augite syenite, phase of Ilimaussaq have very similar REE patterns. The later agpaites of Ilimaussaq, however, are greatly enriched in REEs and show strong negative Eu anomalies; this suggests that substantial feldspar fractionation took place at depth before the emplacement of the agpaites, producing rock suites petrologically similar to those exposed in the YDGC.     

Fluid geochemistry in the Ivigtut cryolite deposit, South Greenland

The 1.27 Ga old Ivigtut (Ivittuut) intrusion in South Greenland is world-famous for its hydrothermal cryolite deposit [Na₃AlF₆] situated within a strongly metasomatised A-type granite stock. This detailed fluid inclusion study characterises the fluid present during the formation of the cryolite deposit and thermodynamic modelling allows to constrain its formation conditions.Microthermometry revealed three different types of inclusions: (1) pure CO₂, (2) aqueous-carbonic and (3) saline-aqueous inclusions. Melting temperatures range between − 23 and − 15 °C for type 2 and from − 15 to − 10 °C for type 3 inclusions. Most inclusions homogenise between 110 and 150 °C into the liquid.Stable isotope compositions of CO₂ and H₂O were measured from crushed inclusions in quartz, cryolite, fluorite and siderite. The δ¹³C values of about − 5‰ PDB are typical of mantle-derived magmas. The differences between δ¹⁸O of CO₂ (+ 21 to + 42‰ VSMOW) and δ¹⁸O of H₂O (− 1 to − 21.7‰ VSMOW) suggest low-temperature isotope exchange. δD (H₂O) ranges from − 19 to − 144‰ VSMOW. The isotopic composition of inclusion water closely follows the meteoric water line and is comparable to Canadian Shield brines. Ion chromatography revealed the fluid's predominance in Na, Cl and F. Cl/Br ratios range between 56 and 110 and may imply intensive fluid–rock interaction with the host granite.Isochores deduced from microthermometry in conjunction with estimates for the solidification of the Ivigtut granite suggest a formation pressure of approximately 1–1.5 kbar for the fluid inclusions. Formation temperatures of different types of fluid inclusions vary between 100 and 400 °C. Thermodynamic modelling of phase assemblages and the extraordinary high concentration in F (and Na) may indicate that the cryolite body and its associated fluid inclusions could have formed during the continuous transition from a volatile-rich melt to a solute-rich fluid.     

Geochemical and isotopic zoning patterns of plagioclase megacrysts in gabbroic dykes from the Gardar Province, South Greenland: implications for crystallisation processes in anorthositic magmas

Chemical and Sr isotopic zoning patterns in plagioclase megacrysts from gabbroic dykes in the Gardar Province can be used to elucidate magma-chamber and emplacement processes. The megacrysts occur either as single crystals or assembled as anorthosite xenoliths. The size of the megacrysts varies from <1 cm to 1 m. They consist of a large core with variable zonation (An_58-39) and a relatively small (<600 µm), normally zoned rim (An_62-27). The contact between core and rim is sharp and marked by a sharp increase in anorthite content which can reach 11 mol% An. This gap is interpreted as having formed during dyke emplacement due to a sudden pressure release. Some of the megacryst cores show a fairly constant composition whereas others exhibit an unusual wavy-oscillatory zoning which has not been reported elsewhere to our knowledge. The oscillatory zoning has wavelengths of up to 2,500 µm and a maximum amplitude of 7 mol% An. It is interpreted as reflecting movements of the crystals in the magma reservoir. The Sr isotopic composition of one crystal shows a radiogenic inner core ((⁸⁷Sr/⁸⁶Sr)_i=0.7044) and a less radiogenic outer core ((⁸⁷Sr/⁸⁶Sr)_i=0.7039-0.7036). The lack of a significant change between outer core and rim ((⁸⁷Sr/⁸⁶Sr)_i=0.7037) is consistent with formation of the more An-rich rim due to pressure release. Variations in the core may be related to movements of the crystal and/or magma mixing. A trace-element profile across a megacryst shows a small increase in Sr and small decreases in Ba and La contents of the recalculated melt composition across the core-rim boundary, whereas P, Ce, Nd and Eu remain constant. Melt compositional changes upon emplacement are therefore considered to be of minor importance. Constant ratios of incompatible trace elements in the megacryst cores indicate a dominant influence of a lower crustal source on trace-element budgets.     

Holocene environmental evolution of the SE Greenland Shelf North and South of the Denmark Strait: Irminger and East Greenland current interactions

Holocene climatic and paleoceanographic development of the SE Greenland Shelf is studied from cores MD99-2317 and MD99-2322, at sites north and south of the Denmark Strait, respectively. Lithofacies, IRD counts, calcium carbonate percentages, benthic and planktic foraminiferal assemblages and oxygen isotope analyses, and summer SSTs reveal significant climate variations in the Holocene driven by declining solar insolation and its interaction with waning continental ice sheets, and changing atmospheric pressure patterns. Large changes in the East Greenland and Irminger Currents and the Greenland Ice Sheet are manifested as a 4-part division of the Holocene. An early Holocene cold interval dominated by melting of the Greenland Ice Sheet and Polar Front retreat extends from 11.8 to 9.5 cal kyr BP. A cold interval from 9.5 to 8.1 cal kyr BP involved episodic cooling of the Irminger Current resulting from the last phases of Laurentide Ice Sheet deglaciation and delayed the Holocene optimum off East Greenland by 3 kyr relative to peak summer solar insolation, which likely helped to limit the early Holocene melting of the Greenland Ice Sheet. The period 8.1–3.5 cal kyr BP represents a climatic optimum interval of maximum Greenland Ice Sheet retreat and strong Irminger Current inflow to the Denmark Strait. Between 6.8 and 3.5 cal kyr BP, the Irminger Current penetrated further North into the Nordic Seas than has been observed in recent decades. This signal is consistent with diminished northerly winds, a weaker Greenland High and contracted subpolar gyre. By 5 cal kyr BP, periods of increased Polar Water and decreasing salinity in the Irminger Current suggest a transition toward expansion of the subpolar gyre and increased Polar Water in the EGC. The Neoglacial interval from 3.5 to 0.2 cal kyr BP was cold and variable with increased freshwater forcing from the Arctic Ocean, advance of the Greenland Ice Sheet and southward advance of the Polar Front. Enhanced northerly winds and a strengthened Greenland High are consistent with thicker and more extensive Polar Water and greatly diminished northward advection of Irminger Current in the Denmark Strait.