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| Titel |
The U-Pb, Hf and O isotopic record of ancient detrital zircons in Zimbabwean sediments - formation, reworking and nature of early Archaean crust |
| VerfasserIn |
Robert Bolhar, Axel Hofmann, Anthony I. S. Kemp, Martin J. Whitehouse, Sandra Wind, Yuexing Feng |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250100499
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| Publikation (Nr.) |
 EGU/EGU2014-16473.pdf |
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| Zusammenfassung |
| Hafnium and oxygen isotopic compositions measured in-situ on U-Pb dated zircon
from different Archaean sedimentary successions belonging to the 2.9-2.8 Ga
Belingwean/Bulawayan Groups and undated Sebakwian Group are presented to better
define the crustal evolution of the Zimbabwe Craton prior to 3.0 Ga. Textural and
compositional criteria were employed to minimize effects arising from Pb loss, metamorphic
overprinting and interaction with low temperature fluids. 207Pb/206Pb age spectra
(concordance > 90%) reveal prominent peaks at 3.8, 3.6, 3.5, and 3.35 Ga, corresponding
to documented geological events both globally and within the Zimbabwe craton.
O isotope compositions of ~ 4 - 10 opoint to both derivation from magmas in
equilibrium with mantle and the assimilation of supracrustal material or interaction with
metamorphic fluids. In ÉHf-time space, 3.8-3.6 Ga grains define an array consistent with
derivation from a mafic to intermediate source reservoir (Lu/Hf ~0.015) that separated
from chondritic mantle at ~ 3.9 Ga. Crustal domains formed after 3.6 Ga depict a
more complex evolution, involving contribution from juvenile mantle sources and
reworking of pre-existing crust. Importantly, initial Hf isotopic compositions document a
protracted history of remelting, without evidence for significant mantle depletion
prior to 3.35 Ga. This suggests that production of earliest crust in the Zimbabwe
Craton did not cause complementary enriched and depleted reservoirs, possibly
because heterogeneous mantle was effectively remixed by rapid convection due to
higher temperatures in the early Archaean or the volume of crust was too small in
volume to influence the isotopic mantle evolution. Similar Hf-O-time relationships
observed in southern West Greenland were used as a basis to propose a transition in
geodynamics 3.2 Ga ago. The absence of detrital zircons with crystallization ages > 3.8 Ga,
along with a simple ÉHf-time array consistent with reworking of a mafic protolith,
suggests the absence of subduction and crustal accretion prior to 3.8 Ga. A progressive
enrichment in 18O in the 3.8 – 3.6 Ga zircons with time, coupled with decreasing ÉHf,
requires a closed-system. This was accomplished by remelting of mafic crust that was
progressively altered by interaction with water at low temperatures. With time,
progressive foundering or burial of this altered crust caused increasing contribution of
18O-enriched crust in magmas. In contrast, vertical arrays in ÉHf-time space document
concomitant juvenile input and crustal reworking, suggesting that a modern-style plate
tectonic regime was established after ~ 3.6-3.3 Ga. Residence times for zircon with
mantle-like and non-mantle-like δ18O range from 0 - 100 myr and 100 – 450 myr
indicating that early crust was able to escape reworking for extended periods of
time. |
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