Search results for "Archean"
showing 10 items of 87 documents
Non‐subduction origin for 3.2 Ga high‐pressure metamorphic rocks in the Barberton granitoid‐greenstone terrane, South Africa
2019
The occurrence of high-pressure (HP) garnet-bearing metamorphic rocks in the southern Barberton granitoid-greenstone terrane (BGGT), South Africa, has been proposed as a key indicator of the onset of modern plate tectonics at ca. 3.2 Ga. Here, we report new zircon/titanite U-Pb ages of garnet-bearing HP metamorphic rocks and associated granitoids in the BGGT that argue against such an interpretation. The results show that HP metamorphism occurred synchronously with granitoid magmatism in the Stolzburg domain, the supposed subducted plate, during two episodes at 3.4 and 3.2 Ga, and that these two episodes of magmatism occurred on both sides of the assumed suture zone, a feature that cannot e…
Subduction or sagduction? Ambiguity in constraining the origin of ultramafic–mafic bodies in the Archean crust of NW Scotland
2016
Abstract The Lewisian Complex of NW Scotland is a fragment of the North Atlantic Craton. It comprises mostly Archean tonalite–trondhjemite–granodiorite (TTG) orthogneisses that were variably metamorphosed and reworked in the late Neoarchean to Paleoproterozoic. Within the granulite facies central region of the mainland Lewisian Complex, discontinuous belts composed of ultramafic–mafic rocks and structurally overlying garnet–biotite gneiss (brown gneiss) are spatially associated with steeply-inclined amphibolite facies shear zones that have been interpreted as terrane boundaries. Interpretation of the primary chemical composition of these rocks is complicated by partial melting and melt loss…
Late Palaeoproterozoic depositional age for khondalite protoliths in southern India and tectonic implications
2016
Abstract SHRIMP dating of detrital zircons from garnet–sillimanite–cordierite paragneisses (khondalites) of the Trivandrum Block, southern India, revealed nanometer-scale Pb isotopic inhomogeneity in most grains that we ascribe to annealing processes during ultra-high-temperature metamorphism at ca. 570 Ma. Our age data for zircons from six representative khondalite samples do not document any Neo- or Mesoproterozoic detrital grains, and we conclude from the concordant ages and discordant minimum 207Pb/206Pb ages that the khondalite precursor sediments were deposited more than 2.1 Ga ago and were subsequently intruded by granitoid rocks at ca. 1765–2100 Ma. Some detrital zircons in the khon…
The ~2730 Ma onset of the Neoarchean Yilgarn Orogeny
2017
The timing of the onset of an orogeny is commonly constrained indirectly, because early orogenic structures are rarely exposed, or are overprinted. Establishing the onset of an Archean orogeny is considerably more challenging, because of the more fragmented geological record and the general lack of consensus about Archean geodynamics. We combine existing tectono-stratigraphic data with new structural and geophysical datasets to establish the onset of the Neoarchean Yilgarn Orogeny (Yilgarn Craton, Western Australia). We show that the surface of the c. 2960–2750 Ma deep-marine Yilgarn greenstone sequence was uplifted, eroded and unconformably overlain by a c. 2730 Ma, syntectonic clastic seq…
Granulites and Palaeoproterozoic lower crust of the Baidarik Block, Central Asian Orogenic Belt of NW Mongolia
2017
Abstract Mafic granulite xenoliths are hosted by garnetiferous charnockites in the Archaean to Palaeoproterozoic Bumbuger Complex of northwestern Mongolia, one of the exotic basement terranes in the Central Asian Orogenic Belt. These rocks crystallized at ca. 1850 Ma under granulite-facies conditions (800 ± 27 °C, 6.8 ± 0.6 kbar) in the lower crust and were partly retrogressed to amphibolite-facies during ascent to higher crustal levels as a result of strong deformation resulting in northwest-trending isoclinal folds. The mafic xenoliths are likely derived from gabbroic protoliths, and geochemical, Hf-in-zircon and Nd whole-rock isotopic data suggest these rocks to have originated from pare…
Photoferrotrophy: Remains of an Ancient Photosynthesis in Modern Environments
2017
Photoferrotrophy, the process by which inorganic carbon is fixed into organic matter using light as an energy source and reduced iron [Fe(II)] as an electron donor, has been proposed as one of the oldest photoautotrophic metabolisms on Earth. Under the iron-rich (ferruginous) but sulfide poor conditions dominating the Archean ocean, this type of metabolism could have accounted for most of the primary production in the photic zone. Here we review the current knowledge of biogeochemical, microbial and phylogenetic aspects of photoferrotrophy, and evaluate the ecological significance of this process in ancient and modern environments. From the ferruginous conditions that prevailed during most …
Sulfur isotope's signal of nanopyrites enclosed in 2.7 Ga stromatolitic organic remains reveal microbial sulfate reduction.
2018
18 pages; International audience; Microbial sulfate reduction (MSR) is thought to have operated very early on Earth and is often invoked to explain the occurrence of sedimentary sulfides in the rock record. Sedimentary sulfides can also form from sulfides produced abiotically during late diagenesis or metamorphism. As both biotic and abiotic processes contribute to the bulk of sedimentary sulfides, tracing back the original microbial signature from the earliest Earth record is challenging. We present in situ sulfur isotope data from nanopyrites occurring in carbonaceous remains lining the domical shape of stromatolite knobs of the 2.7-Gyr-old Tumbiana Formation (Western Australia). The anal…
Isothermal decompression history in the “Western Granulite” terrain, central Tanzania: Evidence from reaction textures and trapped fluids in metapeli…
2008
Abstract The Mozambique Belt (MB) of the East Africa Orogen contains large areas of granulite-facies migmatitic gneisses with Archaean and Palaeoproterozoic protolith ages and that were recycled during the Neoproterozoic Pan-African orogeny. The study area is situated along the Great Ruaha River and within the Mikumi National Park in central Tanzania where migmatitic gneisses and mafic to intermediate granulites are interlayered with Neoproterozoic granulite-facies migmatitic metapelites. Mineral textures suggest isothermal decompression, with the peak mineral assemblage comprising Grt–Bt–Ky–Kfs–Pl–Qtz ± Phn ± Ti-Oxide ± melt and amphibolite-facies retrograde assemblage Grt–Bt–Sil–Ms–Kfs–Pl…
The petrology of two distinct granulite types in the Hengshan Mts, China, and tectonic implications
2005
Abstract The Archean to Proterozoic Hengshan Complex (North China Craton), comprises tonalitic and granodioritic gneisses with subordinate mafic lenses, pegmatites and granites. Amphibolite facies assemblages predominate, although granulite-facies relics are widespread, and greenschist-facies retrogression occurs in km-wide shear zones. Mafic lenses, locally abundant, occur as strongly deformed amphibolite (hornblende+plagioclase) boudins or sheets. In contrast to previously published models we find two series of mafic rocks with distinctly different granulite-facies evolutions. In the north of the Complex, relict high-pressure mafic granulites are garnet+clinopyroxene-bearing rocks with a …
Biogeochemical Cycling of Nitrogen on the Early Earth
2013
Variations in the nitrogen isotope composition of ancient organic matter and associated sediments provide clues for the early evolution of Earth's atmosphere–ocean–biosphere system. In particular, large isotopic variations have been linked to the protracted oxygenation of Earth's atmosphere during the Precambrian. Important problems being investigated include the nature of the variations observed at specific times in Earth's history and the degree of preservation of ancient nitrogen biogeochemical signatures during diagenesis and metamorphism. Interpreting these records in Archean sedimentary environments and their possible implications for the evolution of Earth's early atmosphere, ocean, …