Palaeomagnetic dating of fluid‐flow events in dolomitized rocks along the Highland Boundary Fault, central Scotland

Abstract

Palaeomagnetic and geochemical studies of Cambrian-Ordovician serpentinite in the Highland Border Complex (HBC), a tectonic terrane along the Highland Boundary Fault (HBF) in Scotland, indicate that the HBF was a conduit for fluids in the Carboniferous-Permian. The fluids caused dolomitization, silicification, and haematite authigenesis. Both red dolomitized serpentinite and relatively unaltered serpentinite were sampled at multiple localities. The unaltered serpentinite contains a poorly defined magnetization with westerly declinations that resides in magnetite and has a pole which plots well off the apparent polar wander path. Most specimens of the red dolomitized serpentinite contain a magnetization with southerly declinations and negative inclinations that resides in haematite. A regional fold test suggests that this magnetization post-dates tilting and the pole positions for the different locations fall on the Carboniferous to Permian part of the apparent polar wander path. In some specimens of red dolomitized serpentinite, alternating field (AF) demagnetization prior to thermal treatment removes a component with a similar direction. Dolomitized basement rocks along the fault contain a similar although apparently slightly older magnetization. Fluid inclusion and geochemical studies indicate that the fluids were hydrothermal in origin (110-2408C) and had a range of sources. The Carboniferous-Permian magnetization in haematite is interpreted as a chemical remanent magnetization that formed when warm fluids moved along the fault zone and caused haematite authigenesis. The component removed by AF treatment is interpreted as a thermal resetting of primary magnetite by the fluids. The variability of the palaeomagnetic, fluid inclusion, and stable isotope results suggests that there were probably multiple flow events that caused the alteration. The origin of the fluids could be related to the intrusion of late Carboniferous dikes in central Scotland and/or to reactivation of the HBF in the Carboniferous-Permian.

Remagnetization and fluid flow in the Old Red Sandstone along the Great Glen Fault, Scotland

“…The CRM1 is similar to the Group B magnetization from ORS sediments reported by Tarling (1985) as well as other workers. The CRM1 is also similar to the pole positions for CRMs reported by Elmore et al (2002) for the HBF and Blumstein et al (2005) for the MTZ. The fluids responsible for the hematite authigenesis could have been associated with a dense swarm of Late Carboniferous-Early Permian igneous activity in Scotland (Stewart, 1991), perhaps related to the earliest stages of rifting in the North Atlantic region (Smythe et al, 1995).…”

“…These CRMs can be used to date the fluid migration events along the faults such as the Highland Boundary (HBF) and the Moine Thrust Zone (MTZ) (e.g., Elmore et al, 2002;Blumstein et al, 2005). Little is known, however, about fluid flow along the Great Glen Fault (GGF), another major fault in Scotland.…”

Fluid evolution in base‐metal sulphide mineral deposits in the metamorphic basement rocks of southwest Scotland and Northern Ireland

“…There are also numerous lines of evidence for a more regionally extensive episode of mineralization occurring in central Scotland during Carboniferous times. Palaeomagnetic studies have shown that a low temperature, low salinity, aqueous, hydrothermal fluid demagnetized the basement metamorphic rocks in central Scotland during late Carboniferous to early Permian times (Parnell et al 2000b;Elmore et al 2002). Parnell et al (2000b) recognized this late Carboniferous to early Permian fluid event in the Dalradian limestones of Islay which host all the base-metal sulphide deposits on the island.…”

Detailed internal structure and along-strike variability of the core of a plate boundary fault: the Highland Boundary fault, Scotland

“…2), clay authigenesis must be younger than the emplacement and deformation in the Highland Border Ophiolite and associated reverse motion on the HBF. Based on palaeomagnetic results, Elmore et al (2002) suggest the dolomitisation of serpentinite in footwall damage zone 1 occurred in the Permian at 260 Mya. Clay growth must also be younger than the fossils, which assuming the bryozoans belong to the order Fenestrata (as identified by P. D. Taylor, pers.…”