PALEOMAGNETIC DATING OF A FOCUSED FLUID-FLOW EVENT ALONG THE HIGHLAND BOUNDARY FAULT, SCOTLAND

Cambrian-Ordovician serpentinite in the Highland Border Complex (HBC), a tectonic terrane along the Highland Boundary Fault (HBF) in Scotland, contains a complex multi-component magnetization. Red dolomitized serpentinite was sampled at multiple localities along the HBF and relatively unaltered serpentinite was sampled at two localities. The unaltered serpentinite contains a poorly defined magnetization with westerly declinations. The pole for the westerly magnetization plots well off the APWP. The results of alternating field (AF) and thermal demagnetization, as well as isothermal remanent magnetization (IRM) acquisition and thermal decay, indicate that magnetite dominates the magnetization in the unaltered serpentinite. Most specimens of the red dolomitized serpentinite contain a magnetization with southerly declinations and negative inclinations (D=187^o, I=-13^o) that was acquired in the Carboniferous and resides in hematite. In some specimens of red serpentinite, AF demagnetization prior to thermal treatment removes a component with a similar direction. In addition, some specimens also contain a component with westerly declinations and shallow inclinations similar to the direction in the unaltered serpentinite. IRM acquisition and decay of red serpentinite samples indicate the presence of both magnetite as well as hematite. The red serpentinite contains abundant authigenic hematite and fluid inclusion studies indicate that the fluids responsible for the alteration were hydrothermal in origin (110-240^oC) and had a range of sources, including connate waters of marine origin and meteoric waters. The Carboniferous magnetization in hematite is interpreted as a chemical remanent magnetization that formed when warm, probably basinal fluids moved along the fault zone and caused hematite authigenesis. The component removed by AF treatment is interpreted as a thermal resetting of original magnetite by the fluids. The origin of the westerly magnetization is unknown although it could be related to an earlier episode of diagenesis or deformation.