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Titel |
500 Myr of thermal history elucidated by multi-method detrital thermochronology of North Gondwana Cambrian sandstone (Eilat area, Israel) |
VerfasserIn |
P. Vermeesch, D. Avigad |
Konferenz |
EGU General Assembly 2009
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 11 (2009) |
Datensatznummer |
250022190
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Zusammenfassung |
Following the Neoproterozoic Pan-African orogeny, the Arabian-Nubian Shield (ANS) of
North Africa and Arabia was eroded and then covered by Cambrian sandstones
that record the onset of platform sedimentation. We applied K-feldspar 40Ar/39Ar,
zircon and apatite fission track and apatite (U-Th)/He thermochronology to detritus
from Cambrian sandstones of southern Israel deposited at about 500 Ma. U-Pb
detrital zircon ages from these sandstones predate deposition and record the earlier
Neoproterozoic crustal evolution of the Pan-African orogens. 40Ar/39Ar ages from 50
single grains of K-feldspar yield a Cambrian mean of approximately 535 Ma. The
40Ar/39Ar age spectrum of a multi-grain K-feldspar aliquot displays diffusion behaviour
compatible with >560 Ma cooling later affected by a heating event. Assuming
that the high temperature domains of the K-feldspars have not been affected by
subsequent (hydro)thermal events, and taking previously published K-Ar and Rb-Sr ages
from other parts of the East African Orogen at face value, these ages apparently
record Pan-African thermal resetting below a thick volcano-sedimentary pile similar
to the Saramuj conglomerate in Jordan and/or the Hammamat in Egypt. Detrital
zircon fission track (ZFT) ages cluster around 380 Ma, consistent with previous ZFT
results from Neoproterozoic basement and sediments of the region, revealing that the
Cambrian platform sequence experienced a middle Devonian thermal event and
low-grade metamorphism. Regional correlation indicates that during Devonian time
the sedimentary cover atop the Cambrian in Israel was never in excess of 2.5 km,
requiring an abnormally steep geothermal gradient to explain the complete ZFT
annealing. A basal Carboniferous unconformity can be traced from Syria to southern
Saudi Arabia, suggesting that the observed Devonian ZFT ages represent a regional
tectonothermal event. Similar Devonian ZFT ages were reported from ANS basement
outcrops in the Eastern Desert, 500 km south of Eilat. The detrital apatites we studied
all have extremely rounded cores suggestive of a distant provenance, but some
grains also feature distinct euhedral U-rich apatite overgrowth rims. Authigenic
apatite may have grown during the late Devonian thermal event we dated by ZFT,
coinciding with existing Rb-Sr ages from authigenic clays in the same deposits and
leading to the conclusion that the Devonian event was probably hydrothermal. Like
the ZFT ages, the detrital apatite fission track (AFT) ages were also completely
reset after deposition. Sixty single grain detrital apatite fission track (AFT) ages
group at ~270 Ma with significant dispersion. Inverse modeling of the AFT data
indicate extended and/or repeated residence in the AFT partial annealing zone, in
turn suggesting an episodic burial-erosion history during the Mesozoic caused by
low-amplitude vertical motions. Seven detrital apatite (U-Th)/He ages scatter between 33
and 77 Ma, possibly resulting from extreme compositional zonation associated
with the authigenic U-rich overgrowths. The ~70 Ma (U-Th)/He ages are more
likely to be accurate, setting 1-2 km as an upper limit (depending on the geothermal
gradient) on the post-Cretaceous exhumation of the Cambrian sandstone and showing
no evidence for substantial denudation related to Tertiary rifting of the Red Sea. |
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