Modification of magnetic properties in granite during hydrothermal alteration (EPS-1 borehole, Upper Rhine Graben)

Rock magnetic properties and petrological investigations of the magnetite-bearing Variscan Soultz granite from the EPS-1 borehole (Upper Rhine Graben) provided a significant contribution for the understanding of the “old” (Middle Carboniferous to Permo-Triassic) hydrothermal alteration history. This alteration history is generally subdivided into two stages. Stage I alteration is related to processes during magma cooling and solidification of the plutonic body in Middle Carboniferous times. Stage II alteration is associated with post-emplacement tectonics from Late Carboniferous to Permo-Triassic, such as exhumation of the plutonic body to a paleo-erosion surface, near surface processes beneath the paleo-surface and the influence of the overlying Permo-Mesozoic sedimentary cover. This study focuses on two main scopes: (i) chronological succession of the alteration history of the Soultz granite, and (ii) modification of the magnetic properties in relation to the different geological processes mentioned above. During the stage I alteration the plutonic body solidified, which led to the formation of first permeable fault zones at temperatures ~300 °C, obtained from chlorite-geothermometry. The formation of permeable zones allowed the circulation of O2-rich fluids, which caused the oxidation of magnetite into martite (hematite) within discrete zones. Granites, which are situated further away from a fault zone remained unaffected, and thus are called “fresh” from the magnetic point of view (magnetic susceptibility, k > 10 x 10-3 SI). The k(T) measurements revealed thermomagnetic behavior typical for multi-domain magnetite in fresh granite and an irreversible thermomagnetic behavior in altered granite reflecting the oxidation degree of magnetite. Such degree is related to an alteration index (Al-index) obtained from the difference between k of the heating and the cooling run at room temperature. The AMS (anisotropy of magnetic susceptibilty) fabric in fresh granites is related to the subhorizontal primary magmatic fabric indicating emplacement within a decelerating flow regime. Along with the oxidation of magnetite k was decreased to values < 1 x 10-3 SI. The primary horizontal magnetic fabric changed to a steeply dipping and NW-SE trending magnetic fabric in the altered granite due to a microcrack-controlled oxidation. This oxidation caused elongated and aligned small magnetite relics within the host mineral martite. Paleomagnetic investigations revealed that this first faulting event took place during Middle Carboniferous. During Late Carboniferous, probably until Perm, the plutonic body was affected by unloading and exhumation. The initial fault zones were reactivated and k(T) curves revealed the formation of pyrite and Fe-carbonates. The AMS fabric in wall rocks of the fault zones evolved from a normal to an inverse fabric due to decreased (single-domain) grain sizes of the magnetite relics. At the same time, the AMS fabric in the cataclastic granites was destroyed by brittle deformation. The exhumation of the plutonic body to a paleo-erosion surface resulted in near surface processes within the upper borehole section . These processes caused an oxidation of pyrite, which led to the circulation of strongly acidic fluids. Such acidic fluids led to the decomposition of Fe-carbonates and martite until they were neutralized by mixing with meteoric fluids, which in turn caused the precipitation of abundant fine-grained hematite. This event led to a widespread hematitization of the upper borehole section. The fine-grained hematite shows paleofield vectors typical for Permo-Triassic. In contrast, fault zones at the bottom of the borehole remained unaffected by this near surface processes. Granites from these fault zones display a reduction of martite back to magnetite caused by the infiltration of organic matter from the overlying Permo-Mesozoic sedimentary cover. In such granites, no relevant paleomagnetic information could be obtain.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Heidelberg

Year

2005

Author dc:creator

• Just, Jana

Contributors dc:contributor

• Dr. Agnes Kontny, HD

Repository record source_url

http://www.ub.uni-heidelberg.de/archiv/5386

OAI identifier oai:identifier

oai:archiv.ub.uni-heidelberg.de:5386