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- Shumlyanskyy, Leonid, et al.
(författare)
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Eoarchean rock association in the Dniester-Bouh Domain of the Ukrainian Shield: A suite of LILE-depleted enderbites and mafic granulites
- 2020
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433.
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of a study of an Eoarchean rock assemblage in the Dniester-Bouh Domain of the Ukrainian Shield. This comprises granulite-facies granitoids intercalated with mafic and ultramafic granulites. Zircon U-Pb geochronology indicates enderbite crystallisation at 3786 ± 32 Ma, followed by a subsequent event at ca. 3500 Ma. Several events can be tentatively identified that affected these rocks between ca. 3000 and 2700 Ma. The last zircon growth event took place in response to granulite facies metamorphism and included two separate episodes at ca. 2000 and ca. 1900 Ma. The oldest two zircon populations in enderbites have εHf values around 0, indicating their crystallisation from a protolith with a short crustal residence time. Zircons that crystallised during the 3000–2700 Ma event(s) vary in Hf isotope systematics from εHf ~ 1 at ca. 3000 Ma to εHf ~ −14 at c. 2700 Ma. Paleoproterozoic zircons reveal even more significant variations in εHf value from +6 to –22. Such variations are indicative of juvenile input and mixing with old non-radiogenic Hf.All Eoarchean rocks are depleted in incompatible trace elements and have negative Ta-Nb, P, and Ti anomalies. Compared to the typical TTG associations, enderbites record depletion in felsic components (SiO2, Na2O, K2O, Rb, Th), and enrichment in mafic ones (TiO2, MgO, CaO, V), allowing them to be defined as “mafic” or “depleted” TTG.Geochemical data indicate that mafic and ultramafic rocks of the Dniester-Bouh Domain formed by shallow high-degree melting of the mantle, with the absence of garnet in their source, and the presence of residual Ti-bearing minerals and/or amphibole. In contrast, enderbites were formed from a mixed garnet-bearing amphibolite – eclogite source, i.e. melting over a range of pressures/depths. Our preferred model for the formation of the Eoarchean rock association involves the shallow melting of mantle and formation of basalts and accompanying ultramafic cumulates at a spreading centre, with subsequent underthrusting of one segment of oceanic crust beneath the other, and partial melting of hydrated metamorphosed (eclogitized) mafic rocks in the underthrust plate, leading to the formation of the TTG melts
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| 2. |
- Siliauskas, Laurynas, et al.
(författare)
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What the ca. 1.83 Ga gedrite-cordierite schists in the crystalline basement of Lithuania tell us about the late Palaeoproterozoic accretion of the East European Craton
- 2018
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Ingår i: GFF. - : Taylor & Francis. - 1103-5897 .- 2000-0863. ; 140:4, s. 332-344
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Tidskriftsartikel (refereegranskat)abstract
- ABSTRACTIn the western East European Craton (EEC), southern Lithuania, a suite of fine-grained, thinly bedded rocks of unusual composition has been shown to have originally comprised intermediate and felsic volcanic rocks. They extruded at ca. 1.83 Ga and were hydrothermally altered prior to metamorphism, which converted them into garnet-, gedrite-, anthophyllite-, staurolite- and cordierite-bearing schists. After the rocks have experienced a 630°C and 7 kbar metamorphism, they were uplifted to 15 km (5 kbar) probably at ca. 1.73 Ga. They were reheated to 640°C at ca. 1.50 Ga (monazite age). The monazite age of ca. 1.50 Ga is coeval with the emplacement of the neighboring 1.50 Ga Anorthosite-Mangerite-Charnockite-Granite (AMCG) Mazury complex. The ca. 1.83 Ga volcanic suites in Lithuania and northern Poland, together with the Oskarshamn-Jönköping belt (OJB) in south-central Sweden, may belong to the same chain of volcanic island arcs, and thus provide information on the evolution of the entire western EEC. The ca. 1.50 Ga metamorphic reworking and the replacement of the Mazury AMCG suite may have been triggered by the Danopolonian orogeny further west and, at a larger scale, accretion of the continental margin of Columbia.
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