Abstract
A Middle Miocene pyroclastic (tuffite) dike at Nakaoku, central Kii Peninsula, has a northerly remanent magnetization direction of normal polarity with low unblocking temperatures (<400°C). A previous study has interpreted this direction as a primary thermoremanent magnetization (TRM). We propose an alternative interpretation that it is a thermochemical remanent magnetization (TCRM) acquired through hydrothermal processes. Samples were collected from both rhyolitic welded tuff fragments and the matrix. A detailed rock magnetic study, including the analysis of isothermal remanent magnetization (IRM) acquisition curves, thermal demagnetization of a composite IRM, and high- and low-temperature measurements, as well as ore microscopic observation, indicates that the main magnetic carrier is pyrrhotite. The pyrrhotite occurs not only as anhedral minerals enveloping other small grains, but also as those filling open fractures within lithic fragments. These observations indicate that the pyrrhotite is a secondary mineral, precipitated most likely by hydrothermal activity. We also determined directions for nearby dikes of basaltic andesite and quartz porphyry. Similarly to the tuffite, the basaltic andesite contains pyrrhotite as the magnetic carrier and possesses a northerly direction of normal polarity, probably acquired as TCRM at nearly the same time as the tuffite. On the other hand, the quartz porphyry has a reverse magnetization with a southerly declination and a steep inclination. Magnetite is the main carrier. The reverse direction can be interpreted as a primary TRM representing the paleomagnetic direction at the time of deuteric cooling (∼15 Ma).
