Pondering the Precambrian #5

Proterozoic:

Evidence of anoxic and alkaline oceanic conditions in the Vempalle and Tadpatri formations in India from stromatolites.

NeoProterozoic:

The evolution of sponges is reviewed.

Ediacaran:

There was a very negative and unexplained carbon-13 shift during the aftermath of the Snowball Earth episode.

At the dawn of the Ediacaran, during the retreat of the Ghaub Glaciation, the Ediacaran oceans' oxygen levels fluctuated.

Relatives of Cloudina from China are helping our understanding of the Ediacaran biomineralized metazoan.

Using x-ray tomography, attempts are made to interpret trace fossils made by metazoans during the Ediacaran.

Why did metazoans start growing in size during the Ediacaran?

Could the discovery of certain acanthomorphic acritarchs allow for better correlation of different Ediacaran fossils localities?

A unique fossil of Rangea has been found in ironstone and demonstrates a semi rigid skeleton for the Ediacaran critter.

There was an anoxic ocean volume in Brazil during the Ediacaran.

A transitional fauna from the end Ediacaran was found in Nevada.

Cryogenian:

A very thick layer of sediment from the Cryogenian has been found in Death Valley.  Evidence of glacial activity and rifting is present.

Tonian:

Did animals diverge into different lineages in the Tonian prior to the Snowball Earth episode?

The chemical weathering and Rodinia's continental drift in the Tonian contributed to the lead up to the Cryogenian Snowball Earth episode.  It also had a lot of parallels with the lead up to the Huronian Snowball Earth episode.

The evidence of evolutionary radiation of eukaryotes in Africa at the time of the MesoProterozoic/NeoProterozoic boundary.

Vase shaped microfossils have been found in Arizona, USA from the Tonian.

MesoProterozoic:

Was there an oceanic oxygenation spike during the Ectasian 1.36 billion years ago?

Evidence from Mauritania suggests what the paleoecology of the MesoProterozoic might have been like.

PaleoProterozoic:

Examining a paleoproterozoic gold deposit suggests there were two different sources for its carbon content and some caution about inferring the deposition temperatures from Raman spectroscopy.

Examining a paleosol from the Statherian shows atmospheric carbon dioxide was 4.8x preindustrial levels.

Evidence of continental assembly prior to the supercontinent Columbia from Brazil during the Orosirian.

There is evidence of significant geological turmoil during the Orosirian and is interpreted as a true polar wander.

The iron deposits of the Dongshan in China from the Orosirian show evidence of two different sources for the iron.

Siderian fossils from Western Australia seem to show benthic microbial mats seem to have continued to thrive during the Great Oxidation Event.

The evidence from the first known Snowball Earth episode during the Siderian appears to support the Jormungund scenario (slushball earth).

Archean:

Microbial dissimilatory iron reduction was common across the Archean/Proterozoic Boundary.

Plankton-like micro fossils from Australia have been found in South Africa.

Evidence of anaerobic photosynthetic microbes has been found from the PaleoArchean of Australia.

Did life really start on land?

META:

How did the Earth stay warm enough for life despite the Faint Young Star Paradox?

The importance of methane to prebiotic chemistry.

Current molded, storm damaged, sinuous columnar stromatolites From Ectasian Mesoproterozoic China

Current molded, storm damaged, sinuous columnar stromatolites: Mesoproterozoic of northern China

Authors:

Tosti et al

Abstract:

A thin (~ 50 cm) horizon of sinuous stromatolites occurs within a succession of elongate upright columns in the ~ 1.4 Ga Tieling Formation, near Jixian, China. The upright columns form aligned closely spaced ridges, separated by narrow runnels with signs of current scour. The sinuous and upright stromatolites, and their intervening matrix, were originally mainly composed of carbonate mud. In end-on view, the sinuous columns incline back and forth. Each column consists of well-defined laminae that successively rotate relative to column curvature, maintaining their orientation approximately at right-angles to the column axis. In inclined parts of the columns, laminae are typically asymmetric and the steeper side faces the direction of column inclination. We interpret this column sinuosity to be a response to changes in current-direction, with accreting laminae facing into currents that supplied sediment. We find no evidence for heliotropism (mat growth towards the sun) in these examples. Adjacent columns typically show similar shapes, bending back and forth together, but their angles of curvature and inclination can change laterally from column to column, from near vertical to 45°, over distances of 30 cm. Columns can show breakage and separation of adjacent laminae. Some of this is enhanced by compaction and stress, but the occurrence of sinuous columns on their sides or upturned, in spaces between undisturbed columns, indicates that column curvature developed during growth and that toppling of columns was syndepositional. We infer that sinuosity developed in response to changes in current direction, that column inclination reflects current strength, and that breakage and toppling was produced by strong currents. Curved and sinuous columns could reflect shoaling. This would also have exposed them to storm damage and to the effects of currents that scoured sufficient matrix to locally break and topple columns. Markedly sinuous Mesoproterozoic columns also occur in Siberia and North America, suggesting that similar conditions and processes of stromatolite formation may have been widespread at this time. Formation and preservation of the sinuous stromatolites described here required a combination of conditions that included abundant fine-grained carbonate sediment, microbial mats capable of trapping it, reduced early lithification, and absence of bioturbation.

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.

Evidence for Atmospheric Oxygenation During Ectasian MesoProterozoic?!

Pyrite trace element chemistry of the Velkerri Formation, Roper Group, McArthur Basin: Evidence for atmospheric oxygenation during the Boring Billion

Authors:

Mukherjee et al

Abstract:

The trace element content of sedimentary pyrite in black shales of varying ages has recently been used to construct secular trends of trace element variation in the ocean. The approach also has potential to be used as a proxy for estimating evolution of the redox state of the ocean/atmosphere system through time. Here, we apply a combination of whole-rock chemostratigraphy and laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) analyses of marine pyrite to the carbonaceous mudstones of the Mesoproterozoic (∼1400 Ma) Velkerri Formation, and underlying Corcoran Formation, Roper Group, McArthur Basin to interpret and compare basin water conditions and basinal trace element chemistry at the time of sedimentation.

Our results suggest that the black shales of the Velkerri Formation deposited under different geochemical conditions in comparison to the underlying Corcoran Formation. This proportionate difference is manifested in the form of high total organic carbon (TOC) contents coupled with an increase of trace elements such as P, Mo, Cd, Se, Ni, Se/Co, Ni/Co in the mudstones of Upper Velkerri Formation in comparison to Lower Velkerri and underlying Corcoran Formation. Cobalt on the other hand, exhibits an opposite trend compared to other redox sensitive trace elements (Mo, Se) due to its unique redox chemistry, particularly the cationic nature of its soluble species making Se/Co a useful proxy for redox conditions of the atmosphere. This progressive increment in trace elements (P, Mo, Cd, Se, Ni, Se/Co, Ni/Co) up stratigraphy from Corcoran Formation to Lower Velkerri Formation to Upper Velkerri is herein attributed to an increase in nutrient trace element supply into the marine reservoir, possibly in response to tectonic activity and an increase in oxygen in the atmosphere. This increased nutrient supply subsequently promoted an increase in primary productivity as indicated by high TOC contents in the Upper Velkerri Formation. This positive change in nutrient supply also coincides with an increased supply of sulphate to the ocean (modelled paleo-seawater [SO42−]) as evidenced by previous sulphur isotope studies and development of morphological complexity in eukaryotes based on paleontological observations of previous workers. The totality of evidence from this study and previous studies, suggest that there was a possible oxygenation event around ∼1400 Ma. This is in contrast with the general notion that Mesoproterozoic oxygen levels were low and devoid of significant fluctuations.

Sufficient Oxygen in Paleoatmosphere at the Calymmian/Ectasian MesoProterozoic Boundary for Animals to Breathe

Sufficient oxygen for animal respiration 1,400 million years ago

Authors:

Zhang et al

Abstract:

The Mesoproterozoic Eon [1,600–1,000 million years ago (Ma)] is emerging as a key interval in Earth history, with a unique geochemical history that might have influenced the course of biological evolution on Earth. Indeed, although this time interval is rather poorly understood, recent chromium isotope results suggest that atmospheric oxygen levels were less than 0.1% of present levels, sufficiently low to have inhibited the evolution of animal life. In contrast, using a different approach, we explore the distribution and enrichments of redox-sensitive trace metals in the 1,400 Ma sediments of Unit 3 of the Xiamaling Formation, North China Block. Patterns of trace metal enrichments reveal oxygenated bottom waters during deposition of the sediments, and biomarker results demonstrate the presence of green sulfur bacteria in the water column. Thus, we document an ancient oxygen minimum zone. We develop a simple, yet comprehensive, model of marine carbon−oxygen cycle dynamics to show that our geochemical results are consistent with atmospheric oxygen levels greater than 4% of present-day levels. Therefore, in contrast to previous suggestions, we show that there was sufficient oxygen to fuel animal respiration long before the evolution of animals themselves.

pop sci write up here and here.

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.

Evidence From Tasmania East Antarctica and Laurentia Connected in the Columbia (Nuna) Supercontinent

Mesoproterozoic Tasmania: Witness to the East Antarctica–Laurentia connection within Nuna

Authors:

Mulder et al

Abstract:

Most recent paleogeographic reconstructions of the supercontinent Nuna juxtapose the North Australian craton, Mawson continent (South Australia–East Antarctica), and Laurentia between 1.6 Ga and 1.3 Ga but differ in their relative positioning. The greater than 10-km-thick siliciclastic Rocky Cape Group of Tasmania was deposited in an opening marine basin on the margin of East Antarctica during Nuna breakup. Based on a similar detrital zircon signature and depositional age, the Rocky Cape Group has been correlated with the upper Belt-Purcell Supergroup in Laurentia, thus representing a key tie point within Nuna. Here the detrital zircon age signature of Mesoproterozoic Rocky Cape Group quartzites is investigated by comparing new detrital zircon U-Pb-Hf isotopic data to an extensive compilation of zircon isotopic data from Australia, East Antarctica, and Laurentia. Our analysis demonstrates that the Rocky Cape Group is unlikely to have been sourced from any geological terrane exposed in presentday Australia. Instead, zircon U-Pb-Hf isotopic data from basement terranes in Laurentia and East Antarctica show striking similarities to the Rocky Cape Group detrital signature. Paleocurrent data indicate that the majority of sediment in the Rocky Cape Group was sourced from Laurentia, which was to the southeast (present-day coordinates) of Tasmania, supporting a SWEAT-like (southwest United States–East Antarctica) configuration for Nuna. We suggest that rifting left a thinned continental connection between East Antarctica and Laurentia onto which the lower-middle Rocky Cape Group was deposited between 1.45 and 1.30 Ga.

Orbital Climate Forcing at the Calymmian/Ectasian MesoProterozoic Boundary

Orbital forcing of climate 1.4 billion years ago

Authors:

Zhang et al

Abstract:

Fluctuating climate is a hallmark of Earth. As one transcends deep into Earth time, however, both the evidence for and the causes of climate change become difficult to establish. We report geochemical and sedimentological evidence for repeated, short-term climate fluctuations from the exceptionally well-preserved ∼1.4-billion-year-old Xiamaling Formation of the North China Craton. We observe two patterns of climate fluctuations: On long time scales, over what amounts to tens of millions of years, sediments of the Xiamaling Formation record changes in geochemistry consistent with long-term changes in the location of the Xiamaling relative to the position of the Intertropical Convergence Zone. On shorter time scales, and within a precisely calibrated stratigraphic framework, cyclicity in sediment geochemical dynamics is consistent with orbital control. In particular, sediment geochemical fluctuations reflect what appear to be orbitally forced changes in wind patterns and ocean circulation as they influenced rates of organic carbon flux, trace metal accumulation, and the source of detrital particles to the sediment.

The North China Craton Broke off From Columbia Supercontinent During Ectasian Mesoproterozoic

Detrital zircon U–Pb, Hf isotopes, detrital rutile and whole-rock geochemistry of the Huade Group on the northern margin of the North China Craton: Implications on the breakup of the Columbia supercontinent

Authors:

Liu et al

Abstract:

Supracrustal successions in the Zhaertai–Bayan Obo–Huade rift zone on the northern margin of the North China Craton include the Zhaertai, Bayan Obo and Huade Groups from west to east. The Huade Group, which has been subdivided into four formations, is composed of basal pebbled quartzites, quartzites, schists, phyllites, marbles, slates and diopsidites. Geochemistry of the metasedimentary succession indicates that source rocks of the lower two formations are felsic, displaying passive margin signatures, whereas those of the upper two formations have continental island arc or active continental margin signatures. U–Pb ages of detrital zircons from the group yielded three age populations of 2690–2450, 2150–1710 and 1660–1330 Ma, of which the youngest age population only exist in the upper Huade Group. The dominant 2690–2450 detrital zircons were likely sourced from the granitic rocks in the Yinshan Block. The subordinate 2150–1710 Ma detrital zircons were probably recycled from the metasedimentary units from the Khondalite Belt and/or derived from the similar aged granitic plutons of the Hongqiyingzi “Group”, which are also evidenced by the similar geochemical features of rutiles from the Huade Group and the Khondalite Belt. Minor amounts of 1660–1330 Ma detrital zircons may have derived from the North American and/or Baltica cratons connected to the northern margin of the North China Craton in the Columbia supercontinent. The youngest detrital zircon age peak of ∼1337 Ma in the upper Huade Group suggests that the sedimentation began after ∼1.34 Ga. In combination with the ∼1320 Ma granitoids cross-cutting it, depositional ages of the upper Huade Group can be constrained between ∼1.34 and ∼1.32 Ga. Taking into account the lithostratigraphic features, provenances and depositional ages, a continental rift basin deposit represented by the upper Huade Group on the northern margin of the North China Craton developed between ∼1.34 and ∼1.32 Ga, which indicates that the final breakup of the North China Craton from the Columbia supercontinent happened in the middle Mesoproterozoic.

Thermally Extreme Orogeny & Evolution of the Australian Musgrave Region From the Statherian PaleoProterozoic to the Ectasian MesoProterozoic

The burning heart - the Proterozoic geology and geological evolution of the west Musgrave Region, central Australia

Authors:

Howard et al

Abstract:

he Musgrave Province is one of the most geodynamically significant of Australia’s Proterozoic orogenic belts, lying at the intersection of the continent’s three cratonic elements – the West, North and South Australian Cratons. While remoteness and cultural sensitivity have slowed geological research into this region, recent collaborative programs in Western Australia (the west Musgrave Province) have done much to address this. This Focus Review provides a synthesis of this, and previous, work investigating the Mesoproterozoic to Neoproterozoic geological evolution of the province. The Musgrave Province is a Mesoproterozoic to Neoproterozoic belt dominated by granites formed and deformed during several major events. A cryptic juvenile basement is exposed mainly in the east Musgrave Province as c. 1600 – 1550 Ma orthogneiss and in the west Musgrave Province as isolated outcrops of granulite-facies metagranites of the c. 1575 Ma Warlawurru Supersuite. Zircon Hf-isotopic data suggest an earlier major juvenile crust-forming event at c. 1950–1900 Ma. There is, however, no evidence that the province evolved over Archean crust. The c. 1600 – 1550 Ma period probably involved evolution within a primitive arc setting, perhaps developed on c. 1950–1900 Ma oceanic or oceanic-arc crust. Voluminous calc-alkaline plutonism was accompanied by clastic and volcaniclastic basin formation during the 1345–1293 Ma Mount West Orogeny. This stage traced the evolution of a continental arc reflecting the final amalgamation of the combined North and West Australian Craton with the South Australian Craton. The intervening c. 1400 Ma primitive crust - the Madura Province – on which the proto-Musgrave Province had evolved, was consumed during amalgamation. The thickened crust resulting from this accretion was drastically thinned at the beginning of the c. 1220–1150 Ma Musgrave Orogeny as this central part of the new combined craton entered an extraordinary period of high heat flow characterised by c. 100 m.y. of ultrahigh-temperature metamorphism and high-temperature, anhydrous, alkali-calcic magmatism sourced from MASH chambers developed at the base of the thinned crust. The ridged cratonic architecture and a massive accumulation of high radiogenic heat producing granites within the mid crust perpetuated a thin crustal regime. Voluminous magmatism was again triggered during the c. 1090–1040 Ma Giles Event with the evolution of the magmatism-dominated, Ngaanyatjarra Rift. This event was likely initiated through renewed movement along translithospheric faults that intersected the thermally perturbed Musgrave Province, pinned at a cratonic junction. Mantle-derived bimodal magmatism extended more or less continuously for 50 m.y., producing one of the world’s largest layered mafic intrusions and supervolcano-sized additions of juvenile felsic crust, in the form of alkali-calcic to alkali, A-type, rhyolite deposits. Together, the Albany – Fraser Orogen, which developed over the southern margin of the West Australian Craton, and the Musgrave Province mark the preserved edge of the North and West Australian Craton. These two belts show remarkable chronological links between c. 1345 and 1150 Ma but contrasting histories before and after that period. Their period of shared evolution reflects collision and accretion of the South Australian Craton, but their tectonic setting and basement geology throughout that event were very different.

The Columbia Supercontinent Broke up in the Ectasian MesoProterozoic

Mid-Mesoproterozoic (∼1.32 Ga) diabase swarms from the western Liaoning region in the northern margin of the North China Craton: Baddeleyite Pb–Pb geochronology, geochemistry and implications for the final breakup of the Columbia supercontinent

Authors:

Wang et al

Abstract:

Diabase swarms widely intrude the late Paleoproterozoic to Mesoproterozoic sedimentary rocks in the northern margin of the North China Craton (NCC). In this paper, we present new baddeleyite SIMS Pb–Pb ages and whole-rock geochemical data for representative samples of the diabase swarms intruding the Mesoproterozoic Wumishan and Xiamaling formations in the western Liaoning region. Our aim is to elucidate the age and origin of the diabase swarms and their relationship to the fragmentation of the Columbia supercontinent. The baddeleyite SIMS Pb–Pb dating results show emplacement ages varying from 1325.9 ± 4.1 Ma to 1318.6 ± 3.7 Ma, indicating voluminous basic magmatism leading to the generation of the diabase swarms during the Mid-Mesoproterozoic time. Geochemically, the Mesoproterozoic diabasic rocks have relatively low REEs contents and show weak positive Eu anomalies. Moreover, these diabase samples are characterized by enrichment in light rare earth elements (LREEs), large ion lithophile elements (LILEs; e.g., Rb and Pb) and some high field strength elements (HFSEs; e.g., Th, U, Zr, Ti and Hf), and depletion in heavy rare earth elements (HREEs) and several high field strength elements (HFSEs; e.g., Nb, Sm, Gd and P), with affinities to basalts belonging to within-plate tholeiite series. Taken together, these geochemical characteristics suggest that the primary magmas of the diabases in the western Liaoning region could have been derived by partial melting of a transitional mantle or slightly enriched mantle in a continental extensional setting. In the context of assembly, growth and breakup of the Columbia supercontinent, together with the recognition of large volumes of ∼1.32 Ga diabase swarms in the northern NCC as well as the nearly coeval rift-related anorogenic magmatism recorded elsewhere on the globe, such as Africa, Baltica, Siberia, North America and Greenland, we conclude that the northern margin of NCC experienced the global-scale Mid-Mesoproterozoic rifting events, corresponding to the final breakup of the Columbia supercontinent, probably at around 1.32 Ga.

Ectasian MesoProterozoic Continental Configuration: 1270 Million Years ago

Ectasian MesoProterozoic Continental Configuration: 1380 Million Years ago

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

Was there SAMBA in Columbia (supercontinent) Ectasian MesoProterozoic?

Was there SAMBA in Columbia? Paleomagnetic evidence from 1790 Ma Avanavero mafic sills (Northern Amazonian Craton)

Authors:

Bispo-Santos et al.

Abstract:

The Amazonian Craton represents an important piece in the Paleoproterozoic paleogeography of the Earth. This study presents paleomagnetic data obtained on well-dated (U-Pb) 1790 Ma mafic sills from the Avanavero magmatism in northern Amazonian Craton (Guiana Shield). AF and thermal treatments revealed southeastern, low downward/upward inclination, remanent magnetization directions that are carried by moderate to high-Hc and high-TB Ti-poor titanomagnetite. The site mean directions cluster around the mean Dm = 138.2°, Im = -3.4° (N = 13, α95 = 13.0°), which yields a robust paleomagnetic pole (AV pole) at 27.9°E, 48.4°S (A95 = 9.6°) with a Q-value of 5. The characteristic component disclosed for the Avanavero sill matches that obtained for sediments collected along the baked contact and are distinct from those away from the sill. The Avanavero directions are also significantly different from those obtained for younger units, with ages spanning from 1420 to 520 Ma, suggesting the sills carry a primary remanence. The Avanavero pole helps in constraining the paleogeography of the central pieces of Columbia. It is compatible with the Baltica and Amazonian Cratons SAMBA link in the Columbia Supercontinent at about 1780 Ma, but other configurations are also possible. When compared to other Paleo- to Mesoproterozoic paleomagnetic poles from the southern Amazonian Craton (Central Brazil Shield), our new paleomagnetic pole suggests intracratonic motions within the southern area of the Craton, probably after 1420 Ma ago. We tentatively suggest that these movements are related to the collision of the Paraguá Block with the proto-Amazonian Craton at about 1350-1320 Ma ago.

Isotopic Evidence From Vazante Group in Ectasian MesoProterozoic Brazil Suggests Unexpected Glaciation

Re-Os age constraints and new observations of Proterozoic glacial deposits in the Vazante Group, Brazil

Authors:

Geboy et al

Abstract:

A new Re-Os radiometric age date for an organic-rich shale horizon from the Vazante Group in Brazil, coupled with geological observations, provide evidence for late Mesoproterozoic glacial episodes, conflicting with the general view of greenhouse conditions marked by a eustatic high stand at this time. Field observations of a reverse fault juxtaposing older Mesoproterozoic sedimentary rocks above younger Neoproterozoic strata provide a new stratigraphic framework and reconcile the apparent inversion of U-Pb detrital zircon ages through the succession. Combined, the geochronological, geochemical and stratigraphic evidence suggest that the Vazante Group sediments accumulated along a passive margin of the São Francisco craton and are correlative with the neighboring Paranoá Group. Biomarker, sulfur isotope and iron speciation analyses support the interpretation of a strongly stratified water column during post-glacial transgression and deposition of one of the bituminous shale horizons. The relationship of the glaciogenic Vazante Group to other late Mesoproterozoic successions, such as the non-glacial Atar Group in West Africa and Bylot Supergroup in arctic Canada, however, remains enigmatic