Abstract
Fission track (FT) dating has been conducted for three dacitic welded tuff samples of the Gongenzaki Formation, the lowermost volcanic formation of Tertiary green tuff sequences in the Tsugaru district. The samples were collected from the northern coast of the Kodomari Peninsula. FT ages of 19-23 Ma were determined for internal zircon surfaces, and we interpret these, based on analyses of FT length measurements, as representing emplacement ages of pyroclastic materials. FTs on external surfaces provided ages of 22-31 Ma, one of which is significantly older than that for internal surfaces. Experimental results show that the disagreement is caused by external effect. The onset of deposition of the formation is defined as~23 Ma or older, and the uppermost part can be dated as late Early Miocene (17-20 Ma). U-Pb dating by a LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) reveals that a sample contains zircons having a single age population of about 33 Ma (Early Oligocene), which is much older than the FT age (~23 Ma: early Early Miocene). It is suggested that the zircons were originally crystallized in Early Oligocene time and, nearly 10 million years later, picked up by a dacitic pyroclastic flow or within its source magma.
