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Tectonic significance of the Variscan suture between Brunovistulia and the Bohemian Massif
Plate-tectonic processes at ca. 2.0 Ga: Evidence from >600 km of plate convergence
Archaean granulite-facies paragneisses from central Swaziland: inferences on Palaeoarchaean crustal reworking and a complex metamorphic history
Tectono-Metamorphic and Geochronologic Studies from Sandmata Complex, Northwest Indian Shield: Implications on Exhumation of Late-Palaeoproterozoic Granulites in an Archaean-early Palaeoproterozoic Granite-Gneiss Terrane
Tectono-Metamorphic and Geochronologic Studies from Sandmata Complex, Northwest Indian Shield: Implications on Exhumation of Late-Palaeoproterozoic Granulites in an Archaean-early Palaeoproterozoic Granite-Gneiss Terrane
Hydrothermal origin and age of jadeitites from Sierra del Convento Mélange (Eastern Cuba)
Single zircon U–Pb ages and geochemistry of granitoid gneisses from SW Poland: evidence for an Avalonian affinity of the Brunian microcontinent
The role of geochronology in understanding continental evolution
Abstract Geochronology has become one of the most essential tools in reconstructing processes of continental growth and evolution, and in situ dating of minerals has become common practice through the development of high-resolution ion microprobes and laser ablation inductively coupled plasma mass spectrometry techniques. Zircon has established itself as the most robust and reliable mineral to record magmatic and metamorphic processes. The combination of mineral ages with Sm–Nd, Lu–Hf and O isotopic systematics constrains magma sources and their evolution, and a picture is emerging that supports the beginning of modern-style plate tectonics in the early Archaean. Major fields for future research in geochronology include the search for very old crustal remnants, the establishment of Precambrian supercontinents, reconstruction of magmatic and tectonic processes in accretionary orogens, verification of ancient high-pressure rocks, and the reconstruction of detailed metamorphic histories by dating minerals in their original textural settings.
Zircon Ages from the Baydrag Block and the Bayankhongor Ophiolite Zone: Time Constraints on Late Neoproterozoic to Cambrian Subduction- and Accretion-Related Magmatism in Central Mongolia
Devonian arc-related magmatism in the Tseel terrane of SW Mongolia: chronological and geochemical evidence
Accretionary orogens through Earth history
Abstract Accretionary orogens form at intraoceanic and continental margin convergent plate boundaries. They include the supra-subduction zone forearc, magmatic arc and back-arc components. Accretionary orogens can be grouped into retreating and advancing types, based on their kinematic framework and resulting geological character. Retreating orogens (e.g. modern western Pacific) are undergoing long-term extension in response to the site of subduction of the lower plate retreating with respect to the overriding plate and are characterized by back-arc basins. Advancing orogens (e.g. Andes) develop in an environment in which the overriding plate is advancing towards the downgoing plate, resulting in the development of foreland fold and thrust belts and crustal thickening. Cratonization of accretionary orogens occurs during continuing plate convergence and requires transient coupling across the plate boundary with strain concentrated in zones of mechanical and thermal weakening such as the magmatic arc and back-arc region. Potential driving mechanisms for coupling include accretion of buoyant lithosphere (terrane accretion), flat-slab subduction, and rapid absolute upper plate motion overriding the downgoing plate. Accretionary orogens have been active throughout Earth history, extending back until at least 3.2 Ga, and potentially earlier, and provide an important constraint on the initiation of horizontal motion of lithospheric plates on Earth. They have been responsible for major growth of the continental lithosphere through the addition of juvenile magmatic products but are also major sites of consumption and reworking of continental crust through time, through sediment subduction and subduction erosion. It is probable that the rates of crustal growth and destruction are roughly equal, implying that net growth since the Archaean is effectively zero.
Modern-style plate tectonics can be tracked into the geologic past with petrotectonic assemblages and other platetectonic indicators. These indicators suggest that modern plate tectonics were operational, at least in some places on the planet, by 3.0 Ga, or even earlier, and that they became widespread by 2.7 Ga. The scarcity of complete ophiolites before 1 Ga may be explained by thicker oceanic crust and preservation of only the upper, basaltic unit. The apparent absence of blueschists and ultrahigh-pressure metamorphic rocks before ca. 1 Ga may reflect steeper subduction geotherms and slower rates of uplift at convergent margins. It is unlikely that plate tectonics began on Earth as a single global “event” at a distinct time, but rather it is probable that it began locally and progressively became more widespread from the early to the late Archean.
Proterozoic Tectonostratigraphy and Paleogeography of Central Madagascar Derived from Detrital Zircon U-Pb Age Populations
Abstract In-situ U-Th-Pb analyses by ion-microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon. This approach helps: (a) to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the Rogaland anorthosite-norite intrusive complex; and (b) to constrain the age of M 2 ultrahigh-temperature (UHT) metamorphism and the subsequent retrograde M 3 event. All samples yield magmatic inherited zircon of c. 1035 Ma, some an additional group at c. 1050 Ma. This suggests that loss of Pb by volume diffusion in non-metamict zircon is not an important factor even under extreme crustal conditions. Furthermore, the identical inheritance patterns in aluminous (garnet, cordierite ± osumilite-bearing) migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite. Results for the M 1 metamorphic event at c. 1000 Ma BP are consistent in all samples, including those from outside the orthopyroxene-in isograd. The latter do not show evidence for zircon growth during the M 2 metamorphic episode. Zircon intergrown with or included within M 2 metamorphic minerals (magnetite, spinel, orthopyroxene) give an age of 927 ± 7 Ma (2 σ, n = 20). The youngest observed results are found in zircon outside M 2 minerals, some overgrown by M 3 mineral assemblages (late garnet coronas, garnet + quartz and orthopyroxene + garnet symplectites) and yield a slightly younger pooled age of 908 ± 9 Ma (2 σ, n = 6). These textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the UHT event. These youngest age groups are consistent with the emplacement age of the Rogaland intrusive complex and the last magmatic activity (Tellnes dyke intrusion), respectively. This is direct and conclusive evidence for UHT metamorphism in the regional aureole being caused by the intrusions, and corrects earlier notions that the events are not linked. Trace element behaviour of zircon (Tb/U and Y content) has been tracked through time in the samples and shows variations both within and between samples. This heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use of ‘rules of thumb’ in the interpretation of zircon chemistry, but chemical tracers are useful for recognition of zircon growth or recrystallization during metamorphism.
Neoproterozoic to Paleozoic Geology of the Altai Orogen, NW China: New Zircon Age Data and Tectonic Evolution
Single Zircon Evaporation and SHRIMP Ages for Granulite-Facies Rocks in the Mozambique Belt of Tanzania
Abstract Nd crustal residence ages combined with xenocrystic and detrital zircon ages of Neoproterozoic and early Palaeozoic granitoid gneisses and metasediments from the NE Bohemian Massif (West Sudetes and Erzgebirge) suggest an origin from a basement with significant Grenvillian and Svecofennian–Birimian–Amazonian age components. Large contributions from Archaean crust appear to be missing. Nd model ages of 1.4–1.7 Ga as well as Meso- to Palaeoproterozoic zircon xenocrysts ages for the Lugian Domain (Jizerské, Krkonoše, and Orlica–Sniezník Mountains) are interpreted as evidence for a predominantly Palaeoproterozoic basement that underwent rejuvenation during Meso- and Neoproterozoic– Ordovician magmatic events. Our interpretation of the Nd model ages may be relevant for the western part of the Variscan Belt, where a similar range of Nd model ages characterizes large crustal segments. Samples from the Silesian Domain (Desná Dome, Staré Město Belt, Velké Vrbno Unit) of the Brunia Microcontinent, in thrust contact with the Lugian Domain, yielded Nd crustal residence ages of 1.1–1.3 Ga, which coincide with the oldest ages of xenocrystic zircons. Brunia is interpreted as a predominantly juvenile Grenvillian-age crustal segment, possibly constituting a former arc batholith outboard from the more ensialic setting of the Lugian Domain. Nd crustal residence ages for our 77 whole-rock samples, including 41 new data, as well as 185 xenocrystic and detrital zircon ages from the northern and northeastern Bohemian Massif agree with those of Neoproterozoic rocks from northern South America and support an origin from a northwest Gondwana palaeogeographical location.