Giant Dyke Research Articles

New insights from field observations of the Younger giant dyke complex and mafic lamprophyres of the Gardar Province on Tuttutooq island, South Greenland

The Gardar Province of south Greenland is defined by the products of alkaline igneous magmatism during the Mesoproterozoic. The most laterally extensive Gardar intrusions are a series of giant dyke complexes best exposed on the Tuttutooq archipelago. We present new field observations and a geological map of north-east Tuttutooq island that provide fresh insights into the temporal evolution of the Younger giant dyke complex and two associated ultramafic lamprophyres. Our data demonstrate that distinctive crystallisation regimes occurred in different sectors of the dyke complex, leading to the formation of marginal gabbros and ovoid pod-like domains displaying lamination, modal layering and/or more evolved differentiates. We infer that at least two pulses of magma contributed to the formation of the Younger giant dyke complex. In addition, the relative ages of two ultramafic lamprophyre diatremes are constrained and attributed to two distinct phases of rifting in the Gardar Province.

Petrography, geochemistry and magnetic susceptibility of the Isortoq Fe-Ti-V deposit, Isortoq Giant Dykes, South Greenland

The Isortoq Giant Dykes in the Proterozoic Gardar Province, South Greenland, include the Isortoq South giant dyke and the Isortoq North giant dyke. The fine-grained Fe-Ti-V deposit hosted by the Isortoq South giant dyke, referred to as the Isortoq Fe-Ti-V deposit, is considered a good test site for the use of magnetic susceptibility for the mapping of ore grades. Here, we test this and show that the Fe, Ti and V distribution is controlled by titanomagnetite disseminated throughout fine-grained troctolite. The deposit displays a clear correlation between magnetic susceptibility and Fe, Ti and V grades in bulk samples of consecutive 2 m sections from 11 drill cores, totalling 2671 m in length. We observe that Fe, Ti and V are almost entirely hosted in titanomagnetite, which controls the magnetic susceptibility. Field measurements of the magnetic susceptibility can thus be considered as a reliable exploration tool for this type of mineralisation. We further consider the origins of the deposit by reconnaissance petrography, mineral and bulk rock chemistry of the large mass of aphanitic Fe-rich troctolite in the Isortoq South giant dyke. We suggest that the deposit may represent the base of a basanitic to trachybasaltic magma chamber, in which Fe-rich immiscible melts accumulated, crystallised and fractionated. The processes suggested here may apply to other giant dykes and intrusions of the Gardar Province.

Applicability of AMS technique as a flow fabric indicator in dykes: insight from Nandurbar–Dhule Deccan dyke swarm

Determination of the nature of magma flow (vertical or lateral) from rock fabrics in a dyke swarm holds significant importance in deciphering its emplacement mechanism, depth of magma chamber and its association with mantle plumes. Nandurbar–Dhule dyke swarm from Western India consists of ~ 210 mappable, tholeiitic, large, linear dykes intruding the country rocks of Deccan basalt. We discuss here the development of flow-controlled rock fabrics in these dykes as a function of their thicknesses. Magnetic fabric obtained from anisotropy of magnetic susceptibility (AMS) analysis of the samples collected from these dykes was used as a proxy for rock (flow) fabric. Samples from the central parts of the thicker dykes (strike perpendicular thickness > 10 m) show random and inconsistent development of magnetic fabrics than their marginal counterparts. Petrography suggests the development of relatively clear shape-preferred orientations of the elongated mineral grains only at the margin of a thick dyke. Thinner dykes (strike perpendicular thickness < 10 m) show both consistent magnetic fabrics and clear shape-preferred orientations. We propose that the center of a thick dyke fails to provide (due to high flow velocity, slower cooling rate, lack of friction-induced shearing, and draining back of magma) a favorable condition where (a) flow fabric can efficiently develop and (b) magnetic fabric can effectively mimic the flow fabric. Thinner dykes and the margins of the thicker dykes are, hence, better candidates for observing flow fabrics represented by magnetic fabrics. This work is also motivated by the reservations expressed by earlier researchers in using the AMS technique for magma flow characterization, especially for giant dyke swarms. We have critically evaluated their reservations in light of our results and commented on the applicability of the AMS technique on such studies.

Petrology, geochronology and emplacement model of the giant 1.37 Ga arcuate Lake Victoria Dyke Swarm on the margin of a large igneous province in eastern Africa

A comprehensive description of the petrography, geochemical composition, Sm−Nd data and intrinsic field relationships of a giant arcuate Mesoproterozoic mafic dyke swarm in SW Uganda is presented for the first time. The swarm is ∼100km wide and mainly hosted in the Palaeoproterozoic Rwenzori Belt between the Mesoproterozoic Karagwe-Ankole Belt and the Archaean Uganda Block. The dykes trend NW–SE across Uganda, but can be correlated across Lake Victoria to another set of arcuate aeromagnetic anomalies that continue southwards into Tanzania, resulting in a remarkably large semi-circular swarm with an outer diameter of ∼500km. We propose that this unique giant dyke structure be named the Lake Victoria Dyke Swarm (LVDS). The dykes are tholeiites with Mg numbers between 0.69 and 0.44, and with inherited marked negative Nb and P anomalies in spider diagrams. Two dykes provide Sm−Nd mineral ages of 1368±41Ma and 1374±42Ma, with initial εNd values of −2.3 and −3.2, and 87Sr/86Sr ratios of ∼0.706–0.709. Geotectonic discrimination diagrams for the swarm exhibit more arc type than within-plate tectonic signatures, but this is in accordance with systematic enrichments in LREE, U and Th in the dolerites, more likely due to the involvement of the continental lithosphere during their petrogenesis.The LVDS is coeval with a regional ∼1375Ma bimodal magmatic event across nearby Burundi, Rwanda and NW Tanzania, which can collectively be viewed as a large igneous province (LIP). It also indicates that the nearby Karagwe-Ankole Belt sequences – bracketed between 1.78 and 1.37Ga and assumed by some to have been deposited within intracratonic basins – were capped by flood basalts that have subsequently been removed by erosion. Different geochemical signatures (e.g. LaN/SmN) suggest that most of the arcuate swarm was derived from an enriched SCLM, whereas related intrusions in the centre of this semi-circular segment have more or less enriched asthenospheric mantle source signatures. A model of how the LIP configuration formed, and especially its giant arcuate swarm, requires fortuitous pre-existing structures, an unusually large sub-crustal magma chamber, and/or some very intrinsic rift process. The LIP is apparently related to a global 1.4–1.2Ga rifting event that led to the break-up of the Columbia/Nuna supercontinent.

Tectono-magmatic evolution of the younger Gardar southern rift, South Greenland

The 1300–1140 Ma Gardar period in South Greenland involved continental rifting, sedimentation and alkaline magmatism. The latest magmatism was located along two parallel rift zones, Isortoq–Nunarsuit in the north and the Tuttutooq–Ilimmaasaq–Narsarsuaq zone in the south addressed here. The intrusive rocks crystallised at a depth of troctolitic gabbros. These relatively reduced magmas evolved through marked iron enrichment to alkaline salic differentiates. In the Older giant dyke complex, undersaturated augite syenites grade into sodalite foyaite. The larger, c. 1163 Ma Younger giant dyke complex (YGDC) mainly consists of structureless troctolite with localised developments of layered cumulates. A layered pluton (Klokken) is considered to be coeval and presumably comagmatic with the YGDC. At the unconformity between the Ketilidian basement and Gardar rift deposits, the YGDC expanded into a gabbroic lopolith. Its magma may represent a sample from a great, underplated mafic magma reservoir, parental to all the salic alkaline rocks in the southern rift. The bulk of these are silica undersaturated; oversaturated differentiates are probably products of combined fractional crystallisation and crustal assimilation.&#x0D; A major dyke swarm 1–15 km broad was intruded during declining crustal extension, with decreasing dyke widths and increasing differentiation over time. Intersection of the dyke swarm and E–W-trending sinistral faults controlled the emplacement of at least three central complexes (Narssaq, South Qôroq and early Igdlerfigssalik). Three post-extensional complexes (Tugtutôq, Ilímaussaq and late Igdlerfigssalik) along the former rift mark the end of magmatism at c. 1140 Ma. The latter two complexes have oblate plans reflecting ductile, fault-related strain. The Tugtutôq complex comprises quartz syenites and alkali granites. The Ilímaussaq complex mainly consists of nepheline syenite crystallised from highly reduced, Fe-rich phonolitic peralkaline (agpaitic) magma, and resulted in rocks with very high incompatible element concentrations.&#x0D; Abundant anorthositic xenoliths in the mafic and intermediate intrusions point to a large anorthosite protolith at depth which is considered of critical importance in the petrogenesis of the salic rocks. Small intrusions of aillikite and carbonatite may represent remobilised mantle metasomites. The petrological similarity between Older and Younger Gardar suites implies strong lithospheric control of their petrogenesis. The parental magmas are inferred to have been derived from restitic Ketilidian lithospheric mantle, metasomatised by melts from subducting Ketilidian oceanic crust and by small-scale melt fractions associated with Gardar rifting.&#x0D; There are numerous analogies between the southern Gardar rift and the Palaeogene East African rift.

Analytical predictions for a natural spacing within dyke swarms

Dykes often grow next to other dykes, evidenced by the widespread occurrence of dyke swarms that comprise many closely spaced dykes. In giant dyke swarms, dykes are observed to maintain a finite spacing from their neighbors that is tens to hundreds of times smaller than their length. To date, mechanical models have not been able to clarify whether there exists an optimum or natural spacing between the dykes. And yet, the existence of a natural spacing is at the heart of why dykes grow in swarms in the first place. Here we present and examine a mechanical model for the horizontal propagation of multiple, closely spaced blade-like dykes in order to find energetically optimal dyke spacings associated with both constant pressure and constant influx magma sources. We show that the constant pressure source leads to an optimal spacing that is equal to the height of the blade-like dykes. We also show that the constant influx source leads to two candidates for an optimal spacing, one which is expected to be around 0.3 times the dyke height and the other which is expected to be around 2.5 times the dyke height. Comparison with measurements from dyke swarms in Iceland and Canada lends initial support to our predictions, and we conclude that dyke swarms are indeed expected to have a natural spacing between first generation dykes and that this spacing scales with, and is on the order of, the height of the blade-like dykes that comprise the swarm.

Precise U–Pb ages and geochemistry of Palaeoproterozoic mafic dykes from southern West Greenland: Linking the North Atlantic and the Dharwar cratons

Palaeoproterozoic magmatism throughout the North Atlantic Craton (NAC) has produced dyke swarms of diverse orientations and emplacement ages. In southern West Greenland, previously identified swarms include the ca. 2500Ma Kilarsaarfik dykes, ca. 2215Ma boninitic norite (BN) dykes, and the abundant ca. 2040Ma Kangâmiut and coeval 2050–2030Ma MD3 dykes. Additional insight into the distribution of swarms is provided by new baddeleyite U–Pb ID-TIMS dates interpreted as emplacement ages for six dykes. The geochemistry for a total of 42 samples on 40 dykes (including the dated dykes) provides some constraints on their petrogenesis. Two E–W trending dykes yield ages of 2365±2Ma and 2374±4Ma, representing a hitherto unknown magmatic event. We propose the name Grædefjord dykes for dykes of this age in southern West Greenland. One NE–SW trending dyke is dated at 2209±5Ma, close to a previously published 2214±10Ma age for a N–S trending dyke (member of BN dykes). Three dykes yield MD3 ages of 2053±2Ma (E–W trending), 2049±6Ma (N–S trending) and 2042±2Ma (WNW–ESE trending). Ca. 2.21Ga old and predominantly NE–SE and NNE–SSE trending dolerites (some of which are high-Al basalts) were derived from a relatively depleted mantle and injected during a boninite–dolerite magmatic event, together with nearby BN dykes derived from a harzburgitic, yet highly LREE and LILE enriched, subcontinental lithospheric mantle (SCLM). Ca. 2.05Ga Kangâmiut–MD3 dolerites were derived from a similar ambient asthenospheric mantle and subsequently experienced crustal assimilation during fractional crystallisation.The ca. 2370Ma dykes in the NAC are coeval with the Dharwar Giant Dyke Swarm of the Dharwar Craton (DC) and based on this unique age match it is proposed that the Dharwar and NAC were neighbours in the Palaeoproterozoic. Furthermore, both blocks share 2.22–2.21Ga dykes, and matching the trends of 2.2–2.1 and 2.37Ga swarms between the two blocks allows several plausible reconstructions.

Large igneous provinces (LIPs), giant dyke swarms, and mantle plumes: significance for breakup events within Canada and adjacent regions from 2.5 Ga to the PresentThis article is one of a selection of papers published in this Special Issue on the the theme Lithoprobe—parameters, processes, and the evolution of a continent.Lithoprobe Contribution 1482. Geological Survey of Canada Contribution 20100072.

Large igneous provinces (LIPs) are high volume, short duration pulses of intraplate magmatism consisting mainly of flood basalts and their associated plumbing system, but also may include silicic components and carbonatites. Many LIPs have an associated radiating diabase dyke swarm, which typically converges on a cratonic margin, identifies a mantle plume centre, and is linked to breakup or attempted breakup to form that cratonic margin. We hypothesize that every major breakup margin in Canada can be associated with a LIP, and we attempt to identify this LIP. To this end, we focus mainly on high-precision age determinations and the distribution of diabase dyke swarms, which are uniquely valued for preserving the record of magmatic events. The analysis extends from the Phanerozoic to the Neoarchean, but our most complete information is for the Superior craton. There, events at 2.50–2.45, 2.22–2.17, and 2.12–2.08 Ga (LIP and plume) are linked with rifting and breakup or attempted breakup of the south-southeastern, northeastern, and southern margins, respectively. Events at 2.00–1.97 Ga are probably linked with the northern margin (Ungava promontory), while the Circum-Superior event at ca. 1.88 Ga is linked to the north to northwestern margins during a time of Manikewan Ocean closure. Similar linkages for other cratons of North America improve understanding of the breakup history to help identify which blocks were nearest neighbours to Canadian crustal blocks in Precambrian supercontinents. Such interpretations provide a framework for interpreting other geological features of these margins to further test models for the timing and location of breakup.

The Cleveland Dyke in southern Scotland

Synopsis There have been a number of recent papers on ‘giant dykes’, i.e. dykes exceeding 100 km in length. This paper extends the known length of the Tertiary (Palaeogene) Cleveland Dyke (CD) by about 64 km, giving a total proven length of c. 260 km. The results of a detailed investigation using ground magnetic surveys have discovered 26 additional exposures in southern Scotland, allowing magnetic, petrographic and geochemical measurements and magnetic modelling to be carried out, resulting in one of the most detailed studies of a ‘giant’ dyke. The structure of the CD complex in the Southern Uplands of SW Scotland has been influenced by pre-existing structures, in particular faults and granitic intrusions. It is chemically heterogeneous without systematic trend along its length. This heterogeneity, plus segmentation along strike, numerous offsets, in particular one of 5 km along the Water of Ken, en-échelon structure, narrow width at many localities (2.3–0.8 m) with lack of baking of adjacent country rocks, all argue strongly against lateral intrusion from the Mull intrusive complex, as has been proposed. Instead, aeromagnetic surveys and wide-angle seismic and gravity surveys make it probable that the dyke relates to a regional magma reservoir beneath the Southern Uplands.

The Okavango giant mafic dyke swarm (NE Botswana): its structural significance within the Karoo Large Igneous Province

The structural organization of a giant mafic dyke swarm, the Okavango complex, in the northern Karoo Large Igneous Province (LIP) of NE Botswana is detailed. This N110°E-oriented dyke swarm extends for 1500 km with a maximum width of 100 km through Archaean basement terranes and Permo-Jurassic sedimentary sequences. The cornerstone of the study is the quantitative analysis of N>170 (exposed) and N>420 (detected by ground magnetics) dykes evidenced on a ca. 80-km-long section lying in crystalline host-rocks, at high-angle to the densest zone of the swarm (Shashe area). Individual dykes are generally sub-vertical and parallel to the entire swarm. Statistical analysis of width data indicates anomalous dyke frequency (few data <5.0 m) and mean dyke thickness (high value of 17 m) with respect to values classically obtained from other giant swarms. Variations of mean dyke thicknesses from 17 (N110°E swarm) to 27 m (adjoining and coeval N70°E giant swarm) are assigned to the conditions hosting fracture networks dilated as either shear or pure extensional structures, respectively, in response to an inferred NNW–SSE extension. Both fracture patterns are regarded as inherited brittle basement fabrics associated with a previous (Proterozoic) dyking event. The Okavango N110°E dyke swarm is thus a polyphase intrusive system in which total dilation caused by Karoo dykes (estimated frequency of 87%) is 12.2% (6315 m of cumulative dyke width) throughout the 52-km-long projected Shashe section. Assuming that Karoo mafic dyke swarms in NE Botswana follow inherited Proterozoic fractures, as similarly applied for most of the nearly synchronous giant dyke complexes converging towards the Nuanetsi area, leads us to consider that the resulting triple junction-like dyke/fracture pattern is not a definitive proof for a deep mantle plume in the Karoo LIP.

Frontiers in large igneous province research

Earth history is punctuated by events during which large volumes of mafic magmas were generated and emplaced by processes distinct from “normal” seafloor spreading and subduction-related magmatism. Large Igneous Provinces (LIPs) of Mesozoic and Cenozoic age are the best preserved, and comprise continental flood basalts, volcanic rifted margins, oceanic plateaus, ocean basin flood basalts, submarine ridges, ocean islands and seamount chains. Paleozoic and Proterozoic LIPs are typically more deeply eroded and are recognized by their exposed plumbing system of giant dyke swarms, sill provinces and layered intrusions. The most promising Archean LIP candidates (apart from the Fortescue and Ventersdorp platformal flood basalts) are those greenstone belts containing tholeiites with minor komatiites. Some LIPs have a substantial component of felsic rocks. Many LIPs can be linked to regional-scale uplift, continental rifting and breakup, climatic shifts that may result in extinction events, and Ni–Cu–PGE (platinum group element) ore deposits. Some current frontiers in LIP research include: (1) Testing various mantle plume and alternative hypotheses for the origin for LIPs. (2) Characterizing individual LIPs in terms of (a) original volume and areal extent of their combined extrusive and intrusive components, (b) melt production rates, (c) plumbing system geometry, (d) nature of the mantle source region, and (e) links with ore deposits. (3) Determining the distribution of LIPs in time (from Archean to Present) and in space (after continental reconstruction). This will allow assessment of proposed links between LIPs and supercontinent breakup, juvenile crust production, climatic excursions, and mass extinctions. It will also allow an evaluation of periodicity in the LIP record, the identification of clusters of LIPs, and postulated links with the reversal frequency of the Earth's magnetic field. (4) Comparing the characteristics, origin and distribution of LIPs on Earth with planets lacking plate tectonics, such as Venus and Mars. Interplanetary comparison may also provide a better understanding of convective processes in the mantles of the inner planets. In order to achieve rapid progress in these frontier areas, a global campaign is proposed, which would focus on high-precision geochronology, integrated with paleomagnetism and geochemistry. Most fundamentally, such a campaign could help hasten the determination of continental configurations in the Precambrian back to 2.5 Ga or greater. Such reconstructions are vital for the proper assessment of the LIP record, as well as providing first-order information related to all geodynamic processes.

Hotspots and mantle plumes: global intraplate tectonics, magmatism and ore deposits

Intraplate tectono-magmatic phenomena, including the emplacement of layered intrusions, and the giant dyke swarms, anorogenic (hotspot) volcanism, oceanic plataeux, rifting processes, basin formation, and geomorphological features are discussed in the context of the mantle plume theory. A review of the relationships between mantle plumes and ore deposits focuses on direct links, proxied by the emplacement of mafic-ultramafic magmas (e.g. PGE and Ni–Cu sulphides associated with flood basalts) and indirectly in rift systems where high geothermal gradients are set up in the crust above the plume, induce large scale circulation of hydrothermal fluids, which result in the generation of a wide range of ore deposits. Peak periods in the deposition of iron formations coincide with plume events in the Archeaen and Proterozoic. Passive margins, which evolve from continental breakups and triple junctions, host abundant mineral and hydrocarbon resources.

The palaeomagnetism of (Mesoproterozoic) Eriksfjord Group red beds, South Greenland: multiphase remagnetization during the Gardar and Grenville episodes

The Eriksfjord Group comprises ∼3000 m of lavas and sediments rapidly deposited in a rift which developed within an Andean-type batholith in juxtaposition to the southern margin of the Laurentian Shield in South Greenland at ca. 1300 Ma. The lavas have been shown to preserve a detailed record of the geomagnetic field at the time of eruption, incorporating normal, reversed and transitional directions. This study has examined the magnetic properties of the intervening red sediments. They are found to possess a diagenetic remanence imparted by mediating fluids at later times. The impact of diagenesis is stratigraphically controlled: the base of the rift infill has magnetizations partially resident in magnetite which are either unstable to thermal cleaning or record a single polarity ‘B’ magnetization (D/I=284/67°, 31 samples, α95=5.5°, palaeopole at 244.1°E, 47.5°N, dp/dm=7.5/9.1°). This corresponds in polarity, and closely in direction, to remanence observed in mid-Gardar gabbro giant dykes and dyke swarms emplaced along the axis of the rift system at ca. 1160 Ma; the causative diagenetic magnetite appears to have grown from hydrothermal systems motivated by this magmatism in a sealed reservoir setting within the lower part of the rift infill. The Ilímaussaq alkaline igneous complex was emplaced into the southern extension of the rift at ca. 1130 Ma and possesses a dual polarity magnetization (D/I=327/81°, α95=6.4°, 10 sites). Eriksfjord lavas within the thermal aureole are overprinted to varying degrees by comparable magnetizations with steep inclinations. The mean pole position (283°E, 71°N, dp/dm=12/12°) lies near the apex of an apparent polar wander loop incorporating the Gardar Track (ca. 1300–1140 Ma) and the Keweenawan Track (ca. 1115–1050 Ma). Magnetizations in the Eriksfjord sedimentary succession have not been significantly reset by emplacement of the Ilímaussaq complex, but higher levels of the rift infill are dominated by an ‘A’ magnetization (D/I=305/34°, α95=4.3°, 57 samples, palaeopole at 202.1°E, 32.4°N, dp/dm=2.8/4.9°) resident in haematite. The pole position does not correspond with any part of the Gardar Track, but does correlate with the return Keweenawan Track at ca. 1090 Ma, close to the time of Grenville orogenesis along the bordering southeastern margin of the Laurentian Shield. This remanence is attributed to diagenesis during extensional tectonism linked to the collapse of the Grenville Orogen formerly sited 100–200 km to the south.

Gardar dykes north of the Igaliko Syenite Complex, southern Greenland

An ENE-WSW trending swarm of Gardar dykes, traversing Mellemlandet and G. E Holm Nunataq is principally composed of a 'main series' with compositions ranging from alkali olivine basalt to trachyte and rhyolite, and scarcer phonolitic trachyte associates. The most basic 'main series' magmas were emplaced as several giant dykes up to 650 m wide. Synformally layered gabbroic and anorthositic cumulates are locally developed within these. At Syenitknold internal differentiation within a giant dyke gave rise to syenogabbros, layered syenite cumulates and peralkaline nepheline syenite pegmatites. A large xenolithic mass of exotic feldspathic gabbro within the syenites is ascribed to the foundering of feldspar-rich roofing facies into the underlying magma chamber. Less extreme differentiation in the same giant dyke east of Syenitknold produced syenogabbroic cumulates containing evidence for vigorous convective flow having developed in the cooling intrusion. Smaller (&lt; 40 m wide) and younger dykes are almost invariably of more differentiated character. The commonest dykes ( &lt; 15 m wide) are of benmoreite and trachyte. Dykes with their interiors crowded with plagioclase xenocrysts and anorthositic inclusions are referred to as 'big feldspar dykes' (B.F.D.s). While all compositions from basalt to benmoreite may be involved in the B.F.D.s, the B.F.D. character is typical of the hawaiites and mugearites. Small (typically &lt; 1 m), scarce dykes and sills of highly silica-undersaturated types range from ultramafic lamprophyres to carbonatites. These may be representative of a compositional continuum between 36 and 2 wt % SiO2,. The main swarm is so closely similar to that seen to the WSW, extending through Tugtutoq and the Narssaq and Qagssiarssuk areas, that it is thought to be merelya faulted continuation ofthe latter. Itso, this swarm, c. 15 km across, is at least 140 km long. The magnitude and extent of this alkaline swarm and its individual components, may well be unique: it differs from other swarms (e.g. that of the roughly contemporaneous Nunarssuit-Isortoq swarm) in the size and abundance of the salic dykes within it. It was almost certainly related to extensive fissure eruption of basic to salic lavas. A clockwise change of several degrees between the orientation of early giant dykes and later differentiated dykes is related to a change in the extensional stress direction during the development of the Gardar rift system.

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.

A palaeomagnetic study of Svecofennian basic rocks: Middle Proterozoic configuration of the Fennoscandian, Laurentian and Siberian shields

The Svecofennian mobile zone occupies the bulk of the Fennoscandian shield and represents terrain subjected to profound tectonic activity and granite intrusion at ∼1800 My. This study covers the palaeomagnetism of basic rocks within this belt in Sweden between 65.5 and 67.5°N (gabbros and diorites of Kallax, Niemisel, Sangis, Stora Lulevatten and the Gällivare, Jokkmokk and Voullerim regions) magnetised during uplift and cooling of this belt at ∼1750-1550 My. AF and thermal demagnetisation define a consistent sequence of high to low blocking-temperature components identifying a migration of the geomagnetic field during part of this interval. Together with the Rådmansö gabbro-diorite of central-east Sweden (palaeomagnetic pole 201°E, 36°N) these components yield a comparable sequence of palaeopoles to those derived from uplift magnetisations of the contemporaneous Svecokarelian terrain of Finland. The post-tectonic, Uppsala metabasite suite possesses a magnetite-held (“A”) remanence comparable to Svecofennian uplift magnetisations from elsewhere; within the aureole of the Almunge alkaline complex this has been largely displaced by a low blocking temperature (“B”) remanence, possibly related to a late stage in the Svecofennian uplift cycle. The Hälleforsnäs giant dyke possesses a magnetite-held remanence attributed to initial cooling at 1518 My (palaeomagnetic pole 167°E, 27°N) and at least two high blocking-temperature components. One of these is correlated with the ∼1000-800 My Sveconorwegian mobile activity of southwest Sweden; this latter component is represented as the univectorial remanence in dolerite dykes of this age, and sporadically as a secondary component in the adjacent Svecofennian terrain. The results are compiled with other palaeomagnetic poles from the Fennoscandian shield to derive a generalised apparent polar wandering path for the interval ∼1750-1550 My. They define segments of a large loop which agrees closely with uplift magnetisations from the contemporaneous Hudsonian mobile terrain of the Laurentian shield on a single reconstruction derived from Upper Proterozoic (1450-1200 My) palaeomagnetic data. The two shields thus appear to have formed an integral continental unit during the interval 1750-1200 My. A geological reconstruction of the Siberian and Laurentian shields is also tested and found to yield general agreement with the palaeomagnetic evidence. The major geological implications of the collective reconstruction are an alignment of major tectonic trends and a gradual restriction of anorthosite-Rapakivi magmatism between the termination of the ∼1800 and ∼1100 My mobile episodes.

The Tugtutoq Younger Giant Dyke Complex, South Greenland: Fractional Crystallization of Transitional Olivine Basalt Magma

The Younger Giant Dyke Complex consists of a set of massive coalescing dykes of Proterozoic age (c. 1170 Ma), resulting from intrusion of a suite of transitional olivine basaltic/hawaiitic magmas in a continental rift setting. The suite, compositionally related by low pressure (< 10 kb) olivine-plagioclase fractionation, is believed to have had a deeper level evolution dominated by pyroxene and possibly garnet fractionation. Slow cooling in situ of the interior parts of the dyke complex produced cumulitic suites. Those exposed range from gabbroic to syenitic; residual bodies of riebeckite granite and, very subordinate, feldspathoidal syenite were also generated. The basic magmas had notably low fO2 values, leading to delayed magnetite and clinopyroxene precipitation, relatively iron-rich differentiates and some residual liquids of pantelleritic composition. The basic magmas had high F/Cl values and are inferred to have had low H2O contents. They were also characterized by relatively high K/Rb and low 87Sr/86Sr values; these characteristics imply a mantle source with high F/Cl but depleted in Rb relative to K and Sr. Basaltic magmas responsible for (a) the preceding Older Giant Dyke Complex and (b) a suite of anorthositic xenoliths within the Younger Giant Dyke Complex, are inferred to have been derived from separate primary magma batches independent of those that yielded the Younger Giant Dyke Complex. The giant dykes are the highest-level representatives of a larger basic complex responsible for the extensive linear gravity ‘high’ in the Tugtutôq-Narssaq area.

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.

Palaeomagnetism of the giant dykes of Tugtutoq and Narssaq Gabbro, Gardar· Igneous Province, South Greenland

The earliest phases of the last major episode of magmatism within the Precarnbrian Gardar Igneous Province of South Greenland are NE-SW trending giant dykes. Palaeomagnetic samples from seven localities within the Hviddal syenite giant dyke (Rb-Sr whole rock isochron 1175±9 m.y.) yield a palaeomagnetic pole at 215°E, 33°N (k = 38, A,s = 9.9"J. Thirteen sites from gabbro giant dykes (&lt;1175 m.y., &gt;1168m.y.) give a palaeomagnetic pole at 226°E, 42°N (k = 30, A,s = 9.5°), and four sites from the Narssaq gabbro of comparable age give a pole at 225°E, 32°N (k= 97, A,s = 9.9"). All sites have the same polarity although eight additional sites suggest that a secondary intermediate direction was occupied by the magnetic field at intervals with comparable intensity to the main field. The palaeomagnetic pole positions define an easterly apparent polar movement and are consistent with poles of similar age from the Superior craton of North America on the pre-drift reconstruction.