著者

新城 竜一 伴 雅雄 斎藤 和男 加藤 祐三

出版者

日本岩石鉱物鉱床学会

雑誌

岩鉱 (ISSN:09149783)

巻号頁・発行日

vol.86, no.7, pp.323-328, 1991-07-05 (Released:2008-03-18)

参考文献数

29

被引用文献数

10 11

---

High magnesian andesites from Kume-jima and Iriomote-jima, the Ryukyu islands were dated by the K-Ar method with the basalts from Kobi-sho and Sekibi-sho, the Senkaku islands. The obtained K-Ar ages of the high magnesian andesites are 6.08 ± 0.46 Ma and 13.1 ± 1.1 Ma for the sample (s) from Kume-jima and Iriomote-jima, respectively. The volcanic activity of the high magnesian andesite magma continued intermittently for at least 6 Ma. The K-Ar age of the basalt from Sekibi-sho is 2.59 ± 0.19 Ma. The age of the basalt lava from Kobi-sho is as young as 0.2 Ma or even younger, and indicates that Kobi-sho is a Quaternary volcano.

著者

池田 圭一 早坂 康隆

出版者

日本岩石鉱物鉱床学会

雑誌

岩鉱 (ISSN:09149783)

巻号頁・発行日

vol.89, no.12, pp.454-464, 1994-12-05 (Released:2008-03-18)

参考文献数

26

被引用文献数

6 13

---

Rb-Sr isotopic age determinations were made on the basement rocks of the Northern subzone of the Maizuru Terrane in the western Maizuru City and Oe Town area. The rocks of the area consist mainly of granitic rocks with minor amounts of quartz-diorite, gabbro, diabase, and pelitic schist and amphibolite (Komori metamorphic rocks), being called the Northern Yakuno rocks. They are distinguished from the Southern Yakuno rocks which consist of ophiolitic complex (Yakuno ophiolite) lying in the Southern subzone of the terrane by their lithofacies and whole rock chemistry. The Northern Yakuno rocks are characterized by the abundance of K-feldspar rich granitic rocks, whereas the Southern Yakuno rocks do not contain such rocks. The Northern Yakuno rocks are thrust over the Shimomidani Formation of the Akiyoshi Terrane to the north in the area. The Rb-Sr mineral isochron ages were determined upon six granitic rocks and one quartzdiorite. They can be divided into two age groups, the older (around 300 Ma) and the younger (around 150 Ma). The rocks give whole rock isochron ages of 327 Ma for the older group and 149 Ma for the younger one, whose initial 87Sr/86Sr ratios are 0.70669 and 0.70779 respectively. Whole rock isochron age for the gabbros and diabases is 163 Ma, and initial 87Sr/86Sr ratio is 0.70687. Nearly coincidence between whole rock and mineral isochron ages for each age group explains the existence of two stage igneous activities at those ages in this area, despite their mylonitization and recrystallization. Both whole rock chemistry and the initial 87Sr/86Sr ratio of those rocks indicate that the Northern Yakuno rocks was evolved as an matured crust of island-arc or continent by the late Carboniferous time. In this respect, the evolutional history of the Northern subzone is quite different from that of the Southern subzone in the terrane.

著者

林 正雄

出版者

日本岩石鉱物鉱床学会

雑誌

岩鉱 (ISSN:09149783)

巻号頁・発行日

vol.84, no.5, pp.152-158, 1989-05-05 (Released:2008-03-18)

参考文献数

5

被引用文献数

2 1

---

A method is proposed to describe quantitatively various zircon crystal forms projected either on the plane {100} or {110}. When the crystal planes observable from one direction are assumed to be the same size as those on the opposite side, four indexes can be calculated: the flatness index is the ratio of the total thickness to the total width, the elongation index is the ratio of the prism height to the sum of the total height and the total width, the prism index is the ratio of the total width of {100} to the prism perimeter, and the pyramid index is the ratio of the pyramid height to the height of {211}. All these indexes range from 0 to 1 for almost all zircon, and, regardless of the direction of observation, they remain virtually unchanged. This technique will provide useful clues not only to correlate igneous formations but also to estimate the physico-chemical conditions of the host rock of zircon crystals.

著者

関 陽太郎

出版者

日本岩石鉱物鉱床学会

雑誌

岩石鉱物鉱床学会誌 (ISSN:00214825)

巻号頁・発行日

vol.60, no.1, pp.27-33, 1968-07

著者

大場 司

出版者

日本岩石鉱物鉱床学会

雑誌

岩鉱 (ISSN:09149783)

巻号頁・発行日

vol.88, no.1, pp.1-19, 1993-01-05 (Released:2008-03-18)

参考文献数

15

被引用文献数

5 9

---

Magmatic evolution of Akita-yakeyama volcano, northeastern Japan, has been investigated based on bulk chemistry (major and trace), Sr isotopic features and mineral chemistry in combination with stratigraphical study. The main body of the volcano was built by eruptions of a large amount of hypersthene-augite andesite magma and a minor amount of olivine bearing hypersthene-augite andesite magma as lava flows and pyroclastic falls, and an edifice of stratovolcano was built. No evidence of magma mixing or crustal assimilation can be recognized in the olivine free andesite, but olivine-pyroxene disequilibrium in the olivine bearing andesite indicates magma mixing. After a repose period, a series of eruptions began from the central vent and erupted hypersthene-augite andesite magma as fine ash fall and hypersthene-augite-olivine andesite magma as lithic-rich pyroclastic falls. As stated above the olivine-free andesite has no evidence of magma mixing, but the olivine bearing andesite has the mineralogical and chemical evidence of magma mixing, such as disequiliblium phenocryst assemblage, reverse zoning of pyroxene and dusty plagioclase. Flank eruptions followed and erupted quartz bearing olivine-hypersthene-augite andesite magma, quartz bearing augite-hypersthene andesite magma and augite-hypersthene dacite magma as lithic-rich pyroclastic falls, lava flows and pumice fall. Among them quartz bearing andesites have high 87Sr/86Sr ratios, suggesting the involvement of crustal assimilation. At the end of these eruptions, augitehypersthene dacitic lava dome and olivine bearing augite-hypersthene dacitic lava dome intruded into central crater. Dacite magma of the lava domes has no or little evidence of magma mixing or crustal assimilation. Chemical variation of trace elements (Zr, Rb, Y) from the hypersthene-augite andesite of the main body to the augite-hypersthene dacite of the domes cannot be accounted for by fractional crystallization, and difference of degree of partial melting of the source material is required.

著者

村上 英樹 木股 三善 下田 右 伊藤 英司 佐々木 聡

出版者

日本岩石鉱物鉱床学会

雑誌

岩鉱 (ISSN:09149783)

巻号頁・発行日

vol.87, no.12, pp.491-509, 1992-12-05 (Released:2008-03-18)

参考文献数

37

被引用文献数

2 8

---

合成された純粋な灰長石,それぞれMgO, SiO2に富んだ灰長石を, EPMAと四軸自動X線回折装置を用いて化学組成と格子定数の測定を行い,三宅島産灰長石との比較を行った。複合イオン置換に基づく検討から,天然及び合成の灰長石は, MgをCaMgSi3O8, 過剰のSiは ?? Si4O8端成分として,固溶する可能性が指摘された。さらにMgに対するこの端成分を確認するため, EPMAによるMgの化学シフトを解析した。その結果,合成灰長石に固溶されるMgは,オケルマナイト(Ca2MgSi2O7) 中のMgと同じ四配位席を占有することが示唆された。また, X線単結晶回折法からは, Mgの固溶量が増加するに従って,単位格子の体積が増加する傾向が明らかになった。以上のことから灰長石におけるMgの固溶は,雲母,角閃石,輝石に認められる通常のTschermak置換, Mg+Si ?? ;2Al, とは異なり,四配位席だけでのTschermak置換が,メリライトと同様に灰長石においても成立することを明示している。

著者

伊藤 俊彦

出版者

日本岩石鉱物鉱床学会

雑誌

岩鉱 : 岩石鉱物鉱床学会誌 : journal of mineralogy, petrology and economic geology (ISSN:09149783)

巻号頁・発行日

vol.91, no.6, pp.209-219, 1996-06-05

被引用文献数

1