The Caldwell Group lavas of southern Quebec: MORB-like tholeiites associated with the opening of Iapetus Ocean

1999 ◽  
Vol 36 (6) ◽  
pp. 999-1019 ◽  
Author(s):  
Jean H Bédard ◽  
Ross Stevenson
Keyword(s):  
Incompatible Element ◽  
Crustal Contamination ◽  
Isotopic Data ◽  
High Field Strength ◽  
Element Abundances ◽  
Incompatible Elements ◽  
Iapetus Ocean ◽  
Mid Ocean Ridge ◽  
Source Mantle ◽  
Ocean Ridge

The Caldwell Group belongs to the Internal Nappe Domain of the Humber Zone and consists of basaltic lavas, quartzo-feldspathic sandstones, and mudslates. The lavas are clinopyroxene ± plagioclase ± olivine-phyric tholeiites, and are typically altered to epidote-, chlorite-, carbonate-, and (or) hematite-rich secondary assemblages. In most cases, the high field strength elements do not appear to have been perturbed by the alteration, and preserve magmatic signatures. Most Caldwell basalts exhibit coupled major and trace element variations compatible with low- to medium-pressure ([Formula: see text] 10 kbar, where 1 kbar = 100 MPa) fractional crystallization. Paleotectonic discriminants imply an ocean-floor or normal mid-ocean ridge basalt (N-MORB) affinity. Most basalts have flat N-MORB-normalized profiles, except for the highly incompatible elements (Ba, Th, Nb), which show slight relative enrichment. Melting models suggest that most of these lavas formed by about 20% melting from a mantle slightly less depleted than fertile MORB mantle (FMM). Subpopulations of Caldwell lavas (types 1b and 1a) are characterized by slightly higher incompatible element abundances, with similarly shaped N-MORB-normalized profiles, and can be modeled by slightly smaller degrees of melting (6-15%) of a similar source mantle. The Caldwell basalts erupted in the final stages of Iapetus rifting, when the predominant mantle source involved in melting was the depleted asthenosphere. Isotopic data preclude significant crustal contamination, yet the basalts are associated with sandstones, implying that a mature continental crust was present nearby. Nd isotopic data on the sandstones suggest erosion of an ancient Archean-Proterozoic composite terrane.

scholarly journals Secular oxidation of Archean upper mantle caused by increasing crustal recycling

2021 ◽  
Author(s):  
Lei Gao ◽  
Shuwen Liu ◽  
Peter Cawood ◽  
Jintuan Wang ◽  
Guozheng Sun ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Redox State ◽  
Incompatible Element ◽  
Volcanic Rocks ◽  
Crustal Recycling ◽  
Incompatible Elements ◽  
Mid Ocean Ridge ◽  
Redox Evolution ◽  
Ocean Ridge ◽  
Plate Subduction

Abstract The redox evolution of Archean mantle impacted Earth differentiation, mantle melting and the nature of chemical equilibrium between mantle, ocean and atmosphere of the early Earth. However, how and why it varies with time remain controversial. Archean mantle-derived volcanic rocks, especially basalts are ideal lithologies for reconstructing the mantle redox state. Here we show that the ~3.8-2.5 Ga basalts from fourteen cratons are subdivided geochemically into two groups, B-1, showing incompatible element depleted and modern mid-ocean ridge basalt-like features ((Nb/La)PM ≥ 0.75) and B-2 ((Nb/La)PM < 0.75), characterized by modern island arc basalt-like features. Our updated V-Ti redox proxy indicates the Archean upper mantle was more reducing than today, and that there was a significant redox heterogeneity between ambient and modified mantle presumably related to crustal recycling, perhaps via plate subduction, as shown by B-1 and B-2 magmas, respectively. The oxygen fugacity of modified mantle exhibits a ~1.5-2.0 log units increase over ~3.8-2.5 Ga, whereas the ambient mantle becomes more and more heterogeneous with respect to redox, apart from a significant increase at ~2.7 Ga. These findings are coincident with the increase in the proportions of crustal recycling-related lithologies with associated enrichment of associated incompatible elements (e.g., Th/Nb), indicating that increasing recycling played a crucial role on the secular oxidation of Archean upper mantle.

Composition of the Earth's interior: the importance of early events

2008 ◽  
Vol 366 (1883) ◽  
pp. 4077-4103 ◽  
Author(s):  
Richard W Carlson ◽  
Maud Boyet
Keyword(s):  
Incompatible Element ◽  
Crustal Rocks ◽  
Compositional Model ◽  
Low Viscosity ◽  
Mid Ocean Ridge ◽  
Source Mantle ◽  
Morb Source ◽  
Earth’S Interior ◽  
Ocean Ridge ◽  
Earth's Interior

The detection of excess 142 Nd caused by the decay of 103 Ma half-life 146 Sm in all terrestrial rocks compared with chondrites shows that the chondrite analogue compositional model cannot be strictly correct, at least for the accessible portion of the Earth. Both the continental crust (CC) and the mantle source of mid-ocean ridge basalts (MORB) originate from the material characterized by superchondritic 142 Nd/ 144 Nd. Thus, the mass balance of CC plus mantle depleted by crust extraction (the MORB-source mantle) does not sum back to chondritic compositions, but instead to a composition with Sm/Nd ratio sufficiently high to explain the superchondritic 142 Nd/ 144 Nd. This requires that the mass of mantle depleted by CC extraction expand to 75–100 per cent of the mantle depending on the composition assumed for average CC. If the bulk silicate Earth has chondritic relative abundances of the refractory lithophile elements, then there must exist within the Earth's interior an incompatible-element-enriched reservoir that contains roughly 40 per cent of the Earth's 40 Ar and heat-producing radioactive elements. The existence of this enriched reservoir is demonstrated by time-varying 142 Nd/ 144 Nd in Archaean crustal rocks. Calculations of the mass of the enriched reservoir along with seismically determined properties of the D″ layer at the base of the mantle allow the speculation that this enriched reservoir formed by the sinking of dense melts deep in a terrestrial magma ocean. The enriched reservoir may now be confined to the base of the mantle owing to a combination of compositionally induced high density and low viscosity, both of which allow only minimal entrainment into the overlying convecting mantle.

scholarly journals Sr–Nd–Pb–Hf Isotopic Constraints on the Mantle Heterogeneities beneath the South Mid-Atlantic Ridge at 18–21°S

Minerals ◽  
2020 ◽  
Vol 10 (11) ◽  
pp. 1010
Author(s):  
Yun Zhong ◽  
Xu Zhang ◽  
Zhilei Sun ◽  
Jinnan Liu ◽  
Wei Li ◽  
...  
Keyword(s):  
Isotopic Data ◽  
The South ◽  
Mantle Heterogeneities ◽  
Depleted Mantle ◽  
Mid Ocean Ridge ◽  
Trace Elemental ◽  
Heterogeneous Mantle ◽  
Mid Atlantic Ridge ◽  
St Helena ◽  
Ocean Ridge

In an attempt to investigate the nature and origin of mantle heterogeneities beneath the South Mid-Atlantic Ridge (SMAR), we report new whole-rock Sr, Nd, Pb, and Hf isotopic data from eight basalt samples at four dredge stations along the SMAR between 18°S and 21°S. Sr, Nd, and Pb isotopic data from SMAR mid-ocean ridge basalts (MORBs) at 18–21°S published by other researchers were also utilized in this study. The SMAR MORBs at 18–21°S feature the following ratio ranges: 87Sr/86Sr = 0.70212 to 0.70410, 143Nd/144Nd = 0.512893 to 0.513177, 206Pb/204Pb = 18.05 to 19.50, 207Pb/204Pb = 15.47 to 15.71, 208Pb/204Pb = 37.87 to 38.64, and 176Hf/177Hf = 0.283001 to 0.283175. The 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, and 176Hf/177Hf ratios of these MORBs varied considerably along the SMAR axis. The variable compositions of the Sr–Nd–Pb–Hf isotopes, combined with the corresponding whole-rock major and trace elemental abundances reported in previous studies, suggest that the SMAR MORBs at 18–21°S were probably derived from a heterogeneous mantle substrate related to a mixture of depleted mantle (DM) materials with a small amount (but variable input) of HIMU (high-μ, where μ = 238U/204Pb)- and enriched (EMII)-type materials. The HIMU-type materials likely originated from the proximal St. Helena plume and may have been transported through “pipe-like inclined sublithospheric channels” into the SMAR axial zone. The EMII-type materials possibly originated from a recycled metasomatized oceanic crust that may have been derived from the early dispersion of other plume heads into the subcontinental asthenosphere prior to the opening of the South Atlantic Ocean. In addition, the contributions of subducted sediments, continental crust, and subcontinental lithospheric mantle components to the formation of the SMAR MORBs at 18–21°S may be nonexistent or negligible.

Age and radiogenic isotopic systematics of the Borden carbonatite complex, Ontario, Canada

1987 ◽  
Vol 24 (1) ◽  
pp. 24-30 ◽  
Author(s):  
Keith Bell ◽  
John Blenkinsop ◽  
S. T. Kwon ◽  
G. R. Tilton ◽  
R. P. Sage
Keyword(s):  
Upper Mantle ◽  
Volcanic Rocks ◽  
Crustal Contamination ◽  
Growth Curves ◽  
Carbonatite Complex ◽  
Northern Ontario ◽  
Mantle Origin ◽  
Mid Ocean Ridge ◽  
Ocean Ridge ◽  
Isotope Correlation

Rb–Sr and U–Pb data from the Borden complex of northern Ontario, a carbonatite associated with the Kapuskasing Structural Zone, indicate a mid-Proterozoic age. A 207Pb/206Pb age of 1872 ± 13 Ma is interpreted as the emplacement age of this body, grouping it with other ca. 1900 Ma complexes that are the oldest known carbonatites associated with the Kapuskasing structure. A 206Pb–238U age of 1894 ± 29 Ma agrees with the Pb–Pb age but has a high mean square of weighted deviates (MSWD) of 42. A Rb–Sr apatite–carbonate–mica whole-rock isochron date of 1807 ± 13 Ma probably indicates later resetting of the Rb–Sr system.An εSr(T) value of −6.2 ± 0.5 (87Sr/86Sr = 0.70184 ± 0.00003) and an εNd(T) value of +2.8 ± 0.4 for Borden indicate derivation of the Sr and Nd from a source with a time-integrated depletion in the large-ion lithophile (LIL) elements. These closely resemble the ε values for Sr and Nd from the Cargill and Spanish River complexes, two other 1900 Ma plutons. The estimated initial 207Pb/204Pb and 206Pb/204Pb ratios from Borden calcites plot significantly below growth curves for average continental crust in isotope correlation diagrams, a pattern similar to those found in mid-ocean ridge basalts (MORB) and most ocean-island volcanic rocks, again suggesting a source depleted in LIL elements. The combined Nd and Sr, and probably Pb, data strongly favour a mantle origin for the Borden complex with little or no crustal contamination and support the model of Bell et al. that many carbonatites intruded into the Canadian Shield were derived from an ancient, LIL-depleted subcontinental upper mantle.

scholarly journals Identification of chondritic krypton and xenon in Yellowstone gases and the timing of terrestrial volatile accretion

2020 ◽  
Vol 117 (25) ◽  
pp. 13997-14004 ◽  
Author(s):  
Michael W. Broadley ◽  
Peter H. Barry ◽  
David V. Bekaert ◽  
David J. Byrne ◽  
Antonio Caracausi ◽  
...  
Keyword(s):  
Mantle Plume ◽  
Noble Gases ◽  
Noble Gas ◽  
Mid Ocean Ridge ◽  
Mantle Sources ◽  
Source Mantle ◽  
Morb Source ◽  
Protosolar Nebula ◽  
Ocean Ridge ◽  
Yellowstone Caldera

Identifying the origin of noble gases in Earth’s mantle can provide crucial constraints on the source and timing of volatile (C, N, H2O, noble gases, etc.) delivery to Earth. It remains unclear whether the early Earth was able to directly capture and retain volatiles throughout accretion or whether it accreted anhydrously and subsequently acquired volatiles through later additions of chondritic material. Here, we report high-precision noble gas isotopic data from volcanic gases emanating from, in and around, the Yellowstone caldera (Wyoming, United States). We show that the He and Ne isotopic and elemental signatures of the Yellowstone gas requires an input from an undegassed mantle plume. Coupled with the distinct ratio of129Xe to primordial Xe isotopes in Yellowstone compared with mid-ocean ridge basalt (MORB) samples, this confirms that the deep plume and shallow MORB mantles have remained distinct from one another for the majority of Earth’s history. Krypton and xenon isotopes in the Yellowstone mantle plume are found to be chondritic in origin, similar to the MORB source mantle. This is in contrast with the origin of neon in the mantle, which exhibits an isotopic dichotomy between solar plume and chondritic MORB mantle sources. The co-occurrence of solar and chondritic noble gases in the deep mantle is thought to reflect the heterogeneous nature of Earth’s volatile accretion during the lifetime of the protosolar nebula. It notably implies that the Earth was able to retain its chondritic volatiles since its earliest stages of accretion, and not only through late additions.

Differentiation and source of the Nipissing Diabase intrusions, Ontario, Canada

1993 ◽  
Vol 30 (6) ◽  
pp. 1123-1140 ◽  
Author(s):  
P. C. Lightfoot ◽  
H. de Souza ◽  
W. Doherty
Keyword(s):  
Trace Element ◽  
Crustal Contamination ◽  
Primitive Mantle ◽  
Magma Source ◽  
Magma Type ◽  
Ni Content ◽  
Chemical Signatures ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

Major and trace element data are presented for 2.2 Ga Proterozoic diabase sills from across the Nipissing magmatic province of Ontario. In situ differentiation of the magma coupled with assimilation of Huronian Supergroup roof sediments is responsible for the variation in composition between quartz diabase and granophyric diabase seen within many of the differentiated intrusions. Uniform trace element and isotope ratio signatures, such as La/Sm (2.8 – 3.7) and εNdCHUR (−2.7 to −5.9) characterize chilled margins and undifferentiated quartz diabases. These chemical signatures suggest the existence of a single magma source that was parental to intrusions throughout the magmatic province; this magma has higher La/Sm and lower Ti/Y than primitive mantle and is displaced towards the composition of shales. Most chilled diabases and quartz diabases have a similar Mg# (0.64 and 0.60) and Ni content (98 and 127 ppm), and it is argued that the magma differentiated at depth and was emplaced as a uniform low-Mg magma. The Wanapitei intrusion and Kukagami Lake sill are an exception in that although the quartz diabase has La/Sm similar to the Nipissing magma type, which suggests that they came from the same source, the Mg# (0.68–0.71) and Ni content (130–141 ppm) are higher, which may suggest that they are either slightly more primitive examples of the normal Nipissing magma or that cumulus hypersthene has been resorbed. The light rare earth element enriched signature of the Nipissing magmas was perhaps introduced from the continental crust as the magma migrated from the mantle to the surface, but a remarkably constant and large amount (>20%) of crustal contamination would be required. An addition of 1 –3% shale to the source of a transitional mid-ocean ridge basalt type magma can broadly reproduce the compositional features of the Nipissing magma type. The source characteristics were perhaps imparted during subduction accompanying the terminal Kenoran orogeny.

Element fluxes associated with subduction related magmatism

1991 ◽  
Vol 335 (1638) ◽  
pp. 393-405 ◽  
Keyword(s):  
Trace Element ◽  
Incompatible Element ◽  
Mantle Wedge ◽  
Isotope Ratios ◽  
Noticeable Effect ◽  
Arc Magmas ◽  
Mid Ocean Ridge ◽  
Ocean Ridge ◽  
Arc Magma ◽  
Subducted Sediment

Destructive plate margin magmas may be subdivided into two groups on the basis of their rare earth element (REE) ratios. Most island arc suites have low Ce/Yb, and remarkably restricted isotope ratios of 87 Sr/ 86 Sr = 0.7033, 143 Nd/ 144 Nd = 0.51302, 206 Pb/ 204 Pb = 18.76 , 207 Pb/ 204 Pb = 15.57, and 208 Pb/ 204 Pb = 38.4. However, they also have Rb/Sr (0.03), Th/U (2.2) and Ce/Yb (8.5) ratios which are significantly less than accepted estimates for the bulk continental crust. The high Ce/Yb suites have higher incompatible element contents, more restricted heavy REE, and much more variable isotope ratios. Such rocks are found in the Aeolian Islands, Grenada, Indonesia and Philippines, and their isotope and trace element features have been attributed both to contributions from subducted sediment, and/or old trace element enriched material in the mantle wedge. It is argued that for isotope and trace element models the slab component can usefully be taken to consist of subducted sediment and altered mid-ocean ridge basalts, since these may contain ca. 80% of the water in the subducted slab, and the distinctive trace element features of arc magmas are generally attributed to the movement of material in hydrous fluids. The isotope data indicate that not more than 15% of the Sr and Th in an average arc magma were derived from subducted material, and that the rest were derived from the mantle wedge. The fluxes of elements which cannot be characterized isotopically are more difficult to constrain, but for most minor and trace elements the slab derived contribution in arc magmas is too small to have a noticeable effect on the residual slab.

scholarly journals Basalt derived from highly refractory mantle sources during early Izu-Bonin-Mariana arc development

2020 ◽  
Author(s):  
He Li ◽  
Richard Arculus ◽  
Osamu Ishizuka ◽  
Rosemary Hickey-Vargas ◽  
Gene Yogodzinski ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Philippine Sea Plate ◽  
Island Arcs ◽  
Element Abundances ◽  
Mid Ocean Ridge ◽  
Mantle Sources ◽  
Backarc Basins ◽  
Mariana Arc ◽  
Ocean Ridge ◽  
Aluminous Spinel

Abstract The character of magmatism associated with the early stages of subduction zone and island arc development is unlike that of mature systems, being dominated in the Izu-Bonon-Mariana (IBM) case by low-Ti-K tholeiitic basalts and boninites. Basalts recovered by coring the basement of the Amami Sankaku Basin (ASB), located west of the oldest remnant arc of the IBM system (Kyushu-Palau Ridge; KPR), were erupted at ~49 Ma, about 3 million years after subduction inception. The chain of stratovolcanoes defined by the KPR is superimposed on this basement. The basalts were sourced from upper mantle similar to that tapped following subduction inception, and represented by forearc basalt (FAB) dated at ~52-51 Ma. The mantle sources of the ASB basalt basement were more depleted by prior melt extraction than those involved in the vast majority of mid-ocean ridge (MOR) basalt generation. The ASB basalts are low-Ti-K, aluminous spinel-olivine-plagioclase-clinopyroxene-bearing tholeiites. We show this primary mineralogy is collectively distinct compared to basalts of MOR, backarc basins of the Philippine Sea Plate, forearc, or mature island arcs. In combination with bulk compositional (major and trace element abundances plus radiogenic isotope characteristics) data for the ASB basalts, we infer the upper mantle involved was hot (~1400oC), reduced, and refractory peridotite. For a few million years following subduction initiation, a broad region of mantle upwelling accompanied by partial melting prevailed. The ASB basalts were transferred rapidly from moderate pressures (1-2 GPa), preserving a mineralogy established at sub-crustal conditions, and experienced little of recharge-mix-tap-fractionate regimes typical of MOR or mature arcs.

scholarly journals Evidence of Late Ediacaran Hyperextension of the Laurentian Iapetan Margin in the Birchy Complex, Baie Verte Peninsula, Northwest Newfoundland: Implications for the Opening of Iapetus, Formation of Peri-Laurentian Microcontinents and Taconic – Grampian Orogenesis

2013 ◽  
Vol 40 (2) ◽  
pp. 94 ◽  
Author(s):  
Cees R. Van Staal ◽  
Dave M. Chew ◽  
Alexandre Zagorevski ◽  
Vicki McNicoll ◽  
James Hibbard ◽  
...  
Keyword(s):  
Hanging Wall ◽  
Ultramafic Rocks ◽  
Nous Proposons ◽  
Mafic Rocks ◽  
Metasedimentary Rocks ◽  
Iapetus Ocean ◽  
Icp Ms ◽  
Mid Ocean Ridge ◽  
Continental Lithospheric Mantle ◽  
Ocean Ridge

The Birchy Complex of the Baie Verte Peninsula, northwestern Newfoundland, comprises an assemblage of mafic schist, ultramafic rocks, and metasedimentary rocks that are structurally sandwiched between overlying ca. 490 Ma ophiolite massifs of the Baie Verte oceanic tract and underlying metasedimentary rocks of the Fleur de Lys Supergroup of the Appalachian Humber margin. Birchy Complex gabbro yielded a Late Ediacaran U–Pb zircon ID–TIMS age of 558.3 ± 0.7 Ma, whereas gabbro and an intermediate tuffaceous schist yielded LA–ICPMS concordia zircon ages of 564 ± 7.5 Ma and 556 ± 4 Ma, respectively. These ages overlap the last phase of rift-related magmatism observed along the Humber margin of the northern Appalachians (565–550 Ma). The associated ultramafic rocks were exhumed by the Late Ediacaran and shed detritus into the interleaved sedimentary rocks. Psammite in the overlying Flat Point Formation yielded a detrital zircon population typical of the Laurentian Humber margin in the northern Appalachians. Age relationships and characteristics of the Birchy Complex and adjacent Rattling Brook Group suggest that the ultramafic rocks represent slices of continental lithospheric mantle exhumed onto the seafloor shortly before or coeval with magmatic accretion of mid-ocean ridge basalt-like mafic rocks. Hence, they represent the remnants of an ocean – continent transition zone formed during hyperextension of the Humber margin prior to establishment of a mid-ocean ridge farther outboard in the Iapetus Ocean. We propose that microcontinents such as Dashwoods and the Rattling Brook Group formed as a hanging wall block and an extensional crustal allochthon, respectively, analogous to the isolation of the Briançonnais block during the opening of the Alpine Ligurian–Piemonte and Valais oceanic seaways.SOMMAIRELe complexe de Birchy de la péninsule de Baie Verte, dans le nord-ouest de Terre-Neuve, est constitué d’un assemblage de schistes mafiques, de roches ultramafiques et de métasédiments qui sont coincés entre des massifs ophiolitiques d’ascendance océanique de la Baie Verte au-dessus, et des métasédiments du Supergroupe de Fleur de Lys de la marge de Humber des Appalaches en-dessous. Le complexe de gabbro de Birchy a donné une datation U-Pb sur zircon ID-TIMS correspondant à la fin de l’Édiacarien, soit 558,3 ± 0,7 Ma, alors qu’un gabbro et un schiste tufacé intermédiaire montrent une datation LA-ICP-MS Concordia sur zircon de 564 ± 7,5 Ma et 556 ± 4 Ma, respectivement. Ces datations chevauchent la dernière phase de magmatisme de rift observée le long de la marge Humber des Appalaches du Nord (565-550 Ma). Les roches ultramafiques associées ont été exhumées vers la fin de l’Édiacarien et leurs débris ont été imbriqués dans des roches sédimentaires. Les psammites de la Formation de Flat Point susjacente ont donné une population de zircons détritiques typique de la marge laurentienne de Humber des Appalaches du Nord. Les relations chronologiques et les caractéristiques du complexe de Birchy et du groupe de Rattling Brook adjacent, permettent de penser que ces roches ultramafiques pourraient être des écailles de manteau lithosphérique continental qui auraient été exhumées sur le plancher océanique peu avant ou en même temps que l’accrétion magmatique de roches mafiques basaltiques de type dorsale médio-océanique. Par conséquent, elles seraient des vestiges d’une zone de transition océan-continent formée au cours de l’hyper-extension de la marge de Humber avant l’apparition d’une dorsale médio-océanique plus loin au large dans l’océan Iapétus. Nous proposons que des microcontinents comme de Dashwoods et du groupe de Rattling Brook ont constitués respectivement un bloc de toit et un allochtone crustal d’extension, de la même manière que le bloc Briançonnais a été isolé lors de l’ouverture des bras océaniques alpins de Ligurie-Piémont et de Valais.

Iapetan Oceans: An analog of Tethys?

Geology ◽  
2020 ◽  
Vol 48 (9) ◽  
pp. 929-933 ◽  
Author(s):  
B. Robert ◽  
M. Domeier ◽  
J. Jakob
Keyword(s):  
Second Phase ◽  
Continental Margins ◽  
Deep Time ◽  
Zircon Age ◽  
Continental Extension ◽  
Iapetus Ocean ◽  
Mid Ocean Ridge ◽  
Tethys Ocean ◽  
Late Neoproterozoic ◽  
Ocean Ridge

Abstract The Iapetus Ocean opened during the breakup of Rodinia by the separation of the major continental blocks of Laurentia (LA), Baltica, and Amazonia (AM). Relics of protracted continental extension to rifting from 750 to 530 Ma are observed along those continental margins, including two distinct phases of rifting: (1) at 750–680 Ma, and (2) at 615–550 Ma. Conventionally, the second phase is thought to have led to the opening of the Iapetus, while the first phase marked a failed rifting attempt. We challenge this concept on the basis of a new review of the geological observations from those margins and propose the successive opening of two “Iapetan” ocean basins. First, a “Paleo-Iapetus” opened between LA and AM at ca. 700 Ma, followed by the opening of the “Neo-Iapetus” at 600 Ma, which led to the final disaggregation of the supercontinent Rodinia. This scenario better explains the absence of the second rifting phase in western AM, as well as an otherwise enigmatic late Neoproterozoic detrital zircon age fraction in Phanerozoic sediments along that margin. We further propose that the opening of the Neo-Iapetus led to the detachment of small terranes from LA and their drift toward AM, following subduction of the Paleo-Iapetus mid-ocean ridge and the arrival of a mantle plume around 615 Ma. This could be a direct, deep-time analog of the opening of the Neo-Tethys Ocean in the late Paleozoic.

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