On the demonstration of the normal polarity of remanent magnetization by volcanic rocks containing magnetite or hematite

Abstract

Magnetic and paleomagnetic properties, and magnetic mineralogy of the
Neogene andesitic rocks were previously studied. The author of the article has recently
proposed a model of the sources of normal and reversed RM and tested it applying the
results of the submarine volcanics (Orlický, 2010). The magnetites (cubic phase) and
the titanium rich titanomagnetites (cubic phase) have been considered as the carriers of
only normal remanent magnetization of the rocks. The low-temperature oxidized titanomagnetites
and the Ilmenite-Hematites (Ilm-Hem; rhombohedral phase) carry either the
reversed remanent magnetism (RM) of self-reversed origin, or normal RM, depending on
the proportion of hematite in the Ilm-Hem solid solutions. I have enlarged the original
model in the hematite bearing rocks which carry only normal RM. The paleomagnetic
and magnetic results and the results of magnetic mineralogy of selected Neogene andesites
and rhyolites from central, southern and eastern Slovakia of the Badenian to the Middle
or Lower Sarmatian age (16.05–12.9 M.Y.) have been considered. The selected rocks
with the magnetites and hematites have revealed only the normal RM. Normal partial
thermoremanent magnetization (PTRM) was induced in many magnetites and hematites
containing andesite and rhyolite samples in the laboratory. The occurrence of both these
types of magnetic minerals is quite rare in volcanics. While the magnetite is more frequently
present in the intrusive rocks, the hematite is frequently present in effusive, more
acidic volcanics, e.g. in the rhyolites. Very important outcome is that both magnetite
and hematite are the carriers of only normal polarity of RM in the rocks, regardless their
age.