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The AT2-1 well in the Davis Strait between Canada and Greenland penetrated an approximately 1.2 km thick sequence of alkaline volcanic rocks with some intercalated sediments at depths between 3690 to 4850 m. These volcanic rocks can be mapped on 2D seismic data and constitute a cone-shaped 5 km × 10 km wide and >1.2 km high structural high named the Atammik Volcano. This sequence comprises two distinct parts, an upper part of phono-tephrite to basaltic trachy-andesite and a lower part of tephriphonolite and phonolite. Rock textures and structures testify to a volcanic origin, with the uppermost units showing textural evidence of being subaerially extruded. Zircon crystals found in a sample of phonolite from 4453 m were dated by in situ laser ablation ICP–MS technique to yield ages between 98 and 93 Ma, indicating a maximum age of the formation of the phonolitic volcano of 93 Ma (Turonian). Further, detrital zircons from the clastic material have been dated yielding Archean ages. The gamma ray log indicates three internal cycles within the phonolites, each cycle displaying a stratigraphically upwards decrease in potassium content, suggesting the existence of a longer-lived system undergoing repeated magmatic differentiation and eruption events. The upper volcanic sequence is less evolved and less alkaline than the lower, suggesting a change in primary magma compositions towards progressively higher degrees of melting of the underlying mantle. This fits into a scheme of progressively higher degrees of melting with time, which in a regional context probably corresponds to a rifting event.

Zircon U–Pb geochronology and heavy mineral CCSEM analysis were used to interpret the provenance of Oxfordian–Volgian sandstones of the Hareelv Formation in East Greenland. Six samples were collected from the Blokelv-1 core drilled in southern Jameson Land, and the zircon age distributions and heavy-mineral assemblages are quite uniform. The samples contain a wide spectrum of Archaean to Palaeozoic zircon ages with peak ages at 2.71, 2.49, 1.95, 1.65, 1.49, 1.37, 1.10 and 0.43 Ga when combining all data. The heavy-mineral compositions show derivation from felsic source rocks, some of which were metamorphic. The results reveal that the sediment was derived from the Caledonides, and it is plausible that some or all of the material has experienced several cycles of sedimentation. Devonian and Carboniferous sediments preserved north of the area have zircon age distributions that correspond to those from the Hareelv Formation, and such rocks may have been reworked into the Jameson Land Basin. The provenance signature describes both the gravity-flow sandstones of the Hareelv Formation and the delta-edge sands that are inferred to have fed them. Lithological and provenance contrasts between the sandstones of the Sjællandselv Member and those of the Katedralen Member indicate a shorter transport distance, source to sink, suggestive of proximal topographic rejuvenation in the Volgian.

The Paleoproterozoic Ammassalik Intrusive Complex of south-east Greenland is located at the margin between the Rae Craton to the north and the North Atlantic Craton to the south. The complex is part of the Nagssugtoqidian Mobile Belt emplaced at mid- to lower-crustal level and represents an eastward continuation of the Nagssugtoqidian Orogen of western Greenland. We present new whole-rock geochemical and isotopic data (whole-rock Rb-Sr, Sm-Nd, U-Pb, Lu-Hf and zircon U-Pb, Lu-Hf, O) acquired from the Ammassalik Intrusive Complex, with the aim of reassessing its petrogenesis and evolution. The intrusive rocks and surrounding host-rocks show typical calc-alkaline signatures as well as negative Ta, Nb and Ti anomalies. The new U-Pb age data indicate a protracted magmatic history over ca. 40 Ma, involving at least three successive magmatic pulses between ca. 1910 and 1870 Ma emplaced into a granulite facies biotite-garnet paragneiss with ca. ≤ 1990 Ma protolith ages. Whole-rock trace element and isotope geochemistry indicate that the magmas that formed the Ammassalik Intrusive Complex were derived from a mildly enriched (eNd0 = −0.8 to –3.8; eSr0 = +24 to +42) and deep, garnet-bearing (MREE/HREE fractionated patterns) mantle source, possibly the sub-continental lithospheric mantle. However, the observed decreasing zircon eHf values with time (eHf0 = +2.8 to −12.5) indicate increasing degrees of crustal contamination involving variably 18O-enriched crustal material (δ18O = 7–9‰) into the mafic magmas of the complex. This interpretation suggests that the mantle-derived magma do not have a deep, garnet-bearing source, but that the continental arc signature in the Ammassalik Intrusive Complex igneous rocks may have been inherited from the assimilation of partial melts of the garnet-bearing host-rocks. In addition, based on the new crystallisation ages broadly coeval with the Columbia supercontinent build up, marked by the formation of world-class Ni-Cu deposits globally, it is proposed that the Ammassalik Intrusive Complex and the south-east Greenland region were in direct continuity of the mineralised Kotalahti belt (Svecofennian Province, Finland). Together with the recent discovery of Ni-Cu mineralisation in the Ammassalik Intrusive Complex, our study further supports the high prospectivity of south-east Greenland for orthomagmatic Ni-Cu deposits.

The West Greenland continental margin has been the subject of petroleum exploration by companies and research projects since the 1970s and many data have been acquired since. Licensing rounds issued by the Greenland authorities in 2002 and 2004 offshore southern West Greenland resulted in company licenses which led to data acquisition and three exploration wells. The extensive new data form a basis for updated mapping by means of data, new analyses of the subsurface geology and improved understanding of the stratigraphy and the geological development. The Geological Survey of Denmark and Greenland (GEUS) has recently completed a comprehensive mapping project of the subsurface in an area covering 116 000 km2 offshore southern West Greenland (Fig. 1). The results include maps displaying large structural highs and faults, Cretaceous sedimentary basins and volcanic areas, illustrated by cross-sections through the area. A new seismic stratigraphy with eight mega-units from the seabed to the basement was also defined. In addition, studies from wells of biostratigraphy and petrology were carried out that provide important new information. The new data include extensive 2D seismic data and eight wells including the three exploration wells AT2-1, AT7-1 and LF7-1 drilled in 2011 by Cairn Energy (Fig. 1). Key results of the work are summarised below.

Terrane accretion in the Nuuk region in the North Atlantic Craton in Greenland shares many characteristics with modern accretionary systems, including compelling evidence for regional-scale crustal thickening as well as the presence of terranes with distinct PT histories. The Nuuk region is therefore interpreted to contain a paired metamorphic belt, formed during the convergence and collision of the Faeringehavn terrane with the structurally overlying Tre Brodre and Tasiusarsuaq terranes between ca. 2720 and 2700 Ma. The Tasiusarsuaq terrane exposes a deeply eroded section through the upper plate of the orogen, dominated by granulite and amphibolite facies TTG gneiss and mafic supracrustal rocks. In its centre, nappes of deep crustal granulites (800–950 °C, 9–12 kbar) were exhumed to mid crustal levels at conditions of 660–810 °C and 7–8 kbar. Nappe emplacement occurred during NW-vergent thrusting and the steepening of fabrics between ca. 2760 and 2720 Ma, coeval with the underthrusting of the Faeringehavn beneath the Tasiusarsuaq terrane. Pervasive ductile fabrics and the presence of abundant leucosomes and syntectonic pegmatites suggest that mid-crustal nappe emplacement was melt-assisted. The final collisional event between 2720 and 2700 Ma was associated with a switch from convergence-related thrusting and non-coaxial shearing to co-axial shortening, leading to the development of the subvertical, linear Graedefjord gneiss belt that records the vertical extrusion of material. The Graedefjord gneiss belt is the most prominent of these late-stage ca. 2700 Ma extrusion zones that was previously interpreted to be located at or near a boundary between two tectonic blocks.

Volcanic sequences on ocean islands record the temporal evolution of underlying magmatic systems and provide insights into how silicic crust is produced away from convergent margins. Assimilation has often been suspected to contribute, but the detection of such a process and its evolving maturity during migration across a mantle plume is less well documented. Here we present new major and trace element and Sr-Nd-Pb-U-Th-Ra-Pa isotope data that facilitate comparison of basanite to phonolite evolution on Tenerife (Canary Islands) with that shown by published data from La Palma. On both islands, (230Th/238U) ratios decrease with differentiation from parental magmas with 230Th excess toward different, silicic contaminants in secular equilibrium. On La Palma, this is inferred to reflect assimilation of small amounts of mafic wall rock. On Tenerife, both (230Th/238U) and (231Pa/235U) ratios decrease toward 1 with increasing differentiation, and this is accompanied by a subtle increase in Pb isotope ratios. At the same time, (226Ra/230Th) ratios change from 1 (a hitherto unreported magnitude). The Tenerife assimilant is thus constrained to be a partial melt of syenite formed in equilibrium with residual feldspar. The differences reflect a primarily deeper, more mafic magma system beneath La Palma during its late shield-building stage, whereas recent magmatic evolution at Tenerife occurs primarily at lower temperatures in small, shallower magma systems formed during its post–basaltic shield stage. Differentiation takes millennia or less.

Several recent studies have highlighted the potential of combined 238U–230Th and 235U–231Pa systematics to constrain upwelling rates and the role of recycled mafic lithologies in mantle plume-derived basalts. Accordingly, we present measurements of the 231Pa concentrations from 26 mafic volcanic rocks from Iceland, including off-axis basalts from the Snaefellsnes Peninsula, to complement previously published 238U–230Th–226Ra data. 231Pa concentrations vary from 27 to 624 fg/g and (231Pa/235U) ratios from 1.12 to 2.11 with the exception of one anomalous sample from the Southeast Rift which has a 231Pa deficit with (231Pa/235U) = 0.86. An important new result is that basalts from the Southeast Rift and the Snaefellsnes Peninsula define a trend at relatively low (231Pa/235U) for a given (230Th/238U) ratio. Many of the remaining samples fall in or around the global field for ocean island basalts but those from the Mid-Iceland Belt and the Southwest Rift/Reykjanes Peninsula extend to higher (231Pa/235U) ratios at a given (230Th/238U), similar to mid-ocean ridge basalts. In principle, these lavas could result from melting of peridotite at lower pressures. However, there is no reason to suspect that the Mid-Iceland Belt and the Southwest Rift lavas reflect shallower melting than elsewhere in Iceland. In our preferred model, these lavas reflect melting of garnet peridotite whereas those from the Southeast Rift and the Snaefellsnes Peninsula contain a significant contribution (up to 20%) of melt from garnet pyroxenite. This is consistent with incompatible trace element and radiogenic isotope evidence for recycled oceanic crust in these lavas. There is increasing agreement that the displacement of ocean island basalts to lower (231Pa/235U) ratios at a given (230Th/238U), compared to mid-ocean ridge basalts, reflects the role of recycled mafic lithologies such as garnet pyroxenite as well as higher average pressures of melting. It now seems likely that this interpretation may have more general application to plume-affected MORB that exhibit the same U-series disequilibria characteristics.

We present geological observations and geochemical data for the youngest volcanic features on the slow-spreading Mid-Atlantic Ridge at 8°48'S that shows seismic evidence for a thickened crust and excess magma formation. Young lava flows with high sonar reflectivity cover about 14 km2 in the axial rift and were probably erupted from two axial volcanic ridges each of about 3 km in length. Three different lava units occur along an about 11 km long portion of the ridge, and lavas from the northern axial volcanic ridge differ from those of the southern axial volcanic ridge and surrounding lava flows. Basalts from the axial rift flanks and from a pillow mound within the young flows are more incompatible element depleted than those from the young volcanic field. Lavas from this volcanic area have 226Ra-230Th disequilibria model ages of 1,000 and 4,000 years whereas the older lavas from the rift flank and the pillow mound, but also some of the lava field, are older than 8,000 years. Glasses from the northern and southern ends of the southern lava unit indicate up to 100°C cooler magma temperatures than in the center and increased assimilation of hydrothermally altered material. The compositional heterogeneity on a scale of 3 km suggests small magma batches rising vertically from the mantle to the surface without significant lateral flow and mixing. The observations on the 8°48'S lava field support the model of low frequency eruptions from single ascending magma batches that has been developed for slow-spreading ridges.

U-series disequilibria have provided important constraints on the physical processes of partial melting that produce basaltic magma beneath mid-ocean ridges. Here we present the first 238U–230Th–226Ra isotope data for a suite of 83 basalts sampled between 5°S and 11°S along the South Mid-Atlantic Ridge. This section of the ridge can be divided into 5 segments (A0–A4) and the depths to the ridge axis span much of the global range, varying from 1429 to 4514 m. Previous work has also demonstrated that strong trace element and radiogenic isotope heterogeneity existed in the source regions of these basalts. Accordingly, this area provides an ideal location in which to investigate the effects of both inferred melt column length and recycled materials. 226Ra–230Th disequilibria indicate that the majority of the basalts are less than a few millennia old such that their 230Th values do not require any age correction. The U–Th isotope data span a significant range from secular equilibrium up to 32% 230Th excess, also similar to the global range, and vary from segment to segment. However, the (230Th/238U) ratios are not negatively correlated with axial depth and the samples with the largest 230Th excesses come from the deepest ridge segment (A1). Two sub-parallel and positively sloped arrays (for segments A0–2 and A3 and A4) between (230Th/238U) and Th/U ratios can be modelled in various ways as mixing between melts from peridotite and recycled mafic lithologies. Despite abundant evidence for source heterogeneity, there is no simple correlation between (230Th/238U) and radiogenic isotope ratios suggesting that at least some of the trace element and radiogenic isotope variability may have been imparted to the source regions >350 kyr prior to partial melting to produce the basalts. In our preferred model, the two (230Th/238U) versus Th/U arrays can be explained by mixing of melts from one or more recycled mafic lithologies with melts derived from chemically heterogeneous peridotite source regions.

New data from the Proterozoic basement and scattered Palaeozoic sediments in the Ringkøbing–Fyn High including zircon U–Pb geochronometry, heavy mineral compositions and whole rock geochemistry is presented here to provide a frame of reference for detrital provenance studies. The Ringkøbing–Fyn High is a WNW–ESE trending structural high including subcropping basement rocks, and the results indicate that it is a southerly extension of the Fennoscandian Shield. The zircon data show matching age distribution patterns in crystalline basement rocks obtained from two drill sites, the Glamsbjerg-1 and Grindsted-1 wells. They both record a characteristic Telemarkian accretionary event at 1.51 and 1.48 Ga and a Sveconorwegian metamorphic overprinting at 1.08 Ga. Furthermore, the dominant age intervals in the Glamsbjerg High (1.55–1.48 Ga) and the Grindsted High (1.51–1.44 Ga) suggest that rocks of the Gothian orogeny (that ended at 1.52 Ga) are only present in the eastern part of the Ringkøbing–Fyn High. Thus, the buried basement in central Denmark may be youngest towards the west, which is consistent with the general westward age progression trend in the Sveconorwegian Orogen. The basement breccia in the Arnum-1 well on the southern flank of the Ringkøbing–Fyn High has zircon ages (c. 1.54–1.53 Ga) that resemble those of gneiss in the Glamsbjerg High. The conglomeratic sandstone in the Ringe-1 well on the Glamsbjerg High has a dual age distribution as the matrix has late Palaeoproterozoic to early Mesoproterozoic ages, whereas the granitic clasts have a distinct middle Neoproterozoic age (c. 0.76 Ga) that may indicate an Avalonian source. The quartzite in the Slagelse-1 well on the northern flank of the Ringkøbing–Fyn High has a broad age span with late Palaeoproterozoic to late Mesoproterozoic zircon ages. Supplementary material: Detailed documentation of U/Pb analytical procedures, results and analysed zircon spots are available below.

Over the past 4.5 billion years (Ga) the Earth has undergone a secular change in tectonic regimes. Archaean tectonics was a markedly different process than observed today, involving a hotter planet with a thinner lithosphere, thicker oceanic crust, and smaller proto-continents. The transition to a more modern type of plate tectonics required the initiation of subduction of oceanic lithosphere. The onset of subduction in the Meso- to Neoarchaean (ca. 3.2 to 2.5 Ga) is shown by geochemical signatures indicative of crustal recycling and petrologic evidence of tectonically driven thickening of the continental crust. This transition is recorded in the rock record of the North Atlantic Craton, which preserves a history of crust formation through to terrane accretion and the collision of crustal blocks. Here we present petrologic, chronologic, chemical and geophysical evidence from the Majorqaq Belt, a ~ 100 km wide east–west striking zone, defined by a high gravity anomaly, voluminous metasedimentary units, olivine-rich ultramafic enclaves, and anomalously high K2O, Ni, and MgO stream-sediment chemistry. We use a 207Pb/206Pb zircon LA-ICP-MS age of 2.557 ± 0.005 Ga (2σ) from an upper-amphibolite facies metapelitic migmatite to propose Neoarchaean suturing of the Maniitsoq crustal block to the North Atlantic Craton. Ti-in-zircon temperatures and zircon–garnet REE distribution coefficients constrain zircon crystallisation to 815–729 °C, which is at, or near the solidus during leucosome crystallisation. Our data reveal a clockwise P–T path with a high-pressure assemblage of garnet–kyanite–plagioclase–rutile–biotite–quartz which grew at an apparent thermal gradient of ~ 660 °C/GPa during subduction-driven compression of surface-derived sediments to a minimum-peak-pressure of ~ 1.15 GPa at ~ 760 °C followed by the growth of gedrite–cordierite–sillimanite–ilmenite at minimum-peak-temperature conditions of ~ 810 °C at ~ 0.94 GPa. We propose the evolution of the Majorqaq belt formed from the collision of the Maniitsoq block with the proto-North Atlantic Craton following closure of an oceanic basin with south-dipping subduction, providing a potential source of volatiles for the ca. 2.55 Ga Qorqut granite complex situated ~ 150 km further southwards. These conditions necessitate orogenesis and indicate that plate tectonics was active by ca. 2.56 Ga during the final assembly of the North Atlantic Craton.

The Tasiusarsuaq terrane represents an Archean continental upper plate that was assembled and thickened by a series of igneous and tectonic events between c. 2970 and 2700 Ma. A review of the available data combined with thermodynamic modelling and U-Pb titanite and zircon dating allows detailed insight into the processes of crustal growth during the Meso- and Neoarchean. Following the intrusion of the c. 2970–2950 Ma Fiskenaesset complex in an arc-type setting, the southern part of the Tasiusarsuaq terrane was intruded by an early TTG generation between c. 2920 and 2880 Ma to create the Bjornesund block. Subsequently, from c. 2880 to 2830 Ma, TTG magmatism migrated to the Sermilik block in the central and northern part of the Tasiusarsuaq terrane, whereas monzogranite magmatism occurred in the south. We interpret this pattern of igneous activity to be a result of accretion of the Sermilik block to the Bjornesund block and the formation of a new plate interface behind the accreted terrane. Fabrics related to this early accretionary stage are preserved in the prograde amphibolite facies (580–630 °C; 4–6 kbar) lithologies from the Bjornesund block that have U-Pb titanite ages of c. 2820 Ma. Afterwards, ongoing southwards-directed subduction led to substantial thickening, manifested by high-pressure granulite facies metamorphism (870 °C, 9.0–9.2 kbar) and the intrusion of the c. 2805–2785 Ma Ilivertalik intrusive complex. Convergence and underthrusting by Eoarchean continental crust (Faeringehavn terrane) at c. 2760 to 2720 Ma led to the extrusion of hot, ductile granulite nappes into the mid crust. At this time, large sections of the Tasiusarsuaq terrane underwent extensive amphibolite facies reworking, at conditions ranging from ~725 °C to 5.0–5.5 kbar in the central parts to 6.5–7 kbar and 700 °C in the north. These conditions lasted until the final collision of the terranes at 2720–2700 Ma. Collectively, our model describes the evolution of an Archean large hot orogen that is characterized by continuous convergence rather than intermittent subduction. In many respects, it resembles more recent large hot orogens such as the Grenville, including the accretion of terranes prior to collisional orogeny.

The provenance of the Danish Miocene sands has been deduced from zircon ages and heavy minerals integrated with an existing detailed stratigraphic model of the fluvio-deltaic sand successions that prograded into the North Sea Basin in large delta lobes. The stratigraphic model allowed interpretation of the depositional ages of the samples, which were used to reconstruct the outline of the river pathway system for each time slice. The contribution from individual source areas to a specific position in the Miocene succession was controlled by depositional environment and tectonics. The major rivers each flowed separately to the sea, so mixing between supplies from different sources could only have occurred in the marine environment, where a wave-induced shallow current and a tidally-induced deeper current transported the sediment along the shore in opposite directions. Changes in grain size parameters and mineral assemblages across Denmark showed that the overall sediment transport direction for southern Scandinavia was to the south-west. They also showed that the sediments are immature with regards to transport length, so the high maturity of the heavy mineral suite suggests that kaolinisation has been a widespread phenomenon in the source areas. Rapid uplift and/or extensive denudation may have been revealed by an apparent eastward migration of the dominating provenance area, which was observed in the zircon age data for two consecutive time spans; the early Miocene first tectonic pulse and the intra-early Miocene tectonic phase. Comparison of the zircon age distributions with age markers defined using the published ages of the Scandinavian basement terranes shows that most of the sand came from several Sveconorwegian source areas in south-eastern Norway and south-western Sweden. Contributions from the Svecofennian Orogen, the Caledonian belt and the Oslo Rift are also found and the presence of Archaean zircon grains suggests that contributions to the river systems came from as far away as Finland.

We present new whole-rock major, trace and platinum-group element data, as well as Sm–Nd and Lu–Hfisotope data for meta-volcanic rocks from the Mesoarchaean Graedefjord Supracrustal Belt (GSB), locatedwithin the Tasiusarsuaq terrane, southern West Greenland. We also present new in situ zircon U–Pbisotope data (by LA-ICP-MS) for associated felsic rocks. This region has experienced amphibolite to lowergranulite facies metamorphism, causing re-equilibration of most mineral phases (including zircon).An intrusive tonalite sheet with a zircon U–Pb age of 2888 ±6.8 Ma, yields a minimum age for the GSB.The Sm–Nd and Lu–Hf isotope data do not provide meaningful isochron ages, but the isotope compositionsof the mafic rocks are consistent with the ca. 2970 Ma regional volcanic event, which is documented in pre-vious studies of the Tasiusarsuaq terrane. The major and trace element data suggest a significant crustalcontribution in the petrogenesis of andesitic volcanic rocks in the GSB. The trace element variation of theseandesitic leucoamphibolites cannot be explained by bulk assimilation–fractional-crystallisation (AFC)processes involving local basement. Rather, the observed patterns require binary mixing between basalticand felsic end-member magmas with between 50% and 80% contributions from the latter (depending onthe assumed felsic composition). Hf-isotope constraints point to contamination with pre-existing conti-nental crust with an age of ca. 3250 Ma. Basement gneisses of this age were previously described at twolocalities in the Tasiusarsuaq terrane, which supports the mixing hypothesis. Thus the felsic end-memberlikely represents melts derived from the local basement.Ultramafic rocks (18.35–22.80 wt.% MgO) in GSB have platinum-group element (PGE) patterns that aresimilar to magmas derived from high-degree melting of mantle, but they have relatively enriched traceelement patterns. We propose that the ultramafic rocks represent arc-related picrites or alternativelywere derived by melting of metasomatised sub-continental lithospheric mantle.Overall these new geochemical data from the Mesoarchaean Graedefjord Supracrustal Belt and thepetrogenetic mixing model in particular, are similar to observations from modern continental subductionzone environments, which also require large degrees of mixing with felsic basement melts. Therefore,we propose that the metavolcanic rocks formed in a modern-style subduction zone geodynamic setting,which due to the hotter Archaean mantle conditions allowed for substantial amounts of partial meltingand magma mixing, rather than assimilating pre-existing continental crust. (Less)

With the recent discovery of the giant, deeply eroded, 3 Ga Maniitsoq impact structure in southern West Greenland (Garde 2010), an enigmatic, c. 75 by 15 km large, curvilinear belt of undeformed norite intrusions with Ni-Cu mineralisation was re-interpreted as representing crustally contaminated melts derived from the mantle in the wake of the impact (Fig. 1; Garde et al. 2012). The norite belt (Nielsen 1976; Secher 1983) was discovered in the early 1960s by the mining and exploration company Kryolitselskabet Øresund A/S, and more than one hundred shallow exploration holes were drilled by the company in the period 1965–1971. The mineralisation has subsequently been investigated by Cominco Ltd., Falconbridge Ltd. and NunaMinerals A/S. In 2011, the re-interpretation of the norite belt, and recent availability of improved airborne geophysical exploration tools, prompted the Canadian company North American Nickel Inc. (NAN) to resume exploration.

The Tartoq Group, located in SW Greenland, consists of supracrustal rocks of mainly tholeiitic basaltic composition, including pillow lavas, sills/dykes and gabbros, as well as ultramafic rocks. Metamorphic grade ranges from greenschist fades to granulite facies. The Tartoq Group crops out as a series of blocks and slivers that are imbricated with originally intrusive Mesoarchaean TTG orthogneisses. The supracrustal rocks form part of a SE vergent fold and thrust belt consistent with the imbrication of TTG gneisses and supracrustal rocks along a convergent margin. LA-ICP-MS U-Pb zircon dating of an intrusive TTG sheet yields a minimum age of 2986 +/- 4 Ma for the Tartoq Group. This age is consistent with MC-ICP-MS Lu-Hf and Sm-Nd isotopic whole-rock data for mafic samples from different blocks of the Tartoq Group, which yield errorchron ages of 3189 +/- 65 Ma and 3068 +/- 220 Ma, respectively. The mafic supracrustal rocks of the Tartoq Group have chondrite-normalized REE patterns with La-CN/Sm-CN of 0.67-1.96 and rather flat primitive mantle-normalized multi-element patterns, except for scattered LILE contents, and generally negative Nb-anomalies with Nb/Nb* of 0.26-1.31. Th/Yb varies between 0.06 and 0.47 and Nb/Yb between 0.45 and 4.4 indicative of an arc affinity when compared to rocks from modern settings. The similar geochemistry of the different lithological units, together with their coeval formation, as evident from trace element geochemical trends, supports a co-magmatic origin for the rock assemblage and their formation as imbricated relics of oceanic crust Accordingly, we propose that the Tartoq Group represents remnants of Mesoarchaean oceanic crust, which formed in a suprasubduction zone geodynamic environment (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. (Less)

We present bulk-rock geochemical and U-Pb zircon age constraints on a ∼580 m thick sequence of Mesoarchean metavolcanic rocks from SW Greenland. The rocks were arguably deformed into a tight synform and metamorphosed under amphibolite facies conditions, where relict volcanic structures testify to their igneous origins. The sequence includes picrites, tholeiitic basalts and calc-alkaline andesitic to dacitic schists, interbedded with syn-volcanic mafic and felsic feeder intrusions. Amongst late aplitic intrusions, four conform to a minimum U-Pb age for the entire succession of 2929 ± 5 Ma, which is slightly older than a protolith intrusion age of 2902 ± 4 Ma for the regional grey orthogneisses. The metavolcanic section describes an overall inward increase in SiO2 and Al2O3, coupled with a decrease in MgO, total iron (FeOT) and CaO, across a major discontinuity that separates: (1) A tholeiitic suite with high FeOT, low Al2O3/TiO2 coupled with flat REE-patterns, interpreted as mantle-derived signatures; and (2) a more evolved calc-alkaline suite with relatively low FeOT, high Al2O3/TiO2 and steep REE-patterns, yet relatively high Mg, Ni and Cr, which resemble adakites (or high-Mg andesites). A systematic greater negative Nb-anomaly (Nb/Nb∗) and LREE-enrichment (La/Sm)N up through the metavolcanic section is consistent with an increasing subduction zone signature, and thereby a likely arc setting for all rocks. The overall field relationships argue strongly for the deposition of (i) enriched tholeiites (including basal picrites that could be either lavas or cumulates), (ii) more depleted tholeiites that also intruded the enriched tholeiites, before (iii) an abrupt transition into distinctly different calc-alkaline and adakite-like volcanics that evolved from andesites to dacites. Thus, the Nigerlikasik section records tholeiitic magmatism, succeeded by calc-alkaline adakitic magmatism during the evolution of an Archean island arc. In our single arc model, we discuss the possibilities of the early tholeiitic suite being formed through progressively more hydrous melting of a juvenile mantle wedge and subsequent low-P fractionation of plagioclase-bearing cumulates; whereas the adakitic suite more likely formed through high-P hornblende + garnet fractionation of an enigmatic parent that was possibly sourced from a crustally contaminated mantle.

The Mesoarchean Târtoq greenstone belt, southern West Greenland, consists of tectonically imbricated slices of metamorphosed basalt, gabbro, peridotite, and sedimentary rocks and is intruded by felsic rocks (now mylonites) with well-preserved duplex structures, representing a relict accretionary prism. The Târtoq greenstone belt is a remnant of a supra-subduction zone ophiolite that originated as back-arc basin oceanic crust. Following the initiation of intra-oceanic subduction, the back-arc oceanic crust accreted to the overriding plate, forming an accretionary prism. The felsic mylonites are compositionally akin to Archean tonalite–trondhjemite–granodiorite suites. Field observations, along with geochemical and zircon U–Pb age data, indicate that the protoliths of the felsic mylonites were derived from partial melting of back-arc basalts in the accretionary prism and emplaced along thrust faults between 3012 ± 4 and 2993 ± 6 Ma. It is proposed that the partial melting of the basalts likely occurred in response to ridge subduction. The Upper Cretaceous turbiditic greywackes of the Chugach – Prince William accretionary complex in southern Alaska are intruded by Paleogene felsic dykes. These felsic dykes appear to have been derived from partial melting of subducted and (or) accreted oceanic crust during slab window magmatism. Archean granitoid–greenstone terrains share many geological characteristics of Phanerozoic subduction–accretion complexes such as the Alaskan and Altaid subduction–accretion complexes, consistent with the operation of uniformitarian geological processes in the Archean. The Archean Earth might have been dominated by numerous smaller plates and greater ridge length than today that would have resulted in more frequent ridge-accretionary prism interactions and larger volumes of tonalite–trondhjemite–granodiorite generation in subduction–accretion complexes.

The Neoarchaean Tasiusarsuaq terrane is situated in the North Atlantic craton approximately 50 km south of the capital Nuuk, and extends for 400 km to the south to the Frederikshab Isblink. It mainly consists of granulite facies and commonly migmatitic grey gneiss, narrow supracrustal belts and volumetrically minor late-tectonic granites. The supracrustal belts have a lithological association of picritic, komatiitic and basaltic rocks together with ultramafic dykes and sills, with geochemical characteristics typical of an arc-type setting. The grey gneiss is tonalitic to granodioritic in composition, and was emplaced between ca. 2.92 and 2.82 Ga into crust that is probably represented by the supracrustal rocks. The gneiss, in turn, was intruded by granite dykes at ca. 2830 Ma–2800 Ma, and by tonalitic dykes at ca. 2770 Ma. Both probably are the result of partial melting of the thickened crust. Late-tectonic tonalitic pegmatites intruded at 2673 ± 5 Ma at amphibolite-facies conditions. The structural evolution from the granulite-facies peak to amphibolite-facies levels is characterised by three deformation stages. D 1 fabrics form only relics in low-strain domains of the dominant NW-vergent D 2 fold-and-thrust structures. Syn-tectonic granite and tonalitic dykes date the D 2 thrusting to have occurred between ca. 2800 Ma and 2700 Ma, contemporaneously with near-isobaric cooling. The subsequent D 3 deformation, still in the amphibolite-facies, is marked by E-W to NE-SW shortening, forming upright folds and near-vertical strike slip shear zones at ca. 2670–2630 Ma. The final exhumation of the terrane likely occurred first during the Proterozoic, and was probably mainly driven by erosion. The granulites of the Tasiusarsuaq terrane formed at an orogenic margin in a compressive regime. A first orogenic cycle (2.8–2.7 Ga), here termed the Tasiusarsuaq orogeny, resulted in thrusting of the terrane on a rigid foreland represented by the Faeringehavn terrane in the north. The Tasiusarsuaq granulites were further affected by a second Archaean orogeny (2.67–2.58 Ga), the Kapisilik orogeny, which caused transcurrent deformation and only insignificant exhumation to mid-crustal levels. Significantly, there is no evidence that the exhumation of the granulite facies Tasiusarsuaq terrane was driven by Archaean extension. In contrast to many modern situations, where granulites are exhumed by extension related to magmatic underplating and delamination of the lithosphere, the thickened Tasiusarsuaq terrane was possibly stabilised due to the low density differences and equilibration by melt extrusion during granulite-facies metamorphism. Stabilisation of the crust and lack of lithospheric delamination is, therefore, regarded as being more common in Archaean orogens compared to younger orogens.

We present new geochemical data for three Mesoarchaean supracrustal belts (Ravns Storo, Bjornesund and Perserajoorsuaq) situated north of Frederikshab Isblink in southern West Greenland for which we propose the collective name 'The Ikkattup Nunaa Supracrustal Association'. They comprise mainly amphibolites of tholeiitic basalt composition and leucoamphibolites of calc-alkaline andesite composition. Both lithological units are cut by aplite sheets of tonalite-trondhjemite-granodiorite (TTG) composition with U-Pb zircon ages of c. 2900 Ma. Lu-Hf and Sm-Nd isochrons based on whole rock amphibolite and leucoamphibolite samples yield ages of 2990 +/- 41 Ma and 3020 +/- 78 Ma, respectively, which are within error of the age of the Fiskenaesset Complex situated 5-25 km to the north. Leucoamphibolites from the three supracrustal belts show apparent geochemical mixing trends between tholeiitic amphibolites and TTG gneisses, as the end-members. By assimilation-fractional-crystallisation (AFC) modelling we can show that one group of leucoamphibolites can indeed be explained by contamination of the parental melts by a TTG-like end-member and another group of high P2O5, La and Nb leucoamphibolites can be explained by contamination involving a hypothetical low-silica adakite end-member. However, the leucoamphibolites are juvenile with epsilon Nd-(2970 Ma) from +2.1 to +3.5 and epsilon Hf-(2970 (Ma)) of +3.5 to +4.3. Thus, the mafic source of the felsic contaminant melts must have been derived from a depleted mantle source more or less at the same time (

We report whole-rock elemental and Sm-Nd isotope geochemical data from mafic-ultramafic supracrustal rocks from the Nunatak 1390 area in southern West Greenland. Additionally, we report the metamorphic temperature history for these rocks as derived from tourmaline thermometry on a tourmalinite inlier, as well as in situ U-Pb, Hf and O isotopic data from zircons extracted from tonalite-trondhjemite-granodiorite (TTG) gneisses that intruded the mafic-ultramafic sequence. The supracrustal rocks from the Nunatak 1390 area have a minimum age of c. 2900 Ma defined by U-Pb zircon ages of cross-cutting aplite sheets of TTG composition. The supracrustal sequence comprises mafic rocks with pillow structures and ultramafic rocks with no evidence of their protolith. They all have amphibolite-facies mineral assemblages and a peak metamorphic temperature of approximately 550 degrees C. The mafic sequence has relatively flat trace element patterns (La-N/Sm-N of 0.70-2.4) and mostly negative Nb-anomalies (Nb/Nb* of 0.30-1.0) and resembles modern island arc tholeiites. The mafic sequence can be divided into a high- and low-Ti group, where the former group has lower MgO, and significantly higher contents of incompatible elements such as TiO2, P2O5, Zr, Nb and Th. The ultramafic rocks have major and trace element compositions similar to Ti-enriched/Karasjok-type komatiites described in the literature. However, there are no textural indications that the ultramafic rocks from Nunatak 1390 are komatiites sensu stricto. The low-Ti group of the mafic sequence appears to have been derived from a N-MORB source, whereas the high-Ti group and the ultramafic rocks appear to have been derived from a mantle source that is more enriched than the N-MORB source. However, there is no difference in the initial epsilon Nd of the mafic and ultramafic rocks. Additionally, assimilation-fractional-crystallisation (AFC) modelling is consistent with this enrichment being caused by introduction of juvenile low-silica adalcite (slab-melt) into the mantle source region. Accordingly, we propose that the mafic and ultramafic rocks were derived from a similar type of mantle source, but that the ultramafic rocks were derived from a previously depleted mantle source that was refertilised by slab melts in a subduction zone setting. The high MgO contents of the ultramafic rocks could thus reflect a second stage of partial melting of a refractory mantle in a process similar to that which is suggested for the formation of modern boninites. We propose that the mafic-ultramafic sequence represents an island arc that evolved initially as a juvenile complex (c. 3000 Ma). However, inherited zircon grains in aplites and Hf isotope data recorded by the second intrusive TTG phase (c. 2850-2870 Ma), show that mixing with older pre-existing crust occurred during this event. Because the regional crust is dominated by TTGs of this younger age, our data suggests that it likely formed by accretion and melting of arcs of different ages and/or contamination of juvenile arcs by pre-existing continental crust rather than entirely by juvenile arc differentiation or melting. Our data thus supports melting of thickened mafic crust in an accretionary setting, rather than direct slab melting, as a mechanism for Archaean crust formation. (C) 2012 Elsevier B.V. All rights reserved.

The Mid-Atlantic Ridge between the Ascension and Bode Verde Fracture Zones exhibits anomalous crustal thickness and geochemical compositions, which could reflect the presence of either small, enriched heterogeneities in the upper mantle or a weak, diffuse mantle plume. We report new trace element (106 samples) and Sr, Nd and Pb (double spike) isotope data from 72 ridge axis samples and 9 off-axis seamount samples between 5 and 11°S, aswell asU–Th–Ra disequilibria data for the seamounts. The U-series data constrain the age of one sample from Seamount D, furthest (120 km) east of the shallowest part of the ridge, to be b10,000 yrs old and the samples from the other three seamounts closer to the ridge to be younger than 240,000 yrs. As can be most clearly discerned on a diagram of 208Pb/206Pb vs. 143Nd/144Nd, at least four distinct components are required to explain the geochemical variations along the ridge: 1) a common depleted (D-MORBlike) component in samples near and north of the Ascension Fracture Zone (4.8–7.6°S), representing the most depleted compositions sampled thus far along the mid-Atlantic ridge, 2) an enriched component upwelling beneath Ascension Island and the northern A1 ridge segment (segment numbers increase from A1 to A4 going south from the Ascension Fracture Zone), 3) an enriched component upwelling beneath the A2 ridge segment, and 4) an enriched component upwelling beneath the line of seamounts east of the A3 segment and the A3 and A4 segments. The A1 and the A3+A4 segment lavas form well-defined mixing arrays, which extend from Ascension Island and the A3 seamounts respectively to the depleted D-MORB component, interpreted to reflect local ambient mantle. We propose that the enriched components represent different packages of subducted ocean crust and/or ocean island basalt (OIB) type volcanic islands and seamounts that have either been recycled through 1) the shallowmantle, upwelling passively beneath the ridge systemor 2) the deep mantle via an actively upwelling heterogeneous mantle plume that interacts with the ridge system.

The Fiskenæsset complex in southern West Greenland is part of the North Atlantic craton and is a layered intrusion consisting of gabbro, ultramafic and anorthositic rocks that was deformed during multiple episodes of folding and metamorphism (Myers 1985). We collected late-stage magmatic hornblenditic dykes and adjacent anorthosites and studied these samples integratively with several in situ techniques to determine the igneous and metamorphic history of the Fiskenæsset complex. The work presented here is part of an ongoing joint project between the Greenland Bureau of Minerals and Petroleum and the Geological Survey of Denmark and Greenland (GEUS). Here we report on new radiometric ages and mineral chemistry data from anorthosites from the North Atlantic craton in southern West Greenland (Fig. 1).

A suite of 48 samples, including both historical and prehistoric lavas and some plutonic rocks, have been analysed from the Cumbre Vieja rift, La Palma, Canary Islands. Additionally, mineral–melt partition coefficients have been measured for clinopyroxene, plagioclase, amphibole, titanite and apatite in selected rocks. The lavas range from basanite to phonolite (SiO2 = 41·2–57·5 wt % and MgO = 10–0·8 wt %) in composition and form coherent, curvilinear major and trace element arrays in variation diagrams, irrespective of eruption age. The mafic lavas have typical ocean island incompatible trace element patterns and Sr, Nd and Pb isotope compositions show little variation but have a HIMU-type character. Generation of the parental magmas is inferred to have involved ∼4% dynamic melting of a garnet lherzolite source that may have previously been metasomatized by melts derived from a recycled mafic component containing residual phlogopite. The major process of differentiation to phonotephrite involved fractional crystallization of basanitic magmas that evolved along the same liquid line of descent under similar pressure–temperature conditions. Numerical simulations using the MELTS algorithm suggest that this occurred across a temperature interval from c. 1320 to 950°C at 400 MPa and an oxygen fugacity equivalent to quartz–fayalite–magnetite (QFM), with an initial H2O content of 0·3 wt %. The later stages of differentiation (

K-feldspar is a common detrital mineral in sandstones. It contains little U or Th, and hence the common Pb isotope composition of unaltered K-feldspar is that of the source rock. Pb isotope variations in igneous and metamorphic crustal rocks define broad spatial patterns that make the Pb signature of detrital K-feldspar grains a useful provenance tool. However, it is unclear how robust this signal is, and to what extent it can be modified by weathering, transport, burial diagenesis, and/or exposure to hydrothermal fluids. This study reports an evaluation of the technique, using two granite–arkose pairs (from Helmsdale, northern Scotland, and Shap, northwest England) where the composition of the detrital grains can be compared to feldspars in the granite from which they were derived. The results of this test have informed a pilot study using the Pb isotope composition of detrital K-feldspar in Upper Carboniferous fluvial sedimentary rocks of northern England to constrain the sediment routing. Pb isotope compositions have been measured using an in situ high-resolution single-grain technique that can distinguish original composition, heterogeneity, and subsequent alteration within individual sand grains. The results for the granite–arkose pairs show that the common Pb isotope signature of the detrital K-feldspar grains matches the in situ feldspars in the source granites. The signal is independent of grain size (medium-grained sand to granules), but alteration along a paleo-regolith at Helmsdale has resulted in perturbations in Pb composition and an increase in the errors on the analyses. The application of the technique to arkosic Carboniferous (Serpukhovian–Bashkirian) Millstone Grit Group sandstones in the Pennine Basin reveals two discrete populations of K-feldspars that cannot be distinguished petrographically. Matching of the compositions to potential sourcelands to the north and northeast of the basin suggest a far-traveled (> 500 km) source of K-feldspar from the Archean–Paleoproterozoic Lewisian rocks of northwest Scotland or their extension to the north, but also an equally important proximal source emanating from the Southern Uplands Belt of Scotland in what would have been the lower part of the hinterland drainage basin of the Millstone Grit.

New thermal ionization mass spectrometry U-series disequilibrium data are presented for 24 basaltic to dacitic glasses from active spreading centres in the back-arc Lau Basin (SW Pacific), together with additional inductively coupled plasma mass spectrometry trace element analyses and Sr–Nd–Pb isotope data. Valu Fa Ridge samples, adjacent to the arc front, have high U/Th and (230Th/238U)

Earth's lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time, with some estimates suggesting that over 70% of the present continental crustal reservoir was extracted by the end of the Archaean eon. Patterns of crustal growth and reworking in rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago, suggesting that modern-day tectonic regimes operated during the formation of the earliest crustal rock record. Here we report in situ uranium-lead, hafnium and oxygen isotope data from zircons of basement rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope-time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9-3.5-Gyr-old rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens. These data suggest a transitional period 3.5-3.2 Gyr ago from an ancient (3.9-3.5 Gyr old) crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.

Young silica undersaturated rocks from the island of Fogo, in the southern Cape Verde islands, show striking correlations between rocks types, incompatible trace element (ITE) ratios and Pb-Sr-Nd isotopic compositions that provide new insights into the generation of the HIMU source. The Cape Verde islands are situated on relatively old and thick oceanic lithosphere that, unlike most hotspots world-wide, has remained relatively steady compared to the underlying mantle plume for a considerable period of time. The magmatism has resulted in highly silica undersaturated rocks, including some of the rare cases, globally, of oceanic carbonatites. The lavas studied are mainly primitive (MgO = 5-12%) nephelinites and tephrites, with less silica undersaturation characterising the historic volcanism. ITE ratios range widely, with Zr/Hf = 37-60, Nb/U = 33-92 and Th/U = 2.7-4.2, whereas Nd and Sr isotopic compositions show a restricted range with 143Nd/144Nd = 0.51272-0.51282 and 87Sr/S68r = 0,7034-0.7038. Pb isotope compositions are within the typical range of 'young HIMU' OIB: 2~176 = 18 .8 -19 .4 , 2~176 = 15 .52 -15 .59 , 2~176 = 38.7-39.1. The isotopic data agree well with previously published results (Gerlach et al., 1988), albeit with tighter arrays. Measured oxidation states of iron vary by a factor of two, and show good correlations with rock types and degrees of silica undersaturation, but also with isotopic and ITE ratios. The Fogo arrays are in accordance with a model of binary mixing between two dominating source components beneath the Cape Verde islands, a HIMU and a EMI-type source. The most distinctive feature of the EMI-type source is its low 143Nd/144Nd ~0.51260 and high Ba/La ratios, and it is interpreted to represent delaminated subcontinental lithosphere. The HIMU component in the Cape Verde islands is characterised by high oxygen fugacity and high ratios of U/Pb, U/Th, Zr/Hf and LREE/HFSE, features that are shared with many carbonatites. It also has high present-day 143Nd/144Nd, -0.51305 and negative A7/ 4 values (down to -5.5), the latter indicating it to be much younger than the 1.5-2 Ga source age, typically inferred for the HIMU source. There are well-defined arrays in diagrams of 2~176 versus ~t (238u/Z~ and of 2~176 versus (232Th/2~ that imply concordant ages o f -120 Ma for the two systems (Fig. 1). No such good correlations exist for the Sm-Nd and Rb-Sr systems, so mixing is restricted to the source regions. For the U-Th-Pb arrays to have preserved significant source age information, provides tight constraints on aspects of the melt generation models. The preferred model includes the derivation of the HIMU component from DMM by mantle degassing (due to the high 143Nd/144Nd in the HIMU endmember), and the best-fit model ages for the HIMU component o f -200 Ma broadly overlap the timing of Pangea break-up at this latitude. In such a model, the source region is situated within the oceanic lithosphere, and the contribution from the plume itself is limited. Geophysically, the model is consistent with (1) the considerable thickness of the Cape Verde plate (100-125 km) which would diminish the probability of any extensive melting in the plume, and (2) the fixed position of the ocean plate, that would allow for heat to be conducted from the plume into the lithosphere. The temporal variations are attributed to exhaustion of the HIMU component, possibly combined with progressive thermal erosion of the lithosphere by the rising plume, causing melting to proceed at higher levels in the oceanic lithosphere. The presented model suggests that 'young HIMU'