Anisotropy of magnetic susceptibility of tonalite in the Tanzawa plutonic complex and its implications for emplacement mechanism.

Abstract

The anisotropy of magnetic susceptibility and paleomagnetic direction of the Tanzawa tonalite and related rocks are studied in order to throw light on the emplacement mechanism of tonalite magma. The paleomagnetic directions of samples from syn-plutonic dikes in the Tanzawa main tonalite and dikes in the Jizodaira intrusion reveal that the mass of Tanzawa tonalite did not tilt greatly since its emplacement at 7 Ma. The Tanzawa main tonalite comprises ferromagnetic granite and its anisotropy of magnetic susceptibility is caused by the shape of magnetite clots and/or magnetite grains included in elongated minerals such as hornblende and biotite. On the basis of magnetic foliation, the four successively emplaced units are identified; they are the Kurokuragawa, Murokubogawa, Mizunokisawa, and Ishiwariyama units. Each unit is a laterally zoned pluton with mafic margin grading to felsic core. The dips of magnetic foliations are generally steep especially near the margin of each unit, but those around the roof pendant of the Kurokuragawa unit are rather gentle. The dip of magnetic lineation is steep along the margin of the units, indicating the position of the feeder of magma. On the contrary, the dip of magnetic lineation in the inner part of the unit is gentle. The magnetic foliation and lineation in the Ishiwariyama unit may have been disturbed by the local thermal convection induced by the input of high temperature magma through syn-plutonic dikes. The deformation in the last stage of solidification of magma by the left lateral strike-slip tectonic movement suggests that the tonalite magma was emplaced under transtensional tectonic regime. The space of each unit may have been created by the subsidence of the floor, and magma migrated upward through ring-shaped feeders along the margin of the subsiding floor, resulting in the formation of a lopolith-like pluton.