Archean dome-and-keel structures indicate local thermal anomalies

One of the major unresolved questions in Precambrian geology is the nature of tectonic processes during Earth’s early history and the timing of the transition to modern-style plate tectonics. The Barberton Greenstone Belt (BGB) of South Africa and Eswatini features prominently in this discussion because it represents, along with the Pilbara region of Australia, the prime geological archive of the late Paleoarchean (ca. 3.5-3.2 Ga). This time period may mark the transition from a pre-plate tectonic setting to Phanerozoic-style plate tectonics. The cuspate-lobate geometry of the BGB, together with its general structural make-up (defined by folding style, stratigraphic fill and strain distribution) appears to represent a non-actualistic Archean tectonic style characterized by vertical rather than horizontal displacements, as known from modern plate tectonics. A compilation of geological data from the entire greenstone belt demonstrates its heterogeneity and complex deformation history is compared with own investigations in this work. A critical comparison of suggested tectonic settings to recent observations shows that no pure plate-tectonic scenarios are applicable. The temporal and spatial heterogeneity of deformation, the relative greenstone-down sense of shear along many of its contacts to the adjacent plutons, and the overall synclinal structure of the BGB emphasize a non-plate-tectonic setting dominated by vertical movements. Local subsidence due to folding, tilting and sagging of thick, dense greenstone regions into an incompetent granitoid middle crust during partial convective overturn plausibly explains the rotation of the enormous Onverwacht Anticline, the characteristic folding pattern and the temporally and spatially heterogeneous deformation history of the BGB.

The global tectonic setting(s) of the early Archean is a long-debated topic in geology. There are two major groups of models: uniformitarian models that largely mirror modern plate tectonics and non-uniformitarian models which involve various ‘vertical’ tectonics concepts. These models have been proposed and examined during the last decades based primarily on study of exposed Archean terranes. Isua and Eastern Pilbara are two of the best studied Archean terranes because of their old ages (>3.8-3.6 Ga and >3.5-3.2 Ga), large exposures and relatively good preservation of early Archean lithologies and structures. The uniformitarian model has been widely applied to Isua whereas the vertical tectonics model is largely drawn from the geology of the Eastern Pilbara. This difference in interpretation suggests a few possible scenarios: early Earth had diverse geological systems, or one or both interpretations may be flawed. Plate tectonics appears strongly excluded by the distinctive dome-and-keel structural geometry of the Eastern Pilbara, but it appears possible to question whether Isua may have developed via vertical tectonic processes. To address this question, this project investigates the lithology, geochemistry, geochronology, metamorphism and structural geology of Isua and Eastern Pilbara. Data are compiled from literature sources, new field mapping in Eastern Pilbara, and new whole rock geochemistry of Pilbara ultramafic rocks. Results from the two terranes show that the geometry, lithology, geochemistry, metamorphism styles, and structural patterns are largely similar, with minor differences that can be interpreted as results of local deformation variations. Processes of subduction, arc magmatism, terrane juxtaposition, and formation of core complexes provide most predictions for uniformitarian models, whereas extensive volcanism, TTG doming, and sinking of thick mafic crust are the main processes supplying predictions for non-uniformitarian vertical tectonics models. Although predictions of uniformitarian models can fit much of the assembled data, there are always some key features that cannot be explained. For examples: (1) ultramafic rocks from Pilbara and Isua are chemically consistent with intrusive and extrusive crustal igneous rocks and not consistent with mantle peridotite chemistry, as predicted by plate tectonic models of these terranes. Likewise, (2) the radial dome-and-keel structural geometry of Pilbara cannot be explained via formation of metamorphic core complexes, as predicted by some plate tectonic models, and Isua may represent a similar structural pattern that has undergone later deformation. In contrast, these records are consistent with the predictions of non-uniformitarian vertical tectonics models. Therefore, it appears that a non-uniformitarian vertical tectonics model may

Text S1 and Figure S1 show the physical model used to study the magma cooling process. Figure S2 plots the results of a model with solidified greenstone sequences atop the TTG crust. Figure S3 plots the stress and viscosity fields of the model in Figure 7. Figure S4 plots the results of a model with the eruption of mafic magma on a cold greenstone sequence and a heated TTG crust. Figure S5 is the regime diagram for the formation of dome-and-keel structures. Figure S6 plots the results of a contractional model with large perturbations. Figure S7 plots the results of a model with a 23-km-thick greenstone sequence. Figure S8 plots the viscosity profiles of serpentinized greenstone sequences. Figure S9 rebuilds the model proposed by Robin and Bailey (2009). Figure S10 plots the density of greenstone sequences after serpentinization. Figure S11 compares two models with different plastic yields. Figure S12 shows the results of a model with strong perturbations.<p></p>

Text S1 and Figure S1 show the physical model used to study the magma cooling process. Figure S2 plots the results of a model with solidified greenstone sequences atop the TTG crust. Figure S3 plots the stress and viscosity fields of the model in Figure 7. Figure S4 plots the results of a model with the eruption of mafic magma on a cold greenstone sequence and a heated TTG crust. Figure S5 is the regime diagram for the formation of dome-and-keel structures. Figure S6 plots the results of a contractional model with large perturbations. Figure S7 plots the results of a model with a 23-km-thick greenstone sequence. Figure S8 plots the viscosity profiles of serpentinized greenstone sequences. Figure S9 rebuilds the model proposed by Robin and Bailey (2009). Figure S10 plots the density of greenstone sequences after serpentinization. Figure S11 compares two models with different plastic yields. Figure S12 shows the results of a model with strong perturbations.<p></p>

The Archean accounts for approximately half the recorded history of the earth, comprising over 2,000 m.y. of geologic time. As the first ~800 m.y. are virtually lost from the record, events on the surface of the planet can only be speculated over. It is probable that this formative episode was magmatically turbulent and culminated with the final stages of meteorite bombardment ~3,900 m.y. ago. The next events, for which there is geologic evidence, involved the regions now represented by the high-grade gneiss terranes in which the first mineralization was developed. Ore deposits are not well represented in these regions for one or a combination of the following reasons: (1) mineral concentrations may have been recycled or destroyed during the initial turbulent episode of protocontinental development; (2) mineralization may have been destroyed by later tectono-thermal events that characterize the high-grade mobile belt regimes; or (3) mineralizing events or conditions may not have evolved sufficiently for metal concentrations to have formed in significant amounts. The period of the Archean, beginning ~3,500 m.y. ago and terminating ~2,500 m.y. ago, witnessed the development of the low-grade granite-greenstone terranes of the shield areas. An examination of the principal components of these regions is undertaken following an existing model whereby greenstone sequences are subdivided into a basal unit comprising mainly mafic-ultramafic rock types, followed by an upper mafic to felsic volcanogenic succession, the latter succeeded by an essentially sedimentary group of rocks. The nature and distribution of Archean mineralization associated with the granite-greenstone terranes is described, and it is shown that there is a strong genetic link between mineral types and rock compositions. Evolutionary trends in the nature and geochemistry of magma types are discussed, and these are considered to reflect changing conditions of heat flow, changes in the nature of the atmosphere and hydrosphere, and progressive modification of the earth’s crust resulting from protocontinental nucleation and development. Magma types ranging from komatiitic basalts and peridotites to calc-alkaline series basalts-dacites-rhyodacites-rhyolites suggest that geotectonic conditions have changed not only within individual greenstone belts but also over the time span of the entire Archean. Available information suggests that the island-arc or trench-back arc-marginal basin system provides the closest modern analogue to the ancient greenstone sequences. However, sufficient differences exist to caution against direct correlation with plate tectonic mechanisms operating at present, and it is suggested that the early Archean greenstone sequences may have been involved more with vertical tectonics in response to gravitational differences resulting from sinking volcanic piles and diapirically rising granitoid complexes. In later Archean times conditions might have approached those currently favored for Phanerozoic orogenesis. Finally, it is suggested that Archean ore deposits are essentially secondary in origin, having formed mainly as a result of superimposed processes of igneous intrusion, metamorphism, structural disturbance, and chemical alteration acting on suitable host rocks during or after their formation and deposition.

The southern portion of the Sao Francisco Craton (SSFC), located in Minas Gerais State, underwent a long crustal development that occurred from Mesoarchean to Neoproterozoic times (Carneiro et al., 1998a; 1998b; Teixeira et al., 1996). The Archean rocks of the SSFC, which formed between 3.5 – 2.5 Ga are essentially gneisses and granitoids (TTG suite), a greenstone belt sequence (Rio das Velhas Greenstone Belt - RVGB), an intrusive ultramafic-mafic layered complex (Ribeirao dos Motas Layered Sequence - SARM), and mafic dikes swarms. Locally, the Archean rocks are covered by Proterozoic intracratonic sedimentary basin sequences containing quartzite-carbonate-shale with minor contribution of the bimodal volcanics-quartzite-arkose and volcanics-graywacke and intruded by swarms of mafic dikes and/or granitoid bodies. The SARM comprises alternate layers of main peridotitic and pyroxenitic rocks with minor gabronorite and amphibolite rocks, outcropping as several disrupted bodies (Carneiro et al., 1997). Olivine, spinel, amphibole and pyroxene (orto- and clino-) constitute the peridotite mineralogy. The pyroxenite is constituted by pyroxenes (orto- and clino-), amphibole and spinel. Besides the typical petrographic features and texture commonly found in layered complexes such as: igneous layering, euhedral crystals and cumulus textures, the studied rocks present some sui generis petrographic aspects such as chadacristal of euhedral amphibole included in euhedral oikocristal of pyroxene. Major and minor elements have been performed on 15 whole rock samples from different SARM outcrops. The SARM ultramafic rocks have MgO contents ranging from 23.8 to 34.9 wt% (MgO = 30.08 average wt%); TiO2 = 0.182 average wt%; CaO/Al2O3 ratio of 0.72 (average); Al2O3/TiO2 ratio of 33 (average). Analyses of REE and PGE have been performed on 6 samples. All samples are low in total REE abundance; Pt and Pd average are 28 and 10.6 ppb respectively. These chemical data suggest that SARM magmas were derived from komatiite magmas similar to siliceous high magnesian basalts (SHMB). Furthermore, the higher Pt and Pd (average ppb) suggest that magmas feeding the SARM could be carrying conspicuous amounts of PGE. Regionally, the geochemistry studies reveal great similarity between the SARM and the komatiite rocks of the RVGB (Padilha et al., 1984; Sichel et al., 1983; Schorscher et al., 1992). However, the SARM ultramafic rocks have higher average contents of MgO, Cr, Ni and lower average contents of SiO2, TiO2, K2O and REE. These geochemical features suggest that the RVGB komatiite may have been generated from more evolved SARM magmas. However, it is also possible that the mantle source region for both ultramafic magmas had been depleted during a melting episode that caused extraction of the RVGB komatiite magma. Thus, in spite of the lack of unequivocal geological relationships between SARM and the komatiite rocks of the RVGB, the two sequences referred to above may have had the same magmatic source and, as a consequence, been tectonically related. Assuming that this is true, the SARM rocks are a fraction of the komatiite magma that was encapsulated in the lower crust forming a layered ultramafic-mafic complex. Another portion of the komatiite magma would have reached the surface forming ultramafic volcanic supracrustal sequences of the SGRV. Finally, the SARM geochemical data when compared with those of the Archean Australian layered complexes (Sun et al., 1991; Hoatson et al., 1992) exhibit higher average Pt, Pd, Al2O3/TiO2 ratio, Cr and Ni, while Ti/Sc ratio, Ti, SiO2 and K2O average (wt%) are lower. The higher Pt and Pd values in comparison with those values of the Archean Australian layered complexes, which were presented by Sun et al. (1991) and Hoatson et al. (1992), are suggestive of a potential PGE mineralization in the SARM.

In the Bushveld Complex, the ultramafic (orthopyroxenite/harzburgite with chromitite) layers that host most of the PGE and chromite mineralization in the Upper Critical Zone display well-documented discordant basal contacts with their anorthositic and noritic host rocks. Whilst not so well documented, there is evidence that the upper contacts of these units are also discordant. We review the nature of the contacts between the ultramafic units and adjacent plagioclase-rich lithologies. These include contact phenomena like pegmatoidal lithologies and thin magmatic reaction chromite stringers. We conclude that most, if not all, ultramafic layers were intruded as sills into pre-existing norite/anorthosite cumulates. The sequence of norites and anorthosites that hosts the ultramafic layers was built up by a prior series of multiple tholeiitic (A-type) magma intrusions. The spectrum of lithologies from melanorite through to (mottled) anorthosite represents differing degrees of partial melting in response to these successive magma influxes. Density and competence contrasts between layers of plagioclase-rich rocks in turn provided pathways for sill propagation of subsequent ultramafic (U-type) magmas. The ultramafic magmas further modified the host norites and anorthosites by processes of partial melting and metasomatism. The ultramafic units themselves accumulated as composite sills in response to multiple magma injections. In the western Bushveld Complex, particularly including the Swartklip Sector in the north-western part of the complex, the Merensky Reef is represented by various facies that occur at different levels in the host stratigraphy. This phenomenon has been referred to by the term “regional potholing”, and has been attributed to the erosion of footwall cumulates by new influxes of magma. We suggest that a series of step-and-stair-type transitions of intruding sills to successive stratigraphic levels might more appropriately explain the various facies of the Merensky Reef.

It is debated whether plate tectonics (horizontal tectonics) or single-lid tectonics (vertical tectonics) dominated the Mesoproterozoic Era. Either rifting of the Nuna/Columbia supercontinent or a localized vertical subsidence and tectonism mechanism within a single tectonic plate is likely recorded in Mesoproterozoic basins. This study summarizes detrital zircon samples from the Mesoproterozoic Belt and Purcell Supergroups and Lemhi subbasin of the western United States and Canada and tests competing rift and intracratonic basin models. Rift models take the observed detrital zircon trends to mean that a non-Laurentian (ca. 1.6–1.5 Ga) detrital zircon component becomes completely absent higher in the section, signifying rifting of the Nuna/Columbia supercontinent at ca. 1.4 Ga. Intracratonic models acknowledge this observed shift in provenance but interpret a long-lived intracratonic setting for the basin following an earlier failed rifting event. The fundamental question is whether the Belt basin represents a failed or successful rift. We used statistical comparison of 72 detrital zircon signatures, reported in the literature and presented in this study, to test the rift model. Samples are not evenly distributed across the basin or its stratigraphy. Non-Laurentian grains are spatially restricted to the northwest part of the basin but are present in all groups, suggesting that the apparent loss of the non-Laurentian population is an artifact of sampling bias. Like stratigraphic boundaries and facies changes, mixing trends are gradual, not sharp or sudden, signifying progressive reworking of Proterozoic zircons and transport from all sides. Archean zircons are localized near the edges of Archean blocks, signifying local down-dropping along cratonic margins. The rift model is therefore rejected in favor of the intracratonic model for the Belt basin on the basis of variable mixing between non-Laurentian and Laurentian sources in both pre–Missoula Group and Missoula Group strata. Far away from plate margins, sediment incrementally filled topographic depressions created by densified and thinned Proterozoic crustal blocks, resulting in vertical down-dropping along preexisting sutures with neighboring Archean blocks. More systematic detrital zircon studies are needed in order to accurately quantify provenance trends in space and time. Continued investigation of the Belt basin may reveal underappreciated or unrecognized vertical tectonic processes that may explain Mesoproterozoic rocks more accurately.

The evolution of Archaean greenstone terrains, Eastern Goldfields Province, Western Australia

Generation, ascent and eruption of magma on the Moon: New insights into source depths, magma supply, intrusions and effusive/explosive eruptions (Part 2: Predicted emplacement processes and observations)

Hess9 ideas related to the tectonic significance of ultramafic rocks, ultramafic magmas, evolution of oceanic crust, and H 2 O outgassing, all emanating from his abiding interest in serpentinite. Hess recognized the prime importance of ultramafic rocks in understanding the tectonic development of collisional (Alpine-type) mountain belts. They are not magmatic intrusions, as he proposed, but tectonic remnants of fossil plate boundaries. Hess was the formulator of the concept of sea-floor spreading. His ideas on the nature of oceanic crust changed through the years, but he repeatedly argued that oceanic crust was partly or predominantly serpentinite. Despite the lack of subsequent popularity of this view, he was at least partly right. Serpentinite is a part of oceanic crust, notably in tectonically thinned regions along rifted ridges, near transform faults, and along obliquely rifted continental margins, such as the Mesozoic western Alpine continental margin.

Hess9 ideas related to the tectonic significance of ultramafic rocks, ultramafic magmas, evolution of oceanic crust, and H 2 O outgassing, all emanating from his abiding interest in serpentinite. Hess recognized the prime importance of ultramafic rocks in understanding the tectonic development of collisional (Alpine-type) mountain belts. They are not magmatic intrusions, as he proposed, but tectonic remnants of fossil plate boundaries. Hess was the formulator of the concept of sea-floor spreading. His ideas on the nature of oceanic crust changed through the years, but he repeatedly argued that oceanic crust was partly or predominantly serpentinite. Despite the lack of subsequent popularity of this view, he was at least partly right. Serpentinite is a part of oceanic crust, notably in tectonically thinned regions along rifted ridges, near transform faults, and along obliquely rifted continental margins, such as the Mesozoic western Alpine continental margin.

<p>The Eoarchean Isua Supracrustal Belt (ISB) is one of the few locations where it is possible to study the tectono-metamorphic evolution of a young planet. The ISB is thought to represent meta-volcano-sedimentary units from two different embryotic continental segments/terranes associated with two large TTG bodies of contrasting crystallization age. Until recently, geochemical and metamorphic signatures have been interpreted to be consistent with a subduction-collision event, thereby matching Earth’s active ‘horizontal’ tectonic regime. This interpretation is often cited as evidence that plate tectonics has operated since the Early Archean. New structural, field, isotopic and geochemical data, however, suggest that the ISB is rather a continuous volcano-sedimentary sequence with a rock record that could be explained by ‘vertical’ tectonic models involving extensive volcanic resurfacing and single-plate tectonics. In this work, we present metamorphic data retrieved from a new set of samples from the eastern ISB to evaluate the two contrasting hypotheses. Throughout the ISB, two major Archean medium grade metamorphic events (M<sub>1</sub>, M<sub>2</sub>) can be identified, overprinted partially by near-pervasive low-temperature retrogression. The pre-Ameralik dykes (≈ 3500 Ma) event M<sub>1</sub>, is characterized by a strong foliation and typically lineation that plunges towards the SE with development of amphibolite facies assemblages, with common appearance of syn-tectonic garnet and amphibole porphyroblasts. Phase equilibria modelling, classic and isopleth geothermobarometry show that M<sub>1</sub> evolved as a nearly isothermal prograde metamorphism that culminated in an amphibolite facies peak (0.65 GPa and 550-580 °C) common to the entire belt. M<sub>2</sub>, probably Neoarchean in age, is identified by the frequent appearance of post-tectonic garnet rims with estimated lower grade conditions. Low temperature retrogression is widespread along the ISB, however, it seems more penetrative in the northern area occurring as garnet pseudomorphism and retrograde chlorite commonly mimicking the foliation by replacing biotite, with some samples showing complete chloritization. We argue that the retrogression textures could be responsible for the apparent zones of lower metamorphism previously reported as prograde, a conclusion also supported by our geothermobarometric data, and that the tectonic models supported by previuos interpretations need to be revised. The isothermal prograde path as well as the high geothermal gradient associated with peak conditions (≈ 900 °C/GPa) is consistent with vertical tectonics models during the Eoarchean. This interpretation is in agreement with global data analysis that suggest non-uniformitarian geodynamics in the Early Archean, as well as the viability of early vertical tectonics on the other terrestrial bodies of our solar system. It follows that studies like this can shed light on not just the cooling of early Earth, but also on the cooling of terrestrial planets universally.</p>

Plate tectonic processes in the Atlantic and western Tethyan realm directed the post-Variscan sedimentary and structural evolution of the High Atlas and Middle Atlas intracontinental mountain ranges of Morocco. Plate movements caused a reactivation of an inherited pan-African or Hercynian fault pattern by the variation of stress regimes through time. This resulted in strike-slip as well as vertical tectonics. During times of relative tectonic quiescence eustatic sea-level changes governed the sedimentary development. The most important, often interacting, global tectonic determinants are: taphrogenesis of the NW-African continental margin lasting until the Early Cretaceous (Triassic rifting and subsequent mid-Atlantic spreading), strike-slip-faulting at the Newfoundland-Gibraltar fault zone (Liassic — earliest Eocene), and continental convergence between Europe (Iberia) and Africa which started in the Late Cretaceous and reached its acme in the Neogene. In the realm of the Central High Atlas and the Middle Atlas the interaction of these processes triggered continental rifting (Triassic) and subsequent marine flooding of the intergrown riftgrabens prograding from the Tethys realm (Early Jurassic — earliest Middle Jurassic). After its abortion, the former Atlas rift was filled up with marine sediments (Bajocian — Bathonian), followed by continental redbeds and final uplift (late Mid Jurassic — late Early Cretaceous). Eustatic sea-level changes mostly governed the sedimentary evolution from Aptian to latest Mid Eocene. After a first weak uplift of the central High Atlas during the Senonian major uplift of the intracontinental chains commenced at the Mid/ Late Eocene transition. Diastrophism of the Atlas ranges during the Miocene and Pliocene coincided with the main orogenic movements of the Betico-Rifean arc.

It is proposed that the upper mantle transition region, 220 to 670 km, is composed of eclogite which has been derived from primitive mantle by about 20 percent partial melting and that this is the source and sink of oceanic crust. The remainder of the upper mantle is garnet peridotite which is the source of continental basalts and hotspot magmas. This region is enriched in incompatible elements by hydrous and CO2 rich metasomatic fluids which have depleted the underlying layers in the L.I.L. elements and L.R.E.E. The volatiles make this a low-velocity, high attenuation, low viscosity region. The eclogite layer is internally heated and its controls the convection pattern in the upper mantle. Plate tectonics is intermittent. The continental thermal anomaly at a depth of 150-220 km triggers kimberlite and carbonatite activity, alkali and flood basalt volcanism, vertical tectonics and continental breakup. Hot spots remain active after the continents leave and build the oceanic islands. Mantle plumes rise from a depth of about 220 km. Midocean ridge basalts rise from the depleted layer below this depth. Material from this layer can also be displaced upwards by subducted oceanic lithosphere to form back-arc basins.