Origin of the Magnetization of the Wichita Mountains Granites, Oklahoma

Summary. The magnetization of the Cambrian Wichita Mountains basement complex consists of several components, two of which are shown to be significant. The residual primary remanence is associated with the original titanomagnetite and the dominant secondary component is carried by minerals of the ilmenite-haematite series. The latter has been acquired at temperatures up to 300°C as a result of the regional hydrothermal alteration during late Palaeozoic. The secondary remanence is of thermochemical origin and carries a memory of a variable ambient field direction which may be modelled as a resultant of the geomagnetic field direction contemporaneous with the hydrothermal process and of the direction of the magneto-static field of the primary titanomagnetite. Although the destruction of the original presumably Cambrian remanence has been extreme, in a few cases its direction appears to have been recovered. The results are complicated by the possibility of a self-reversal of secondary remanence as well as geomagnetic field reversal. An issue unresolved by the investigation relates to the possibility of generation of magnetite by heat treatment of rock material in the laboratory. It is generally concluded that it is difficult to recover the original cooling thermoremanence direction in an igneous intrusion which has been subjected later in its life to extreme conditions of hydrothermal alteration.