抄録
中奥地域の中期中新世火砕岩岩脈は300〜400℃で消磁される正帯磁残留磁化をもつ.この磁化は初生の熱残留磁化と解釈されてきた.本論で筆者らはこれが熱水作用に伴って生じた熱化学残留磁化であるという解釈を提示する.詳細な磁気測定と反射顕微鏡観察は磁化の担い手がピロタイトであることを示す.ピロタイトは火砕岩の定置後に熱水変質によってその場でできた二次生成物である.火砕岩岩脈の近傍に産する玄武岩質安山岩もピロタイトに担われる正帯磁磁化を持ち,これも火砕岩と同時期に獲得された熱化学残留磁化と推定される.他方,石英斑岩岩脈は南向きの偏角と深い伏角をもつ逆帯磁磁化をもち(担い手はマグネタイト),これは初生熱残留磁化と考えられる.石英斑岩は15Ma前後の逆磁極期に,伏角が深い時に貫入・冷却したと判断される.
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℃). 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-telmperature 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).
