ニュージーランド北島, オハクネ火口産高マグネシア質安山岩と同質デイサイト

概要

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The Ohakune crater is one of the parasitic vents around Ruapehu volcano. Olivine andesite and bronzite dacite occur in and around the crater, the eruption of which took place explosively on the flat laharic plain surrounding Ruapehu volcano. The Ohakune crater rocks ' contain some phenocrysts of Mg-rich olivine (Fo88) or orthopyroxene (En89) , indicating their derivation from the mantle peridotite. The olivine andesites have high contents of MgO (7.3-6.6%) and SiO_2 (55.5-57.4%), similar to magnesian andesites from some island arcs and continental margins. The bronzite dacite also has MgO (5%) despite its relatively high SiO_2 (63.7%). The high-Mg olivine andesites fall near the area of parental basalts and andesites of very low-K and -Rb in the linear trend of the Tongariro volcanic rocks in the K-Rb relation, whereas the high-Mg dacite is plotted on the area of relatively high-K and -Rb in the same linear trend. The dacite may have formed by contamination ( or mixing) of the olivine andesite-forming magma with a high-K and -Rb, felsic crustal material.
静岡大学の論文