スフェーンを用いたフィッショントラック法による年代測定 塙深成岩体の例

Abstract

Fission track age of sphene of five samples from the Hanawa pluton near the Tanakura Shear Zone, northeastern Honsyu, Japan was able to be measured by grain-by-grain method, although the number of spontaneous track (density) of sphenes is variable among rock samples and sphene crystals capable of measurement are a little among mineral grains of a sample. The ages were determined comparing with age standard sample (M. D. C., 98.7 Ma). Mineral grains were burned in a tephron resin was etched by 50M(1 M=1 mol dm-3) NaOH(130°C) after. The spontaneous track density was measured on the polished surface and the induced track density on the mica detecter after the thermal neutron irradiation. Since the tracks induced by 232Th and 238U occupy considerable amounts on the irradiation at a reacter with a low Cd ratio, age data were corrected by Th/U ratio. Th and U contents in sphene measured by NAA range from several ten to several hundred ppm corresponding to 0.089-4.0 in their ratio (Th/U). As a results of the correction by Th/U, the age data became 2% younger than before. The ages of the mylonite-like subfacies, large sphene bearing subfacies, and fine-grained quartz diorite-tonalite facies are 67.4, 91.9 and 92.5, 92.5 and 93.1 Ma respectively. The K-Ar ages of hornblendes in large sphene-bearing subfacies are 10% younger than those of the fine-grained quartz diorite-tonalite facies (Shibata and Uchiumi, 1983), while F. T. age of sphene of both facies are consistent. This is caused by the gap of the closure temperature in both methods. More younger age (67 Ma) of the mylonite-like subfacies is probably due to shear heating effect along the Tanakura Shear Zone.