佐渡島小木玄武岩中に包有されるはんれい岩類

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  • サドガシマ ショウモク ゲンブガン チュウ ニ ツツミ ユウサレル ハンレイガンルイ
  • Gabbroic inclusions in the middle Miocene Ogi Basalt Member, Sado Island, Japan.

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Abstract

Petrography and mineral chemistry were studied for gabbroic inclusions in the middle Miocene Ogi Basalt, Sado Island, Japan. The gabbroic inclusions are mainly olivine gabbro and gabbronorite, which are divided into three types based on a mafic-mineral assemblage: Ol-Cpx type with olivine and clinopyroxene, Ol-2Px type with olivine, clinopyroxene, and orthopyroxene, and 2Px type with clinopyroxene and orthopyroxene. Each mineral has similar range in composition. However, some Ol-Cpx type samples rich in modal olivine are characterized with the highest Fo# and NiO content of olivine and highest Mg# and Cr_2O_3 content of clinopyroxene. Spinel included in the olivine also has high Cr#. Primitive basaltic magma is expected as their parent. As far as based on a similarity in mineral chemistry, the gabbroic inclusions are cognate with olivine basalt composing the Ogi Basalt. Consequently, the gabbroic inclusions may represent a plutonic facies of new crust originated by basaltic magmatism during the back-arc rifting to form the Japan Sea.

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