Pondering the Precambrian #39

Proterozoic:

NeoProterozoic:

Global microfossil changes across the Ediacaran/Cambrian are characterized.

A new method of characterizing oxygen availability across the Ediacaran/Cambrian boundary has been found.

The lack of attention to taphonomy can mislead about the late Ediacaran fossils.

Data from Argentina covers the Ediacaran's oxygenation event.

Simulations of Ernettia suggest it was a filter feeder and fed better in groups.

Has there been a case of lateral gene transfer detected in bilaterians?

Ancient paralogies suggest jellyfish are a sister group to all other metazoans.

Green algae transitioned to macroscopic growth multiple times, but only as recently as the NeoProterozoic.

There is evidence of strange magmatism in the Sahara from the Ediacaran 580 million years ago.

An astronomical time scale for the middle/upper Doushantuo Formation has been established.

Braided rivers were not the rule prior to the evolution of plant life despite previous thought.

The Paleo Hunan Ocean was completely closed by 830 million years ago during the Tonian.

Evidence of two carbon excursions are detected in Tonian deposits in Namibia.

MesoProterozoic:

A new model suggests a sluggish, tepid mesoproterozoic ecosystem.

A very large igneous magmatic province has been found in South Africa from Stenian of South Africa.  These provinces are associated with mass extinctions like the Permian and Triassic/Jurassic.

An impact has been found in Scotland dating from the Mesoproterozoic.

Was anaerobic photosynthesis the reason for the delayed rise in oxygen in Earth's atmosphere?

PaleoProterozoic:

Iron stones were laid down by iron oxidizing bacteria during the Statherian.

There is evidence of deep subduction from North China from the Paleoproterozoic.

There is also evidence of a significant back arc continental collision during the Orosirian in the North China Craton.

There is evidence, according to Retallack, of macroscopic terrestrial life from the Orosirian.

The shape of the Dhala Crater in India from the Rhyacian is reconstructed.

Paleoproterozoic dolomites shed some light on the evolution of marine chemistry.

Archean:

The early Earth's oceans may not have been as hot as originally thought.

Compared to later granites, Archean granites had relatively unstable compositions.

Archean sulfur isotopes from Australia's Fraser Zone have unexplained ratios and amounts.

Did the subduction of the oceanic basins take place during the mesoarchean?

Could continents have existed from the dawn of the Archean, during the Eoarchean?

Eoarchean hydrothermal vent boron deposits give some insight to the origin of life.

Origin of Life:

Peptides can form without amino acids.

Could microscopic bubbles (interfaces) have helped kickstart life?

Pondering the Precambrian #27

Proterozoic:

NeoProterozoic:

Did a massive volcanic eruption trigger the Gaskiers Glaciation?

Deposits from British Columbia may be evidence of deep sea conditions just prior to the Gaskiers Glaciation.

Fossil fats from the Cryogenian may actually be from algae instead of sponges.

Amoebozoa diversified earlier than expected during the Tonian 750 million years ago.

How were banded iron formations created in Egypt from the NeoProterozoic?

Was the Cambrian Explosion really that big a deal? Or did it really originate in the Ediacaran?

First record of carbonates with spherulites and cone-in-cone structures from the Ediacaran of Norway has been found.

MesoProterozoic:

The Yili Block was likely located in the NW margin of the supercontinent Rodinia.

PaleoProterozoic:

The apparent cyclical deposition of the Dales Gorge Member banded iron formation appears to be related to sea level rise and fall or tectonic activity.

The Vempalle Formation dolomites of India appear to have more in common with Phanerozoic carbonates than Proterozoic ones.

There's a sedimentation record from 2.27 BYA to 1.96 BYA in West Africa with evidence of volcanic ash and activity in the layers.

The first drillings into the Temagami Anomaly have uncovered potential links to the Sudbury Impact.

The Francevillian Biota of Rhyacian Gabon seems to have had something able to move.  Scientists have found evidence of trackways from shallow, oxygenated waters.

Archean:

Evidence from Mauritania suggests the Archean geophysics were as complicated as today.

Evidence of a breakup of a continent from the Archean.

There appears to be evidence of major crustal growth circa 2.6 BYA and 3.2 BYA.

There is evidence plate tectonics were active, despite prior theory, during the Archean.

MesoArchean:

There is evidence of a continental margin at the tail end of the MesoArchean in North China.

A subduction zone has been found from the MesoArchean/NeoArchean boundary.

Evidence of a continental rift was found in Hainan, China from the MesoArchean.

There is evidence of ephemeral oxygen oases in the Mesoarchean Ocean.

PaleoArchean:

South African barite deposits were not laid down in a marine environment, but from a spring.

Life was thriving during the paleoarchean 3.5 billion years ago.

EoArchean:

How the first continents formed in the EoArchean.

Hadean:

Was the Theia impact the reason for the heterogeneity of Earth's crust?

Origin of Life:

Was the repeated drying and wetting cycle of the margin of water the source of energy for the original polymerization of amino acids et al?

Did hydrogen peroxide play a crucial role in the origin of life?

Pluripotency and the origin of multicellular life.

Could the RNA world hypothesized never have existed?

Shallow pools of water might have been the original location for the origin of life, not the sea.

The Mawson Continent Formed During the Nuna/Columbia to Rodinia Transition

Mesoarchean and Paleoproterozoic history of the Nimrod Complex, central Transantarctic Mountains, Antarctica: Stratigraphic revisions and relation to the Mawson Continent in East Gondwana

Authors:

Goodge et al

Abstract:

High-grade metamorphic and igneous rocks originally mapped as the Nimrod Group represent the only known crystalline basement of the East Antarctic shield exposed in the Transantarctic Mountains. SHRIMP U–Pb age data from zircon show that this assemblage preserves multiple geologic events spanning 2.5 b.y. of Archean to early Paleozoic history, culminating in thermomechanical reworking and active-margin magmatism during the Ross Orogeny. New age data from igneous and metamorphic rocks, as well as from detrital zircons in metasedimentary quartzites, refine the Mesoarchean to Paleoproterozoic history and indicate the need to abandon the stratigraphic term Nimrod Group and its five formations. In its place, we redefine only the igneous and high-grade gneissic parts of the assemblage as the Nimrod Complex, and all other metasedimentary rocks exposed in the Miller and Geologists ranges are included in a newly defined Argosy Schist. Within the Nimrod Complex, the oldest layered gneisses represent magmatic Mesoarchean crust formed between about 3150 and 3050 Ma. Correspondence of U–Pb zircon crystallization ages with whole-rock Sm–Nd model ages indicates formation from juvenile mantle melts. Magmatism at ca. 3100 Ma was followed closely by high-temperature metamorphism, recorded by zircon crystallization at 2955–2900 Ma, likely due to high advective heat transfer and/or thermal insulation of newly stabilized crust. Metaigneous units with ages of ca. 2500 Ma indicate a late Neoarchean period of anatexis and/or magmatism, although the geologic context for these events is uncertain. Gneissic, eclogitic and metaigneous rocks record an important period of deep-crustal metamorphism, thickening and magmatism between 1730 and 1700 Ma, referred to as the Nimrod Orogeny, that may have played an early role in assembly of East Gondwana cratons. A new lithodemic unit, Argosy Schist, consists of interlayered mica schist, quartzite, amphibolite, and calc-silicate schist. Quartzites from the schist unit have two distinctive detrital-zircon provenance characteristics; one type has populations that correlate with dated gneissic and metaigneous units of the Nimrod Complex, indicating a proximal provenance, whereas a second type has detrital zircon ages resembling those from Neoproterozoic Beardmore Group sandstones, indicating a common broader East Antarctic shield provenance. Their maximum depositional ages range widely from about 2000 to 900 Ma, leaving unresolved their age and geologic relationship to either the Nimrod Complex or Beardmore Group, yet they comprise a metasedimentary assemblage that is distinctive from the Nimrod Complex. Recent findings of Mesoarchean gneisses in the Gawler Craton, geochronology from Terre Adélie, and our new age data from the Nimrod Complex thus highlight the importance of ca. 3.1, 2.5 and 1.7 Ga events in formation of the composite East Antarctic shield adjacent to the modern Transantarctic Mountains. Correlative crustal units may extend to the southern Prince Charles Mountains and Shackleton Range. Collectively, this extensive crustal province, referred to as the Mawson Continent, represents an elongate belt of primary Mesoarchean crust that experienced major reworking at ca. 2.5 and 1.7 Ga. The latter period includes both high- and low-P/T petrologic signatures, providing evidence of crustal thickening and magmatism that may signify collisional processes related to Paleoproterozoic cratonal amalgamation during formation of the Nuna supercontinent.

Evidence of Atmospheric Sulfur During the MesoArchean

An atmospheric source of S in Mesoarchaean structurally-controlled gold mineralisation of the Barberton Greenstone Belt

Authors:

Agangi et al

Abstract:

The Barberton Greenstone Belt of southern Africa hosts several Mesoarchaean gold deposits. The ores were mostly formed in greenschist facies conditions, and occur as hydrothermal alteration zones around extensional faults that truncate and post-date the main compressional structures of the greenstone belt. Ore deposition was accompanied by the intrusion of porphyries, which has led to the hypothesis that gold may have been sourced from magmas. Because the transport of Au in the hydrothermal fluids is widely believed to have involved S complexes, tracing the origin of S may place strong constraints on the origin of Au. We measured multiple S isotopes in sulfide ore from Sheba and Fairview mines of the Barberton Greenstone Belt to distinguish “deep” S sources (e.g. magmas) from “surface” S sources (i.e. rocks of the volcano-sedimentary succession that contain S processed in the atmosphere preserved as sulfide and sulfate minerals). Ion probe (SIMS) analyses of pyrite from ore zones indicate mass-independent fractionation of S isotopes (Δ33S = −0.6‰ to +1.0‰) and the distribution of the analyses in the Δ33S–δ34S space matches the distribution peak of previously published analyses of pyrite from the entire volcano-sedimentary succession. Notwithstanding that the H2O–CO2 components of the fluids may have been introduced from a deep source external to the greenstone belt rocks, the fact that S bears an atmospheric signature suggests the hypothesis that the source of Au should also be identified in the supracrustal succession of the greenstone belt. Our findings differ from conclusions of previous studies of other Archaean shear-hosted Au deposits based on mineralogical and isotopic evidence, which suggested a magmatic or mantle source for Au, and imply that there is no single model that can be applied to this type of mineralisation in the Archaean.

Evidence of Bacteria Producing Soil at the PaleoArchean/MesoArchean Boundary

Biogenic overgrowth on detrital pyrite in ca. 3.2 Ga Archean paleosols

Authors:

Sabhan et al

Abstract:

Regionally traceable paleosols in the lower Moodies Group of the Barberton greenstone belt (ca. 3.22 Ga, northeastern South Africa and Swaziland) contain locally abundant silicified nodules, originally composed of pedogenic carbonates and sulfates, interbedded with heavy-mineral laminae dominated by pyrite. Pyrite grains show rounded detrital cores and secondary idiomorphic rims with trace element concentrations and δ34S ratios clearly different from those of the cores. While cores have low Co and Ni concentrations and high Co/Ni ratios, rims show as much as 5.5 wt% of these elements and low Co/Ni ratios, reflecting the weathering of nearby ultramafic rocks. In-situ sulfur isotope analyses of pyrite cores show δ34SVCDT (Vienna Canyon Diablo troilite) values between +5‰ and –5‰, while the rims show δ34VCDT values between –20‰ and –24.5‰, suggesting biogenic fractionation of sulfur. The close spatial association and microtextural evidence for nearly contemporaneous formation of the pedogenic sulfate nodules and the secondary pyrite rims suggests microbial processing of sulfur in the paleosols, which provided reduced and 34S-depleted sulfur for the growth of authigenic pyrite. This indicates that vadose-zone soil-forming processes in the Archean involved not only physical and chemical modification of moist, unconsolidated sediment in a terrestrial environment but also already included its microbiological modification.

Is an Unusual Manganese Deposit Evidence of Precambrian Aerobic Photosynthesis?

Unusual manganese enrichment in the Mesoarchean Mozaan Group, Pongola Supergroup, South Africa

Authors:

Ossa ossa et al

Abstract:

An unusual sediment-hosted manganese deposit is described from the Mesoarchean Mozaan Group, Pongola Supergroup, South Africa. MnO contents up to 15 wt.% were observed in marine clastic and chemical sedimentary rocks. Mn enrichment is interpreted to have resulted from the hydrothermal alteration of manganiferous shale and BIF parent rocks, the primary MnO contents of which are as high as 8.5 wt.%. A detailed mineralogical and petrographic study shows that these parent rocks are characterized by manganoan siderite, ferroan rhodochrosite and other Mn–Fe-rich mineral phases, such as kutnohorite and Fe–Mn-chlorite. Their hypogene alteration gave rise to a diversification of mineral assemblages where ferroan tephroite, calcian rhodochrosite, rhodochrosite, pyrochroite, pyrophanite, cronstedtite, manganoan Fe-rich chlorite and manganoan phlogopite partially or totally replaced the previous mineral assemblage. Thermodynamic modeling performed on chlorite phases associated with the described mineral assemblages illustrates a decrease of average crystallization temperatures from ca. 310 °C during early metamorphic stages to ca. 250 °C during a hydrothermal stage. Mineral transformation processes were thus related to retrograde metamorphism and/or hydrothermal alteration post-dating metamorphism and gave rise to progressive Mn enrichment from unaltered parent to altered rocks. The timing of hypogene alteration was constrained by 40Ar/39Ar dating to between about 1500 and 1100 Ma ago, reflecting tectonic processes associated with the Namaqua-Natal orogeny along the southern Kaapvaal Craton margin. Manganiferous shale and BIF of the Mozaan Group may represent the oldest known examples of primary sedimentary Mn deposition, related to oxidation of dissolved Mn(II) by free oxygen in a shallow marine environment. Oxygenic photosynthesis would have acted as a first-order control during Mn precipitation. This hypothesis opens a new perspective for better constraining secular evolution of sediment-hosted mineral deposits linked to oxygen levels in the atmosphere-hydrosphere system during the Archean Eon.

Hints of a Higher Than Expected Oxidation From During MesoArchean

Uranium irradiation history of carbonado diamond; implications for Paleoarchean oxidation in the São Francisco craton

Authors:

Magee et al

Abstract:

Carbonado is a porous polycrystalline diamond rock found in central African and Brazilian placer deposits. It contains unsupported radiogenic isotopes of He, Ne, Kr, Xe, and Pb. Here we show that these, and the radiation-related defects introduced to the diamond structure, are a result of uranium precipitation, with no isotopic or spectroscopic evidence of Th enrichment. The daughter products are unsupported due to Proterozoic U remobilization. Combining existing carbonado Pb isotope data with recent studies of the geochronology of the tectonic evolution of the São Francisco craton (eastern South America) reveals that the most likely scenario is Paleoarchean uranium enrichment of carbonado, followed by Mesoproterozoic uranium dissolution. Under all possible scenarios, the carbonado radiation damage history requires U mobilization in the Mesoarchean or late Paleoarchean. This is consistent with recent studies of South Africa and India Mesoarchean paleosols, which also show evidence for local oxygen activity greater than that of the Archean atmosphere and ocean. While those studies rely on whole-rock trace element and transition metal stable isotope measurements, this combination of crystallographic defects, sedimentary geochronology, and radiogenic isotopes supports the same conclusions of nonmarine, near-surface Archean oxygen enhancement.

Evidence of the First Continental Kernels From Archean China

U-Pb age and Hf isotopes of detrital zircons from the Southeastern North China Craton: Meso- to Neoarchean episodic crustal growth in a shifting tectonic regime

Auhtors:

Liu et al

Abstract:

The North China Craton (NCC) represents one of only a few cratonic nuclei on the globe with a geological history extending back to the Eoarchean. However, extensive ca. 2.5 Ga crustal reworking has destroyed a considerable portion of the pre-existing crustal record, hindering the investigation of tectonothermal evolution prior to 2.5 Ga. The Huoqiu Complex (HQC), located at the southeastern margin of the NCC, preserves the vestiges of crustal components that survived the ca. 2.5 Ga tectonothermal events, which provide the opportunity to investigate the Meso- to Neoarchean episodic crustal evolution of the NCC. Here we present results from in-situ detrital zircon U-Pb dating and Hf isotope analyses on zircons from three paragneisses in three drill cores that cut through the basement of the HQC. In combination with published data, the concordant age spectra of the detrital zircons in the paragneisses yield 207Pb/206Pb ages of 2343-3997 Ma that cluster into two principal age populations with peaks at 3015 and 2755 Ma. One zircon grain dated at 3997 ± 8 Ma with 98% concordance provides new evidence for 4.0 Ga components in the NCC. The εHf(t) values of all zircons range from -5.2 to + 6.5, with most of the spots (n = 31 of 47) showing positive values, indicating at least two episodes of juvenile continental crustal growth at 3.01 Ga and 2.75 Ga. The older episode is recorded only in few ancient cratons, suggesting limited crustal accretion occurred globally at a time of subdued mantle-derived magmatism. In contrast, the younger episode is coincident with a global rise in magmatic activity in the early Neoarchean. The geochemical and geochronological data suggest that the 3.01 Ga juvenile crust was likely generated in an island-arc subduction system, whereas the 2.75 Ga crustal rocks were probably formed during magmatic underplating and subsequent partial melting of lower crustal mafic rocks. Consequently, a tectonic transition is suggested from a compressive to an extensional setting along the southeastern margin of the NCC between 3.01 and 2.75 Ga. This sequence of events heralds a shift, from a mixture of net crustal growth and crustal reworking during multiple short-lived magmatic pulses, to fragmentation and dispersal of the early continental nucleus within 260 Ma.

Evidence of Successions of Different Microbial Biota From MesoArchean South Africa

Sedimentology and facies analysis of Mesoarchaean stromatolitic carbonate rocks of the Pongola Supergroup, South Africa

Authors:

Shishi et al

Abstract:

The c. 3.0 Ga old Chobeni Formation (Nsuze Group, Pongola Supergroup) contains several carbonate successions interpreted to have been deposited in a tide-dominated, shallow-marine environment. Facies consist of mixed siliciclastic and carbonate rocks and include subtidal cross-bedded sandstone and oncolitic dolostone, intertidal wave-rippled oodolarenite, and supratidal wrinkly-laminated dolarenite–dololutite. Sedimentary facies are partially arranged in a shallowing-upward pattern reflecting cyclic variations of Mesoarchaean sea level. Microbialites occur in a variety of sedimentary sub-environments and include laminated as well as structureless, lenticular-domal types. Differences in microbialite morphology, macrofabric, and size suggest potentially distinct assemblages of microorganisms responding to different physicochemical and environmental conditions. Microbialite textures are consistent with in situ carbonate precipitation and to a lesser extent by trapping and binding of sediment. Despite their great age, the Pongola carbonates are compositionally and texturally very similar to much younger carbonate successions preserved in the geological record.

Archaean Volcanic Sediment From MesoArchean to NeoArchean in Finland

U-Pb geochronology of Archaean volcanic-sedimentary sequences in the Kuhmo greenstone belt, Karelia Province–multiphase volcanism from Meso- to Neoarchaean and an Archaean depositional basin?

Authors:

Lehtonen et al

Abstract:

The Kuhmo greenstone belt in eastern Finland is one of the most studied areas of the Karelia Province. A single-grain zircon U-Pb study of andesitic, trachyandesitic and dacitic volcanic rocks from the belt demonstrates multiple widely spaced ages of the rocks. The results show that the volcanic activity took place as two major episodes at ca. 2847–2836 Ma and ca. 2799–2792 Ma, both containing komatiitic members as well as volcanic-sedimentary units. Based on the geochronological results combined with previously published data, we propose a chronostratigraphic interpretation for the Kuhmo greenstone belt, dividing it into three units: the Nuolikangas and Siivikkovaara volcanic units, and the sedimentary Ronkaperä unit. The results support the previous interpretation that age period of 2.80–2.79 Ga was an important crustal forming episode in the Kuhmo area. The zircon populations in the uppermost sedimentary rocks contain greater than 3.0 Ga zircon grains that are older than the volcanic rocks found in the greenstone belt. The youngest detrital zircon populations, ca. 2.73 Ga and 2.70 Ga in a conglomerate and quartz sandstone, respectively, imply that they were deposited at least ca. 60–90 Ma after the last volcanic phase. Based on the detrital zircon record, it can be interpreted that during the deposition of the detritus of the sedimentary rocks belonging to the Ronkaperä unit, the Karelia subprovinces were likely juxtaposed together and acted as a source provenance for the detritus.

First Evidence of MesoArchean Continental Crust?

Evolution history of the Neoproterozoic eclogite-bearing complexof the Muya dome (Central Asian Orogenic Belt): constraints from zircon U-Pb age, Hf and whole-rock Nd isotopes

Authors:

Shatsky et al

Abstract:

U-Pb dating and Hf-isotope analysis of zircons and whole-rock Nd-isotope analyses were carried out on country rocks of the eclogite-gneiss complex of the North Muya dome in the Anamakit-Muya zone of the Baikal Muya accretionary fold belt. Zircons from garnet-biotite gneisses (Qtz + Kfsp + Pl + Bt + Grt) and garnet-biotite-muscovite schist (Pl + Kfsp + Bt + Mu + Grt + Qtz) were dated using the LA-ICP-MS technique. Based on U-Pb isotope data and CL images zircon grains were divided into three groups: detrital, magmatic and metamorphic zircons. Metamorphic zircons display no zoning or the cloudy zoning. The grains morphology together with the well-developed oscillatory zoning clearly identifies the igneous origin of magmatic zircons. The metamorphic zircons (ages 576-680 Ma) have Th/U ratios varying from 0.271 to 0.004, whereas the ratio in magmatic zircons ranges from 0.779 to 0.11. Magmatic zircons from granite-gneisses of the North Muya dome exhibit a relatively narrow spread in the crystallization age with the major peak at ca 764 Ma. Younger ages are interpreted as due to the partial resetting of U-Pb system during the subsequent metamorphic evolution. Detrital zircons from two-mica schist sample Mu-93-10 give ages of 1.88-2.66 Ga. The oldest detrital zircon from this sample plots near concordia and has a Pb207/206Pb age of 3.2 Ga. Zircons from this sample are characterized by the widest scatter of ɛHf(t) values (from +13.9 to -15.3) and View the MathML sourceTDMC model ages (0.82-3.86 Ga). Zircons from other samples have a much narrower ranges of ɛHf(t) (+11.6 to -0.7) and View the MathML sourceTDMC (0.85-1.52 Ga). The involvement of older crustal material is also evident from the whole-rock Nd isotopic compositions. The gneisses and schists exhibit a range of Nd isotopic compositions with ɛNd(t) values ranging from -3.5 to +3.6 and tNd(DM) from 1.64 to 1.09 Ga. The integration of the Hf-isotope data with the age spectra provides with the first evidence for the existence of Mesoarchean crust in the Baykal-Muya sector of the Central Asian Orogenic Belt.

When Did Plate Tectonics Start?

The mystery of what kick-started the motion of our earth's massive tectonic plates across its surface has been explained by researchers at the University of Sydney.

"Earth is the only planet in our solar system where the process of plate tectonics occurs," said Professor Patrice Rey, from the University of Sydney's School of Geosciences.

"The geological record suggests that until three billion years ago the earth's crust was immobile so what sparked this unique phenomenon has fascinated geoscientists for decades. We suggest it was triggered by the spreading of early continents then eventually became a self-sustaining process."

link.

Was There a Great Oxidation Event (or two) During the MesoArchean?

Oxygenation of the Archean atmosphere: New paleosol constraints from eastern India

Authors:

Mukhopadhyay et al

Abstract:

It is widely believed that atmospheric oxygen saturation rose from less than 10–5 present atmospheric level (PAL) in the Archean to greater than 10–2 PAL at the Great Oxidation Event (GOE) at ca. 2.4 Ga, but it is unclear if any earlier oxygenation events occurred. Here we report U-Pb zircon data indicating that a pyrophyllite-bearing paleosol, from Keonjhar in the Precambrian Singhbhum Craton of eastern India, formed between 3.29 and 3.02 Ga, making it one of very few known Archean paleosols globally. Field and geochemical evidence suggests that the upper part of the paleosol was eroded prior to unconformable deposition of an overlying sequence of shallow-marine siliciclastic sediments. A negative cerium anomaly within the currently preserved level of the paleosol indicates that ancient oxidative weathering occurred in the original upper soil profile. The presence of redox-sensitive detrital uraninite and pyrite together with a complete absence of pyrophyllite in the overlying sediments indicate that the mineralogical and geochemical features of the paleosol were established prior to the unconformable deposition of the sediments and are not related to subsequent diagenetic or hydrothermal effects. We suggest that a transient atmospheric oxygenation event occurred at least 600 m.y. prior to the GOE and ∼60 m.y. prior to a previously documented Archean oxygenation event. We propose that several pulsed and short-lived oxygenation events are likely to have occurred prior to the GOE, and that these changes to atmospheric composition arose due to the presence of organisms capable of oxygenic photosynthesis.

Indo-Madagascar Connectivity in the PreCambrian From the Western Dharwar Craton

Tectonic restoration of the Precambrian crystalline rocks along the west coast of India: Correlation with eastern Madagascar in East Gondwana

Authors:

Rekha et al

Abstract:

New structural–mineralogical data and U–Th-total Pb monazite chemical ages in 27 samples in a 430 km long corridor along the west coast of India are combined with existing data to reconstruct the tectonic set up of the Meso/Neoarchean crystalline rocks in the Western Dharwar Craton (WDC). The data helps to delineate two NW-trending Paleoproterozoic ductile shear zones that limit the southern and the northern margins of the WDC. The southern shear zone (metamorphic age: 2.3–2.4 Ga) separates the greenschist facies supracrustal belts (2.5 and 3.3 Ga), foliated granitoids (2.5 and 2.9 Ga) and amphibolite facies anatectic gneisses ( greater than 3.0 Ga) of the WDC from the greater than 2.9 Ga granulite facies ortho/para-gneisses of the Coorg Block. This shear zone is correlated with the ∼2.4 Ga Betsimisaraka suture zone in east-central Madagascar that demarcates the accretion zone between the Antongil Block (≈WDC) and the granulite facies lithologies of the Antananarivo domain (≈Coorg Block). The northern shear zone system (metamorphic age: 2.2–1.8 Ga) extending NW into Madagascar possibly exists as a hitherto undiscovered tectonic zone forming the basement of the Mesoproterozoic Sahantaha Formation underlying the Neoproterozoic Bemarivo Belt supracrustals in NE Madagascar.

Within the WDC, the Meso/Neoproterozoic ages retrieved from poorly-defined margins in monazite are uncommon, dispersed within the craton, and do not define localized zones within the craton. The chemical ages of metamorphic monazites formed at greenschist/amphibolite facies conditions preclude metamorphism–deformation associated with accretion of crustal blocks within the WDC during the Rodinia assembly.

Subduction-Driven Plate Tectonics MAY Date From MesoArchean

Petrogenesis of Neoarchean TTG rocks in the Yangtze Craton and its implication for the formation of Archean TTGs

Authors:

Wu et al

Abstract:

The Yangtze Craton is one of the most important cratonic blocks in eastern Asia. However, its early evolution has not been well constrained yet, because of the poor exposure of Archean basement rocks. In this contribution, a combined study of zircon U-Pb age, Hf-O isotope compositions, as well as whole rock element compositions was carried out for gray gneiss rocks from the Douling complex in the northwestern part of the Yangtze Craton. Zircon U-Pb dating for two gneiss samples yielded weighted mean ages of 2509 ± 21 and 2496 ± 15 Ma. The gray gneisses have high SiO2 (63.64-73.74%), Na2O (3.66-6.21%), but low K2O (0.76-2.93%) contents, and belong to the trondhjemite, tonalite and granodiorite series rocks. They are characterized by enrichments of LILEs and LREEs, but notable depletions in Nb–Ta and Ti, as well as heavy REEs and Y. All the samples plot into the fields of typical Archean TTG rocks in the (La/Yb)N vs. (Yb) N and Sr/Y vs. Y diagrams. Therefore, the gray gneisses in the Douling complex are Neoarchean TTG rocks. The relatively high Nb/Ta and Zr/Sm ratios of 19.6-46.0 and 14.5-143.4 suggest their formation under eclogite-facies conditions. They have low ɛHf(t) values of -3.21 and -3.39 and Hf model ages of 3194 ± 26 and 3205 ± 24 Ma, indicating their derivation from ca. 3.2 Ga juvenile mafic rocks. Their δ18O values of 6.41 ± 0.12 and 5.44 ± 0.12 ‰ suggest that surface rocks were differently recycled into their sources. Collectively, the TTG rocks in the Douling complex were generated by partial melting of thickened mafic crust rather than direct partial melting of oceanic crust in a convergent plate boundary. A compilation of available zircon U-Pb and Hf isotope compositions of the Yangtze Craton reveals that there are quite differences of Hf isotope compositions in Archean. The ca. 3.8-3.4 Ga zircons exhibit ɛHf(t) values of -5.8 to 0.0. These chondritic to slightly enriched hafnium isotope compositions imply that their host rocks were formed by protracted intra-crustal reworking of ancient protoliths without juvenile input. By contrast, zircons with ages younger than ca. 3.2 Ga register both negative and positive ɛHf(t) values up to the depleted mantle, providing clear evidence for the addition of juvenile materials. These imply that the subduction-driven plate tectonics might have been operated since ca. 3.2 Ga in the Yangtze Craton.

Evidence of a MesoArchean Impact in Greenland?

The Finnefjeld domain, Maniitsoq structure, West Greenland: Differential rheological features and mechanical homogenisation in response to impacting?

Authors:

Garde et al

Abstract:

The 35 by 50 km large, Mesoarchaean Finnefjeld domain near Maniitsoq in the North Atlantic craton of southern West Greenland constitutes the central part of the previously proposed, deeply eroded Maniitsoq impact structure with an age of 3.0 Ga. The Finnefjeld domain is an exceedingly homogeneous, quartzo-feldspathic rock mass which superficially appears to be a late-orogenic, deep-crustal, intrusive granitoid pluton, and which was described as such for decades. However, new observations confirm and qualify the first observations from 1962 of these rocks as ‘cataclastic’. The Finnefjeld domain is characterised by a highly unusual, mixed rheological behaviour. Plagioclase displays brittle behaviour with cataclasis, quartz was ductilely deformed, and K-feldspar was melted. The deformation and homogenisation of the Finnefjeld domain was caused by an intense event of heating and deformation, which was coseismic in nature and comprised numerous increments of pure shear strain. This type of intense, brittle, regional deformation and concomitant direct mineral melting in the deep crust is unknown from endogenic orogenic events and is ascribed to deep-crustal effects of impacting.

MesoArchean Evidence of the Amalgamation of the North American Superior Province

Structural and metamorphic evidence for Mesoarchaean subduction in the Finlayson Lake greenstone belt, Superior Province, Ontario

Authors:

Backeberg et al

Abstract:

The unique structural architecture of Archaean terranes has generated competing models for early earth tectonics. Understanding the structural and metamorphic history of an individual terrane allows us to compare the deformation path to that predicted by tectonic models, and determine the best model for matching field observations. The Finlayson Lake greenstone belt is a Mesoarchaean terrane lying between three different gneiss terranes in the south-central Wabigoon subprovince in Canada. The belt has been interpreted as either a synformal keel sagducted between rising gneiss diapirs or as three fault-bounded allochtonous sub-belts of different ages. We present a detailed structural field study to define the deformation history of the Finlayson Lake greenstone belt and show that it is not consistent with either previous hypothesis. The Finlayson Lake greenstone belt is a single volcanic package that incorporates detritus from exposed felsic terranes similar in age and composition to the adjacent 3.0 Ga Marmion tonalites. The geometry of the greenstone belt is defined by two outward-facing palaeo way-up orientations (anticlinal) and geothermobarometry records a clockwise metamorphic path with a deep prograde event at 820 ± 40 MPa (27-30 km) and 600 ± 45°C, followed by peak metamorphism at 635 ± 165 MPa (21-23 km) and 625 ± 25°C. The deformation history records sinistral transpression during peak metamorphism and continued flattening during retrogression and exhumation from ductile to brittle regimes. The structural and metamorphic results are comparable with modern subduction-accretion style settings. The intensity of both retrogressive and brittle deformation fabrics during exhumation decrease from east to west away from the eastern boundary shear zone, the Marmion Shear Zone. Stronger deformation along the eastern margin of the Finlayson Lake greenstone belt, adjacent to the Marmion Shear Zone, suggest reactivation during exhumation and is likely related to the 2.7 Ga amalgamation of the Superior Province.

Evidence of Plate Tectonics in the MesoArchean From India

Preserved and modified mid-Archean crustal blocks in Dharwar craton: Seismological evidence

Authors:

Bora et al

Abstract:

We report significant lateral variability in shear wave velocity and Moho depth in the Archean crust, beneath the Dharwar craton, using earthquake waveform data recorded over 50 broadband seismographs. The craton is a continuously exposed Archean continental fragment divided into the west Dharwar craton (WDC) of age 2.7-3.36 Ga, and the east Dharwar craton (EDC) of age, dominantly, 2.5 Ga. The craton progressively transitions into the Southern Granulite Terrain (SGT) with age of metamorphism around 2.6 Ga.

The inversion and modeling of receiver function data reveal significant variation of Moho depth, viz., 38-54 km in the WDC, and 40-46 km in the SGT and 32-38 km in the EDC. The average shear wave velocity of crust beneath the WDC is ∼3.85 km/s as compared to ∼3.6 km/s in the EDC. We infer highly variable thickness (16-30 km) of mafic cumulate (Vs ≥4.0 km/s and Vp ≥7.0 km/s) beneath the WDC, in contrast with a thin one (less than 5 km) beneath the late Archean EDC. The 3.36 Ga greenstone belt in the WDC has maximum basal layer thickness of ∼30 km. These results suggest the intermediate-mafic composition and exceptional thickness of crust beneath the WDC (greater than 50km) as compared to felsic to intermediate composition for the EDC crust with almost flat Moho (down to ∼38km). These results suggest preserved mafic crustal root beneath the middle Archean terrain in the WDC that has remained inert since then. On the other hand, felsic to intermediate composition of crust with a nearly-flat Moho beneath the late Archean EDC could be a consequence of regional delamination of lower crust. The preserved distinct Moho topography across the Archean terrains suggests it as compositional boundary. Considering the surface exposure of 15-20km crust, based on P-T condition, in the granulite segment of the WDC, we speculate a Himalaya-like crustal thickness (50-70km) beneath the middle Archean crust pointing towards a plate tectonic-like scenario at ∼3.0 Ga

Evidence of the Kola Continent From the Meso-NeoArchean

The Meso-Neoarchaean Belomorian eclogite province: Tectonic position and geodynamic evolution

Authors:

Mints et al

Abstract:

The aim of this paper is to review the main features of the Meso-Neoarchaean Belomorian eclogite province (BEP) in the northeastern Fennoscandian Shield, including regional and local geology, geochemistry, petrology and geochronology and to compare the Belomorian eclogites with Precambrian eclogites elsewhere. Two eclogite associations have been recognized within Belomorian TTG gneisses: (1) the subduction-type Salma association and (2) Gridino eclogitized mafic dykes. Protoliths of the Salma eclogites represent a sequence comprising gabbro, Fe–Ti gabbro and troctolites, formed at ~ 2.9 Ga in a slow-spreading ridge setting (like the Southwest Indian Ridge). The main subduction and eclogite-facies events occurred between ~ 2.87 and ~ 2.82 Ga. Injection of mafic magma into an active continental margin setting, recorded by the Gridino dyke swarm, is attributed to subduction of a mid-ocean ridge, commencing at 2.87 Ga. Crustal delamination of the active margin and subsequent involvement of the lower crust in subduction between 2.87 and 2.82 Ga ago caused high-pressure metamorphism of the Gridino dykes, culminating in eclogite-facies conditions between 2.82 and 2.78 Ga and accompanying amalgamation of the Karelia, Kola and Khetolamba blocks and formation of the Mesoarchaean Belomorian accretionary–collisional orogen. The clockwise P–T paths of the Salma and Gridino associations cross the granulite-facies P–T field. Detailed metamorphic studies indicate a complicated post-eclogite history with thermal events and fluid infiltration, related to plume activity at 2.72–2.70, ~ 2.4 and ~ 1.9 Ga. The eclogite assemblages were exhumed to mid-to-lower crustal depths at ~ 1.7 Ga, while erosion or younger tectonic events were responsible for final exhumation to the surface. Comparison of P–T–t paths and data for peak metamorphic parameters demonstrates the general similarity of the Archaean and Palaeoproterozoic eclogites worldwide and their association with anomalously “hot” environments. The occurrence of high-T conditions during eclogite-facies metamorphism can be attributed to either subduction of a mid-ocean ridge (Archaean, BEP) or to interaction with mantle plumes (Proterozoic).

Evidence of a Microcontinent and Modern Plate Tectonics in MesoArchean India

An exotic Mesoarchean microcontinent: The Coorg Block, southern India

Authors:

M. Santosh, Qiong-Yan Yang, E. Shaji, T. Tsunogae, M. Ram Mohan and M. Satyanarayanan

Abstract:

Sandwiched between the Dharwar Craton in the north and the Neoarchean – Proterozoic crustal blocks to the south, the Coorg Block in southern India is composed dominantly of a suite of arc magmatic rocks including charnockites, TTG (tonalite-trondhjemite-granodiorite)-related granitoid suite and felsic volcanic tuffs together with minor accreted oceanic remnants along the periphery of the block. Coeval mafic and felsic magmatism with magma mixing and mingling in an arc setting is well represented in the block. Here we present the petrology, geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes of all the major lithologies from this block. Computation of metamorphic P-T conditions from mineral chemical data shows consistent granulite-facies P-T conditions of 820-870 °C and up to 6 kbar. Our geochemical data from major, trace and REE on representative samples of the dominant rock types from the Coorg Block corroborate an arc-related signature, with magma generation in a convergent margin setting. The zircon data yield weighted mean 207Pb/206Pb ages of 3153.4 ± 9 to 3184.0 ± 5.5 Ma for syenogranites, 3170.3 ± 6.8 Ma for biotite granite, 3275 ± 5.1 Ma for trondhjemite, 3133 ± 12 to 3163.8 ± 6.9 Ma for charnockites, 3156 ± 10 to 3158.3 ± 8.2 for mafic enclaves, 3161 ± 16 Ma for diorite and 3173 ± 16 Ma for felsic volcanic tuff. An upper intercept age of 3363 ± 59 Ma and a lower intercept age of 2896 ± 130 Ma on zircons from a charnockite, as well as an evaluation of the Th/U values of the zircon domains against respective 207Pb/206Pb ages suggest that the Mesoarchean magma emplacement which probably ranged from greater than 3.3 to 3.1 Ga was immediately followed by metamorphism at ca. 3.0 to 2.9 Ga. The ages of magmatic zircons from the charnockites and their mafic granulite enclaves, as well as those from the volcanic tuff and biotite granite, are all remarkably consistent and concordant marking ca. 3.1 Ga as the peak of subduction-related crust building in this block, within the tectonic milieu of an active convergent margin. The majority of zircons from the Coorg rocks show Hf isotope features typical of crystallization from magmas derived from juvenile sources. Their Hf crustal model ages suggest that the crust building might have also involved partial recycling of basement rocks as old as ca. 3.8 Ga. The crustal blocks in the Southern Granulite Terrane in India preserve strong imprints of major tectonothermal events at 2.5 Ga, 2.0 Ga, 0.8 Ga and 0.55 Ga associated with various subduction-accretion-collision or rifting events. However, the Coorg Block is exceptional with our data suggesting that none of the above events affected this block. Importantly, there is also no record in the Coorg Block for the 2.5 Ga pervasive regional metamorphism that affected all the other blocks in this region. The geochronological data raise the intriguing possibility that this block is an exotic entity within the dominantly Neoarchean collage in the northern domain of the Southern Granulite Terrane of India. The Mesoarchean arc-related rocks in the Coorg Block suggest that the magma factories and their tectonic architecture in the Early Earth were not markedly different from those associated with the modern-style plate tectonics.