Fragmentation Of Rodinia Research Articles

Basin filling and magmatic response to the migration of two-stage rifts: New insights into the late Neoproterozoic tectonics of the northern Tarim Craton

The spatiotemporal evolution of late Neoproterozoic sedimentary basins and their tectono-magmatic context provide essential information regarding the continental rift process of the Tarim Craton during the fragmentation of the Rodinia supercontinent. However, the incomplete exposure of strata and magmatism in the distal continental margin hinders the understanding of different rift phases that trigger continental breakup. The Sawapuqi region is currently located at the northernmost margin of the Tarim Craton and presents an uninterrupted sedimentary sequence and volcanic rocks from the latest Precambrian to the Cambrian period. Herein, we perform an integrated analysis of the Sawapuqi sub-basin and provide critical insights into the Ediacaran stratigraphic and tectono-magmatic evolution that shed light on the final rifting phase. The Sawapuqi sub-basin preserves regressive and transgressive successions, ranging from thick-bedded siltstones and siliceous rocks at the lower unit of the Sugetbrak Formation, through shore gravel-bearing laminated sandstone accumulation (upper unit), and restricted platform stromatolitic carbonates (Qigebrak Formation), to Cambrian mature passive margin sedimentation. An alkaline basaltic layer is interbedded within the coarsening-upward sedimentary packages of the early Ediacaran, recording the final rift-related magmatic pulse in the northwestern Tarim margin. The geochemical signatures of these basalts suggest a relatively shallow-depth derivation from the enriched mantle reservoir in an intraplate setting. The detrital zircon provenance of the coarse-grained siliciclastic to pebbly feldspathic debris sandstone sequence of the upper Sugetbrak Formation indicates the hybrid sediment infills from local metamorphic basements and Neoproterozoic igneous rocks, with progressively increased contribution from the coeval granites in the northern periphery of the Tarim Craton. The stratigraphic evolution from syn-rift clastic rocks to post-rift carbonates and the transition of detrital provenance observed in the Sawapuqi sub-basin suggest the development of a breakup succession at the end of the early Ediacaran period. The margin-localized continental extension and mantle-derived volcanism during the early Ediacaran differed from the distributed extension and bimodal magmatism during the Cryogenian, indicating two-stage extensional modes and subsidences prior to the lithospheric breakup. The rift system of the latter stage, which migrated oceanwards, eventually led to the continental breakup in the northwestern margin of the Tarim Craton. Comparable late Neoproterozoic stratigraphy and magmatic pulses between the Tarim Craton and Yili-Central Tianshan Block (YCTB) support their reconstructions as two conjugate rifted margins. This study indicates that the rift-drift tectonics triggered by this prolonged episodic rifting between the Tarim Craton and the YCTB was closely associated with the final fragmentation of Rodinia.

Indian carbonatites in the global tectonic context

Chrono-tectonic settings of the carbonatite occurrences of India are reviewed with a focus on the “big picture” of carbonatite emplacements in the Indian plate in relation to global tectonic events associated with the amalgamation and breakup of supercontinents. Four chrono-tectonic domains, namely, Southern domain, Southeastern domain, Northeastern domain and Northwestern domain, are delineated based on the geographical distribution, tectonic settings and temporal relationships amongst the carbonatite complexes. The Southern domain comprises two sub-domains – Paleoproterozic and Neoproterozoic. The Paleoproterozoic sub-domain is related to extension due to relaxation after the Southern Granulite Terrain-Dharwar accretion, while the Neoproterozoic sub-domain is related to rifting related to the fragmentation of Rodinia. The Southeastern domain is related to the Mesoproterozoic fragmentation of Columbia. The Northeastern domain is related to the Mid-Cretaceous breakup of Greater India from Australia-Antarctica driven by the Kerguelen mantle plume that also produced the Rajmahal-Sylhet Large Igneous Province (LIP). The Northwestern domain is related to the Late-Cretaceous Indo-Seychelles-Madagascar split and the passage of Greater India over the reunion hotspot, which also produced the Deccan LIP.

Inverted South China: A novel configuration for Rodinia and its breakup

Abstract Disentangling records of Rodinia fragmentation and true polar wander remains a challenge for understanding late Tonian plate tectonics. The ca. 760 Ma lower member of the Liántuó Formation, South China, yields a primary paleomagnetic remanence that passes both the fold and reversal tests. This new result and recently reported ca. 800 Ma data from elsewhere in South China suggest a new interpretation of its apparent polar wander path, whereby pre–770 Ma poles have inverted absolute polarity relative to traditional interpretations. Based on this inversion, and an interpretation of several oscillations of true polar wander documented by global data during 810–760 Ma, we propose a novel reconstruction for Rodinia and its breakup. Our reconstruction places the South China, India, and Kalahari cratons to the southwest of Laurentia, with connections that might have been established as early as ca. 1000 Ma. Our model also suggests that initial rifting of Rodinia occurred at ca. 800 Ma via fast northward motion of the India craton and South China.

Neoproterozoic extension and the Central Iapetus Magmatic Province in southern Mexico – New U-Pb ages, Hf-O isotopes and trace element data of zircon from the Chiapas Massif Complex

Final fragmentation of Rodinia occurred during the Ediacaran Period as Amazonia, Baltica and Laurentia drifted apart to form the Iapetus Ocean. Accompanying rift-related mafic dyke swarms of the Central Iapetus Magmatic Province (CIMP) were emplaced between 0.62 and 0.55 Ga, which are preserved in Laurentia and Baltica, whereas no coeval mafic rocks are known from Amazonia. First evidence for the CIMP extending into Oaxaquia (Rodinia-type basement of Mexico) was reported as tholeiitic dykes that intruded the Novillo gneiss, NE Mexico, at 619 ± 9 Ma. In Chiapas, SE Mexico, amphibolite dykes that are chemically similar to the Novillo dykes intruded anorthosite and gneiss. In this paper, a new dating approach to obtain mafic dyke intrusion ages is presented by targeting contact metamorphic zircon with the UPb method, employing Secondary Ion Mass Spectrometry. Zircon that crystallized in anorthosite at intrusive contacts to mafic dykes and at temperatures exceeding 700 °C (Ti-in-zircon thermometry) yields ages between 615 ± 7 Ma and 608 ± 12 Ma, reflecting the time of dyke intrusion. Zircon chemical and isotopic (Hf, O) characteristics suggest a diachronous sequence of metamorphic processes involving Zr release from FeTi oxides, breakdown and recrystallization of other phases, and fluid-mitigated reactions during Ordovician metamorphism. Zircon δ18O values of granulites from Oaxaquia range from +6.2‰ to +9.8‰, whereas Tonian (~0.92 Ga) metamorphic zircon from SE Chiapas yielded low δ18O values from +2.0‰ to +2.8‰ that are explained by the reactivation of major tectonic boundaries during Tonian gravitational collapse. The observations increase the known extent of the CIMP in Mexico, suggesting that a Neoproterozoic superplume was still active during the Early Ediacaran producing a Large Igneous Province that extended over Amazonia, Baltica and Laurentia. The results further suggest that Oaxaquia at the northern edge of Amazonia formed the conjugate margin of Baltica during rifting.

Caught between two continents: First identification of the Ediacaran Central Iapetus Magmatic Province in Western Svalbard with palaeogeographic implications during final Rodinia breakup

The final fragmentation of Rodinia occurred during the Ediacaran period as the continental blocks of Baltica and Laurentia, as well as Amazonia and West Africa, rifted and drifted apart. It was along this progressively rifted margin that the Iapetus Ocean opened and subsequently closed, creating the Caledonian orogeny (sensu lato), some 100 and 200 million years later. The 0.62–0.54 Ga Central Iapetus Magmatic Province (CIMP), accompanying this break-up, is variously manifested in northern Europe and north-eastern North America as mafic dykes and sills, volcanic rocks, as well as carbonatites. This magmatism is interpreted to be broadly coincident with the so-called Marinoan and Gaskiers glaciations, and the subsequent evolution of metazoans. While the various Precambrian basement blocks of Svalbard are known to carry a Caledonian overprint, the proximity of various parts of Svalbard to either Baltica or Laurentia during final Rodinia breakup in the Ediacaran has yet to be established. Petrographic, geochronological and geochemical data from the variably deformed and metamorphosed coarse-grained mafic units of Oscar II Land (OIIL) from Spitsbergen, Svalbard, are presented here to determine the source of the magma and to elucidate on their palaeogeographic affinity with adjacent crustal blocks/units. An alkali and OIB-like affinity of the mafic units is confirmed by their whole-rock geochemical composition. An ID-TIMS U-Pb baddeleyite crystallisation age determination of 560 ± 12 Ma, obtained from one such mafic unit, contrasts with the published Caledonian (sensu lato) age determinations. These data for OIIL compare well with the broadly coeval and geochemically similar 570–560 Ma Norwegian Seiland Igneous Province, which may have formed a part of Baltica. Temporally and compositionally similar mafic units also occur within the North American Appalachian Belt, as well as the Sept-Îles intrusion, which is part of Laurentia. However, any links between these intrusions with those of Svalbard are yet to be unequivocally demonstrated.

Evidence of early Archean crust in northwest Gondwana, from U–Pb and Hf isotope analysis of detrital zircon, in Ediacaran surpacrustal rocks of northern Spain

The Mora Formation (Narcea Group) is one of the oldest Precambrian supracrustal successions in northern Spain. Here, we use U–Pb and in situ Hf isotope analysis on detrital zircon to determine its age and provenance. The youngest U–Pb dates constrain the maximum depositional age of the Mora Formation at 565 ± 11 Ma. Results indicate: (1) a dominant Ediacaran zircon population (33%; 565–633 Ma, Cadomian) within a spectrum of Neoproterozoic ages (40%; 636–996 Ma); and (2) smaller Mesoproterozoic (5%; 1004–1240 Ma), Palaeoproterozoic (11%; 1890–2476 Ma) and Archean (11%; 2519–3550 Ma) populations. Results here do not point to one specific cratonic source area; instead, detritus may have been derived from the West African craton and Amazonia, or even the concealed Iberian basement. The lack of 1.3–1.8 Ga grains suggests exclusion of the Sahara Craton as a major source, but this is not certain. This mixed composition favours a complex source history with reworking of detritus across terrane/craton boundaries. Hafnium isotope compositions indicate a range of crustal and juvenile sources, with initial eHf values between −15.8 and 11.1, and Hf model ages from 0.8 to 3.7 Ga. For Neoproterozoic zircons (80%), juvenile components (eHf(i) ~+10) may be related to Rodinia fragmentation and the onset of an active margin setting leading to the Cadomian orogeny. Palaeoproterozoic to Paleoarchean grains (20%) all have negative eHf values and Meso- to Eoarchean Hf model ages. This indicates an early (Archean) sialic crustal component for northwestern Gondwana.

New U-Pb constraints on the age of the Little Dal Basalts and Gunbarrel-related volcanism in Rodinia

Diabase exposed amongst outcrops of the Little Dal Basalts (LDB) between the Coates Lake Group and overlying the Little Dal Group in the Mackenzie Mountains of northern Canada has a geochemical signature that is characteristic of the LDB and associated Tsezotene sills. Marginally concordant magmatic zircons from a sample of the diabase yielded CA-ID-TIMS weighted mean 207Pb/206Pb and 206Pb/238U dates of 778.4±1.8Ma (2σ) and 775.10±0.54Ma (2σ), respectively. These dates provide the first direct constraint on the age of the LDB and confirm a temporal relationship to the widespread Gunbarrel magmatic event that affected western North America during the protracted breakup of the supercontinent Rodinia. Hafnium isotopes were measured on three grains and calculated values of εHf at 775Ma are 9.5±0.4, 9.7±0.2 and 9.3±0.2 (2σ), consistent with a mantle plume genesis.Gunbarrel magmatism was not temporally related to the final fragmentation of Rodinia and the establishment of the west Laurentian continental margin (rift-drift transition), which did not occur until after 720Ma. However, the age and stratigraphic position of the LDB suggest the Gunbarrel event may have thermally weakened the lithosphere leading to a period of intracontinental rifting and the deposition of the Coates Lake Group rift-sediments prior to 732Ma.The sample of diabase also yielded a single 207Pb/206Pb 1565.7±1.5Ma xenocrystic zircon with a model Th/U ratio of 0.47 indicative of a crustal origin. Although there is limited evidence for felsic magmatism, metamorphism, and orogenesis of this age in western Laurentia, a range of felsic volcanic rocks and/or intrusive suites of this age do occur in southern, central and eastern Australia, supporting existing reconstructions of the Nuna/Columbia supercontinent that place the northwest margin of Laurentia adjacent to Australia.

Geochemical and geochronological constraints on distinct Early-Neoproterozoic and Cambrian accretionary events along southern margin of the Baydrag Continent in western Mongolia

New geological, geochemical and geochronological data characterize architecture and geodynamic evolution of the southern Baydrag continental margin in Mongolia. The structurally deepest Zamtyn Nuruu Complex is composed of highly deformed orthogneisses and amphibolites intruded by syn-tectonic diorites and gabbros. Geochemical affinity of the amphibolites varies from continental-arc to within-plate tholeiites, while the Grenvillean (948±6 and 941±11Ma) orthogneisses show features typical of magmatic-arc origin. The syn-metamorphic gabbros and diorites intruded coevally with the regional thermal event at 542±4Ma. Zircon Hf isotope and whole-rock geochemical data point to a primitive source of mafic rocks, most likely formed in Early Cambrian arc developed on Grenvillean basement. The overlying Alag Khadny subduction mélange contains eclogites and amphibolites with MORB affinity while lenses of orthogneisses (953±12Ma: zircon, 939±5Ma: monazite) reveal late- to post-collisional signature. The subduction event is dated by zircon at ca. 538±20Ma. The matrix of the mélange consists of metapelites and carbonates, most likely fragments of Neoproterozoic to Early Cambrian carbonate platform scrapped off the subducting plate during the mid-Cambrian accretion. The hanging-wall Khan-Taishir–Erdene Uul ophiolite is formed by Neoproterozoic pillow lavas, red cherts and associated ultramafic rocks, with gabbros dated at 973±12Ma. Taken together, the new data allow placing south Mongolian continental fragments into a geodynamic model of Rodinia formation, fragmentation and Palaeo-Pacific subduction initiation. The Zamtyn Nuruu Grenvillean arc is interpreted as a result of Mirovoi Ocean subduction beneath Rodinian margin, while the Early Cambrian diorites and gabbros testify initiation of supra-subduction magmatism of Palaeo-Pacific plate beneath continental fragment of Gondwanan, most likely Tarim affinity. These two events were separated by the development of Neoproterozoic passive margin probably associated with Rodinia fragmentation.

Paleomagnetic evidence for a large rotation of the Yukon block relative to Laurentia: Implications for a low-latitude Sturtian glaciation and the breakup of Rodinia

Understanding the tectonic history of the supercontinent Rodinia is crucial for testing proposed links among Neoproterozoic tectonics, supercontinent cycles, climate, and biogeochemistry. The Neoproterozoic Mount Harper volcanics of the Ogilvie Mountains, Yukon, Canada, interfinger with Sturtian-age (ca. 717−660 Ma) glacial deposits that were deposited in narrow, fault-bounded basins related to the breakup of Rodinia. Here, we present new paleomagnetic data from the Mount Harper volcanics and isolate four paleomagnetic directions: a low-temperature direction recording the present geomagnetic field, a mid-temperature direction consistent with a Cretaceous overprint, and two high-temperature directions, one of which is carried by hematite and likely represents a chemical overprint, and the other of which is carried by magnetite and likely is a primary direction. This primary pole passes the fold and conglomerate tests and includes a reversal but is 50° away from the coeval 721−712 Ma Laurentian Franklin large igneous province pole. This difference can be reconciled using a 50° counterclockwise rotation of the Yukon block relative to Laurentia. The prerotation reconstruction of the Yukon block relative to Laurentia aligns Neoproterozoic fault orientations and facies belts between the Wernecke and Mackenzie Mountains, rectifies paleoflow measurements in Mesoproterozoic and Paleoproterozoic strata, and realigns the orientation of the ca. 1260 Ma Bear River dikes with the Mackenzie dike swarm of northern Canada. This reconstruction also facilitates future studies that relate Neoproterozoic sedimentary and structural patterns to the fragmentation of Rodinia. Finally, this low-latitude pole supports the snowball Earth interpretation of the ca. 717 Ma Sturtian glacial deposits.

Cryogenian intraplate magmatism along the buried southern Laurentian margin: Evidence from volcanic clasts in Ordovician strata, Marathon uplift, west Texas

Abstract Critical evidence bearing on the breakup history of the supercontinent Rodinia near the end of the Proterozoic comes from widespread Cryogenian–Cambrian intraplate igneous assemblages present along the margins of cratonic blocks released during Rodinia fragmentation and now distributed around the globe. This magmatism occurred over a long time span (780–540 Ma) prior to and during final stages of Rodinia breakup along the eastern, western, and northern margins of the Laurentia craton, which forms the centerpiece of Rodinia in many reconstructions. Whether similar protracted magmatism occurred prior to the rift-drift transition along the southern Laurentian margin has remained uncertain because of deep burial beneath younger strata. We present geochemical and geochronological data from volcanic clasts within shelf-derived Ordovician turbidites and debris-flow deposits now exposed in allochthonous thrust slices in the Marathon uplift, west Texas (USA), that document one or more episodes of intraplate magmatism extending back at least to 706 Ma along this part of the ancient margin. These data raise the possibility that Laurentia may have been completely encircled by intraplate igneous activity prior to Rodinia breakup, with implications for the driving forces leading to supercontinent fragmentation and factors controlling the sites of ocean-basin formation during that process.

Depositional age, provenance, and tectonic and paleoclimatic settings of the late Mesoproterozoic–middle Neoproterozoic Mbuji-Mayi Supergroup, Democratic Republic of Congo

The late Mesoproterozoic–middle Neoproterozoic period (ca. 1300Ma–800Ma) heralded extraordinary climatic and biological changes related to the tectonic changes that resulted in the assembly (~1.0Ga) and the break-up of Rodinia (880Ma–850Ma). In the Democratic Republic of Congo, these changes are recorded in the Mbuji-Mayi Supergroup which was deposited in the SE–NW trending siliciclastic-carbonate failed-rift Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin. New LA-ICP–MS U–Pb laser ablation data on detrital zircon grains retrieved from the lower arenaceous-pelitic sequence (BI group) together with C and Sr isotopic data on carbonates from the upper dolomitic-pelitic sequence (BII group) and an 40Ar/39Ar age determination on a dolerite give a new depositional time frame between 1174±22Ma and ca. 800Ma for the Mbuji-Mayi Supergroup. The upper age limit is based on the assumption that the transition between the BIIb and BIIc subgroups recorded the Bitter Springs anomaly. In terms of tectonic and paleoclimatic settings, the BII group was deposited in the eastern passive margin of the Congo Craton during warm periods interlaced with temporarily dry and wet seasons, suggesting greenhouse conditions during the fragmentation of Rodinia.

Neoproterozoic ironstones in northern Namibia: Biogenic precipitation and Cryogenian glaciation

The precipitation of Cryogenian ironstones has been attributed to a spectrum of mechanisms ranging from virtually instantaneous “rusting of the seas” in response to post-snowball Earth ice meltback, to localised hydrothermal activity during fragmentation of Rodinia. The former model presupposes that ironstone deposition took place following peak glaciation. In the Chuos Formation of the Otavi Mountain Land, northern Namibia, ironstone facies precede, and are vertically gradational into, diamictites. Evidence for glaciation in the diamictites includes 1) dropstone textures, 2) subglacially deformed and attenuated, thinly stratified diamictites, supported by 3) the co-occurrence of soft-sediment striations. The underlying ironstones contain evidence for tractional processes (large-scale cross bedding) and biogenic growth (stromatolites) in strata rich in magnetite and hematite. Using the analogy of acidophile biomats in modern acid mine drainage environments, where photosynthetic bacteria construct stromatolites, fixing CO2 and Fe intracellularly, it is suggested that Cryogenian acidophile biomats did likewise, triggering ironstone precipitation and local CO2 drawdown, thereby facilitating concomitant glaciation.

Two Cryogenian glacial successions compared: Aspects of the Sturt and Elatina sediment records of South Australia

Two Cryogenian glacial sequences, the Sturt and Elatina, occur within the Adelaide Rift Complex, S Australia, and are part of an ∼8km thick succession recording Rodinia fragmentation and subsequent passive margin development. We compare and contrast these glacial successions, presenting new data, including detailed logged sections, for both glacial sequences. Facies analysis reveals a total of six facies associations, recording a range of glacial and glacially related processes. These are a diamictite facies association (ice-proximal debris flows), a sheet sandstone facies association (fluvial deposits), a flaser and rhythmically bedded facies association (tidal flat), a hummocky cross-stratified sandstone facies association (shoreface to offshore transition zone deposits) and an underflow facies association (recording offshore deposition). A sixth facies association, a ferruginous lonestone-bearing siltstone, is restricted to the top of the Elatina Formation and records rapid environmental amelioration and emergence from glaciation. In the central Flinders Ranges, the Sturt succession commences with a glacial erosion surface (GES), which in agreement with previous workers, was probably enhanced by rifting. This GES is overlain, in ascending order, by the Pualco Tillite, Holowilena Ironstone and Wilyerpa Formation, with the Warcowie Dolomite Member at the base of this latter formation. A second GES is preserved within this latter unit, and is overlain by a stratigraphic motif that records an alternation of offshore transition zone and offshore deposits. Significant base level falls within the Wilyerpa Formation are identified by fluvial deposits sitting disconformably upon offshore deposits. In contrast, within the Elatina Formation, a single GES is proposed, which defines the base of the formation. The Elatina Formation is split into two informal members on the basis of analysis of sediment stacking patterns and the recognition of a sharp stratigraphic surface that divides the formation. Apart from a locally preserved basal diamictite, the lower member consists of fluvial deposits, whereas the upper member is dominated by a mixture of tidal flat sediments and debris flow deposits. The boundary between these units is therefore interpreted as a hybrid surface recording tidal ravinement in some areas and a glacial advance in others. Deglaciation left contrasting deposits: the Sturt deglaciation culminated in deposition of black shale of the Tindelpina Shale Member of the Tapley Hill Formation, seemingly as a sheet-like deposit over the Adelaide Rift Complex and beyond into the Amadeus Basin of central Australia. The Elatina deglaciation, meanwhile, began first with deposition of the sixth facies association (ferruginous dropstone-bearing siltstone), then proceeded with deposition of cap dolostone of the Nuccaleena Formation.

Dating of volcanism and sedimentation in the Skelton Group, Transantarctic Mountains: Implications for the Rodinia-Gondwana transition in southern Victoria Land, Antarctica

The Ross orogen in southern Victoria Land, Antarctica, is composed of Skelton Group metasedimentary rocks and varied plutons of the Granite Harbor intrusives. In the Baronick Glacier area, metasediments are dominated by thick sequences of coarse conglomerate, interbedded with pisolitic limestones. Clasts within metaconglomerates are predominantly volcanic, ranging in composition from basanite to rhyolite. Baronick basaltic clast compositions are mildly alkalic and ocean-island basalt–like, compatible with eruption in a continental-margin rift environment. Sr-Nd isotope ratios suggest rhyolites and quartz-trachytes were produced by assimilation and fractional crystallization mechanisms. Two rhyolites yielded U-Pb zircon ages determined by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) of 650.1 ± 4.0 Ma and 648.5 ± 3.2 Ma (both 2σ). Studies of southern Victoria Land Skelton Group metasediments indicate that, although dominated by Grenville-aged detrital zircons, they contain an important component dated between 630 and 680 Ma, overlapping the age of volcanism in both the Baronick Glacier and central Transantarctic Mountain areas. The distribution of 650 Ma, rift-related volcanism in the southern Victoria Land segment of the Ross orogen, and the absence of significant older, but post-Grenville, extension-related zircon populations, is interpreted to reflect initiation of Rodinia fragmentation on the Antarctic margin at this time. It is proposed that rifting along the paleo-Pacific margin was diachronous, with the Antarctic segment of the rift opening substantially earlier than in Australia. Based on the dates of subduction-related calc-alkaline plutons formed during the Ross and Delamerian orogenies, the paleo–Pacific Ocean in Antarctica may also have closed earlier than in Australia, effectively defining the time of Gondwana assembly.

The Guarguaraz Complex and the Neoproterozoic–Cambrian evolution of southwestern Gondwana: Geochemical signatures and geochronological constraints

The Guarguaraz Complex, in western Argentina, comprises a metasedimentary assemblage, associated with mafic sills and ultramafic bodies intruded by basaltic dikes, which are interpreted as Ordovician dismembered ophiolites. Two kinds of dikes are recognized, a group associated with the metasediments and the other ophiolite-related. Both have N-MORB signatures, with ε Nd between +3.5 and +8.2, indicating a depleted source, and Grenville model ages between 0.99 and 1.62 Ga. A whole-rock Sm–Nd isochron yielded an age of 655 ± 76 Ma for these mafic rocks, which is compatible with cianobacteria and acritarchae recognized in the clastic metasedimentary platform sequences, that indicate a Neoproterozoic (Vendian)–Cambrian age of deposition. The Guarguaraz metasedimentary–ophiolitic complex represents, therefore, a remnant of an oceanic basin developed to the west of the Grenville-aged Cuyania terrane during the Neoproterozoic. The southernmost extension of these metasedimentary sequences in Cordón del Portillo might represent part of this platform and not fragments of the Chilenia terrane. An extensional event related to the fragmentation of Rodinia is represented by the mafic and ultramafic rocks. The Devonian docking of Chilenia emplaced remnants of ocean floor and slices of the Cuyania terrane (Las Yaretas Gneisses) in tectonic contact with the Neoproterozoic metasediments, marking the Devonian western border of Gondwana.

Age, provenance and post-deposition metamorphic overprint of detrital zircons from the Nathorst Land group (NE Greenland)—A LA-ICP-MS and SIMS study

LA-ICP-MS and SIMS U–Pb analyses have been performed on detrital zircon grains from four heavy mineral rich metasediments collected at different levels of the Nathorst Land Group (Eleonore Bay Supergroup, Greenland). Zircons from high-grade samples collected in the sillimanite and migmatite zones exhibit modifications to their structure, which are lacking in grains from a low-grade sample (0.3 GPa and 350–400 °C). In the sillimanite zone (0.5 GPa, 650 °C), thin discontinuous rims (<20 μm) plating detrital zircon grains document a metamorphic overgrowth during the Caledonian event dated at 428 ± 25 Ma (2 σ). In the migmatite zone (0.4 GPa, 700 °C), zircons underwent severe recrystallisation processes but no new zircon growth. Ilmenite, which constitutes over 50% of the heavy mineral layers, underwent recrystallisation in both high-grade samples and is likely to represent the main source of Zr available for growth of zircon rims in the sillimanite zone. However, in the migmatite zone sample, the metamorphic conditions allowed titanite overgrowth around ilmenite, which acted as a sink for Zr and inhibited new zircon growth. 207Pb/ 206Pb ages for 152 detrital zircons broadly range between 2800 and 990 Ma. The detrital zircon age signature is characterized by the large predominance of late Paleoproterozoic (1.85–1.60 Ga) grains in all analysed samples and by lesser amounts of Archean (2.7–2.8 Ga) and Mesoproterozoic (1.2–1.0 Ga) zircons. The overall age range indicates that detritus can be sourced from the Labradorian and Makkovikian provinces of northeastern Laurentia. The youngest grain analysed (987 ± 18 Ma) indicates that deposition took place during the Neoproterozoic, in a post-Grenvillian sedimentary environment and was likely coeval with the Mid-Neoproterozoic episode of aborted rifting that affected the eastern margin of Laurentia. The lack of detritus originating from Amazonia or Baltica, placed adjacent to Laurentia during the early stages of Rodinia fragmentation, suggests that these two continental landmasses did not constitute a topographic high during the Neoproterozoic or that the Mid-Neoproterozoic rifting opened toward an open ocean to the north of a combined Laurentia/Baltica, thus resulting in a general south to north direction of transport of the sediments.

‘Zipper-rift’: a tectonic model for Neoproterozoic glaciations during the breakup of Rodinia after 750 Ma

The ‘Snowball Earth’ model of Hoffman et al. [Science 281 (1998) 1342] has stimulated renewed interest in the causes of glaciation in Earth history and the sedimentary, stratigraphic and geochemical response. The model invokes catastrophic global Neoproterozoic refrigerations when oceans froze, ice sheets covered the tropics and global temperatures plummeted to −50 °C. Each event is argued to be recorded by tillites and have lasted up to 10 million years. Planetary biological activity was arrested only to resume in the aftermath of abrupt and brutal volcanically generated ‘greenhouse’ deglaciations when global temperatures reached +50 °C. The ‘Cambrian explosion’ is regarded by some as a consequence of post-Snowball glacioeustatic flooding of continental shelves. We shall show by a systematic review of the model that it is based on many long standing assumptions of the character and origin of the Neoproterozoic glacial record, in particular, ‘tillites’, that are no longer valid. This paper focusses on the sedimentological and stratigraphic evidence for glaciation in the light of current knowledge of glacial depositional systems. By integrating this analysis with recent understanding of the tectonic setting of Neoproterozoic sedimentary basins, an alternative ‘Zipper-rift’ hypothesis for Neoproterozoic glaciations is developed. The ‘Zipper-rift’ model emphasises the strong linkage between the first-order reorganisation of the Earth's surface created by diachronous rifting of the supercontinent Rodinia, the climatic effects of uplifted rift flanks and the resulting sedimentary record deposited in newly formed rift basins. Initial fragmentation of Rodinia commenced after about 750 Ma (when the paleo-Pacific Ocean started to form along the western margin of Laurentia) and culminated sometime after 610 Ma (with the opening of the paleo-Atlantic Ocean on the eastern margin of Laurentia). Breakup is recorded by well-defined ‘tectonostratigraphic’ successions that were deposited in marine rift basins. The base of each succession is characterised by coarse-grained synrift strata consisting of mass flow diamictites and conglomerates (many of the ‘tillites’ of the older literature). These facies are interbedded with large olistostromes and contain clastic carbonate debris derived from landsliding of fault scarps along rifted carbonate platforms. Diamictites and conglomerates are dominantly the product of subaqueous mass flow and mixing of coarse and fine sediment populations (the term mixtite has been used in the past). These facies are not uniquely glacial and are produced regardless of climate and latitude. Synrift deposits commonly pass up into thick slope turbidites recording enhanced subsidence and are capped by uppermost shallow marine strata that record a reduction in subsidence rates and overall basin shallowing. Tectonostratigraphic ‘cycles’ can attain thicknesses of several kilometres, but have been commonly misinterpreted as recording globally synchronous ‘glacioeustatic’, falls and rises in sea level. In fact, eustatic sea-level changes in rift basins are suppressed as a result of a strong tectonic control on relative water depths. The great length of newly formed rifted margins around the long perimeter of Laurentia (<20,000 km) ensured that deposition of tectonostratigraphic successions occurred diachronously as in the manner of a zipper between approximately 740 and 610 Ma. Some successions show a definite glacial influence on sedimentation as a consequence of latitude or strong tectonic uplift, but many do not. Regardless, all deposits record a fully functioning hydrological cycle entirely at odds with a supposed fully permafrozen planet. Mapping of those deposits where a definite glacial imprint is apparent indicates that Neoproterozoic glaciation(s) were likely regional or hemispheric in scope and latitudinally constrained. They were perhaps no more severe than other glaciations recorded in Earth history. The regional distribution of ice centres is argued to have been influenced by tectonic topography created by large mantle plumes and by rift shoulder uplift. Paleomagnetic data indicative of tropical glaciation are, in our view, ambiguous because of uncertainty as to when such paleomagnetic characteristics were acquired. A lower solar luminosity may have played a role in lowering snow line elevations and displacing glaciation into latitudes lower than those of Phanerozoic glaciations. Global tectonic and volcanic activity, especially the rapid burial or organic carbon in new rift basins, may explain extreme shifts in C-isotopic values evident in late Neoproterozoic strata.

Vestiges of the Mesoproterozoic Events in the Neoproterozoic Mozambique Belt: the East African Perspective in the Rodinia Puzzle

Most of the geological and palaeogeographical models consider the Neoproterozoic supercontinent Gondwana (∼650-550 Ma) as the direct offspring of the disintegrated Mesoproterozoic supercontinent Rodinia (∼1300-750 Ma). One of the main classical sutures along which the dispersing Rodinia fragments were fused into a new supercontinent (Godwana) is identified as the Mozambique belt of East Africa. The calc-alkaline magmatism (∼1200-950 Ma) in northern Mozambique, southern Malawi and southern Tanzania is regarded as the sole evidence for fragmentation of Rodinia, which is traced within this Neoproterozoic orogenic belt. There are no unequivocal Mesoproterozoic (Kibaran) sediments in this orogen. Concrete evidence for Kibaran metamorphism and deformation is missing. Thus, these solitary documented Kibaran magmatic vestiges in the belt do not ascribe to a true complete orogenesis, which involved the disintegration and dispersal of Rodinia. Consequently, the available sparse Mesoproterozoic (Kibaran) geological and isotopic data from the Mozambique belt of East Africa contentiously suggest its involvement in the aggregation of the supercontinent Rodinia at about 1300-1100 Ma ago.

From Rodinia to Gondwana: A Review of the Available Evidence from South America

In this work we will examine the available evidence for the position of the cratonic masses that formed West Gondwana, in the Neoproterozoic, such as Amazonia, West Africa, São Francisco-Congo, Rio de La Plata and Kalahari. Some emphasis will be given to South America, and the smaller continental masses or microcontinents, that are usually forgotten in the exercise of global reconstruction, will also be considered. Among others, such tectonic units are the Central Goiás massif, the Borborema/Trans-Saharan block, as well as the Luiz Alves, Paraná and Pampia cratonic fragments. Taking into account the relevant paleomagnetic and geochronological data, it is necessary to consider a large time interval for the process of break up of Rodinia. Signals of extension and fracturing are as old as 1000-1100 Ma, when mafic dyke swarms are observed on the São Francisco-Congo craton, and final break out is reported as young as 750 Ma. On the other hand, the formation of intra-oceanic magmatic arcs within the large Goiás Ocean occurred prior to 930 Ma ago. Moreover, for the make up of Gondwana, the first continental collisions are reported at ca. 800 Ma, and the most important accretion episodes are the ones between 650 and 500 Ma, corresponding to the Brasiliano/Pan-African Orogenic Cycle. Therefore, it is apparent that fragmentation of Rodinia is partially synchronous with agglutination of Gondwana.

Puncoviscana folded belt in northwestern Argentina: testimony of Late Proterozoic Rodinia fragmentation and pre-Gondwana collisional episodes: a reply

Puncoviscana folded belt in northwestern Argentina: testimony of Late Proterozoic Rodinia fragmentation and pre-Gondwana collisional episodes: a reply