Evidence of Ectasian/Stenian MesoProterozoic River Morphology

Deeply channelled Precambrian rivers: Remote sensing and outcrop evidence from the 1.2 Ga Stoer Group of NW Scotland

Authors:

Ielpi et al

Abstract:

The current paradigm on Precambrian fluvial sedimentology assumes that pre-vegetation environments did not allow for the establishment of deep, stable channels. However, few studies have documented a continuum of fluvial-depositional architectures along km-scale transects where clusters of channel bodies can be observed in their entirety. The Stoer Group consists of a 1.2 Ga rift-basin fill, its type area occurring at Stoer Peninsula, NW Scotland. Ground observations on classic coastal sections are integrated with remote sensing on a restored transect 7 km wide, representing 400 m of stratigraphy. The transect spans a set of palaeovalleys carved on high-relief gneissic basement. Remote sensing and ground-based sedimentology unveil a set of depositional domains, including: (i) perennial channels filled with downstream-lateral accretionary bars; (ii) poorly drained muddy floodbasins; (iii) well-drained sandy floodbasins containing splays and distributary channels, at times meandering; (iv) gravelly piedmonts composed of talus cones and fluvial-fan deposits; and (v) aeolian fields comprising dunes, ponded interdunes, and sandsheets. These depositional domains reveal an original geomorphic complexity higher than previously recognized for Precambrian terrestrial environments.

The occurrence of clustered channel bodies alongside or within both muddy and sandy depositional domains indicates that stable fluvial channels developed independently from the cohesive nature of the substrate. Their development was possibly aided by drainage focusing along valley thalwegs. Three types of fluvial channels have been identified: laterally extensive, multistorey channels akin to mobile, weakly sinuous rivers; vertically stacked, multistorey channels reminiscent of confined, weakly sinuous rivers; and laterally extensive, multilateral channels, indicative of highly mobile, moderately sinuous rivers. Eighty-four preserved channel fills display width-to-thickness ratios fully overlapping with those of post-vegetation and modern braided rivers, disproving the commonly held notion that pre-vegetation rivers could only generate sheet-like sandbodies. This study provides new insights on Precambrian fluvial styles, and underscores the potential of remote-sensing methods to analyze very large exposures of fluvial rocks.

Pondering the Precambrian #1

Archean:

A study used diamonds to suggest the start of plate tectonics was approximately 3.5 billion years ago during the PaleoArchean.

Carbon deposited in the PaleoArchean Australia appears to be biological in origin.

Another study claims plate tectonics started 3 billion years ago during the MesoArchean through the examination of granites and magnesium.

The Helen Iron Formation in Canada was formed during the NeoArchean, 2.75 billion years ago, in moderately deep water when underwater volcanism ceased for a time.

There is evidence of NeoArchean plate tectonics in India

PaleoProterozoic:

Oxygen levels during the PaleoProterozoic Great Oxygenation Event may have been higher than the following Proterozoic steady state.

MesoProterozoic:

Evidence of freshwater stromatolites have been found from the Stenian of Michigan in the Copper Harbor Conglomerate.

There is paleoclimatic evidence also from the Stenian of Michigan.  The paleosols appear to indicate it was a floodplain.

NeoProterozoic:

Evidence from China gives insight into marine plate subduction during the Tonian.

Evidence from the Doushantuo Formation in China suggest the Ediacaran there had a carbon anomaly and the phosphated embryos found predate the Gaskiers Glaciation.

Fresh Water Cyanobacteria Fossils From Stenian MesoProterozoic/Tonian NeoProterozoic

Palaeoecology of a billion-year-old non-marine cyanobacterium from the Torridon Group and Nonesuch Formation

Authors:

Miles et al

Abstract:

A new chroococcalean cyanobacterium is described from approximately 1-billion-year-old non-marine deposits of the Torridonian Group of Scotland and the Nonesuch Formation of Michigan, USA. Individual cells of the new microfossil, Eohalothece lacustrina gen. et sp. nov., are associated with benthic microbial biofilms, but the majority of samples are recovered in palynological preparations in the form of large, apparently planktonic colonies, similar to extant species of Microcystis. In the Torridonian, Eohalothece is associated with phosphatic nodules, and we have developed a novel hypothesis linking Eohalothece to phosphate deposition in ancient freshwater settings. Extant cyanobacteria can be prolific producers of extracellular microcystins, which are non-ribosomal polypeptide phosphatase inhibitors. Microcystins may have promoted the retention and concentration of sedimentary organic phosphate prior to mineralization of francolite and nodule formation. This has a further implication that the Torridonian lakes were nitrogen limited as the release of microcystins is enhanced under such conditions today. The abundance and wide distribution of Eohalothece lacustrina attests to the importance of cyanobacteria as oxygen-producing photoautotrophs in lacustrine ecosystems at the time of the Mesoproterozoic–Neoproterozoic transition.

Seemingly Deep Water Seep Carbonate Mounds are Really From a Stenian MesoProterozoic Alkali Lake

Deep-water seep-related carbonate mounds in a Mesoproterozoic alkaline lake, Borden Basin (Nunavut, Canada)

Authors:

Hahn et al

Abstract:

The Mesoproterozoic (1.1 Ga) Borden Basin contains extremely large, deep-water dolostone seep mounds (Ikpiarjuk Formation) whose distribution is controlled by faults. Four mounds were investigated along measured stratigraphic sections. Petrographic study revealed several depositional components, and a mixture of at least two distinct carbonate sources. Stable isotope data showed no significant methane contribution to the carbonate phases. Detritus-corrected REE + Y patterns, obtained using solution ICP-MS, depict binary mixtures between basin-water-derived precipitates and seep-fluid-derived carbonate. The purest pelagic REE + Y signal is from mound tops, suggesting that mound accumulation ceased when the seep fluid waned. The REE + Y pattern of the pelagic precipitates resembles that of modern alkaline lake water. The shale-normalised pattern of the basin water is LREE-depleted, has a positive Ce anomaly and pronounced Y-excess, but lacks the La and Gd anomalies typical of seawater. The seep-fluid-related dolomite has flat shale-normalised REE + Y patterns, no Ce anomaly, and a negative Eu anomaly. This combination of characteristics points to circum-neutral (Ca and Mg-bearing?) fluids that interacted with the underlying basement before seeping into the lake bottom through faults. The chemostratigraphic patterns of the mounds result from the relative contribution of elements from the basin water vs. seep fluids. When combined with published geochemical data for coeval black shale surrounding the mounds, the new data suggest a lacustrine setting, surrounded by catchments with a preponderance of moderately to strongly weathered alkali basalt whose runoff drove the lake to alkalinity. Seep fluid was probably evaporatively concentrated basin water that acquired new geochemical characteristics both during evaporation and through water-rock interaction in the subsurface. The new understanding of this depositional stage of the Borden Basin highlights the importance of lacustrine deposits in the Mesoproterozoic, and presents an obvious impediment to using carbonate stable isotope or trace element geochemistry to reconstruct global atmosphere-hydrosphere conditions for this time for any units that cannot be demonstrated conclusively to be of marine origin.

Evidence of a Stenian MesoProterozoic Impact Discovered in Scotland

Precambrian reidite discovered in shocked zircon from the Stac Fada impactite, Scotland

Authors:

Reddy et al

Abstract:

Terrestrial impact events have had a profound influence on Earth's geological, geochemical, and biological evolution. However, the record of Precambrian impacts is poorly constrained due to the dynamic nature of plate tectonics, erosion, and deposition of younger rocks that may destroy or cover the evidence. Here we report the first Precambrian occurrence of the rare mineral reidite (ZrSiO4) within grains of shocked zircon in the ca. 1.18 Ga Stac Fada Member (Stoer Group), northwestern Scotland. The reidite, preserved as less than 2-μm-wide lamellae, is unambiguous evidence of shock pressures in excess of ∼30 GPa and confirms the impact origin for the Stac Fada deposit. The reidite lamellae are locally deformed, and sites of deformation record its decomposition to baddeleyite (ZrO2) and amorphous silica, the first natural example of this transformation. The findings demonstrate that reidite and baddeleyite may form and be transported in high-energy ejecta without physical or chemical breakdown and are stable during sedimentary diagenesis and low-grade metamorphism. Thus, reidite may be preserved over time scales exceeding 1 b.y., establishing the use of reidite within detrital shocked zircon from Precambrian strata as a viable and valuable means of recognizing and characterizing ancient terrestrial impact events.

Complex Life Cycles and Sexual Reproduction Evident in Ectasian/Stenian MesoProterozoic MicroFossils

Affinity, life cycle, and intracellular complexity of organic-walled microfossils from the Mesoproterozoic of Shanxi, China

Authors:

Agic et al

Abstract:

Light microscope and scanning electron microscope observations on new material of unicellular microfossils Dictyosphaera macroreticulata and Shuiyousphaeridium macroreticulatum, from the Mesoproterozoic Ruyang Group in China, provide insights into the microorganisms’ biological affinity, life cycle and cellular complexity. Gigantosphaeridium fibratum n. gen. et sp., is described and is one of the largest Mesoproterozoic microfossils recorded. Phenotypic characters of vesicle ornamentation and excystment structures, properties of resistance and cell wall structure in Dictyosphaera and Shuiyousphaeridium are all diagnostic of microalgal cysts. The wide size ranges of the various morphotypes indicate growth phases compatible with the development of reproductive cysts. Conspecific biologically, each morphotype represents an asexual (resting cyst) or sexual (zygotic cyst) stage in the life cycle, respectively. We reconstruct this hypothetical life cycle and infer that the organism demonstrates a reproductive strategy of alternation of heteromorphic generations. Similarly in Gigantosphaeridium, a metabolically expensive vesicle with processes suggests its protective role as a zygotic cyst. In combination with all these characters and from the resemblance to extant green algae, we propose the placement of these ancient microorganisms in the stem group of Chloroplastida (Viridiplantae). A cell wall composed of primary and secondary layers in Dictyosphaera and Shuiyouisphaeridium required a high cellular complexity for their synthesis and the presence of an endomembrane system and the Golgi apparatus. The plastid was also present, accepting the organism was photosynthetic. The biota reveals a high degree of morphological and cell structural complexity, and provides an insight into ongoing eukaryotic evolution and the development of complex life cycles with sexual reproduction by 1200 Ma.

Stenian MesoProterozoic Ocean had Heterogenous Redox Conditions

Heterogeneous Redox Conditions and a Shallow Chemocline in the Mesoproterozoic Ocean: Evidence from Carbon-Sulfur-Iron Relationships

Authors:

Gilleaudeau et al

Abstract:

Oxygenation of Earth's oceans in the Proterozoic was a protracted process that began prior to the Great Oxidation Event (GOE) and culminated in deep ocean ventilation following Neoproterozoic glacial episodes, with both of these major thresholds accompanied by profound changes in the evolution of Earth's biosphere. Our understanding of the redox structure of Mesoproterozoic oceans, however, is considerably less certain. Limited proxy investigations of oceanic redox in the Mesoproterozoic have focused largely on early Mesoproterozoic shelf to basinal settings, with comparatively little attention paid to epeiric sea environments that dominate the late Mesoproterozoic sedimentary record. Here we report multi-proxy shale redox data from epicratonic and pericratonic environments of the 1.1 Ga Atar and El Mreiti groups, Taoudeni Basin, Mauritania. These strata were deposited across the West African craton during global sea level highstand, and provide insight into the redox structure of Mesoproterozoic epeiric seas. Iron speciation, pyrite sulfur isotope, and trace metal concentration data suggest a shallow chemocline and highly heterogeneous redox conditions, with fundamental heterogeneities in sulfate concentration and the extent and persistence of euxinia between onshore and offshore environments. We suggest that a protracted increase in biospheric oxygenation in the late Mesoproterozoic was accompanied by unstable and heterogeneous redox conditions in shallow, nearshore areas, contributing to the fragmentary pattern of early eukaryotic evolution and diversification.

Proterozoic Supercontinental Restorations From the Siberian Craton

Proterozoic supercontinental restorations: Constraints from provenance studies of Mesoproterozoic to Cambrian clastic rocks, eastern Siberian Craton

Authors:

Khudoley et al

Abstract:

The Mesoproterozoic–Neoproterozoic sedimentary succession of the eastern part of the Siberian Craton consists of several unconformity-bounded, kilometer-scale siliciclastic-carbonate cycles. The overlying Lower Cambrian rocks are often compositionally similar to the uppermost units of the Neoproterozoic succession.

Twenty-nine samples were collected for U–Pb detrital zircon study and 27 samples were analyzed for whole-rock Sm–Nd isotopes. In total, 1491 detrital zircon grains were dated and 1148 grains were selected for provenance interpretation. Samples from the Uchur and Aimchan groups only contain detrital zircons of Paleoproterozoic and Archean age. Samples from the Kerpyl Group located on the Siberian Craton contain Paleoproterozoic and Archean grains as well, but samples from the Kerpyl Group in the Sette-Daban Ridge have significant numbers of Mesoproterozoic detrital zircons. Mesoproterozoic detrital zircons predominate in samples from the Uy Group. In the northern part of the study area, samples from the uppermost Neoproterozoic and Lower Cambrian strata contain numerous ca. 790–590 Ma detrital zircons, whereas in the southern part of the study area only Paleoproterozoic and Archean grains have been found. The whole-rock Sm–Nd isotopic values of clastic rocks show that most samples have isotopic signatures typical for the Siberian Craton basement, whereas some samples from the Kerpyl Group and younger rock units have isotopic signatures typical of the Grenville Orogen.

Most of the Archean and Paleoproterozoic detrital zircons were eroded from the basement of the Siberian Craton, although some ca. 2080–2030 Ma detrital zircons are likely to have a non-Siberian provenance. However, rocks younger than ca. 1700 Ma are not known in the Siberian Craton basement and all Mesoproterozoic and younger grains must therefore have a non-Siberian provenance.

The detrital zircon age distributions and whole-rock Nd isotopic signatures of many samples from the Kerpyl Group and younger units are very close to those of the Grenville Orogen in North America, suggesting that erosion of the latter contributed to clastic deposition along the Siberian margin. Three paleocontinental restorations proposed by Sears and Price (1978, 2003), Rainbird et al. (1998) and Pisarevsky and Natapov (2003) are invoked to explain the occurrence of Grenville-age detrital zircons in the Siberian sedimentary succession. The provenance of ca. 790–590 Ma detrital zircons is most likely to be located within the Central Taimyr accretionary belt formed along the northern margin of the Siberian Craton in the Neoproterozoic.

North America's Stenian MesoProterozoic Midcontinent Rift Explored

An international team of geologists has a new explanation for how the Midwest's biggest geological feature -- an ancient and giant 2,000-mile-long underground crack that starts in Lake Superior and runs south to Oklahoma and to Alabama -- evolved.

Scientists from Northwestern University, the University of Illinois at Chicago (UIC), the University of Gottingen in Germany and the University of Oklahoma report that the 1.1 billion-year-old Midcontinent Rift is a geological hybrid, having formed in three stages: it started as an enormous narrow crack in the Earth's crust; that space then filled with an unusually large amount of volcanic rock; and, finally, the igneous rocks were forced to the surface, forming the beautiful scenery seen today in the Lake Superior area of the Upper Midwest.

The rift produced some of the Midwest's most interesting geology and scenery, but there has never been a good explanation for what caused it. Inspired by vacations to Lake Superior, Seth and Carol A. Stein, a husband-and-wife team from Northwestern and UIC, have been determined to learn more in recent years.

link.

Geochronology of Meso/NeoProterozoic Carbonate Platforms in Brazil

Meso-Neoproterozoic isotope stratigraphy on carbonates platforms in the Brasilia Belt of Brazil

Authors:

Alvarenga et al

Abstract:

Carbonate platforms were present worldwide during the Proterozoic Eon, and variations in their C and Sr isotope ratios are commonly used as a correlation tool particularly through the Neoproterozoic, when rapid secular change in marine C and Sr isotope values permit distinction between Cryogenian glacial events. In central Brazil, the late Mesoproterozoic and Neoproterozoic eras are represented by a thick succession of sedimentary rocks that were deposited, and later deformed, along the eastern margin of the São Francisco Craton. These strata are divided into the three major groups: (1) the Paranoá Group, which consists of a succession of sandstone, siltstone, rhythmite, and selected intervals of carbonate, (2) the Macaúbas Group, which consists of a glacial diamictite (Jequitaí Formation), and (3) the Bambuí Group with an important carbonate-bearing succession that includes characteristic “cap carbonate” facies in its lower strata. Carbonate facies of Paranoá and Bambuí Groups typically occur in unconformable contact, and when the diamictite of Jequitaí Formation is absent, it can be difficult to determine the stratigraphic position of these lithological similar groups. Furthermore, uncertainties in the age of the Bambuí Group has lead to several distinct interpretations regarding the age of the Jequitaí glacial diamictites.

We investigated the C, O, and Sr isotopes and chemical composition of carbonate rocks in five measured sections, including both pre- and post-glaciation carbonate successions. The δ13C (‰ pdb) values in the upper Paranoá Group occur in a narrow interval between +0.6 and +3.6, whereas the post-glacial Bambuí Group begins with substantially negative values (as low as −5.7‰) in cap dolomite facies and rises to values up to +11 permil in limestone of the upper Sete Lagoas Formation. Similarly, carbonate rocks of the Paranoá and Bambuí groups are distinct in terms of their 87Sr/86Sr ratio. Generally non-radiogenic ratios between 0.7056 and 0.7068 are recorded in the upper Paranoá Group, and ratios between 0.7074 and 0.7080, occurring within the Bambuí Group.

The stratigraphic pattern of the C and Sr ratios indicates distinctive isotopic characteristics for these two carbonate successions. The isotopic data for the cabonates in the Paranoá Group are consistent with a sedimentation age in upper Mesoproterozoic or lowermost Neoproterozoic, preceding the first Cryogenian glaciation. Carbonate facies and the isotopic data for the lower Bambuí Group suggest a relationship with the second Cryogenian glaciation.

Geological Record of Death Valley From the Stenian Mesoproterozoic to the Ediacaran NeoProterozoic

Detrital zircon provenance and paleogeography of the Pahrump Group and overlying strata, Death Valley, California

Authors:

Mahon et al

Abstract:

The Mesoproterozoic and Neoproterozoic Pahrump Group of Death Valley, California spans ca. 1300 to 635 Ma and provides a greater than 500 million-year record of geologic events in southwestern Laurentia. The strata analyzed include preserved sequences separated by unconformities recording syn-Rodinia basin development (Crystal Spring Formation); Rodinia stability; regional extension culminating in Neoproterozoic rifting of the Laurentian margin of Rodinia (Horse Thief Springs through Johnnie Formations); and multiple phases of glacial sedimentation and subsequent cap carbonate deposition (Kingston Peak Formation and Noonday Dolomite). U-Pb detrital zircon analyses were conducted on samples from the entire Pahrump Group and the Noonday Dolomite in the southeastern Death Valley region (20 samples, 1,945 grains) to further constrain hypotheses for regional basin development during the development of the southwestern Laurentian margin.

Our interpretation of provenance data expands upon and clarifies previous models defining a series of tectonostratigraphic units including: A) the less than 1400 Ma basal conglomerate of the Crystal Spring Formation, comprised of metasedimentary quartzite clasts, and exhibiting a unimodal detrital zircon sample distribution at 1690 Ma with northerly source; B) the ca. 1320-1080 Ma Crystal Spring Formation exhibiting unimodal zircon distributions derived from southerly, local Paleoproterozoic basement sources punctuated by a ca. 300 Ma duration unconformity; C) the ca. 780-740 Ma sequence of the Horse Thief Springs Formation, Beck Spring Dolomite, and KP1 unit of Kingston Peak Formation deposited in a marine basin with mixed southwestern Laurentian provenance; D) a ca. 710-635 Ma glaciogenic sequence (KP2-KP4 members of Kingston Peak Formation), recording the onset of Rodinia rifting, and Sturtian and Marinoan “Snowball Earth” intervals. Provenance data suggest derivation from erosion and recycling of older Pahrump Group strata; E) the ca. 635 Ma cap dolostone of the Sentinel Peak Member of the Noonday Dolomite, representing post-glacial drainage reorganization with more regional provenance; and followed by F) the <635 500="" a="" addition="" age="" analysis="" and="" basement="" bimodal="" blockquote="" ca.="" complement="" data="" derivation="" distributions="" dolomite="" for="" from="" in="" indicating="" laurentian="" local="" ma="" margin.="" marked="" member="" new="" noonday="" of="" overlying="" previous="" provenance="" radcliff="" result="" shift="" showing="" sources.="" southwestern="" strata="" studies="" synthesize="" the="" these="" to="" units="">

Evidence of Rodinia's Assembly From Stenian MesoProterozoic South Africa

Dykes of the 1.11 Ga Umkondo LIP, Southern Africa: Clues to a complex plumbing system

Authors:

de Kock et al

Abstract:

The Umkondo large igneous province (LIP) is represented by widespread (∼2.0 × 106 km2) mafic intrusions that were rapidly emplaced (1112-1108 Ma) into the Kalahari craton of southern Africa and the formerly adjacent Grunehogna Province of Antarctica during Rodinia assembly. Very few Umkondo-aged dykes have been identified before, resulting in a poor understanding of this LIP's plumbing system and origin. Here we report six new ∼1110 Ma U-Pb TIMS baddeleyite ages for various dolerite dykes, which, when coupled with geochemistry from some of the dykes, suggest association with the Umkondo LIP. The distribution of dykes defines distinct radiating swarms, which locate two separate magmatic centers on the northern margin of the Kalahari craton, and a third less robust centre on the SE margin. The Umkondo intrusions’ geochemistry indicates significant partial melting of the sub-continental mantle lithosphere (SCLM) and requires a transient thermal anomaly in the mantle. A viable model sees a mantle plume ascend beneath the craton and split into different portions that moved and ascended to different lithospheric thin-spots along the margins of the craton. As an alternative, the rise in mantle temperature associated with continental aggregation at this time is considered sufficient to cause partial melting of the SCLM without any plume involvement. Specific features of the Umkondo LIP's plumbing system are supportive of either model, and an approach of multiple working hypotheses is recommended

Keweenawan Midcontinent Rift: the Attempt to Destroy North America in the Stenian MesoProtoerozoic

Magmatic activity and plate motion during the latent stage of Midcontinent Rift development

Authors:

Swanson-Hysell et al

Abstract:

The Keweenawan Midcontinent Rift of North America records significant continental rifting between ca. 1110 and 1085 Ma, and preserves the most detailed paleomagnetic record of plate motion of any continent in Precambrian time. U/Pb dates from extrusive and intrusive rocks of the western Lake Superior Basin suggest a latent stage of reduced magmatic activity from ca. 1106 to 1100 Ma that places constraints on the dynamics of rift development and the record of plate motion. However, it has remained unclear whether this stage is a feature of the entire >2500-km-long rift. The succession of picritic and basaltic lava flows at Mamainse Point in the eastern Lake Superior Basin may be the most continuous and best exposed record of rift-related volcanism and magnetic reversals, but its age and duration relative to the latent stage has been uncertain due to a lack of radioisotopic dates. We present a weighted mean 206Pb/238U date of 1100.36 ± 0.25 Ma on zircon crystals isolated from a newly discovered tuff within the upper reversed polarity portion of the stratigraphy below the Great Conglomerate. This date indicates that eruptive activity at Mamainse Point continued during the interval of diminished magmatic activity in the western Lake Superior Basin. This result strengthens the chronostratigraphic framework of rift development while explaining the preservation of additional geomagnetic reversals at Mamainse Point and the record of progressively decreasing paleomagnetic inclination that is indicative of rapid paleogeographic change.

Evidence of Continental Drift for Laurentia and Australia From the Stenian MesoProterozoic

Age and paleomagnetism of the 1210 Ma Gnowangerup–Fraser dyke swarm, Western Australia, and implications for late Mesoproterozoic paleogeography

Authors:

Pisarevsky et al

Abstract:

Dolerite dykes of the Gnowangerup–Fraser Dyke Suite are subparallel to the southern and southeastern margins of the Yilgarn Craton. We collected oriented samples for paleomagnetic study from 19 dykes along the Phillips and Fitzgerald Rivers and north of Ravensthorpe. Alternating-field (AF) demagnetization revealed a stable two-polarity remanence in 14 dykes, and the primary nature of the magnetic directions is supported by a positive baked-contact test and by rock-magnetic evidence. U–Pb zircon and baddeleyite ages for two dykes confirm that the Gnowangerup–Fraser dykes are part of the 1210 Ma Marnda Moorn Large Igneous Province. The mean paleomagnetic pole, at 55.8°N, 323.9°E, A95 = 6.5°, is almost identical to the previously reported VGP of the 1212 Ma Fraser dyke, also supported by a positive baked-contact test. The combined robust paleopole places the West Australian Craton in a near-polar position at 1210 Ma. Comparison with coeval Laurentian paleopoles indicates that Laurentia and Australia were widely separated at that time. We present a paleomagnetically permissible drift model for these two continents between 1210 and 1070 Ma. One dyke yields a stable remanence with a VGP similar to the paleopole for the 755 Ma Mundine Well dykes indicating that this dyke may have been emplaced during the same event at c. 755 Ma. Differences in lengths and shapes of late Mesoproterozoic Apparent Polar Wander Paths of several continents suggests that a large supercontinent did not exist between about 1300 and 1050 Ma. This may have been a transitional time between the final breakup of Nuna and the assembly of Rodinia.

Was There a Massive Mantle Plume at the Ectasian-Stenian MesoProterozoic?

1.23 Ga mafic dykes in the North China Craton and their implications for the reconstruction of the Columbia supercontinent

Authors:

Wang et al

Abstract:

The Proterozoic world was shaped by the Paleo-Mesoproterozoic Columbia and Neoproterozoic Rodinia supercontinents. The North China Craton (NCC) is an integral component of Columbia supercontinent assembly, but the lack of rock records in the transitional period between Columbia and Rodinia in the late Mesoproterozoic (1.3 – 1.2 Ga) has resulted in its exclusion from models that trace the Columbia – Rodinia transition. The paleogeographic position of the NCC is also elusive, with India, Baltica, and Siberia as potential neighbours during the early evolution of Columbia. Here we report the discovery of a suite of ~ 1.23 Ga mafic dykes covering an area of ~ 0.6 × 106 km2 of the NCC. These mafic rocks can be classified into both alkaline and subalkaline groups. The former group may have been derived from lower degrees of partial melting of a depleted asthenospheric mantle with limited involvement of a lithospheric mantle component, whereas the latter group can be modeled by higher degrees of partial melting of a subduction-modified enriched lithospheric mantle. Considering the large areal extent of the 1.23 Ga mafic dykes, and their dominantly OIB (Ocean Island Basalt)-like geochemical features, a Mesoproterozoic mantle plume regime is invoked for the NCC. Compiling information on global ~ 1.27-1.21 Ga mafic dykes, flood basalts and layered intrusions, we establish a Mesoproterozoic hotspot track, and consider the NCC to have been located between Laurentia and Baltica. Combined with recent paleomagnetic and geological data, we infer that the Laurentia-NCC-Baltica connection may have existed since the late Paleoproterozoic. We further propose that both plate tectonic (introversion or extroversion) and mantle plume regimes played vital roles during the supercontinent transition.

Marnda Moorn LIP Generated by Plume-Lithosphere Interaction in Ectasian/Stenian NeoProterozoic Australia

Genesis of the 1.21 Ga Marnda Moorn large igneous province by plume–lithosphere interaction 

Authors:

Wang et al

Abstract:

The 1.21 Ga Marnda Moorn large igneous province (LIP) of the Yilgarn Craton is important for understanding the final breakup of the Nuna (Columbia) supercontinent. However, its petrogenesis is poorly understood owing to the lack of geochemical data. We conducted geochemical analyses of the Gnowangerup-Fraser Dyke Suite, a major part of the Marnda Moorn LIP, and report the first geochemical and Nd isotope data for this LIP. Results of a complementary paleomagnetic study of these dykes will be published elsewhere. Most of the studied dykes consist of predominately tholeiitic and OIB-like dolerite (Group 1) and one arc-like and more felsic dyke (Group 2). Group 1 samples have incompatible trace element compositions similar to those of tholeiitic Hawaiian plume-induced OIB and typical asthenospheric mantle-derived Nd isotopes with ɛNd(t) varying from +3.7 to +7.5, produced mainly within the spinel stability field (below 75 km depth). Their source region most likely contains recycled oceanic crust. Samples from the Group 2 dyke are characterized by extremely unradiogenic Nd isotopes with ɛNd(t) of about -12, strong depletion of Nb-Ta-Zr-Hf-Ti, chondritic Nb/Ta ratios (20–18), oversaturated silica, and strong deficiencies in CaO, FeOt, TiO2, and Ni. This implies that the dyke was produced by partial melting of enriched sub-continental lithospheric mantle. The coexistence of OIB- and arc-like end-members but mainly Hawaiian OIB-like tholeiitic mafic dykes, interpreted large-scale asthenosphere upwelling in a very short time, and the large volume of mafic magma, favour a plume origin for the Marnda Moorn LIP. The geochemical and emplacement characteristics are attributed to relief of the lithosphere–asthenosphere boundary across the Yilgarn craton and a complex interplay between the plume, heated lithosphere, normal asthenosphere, and recycled components. We propose a two-stage melting model to explain the geochemical composition and emplacement of the Marnda Moorn LIP. Our plume-lithosphere interaction model is consistent with the occurrence of synchronous ultrahigh-temperature events in the Musgrave Province of central Australia

Evidence of at Least Three Proterozoic Islands Arcs in the Northern Arabian–Nubian Shield

Three successive Proterozoic island arcs in the Northern Arabian–Nubian Shield: Evidence from SIMS U–Pb dating of zircon

Authors:

Eyal et al.

Abstract:

Four isolated metamorphic complexes located within post-collisional granitoids occupying up to 70% of the total area, were distinguished in Sinai (Egypt) and Elat area (southern Israel), the northernmost part of the Arabian–Nubian Shield. The metamorphic rocks include metasediments, felsic and mafic metavolcanic rocks intruded by granitic, dioritic, and gabbroic plutons, all subjected to penetrative deformation.

We present new SIMS U–Pb dating of zircons from 13 rock units comprising metasediments, volcanic rocks, gneisses and plutons from three metamorphic complexes (Sa'al, Feiran–Solaf, and Kid). In addition we present a SIMS U–Pb titanite age of a granitic gneiss previously dated using zircon. On the basis of the new and published U–Pb data, three successive Meso- to Neoproterozoic island arcs formed during a period of ca. 500 My are recognized. The Sa'al arc (represented by the oldest arc rocks in the ANS) evolved from 1.03 to 0.93 Ga (100 My); the Feiran–Elat arc developed from ca. 870 to 740 Ma (130 My), and the Kid arc formed from ca. 640 to 620 Ma (20 My). Evidence for an older, ca. 1.1 Ga, pre-Sa'al island arc was established from the zircon xenocryst population, though no exposures of rocks of this age were found. In the Sa'al and Kid arcs both volcanic and sedimentary rocks are preserved, whereas in the Elat–Feiran arc volcanic rocks are missing. We suggest that at ~ 700 Ma the Elat−Feiran arc was subjected to rifting that resulted in separating of the Qenaia block and its movement to the SE.

Stenian Mesoproterozoic Ngaanyatjarra Rift in Australia was Driven by Magmatism

Magmatism-dominated intracontinental rifting in the Mesoproterozoic: The Ngaanyatjarra Rift, central Australia

Authors:

1. Alan R.A. Aitken (a)
2. R. Hugh Smithies (b)
3. Mike C. Dentith (a)
4. Aurore Joly (a)
5. Shane Evans (a)
6. Heather M. Howard (b)

Affiliations:

a. Centre for Exploration Targeting, University of Western Australia, M006, 35 Stirling Highway, Perth, Western Australia, Australia

b. The Geological Survey of Western Australia, Mineral House, 100 Plain Street, East Perth, Western Australia, Australia

Abstract:

The Late Mesoproterozoic (1085–1040 Ma) Ngaanyatjarra Rift, previously referred to as the Giles Event, is the dominant component of the Warakurna Large Igneous Province (LIP) that affected much of central and western Australia. This rift is well preserved and provides excellent examples of rift structure at a variety of crustal levels and times in the rift's evolution. Geological knowledge is integrated with geophysical interpretations and models to understand the crustal structure and evolution of this rift. Two phases are identified: an early rift stage (1085–1074 Ma) that is characterised by voluminous magmatism within the upper crust and relatively little tectonic deformation; and a late rift stage that is characterised by tectonic deformation, synchronous with the deposition of a thick pile of volcanic and sedimentary rocks (1074–1040 Ma). Compared to modern rift examples, this rift is unusual in that the crust was thickened by ~ 15 km and overall extension was very limited. However, its structure and evolution are very similar to the near-contemporaneous Midcontinent Rift, which shows the addition of a similar quantity of magmatic material as well as crustal thickening and limited extension. For these Mesoproterozoic rifts, we suggest that magmatism was the dominant process, and that the extension observed was a response to magmatism-induced crustal thickening and the gravitational collapse of the crustal column. Other Proterozoic rifts show similar characteristics (e.g. Transvaal Rift), whereas most Phanerozoic rifts are dissimilar, showing instead a dominance of extension, with magmatism largely a result of this extension. This change in the style of rifting from the Precambrian to the Phanerozoic may relate to the influence of a typically cooler and stronger lithosphere, which has caused stronger strain localisation and a greater role for extension as the controlling factor in rift evolution.