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Abstract
Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 67 (1997)No.
5. (September), Pages 923-934
Mineralogical, Geochemical (REE), and Isotopic (K-Ar, Rb-Sr, d18O)
Evolution of the Clay Minerals from Faulted, Carbonate-rich, Passive Paleomargin
of Southeastern Massif Central, France
Norbert Clauer (1), Francis Weber (1), Francois Gauthier-Lafaye
(1), Theofilos Toulkeridis (1, 2), Jean-Pierre Sizun (1)
ABSTRACT
Mineralogical, geochemical (REE), and isotopic (K-Ar, Rb-Sr, d18O)
data of < 0.2 µm clay fractions from core samples have been used
to enhance our understanding of the evolution of the faulted, carbonate-rich,
passive paleomargin in the southeastern French Massif Central. The fault
system described at the lower part of the sedimentary sequence was active
at 190 ± 20 Ma during a rifting tectono-thermal activity with important
migration of hot fluids. It was active again afterwards, probably after
maximum burial occurring at 110-120 Ma, as a result of the compressive
Alpine activity in the region. This late shearing induced an overall throw
of at least 1160 m and subsequent erosion of as much as 2000 m of s diments.
However, it probably did not induce major fluid movements, which means
in turn that the mineralizing fluids responsible for economic ore deposits
in the margin relate to the 190-Ma-old Liassic hydrothermal event.
The Rb-Sr, K-Ar, and oxygen isotopic systematics of the clay fractions
suggest that late faulting activity occurred under very low water-to-rock
conditions, which did not favor widespread fluid migration. The clay material
of the fault yields unreasonably widely scattered Rb-Sr and K-Ar dates,
from 70 to 183 Ma, because of variable contents of radiogenic 40Ar
and some unrealistic initial 87Sr/86Sr ratios, even
below 0.700. The 190 Ma illites that crystallized in the fault at 200-210°C
also have abnormally low d18O values
of about +13.3
,
leading to a d18O value of the pa
eofluids as high as +8.1.
The 87Sr/86Sr ratios and REE distributions of acid-leached
clay residues delineate two types of fluids that have probably interacted
with the clay minerals. One, with an 87Sr/86Sr ratio
of 0.7250 and a humped REE distribution pattern, is believed to be of hydrothermal
origin, and the other, with an 87Sr/86Sr ratio of
0.7082 and a flat REE pattern, might be of recent continental origin.
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