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Geology of the Early Earth – Geodynamic Constraints from Cratons

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A special issue of Geosciences (ISSN 2076-3263). This special issue belongs to the section "Geophysics".

Deadline for manuscript submissions: closed (30 June 2018) | Viewed by 46048

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Dr. Kristoffer Szilas

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Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark
Interests: geology of greenland; archaean tectonics; ultramafic intrusions; mineral deposits
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Special Issue Information

Dear Colleagues,

This Special Issue of Geosciences will focus on the early Earth, and specifically on contributions that can improve our understanding of the geodynamic processes that operated during the Archaean Eon (4000 to 2500 Ma).

Currently, it is debated when and how modern-style plate tectonics was initiated, as well as by what mechanism the cratons formed. Thus, we are interested in a wide range of papers that address the above problems from several different perspectives, and by employing various methods ranging from geophysical, numerical, experimental, petrological, and geochemical studies. We also welcome field-based studies that can provide first-order constraints, upon which our models should always be based.

I therefore encourage you all to submit your most recent work which can contribute to new insights about the early Earth.

Dr. Kristoffer Szilas
Guest Editor

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Keywords

Archaean
Cratons
Geodynamics
Plate tectonics
Mantle plumes
Greenstone belts
Supracrustal belts
Petrology
Geochemistry

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Published Papers (9 papers)

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Research

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20 pages, 6257 KiB

Open AccessArticle

Titanian Clinohumite-Bearing Peridotite from the Ulamertoq Ultramafic Body in the 3.0 Ga Akia Terrane of Southern West Greenland

by Ikuya Nishio, Tomoaki Morishita, Kristofer Szilas, Graham Pearson, Ken-Ichiro Tani, Akihiro Tamura, Yumiko Harigane and Juan Miguel Guotana

Geosciences 2019, 9(4), 153; https://doi.org/10.3390/geosciences9040153 - 1 Apr 2019

Cited by 12 | Viewed by 5052

Abstract

A titanian clinohumite-bearing dunite was recently found in the Ulamertoq ultramafic body within the 3.0 Ga Akia Terrane of southern West Greenland. Titanian clinohumite occurs as disseminated and discrete grains. Titanian clinohumite contains relatively high amounts of fluorine, reaching up to 2.4 wt.%. The high-Fo content of olivine (Fo₉₃) coupled with low Cr/(Cr + Al) ratio of orthopyroxene implies that the dunite host is not of residual origin after melt extraction by partial melting of the primitive mantle. Olivine grains are classified into two types based on abundances of opaque mineral inclusions: (1) dusty inclusion-rich and (2) clear inclusion-free olivines. Opaque inclusions in coarse-grained olivines are mainly magnetite. Small amounts of ilmenite are also present around titanian clinohumite grains. The observed mineral association indicates partial replacement of titanian clinohumite to ilmenite (+magnetite) and olivine following the reaction: titanian clinohumite = ilmenite + olivine + hydrous fluid. The coexistence of F-bearing titanian clinohumite, olivine, and chromian chlorite indicates equilibration at around 800–900 °C under garnet-free conditions (<2 GPa). Petrological and mineralogical characteristics of the studied titanian clinohumite-bearing dunite are comparable to deserpentinized peridotites derived from former serpentinites. This study demonstrates the importance of considering the effects of hydration/dehydration processes for the origin of ultramafic bodies found in polymetamorphic Archaean terranes. Full article

(This article belongs to the Special Issue Geology of the Early Earth – Geodynamic Constraints from Cratons)

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21 pages, 11434 KiB

Open AccessArticle

Pressure–Temperature History of the >3 Ga Tartoq Greenstone Belt in Southwest Greenland and Its Implications for Archaean Tectonics

by Vincent Van Hinsberg, Catherine Crotty, Stan Roozen, Kristoffer Szilas and Alexander Kisters

Geosciences 2018, 8(10), 367; https://doi.org/10.3390/geosciences8100367 - 30 Sep 2018

Cited by 8 | Viewed by 4692

Abstract

The Tartoq greenstone belt of southwest Greenland represents a well-preserved section through >3 Ga old oceanic crust and has the potential to provide important constraints on the composition and geodynamics of the Archaean crust. Based on a detailed structural examination, it has been proposed that the belt records an early style of horizontal convergent plate tectonics where elevated temperatures, compared to the modern-day, led to repeated aborted subduction and tonalite–trondhjemite–granodiorite (TTG) type melt formation. This interpretation hinges on pressure–temperature (P–T) constraints for the belt, for which only preliminary estimates are currently available. Here, we present a detailed study of the pressure–temperature conditions and metamorphic histories for rocks from all fragments of the Tartoq belt using pseudosection modelling and geothermobarometry. We show that peak conditions are predominantly amphibolite facies, but range from 450 to 800 °C at up to 7.5 kbar; reaching anatexis with formation of TTG-type partial melts in the Bikuben segment. Emplacement of the Tartoq segments into the host TTG gneisses took place at approximately 3 Ga at 450–500 °C and 4 kbar as constrained from actinolite–chlorite–epidote–titanite–quartz parageneses, and was followed by extensive hydrothermal retrogression related to formation of shear zone-hosted gold mineralisation. Tourmaline thermometry and retrograde assemblages in mafic and ultramafic lithologies constrain this event to 380 ± 50 °C at a pressure below 1 kbar. Our results show that the convergent tectonics recorded by the Tartoq belt took place at a P–T gradient markedly shallower than that of modern-day subduction, resulting in a hot, weak and buoyant slab unable to generate and transfer ‘slab pull’, nor sustain a single continuous downgoing slab. The Tartoq belt suggests that convergence was instead accomplished by under-stacking of slabs from repeated aborted subduction. The shallow P–T path combined with thermal relaxation following subduction stalling subsequently resulted in partial melting and formation of TTG melts. Full article

(This article belongs to the Special Issue Geology of the Early Earth – Geodynamic Constraints from Cratons)

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32 pages, 10521 KiB

Open AccessArticle

Assessing the Validity of Negative High Field Strength-Element Anomalies as a Proxy for Archaean Subduction: Evidence from the Ben Strome Complex, NW Scotland

by George L. Guice, Iain McDonald, Hannah S. R. Hughes, Denis M. Schlatter, Kathryn M. Goodenough, John M. MacDonald and John W. Faithfull

Geosciences 2018, 8(9), 338; https://doi.org/10.3390/geosciences8090338 - 8 Sep 2018

Cited by 23 | Viewed by 6670

Abstract

The relative depletion of high field strength elements (HFSE), such as Nb, Ta and Ti, on normalised trace-element plots is a geochemical proxy routinely used to fingerprint magmatic processes linked to Phanerozoic subduction. This proxy has increasingly been applied to ultramafic-mafic units in Archaean cratons, but as these assemblages have commonly been affected by high-grade metamorphism and hydrothermal alteration/metasomatism, the likelihood of element mobility is high relative to Phanerozoic examples. To assess the validity of HFSE anomalies as a reliable proxy for Archaean subduction, we here investigate their origin in ultramafic rocks from the Ben Strome Complex, which is a 7 km² ultramafic-mafic complex in the Lewisian Gneiss Complex of NW Scotland. Recently interpreted as a deformed layered intrusion, the Ben Strome Complex has been subject to multiple phases of high-grade metamorphism, including separate granulite- and amphibolite-facies deformation events. Additional to bulk-rock geochemistry, we present detailed petrography, and major- and trace-element mineral chemistry for 35 ultramafic samples, of which 15 display negative HFSE anomalies. Our data indicate that the magnitude of HFSE anomalies in the Ben Strome Complex are correlated with light rare earth-element (LREE) enrichment likely generated during interaction with H₂O and CO₂-rich hydrothermal fluids associated with amphibolitisation, rather than primary magmatic (subduction-related) processes. Consequently, we consider bulk-rock HFSE anomalies alone to be an unreliable proxy for Archaean subduction in Archaean terranes that have experienced multiple phases of high-grade metamorphism, with a comprehensive assessment of element mobility and petrography a minimum requirement prior to assigning geodynamic interpretations to bulk-rock geochemical data. Full article

(This article belongs to the Special Issue Geology of the Early Earth – Geodynamic Constraints from Cratons)

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19 pages, 8371 KiB

Open AccessArticle

Contrasting Textural and Chemical Signatures of Chromitites in the Mesoarchaean Ulamertoq Peridotite Body, Southern West Greenland

by Juan Miguel Guotana, Tomoaki Morishita, Ryoko Yamaguchi, Ikuya Nishio, Akihiro Tamura, Kenichiro Tani, Yumiko Harigane, Kristoffer Szilas and D. Graham Pearson

Geosciences 2018, 8(9), 328; https://doi.org/10.3390/geosciences8090328 - 3 Sep 2018

Cited by 10 | Viewed by 5457

Abstract

Peridotites occur as lensoid bodies within the Mesoarchaean orthogneiss in the Akia terrane of Southern West Greenland. The Ulamertoq peridotite body is the largest of these peridotites hosted within the regional orthogneiss. It consists mainly of olivine, orthopyroxene, and amphibole-rich ultramafic rocks exhibiting metamorphic textural and chemical features. Chromitite layers from different localities in Ulamertoq show contrasting characteristics. In one locality, zoned chromites are hosted in orthopyroxene-amphibole peridotites. Compositional zonation in chromites is evident with decreasing Cr and Fe content from core to rim, while Al and Mg increase. Homogeneous chromites from another locality are fairly uniform and Fe-rich. The mineral chemistry of the major and accessory phases shows metamorphic signatures. Inferred temperature conditions suggest that the zoned chromites, homogeneous chromites, and their hosts are equilibrated at different metamorphic conditions. In this paper, various mechanisms during the cumulus to subsolidus stages are explored in order to understand the origin of the two contrasting types of chromites. Full article

(This article belongs to the Special Issue Geology of the Early Earth – Geodynamic Constraints from Cratons)

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16 pages, 4433 KiB

Open AccessArticle

The ca. 2785–2805 Ma High Temperature Ilivertalik Intrusive Complex of Southern West Greenland

by Tomas Næraa, Thomas F. Kokfelt, Anders Scherstén and Andreas Petersson

Geosciences 2018, 8(9), 319; https://doi.org/10.3390/geosciences8090319 - 24 Aug 2018

Cited by 2 | Viewed by 2966

Abstract

Ferroan granitoid intrusions are rare in the Archaean rock record, but have played a large role in the evolution of the Proterozoic crust, particular in relation to anorthosite-mangerite-charnockite-granite suites. Here we discuss the petrogenesis of the ca. 2785–2805 Ma ferroan Ilivertalik Intrusive Complex, which has many geochemical similarities to Proterozoic iron rich granitoids. We present major and trace element whole rock chemistry and combined in-situ zircon U-Pb, Hf and O isotope data. The intrusive complex divides into: (i) minor tabular units of mainly diorite-tonalite compositions, which are typically situated along contacts to the host basement and (ii) interior larger, bodies of mainly granite-granodiorite composition. Geochemically these two unites display continuous to semi-continuous trends in Haker-diagrams. Whole rock REE enrichment display increases from Yb to La, from 10–25 to 80–100 times chondrite, respectively. The diorite-tonalite samples are generally more enriched in REE compared to the granite-granodiorite samples. The complex has hafnium isotope compositions from around +1.5 to −2.5 epsilon units and δ¹⁸O compositions in the range of 6.3 to 6.6‰. The complex is interpreted to be derived from partial melting in a crustal source region during anomalously high crustal temperatures. Full article

(This article belongs to the Special Issue Geology of the Early Earth – Geodynamic Constraints from Cratons)

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17 pages, 3809 KiB

Open AccessFeature PaperArticle

Boninites in the ~3.3 Ga Holenarsipur Greenstone Belt, Western Dharwar Craton, India

by Tarun C. Khanna, V. V. Sesha Sai, S. H. Jaffri, A. Keshav Krishna and M. M. Korakoppa

Geosciences 2018, 8(7), 248; https://doi.org/10.3390/geosciences8070248 - 5 Jul 2018

Cited by 5 | Viewed by 5124

Abstract

In this contribution, we present detailed field, petrography, mineral chemistry, and geochemistry of newly identified high-Si high-Mg metavolcanic rocks from the southern part of the ~3.3 Ga Holenarsipur greenstone belt in the western Dharwar craton, India. The rocks occur as conformable bands that were interleaved with the mafic-ultramafic units. The entire volcanic package exhibits uniform foliation pattern, and metamorphosed under greenschist to low grade amphibolite facies conditions. The rocks are extremely fine grained and exhibit relict primary igneous textures. They are composed of orthopyroxene and clinopyroxene phenocrysts with serpentine, talc, and amphibole (altered clinopyroxene). Cr-spinel, rutile, ilmenite, and apatite occur as disseminated minute grains in the groundmass. The mineralogical composition and the geochemical signatures comprising of high SiO₂ (~53 wt. %), Mg# (~83), low TiO₂ (~0.18 wt. %), and higher than chondritic Al₂O₃/TiO₂ ratio (~26), reversely fractionated heavy rare earth elements (REE) (Gd_N/Yb_N ~ 0.8), resulting in concave-up patterns, and positive Zr anomaly, typically resembled with the Phanerozoic boninites. Depletion in the high field strength elements Nb, and Ti relative to Th and the REE in a primitive mantle normalized trace element variation diagram, cannot account for contamination by pre-existing Mesoarchean continental crust present in the study area. The trace element attributes instead suggest an intraoceanic subduction-related tectonic setting for the genesis of these rocks. Accordingly, the Holenarsipur high-Si high-Mg metavolcanic rocks have been identified as boninites. It importantly indicates that the geodynamic process involved in the generation of Archean boninites, was perhaps not significantly different from the widely recognized two-stage melt generation process that produced the Phanerozoic boninites, and hence provides compelling evidence for the onset of Phanerozoic type plate tectonic processes by at least ~3.3 Ga, in the Earth’s evolutionary history. Full article

(This article belongs to the Special Issue Geology of the Early Earth – Geodynamic Constraints from Cratons)

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Review

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12 pages, 1482 KiB

Open AccessReview

Highlights on Geochemical Changes in Archaean Granitoids and Their Implications for Early Earth Geodynamics

by Jaana Halla

Geosciences 2018, 8(9), 353; https://doi.org/10.3390/geosciences8090353 - 17 Sep 2018

Cited by 25 | Viewed by 4374

Abstract

The Archaean (4.0–2.5 Ga) continental crust is mainly composed of granitoids, whose geochemical characteristics are a function of their formation mechanisms and components, as well as physical conditions of their source. Therefore, revealing changes in Archaean geodynamic processes requires understanding of geochemical changes in Archaean granitoids. This paper compares key geochemical signatures in granitoid occurrences from the Eoarchaean to Neoarchaean Eras and aims to highlight changes or variations in their geochemical signatures. The study is performed by exploring and comparing geochemical and geochronological datasets of Archaean granitoids compiled from literature. The results show that two end-members of sodic TTGs (tonalite–trondhjemite–granodiorite) occur throughout the Archaean: low- and high-HREE (heavy rare earth elements) types. A profound change in granitoid geochemistry occurred between 3.0 and 2.5 Ga when multi-source high-K calc-alkaline granitoid batholiths emerged, possibly indicating the onset of modern-type plate tectonics. Full article

(This article belongs to the Special Issue Geology of the Early Earth – Geodynamic Constraints from Cratons)

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20 pages, 7213 KiB

Open AccessFeature PaperReview

A Geochemical Overview of Mid-Archaean Metavolcanic Rocks from Southwest Greenland

by Kristoffer Szilas

Geosciences 2018, 8(7), 266; https://doi.org/10.3390/geosciences8070266 - 19 Jul 2018

Cited by 13 | Viewed by 5587

Abstract

The present contribution reviews bulk-rock geochemical data for mid-Archaean (ca. 3075–2840 Ma) metavolcanic rocks from the North Atlantic Craton of southwest Greenland. The data set includes the most recent high quality major and trace element geochemical analyses for ten different supracrustal/greenstone belts in the region. When distilling the data set to only include the least altered metavolcanic rocks, by filtering out obviously altered samples, mafic/ultramafic cumulate rocks, late-stage intrusive sheets (dolerites) and migmatites, the remaining data (N = 427) reveal two fundamentally distinct geochemical suites. The contrasting trends that emerge from the filtered geochemical data set, which best represents the melt compositions for these mid-Archaean metavolcanic rocks are: (1) tholeiitic (mainly basaltic) versus (2) calc-alkaline (mainly andesitic). These two rock suites are effectively separated by their La/Sm ratios (below or above three, respectively). It is demonstrated by geochemical modelling that the two contrasting suites cannot be related by either fractional crystallization or crustal assimilation processes, despite occurring within the same metavolcanic sequences. The tholeiitic basaltic rocks were directly mantle-derived, whereas the petrogenesis of the calc-alkaline andesitic rocks involve a significant (>50%) felsic component. The felsic contribution in the calc-alkaline suite could either represent slab-melt metasomatism of their mantle source, mafic-felsic magma mixing, or very large degrees of partial melting of mafic lower crust. At face value, the occurrence of andesites, and the negative Nb-Ta-Ti-anomalies of both suites, is consistent with a subduction zone setting for the origin of these metavolcanic rocks. However, the latter geochemical feature is inherent to processes involving crustal partial melts, and therefore independent lines of evidence are needed to substantiate the hypothesis that plate tectonic processes were already operating by the mid-Archaean. Full article

(This article belongs to the Special Issue Geology of the Early Earth – Geodynamic Constraints from Cratons)

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15 pages, 1556 KiB

Open AccessReview

Hf-Nd Isotopes in Archean Marine Chemical Sediments: Implications for the Geodynamical History of Early Earth and Its Impact on Earliest Marine Habitats

by Sebastian Viehmann

Geosciences 2018, 8(7), 263; https://doi.org/10.3390/geosciences8070263 - 16 Jul 2018

Cited by 2 | Viewed by 5227

Abstract

The Hf-Nd isotope systems are coupled in magmatic systems, but incongruent Hf weathering (‘zircon effect’) of the continental crust leads to a decoupling of the Hf-Nd isotope systems in low-temperature environments during weathering and erosion processes. The Hf-Nd isotope record was recently dated back from the Cenozoic oceans until the Archean, showing that both isotope systems were already decoupled in seawater 2.7 Ga ago and potentially 3.4 Ga and 3.7 Ga ago. While there might have existed a hydrothermal pathway for Hf into Archean seawater, incongruent Hf weathering of more evolved, zircon-bearing uppermost continental crust that was emerged and available for subaerial weathering accounts for a significant decoupling of Hf-Nd isotopes in the dissolved (<0.2 µm) and suspended (>0.2 µm) fractions of Early Earth’s seawater. These findings contradict the consensus that uppermost Archean continental crust was (ultra)mafic in composition and predominantly submerged. Hence, Hf-Nd isotopes in Archean marine chemical sediments provide the unique potential for future research to trace the emergence of evolved continental crust, which in turn has major implications for the geodynamical evolution of Early Earth and the nutrient flux into the earliest marine habitats on Earth. Full article

(This article belongs to the Special Issue Geology of the Early Earth – Geodynamic Constraints from Cratons)

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