Modern-style Plate Tectonics Research Articles

U–Pb and Lu–Hf isotope record of detrital zircon grains from the Limpopo Belt – Evidence for crustal recycling at the Hadean to early-Archean transition

Detrital zircon grains from Beit Bridge Group quartzite from the Central Zone of the Limpopo Belt near Musina yield mostly ages of 3.35–3.15 Ga, minor 3.15–2.51 Ga components, and numerous older grains grouped at approximately 3.4, 3.5 and 3.6 Ga. Two grains yielded concordant Late Hadean U–Pb ages of 3881 ± 11 Ma and 3909 ± 26 Ma, which are the oldest zircon grains so far found in Africa. The combined U–Pb and Lu–Hf datasets and field relationships provide evidence that the sedimentary protolith of the Beit Bridge Group quartzite was deposited after the emplacement of the Sand River Gneisses (3.35–3.15 Ga), but prior to the Neoarchean magmatic-metamorphic events at 2.65–2.60 Ga. The finding of abundant magmatic zircon detritus with concordant U–Pb ages of 3.35–3.15 Ga, and 176Hf/ 177Hf of 0.28066 ± 0.00004 indicate that the Sand River Gneiss-type rocks were a predominant source. In contrast, detrital zircon grains older than approximately 3.35 Ga were derived from the hinterland of the Limpopo Belt; either from a so far unknown crustal source in southern Africa, possibly from the Zimbabwe Craton and/or a source, which was similar but not necessarily identical to the one that supplied the Hadean zircons to Jack Hills, Western Australia. The Beit Bridge Group zircon population at >3.35 Ga shows a general εHf t increase with decreasing age from εHf 3.9Ga = −6.3 to εHf 3.3–3.1Ga = −0.2, indicating that Hadean crust older than 4.0 Ga ( T DM = 4.45–4.36 Ga) was rejuvenated during magmatic events between >3.9 and 3.1 Ga, due to a successive mixing of crustal rocks with mantle derived magmas. The existence of a depleted mantle reservoir in the Limpopo’s hinterland is reflected by the ∼3.6 Ga zircon population, which shows εHf 3.6Ga between −4.6 and +3.2. In a global context, our data suggest that a long-lived, mafic Hadean protocrust with some tonalite–trondhjemite–granodiorite constituents was destroyed and partly recycled at the Hadean/Archean transition, perhaps due to the onset of modern-style plate tectonics.

When and how did plate tectonics begin? Theoretical and empirical considerations

Plate tectonics is the horizontal motion of Earth’s thermal boundary layer (lithosphere) over the convecting mantle (asthenosphere) and is mostly driven by lithosphere sinking in subduction zones. Plate tectonics is an outstanding example of a self organizing, far from equilibrium complex system (SOFFECS), driven by the negative buoyancy of the thermal boundary layer and controlled by dissipation in the bending lithosphere and viscous mantle. Plate tectonics is an unusual way for a silicate planet to lose heat, as it exists on only one of the large five silicate bodies in the inner solar system. It is not known when this mode of tectonic activity and heat loss began on Earth. All silicate planets probably experienced a short-lived magma ocean stage. After this solidified, stagnant lid behavior is the common mode of planetary heat loss, with interior heat being lost by delamination and “hot spot” volcanism and shallow intrusions. Decompression melting in the hotter early Earth generated a different lithosphere than today, with thicker oceanic crust and thinner mantle lithosphere; such lithosphere would take much longer than at present to become negatively buoyant, suggesting that plate tectonics on the early Earth occurred sporadically if at all. Plate tectonics became sustainable (the modern style) when Earth cooled sufficiently that decompression melting beneath spreading ridges made thin oceanic crust, allowing oceanic lithosphere to become negatively buoyant after a few tens of millions of years. Ultimately the question of when plate tectonics began must be answered by information retrieved from the geologic record. Criteria for the operation of plate tectonics includes ophiolites, blueschist and ultra-high pressure metamorphic belts, eclogites, passive margins, transform faults, paleomagnetic demonstration of different motions of different cratons, and the presence of diagnostic geochemical and isotopic indicators in igneous rocks. This record must be interpreted individually; I interpret the record to indicate a progression of tectonic styles from active Archean tectonics and magmatism to something similar to plate tectonics at ∼1.9 Ga to sustained, modern style plate tectonics with deep subduction—and powerful slab pull—beginning in Neoproterozoic time.

Geodynamic settings of mineral deposit systems

Mineral deposits represent extraordinary metal concentrations that form by magmatic, magmatic–hydrothermal or hydrothermal processes in geodynamic environments typified by anomalously high thermal and/or mechanical energy near plate boundaries. As they require the conjunction of specific environmental conditions to form, particular mineral deposit types tend to occupy specific geodynamic niches. The temporal distributions of mineral deposit types reflect both formational and preservational processes. In the Archaean and Palaeoproterozoic, these were linked because of preservation in continental crust connected to thick buoyant subcontinental lithospheric mantle (SCLM), but were decoupled by the Neoproterozoic and Phanerozoic as a result of evolution to thinner, increasingly dense SCLM. The transition marks a change from mantle plume-influenced plate tectonics to modern-style plate tectonics, with broadly coincident environmental changes and a major impact on the nature and abundance of preserved mineral deposit types. As mineral deposits represent an integral part of tectonic process, they are essential indicators of that process and geodynamic settings, and should be incorporated into any holistic tectonic terrane analysis. Their distribution also provides a particularly critical test on ancient continental reconstructions derived from palaeomagnetic data. Conversely, such reconstructions provide a first-order targeting tool for the conceptual exploration required to discover new mineral provinces and deposits under cover.

The Teton – Wind River domain: a 2.68–2.67 Ga active margin in the western Wyoming Province

The Archean rocks in western Wyoming, including the Teton Range, the northern Wind River Range, and the western Owl Creek Mountains, preserve a record of a 2.68–2.67 Ga orogenic belt that has many of the hallmarks of modern plate tectonics. A 2683 Ma tholeiitic dike swarm is undeformed and unmetamorphosed in the western Owl Creek Mountains. In the Wind River Range, these dikes have been deformed and metamorphosed during thrusting along the west- to southwest-directed Mount Helen structural belt, which was active at the time that the 2.67 Ga Bridger batholith was emplaced. In the northern Teton Range, the Moose Basin gneiss, which contains relict granulite-facies assemblages, appears to have been thrust upon the amphibolite-grade layered gneiss. The syntectonic Webb Canyon orthogneiss was intruded into the thrust at or before 2673 Ma. We interpret these relations, along with isotopic data indicating that the layered gneiss in the Teton Range consists of juvenile components, to indicate that the western Wyoming Province was the site of active margin tectonics at 2.68–2.67 Ga. This involved a magmatic arc in the present Wind River Range and back-arc spreading in the Owl Creek Mountains. The immature, juvenile layered gneiss in the Teton Range probably represents an accretionary prism or fore-arc basin onto which high-pressure rocks containing a mature sedimentary sequence were thrust at 2.67 Ga. Although it may be questioned as to when modern-style plate tectonics began in other cratons, it was certainly operating in the Wyoming Province by 2.67 Ga.

Controls on the heterogeneous distribution of mineral deposits through time

Abstract Mineral deposits exhibit heterogeneous distributions, with each major deposit type showing distinctive, commonly unique, temporal patterns. These reflect a complex interplay between formational and preservational forces that, in turn, largely reflect changes in tectonic processes and environmental conditions in an evolving Earth. The major drivers were the supercontinent cycle and evolution from plume-dominated to modern-style plate tectonics in a cooling Earth. Consequent decrease in the growth rate of continental crust, and change from thick, buoyant sub-continental lithospheric mantle (SCLM) in the Precambrian to thinner, negatively buoyant SCLM in the Phanerozoic, led to progressive decoupling of formational and preservational processes through time. This affected the temporal patterns of deposit types including orogenic gold, porphyry and epithermal deposits, volcanic hosted massive sulphide (VHMS), palaeoplacer Au, iron oxide, copper gold (IOCG), platinum group elements (PGE), diamond and probably massive sulphide SEDEX deposits. Sedimentary mineral deposits mined for redox-sensitive metals show highly anomalous temporal patterns in which specific deposit types are restricted to particular times in Earth history. In particular, palaeoplacer uranium, banded iron formation (BIF) and BIF-associated manganese carbonates that formed in the early Precambrian do not reappear in younger basins. The most obvious driver is progressive oxidation of the atmosphere, with consequent long-term changes in the hydrosphere and biosphere, the latter influencing the temporal distribution and peak development of deposits such as Mississippi Valley types (MVT), hosted in biogenic sedimentary rocks.

Stabilisation of Archaean lithospheric mantle: A Re [sbnd]Os isotope study of peridotite xenoliths from the Kaapvaal craton

Os isotopic compositions of lithospheric peridotite xenoliths erupted by kimberlites in the Kaapvaal craton are almost exclusively less radiogenic than estimates of Bulk Earth ( 187Os/ 188Os as low as 0.106) and require long-term evolution in low Re/Os environments. Using Re depletion model ages which assume complete Re removal during formation, the data indicate that cratonic lithosphere stabilisation occurred at, at least, 3.5 Ga, when the lithosphere was over 200 km thick. This thick lithosphere persisted into the Phanerozoic to be sampled by kimberlites. Younger, Proterozoic and Phanerozoic Re depletion ages are interpreted to be largely the result of open system behaviour involving Re addition by metasomatic processes. Some of the younger ages may represent the addition of new lithospheric material during periods of major igneous activity. A mid-Archaean age for the Kaapvaal cratonic mantle concurs with Archaean Re Os ages found in samples of Siberian and Wyoming cratonic mantle. Both shallow (spinel facies) and deep (diamond facies) Kaapvaal peridotites have similar ages (3.3–3.5 Ga) suggesting that 150 km of mantle lithosphere may have accumulated very rapidly. Os isotope estimates for the timing of separation and stabilisation of Kaapvaal cratonic mantle overlap the main period of cratonic crust building and stabilisation (3.5-2.7 Ga). A similar overlap between crust and mantle stabilisation is evident for the Siberian craton. Archaean lithospheric mantle is compositionally different to that formed post-Archaean. The Kaapvaal peridotites have very low FeO compared to post-Archaean peridotites and show a large spread in Mg/Si. Some samples are anomalously Si-enriched compared with post-Archaean mantle samples. This compositional distinction and the varied Nd Os isotope systematics are difficult to explain in terms of accepted models involving ancient melt depletion and subsequent metasomatism. Crystal segregation/cumulate processes have been suggested as a mechanism for forming the compositional range observed in Kaapvaal peridotites. This type of process may have occurred during harzburgite crystallisation from high-degree (> 50%) mantle melts associated with Archaean plume activity. A role for hot mantle plumes in generating the thick lithospheric keels beneath the Kaapvaal and Siberian cratons is supported by the possibility of their rapid formation and their thermal stability with respect to post-Archaean lithosphere. The coincidence of mid-Archaean cratonic mantle differentiation with periods of major crust building and stabilisation on the Kaapvaal and Siberian cratons suggests a link between crust generation and stabilisation and lithospheric mantle formation in the Archaean. Thermal energy from the plume may have been the impetus for major crust building at the time of lithosphere stabilisation, possibly by underplating of basaltic magmas. Direct involvement of mantle plumes in episodes of major mantle and possibly crust differentiation would imply that modern style plate tectonics may not have been the primary mechanism of planetary differentiation in the early Earth. Archaean ages for peridotites originating up to 200 km deep suggest that the mechanical boundary layer beneath continents is at least this thick.

Precambrian tectonics and the physicochemical evolution of the continental crust. II. Lithosphere delamination and ensialic orogeny

Precambrian ensialic orogeny driven by gravitational instability of the lower crust and underlying lithospheric mantle as a consequence of decaying geotherms has been proposed as an alternative cause for tectonism than modern plate tectonics for the Precambrian. Based on a depleted mantle lithosphere, calculations show that this is unlikely. As a consequence, Precambrian orogenic events, whether at ancient plate margins or in an ensialic regime, had to be caused by modern style plate tectonics. Although the process of Wilson-cycle plate tectonics operated throughout Earth's history, there is evidence to suggest that the products of this process—plate margin volcanism and metamorphism—have changed significantly through time due to a secular decrease in heat flow. There is good evidence that some Proterozoic terranes initially developed in an ensialic extensional setting (Etheridge et al.). However, it is proposed here that the cause of later compressive orogenies in these belts must have been transfer of stresses from distant active plate margins and subduction zones. This conclusion necessarily requires that some Precambrian mobile belts must represent sutures which were formerly ancient active plate margins. Although much of the outcrop and metamorphic evidence is ambiguous in determining the ultimate driving force for compressive orogenesis, there is abundant independent evidence to support subduction in the Precambrian when mantle xenoliths, refractory inclusions in diamonds and the trace element compositions of continental basalts are considered. Isotopic data indicate even Archaean ages for some of these peridotite, harzburgite and eclogite xenoliths and diamond inclusions. In addition, the occurrence of high field strength (HFS) element depletions in many continental mafic dykes and volcanics ranging in age back to the Archaean, can only be satisfactorily explained by this geochemical signature ultimately being caused by subduction. This is correct irrespective of whether the crustal influence is due to subduction of sediment into the mantle or in situ crustal assimilation by ascending basalts. Alternative models for HFS element depletions involving delamination of lower crustal eclogite back into the mantle and its subsequent partial melting are rejected. It is argued that subcontinental lithosphere has remained attached to Precambrian shields as an integral part of moving plates in the Earth's history. Later reactivation of this lithospheric mantle beneath continents may explain the occurrence of HFS element depleted continental basalts, yet their absence from modern oceanic regions.

Tectonic evolution in the Archaean and Proterozoic

Evidence for continental motion since at least 3.3 Ga and at minimum average velocities comparable to those of today leaves little doubt that Precambrian tectonic styles are essentially comparable to those of the Phanerozoic plate-tectonic regime and were governed by deformation along and within continental plates. The detailed mechanisms for these motions, however, remain a matter of considerable dispute. For the earliest Archaean, from > 4.2 Ga to perhaps 3.9 Ga, plate tectonics appear unlikely because of thin and “soft” lithosphere: high heat flow probably favoured global hotspot activity with crustal growth of Iceland type through vertical magmatic accretion. The tectonic environment for the generation of about 3.9-2.5 Ga granite-gneiss-greenstone terrains with voluminous production of tonalite-trondhjemite-granodiorite magmas is still uncertain, and both plate margin and intraplate scenarios are possible. Deformation styles in greenstones and adjacent high-grade orthogneisses with intercalated shallow-water supracrustal assemblages suggest extensive horizontal shortening and crustal interstacking. This probably resulted from collisional and/or rotational motion and produced significant intracrustal melting as early as 3.9 Ga. By the end of the Archaean, and locally much earlier, crustal thicknesses reached 35–50 km or more, and the extraordinary high global crust-production rate between about 2.8 and 2.5 Ga may be ascribed to both subduction-related lateral accretion and to extensive magmatic underplating, which finally led to large, stable cratons. The apparent worldwide gap in major tectonic and magmatic activity during the early Proterozoic between about 2.5 and 2.2 Ga is an artifact due to lack of precise age data; both intracontinental as well as arc-type magmatic rocks were produced, though only locally. The voluminuous and worldwide crust-formation event at about 2.1-1.8 Ga provides the first evidence of modern-style plate tectonics, many magmatic rocks, such as in the Canadian Trans-Hudson orogen and in the Svecofennian of southwest Finland, have arc-type geochemical signatures, and both obducted ophiolites and blueschist assemblages have recently been discovered. In addition, the Proterozoic is characterized by long, linear mobile belts whose origin and evolution are still under debate. Many of these belts have tectonic features comparable to Phanerozoic collisional orogens, but in Australia and parts of southern Africa some Proterozoic belts appear to have formed through crustal stretching without lithospheric separation, and subsequent shortening leading to orogenic deformation may have been due to delamination and crustal subduction. Towards the end of the Proterozoic, evidence for modern-style plate tectonics becomes abundant through the recognition of island arc accretion, ophiolite obduction, foreland thrust and fold belts and exotic terranes. One of the best documented example for this tectonic environment is the Arabian-Nubian shield. Although it is doubtful whether modern plate tectonic settings can serve as analogues for all Precambrian tectonic styles, it is clear that global crustal evolution since the early Archaean was governed by the motion of plates which repeatedly came together to form supercontinents and then dispersed again. We shall not understand the detailed mechanism of Precambrian crustal tectonics before we learn how to interpret recent large-scale crustal deformation and its causes.