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.

Pondering the Precambrian #4

NeoProterozoic:

Ediacaran:

From Brazil, Ediacaran vase shaped fossils have been found.  is this the earliest known protist fossils yet?

How are the various frond critters of the Ediacaran related to each other?

How did the Ediacaran critter Dickinsonia grow?

A mixed Cloudinia-Corumbella-Namacalathus assemblage shows increasing ecological complexity over the course of the Ediacaran.

Cryogenian:

Did a freshwater layer exist on the world's oceans and persist after the Snowball Earth episodes like the one in the Cryogenian?

As the Snowball Earth ended, how much oxygen weathering took place and what were the biotic impacts?

Tonian:

There was a huge Andes-like mountain range on the northwest of Rodinia that may have lasted for 100 million years during the Tonian 800 million years ago.

MesoProterozoic:

Wind patterns have been inferred from dunes from Calymmian Brazil.

The diversity of Eukaryote microfossils of Calymmian China is impressive.

PaleoProterozoic:

In Statherian China, there is evidence of a sillicified microbiota from the Dahongyu Formation.

The Sudbury Impact appears to have caused long lived volcanic eruptions during the Orosirian PaleoProterozoic.  Since it was one of 150 impacts within a relatively short period, combined with this above volcanism, it should be no surprise life didn't recover to take a second stab at complexity for a billion years.

The Sudbury Basin continued to have geothermal heat during the Huronian snowball earth.

Can the Rhyacian/Siderian Glaciations (huronian snowball earth) be dated based on subglacial hydrothermal activity?

Beginning in the Siderian, ancient carbon was subsumed into the Earth's mantle.

Did anaerobic oxygenic photosynthesis (read the paper) come about in cyanobacteria prior to modern aerobic photosynthesis?

During the Siderian, Earth had a hazy, methane filled atmosphere.

Did eukaryotes arise during the Siderian?

Archean:

Lenticular organisms from South Africa are related to the Pilbara forms.

Did life arise during the EoArchean WITHOUT using phosphate?

Fossils were found from the Eoarchean 3.77 billion years ago in Quebec, Canada.

How did the crust form?

Hadean:

The Earth probably began with a solid shell for a crust, like Mars.

META:

New branches have been found in Archaea.

Iron eating, methanogen organisms probably kept the Earth warm for its first 2 billion years.

Mineral self assembly was common in the early years of the Earth.

Where was the Nuna/Columbia Supercontinent?

Mesoproterozoic geomagnetic reversal asymmetry in light of new paleomagnetic and geochronological data for the Häme dyke swarm, Finland: Implications for the Nuna supercontinent

Authors:

Salminen et al

Abstract:

Baltica represents one of the key continents of the Mesoproterozoic supercontinent Nuna forming the core of it together with Laurentia and Siberia. This study presents new geochronological and paleomagnetic data obtained for the Häme diabase dyke swarm in southern Finland. New U-Pb (baddeleyite) ages 1642 ± 2 Ma and 1647 ± 14 Ma for two reversely magnetized dykes are acquired. Demagnetization revealed a dual polarity remanent magnetization direction carried by magnetite. The combined normal (N) and reversed (R) polarity direction for 11 dykes (=sites) is D = 355.6°, I = −09.1° (k = 8.6 and α95 = 16.6°) yielding a paleomagnetic pole at 23.6°N, 209.8°E (K = 10.6 and A95 = 14.7°) with Van der Voo value Q = 7. N and R magnetized units for the Häme dyke swarm show asymmetry in declination values, probably caused by an age difference between the dykes. The Geocentric Axial Dipole (GAD) model indicates that all geomagnetic reversals should be symmetric (in inclination), yet it has been noted that this is not always the case (e.g. 1.57 Ga Satakunta and Åland dykes in Baltica). By analyzing global dual polarity paleomagnetic data we show that the stationary GAD model is a valid assumption at 1.7–1.4 Ga and that the asymmetry between some normal and reversed polarities in global dual-polarity data sets appears randomly over time, and does not follow a global trend. Furthermore, we show that in the case of Åland and Satakunta dykes an unremoved secondary magnetization component could explain the obtained asymmetry. The GAD assumption is used to reconstruct the core of Nuna on equatorial latitudes using new data for Häme dykes. Paleomagnetic evidence suggest that maximum assembly of Nuna occurred at 1.5 Ga and the dispersal of the core is proposed to be associated with coeval 1.38–1.27 Ga magmatism in its core continents.

Was Delayed Intercontinental Seas' Euxinia A Vital for Eukaryotes to Survive During the Statherian PaleoProterozoic?

Delayed euxinia in Paleoproterozoic intracontinental seas: Vital havens for the evolution of eukaryotes?

Authors:

Spinks et al

Abstract:

Increased flux of sulfate to the oceans in the aftermath of the Great Oxidation Event (GOE) ∼2.4 billion years ago (Ga) caused major changes in seawater chemistry, which eventually contributed to the cessation of iron formation deposition ∼1.8 Ga. It is generally accepted that this engendered heterogeneous stratified redox conditions, with anoxic and sulfidic (euxinic) conditions in shallow open-marine environments and anoxic ferruginous conditions in deeper environments. However, the redox evolution of intracontinental marine basins following the cessation of iron formation deposition remains poorly understood.

Here, we report contrasting paleoredox conditions in two shale units of the lower McArthur Basin, northern Australia, soon after the cessation of iron formation deposition ∼1.84 Ga. Our data shows that the ∼1.78 Ga McDermott Formation was deposited in a sulfur-limited, anoxic shallow-marine environment, whereas the younger ∼1.73 Ga Wollogorang Formation was deposited in a euxinic shallow-marine environment. This implies a delay in the development of euxinia in a shallow intracontinental basin following the onset of euxinia in the open marine realm. Since bioessential metals are sequestered by pyrite deposition under euxinic conditions, protracted low-sulfidic conditions in 1.78 Ga intracontinental shallow environments could have provided vital niches for nitrogen-fixing prokaryotes and eukaryotes. Thus the ability for localized Paleoproterozoic intracontinental basins to remain non-euxinic after the onset of euxinia in shallow open-marine shelves highlights the importance of intracontinental environments to the evolution and diversification of microbial life, perhaps throughout the wider Proterozoic.

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.

Digging Into Statherian PaleoProterozoic/ Calymmian MesoProterozoic Eukaryotic Fossil Valeria

A biomechanical analysis of the early eukaryotic fossil Valeria and new occurrence of organic-walled microfossils from the Paleo-Mesoproterozoic Ruyang Group

Authors:

Pang et al

Abstract:

The Paleo-Mesoproterozoic Ruyang Group of North China hosts early eukaryotic fossils such as Dictyosphaera, Shuiyousphaeridium, and Valeria, and thus offers valuable insights into the early evolution of single-celled eukaryotic life. In this paper, we report several additional forms of organic-walled microfossils from the Ruyang Group, including Plicatidium latum, Spiromorpha sp., and an unnamed form. V. lophostriata from the Ruyang Group is investigated using transmitted light microscopy, scanning electron microscopy, transmission electron microscopy, and biomechanical analysis. V. lophostriata is reconstructed as a spherical vesicle with two hemispherical halves bearing concentric striations resembling latitudinal circles. The formation of striations could be explained using the Belousov-Zhabotinsky reaction model or the Turing reaction-diffusion model. A biomechanical analysis using the thin-walled spherical pressure vessel model suggests that the concentric striations of V. lophostriata may have functioned as a mechanism to guide biologically programmed excystment through medial split. Our analysis provides essential paleontological data to better understand the functional biology and life cycles of early eukaryotes such as Valeria.

Using Ancient Dynamin Segments to Understand Mitochondrial Endosymbiosis During the Statherian PaleoProtoerozoic

Ancient dynamin segments capture early stages of host–mitochondrial integration

Authors:

Purkanti et al

Abstract:

Eukaryotic cells use dynamins—mechano-chemical GTPases—to drive the division of endosymbiotic organelles. Here we probe early steps of mitochondrial and chloroplast endosymbiosis by tracing the evolution of dynamins. We develop a parsimony-based phylogenetic method for protein sequence reconstruction, with deep time resolution. Using this, we demonstrate that dynamins diversify through the punctuated transformation of sequence segments on the scale of secondary-structural elements. We find examples of segments that have remained essentially unchanged from the 1.8-billion-y-old last eukaryotic common ancestor to the present day. Stitching these together, we reconstruct three ancestral dynamins: The first is nearly identical to the ubiquitous mitochondrial division dynamins of extant eukaryotes, the second is partially preserved in the myxovirus-resistance-like dynamins of metazoans, and the third gives rise to the cytokinetic dynamins of amoebozoans and plants and to chloroplast division dynamins. The reconstructed sequences, combined with evolutionary models and published functional data, suggest that the ancestral mitochondrial division dynamin also mediated vesicle scission. This bifunctional protein duplicated into specialized mitochondrial and vesicle variants at least three independent times—in alveolates, green algae, and the ancestor of fungi and metazoans—accompanied by the loss of the ancient prokaryotic mitochondrial division protein FtsZ. Remarkably, many extant species that retain FtsZ also retain the predicted ancestral bifunctional dynamin. The mitochondrial division apparatus of such organisms, including amoebozoans, red algae, and stramenopiles, seems preserved in a near-primordial form.

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.

Paleo-position of the North China Craton Within the supercontinent Columbia

Paleo-position of the North China craton within the supercontinent Columbia: Constraints from new paleomagnetic results

Authors:

Xu et al

Abstract:

Several new paleomagnetic and geological studies focused on the reconstruction of the North China Craton (NCC) within the Paleo-Mesoproterozoic Columbia supercontinent. In spite of these new data, there are still widely divergent opinions regarding supercontinental reconstructions. In addition to qualitative correlations of orogenic belts, rift basin ages, stratigraphy and distribution of igneous provinces, paleomagnetic data can provide key constraints on the positioning of individual cratons on the globe. In this paper, we report a detailed paleomagnetic study on the coeval ∼1780 Ma mafic dyke swarm and Xiong’er volcanic province, which extended for more than one thousand kilometers in the central NCC. Rock magnetic studies, including thermomagnetic curves, hysteresis loops and the progressive acquisition of isothermal remanence conducted in selected samples, indicate that the dominant magnetic carriers are PSD magnetite. Stepwise thermal demagnetization isolated higher-temperature components directed to NNE/SSW with shallow inclinations from 37 sampling sites (16 sites in Yinshan area, 13 sites in Taihang area and 8 sites in Xiaoshan area). A baked contact test conducted on two Yinshan dykes intruded by a younger dyke demonstrates the magnetization in the Yinshan dykes pre-dates 1320 Ma. The existence of dual-polarity magnetizations in both Taihang and Xiong’er areas support our contention that the ChRM was acquired during the cooling of the magma. The primary origin of the ChRM is also supported by a positive fold test on the Xiong’er data, and a coherent regional test between the results from the Taihang and Xiong’er areas. Two different site-mean directions were compiled from these new results along with previous publications. The first direction, from the Taihang and Xiong’er areas, yields Declination (D)/Inclination (I) = 12.4°/−3.7° (κ = 20.5, α95 = 4.3°, N = 57 sites). The second, from the Yinshan area is at (D) 36.7°/(I)−12.4° (κ = 86.8, α95 = 2.7°, N = 32 sites). We argue that the difference is due to Mesozoic and/or Cenozoic vertical-axis rotation of the Taihang and Xiong’er areas with respect to the fixed Yinshan-Ordos basin. The corresponding paleopoles for the Yinshan dykes falls at 245.2°E/35.5°N (A95 = 2.4°). A comparison between the NCC, Laurentia, Siberia and Baltica is consistent with possible links between these four blocks in a perhaps, even larger, continent. The proximity of Australia and India to the NCC is also evaluated.

Evidence of Continental Growth in the North China Craton From Neoarchean (MesoArchean?) to Statherian Paleoproterozoic

Neoarchean to Paleoproterozoic continental growth in the southeastern margin of the North China Craton: Geochemical, zircon U-Pb and Hf isotope evidence from the Huoqiu complex

Authors:

Liu et al

Abstract:

The Huoqiu complex in the southeastern margin of the North China Craton (NCC) is dominated by Neoarchean grey gneisses, amphibolites and voluminous metasediments. Here we report the occurrence of Neoarchean to Paleoproterozoic rocks from drill core samples. The gneisses are similar to TTG (tonalite-trondhjemite-granodiorite) in composition and show close spatial association with amphibolites. Geochemical characteristics such as high Sr/Y and (La/Yb)N with steep REE patterns and trace element modeling suggest that these rocks were generated by partial melting of hydrous meta-basalts (amphibolites) at the base of a thickened mafic continental crust, leaving a rutile-bearing eclogite residue. LA-MC-ICP-MS U-Pb age data from magmatic zircon grains show protolith emplacement ages of 2.76-2.71 Ga. Subsequently, widespread migmatization took place at 1.91-1.82 Ga, generating voluminous migmatites and high-K granites. Hf isotopic compositions of zircon grains from the amphibolite and gneiss show εHf(t) values of 2.4-15.5 and − 3.0-1.5, respectively. The tDM2(Hf) model ages of the gneisses range from 2.87 to 3.14 Ga, and are identical to the tDM1(Hf) ages of amphibolites (2.84-3.16 Ga) within analytical uncertainty, suggesting that the gneisses formed by partial melting of amphibolite, and attest to large-scale reworking of the ancient continental crust during Neoarchean. The zircon grains from the granites define two groups with regard to their Hf isotopic composition. The older group (1916 ± 42 Ma) has εHf(t) values and tDM2(Hf) ages of − 10.5-2.4 and 2.40-3.20 Ga, respectively, whereas the younger one (1823 ± 41 Ma) shows a large variation in εHf(t) values ranging from − 18.1 to 12.5, with tDM2(Hf) model ages of 1.70-3.59 Ga. A couple of zircon grains from the younger group display consistent U-Pb ages and tDM2, indicating accretion of juvenile crust from depleted mantle sources during 1.82 to 1.91 Ga. However, the dominant Hf isotope features are consistent with the reworking of preexisting continental crust. We therefore infer that only limited accretion of juvenile crust occurred during this time, and that the Paleoproterozoic (1.82 to 1.91 Ga) tectonics in the southeastern margin of the NCC witnessed extensive reworking of older continental crust.

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.

Statherian PaleoProterozoic Continental Configuration: 1650 Million Years ago

Statherian PaleoProterozoic Continental Configuration: 1720 Million Years ago

Statherian PaleoProterozoic Continental Configuration: 1770 Million Years ago

Evidence of a River Delta From Statherian PaleoProterozoic Australia

Riverine mixing and fluvial iron formation: A new type of Precambrian biochemical sediment

Authors:

Pufahl et al

Abstract:

Precambrian iron formations are biochemical sediments that record ocean chemistry and circulation on the early Earth. The appearance of large, economically important continental margin iron formation reflects the creation of extensive continental shelves and oxygenation of the ocean-atmosphere system near the end of the Archean. Exhalative iron formation contains a record of hydrothermal vent chemistry through time. We introduce here fluvial iron formation, a new type of Fe-rich microbial-biochemical sediment that formed by mixing river discharge and seawater in coastal environments. The Paleoproterozoic Chiall Formation (ca. 1.8 Ga), Earaheedy Basin, Western Australia, contains laminated and granular hematitic iron formation in delta channel deposits. Where mixing occurred in adjacent peritidal settings, laminated iron formation and hematitic oncoids formed. Because fluvial iron formations precipitated at the interface between terrestrial and marine realms, the locus of known Fe precipitation processes is shifted landward into paleoestuarine settings and reflects Fe derived from both terrestrial weathering and coastal upwelling, providing a new window into ocean-atmosphere evolution.

North China Craton Shows Rifting Within Statherian PaleoProterozoic Supercontinent Columbia

Late Paleoproterozoic geodynamics of the North China Craton: Geochemical and zircon U-Pb-Hf records from a volcanic suite in the Yanliao rift

Authors:

Wang et al

Abstract:

The Yanliao rift zone along the northern margin of the North China Craton exposes a suite of volcanic rocks comprising trachybasalts and trachytes interlayered with dolostones in the Tuanshanzi Formation of the Changcheng Group. The overlying Dahongyu Formation is composed of olivine basalts, trachybasalts, trachyandesites, trachytes, and minor sub-alkaline basalts, intercalated with a lower sandstone and an upper dolostone sequence. LA-ICP-MS zircon U-Pb age data from the volcanics reveal that the Tuanshanzi Formation was deposited at ca. 1670 Ma, and the Dahongyu Formation at ca. 1625–1664 Ma. Combined with previous studies, these new data indicate that the lowest boundary age of the Changcheng Group, marking the timing of initiation of the Yanliao rift, is ca. 1680 Ma. Except for the sub-alkaline basalts, the other alkaline volcanic rocks of the two formations are characterized by high Nb/Y ratios (1–3), and possess SiO2 contents of 43.3-60.5 wt.%, K2O of 0.72-15.69 wt.%, MgO of 0.10-8.56 wt.%, and with Mg# of 2–64. These rocks also display strongly fractionated REE patterns and generally positive Nb and Ta anomalies, with high (Nb/La)PM and (Nb/Th)PM values of 0.85-1.39 and 0.96-2.05, respectively. They show positive zircon εHf(t) values (+ 0.1 to + 1.8 for the trachybasalt sample 11PG31-2) and whole-rock εNd(t) values (+ 0.83 to + 2.25). These geochemical features, together with trace element modeling, suggest that the alkaline volcanic rocks were generated by fractional crystallization of magma derived from low-degree partial melting of an OIB-like depleted asthenospheric mantle source, with only minor involvement of melts generated from the overlying enriched lithospheric mantle source. In contrast, the sub-alkaline basalts are characterized by low Nb/Y ratios of 0.09-0.19, and show fractionated REE patterns and pronounced negative Nb-Ta anomalies with low (Nb/La)PM and (Nb/Th)PM values of 0.07-0.21 and 0.10-0.33, respectively. In combination with the positive zircon εHf(t) values (+ 0.8- + 4.3), we infer that these rocks were derived from the partial melting of a depleted lithospheric mantle source previously metasomatized by fluids from subducted slab. Combined with previous studies on the 1680–1780 Ma magmatic rocks along the northern margin of the NCC, our study identifies a late Paleoproterozoic asthenospheric upwelling in this region. The generation of the Yanliao rift and the eruption of these volcanic rocks were possibly triggered by the delamination of the continental lithosphere in a post-orogenic setting following the final amalgamation of the North China Craton within the Columbia supercontinent.

Evidence From India of a Carbon Isotope Excursion During the Statherian PaleoProterozoic

New age constraints for the Proterozoic Aravalli–Delhi successions of India and their implications

Authors:

N. Ryan McKenzie, Nigel C. Hughes, Paul M. Myrow, Dhiraj M. Banerjee, Mihir Deb and Noah J. Planavsky

Abstract:

Proterozoic sedimentary successions of India are important archives of both the tectonic history of the Indian subcontinent and the geochemical evolution of Earth surface processes. However, the lack of firm age constraints on many of these stratigraphic units limits their current utility. Here, we present new detrital zircon age data from strata of the southern Aravalli–Delhi Orogenic Belt (ADOB) and the Rajasthan Vindhyan successions. The Alwar Group of the southern Delhi Supergroup yielded a large population of ∼1.2 Ga detrital zircon grains, which refutes the 1.9–1.7 Ga age assertion for this unit. Detrital zircon age distributions from the southern Alwar Group differ strongly from the Alwar Group of the “North Delhi Belt”, demonstrating miscorrelation between these two regions. The Jhamarkotra Formation of the Lower Aravalli Group contains a large population of 1.9–1.7 Ga detrital zircon grains. Therefore, the unit cannot be ∼2.1 Ga as traditionally assumed. Age distributions of the Aravalli and Delhi supergroups are similar to those of the lower and upper Vindhyan successions, and we postulate contiguous sediment sources for both regions, with strata of the tectonically deformed ADOB representing the distal margin equivalents of the Vindhyan successions. Additionally, a late Paleoproterozoic age for the Jhamarkotra Formation nullifies the hypothesis that the markedly positive carbonate δ13C values in this unit are linked to the 2.3–2.0 Ga Lomagundi–Jatuli positive isotope excursion. The potential of a large late Paleoproterozoic (ca. 1.7 Ga) positive δ13C excursion contrasts with the long-held view of a prolonged period of carbon isotope stasis during the so-called ‘boring billion’.

Where India Was in the Columbia (Nuna) Supercontinent

Geologically constraining India in Columbia: the age, isotopic provenance and geochemistry of the protoliths of the Ongole Domain, Southern Eastern Ghats, India

Authors:

1. Bonnie Henderson (a)
2. Alan S. Collins (a)
3. Justin Payne (a)
4. Caroline Forbes (a)
5. Dilip Saha (b)

Affiliations:

a. Tectonics Resources and Exploration (TRaX), School of Earth and Environmental Sciences, The University of Adelaide, Adelaide, SA 5005, Australia

b. Geological Studies Unit (GSU), Indian Statistical Institute, Kolkata, India

Abstract:

The Ongole Domain in the southern Eastern Ghats BeBeltlt of India formed during the final stages of Columbia amalgamation at ca. 1600 Ma. Yet very little is known about the protolith ages, tectonic evolution or geographic affinity of the region. We present new detrital and igneous U-Pb-Hf zircon data and in-situ monazite data to further understand the tectonic evolution of this Columbia-forming orogen.

Detrital zircon patterns from the metasedimentary rocks are dominated by major populations of Palaeoproterozoic grains (ca. 2460, 2320, 2260, 2200-2100, 2080-2010, 1980-1920, 1850 and 1750 Ma), and minor Archaean grains (ca. 2850, 2740, 2600 and 2550 Ma). Combined U-Pb ages and Lu-Hf zircon isotopic data suggest that the sedimentary protoliths were not sourced from the adjacent Dharwar Craton. Instead they were likely derived from East Antarctica, possibly the same source as parts of Proterozoic Australia. Magmatism occurred episodically between 1.64 and 1.57 Ga in the Ongole Domain, forming felsic orthopyroxene-bearing granitoids. Isotopically, the granitoids are evolved, producing εHf values between -2 and -12. The magmatism is interpreted to have been derived from the reworking of Archaean crust with only a minor juvenile input. Metamorphism between 1.68-1.60 Ga resulted in the partial to complete resetting of detrital zircon grains, as well as the growth of new metamorphic zircon at 1.67 and 1.63 Ga. In-situ monazite geochronology indicates metamorphism occurred between 1.68-1.59 Ga.

The Ongole Domain is interpreted to represent part of an exotic terrane, which was transferred to proto-India in the late Palaeoproterozoic as part of a linear accretionary orogenic belt that may also have included south-west Baltica and south-eastern Laurentia. Given the isotopic, geological and geochemical similarities, the proposed exotic terrane is interpreted to be an extension of the Napier Complex, Antarctica, and may also have been connected to Proterozoic Australia (North Australian Craton and Gawler Craton).