阿武隈山地東縁の石炭紀および白亜紀アダカイト質花崗岩類とその地質学的意義

Abstract

The ‘Wariyama Sheared Granodiorite’ occurs along the east side of the Futaba Fault with the Wariyama uplift zone, Miyagi, Japan. Rocks from the ‘Wariyama Sheared Granodiorite’ can be geochemically classified as typical adakite, and further subdivided into medium-grained and K2O- and Sr-poorer rocks, and coarse-grained and K2O- and Sr-richer rocks. Zircons from the former yield U–Pb ages of 308 ± 3 and 302 ± 4 Ma, whereas zircons from the latter have ages of 118 ± 2 and 117 ± 1 Ma. On the basis of these results, the ‘Wariyama Sheared Granodiorite’ can be redefined. The late Carboniferous granite is renamed the Wariyama Granite as in the past, but we use the term ‘granite’ in a broad sense as describing the rock type. The Early Cretaceous granite is newly defined as the Takase Granite. The Takase Granite crops out on the eastern side of the central region of the Wariyama Granite with faulted contact. The Takase Granite is divided into the SiO2-poor Takase Granite A and the SiO2-rich Takase Granite B. The U–Pb zircon age of 300 Ma obtained here corresponds to an interval of five major granitic events, and the timing of the onset of Permian granitic magmatism (280 Ma) may actually be ca. 300 Ma. Typical adakites are considered to be the product of slab melting, which occurred in response to unusual tectonic conditions, such as subduction of young lithosphere and/or an active ridge at ca. 300 Ma.