5:15 PM - 6:30 PM
[SMP43-P15] The source rock age of Renge metamorphic rock in the Omi-area, Itoigawa city.
Keywords:Renge metamorphic rock, U-Pb age
The sample used for analysis is a Grt glaucophanite, occurring as a layer or lenticular form surrounded by politic schists (Grt-Ms schist). It comprises of euhedral~subhedral garnet porphyroblast (1-4mm) and large amount of euhedral glaucophane. Chlorite occurs in the altered domain. This sample contains a lot of zircon, which has bright rim and dark core under the CL image. About 200 zircon grains were hand picked and U-Pb analysis was carried out using an Agilent 7500a LA-ICPMS at Niigata University.
Off the 115 zircons analyzed, 34 spot gave concordant ages. Zircon core age is about 420-690Ma and the most prominent peak is seen in the range of about 450-460Ma. In contrast, some of the zircon core show about 1200-1460Ma.
The U-Pb results in this study (450-460Ma) are older than 280-340Ma of K-Ar age (Kunugiza et al., 2004) and 380Ma of eclogite metamorphism (Tsujimori, 2010). However, the sample has basaltic chemical composition. But it is hard to conclude that the protolith of this sample is basalt, because it contain large amount of zircon and those zircon shows a large spread of ages. Therefore it is necessary to examine the sedimentation history and tetonic evolution of the Omi area in more detail, which is ongoing.