Assembly Of Nuna Research Articles

New insights into Paleoproterozoic tectonics of the Yangtze Block in the context of early Nuna assembly: Possible collisional granitic magmatism in the Zhongxiang Complex, South China

The newly discovered Zhongxiang Complex preserving extensive Paleoproterozoic granitoids plays a pivotal role in understanding the early tectonic processes of the Yangtze Block, South China, in the context of the Nuna supercontinent cycle. New zircon U-Pb age and Hf-isotope coupled with whole-rock chemical and Nd-isotope results from three granitic intrusions in the southern Zhongxiang Complex document a magmatic record of significant late Paleoproterozoic tectonic convergence in the region. Geochronological data show that granites from these intrusions were emplaced in a relatively long period at 2.0–1.93 Ga. The granites have variably high SiO2 contents of 74.01–77.19 wt%, and are weakly to strongly peraluminous with high ASI (alumina saturation index) values (>1.05). They are generally magnesian, showing fractionated S-type granite affinities. They have negative zircon εHf(t) of −9.69 to −17.11 and negative whole-rock εNd(t) of −7.43 to −11.69, corresponding to two-stage Hf model ages of 3.24–3.67 Ga and two-stage Nd model ages of 3.0–3.37 Ga, respectively, indicating a derivation of their parental magmas from remelting of ancient crustal components (i.e., the Yangpo “Group” paragneisses). The moderate to strong LREE/HREE fractionation ((La/Yb)N > 22, average 73.15) and the relatively low Sr/Y and negative Eu anomalies of the least felsic samples further imply a residual assemblage probably involving both garnet and plagioclase, corresponding to middle-lower crustal levels. This magmatic event most likely occurred at a convergent continental margin related to continental collision, which, in conjunction with broadly coeval metamorphism and the succeeding A-type granitic magmatism in the region, delineates a possible late Paleoproterozoic tectonic transition from compression to extension, signifying a cycle from collision (2.0–1.93 Ga) to post-collision (1.85 Ga) in the Zhongxiang Complex. Such a collisional orogenesis was potentially linked to the Paleoproterozoic internal assembly of the eastern part of the Yangtze Block, presumably as part of the early assembly of the Nuna supercontinent.

New insights on the Orosirian carbon cycle, early Cyanobacteria, and the assembly of Laurentia from the Paleoproterozoic Belcher Group

The Orosirian Period (2050–1800 Ma) of the Paleoproterozoic Era represents an interval between the Great Oxidation Event and Lomagundi-Jatuli Excursion, and the apparent environmental stability of the late Paleoproterozoic to mid-Mesoproterozoic (1800–1300 Ma). Here, we present stratigraphic data, carbon isotope chemostratigraphy, element abundances, and U–Pb zircon depositional ages from the Belcher Group in subarctic Canada, providing an unparallelled global reference section for the Orosirian Period. U–Pb dates of 2018.5 ± 1.0 Ma and 2015.4 ± 1.8 Ma for two tuffs in the Kasegalik Formation (lowest Belcher Group) provide the first robust age constraints for Earth's earliest unambiguous cyanobacterial fossil, Eoentophysalis belcherensis. A tuffaceous shale ∼4 km stratigraphically higher, at the contact of the Flaherty and Omarolluk formations (1854.2 ± 1.6 Ma), provides an age for foreland basin development associated with the collision of the Superior and Hearne cratons. Carbonate carbon isotope data show smooth fluctuations ranging from −2.3 to +3.6‰, reaching a maximum near ca. 1.88 Ga. This trend toward heavier carbon isotope values is interpreted as the result of global orogenesis during Nuna assembly that elevated nutrient supply and sedimentation rates, leading to an increase in the proportion of organic carbon burial relative to carbonate carbon burial.

Early Paleoproterozoic magmatism in the Yangtze Block: Evidence from zircon U-Pb ages, Sr-Nd-Hf isotopes and geochemistry of ca. 2.3 Ga and 2.1 Ga granitic rocks in the Phan Si Pan Complex, north Vietnam

Our understanding of the early evolution of the Yangtze Block is limited by the sparsely dispersed nature of pre-Neoproterozoic exposures. New, integrated petrographic, zircon U-Pb age and Hf-Nd isotope analyses, and whole-rock geochemical data for early Paleoproterozoic granites in the Phan Si Pan Complex provides new insights into the evolution of the Yangtze Block as well as its role in the Pre-Nuna supercontinent. LA-ICP-MS zircon U-Pb dating of magmatic zircons from quartz monzonite and gneissic granite yielded 207Pb/206Pb ages of 2306 ± 12 Ma and 2096 ± 15 Ma, respectively. Zircons from the quartz monzonite have εHf(t) values ranging from −4.1 to −2.1, corresponding to TDM2 model ages of 3002–2890 Ma, whereas zircons in the gneissic granite have εHf(t) values between −0.95 and +1.72 and corresponding TDM2 model ages of 2660–2516 Ma, which are consistent with their whole-rock Nd isotope values. Geochemically, the quartz monzonites are I-type granites. Combined with their relatively high Sr/Y ratios and low Y concentrations, as well as fractionated REE patterns with relatively high LREE but low HREE concentrations, they were probably generated by partial melting of the thickened middle-lower crust under elevated temperature. Geochemical and isotopic signatures suggest that the ca. 2.1 Ga gneissic granites are high-K calc-alkaline, ferroan A-type granites formed by partial melting of juvenile crustal source at high temperature and low pressure with little involvement of ancient crustal material. The Phan Si Pan complex has a distinct early Paleoproterozoic crustal evolution history compared with the other crustal provinces of the Yangtze Block, suggesting independent histories that were not unified until the late Paleoproterozoic during the assembly of Nuna. Moreover, the magmatism and tectonic evolution of the north Vietnam region is broadly similar to that of the Arrowsmith Orogen of the Rae craton in Laurentia suggesting a potential spatial linkage. The geologic record of the Yangtze Block does not support an early Paleoproterozoic shutdown of plate tectonics.

Paleomagnetism of the Hart Dolerite (Kimberley, Western Australia) – A two-stage assembly of the supercontinent Nuna?

We present new paleo- and rock magnetic results from the ca. 1792 Ma Hart Dolerite sills that intrude the strata of the Kimberley craton, Western Australia. From 24 sites sampled, 23 are directionally clustered and the site mean directions were used to calculate a grand mean direction. Ten of the 23 sites have 95% confidence intervals (α95) less than 16°. A positive reversal test, dissimilarity of the corresponding paleopole from known younger Australian poles, and a previously reported positive baked contact test, indicate that our paleomagnetic results are of primary origin, and the results are consistent with the geologic interpretation that the Hart Dolerite sills represent a multiphase magmatic emplacement over the duration of several million years. The new ca. 1792 Ma paleopole for the Kimberley craton is applicable to the entire North Australian Craton. A comparison of Paleoproterozoic poles from the North and West Australian cratons supports the occurrence of a previously proposed 40° intracontinental rotation between the two cratons during latest Neoproterozoic time. Furthermore, comparing these Australian poles with similar aged Laurentian poles shows that the apparent polar wander paths of the two continents have a similar history between ca. 1800 and 1730 Ma and between ca. 1650 and 1400 Ma. To achieve a fit within uncertainty limits, an additional relative latitudinal motion is necessary during the transition interval (1730–1650 Ma). We suggest that a two-stage evolution occurred between Laurentia and Australia during the assembly of the supercontinent Nuna. Australia and Laurentia traveled together between ca. 1800 Ma until ca. 1730 Ma in a semi-stable configuration. During 1730–1650 Ma, they underwent a net relative latitudinal motion, leading to the final assembly of Nuna after ca. 1650 Ma.

Zircon U–Pb age and Hf isotope evidence for an Eoarchaean crustal remnant and episodic crustal reworking in response to supercontinent cycles in NW India

Scattered T DM2 (3.8–3.2 Ga) for 3.28–2.99 Ga zircons from the Proterozoic Delhi Supergroup in northwestern India provide evidence for generation of juvenile crust and reworking of older crust. Depleted mantle-like ε Hf( t ) values (+7.2 to +5.6) for 2.86–2.71 Ga zircons indicate that generation of juvenile crust occurred during this period and ceased at 2.71 Ga. Extensive magmatism at 2.66–2.34, 2.11–2.01 and 1.60–1.37 Ga was dominated by reworking of pre-existing crust with variable ages, and the last two periods were accompanied by formation of juvenile crust. An Eoarchaean age of 3671 ± 15 Ma represents the oldest age found in NW India. Zircons formed during supercontinent assembly have positive to negative ε Hf( t ) values, suggesting involvement of juvenile and ancient crust, whereas largely positive ε Hf( t ) values for zircons crystallized subsequent to supercontinent amalgamation suggest involvement of predominantly juvenile crust. Correlation of detrital age patterns and tectonomagmatic events indicates a conjugate position for northern Indian and the Cathaysia Block of South China during the assembly of Nuna. The South China Block remained juxtaposed to India until its separation from Pangaea in the late Palaeozoic. Supplementary material: Supplementary data, including detailed metadata related to laboratory and sample preparation methods, U-Pb and Lu-Hf isotopic compositions of the analyzed samples and standards are available at https://doi.org/10.6084/m9.figshare.c.3711847

Mesoproterozoic geomagnetic reversal asymmetry in light of new paleomagnetic and geochronological data for the Häme dyke swarm, Finland: Implications for the Nuna supercontinent

Baltica represents one of the key continents of the Mesoproterozoic supercontinent Nuna forming the core of it together with Laurentia and Siberia. This study presents new geochronological and paleomagnetic data obtained for the Häme diabase dyke swarm in southern Finland. New U-Pb (baddeleyite) ages 1642±2Ma and 1647±14Ma for two reversely magnetized dykes are acquired. Demagnetization revealed a dual polarity remanent magnetization direction carried by magnetite. The combined normal (N) and reversed (R) polarity direction for 11 dykes (=sites) is D=355.6°, I=−09.1° (k=8.6 and α95=16.6°) yielding a paleomagnetic pole at 23.6°N, 209.8°E (K=10.6 and A95=14.7°) with Van der Voo value Q=7. N and R magnetized units for the Häme dyke swarm show asymmetry in declination values, probably caused by an age difference between the dykes. The Geocentric Axial Dipole (GAD) model indicates that all geomagnetic reversals should be symmetric (in inclination), yet it has been noted that this is not always the case (e.g. 1.57Ga Satakunta and Åland dykes in Baltica). By analyzing global dual polarity paleomagnetic data we show that the stationary GAD model is a valid assumption at 1.7–1.4Ga and that the asymmetry between some normal and reversed polarities in global dual-polarity data sets appears randomly over time, and does not follow a global trend. Furthermore, we show that in the case of Åland and Satakunta dykes an unremoved secondary magnetization component could explain the obtained asymmetry. The GAD assumption is used to reconstruct the core of Nuna on equatorial latitudes using new data for Häme dykes. Paleomagnetic evidence suggest that maximum assembly of Nuna occurred at 1.5Ga and the dispersal of the core is proposed to be associated with coeval 1.38–1.27Ga magmatism in its core continents.

Metallogeny and its link to orogenic style during the Nuna supercontinent cycle

Abstract The link between observed episodicity in ore deposit formation and preservation and the supercontinent cycle is well established, but this general framework has not, however, been able to explain a lack of deposits associated with some accretionary orogens during specific periods of Earth history. Here we show that there are intriguing correlations between styles of orogenesis and specific mineral deposit types, in the context of the Nuna supercontinent cycle. Using animated global reconstructions of Nuna's assembly and initial breakup, and integrating extensive databases of mineral deposits, stratigraphy, geochronology and palaeomagnetism we are able to assess spatial patterns of deposit formation and preservation. We find that lode gold, volcanic-hosted-massive-sulphide and nickel–copper deposits peak during closure of Nuna's interior ocean but decline during subsequent peripheral orogenesis, suggesting that accretionary style is also important. Deposits such as intrusion-related gold, carbonate-hosted lead-zinc and unconformity uranium deposits are associated with the post-assembly, peripheral orogenic phase. These observations imply that the use of plate reconstructions to assess orogenic style, although challenging for the Precambrian, can be a powerful tool for mineral exploration targeting. Supplementary material: Supplementary material including (1) tables (S1–S3) of Euler poles and palaeopoles used, summary of Nuna orogens; (2) a figure (S1) of modelled plate velocities; (3) mp4 files (S1 & S2) of the model with age data; ore deposits and VGPs; and (4) a zip file (S1) of the Gplates model is available at http://www.geolsoc.org.uk/SUP18822 .

Australia and Nuna

Abstract The Australian continent records c. 1860–1800 Ma orogenesis associated with rapid accretion of several ribbon micro-continents along the southern and eastern margins of the proto-North Australian Craton during Nuna assembly. The boundaries of these accreted micro-continents are imaged in crustal-scale seismic reflection data, and regional gravity and aeromagnetic datasets. Continental growth ( c. 1860–1850 Ma) along the southern margin of the proto-North Australian Craton is recorded by the accretion of a micro-continent that included the Aileron Terrane (northern Arunta Inlier) and the Gawler Craton. Eastward growth of the North Australian Craton occurred during the accretion of the Numil Terrane and the Abingdon Seismic Province, which forms part of a broader zone of collision between the northwestern margins of Laurentia and the proto-North Australian Craton. The Tickalara Arc initially accreted with the Kimberley Craton at c. 1850 Ma and together these collided with the proto-North Australian Craton at c. 1820 Ma. Collision between the West Australian Craton and the proto-North Australian Craton at c. 1790–1760 Ma terminated the rapid growth of the Australian continent.

Is the rate of supercontinent assembly changing with time?

To address the question of secular changes in the speed of the supercontinent cycle, we use two major databases for the last 2.5Gyr: the timing and locations of collisional and accretionary orogens, and average plate velocities as deduced from paleomagnetic and paleogeographic data. Peaks in craton collision occur at 1850 and 600Ma with smaller peaks at 1100 and 350Ma. Distinct minima occur at 1700–1200, 900–700, and 300–200Ma. There is no simple relationship in craton collision frequency or average plate velocity between supercontinent assemblies and breakups. Assembly of Nuna at 1700–1500Ma correlates with very low collision rates, whereas assemblies of Rodinia and Gondwana at 1000–850 and 650–350Ma, respectively correspond to moderate to high rates. Very low collision rates occur at times of supercontinent breakup at 2200–2100, 1300–1100, 800–650, and 150–0Ma. A peak in plate velocity at 450–350Ma correlates with early stages of growth of Pangea and another at 1100Ma with initial stages of Rodinia assembly following breakup of Nuna. A major drop in craton numbers after 1850Ma corresponds with the collision and suturing of numerous Archean blocks.Orogens and passive margins show the same two cycles of ocean basin closing: an early cycle from Neoarchean to 1900Ma and a later cycle, which corresponds to the supercontinent cycle, from 1900Ma to the present. The cause of these cycles is not understood, but may be related to increasing plate speeds during supercontinent assembly and whether or not long-lived accretionary orogens accompany supercontinent assembly. LIP (large igneous province) age peaks at 2200, 2100, 1380 (and 1450?), 800, 300, 200 and 100Ma correlate with supercontinent breakup and minima at 2600, 1700–1500, 1100–900, and 600–400Ma with supercontinent assembly. Other major LIP age peaks do not correlate with the supercontinent cycle. A thermochemical instability model for mantle plume generation can explain all major LIP events by one process and implies that LIP events that correspond to the supercontinent cycle are independent of this cycle.The period of the supercontinent cycle is highly variable, ranging from 500 to 1000Myr if the late Archean supercratons are included. Nuna has a duration of about 300Myr (1500–1200Ma), Rodinia 100Myr (850–750Ma), and Gondwana–Pangea 200Myr (350–150Ma). Breakup durations are short, generally 100–200Myr. The history of angular plate velocities, craton collision frequency, passive margin histories, and periodicity of the supercontinent cycle all suggest a gradual speed up of plate tectonics with time.

Reconstructing pre-Pangean supercontinents

Twenty-fi ve years ago, initial plans for reconstructing the Rodinia supercontinent were being drafted, based on the growing recognition of correlatable mid-Neoproterozoic (0.8–0.7 Ga) rifted passive margins, many of which were established on the eroded remnants of late Mesoproterozoic (1.3–1.0 Ga) orogenic belts. The 1990s witnessed a surge of interest in Rodinia, with many regional studies of tectonostratigraphy and U-Pb geochronology generally conforming to the “inside-out” reconstruction model: juxtaposition of west Laurentia with east Australia/ Antarctica, north Laurentia with Siberia, and east Laurentia with Baltica and cratons that would later form West Gondwana. This standard model of Rodinia appeared to be converging toward a solution with only minor variations by the turn of the millennium, but new paleomagnetic data and tectonostratigraphic information obtained in the succeeding decade chipped away at various aspects of the reconstruction; several cratons seemed to require exclusion from the supercontinent (thus questioning its very validity), or the landmass might have assembled much later (≤0.9 Ga) than originally envisaged (thus weakening the link to global Mesoproterozoic orogenesis). Although a consensus model of Rodinia’s assembly and fragmentation has arisen from the International Geoscience Programme Project 440 working group, the reconstruction is supported by rather sparse defi nitive-quality data. As the quest for Rodinia matures to a third decade of scrutiny, the search for its predecessor Nuna (a.k.a. Hudsonland or Columbia) is only now reaching a stage of global synthesis between tectonostratigraphic and paleomagnetic data. According to most defi nitions, Nuna assembled at 1.9–1.75 Ga, or perhaps as late as 1.6 Ga, and fragmented during the interval 1.5–1.2 Ga. Because mafi c dike swarms are ideal targets for paleomagnetic study, and because they are now amenable to routine dating by U-Pb on baddeleyite, the global abundance of Paleo-Mesoproterozoic dike swarms might make Nuna more imminently solvable than Rodinia. Prior to the assembly of Nuna, various “supercraton” connections such as Vaalbara, Superia, and Sclavia are only beginning to take form. Unmetamorphosed, early Paleoproterozoic (2.5–2.0 Ga) mafi c dike swarms are commonplace features across the interiors of Archean cratons, and their joint paleo magnetic and geochronologic study can help reassemble the cratons into their supercraton parent landmasses. Progressively older geologic times require consideration of a greater number of potentially independent terranes, each needing individual kinematic constraints. Furthermore, the initial stabilizing events of most extant cratons during Neoarchean time (3.0–2.5 Ga) therefore render global reconstructions older than that interval improbable.