Thursday, 14 May 2020

Identification of the Ediacaran Central Iapetus Magmatic Province in Western Svalbard.

The Ediacaran period of Earth history witnessed significant environmental and evolutionary change, with the emergence of the first Metazoans during oxygenation of the Ediacaran oceans. This period also coincided with the aftermath of the global Sturtian and Marinoan glaciations, and is broadly coeval with the emplacement of the Central Iapetus Magmatic Province. This large igneous province accompanied the final breakup of the Rodinia supercontinent into Amazonia, Baltica, Laurentia and West Africa from about 615 to 550 million years ago. The margins of all these blocks were subjected to multiple rifting events and several phases of Central Iapetus Magmatic Province-related early sub-alkaline as well as later alkali mafic magmatism. The separation of Laurentia and Baltica and the opening of the Iapetus Ocean, took place during this time, and constitutes, along the Appalachian-Caledonian orogenic belt, the beginning of the first well-documented Wilson Cycle. The remnants of the Central Iapetus Magmatic Province are exposed today in north-eastern North America, north-western Europe, and into north-western Africa and Central America It occurs as numerous mafic dykes and sills, carbonatites and other intrusive rocks, as well as more limited extrusive rocks. A mantle plume is considered by some authors to be the cause of such magmatism and rifting.

In a paper published in the journal Precambrian Research on 25 January 2020, Ashley Gumsley of the Institute of Geophysics of the Polish Academy of Sciences, Geoffrey Manby of the Earth Sciences Department at the Natural History Museum, Justyna Domańska-Siuda and Krzysztof Nejbert of the Faculty of Geology at the University of Warsaw, and Krzysztof Michalski, also of the Institute of Geophysics of the Polish Academy of Sciences, present new (i) petrography, (ii) uranium-lead baddeleyite isotope dilutionthermal ionisation mass spectrometry geochronology, and (iii) whole-rock geochemistry for the Central Iapetus Magmatic Province occurrences in the Caledonian nappes of the Southwestern Basement Province or Western Terrane of Oscar II Land, in western Svalbard. 

Svalbard’s place within the Rodinia supercontinent has been a matter of long debate, with different authors proposing affinities with either Laurentia or Baltica. Gumsley et al. aimed to better define these complex affinities based on temporal and geochemical relationships of the Central Iapetus Magmatic Province in the Caledonian basement of Svalbard, to help determine possible magmatic sources and tectonic settings, as well as the palaeogeography of the Southwestern Basement Province of Svalbard within Rodinia. Although published accounts of the petrogenesis of mafic magmatic events in Greenland and Svalbard in the late Neoproterozoic realm are few, data from the Scandinavian Caledonides, Scotland, and the Appalachians of North America, are more extensive. The mafic units in these areas, in common with those of the study area, are characterised by variable greenschist- to lower amphibolite-facies grade metamorphism. Gumsley et al.'s aim was, therefore, to discuss the petrology and geochronology of the Central Iapetus Magmatic Province fragments in the Caledonian nappes of the Southwestern Basement Province of Svalbard in an attempt to shed new light on Svalbard’s possible place within Baltica or Laurentia during the opening of the Iapetus Ocean.

The Svalbard archipelago showing the geological distribution of the Caledonian basement and younger cover rocks, as well as the study area (inset) on Spitsbergen in the Southwestern Basement Province of Oscar II Land. Gumsley et al. (2020).

The Caledonian basement of Svalbard is divided into three main crustal units: the Eastern, Central and Western terranes or the Northeastern, Northwestern and Southwestern basement provinces as outlined in the Geoscience Atlas of Svalbard and used by Gumsley et al.. It was postulated that before final Caledonian amalgamation, each of the terranes/provinces developed separately along with east and northeastern Greenland, as well as in the Canadian Arctic (Pearya Terrane) and Scandinavia.

Gumsley et al.'s study is focused on variably metamorphosed and deformed mafic units from the northern part of the Southwestern Basement Province-area of Oscar II Land. Several authors have postulated an affinity of the Southwestern Basement Province with the Pearya Terrane on Ellesmere Island in the Canadian Arctic, while others suggest a non-Laurentia relationship, i.e., with Baltica. The southern part of the Southwestern Basement Province, Wedel Jarlsberg Land, can additionally be divided into smaller terranes separated by strike-slip terrane boundaries. It has been postulated that the northern terrane of Wedel Jarlsberg Land shares affinities with the Pearya Terrane, whereas the southern terrane resembles the Timanide Belt of north-eastern Europe (Scandinavia).

Geological map of Oscar II Land and Prins Karls Forland within the Southwestern Basement Province of Spitsbergen on Svalbard, showing the sampling localities. Gumsley et al. (2020).

Within the Southwestern Basement Province, several variably metamorphosed and deformed mafic intrusive and extrusive units occur at different stratigraphic levels. The ages of these intrusions and extrusions have been assumed to be about 600 nillion years, based on their intercalation with metamorphosed Cryogenian and Ediacaran sedimentary rocks, especially the glacial diamictites connected with the Sturtian and Marinoan global glaciations.. However potasium-argon and argon-argon ages have been documented in a variety of minerals likely indicative of metamorphism and/or magmatism between about 584 and about. 556 million years ago in Wedel Jarlsberg Land. 

In the southern part of Oscar II Land, in the Kinnefjellet-Ommafjellet and Konowfjellet areas, mafic units are found in the Neoproterozoic St. Jonsfjorden Group metamorphosed carbonates, sandstones and shales. In the northern part of Oscar II Land, they occur in the Mesoproterozoic Kongsvegen Group of the Engelskbukta area, where they have been found to consist of metamorphosed cumulate-like gabbros as well as garnet-bearing amphibolites. West of Oscar II Land, the Meso- to Neoproterozoic Pinkie Group and Peachflya Group of Prins Karls Forland, respectively, also contain mafic units varying from those with well-preserved igneous textures to those with a well-developed, metamorphic foliation. Variably metamorphosed mafic units also occur south of Oscar II Land in Wedel Jarlsberg Land, with a composition of sub-alkaline basaltic andesites in southern Wedel Jarlsberg Land from the Neoproterozoic Deilegga Group and Sofiebogen Group, while those in northern Wedel Jarlsberg Land are alkaline basalts and cumulates. These Cryogenian-Ediacaran mafic units of Wedel Jarlsberg Land, which are considered by Gumsley et al. to have been products of similar rifting events, are given further consideration, together with the units from Oscar II Land.

In Oscar II Land, on the north shore of St. Jonsfjorden, samples were taken from a weakly metamorphosed and deformed gabbro and other nearby mafic sheets that intruded into the metamorphosed carbonates and phyllites of the Neoproterozoic St. Jonsfjorden Group near Konowfjellet. To the south of St. Jonsfjorden, in the Kinnefjellet-Omafjellet area, samples were also taken of more metamorphosed mafic units of the same composition that also intrude the Neoproterozoic metamorphosed and deformed carbonates and phyllite successions, also of the St Jonsfjorden Group. The sampled units on the north coast of St Jonsfjorden form two large lensoid bodies lying within the main Caledonian schistosity. The contact of the main gabbro body with the St Jonsfjorden Group is not exposed along the coastal cliff section, but on the elevated strand flat above, small exposures of phyllites that crop-out in a curved pattern defining the boudin-like shape of the unit. The gabbro is heavily fractured and cut by a series of west-dipping faults that are commonly associated with metre-scale alteration zones. Internally, the primary igneous textures are well preserved, although the mineralogy has been partially replaced by metamorphic assemblages. An alternation of light coloured plagioclase feldspar bands with more dark green mafic bands is apparent, and the steep south-west dipping banding is occasionally traversed by ductile shear zones that are superimposed by more brittle shears. The whole gabbro is cut by several meter-wide brittle fracture zones causing red-staining and carbonate veining. Another sampled dolerite body lies structurally above and to the west of the gabbro, and is patchily exposed on the western flank of Konowfjellet and down to the strand flat. These intrusions lie on the overturned limb of a mesoscale east vergent synformal fold. The dolerites of the Kinnefjellet-Omafjellet area have many deformational features. They occur, like the Konowfjellet gabbro, as lensoid or boudin-like bodies. The boudins are internally disrupted by numerous slickensided shears, with varying senses of slip, into rough sigmoidal lenses often truncated at high angles by conjugate fractures. Primary igneous textures are largely well preserved internally, although again the primary magmatic minerals are extensively replaced by metamorphic assemblages. Ductile shear zones with strongly aligned plagioclase feldspar and mafic minerals often border both the main dolerite bodies and may define the margins of the lesser internal lenses. Quartz and calcite veins that are often folded are a common feature of these dolerites, demonstrating the extent of late intrusion and/or syn-metamorphic fluid phase infiltration. The structural setting of the metamorphosed mafic rocks in the Kinnefjellet-Omafjellet area appears to be map-scale isoclinal folds with north-trending axial traces and west-dipping axial surfaces These folds show some limited late- to post-fold (Caledonian?) thrusting to the west, possibly associated with the late north- to south-trending synformal flexuring, that extends southward as far as Konowfjellet.

Field photographs of the sampled Oscar II Land metamorphosed mafic units in western Svalbard. (a) An alternation of light coloured plagioclase feldspar bands with more dark green mafic bands in the gabbro of the Konowfjellet area. (b) Several meterwide brittle fracture zones along which the gabbro shows red-staining and carbonate veining. (c) Dolerites of the Kinnefjellet-Omafjellet area often occur as lensoid or boudin-like bodies. (d) The dolerites are often internally disrupted by numerous slickensided shears, with varying senses of slip, into rough sigmoidal lenses often truncated at high angles by conjugate fractures. Gumsley et al. (2020).

Petrographic analyses of thin sections were undertaken at the Faculty of Geology in the University of Warsaw using an Olympus BX-51 optical microscope. Mineral chemical analyses of the main rock-forming and accessory minerals were carried out at the Inter-Institutional Laboratory of Microanalyses of Minerals and Synthetic Substances at the University of Warsaw, using a CAMECA SX-100 electron microprobe. The analytical conditions employed an accelerating voltage of 15 kV, a beam current of 20 nA, counting times of 4 seconds for the peaks and background, and a beam diameter of 1–5 μm.

The metamorphosed mafic units of the study area occur predominantly in the form of mafic sills that have been highly deformed, boudinaged and subjected to Caledonian greenschist facies metamorphism. The deformation and metamorphic fabrics are readily visible on macroscopic and microscopic scales. Many metamorphic transformational macro- and micro-structures are present, ranging from well preserved primary magmatic aphyric-interstitial doleritic textures in the Konowfjellet gabbros to the highly recrystallised dolerites cropping out to the south of St. Jonsfiorden and Isfiorden, where the synmetamorphic S₁ foliation is well developed. The Konowfjellet gabbros, from which the baddeleyite was extracted, are medium- to coarse-grained, and display the best-preserved magmatic textures. The intrusion exhibits many, up to 20 cm, alternations
of leucocratic to melanocratic layers reflecting the fractionation processes that accompanied the emplacement of this relatively large volume of mafic magma. The lack of a noticeable synmetamorphic tectonic foliation is a function of its highly competent nature in contrast to the host phyllites and metamorphosed carbonates. There are a few sites where ductile shear zones are cut by later brittle fracture. The eastern part of the gabbro section does, however, exhibit zones with a discernible schistosity.

Microscopy images of the Oscar II Land mafic units in western Svalbard. (a) Coarse- and (b) medium-grained primary magmatic textures altered at greenschist facies observed in transmitted plane-polarised light microscopy. (c) Albitised plagioclase feldspar with chlorite. (d) Primary magmatic textures with some deformational realignment along the foliation. Abbreviations: Ap, apatite; Chl, chlorite; Cpx, clinopyroxene; Fsp, feldspar; Ilm, ilmenite; Krs, kaersutite. Gumsley et al. (2020).

The primary mineral assemblage includes clinopyroxene, green hornblende and kaersutite, plagioclase feldspar (altered to albite), biotite, apatite, ilmenite, magnetite, and pyrrhotite. The fine-grained secondary minerals consist of intergrowths of chlorite, albite, actinolite, epidote, titanite, anatase/brookite, quartz, carbonates (calcium and magnexium), and growths of sulphides, including pyrrhotite, pentlandite, chalcopyrite, sphalerite, covellite, and pyrite. Goethite pseudomorphs after the sulphides are also common. The biotite grains often appear as corona-like aggregates rimming the magmatic iron-titanium oxides, while kaersutites, which are largely unaltered, are found as grains ranging from 50 μm to a several mm in size. The textural relationships of both phases suggest a subsolidus origin. The occurrence of kaersutite, identified within non-recrystallised and recrystallised dolerites, together with the high sodium oxide and potassium oxid contents of the clinopyroxenes, indicates that the samples were of alkaline basaltic composition. The Kinnefjellet-Omafjellet metadolerites often retain their magmatic textures with some deformational re-alignment along the S₁ foliation and minor folds. 

Baddeleyite grains were separated from the sample G2 using standard cutting, crushing and pulverising techniques before high-density water-based accessory mineral separation on a Wilfley Table at the Department of Geology at Lund University. The best-quality grains were selected for uranium-lead geochronology. Baddeleyite is a zirconoim oxide mineral which can incorporate trace amounts of uranium, but not lead, during its formation. uranium decays into lead at a known rate; since lead will not have been present in the original crystal, it is possible to calculate the age of a baddeleyite grain from the ratio between these elements). 

Five fractions were analysed from the separated baddeleyite crystals, each composed of between four and five large good quality light brown baddeleyite crystals, up to 60 μm in length. The fractions analysed are concordant to slightly discordant (1–4%) and define a regression line between an upper intercept date at 560 ± 12 million years ago and a lower intercept date at 89 ± 330 million years ago. The upper intercept date of about 560 Ma iwas interpreted as the crystallisation age of the sample.

For whole-rock geochemistry and isotope geochemistry, 20 samples of the metamorphosed hypabyssal rock were selected from the most homogenous parts of the sampled units, after the weathered material had been removed. They were then hand crushed and milled in an agate ring mill at the Natural History Museum in London, United Kingdom. The resultant material was then coned and quartered, before being dispatched for analyses of major, minor and trace elements also at the Museum. Major, minor and trace element analyses were made on glass beads prepared from the powdered samples with a sample-to-flux (lithium tetraborate) ratio of 1:10, and the resulting molten bead was rapidly digested in concentrated nitric acid for inductively coupled plasma atomic emission spectroscopy and weak hydrofluoric acid-nitric acid-perchloric acid solution for inductively coupled plasma mass spectrometry. Inductively coupled plasma atomic emission spectroscopy was used for major and minor elements, while inductively coupled plasma mass spectrometry was used for trace elements, including rare earth elements.

The metamorphosed coarse-grained mafic units in this study from the Oscar II Land region, plot in the alkaline field in a total alkali-silica diagram, varying in silica dioxide from 39.58 to 51.44 percent by weight for the mafic units. There is between 2.51 and 8.60 percent by weight alkalis (sodium oxide and potassium oxide). The presence of metamorphic hydrous phases and evidence of water and carbon dioxide fluid infiltration in many samples makes it likely that the alkali elements have been mobilised during greenschist facies metamorphism. However, the variably metamorphosed mafic units occur within the alkali basalt field in the zirconium/titanium-niobium/yttrium diagram. 

The trace element data for the analysed Oscar II Land rocks were plotted on a primitive mantle normalised multi-element plot. The mobile elements such as rubidium and barium are excluded from further interpretation. No discernible negative niobium-tantalum anomalies exist, which would be indicative of subduction settings or crustal contamination. The Wedel Jarlsberg Land mafic volcanic samples are not as enriched as those from Oscar II Land, and generally they are similar to continental crustal values (with the exception of a positive lead anomaly, which was not measured), perhaps as a result of assimilation of crustal material during their ascent, whereas several of the mafic alkali rocks of Wedel Jarlsberg Land from an earlier study are more similar to the rocks from Oscar II Land in Gumsley et al.'s study. The high thorium, uranium, and cerium values of the Oscar II Land samples are also ocean island basalt-like, further suggesting little assimilation of crustal material in the Oscar II Land area. Cerium is slightly more enriched in the Wedel Jarlsberg Land volcanic rocks than for those from Oscar II Land. The lead contents of the Oscar II Land mafic units are variably depleted. The mafic units, however, are variably depleted or enriched in strontium, whereas in the chondrite-normalised rare earth element diagram, the trends for the Oscar II Land units are parallel with enrichment of light rare earth elements over heavy rare earth elements, which is a characteristic of alkali (ocean island basalt-like) basalts. The lack of significant europium anomalies suggests that low-pressure fractional crystallisation of plagioclase feldspar was not important in any samples. One definitive grouping of Wedel Jarlsberg Land cumulate rare earth elements follow a similar pattern to the Oscar II Land samples, indicated in their ocean island basalt-like character, whereas the Wedel Jarlsberg Land volcanic rocks are slightly more depleted.

The Meso- to Neoproterozoic variably metamorphosed and deformed successions of the Kongsvegen and St. Jonsfjorden groups of the Southwestern Basement Province in Oscar II Land are dominated by alternations of metamorphosed clastics, as well as metamorphosed carbonates, including a sequence of metamorphosed carbonates predating the metamorphosed Neoproterozoic diamictites, all of which accumulated on a continental margin subject to periodic subsidence and rifting in Oscar II Land, as well as Prins Karls Forland to the west and Nordenskiøld Land and Wedel Jarlsberg Land to the south, where similar successions are found. Given that correlatives of the diamictites are found from Prins Karls Forland to Oscar II Land, through to Nordenskiøld Land and Wedel Jarlsberg Land in the whole Southwestern Basement Province, it seems reasonable to assume that they accumulated on the same continental margin in the Neoproterozoic, despite some probably different provenance affinities from the Mesoproterozoic and earlier, with various exotic terranes docking later. The reported 560 ± 12 million year uranium-lead baddeleyite crystallisation age for the St. Jonsfjorden alkali gabbro within the St Jonsfiorden Group confirms that at least some of the pre-Caledonian intrusive bodies of Oscar II Land were emplaced after deposition of at least some of diamictites in the area. However, different generations of diamictites are likely present in the area, reflecting those known in the Cryogenian and Ediacaran glacial periods, connected with the Sturtian, Marinoan and potentially the Gaskiers global glaciations, which all occurred before about. 560 million years ago. The mafic units may also be different in age, considering the different alkali and tholeiitic geochemical affinities of the units in Oscar II Land and Wedel Jarlsberg Land, with only part of the alkali mafic units from Wedel Jarlsberg Land comparing with those known from St. Jonsfjorden reported in Gumsley et al.'s study.

Trace element ratios of contrasting and similar compatibilities are considered by Gumsley et al. to define the nature of the source of the magma, and for example, any contamination. The lanthanum/niobium ratio, for example, can be used to determine the extent to which the magma has interacted with the continental crust during its extraction and ascent. The lanthanum/niobium of all the mafic units in Gumsley et al.'s study ranges from about 1.13 to about 0.74, which is significantly less than the average for the continental crust (1.5–2.2). The niobium/tantalum ratios for most of the Oscar II Land mafic units lie between about 14.8 and about 18.6, and show little deviation from chondrite or mantle values, with continental crustal values around 12–13. Using the thorium/ytterbium-niobium/ytterbium diagram, all the samples appear along the mid-ocean ridge basalt–oceanic island basalt array of oceanic basalts, with an enriched oceanic island basalt-like source, with little indication of a crustal component. Of the Wedel Jarlsberg Land volcanic and hyperbassal rocks, the Chamberlindalen-area alkali cumulates of northern Wedel Jarlsberg Land in the Neoproterozoic Sofiebogen Group plot within the oceanic island basalt-like end member of the enriched mid-ocean ridge basalt-oceanic island basalt array. Other mafic units from southern Wedel Jarlsberg Land (Jens Erikfjellet-area) in the Sofiebogen Group appear to plot towards the middle continental crust field, indicating derivation from magmas that have assimilated at least some crustal material. Continental- and mid-ocean ridge basalt-like mafic units have zirconium/hafnium ratios that lie within 36–38 and 35–38 ranges, respectively, Chondrite zirconium/hafnium ratios, which range from 38 to 52, contrast with those for the Oscar II Land mafic units, which are in the 40–116 range. Such high values may be generated by the addition of partial melts from the spinel- and garnet-lherzolite mantle. Additionally, all but one of the Wedel Jarlsberg Land samples range from 39 to 50, and are thus in the chondrite field.

While the trace element data suggests that there has been little crustal assimilation, decoupling of elements that have similar behaviour (e.g., zirconium, hafnium) is evident. Partial melting of heterogeneous mantle could account for some of the observed elemental anomalies, as could episodes of partial melting and melt rock reactions.

The trace and rare earth element data show that the sampled Oscar II Land mafic units are largely of alkaline, oceanic island basalt-like composition. The rocks were derived from a polybaric spinel- to garnet-facies mantle source, as were those samples from Wedel Jarlsberg Land. The alkaline mafic rocks of Oscar II Land, together with tholeiitic to alkaline mafic units of Wedel Jarlsberg Land, appear to be products of protracted riftgenerated magmatic events sourced from the same, possibly heterogeneous, sub-lithospheric mantle. The more extensively exposed mafic units of the more magma-rich crust of Wedel Jarlsberg Land appear to display systematic, south to north, changes in their geochemical signatures. These changes could be attributed to a south to north decreasing depths (and hence pressures) of melt generation, or simply to different generations of magmatism, as is seen in both Laurentia and Baltica.

The roughly 560 million year crystallisation age on the St. Jonsfjorden gabbro in the Southwestern Basement Province of western Svalbard in this study identifies this magmatic event with the terminal stages of the Ediacaran Central Iapetus Magmatic Province. The margins of the Iapetus Ocean, which opened up between the Ediacaran and Cambrian, is host to many (mostly variably deformed and metamorphosed) mafic units connected with the Central Iapetus Magmatic Province. Mostly, these mafic units are reasonably well studied along the Laurentia and Baltic margins although further uranium-lead geochronology is certainly needed, with the Central Iapetus Magmatic Province linked to the final breakup of the Rodinia supercontinent. The Central Iapetus Magmatic Province-related volcanic rocks and associated feeders in the form of dyke swarms, sill provinces and layered intrusions cover a vast area over 9 million square kilometres. The Central Iapetus Magmatic Province has several pulses between about 619 and about 550 million years ago, which are linked to the progressive breakup of eastern Laurentia and Baltica mostly, and may actually make up several independent large igneous provinces associated with different stages of rifting between the northern and southern Iapetus Ocean. The earliest pulse at 619–596 million years ago consists of the 615 ± 2-million-year-old Long Range Dyke Swarm of Laurentia in Labrador (Canada), but is manifest also in the 616 ± 3-millio-year-old Egersund Dyke Swarm in southern Norway of Baltica. The 610–596-millio-year-old Scandinavian Dyke Swarm of the allochotonous Caledonides nappes in Sweden and Norway, as well as in the 601 ± 4-millio-year-old Tayvallich volcanic rocks of Dalradian in Scotland, provide further evidence of the Central Iapetus Magmatic Province away from the margins of the Iapetus between Laurentia and Baltica. These nappes in Scandinavia were likely thrust on to Baltica during the Caledonian Orogeny. Such Central Iapetus Magmatic Province-related mafic magmatism is also documented from carbonatites in Greenland, but whether they are part of the 615–596 million years or 590–577 million years ago magmatic pulse remains uncertain due to the lack of precise uranium-lead age determinations. The 619 ± 9-millio-year-old Novillo Dyke Swarm in Mexico, which formed part of Amazonia, indicates that this part of Amazonia was likely part of the same rift zone with the bulk of the Central Iapetus Magmatic Province between Laurentia and Baltica. The 590–577 million years ago magmatic pulse is documented mostly in the Grenville Dyke Swarm and Adirondack Dyke Swarm of eastern Laurentia, which occur in the failed arms of a rift. The third magmatic pulse at 571–550 millio years ago is very significant in eastern Laurentia, consisting of the 565 ± 4-millio-year-old Sept Iles layered intrusion, as well as the 551 ± 3-millio-year-old Skinner Cover and 564 ± 9-millio-year-old Cactoctin volcanic rocks and their associated feeders, with many other occurrences towards and in the Sutton mountains between Canada and the USA. This led to the proclamation of the so-called ‘Sutton Mantle Plume’ centred on this triple point, a product of socalled deep mantle melts. Coeval magmatism with similar geochemistry occurs in the Seiland Igneous Province of the Norway Caledonides at 574–523 million years ago in the Kalak Nappe. The peak of magmatism, however, was at 570–560 million years ago. The parental magmas to the Seiland Igneous Province include picrites similar to oceanic island basalts, and are often explained as deep, hot mantle plume melts, whereas the origin of the dominantly tholeiitic basalts of the dyke complex is debated. The Oscar II Land gabbros of the Southwestern Basement Province of Svalbard are coeval with this third pulse of magmatism in North America and Scandinavia. Other more mid-oceanic ridge-like basalts of this age are likened to a third stage of rifting in both North America and Scandinavia Similar magmatism is also documented within southwest Baltica, with the roughly 570-million-year-old Volyn flood basalts. Coeval ages also occur in the volcanic rocks of the Ouarzazate Group at 570–560  million years ago in Morocco, which forms part of the West African Craton. Lastly, carbonatites and kimberlites were also emplaced throughout the Central Iapetus Magmatic Province event and have been identified across most of the area covered by Central Iapetus Magmatic Province, both in Baltica and Laurentia, but usually on prexisting continental crust.

The location of the three temporal phases of magmatic rocks of the Central Iapetus Magmatic Province within the Laurentia and Baltica crustal blocks of the Rodinia supercontinent, highlighting the positions of western and eastern Svalbard. Numerical symbols: (1) Volhyn volcanics, (2) Winter Coast volcanics, (3) Seiland Igneous Province, (4) Scandinavian Dyke Complex, (5) Egersund Dyke Swarm, (6) Dalradian volcanics, (7) area studied by Gumsley et al., (8) Long Range Dyke Swarm, (9) Grenville Dyke Swarm, (10) Adirondack Dyke Swarm, (11) Sept Iles intrusion, (12) Skinner Cove volcanics, (13) Callander Complex and (14) Catoctin volcanics. Note the shaded yellow areas denoting the Caledonian nappes, some of which have been determined to be ‘exotic’ terranes, especially within Baltica. Gumsley et al. (2020).

The earlier pulses of the Central Iapetus Magmatic Province appear to have compositions similar to typical tholeiitic continental flood basalts, whereas the later pulses are more alkali, more similar to ocean island basalts. A mantle plume has been defined for the whole of the  Central Iapetus Magmatic Province based on protracted rifting, associated with the dyke swarms and sill provinces, as well as regional uplift, and regional geochemical variation in the later oceanic island basalt-like pulses, which will include the roughly. 560-millio-year-old St. Jonsfjorden gabbro. Constructing a 1000 km in diameter circle about this plume centre, however, only encompasses part of the overall event, which can be traced over maximum distance of about 4500 km. Whether this entire distribution of magmatism can be linked to a single 615–550 million year ago plume centre is unknown. However, by way of comparison, the roughly 200-millio-year-old Central Atlantic Magmatic Province event is of similar scale (e.g., approximately 5000 km across), and can be linked to a single plume centre.

Although the timing of the Ediacaran Oscar II Land and Wedel Jarlsberg Land appear to display systematic, south to north, changes in their geochemical signatures. These change mafic magmatism is shown above to roughly coincide with the Central Iapetus Magmatic Province and opening of the Iapetus Ocean, as well as and the final break-up of Rodinia into the Laurentia and Baltica crustal blocks, the placement of the Southwestern Basement Province of Svalbard in the Iapetus Ocean realm remains controversial. Indeed, the placement of all three of the pre-Caledonian terranes of Svalbard (i.e., the Southwestern Basement Province, Northwestern Basement Province and Northeastern Basement Province) within pre-Caledonian plate reconstructions is vague. These terranes are defined according to their supposedly different sedimentary, magmatic and tectonic histories along major north-trending faults. Gumsley et al.'s study concerns the Southwestern Basement Province only; however, even within the Southwestern Basement Province between Oscar II Land and Wedel Jarlsberg Land, different terrane affinities are suggested. These affinities have led to comparisons with either Laurentia, Baltica, Pearya or further afield with either the Timanides or Gondwana terranes being suggested. Using the magmatic barcode method utilising dyke swarms, sill provinces and flood basalts as indicators of large igneous provinces, the mafic magmatism defined within Gumsley et al.'s study is coeval with the last phase of mafic magmatism occurring within the Central Iapetus Magmatic Province. This includes the 570–560-millio-year-old Seiland Igneous Province, thrusted on to Baltica in the Kalak Nappe, and various intrusions within the Appalachian Belt and basement of Laurentia, including the 565 ± 5-million-year-old Sept Îles intrusion, all of which are geochemically alkali, like the gabbros of St. Jonsfjorden in this study. The Sept Îles intrusion is, however, the only known ‘in-situ’ magmatic province of this age, intruding into the basement of Laurentia, within the Mesoproterozoic Grenville Province after its accretion to Laurentia. The Seiland Igneous Province, however, is located within the Kalak Nappe of the Caledonides, and is considered an exotic terrane to Baltica by some authors, while others argue against such an interpretation. There are what appear to be Gondwanan elements in the Kalak and Seve nappes from the Caledonides that appear similar with those of the Southwestern Basement Province of Svalbard, as well as others that are similar with Laurentia. This highlights the similarities between the various Sveconorwegian and Grenvillean terranes and orogens across the North Atlantic region, and between the various nappes of the Scandinavian Caledonides and Svalbard, with these orogens potentially extending much further to the north, into the Arctic. Successor basins to the Grenvillean-Sveconorwegian orogenies were likely developed across the region. The continuity between North America and Scandinavia across the Grenville Orogeny and the Sveconorwegian Orogeny, however, has been challenged by several authors, based on various geological and paleomagnetic constraints, with Baltica even been considered to be ‘upside down’ relative to Baltica by some authors, although this is not a well-accepted hypothesis. This would allow the Southwestern Basement Province of Svalbard and the Kalak and Seve nappes of Scandinavia to easily accommodate any Laurentian affinities, and would allow the St. Jonsfjorden gabbros in the Southwestern Basement Province of Svalbard and the Kalak-Seve nappes (including the Seiland Igneous Province) of the Caledonides to be located within the same rift zone centred on the Sutton mountains at about 560 million years ago in Laurentia. However, the presence of the Long Range Dyke Swarm (in Laurentia) and Egersund Dyke Swarm (in Baltica) at about. 0.61 billion years ago would appear to suggest a more traditional configuration between Laurentia and Baltica, with all the Caledonide and Appalachian provinces appearing contiguous, or at least in close proximity from the continental margins of both Laurentia and Baltica. It is also worth noting that the Pearya Terrane of Ellesmere Island in the Canadian Arctic also includes mafic rocks of similar composition to the St. Jonsfjorden gabbros in Oscar II Land, hosted in similarly metamorphosed and deformed Neoproterozoic sedimentary rock, although they lack age constraints at present. Therefore, any links between the oceanic island basalt-like rocks of the Southwestern Basement Province terranes of Svalbard, and either Laurentia or Baltica remain ambiguous at present. This is despite the confirmation of affinity to Rodinia of the Southwestern Basement Province of Svalbard, with the Southwestern Basement Province lying in the same terminal rift zone as the bulk of Central Iapetus Magmatic Province-related magmatic material between 610 and 550 million years ago.

See also...

https://sciencythoughts.blogspot.com/2020/05/filamentous-connections-between.htmlhttps://sciencythoughts.blogspot.com/2019/07/acritarchs-from-ediacaran-of-camaqua.html
https://sciencythoughts.blogspot.com/2019/02/ediacaran-fossils-from-taroudant-group.htmlhttps://sciencythoughts.blogspot.com/2019/01/putative-discoidal-fossils-from.html
https://sciencythoughts.blogspot.com/2019/01/using-ordovician-tafilalt-lagerstatte.htmlhttps://sciencythoughts.blogspot.com/2019/01/reconstructing-ediacaran-nilpena.html
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