北部フォッサマグナ，関田山脈–志久見川流域に分布する前期更新世火山岩類の岩石学的特徴
─ソレアイト系列火山岩類の形成過程─

抄録

  The Sekita Mountains and Torikabuto Volcano District lies at the northeastern end of the Central Upheaval Zone located within the North Fossa Magna region. In this area, early Pleistocene volcanic rocks ranging basaltic to dacitic compositions are widely distributed. These volcanic rocks are subdivided into four distinct groups, based on their geological and geochemical characteristics, the Sekita Mountains (SM), the Shikumi River Basin (SRB), the Kenashi (KN) volcano and the Torikabuto (TK) volcano.
  In this paper we report petrographical, mineralogical, whole rock geochemical, and Sr- and Nd- isotopic characteristics of the SM and SRB volcanic rocks. The SM and SRB volcanic rocks are divided into group 1 and group 2, and these volcanic rocks are composed mainly of basalt, andesite and dacite belonging to the tholeiitic rock series. The ages of SM and SRB rocks range from ∼2 Ma to 1.3 Ma. Our data can concluded as the followings. 1) The SM and SRB volcanic rocks were formed by fractional crystallization processes of tholeiitic magma, involving plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxides. 2) The Sr- and Nd-isotopic compositions of the SM and SRB tholeiitic volcanic rocks are nearly identical to that of the TK calc-alkaline volcanic rocks, suggesting a genetic link between the SM, SRB and TK rocks. 3) The tholeiitic primary magmas for the SM and SRB volcanic rocks and the calc-alkaline primary magmas for the TK rocks would be formed by partial melting processes of the mantle source materials having similar Sr- and Nd-isotopic composition. We interpret that the tholeiitic rocks are products of crystal differentiation at middle crustal level (∼5 kbar), while the calc-alkaline rocks are derived by fractional crystallization at shallower level (2–3 kbar).