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| Titel |
Early Archean crust in the Ukrainian Shield - U-Pb and Hf isotopic constraints |
| VerfasserIn |
S. Claesson, E. Bibikova, L. Shumlyanskyy, C. J. Hawkesworth |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250062404
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| Zusammenfassung |
| The Ukrainian Shield includes early Archean components which have been strongly
reworked in both Archean and Proterozoic time. In the Podolian Domain in the west, the
oldest rocks occur in the Dniestr-Bug formation composed of dominating granulite facies
granitoids (enderbites), mafic and supracrustal rocks. Zircon from these rocks commonly
have complex internal structures, reflecting multiple episodes of growth and recrystallisation
in both Archean and Paleoproterozoic time. U-Pb SIMS dating of enderbite zircon has
identified a group with 207Pb/206Pb ages of c. 3.65 Ga, and also older zircon up to 3.75 Ga.
Dniestr-Bug rocks interpreted as metasediments also have yielded ages up to c 3.75
Ga.
In Hf-time space, most enderbite zircon analyses form a well defined array with a slope
corresponding to 176Lu/177Hf close to zero, which intersects the CHUR composition at 3.75
Ga and an assumed DM evolution curve at c. 3.9 Ga. Some analyses plot on the young side
of this array. We suggest that zircon along this Hf-age array crystallized during a
magmatic event at c. 3.6 Ga, or earlier, and that individual zircon crystals since then
have been individual small closed Lu-Hf reservoirs. At the same time, the crystals
have been open to Pb loss during the metamorphic reworking which has caused
recrystallisation and alteration of primary magmatic zircon, and growth of new
zircon.
The crustal provenance age of the material which was melted to produce the enderbite
magma may be constrained by assumed compositions of this crustal precursor and of the
mantle from which it was derived. For a mafic primary crust with 176Lu/177Hf=0.022, derived
from a depleted mantle source, the crustal provenance age is c. 4.2 Ga. A more differentiated
primary crust, less depleted mantle source, or older age of the enderbite result in younger
model ages for this primary crust.
In the Azov Domain in the east, ages for detrital zircon from the Soroki and Fedorovka
greenstone belts vary from c. 3.6 to 2.6 Ga. A 3.5-3.6 Ga age group demonstrates that
Paleoarchean crust was widely distributed, and zircon cores older than 3.7 Ga have been
identified. Soroki greenstone belt data forms two groups in Hf-time space, with a few
scattered results. The rocks in which these zircons crystallised may be derived from two
batches of primary continental crust, with maximum model ages estimated to c. 4.1 Ga and
3.4 Ga respectively. |
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