Paleomagnetism of the Middle Cretaceous Iritono Granite in the Abukuma Region, Northeast Japan: Implications for the CNS Geomagnetic Field Intensity

We have studied the paleomagnetism of the middle Cretaceous Iritono granite of the Abukuma massif in northeast Japan. Paleomagnetic samples were collected from ten sites of the Iritono garnite (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 (Bc > 50-90 mT) or high blocking temperature (Tb > 350-560 C) component and (2) L component, relatively low Bc or low Tb 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, 95 = 2.7, N = 12). This direction is different from the geocentric axial dipole field at the present latitude and the typical direction of the Cenozoic remagnetization in northeast Japan. Since rock magnetic properties indicate that H component of the Iritono granite is carried mainly by magnetite inclusions in plagioclase, this component probably retains a primary one. The geomagnetic secular variation could be averaged since the cooling time of the representative blocking temperatures width of H component is estimated to be 40 kyr from the granite body size of about 2 km. 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 deg in latitude. On the other hand, 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, 95 = 3.3, N = 9). Some thermal event might occur at lower temperature than pyrrhotite Cuire point during the middle Cretaceous to early Cenozoic time to have resulted in partial remagnetization. The stable primary component (H component) is thought to be carried mainly by single-domain magnetite inclusions of granite plagioclase, probably due to the exsolution during the formation, thus paleointensity measurements have been also conducted for Iritono granite samples with less pyrrhotite. The Coe version of the Thellier method yielded 54.5 +/-4.8 micro-T (N = 11), while the LTD-DHT Shaw method gave 67.1 +/-3.1 micro-T (N = 5). The relatively high mean of the LTD-DHT Shaw data may be caused by more effective removal of L component since the inclination of the average direction is shallower in the LTD-DHT Shaw measurements (Im = 22.1) than in the Thellier ones (Im = 28.2). From the LTD-DHT Shaw results, the corresponding VDM is calculated as high value of 16.5 +/-0.8 ZAm2. Since the cooling rate of granite is extremely lower than that in the laboratory, however, some correction is required. If the cooling time of 40 kyr in nature and 100 sec in the laboratory are assumed for single-domain magnetite particles of Tb = 500 C, overestimation of the field intensity is calculated as a factor of 1.4. Applying this value, VDM is reduced to be 11.8 ZAm2. We will also discuss its implication for the CNS geomagnetic field intensity.