Gardar Province Research Articles

The palaeomagnetism of middle Proterozoic dyke swarms of the Gardar Province and Mesozoic dykes in SW Greenland

SUMMARY The Precambrian metamorphic basement in SW Greenland was subjected to widespread emplacement of dolerite dykes between circa 1290 and 1160 Ma; during this period the regional stress field rotated in a counter-clockwise sense. A regional palaeomagnetic study of 59 of these dykes and contacts is reported. Whilst contact tests generally indicate primary magnetizations, two- and three-component structures in some dykes record partial overprinting at later stages of the igneous cycle. Component directions are distributed from shallow negative westerly to steep NW positive. Over most of the outcrop the former magnetizations are found in the earlist (E-W trending) dykes. Steepening and rotation of magnetization directions correlates with rotation of dyke trends towards later (NE-SW) trends. The exception occurs where dykes are deflected in a sigmoidal way through an older alkaline complex. The quasi-continuous nature of dyke emplacement records migration of the palaeofield direction between circa 1290 and 1160 Ma and representative sequential pole positions are (A1) 215.3dE, 3.1dN (21 dykes, dp/dm= 4.8/9.6d), (A2) 220.4dE, 7.4dN (5 dykes, dp/dm= 4.3/8.5d) and (A3) 222.3dE, 33.5dN (30 dykes, dp/dm= 4.5/7.0d). Palaeomagnetic poles from the Gardar Igneous Province define the major part of a double APW loop anchored by dated poles from igneous complexes and executed between 1300 and 1140 Ma. This interval appears to have been dominated by one field polarity. Palaeomagnetic results are also reported for five dykes belonging to the Mesozoic coast parallel swarm, and palaeomagnetic results from these dykes in south and south-west Greenland are reassessed. They define a dual-polarity axis of intermediate inclination (D/I = 336/51d, 13 sites, palaeomagnetic pole (TD1) at 168.3dE, 55.6dN. dp/dm= 4.7/6.9d) plus a steeper single-polarity group (mean direction D/I = 329/69d, 14 sites, palaeomagnetic pole (TD2) at 207.7dE, 68.6dN, dp/dm= 6.6/7.8d). The TD1 pole corresponds to North American apparent polar wander at circa 165 Ma compatible with the 168-138 Ma age constraint, whilst the TD2 pole correlates with near-static APW between 129 and 88 Ma. It appears to record an episode of dyke emplacement across south Greenland during the Cretaceous Normal Superchron shortly before commencement of sea-floor spreading between Greenland and Labrador.

Timing of Proterozoic magmatism in the Gardar Province, southern Greenland

The Gardar Province in southern Greenland is believed to have experienced three episodes of rifting, all associated with the extrusion of large volumes of alkalic magma. A nepheline-syenite of the Motzfeldt Center yields a U-Pb apatite age of 1.35 ± 0.01 Ga. An augite-syenite from the Ilimaussaq Complex yields a Sm-Nd pyroxene, alkali feldspar, magnetite age of 1.13 ± 0.05 Ga. On the basis of field relationships, the Eriksfjord Formation lavas have been interpreted as the oldest manifestation of Gardar magmatism. Sm-Nd plagioclase, pyroxene, wholerock ages of 1.17 ± 0.03 Ga and 1.20 ± 0.03 Ga for the middle member of the Eriksfjord Formation, however, indicate that these lavas may be part of the youngest magmatic activity. The available chronological data indicate that the Gardar Province was active for at least 200 m.y., from 1.35 to 1.12 Ga. It is difficult to reconcile this protracted history with plume models for the genesis of such alkaline flood-basalt provinces.

Contrasting Pb isotopic compositions in two intrusive complexes of the Gardar Magmatic Province of South Greenland

Pb isotopic compositions were determined in rocks from two Proterozoic alkaline complexes in the Gardar Province of South Greenland. Both complexes, Kûngnât Fjeld and Tugtutôq Younger Giant Dyke, embrace a comparable range of compositions (alkali gabbro-ferrosyenogabbro-syenite-granite) but the two lie in contrasting crustal environments. Kûngnât lies within the southern marginal part of the Archaean craton; Tugtutôq lies off the craton in early Proterozoic (Ketilidian) granites. There are significant initial isotopic Pb differences between the two as well as marked Pb isotopic variations within each complex. These latter are attributed to increasing degrees of crustal contamination accompanying crystal fractionation processes whereas the overall differences between the two complexes reflect the differing isotopic signatures of the country rocks, viz. the relatively unradiogenic Archaean host rocks for Kûngnât and the more radiogenic Proterozoic granitic hosts for Tugtutôq.

The Proterozoic Gardar rift zone, south Greenland: comparisons with the East African Rift System

Abstract The late Tertiary to Recent Kenya rift valley and the deeply dissected, Proterozoic, Gardar province of south Greenland are both parts of continental rift systems thousands of kilometres long. Both were generated within stabilized orogenic belts skirting cratonic forelands, in regions in which repeated reactivation of earlier structures has occurred. Both may represent failed attempts at continental separation. Mafic magmatism in Kenya has ranged from strongly silica-undersaturated, melanephelinitic types through basanites to mildly alkaline and transitional basalts. Gardar activity was dominated by mildly alkaline and transitional basalts; basanites and nephelinites are rare or absent, the most undersaturated representatives being ultramafic lamprophyres. Very primitive (possibly primary) magmas are scarce in both rifts. Trace element chemistry suggests that the majority of the mafic rocks have lithospheric signatures, and that asthenospheric magmas had no direct access to the crust. The primary magmas were generated as small-degree melts in mantle sources which had experienced earlier depletion events and which were subsequently metasomatized. An important, mantle-derived feature of the Gardar basalts is their strongly aluminous nature, which appears to have generated an abundance of anorthositic bodies in the crust; this feature is not shown in the Kenya rift. Considerable amounts of underplating are inferred to characterize both rifts, limiting crustal thinning in regions of lithospheric extension. Both rift zones have extensive axial dyke swarms, which are particularly focused on central complexes/caldera volcanoes. Kenya apparently lacks, however, the massive dykes, up to several hundred metres wide, of the Gardar province.

The palaeomagnetism of major (middle proterozoic) igneous complexes, South Greenland and the Gardar apparent polar wander track

The Gardar Province comprises a major rift-related igneous suite emplaced in three pulses at ca. 1300–1280 Ma, ca. 1220 Ma and 1160–1125 Ma. This study reports a palaeomagnetic study of the northeastern sector of the present outcrop and incorporates igneous units belonging to the first and last episodes. Early lavas from the basal part of the rift infill record a reversal of magnetisation and define a palaeomagnetic pole at 207°E, 10°N ( 5 sites, dp/dm= 7 14° ). This primary magnetisation record has been partially overprinted in the aureoles of large alkaline complexes. The latter plutons include the North Qoroq intrusion (1291 ± 61 Ma) yielding a pole position at 203°E, 13°N ( 65 samples from 12 sites, dp/dm= 6 12° ), the Motzfeldt intrusion (1291 ± 31 Ma) yielding a pole position at 212°E, 10°N ( 25 samples, dp/dm= 6 12° ), and the Narssarssuaq Stock yielding a pole position at 207°E, 9°N ( 23 samples, dp/dm= 4 7° ). The younger South Qoroq complex (1160 ± 8 Ma) yields a pole position at 216°E, 42°N ( 67 samples from 9 sites, dp/dm= 11 16° ). Positive contact tests indicate primary cooling-related magnetisations. These bodies are predominantly of single polarity equivalent to W + directions of magnetisations although reversals are recorded. The majority of a suite of NE-SW trending dykes emplaced at ca. 1154–1143 Ma are magnetised in a similar direction [ D/I= sol3 51° , pole position at 231°E, 33°N ( 18 dykes, dp/dm= 5 7° )] to the slightly-older South Qoroq complex. Together with earlier results from the Gardar Province, these data define a motion of the region from shallow to equatorial and then back to intermediate palaeolatitudes between 1290 and 1140 Ma. Pole positions trace out an apparent polar wander path comprising a small loop followed by a long track. This track is the outward path of the “Great Logan” palaeomagnetic loop for which data from the Keweenawan Province of North America (ca. 1100–1050 Ma) define the return path. The loop was executed between ca. 1200 and 1050 Ma following a near-static interval between ca. 1290 and 1200 Ma. This long static interval was also characterised by constant field polarity. Asymmetry of geomagnetic field reversals is recognised in early Gardar events; it is not apparent in younger Gardar data although it recurs in Keweenawan times.

Feldspar megacrysts and anorthosite xenolith-bearing dykes in the Narssarssuaq area, South Greenland

Numerous dyke intrusions are found in the Narssarssuaq area of the Gardar province, a Mid-Proterozoic intracontinental rift system. Ten to fifteen percent of these dykes, which range in composition from trachybasalt to phonolite and rhyolite, contain significant proportions of feldspar megacrysts and occasionally anorthosite xenoliths. Two groups of dykes are distinguished; the older group is more alkaline, richer in incompatible elements and contains more anorthosite xenoliths than the younger. It is probable that the main reason for the differences is variation in magma production through time and from one area to another. Chemical zonation in the dykes reflects compositional gradients in the feeding magma reservoirs; the magma reservoirs acting as open systems in which crystal fractionation was an important controlling process. The anorthosite xenoliths are not strictly cognate with their hosts, but were derived from comparable alkaline magmas with a composition roughly corresponding to the most primitive of the dykes. The plagioclase megacrysts were presumably formed at an early stage of the development of the magma chambers. Rb-Sr dating of one of the dykes from the older group of dykes gives an age of 1206 ± 20 Ma and an initial 87Sr/86Sr ratio of 0.7028 ± 0.0001 supporting a low degree of contamination with upper crustal Sr.

Biotite Equilibria and Fluid Circulation in the Klokken Intrusion

Chemical variation in biotite from the KJokken gabbro-syenite intrusion in the Proterozoic Gardar province in South Greenland has been investigated by electron probe and, for F and Li, ion microprobe. Most mica occurs in small amounts as fringes on ilmenomagnetite or fayalite, rarely as an intercumulus or poikilitic phase. The micas range continuously from Phlog70Ann30 in a gabbro, to Phlog4Ann96 in the most evolved (slightly persilicic and peralkaline) syenite. In the syenites Fe-Mg partitioning between biotite and olivine can be described by a single distribution coefficient, K6 = Xfi X%? /X^-Xf°'~3, suggesting that these reactant phases mix ideally at the reaction T. Experimental data for Fe-Mg exchange via aqueous chloride solutions (Schulien, 1980) imply low T (~ 320 °Q. F was absent in the experiments and may significantly affect the exchange equilibrium. K6 in the gabbros is ~ 1, consistent with equilibrium via a fluid depleted in F because of crystallization of large amounts of amphibole. Al, Mn, and Ti vary regularly throughout the series and can be used as markers of cryptic variation in the layered syenites. (Al + Si): 22 O is always in the range 7-7—7-85. A1/(A1 + Si) decreases from ~O31 in gabbros to 0-25 in the most Fe-rich micas. Li is always 300 °C subsolidus even when texturally intercumulus. Stable isotope data are consistent with the separation and retention of a deuteric fluid during the final stage of magmatic crystallization. Klokken was not generally subject to the pervasive, long-range (in both distance and time) dydrothermal interactions demonstrated in

Cathodoluminescence and microporosity in alkali feldspars from the Blå Måne Sø perthosite, South Greenland

AbstractSamples from a traverse across the Blå Måne Sø perthosite unit in the Tugtutôq Central Complex of the Gardar province, South Greenland have been examined for cathodoluminescence characteristics, microporosity and δ18O isotopic values. Reddening of cathodoluminescence colours in alkali feldspars (normally blue) from the unit may be correlated with increased microporosity of the feldspars as determined using scanning electron microscopy. δ18O values of all samples lie within the range of values expected of juvenile fluids, independent of the level of alteration indicated by cathodoluminescence studies.Observations are consistent with previous suggestions that levels of alkali feldspar microporosity and levels of fluid alteration (as determined by cathodoluminescence of alkali feldspars) are related phenomena. Oxygen isotope ratios suggest that the fluid is largely juvenile in origin, with, perhaps, some meteoric (low δ18O) component.

Compositional zoning in hydrothermal aegirine from fenites in the Proterozoic Gardar Province, South Greenland

Compositionally aoned Ti-bearing aegirine is a common constituent in fenites and hydrothermal veins and vugs associated with nepheline syenites from the mid-Proterozoic Gardar Province of South Greenland. A typical assemblage in these mineralization includes quartz, albite, microcline, aegirine, calcite, stilpnomelane, apatite, monazite and Nb-Fe-Ti-Mn oxides, with a wide range of unusual REE-Zn-Be-Ba-Mn-silicates as accessory phases. Individual aegirine crystals display evidence of sequential growth, concentric- and sector-zoning that represent large variations in wt. % TiO 2 (0.1-3.0), Na 2 O (11.6-12.8), CaO (0.0-2.0), FeO t (25.4-30.9) and ZrO 2 (0.0-0.74). Concentrations of Al 2 O 3 (<1.34 but typically less than 0.7 wt. %) and MgO (<0.76 but typically less than 0.4 wt. %) are both very low. In terms of pyroxene end-members, these compositions correspond to from 70-95 mole % NaFe 3+ [Si 2 O 6 ] (Aegirine), <1-6 mole % NaAl[Si 2 O 6 ] (Jadeite), <1-8 mole % Ca (Mg,Fe 2+ )[Sii2O 6 ] (Enstatite-Ferrosillite), and from <mole % of an unnamed end-member having the composition Na(Mg,Fe 2+ ) 0.5 (Ti,Zr) 0.5 [Si 2 O 6 ]. As such, the principal chemical variation in the analyzed crystals involves the substitution : VI (Fe 2+ ,Mg)+ VI (Ti 4+ =2 VI (Fe 3+ ) in the M1-position. Textural relationships, together with the chemical inhomogeneities of the pyroxenes, suggest rapid nonequilibrium crystallization from a hydrothermal fluid of changing composition and/or temperature

Conversion of nepheline to sodalite during subsolidus processes in alkaline rocks

AbstractCathodoluminescence (CL) petrography of nepheline syenites of the Igaliko complex, Gardar province, South Greenland shows that sodalites possess embayed contacts against nepheline, and have formed by a process of metasomatic replacement. This texture is demonstrated clearly by CL, since sodalite luminesces bright orange and nepheline is poorly luminescent. The transformation from nepheline to sodalite results in a volume change which leads to a network of fractures in which deep-blue luminescent fluorite is precipitated. Fluorite is formed since the chlorination process involved in the transformation causes localised reductions of the salinity of the fluid and therefore a decrease in the solubility of fluorite. Sodalite-fluorite textures observed using CL allow sodalites of secondary origin in alkaline igneous rocks to be identified.Nephelines and sodalites, when observed using scanning electron microscopy, possess small micropores. By analogy with recent work on alkali feldspars, pervasive alteration of nephelines may occur by fluid flow assisted by a permeable micropore network.

High-technology metals in alkaline and carbonatitic rocks in Greenland: recognition and exploration

High-technology metals such as Zr, Y, Nb, Ta, ree and Ga are generally associated with alkaline and carbonatitic rocks. The recognition of alkaline magmatic provinces or carbonatitic complexes is, therefore, a first step in the exploration for high-technology metals. Low-density exploration data over large parts of South and West Greenland including airborne gamma-ray spectrometry and stream sediment surveys clearly delineate the extension of a major Proterozoic alkaline igneous province and two Phanerozoic carbonatite complexes. On a reconnaissance scale, Nb in stream sediment is the best indicator and gave distinct anomaly patterns over all of the alkaline and carbonatitic intrusive complexes. The anomaly patterns for Y, Zr, Ga, Mn, Sr, Zn, U, and Th also reflect the alkaline/carbonatite intrusions but they are less regular and consistent. In the alkaline Gardar igneous province in South Greenland, two prospects in the Ilimaussaq intrusion relate to rock units rich in eudialyte (Zr, Y, ree). Some of the eudialyte-rich zones were located by geological mapping and visual recognition of this red mineral; elsewhere in the fine-grained eudialyte-bearing rocks, analyses of chip samples and drilling were used. In the Motzfeldt intrusion of the Gardar province, zones of intense hydrothermal alteration are enriched in Nb, Ta, Zr, ree, U and Th. The mineralised zones were located by helicopter-borne gamma spectrometry and subsequently grid sampled to assess Nb and Ta contents. In the Sarfartôq carbonatite complex, alteration zones with massive pyrochlore (Nb) were located by airborne and ground radiometry combined with detailed stream sediment sampling. In the Qaqarssuk carbonatite, minor pyrochlore concentrations and ree-bearing veins have been discovered during exploration for apatite. Based on this experience it is concluded that alkaline and carbonatite complexes are easily recognised in low-density geochemical data. Ore zones have been located using radiometric methods combined with geological mapping and detailed geochemical surveying.

Zirconium and niobium-bearing ilmenites from the Igaliko dyke swarm, South Greenland

AbstractIlmenites from alkaline basic dykes in the Gardar province, south Greenland have Zr contents up to 3850ppm and Nb contents up to 1030ppm. These elements substitute for Ti in the octahedral site. These are amongst the highest recorded Zr and Nb contents for ilmenites, giving distribution coefficients (Kdilmenite/matrix) for Zr up to 11.6 and Nb up to 70.3. Fractionation of ilmenite may thus have a dramatic effect on residual Zr and Nb contents. These effects are discussed in relation to the use of trace element tectonomagmatic discrimination diagrams.

An Anorthosite Suite in a Ring-Complex: Crystallization and Emplacement of an Anorogenic Type from Abontorok, A r, Niger

Laminated anorthosite grading outwards into leucogabbro, gabbro, and monzogabbro occurs in a 2.6-km-diameter funnel-shaped intrusion, cut by a quartz alkali syenite plug and concentric syenite and granite ring-dykes. The anorthosite-gabbro series is laminated but not modally or otherwise texturally layered. The lamination, defined by large tabular plagioclase crystals, forms a set of inwarddipping cones, the dips of which decrease from 60–45° in the central anorthosite to < 25° in the outer gabbros. Rocks close to the outer contact are medium-grained isotropic gabbros. Plagioclase, forming >80% of the series, generally has homogeneous labradorite cores (An62–58 in the whole series) and thin strongly zoned rims, which follow progressively longer solidus paths from the anorthosites to the gabbros. All rocks contain a late-magmatic alkali feldspar. Plagioclase is the main or only cumulus phase, the anorthosites being ad- to mesocumulates and the gabbros orthocumulates. Olivine (FO49–41) is more abundant than clinopyroxene in most of the series. Depending on quartz content, the syenites and granites are hypersolvus or subsolvus and the depth of crystallization was calculated to be 5 ± 2 km. A Rb/Sr isochron for the syenites and granites gave an age of 399 ± 10 Ma with an initial strontium isotopic ratio of 0.7084 ± 0.0005. Ten samples from the anorthosite-gabbro scries have an average calculated initial ratio of 0.70582 ± 0-00004 at − 400 Ma, showing that the two series are not comagmatic. The anorthosite-gabbro series has parallel REE trends (LaN/YbN ˜ 7–10) with decreasing positive Eu anomalies and increasing total REE contents from anorthosite to gabbro; two monzogabbros have almost no Eu anomaly. The liquid calculated to be in equilibrium with the lowest anorthosite has almost no Eu anomaly and its normalized REE pattern lies just above those for the monzogabbros. The syenites and granites have complementary REE patterns with negative Eu anomalies. The inferred parental magma was alkalic and leucotroctolitic with high TiO2 P2O5, Sr and K/Rb and with low MgO, very similar to parental magmas in the Gardar province, South Greenland. It was probably produced at depth by settling of olivine and clinopyroxene but not of plagioclase, which accumulated by flotation. It is suggested that plagioclase crystals from this lower chamber were progressively entrained (from 0% in the gabbros to 30–40% in the anorthosites), giving rise to the flow lamination in the upper chamber. The magma in the lower chamber may have been layered, because the plagioclase cores in the anorthosite are considerably richer in Or than those in the leucogabbros or gabbros. Overall convection did not occur in the upper chamber, whereas compositional convection occurred in the more slowly cooled central anorthositic adcumulates.

Comment on the paper "Pseudoleucite from the Gardar of South Greenland" by Stephen P. Hesselbo

This paper (Hesselbo, 1986) reports the first oc­currence of pseudoleucite in the Precambrian Gardar province of East Greenland. In summa­rizing previous work on the "pseudoleucite prob­lem" the author states that there is a "lack of proper descriptions of pseudoleucite intergrowth textures" and that "photomicrographs of unal­tered intergrowths of nepheline and K-feldspar illustrated by Fudali (1963, p. 1102) are the only ones published"....

Mid-Proterozoic alkaline magmatism in southern Greenland: the Gardar province

Summary The Gardar province ( c. 1320–1120 Ma old) relates to repetitive rifting within the Mid-Proterozoic continent and the attendant large-scale generation of alkali and transitional olivine basaltic and hawaiitic magmas. Lavas from the earliest episodes are preserved in down-faulted units (the Eriksfjord formation), whereas younger basic magmatism is recorded only as intrusions, mainly dykes. The province shows a compositional continuum from basaltic to salic alkaline compositions. The latter are represented among the early lavas, as dykes and, most prominently, as central-type complexes commonly involving layered (cumulate) syenitic intrusions. Most of the central complexes display either a silica-oversaturated trend (augite syenite-quartz syenite-(A-type) alkali granite) or a silica-undersaturated trend (augite syenite-pulaskite-foyaite). Strongly peralkaline products are present in each association with agpaitic rocks occurring as differentiates at Ilímaussaq and Motzfeldt. The oversaturated and undersaturated complexes, irrespective of age, define southerly and northerly zones respectively in the province. ‘Giant-dyke complexes’, produced in late-Gardar rifting events (contemporaneous with Keweenawan activity in N America) are important in providing a link between simple dilatational dykes and the central-type complexes. An ultramafic lamprophyre-carbonatite association plays a subordinate role in the province, and carbonatites are also associated with some of the salic complexes. The carbonatites at Grønnedal-Ika are thought to be residual from CO 2 -rich phonolitic magmas, whereas carbonate (siderite)-fluoride rocks at Ivigtut crystallized from volatile-rich residues associated with an alkali granite. The salic alkaline magmas were produced from alkali or transitional olivine basaltic parent magmas, with crystal fractionation being the dominant mechanism. Generally low 87 Sr/ 86 Sr (i) ratios in the alkaline complexes suggest little assimilation of old radiogenic crustal materials. The basaltic magmas were characterized by high Al 2 O 3 /CaO and FeO/MgO ratios, and it is inferred that they themselves were residues from more magnesian parent magmas that had undergone extended olivine and clinopyroxene fractionation at sub-crustal levels. The primary magmas may have been derived from garnet lherzolite source rocks with high garnet to clinopyroxene ratios. Late-Gardar basic magmas of the Tugtutôq-Ilímaussaq-Nunataq zone are geochemically distinct from all other Gardar basic magmas in being enriched in P, Ba, Nb and light rare earth elements. This feature may be due to metasomatic enrichment of source rocks along a lineament sub-parallel to the Archaean craton margin. Gardar alkaline magmas were rich in F, Cl and C compounds (oxides and/or hydrocarbons) while being relatively anhydrous. Many crystallized under low f O 2 conditions. The fundamental cause of the repetitive alkaline magma generation may have been the ascent of F-, Cl- and C-rich fluids from the deep mantle, leading to partial melting of the lithospheric mantle close to the boundary between Archaean and Proterozoic crustal units.

The Tugtutôq older giant dyke complex: mineralogy and geochemistry of an alkali gabbro-augite-syenite-foyaite association in the Gardar Province of South Greenland

AbstractThe Older Giant Dyke Complex is a differentiated alkaline intrusion of Proterozoic age (1154±16 Ma) and is the earliest of the late Gardar intrusions in the Tugtutôq-Ilímaussaq region. The dyke is approximately 20 km long by 0·5 km broad and comprises (i) marginal ‘border group’ rocks of alkali olivine gabbro, grading inwards to ferro-syenogabbro and (ii) an axial ‘central group’ of salic rocks ranging from augite syenite in the WSW to sodalite foyaite in the ENE.Chilled margins contain plagioclase (An53), olivine (Fo53), magnetite, ilmenite, and apatite as liquidus phases and later-crystallized augite (Di69Hd27Ac4) and biotite (Annite32). The coexisting Fe-Ti oxides indicate fO2 and T values just below the synthetic QFM buffer curve. In the border group, plagioclase cores zone into anorthoclase and soda-sanidine rims, olivines reach Fo16, pyroxenes Di32Hd59Ac9, and biotites Annite86. Interstitial pargasitic amphibole appears close to the innermost margins. In the central group, feldspars are all perthitic alkali feldspars and nepheline becomes a major, early crystallizing phase. Olivines range from Fo10-Fo4, in the augite-syenites where they coexist with ferro-salites Di50Hd47Ac3, but olivine is absent from foyaitic assemblages in which the pyroxenes range through aegirine-augite to pure aegirine. Interstitial amphiboles range from ferro-pargasite or hastingsite to katophorite and thence towards arfvedsonite, but are absent from the most differentiated rocks, whereas biotite occurs throughout the entire group in the range Annite71-Annite100.The parental magma, represented by the chilled margins, was a relatively anhydrous alkali olivine-basalt with an initial 87Sr/86Sr ratio of 0.70326. Its high Ti, P, Ba, and F contents are inferred to be features inherited from a primary magma, derived from the mantle as a small partial melt fraction which involved significant amounts of fluor-apatite and phlogopite. While all lithologies are considered as differentiates from this parental magma, there is both a well-defined field junction and a compositional hiatus between the border group and the central group rocks. Mineralogical considerations and REE patterns suggest that the later, more salic (?benmoreitic) magma from which the central group crystallized, related to the parental magma by ol-fsp-ap-FeTi oxide fractionation. Congelation in both border group and central group occurred by side-wall crystallization, but the salic magma became compositionally stratified, with upward concentration of alkalis and volatiles to produce a phonolitic upper facies which is preserved at the ENE end of the intrusion owing to subsequent axial tilting.

Combined Raman microprobe spectrometry and microthermometry of fluid inclusions in minerals from igneous rocks of the Gardar province (south Greenland)

Raman microprobe spectrometry has been applied to analysis of the gaseous phase in individual fluid inclusions in minerals from alkalic igneous rock types and associated hydrothermal veins from the Gardar province, south Greenland. Quantitative analyses are given for CO 2, CH 4, C 2H 6, H 2 and H 2S. Traces of CO were observed in one CO 2 inclusion. In conjunction with data from microthermometry, the results permit calculation of the bulk chemical composition of entrapped fluids and evaluation of the possible pressure, temperature, oxygen fugacity and sulphur fugacity during entrapment of the individual fluid inclusions. Calculated oxygen fugacity values correspond to those of the NNO and QFM buffer reactions at temperatures of 300–400°C and pressures of 0.5–1.0 kbar. Calculated sulphur fugacity values are within the range of normal ore forming solutions (log f S 2 : −6 to −10).

Composition of fluid inclusions in granites and quartz syenites from the Gardar continental rift province (South Greenland)

Fluid inclusions in quartz from granites and quartz syenites from the Precambrian Gardar province (south Greenland) vary in composition from CO2-CH4, through CO2-CH4-H2O mixtures, to aqueous saline fluids. The inclusions primarily represent fluids present during most-magmatic stages. Aqueous inclusions are the dominant type in all examined complexes. Salinities vary from 1 to 64 weight per cent, but late inclusions contain less than 40 equivalent weight per cent NaCl and have densities from 0.9 to 1.15 g/cm3. Boiling of the aqueous fluids does not appear to have played a prominent role in the generation of the higher salinity fluids. Early CO2-CH4 fluids contain less than 5 mole per cent CH4 and have densities from 0.75 to 1.0 g/cm3, but later fluids may contain up to about 80 mole per cent CH4 and have lower densities. Conditions for entrapment of most of the examined inclusion fluids at 250° to 400 °C and about 1-2 kbar are indicated.

Uranium districts defined by reconnaissance geochemistry in South Greenland

A reconnaissance exploration survey over 14 000 km 2 of Precambrian terrain in South Greenland using stream-sediment and stream-water samples delineated a central uranium district of 2000 km 2 with enhanced uranium levels and smaller anomalous zones in the south of the field area. The area is underlain by Archean and Proterozoic gneisses, granites and metasediments all of which have been intruded by late Proterozoic alkaline intrusions (Gardar Province). The terrain is mountainous and the streams are either steep torrents or impeded drainages typical of glaciated terrains with boggy organic rich sediments. The central uranium district was defined by a high uranium background in both stream sediments (5–20 ppm) and stream waters (0.5–1.0 ppb) and a markedly higher frequency of very anomalous values in the order of 50–100's ppm in the stream sediments and 1–10 ppb in the stream waters. An areal correlation of uranium, in this district, with high pH and conductivity in the stream water in addition to a higher organic content noted in the stream sediment raised the question of a possible enhancement of uranium values due to secondary environmental effects. On the other hand, an areal correlation of uranium with niobium and other trace elements characteristically associated with alkaline rocks, and the geographic proximity of this uraniferous district to the alkaline intrusions suggested a genetic relationship between uranium mineralization and the alkaline igneous activity. Limited follow-up work located 8 pitchblende occurrences in this extensive district. The pitchblende is in veins which contain quartz, calcite, iron oxide, fluorite and minor sulphides. The isotopic (U-Pb) age of the pitchblende, which ranges from 1180-1090 Ma, corresponds to the late stages of Gardar alkaline igneous activity. It is concluded, therefore, that the reconnaissance geochemistry reflects a district-wide hydrothermal event related to the late volatile differentiates derived from the highly fractionated alkaline magma. A combination of primary and secondary features have complemented each other in enhancing the geochemical reconnaissance data and emphasized its importance but has not materially altered the interpretation. The south of the field area also has a relatively high uranium background in both the sample media with some discrete anomalous zones, usually with a slightly lower order of magnitude than the central area, but still with a distinct contrast of 5–10 times. Fine-grained uraninite has been found in the area occurring as disseminated grains in pegmatitic elements as in the central district. Isotopic ratios (U-Pb) suggest an age of 1728 ± 30 Ma which probably reflects the long cooling of the granite. It is concluded that the geochemical reconnaissance data delineated two uranium metallogenic districts characterized by distinctly different types of uranium mineralization. It is suggested that South Greenland may be part of a much wider uranium geochemical province which includes parts of Labrador. To the present plate-tectonic models, which suggest such a connection (Le Pichon et al., 1977), must be added the comparable reconnaissance geochemical results (G.S.C. Open Files nos. 748 and 749), and the similar 1730 Ma age of the Kitts uranium mineral occurrence in Labrador (Gandhi, S.S , 1978) to that of the uraninite found in the south of the field area in Greenland.

Introduction to the Ilímaussaq intrusion with a summary of the reported investigations

The Ilimaussaq intrusion belongs to the late Precambrian Gardar province of South Greenland (fig. 1). The province ean be regarded as a cratogenic rift province with similarities to the East African rift system and the Oslo igneous provinee (Upton, 1974; Emeleus &amp; Upton, 1976). The provinee consists of continental sandstones, and a variety of volcanic and plutonic igneous rocks many of which are alkaline. An important series of faults have influenced the position of the plutonic centres and presumably facilitated the rise of magma into the upper erust. The alkaline roeks evolved towards Si-rich melts such as comendites and alkali granites, and towards Si-poar melts such as phonolites and nepheline syenites. Both types of evolution are present at Ilimaussaq (fig. 2).