Thermal Demagnetization of Some Precambrian Igneous Rocks of Missouri

Abstract

The results of a deliberate search for stable secondary magnetization in Precambrian igneous rocks of St. Francois Mountains, Southeast Missouri, U. S. A., are described. These rocks, at least one billion years old, possess stable components of natural remanent magnetization, which are not always of thermoremanent origin. The results of thermal demagnetization and of some subsidiary experiments show that rhyolite porphyries of Flint Hill have been remagnetized by strong fields due to lightning and are of little use for paleomagnetic investigations. Similar experiments conducted on felsites and bedded tuffs of Johnson Shut-Ins area reveal the presence of stable and significant magnetization of a direction similar to that found by Scharon, Hayes, and Anderson for the whole of St. Francois Mountains. The granite porphyry from the same area carries a stable component of natural remanent magnetization, which is of secondary origin, but is of little significance for the paleomagnetic studies of the whole system of rock units.It is concluded that the rock units investigated carry no stable secondary magnetization which masks their original magnetization and leads to wrong conclusions about the direction of Precambrian geomagnetic field. From this it is inferred that the natural remanent magnetization of all other rock units of St. Francois Mountains is unlikely to be masked by stable secondary components.