Evidence of Paleoproterozoic plate tectonics: eclogitic subduction (original) (raw)
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Scientific Reports, 2018
Knowing which geodynamic regimes characterised the early Earth is a fundamental question. This implies to determine when and how modern plate tectonics began. Today, the tectonic regime is dominated by mobile-lid tectonics including deep and cold subduction. However, in the early Earth (4.5 to 2 Ga) stagnant-lid tectonics may also have occurred. The study of high pressure-low temperature (HP-LT) metamorphic rocks is important, because these rocks are only produced in present-day subduction settings. Here, we characterize the oldest known HP-LT eclogite worldwide (2089 ± 13 Ma; 17-23 kbar/500-550 °C), discovered in the Democratic Republic of the Congo. We provide evidence that the mafic protolith of the eclogite formed at 2216 ± 26 Ma in a rift-type basin, and was then subducted to mantle depths (>55 km) before being exhumed during a complete Wilson cycle lasting ca. 130 Ma. Our results indicate the operation of modern mobile-lid plate tectonics at 2.2-2.1 Ga.
Journal of African Earth Sciences, 2019
High-to ultrahigh-pressure metamorphic assemblages consisting of garnet-omphacitic clinopyroxene bearing mafic rocks have been identified within the Paleoproterozoic Nyong Group in SW Cameroon, at the northwestern margin of the Archean Congo craton. These rocks were investigated in detail and for the first time evidence for eclogite facies metamorphism at ca 25 kbar and 850°C is provided. A clockwise P-T path with nearly isothermal decompression (ITD) is deduced from mineral zoning and textural relationships characterized by mineral recrystallization and multi-layered coronitic overgrowths of plagioclase and clinopyroxene surrounding garnet porphyroblasts. These P-T conditions imply a burial depth greater than 90 km, at lower geothermal gradient of ca 10°C/km. The geochemical signature of ten representative rock samples show that two groups of eclogite facies rocks genetically originate from mostly basaltic and basaltic andesite compositions, with a characteristic upper mantle-derived tholeiitic trend. Moreover, their chondrite and MORB normalized REE and trace element concentrations are characterized by nearly flat REE patterns with very little to no Eu anomaly, (La/Sm) N ≥ 1 and Zr/Nb ≤ 10, as well as a gradual depletion from LREE to HREE with also very little to no Eu anomaly, but (La/Sm) N < 1, Zr/Nb > 10 and negative anomalies in Th, K, Nb, Ta, Sr, Zr and Ti consistent with midocean ridge basalt (MORB) contaminated by a subduction component or by a crustal component. Previous available geochronological data coupled with our new petrological, mineralogical and geochemical findings clearly indicate that the eclogite facies metabasites from the Eburnean Nyong Group between 2100 and 2000 Ma represent one of the oldest subducted oceanic slab or trace of a suture zone so far recorded within the West Central African Fold Belt (WCAFB). The geodynamic implications of these eclogites suggest a subduction-related
Nature Communications , 2021
(Authors: Buntin S., Artemieva I.M., Malehmir A., Thybo H., et al.) The nature of the lower crust and the crust-mantle transition is fundamental to Earth sciences. Transformation of lower crustal rocks into eclogite facies is usually expected to result in lower crustal delamination. Here we provide compelling evidence for long-lasting presence of lower crustal eclogite below the seismic Moho. Our new wide-angle seismic data from the Paleoproterozoic Fennoscandian Shield identify a 6-8 km thick body with extremely high velocity (Vp~8.5-8.6 km/s) and high density (>3.4 g/cm 3) immediately beneath equally thinned high-velocity (Vp~7.3-7.4 km/s) lowermost crust, which extends over >350 km distance. We relate this observed structure to partial (50-70%) transformation of part of the mafic lowermost crustal layer into eclogite facies during Paleoproterozoic orogeny without later delamination. Our findings challenge conventional models for the role of lower crustal eclogitization and delamination in lithosphere evolution and for the long-term stability of cratonic crust.
Long-lived Paleoproterozoic eclogitic lower crust
Nature Communications
The nature of the lower crust and the crust-mantle transition is fundamental to Earth sciences. Transformation of lower crustal rocks into eclogite facies is usually expected to result in lower crustal delamination. Here we provide compelling evidence for long-lasting presence of lower crustal eclogite below the seismic Moho. Our new wide-angle seismic data from the Paleoproterozoic Fennoscandian Shield identify a 6–8 km thick body with extremely high velocity (Vp ~ 8.5–8.6 km/s) and high density (>3.4 g/cm3) immediately beneath equally thinned high-velocity (Vp ~ 7.3–7.4 km/s) lowermost crust, which extends over >350 km distance. We relate this observed structure to partial (50–70%) transformation of part of the mafic lowermost crustal layer into eclogite facies during Paleoproterozoic orogeny without later delamination. Our findings challenge conventional models for the role of lower crustal eclogitization and delamination in lithosphere evolution and for the long-term stabi...
Precambrian Research, 2014
Neoproterozoic eclogite and garnet amphibolites, representing retrogressed eclogites, from the Tassendjanet-Tidéridjaouine terrane in the Pan-African Western Hoggar (Tuareg shield, Southern Algeria) form lenses enclosed in 1.8 Ga subalkaline Paleoproterozoic orthogneisses. They lie along a major sinistral shear zone marking the boundary with the Archean/Paleoproterozoic granulitic In Ouzzal terrane and are associated with high-pressure metasediments. According to thermodynamic calculations, peak condition of the eclogitic stage is 650 • C, 20-22 kbar followed by post-peak heating and exhumation at 730 • C, 10-14 kbar and cooling to 610 • C, 7-10 kbar. Their major, trace-element and isotopic compositions are consistent with emplacement of former basalts in the shoulder of a 700-800 Ma continental rift that evolved to an oceanic basin. The eclogite and garnet amphibolites were thus part of the continental portion of a subducting slab that was pulled down by a denser oceanic lithosphere. Sheath and recumbent folds associated with the exhumation of HP units are verging toward the west, meaning that the former slab was dipping to the east. U-Pb zircon dating on metamorphic zircons bordering ilmenite and rutile in the eclogite gave an age of 623 ± 2 Ma, interpreted as the syn-collisional exhumation stage of the high-pressure unit. This event was rapidly followed by transpressional tectonics and abundant magmatism linked to the northward motion of the adjacent In Ouzzal terrane.
Geology, 2024
Post-Archean secular changes in continental crust composition, which provide key evidence for the evolution of plate tectonics, remain uncertain, particularly regarding the lower crust. Here, by digitizing 18,000 km of seismic profiles, we demonstrate a change in bulk crustal composition at the Proterozoic-Phanerozoic transition. We document that a mafic crustal layer is preserved in Proterozoic orogens but generally absent in Phanerozoic orogens. We explain this fundamental shift by a change in the global subduction style, where continental collision became important in the Phanerozoic. Densification of the lower crust by widespread eclogitization, triggered by continental collision and subduction, led to massive recycling of mafic lower crust into the mantle, leaving behind buoyant felsic crust and promoting the rise of continents, which led to the emergence of large continental areas above sea level and the related Neoproterozoic oxidation event, followed by the explosion of life in the Phanerozoic.
Late Miocene coesite-eclogite exhumed in the Woodlark Rift
Geology, 2008
Late Miocene-Pliocene eclogites were exhumed in the Woodlark Rift of eastern Papua New Guinea, an actively extending region west of the Woodlark Basin seafl oor spreading center. We report the discovery of coesite in late Miocene eclogite from the lower plate of one of the D'Entrecasteaux Islands metamorphic core complexes within the Woodlark Rift. Zircon crystallization temperatures (650-675 °C) and 238 U/ 206 Pb age (ca. 8 Ma), and rutile thermometry (695-743 °C) combined with garnet-pyroxene thermometry (600-760 °C) and garnet-pyroxenephengite barometry (18-27 kbar), indicate that the coesite-eclogite was exhumed from mantle depths (≥90 km) to the Earth's surface at plate tectonic rates (cm yr-1). This late Miocene coesiteeclogite is the youngest exhumed ultrahigh-pressure (UHP) rock on Earth, and its preservation ahead of the westward-propagating seafl oor spreading center forces reevaluation of models for UHP exhumation, as well as the geologic and tectonic evolution of the Woodlark Rift.
Do Cenozoic analogues support a plate tectonic origin for the Earth’s earliest continental crust?
Archean continental crust largely comprises the trondhjemite, tonalite, and granodiorite/ dacite (TTG/D) suite of igneous rocks. Formation of the earliest Archean (>3.5 Ga) TTG/Ds is controversial, being attributed to either subduction zone processes with active plate tectonics or thermochemical mantle convection with no plate tectonic processes. A suite of Cenozoic adakite-like lavas in Jamaica has geochemical compositions comparable to early Archean TTG/D. The data indicate that the adakites were generated by underthrusting (or subducting) and partial melting of oceanic plateau crust beneath Jamaica. This setting is analogous to proposed plate tectonic processes in the early Archean where hot, thick, and more buoyant Archean oceanic crust underthrusts adjacent plates. The new adakite data imply that earliest Archean TTG/D continental crust could have formed above primitive subduction zones.