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| Titel |
U-Pb Geochronology of Hydrous Silica (Siebengebirge, Germany) |
| VerfasserIn |
Frank Tomaschek, Alexander Nemchin, Thorsten Geisler, Alexander Heuser, Renaud Merle |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250112226
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| Publikation (Nr.) |
 EGU/EGU2015-12377.pdf |
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| Zusammenfassung |
| Low-temperature, hydrous weathering eventually leads to characteristic products
such as silica indurations. Elevated U concentrations and the ability of silica to
maintain a closed system permits silica to be dated by the U-Pb method, which, in
turn, will potentially allow constraining the timing of near-surface processes. To
test the feasibility of silica U-Pb geochronology, we sampled opal and chalcedony
from the Siebengebirge, Germany. This study area is situated at the terminus of the
Cenozoic Lower Rhine Basin on the Rhenish Massif. The investigated samples include
silicified gravels from the Mittelbachtal locality, renowned for the embedded wood
opal. Structural characterization of the silica phases (Raman spectroscopy) was
combined with in situ isotopic analyses, using ion microprobe and LA-ICPMS
techniques.
In the Siebengebirge area fluviatile sediments of Upper Oligocene age were covered by an
extended trachyte tuff at around 25 Ma. Silica is known to indurate some domains within the
tuff and, in particular, certain horizons within the subjacent fluviatile sediments
(“Tertiärquarzite”). Cementation of the gravels occurred during at least three successive
growth stages: early paracrystalline silica (opal-CT), fibrous chalcedony, and late
microcrystalline quartz. It has traditionally been assumed that this silica induration reflects
intense weathering, more or less synchronous with the deposition of the volcanic
ashes.
Results from U-Pb geochronology returned a range of discrete 206Pb-238U ages,
recording a protracted silicification history. For instance, we obtained 22 ± 1 Ma for opal-CT
cement from a silicified tuff, 16.6 ± 0.5 Ma for silicified wood and opal-CT cement in
the fluviatile gravels, as well as 11 ± 1 Ma for texturally late chalcedony. While
silicification of the sampled tuff might be contemporaneous with late-stage basalts, opaline
silicification of the subjacent sediments and their wood in the Mittelbachtal clearly
postdates active Siebengebirge volcanism, and the clastic sedimentation by about 8
Myr.
To account for the age discrepancies, opal-CT formation might be a local and
episodic phenomenon, reflecting progressive denudation of the trachyte tuff cover.
Alternatively, the dominant silicification event of the Mittelbachtal silcretes could
be of regional significance (Middle Miocene Climatic Optimum). Our relatively
fast approach by LA-ICPMS analysis will be used to further expand the database. |
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