Secondary remanent magnetization carried by magnetite inclusions in silicates: a comparative study of unremagnetized and remagnetized granites

Magnetic carriers in remagnetized Cretaceous granitic rocks of northeast Ja pan were studied using paleomagnetism, rock magnetism, optical microscopy a nd scanning electron microscopy (SEM) by comparison with unremagnetized gra nitic rocks. The natural remanent magnetization (NRM) of the remagnetized r ocks is strong (0.3-1.7 A/m) and shows a northwesterly direction with moder ate inclination (NW remanence), whereas the unremagnetized rocks preserve w eak NRM (<0.5 A/m) with westerly and shallow direction (W remanence). Altho ugh thermal demagnetization shows that both NRMs are carried by magnetite, the remagnetized rocks reveal a higher coercivity with respect to alternati ng field demagnetization (20 mT < median destructive field) than the unrema gnetized rocks (<10 mT). On the basis of the modified Lowrie-Fuller test an d hysteresis parameters, the unremagnetized rocks carry multi-domain grains whereas the magnetic carrier of the remagnetized rocks is described as a m ixture of multi- and single-domain grains. Optical examination reveals that although common magnetic particles in the unremagnetized and remagnetized rocks are titanomagnetite grains with ilmenite lamellae larger than 30 mu m , numerous pure-magnetite grains finer than 10 mu m are characteristic of t he remagnetized ones. Studies by SEM equipped with an energy-dispersive ana lytical system show that these fine pure-magnetite grains occur as discrete particles within actinolitized parts where the Fe/Mg mole ratio is one thi rd of primary hornblende. These observations suggest that the NW remanence in the remagnetized rocks resides in the fine pure-magnetite inclusions sec ondarily formed from released Fe ions during the alteration of hornblende t o actinolite, The NW remanence is a chemical remanent magnetization which w as acquired during the alteration of rocks at some time between 62 and 15 M a. The originally formed coarse-grained titanomagnetite observed in both un remagnetized and remagnetized rocks carry the W remanence of a thermoremane nt magnetization acquired in the Cretaceous time. (C) 2000 Elsevier Science B.V. All rights reserved.