Precambrian Supercontinents Research Articles

Precambrian geological history of the Tarim Craton (NW China) involving the assembly and fragmentation of the Columbia and Rodinia supercontinents: review and synthesis

The Tarim Craton (NW China) is a significant archive of the tectonic events that occurred during the assembly and break-up of the Precambrian supercontinents Columbia and Rodinia. It provides a comprehensive record of crustal development during the Proterozoic. We review and synthesize the magmatic, metamorphic and stratigraphic records of the Precambrian Tarim Craton and delineate the geochronology of significant geological events that led to the formation of this major cratonic block. The extant geophysical and geological data show that the Tarim Craton consists mainly of the South and North Tarim blocks. The record of late Paleoproterozoic tectonothermal events is displayed well in and across the craton, involving the amalgamation of these two blocks to form its unified crystalline basement during the build-up of Columbia. However, the record of mid-Neoproterozoic events during the assembly of Rodinia is only exposed around its periphery. The Proterozoic record of the craton includes Mesoproterozoic–early Neoproterozoic low-grade metamorphic rock units and a late Neoproterozoic sedimentary cover. The early Mesoproterozoic stratigraphy in the SW, NE and SE of the Tarim Basin suggests a period of tectonic stability following the amalgamation of the North and South Tarim blocks in the late Paleoproterozoic, indicating the completion of its cratonic build-up.

The Precambrian continental record: A window into early Earth environments

The Precambrian was characterized by unique palaeoenvironmental conditions in the Earth’s atmosphere, biosphere and geosphere. This study presents a global quantitative analysis of Precambrian sedimentary successions of aeolian, alluvial, fluvial, lacustrine and glacigenic origins, examined in the broader context of Earth evolution. In the Precambrian, an apparent scarcity of aeolian successions is observed. This may be linked to: (1) differences in atmospheric density, which controlled wind erosion and sedimentation; (2) different astronomical configurations, which may have influenced tides and atmospheric circulation, thereby affecting sand availability and the width of subtropical zones; (3) potentially hotter and more humid climates, restricting dry-sand availability; (4) a lack of vascular vegetation that could prevent reworking of aeolian deposits; (5) poor preservation potential; (6) misinterpretation of the Precambrian record. Mixed aeolian-alluvial strata are more abundant, perhaps because their preservation in the geological record was favoured by water tables sustained by incursions of alluvial systems into otherwise aeolian dominated environments. Aeolian deposits were preferentially accumulated during phases of supercontinental breakup, where rapidly subsiding rift basins provided accommodation suitable for preservation. Other than in the Neoproterozoic record, where glacigenic deposits dominate, alluvial strata are the most common and thickest type of continental deposit in the Precambrian. Precambrian braided alluvial systems were more widespread than in the Phanerozoic. Major alluvial systems formed preferentially during phases of supercontinent assembly, whereby alluvial systems drained major orogens, and long drainage pathways developed from supercontinent interiors to coastlines. In the Paleoproterozoic, ephemeral, saline to partly arid lakes developed extensively in the desertic interior of Columbia. Glacial deposits preferentially formed in the breakup phase of supercontinental cycles; this supports theories invoking enhanced chemical weathering of uplifted rift shoulders as a driver of carbon dioxide sequestration, global cooling, and glaciation. Overall, the number of identified continental successions increases towards the Precambrian-Phanerozoic boundary. This may be an artefact of an increasingly more complete stratigraphic record as time progresses. However, the abundance of continental successions varies on a quasi-periodic cycle of 500 – 700 Myr, with peaks coinciding with the tenure and breakup of Precambrian supercontinents.

Crustal evolution and tectonomagmatic history of the Indian Shield at the periphery of supercontinents

River sand detrital zircons from several rivers flowing through the Peninsular Indian cratons were analyzed for U-Pb, Lu-Hf, and O isotopes to characterize the Precambrian crustal evolution of the Indian Shield and to constrain its role in the early supercontinent cycles. Analyzed river sand zircon samples exhibit a prominent age grouping at 2.7–2.4 Ga and additional peaks at 1.8–1.7 Ga, 1.0–0.8 Ga, and 0.6–0.5 Ga. The time-related Hf and O isotope trends of the Indian zircons display a slight offset from the global trends. The age peaks and distinct Hf-O isotopic compositions of the Indian zircons also lack coherence with the global patterns associated with supercontinent cycles, implying a discrete crustal evolutionary history for the Indian Shield. Their contrasting εHf-age trajectories indicate that the Indian Shield was accreted to the Columbia/Nuna supercontinental framework through a collision event that postdated the 2.1–1.8 Ga global-scale orogeny, and was part of a long-lived subduction system along the margin of Rodinia. This implies that the Indian Shield occupied a peripheral paleo-position during the assembly of the two Precambrian supercontinents. The ca. 2.6–2.4 Ga Indian river sand zircons have remarkably low δ18O values that are distinct from the global zircon O isotopic record. This period coincides with the fragmentation of the Archean supercraton and the flaring-up of a subaerial Large Igneous Province that facilitated the generation of 18O-depleted magmas. Depending on whether the 2.6–2.4 Ga zircons with low δ18O values (<4.7 ‰) are considered or not, approximately 2.77 ‰ or 1.78 ‰ rise in the zircon δ18O values occurred between ca. 2.4 Ga and ca. 1.87 Ga (decoupled from zircon Lu-Hf isotopes), further suggesting that the fine-grained sediments were enriched in 18O after the Great Oxidation Event. These high δ18O sediments were subsequently incorporated into the magmatic systems resulting in elevated δ18O in the zircons crystallizing from such melts. Nevertheless, a ca. 0.9 Ga peak of >10 ‰ in δ18O system was followed by a pronounced δ18O drop at ca. 0.759 Ga, suggesting that δ18O of recycled sedimentary reservoirs was not the only controlling factor for zircon δ18O characteristics. Variations in the Hf-O data for Indian and global zircons could be attributed to the initiation of the supercontinent cycle at ca. 2.0 Ga. A significant volume of the juvenile crust was added during the 3.0–2.7 Ga mafic magmatism in the Indian Shield, as underlined by the Hf model ages of all the zircon grains and those with mantle-like δ18O signatures. A gradual addition of continental crust during ca. 3.6 Ga to 3.1 Ga can also be deduced, with the oldest crust being derived from the mantle at ca. 4.4 Ga.

Provenance, Age, and Tectonic Settings of Rock Complexes (Transangarian Yenisey Ridge, East Siberia): Geochemical and Geochronological Evidence

The tectonic evolution of the Siberian Cratonic margins offers important clues for global paleogeographic reconstructions, particularly with regard to the complex geological history of Central Asia and Precambrian supercontinents Columbia/Nuna and Rodinia and its subsequent breakup with the opening of the Paleo-Asian Ocean. Here, we present an overview of geochemical, petrological, and geochronological data from a suite of various rocks to clarify the age, tectonic settings, and nature of their protolith, with an emphasis on understanding the tectonic history of the Yenisey Ridge fold-and-thrust belt at the western margin of the Siberian Craton. These pre-Grenville, Grenville, and post-Grenville episodes of regional crustal evolution are correlated with the synchronous successions and similar style of rocks along the Arctic margin of Nuna-Columbia and Rodinia and support the possible spatial proximity of Siberia and North Atlantic cratons (Laurentia and Baltica) over a long period ~1.4–0.55 Ga.

ANOROGENIC MAGMATISM OF THE YENISEY RIDGE: EVIDENCE FOR BREAKUP OF THE PRECAMBRIAN SUPERCONTINENTS

Here we report the occurrence of A-type granites with geochemical features indicating intraplate setting from the Yenisey Ridge. Zircon U–Pb analyses coupled with in situ U–Th–Pb geochronology of monazite constrain timing of emplacement of the rift–related granitoids and suggest two consequential breakup events at 1380 Ma and 800–720 Ma. These episodes of regional crustal evolution are correlated with the breakup ages of the Precambrian supercontinents Nuna and Rodinia supercontinents and supports the spatial proximity of Siberia and North Atlantic cratons (Laurentia and Baltica) over the long period 1.38–0.72 Ga.

The implication of two episodic Precambrian supercontinents convergence events from enriched mantle beneath the Yangtze Block: Constraints from the Zhangbaling mafic rocks

The Yangtze Block bears witness to a prolonged Precambrian evolutionary history and records the assembly of Precambrian supercontinents. Significantly, compared with the widely recognized ancient continental crustal growth and reconstruction, subduction-related mafic magmatism contains detailed possess of crust-mantle interaction in convergent plate boundaries. This paper reports Neoproterozoic mafic igneous rocks, represented by the 808–787 Ma amphibolites, from the Zhangbaling Uplift, northeastern sector of the Yangtze Block. Their prominent arc-affinity trace-element geochemical compositions with high concentration of LILE, the trough of Th, U, Zr and Hf in the spider diagram as well as natural δ18O values (5.3–6.5‰), indicate an interaction between the lithospheric mantle wedge peridotite and slab-derived fluids during early Neoproterozoic subduction event, and deficient in the involvement of felsic melts from crustal materials and sediments. Analyses of radioisotopes for amphibolites yield extremely low zircon εHf(t) (−25.3 to −3.6) and whole-rock εNd(t) (−17.1 to −10.4) values respectively, corresponding to an enriched mantle source. However, in consideration of the fact that the recycled slab-derived fluids is difficult to alter the isotopic composition (especially hafnium) of lithospheric mantle compared with its geochemical characteristics. Meanwhile, on the compilation of all Precambrian subduction-related magmatism in the Yangtze Block, it appears that the enriched isotopic signature in amphibolites is mainly inherited from the Paleoproterozoic (ca. 2.0 Ga) lithospheric mantle beneath the Yangtze Block, and extremely negative εHf(t) values (–25.3 to –18.4) further reflect prominent crust-mantle interaction correspond to the assembly of Columbia supercontinents. Therefore, the Zhangbaling mafic rocks witness the modification of lithospheric mantle by two episodic subduction events during Paleoproterozoic and Neoproterozoic respectively, which provide new evidence for the Yangtze Block to get involved in the reconstruction of Columbia and Rodinia supercontinents.

Revisiting to the Neoproterozoic tectonic evolution of the Tarim Block, NW China

The Tarim Block comprises two terranes, namely, the Northern Tarim terrane (NTT) and the Southern Tarim terrane (STT). The distinct geological features of the Pre- Cryogenian basement and igneous activity between these two terranes that share common Cryogenian cover sequences strongly suggest that they underwent independent pre-Cryogenian evolutions. However, owing to the likely position of the Tarim Block on the margin of Rodinia, the Neoproterozoic igneous rocks and sedimentary sequences of this block should provide important insights into the geodynamics of the evolution of this supercontinent. In this contribution, our new geochronological data show that the earliest glacial deposit in the Tarim Block, namely, the Beiyixi tillite, was deposited a little later than 720 Ma, which is consistent with the timing of the global Sturtian glaciation. The other two glacial deposits, namely, the Altungol–Tereenken and Hankachocugh tillites, were deposited at 685–635 Ma and later than 591 Ma, consistent with the global Marinoan and Gaskiers glaciations, respectively.Combining our field observations and new geochronological data with a comprehensive synthesis of the Precambrian basement rock units, metamorphic events, and igneous activities in both the NTT and STT, we construct a detailed late Mesoproterozoic to Cryogenian evolution for the Tarim Block, and discuss its relationship to Precambrian supercontinent evolution. We suggest that the STT might be a continental fragment that separated and drifted from the Congo Craton during the initial breakup of Rodinia and subsequently docked along the northern fringe of Australia a little later than ca. 800 Ma. In contrast, the NTT might be a continental fragment that detached and drifted from North China–India during the breakup of the Columbia supercontinent. During the process of breakup of Columbia and the assembly of Rodinia, the NTT amalgamated with the STT a little earlier than 800 Ma, owing to circum-Rodinia subduction between 1000 and 760 Ma. However, upwelling of the Rodinia plume could have affected subduction along the northern margin of the Tarim Block, as demonstrated by a voluminous ca. 770 Ma mafic dyke swarm. The Rodinia plume terminated the subduction along the northern margin of the Tarim Block at ca. 740 Ma, suggesting that the plume could have played a key role in the breakup of Rodinia. This conclusion is supported by Cryogenian–Cambrian sedimentary sequences with passive-continental-margin affinity in the Tarim Block, indicating the detachment and movement of this block from Rodinia. Both the characteristics of Cryogenian–Cambrian basins and coeval igneous activity argue strongly against long-term subduction along the northern margin of the Tarim Block.

Low-grade Sandow Group metasediments of the Denman Glacier area (East Antarctica): Chemical composition, age and provenance from U–Pb detrital zircon data, with some palaeotectonic implications

The Sandow Group in the Denman Glacier area consists of low-grade supracrustal (post-cratonic) strata. Such rocks are scarce and poorly studied in East Antarctica, but are significant for a better understanding of the geological history and tectonic evolution of the Precambrian supercontinents. We report U–Pb (LA-ICP-MS) detrital zircon ages which mostly fall into two groups of ca 1350−900 Ma and ca 1800−1500 Ma, which correspond to those of crystalline rocks exposed in the western Australo-Antarctica. The youngest zircons with magmatic zoning yield ages of ca 950–900 Ma which define the maximum age of sedimentation. Chemical and neodymium isotopic compositions of the Sandow Group rocks indicate derivation from a Proterozoic largely granitic source region. High-Ti mafic volcanic activity accompanied sedimentation, and therefore the Sandow Group should be considered a volcanic-sedimentary succession. The Sandow Group was presumably accumulated in a continental (pull-apart?) basin formed in the Neoproterozoic in relation to proto-Darling Fault system activity with its continuation into Antarctica. The Sandow Group may have its correlatives in Western Australia where sedimentary successions (e.g., Moora and Badgeradda Groups) occur along the proto-Darling Fault system.

Zircons from the Tarim basement provide insights into its positions in Columbia and Rodinia supercontinents

The positions of the Tarim Craton in two Precambrian supercontinents Columbia and Rodinia still remain unknown or controversial, as the craton is largely covered by desert in the central part and only sparse Precambrian basement rocks are scattered at its margins. In this contribution, we attempt to use new in situ zircon U-Pb ages and Hf isotopic data, combined with comprehensive compiled data from Precambrian basement rocks at its margins, to provide new insights into positions in two Precambrian supercontinents. Detrital zircon profiles of northern Tarim and southern Siberia are characterized by a peak at 2.0–1.8 Ga and the lack of 1.7–1.1 Ga zircons. Moreover, both the 2.0–1.8 Ga detrital zircons show remarkable similarities dominated by mostly negative εHf(t) values ranging from −15 to 0. In addition, 2.0–1.8 Ga collisional granitic rocks are exposed in two terranes. These suggest that northern Tarim likely collided with southern Siberia at 2.0–1.8 Ga during the assembly of Columbia. In southern Tarim, Neoproterozoic magmatic rocks were closely related with the evolution of Rodinia. The ~0.9 Ga Ma gneissic granites in Altyn Tagh area were formed in collisional settings due to the collision between southern Tarim and northern India during the assembly of Rodinia, which is supported by distinctly comparable zircon age spectra and Hf isotopes from Neoproterozoic strata in two terrenes. This model considers the Tarim Craton as a single block in the period from Columbia to Rodinia, which can explain different tectonomagamtic events in northern and southern Tarim.

A-type granites in the western margin of the Siberian Craton: Implications for breakup of the Precambrian supercontinents Columbia/Nuna and Rodinia

The tectonic evolution of the Siberian Cratonic margins offers important clues for global paleogeographic reconstructions, within the complex continental collage of Central Asia. The Yenisey Ridge fold–and–thrust belt at the western margin of the Siberian Craton forms part of the Central Asian Orogenic Belt (CAOB) and is a key to understand the Precambrian tectonic evolution of the Siberian Craton and crustal growth in the CAOB, the world's largest Phanerozoic accretionary orogenic belt. Here we report the occurrence of A-type granites with geochemical features indicating intraplate setting from the Yenisey Ridge and provide evidence for rift-related magmatism. Zircon SHRIMP U–Pb analyses coupled with in situ U–Th–Pb geochronology of monazite constrain the timing of emplacement of the rift–related granitoids and suggest two consequential breakup events. The magmatic events at 1380 Ma and 800–720 Ma along the western margin of the Siberian Craton and other continental blocks can be associated with the breakup of the Precambrian supercontinents Nuna-Columbia (1.8–1.3 Ga) and Rodinia (1.2–0.7 Ga). These pre-Grenville and post-Grenville episodes of regional crustal evolution are correlated with the synchronous successions and similar style of rocks along the Arctic margin of Nuna–Columbia and Rodinia and supports the spatial proximity of Siberia and North Atlantic cratons (Laurentia and Baltica) over the long period 1.38–0.72 Ga. Our data confirm the proposed Neoproterozoic paleogeographic reconstructions of Columbia and Rodinia as constrained from the large igneous province (LIP) record.

Revisiting the Precambrian evolution of the Southwestern Tarim terrane: Implications for its role in Precambrian supercontinents

The Tarim block in NW China consists of a Precambrian basement and late Neoproterozoic to Paleozoic cover sequences. In order to better understand its roles in Precambrian supercontinent evolution, we used SHRIMP and LA-ICPMS U-Pb methods to date late Paleoproterozoic rifting-related mafic dykes and Mesoproterozoic igneous activity in the Precambrian of the southwestern Tarim terrane (STT): 1) WNW trending mafic dyke swarm emplaced at ca. 1785 Ma; 2) Mesoproterozoic bimodal volcanic sequence (the lower member) and marble- clastic-quartzite sequences (the upper member) deposited at ca. 1525 Ma and 1500–1400 Ma, respectively; 3) late Mesoproterozoic granodiorite-augen granite-leucogranite complex emplaced at ca. 1117 Ma. Geochemistry revealed that the 1784 ± 12 Ma mafic dykes were derived from an E-MORB – like mantle source suggesting an extensional setting, and likely linked with coeval major large igneous provinces (LIPs) on several crustal blocks which collectively suggest a pulse of rifting associated with assembly of the Columbia supercontinent. The obtained U-Pb age of 1525 ± 4 Ma from a meta-rhyolite in the bimodal sequence matches a recently discovered 1525–1529 Ma Large Igneous Province from the southern West African craton (Essakane-Sambarabougou swarm) and formerly adjacent Amazonian craton (Kayser event). Geochemistry of the ca. 1117 Ma granitic intrusions defines their A-type signature. These may be linked to coeval intraplate mafic magmatism of a reconstructed 1110 Ma LIP involving Kalahari, Congo, India (Bundelkhand block), Amazonia cratons. In line with these three newly recognized pulses of Precambrian igneous activities in STT and the significant diachroneity of the Precambrian tectono-magnetic events between the STT and the Northern Tarim terrane (NTT), we argue that the Precambrian basement of the Tarim were most likely composed of two distinct terranes (i.e., the STT and the NTT). These originated from different Precambrian nuclei and they did not amalgamate as the Tarim block until to the early middle Neoproterozoic.

Coupled Precambrian crustal evolution and supercontinent cycles: Insights from in-situ U-Pb, O- and Hf-isotopes in detrital zircon, NW india

Oxygen and hafnium isotopic compositions, measured in-situ on U-Pb dated zircon grains from Paleoproterozoic to early Cambrian successions in NW India have implication for regional crustal evolution and supercontinent cycles. Analyzed zircon grains have high Th/U ratios (>0.1), display strongly fractionated REE patterns, metamorphic overprint, and evidence of interaction with low temperature fluids. Their positive Ce and negative Eu anomalies preclude any Pb loss after zircon crystallization. The U-Pb age spectra (concordance between 90 and 110%) indicate prominent peaks at 2.6 to 2.4 Ga, 1.9 to 1.7 Ga, 1.6 to 1.5 Ga, 1.2 to 1.0 Ga and 0.9 to 0.7 Ga that coincide with the assembly and breakup of Precambrian supercontinents. The Hf model ages of zircon grains with mantle like δ18O values reveal continuous generation of the continental crust from 3.3 to 1.3 Ga in NW India with major episodes during 3.3 to 2.7 Ga and 1.7 to 1.5 Ga. These ages correspond well with the 3.4 to 2.9 Ga and 2.2 to 1.6 Ga age peaks recognized in detrital zircon populations from eastern Australia and North America, underlining the significance of these time brackets in continental crust generation during the global continental evolution. Magmatic episodes at 1.9 to 1.7, 1.2 to 1.0 and 0.9 to 0.7 Ga are considered to represent crustal reworking rather than juvenile addition and the former two phases correspond with periods of supercontinent assembly. However, a progressive depletion in 18O from supra-mantle to mantle values in the 1.7 to 1.5 Ga zircons, coupled with their mantle-like eHf(t) values, indicate at least some juvenile input. Moreover, the 1344 to 1120 Ma zircon grains with low δ18O (3.7–1.5‰) but high eHf(t) (+8.1− +1.9 with one exception of −2.5) values signify rapid reworking of mantle derived materials in an extensional setting during this period. The 0.9 to 0.7 Ga peak, corresponding to the fragmentation of Rodinia supercontinent, documents crustal reworking that is in contradiction to the generally considered juvenile crustal addition in extensional setting associated with supercontinent breakup.

A newly identified Precambrian terrane at the Pamir Plateau: The Archean basement and Neoproterozoic granitic intrusions

A number of Precambrian terranes are distributed in the Tibetan Plateau, including the Lhasa, Sibumasu, Qiangtang, Qaidam, Qilian and Tianshuihai terranes. These terranes exhibit distinct features in rock associations and tectonothermal history. They are important for understanding the evolution of the Tethys Ocean and the assemblage and breakup of the Precambrian supercontinents. In this contribution, we identify a Precambrian terrane at the southeastern section of the Pamir Syntax and we name it as Mazar terrane. Systematic zircon U-Pb dating shows that the basement, mainly composed of metamorphic bimodal volcanic rocks and gneiss/schist, deposited at ca.2.5 Ga and was overprinted by ca.2.0 Ga amphibolite-facies metamorphism. The Neoproterozoic granitic intrusions into the basement, emplaced at 840–835 Ma. Geochemistry of the granitic intrusions defines two distinct types of granitic rocks, including the trondhjemite-tonalites and granodiorites. The sodic trondhjemite- tonalites show TTG-like or adakitic signatures such as enrichment of LREE and LILEs, depletion of HREE and HFSE, resulting in their high Sr/Y and (La/Yb)N ratios. The granodiorites exhibit typical signatures of the calc-alkaline I-type granites. Both trondhjemite-tonalites and granodiorites have coupled unradiogenic whole-rock Nd and zircon Hf isotope compositions. εNd(T) and εHf(T) values of the trondhjemite- tonalites range from −9.5 to −8.7 and from −14 to −10, respectively. The granodiorites have slightly more radiogenic Nd-Hf isotope compositions with εNd(T) values ranging from −5.5 to −4.4 and εHf(T) values ranging from −10 to −7. According to elemental and isotope signatures of the granitic intrusions, the trondhjemite-tonalites were derived from partial melting of a mafic crust at depth ≥30 km with a garnet amphibolite residue whereas the granodiorite was formed by partial melting of a mafic crust at a shallower level and mantle-derived magma could be involved in the genesis of the granodiorite. In line with their geochemistry and the fact that the granodiorite is slightly younger than that of the trondhjemite-tonalites, we propose that those granitic intrusions were most likely formed in an extensional environment, and that they could be part of the earliest igneous activity related to the initial breakup of the Rodinia supercontinent. Our study, in combination with previous works, suggests that the Mazar terrane, as well as the Qilian, Qaidam, Tianshuihai terranes, is most likely continental fragments drifted from the Yangtze Block during the breakup of the Rodinia supercontinent.

Proterozoic tectonics of Hainan Island in supercontinent cycles: New insights from geochronological and isotopic results

Hainan Island in southwestern south China has 1.43Ga crystalline rocks as part of the Proterozoic basement rocks for the Cathaysia Block. Understanding Proterozoic tectonics of the island is thus essential for constraining the paleogeographic positions of the Cathaysia Block in Precambrian supercontinents. We report here new geochronological and Hf–O isotopic results for Mesoproterozoic gneisses, metavolcanic and metasedimentary rocks, and quartz sandstones from Hainan Island. Together with published data from the region, we divide Hainan Mesoproterozoic rocks into three major tectonostratigraphic units: (1) the amphibolite-facies Baoban Complex, consisting mainly of 1.43Ga volcanic/plutonic rocks and sedimentary rocks formed in a continental rift setting but subsequently experienced high-grade metamorphism during 1.3–1.0Ga; (2) the greenschist-facies Shilu Group, consisting of 1.43Ga rift sediments with 1.44Ga tuff in the “fifth layer” that are coeval with the protolith of the Baoban Complex, and ca. 1.0Ga foreland basin sediments in the “sixth layer”, all underwent upper greenschist-facies metamorphism soon after the deposition of the “sixth layer”; (3) the Shihuiding Formation that unconformably overlies the Shilu Group, consisting of <1.1Ga (most likely ⩽1.0Ga) coastal marine to lagoon sediments deposited in a post-orogenic setting. Inter-continental provenance and magmatic comparisons suggest a united Tasmania-Cathaysia-Laurentia landmass during the Mesoproterozoic, likely all received similar-sourced rifting sediments from Cathaysia-like basements during the Nuna breakup at ∼1.43Ga. The 1.3(?)–0.9Ga Grenville-aged Sibao orogeny welded the Yangtze Block with the Cathaysia-Laurentia landmass, bringing Tasmania, Yangtze and the Cathaysia-Laurentia landmass together. Sedimentary detritus were shed from the orogen to basins covering Hainan Island, western Yangtze, Tasmania and western Laurentia during the assembling of Rodinia.

Paleomagnetism of the Amazonian Craton and its role in paleocontinents

ABSTRACT: In the last decade, the participation of the Amazonian Craton on Precambrian supercontinents has been clarified thanks to a wealth of new paleomagnetic data. Paleo to Mesoproterozoic paleomagnetic data favored that the Amazonian Craton joined the Columbia supercontinent at 1780 Ma ago, in a scenario that resembled the South AMerica and BAltica (SAMBA) configuration. Then, the mismatch of paleomagnetic poles within the Craton implied that either dextral transcurrent movements occurred between Guiana and Brazil-Central Shield after 1400 Ma or internal rotation movements of the Amazonia-West African block took place between 1780 and 1400 Ma. The presently available late-Mesoproterozoic paleomagnetic data are compatible with two different scenarios for the Amazonian Craton in the Rodinia supercontinent. The first one involves an oblique collision of the Amazonian Craton with Laurentia at 1200 Ma ago, starting at the present-day Texas location, followed by transcurrent movements, until the final collision of the Amazonian Craton with Baltica at ca. 1000 Ma. The second one requires drifting of the Amazonian Craton and Baltica away from the other components of Columbia after 1260 Ma, followed by clockwise rotation and collision of these blocks with Laurentia along Grenvillian Belt at 1000 Ma. Finally, although the time Amazonian Craton collided with the Central African block is yet very disputed, the few late Neoproterozoic/Cambrian paleomagnetic poles available for the Amazonian Craton, Laurentia and other West Gondwana blocks suggest that the Clymene Ocean separating these blocks has only closed at late Ediacaran to Cambrian times, after the Amazonian Craton rifted apart from Laurentia at ca. 570 Ma.

Long-lived connection between southern Siberia and northern Laurentia in the Proterozoic

Precambrian supercontinents Nuna-Columbia (1.7 to 1.3 billion years ago) and Rodinia (1.1 to 0.7 billion years ago) have been proposed. However, the arrangements of crustal blocks within these supercontinents are poorly known. Huge, dominantly basaltic magmatic outpourings and intrusions, covering up to millions of square kilometres, termed Large Igneous Provinces, typically accompany (super) continent breakup, or attempted breakup and offer an important tool for reconstructing supercontinents. Here we focus on the Large Igneous Province record for Siberia and Laurentia, whose relative position in Nuna-Columbia and Rodinia reconstructions is highly controversial. We present precise geochronology—nine U–Pb and six Ar–Ar ages—on dolerite dykes and sills, along with existing dates from the literature, that constrain the timing of emplacement of Large Igneous Province magmatism in southern Siberia and northern Laurentia between 1,900 and 720 million years ago. We identify four robust age matches between the continents 1,870, 1,750, 1,350 and 720 million years ago, as well as several additional approximate age correlations that indicate southern Siberia and northern Laurentia were probably near neighbours for this 1.2-billion-year interval. Our reconstructions provide a framework for evaluating the shared geological, tectonic and metallogenic histories of these continental blocks. The configurations of the ancient supercontinents are poorly known. Analysis of the ages of giant magma intrusions that affected both Siberia and Laurentia shows that the two continents were connected, possibly for as long as 1.2 billion years.

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).

Geochemistry, mineralogy and petrology of the Eocene potassic magmatism from the Milk River area, southern Alberta, and Sweet Grass Hills, northern Montana

New whole-rock geochemical, quantitative mineralogical and electron-microprobe data are presented from lamproite, olivine minettes and mafic trachyte of the Milk River area, southern Alberta, and latites, trachytes and high-K rhyolite of the Sweet Grass Hills, northern Montana. These rocks comprise dikes, small plugs and a pipe complex associated with mainly syenite and hornblende monzonite plutons that core several buttes in the Sweet Grass Hills and mark the northern extent of the Cenozoic Montana Alkaline Province. Minerals restricted to peralkaline lamproite at the 49th Parallel locality include Ti-rich phlogopite–annite with Fe3+ in tetrahedral coordination, Na-poor sanidine, Ba-rich and Cl-poor fluorapatite, K-rich titano-ferronyböite/ferro-obertiite, LREE-rich strontian fluorapatite (SrO=17.5–25.2wt.%, Ce2O3=2.9–3.3wt.%), ilmenite, strontian barite, barian celestine, and unidentified Zr–Ti phase (TiO2=76.7–88.0wt.%, ZrO2=2.1–7.2wt.%) perhaps replacing primary Zr–Ti silicates. The strontian fluorapatite has the highest SrO content thus far reported in the literature for apatites from lamproites. Olivine minettes and mafic trachyte of the Milk River area contain augite, Al-diopside, Ti-rich magnetite, sanidine with higher Na, hyalophane, Cl-bearing fluorapatite with negligible Ba, ferroan magnesite phenocrysts, and primary calcite, all of which are notably absent in the 49th Parallel lamproite. Mica displays distinct core-to-rim zoning trends in these rocks, characterized by increasing FeOt and TiO2 at decreasing Al2O3 in lamproite and increasing FeOt and TiO2 at increasing Al2O3 in minettes and trachytes. The 49th Parallel lamproite has higher Ti, K, P, Ba, Rb, Sr, Nb, Th, U, Zr, Hf and REE concentrations, higher K/Al, (Ce/Yb)CN, (La/Sm)CN, (Gd/Yb)CN, Rb/Cs, Ce/Pb, Th/Ta, Th/Yb, and lower Ba/La, Ba/Th and Nb/U relative to minettes of the Milk River area. Major-element, least-squares mass balance and trace-element models call for the origin of latites, trachytes and high-K rhyolite in the Sweet Grass Hills complex by two-stage fractional crystallization of primitive olivine minette magmas concurrent with negligible, if any, assimilation of crustal materials. Oscillatory and reverse zoning in phenocrysts and hybrid phenocryst assemblages record recharge of more evolved magma chambers with primitive minette magmas. The calculated evolutionary models are inconsistent with the compositional variations displayed by the olivine minettes and lamproite of the Milk River area. These rocks represent distinct low-degree partial melts of sub-lithospheric mantle enriched in K and volatiles by the recent (<100Ma) plume activity or convective upwelling of the upper mantle above subducted Farallon slab, which underwent variable assimilation of the ancient, LREE-Ba–Sr enriched and HFSE depleted Wyoming–Medicine Hat lithosphere during the post-Laramide extension. The olivine minettes derived from more oxidized, carbonated region of the mantle source, compared to the more reduced source of the 49th Parallel lamproite. The extremely negative εNd values, unradiogenic Pb and moderately radiogenic Sr isotopic compositions, correlated with LREE, Ba enrichment and relative Nb, Ta, Ti and other HFSE depletions of the Cenozoic K-rich rocks of the MAP, may reflect carbonatite metasomatism of the lithospheric mantle associated with the widespread Mesoproterozoic (1.5–1.4Ga) anorogenic magmatism and intracontinental rifting related to mantle upwelling and plume activity beneath a thick Precambrian supercontinent.

Pre-Rodinia supercontinent Nuna shaping up: A global synthesis with new paleomagnetic results from North China

The existence of a pre-Rodinia Precambrian supercontinent, variously called Nuna or Columbia, has been widely speculated in the past decade, but the precise timing of its existence and its configuration have been uncertain due to the lack of unequivocal paleomagnetic and geological constraints. Here we report high-quality paleomagnetic results from the well dated ∼1780Ma Xiong'er Group in southern North China Block (NCB). A total of 110 paleomagnetic samples from 14 sites were collected and subjected to stepwise thermal demagnetization. After removing a low temperature component (CL) of viscous magnetic remanence acquired in recent geomagnetic field, a high temperature component (CH), carried by hematite and magnetite in redbeds and volcanic samples, has been isolated. It gives a mean direction of (D=18.4°, I=−3.7°, α95=7.6°, N=14) after bedding correction, and a corresponding paleomagnetic pole at 50.2°N, 263.0°E (A95=4.5°). The CH passed a reversal test and was interpreted as a primary remanence. This new pole plus three other high-quality poles from the NCB that have been more precisely dated at 1769±3Ma, 1560–1440Ma and 1437±21Ma define a 1780–1440Ma apparent polar wander path (APWP) for the NCB. This, together with an update of global high quality paleomagnetic dataset, allows us to demonstrate that the pre-Rodinia supercontinent Nuna likely existed at least between ∼1780Ma and ∼1400Ma. Our paleomagnetism-based global reconstruction, for the first time, quantitatively assembles all major cratons together; it encompasses previously proposed regional links including the SAMBA connection between Baltica, Amazonia and Western Africa (Johansson, 2009), connections between Laurentia, Baltica and Siberia at the core of Nuna (Evans and Mitchell, 2011), the proto-SWEAT connection between Laurentia, East Antarctica and Australian blocks (Payne et al., 2009), and the NCB–India connection (Zhao et al., 2011).

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.