Paleomagnetism of the middle Cretaceous Iritono granite in the Abukuma region, northeast Japan

Abstract

We have studied the paleomagnetism of the middle Cretaceous Iritono granite of the Abukuma massif in northeast Japan together with 40Ar– 39Ar dating. Paleomagnetic samples were collected from ten sites of the Iritono granite (102 Ma 40Ar– 39Ar age) and two sites of its associated gabbroic dikes. The samples were carefully subjected to alternating field and thermal demagnetizations and to rock magnetic analyses. Most of natural remanent magnetizations show mixtures of two components: (1) H component, high coercivity ( B c > 50–90 mT) or high blocking temperature ( T b > 350–560 °C) component and (2) L component, relatively low B c or low T b component. H component was obtained from all the 12 sites to give a mean direction of shallow inclination and northwesterly declination ( I = 29.9°, D = 311.0°, α 95 = 2.7°, N = 12). This direction is different from the geocentric axial dipole field at the present latitude ( I = 56.5°) and the typical direction of the Cenozoic remagnetization in northeast Japan. Since rock magnetic properties indicate that the H component of the Iritono granite is carried mainly by magnetite inclusions in plagioclase, this component probably retains a primary one. Thus the shallow inclination indicates that the Abukuma massif was located at a low latitude (16.1 ± 1.6°N) about 100 Ma and then drifted northward by about 20° in latitude. The northwesterly deflection is attributed mostly to the counterclockwise rotation of northeast Japan due to Miocene opening of the Japan Sea. According to this model, the low-pressure and high-temperature (low- P/high- T) metamorphism of the Abukuma massif, which has been well known as a typical location, would have not occurred in the present location. On the other hand, the L component is carried mainly by pyrrhotite and its mean direction shows a moderate inclination and a northwesterly declination ( I = 42.8°, D = 311.5°, α 95 = 3.3°, N = 9). Since this direction is intermediate between the H component and early Cenozoic remagnetization in northeast Japan, some thermal event would have occurred at lower temperature than pyrrhotite Curie point (∼ 320 °C) during the middle Cretaceous to early Cenozoic time to have resulted in partial remagnetization.