Wavy extinction in quartz is a good indicator for the magnitude of deformation. The intensity of wavy extinction (IWE) in quartz was quantitatively measured using polarized optical microscope, digital camera and NIH Image. Quartz-grain showing a maximum interference color is appropriate for the measurement of IWE. A traverse for the measurement is set across the parts showing distinctive wavy extinction, where subgrain boundaries in deformed quartz generally intersect with the line. The brightness of each part showing wavy extinction is measured at the extinction position of a subgrain. A gradation of brightness is treated as 256 gradient in computer. IWE is defined by the formula, IWE=tan (magnitude of brightness/1000*length of the line for measurement), where the length is scaled by millimeter. Based on this method, we can give an intensity of wavy extinction to deformed quartz grains, and it has been shown that the mylonitic and weakly mylonitized granitic body adjacent to the Tanakura shear zone in northeastern Japan can be divided into several zones of deformation intensity. This method can be applicable to many kinds of deformed minerals to describe the degree of deformation.

View full abstract

Whole rock chemistry (major elements) and mineral chemistry (biotite, amphibole and plagioclase) of the Nihonkoku Mylonite and Iwafune Granite, analyzed using XRF and EPMA, are presented.
     The Nihonkoku Mylonite originated mainly from granitic rocks and small amount of sedimentary rocks of the Ashio Belt. It is composed of gneissose hornblende-biotite granodiorite, gneissose biotite granodiorite, gneissose biotite granite and biotite-muscovite schist. The gneissose biotite granite grades into massive biotite granite of the Iwafune Granite, which is distributed widely on the southwest side of the Nihonkoku Mylonite Zone.
     Whole rock and mineral chemistries of the gneissose biotite granite of the Nihonkoku Mylonite overlap with those of massive biotite granite of the Iwafune Granite. On the basis of gradual field relations and petrographic features, the Iwafune Granite is regarded as one of the original rocks of the Nihonkoku Mylonite. Bulk and mineral chemistries of massive biotite granite, located on the northeast side of the Nihonkoku Mylonite Zone, are comparable with those of the Iwafune Granite. Based on this chemistry, together with petrographic similarities, the massive biotite granite located on the northeast side of the Nihonkoku Mylonite Zone is regarded as a member of the Iwafune Granite. This means that the Nihonkoku Mylonite Zone can not be the northern extension of the Tanakura Tectonic Line, because both sides of the mylonite zone are occupied by the Iwafune Granite in the Ashio Belt.
     Whole rock chemistries of the Nihonkoku Mylonite and Iwafune Granite represent a nearly straight trend for every oxide except for Na₂O on the Harker's diagram. The range of SiO₂ contents increases systematically from gneissose hornblende-biotite granodiorite through gneissose biotite granodiorite, gneissose biotite granite to massive biotite granite (Iwafune Granite). Mg/(Mg+Fe) ratio of biotite and anorthite content of plagioclase decrease systematically from the gneissose hornblende-biotite granodiorite through gneissose biotite granodiorite, gneissose biotite granite to massive biotite granite (Iwafune Granite). These systematic chemical features suggest that each rock facies of the Nihonkoku Mylonite and the Iwafune Granite is comagmatic in origin.
     Elongated aggregates of fine-grained biotite crystals, which form the foliation of the Nihonkoku Mylonite, are recrystallized products from mylonitization. These fine-grained crystals of biotite are poor in Ti and rich in Al, compared to the original coarse-grained crystals. The decrease in Ti indicates that the temperature of recrystallization through mylonitization was lower than the crystallization temperature of original magmatic biotite.

View full abstract

Minute hexagonal platy crystals in druse of pyroxene andesite from Taihakusan, Sendai were examined by polarized optical microscope, SEM, EPMA and X ray studies. Although the mineral looks like trydimite, X ray analyses (microdiffraction and precession camera) demonstrate it to be cristobalite. It is assumed that the cristobalite crystals with fine stacking faults and polysynthetic twins were produced in the druse from a gas phase after andesite was solidified.

View full abstract