HR: 0830h
AN: GP11D-0291 [PDF]
TI: Magnetic Fabric Development During Hydrothermal Alteration And Brittle Deformation In Granite From The
EPS-1 Drilling, (Soultz-sous-Forts, France)
AU: * Just, J
EM: jana.just@urz.uni-heidelberg.de
AF: University of Heidelberg
Geol.-Pal. Institute, INF 234, Heidelberg, 69120
AU: Kontny, A
EM: agnes.kontny@urz.uni-heidelberg.de
AF: University of Heidelberg
Geol.-Pal. Institute, INF 234, Heidelberg, 69120
AU: deWall, H
EM: deWall@urz.uni-wuerzburg.de
AF: University of Wrzburg, Pleicherwall 1, Wuerzburg, 97070
AB:
The EPS-1 drilling occurs within the Upper Rhine Graben and penetrates Variscan granites from 1420 to 2200 m depth. According
to studies of Traineau et al. (1991) two main hydrothermal alteration stages were determined. An early propyllitic and
pervasive alteration took place during cooling and uplift history of the pluton. The multiphase evolution of the Cenozoic
rift was accompanied by an intense brittle deformation of the pluton causing a vein alteration within discrete fault zones
and their wall rocks.
We present a rock-magnetic and fabric study of the fresh, pervasive and vein-altered granite comparing petrofabrics, fracture
patterns and the anisotropy of magnetic susceptibility (AMS). AMS is caused by the directional dependence of the magnetic
susceptibility based on the distribution of magnetic minerals, mostly Fe oxides. The magnetic foliation in the fresh granite
is interpreted as a magmatic flow fabric which is oriented almost horizontal in the deeper part and reveal a steeper
dip-values in the upper part according to wrench tectonics (Ziegler, 1986) during Variscan time. The AMS ellipsoids of the
fresh granites show only oblate shapes with relative high magnetic anisotropy due to the association of clustered magnetite
with Fe-Mg silicates describing the weak magmatic foliation. The magnetic foliation in the pervasive altered granite reveals
similar orientations as the fresh granite with steeper dip-values indicating formation in a similar paleostress field. This
alteration caused a major transformation of magnetite to hematite, started with the growth of hematite platelets relative to
the \{111\} planes of magnetite expressed by low magnetic anisotropies. The magnetic foliation of the vein-altered granite is
randomly oriented according to the multiphase Cenozoic rifting with successive compressional and extensional paleostress
directions. New formed minerals like hematite show crystal growth in the veins but also due to the alignment of altered
minerals. Both pervasive and vein-altered granites display oblate as well as prolate ellipsoids. The decreasing effect of
magnetite and the increasing influence of new formed minerals like hematite, illite and Fe carbonates on the shape and
orientation of the AMS ellipsoid will be demonstrated using AMS measurements performed at different temperatures.
Traineau et al., 1991. Geotherm. Sci. Techn., 3(1-4), 1-29.
Ziegler et al., 1986. Tectonopysics, 126, 303-328.
DE: 1518 Magnetic fabrics and anisotropy
DE: 3022 Marine sediments--processes and transport
DE: 8030 Microstructures
DE: 8434 Magma migration
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting