Central Iapetus Magmatic Province Research Articles

A depleted mantle source for Neoproterozoic continental rifting in the Seve Nappe Complex, Kebnekaise region, northern Swedish Caledonides

The Central Iapetus Magmatic Province (CIMP) is a large igneous province (LIP) emplaced in the Baltican and Laurentian paleocontinents that marks the onset of the Caledonian Wilson Cycle. Paleozoic magmatism of the CIMP is preserved in both northeastern America and northern Europe. This study investigates rocks belonging to the hyper-extended margin of Baltica currently found in the Seve Nappe Complex of the Scandinavian Caledonides. Specifically, U-Pb zircon geochronology and whole-rock geochemistry are applied to a migmatitic variety of the Vierručohkka amphibolite of the Mårma Terrane, to the Aurek gabbro, and amphibolite of the Aurek Assemblage exposed in the Seve Nappe Complex in the Kebnekaise region, northern Swedish Caledonides. U-Pb zircon geochronology yields crystallization ages of 626 ± 7 Ma for the protolith of the Vierručohkka amphibolite, and 614 ± 2 Ma and 609 ± 1 Ma for the emplacement of the Aurek gabbro and amphibolite protolith, respectively. A younger age of 599 ± 3 Ma is recorded in the Vierručohkka amphibolite and is interpreted as the age of partial melting and migmatization. The geochemical signatures of the rocks demonstrate crustal assimilation during the emplacement of their protoliths and modification due to prograde metamorphic processes during Caledonian subduction. The Vierručohkka amphibolite and the Aurek Assemblage samples display upper and lower crustal assimilation, respectively. Trace elements (Dy, Sm, Lu, and Y) record the growth of metamorphic garnet, while elevated TiO2 contents record the crystallization of metamorphic rutile. Nevertheless, high field strength elements (HSFE) and ∆Nb suggest a depleted mantle source for the magmas of the protoliths of the investigated rocks. Altogether, geochemical data indicate that the igneous activity recorded in the Vierručohkka amphibolite and the Aurek Assemblage between c. 626–609 Ma is related to continental rifting processes associated with the opening of the Iapetus Ocean.

The Architecture of a Root Zone of a Large Magmatic Conduit System From High Resolution Magnetic, Gravity and Petrophysical Data: The Reinfjord Ultramafic Complex

AbstractThe Seiland Igneous Province (SIP) is a large province of mafic and ultramafic (UM) complexes interpreted to be relics of a giant plumbing system feeding the Ediacaran Central Iapetus Magmatic Province. The Reinfjord Ultramafic Complex (RUC) is one of the four major ultramafic complexes of the SIP. The RUC has a younger dunite core surrounded by wehrlite and lherzolite embedded in country rocks consisting of layered gabbros with sub‐horizontal layering and metamorphosed sedimentary rocks. Here, we develop a 3D subsurface model for the RUC using high‐resolution magnetic and gravity data and extensive petrophysical measurements from oriented surface samples and drill core samples. Our model indicates that the RUC narrows in depth, extending a minimum of 1.4 km below sea level, and plunges eastwards below the country rock. This model allows us to decipher the lithologic heterogeneities, and the depth and lateral extent of ultramafic rocks, which we interpret in the context of the geologic history of the area. The RUC is spatially separated from other UM complexes of the SIP and the result of this study indicates a smaller depth extent. Combining these findings with the previously reported distribution of the SIP rocks based on the regional gravity data, we propose that the uplift of the crustal block hosting the RUC is larger than for ultramafic complexes in the northwestern part of the SIP.

Ediacaran-Ordovician tectonic and geodynamic drivers of Great Unconformity exhumation on the southern Canadian Shield

The Great Unconformity erosion surface between Paleozoic sedimentary rocks and Archean-Proterozoic basement has been related to erosion occurring during Snowball Earth glaciations, eustatic sea level fluctuations, and a range of tectonic and/or geodynamic mechanisms. Each class of mechanism predicts distinct timing and spatial patterns of exhumation. Snowball Earth glacial erosion is limited to 717-635 Ma and concentrated in narrow ice streams on continental margins. Sea-level related erosion is unconstrained in time but also spatially limited to continental margins. Tectonic and geodynamic mechanisms, in contrast, can result in exhumation distributed more broadly in time and space. We combine new zircon and apatite (U-Th)/He thermochronology data (ZHe, AHe) with independent paleodepth information from a continental interior location in southeastern Ontario, Canada to constrain the timing, magnitude, and regional pattern of exhumation associated with the Great Unconformity along a ∼650 km-long transect across the southern Canadian Shield. Here, the unconformity is defined by Middle Ordovician carbonates atop Archean-Proterozoic basement. ZHe analyses for seven basement samples display a range of dates from 960 ± 20 Ma to 37.5 ± 0.9 Ma that correlate negatively with radiation damage. AHe dates are ∼300-200 Ma and generally consistent across samples regardless of radiation damage. Independent evidence supports emplacement of both the 590 +2/-1 Ma Grenville dikes and the 577 ± 1 Ma Callander Complex in the study region at depths ≥6 km. The combined data require ≥6 km of exhumation between ca. 590-577 Ma and 470 Ma in the middle of our transect, with multi-km erosion up to ≥5 km elsewhere in the study area, well after the ca. 717-635 Ma Snowball Earth glaciations. These outcomes expand the spatial extent of an Ediacaran to early-Paleozoic exhumation signal previously inferred elsewhere across the Shield to ∼1.1 million km2. Thick Ediacaran successions on the Laurentian margins are complementary depositional signals of this erosion. The enormous spatial extent of Great Unconformity exhumation across the continental interior of the Canadian Shield is incompatible with glacial erosion and eustatic sea-level change as the primary causes. Instead, we attribute exhumation to tectonic and geodynamic mechanisms, which may include isostatic rebound, dynamic topography, plume activity associated with the Central Iapetus Magmatic Province, rifting during opening of the Iapetus Ocean, and development of the Transcontinental Arch.

The early Ediacaran Brachina sequence, Flinders Ranges, South Australia: its age, formation and plate tectonic setting

Throughout the Flinders Ranges of South Australia, the red bed Brachina sequence comprises the lower of two regressive, coarsening-upward successions that together constitute the Ediacaran-aged Wilpena Group. Detailed examination of the Brachina sequence has identified evidence of ice-rafting, supporting the contention that frigid climatic conditions continued well after cessation of the Marinoan Glaciation, as well as features postulated to be harbingers of the macroscopic metazoans that occur in abundance higher in the local stratigraphy. Although tuffaceous and lying stratigraphically between deposits of the Marinoan Glaciation and the Acraman Impact/Gaskiers Glaciation, no absolute age dates exist for the sequence. However, through correlation to the global carbon isotope profile, the timespan of Brachina sequence deposition can be constrained to between 635 and 586 Ma, when the regression climaxed with terrestrial deposition upon a local disconformity. The tuffaceous input is timed to have begun at 620 Ma, essentially coincident with initial formation of the passive volcanic continental margin of southeastern Australia and initiation of both the Volyn Large Igneous Province and activity within the Central Iapetus Magmatic Province, all components marking the demise of Rodinia and initial amalgamation of Gondwana.

Ediacaran magmatism and rifting along the northern margin of the Tarim craton: Implications for the late Neoproterozoic Rodinia configuration and breakup

Abstract The Tarim craton in modern Central Asia was an important component of the supercontinent Rodinia in the Neoproterozoic, although its paleogeography in Rodinia during that era is still controversial. Here, we present new stratigraphic, geochemical, and geochronological data from the Neoproterozoic sedimentary and volcanic rock successions along the northwestern margin of the Tarim craton and discuss the significance of these data and our interpretations for its tectonic evolution and paleogeographic position within Rodinia. The Lower Ediacaran sedimentary sequence (Sugetbrak Formation) in northwest Tarim includes terrestrial and shallow-marine clastic rocks intercalated with two discrete basaltic lava flows near the top. The Upper Ediacaran sedimentary sequence conformably overlying the volcanic and clastic rocks consists mainly of stromatolitic dolomite (Chigebrak Formation), representing a transgressive shallow-marine environment. Previous U-Pb zircon dating of the basaltic lava flows has constrained the timing of their eruption in the early Ediacaran (615 Ma). Detrital zircon U-Pb dating of a feldspar-quartz-sandstone unit situated between the two lava flows revealed an oldest age of 2517 ± 18 Ma and a youngest age of 612 ± 6 Ma, with a majority of zircon grains (n = 42) dated at 891–754 Ma (Tonian). A quartz-sandstone unit above the upper lava flow revealed an oldest age of 2724 ± 15 Ma and a youngest age of 607 ± 8 Ma, with a missing age group of 891–800 Ma. These data and observations indicate: (1) a major switch in the depositional setting from a terrestrial (synrifting) to shallow-marine environment following the eruption of the upper lava unit; and (2) an abrupt disappearance of the source rocks of the 891–800 Ma zircons and sediments from the provenance of the post-615 Ma (postrifting) sedimentary sequence. The basaltic rocks have low SiO2 and MgO but high total Fe2O3 and TiO2 contents (2.34–3.19 wt%), analogous to high-Ti basalts and continental flood basalts. Their Ti/V ratios (65–88), low Th/Nb ratios (~0.1), and high TiO2/Yb ratios (~1.1) are similar to those of ocean-island basalt (OIB). Combined with their Sm/Yb and La/Sm ratios and Sr-Nd-Pb-Hf isotope values, we infer that magmas of the Sugetbrak basalts were likely derived from partial melting of an enriched mantle source (EM I) in a transitional spinel-garnet lherzolite field. This petrogenetic evolution was a result of mantle plume–influenced rift magmatism during the Ediacaran breakup of Rodinia. The Central Tianshan terrane, which was attached to the northern Tarim craton until the Ediacaran Period, was the provenance of 891–800 Ma (Tonian) zircons in the synrift sedimentary succession. As the Central Tianshan terrane broke away from the Tarim craton after ca. 615 Ma, Tonianaged zircons were no longer available to the depocenter of the postrift sedimentary sequence. The transition from rifting to drifting between the Tarim craton and the Central Tianshan terrane marked the final breakup of Rodinia, a global event that was possibly driven by mantle plume activities coeval with the development of the Central Iapetus magmatic province in Laurentia during the Ediacaran.

Vestiges of the Pre-Caledonian Passive Margin of Baltica in the Scandinavian Caledonides: Overview, Revisions and Control on the Structure of the Mountain Belt

The Pre-Caledonian margin of Baltica has been outlined as a tapering wedge with increasing magmatism towards the ocean–continent transition. It is, however, well known that margins are complex, with different and diachronous evolution along and across strike. Baltica’s vestiges in the Scandes have complexities akin to modern margins. It included a microcontinent and magma-poor hyperextended and magma-rich segments. It was probably up to 1500 km wide before distal parts were affected by plate convergence. Characteristic features are exhumed mantle peridotites and their detrital equivalents, some exposed to the seafloor by the pre-orogenic hyperextension. A major change in the architecture of the mountain belt occurred across the NW–SE trending Sveconorwegian front in the Baltican basement. This coincided with the NE termination of the Jotun-Lindås-Dalsfjord basement nappes, the remains of the Jotun Microcontinent (JMC) formed by hyperextension prior to the orogeny. Mantle with ophicalcite breccias exhumed by hyperextension are covered by deep-marine sediments and local conglomerates. Baltican basement slivers are common in the transitional crust basins. Outboard the JMC, the margin was magma-rich. The main break-up magmatism at 605 ± 10 Ma was part of the vast Central Iapetus Magmatic Province. The along-strike heterogeneity of the margin controlled diachronous and contrasting tectonic evolution during the later Caledonian plate convergence and collision.

On the origins of the Iapetus Ocean

The Iapetus Ocean opened during the fission of the supercontinent Rodinia, from the breakup of three of its core continental constituents: Laurentia, Baltica and Amazonia. The timing of Iapetus opening is still much debated, with estimates ranging from 700 to 550 Ma. Similarly debated is exactly how Laurentia, Baltica and Amazonia were positioned relative to each other immediately before their breakup. In this study, we reconsider the timing and framework of Iapetus opening by integrating the fragmentary mid-Neoproterozoic to early Cambrian observational records from these continents. We first demonstrate that paleomagnetic data, despite being both sparse and probably contaminated by some global-scale, non-uniformitarian process in Ediacaran time, support the existence of a wide ocean between these continents by 575 Ma. However, the paleomagnetic data alone are insufficient to allow the formulation of more specific conclusions concerning the timing and paleogeography of Iapetus opening. We therefore conduct an extensive review of the mid-Neoproterozoic to Cambrian geology of eastern Laurentia, western Baltica and western Amazonia which, jointly interpreted with the paleomagnetic constraints, allow us to construct a self-consistent and geodynamically feasible plate tectonic model. In this model, the breakup of Laurentia, Baltica and Amazonia was polyphase, involving the spalling of multiple marginal terranes from Laurentia and the successive opening of several oceans, including a composite ‘Iapetus Ocean’. The first phase of continental breakup occurred between eastern Laurentia and western Amazonia at 750–700 Ma, leading to the opening of the Puncoviscana Ocean. This was followed by the opening of the eastern branch of the Iapetus Ocean, between Laurentia and Baltica, at ~590 Ma, which may have been instigated by emplacement of the Central Iapetus Magmatic Province. The western branch of Iapetus subsequently opened at ~550 Ma by the detachment of marginal terranes from eastern Laurentia, following a protracted phase of rifting. We contend that our preferred scenario is the simplest solution given the presently available evidence but throughout this review we underline key outstanding questions and the attendant uncertainties in our preferred model.

Constraints on the ages of the crystalline basement and Palaeozoic cover exposed in the Cordillera real, Ecuador: 40Ar/39Ar analyses and detrital zircon U/Pb geochronology

Gabbros and ultramafic rocks of the Huarguallá Gabbro unit exposed in faulted slivers along the western Cordillera Real of Ecuador crystallised between 623and531 Ma (40Ar/39Ar dates), were derived from asthenospheric sources with minor crustal contamination, and form part of the Central Iapetus Magmatic Province. These rocks formed in an early rift environment during the opening of the Iapetus Ocean, and represent the only igneous record of Iapetus rifting north of the Huancabamba deflection (5°S) in South America. The age and composition of the Huarguallá Gabbro unit is consistent with the reconstruction of Tegner et al. (2019), which juxtaposes Baltica and northwestern Gondwana within Panotia. 206Pb/238U dates of detrital zircons combined with fossil assemblages shows that the Chiguinda unit of the Cordillera Real, and La Victoria Unit of the Amotape Complex were deposited during the Carboniferous. These new data, combined with previous studies of magmatism and sedimentation from southern Peru, Colombia and Venezuela, imply that the rocks of the Cordillera Real were in the Ordovician and Carboniferous back-arcs, while the arcs occur in conjugate margins that separated during the Triassic rifting of Pangaea. Faulted remnants of Ordovician arc rocks in the Cordillera Central of Colombia are probably allochthonous, and have been displaced from an Ordovician margin that did not face the rifted crustal sections.

The clastic record of a Wilson Cycle: Evidence from detrital apatite petrochronology of the Grampian-Taconic fore-arc

Detrital apatite deposited before, during and after Grampian-Taconic arc accretion in western Ireland reveals dramatic changes in source provenance and provides a detailed detrital petrochronological record of a Wilson Cycle sited on the Laurentian margin. These determinations are made possible by recent improvements in source rock type diagnosis by apatite trace element analysis. In addition, by interrogating the proportion of apatite derived from peraluminous vs metaluminous melt and felsic vs mafic melt in detritus, temporal changes can be seen in the relative proportions of these magma compositions over time. We present combined trace element and U-Pb age data for detrital apatite in clastic formations from the South Mayo Trough, a uniquely well-preserved fore-arc basin accreted to the Laurentian margin during the Grampian-Taconic orogeny (∼ 475 – 460 Ma), which records the early phase of closing of the Iapetus Ocean. Apatite deposited before collision is derived from the Laurentian margin and mafic igneous rocks of the Central Iapetus Magmatic Province associated with the Neoproterozoic break-up of Rodinia / Pannotia leading to the birth of the Iapteus Ocean. Following the onset of ocean closure in the Cambrian, igneous apatite derived from the nascent Grampian-Taconic arc is detected. Apatite derived from felsic and peraluminous melts becomes progressively more dominant over apatite from more mafic and metaluminous sources as the arc evolves from an intra-oceanic to continental arc setting after collision. The subsequent Grampian-Taconic unroofing history recorded by detrital apatite records the transition from ancient Laurentian margin- and intra-oceanic arc-derived detritus, to the exhumation of the high-grade metamorphic core of the Grampian-Taconic orogen and the exposure of post-collisional continental-arc granitoids. Our results are in broad agreement with both the Grampian fore-arc provenance record (heavy mineral analysis, detrital zircon U-Pb and white mica Ar-Ar) and the Laurentian crustal reworking record derived from the geochemistry of volcaniclastic units in the South Mayo Trough. However, we also obtain novel provenance information undetected by detrital zircon in the South Mayo Trough, such as apatite derived from the Central Iapetus Magmatic Province. Critically this information is obtained from a single mineral system, which eliminates the need to consider differing mineral stabilities during weathering, transport and diagenesis. The high preservation potential for apatite in similar tectonic environments illustrates the potential to employ detrital apatite as a record of Earth's lithological composition in deep time.

Neoproterozoic extension and the Central Iapetus Magmatic Province in southern Mexico – New U-Pb ages, Hf-O isotopes and trace element data of zircon from the Chiapas Massif Complex

Final fragmentation of Rodinia occurred during the Ediacaran Period as Amazonia, Baltica and Laurentia drifted apart to form the Iapetus Ocean. Accompanying rift-related mafic dyke swarms of the Central Iapetus Magmatic Province (CIMP) were emplaced between 0.62 and 0.55 Ga, which are preserved in Laurentia and Baltica, whereas no coeval mafic rocks are known from Amazonia. First evidence for the CIMP extending into Oaxaquia (Rodinia-type basement of Mexico) was reported as tholeiitic dykes that intruded the Novillo gneiss, NE Mexico, at 619 ± 9 Ma. In Chiapas, SE Mexico, amphibolite dykes that are chemically similar to the Novillo dykes intruded anorthosite and gneiss. In this paper, a new dating approach to obtain mafic dyke intrusion ages is presented by targeting contact metamorphic zircon with the UPb method, employing Secondary Ion Mass Spectrometry. Zircon that crystallized in anorthosite at intrusive contacts to mafic dykes and at temperatures exceeding 700 °C (Ti-in-zircon thermometry) yields ages between 615 ± 7 Ma and 608 ± 12 Ma, reflecting the time of dyke intrusion. Zircon chemical and isotopic (Hf, O) characteristics suggest a diachronous sequence of metamorphic processes involving Zr release from FeTi oxides, breakdown and recrystallization of other phases, and fluid-mitigated reactions during Ordovician metamorphism. Zircon δ18O values of granulites from Oaxaquia range from +6.2‰ to +9.8‰, whereas Tonian (~0.92 Ga) metamorphic zircon from SE Chiapas yielded low δ18O values from +2.0‰ to +2.8‰ that are explained by the reactivation of major tectonic boundaries during Tonian gravitational collapse. The observations increase the known extent of the CIMP in Mexico, suggesting that a Neoproterozoic superplume was still active during the Early Ediacaran producing a Large Igneous Province that extended over Amazonia, Baltica and Laurentia. The results further suggest that Oaxaquia at the northern edge of Amazonia formed the conjugate margin of Baltica during rifting.

GRANITES OF THE NORTHERN TIMAN – PROBABLE INDICATORS OF NEOPROTEROZOIC STAGES OF RODINIA BREAKUP

The Northern Timan is an uplifted block of Late Precambrian basement of the Timan Ridge, where Neoproterozoic sedimentary-metamorphic rocks of the Barmin Group are cut by intrusive rocks of different composition and all unconformably overlain by Lower Silurian limestone. To determine the age of granites, U-Pb dating of zircons was carried out using secondary ion mass spectrometry (SIMS). Two episodes of Neoproterozoic granite magmatism were established. Granite rocks of the Bolshoy Kameshek (613 ± 6 Ma) and Cape Bolshoy Rumyanichny (614 ± 11 Ma) plutons are interpreted to be associated with the formation of Central Iapetus Magmatic Province and record the Ediacaran stage of Rodinia breakup. The granites of the Sopki Kamennyie pluton (723‒727 Ma) formed in Cryogenian time and are assumed to represent an earlier episode of Rodinia breakup. Their ages correlate with the age of the Franklin LIP that existed in Northern Laurentia and is believed to have spread to South Siberia.

Mixing of heterogeneous, high-MgO, plume-derived magmas at the base of the crust in the Central Iapetus Magmatic Province (Ma 610-550): Origin of parental magmas to a global LIP event

High-MgO (>12 wt%) magmas represent some of the most primary and high temperature melts from mantle plumes. The compositional diversity of high-MgO magmas gained by fractional melting within inhomogeneous mantle sources, is often overprinted by wall rock assimilation, magma mixing and fractional crystallisation within magma chambers at various depths within the crust. The deepest intrusions, at the base of the crust, are the first stop for magmas as they leave the mantle and such localities offer vital insights into the crustal processes that modify high-MgO melts. The Seiland Igneous Province (SIP) represents a rare exposure of a deep crustal magma conduit system that transported large volumes of mantle-derived melts through the lower continental crust. In this work, trace element compositions of clinopyroxenes in dunite, wehrlite and olivine clinopyroxenite samples from the Reinfjord intrusion were measured using laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The melts calculated to be in equilibrium with the clinopyroxenes represent derivatives of mantle-derived magmas and have steep rare earth element (REE) profiles with La/YbPM of 8.4–14., Sm/YbPM of 3.5–4.8, and negative anomalies in Nd, Zr and Hf. Assimilation, recharge and fractional crystallisation modelling shows that the ultramafic cumulates in the Reinfjord intrusion formed through concurrent fractional crystallisation and repetitive recharge (and mixing) of new primitive magmas from the mantle with very limited input from crustal sources. The recharge and mixing model is strongly supported by field and petrographic data. Two end-member ultramafic magmas are identified; the first end-member melt composition (MELTCPX0) is similar to the melt compositions that are in equilibrium with the most trace element depleted clinopyroxenes from Reinfjord. The second end-member melt composition (MELTPIC0) is similar to a number of picrite dykes found throughout the SIP, which have less steep LREE/HREE and lack strong negative anomalies in Zr and Hf. The REEBOX PRO melting model (Brown and Lesher, 2016) was used to forward model adiabatic decompression melting of lithologically homogeneous and heterogeneous sources containing anhydrous/hydrous peridotite ± pyroxenite. It was found that MELTPIC0 formed by melting of a peridotite source with up to 10% pyroxenite component and mantle potential temperatures of up to 1450 °C. MELTCPX0, however, formed by melting of a carbonate- metasomatised peridotite with potential temperatures of up to 1630 °C. These results indicate a compositionally and thermally inhomogeneous mantle plume beneath the SIP.

Caught between two continents: First identification of the Ediacaran Central Iapetus Magmatic Province in Western Svalbard with palaeogeographic implications during final Rodinia breakup

The final fragmentation of Rodinia occurred during the Ediacaran period as the continental blocks of Baltica and Laurentia, as well as Amazonia and West Africa, rifted and drifted apart. It was along this progressively rifted margin that the Iapetus Ocean opened and subsequently closed, creating the Caledonian orogeny (sensu lato), some 100 and 200 million years later. The 0.62–0.54 Ga Central Iapetus Magmatic Province (CIMP), accompanying this break-up, is variously manifested in northern Europe and north-eastern North America as mafic dykes and sills, volcanic rocks, as well as carbonatites. This magmatism is interpreted to be broadly coincident with the so-called Marinoan and Gaskiers glaciations, and the subsequent evolution of metazoans. While the various Precambrian basement blocks of Svalbard are known to carry a Caledonian overprint, the proximity of various parts of Svalbard to either Baltica or Laurentia during final Rodinia breakup in the Ediacaran has yet to be established. Petrographic, geochronological and geochemical data from the variably deformed and metamorphosed coarse-grained mafic units of Oscar II Land (OIIL) from Spitsbergen, Svalbard, are presented here to determine the source of the magma and to elucidate on their palaeogeographic affinity with adjacent crustal blocks/units. An alkali and OIB-like affinity of the mafic units is confirmed by their whole-rock geochemical composition. An ID-TIMS U-Pb baddeleyite crystallisation age determination of 560 ± 12 Ma, obtained from one such mafic unit, contrasts with the published Caledonian (sensu lato) age determinations. These data for OIIL compare well with the broadly coeval and geochemically similar 570–560 Ma Norwegian Seiland Igneous Province, which may have formed a part of Baltica. Temporally and compositionally similar mafic units also occur within the North American Appalachian Belt, as well as the Sept-Îles intrusion, which is part of Laurentia. However, any links between these intrusions with those of Svalbard are yet to be unequivocally demonstrated.

A Mantle Plume Origin for the Scandinavian Dyke Complex: A “Piercing Point” for 615 Ma Plate Reconstruction of Baltica?

AbstractThe origin of Large Igneous Provinces (LIPs) associated with continental breakup and the reconstruction of continents older than ca. 320 million years (pre‐Pangea) are contentious research problems. Here we study the petrology of a 615–590 Ma dolerite dyke complex that intruded rift basins of the magma‐rich margin of Baltica and now is exposed in the Scandinavian Caledonides. These dykes are part of the Central Iapetus Magmatic Province (CIMP), a LIP emplaced in Baltica and Laurentia during opening of the Caledonian Wilson Cycle. The >1,000‐km‐long dyke complex displays lateral geochemical zonation from enriched to depleted basaltic compositions from south to north. Geochemical modelling of major and trace elements shows these compositions are best explained by melting hot mantle 75–250 °C above ambient mantle. Although the trace element modelling solutions are nonunique, the best explanation involves melting a laterally zoned mantle plume with enriched and depleted peridotite lithologies, similar to present‐day Iceland and to the North Atlantic Igneous Province. The origin of CIMP appears to have involved several mantle plumes. This is best explained if rifting and breakup magmatism coincided with plume generation zones at the margins of a Large Low Shear‐wave Velocity Province (LLSVP) at the core mantle boundary. If the LLSVPs are quasi‐stationary back in time as suggested in recent geodynamic models, the CIMP provides a guide for reconstructing the paleogeography of Baltica and Laurentia 615 million years ago to the LLSVP now positioned under the Pacific Ocean. Our results provide a stimulus for using LIPs as piercing points for plate reconstructions.

Portrait of a giant deep-seated magmatic conduit system: The Seiland Igneous Province

The Seiland Igneous Province (SIP), Northern Norway, contains >5000km2 of mafic and ultramafic intrusions with minor alkaline, carbonatite and felsic rocks that were intruded into the lower continental crust at a depth of 25 to as much as 35km.The SIP can be geochemically and temporally correlated to numerous dyke swarms throughout Scandinavia at 560–610Ma, and is linked to magmatic provinces in W-Greenland and NE-America that are collectively known as the Central Iapetus Magmatic Province (CIMP).Revised mapping show that the SIP exposes 85–90% layered tholeiitic- alkaline- and syeno-gabbros, 8–10% peridotitic complexes, 2-5% carbonatite, syenite and diorite that formed within a narrow (<10Ma) time frame in the Ediacaran (560–570Ma). Large peridotite complexes were emplaced into the still hot and unconsolidated gabbro (no dating available) and are regarded as the main-conduit systems.Gravimetric data implies an average thickness of igneous rocks of 4–5km and also features six deep lithospheric roots of ultramafic rocks extending min 9km into the crust. Together, the root structures represent the main volcanic conduits conveying thousands of km3 of mafic-ultramafic melts from the asthenosphere to the lithosphere.The ultramafic complexes were predominantly emplaced into the layered gabbros at four major igneous centres, respectively, Nordre Brumandsfjord, Melkvann, Kvalfjord and Reinfjord. All complexes are situated in a right-way-up position and are steep sided forming large plugs. A marginal hybrid zone forms at the contact with country-rock and transitions gradually from olivine-mela-gabbro over pyroxenites that grades in to an olivine-clinopyroxenite zone, which is followed by a wehrlite zone and, finally, the centre of the complexes comprises pure dunite. From pyroxenite to dunite, olivine changes from Fo72 to Fo85 and clinopyroxene from Di80 to Di92 i.e. the complexes observe a reverse fractional crystallisation sequence with time.Parental melt compositions modelled from early dykes indicate komatiitic to picritic melts with 16–22wt% MgO, Cr of 1594ppm and Ni of 611ppm, which were emplaced at 1450–1500°C. Melt compositions calculated from clinopyroxene compositions from Reinfjord are OIB-like with LREE enriched over HREE.The high abundance of carbonatites and lamproites demonstrates the volatile-rich nature of the mantle source region and is further corroborated by the unusually high abundance of magmatic sulphides (0.5–1%) and carbonated and hydrous assemblages (c. 1%) throughout the region. In Reinfjord, they are also closely associated with PGE-Cu-Ni reef deposits.Essentially, the ultramafic complexes in the SIP comprises deep-seated transient magma chambers that facilitated mixing and homogenisation of a rich diversity of fertile asthenospheric melts en route to the upper parts of the continental crust.

Large igneous provinces (LIPs) and carbonatites

There is increasing evidence that many carbonatites are linked both spatially and temporally with large igneous provinces (LIPs), i.e. high volume, short duration, intraplate-type, magmatic events consisting mainly of flood basalts and their plumbing systems (of dykes, sills and layered intrusions). Examples of LIP-carbonatite associations include: i. the 66 Ma Deccan flood basalt province associated with the Amba Dongar, Sarnu-Dandali (Barmer), and Mundwara carbonatites and associated alkali rocks, ii. the 130 Ma Parana-Etendeka (e.g. Jacupiranga, Messum); iii. the 250 Ma Siberian LIP that includes a major alkaline province, Maimecha-Kotui with numerous carbonatites, iv. the ca. 370 Ma Kola Alkaline Province coeval with basaltic magmatism widespread in parts of the East European craton, and v. the 615–555 Ma CIMP (Central Iapetus Magmatic Province) of eastern Laurentia and western Baltica. In the Superior craton, Canada, a number of carbonatites are associated with the 1114–1085 Ma Keweenawan LIP and some are coeval with the pan-Superior 1880 Ma mafic-ultramafic magmatism. In addition, the Phalaborwa and Shiel carbonatites are associated with the 2055 Ma Bushveld event of the Kaapvaal craton. The frequency of this LIP-carbonatite association suggests that LIPs and carbonatites might be considered as different evolutionary ‘pathways’ in a single magmatic process/system. The isotopic mantle components FOZO, HIMU, EM1 but not DMM, along with primitive noble gas signatures in some carbonatites, suggest a sub-lithospheric mantle source for carbonatites, consistent with a plume/asthenospheric upwelling origin proposed for many LIPs.