Formation Of Supercontinent Research Articles

Two stages of Neoproterozoic magmatism in the evolution of the Bureya continental massif of the Central Asian Fold Belt

U-Pb geochronological studies have shown at least two stages of Neoproterozoic magmatism, 940-933 and 804-789 Ma, throughout the geologic evolution of the Bureya continental massif in the east of the Central Asian Fold Belt. The first stage (940-933 Ma) was marked by the formation of the protoliths of metagabbroids and amphibole-biotite gneiss-granites of the Bureya River basin. The geochemical features of these rocks suggest that they formed, most probably, in an island-arc setting on the continental basement or on an active continental margin. The second stage (804-789 Ma) of magmatism was manifested as the formation of biotite leucogranites and hastingsite-lepidomelane granites (A-type granites) in the Chepkan River basin. The latter rocks are similar in geochemical features to within-plate granitoids. The established stages of magmatism are the most ancient for the continental massifs in the east of the Central Asian Fold Belt. It is not ruled out that these magmatism stages were related to the stages of formation and breakup of the Rodinia supercontinent.

Singularity analysis of magmatic flare-ups caused by India – Asia collisions

Frequency distribution (histogram) calculated on the basis of igneous and detrital zircon U-Pb ages has been commonly utilized to interpret the age (range) of magmatic events. The temporal properties revealed by these types of data have also been integrated with other types of isotope data (e.g., neodymium, hafnium, and oxygen) to describe the high magmatic addition rate (MAR) and its association with the growth or reworking of the continental crust. Major peaks are picked to associate with pulses of high-volume magmatic flare-ups related to episodic evolution of continental crust. With the development of modern isotope identification technology which results both in reduction of analytical error of isotope data and rapid accumulation of massive high-quality data, the temporal and frequency variances of these types of data ought to be quantitatively analyzed to study magmatic evolution with respect to tectonic background. A new fractal density concept and a local singularity analysis (LSA) method have been recently and successfully applied to analyze the geometric property of the age peaks in global zircon U-Pb age database by power law models. The anomalies of age peaks identified are linked to deeply rooted avalanches associated with short spurts of convection during formation of supercontinents and continent crust growth. In this paper, the method is further used to analyze a small dataset of U-Pb ages from Gangdese arc to characterize the causational relationship of age peak and India-Asia collision. The results show that the age density around peak at 51Ma can be fitted by power law functions. Both the scaling range of the age distribution and the exponents of the power law functions observed from the data suggest that the age peak may reflect magmatic flare-ups which would have been caused by superimposing of subduction, exhumation and slab breakoffs. It has been demonstrated that the LSA can be used as a new way to quantitatively characterize magmatic flare-ups based on U-Pb age data from a fractal density point of view.

Petrology and U–Pb geochronology of zircon in a suite of charnockitic gneisses from parts of the Chotanagpur Granite Gneiss Complex (CGGC): evidence for the reworking of a Mesoproterozoic basement during the formation of the Rodinia supercontinent

Abstract Understanding the evolution of the Chotanagpur Granite Gneiss Complex (CGGC) of the East Indian Shield is crucial to decipher the role of the Indian Shield in the formation of the Rodinia supercontinent. The area around Deoghar–Dumka exposes a suite of granulite-facies orthogneisses (variably retrogressed to amphibole–biotite gneiss) that enclose remnants of Palaeoproterozoic metasedimentary and meta-igneous rocks. Results from mineral chemistry, laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS) U–Pb dating of zircon and limited bulk-rock compositions of the studied rocks suggest that the magmatic protoliths of the felsic orthogneisses had A-type chemistry, and that these were emplaced at approximately 1450 Ma presumably in a continental rift setting. Intense deformation and metamorphism of the felsic rock culminated at approximately 9 kbar and 850°C along an apparent geothermal gradient of 26°C km −1 . These peak metamorphic conditions were successively followed by initially a steeply decompressive and then a weakly decompressive retrograde pressure–temperature path. The shape of the retrograde pressure–temperature path and the estimated geothermal gradient at the metamorphic peak are interpreted to be the products of continent–continent collision; U–Pb dates of metamorphic zircon overgrowths suggest an age of approximately 943 Ma for the collisional event. This study demonstrates that ‘Grenville-age’ orogenesis thoroughly reworked the approximately 1450 myr-old basement of the CGGC during the formation of the Rodinia supercontinent.

Reaction textures, pressure–temperature paths and chemical dates of monazite from a new suite of sapphirine–spinel granulites from parts of the Eastern Ghats Province, India: insights into the final amalgamation of India and East Antarctica during the formation of Rodinia

Abstract A suite of Mg–Al granulites from two new localities in the Eastern Ghats Province are investigated to put constraints on: (a) the thermal and baric evolution of these rocks; (b) the timing of high-grade metamorphisms (chemical dating of monazite); (c) the tectonic setting where the high-grade metamorphisms occurred; and (d) a possible link between India and East Antarctica during the formation of the Rodinia supercontinent. Supporting the proposition of polymetamorphism over single metamorphism, our study documents at least two distinct phases of high-grade metamorphism that occurred in two contrasting tectonic settings. Reconstructed pristine spinel composition from oxide aggregates, the Al content of coronitic orthopyroxene over sapphirine and spinel, and the constraints of the FeO–MgO–Al 2 O 3 –SiO 2 (FMAS) topology in the FMAS system document temperatures in excess of 1070°C at 8–9 kbar pressure (>1100°C GPa −1 ). This study shows that such an extreme metamorphic condition was reached along a counter-clockwise P – T trajectory presumably in an extensional setting at approximately 1.2 Ga. The eventual collision of India and East Antarctica reworked the near-isobarically cooled assemblages of the first event, and triggered exhumation of the former lower crust to the upper-crustal depth along a steeply decompressive trajectory during the formation of the Rodinia supercontinent ( c. 0.95–0.90 Ga). Supplementary material: Representative electron microprobe analyses of monazite in wt%, calculated apparent ages and ±2σ error are available at https://doi.org/10.6084/m9.figshare.c.3771044

Statistical petrology reveals a link between supercontinents cycle and mantle global climate

The breakup of supercontinents is accompanied by the emplacement of continental flood basalts and dike swarms, the origin of which is often attributed to mantle plumes. However, convection modeling has showed that the formation of supercontinents result in the warming of the sub-continental asthenospheric mantle (SCAM), which could also explain syn-breakup volcanism. Temperature variations during the formation then breakup of supercontinents are therefore fundamental to understand volcanism related to supercontinent cycles. Magmatic minerals record the thermal state of their magmatic sources. Here we present a data mining analysis on the first global compilation of chemical information on magmatic rocks and minerals formed over the past 600 million years: a time period spanning the aggregation and breakup of Pangea, the last supercontinent. We show that following a period of increasingly hotter Mg-rich magmatism with dominant tholeiitic affinity during the aggregation of Pangea, lower-temperature minerals crystallized within Mg-poorer magma with a dominant calc-alkaline affinity during Pangea disassembly. These trends reflect temporal changes in global mantle climate and global plate tectonics in response to continental masses assembly and dispersal. We also show that the final amalgamation of Pangea at ~300 Myr led to a long period of lithospheric collapse and cooling until the major step of Pangea disassembly started at ~125 Myr. The geological control on the geosphere magma budget has implications on the oxidation state and temperature of the Earth’s outer envelopes in the Phanerozoic and may have exerted indirect influence on the evolution of climate and life on Earth.

Recycling of subducted crustal components into carbonatite melts revealed by boron isotopes

The global boron geochemical cycle is closely linked to recycling of geologic material via subduction processes that have occurred over billions of years of Earth’s history. The origin of carbonatites, unique melts derived from carbon-rich and carbonate-rich regions of the upper mantle, has been linked to a variety of mantle-related processes, including subduction and plume–lithosphere interaction. Here we present boron isotope (δ11B) compositions for carbonatites from locations worldwide that span a wide range of emplacement ages (between ∼40 and ∼2,600 Ma). Hence, they provide insight into the temporal evolution of their mantle sources for ∼2.6 billion years of Earth’s history. Boron isotope values are highly variable and range between −8.6‰ and +5.5‰, with all of the young ( −4.0‰), whereas most of the older carbonatite samples record lower B isotope values. Given the δ11B value for asthenospheric mantle of −7 ± 1‰, the B isotope compositions for young carbonatites require the involvement of an enriched (crustal) component. Recycled crustal components may be sampled by carbonatite melts associated with mantle plume activity coincident with major tectonic events, and linked to past episodes of significant subduction associated with supercontinent formation. The origin of carbon-rich magmas is unclear. Boron isotopic analysis of carbonatite magmas that formed over the past 2.6 billion years reveals a link to carbon recycled during tectonic plate subduction.

The future of Earth's oceans: consequences of subduction initiation in the Atlantic and implications for supercontinent formation

AbstractSubduction initiation is a cornerstone in the edifice of plate tectonics. It marks the turning point of the Earth's Wilson cycles and ultimately the supercycles as well. In this paper, we explore the consequences of subduction zone invasion in the Atlantic Ocean, following recent discoveries at the SW Iberia margin. We discuss a buoyancy argument based on the premise that old oceanic lithosphere is unstable for supporting large basins, implying that it must be removed in subduction zones. As a consequence, we propose a new conceptual model in which both the Pacific and the Atlantic oceans close simultaneously, leading to the termination of the present Earth's supercycle and to the formation of a new supercontinent, which we name Aurica. Our new conceptual model also provides insights into supercontinent formation and destruction (supercycles) proposed for past geological times (e.g. Pangaea, Rodinia, Columbia, Kenorland).

Large‐scale tectonic cycles in Europe revealed by distinct Pb isotope provinces

AbstractLead isotopic systematics of U‐poor minerals, such as sulfides and feldspars, can provide unique insights into the origin and evolution of continents because these minerals “freeze in” the Pb isotopic composition of the crust during major tectonothermal events, allowing the history of a continent to be told through Pb isotopes. Lead model ages constrain the timing of crust formation while time‐integrated U/Pb, Th/Pb, and Th/U ratios shed light onto key geochemical processes associated with continent formation. Using ∼6800 Pb isotope measurements of primarily lead ores and minor K‐feldspar, we mapped out the Pb isotope systematics across Europe and the Mediterranean. Lead model ages define spatially distinct age provinces, consistent with major tectonic events ranging from the Paleozoic to the Proterozoic and latest Archean. However, the regions defined by time‐integrated U/Pb and Th/Pb ratios cut across the boundaries of age provinces, with high U/Pb systematics characterizing most of southern Europe. Magmatic influx, followed by segregation of dense sulfide‐rich mafic cumulates, resulted in foundering of U‐ and Th‐poor lower crust, thereby changing the bulk composition of the continental crust and leading to distinct time‐integrated U‐Th/Pb provinces. We show that the tectonic assembly of small crustal fragments leaves the crust largely undifferentiated, whereas the formation of supercontinents results in fundamental changes in the composition of the crust, identifiable in time and space by means of Pb isotope systematics. Observations based on Pb isotopes open up a new perspective on possible relationships between crustal thickness and geodynamic processes, in particular the role of crustal foundering into the mantle and the mechanisms responsible for the existence of cratons.

Control of lithospheric inheritance on neotectonic activity in northwestern Canada?

Abstract Lithospheric inheritance is thought to affect the location and reactivation of tectonic structures through successive cycles of supercontinent formation and dispersal; however, its relation to neotectonic activity remains unclear. In northwestern Canada, abundant seismicity throughout the northern Canadian Cordillera (NCC) is geographically confined by several crustal-scale boundaries, yet its southern extent terminates abruptly along the inferred westward extension of a Late Cretaceous rifted margin boundary called the Liard transfer zone (LTZ). We use seismic data to show that the uppermost mantle beneath the Cordillera exhibits a sharp north-south contrast in fabric across the LTZ. South of the LTZ, fast axes of seismic wave propagation align closely with the lithospheric mantle fabric orientation of the adjacent Canadian shield. North of the LTZ, fast axes are reoriented subparallel to the motion of the Pacific plate and follow the strike of the large dextral strike-slip Tintina and Denali faults. We attribute changes in anisotropic delay times across the Tintina and Denali faults to localized shear within the lithosphere; this implies that the crust and lithospheric mantle remained mechanically coupled during shearing. We propose that the contrast in uppermost mantle structure across the LTZ reflects a change in the nature and origin of the lithospheric mantle from inherited rifted margin structures, which affects the stability of the lithosphere and limits the extent of seismic activity within the NCC. These results indicate that neotectonic activity in modern Cordilleras is controlled in part by inherited upper mantle structures.

Formation of a future supercontinent through plate motion–driven flow coupled with mantle downwelling flow

Abstract Series of high-resolution numerical simulations of three-dimensional mantle convection were performed to examine the interaction between the drifting continental lithospheres and the underlying mantle structure for 250 m.y. from the present, and to predict the configuration of the future supercontinent. The density anomaly of the mantle interior was determined by the seismic velocity anomaly from global seismic tomography data sets, which contain well-resolved subducting slabs. The present-day plate motion was imposed for the first stage of the simulation as a velocity boundary condition at the top surface boundary, instead of a shear stress–free condition. The switching time from the plate motion boundary to shear stress–free conditions was taken as a free parameter. The results revealed that Australia, Eurasia, North America, and Africa will merge together in the Northern Hemisphere to form a new supercontinent within ∼250 m.y. from the present. The continental drift was assumed to be realized by plate-scale mantle flow, rather than large-scale upwelling plumes. That is, continuously moving plates at the surface for the first stage of the simulation are mechanically coupled with the subducting slabs in the mantle; this enhances the underlying mantle downwelling flow. As a result, persistent continental drift can be reproduced for long future time periods even though top surface boundary conditions may switch in response to shear stress–free conditions. The configuration of the numerically reproduced future supercontinent in this study is broadly consistent with the hypothetical model of Amasia as indicated by previous findings from geological correlations and a paleogeographic reconstruction.

Early Mesoproterozoic (1.6–1.5 Ga) granulite facies events in the Ongole domain: geodynamic significance and global correlation

AbstractThe late Palaeoproterozoic to early Mesoproterozoic Ongole domain of the Eastern Ghats Belt in southeastern India is composed of a suite of charnoenderbites with enclaves of metapelitic and mafic granulites. Geochemical data for all meta‐igneous rocks are typical for continental magmatic arc suites. The rocks preserve mineral assemblages and reaction textures indicating two separate metamorphic events: an initial counter‐clockwise medium‐pressure ultrahigh temperature (MP−UHT) event (>900 °C at 6.5–7.0 kbar), attributed to the magmatic accretion of the arc, which was followed by a clockwise HP‐MT event (~700 °C at 9 kbar), attributed to its tectonic crustal thickening. Zircon dating of charnoenderbites yield magmatic crystallization ages between 1.75 and 1.70 Ga and metamorphic ages between 1.62 and 1.60 Ga. Syn‐metamorphic enderbites yield magmatic and metamorphic zircon ages of c. 1.60 Ga, overlapping with all other metamorphic rim ages. In contrast, texturally controlled monazite dating of two chemically distinct monazite generations reveals two metamorphic events at c. 1.62 and c. 1.54 Ga. These data suggest that the Ongole domain was formed as a magmatic arc during the Palaeoproterozoic (c. 1.75–1.70 Ga), and subsequently experienced UHT metamorphism accompanied by syn‐metamorphic intrusions at c. 1.60 Ga, suggesting magmatic underplating as the metamorphic heat source. The subsequent crustal thickening at c. 1.54 Ga was due to the collision of the Ongole magmatic arc with the Indian craton. The UHT metamorphism and the related orogenic processes of the Ongole domain fall into the assumed late Palaeoproterozoic to late Mesoproterozoic ‘orogenic and UHT time gap’ (1.8–1.1 Ga) regarded by some studies as an orogenic quiescent period after the formation of the Palaeoproterozoic supercontinent known as Columbia or Nuna at c. 2.1–1.8 Ga. Coeval orogenic processes at c. 1.7–1.5 Ga occurring along accretionary margins in different fragments of Columbia (India, Australia, Antarctica and southwest Africa) indicate a protracted formation of this supercontinent.

Geochemistry and geochronology of mylonitic metasedimentary rocks associated with the Proterozoic Miaowan Ophiolite Complex, Yangtze craton, China: Implications for geodynamic events

Constraining the timing of tectonothermal events in the Proterozoic Miaowan Ophiolite Complex (MOC) and associated rocks in the southern Huangling dome, Yangtze craton, is critical for understanding late Mesoproterozoic and Neoproterozoic tectonic evolution (ca. 1.2–0.9Ga) of South China and its relationship with the formation of the Rodinian supercontinent. The MOC consists of metamorphosed dunite and harzburgite, isotropic and layered gabbros, diabasic sheeted dike, basalt, plagiogranite, and calc-silicate-bearing siliceous and carbonaceous mylonitic rocks. In this study, we present new field, petrographic, geochemical, geochronological and isotopic data for these metasedimentary rocks and mylonitic metasandstone between the ophiolite and underlying flysch sequence, to unravel their origin and tectonic significance for the Yangtze craton. The MOC and associated sedimentary rocks underwent amphibolite facies metamorphism. Geochemical data and field relationships indicate that the mylonitic metasandstone was derived from both the autochthonous rocks of the underlying Yangtze craton and the allochthonous MOC during the accretion of the ophiolite to the craton. The calc-silicate-bearing siliceous and carbonaceous rocks are interpreted as tectonic slices of metasomatized and mylonitized chert and limestone, respectively, deposited on the basaltic crust in a Neoproterozoic ocean. Cores of igneous detrital zircons in the calc-silicate-bearing siliceous and carbonaceous mylonitic rocks have yielded two distinct age groups including a 967–1105Ma (Mean=1009Ma) group and a 1011–1095Ma (Mean=1054Ma) group. The majority of initial εHf(t) values (+9.2 to +14.1) in the zircon cores are similar to those of zircons from the gabbro, diabase and plagiogranite in the MOC, indicating that zircons in the mylonitic rocks were mainly derived from the MOC. Metamorphic overgrowth ages in zircon rims suggest that the latest tectonothermal event in the Precambrian basement rocks of the Yangtze craton took place between 942 and 935Ma. These metamorphic ages are comparable to those of the Grenvillian-aged collisional events recorded in orogenic belts worldwide. Accordingly, we suggest that this youngest tectonothermal event represents the latest amalgamation time of the heterogeneous tectonic blocks of the Yangtze craton, which were associated with the assembly of the Rodinian supercontinent.

Ancient plate kinematics derived from the deformation pattern of continental crust: Paleo- and Neo-Tethys opening coeval with prolonged Gondwana–Laurussia convergence

The formation and destruction of supercontinents requires prolonged convergent tectonics between particular plates, followed by intra-continental extension during subsequent breakup stages. A specific feature of the Late Paleozoic supercontinent Pangea is the prolonged and diachronous formation of the collisional belts of the Rheic suture zone coeval with recurrent continental breakup and subsequent formation of the mid-ocean ridge systems of the Paleo- and Neo-Tethys oceans at the Devonian and Permian margins of the Gondwana plate, respectively. To decide whether these processes are causally related or not, it is necessary to accurately reconstruct the plate motion of Gondwana relative to Laurussia. Here we propose that the strain pattern preserved in the continental crust can be used for the reconstruction of ancient plate kinematics. We present Euler pole locations for the three fundamental stages of the Late Paleozoic assembly of Pangea and closure of the Rheic Ocean: (I) Early Devonian (ca. 400Ma) collisional tectonics affected Gondwana at the Armorican Spur north of western Africa and at the promontory of the South China block/Australia of eastern Gondwana, resulting in the Variscan and the Qinling orogenies, respectively. The Euler pole of the rotational axis between Gondwana and Laurussia is positioned east of Gondwana close to Australia. (II) Continued subduction of the western Rheic Ocean initiates the clockwise rotation of Gondwana that is responsible for the separation of the South China block from Gondwana and the opening of Paleo-Tethys during the Late Devonian. The position of the rotational axis north of Africa reveals a shift of the Euler pole to the west. (III) The terminal closure of the Rheic Ocean resulted in the final tectonics of the Alleghanides, the Mauritanides and the Ouachita–Sonora–Marathon belt, occurred after the cessation of the Variscan orogeny in Central Europe, and is coeval with the formation of the Central European Extensional Province and the opening of Neo-Tethys at ca. 300Ma. The Euler pole for the final closure of the Rheic Ocean is positioned near Oslo (Laurussia). Thus, the concomitant formation of convergent and divergent plate boundaries during the assembly of Pangea is due to the relocation of the particular rotational axis. From a geodynamic point of view, coupled collisional (western Pangea) and extensional tectonics (eastern Pangea) due to plate tectonic reorganization is fully explained by slab pull and ridge push forces.

Mantle evolution in the Variscides of SW England: Geochemical and isotopic constraints from mafic rocks

The geology of SW England has long been interpreted to reflect Variscan collisional processes associated with the closure of the Rhenohercynian Ocean and the formation of Pangea. The Cornish peninsula is composed largely of Early Devonian to Late Carboniferous volcanosedimentary successions that were deposited in pre- and syn-collisional basins and were subsequently metamorphosed and deformed during the Variscan orogeny. Voluminous Early Permian granitic magmatism (Cornubian Batholith) is broadly coeval with the emplacement of ca. 280–295Ma lamprophyric dykes and flows. Although these lamprophyres are well mapped and documented, the processes responsible for their genesis and their relationship with regional Variscan tectonic events are less understood.Pre- to syn-collisional basalts have intra-continental alkalic affinities, and have REE profiles consistent with derivation from the spinel–garnet lherzolite boundary. εNd values for the basalts range from +0.37 to +5.2 and TDM ages from 595Ma to 705Ma. The lamprophyres are extremely enriched in light rare earth elements, large iron lithophile elements, and are depleted in heavy rare earth elements, suggesting a deep, garnet lherzolite source that was previously metasomatised. They display εNd values ranging from −1.4 to +1.4, initial Sr values of ca. 0.706, and TDM ages from 671Ma to 1031Ma, suggesting that metasomatism occurred in the Neoproterozoic.Lamprophyres and coeval granite batholiths of similar chemistry to those in Cornwall occur in other regions of the Variscan orogen, including Iberia and Bohemia. By using new geochemical and isotopic data to constrain the evolution of the mantle beneath SW England and the processes associated with the formation of these post-collisional rocks, we may be able to gain a more complete understanding of mantle processes during the waning stages of supercontinent formation.

Black shales and other sediments with high organic matter contents in Phanerozoic climatic cycles: Communication 2. Black shales during the Pangea existence

According to recent concepts, the Earth surface was permanently transformed during its geological history. Some stages of its evolution were marked by the convergence of separate continental blocks to result in the formation of supercontinents, which resisted successfully centrifugal processes. Other stages were characterized by the opposite tendency: after their long existence, the supercontinents became disintegrated into several large and small blacks, the motion of which was accompanied by opening of new sea basins and closure of former basins with the oceanic crust. The second half of the Paleozoic was marked by amalgamation of large continental blocks. In the Devonian, collision between Laurentia and Baltica culminated in the formation of the Euroamerica continent. After the closure of the Ural paleocean in the terminal Carboniferous–initial Permian, it was united with the Siberian and Kazakhstan continental blocks. These events provided the prerequisites for the formation of a new supercontinent (Pangea), which acquired its final configuration at the end of the Permian. One of its segments located mainly south of the equator included Gondwana. Another segment located northward included Euroamerica, Kazakhstan, Siberian, and two China continental blocks. During its geological history, Pangea suffered many dramatic events including several extinctions of organisms. The most significant event took place in the terminal Permian–initial Triassic and at the transition between the Triassic and Jurassic periods.

Using palaeomagnetism to determine late Mesoproterozoic palaeogeographic history and tectonic relations of the Sinclair terrane, Namaqua orogen, Namibia

Abstract The Sinclair terrane is an important part of the Namaqua orogenic province in southern Namibia containing well-preserved Mesoproterozoic volcano-sedimentary successions suitable for palaeomagnetic and geochronological studies. The Guperas Formation in the upper part of the Sinclair stratigraphic assemblage contains both volcanic and sedimentary rocks cut by a bimodal dyke swarm with felsic members dated herein by U–Pb on zircon at c. 1105 Ma. Guperas igneous rocks yield a pre-fold direction and palaeomagnetic pole similar to that previously reported. Guperas sedimentary rocks yield positive conglomerate and fold tests, with a maximum concentration of characteristic remanence directions at 100% untilting. The combined Guperas data generate a palaeomagnetic pole of 69.8° N, 004.1° E ( A 95 =7.4°, N= 9). The 1105 Ma post-Guperas dykes yield stable remanence directions with positive baked-contact tests and a palaeomagnetic pole at 62.3° N, 031.9° E ( A 95 =6.9°, N= 26), which is coincident with that of the Kalahari-wide Umkondo large igneous province, demonstrating tectonic coherence of the Sinclair terrane with the Kalahari craton at the time of dyke emplacement. These results show that palaeomagnetic and geochronological studies of the Sinclair terrane can provide kinematic constraints on the tectonic evolution of the Namaqua–Natal–Maud orogenic belt and its role in the formation of the Rodinia supercontinent.

Age and sources of Precambrian zircon–rutile deposits in the Kokchetav sialic massif (northern Kazakhstan)

The U–Pb geochronological data on detrital zircons from placers confined to Neoproterozoic quartzite–schist sequences, which are widespread in the Kokchetav massif of northern Kazakhstan, are discussed. Detrital zircons (332 grains in total) originate from the ore occurrences in the central, northern, and western parts of the massif. The concordant ages of detrital zircons from all the examined occurrences largely correspond to intervals of 1017–1528, 1628–1946, and 2653–2739 Ma. The obtained data imply that material of quartzite–schist sequences of the Kokchetav massif was provided by Mesoproterozoic, Paleoproterozoic, and Neoarchean rock complexes. The lower age limit determined for these sequences is approximately 1.06 Ga. The dates obtained for detrital zircons are most consistent with events that took place in Laurentia. They correspond to the formation and breakup of the Columbia/Nuna supercontinent (approximately 1650–1580 and 1450–1380 Ma ago, respectively) and formation of the Rodinia supercontinent in the period of 1300–900 Ma ago.

Black shales and other sediments with high organic matter contents in Phanerozoic climatic cycles: Communication 1. Black shales at the Gondwanan stage of biosphere evolution

The terminal Neoproterozoic was marked by the formation of a supercontinent, which was located during a long period at high latitudes of the Southern Hemisphere. The supercontinent named Gondwana united present-day Africa, Australia, Antarctica, South America, Arabia, Hindustan, and several other small continental blocks. It was separated from other, smaller continental blocks by ocean basins with the thinned continental and oceanic crust with the Panthalassa Ocean being largest among them. The Gondwanan stage, which comprised the Vendian and significant part of the Paleozoic, was one of the longest stages defined in the Neoproterozoic and Phanerozoic geological history lasting over 300–330 Ma. During this stage, the continental sector of the Earth’s crust included the Laurentia, Baltica, Siberia, Kazakhstan, and North China (Tarim) blocks, in addition to Gondwana, which were largely located at low latitudes of the Northern Hemisphere. In the Early Paleozoic, main tectonic events were related to the breakup in peripheral parts of Gondwana with the separation of small continental massifs, which moved in the northerly direction. Their collision with other continents was accompanied by the folding and formation of mountainous structures with corresponding foredeeps. For example, the Taconian phase of fold—thrust deformations resulted from the convergence of Laurentia with the Avalon block, which was separated from the northern margin of Gondwana in the Late Cambrian or Early Ordovician. The movement of Gondwana toward the equator and its amalgamation with other continents and smaller continental blocks in the terminal Cambrian resulted in the formation of the Pangea supercontinent, whose location near the equator for more than 150 Ma up to its disintegration at the terminal Jurassic—initial Cretaceous transition was responsible for significant global changes in the floral and faunal communities. Paleozoic epochs of the relatively stable warm climate alternated with shorter periods, which were characterized by global cooling due to expansion of the ice sheet in Gondwana. These periods are reflected in frequent migrations of climatic belts and rapid sea level oscillations, which were accompanied by changes in boundaries of facies zones and extinctions of biotic communities. Distribution patterns of black shales and other organic-rich sediments in sections imply cyclic climatic variations during the entire Paleozoic. Duration of individual cycles, which included a long phase of the dominant warm or even hot climate and shorter cooling phase crowning the cycle, was as long as 47 Ma. Two such cycles terminated with glaciations; others, with a less significant growth of the Gondwanan ice sheet.

Late Triassic paleomagnetic result from the Baoshan Terrane, West Yunnan of China: Implication for orientation of the East Paleotethys suture zone and timing of the Sibumasu-Indochina collision

In order to better understand the paleogeographic position of the Baoshan Terrane in the northernmost part of the Sibumasu Block during formation of the Pangea supercontinent, a paleomagnetic study has been conducted on Late Triassic basaltic lavas from the southern part of the Baoshan Terrane in the West Yunnan region of Southwest China. Following detailed rock magnetic investigations and progressive thermal demagnetization, stable characteristic remanent magnetizations (ChRMs) were successfully isolated from Late Triassic Niuhetang lava flows. The ChRMs are of dual polarity and pass fold and reversal tests with magnetic carriers dominated by magnetite and subordinate oxidation-induced hematite; we thus interpret them as a primary remanence. This new paleomagnetic result indicates that the Baoshan Terrane was located at low paleolatitudes of ∼15°N in the Northern Hemisphere during Late Triassic times. Together with available paleomagnetic data from the Baoshan Terrane and surrounding areas, a wider paleomagnetic comparison supports the view that the East Paleotethys Ocean separated the Sibumasu and Indochina blocks and closed no later than Late Triassic times. We argue that the currently approximately north-to-south directed Changning-Menglian suture zone is very likely to have been oriented nearly east-to-west at the time of the Sibumasu-Indochina collision.

Influences on the positioning of mantle plumes following supercontinent formation

AbstractSeveral mantle convection studies analyzing the effects of supercontinent formation and dispersal show that the genesis of subcontinental plumes results from the formation of subduction zones at the edges of the supercontinent rather than from the effect of continental thermal insulation or thermochemical piles. However, the influence of subduction zone location on the position of subcontinental plumes has received little attention. This study analyzes 2‐D and 3‐D numerical models of supercontinent formation (in an isochemical mantle) to assess the role of subduction and mantle viscosity contrast in the generation of subcontinental mantle plumes. We find that once a critical supercontinent width is reached, plumes do not form under the center of a supercontinent. In studies featuring a low viscosity lower mantle, the surface positions of the initial plumes (arriving within 90 Myr of supercontinent assembly) become locked beneath the continent at a distance 2000–3000 km from the continental margin. However, the broad downwellings in simulations that feature a high‐viscosity lower mantle trigger plumes at a greater distance from the continental margin subduction. For all mantle viscosity profiles, subcontinental plumes show dependence on the location of supercontinent margin subduction. As theories differ on the role of core‐mantle boundary chemical piles in plume formation, it is significant that our isochemical models show that the formation of subduction zones at the margins of a supercontinent has a profound effect on subcontinental mantle dynamics. Our results may help to explain what determined the eruption sites of past (and future) large igneous provinces.