Petrology of Oligocene volcanic rocks from the middle part of the Monzen Formation, Oga Peninsula, northeastern Japan.

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  • 男鹿半島，漸新統門前層中部の火山岩類の岩石学
  • オガ ハントウ ゼンシントウモンゼンソウ チュウブ ノ カザンガンルイ ノ ガンセキガク

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Abstract

Oligocene volcanic rocks from the middle part of the Monzen Formation in the Oga Peninsula, northeastern Japan consist of lava flows and pyroclastic rocks. These volcanics are composed of basalts and andesites which can be divided into four groups based on differences in their TiO2 contents and petrographic characteristics ; high TiO2 basalt group (HTG), low TiO2 basalt and andesite group (LTG), quartz bearing medium TiO2 andesite group (MTG) and porphyritic andesite group (PTG).Olivine basalts of the HTG have geochemical characteristics similar to those of basaltic rocks from continental rift zone, whereas other basalts of the HTG and LTG have characteristics of both continental rift zone basalts and of island arc basalts. These compositional differences of basaltic rocks can be attributed mainly to differences in the geochemical characteristics of their source mantle. The variations in HFS (high field strength) element contents among olivine basalts in the HTG may have resulted from either differences in degree of partial melting of source mantle or differences in degree of fractional crystallization mainly of clinopyroxene from the primary basaltic magma.Andesitic rocks from the study area are characterized by the wide variations in Zr/Nb ratio and initial 143Nd/144Nd ratio (Nd I value), suggesting that the production of these andesitic rocks were affected by assimilation of crustal materials with low Zr/Nb ratio and low 143Nd/144Nd ratio. The accidental materials such as granitic fragments, quartz crystals having corroded form and phenocrystic plagioclases having core part with very low An% are contained in the MTG and LTG andesites. These features indicate that granitic rocks probably constituting wall rocks around the magma chamber are the candidate for the assimilants.

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