Large Igneous Provinces

The Matachewan Large Igneous Province (LIP) is interpreted to have formed during mantle plume-induced continental break-up during the early Proterozoic. When the Matachewan LIP is reconstructed to its original configuration, the dyke swarms,
layered intrusions and flood basalt provinces emplaced over the lifetime of the province comprise one of the largest magmatic provinces recognised in the geological record. New geochemical data allow, for the first time, the Matachewan
LIP to be treated as a single, coherent entity. The major and trace element data presented here show that Matachewan LIP suites share a common tholeiitic composition and trace element geochemistry characterised by enrichment in the most incompatible elements and depletion in the less incompatible elements. This signature, ubiquitous in early Proterozoic continental magmatic rocks, may indicate that the Matachewan LIP formed through contamination or mixing of its primary magmas with crustal material or that the early Proterozoic mantle had a fundamentally different composition to the modern mantle.

Aside from the radiating geometry of the dyke swarms, a plume origin for the Matachewan LIP is implied by the geochemistry of some of the suites, used here to constrain the potential temperature of the magmatism. Comparison of these potential
temperatures with the temperature of the early Proterozoic upper mantle, shows that the province is the product of anomalously hot magmatism as predicted by mantle plume theory. Geochemical data from coeval intrusions suggest that the plume head was compositionally heterogeneous and sampled material from both depleted and enriched mantle. Sr-Nd-Pb isotopic data show that this source heterogeneity dictates the Ni-Cu-PGE potential of the related intrusions.

The enormity of the Matachewan LIP cannot be overstated and its potential impact on the early Proterozoic global environment was likely immense – it may even have been the trigger to the irreversible oxygenation of our planet.

Rare minerals, chloriane amphiboles and chloriane biotites were discovered in rocks of  Palaeoproterozoic (Jatulian) traps of Karelia: in gabbro-dolerites of layered Kojkarsko-Svjatnavolok sill and in coarse-grained albitic (“karjalitic”) enclave in basaltic flow, which is exposed in village Girvas. Amphiboles detected as chlorinehastingsites (AlVI  < Fe3+, Fe/(Fe2+ + Mg) > 0,6; Cl from 2,03 to 5,50%), chlorinebiotites attributed to phlogopite-annite series with Fe/Mg ≥ 0,7 and Cl content ranging from 1,44 to 3,41%. These minerals were formed at the latest subsolidus stage of crystallization of Karelian traps. The composition of them indicates high chlorine activity in late- postmagmatic fluid, which could be considered as favorable factor when evaluation of PGE metal mineralization, well known in trap layered intrusions, would be conducted.

[by Petrov O.V. et al.]

We present a new model of the crustal and tectonic structure of the Arctic region north of 60° N latitude, constrained as a part of the international Atlas of Geological Maps of the Circumpolar Arctic under the aegis of the Commission for the Geological Map of the World. The region is largely formed by (i) Archean– Paleoproterozoic shields and platforms, (ii) orogenic belts of the Neoproterozoic to the Late Mesozoic ages over-lain by platform and basin sediments, (iii) Cenozoic rift structures formed in part as a consequence of seafloor spreading in the North East Atlantic Ocean with propagation into the Central Arctic Ocean along the Gakkel Ridge, (iv) deep-water ocean basins and shallow-water shelves of the North Atlantic and Arctic Oceans, and (v) associated large igneous provinces (LIPs). We present a series of maps for the Circumpolar Arctic which include maps of the depth to Moho and thickness of the sedimentary cover and presents tectonic regionalization based on 18 major crustal types (oceanic, transitional , and continental) recognized in the Arctic. A 7600 km-long crustal geotransect across the region illustrates the details of its crustal and tectonic structure. We discuss geological, geophysical and geochemical datasets on which the new regional model is based. These include magnetic and gravity maps of the Circumpolar Arctic and a wide range of existing and recently acquired bathymetric, geophysical (including seismic), geological, isotope and geo-chronological data (illustrated by new age data for seabed and bedrock samples from the New Siberian Islands). The analysis of these data in the framework of regional tectonic evolution allows for correlating onshore and offshore lithospheric fragments and for recognizing regions with oceanic, transitional, and continental crust in the Circumpolar Arctic.

Chromite is a ubiquitous accessory mineral in the olivine-pyroxene cumulate bodies that host massive and disseminated nickel sulphide mineralization in intrusions of the Kabanga-Musongati-Kapalagulu Alignment in East Africa. Its composition is related to the conditions of emplacement and petrologic evolution of its host magma in a spectrum of intrusions ranging from classical lopolithic layered intrusions to groups of smaller, discrete sill-like chonoliths.
The Kapalagulu lopolithic intrusion, emplaced into polymetamorphosed Archæan-Palæoproterozoic crust, contains abundant chromite with relatively oxidized compositions, whereas chromites from the highly-mineralized Kabanga chonolith intrusions, emplaced into graphitic and sulphidic schists, are strongly reduced in terms of their Fe3+/Fetotal ratio. Ni in chromite correlates with Ni in olivine: Ni in both is depleted in the more strongly sulphide-mineralized intrusions. The Musongati intrusion, also emplaced through graphitic schists, but much larger and less-well mineralized in sulphides than Kabanga, has chromites intermediate in character. The compositions of the chromites can be used to determine the petrologic history of the intrusions, and may prove to be a useful exploration tool in such mineralized belts.

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright a b s t r a c t As part of the Karoo large igneous province (LIP), the ∼ 700 km-long and ∼ 182 Ma-old Lebombo monocline outcrops inland of a younger Mozambique Lowland cover that separates it from the Indian Ocean. Detailed mapping of a particularly well exposed Olifant River traverse across this monocline correlates basaltic and rhyodacitic lava units within the Lebombo Group to at least three hosted generations of intrusions within the monocline-parallel Northern Lebombo dyke swarm, implying that a much denser feeder dyke swarm is covered by younger flows. Measured attitudes and thicknesses of up to 197 lavas and 791 dykes statistically quantify a structural cross section, from which a properly weighted lava log of nearly 5 km basalts inter-bedded with an overlying N 3.5 km of rhyodacites is constructed. The geological section is compatible with a slightly deeper (sub-lava) section through the innermost ∼ 30 km's of a relatively narrow East Greenland margin, to which the Lebombo monocline can be rigorously compared. In both cases, field relationships are consistent with early tectonic extension being overwhelmed by dyke dilation within margin-parallel swarm segments that may have been injected laterally from central magma chambers. Thus, it is argued that the Lebombo monocline is part of a successfully rifted margin that, furthermore, belongs to a distinct class of highly volcanic and relatively narrow margins, which only form proximal to classical triple rift LIP centers, where an elevated magma flux sustains a magmatic mode of extension. For the Karoo LIP, this conforms with (1) a tight Gondwana reconstruction of Africa with Antarctica, (2) successful breakup during intense ∼ 182 Ma emplacement of the Lebombo Group, (3) triple rift formation, together with a short-lived (∼ 181–178 Ma) Okavango dyke swarm, and (4) anomalously thick igneous crust formation during continued ∼ 2.5 cm/yr separation into a loose ∼ 155 Ma plate configuration of Africa and Antarctica.

Gorgona Island, Colombia is remarkable not only because it contains the only Phanerozoic komatiites, but also because it has mafic to ultramafic lavas with a wide range of compositions, from moderately enriched to extremely depleted (relative to Bulk Earth). The komatiite flows are, in many respects similar to Archaean komatiites; they formed from MgO-rich (18%) liquids and have upper spinifex zones and lower cumulate zones. The cumulate zones of Archaean komatiites contain many solid grains, in contrast more than 90% of the olivine in the Gorgona cumulates is highly skeletal. This combined with the fact that the Gorgona cumulate zones are thinner than those in Archaean komatiites, suggests that the komatiite magma became strongly superheated en route to the surface.The komatiites have trace element contents intermediate between those of the basalts and the ultramafic tuffs. Some basalts have isotope compositions indicative of long-term enrichment in incompatible elements, whereas other basalts and ultramafic volcanics have isotopic signatures that imply corresponding depletion. It is apparent that the plume source region of the Gorgona magmas was markedly heterogeneous, with at least two source components contributing to the observed variation in composition. This heterogeneity may have resulted from the incorporation of different components into the plume source, or it may be the result of complex melting and melt extraction processes during the ascent of a heterogeneous plume.Despite earlier suggestions that there may have been a significant age gap between depleted komatiite and basalt flows and the enriched basalts, new 40Ar-39Ar dating of basalts and gabbros are more consistent with all being generated at 87 Ma during formation of the Caribbean/Colombian plateau, possibly at the Galapagos hotspot.

The Central Atlantic Magmatic Province (CAMP) is a large igneous
province (LIP) composed of basic dykes, sills, layered intrusions and lava flows emplaced before Pangea break-up and currently distributed on the four continents surrounding the Atlantic Ocean. One of the oldest, best preserved and most complete sub-provinces of the CAMP is located in Morocco. Geochemical, geochronologic, petrographic and magnetostratigraphic data obtained in previous studies allowed identification of four strato-chemical magmatic units, i.e. the Lower, Intermediate, Upper and Recurrent units. For this study, we completed a detailed sampling of the CAMP in Morocco, from the Anti Atlas in the south to the Meseta in the north. We provide a complete mineralogical, petrologic (major and trace elements on whole-rocks and minerals), geochronologic (40Ar/39Ar and U–Pb ages) and geochemical set of
data (including Sr–Nd–Pb–Os isotope systematics) for basaltic and basaltic–andesitic lava flow piles and for their presumed feeder dykes and sills. Combined with field observations, these data suggest a very rapid (<0.3Ma) emplacement of over 95% of the preserved magmatic rocks. In particular, new and previously published data for the Lower to Upper unit samples yielded indistinguishable 40Ar/39Ar (mean age
= 201.2+/-0.8Ma) and U–Pb ages (201.57+/-0.04Ma), suggesting emplacement coincident with the main phase of the end-Triassic biotic turnover (c.201.5 to 201.3Ma). Eruptions are suggested to have been pulsed with rates in excess of 10km3/year during five main volcanic pulses, each pulse possibly lasting only a few centuries. Such high eruption rates reinforce the likelihood that CAMP magmatism triggered the end-Triassic climate change and mass extinction. Only the
Recurrent unitmay have been younger but by no more than 1Ma. Whole-rock and mineral geochemistry constrain the petrogenesis of the CAMP basalts. The Moroccan magmas evolved in mid-crustal reservoirs (7–20 km deep) where most of the differentiation occurred. However, a previous stage of crystallization probably occurred at even greater depths. The four units cannot be linked by closedsystem fractional crystallization processes, but require distinct parentalmagmas and/or distinct crustal assimilation processes. EC-AFC modeling shows that limited crustal assimilation (maximum c.5–8% assimilation of e.g. Eburnean or Pan-African granites) could explain some, but not all the
observed geochemical variations. Intermediate unit magmas are apparently the most contaminated and may have been derived from parental magmas similar to the Upper basalts (as attested by indistinguishable trace element contents in the augites analysed for these units). Chemical differences between Central High Atlas and Middle Atlas samples in the Intermediate unit could be explained by
distinct crustal contaminants (lower crustal rocks or Pan-African granites for the former and Eburnean granites for the latter). The CAMP units in Morocco are likely derived from 5–10% melting of enriched peridotite sources. The differences observed in REE ratios for the four units are attributed to variations in both source mineralogy and melting degree. In particular, the Lower basalts require a garnet peridotite source, while the Upper basalts were probably formed from a shallower melting region straddling the garnet–spinel transition. Recurrent basalts instead are relatively shallow-level melts generated mainly from spinel peridotites. Sr–Nd–Pb–Os isotopic ratios in the CAMP units from
Morocco are similar to those of other CAMP sub-provinces and suggest a significant enrichment of the mantle-source regions by subducted crustal components. The enriched signature is attributed to involvement of about 5–10% recycled crustal materials introduced into an ambient depleted or PREMA-type mantle, while involvement of mantle-plume components like those sampled by present-day Central Atlantic Ocean Island Basalts (OIB, e.g. Cape Verde and Canary Islands) is not
supported by the observed compositions. Only Recurrent basalts may possibly reflect a Central Atlantic plume-like signature similar to the Common or FOZO components.

The NNW- to NNE-trending Kaminak diabase dykes, exposed in the Hearne craton of the western Churchill
Province, Canada, are components of a Neoarchean–Paleoproterozoic LIP that forms an important pin in
plate tectonic reconstructions impacting models for the assembly of the Nuna supercontinent. This study
provides the first detailed information on the age, geochemistry and isotopic composition of these dykes.
A precise U–Pb baddeleyite date of 2497.6 ± 1.1 Ma, obtained for one dyke sample, provides the best
estimate for the timing of Kaminak mafic magmatism. The dykes are tholeiitic basalts to basaltic andesites
with consistent compositions over an areal extent of ∼10,000 km2. Major and compatible trace element
variations indicate that the dykes were not in equilibrium with anhydrous peridotitic mantle and
evolved through fractionation of Pl + Cpx ± Ol from parental melts in the deep crust. All dykes record
remarkably similar, incompatible element enriched, closely parallel multi-element profiles with variable
HFSE troughs, comparable to Phanerozoic low-TiO2 continental tholeiites. Time-corrected Nd isotopic
compositions overlap, or are slightly below bulk Earth (εNd2500 Ma = +0.4 to −0.8), and yield TDM values
(2.75–2.87 Ga) comparable to those of igneous crust in the Hearne craton. Implied short residence times,
along with the geological, geochemical and isotopic data, are interpreted to imply that parental magmas
of the dykes were low-TiO2 tholeiites generated via contamination of primary, depleted N-MORB-like
magmas by low-degree, calc-alkaline lamprophyre melts in the continental lithospheric mantle. The
melts homogenized in chambers near the Moho and underwent extensive fractional crystallization
(Pl + Cpx ± Ol), and possibly minor assimilation of lower crust, prior to injection into the upper crust.
Paucity of evidence for contemporaneous lithospheric rifting, homogeneous lithogeochemical and Nd isotopic
compositions, and petrochemical variations distinct from the Matachewan and Mistassini swarms
of the Superior Province suggest the Kaminak dykes are the magmatic products of a Late Archean–Early
Paleoproterozoic mantle thermal anomaly that extended, but did not break apart the Hearne craton. The
results of this study do not support direct correlation of the Kaminak and Matachewan or Mistassini dyke
swarms. Further evaluation of the proposed linkage of the Hearne and Superior cratons prior to Nuna
supercontinent assembly requires additional geochronological, lithogeochemical and radiogenic isotopic
constraints.

Large igneous provinces are conspicuous features of late Phanerozoic geology, and include continental flood basalts, rifted continental margin volcanic sequences and oceanic plateaus. The latter are formed in an environment which typically recycles back into the mantle on a time scale of < 200 m.y., but because comparisons have recently been made between oceanic plateaus and Precambrian greenstone belt sequences, new questions arise about their formation, their fate and their preservation. Here we review some critical aspects of three oceanic plateaus, Ontong Java, Kerguelen and the Caribbean/Colombian obducted plateau, and comment on their make-up and the factors governing their preservation, with particular relevance to ancient terranes.Many large igneous provinces can be linked to mantle plumes. Where plumes ascend beneath spreading ridges, their energy is transformed into a large melt volume, producing over-thickened plateau crust. Where the spreading rate is low in relation to magma supply, the plateau may become subaerial (e.g. Iceland), but with fast spreading the plateau remains submarine. Thicker lithosphere may result in plume incubation before magma extrusion, and there are many intermediate situations where plumes could readily break through thin lithosphere (oceanic or continental). Because magma supply exceeds extension rate, plateaus may be characterised by thick sequences of flows and sills rather than the sheeted dykes typical of Phanerozoic ophiolites. Precambrian greenstones could represent imbricated oceanic plateaus, or plumes penetrating thin continental lithosphere.The initial high temperature and the buoyant nature of the depleted refractory keel of plateaus contributes to their preservation relative to normal oceanic crust. When they collide with active margins they choke the subduction zone, causing subduction “flip” or “backstep” and the development of extensive calc-alkaline arc volcanism on top of the plateau sequences. However, after ⪢ 100 m.y. they are potentially negatively buoyant, so if fluids become available to promote transformation of the deeper zones to eclogite, they may be able to spontaneously subduct.

During the Pliensbachian–Toarcian interval of the Early Jurassic, there is a well-known second order marine extinction that is observable at the species and genus levels. Ammonite diversity data from successions throughout Europe and parts of the Arctic suggest that this extinction may have been multi-phased with di- versity declining over six separate intervals. The main-phase of decline begins at the Pliensbachian–Toarcian boundary and extends into the Lower Toarcian, to a level that is correlative with the Tenuicostatum/ Serpentinum Zone boundary. To date, only the main-phase of extinction has been demonstrated as being global in extent and affecting multiple taxonomic groups. This multi-phased extinction has been attributed to regional and global controlling mechanisms that are associated with the Volcanic Greenhouse Scenario which links the eruption of the Karoo–Ferrar large igneous province (LIP) to global warming and mass extinction.
We compare stratigraphic ranges of ammonite and foraminiferal species in Pliensbachian–Toarcian successions of western North America to the record in Europe and parts of the Arctic in order to test the geographic extent of the multiple phases of extinction. Our results show six intervals of species level decline that correlate with those recognized in Europe: 1) middle of the Lower Pliensbachian (middle Whiteavesi–middle Freboldi Zones), 2) middle of the Upper Pliensbachian (upper Kunae–lower Carlottense Zone), 3) Pliensbachian/Toarcian boundary into the Lower Toarcian (upper Carlottense–middle Kanense Zones), 4) Middle Toarcian (upper Planulata–lower Crassicosta Zones), 5) upper Middle–lower Upper Toarcian (middle Crassicosta–Hillebrandti Zones) and 6) Upper Toarcian (lower Yakounensis Zone).
Recognition of this multi-phased event in three separate ocean basins (paleo Pacific, paleo Arctic, and Tethys Oceans), in at least two taxonomic groups, greatly expands the known geographic extent of this multi-phased event and argues for a controlling mechanism that is global in its reach. In relation to the Volcanic Greenhouse Scenario, our study shows that four of the six pulses of extinction occur within the main-phase of Karoo magmatism. The decline in the Early Pliensbachian, previously thought to be separate from this event, occurs within error range of the onset of Karoo magmatism and the decline in the Late Toarcian coincides with the later stages of magmatism. These observations extend the known duration of this multi-phased extinction event to the Early Pliensbachian and support the Volcanic Greenhouse Scenario, specifically the eruption of the Karoo–Ferrar LIP, as a preeminent factor driving the multi-phased extinction of the Pliensbachian–Toarcian.

The Early Jurassic Toarcian oceanic anoxic event (T-OAE) and concurrent negative carbon-isotope (δ13C) excursion have recently been attributed to either the release of methane (CH4) clathrates or thermogenic CH4 gas associated with the Karoo-Ferrar large igneous province (LIP) into coals and organic-rich shales. 12C-enriched thermogenic CH4 production associated with the Karoo-Ferrar would result in residual material being 12C-depleted nearer the intrusions. In this study, geochemical analyses (carbon isotopes, volatile matter (VM), vitrinite reflectance (Ro)) are reported for two coal transects associated with dykes intruding the No. 4L coal in the Highveld Coalfield, Karoo Basin, South Africa. VM decreases from over 35% to around 15% in one transect, and the second transect shows a less pronounced decrease (from ~ 25% to ~ 16%). Accompanying the decrease in VM content is an increase in Ro from background levels of around 0.7% to over 4% adjacent to the dyke; used as a palaeo-geothermometer, Ro values indicate background temperatures of ~100 °C increasing to N300 °C close to the contact. Despite changes in VM and Ro, there are no significant changes in δ13C, certainly not of the magnitude that would be expected associated with large-scale thermogenic CH4 generation. These and other Gondwanan coals have low vitrinite and liptinite contents (components more prone to CH4 generation), in part explaining the modest decreases in VM adjacent to the dykes. This, combined with the relatively narrow metamorphic aureole surrounding the intrusions and the likelihood that at least some of the volatiles generated by the intrusion were trapped as coalbed CH4 or condensed as pyrolytic carbon, suggests only limited CH4 release. In addition, based on original estimates of moisture contents in these coals and the depth at time of intrusion (1,000–2,000 m) the dykes would have lost most of their energy heating and evaporating water, thus having very little remaining energy to generate thermogenic CH4.

source. The results indicate that the spatial-compositional variation of the ENA basaltic rocks is likely related to differing amounts of melting of mantle sources that reflect the uniqueness of their regional accreted terranes (Carolinia and West Avalonia) and their post-accretion, pre-rift structural histories.

Although subparallel swarms of dikes are thought to be the primary feeders to voluminous volcanic eruptions, increasing recognition of volumetrically significant sill complexes suggests that they too play an important role in magma ascent through the shallow crust. However, the extent to which sills and interconnected, sill-fed dikes actually transport magma to the earth&#39;s surface in many large igneous provinces (LIP) is presently unclear. By analyzing field relationships and dimensions of intrusions of the Ferrar LIP in South Victoria Land, Antarctica, we show that sill-fed dikes were the likely feeders for voluminous flood basalt eruptions. These intrusions are small but numerous, with cumulative dimensions equivalent to a feeder network 308,000 km long and 1.8 m wide. Due to the tremendous aerial extent of this intrusive network, each individual dike-feeder segment would only be required to actively feed magma for 2 to 3 days on average to erupt the 70,000 km3 of flood lavas ...