男鹿半島、漸新統門前層中部の火山岩類の岩石学
| Jazyk: | japonština |
|---|---|
| Rok vydání: | 2000 |
| Předmět: | |
| Zdroj: | 地質学雑誌. 106(4):280-298 |
| ISSN: | 0016-7630 |
| Popis: | 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 TiO_2 contents and petrographic characteristics；high TiO_2 basalt group (HTG), low TiO_2 basalt and andesite group (LTG), quartz bearing medium TiO_2 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. 男鹿半島西部地域に分布する漸新統門前層中部の火山岩類について，岩石学的検討を行った．門前層中部の火山岩類は，9タイプの斑晶鉱物組み合わせをもつ玄武岩～安山岩からなる．これらの火山岩類をTiO_2含有量と記載岩石学的特徴の違いに基づいて4つの岩石グループに区分した．玄武岩にはリフト帯に産する玄武岩と類似する微量元素比をもつものと，リフト帯と島弧の玄武岩の中間的な性質をもつものがある．これらの性質の違いは，Nd同位体比の検討から起源マントルの違いを反映したものと考えられる．安山岩には花崗岩質の岩片，これに由来するとみられる融食した石英とAn組成の著しく低い斜長石などが含まれることや，安山岩は幅広いNdI値とZr/Nb比を有することから，安山岩の形成には，これらの値と比の低い花崗岩質岩石の同化作用の関与があったことを論じた． |
| Databáze: | OpenAIRE |
| Externí odkaz: |
|