Archean Tectonics Research Articles

About this title - The Consummate Geoscientist: A Celebration of the Career of Maarten de Wit

Papers in this special publication mirror Maarten's career as a leading expert on Gondwana evolution and Archean tectonics. Maarten also contributed to our understanding of Archean life, African erosional history and South African shale-gas development, and was a vigorous supporter of the advancement of South African students from varied backgrounds.

Isotope evidence for Archean accordion-tectonics in the Superior Province

Archean cratons preserve the oldest continental crust, but their tectonic mode of formation and assembly remain key unknowns. The Superior Province is Earth’s largest Archean craton and comprises Eo-Neoarchean plutonic-gneiss terranes separated by Meso-Neoarchean granite-greenstone terranes and metasedimentary belts. Resolving whether the plutonic-gneiss terranes represent dismembered fragments of a once contiguous proto-craton or a series of unrelated accreted crustal fragments is critical to understanding the early evolution of the Superior Province and Archean tectonics. We present zircon U-Pb-Hf isotopic data from the Tannis and Cedar Lake TTG gneisses, which record the early crustal evolution of the Winnipeg River plutonic-gneiss terrane and are some of the oldest rocks in the western Superior Province. The gneisses yield a large range of zircon εHf(t) signatures (−6 to +4) at igneous formation (ca. 3.3–3.25 Ga) indicating coeval crustal growth and reworking of isotopically depleted and evolved sources. Crustal reworking of Eo-Paleoarchean sources is supported by sub-chondritic ca. 3.5–3.4 Ga zircon xenocrysts in the Cedar Lake gneiss. The early crustal evolution of the central Winnipeg River terrane is similar to the Hudson Bay and Minnesota River Valley terranes, on the northern and southern margins of the Superior Province, respectively. Correlation of the early history amongst the three plutonic-gneiss terranes supports a tectonic model in which a once coherent Eo-Paleoarchean proto-craton disaggregated into three fragments during formation of intervening granite-greenstone terranes in the Mesoarchean. These fragments reaggregated in the Neoarchean. The complex history of the Superior Province highlights that Archean cratons are not simple entities that formed in a single event, but may have experienced times of cratonic breakup and reassembly.

Vertical tectonics of a Neoarchean gneiss dome in the eastern North China Craton

Archean gneiss domes are a very common structure in old cratons, these are different from the structural patterns in the post‐Archean linear orogenic belts related to subduction and collision. There are two tectonic models in Archean tectonics: horizontal tectonism and vertical tectonism. The formation mechanisms of Archean gneiss domes are still debated. Here we present data from the Anziling gneiss dome, a typical gneiss dome structure from the eastern North China Craton, which was formed dominantly by vertical tectonism during Neoarchean times (2554–2452 Ma). The Anziling gneiss dome is located in the east of Shuangshanzi, East Heibei Province and consists of the Anziling gneiss (a diopside‐monzonitic gneiss) in the center outwards surrounded by the Jiguanshan gneiss (hornblende‐plagioclase gneiss) and the Niuxinshan gneiss (trondhjemitic gneiss), together showing an irregular elliptical shape with a NE‐ward extending tail on the surface. But in the cross‐section, it represents a deflective dome, both north‐western and south‐eastern limbs dip approximately to the west. The dome suffered inhomogeneous ductile shear deformation, much stronger with more displacement in the north‐western half (especially close to the margin) than that in the eastern half. The core area shows only a relatively weak deformation. Therefore, we suggest that the Anziling gneiss dome was formed by the TTG (trondhjemite, tonalite, granodiorite) pluton rising obliquely up towards the east by diapirism in the first stage, followed by sub‐horizontal emplacement towards the north and then to the north‐east associated with clockwise rotation in the second stage. The Anziling gneiss dome and its neighbouring Shuangshanzi shear zone were formed simultaneously, indicating that the Anziling gneiss dome was driven by both gravitational forces and magma diapirism.

The Malolotsha Klippe: Large‐Scale Subhorizontal Tectonics Along the Southern Margin of the Archean Barberton Greenstone Belt, Eswatini

AbstractWhether Archean tectonics were horizontally or vertically dominated is controversially discussed because arguments bear on the kinematics and thermal state of the Archean mantle and constrain the mode of formation of the earliest continental crust. Highly deformed strata of Archean greenstone belts figure prominently in this debate because they record long periods of time and multiple deformation phases. Among the best‐preserved greenstone belts counts the Barberton Greenstone Belt (BGB) of southern Africa. Geological mapping of part of the southern BGB in Eswatini (Swaziland), combined with U‐Pb zircon dating, shows that the region preserves a tightly re‐folded imbricate thrust stack in which metavolcanic and ‐volcaniclastic strata of the Onverwacht Group, deposited at 3.34–3.29 Ga, have been thrust on top of ca. 3.22 Ga siliciclastic strata of the Moodies Group. The structurally highest element, the Malolotsha Syncline, forms a tectonic klippe of substantial size and is >1,450 m thick. Forward modeling of a balanced cross section indicates that this thrust stack was part of a northwestward‐verging orogen along the southern margin of the BGB and records a minimum horizontal displacement of 33 km perpendicular to its present‐day faulted, ductily strained and multiply metamorphosed margin. Because conglomerate clasts indicate a significantly higher degree of prolate strain which extends further into the BGB than at its northern margin, late‐stage tectonic architecture of the BGB may be highly asymmetrical. Our study documents that the BGB, and perhaps other Archean greenstone belts, preserves a complex array of both vertically‐ and horizontally‐dominated deformation styles that have interfered with each other at small regional and short temporal scales.

Structural evolution and U-Pb geochronology of the metasedimentary Nemiscau subprovince, Canada: implications for Archean tectonics in the Superior Province

The Nemiscau subprovince is a metasedimentary rocks-dominated sequence of the Archean eastern Superior Province. It is bounded by the gneissic and tonalite-trondhjemite-granodiorite (TTG) rocks-dominated La Grande and Opatica subprovinces. The Nemiscau consists of variably migmatized metasedimentary rocks and felsic to intermediate gneisses and plutonic suites. Mafic-to-ultramafic metavolcanic rocks occur along its northern and southern boundaries. Previous structural and metamorphic studies suggested that it was the result of subduction-related, accretionary and collisional tectonics with adjacent plutonic terranes during the Kenorean orogeny. This study integrates various sets of structural, metamorphic, and U-Pb geochronological data suggesting a long-lasting tectonometamorphic evolution between ca. 2843 and 2598 Ma. Four tectonometamorphic events have been recognized. The first event (D1) is only locally preserved and occurred shortly after the deposition of the oldest volcanic sequences of the Nemiscau ( ca. 2756–2736 Ma) under amphibolite facies metamorphic conditions (M1). The kinematic analysis of shear zones bounding the supracrustal sequences of both the Nemiscau and La Grande subprovinces suggests their relative crustal sinking as compared to TTG of the La Grande and Opatica subprovinces, shortly after deposition (at ≤2724 and ≤2706 Ma, respectively). Sedimentation was followed by regional dip-slip-dominated (D2) deformation between ca. 2704 and 2671 Ma, coeval with extensive high-grade granulite facies metamorphism (M2) and anatexis in the Nemiscau subprovince from ca. 2697 to 2685 Ma, followed by exhumation between ca. 2677 and 2671 Ma. This D2 event was followed by regional-scale dextral strike-slip shearing (D3) from ca. 2658 until 2621 Ma at amphibolite facies metamorphism (M3). The youngest deformation event, D4, was accompanied by strain localization along brittle-to-ductile conjugated shear zones and waning crustal cooling from amphibolite to greenschist facies conditions at ca. 2598 Ma and younger. It is suggested that the Nemiscau, La Grande, and Opatica subprovinces represent a single composite terrane, and that their mutual boundaries do not correspond to “collisional sutures” between different crustal blocks or microcontinents. The Nemiscau subprovince is interpreted as a sedimentary sequence unconformably overlying a ca. 2760–2756 Ma (and older) basement made up of volcanic and crosscutting TTG. In terms of tectonic evolution, the overall structural architecture and isotopic ages of the Nemiscau, La Grande, and Opatica subprovinces support a tectonic model in which the vertical transfer of crustal material occurred during the early stages (D2) of regional deformation, which evolved into a predominant lateral crustal flow of the ductile crust during later stages (D3–D4). A non-uniformitarian tectonic model for the Archean more adequately accounts for synchronous vertical and horizontal tectonism as preserved in the Nemiscau basin.

The Abitibi-Opatica transition, Superior Province, Quebec, Canada: Structural analysis, 40Ar/39Ar thermochronology and implications for Archean tectonics

We present a structural and metamorphic study of the Abitibi greenstone belt (AGB) and Opatica Plutonic Belt (OPB) of the Archean Superior Province. The AGB-OPB contact is considered as an archetype example of Archean subduction, based on a LITHOPROBE seismic profile showing a North-dipping lithospheric-scale reflector interpreted as the vestige of subduction. Our mapping indicates that the AGB overlies the OPB, and that the AGB-OPB contact does not show evidence of significant shear deformation, as expected for a major upper plate-lower plate boundary. There is no metamorphic break at the AGB-OPB transition but rather a progressive increase of metamorphic grade toward the OPB. Therefore, we think that the OPB simply exposes the deepest part of a composite AGB-OPB sequence. 40Ar/39Ar ages suggest that the AGB and OPB rocks were exhumed from amphibolite-facies conditions at ∼ 2678 Ma and ∼ 2668 Ma, respectively, and then that both terranes share the same succession of thermal disturbances at c. 2650 Ma and 2600 Ma, the latter corresponding to shearing along the Nottaway River Shear Zone. We suggest that progressive cooling of both assemblages was accompanied by strain localisation along strike-slip shear zones and occurred when lateral flow of the lower crust became predominant over the convective overturn and vertical crustal material transfer. Comparison with adjacent areas suggests that regional metamorphism has been coeval over a large region, which is consistent with pervasive deformation and slow cooling as expected for vertical tectonic models and diapiric magmagenesis and ascent during the Archean.

Paleo- to Mesoarchean crustal growth in the Karwar block, southern India: Constraints on TTG genesis and Archean tectonics

In this study we present field relations, petrology, whole-rock major, trace and rare earth element geochemistry, zircon U-Pb ages, whole-rock Sr and Nd isotopes, and <i>in situ</i> zircon Hf and O isotopes from the Karwar block, western peninsular India. The rocks consist predominantly of tonalite-trondhjemite-granodiorite (TTG), granite and amphibolite. The felsic rocks are grouped into three: 1. TTG-I characterised by low K₂O, high Na₂O and Al₂O₃, low Sr/Y and La/Yb ratios, slightly enriched HREEs, negative Sr, Eu and Ti anomalies, a 3.2 Ga crystallisation age, and 3.60 Ga and 3.47 Ga inherited zircons; 2. TTG-II with lower SiO₂, higher Sr/Y and La/Yb ratios, stronger REE fractionation with no HREE enrichment, negative Nb and Ta anomalies, a 3.2 Ga crystallisation age, but no inheritance; 3. Granites with high SiO₂ and K₂O, low Na₂O and Al₂O₃, very low Sr/Y and La/Yb ratios, weak REE fractionation with enriched REEs, negative Sr, Eu and Ti anomalies and a 2.94 Ga crystallisation age. The TTG-I formed from a mantle source, but with a significant component of older crustal material, whereas the TTG-II originated mostly from a mantle-derived juvenile magma. The granite evolved from an enriched source containing a relatively large amount of older crustal material. The precursors of TTG-I and -II are similar to mid-ocean ridge basalts (MORB), whereas the granites are similar to volcanic arc/within-plate sources and the amphibolites are remnants of gabbros/basalts. An initial 3.6 Ga crust likely formed by the underplating of an accreted oceanic plateau-like or island arc-like crust. TTG-I was produced by subduction and slab melting at a moderate depth, induced melting of mafic lower crust and older upper crust at 3.2 Ga. TTG-II formed at 3.2 Ga by subduction and with a higher degree of slab melting at a greater depth than TTG-1, together with more effective mixing with mantle peridotite, followed by intrusion and induced melting of mafic lower crust. Basaltic magmatism at 3.0 Ga and subsequent metamorphism to amphibolite resulted in extensive and thicker crust. Assimilation and melting of TTG crust at a shallow depth during the emplacement of a mantle-derived magma produced the 2.94 Ga granites. The presence of inherited zircons, combined with whole-rock major and trace elements, Nd isotopes and <i>in situ</i> zircon Hf and O isotopes, indicates that older crustal material was incorporated into the source magma of TTG-I and that the Karwar block originally contained 3.60 to 3.47 Ga crust that was subsequently reworked during the Paleo- and Mesoarchean.

Geochemistry of Mafic Rocks from the Birimian Basement of Doropo (Northeast of C&amp;#244;te d’Ivoire): Petrogenetic and Geodynamic Implications

The northeastern region of Côte d’Ivoire is characterised by a granitic basement mainly composed of biotite granite rocks. According to mapping work in the Gbabédjou and Doropo areas, these Birimian granitoids are cut by gabbro dykes and amphibolite enclaves, which are the subject of this study. In order to better understand the role and the implication of mafic rocks in the Doropo basement emplacement, a multidisciplinary methodology integrating microscopic observations and geochemical analyses of major and trace elements was carried out on 4 samples considered representative of the outcrops studied. Green hornblende, clinopyroxene, and accessory sphene minerals are found in mafic mineral phases, according to petrographic research. Whole-rock analyses reveal that mafic samples with TiO2 contents 1.1 ratios giving them an orogenic granite nature (I-type). Their REE patterns are moderately fractionated (La/Sm)N = 2.66 - 6.13 and (La/Yb)N = 11.17 - 43.70) with a very negative Eu anomaly (Eu/Eu* = 0.75 - 0.97). The multi-element diagrams are characterized by negative Nb-Ta anomalies and geotectonic studies have identified them as volcanic arc formations. All these characteristics allowed us to distinguish the Doropo mafic rocks as formations originating from the juvenile continental crust, emplaced under the Archean tectonics model with significant crustal contamination in the source. Magma driven by mantle diapir has been injected at the base of the continental crust and the heat induces the partial melting of the overlying crust giving rise to mixed liquids. This magma now enriched in LILE was immediately drained to the upper crust to form the mafic rocks from the studied area.

Timescale of Short‐Term Subduction Episodicity in Convection Models With Grain Damage: Applications to Archean Tectonics

AbstractThe style of subduction that prevailed on the early Earth, or even whether subduction was prevalent at all, is an important question in the evolution of Earth's crust, mantle, and surface environment. Here, two‐dimensional numerical convection models, that include grain size evolution to generate weak plate boundaries, reveal a clear transition in subduction behavior with increasing internal heating rate. Sustained subduction with a coherent slab gives way to a style where slabs periodically detach and sink rapidly into the deep mantle, with increasing internal heating rate. In this latter, “drip‐like” subduction regime, repeating cycles of slab growth by subduction, followed by necking and detachment of the lower portion of the slab, are seen. These cycles are termed “slab detachment cycles,” and similar behavior has been seen in regional scale subduction models of the early Earth. Fourier analysis is used to constrain the timescale of slab detachment cycles, and a simple scaling law for this timescale is developed. Applying the scaling law to the early Earth indicates that slab detachment cycles can occur on timescales of &lt;10 Myr, even as low as &lt;5 Myr if the lithosphere is thick and mantle temperature is &gt;1900 K. These cycles may thus be capable of explaining repeating sequences of rocks with “arc” and “non‐arc” signatures seen in some Archean cratons. The drip‐like subduction regime could also have significant implications for the generation of the tonalite‐trondhjemite‐granodiorite (TTG) suite of rocks and exhumation of high pressure metamorphic rocks, two important features of the early Earth geologic record.

Archean tectonics in the sangmelima granite-greenstone terrains, Ntem Complex (NW Congo craton), southern Cameroon.

Structural study of in the Sangmelima granite-greenstone terrains (SGB) provide a new overview of the evolution of the Ntem Complex (South Cameroon), the northwestern part of the Congo Craton. The regional deformation is marked by a strong steep dips foliation and steeply plunging mineral-stretching. D1-related structures are folding and shearing by a D2 deformation that is synchronous with extraction granitic dykes. D1–D2 structures were developed during regional E-W sub-horizontal shortening under ductile conditions associated with steeply dipping principal stretching in a transpressive regime, progressive strain localization along strike-slip shear zones. Syn-D1 mineral assemblages and high strain microstructures suggest that ductile deformation D1 occurred under high-grade metamorphic conditions, whereas D2 microstructures suggest a brittle deformation. The strain patterns in the SGB recorded a long-lasting Archean tectonic history within the Archean Ntem Complex (ca. 2.83–2.72Ga). Finally, some Archean structures were reactivated during a brittle deformation (D3), likely during the Eburnean event.

SHRIMP U‐Pb of Geochronology of Zircon in North China Craton: Revealing the Late Archean Tectonic Events in the Jining Area

SHRIMP U‐Pb of Geochronology of Zircon in North China Craton: Revealing the Late Archean Tectonic Events in the Jining Area

Neoarchean Synmagmatic Crustal Extrusion in the Transpressional Yilgarn Orogen

AbstractPositive feedback mechanisms between magmatism and transpression allow efficient extraction, transfer, and delivery of synorogenic granitic magma. Although many Archean orogens are dominantly transpressional and record voluminous production of synorogenic granites, the role of crustal magmatism in Archean tectonics has been so far little explored. We detail here the structural evolution of the Ballard shear zone, a major synorogenic, inclined transpressional shear zone from the upper‐crustal granite‐greenstone system of the Neoarchean Yilgarn Orogen (Western Australia). We show that the bulk of the three‐dimensional deformation was efficiently partitioned between greenstones and crystallizing granitic sheets, with the latter accommodating large amounts of orogen‐parallel strain through hypersolidus viscous flow. Other large synorogenic Yilgarn shear zones show a similar structural evolution, allowing the formulation of a general tectonomagmatic model. The positive feedback between protracted magmatism and transpression promoted lower‐ to middle‐crustal extraction and upward transfer of syntectonic magma. In upper‐crustal sink regions, the rheological dichotomy between cold and strong greenstones and hot and weak sheets of crystallizing granite allowed efficient kinematic partitioning, resulting in the extrusion of the partially molten shear zone cores. This sequence of events occurred repeatedly throughout the orogeny, following magma pulses, with the bulk of the shearing along each shear zone waning with the cooling of each syntectonic pluton. Synorogenic magmatism played therefore a major role in promoting and localizing transpressional deformation, and ultimately in shaping the architecture of the whole Yilgarn orogen.

Lithosphere differentiation in the early Earth controls Archean tectonics

The processes that operated on the early Earth and the tectonic regimes in which it was shaped are poorly constrained, reflecting the highly fragmentary rock record and uncertainty in geodynamic conditions. Most models of early Earth geodynamics invoke a poorly mobile lid regime, involving little or episodic movement of the lithosphere, above a convecting mantle. However, such a regime does not reconcile with the record of Archean tectonics, which displays contrasting environments associated with either non-plate tectonics or plate tectonics. Here, we propose a regime for the early Earth in which progressive melt extraction at sites of divergence led to the formation of large portions of stiffer lithospheric lid, called proto-plates. These proto-plates enabled stress propagation to be focussed at their margins, which were then the locus for extension as oppose to shortening, under-thrusting and thickening to form adjoining proto-cratons. We test this hypothesis embedding lithospheric stiffening during melt extraction in thermo-mechanical models of mantle convection, under conditions that prevailed in the Archean. We demonstrate the emergence of migrating, rigid proto-plates in which regions of prolonged focused compression coexist with remnants of the stagnant lid, thereby reproducing the widespread dichotomy proposed for the Archean tectonic record. These diverse tectonic modes coexist in a single regime that is viable since the Hadean and lasted until the transition to modern plate tectonics.

The Late Archean Tectonic Events in the Jining Area of North China Craton: New Evidence from Zircon SHRIMP U‐Pb Geochronology of Charnockite

The Late Archean Tectonic Events in the Jining Area of North China Craton: New Evidence from Zircon SHRIMP U‐Pb Geochronology of Charnockite

Neoarchean tectonics: Insight from the Baijiafen ductile shear zone, eastern Anshan, Liaoning Province, NE China

The North China Craton is one of the major Archean to Paleoproterozoic cratons in the world and oldest craton in China, which preserves a large amount of ancient basement and abundant structures showing the early earth tectonics. The controversy over the Archean tectonic regimes has lasted several decades centering around horizontal and vertical tectonics, the two classical tectonic models for Archean times. Thus, more studies of the early crustal growth and tectonic evolution are requisite for better understanding geodynamic regimes in the early Precambrian. This study provides an example for revealing of Archean tectonics. The NWN-trending, ENE-dipping Baijiafen ductile shear zone is located in the eastern Anshan of the northeastern North China Craton and mainly comprises two types of gneisses, including the Chentaigou porphyritic granitic gneiss and the Baijiafen trondhjemitic gneiss. In this study, we have carried out a detailed study on the macrostructure, microstructure and fabric characteristics of the two main types of deformed gneisses within the shear zone. The ribbon structures formed by intensely elongated quartz grains are widespread in these gneisses. Well-developed mineral stretching lineations and asymmetric fabrics indicate an ESE-directed downward shearing. The quartz c-axis fabric patterns obtained by electron backscatter diffraction technique imply low to middle temperature non-coaxial deformation with active rhomb 〈a〉 slip and basal 〈a〉 slip. Deformation behaviors of minerals and quartz crystallographic preferred orientations demonstrate that the rocks underwent mylonitization at a temperature of 400–500°C under greenschist facies metamorphic conditions. Dislocation creep is the main rock deformation mechanism within the shear zone. Finite strain measurement results suggest that the strain types of the shear zone are generally related to elongate-plane deformation, and the tectonites change from L-S- to LS-type across the shear zone. The strain intensity increases obviously towards east, and the Baijiafen gneiss unit is located within the high-strain zone. Kinematic vorticity values of gneisses indicate that the deformed rocks were produced by steady-state simple-shear dominated general shear. Based on abundant structural evidence and previous studies, we infer that the deformation of the Baijiafen ductile shear zone may have resulted from the downward adjoining rock flow during the sagduction of the banded iron formations and synchronous Archean granite dome emplacement, supporting a vertical tectonic regime in Archean times.

Making Archean cratonic roots by lateral compression: A two-stage thickening and stabilization model

Archean tectonics was capable of producing virtually indestructible cratonic mantle lithosphere, but the dominant mechanism of this process remains a topic of considerable discussion. Recent geophysical and petrological studies have refuelled the debate by suggesting that thickening and associated vertical movement of the cratonic mantle lithosphere after its formation are essential ingredients of the cratonization process. Here we present a geodynamical study that focuses on how the thick stable cratonic lithospheric roots can be made in a thermally evolving mantle. Our numerical experiments explore the viability of a cratonization process in which depleted mantle lithosphere grows via lateral compression into a >200-km thick, stable cratonic root and on what timescales this may happen. Successful scenarios for craton formation, within the bounds of our models, are found to be composed of two stages: an initial phase of tectonic shortening and a later phase of gravitational self-thickening. The initial tectonic shortening of previously depleted mantle material is essential to initiate the cratonization process, while the subsequent gravitational self-thickening contributes to a second thickening phase that is comparable in magnitude to the initial tectonic phase. Our results show that a combination of intrinsic compositional buoyancy of the cratonic root, rapid cooling of the root after shortening, and the long-term secular cooling of the mantle prevents a Rayleigh-Taylor type collapse, and will stabilize the thick cratonic root for future preservation. This two-stage thickening model provides a geodynamically viable cratonization scenario that is consistent with petrological and geophysical constraints.

Archean tectonics and crustal evolution of the Biligiri Rangan Block, southern India

The Southern Granulite Terrain in India is a collage of crustal blocks ranging in age from Archean to Neoproterozoic. This study investigate the tectonic evolution of one of the northernmost block – the Biligiri Rangan Block (BRB) through a multidisciplinary approach involving field investigation, petrographic studies, LA-ICPMS zircon U–Pb geochronology, Hf isotopic analyses, metamorphic P–T phase diagram computations, and crustal thickness modeling. The garnet bearing quartzofeldspathic gneiss from the central BRB preserve Mesoarchean magmatic zircons with ages between 3207 and 2806Ma and positive ɛHf value (+2.7) which possibly indicates vestiges of a Mesoarchean primitive continental crust. The occurrence of quartzite-iron formation intercalation as well as ultramafic lenses along the western boundary of the BRB is interpreted to indicate that the Kollegal structural lineament is a possible paleo-suture. Phase diagram computation of a metagabbro from the southwestern periphery of the Kollegal suture zone reveals high-pressure (∼18.5kbar) and medium-temperature (∼840°C) metamorphism, likely during eastward subduction of the Western Dharwar oceanic crust beneath the Mesoarchean BRB. In the model presented here, slab subduction, melting and underplating processes generated arc magmatism and subsequent charnockitization within the BRB between ca. 2650Ma and ca. 2498Ma. These results thus reveal Meso- to Neoarchean tectonic evolution of the BRB. The spatial variation of crustal thickness, derived from flexure inversion technique, provides additional constraints on the tectonic linkage of the BRB with its surrounding terrains. In conjunction with published data, the Moyar and the Kollegal suture zones are considered to mark the trace of ocean closure along which the Nilgiri and Biligiri Rangan Blocks accreted on to the Western Dharwar Craton.

Tectonics and geodynamics of granulite-gneiss complexes in the East European Craton

The Neoarchean and Paleoproterozoic granulite-gneiss complexes of the northern East European Craton, including their tectonic setting, accompanying igneous and sedimentary rocks, character of metamorphism and deep structure are considered in this paper. A similar approach was applied to the description and interpretation of the granulite-gneiss complexes of the Grenville-Sveconorwegian Orogen localized in the present-day structure of the European and North American continents. One of the most important results of this study is the recognition of a new type of tectonic structures called intracontinental oval orogens. These orogens were formed in the inner domain of the Neoarchean-Proterozoic supercontinent (2.8–0.85 Ga), which is called Lauroscandia. The extensional setting, which was initiated by a series of mantle plumes, locally passing to spreading but did not lead to the eventual breakup of the supercontinent and was followed by intracontinental collisional compression accompanied by closure of short-lived local oceans, subduction, and obduction. The Neoarchean and Proterozoic granulite-gneiss belts and areas of the East European Craton are components of the Karelian-Belomorian, Kola, Volgo-Uralia, Lauro-Russian, and Grenville-Sveconorwegian intracontinental oval orogens formed in the inner domain of Lauroscandia as a result of mantle-plume and plate-tectonic processes initiated by mantle superplumes. The geodynamic settings and tectonic processes in the Neoarchean-Proterozoic supercontinent differed markedly from both the Archean tectonics of miniplates and the Phanerozoic plate tectonics.

The Archean magmatic-hydrothermal system of Lac Shortt (Au-REE), Abitibi, Canada: Insights from carbonate fingerprinting

Trace element and isotopic data from the Lac Shortt (Abitibi, Canada) gold and rare earth element (REE) deposit sheds light on the cogenetic association between alkali magmatism (syenites and lamprophyres), carbonatation (magmatic and carbothermal) and gold – REE mineralization. Magmatic and carbothermal carbonates from comagmatic lithologies from the Lac Shortt gold-REE deposit were analyzed for major, minor and trace elements concentrations as well as for their 87Sr/86Sr0, δ13C and δ18O isotopic ratios. Carbonates from Lac Shortt have δ13C values ranging from −4.89 to 2.38 ‰ VPDB and δ18O values ranging from 8.28 to 15.02. The lightest values are found in carbonate from the carbonatites and the heaviest values are associated with the ore-carbonates. Carbonates from the various lithologies lie between these two extremes. Initial 87Sr/86Sr ratios from the carbonates cluster tightly around 0.7013–0.7016, i.e., depleted Archean mantle signatures, although some have slightly more radiogenic values, from 0.7026 to 0.7029. The small variation in the strontium isotope ratios compared to the C and O isotopic values suggests that carbonate anions (CO32−) were decoupled from the cations during hydrothermal alteration. Cations such as Sr2+ and Ca2+ in carbonates from the high grade ore and from magmatic and hydro-carbothermal carbonatite appear to originate from the Late-Archean, depleted mantle. Since gold is a cation and is strongly associated with carbonatite magmatism and associated carbo-hydrothermal carbonatation, we suggest gold too has a depleted mantle origin. The carbonates are enriched in mobile high field strength elements (HFS; Ba, Sr) elements, depleted in immobile HFS elements (Th, U, Ta, Nb, Zr, Ti) and have enriched- to slightly depleted REE patterns with flat to strong light-REE enrichments. These trace element concentrations support the fact that all the lithologies were comagmatic and that the ore itself was genetically related to these lithologies. Based on present day knowledge of Archean tectonics of Abitibi, post collision extension tectonics and mantle delamination, isotope geochronology, on the magmatic-hydrothermal system of Lac Shortt itself and on the present paper's carbonate fingerprinting, we propose that Lac Shortt calc-alkaline dioritic magmatism-hydrothermalism was followed by alkaline syenitic, and eventually carbonatitic magmatism-hydro-carbothermalism, and that gold and REE originated from the carbonatite magmatic-carbo-hydrothermal system and its mantle roots, which agrees with mass balance calculations. The depleted-mantle-rooted, carbonatite exsolved a hydro-carbothermal fluid enriched in REE and gold which fenitized and oxidized the syenite and out of which the gold and REE mineralization was deposited.

Deformation coupling between the Archean Pukaskwa intrusive complex and the Hemlo shear zone, Superior Province, Canada

Abstract Archean greenstone belts typically form narrow sheared basins separating bulbous tonalo–trondjhemo–granodioritic (TTG) intrusive complexes. The role played by gravity in the development of such dome-and-keel structures constitutes a key question in Archean tectonics. The Pukaskwa intrusive complex (PIC)–Hemlo greenstone belt system stands as a remarkable example of the dome-and-keel architecture that commonly occurs in Archean terrains. Abundant strain markers in the greenstone belt and in the Hemlo shear zone (HSZ) attest of late sinistral strike-slip kinematics (D2) whereas, in general, the quartzofeldspathic coarse-grained rocks of the Pukaskwa intrusive complex bear little macroscopically visible kinematic indicators, most likely due to pervasive recrystallization. The PIC consists dominantly of a heterogeneous assemblage of TTG plutonic rocks and gneisses, which overall are less dense than the greenstone rocks. The study of anisotropy of magnetic susceptibility (AMS), based on 120 stations and 1947 specimens from the PIC, reveals east–west trending prolate and plano-linear fabrics across the northern margin of the complex, i.e., along the HSZ. Since geotherms were higher in the Archean than in the present, the effective viscosity of the TTG units would have been sufficiently low to allow their diapiric ascent through denser greenstone rocks. Here we propose an alternative model where thrust tectonics is responsible for the early structuration of the PIC. Later transpressive tectonics causes strain localization along internal strike-slip shear zones and along lithological boundaries.