Fluid/rock Interaction History of a Faulted Rhyolite-Granite Contact, Eastern Rhine Graben Shoulder, SW-Germany : Alteration Processes Determined by Sr- Pb-Isotopes, Th/U-Disequilibria and Elemental Distributions

The alteration history of the Schauenburg fault has been investigated with isotope and element distributions across the faulted granite-rhyolite contact. The Schauenburg fault zone is situated along the eastern Rhine Graben shoulder, close to Heidelberg (SW-Germany). Major and trace element analyses allow to obtain information concerning the chemical changes induced by alteration. Differences are partly petrographic because the profile crosses the granite-rhyolite contact, but they are also due to different alteration levels induced by fluid circulation along the fault system which has drained the alteration processes. The granite-rhyolite contact constitutes the primary structure. Only the most incompatible elements (Si, Al, Zr, Hf) retain their original signatures and reflect a mixing between typical granite and rhyolite lithologies across the altered zones (cataclasite). The more mobile elements show a different composition within the altered zones (cataclasite) notably a high leaching of cations. The geochemical tracers also suggest at least one strong hydrothermal event with reducing conditions in the altered zones. The isotopic analyses delivered qualitative and temporal information. The use of several isotopic systems, Rb/Sr -, U/Pb-isotopes and Th/U disequilibria, reveals a complex history of polyphase fluid/rock interaction following the Permian volcanic extrusion, showing notable disturbances during the late Jurassic hydrothermal activities, the Tertiary rifting of the Rhine Graben and more recent Quaternary alteration. The granite zone of the sampling profile has underwent an event which set up a new Rb-Sr isotopic composition and reset the Rb/Sr system which originatly corresponded to the Carboniferous intrusion ages. The Rb-Sr data of the granite samples produce a whole rock isochron of 152 ± 5,7 Ma (2s error) in good agreement with the well-known late Jurassic hydrothermal event (135-160 Ma). The rocks evolution lines for Pb support a Tertiary hydrothermal event (54 Ma ± 16; 1s error), potentially connected with the development of the Rhine Graben. The profile samples have undergone uranium and thorium redistribution processes which have occurred within the last ~ 106 years. The samples of the altered zones record a more complex history of uranium exchange with the aqueous phase. This uranium exchange is proportional to the porosity. The best approximation is reached for an exchange coefficient (lE) for uranium ranging from 2,5 E-06 [a-1] in the middle of the altered zones to 2,5 E-05 [a-1] on the sides of the altered zones.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Heidelberg

Year

2002

Author dc:creator

• Marbach, Thierry

Contributors dc:contributor

• Augusto (Prof. Dr.), Mangini

Repository record source_url

http://www.ub.uni-heidelberg.de/archiv/3117

OAI identifier oai:identifier

oai:archiv.ub.uni-heidelberg.de:3117