[S-EM18] Geomagnetism, paleomagnetism and rock magnetism

[SEM18-05] Brunhes-Matuyama contact aureole surrounding the Kurobegawa Granite

*Koji Fukuma¹, Hisatoshi Ito², Mami Takehara³, Ryuji Yamada⁴, Takashi Kubo⁵ (1.Department of Environmental System Science, Faculty of Science and Engineering, Doshisha University, 2.Central Research Institute of Electric Power Industry, 3.National Institute of Polar Research, 4.National Research Institute for Earth Science and Disaster Prevention, 5.MAIBUNKAN)

Keywords:contact test, granite, low temperature demagnetization, tectonics

Contact test is most powerful paleomagnetic field test to give tight constraints on the timing of magnetization acquisition for both country and intrusive rocks. Especially when the country rock is normally magnetized and later thermally metamorphosed during a reversed polarity chron and vice versa, totally or partially remagnetized zones in the country rock can be unequivocally identified as a function of distance from the contact. Here we show an example of contact test for a youngest pair of country and intrusive rocks emplaced during the Matuyama and Brunhes chron respectively. The Quaternary Kurobegawa Granite, which is the youngest granite with U-Pb ages of about 0.8 Ma, forms a volcanic-plutonic complex with the Jiigatake Volcanic Rocks dated about 1.6 Ma by several geochronological methods. Paleomagnetic analyses including contact test for the volcanic-plutonic complex contribute to elucidate the emplacement and deformation processes related to the building of the Hide Mountains. We collected oriented blocks at 14 sites of the Kurobegawa Granite and 6 sites of the Jiigatake Volcanic Rocks. As we expected multidomain grains carry significant portions of natural remanent magnetization, we applied low temperature demagnetization with liquid nitrogen and then performed thermal demagnetization at every 25 degC step from room temperature up to 600 degC with the automated magnetometer-demagnetizer system Tspin. Low temperature demagnetization effectively erased 10-75% of initial magnetization and the middle to high temperature characteristic remanent magnetizations were retrieved at most sites except for some samples struck by lightning. Characteristic remanence directions from a single site are well clustered and the site-mean directions are of normal polarity without exception for the Kurobegawa Granite. The primary magnetizations postdated the age ~0.8 Ma of U-Pb geochronology of a closure temperature 900 degC and are presumed to be acquired during the Brunhes chron (< 0.77 Ma). In the contact aureole of the Jiigatake Volcanic Rocks metamorphosed by the Kurobegawa Granite, we found one totally remagnetized normal-polarity site and two partially remagnetized sites having hybrid normal and reversed magnetizations. This positive contact test suggests that the characteristic magnetization of the Kurobegawa Granite is not of recent viscous origin although the direction is not distinguishable from that of the present geomagnetic field. Neither large-scale tilting nor deformation was found out from the site-mean directions of the Kurobegawa Granite.