Baddeleyite Ages Research Articles

Paleomagnetic and geochronological studies on Paleoproterozoic diabase dykes of Karelia, East Finland—Key for testing the Superia supercraton

Paleomagnetic results are presented for two Paleoproterozoic mafic dykes in the Taivalkoski area in northern Karelia Province of the Fennoscandian shield where, based on K-Ar data, the crust has seen minimal effects of the otherwise pervasive 1.8–1.9Ga Svecofennian orogeny. Within this study a new U–Pb baddeleyite age of 2339±18Ma has been determined for one of the E–W trending dykes (dyke AD13).The paleomagnetic results show that a strong Svecofennian overprinting is pervasive in the area. Upon thermal or AF demagnetization four remanence directions were obtained. Most typical are the secondary Svecofennian remanence direction A (intermediate down to the NNW) and remanence direction B (intermediate down to the NNE). Component D (D=115.4°, I=50.5°, α95=2.6°) yielding a virtual geomagnetic pole (VGP) D (Plat=−19.5°N, Plon=263.3°, A95=3.1°) is obtained from baked rocks for dyke WD, and based on a positive baked contact test is interpreted to represent the primary magnetization dating from about 2.4Ga. Dyke AD13 carries only secondary A and B components, its unbaked host migmatites carry reversed A (AR) component, and the baked host rock carries a component D′ (D=134.5°, I=−7.3°, α95=8.8°), which yields a VGP pole D′ (Plat=−20.4°N, Plon=257.3°, A95=7.6°), possibly representing magnetization at 2.3Ga.The new paleomagnetic data from the Karelia Province compared to similar-aged paleomagnetic data from the Superior Province does not support the recently proposed Superia configuration, based upon dyke swarm trajectories.

Reply to Comment on “U–Pb baddeleyite ages and geochemistry of dolerite dykes in the Bas-Drâa inlier of the Anti-Atlas of Morocco: Newly identified 1380 Ma event in the West African Craton” by André Michard and Dominique Gasquet

Baddeleyite (ZrO2) is today recognized as the key mineral for dating the emplacement of mantle-derived igneous rocks, primarily gabbro and dolerite dykes and sills. The comment by Michard and Gasquet calls for a discussion on the significance of baddeleyite U–Pb TIMS dates of two dykes in the Bas-Drâa inlier, dated at 1381±8 and 1384±6Ma, presented in El Bahat et al. (2013-in this issue). They suggest that the U–Pb systematics were “seriously perturbed in the Bas-Drâa region” with the main argument being a lack of age matches between the ca. 1385Ma dykes in Bas-Drâa and the 2040, 1650 and 885Ma dykes in adjacent inliers to the east. To infer an older age for these dykes, as suggested by Michard and Gasquet, and explain the ca. 1385Ma TIMS results would require two separate events of perturbation, in the Mesoproterozoic and a subsequent Variscan event. However, such an alternative is improbable from basic theoretical reasoning using U–Pb isotopic systematics and our current understanding of the behaviour of baddeleyite in metamorphic systems. In addition, we present new LA-ICPMS U–Pb age data of baddeleyite from one of the dated dykes from Bas-Drâa. The new LA-ICPMS U–Pb age of 1416±7Ma is slightly but significantly older than our TIMS 1384±6Ma upper intercept age, nevertheless confirming this dyke could not correspond to any of the other mafic intrusions dated in the inliers to the east. The LA-ICPMS result also contributes to a deeper understanding on the effects of secondary processes in baddeleyite in general, and suggests a modified methodological approach for samples with complex geological histories.

Comment on “U–Pb baddeleyite ages and geochemistry of dolerite dykes in the Bas Drâa Inlier of the Anti-Atlas of Morocco: Newly identified 1380 Ma event in the West African Craton” by El Bahat et al.

Two important papers have been recently published in Lithos by the same group of authors, both dealing with the pre-Palaeozoic basic dykes that cut across the basement of the Anti-Atlas mountain belt. Kouyate et al. (2013) consider the Precambrian inliers of the central part of the belt, namely the Zenaga, Iguerda and Ait Melloul inliers, whereas El Bahat et al. (2013) consider the Bas Draa inlier in the south-westernmost part (Fig. 1). These papers report U–Pb baddeleyite (and scarce zircon) dating on samples from the numerous basic dykes crosscutting the Palaeoproterozoic basement of the Central and Western Anti-Atlas. Our comment concerns more particularly El Bahat et al.'s (2013) paper. Surprisingly, the ages obtained from the Bas Draa samples are markedly distinct from those of the Zenaga, Iguerda and Ait Melloul inliers. In the latter group, Kouyate et al. (2013) obtain one date at 2040 ± 2, three other dates around 1655 Ma (1656 ± 9, ca. 1655 and 1654 ± 16 Ma) and finally two dates at ca. 885 Ma. In contrast, El Bahat et al. (2013) obtain two dates of 1380 ± 9 and 1384 ± 6 Ma in the Bas Draa inlier, although using the same U–Pb TIMS method on structurally and geochemically equivalent material. We will discuss the possible origin of this puzzling contrast— a discussion which is overlooked in El Bahat et al. (2013) study. It is possible that the ca. 1380 Ma-old dykes did not intrude the Central Anti-Atlas inliers or were not sampled by Kouyate et al. (2013), or conversely that none of the 2040, 1655 and 885 Ma-old

Mesoproterozoic intraplate magmatic ‘barcode’ record of the Angola portion of the Congo Craton: Newly dated magmatic events at 1505 and 1110 Ma and implications for Nuna (Columbia) supercontinent reconstructions

In the Angola portion of the Congo Craton, the only Proterozoic large igneous province (LIP) dated prior to this study was the 1380–1370Ma (Kunene Intrusive Complex and related units). U–Pb TIMS ages on baddeleyite from dolerite sills and gabbro-noritic dykes, has revealed two additional Mesoproterozoic intraplate events: at ca. 1505 and ca. 1110Ma, that are each proposed to be part of the plumbing system for LIPs. The identification of these three Mesoproterozoic magmatic events (ca. 1505, 1380, and 1110Ma) represent an initial magmatic ‘barcode’ for this portion of Congo Craton (and formerly connected São Francisco Craton), which can be compared with the magmatic ‘barcode’ record of other blocks to identify former nearest neighbors in the Precambrian supercontinent Nuna (also known as Columbia).Specifically, a 1502±5Ma U–Pb TIMS baddeleyite age has been obtained for the prominent Humpata dolerite sill which is part of a wider sill province in SW Angola portion of the Congo Craton. The combined presence of both 1505Ma and 1380Ma magmatism in the Congo–São Francisco reconstructed craton is a match with similar ages published for two intraplate magmatic provinces in northern Siberia and suggests a nearest-neighbor relationship in the supercontinent Nuna in which northern Siberia is juxtaposed adjacent to the western São Francisco portion of the reconstructed São Francisco–Congo Craton.In addition, a precise U–Pb TIMS baddeleyite age of 1110±2.5Ma was obtained for a prominent NNW–NNE trending gabbro-noritic (GN) dyke swarm in southeastern Angola, but this age is currently unknown in Siberia suggesting that the breakup of Congo–São Francisco Craton from Siberia happened earlier, perhaps in association with the 1380Ma event. This 1110Ma age is however, a precise match with that of the Umkondo large igneous province (LIP) of the Kalahari Craton, and also with mafic intraplate magmatism on other blocks such as the Bundelkhand Craton (India) and the Amazonian Craton. We provisionally consider these three cratons to have been nearest neighbors to the Congo–São Francisco Craton at this time and to have shared this 1110Ma magmatic event as a LIP node. There is also an age match with the early part of the Keweenawan event (in the interior of the Laurentia); however, on previously discussed paleomagnetic grounds the Keweenawan event is likely to have been distant and unrelated (and on the other side of the Grenville orogen).

Protoliths of Paleoproterozoic calciphyres from the Irkut block (Sharyzhalgai uplift of the Siberian craton): composition and origin

The paper presents data on high-grade silicate–carbonate rocks (calciphyres) from the Irkut block (Sharyzhalgai uplift, southwestern Siberian craton). Their origin and age were determined from the rock characteristics, U–Pb dating, REE content, and Hf isotope composition of zircon. The calciphyres occur both as independent section fragments and as interbeds within Paleoproterozoic garnet-bearing and high-alumina (cordierite- and sillimanite-bearing) gneisses. They were produced by metamorphism of terrigenous-carbonate sediments. The terrigenous sediments range in maturity from arenites and wackes to argillaceous rocks; this is consistent with the reconstruction of the sedimentary protoliths of paragneisses, which are predominant in the metasedimentary rocks. The petrogeochemical features of the calciphyres, their LREE enrichment relative to “pure” carbonate rocks, and a distinct Eu anomaly were inherited from the terrigenous component of calc-silicate sediments. The Nd model age (2.4–2.7 Ga) of the calciphyres and the value THf(DM-2st) = 2.5–3.0 Ga for zircon from these rocks indicate that carbonate accumulation was accompanied by the supply of terrigenous material, which formed during the erosion of Archean and Paleoproterozoic crust. Zircon from the calciphyres is similar to metamorphic zircon in REE patterns and Th/U ratios. It might have been of detrital origin and then recrystallized during high-temperature metamorphism. Terrigenous-silicate rocks were metamorphosed at ca. 1.87 Ga. This is close to the previous age estimates for the terrigenous rocks metamorphism (1.85–1.86 Ga) and the age of baddeleyite from apocarbonate metasomatic rocks (1.86 Ga).

Avanavero mafic magmatism, a late Paleoproterozoic LIP in the Guiana Shield, Amazonian Craton: U–Pb ID-TIMS baddeleyite, geochemical and paleomagnetic evidence

The Avanavero Large Igneous Province (LIP) constitutes the most important Paleoproterozoic mafic magmatism event in the Guiana Shield, northern Amazonian Craton. It comprises voluminous dykes and sills, the latter intruded into regional sedimentary cover successions such as the Roraima Supergroup and Urupi Formation. Roughly contemporary mafic magmatism such as the Crepori Dolerite occurs in the southern part of the Amazonian Craton (Central Brazil Shield). This study reports new geochemical data for samples from the Avanavero Dolerite and the coeval Quarenta Ilhas Dolerite, as well as reassessing published information on roughly contemporaneous mafic dykes in the shield to address issues related with the tectonic significance of such an intraplate igneous event and paleogeographic reconstructions. The Avanavero magmatism is tholeiitic and is geochemically similar to E-MORB and subcontinental lithospheric mantle basalts. New U–Pb baddeleyite ages of 1795±2Ma and 1793±1Ma, respectively, from occurrences in both the Pakaraima and Urupi Blocks at the north and south portions of the Guiana Shield confirm that they belong to the Avanavero LIP. These two ages are within error of a U–Pb age of 1794±4Ma for an Avanavero dyke in Guyana. Slightly younger published U–Pb ages range from about 1780–1787Ma, including the Crepori Dolerite in Central Brazil Shield (southern Amazonian Craton). This may indicate that two pulses of magmatism could be associated with the Avanavero event. The paleomagnetic data favour a Laurentia/Baltica/Amazonia link at 1.79Ga, and this large landmass may have constituted the core of the Columbia supercontinent during Late Proterozoic times.

The Ahmeyim Great Dyke of Mauritania: A newly dated Archaean intrusion

A precise U–Pb baddeleyite age of 2733±2Ma has been obtained for the Ahmeyim Great Dyke of Mauritania that intruded into the Tasiast–Tijirit Terrane of the Reguibat Shield, NW Mauritania. This dyke is approximately 1500m wide at the sampling area and extends for more than 150km NNE/SSW. Major and trace element geochemistry of the dyke indicates that the magmas that formed this intrusive body were sub-alkaline, tholeiitic and boninitic, and the presence of a negative Nb anomaly indicates the involvement of subducted oceanic lithosphere during magma genesis, most likely an inherited signature from earlier subduction events and the Mesoarchaean collision of the Tasiast–Tijirit and Choum–Rag el Abiod Terranes. A palaeomagnetic study was also undertaken on samples collected from two different sections across the dyke. However, no within- or inter-site grouping of any palaeomagnetic directions could be identified, thus precluding any palaeographic interpretation. The Ahmeyim Great Dyke is interpreted to be part of the feeder system for a 2733Ma Large Igneous Province (LIP); tholeiitic–komatiitic greenstone belts of this age are absent in the West African Craton (WAC) but are present on many other blocks. However, additional constraints are required to reliably link the Ahmeyim Great Dyke with any other such LIP-type greenstone belts in late Archaean supercontinent reconstructions. The magmas that formed the Ahmeyim Great Dyke were boninitic; this, combined with evidence of crustal contamination, the scale of the dyke and its potential link (as a feeder) to greenstone belts of tholeiitic–komatiitic affiliation within other crustal blocks suggests that it, and cogenetic magmatic units elsewhere, may be prospective for magmatic Ni–Cu–PGE sulphide exploration.

U–Pb (ID-TIMS) baddeleyite ages and paleomagnetism of 1.79 and 1.59 Ga tholeiitic dyke swarms, and position of the Rio de la Plata Craton within the Columbia supercontinent

The Tandilia Terrane (southernmost fringe of the Rio de la Plata Craton) is an igneous and metamorphic complex produced by an accretionary orogeny (2.25–2.02Ga). Calc-alkaline acidic dykes with E–W strike and a major shear zone with similar orientation are related with the late orogeny stage, as supported by field relations. In a previous study the acid dykes gave 40Ar–39Ar ages of 2007±24Ma to 2020±24Ma. A N and NW trending tholeiitic dyke swarm (Tandil swarm) is also present in the Tandilia Terrane. One sample from the NW-trending subset previously gave a U–Pb (ID-TIMS) baddeleyite age of 1588±11Ma. New precise U–Pb (ID-TIMS) baddeleyite dating of both N- and NW-trending Tandil dykes yielded crystallization ages of 1589±3Ma, 1588±3Ma and 1588±3Ma. Significantly older tholeiitic dykes known as the Florida swarm occur in the Northern Rio de la Plata Craton, for which a U–Pb (ID-TIMS) baddeleyite age of 1790±5Ma was previously reported. Consequently intermittent rifting (1.79, 1.59Ga) took place after tectonic stabilization of the late Paleoproterozoic lithosphere (proto-Rio de la Plata Craton). The available geochemical data for the 1.59Ga Tandil dykes define low- and high-TiO2 trends, although, only the low-TiO2 subgroup is firmly dated. Both the Tandil and Florida dykes have geochemical and Nd–Sr characteristics consistent with derivation from heterogeneous mantle sources that underwent metasomatic effects.The Tandil dykes may be linked with the 1.57±0.02Ga Capivarita anorthosite which occurs to the east of the northern part of the craton. Correlatives on other crustal blocks may include those in Baltica such as bimodal rock association (including the Breven-Hällefors and Åland-Åboland diabase dykes) and in the reconstructed Gawler Craton/NW Laurentia dolerites, bimodal magmatism and IOCG deposits. Contemporary within-plate bimodal associations are also present in the SW Amazonian Craton.Paleomagnetic data for the 1790Ma Florida dykes allows three possible reconstruction scenarios for the position of Rio de la Plata Craton: i) in the southern hemisphere linked to the Pampia terrane, and the Amazonia+Rio Apa landmass as close as they are in present day; ii) in the northern hemisphere, as a nearest neighbor to the reconstructed NW Laurentia and Gawler blocks (proto-Australia); or iii) in the northern hemisphere near the boundary of Amazonia (proto-Amazonian Craton) and Baltica. Positions 2 and 3 are consistent with all three criteria: paleomagnetic poles and proximity to matching ca. 1790Ma and 1590Ma mafic magmatism. Our data are consistent with the idea that the Columbia supercontinent started major break up at 1.59Ga.

The ca. 1380 Ma Mashak igneous event of the Southern Urals

A review of the geochronology, geochemistry and distribution of the 1380Ma Mashak Large Igneous Province (LIP) of the eastern margin of the East European craton indicates a potential link to a major breakup stage of the Mesoproterozoic supercontinent Columbia (Nuna), link to a major stratigraphic boundary (Lower–Middle Riphean), and economic significance for hydrocarbons and metallogeny. Specifically, the Mashak event likely has much greater extent than previously realized. Two U–Pb baddeleyite (ID TIMS) age determinations on dolerite sills obtained from borehole (Menzelinsk–Aktanysh-183) confirm the western extent of the Mashak event into the crystalline basement of the East European Craton (1382±2Ma) and into the overlying Lower Riphean sediments (1391±2Ma), and the imprecise ages reported elsewhere indicate the possible extension into the Timan region, with an overall areal extent of more than 500,000km2 (LIP scale). It has tholeiitic compositions and is associated with breakup on the eastern margin of the craton – in addition, precise SHRIMP zircon ages of 1386±5Ma and 1386±6Ma (this paper) provide confirmation of previous approximate 1380–1383Ma zircon age determination of the same formation, and suggest an age of ca. 1.4Ga for the Lower/Middle Riphean boundary which was formerly considered to be 1350±10Ma. Contemporaneous magmatic rocks in the northeastern Greenland part of Laurentia (Zig‐Zag Dal and Midsommerso formations) and Siberia (Chieress dykes and other dolerites) together with the Mashak event are suggested to be fragments of a single huge LIP and to correspond to breakup stage of the Columbia (Nuna) supercontinent. The Mashak LIP also has some significance, at least in Volgo-Uralia, for hydrocarbons and metallogeny.

U–Pb baddeleyite ages and geochemistry of dolerite dykes in the Bas Drâa Inlier of the Anti-Atlas of Morocco: Newly identified 1380 Ma event in the West African Craton

In the Bas-Drâa Inlier (Anti-Atlas, Morocco), the Paleoproterozoic basement which is cut by the Ediacaran Taourgha granite is also crosscut by numerous dykes of a variety of trends, mostly of uncertain age. Two doleritic dykes are dated by the ID-TIMS U–Pb method on baddeleyite and yield emplacement ages of 1381±8Ma (MSWD=0.84) and 1384±6Ma (MSWD=1.4) determined for a N135°E and a N40°E trending dyke, respectively. These dates represent the first geochronological evidence of a Mesoproterozoic magmatic event in the Anti-Atlas. This magmatic event falls in the previously considered ca 1.7–1.0Ga (Mesoproterozoic) gap in geological activity in the Anti-Atlas. The poorly dated Taghdout and Taarotihate sequences could represent remnants of the ca. 1380Ma magmatism and rift-related sedimentation. The Mesoproterozoic sedimentary succession of the Atar Group in the Taoudeni basin (Mauritania) could also represent a good candidate for rift-related sedimentation but it postdates the 1380Ma magmatic event by 270Ma. The dated 1380Ma dykes are transitional to mildly alkaline basalts, not unlike some Hawaiian lavas. However, these dykes have a distinct negative Nb anomaly (a common features in many Large Igneous Provinces, LIPs), and this requires interaction with the lithosphere. This interaction may have occurred at the level of the lithospheric mantle or the crust. These newly dated 1380Ma dykes may converge to the north, speculatively suggesting a magmatic center (associated with a 1380Ma mantle plume?) along the northern margin, and possibly linked to rifting and possible breakup on that margin, and also to a regional uplift that largely removed the evidence of a 1380Ma cover sequence.Contemporaneous 1380–1390Ma magmatism is reported elsewhere on other crustal blocks, and that in northeastern Laurentia (northern Greenland), northern Siberia (Anabar shield), and Baltica (southern Urals) can be reconstructed with that of the Bas Drâa Inlier (Anti Atlas region of the West African Craton, WAC) into a single Large Igneous Province (LIP) extending over an area of >1millionkm2, and associated with the final fragmentation of the Columbia (Nuna) supercontinent.

The Hlagothi Complex: The identification of fragments from a Mesoarchaean large igneous province on the Kaapvaal Craton

In this paper, we present geochronological, geochemical and palaeomagnetic results from the Hlagothi Complex and a NW-trending dolerite dyke swarm on the southeastern region of the Kaapvaal Craton in northern KwaZulu-Natal, South Africa. The Hlagothi Complex consists of layered sills of meta-peridotite, pyroxenite and gabbro intruding into the Pongola Supergroup. U–Pb baddeleyite ages on the Hlagothi Complex and a NW-trending dyke of 2866±2Ma and 2874±2Ma, respectively, reveal a ca. 2.87Ga magmatic event on the southeastern Kaapvaal Craton. The geochemical signature of the Hlagothi Complex recognises two discrete groupings, with a magmatic source that is chemically distinct from those of the older rift-related Nsuze and Dominion groups. Additional units on the Kaapvaal Craton can be linked with this new ‘Hlagothi’ event based on spatial and temporal association, and geochemistry: 1) the Thole Complex, 2) parts of the Usushwana Complex, and 3) flood basalts within the Mozaan Group and Central Rand Group. The association between all these units suggests a previously unrecognised large igneous province in the southeastern Kaapvaal Craton. Our palaeomagnetic data identifies a possible primary magnetisation within the least-altered lithologies of the Hlagothi Complex (with a virtual geographic pole at 23.4°N, 53.4°E, dp=8.2° and dm=11.8°). The bulk of samples however, displayed two episodes of remagnetisation. These are likely to be associated with 2.85 to 2.75Ga aged granitoids across the southeastern Kaapvaal Craton, and tectonic activity in the nearby Meso- to Neoproterozoic Namaqua–Natal mobile belt. A short-lived (≤8Ma) mantle plume is proposed to have caused the ca. 2.87Ga magmatism, and also may well have controlled sedimentation within the Pongola–Witwatersrand basin. Volcanism during uplift would have been fed through a series of feeder dykes and sills, of which the Hlagothi Complex and NW-trending dykes are part of.

First precise U–Pb baddeleyite ages of 1500 Ma mafic dykes from the São Francisco Craton, Brazil, and tectonic implications

The São Francisco Craton (SFC) is an Archaean–Proterozoic craton that hosts a significant number of mafic intrusions. Previous attempts to date the igneous emplacement of these rocks are limited to a few poor precision Rb–Sr and Sm–Nd isochrons. Here, dykes from the Curaçá belt, in the northeastern portion of the craton, as well as from the Chapada Diamantina region in the central part, were dated using U–Pb on baddeleyite (ID-TIMS). These dykes yielded similar baddeleyite U–Pb ages of 1506.7±6.9Ma (Curaçá) and 1501.0±9.1Ma (Chapada), with converging trends indicating a possible magmatic centre located near the western margin of the São Francisco Craton. The two dyke swarms are subalkaline, and have light rare earth enriched chondrite-normalized patterns but the Curaçá dykes have lower Mg-number and higher abundances of most incompatible trace elements than the Chapada dykes. Geochemical modelling and trace element ratios suggest that the two dyke swarms had different petrogenesis, with the Chapada dykes being derived from more depleted mantle sources and the Curaçá dykes from enriched sources, however the mantle sources of both dyke swarms appear to have had contributions from an enriched plume. Current geochronological and palaeomagnetic data suggest a coherent São Francisco–Congo block from at least the late Mesoproterozoic until the opening of the South Atlantic (ca. 130Ma ago). The discovery of ca. 1500Ma intrusions in the São Francisco Craton is a major step forward to a more complete barcode record for the SFC–Congo Craton, which can be used in future palaeo-continental reconstructions.

U–Pb baddeleyite and zircon ages of 2040 Ma, 1650 Ma and 885 Ma on dolerites in the West African Craton (Anti-Atlas inliers): Possible links to break-up of Precambrian supercontinents

Precambrian inliers of the Anti-Atlas belt in the southern part of Morocco contain numerous dolerite dyke and sill swarms which were previously poorly dated. Four dykes and two sills dated by the U–Pb TIMS method on baddeleyite and zircon provide the first steps toward a magmatic ‘barcode’ for the West African Craton (WAC) and constraints on the timing of breakup of the WAC from Precambrian supercontinents. A 2040±2Ma (U–Pb zircon) age for a WNW dyke in the Zenaga inlier, matches the published age of a dyke in the Tagragra of Tata inlier, and also those of Eburnean granites observed in several inliers, which are collectively interpreted to represent ca. 2040Ma bimodal magmatism due to a mantle plume. Based on the presence of matching 2040Ma ages, the WAC may have been connected to the North Atlantic Craton at the initial stage of fragmentation of a late Archean continent. U–Pb baddeleyite ages of 1656±9Ma and ca. 1655Ma from sills in the Zenaga inlier and 1654±16Ma from a NE-trending dyke in the Agadir Melloul inlier are similar to intraplate magmatic ages in eastern and northern Baltica, and support the SAMBA reconstruction (part of the Nuna supercontinent) of the WAC adjacent to Baltica. Approximate U–Pb ages of 885Ma for two dykes in the Iguerda-Taïfast and Zenaga inliers date a NE trending swarm (named herein the Iguerda-Taïfast swarm) which is connected to the initial breakup of the supercontinent Rodinia, and a specific link with the São Francisco/Congo and North China craton is considered.

Os, Nd, O and S isotope constraints on country rock contamination in the conduit-related Eagle Cu–Ni–(PGE) deposit, Midcontinent Rift System, Upper Michigan

The Eagle Cu–Ni–(PGE) deposit is hosted by mafic to ultramafic intrusive rocks associated with the Marquette–Baraga dike swarm in northern Michigan. Sulfide mineralization formed in a conduit system during early stages in the development of the ∼1.1Ga Midcontinent Rift System. The conduit environment represents a prime location for melt–rock interaction. In order to better assess the extent of country rock contamination in the Eagle system, a combined trace element, Nd, Os, O and S isotope study of country rocks, sulfide-bearing igneous rocks and massive sulfide was undertaken.Both the Eagle and the weakly mineralized East Eagle intrusion show trace element patterns that are similar to those of picritic basalts that formed during early stages of rift development. The trace element, Os, Nd, and O isotopic values of the igneous rocks are consistent with <5% of bulk contamination by Paleoproterozoic and Archean country rocks. Both the Re–Os and Sm–Nd system provide isochrons that are in agreement with the 1107Ma U–Pb baddeleyite age of the intrusive rocks. Calculated γOs(1100) and εNd(1100) values for the magmas are +34 and −2. δ18O values of pyroxene in feldspathic pyroxenite range from 6.5‰ to 6.6‰ and provide the only indication that bulk contamination may locally have exceeded 20%. Sulfur isotopic values of disseminated and massive sulfide in the Eagle intrusion range from 0.3‰ to 4.6‰. The δ34S values are much lower than those that characterize most of the country rocks, but could still be indicative of a contribution of S from country rocks of up to ∼50%. Δ33S values of the disseminated and massive sulfides range from −0.10‰ to 0.09‰ indicating a source in Paleoproterozoic country rocks. Semi-massive sulfide in the Eagle deposit has δ34S values between 2.2‰ and 5.3‰, and Δ33S values show a broad range between −0.86‰ and 0.86‰ indicating a major contribution from an Archean source. Isotopic data from the Eagle deposit strongly indicate that multiple sources of sulfur were involved in the generation of the Ni–Cu–(PGE) mineralization, and magmas which traversed variable paths through the mantle and crust were focused and utilized the same conduit at the level of the Eagle deposit. Our results emphasize the fact that the sulfur isotopic values of immediate country rocks may not be appropriate as end-member values for mixing calculations; sulfur derivation from deeper in the conduit system may have involved rocks with very different sulfur isotope ratios. In addition, sulfur isotopic exchange reactions between passing magma and accumulated crystals or sulfide liquid in the conduit may have led to decreased sulfur isotope variability and δ34S values near those of mantle sulfur.

Correction to “Uranium‐lead age of baddeleyite in shergottite Roberts Massif 04261: Implications for magmatic activity on Mars”

Correction to “Uranium‐lead age of baddeleyite in shergottite Roberts Massif 04261: Implications for magmatic activity on Mars”

A Paleoproterozoic giant radiating dyke swarm in the Dharwar Craton, southern India

Identical high precision U‐Pb baddeleyite ages, together with paleomagnetic and geochemical data, on mafic dykes occurring over an area of 140,000 km2, define a Paleoproterozoic giant dyke swarm at ca. 2.367 Ga in the Dharwar craton, south India, referred to here as the Dharwar giant dyke swarm. All six U‐Pb ages on these dykes are identical within error and suggest emplacement of this swarm within a geologically short time span of ∼5 Myr. A systematic southward progression in the trend of dykes from N48°E to N90°E, defines a fan angle of about 40° with convergence to a focal point about 300 km west of the present‐day Dharwar craton boundary, resulting in a spectacular radiating dyke swarm extending across the entire eastern Dharwar craton. The large areal extent, radiating dyke pattern and short duration imply a mantle plume origin for the Dharwar giant dyke swarm. Despite their large areal distribution, all dykes in this swarm are geochemically coherent and have similar primitive mantle‐normalized trace element patterns and rare earth element characteristics. Although the NE part of the swarm is magnetically overprinted, a remanence survives that has the same direction as primary magnetizations from dykes in the southern part of the swarm.

Uranium-lead age of baddeleyite in shergottite Roberts Massif 04261: Implications for magmatic activity on Mars

[1] The issue of the duration of Martian magmatic activity is one of the controversial debates recently: whether the young radiometric ages of ∼180 Ma for shergottites, a class of Martian meteorites, reflect the timing of crystallization from a magma or rather record later events related to shock metamorphism. In order to address the timescale over which Mars was geologically active, we have undertaken U-Pb isotopic studies on baddeleyite in the Roberts Massif (RBT) 04261 shergottite. Baddeleyite in RBT 04261 is usually associated with ilmenite and shows monoclinic structure, suggesting that the baddeleyite could have formed by crystallization from a residual liquid and not by shock metamorphism. In situ U-Th-Pb isotopic analyses of baddeleyite yield a young238U-206Pb age of ∼200 Ma. Since the U-Pb system of baddeleyite is considered to be more resistant to resetting during reheating events, and olivine in RBT 04261 preserves an igneous calcium zoning, the young age of baddeleyite could be interpreted as a crystallization age of RBT 04261. The present results imply that Martian magma was still forming only 200 Ma ago, and that Mars had been geologically active until the recent past.

U–Pb baddeleyite ages, distribution and geochemistry of 925 Ma mafic dykes and 900 Ma sills in the North China craton: Evidence for a Neoproterozoic mantle plume

Numerous Neoproterozoic mafic dykes, referred to as the Dashigou swarm, are identified in the central and southeastern parts of the North China craton (NCC). They are 305–010° trending dykes, with widths of ~10–100m and exposed lengths of several to >10km. Precise U–Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS) measurements on baddeleyite grains separated from three dykes yield 207Pb/206Pb average ages of 924.0±3.7Ma (Dashigou dyke), 921.8±2.6Ma (Yangjiaogou dyke) and 925.8±1.7Ma (Taohuagou dyke). Baddeleyite grains from a late-stage pegmatite vein in the Dashigou dyke were analyzed by secondary ion mass spectrometry (SIMS) methods. These yield a 207Pb/206Pb average age of 920.4±5.7Ma. The Dashigou dyke swarm exhibits an overall radiating geometry (the overall fan angle is about 60°), with a focal point located along the southern margin of the eastern NCC, where a large ca. 900Ma sill swarm was previously recognized. The rift system hosting these slightly younger sills, named the Xu–Huai Rift System, could represent two breakup-parallel arms of a rift–rift–rift triple junction related to the initiation of the magma center that produced the Dashigou dykes. The sills have similar characteristics and could be cogenetic with the Dashigou dykes.The Dashigou dykes are coarse-grained, composed mainly of clinopyroxene and plagioclase feldspar, with or without olivine. One of the most primitive dykes has 47.79wt.% SiO2, 6.41wt.% MgO, 1.38wt.% TiO2, 17.77wt.% Al2O3, 10.47wt.% CaO and 0.62wt.% K2O. It shows slight enrichment in light rare earth elements and a slightly positiveEu/Eu* anomaly (1.1), and is slightly depleted in high field-strength elements compared to neighboring elements on a primitive mantle-normalized spidergram. The Dashigou dykes show some similarities with enriched-mid ocean ridge basalts (E-MORB) or ocean island basalts (OIB) and have εNdt values of +1.8 to +3.1 and 87Sr/86Srt values of 0.7019–0.7047 (t=920Ma). All these characteristics indicate that they are not likely derived from the ancient lithospheric mantle under the NCC, but rather from a mantle source below, in the asthenosphere. This 925–900Ma magmatism represents a second sub-lithospheric mantle upwelling event following the 1780–1730Ma event that occurred shortly (~70Ma) after the formation of the subcontinental lithospheric mantle (SCLM). Therefore the SCLM of the NCC was multiply metasomatized during asthenospheric upwellings (at least at 1780–1730 and 925–900Ma), before most of its eastern part was finally removed during the Mesozoic.Collectively, these ~925Ma dykes and ~900Ma sills constitute a large igneous province (LIP) with an areal extent of about 0.5Mkm2 and a diameter of about 1000km. This LIP probably resulted from a Neoproterozoic mantle plume centered along the present southern margin of the eastern NCC, and probably resulted in the break-off and rifting away of a separate crustal block. We speculate that this conjugate block could have been the combined São Francisco–Congo craton on the basis of precisely matched ages for the Bahia dykes (São Francisco craton) and Gangila–Mayumbian volcanic associations (western part of the Congo craton).

The oldest oil accumulation in China: Meso-proterozoic Xiamaling Formation bituminous sandstone reservoirs

Abstract Based on analysis on the coexistent relationship between igneous rock intrusion and bituminous sandstone of the Xiamaling Formation in northern Hebei (Qnx), the accumulation time of the Xiamaling Formation bituminous sandstone reservoir is defined by the methods of organic petrology and isotope chronology. According to field investigation, observation of thin-section and the shallow drilling show, and based on the previous results of oil seepages and bitumen spots in the Qnx base sandstone, thirty-one new oil seepages (bitumen spots), including twenty-one visible black bitumen spots, were discovered in the Qnx. The distribution of paleo-reservoirs in the Qnx bituminous sandstone was identified, and 4 paleo-reservoirs, i.e. Shuangdong paleo-reservoir in Pingquan, Longtangou paleo-reservoir in Lingyuan, Lujiazhuang paleo-reservoir and Tashan paleo-reservoir in Kuancheng, have been named. The reserves of the paleo-reservoirs are 0.7-1.0 billion tons, which are calculated by the volume method with the solid bitumen content. Organic petrology analysis shows that two kinds of solid bitumen with diferent reflectivity characteristics were developed. Class structure of natural coke is discovered in solid bitumen, which is the first direct evidence to prove that severe thermal alteration occurs in the Qnx base sandstone bitumen. The age of Qnx gabbro-diabase baddeleyite by SHRIMP U-Pb dating is 1 327 Ma±2 Ma, which indicates that the accumulation time of the Xiamaling Formation bituminous sandstone is 1 400 Ma to 1 327 Ma. So, it is the oldest oil accumulation in China.

Lifetime of an ocean island volcano feeder zone: constraints from U–Pb dating on coexisting zircon and baddeleyite, and 40Ar/39Ar age determinations, Fuerteventura, Canary IslandsThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

High-precision isotope dilution – thermal ionization mass spectrometry (ID–TIMS) U–Pb zircon and baddeleyite ages from the PX1 vertically layered mafic intrusion Fuerteventura, Canary Islands, indicate initiation of magma crystallization at 22.10 ± 0.07 Ma. The magmatic activity lasted a minimum of 0.52 Ma. 40Ar/39Ar amphibole dating yielded ages from 21.9 ± 0.6 to 21.8 ± 0.3, identical within errors to the U–Pb ages, despite the expected 1% theoretical bias between 40Ar/39Ar and U–Pb dates. This overlap could result from (i) rapid cooling of the intrusion (i.e., less than the 0.3 to 0.6 Ma 40Ar/39Ar age uncertainties) from closure temperatures (Tc) of zircon (699–988 °C) to amphibole (500–600 °C); (ii) lead loss affecting the youngest zircons; or (iii) excess argon shifting the plateau ages towards older values. The combination of the 40Ar/39Ar and U/Pb datasets implies that the maximum amount of time PX1 intrusion took to cool below amphibole Tc is 0.8 Ma, suggesting PX1 lifetime of 520 000 to 800 000 Ma. Age disparities among coexisting baddeleyite and zircon (22.10 ± 0.07/0.08/0.15 Ma and 21.58 ± 0.15/0.16/0.31 Ma) in a gabbro sample from the pluton margin suggest complex genetic relationships between phases. Baddeleyite is found preserved in plagioclase cores and crystallized early from low silica activity magma. Zircon crystallized later in a higher silica activity environment and is found in secondary scapolite and is found close to calcite veins, in secondary scapolite that recrystallised from plagioclase. close to calcite veins. Oxygen isotope δ18O values of altered plagioclase are high (+7.7), indicating interaction with fluids derived from host-rock carbonatites. The coexistence of baddeleyite and zircon is ascribed to interaction of the PX1 gabbro with CO2-rich carbonatite-derived fluids released during contact metamorphism.