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      Single crystal U–Pb zircon age and Sr–Nd isotopic composition of impactites from the Bosumtwi impact structure, Ghana: Comparison with country rocks and Ivory Coast tektites

      Chemical Geology
      Elsevier
      sr–nd isotopes, impactite, u–pb zircon, ghana, bosumtwi crater

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          Abstract

          The 1.07 Myr old Bosumtwi impact structure (Ghana), excavated in 2.1–2.2 Gyr old supracrustal rocks of the Birimian Supergroup, was drilled in 2004. Here, we present single crystal U–Pb zircon ages from a suevite and two meta-graywacke samples recovered from the central uplift (drill core LB-08A), which yield an upper Concordia intercept age of ca. 2145 ± 82 Ma, in very good agreement with previous geochronological data for the West African Craton rocks in Ghana. Whole rock Rb–Sr and Sm–Nd isotope data of six suevites (five from inside the crater and one from outside the northern crater rim), three meta-graywacke, and two phyllite samples from core LB-08A are also presented, providing further insights into the timing of the metamorphism and a possibly related isotopic redistribution of the Bosumtwi crater rocks. Our Rb–Sr and Sm–Nd data show also that the suevites are mixtures of meta-greywacke and phyllite (and possibly a very low amount of granite). A comparison of our new isotopic data with literature data for the Ivory Coast tektites allows to better constrain the parent material of the Ivory Coast tektites (i.e., distal impactites), which is thought to consist of a mixture of metasedimentary rocks (and possibly granite), but with a higher proportion of phyllite (and shale) than the suevites (i.e., proximal impactites). When plotted in a Rb/Sr isochron diagram, the sample data points ( n = 29, including literature data) scatter along a regression line, whose slope corresponds to an age of 1846 ± 160 Ma, with an initial Sr isotope ratio of 0.703 ± 0.002. However, due to the extensive alteration of some of the investigated samples and the lithological diversity of the source material, this age, which is in close agreement with a possible “metamorphic age” of ∼ 1.8–1.9 Ga tentatively derived from our U–Pb dating of zircons, is difficult to consider as a reliable metamorphic age. It may perhaps reflect a common ancient source whose Rb–Sr isotope systematics has not basically been reset on the whole rock scale during the Bosumtwi impact event, or even reflect another unknown geologic event.

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          Most cited references60

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          Age and provenance of the target materials for tektites and possible impactites as inferred from Sm-Nd and Rb-Sr systematics

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            Shock-metamorphosed zircon in terrestrial impact craters

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              Rubidium-strontium age of the bosumtwi crater area, ghana, compared with the age of the ivory coast tektites.

              Rocks from the vicinity of Bosumtwi crater, Ghana, and a repre-sentative collection of Ivory Coast tektites have been analyzed mass spectrometrically for rubidium, strontium, and strontium isotopic composition. The data from the rocks of the crater area yield an age of 1.97 x 10(9) years (lambda(gb) = 1.47 chi 10(-11) year(-1)). The data for the Ivory Coast tektites fall on this isochron. This identity of age values for the Ivory Coast tektites and the Birrimian basement rocks of West Africa strongly supports the hypothesis of terrestrial formation for these tektites. The evidence available at present suggests that the Ivory Coast tektites are most probably the fusion products of meteoritic impact at the Bosumtwi crater site.
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                Author and article information

                Journal
                Chem Geol
                Chem. Geol
                Chemical Geology
                Elsevier
                0009-2541
                August 2010
                August 2010
                : 275
                : 3-4
                : 254-261
                Affiliations
                [a ]Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
                [b ]State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, China
                [c ]Natural History Museum, Burgring 7, A-1010 Vienna, Austria
                Author notes
                [* ]Corresponding author. Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria. Tel.: +1 519 697 4539; fax: +1 519 488 4721. ludovic.ferriere@ 123456uwo.ca
                [1]

                Present address: Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street, London, ON, Canada N6A 5B7.

                Article
                CHEMGE15975
                10.1016/j.chemgeo.2010.05.016
                2949568
                21037794
                aedb17ef-636e-496d-88e1-2ba2b5b67af0
                © 2010 Elsevier B.V.

                This document may be redistributed and reused, subject to certain conditions.

                History
                : 14 January 2010
                : 14 May 2010
                : 20 May 2010
                Categories
                Article

                Geosciences
                sr–nd isotopes,impactite,u–pb zircon,ghana,bosumtwi crater
                Geosciences
                sr–nd isotopes, impactite, u–pb zircon, ghana, bosumtwi crater

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