紀伊半島中央部,中期中新世宮滝岩脈 : 火道内でのマグマ・ミングリングとその意義(<特集>紀伊半島における中新世火成作用とテクトニクス)  [in Japanese] The Middle Miocene Miyataki dile, central Kii Peninsula, southwest Japan : magma mingling observed in dike conduit and its implications(<Special Issue>Miocene igneous activity and tectonics in the Kii Peninsula)  [in Japanese]

紀伊半島中央部,奈良県吉野町に露出する宮滝岩脈の形成過程を再検討した.同岩脈は従来,多量の岩片を含むため火砕岩であると考えられてきた.今回,野外および鏡下での観察により,宮滝岩脈は流紋岩と安山岩が互いにミングリングし,また包有物として含まれる岩石からなることが明らかとなった.さらに花崗岩が流紋岩・安山岩に包有されるとともに,流紋岩中の花崗岩は安山岩に包有された上で包有されている.いずれの包有物とも不定形状をなし高温時に塑性変形を受けたことを示している.これらのことから,流紋岩・安山岩両マグマは花崗岩体と岩脈定置前に一旦ミングリング・断片化した後,さらにミングリングを続けながら上昇して宮滝岩脈を形成したと推定できる.

We reexamined the Miocene Miyataki dike exposed at central Kii Peninsula, southwest Japan to infer the emplacement process of the dike. Although the dike has been regarded as of pyroclastic origin, our detailed field and petrographical investigations reveal that the Miyataki dike is composed of andesite mingled with rhyolite. In addition, granite enclaves also occur both in andesite and rhyolite, and those in rhyolite are often mantled by andesite. Shapes of rhyolite, andesite and granite enclaves are irregular and amoeboidal, indicative of ductile deformation during dike emplacement. These evidences show that andesite and rhyolite magmas and granite body could mingle with one another before dike emplacement, and that after the interaction, mingled magma rose to form the Miyataki dike. Whole rock compositions also support this mingling model for the Miyataki dike. Therefore, the facies and structures observed in the Miyataki dike are considered to be formed by a physical mingling processes in dike conduit.