Seminars

a continental crust with a felsic composition is unique to the Earth. The Archean continental cratons consist mainly of orthogneisses and supracrustals that were metamorphosed from tonalite-trondhjemite-granodiorite (TTG) plutons and mafic-ultramafic volcanic rocks with minor sedimentary rocks, respectively. Available data suggest that these rock assemblages were derived from oceanic crust or its partial melts. The oceanic crust is generally divisible into ocean basins, mid-ocean ridge, island arc and oceanic plateau (ocean island), of which the former has an average crustal thickness ranging from 5-10 km that is too thin to generate Archean TTG plutons. Therefore, continents with felsic composition must have been originated either from island arcs under a plate tectonic regime or from oceanic plateaus derived from mantle plume. The formation of Archean TTG rocks can be well explained by the island arc model in which the Archean high-pressure-type TTG rocks is considered to have been derived from the partial melting of subducted slabs, whereas the Archean low-pressure-type TTG rocks equivalent to calc-alkaline granitoids were derived from the partial melting of juvenile basaltic crust which itself formed by the partial melting of the mantle wedge with addition of fluids released from the subducted slabs. However, the island arc model is failure in explaining the absence of andesites from some Archean greenstone terranes, the presence of komatiites with temperatures of 1600ºC, nearly coeval emplacement of TTG plutons on a cratonic scale, dome-and-keel structures and anticlockwise P-T paths involving isobaric cooling that characterize the deformation and metamorphism of Archean continental cratons. In contrast, the mantle-plume oceanic plateau model can reasonably interpret the origin of the bimodal volcanic assemblages in Archean greenstones of which the tholetiites and komatiites were derived from partial melting of the head and tail of a mantle plume, respectively, whereas the felsic dacite, rhyolitic dacite and rhyolite were derived from the partial melting of the crust. According to the oceanic plateau model, Archean TTG magmas were derived from the partial melting of basaltic rocks from the lower part of oceanic plateau, which can rationally explain why voluminous Archean TTG magmas were produced within a short period and emplaced over the whole craton without any accretionary patterns. The mantle-plume oceanic plateau model can also well explain the Archean dome-and-keel structure, anticlockwise P-T paths, and absence of blueschist and paired metamorphic belts that are considered to be earmarks of modern island arcs. However, an oceanic plateau model is failure in explaining sources of enough H2O for the aqueous partial melting of basaltic rocks to generate TTG magmas.

review sedimentary, igneous and metamorphic proxies along with paleomagnetic data to infer both the development of rigid lithospheric plates and their independent relative motion, and conclude that significant changes in Earth behaviour occurred in the mid- to late Archean, between 3.2 Ga and 2.5 Ga. These data include: sedimentary rock associations inferred to have accumulated in passive continental margin settings, marking the onset of seafloor spreading; the oldest foreland basin deposits associated with lithospheric convergence; a change from thin, new continental crust of mafic composition to thicker crust of intermediate composition, increased crustal reworking and the emplacement of potassic and peraluminous granites, indicating stabilization of the lithosphere; replacement of dome and keel structures in granite-greenstone terranes, which relate to vertical tectonics, by linear thrust imbricated belts; the commencement of temporally paired systems of intermediate and high dT/dP gradients, with the former interpreted to represent subduction to collisional settings and the latter representing possible hinterland back-arc settings or ocean plateau environments. Paleomagnetic data from the Kaapvaal and Pilbara cratons for the interval 2780–2710Ma and from the Superior, Kaapvaal and Kola-Karelia cratons for 2700–2440Ma suggest significant relative movements. These changes in the behaviour and character of the lithosphere to be consistent with a gestational transition from a non-plate tectonic mode, arguably with localized subduction, to the onset of sustained plate tectonics.

collision orogenesis. Using a dataset of 564 localities with robust peak pressure (P), temperature (T) and age determinations, metamorphism is classified into three types using the thermobaric ratios (T/P), since this ratio varies both spatially and temporally in orogens. Each type of metamorphism is associated with a specific tectonic setting (from low to high T/P: subduction, mountain belt and orogenic hinterland). Orogens younger than 850 Ma record bimodal metamorphism (paired belts with lower and higher T/P), whereas metamorphism at lower T/P is rare in older orogens. A strong case can be made for plate tectonics back to the Neoproterozoic and, probably, back to the early Paleoproterozoic, when the breakup of several supercratons led to the reconfiguration of the fragmented continental lithosphere in the supercontinent Columbia. The suturing orogenic belts in Columbia yield the earliest record of bimodal metamorphism; they also preserve seismic evidence of probable subduction. Significant changes in the crustal record of metamorphism since c. 3.0 Ga, confirmed by sequential analysis (by cumulative sum) of T/P for the dataset as a whole, suggest three geodynamic cycles: prior to the early Paleoproterozoic tectono-magmatic lull (TML); between the TML and the breakup of Rodinia; and, since the breakup of Rodinia. Earth records anomalous metamorphism (hottest) and magmatism (anorthosites and rapakivi granites) between the formation of Columbia and the breakup of Rodinia⏤a period some have called the 'Boring Billion'; this may be due to reduced subduction globally and a period of decreased plate tectonic activity. Modelling indicates that a plate tectonic slowdown after the completion of Columbia led to a warmer mantle and hotter crustal metamorphism, and to stable conditions that were just right to generate anorthosite magmas. But, what caused the slowdown? Based on the main controls on variations in plate velocities, the emergence and evolution of plate tectonics on Earth could have been related to the rise of the continents and the accumulation of sediments at continental edges, and subsequently in trenches, to lubricate and stabilize subduction. Grounded ice sheets reached sea level at all latitudes during two long-lived glacial epochs, in the Siderian and Cryogenian; each produced a substantial volume of sediments in trenches to lubricate subduction. Cases of HP and UHP metamorphism correspond closely with glaciations. This correlation suggests that deep subduction of the continental crust was enabled by sediment lubrication of the plate interface. Conversely, the c. 1000-Myr-long break in the record of HP–UHP metamorphism (only one known occurrence) corresponds to the enigmatic Proterozoic glacial gap (aka the ‘Boring Billion’). Thus, there is an unanticipated connection between the climate of Earth’s surface and the extreme depths to which continental crust can be subducted. Prior to a Neoarchean transition to plate tectonics, subduction may have started multiple times, creating conflicting signals in the geological record, but may not have been stable on a hotter Earth precluding an earlier transition.

dynamic evolution of zircon age peaks through time. This analysis demonstrates that zircon age peaks from ancient sedimentary successions are often out of phase with the detrital zircon record obtained from modern sediments. The growth and diminishment of the zircon age peaks through time implies the presence of continental crust whose age is not proportionately represented in the modern record, and therefore that the current crustal archive is biased. However, when the detrital zircon record is viewed in terms of its evolution through time, that is taken as a time-lapse view of continental growth, it appears there never was a time in Earth history without an associated zircon age peak. The analysis of detrital zircon age peaks presented herein also reveals an evolution that can be broadly divided into three temporal groupings that broadly correspond with phases of Earth’s tectonic evolution, namely pre-supercontinent continental growth (pre-2.1 Ga), Earth’s middle age (2.1-0.8 Ga), and post-onset of modern-day plate tectonics (post-0.8 Ga). These three groupings each display increasing degrees of zircon age diversification with time, and are a likely result of a net increase of preserved continental crust through time. The presence of these three tectonic states in multiple geologic proxies (detrital zircon ages, changing styles of metamorphism, paleogeography/supercontinents) suggests that while the growth of the continental crust is continuous, the tectonic processes that shape the long-term preservation of the crust have evolved over geologic time (published in American Journal of Science, 2020).