Abstract Carbonado and yakutite are both porous aggregates of polycrystalline micrometre-size diamond, with very different characters from those of monocrystalline diamond. The genesis of carbonado is very controversial, whereas yakutite is thought to have been formed by meteorite impact. Neutron activation analyses of trace elements in carbonado and yakutite indicate that their rare earth element ( REE ) abundance patterns have common characteristics: heavy REEs are not much depleted and a negative Eu anomaly is observed. These patterns are quite different from those of kimberlite and monocrystalline diamond and are similar to those of crustal materials such as shale, supporting the hypothesis of a crustal origin for carbonado and yakutite.
Sm-Nd and Rb-Sr isotopic compositions and REE abundances were measured on the Ishikawa composite mass, which is one of the older granitic rocks in the Abukuma mountains. Three mineral and two whole-rock samples define a Sm-Nd isochron yielding an age of 111±42 Ma with an initial 143Nd/144Nd ratio of 0.51251±3. Eight whole-rock samples give a Rb-Sr isochron age of 106±16Ma with an initial 87Sr/86Sr ratio of 0.70518±15. These ages are interpreted to represent the time of intrusion, supporting the Jurassic age of the Gosaisho metamorphic rocks confirmed by the fossil evidence. The Nd model ages of the Jshikawa mass relative to CHUR are as young as intrusion ages, whereas those relative to DM are 600-800Ma. On the basis of the REE patterns and εNd-εSr relationship, the Ishikawa mass is thought to be derived from the source material similar to CHUR. It is therefore suggested that the intrusion of the Ishikawa mass occurred subsequently to the differentiation of the source magma from the mantle.
A concealed fault named “the Gifu-Ichinomiya Line” along the eastern margin of the Nobi Plain was stratigraphically reexamined. This fault has been regarded as an active fault concealed by Cenozoic strata of the Nobi Plain, which runs in the NNW-SSE direction, and connects Gifu with Ichinomiya and Inazawa. However, the exact location of the fault has not been well defined. We focused our attention on the vertical position of a pumice bed of 80, 000 to 90, 000 years old that appears in bore-hole records around the site, because the pumice bed is a key to stratigraphic correlation. The correlation of 97 bore-hole records showed a throw of the pumice bed amounting to 5.6m in average. The strike of this throw parallels to the JR-Tokaido Line and runs about 1, 100m to the east of the line. The vertical displacements were also recognized for two gravel beds (G1 and G2) above and under the pumice bed. The throw for the deeper strata tends to be larger. We regard this throw as an active fault that moved at the Nobi earthquake of 8.0 magnitude in 1891 and other earthquakes repeated during the Quaternary.Recently, Aichi Prefecture carried out stratigraphic examinations of shallow well records and geophysical prospecting and reported that neither fault nor flexure is realized around the presumed site of the fault in question. On the basis of the results, Governmental Earthquake Research Committee concluded the fault to be nonexistent. However, the survey by Aichi Prefecture exploited exclusively the area to the west of the fault disclosed from our stratigraphic work and scarcely covered the site of the fault mentioned above. Therefore, the survey by Aichi Prefecture is meaningless in the examination of the Gifu-Ichinomiya fault, and their conclusion is erroneous.
