Department of Geoscience, Joetsu University of Education
National Institute of Polar Research
抄録(英)
The melting relationships of five syenites from the Yamato and Sør Rondane Mountains were determined in the temperature range of 650-850℃, under the water pressure of 1-2 kbar and oxygen fugacities of the FMQ buffer. The syenites studied are samples from the Yamato Mountains (Nos. 73120904 : Y904 and 73120405 : Y405) and from the Sør Rondane Mountains (Nos. 85012101A : SR-A, 85012103B : SR-B and 85012103C : SR-C). They are composed of porphyritic K-feldspar, plagioclase, quartz, biotite, Ca-amphibole or Ca-Na amphibole, clinopyroxene, sphene, ilmenite and rarely magnetite. Though the syenites from both regions consists of the same mineral assemblages, the compositions of amphiboles are various as follows; Ca-amphibole in the syenite (Y904) from the Yamato Mountains is close to edenitic. On the other hand, the bluish green hornblende in the mafic layer (SR-B) and the leucocratic layer (SR-C) from the Sør Rondane Mountains is richteritic actinolite. In the porphyritic syenite (Y405) and the leucocratic syenite (SR-A) intruding the layered syenites (SR-B and SR-C), the Ca-amphibole is pargasitic hornblende. The solidus temperatures of the syenites from the Sør Rondane Mountains are about 770℃ at 1 kbar. With increasing temperature, plagioclase (albite) and quartz disappeared in the lower temperature range than K-feldspar, hornblende, biotite and clinopyroxene. Clinopyroxene, biotite and K-feldspar are stable at 850℃ under the water pressure of 1 kbar. Richteritic actinolite in two layered syenites (SR-B and SR-C) becomes unstable at slightly higher temperature than solidus at 1 kbar. Pargasitic hornblende from syenite (SR-A) is stable at 825℃ and 1 kbar, and it disappears at 775℃ and 1.7 kbar. Representative results of syenites from the Yamato Mountains at 1 kbar are : clinopyroxene quartz syenite (Y904) begins to melt at 680℃, quartz and plagioclase disappear at 710℃, edenitic hornblende disappears at 775℃, K-feldspar disappears at 825℃, clinopyroxene hornblende quartz syenite (Y405) begins to melt at 760℃, plagioclase and quartz disappear at 780℃, K-feldspar disappears at 830℃, hornblende appears at 850℃. In both rocks, clinopyroxene and biotite remain at 850℃. The composition of edenitic hornblende shifts to actinolite-pargasite join with increasing temperature under water pressure. The result suggests that edenite end member is unstable under high water pressure in a preliminary synthesized experiment. Pargasitic hornblende from two syenites (SR-A and Y405) are both stable at 825℃ and 1 kbar. At 1.6 kbar the hornblende (Y405) is stable at 825℃, while the other hornblendes is unstable at 800℃.All rocks investigated are at least 40% melted at temperatures 50℃ above the solidus. The approximate K-feldspar content of the syenite is about 30%. The compositions of K-feldspar are Ab_<40>Or_<60> and Ab_5Or_<95> at 800℃ and 700℃, respectively. As compared to biotite, the behavior of K-feldspar is important to concentrate K_2O content of syenite.
雑誌書誌ID
AA1072335X
雑誌名
Proceedings of the NIPR Symposium on Antarctic Geosciences
EXPERIMENTAL STUDY OF THE SYENITIC ROCKS FROM THE YAMATO AND SØR RONDANE MOUNTAINS, EAST ANTARCTICA
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KJ00002368250
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概要
