Gardar Province Research Articles

A palaeomagnetic survey of the Jotnian dolerites of central?east Sweden

Summary. The Jotnian igneous episode of central-east Sweden is represented by dolerite sills distributed over an area of 5000 km2. After detailed af and thermal cleaning of 265 samples from 46 sites in these dolerites, 43 of the sites yield a mean direction of magnetization D=47.8, I= -45.5' (a%= 2.6O) and a palaeomagnetic pole at 156.5 E, 7.5 S. The results are combined with other data from the same region to derive an overall mean palaeomagnetic pole for the Jotnian igneous episode. All samples have the same polarity and directions of magnetization agree closely with dolerites attributed to the Jotnian episode from elsewhere in the Baltic Shield. The Jotnian igneous intrusions of Sweden are dated 1247-1213 Ma and are broadly contemporaneous with the Mackenzie magmatic episode in North America (1205-1 189 Ma) and dyke intrusions within the Gardar Province of South Greenland (1 250-1 21 2 Ma). These igneous events all have the same polarity on the continental reconstruction derived from 2000-1200 Ma palaeomagnetic data with the Laurentian and Baltic Shields in juxtaposition.

Subject index

The persodic, 1.16 Ga old Ilímaussaq intrusion consists of alkali granites, syenites, and agpaitic nepheline syenites. These rocks are cut by late-magmatic veins and lenses consisting of albite, aegirine, ussingite (Na2AlSi3O8(OH)), fluorite, and occasionally quartz. Oxygen-isotope compositions of quartz indicates both orthomagmatic (δ18Oqtz = 8.5‰) and country rock-influenced (δ18Oqtz ~ 10‰) origin. While albite and aegirine do not contain fluid inclusions suitable for investigations, ussingite contains secondary hydrocarbon fluid inclusions, fluorite contains brine inclusions of primary and secondary origin with up to 26.3 wt.% NaClequiv., and quartz contains predominantly secondary brine inclusions with up to 29.7 wt.% NaClequiv. or CH4–H2O–NaCl mixtures. These fluids are interpreted to reflect fluids in equilibrium with late-stage melts at Ilímaussaq at ≤ 450 °C.The carbon- and hydrogen-isotope composition of CH4 in ussingite (δ13C = − 6 to − 3‰, δD = ~− 121‰) is indicative of a magmatic abiogenic origin. However, the isotopic composition of CH4 in fluid inclusions in quartz veins resembles the signature of thermogenic CH4 (δ13C = − 43 to − 23‰, δD = − 176 to − 121‰), but the higher hydrocarbons are mostly 13C-depleted in relation to CH4 (up to 7.3‰), which is typical of abiogenically-derived hydrocarbons.Ion-chromatography of fluid inclusion leachates from the late-stage veins reveals Cl/Br ratios of ~ 100. As such values seem to be typical of peralkaline magmatic rocks, at least in the Gardar Province of South Greenland, it is suggested that this ratio is typical of Gardar magmatic fluids and may be characteristic of the Cl/Br ratio of the lithospheric mantle from which these alkaline melts are derived.

Age and origin of the major syenite centers in the Gardar province of south Greenland: Rb-Sr studies

Rb-Sr whole-rock isochron data for nine intrusive centers (including the major oversaturated and undersaturated syenite bodies) in the Precambrian Gardar alkalic igneous province of south Greenland complete a systematic isotopic study of the major complexes of the region (15 distinct igneous centers). The new data show that magmatic activity occurred throughout the region over a period in excess of 150 m.y. and that there is a clear distinction between two major magmatic events — an early Gardar (∼1,300 m.y. old) event and a more widespread late Gardar (∼1,160 m.y. old) event, which also involved extensive east-northeast-trending dike emplacement. Two relatively small intrusions in the western part of the province have middle Gardar (∼1,250 m.y. old) ages. There does not appear to be any relationship between emplacement age and geographic location, mode of emplacement, or the recognized petrologic division into saturated or undersaturated types; indeed, some adjacent complexes with strikingly similar petrologic characteristics have originated during different events, and, conversely, adjacent complexes with very different compositions are chronologically indistinguishable with present techniques. Initial 87 Sr/ 86 Sr ratios for the Gardar rocks are broadly divisible into three groups. Low (0.702 to 0.704) ratios are characteristic of most of the intrusions in the province, including most of the syenites (both undersaturated and oversaturated) and gabbros —which indicates a primitive (mantle) origin for the parental Gardar magmas. Intermediate ratios (0.704 to 0.707) are observed in fewer centers, particularly in the Nunarssuit complex (gabbro, oversaturated syenites, and granites). For these centers, the possibility of closed-system enrichment in 87 Sr (by decay of 87 Rb during a complex molten history extending throughout 150 m.y. of Gardar time) is rejected as unlikely, as is a direct origin by partial melting of older crust; field, chemical, and isotopic evidence presented here strongly support a bulk-assimilation hypothesis. Only two complexes have high (>0.707)ratios; both show evidence of introduction of radiogenic 87 Sr into their magmas at a late stage of development through selective leaching of radiogenic 87 Sr from older crust, and both of these complexes are associated with mineralization.

Chronology of the Red Wine alkaline province, central Labrador

An 11-point Rb–Sr whole-rock isochron for the regionally metamorphosed Red Wine alkaline province gives an age of intrusion of 1345 ± 75 Ma (errors quoted at 2σ) and an initial 87Sr/86Sr ratio of (1.7021 ± 0.0103. Two 5-point mineral isochrons give ages of ~1000 Ma that represent metamorphic 'resetting' of the alkaline rocks. The whole-rock intrusion age compares closely with the early stage of magmatism in the Gardar Province of south Greenland where un-metamorphosed agpaitic igneous rocks, similar to those of the Red Wine Province, occur. In both provinces, alkaline plutonic rocks are associated, both spatially and chronologically, with thick sequences of continental sediments and basaltic lavas, and the new age data lend strong support to the supposition that the Gardar and Red Wine rocks are parts of the same pre-drift magmatic province and inferred rift system. The Gardar Province has, however, escaped the effects of the Grenville regional metamorphism which severely affected the Red Wine rocks.

Palaeomagnetism of the dyke swarms of the Gardar Igneous Province, south Greenland

In the western part of the Gardar Igneous Province of southern Greenland, lamprophyre dykes intruded at ca. 1276-1254 m.y. RbSr biotite ages yield a palaeomagnetic pole at 206.5°E,3°N (nine sites, d ψ = 5.1°, d χ = 10.1°) Slightly younger dolerite dykes with RbSr biotite ages in the range 1278-1263 m.y. give a pole at 201.5°E,8.5°N (24 sites, d ψ = 4.7°, d χ = 9.4°), and the syeno-gabbro ring dyke of the Kûngnât complex (RbSr isochron age 1245 ± 17 m.y.) cutting both of these dykes swarms, gives a pole at 198.5°E, 3.5°N (four sites, d ψ = 2.3°,d χ = 4.4°). All these rock units have the same polarity and the poles are identical to those from Mackenzie and related igneous rocks of North America (1280-1220 m.y.) after closure of the Davis Strait; they confirm that this part of the Gardar Province is a lateral extension of the Mackenzie igneous episode within the Laurentian craton. In the Tugtutôq region of the eastern part of the Gardar Province 47 NNE-trending dykes of various petrologic types, and intruded between 1175 ± 9 and 1168 ± 37 m.y. (RbSr isochron ages) yield a palaeomagnetic pole at 223.9° E, 36.4°N (d ψ = 4.1°, d χ = 6.1°). Fifteen other dykes in this swarm were intruded during a transitional phase of the magnetic field which, however, does not appear to have achieved a complete reversal over a period of several millions of years. The majority of dykes studied are highly stable to AF and thermal demagnetisation and contain single high blocking temperature components with single Curie points in the range 380–560°C. Palaeomagnetic poles from the Gardar Province between ca. 1330 and 1160 m.y. in age define the earlier part of the Great Logan apparent polar-wander loop; they correlate closely with contemporaneous North American results and confirm the coherence of the Laurentian craton in Upper Proterozoic times.

Magnetic stratigraphy and magnetic-petrologic properties of Precambrian Gardar lavas, South Greenland

The Gardar lavas ( 1310 m.y.) within the Eriksfjord Formation of South Greenland were a prelude to major hypabyssal magmatism within the Gardar alkaline igneous province. This paper reports a palaeomagnetic study of the lower two-thirds of the lava stratigraphy comprising three groups of flows separated by sediments. The lower group (palaeomagnetic pole at 239°E, 7°N, 7 sites, dΨ = 7.6°, dχ = 14.1°) includes one reversal of magnetisation and two intermediate directions, and the upper group (pole at 222°E, 16°N, 24 sites, dΨ = 4.8°, dχ = 9.0°) includes four reversals and two intermediate directions. The middle group of 13 lavas yields anomalous directions with respect to the other groups and appears to have been erupted during a polarity excursion of uncertain significance. Opaque petrographic analysis reveals all stages of deuteric oxidation (800–500°C) plus lower-temperature hydrothermal alteration to epidote grade. In addition to high reversal frequency, the succession illustrates asymmetry of the dipole source, and there is a suggestion that lavas with easterly directed magnetisations have higher deuteric oxidation states than those with westerly directed magnetisations. Collective palaeomagnetic results from the Gardar Province are linked to a number of Rb Sr isochrons, and comparison with contemporaneous North American data confirms the 500-fathom continental reconstruction for Proterozoic times. The poles define a movement of the Laurentian craton from low palaeolatitudes (1400–1220 m.y.) to high palaeolatitudes (1168–1070 m.y.) with a quasi-static interval at 1280–1220 m.y. coinciding with the Mackenzie igneous episode. The youngest Gardar data correlate with the oldest Keweenawan palaeomagnetic poles and restrict deposition of this group to the interval ca. 1160–1040 m.y.

Petrology of the late lavas of the Eriksfjord Formation, Gardar province, South Greenland

The uppermost 740-1300 m of the c. 3500 m thick Precambrian Eriksfjord Formation in South Greenland are composed of plagioclase- and olivine-porphyritic basalts and hawaiites of a transitional alkaline type, slightly nepheline-normative trachybasalts, trachyandesites and trachytes and a few flows of phonolite. One ultramafic lava also occurs. Evidence for acid volcanism is restricted to a single small occurrence. The rocks show varying degrees of contact metamorphism and hydrothermal alteration caused by the later Gardar intrusions. Major and trace elements show that fractional crystallization of plagioclase and olivine and minor amounts of clinopyroxene, hypersthene (?) and Fe-Ti oxides possibly governed the differentiation in the basic rocks at elevated pressures. The salic lavas belong to several lineages representing a large range in Na2O/K2O ratios. The covariance of the less mobile trace elements Zr, Nb, Y and Nd suggest that they are not derived from magmas represented by the basalts, but they may have been formed by differentiation of another type of basaltic magma.

Origin of anorthosites, gabbros and potassic ultramafic rocks from the Gardar Prqvince, South Greenland: Sr isotopic ratio studies

Precise initiaJ 87Sr ratio data (2a~0.00006) for five xenolithic anorthosites and five host gabbros from the late phase (~ 1170 m. y.} of activity in the Narssaq area of the Gardar Province, South Greenland, show that the two rock-types had an· identical mean initial B7Sr/B6Sr ratio of 0.70297 at 1170 m.y. The samples de1me a line of 1150 ± 142 m. y. which is in agreement wtih their known age. The concordance of initial B7Sr/B6Sr ratios strongly suggests a related origin and favours models for differentiation of Gardar magmas which involve plagioclase fractionation. Initial B1Sr/B6Sr ratio data for rocks and minerals from small potassic ultramafic bodies belonging to the same magmatic cycle and intruding the gabbros are also uniformly low (~ 0.703). These values axe similar to the low initial B7Sr/B6Sr ratios obtained for other, uncontaminated Gardar intrusions and make any significant crustal contamination of the ultramafic magmas improbable-

Rb-Sr isotopic evidence for the age and origin of the Ivigtut Granite and associated cryolite body, South Greenland

A Rb-Sr whole-rock isochron age of 1,248 + or - 25 m.y. (errors quoted at 2Sigma ) for the granite intrusion at Ivigtut shows that it is significantly younger than the nearby Gronnedal-Ika nepheline syenite-carbonatite complex but older than most of the major Gardar intrusions to the south and east. The initial 87 Sr/ 86 Sr ratio (0.7125 + or - 0.0048) determined for the granite is the highest reported in the Gardar Province and is largely due to the introduction of radiogenic 87 Sr in an interstitial cryolite phase ( approximately 0.720) after the bulk of the granite was already solid. The main cryolite orebody itself had an initial 87 Sr/ 86 Sr ratio ( approximately 0.709) slightly lower than the granite and was thus not directly responsible for the 87 Sr enrichment of the granite.

A simple-shear model for the ductile deformation of high-level intrusions in South Greenland

Major ring-intrusions in the Gardar Province are frequently elliptical in cross-section and are located at the intersections of regional dyke swarms with sinistral wrench-fault systems. Fault movement was contemporaneous with igneous intrusion and the ellipticity may be explained by ductile deformation of hot intrusions undergoing large-scale simple-shear. Geometrical measurements on four of the intrusions give shear-strain values ( γ ) ranging from 0.6–1.2. Away from the intrusions, deformation is entirely by brittle fracture and the amount of fault movement correlates well with the sum of brittle and ductile movement within the intrusions.

Volcanism and structure of continental rift valleys

T he common image invoked by the term “riff” is of extensive linear depressions embellished by the constructional features of recent volcanic activity. It is to the credit of Drs Pinkerton and Macdonald, both of Lancaster University, that they interpreted "rift" in an imaginative way and organized a group of speakers who, although falling under the general generic heading of “rifters”, dealt with the subject over a long time scale and wide spatial distribution. Drs Williams and Banks were conventional enough in that their subjects were respectively the geology and geophysics of the East African rift system in Kenya. Professor King summarized the geology in and around the non-volcanic Baikal rift. Dr Upton presented the fascinating story of the intense faulting and associated magmatism of the late Precambrian Gardar province of South Greenland whereas Dr Howell Francis injected new data and enthusiasm into the magmatic history of the Midland Valley of Scotland. Professor Bailey, painting on a wider canvas, considered the causes of African rifting and magmatism. Dr Williams, dealing with the distribution, composition and age of the Cenozoic volcanics both within and away from the Kenya rift, emphasised the growing complexity of the story as more data become available. Lavas that at one time had been considered to belong to the same phase, had been shown to differ in both composition and age. There was no simple compositional progression in either space or time, this lack of obvious correlation applied also between volcanic structures and those in the underlying

Discussion

Professor W. S. P itcher said that he was not as perplexed as the author by the apparent absence of fabrics resulting from the deformation of hot intrusions undergoing large-scale simple shear. In general there was much evidence of the early formation of fractures during the cooling history of granitoids and of their subsequent healing and obliteration by continuing crystallisation and recrystallisation. One of the best indicators of the former presence of early joint systems was the presence of synplutonic dyke swarms and the question was asked whether such synplutonic phenomena had been noted in the ring-intrusions of the Gardar Province. The A uthor thanked Professor Pitcher for his reassuring comments which are particularly helpful in explaining the dilational emplacement of dykes within the ductile medium of the South Qôroq Centre rocks, and the subsequent ductile deformation of both dykes and host. The South Qôroq dykes themselves may be almost analogous to the synplutonic phenomena mentioned by Professor Pitcher since they are closely related in time to the major intrusion and some have close petrological similarities to the rocks of the South Qôroq Centre rather than to those of the main regional dyke swarm. However, the author is not aware of any detailed synplutonic features in the Province although it is not impossible that these exist.

Alkali loss and retention in an iron-rich peralkaline phonolite dyke from the Gardar province, south Greenland

An unusually iron-rich, strongly peralkaline dyke occurring near the Ilimaussaq intrusion, south Greenland, is described. The dyke has developed two facies characterised respectively by the mafic assemblages pyroxene+amphibole+aemigmatite, and pyroxene+magnetite. The differences can be explained as an effect of loss of up to 4% Na 1O from the magnetite-bearing facies (relative to the magnetite-free facies). The effect is in accordance with the phase relations at 1 atm in the synthetic system Na 2OAl 2O 2Fe 2O 2SiO 2. Continued alkali loss and magnetic precipitation will change the differentiation trend from agpaitic with iron enrichment towards miaskitic with iron depletion. It is suggested that the extreme degree of iron enrichment in the dyke resulted from the coincidence of low oxygen fugacity and strong peralkalinity tending to stabilise both ferrous and ferric iron in the liquid.

Development of Precambrian Shield in Southwestern Greenland, Labrador, and Baffin Island: ABSTRACT

The Precambrian rocks on either side of Davis Strait show a similar pattern of events and can be interpreted as having formed part of a single shield. Eight major stages in the development of this shield are suggested. (1) Formation of an extensive early crust before 3,000 m.y. ago, relicts of which are now preserved as migmatite and high-grade gneiss in the Archean block of eastern Labrador and southwest Greenland. (2) Deposition of greenstone belts between 2,700 and 3,000 m.y. ago. (3) Plutonic activity in the period 2,500-2,900 m.y. ago, affecting both the greenstone belts and the major part of the basement on which they lie. (4) Intrusion of numerous basic dike swarms in the general period of 2,000-2,600 m.y. ago. (5) Lower Proterozoic (Aphebian) geosynclinal rocks we e deposited on the consolidated Archean basement and were involved in an orogeny known in Canada as Hudsonian and in Greenland as Ketilidian and Nagssugtoquidian. (6) Post-orogenic magmatism, particularly prominent in areas affected by Hudsonian metamorphism, extends from southern Greenland through Labrador. This produced chiefly anorthosites, adamellitic granites, monzonites, and norites, that were probably emplaced between 1,400 and 1,700 m.y. ago although the areas in which they occur commonly remained thermally active to 1,200 m.y. ago or later. (7) Emplacement of the post-orogenic rocks was accompanied and followed, in southern Greenland and in parts of Baffin Island, by graben faulting, deposition of molasse sediments, and widespread intrusion of basic dikes. The majority of these ikes are tholeiitic; however, locally (for example, in the Gardar Province of southern Greenland), alkaline and peralkaline intrusions took place 1,100-1,300 m.y. ago. (8) The Archean and Proterozoic rocks in the southern part of the Canadian shield were metamorphosed and tectonically worked by the Grenville orogeny about 900-1,100 m.y. ago. The only effect of this orogeny in southern Greenland was a slight updating of earlier rocks from areas close to major faults so that they yield K/Ar ages of about 900-1,000 m.y. End_of_Article - Last_Page 2472------------

Mechanics of Flow Differentiation in Ultramafic and Mafic Sills: A Reply

sonal communication) as an example of dikes whose formation may be explained by the process of flow differentiation. The use of the hyphen between and was intended to indicate the end-member relationship between alkali, bostonite-type dikes and calcic, anorthositetype dikes. Valentine (1965, 1967, personal communications) describes bostonite dikes in the Gardar province of Greenland as characteristically buff to pink in color, predominantly of alkali feldspar, while the anorthosite dikes are dark in color, very poor in ferromagnesium minerals, but extremely rich in oligoclase to labradorite plagioclase feldspar. The bostonite and anorthosite dikes show similar characteristic distributions of the same host rock and inclusion minerals. The inclusion minerals in both the bostonite and anorthosite dikes are feldspar phenocrysts. Most commonly, the margins of these dikes are either totally free of or very poor in phenocrysts. In general, the size of the phenocrysts increases from the margin toward the center of the dikes; the maximum concentration of feldspar phenocrysts is in the axial region of the dikes.

On the magmatic evolution of the alkaline igneous province of South Greenland

In 1938 E. Wegmann divided the rocks of southern Greenland into and old basement and a younger formation. The basement consists of the remains of a mountain chain for which Wegmann introduced the name the Ketilides. The younger rocks, which are separated from the Ketilidian orogeny by a long period of denudation, were grouped in the Gardar formation. The Gardar "formation" comprises supracrustal rocks, a great number of dykes and a limited number of plutonic bodies. In this article an outline of the geology and the magmatic evolution of the Gardar province will be given.

The Puklen intrusion, Nunarssuit, SW Greenland, a preliminary account of a new Gardar intrusion

The Puklen Intrusion is a small elongate complex situated in Nunarssuit, SW Greenland. It was emplaced in Ketilidian basement, and cut off and hornfelsed Gardar dolerites. The four component members of the intrusion are augite syenite, quartz syenite, soda granite and granophyre, emplaced in that order. It seems that these components represent a differentiation sequence from augite syenite. This is of interest owing to the widespread occurrence of augite syenite (with fayalite) in the Gardar province of SW. Greenland.