Baddeleyite Ages Research Articles

Age and Petrogenesis of Paleoproterozoic Diorites from Dikes of the Baikal Uplift of the Siberian Craton

Abstract ––We carried out a detailed geological, geochronological, geochemical, and isotope study of diorites from a dike located in the central part of the Baikal uplift of the Siberian craton. The geochemical and isotope data obtained for diorites of the studied intrusion were compared with coeval mafic and intermediate igneous rocks of the southern part of the Siberian craton. The U–Pb (ID-TIMS) baddeleyite age of 1862 ± 7 Ma has been estimated for diorite from a dike located in the area of the Onguren Village. The obtained data are the first reliable age determination for the Paleoproterozoic mafic and intermediate igneous rocks of the Baikal uplift, which are part of the South Siberian postcollisional magmatic belt. The dike is of NE strike. The dike rocks correspond in chemical composition to diorites and are highly differentiated varieties (mg# = 36.5–37.4). There are no significant variations in the composition of diorites in the marginal and central parts of the dike. The rocks are characterized by low contents of TiO2, P2O5, and Nb, high contents of Th, Zr, Ba, and LREE, and low negative values of εNd(T) (–5.9 to –6.2). We assume that the diorites formed from the enriched subcontinental lithospheric mantle. The obtained geochemical and isotope data show similar negative values of εNd(T) from –4.3 to –11.6 for most of Paleoproterozoic mafic and intermediate igneous rocks of the South Siberian postcollisional magmatic belt within the Aldan Shield, the Baikal uplift, and the Irkut block of the Sharyzhalgai uplift. These rocks correspond in geochemical features to rocks resulted from the melting of subduction-modified lithospheric mantle sources in the postcollisional extension setting at the final Paleoproterozoic stage of formation of the Siberian craton.

Placing constraints on the age and origin of basaltic dykes on the Ghaap Plateau, Griqualand West, South Africa

Abstract Well-preserved strata of the Ghaap Plateau along the western margin of the proto-Kalahari Craton have been intruded by a large number of mafic dykes. Owing to the scarcity of outcrops, these dykes are generally not well documented and intrusion ages remain poorly understood. In order to address this lack of knowledge, a total of seventeen Ghaap Plateau dykes that included seven north-northeast to northeast-trending dykes, eight northwest-trending dykes and two east to east-northeast-trending dykes were sampled for this study. Here, we report on the petrography and whole rock geochemistry of these dykes. We also report three new U-Pb baddeleyite ages along with some palaeomagnetic data. The mineralogy of the dykes is dominated by clinopyroxene and plagioclase showing varying degrees of alteration. Whole rock geochemical data identify the dykes as basalts with MORB, E-MORB or within plate-like signatures, suggesting an asthenospheric magma source. Our study indicates that the Ghaap Plateau dykes represent several magmatic events of markedly different age. Based on their whole rock geochemical composition, all but one of the dyke samples are tentatively arranged into four distinct groups (Groups A to D). Group A dykes (four dykes) are compositionally similar to the Umkondo Large Igneous Province (LIP). Two north-northeast-trending dykes of this Group yielded overlapping U-Pb baddeleyite ages of 1 103 ± 84 Ma and 1 112 ± 55 Ma, thereby adding to the known footprint of the Umkondo LIP along the western margin of the proto-Kalahari craton. In Group B (four dykes), one northeast-trending dyke yielded a U-Pb baddeleyite age of 1 929 ± 17 Ma. Its palaeomagnetic signature (Lat. 24.5°, Long. 317.6°, A95= 14.20°) is supported by a positive baked contact test and agrees well with the ~1.93 Ga Hartley LIP palaeopole. Group B dykes are thus interpreted to be related to the Hartley LIP, with our new 1 929 ± 17 Ma age supporting a wider extent for the Hartley magmatism than previously known. A new palaeopole for the Hartley LIP (Lat. 22.3°, Long. 327.4°, A 95 = 9.25°) was calculated by combining the data of our 1 929 Ma dyke with data reported previously from other sites. Whole rock geochemical data for the remaining dykes are consistent with the Karoo magmatism (two east to east-northeast-trending dykes of Group D) or the ~2.4 Ga Ongeluk LIP (north-trending dyke). The final four dykes (Group C) are difficult to match with other known mafic units. Precise geochronology is needed to confirm the exact age of these unassigned dykes.

Detrital zircon ages and proposed provenance of the Koegas Subgroup of the Ghaap Group, and overlying Makganyene Formation, of the Postmasburg Group, Transvaal Supergroup

Abstract The late Archaean to early Palaeoproterozoic strata of the Transvaal Supergroup of southern Africa is renowned for hosting geological units that preserve some the Earth’s most significant geological events. The glaciogenic Makganyene Formation is one such significant unit, given that it has been associated with the Snowball Earth Event. The maximum age of deposition of this formation, and subsequent timing of this event, has come into question, mostly because of concordant detrital zircon ages as young as ~2.2 Ga reported by Beukes et al. (2013). These ages are younger than the recently revised ca. 2.43 Ga baddeleyite age inferred for the overlying Ongeluk Formation and subsequently led to a significant revision of a long-held correlation between the upper Postmasburg- and Pretoria groups of the respective sub-basins of the Transvaal Supergroup. The primary objective of this study is to investigate the mode of occurrence of the ~2.2 Ga zircons in the Makganyene Formation as well as selected formations of the underlying Koegas Subgroup. Here, we report a total of 183 near concordant U-Pb zircon ages for the Koegas Subgroup (Pannetjie- and Naragas formations) and 967 such ages for the Makganyene Formation, extracted from seventeen samples from across the outcrop area for these units. Sensitive High Resolution Ion MicroProbe (SHRIMP) as well as three different Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) techniques were used for U-Pb age measurements. We assessed the quality of the respective data sets and possible shortcomings of the techniques to constrain the maximum age of deposition of the units and to infer possible source areas for the detritus. In contrast to zircon ages determined for the Koegas Subgroup, zircons younger than ~2.4 Ga are a prominent feature of the Makganyene Formation. Upon careful consideration of each data set, we concluded that these so-called young grains have likely suffered significant Pb-loss and that a possible overcorrection for common Pb during data reduction could not be ruled out. Although the age distribution generated using the respective techniques were comparable, none of the four techniques were successful in shedding light on the reliability of the ~2.2 Ga ages. The maximum age of deposition of the Makganyene Formation could not be constrained with confidence and therefore the revised correlation between the Postmasburg- and Pretoria groups is not contested. The detrital zircon age distribution of the Koegas-Makganyene succession was however found to be remarkably similar, with a major ~2.5 Ga age fraction and subordinate fractions at ~2.65 to ~2.9 Ga and older than ~3.0 Ga. The major ~2.5 Ga zircon age fraction points towards a prominent, likely orogenic source of detritus. We assign the origin of the major ~2.5 Ga and older zircon age fractions to a Rae-type craton, which we proposed to have been attached to the north of the Vaalbara Craton at time of deposition of these units and consider a tectono-magmatic event characteristically associated with this family of cratons as the possible cause of alteration and Pb-loss in zircon grains dated at ~2.2 Ga.

Late Tonian (ca. 780–750 Ma) bimodal magmatism in the Ogcheon Metamorphic Belt, Korea, at the eastern margin of North China Craton

Neoproterozoic rift-related rocks of the North China Craton (NCC), including the Korean Peninsula, provide crucial information for global supercontinent reconstruction during the Rodinia breakup and Gondwana assembly. Here, we report not only zircon and baddeleyite U-Pb ages but also zircon Hf isotopic and bulk rock compositions of amphibolites in the Ogcheon Metamorphic Belt (OMB), Korea. Zircon occurs as either primary phase or secondary one replacing igneous baddeleyite. Primary zircons analyzed from three amphibolites yielded the weighted mean 206Pb/238U ages of 755 ± 5 Ma (n = 19), 772 ± 3 Ma (n = 9), and 779 ± 13 Ma (n = 12), respectively; this result is corroborated by the baddeleyite age of ∼760–745 Ma. Secondary zircons define discordia lines with lower intercepts corresponding to the Jurassic or Cretaceous age of granites emplaced adjacent to the amphibolite. In conjunction with the occurrence of coeval (ca. 762–745 Ma) metatrachytes, our result suggests that late Tonian bimodal magmatism has lasted for ∼30 m.yr. in the OMB. Moreover, positive εHf(t) values (4.9–21.4) of zircon in amphibolites indicate a depleted mantle source of mafic magma, whereas trace element compositions attest to an intraplate setting for the crystallization. A compilation of Nb-Th and Zr–Zr/Y data suggests that the intraplate setting is also pertinent to ca. 950–750 Ma mafic rocks from the Gyeonggi Marginal Belt, Korea, as well as the NCC. Such a tectonomagmatic correlation is corroborated by detrital zircon age spectra of metasedimentary rocks in the OMB equivalent to those of early Tonian rocks in the NCC. In marked contrast to tectonic scenarios conjecturing a linkage between the OMB and the Nanhua rift, South China Craton (SCC), we conclude that the Ogcheon Belt provides rare geologic-geochronologic evidence for late Tonian breakup of the Rodinia supercontinent at the eastern NCC margin.

The Greater Cederberg–False Bay Large Igneous Province in South Africa: A southern node of widespread magmatism associated with South Atlantic rifting

Abstract A new 130 to 140 Ma mafic dyke swarm, is identified in western South Africa. It consists of the previously undated Cederberg dyke swarm (CDS), for which we report U-Pb ID-TIMS baddeleyite ages of 131.4 ± 4.5 Ma (Knersvlake subswarm) and 133.0 ± 1.5 Ma (Doring-Tanqua subswarm). 40Ar/39Ar dates on these two samples and two additional dates of the Doring – Tanqua subswarm cluster between 128.5 ± 1.4 Ma and 132.2 ± 1.5 Ma. We also report 40Ar/39Ar ages of 139.3 ± 3.5 Ma for an east-west trending dyke located further north: 27 km south of Kleinsee, and 140.3 ± 1.2 Ma for an east-west dyke near Garies. Together, these eight ages robustly date the emplacement of the northern part of a Greater Cederberg-False Bay Dyke Swarm (GCFDS) at ca. 130 to 140 Ma. Trace and rare earth element datareported herein suggest these dykes are compositionally E-MORB basalts that underwent modification either via subduction-modified lithospheric mantle, or by continental crust, or a combination thereof, and are petrogenetically similar to the ca. 132 Ma False Bay dykes around Cape Town. Therefore, we propose to unify all these coeval and compositionally similar dykes into one large igneous province (LIP) termed the Greater Cederberg-False Bay Large Igneous Province (GCF-LIP).

KENGEDE MAFIC DYKE SWARM AND EXPANSION OF THE 1.50 Ga KUONAMKA LARGE IGNEOUS PROVINCE OF NORTHERN SIBERIA

Within the Anabar shield in the northern part of the Siberia, Late Precambrian mafic igneous units are widespread, which form dyke swarms of different ages of different trends. This paper presents new data on the composition, structure and U-Pb dating of the E-W trending Kengede dyke swarm. Three new U-Pb ID-TIMS baddeleyite ages (1496±7, 1494±3 and 1494±5 Ma) were obtained from three dykes, indicating that the Kengede swarm is part of the 1500 Ma Kuonamka large igneous province (LIP). The previously recognized Kuonamka Large Igneous Province (LIP) extends for 700 km from the Anabar shield to the Olenek uplift in the northern part of the Siberia and is potentially linked to coeval dykes and sills of the São Francisco craton and the Congo craton. The newly dated Kengede swarm is parallel to but offset by 50 km from the previously dated 1501±3 Ma Kuonamka swarm, and the identification of these two subparallel dyke subswarms of the Kuonamka LIP supports the earlier interpretation that mantle plume centre was located along the extrapolated trend of the dykes near the eastern or western margin of the Siberia. The paper examines features of sulfide Cu-Ni mineralization in dolerites of the Kengede and East Anabar dyke swarms and discusses potential Cu-Ni-sulfide mineralization linked to the Precambrian mafic dyke swarms of different ages in the north-east of the Siberia.

Paleogeography of India at ∼1.8 Ga: New constraints from baddeleyite geochronology and paleomagnetism of mafic dykes from the Dharwar craton

The paleogeographic position of Dharwar craton/India within Paleoproterozoic supercontinent configurations has been well demonstrated using highly reliable key paleopoles from ∼2.37 Ga to ∼1.86 Ga mafic events in recent studies. Here, we report a paleomagnetic pole at ∼1.8 Ga along with a precise Pb-Pb baddeleyite age. The emplacement age of an NW-SE trending dolerite dyke yields a weighted mean age of 1799 ± 0.8 Ma (MSWD = 0.7), which coincides with earlier reported U-Pb baddeleyite age (1794 ± 7 Ma) and confirms the existence of significant emplacement of Pebbair dyke swarm at ∼1.8 Ga. Paleomagnetic results from eleven sites shows mean D = 306°, I = 31°, α95 = 10° (k = 23), and corresponding paleopole is calculated at 38°N, 357°E (dp = 6.1° and dm = 11°). These remanent directions are carried by the pseudo-single domain (PSD) magnetite. This pole passes the paleosecular variation test of the geomagnetic field and fulfills the reliability criteria (R = 5) of Meert et al. (2020), so, suggested to be a key pole for Dharwar craton/ India (∼1.8 Ga). The paleopositon of Dharwar craton is occupied at shallow paleolatitude and drifted towards the north with a minimum velocity (∼5.7 cm/yr) from ∼1.86 Ga to ∼1.80 Ga. The paleogeographic reconstruction model at ∼1.8 Ga has been attempted with available coeval paleopoles from other global cratons. The Dharwar craton (South India block), the Amazonia craton, and the Baltica craton have assembled at ∼1.86 Ga, and North Australia, Sao Francisco, Rio de la Plata cratons came in close to the South India block, Amazonia, and Baltica assembly at ∼1.80 Ga.

Identification of the ca. 720 Ma Irkutsk LIP and its plume centre in southern Siberia: The initiation of Laurentia-Siberia separation

Widespread 720 Ma magmatism has been linked with the break-up of Rodinia and the onset of the Sturtian ‘Snowball Earth’ event. We report a new U-Pb baddeleyite age from the Baikal dyke subswarm in southern Siberia which increases the known extent of the 720 Ma Irkutsk LIP and confirms a previous hypothesis that the Baikal and Sayan subswarms converge at the southern tip of the Irkutsk promontory. Together they define a mantle plume centre with direct links to the 720 Ma Franklin plume centre of northern Laurentia, thus constraining the paleo-reconstruction of southern Siberia and northern Laurentia. It is inferred that this combined 720 Franklin – Irktusk LIP event is associated with the breakup of southern Siberia from northern Laurentia during fragmentation of the Rodinia supercontinent. Expansion of 720 Ma magmatism into Siberia greatly increases the scale of the Franklin-Irktusk LIP.

Precise Pb-Pb baddeleyite ages and petrogenesis of the NW-SE trending Paleoproterozoic mafic dyke swarm from the Bundelkhand craton, India: Implications to the enriched sub-continental lithospheric mantle

Based on a similar structural trend, the NW-SE trending mafic dykes from the Bundelkhand craton were assigned an emplacement age of 1979 ± 8 Ma. Many geochemistry and paleomagnetic studies on these dykes have proposed the presence of more than one mafic dyke event within the NW-SE trending mafic dyke swarm. Still, until now, this suggestion was not supported by precise geochronology. This study presents TE-TIMS Pb-Pb baddeleyite ages of 2001.5±1.3 Ma, 2005.0±0.8 Ma, and 2004.5±0.7 Ma from the NW-SE trending dykes of Ajaygarh dyke swarm located towards the southeast margin of the Bundelkhand craton. The mean age of 2004.3±3.5 Ma is inferred as the emplacement age of these dykes. This dyke event is hitherto unknown and the oldest in the Bundelkhand craton. Further, we present the major and trace element geochemistry data on 34 samples from NW-SE trending dykes. These results suggest the presence of three geochemically distinct groups of dykes from NW-SE trending dykes. Further, the geochemical modeling of studied dykes suggests their derivation from subcontinental lithospheric mantle sources enriched by subduction-derived fluids at different depths and times. These dykes were emplaced as part of a sporadically spread global crustal extension event.

Mesoproterozoic (ca. 1.26 Ga) Srednecheremshansk mafic–ultramafic intrusion in the southern Siberia: Signature of the Mackenzie event in Siberia

Several published Mesoproterozoic paleogeographic reconstructions suggest proximity of northern Laurentia and southern Siberia. However, the apparent absence of the traces of the ca. 1.27 Ga giant Mackenzie magmatic event in southern Siberia was somewhat contradictory to this hypothesis. Here we present geochronological, mineralogical, geochemical, Nd isotopic, and paleomagnetic data from the Srednecheremshansk dyke-shaped intrusion, which was recently found in the Sharyzhalgay uplift of the southern part of the Siberian craton and which can be related to the Mackenzie event. The plagioclase-bearing peridotite of this intrusion yielded U-Pb (ID-TIMS) baddeleyite concordia age of 1260 ± 3 Ma, which is interpreted as the time of their emplacement. This age is close to the previously published baddeleyite age of gabbro from the same intrusion (1258 ± 5 Ma). The Srednecheremshansk intrusion is composed of ultramafic and mafic rocks. The main rock-forming minerals of the intrusion are olivine, orthopyroxene, clinopyroxene, phlogopite, and plagioclase in various proportions. Sulfide mineralization of the intrusion is represented by pentlandite nodules. The chemical composition of Srednecheremshansk ultramafic and mafic rocks correspond to subalkaline peridotite gabbro and gabbro. These rocks are characterized by negative ɛNd(t) values range from −6.3 to −6.9. Ultramafic and mafic rocks have similar geochemical and isotopic characteristics, indicating their generation from a single subcontinental lithospheric mantle source. The chemical composition of the Srednecheremshansk intrusion is similar to those of the ca. 1.27 Ga Muskox mafic–ultramafic intrusion of the Mackenzie Large Igneous Province in northern Laurentia. Paleomagnetic data permit a variety of possible Laurentia-Siberia reconstructions in Mesoproterozoic. The combination of geochronological, geochemical, and paleomagnetic data allows the relation of the Srednecheremshansk intrusion with the Mackenzie magmatic event.

The Puduhush gabbro in Griqualand West, South Africa: extendingca.1.89 to 1.83 Ga intraplate magmatism across the proto-Kalahari Craton

AbstractThe Puduhush gabbro is located on the western margin of the proto-Kalahari Craton in Southern Africa. This gabbro intrudes the Volop Formation, which conformably overlies the Hartley Formation lava of the late Palaeoproterozoic Olifantshoek Group. Here we report a new U-Pb ID-TIMS baddeleyite age as well as petrographic, whole-rock geochemical and palaeomagnetic results for the Puduhush gabbro. The gabbro shows a well-preserved sub-ophitic texture between clinopyroxene and plagioclase, with minor amounts of amphibole, olivine, biotite and Fe-Ti oxides. The new U-Pb ID-TIMS baddeleyite age of 1 881 ± 1 Ma reported here for the Puduhush gabbro, together with existing ages for the Hartley Formation, define a ca.1 916 to 1 881 Ma age bracket for the Volop Formation. Our 1 881 ± 1 Ma age is also within error of ages reported for the oldest episode (so-called Episode 1) of the ca.1.89 to 1.83 Ga magmatism in the eastern and northern parts of the proto-Kalahari Craton. Our geochemical results also suggest compositional similarities between the Puduhush gabbro and Episode 1 magmatism, particularly the post-Waterberg sills. The virtual geomagnetic pole calculated here for the Puduhush gabbro (VGP: 1.6°N; 352.0°E; A95 = 14.2°) is consistent with the Episode 1 pole. All data are therefore combined to produce a new palaeomagnetic pole (11.7°N; 8.8°E, A95 = 9.3°) for Episode 1 magmatism. The present study provides the first evidence that the ca.1.89 to 1.83 Ga magmatism had a wider footprint that previously thought, extending to the western margin of the proto-Kalahari Craton. This wide-scale magmatism, previously proposed to be related to a back-arc extension setting, is here reinterpreted in the context of a mantle plume. Our results are consistent with the lithostratigraphic-based notion that at least parts of the red-bed successions (i.e., Olifantshoek and Waterberg Groups) that are hosts to the ca.1.89 to 1.83 Ga magmatism could be correlative units, representing an extensive sedimentary sequence that once covered large expanses of the proto-Kalahari Craton.

Uranium-Lead Systematics of Lunar Basaltic Meteorite Northwest Africa 2977.

Northwest Africa (NWA) 2977 is a lunar basaltic meteorite that was found in 2005 and has been classified as an olivine cumulate gabbro. This meteorite contains a shock melt vein (SMV) induced by an intense shock event. We report herein on an in-situ analysis of phosphates in the host gabbro and the shock vein for the U-Pb dating of NWA 2977 using an ion microprobe, NanoSIMS. The majority of the analyzed phosphates, in both the SMV and host-rock, lie on a linear regression in 238U/206Pb-207Pb/206Pb-204Pb/206Pb three-dimensional space, indicating a total Pb/U isochron age of 3.15±0.12 Ga (95% confidence level), which is consistent ages determined in previous isotopic studies of NWA 2977 (Sm-Nd age of 3.10±0.05 Ga, Rb-Sr age of 3.29±0.11 Ga, and Pb-Pb baddeleyite age of 3.12±0.01 Ga), and identical to the age of the U-Pb phosphate in a paired meteorite NWA 773, 3.09±0.20 Ga, derived from our dataset. There was no clear difference in the formation age between the phosphates found in the SMV and host-rock, although the shape and size of the grains and the Raman spectra show the evidence of intense shock metamorphism. Based on these findings, the cooling rate of the phosphate was very rapid, constrained to be larger than 140 K/s.

New U-Pb baddeleyite ID-TIMS ages from the intrusive high-Ti-Sr rocks of the Southern Paraná LIP, Brazil: Implications for correlations with environmental disturbances during the Early Cretaceous

Large volumes of mafic igneous rocks are commonly emplaced during Large Igneous Province (LIP) eruptions, and these mafic rocks are often contemporaneous with periods of environmental disturbances, such as global ocean anoxia, and as a result, mass extinctions. The Paraná-Etendeka LIP is no exception, and has been previously correlated with, and interpreted to be the cause of the Valanginian oceanic anoxic event (OAE), a small global environmental disturbance. Here, we present new U-Pb ID-TIMS baddeleyite ages from high-Ti-Sr mafic intrusive rocks from the Paraná LIP, in Brazil. While these data are potentially complicated by the presence of Pb-loss and inheritance, it is possible to interpret geologically meaningful ages from them. The first high-precision age is reported for the type-locality of the Florianópolis Dyke Swarm, in North Santa Catarina Island, which yields an age of 132.53 ± 0.40/0.40/0.42 Ma. We also report an age of 132.07 ± 0.27/0.30/0.33 Ma for a dolerite sill, which intrudes organic-rich sedimentary rocks of the Paraná Basin. The emplacement of high-Ti-Sr magmas at ca. 132 Ma suggests that there was up to 2 Myr of intrusive magmatism in the southern part of the Paraná LIP. Further investigation on the mafic intrusive magmatism from the Paraná LIP through robust high-precision geochronology is required to elucidate the proposed linkage more clearly with environmental changes during the Early Cretaceous.

A positive syn-fold test from the Neoarchaean Klipriviersberg Group of South Africa: Quo vadis Vaalbara?

Abstract The existence of Vaalbara, the combined Neoarchaean to Palaeoproterozoic Kaapvaal-Pilbara supercraton, is questionable during the early Neoarchaean when scrutinised through the lens of recent Australian and South African palaeomagnetic data. Remarkably similar ~2.7 to 2.5 Ga geological successions (with near bed-for-bed correlatability) support a coherent Vaalbara at the end of the Neoarchaean. Here we report palaeomagnetic and rock magnetic results from the Klipriviersberg Group of South Africa, which is the oldest rock sequences used to define Vaalbara originally. A positive syn-fold test illustrated a high-temperature remanence component acquired during the formation of the Witwatersrand syncline. This fold structure predates the Vredefort Impact Structure and its formation is synchronous with the deposition of the Mesoarchaean Central Rand Group and extrusion of the Klipriviersberg Group. The studied rocks of the Klipriviersberg Group are not directly dated, but most are likley younger than 2 780 to 2 789 Ma, based on detrital zircon ages from the lowermost Ventersdorp Supergroup and U-Pb baddeleyite ages for mafic sills that intrude the Witwatersrand Supergroup that are regarded as feeders of the Kliprivierberg Group lavas, but older than the overlying 2 720 to 2 750 Ma Platberg Group. The Klipriviersberg Group pole is at 27.7°S, 32.7°E with an A95 of 11°. A comparison of Meso- to Neoarchaean palaeopoles from the Kaapvaal and Pilbara cratons suggests their shared drift path traversing the polar circle and thus supports the existence of Vaalbara across the 2.78 to 2.70 Ga interval.

Corrigendum to “Pb-Pb baddeleyite ages of mafic dykes from the Western Dharwar Craton, Southern India: A window into 2.21–2.18 Ga global mafic magmatism” [J. Asian Earth Sci. 191 (2020) 104221

Corrigendum to “Pb-Pb baddeleyite ages of mafic dykes from the Western Dharwar Craton, Southern India: A window into 2.21–2.18 Ga global mafic magmatism” [J. Asian Earth Sci. 191 (2020) 104221

1.79–1.75 Ga mafic magmatism of the Siberian craton and late Paleoproterozoic paleogeography

The paper presents new geological, geochronological, geochemical, and Nd isotopic data on the late Paleoproterozoic dolerites of the Siberian craton. U-Pb baddeleyite ages of the Doros dolerites (Aldan shield, south-eastern Siberia) and East Anabar dolerites (Anabar shield, northern Siberia) are 1757 ± 4 and 1774 ± 6 Ma, respectively. This new geochronological data together with published 1.76–1.75 Ga ages for Timpton-Algamay dolerites of the Aldan shield and Chaya dolerites of the Baikal uplift (southern Siberia) revealed a 20 my difference between this magmatism in the north and south Siberia. The Doros dolerites in their chemical compositions correspond to sub-alkaline basaltic andesites, but the East Anabar dolerite is chemically close to basaltic trachyandesite. The Doros dolerites demonstrate negative and close to zero ɛNd(t) values (from −7.0 to +0.1), which correlate well with SiO2 and Mg#. All Doros dolerites have Nb–Ta and Ti anomalies in multielement spectra. The geochemical and Nd isotopic data suggest that the Doros dolerites have been formed by the mixing of mantle and crustal material. The East Anabar dolerites are characterized by a negative ɛNd(t) value of −3.7, negative Nb–Ta anomaly in multielement spectra, high concentrations of TiO2 and P2O5. The 1775 Ma East Anabar dolerites could be generated from a subcontinental lithospheric mantle source, possibly with some mantle plume interaction (possibly centred at the convergence with the 1.76–1.75 Ga fanning Aldan swarm). Geochemistry and Nd isotope systematics of all 1.78–1.75 Ga mafic dykes and intrusions of the Siberian craton indicate the subcontinental lithospheric mantle source or mantle source contaminated by crustal material. Geochronological data from the 1.79–1.75 Ga magmatic rocks of Siberia and other continents suggest continuous magmatism over this interval in some continents, but a series of short magmatic events/pulses (from one to four) separated by intervals of quiescence in other continents. We locate the analysed dykes and sills on new 1750 Ma and 1790 Ma global paleogeographic reconstructions. Analysis of 1.79–1.75 Ga geochemical data on mafic intrusions from Late Paleoproterozoic continents suggests the prevalence of subcontinental lithospheric mantle source for the mafic intrusions over the pure mantle plume source.

Petrogenesis of the Paleoproterozoic Näränkävaara layered intrusion, northern Finland, Part II: U-Pb ID-TIMS age and Sm-Nd isotope systematics

Several mafic-ultramafic layered intrusions were emplaced in the NE Fennoscandian Shield during a magmatic episode at 2.44 Ga. The Paleoproterozoic Näränkävaara layered intrusion, northern Finland, is one of the largest ultramafic bodies in the Fennoscandian Shield, with a surface area of 25 km x 5 km and a magmatic stratigraphic thickness of ~3 km. The intrusion comprises a 1.3 km-thick peridotitic–dioritic layered series (2436 ± 5 Ma) with two peridotitic reversals, and a 1.5–2 km thick basal dunite series mainly composed of olivine adcumulates (dated here). The intrusion has been studied since the 1960’s, but several questions regarding its structure and petrogenesis remain. The basal dunite shows several lithological features typical of komatiitic rather than intrusive olivine cumulates; namely, >1 km-thick “extreme” olivine adcumulates, some showing textures with bimodal grain sizes, oscillating variations in Mg# with stratigraphic height, and poikilitic chromite. With Archean greenstone belts nearby, it was previously hypothesized that the basal dunite series could represent an Archean komatiitic wall rock to the Paleoproterozoic layered series. However, our new U-Pb ID-TIMS baddeleyite age of 2441.7 ± 0.9 Ma for the basal dunite series shows that the basal dunite and layered series of the Näränkävaara intrusion are co-genetic. New whole-rock Sm-Nd isotope data from key stratigraphic units (initial εNd at 2440 Ma of -3.5 to -1.7) indicate that the intrusion was constructed from repeated emplacement of LREE-enriched high-MgO basaltic magmas that were mantle-derived and contaminated by crust, similarly to other Fennoscandian 2.44 Ga intrusions. The parental magmas show similar compositions regardless of stratigraphic position, suggesting that most wall rock contamination and homogenization had occurred before emplacement, with in situ contamination being a relatively minor process. The open-system features of the basal dunite suggest that it may have formed (at least partly) as a feeder channel cumulate, possibly related to the ~100 km long Koillismaa-Näränkävaara Layered Igneous Complex. The Näränkävaara parental magmas show variably depleted metal ratios and could have potential for orthomagmatic mineral deposits, given the availability of S-rich wall rocks.

Large igneous provinces of the Amazonian Craton and their metallogenic potential in Proterozoic times

Abstract This paper overviews the Proterozoic large igneous provinces (LIPs) of the Amazonian Craton, characterized by large volumes of extrusive and intrusive magmatic rocks. We reassess the geologic, geochronological and geochemical information to stablish three intracontinental felsic volcanic–plutonic igneous belts (i.e. SLIPs), namely: Orocaima (1.98–1.96 Ga), Uatumã (1.88–1.87 Ga) and Alta Floresta (1.80–1.79 Ga). The Avanavero LIP (1.79–1.78 Ga), as well as the Rincón del Tigre-Huanchaca LIP (1.11 Ga) are also revisited. The relationships of these events to intraplate settings through time and space are apparent. We examine the main characteristics of each magmatic event in light of the U–Pb zircon and baddeleyite ages and coupled isotopic–geochemical constraints, the geodynamic significance and metallogenetic potential. The Uatumã and Alta Floresta SLIPs host the most important mineral resources within the Amazonian Craton. Global barcode matches of the Proterozoic SLIP/LIP events of Amazonia are also addressed, as well as their possible links with geological timescale periods: the Orosirian, Statherian and Stenian boundaries. We also evaluate the available palaeomagnetic data to address issues related to the barcode match of such SLIP/LIP events in the context of supercontinent cycles.

Dating martian mafic crust; microstructurally constrained baddeleyite geochronology of enriched shergottites Northwest Africa (NWA) 7257, NWA 8679 and Zagami

Baddeleyite (monoclinic; m-ZrO2) is a widespread accessory phase within shergottites. However, the effects of shock loading on baddeleyite U-Pb isotopic systematics, and therefore its reliability as a geochronometer within highly shocked lithologies, are less well constrained. To investigate the effects of shock metamorphism on baddeleyite U-Pb chronology, we have conducted high-resolution microstructural analysis and in-situ U-Pb isotopic measurements for baddeleyite within enriched basaltic shergottites Northwest Africa (NWA) 7257, NWA 8679 and Zagami. Electron backscatter diffraction (EBSD) analyses of baddeleyite reveal significant microstructural heterogeneity within individual thin sections, recording widespread partial to complete reversion from high-pressure (≥3.3 GPa) orthorhombic zirconia polymorphs. We define a continuum of baddeleyite microstructures into four groupings on the basis of microstructural characteristics, including rare grains that retain magmatic twin relationships. Uncorrected U-Pb isotopic measurements form Tera-Wasserburg discordia, yielding new 238U-206Pb discordia ages of 195 ± 15 Ma (n = 17) for NWA 7257 and 220 ± 23 Ma (n = 10) for NWA 8679. Critically, there is no resolvable link between baddeleyite microstructure and U-Pb isotope systematics, indicating negligible open-system behaviour of U-Pb during zirconia phase transformations. Instead, we confirm that high post-shock temperatures exert the greatest control on Pb mobility within shocked baddeleyite; in the absence of high post-shock temperatures, baddeleyite yield robust U-Pb isotope systematics and date the age of magmatic crystallization. Low bulk post-shock temperatures recorded within Zagami (≤220 °C), and suggested within NWA 7257 and NWA 8679 by baddeleyite microstructure and other petrological constraints, confirm that the previously derived baddeleyite age of Zagami records magmatic crystallization, and provide greater age diversity to 225 Ma to 160 Ma enriched shergottites. While our data yield no resolvable link between microstructure and U-Pb isotopic composition, we strongly recommend that microstructural analyses should represent an essential step of baddeleyite U-Pb chronology within planetary (e.g., martian, lunar, asteroidal) and shocked terrestrial samples, allowing full contextualisation prior to destructive isotopic techniques. Microstructurally constrained in-situ U-Pb analyses of baddeleyite thus define new opportunities for the absolute chronology of martian meteorites and, more broadly, shocked planetary materials.

LA–SF–ICP-MS U–Pb baddeleyite and zircon geochronology applied to Cretaceous alkaline rocks of the São Paulo state– Brazil

The laser ablation sector field inductively coupled plasma mass spectrometry (LA–SF–ICP-MS) U–Pb geochronological method is frequently used to date minerals, in particular zircon. In turn, baddeleyite is the ideal geochronometer when it comes to silica-undersaturated rocks, such as alkaline rock types. In this study, we describe a series of protocols to separate baddeleyite crystals and date them by the LA–SF–ICP-MS method in the Isotopic Geology Laboratory of the Geoscience Institute (University of Campinas), starting from sample crushing and milling and the calibration of the Wilfley table. The method includes the use of Kovdor and Phalaborwa primary and secondary reference materials, respectively. Laser operation conditions were spot size of 25 μm and ablation time of 60 s (20 for local blank and 40 for baddeleyite signal), at a fluence of 20 L per minute. Cretaceous alkaline rocks associated with the Paraná-Etendeka magmatic province were analyzed, comprising Jacupiranga, Pariquera-Açu, and Cananéia intrusions, all related to the Guapiara lineament. The results yielded concordant baddeleyite ages of 115–120 Ma for Jacupiranga carbonatite, around 110 Ma for Pariquera-Açu shonkinite, and 81 ± 2 Ma and 83 ± 0.4, respectively for baddeleyite and zircon from the Cananéia syenite. Despite the difficulties in baddeleyite separation, the LA–SF–ICP-MS U–Pb ages have proven to be robust for the young alkaline complexes related to the Serra do Mar Province, such as those occurring along the São Jerônimo-Curiuva Lineament, avoiding the concerns about thermal events that could reset, e.g. K–Ar and Ar–Ar systems. Field relations show a hierarchy of intrusive pulses and our data show that, temporarily, the Jacupiranga Complex is active in a younger time interval, 115 to 120 Ma, than previously established.