Pondering the Precambrian #40

Proterozoic:

NeoProterozoic:

Palaeopascichnus jiumenensis can be used to determine the Ediacaran/Cambrian boundary.

There's a new way to determine oxygen levels during paleo time periods.  It seems to check out in the Neoproterozoic-Cambrian transition based on comparison to other methods.

A bilateralan metazoan (animal) fossil and trackway was found in China dating from the Ediacaran.

The rise of siliceous sponges helped shift the Ediacaran ecosystems to the modern Cambrian. 

The differences between the Weng'an and Kuanchuanpu biotas is largely due to how they were preserved.

Acritarchs from the Ediacaran Doushantuo Formation allow for biostratigraphic correlation with the Tanarium conoideum–Cavaspina basiconica Assemblage Zone.

There appears to have been long term denudation of the continents from the middle Ediarcaran based on Nd isotope traces in carbonates.

A new formation has been found dating from the Cryogenian with evidence of bacterial and algae life in South China.

How much hydrothermal activity was there during the Marinoan Glaciation (snowball earth episode) during the Cryogenian?

There is evidence of a large igneous province in the Tonian.

MesoProterozoic:

Horodyski moniliformis, regarded by some as a eukaryote fossil from the Calymmian, has been declared a pseudofossil.

PaleoProterozoic:

Plate tectonics have evolved over the last 2.5 billion years.

Is there evidence for the oxygen overshoot hypothesis from the Orosirian?

There was a paleocontinent named 'Atlantica' proposed.  Recent research suggests it was configured rather differently than proposed.

Evidence has arisen supporting a snowball earth scenario during the Huronian glaciation.

An asteroid impact has been IDed from the Rhyacian in Australia.

Did the supercontinental cycle begin in the PaleoProterozoic?

Archean:

There is evidence of a PaleoArchean continent.

Origin of Life:

Prebiotic amino acids can bind to lipid membranes and stabilize them.

When did eukaryotes originate?  Do we have a way to understand that?

Did the lack of nitrogenase limit early life?

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.

Anomalous Supply of Bioessential Molybdenum From the Orosirian PaleoProterozoic to Calymmian MesoProterozoic

Anomalous supply of bioessential molybdenum in mid-Proterozoic surface environments

Authors:

Parnell et al

Abstract:

Granites aged 1.9 Ga to 1.5 Ga exhibit molybdenite mineralization globally. Sandstones deposited during the mid-Proterozoic have a provenance dominated by 1.9 to 1.7 Ga basement. The mid-Proterozoic surface environment was, consequently, receiving detritus from the molybdenum-rich granites. Thus there was a supply of molybdenum available in terrestrial or shallow marine environments at a time when molybdenum was required to support the evolution of multicellular life.

Formation of the Cratons During the Orosirian PaleoProterozoic

Zircon U-Pb ages of Paleoproterozoic mafic granulites from the Huai’an terrane, North China Craton (NCC): Implications for timing of cratonization and crustal evolution history

Authors:

Zhang et al

Abstract:

The North China Craton (NCC) is considered to have been incorporated into the Columbia supercontinent during late Paleoproterozoic. The Huai’an terrane located in north-central part of the NCC is an importan tectonic window to investigate the Paleoproterozoic crustal evolution history of this craton, particularly due to the wide occurrence of high-grade metamorphic rocks including retrograded eclogites and high-pressure granulites. Available data show two controversial peak metamorphic ages of these rocks as 1.95–1.90 Ga or ∼1.85–1.80 Ga. Thus, determining the precise timing of metamorphism of the high-pressure granulites is important in reconstructing the collisional history. In this study, we present zircon U-Pb ages, bulk chemistry, and mineral inclusions in zircons and matrix mineral assemblages from the high-pressure granulites to evaluate the timing of metamorphism. Our results reveal ca. 2.05–2.0 Ga ages from magmatic zircons in these rocks representing the timing of emplacement of mafic magmas. The metamorphic zircons yield two groups of ages at 1.95–1.90 Ga and ∼1.85∼1.80 Ga. Clinopyroxene, plagioclase, titanite, and apatite occur as common mineral inclusions within the ∼1.85–1.80 Ga metamorphic zircons. The chemical compositions of clinopyroxene and plagioclase occurring as inclusions are very similar to those of the rim compositions of those minerals in the matrix, suggesting their formation during retrograde granulite metamorphism, and are distinctly different from those of the core domains of the minerals in the matrix which represent high-pressure granulite metamorphism. These results indicate that the metamorphic zircons formed during retrograde granulite metamorphism dated as ∼1.85–1.80 Ga and the age range of ∼1.95–1.90 Ga corresponds to high-pressure granulite metamorphism. The dual nature of the geological and geochronological features suggest that the Archean gray gneisses in the Huai’an terrane represent the basement rocks detached from the Khondalite Belt of the NCC after collision. Integrating with published data, a double-side collision with scissor-like suturing model is proposed for the Paleoproterozoic cratonization of the NCC.

Evidence of Local Volcanic Eruptions From Orosirian PaleoProterozoic Tanzania

Age and geochemistry of coeval felsic volcanism and plutonism in the Palaeoproterozoic Ndembera Group of southwestern Tanzania: Constraints from SHRIMP U-Pb zircon and Sm-Nd data

Authors:

Bahame et al

Abstract:

The Ndembera metavolcanic rocks represent a continuum of compositions ranging from intermediate to more siliceous calc-alkaline trachyandesites-dacites-trachytes-rhyolites which have been intruded by largely metaluminous, calc-alkaline I-type granites in the Palaeoproterozoic Usagaran Belt of southewestern Tanzania. SHRIMP U-Pb zircon age data show that the Ndembera metavolcanic rocks were extruded at 1871 ± 30 Ma, an event that was largely coeval with the emplacement of cross-cutting microcline-biotite rich granites at 1896 ± 29 Ma.

Despite some compositional differences, both the granites and metavolcanic rocks share similar geochemical features including very coherent REE patterns characterized by enrichment of the LREE relative to the HREE (La/YbCN = 13.6-32.8 (average = 23.9) for metavolcanic rocks and La/YbCN = 10.8-38.2 (average = 21.8) for granites), negative Eu anomalies (Eu/Eu* = 0.45-1.01 (average = 0.83) for volcanic rocks and Eu/Eu* = 0.48-0.70 (average = 0.60) for granites), negative Ta, Ti and Nb anomalies (Nb/Lapm = 0.14-0.29 (average = 0.20 for volcanic rocks and Nb/Lapm = 0.01-0.37 (average = 0.16) for granites). The metavolcanic rocks have ɛNd(1871Ma) values of -0.03 to -4.89 and TDM ages of 2243 - 2714 which are broadly similar to those of the granites (ɛNd(1896 Ma) values of -3.56 to -4.59 and TDM ages of 2469 - 2646 Ma).

The geochemical similarities between the Ndembera volcanic rocks and granites, coupled with their similar emplacement age, suggest that the two rock suites were derived by the same processes in the same tectonic setting from an isotopically similar source. These geochemical features coupled with the paucity of mafic rocks suggest the derivation of the pluto-volcanic suite by crustal anatexis of basic meta-igneous rocks mixed with a minor metasedimentary component in an intra-continental setting. The more siliceous compositions in the suite may have formed by subsequent fractionation involving hornblende, plagioclase, biotite, titanite and apatite as indicated by the negative correlation between SiO2 and MgO, Fe2O3, Al2O3, CaO, TiO2 and P2O5, low Sr contents and negative Eu and Ti anomalies.

The ∼1.87 Ga coeval felsic volcanic and granitic magmatic event documented in the Ndembera Group is also a feature of the nearby Ubendian belt to the west pointing to a Palaeoproterozoic regional thermal event that occurred in southwestern and western Tanzania.

Academic Bun Fight Over Sulfur Cycling bacteria: Serial Convergence or 1.8 Billion Year Evolutionary Stasis?

Putative extremely long evolutionary stasis in bacteria might be explained by serial convergence

Authors:

Dvorak et al

Abstract:

In a recent paper, Schopf et al. (1) analyzed 1.8-Ga-old fossil sulfur bacteria and found an intriguing morphological similarity between fossil and modern species. Moreover, the authors showed that the deep-water sulfur cycling environment, where these bacteria reside, has not significantly changed throughout time. Thus, the authors hypothesize that this phenomenon is a result of an extreme evolutionary stasis in these bacteria. Such a static evolution is termed hypobradytelic and it has also been described in some cyanobacteria (2), where an evolutionary stasis is expected to be more than 2 Ga. However, these conclusions rely only on geological and morphological evidence.

Counterattack!

Reply to Dvořák et al.: Apparent evolutionary stasis of ancient subseafloor sulfur cycling biocoenoses

Authors:

Schopf et al

Abstract:

We thank Dvořák et al. for their comment (1) on our paper (2), in which we compare sulfur-cycling ∼1.8- and ∼2.3-Ga fossil communities with their modern counterparts and report that the community fabric of the fossil and modern microbes, as well as their organismal and cellular morphology, their interlinked energy-production via anaerobic sulfate-reduction and sulfur species oxidation, and their use of sulfate and nitrate to fuel this sulfur cycle appear to have remained unchanged over a segment of geological time equivalent to half the age of the Earth.

A Heterodox Position for Continent Atlantica During the Orosirian PaleoProterozoic

Paleomagnetic study on mid-Paleoproterozoic rocks from the Rio de la Plata craton: Implications for Atlantica

Authors:

Rapalini et al

Abstract:

The first successful paleomagnetic study on middle Paleoproterozoic rocks from the Rio de la Plata craton is reported. Samples collected from the Soca and Isla Mala granitic bodies, located in southern Uruguay, provided characteristic remanences that were used to compute the first paleomagnetic poles for the craton for ca. 2.05–2.02 Ga. The poles were complemented by a virtual geomagnetic pole from the slightly older Marincho and Mahoma complexes. The paleomagnetic results suggest fast apparent polar wander at high paleolatitudes for the Rio de la Plata craton. Comparison with coeval poles from the Guiana, Congo–São Francisco and West African cratons indicates that a configuration of Atlantica that resembles their Western Gondwana fit is not supported by paleomagnetic data. The geologic similarities in these four cratons are supportive of a major crustal forming event between 2.2–2.0 Ga. A modified configuration for Atlantica is proposed that is consistent with our new (and older) paleomagnetic data. Atlantica was assembled at 2.1–2.05 Ga at polar latitudes and drifted towards the equator soon afterwards.

The Paleo Recipe for Complex Life on Earth

Let's step back through deep time to consider an intriguing event. One which has been written about, but far from sufficiently and its existence could have some pretty profound implications about Earth and life's history.

 

Looking back to the Precambrian.  

The Earth is racked by global glaciations.  Ones which have been called the Snowball Earth.  They stretch over the entire planet from the poles through the equator.  There is some dispute whether or not the oceans were completely frozen over (a slushball earth vs snowball), but the glaciations are acknowledged as real.

The monstrous glaciation is understood to have been triggered by a sudden drop in the atmospheric carbon dioxide levels.  This, in turn, is believed to have been caused by photosynthetic organisms drawing down the CO2 levels while spiking the oxygen levels (relatively speaking).  Yes, the snowball earth events are, like in the Eocene's Azolla Event, examples of biogenic climate change.

Within 100 million years of the end of the Snowball Earth complex life arose.  Except it would vanish from the fossil record, probably having gone extinct and changing the biological fate of the Earth.

Wait.  You thought I was talking about the Cryogenian and Ediacaran?

After all, the Cryogenian's Marinoan Glaciation (or maybe Sturtian or Kaigas), NeoProterozoic Oxygenation Event, Ediacaran with its biota and then Cambrian do parallel all of the above.  Except that complex life is obviously still around and the Phanerozoic is quite biologically diverse, to say the least.

But, no, despite the parallels, I am not talking about the rise of modern complex life.  Rather I am talking about events which took place more than a billion years earlier, during the PaleoProterozoic.

Oxygen, Glaciers and Complex Life, oh my!

There has been up talk of the Great Oxygenation Event.  This is when life puped out oxygen in sufficiently large quantities oxygen were poured into the atmosphere by photosynthetic life, namely something like the cyanobacteria.  This may have been one of the first mass extinctions because obligate anaerobes cannot survive in the presence of oxygen, for one, and for the second, it radically changed the climate.

By pulling down the carbon dioxide and releasing free oxygen, the cyanobacteria tipped the earth from a greenhouse climate to an icehouse climate.  This in turn, without the 'modern' feedback loops, turned into what is suspected to be the earliest Snowball Earth, during the Siderian and Rhyacian.  This glaciation, called the Huronian, lasted (as far as we can tell) for 300 million years, from the mid Siderian to about 50 million years before the end of the Rhyacian. 

In 2010, a remarkable find was brought to light from Gabon, Africa.  There appeared to be colonial, or even possibly true complex life in the Rhyacian.  Further study of the fossils strongly suggested they were some form of complex life.  However, strangely, not like our complex life which seems to have arisen piecemeal during the Cryogenian into the Ediacaran and exploded during the Cambrian.  The timing of the appearance of the Francevillian Biota

Reflections in the Mirror: Parallel Events, Evolution? | ?noitulovE, stnevE lellaraP

The Francevillian Biota has several biotic members.  Its easy to see some physical parallels between the Ediacaran Biota members and those above.  There are forms which are reminicent but not the same as the Edaicaran discoid, Dickinsonia and rangeomorph fossils.  They are not the same, by any means, but the suggested parallels are interesting.  No, I am not saying the Ediacaran forms evolved form the Rhyacian!  Quite the contrary!  Rather, I am suggesting there is an event of parallel evolution taking place.  Independently.  Form follows function.  However, this is more suggestively so than closely (unlike the classic of sharks, ichthyosaurs and dolphins), but what's even better of a parallel is the events which led up to the both the evolution of the Francevillian and Ediacaran Biotas.

The events of the two different time frames, Siderian/Rhyacian & Cryogenian/Ediacaran (global glaciations, O2 spike and the seeming rise of complex life) is pretty exciting.  Is this what it would take to have Earth-evolved life to produce complex life forms?  Perhaps.  Its worth looking into.  However, the huge question becomes, if complex life evolved 2.05 billion years why is there a hiatus for over a billion years.  Those years sometimes being called the Boring Billion (more properly, the MesoProterozoic).  During that time frame, we have evidence of microbial life, but nothing like an animal or plant.  So?  What happened?  It must have gone extinct.  There are two possible explanations, assuming this is in fact complex life in the Francevillian Biota.

One is the Francevillian Biota didn't produce a proper Gaia-like feedback system and they tipped over into a feedback loop which caused them to ruin their own environment.  Or rather they ran out of their nutrients.  Or failed to develop a self sustaining ecosystem.  Its a biological failure then.  Producing Gaia is hard.  At best its 50-50 proposition of true.

The second possible reason is even more compelling.  Why?  Because of the time and energy involved.  27 million years after the Francevillian fossils were deposited, one of the biggest asteroid impacts the Earth has seen since the Late Heavy Bombardment took place.  And 150 million years after that, a comparable asteroid smashed into the world.  Both of these impacts dwarf the Chicxulub Crater.

Say Hello to my Little Friend.  Ahem, BFG.  Twice.

It has already been established giant impacts can cause mass extinctions.  The impact in Chicxulub, Mexico 66 million years ago, wiped out 50% of life on the planet.  The KT or K-Pg Extinction made way for our modern ecology and our own evolution.  The Chicxulub impact was the equivalent of over 100 teratons of explosives, roughly over 1000 times the combined nuclear arsenals on Earth.  There are two impacts which followed the Francevillian Biota which were significantly more powerful.

The Vredefort Impact took place 'first.'   The astrobleme for Vredefort is in southern Africa and dates to 2.023 billion years ago.  The crater it left was 300 km in diameter: Chicxulub is 'only' 180 km in diameter.  The impact would have been significantly more energetic than Chicxulub.  This is guaranteed mass extinction.  In fact, its a significantly more devastating mass extinction because the Francevillian Biota is almost assuredly younger and less robust than when our lineage dating from the Cryogenian took it in the shorts from Chicxulub.  Had Chicxulub happened before the Cambrian Exoplosion, its probable there wouldn't be anything remotely complex on the planet.  Vredefort was worse.

As if Vredefort was not enough, 150 million years later, circa 1.849 billion years ago, came the Sudbury Impact.  It was of a comparable energy and bollide size. If any thing complex survived the Vredefort, which would be doubtful, the Sudbury almost assuredly finished them off.  The two impacts, btw, bracket the Orosirian Period.

Interestingly, a paper which just came out supports the scenario.  Although the authors are studying about two different impacts at an earlier time, they found geological evidence of the upper sea boiling from the devastation wrought.  The Sudbury and the Vredefort Impacts both have the potential to done the exact same thing.  Given the Francevillian Biota was a shallow water assemblage, its entirely likely they were wiped out in one or both of the events.

The surviving largely microbial life would trundle on in a relatively stable environment for another billion years thence.

Every Earth Deserves a Second Chance

Assuming the Francevillian Biota was, in fact, complex life, Earth would need another billion years until the Ediacaran to take another stab at complex life.  This time it would be successful and continue into the present, despite mass extinctions.  Fortunately, there have been no bollides of comparable mass or energy as the Vredefort or Sudbury since.  Their descendents are us.

The hypothesis we had extremely distant cousins makes for some interesting conjectures.

• Under the right stimulus, it seems complex life arises when working with earth evolved organisms.  However, it does seem to require those proper chain of events.  

• Additionally, as soon as life had a chance, it also appears to (inadvertently) triggered the events which led up to complex life.  This, in turn, suggests complex life may be common where it is possible to evolve.  

• However, it is also possible to wipe out complex life without requiring too crazy of events (or intelligent intervention).

• It seems likely Mars will definitely not have any traces of complex life.  If the Earth was still getting battered at 1.85 billion years ago, that is well past the point when Mars' atmosphere thinned.  Additionally, the Hellas Basin is an impact crater which is 2,300 km in diameter.  If Vredefort was tough, then Hellas was hell and the likelihood of anything surviving through that impact is almost nil.  The Hellas Impact took place ~3.8 billion years ago.  For that matter, the Argyre Planitia is also another impact basin and it is 1,800 km in diameter, if not as deep as the Hellas.  Its highly unlikely any life survived past these events.  There's a slim chance Earth could have reintroduced life, but by the time the Francevillian Biota arose, Mars had something akin to its present atmosphere.

The story outlined above, the hypothesis which I've hung out there in my first paleo post in some time, hangs together quite well in view of the science.  However, it needs to be tested.  We can only test the hypothesis by finding other locales similar to Franceville.  Likewise, a lot of detailed examination of the Francevillian Biota as known is needed. Likewise, deposits from after Vredefort and in similar paleoenvironmental conditions are needed from the Orosirian.  

Then what's needed is to understand why the MesoProterozoic didn't have any glaciations and, thus, did not produce the complex life as we (squintingly) know it earlier.

Orosirian Paleoproterozoic Fe-rich Mafic Sills From the Zhongtiao Mountains

Geochronology and geochemistry of the Paleoproterozoic Fe-rich mafic sills from the Zhongtiao Mountains: Petrogenesis and tectonic implications

Authors:

Li et al

Abstract:

Despite the existence of the Trans-North China Orogen (TNCO) in the middle part of the North China Craton (NCC) is widely accepted, its formation timing and mechanism are still controversial. A suite of Paleoproterozoic Fe-rich mafic sills in the Zhongtiao Mountains (ZTM) in the southern TNCO has been investigated to constrain the formation of the TNCO. Our new zircon U–Pb dating on these sills has yielded a Paleoproterozoic age of 1897 ± 30 Ma, and our new geochemical data have suggested an arc-affinity with significant Nb, Ta and Ti depletions. Geochemical evidence further reveals that the mantle source of the ZTM Fe-rich mafic sills was mainly composed of spinel lherzolite, and may have been metasomatized by reduced subduction-related fluids. With these age and geochemical data, we propose a genetic model for the ZTM Fe-rich mafic sills: (1) Subduction-related fluids may have transported Fe from the subducted plate and the overlying mantle wedge to generate the Fe-enriched source at the upper part of mantle wedge; (2) Partial melting of the Fe-enriched source may have generated the primary magmas of the Fe-rich mafic sills, which has evolved under a low oxygen fugacity (fO2) condition and eventually formed the Fe-rich mafic sills. The ZTM Fe-rich mafic sills have moderate Ti/V values (28–36) and La/Nb values (2.3–3.3), suggesting that they may have formed in a back-arc setting. The existence of a back-arc basin at ∼1900 Ma in the ZTM indicates that the TNCO was formed during the ∼1850 Ma final collision between the Western- and Eastern blocks of the NCC.

Evidence of the Oblique Collision of Volgo-Sarmatia and Fennoscandia Continents During the Assembly of PaleoProterozoic Supercontinent Columbia

Trans-Baltic Palaeoproterozoic correlations towards the reconstruction of supercontinent Columbia/Nuna

Authors:

Bogdanova et al

Abstract:

A comparative study of the Palaeoproterozoic accretionary the central and southern parts Svecofennian orogen in the Baltic/Fennoscandian Shield and the platform area to the east and south of the Baltic Sea indicates that at least of these parts of the orogen are built up of several NW-SE trending, 100 to 300 km wide tectonic megadomains separated from each other and complicated by major zones of mostly dextral shearing. The generation of these zones occurred successively between 1.86 and 1.75 Ga, concomitantly with continuing crustal accretion consistently younging toward the southwest. Even considering the distorting presence of a number of microcontinents, this indicates the one-time existence and repeated episodic activity of a master subduction zone stepwise falling back toward the present south-southwest. At 1.82–1.80 Ga, the continent-continent oblique collision of megacontinent Volgo-Sarmatia with the preexisting Fennoscandia interrupted the accretionary growth of the crust in the Svecofennian orogen. In the west, the system of Svecofennian tectonic domains and shear zones is delimited by 1.70–1.55 Ga rock belts marking part of the Laurentia-Greenland-Baltica margin of Columbia. Altogether, the present U-Pb zircon datings and studies of key rocks and structures in the South Baltic region allow more detailed Trans-Baltic correlation and the creation of new integrated models of the structural and tectonic evolution of the Svecofennian orogen in particular and northern Europe in general. The new findings will be important also in the continuing study of supercontinent formation and supercontinent cycles, and the drifting of Palaeoproterozoic protocontinents during the assembly of Columbia/Nuna.

Sudbury Crater in Canada Confirmed to be Orosirian/Statherian Paleoproterozoic Cometary Impact

On the track of the elusive sudbury impact: geochemical evidence for a chondrite or comet bolide

Authors:

Petrus et al

Abstract:

Siderophile and lithophile trace element data for 69 samples from the Sudbury impact crater fill (Onaping Formation) and quartz diorite offset dikes help constrain the sources of the established moderately elevated platinum group element signature associated with the impact structure. The siderophile element distribution of the crater fill requires contributions from bulk continental crust, mafic rocks and a chondritic component. A mantle component is absent, but the involvement of mid to lower crust is implied. After considering post-impact hydrothermal alteration, melt heterogeneity, and mafic target admixture, the projectile elemental ratios were determined on a more robust data subset. Chondrite discrimination diagrams of these ratios identify an ordinary or enstatite chondrite as the most probable source of meteoritic material in the Sudbury crater fill. However, the relative and absolute siderophile element distributions within the impact structure as well as bolide size models are congruent with the bolide being a comet that had a chondritic refractory component.

West African Sedimentation From the Orosirian PaleoProterozoic From an Enigmatic Source

New insights on Proterozoic tectonics and sedimentation along the peri-Gondwanan West African margin based on zircon U-Pb SHRIMP geochronology

Authors:

De Waele et al

Abstract:

New mapping and age data from rocks of the West African Craton and its western margin, as well as a review of published age data for the region indicate that the southwestern margin of the craton is composed of an Archaean basement of TTG gneisses and granitoid rocks that formed over a long period of time between 3.54 and 2.64 Ga. The age data refute the previously held notion of two craton-forming events, the ∼3.0 Ga Leonian and ∼2.7 Ga Liberian cycles, but instead indicate a pulse at ∼3.4 Ga, followed by near-continuous crustal growth between ∼3.05 and 2.64 Ga. Age data for the Kenema Assemblage, a strip of Archaean rocks separated from the main Archaean exposure further east by Neoproterozoic cover, suggest that this represents a tectonic window.

To the west of the Archaean craton, the Rokel-Kasila Belt occurs, which incorporates two tectonic terranes: the Palaeoproterozoic Kasila terrane, accreted to the Archaean craton and comprised of granulite-facies paragneiss units of the Kasila Group, and the Meso- to Neoproterozoic Marampa terrane, thrust on top of the Archaean basement, and comprised of greenschist-facies schists and metabasites of the Marampa Group. A metavolcanic unit in the Kasila Group provides an age of 1941 ± 4 Ma interpreted to date the emplacement of the Kasila succession. Xenocrystic zircon in the sample suggest the presence of cryptic older crust with components of 2.7-2.6 Ga and 2.2 Ga, and the Kasila Terrane is interpreted to represent a Ganderian-type peri-Gondwanan terrane left attached to the West African Craton. Detrital age data on two schist units of the Marampa Group suggest a maximum age of deposition of between 1076-1030 Ma. The detrital age peaks indicate source terranes in part consistent with a West African affinity, but also comprising significant sources of between 2.0 and 1.0 Ga for which no suitable source terranes are known in West Africa. We suggest that at the time the Marampa Group was deposited along the West African margin at ∼1.05 Ga, a source terrane with significant matching components of Palaeoproterozoic and Mesoproterozoic source rocks, was present to provide the sedimentary input, possibly the Amazonian Craton.

Swedish Knaften-Barsele arc Formed, Accreted onto Karelian Continent During Orosirian PaleoProterozoic

Hafnium isotope evidence for early-Proterozoic volcanic arc reworking in the Skellefte district (northern Sweden) and implications for the Svecofennian orogen

Authors:

Guitreau et al

Abstract:

The Skellefte district is a seemingly juvenile and heavily mineralized crustal domain in northern Sweden that formed between 1.90 and 1.87 Ga. It is commonly interpreted as a volcanic arc deposited on a basement (known variously as the Bothnian or the Knaften-Barsele group) that could be represented by older rocks (1.96-1.94 Ga) found in the vicinity. In order to understand the potential genetic relationship between the arc and the basement, Hf and Pb isotopes in magmatic zircons from key lithologies were measured by solution multi-collector inductively-coupled plasma mass spectrometry. It is shown that both geological groups display similar Hf isotope compositions, which translate into decreasing ɛHf with time. Overall, the data are compatible with reworking of the Knaften-Barsele arc to produce the Skellefte rocks over a short time interval from 1.90 to 1.87 Ga in a context of crustal extension with ongoing subduction. When the data presented here are integrated with general models of tectonic evolution of the Svecofennian orogen, they fit a scenario in which the juvenile Knaften-Barsele arc formed between 1.96 and 1.94 Ga and became accreted onto the Karelian continent located further north at about 1.92-1.91 Ga. Systematic north to south variations in Pb, Nd, and Hf isotope compositions throughout the Svecofennides, interpreted as resulting from an increase in Archean crust involvement towards the south, indicate a genetic link between the Proterozoic crustal domains of Sweden and Finland.

Evidence of a Very Slow Motion Continental Collision Between the Rhycian to Orosirian PaleoProterozoic

Geochronology and geochemistry of the Paleoproterozoic meta-basalts from the Jiao-Liao-Ji Belt, North China Craton: Implications for petrogenesis and tectonic setting

Authors:

Li et al

Abstract:

Geochronology, geochemical and isotopic studies were carried out on the Paleoproterozoic meta-basalts and related rocks from the Jiao-Liao-Ji Belt, North China Craton, to understand the mantle source characteristics and the geodynamic setting. U-Pb isotopic dating using the LA-ICPMS method on zircons from the metamorphosed volcanic rocks reveals that they formed at ca. 2204 - 2158 Ma and were metamorphosed at ca. 1895 - 1919 Ma, respectively. The meta-basalts are of calc-alkali affinity and have arc-like geochemical compositions. They are enriched in LREEs ((La/Yb)N = 3.5 - 5.1) and LILEs (Sr, Ba, Rb), depleted in HFSEs (Nb, Ta, Zr, Hf, Ti), and show variable high ɛNd(t) values (+2.4 to +4.5). The basaltic rocks experienced significant fractional crystallization and minor crustal contamination during magma evolution. Modeling suggests that the parent magma of the meta-basalts was derived from 20% partial melting of spinel-garnet lherzolites that were previously metasomatized by subduction zone fluids/melts in the Late Archean. Petrological, geochronological and geochemical data suggest that the Paleoproterozoic volcanic rocks formed in an active continental margin setting and were subsequently deformed and metamorphosed to amphibolite facies due to the arc-continental collision at ca. 1.9 Ga, rather than in a continental rifting setting as previously thought by most workers.

The Evolution of the Orosirian Paleoproterozoic Arctic Canada

Sedimentological and geochemical basin analysis of the Paleoproterozoic Penrhyn and Piling groups of Arctic Canada

Authors:

Partin et al

Abstract:

Paleoproterozoic metasedimentary successions on the western Churchill Province record a history of regional tectonic events spanning the breakup and amalgamation of supercontinents. Metasedimentary successions in the northeastern part of the Trans-Hudson orogeny, including the Penrhyn and Piling groups of Melville Peninsula and Baffin Island, respectively, have been understudied with respect to their basin evolution records. We present new sedimentological, geochemical, and geochronological data for the Penrhyn and Piling groups that record deposition on the Rae craton. New detrital zircon U-Pb age data indicates that deposition of the Penrhyn Group started after ∼1897 Ma and post-dates deposition of most of the Piling Group. The new maximum age constraints and stratigraphy of the Penrhyn Group do not support previous lithostratigraphic correlation with the Piling Group. Deposition of the lower Piling Group occurred in an epicontinental basin setting. The occurrence of detritus from the Meta Incognita microcontinent in the upper Piling Group (Bravo Lake and Longstaff Bluff formations) is interpreted to signal the onset of foreland basin deposition. The details of sedimentation in the Piling foreland basin provide an additional constraint to discerning the polarity of subduction between the Rae craton and Meta Incognita microcontinent that closed the Piling basin. We conclude that the Piling foreland system shows characteristics of a pro-foreland basin, implying that the Rae craton was the lower plate in the collision with the Meta Incognita microcontinent. Finally, with new stratigraphic and detrital zircon geochronological data for the Penrhyn and Piling groups, we provide a comparison with other sedimentary successions on the Rae craton whose histories were influenced by similar regional tectonic events.

Crustal Formation in the NeoArchean and PaleoProterozoic of Tanzania

Neoarchean and Paleoproterozoic crust formation in the Ubendian Belt of Tanzania: Insights from zircon geochronology and geochemistry

Authors:

Kazimoto et al

Abstract:

LA-ICP-MS U-Pb zircon geochronological and geochemical data of meta-igneous and metasedimentary rock types of the Katuma Block of the Paleoproterozoic Ubendian Belt in Tanzania are used to unravel the crustal evolution of this metalliferous terrain. The protoliths of the metabasites and orthogneisses previously considered to be Paleoproterozoic are in fact mostly Neoarchean in age (2713 ± 11 Ma to 2638 ± 5 Ma), from which the oldest rocks experienced their first metamorphism during the same Neoarchean orogenic cycle at ca. 2650 Ma. A second event of mafic magmatism (2021 ± 11 Ma) was concomitant with the migmatization of the Neoarchean orthogneisses and was succeeded by granitic intrusions at 1990–1940 Ma. All rocks of the Katuma Block experienced their main metamorphic reworking during several Paleoproterozoic orogenic events, which were recognized by dating of various metamorphic zircon growth zones and the age of magmatic events dated at ca. 2050, 1960 and 1880 Ma. The detritus of the high-grade metasedimentary rocks derived from Neoarchean (Katuma Block or Tanzania Craton?) and Paleoproterozoic provenances and the minimum age for the deposition is constrained by its first metamorphism at ca. 1960 Ma. The Neoarchean and Paleoproterozoic metabasites, gabbronorites and orthogneisses are sub-alkaline in composition displaying a REE and trace element geochemistry akin to those of rocks formed in modern-arc settings. On the basis of the geochemical data, the presence of eclogites, deformation and metamorphic ages, we suggest that in Paleoproterozoic time the Katuma Block was again at an active continental margin, below which a Paleoproterozoic oceanic lithosphere was subducting.

Evidence of a Continental Collision Within the North China Craton During the Late Orosirian PaleoProterozoic

Palaeoproterozoic metamorphic evolution and geochronology of the Wugang block, southeastern terminal of the Trans-North China Orogen

Authors:

Lu et al

Abstract:

Amphibolites of the Wugang block in the Taihua metamorphic complex, which is located in the southeastern terminal of the Palaeoproterozoic Trans-North China Orogen (TNCO), preserve three generations of metamorphic mineral assemblages, i.e., the prograde (M1), peak (M2) and retrograde (M3) assemblages, forming during the Palaeoproterozoic tectono-metamorphic event. These amphibolites record metamorphic P–T conditions of 4.8–6.7 kbar/720–750 °C (M1), 9.0–10.6 kbar/710–780 °C (M2) and 7.1–8.5 kbar/700–780 °C (M3), which define clockwise P–T paths involving isothermal decompression (ITD). The metamorphic peak reached upper amphibolite facies, and high resolution in situ SIMS U–Pb dating of metamorphic zircons suggests that the peak metamorphism took place at 1.96–1.92 Ga in the Wugang block. The present new data suggest that the Wugang block was also involved in the continental collision between the Eastern and Western Blocks of the North China Craton (NCC) in the Palaeoproterozoic. Combining metamorphic and geochronological data concerning the TNCO obtained by previous scholars, it may be anticipated that the middle and southern sections of the TNCO were involved in a long-lived continental collision, and the collision event can be possibly traced back to as early as ∼1.96 Ga and lasted to as late as ∼1.80 Ga.

Evidence of the Growth of PaleoProterozoic Orosirian Svecofennian Ocean From Karelian Craton, Russia

1.92 Ga kimberlitic rocks from Kimozero, NW Russia: their geochemistry, tectonic setting and unusual field occurrence

Authors:

Priyatkina et al

Abstract:

The Kimozero body is inferred to represent the oldest known diamond-bearing kimberlite. It was emplaced into a basaltic sequence as a gently dipping sheet-like unit, comprising several rock varieties with textures indicating a supracrustal origin. Minor subvolcanic bodies are represented by several narrow dykes and diatremes, which formed during at least two intrusive phases. The kimberlitic rocks have been exposed to extensive interactions with hydrous and CO2 rich fluids, which led to extreme changes in the chemical compositions of the rocks, making it difficult to identify their origin. Geochemical data indicate a within-plate anorogenic tectonic setting of the Kimozero magmatic body. Trace element compositions show that the rocks have a clear geochemical affinity to orangeite (micaceous group II kimberlite) and olivine lamproite. Nd and Sr isotope compositions of the Kimozero rocks show that they were derived from a mantle source resembling Bulk Silicate Earth. New zircon U-Pb isotope data indicate emplacement age of the kimberlite at 1.92 Ga, suggesting that the rocks were associated with a late stage of Paleoproterozoic rifting of the Karelian Craton, related to the formation and growth of the Svecofennian Ocean

Evidence of Climate Change During Orosirian PaleoProterozoic

Evidence for climate shifts in the ~ 2.0 Ga upper Makgabeng Formation erg, South Africa

Authors:

Heness et al

Abstract:

Eolian ergs and associated environs are sensitive to short- and long-term climatic changes. In one of the oldest erg deposits, the ~ 2.0 Ga Makgabeng Formation, vertical changes in facies associations reflect one of the earliest records of short-term climatic shifts in a continental setting.

The Makgabeng Formation is separated into lower and upper erg deposits by a playa or saline pan deposit. The lower erg deposit consists of dune sets with thin lenses of dry and deflationary interdune strata that transitions vertically to thicker damp to wet interdune strata. A laterally persistent playa deposit in the middle of the section consists of mudstone with deep and shallow penetrating mud cracks and subordinate siltstone and sandstone interbeds. Above this lower mudstone interval, the playa strata are sandy. Overlying the playa deposit, the upper erg deposit consists of thick eolian sets with thin lenses of dry interdune deposits. Grain size change near the top of the upper erg deposit corresponds to the appearance of fluvial, sheet flood, eolian cross-beds sculpted by mass flows, and thin playa deposits.

Vertical stacking of facies associations demonstrates temporal shifts in precipitation and fluctuating water tables. Fluvial and playa deposits record high water tables whereas low-water tables are reflected in the core erg. The transition from a lower to higher water table is recorded by wet interdune interspersed within the dune strata towards the top of this interval. Rapid climatic amelioration occurred near the termination of the Makgabeng erg resulting in impingement of ephemeral river systems, development of playas, and generation of massive sand flows. This 2.0 Ga erg demonstrates the impact of climate change on erg development, resulting from shifts in monsoonal impingement through time.

The Position of Ordos Block (India) in the Orosirian Paleoproterozoic Supercontinent Columbia

Possible southwestward extrusion of the Ordos Block in the Late Paleoproterozoic: Constraints from kinematic and geochronologic analysis of peripheral ductile shear zones

Authors:

Gong et al

Abstract:

The Paleoproterozoic is a pivotal period of the tectonic evolution of the North China Craton (NCC). Various models have been postulated by previous researchers through the study of petrology and metamorphism, but the lack of structural analyses has hampered further understanding of the tectonic evolution of the NCC. A series of well-exposed ductile shear zones developed on the northern, northwestern and eastern edges of the Ordos Block in the western part of the NCC. We carried out detailed field studies along with isotopic dating focused on the kinematics and geochronology of the ductile shear zones. On the north margin of the Ordos block, the Wulashan-Daqingshan ductile shear zone is characterized by an E-W trending shearing foliation that steeply dips 75-80° to the north or south, with subhorizontal stretching lineation plunging 10-15° toward the east or west and dextral strike-slip shearing kinematics. On the west margin of the Ordos block is the Zongbieli ductile shear zone with top-to-the-NW thrusting kinematics. On the east margin of the Ordos block is the Xueling ductile shear zone with top-to-the-SE thrusting accompanied by sinistral strike-slip shearing kinematics.

The 40Ar-39Ar ages of deformed minerals from mylonitic high-grade metamorphic rocks and LA-ICP-MS U-Pb dating results of zircons from syn-tectonic anatectic granites reflect that the ductile shear zones surrounding the Ordos block deformed between 1.85 and 1.81 Ga. The similar deformation ages indicate that the ductile shear zones might be genetically correlated. The geometry, kinematics and geochronology characteristics of the Wulashan-Daqingshan ductile shear zone on the north margin, the Zongbieli ductile shear zone on the west margin, and the Xueling ductile shear zone combined with the Zhujiafang sinistral ductile shear zone on the east margin defined the possible southwestward extrusion of the Ordos block in the late Paleoproterozoic, which might have resulted from the westward subduction-collision of the Eastern Block under the Western Block at approximately 1.85 Ga and which is correlated with the amalgamation of the Columbia supercontinent