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Title Transcription
オキ ドウゴ ノ チョウシ アラキガワ リュウイキ ニ ブンプ スル オキ ヘンセイガン ノ ガンソウ キサイ ト ヘンセイ ヘンレキ
Title Alternative (English)
Petrography and P-T History of Oki Metamorphic Rocks from Choshi and Araki Areas, Oki-Dogo Island
File
language
jpn
Author
Hamada, Koichi
Fujihara, Natsuko
Tsunogae, Toshiaki
Description
Clockwise P-T path was constructed using petrographic and mineralogic data of high-T/low-P metamorphic rocks from Oki-Dogo Island, southwest Japan. The studied area is composed predominantly of Cretaceous pelitic gneiss with minor mafic and calcareous gneisses in upper amphibolite to lower granulite grade. There occur several types of leucocratic veins or pods associated with pelitic gneiss. Sillimanite-bearing leucosome (L2) is identified as thin and folded bands in Al-rich pelitic gneiss. In the L2 Ieucosome, sillimanite is present as needle-like aggregates associated with K-feldspar and quartz. We inferred the following dehydration melting reaction [1] for the assemblage.
Ms + Qtz + Pl - Sil + Kfs + L [1]
Garnet-bearing leucosome (L3) , which is younger than L2 because of its less-deformed character, Is composed of quartz, K-feldspar, plagioclase, and garnet. Garnet typically includes quartz and biotite, suggesting a progress of the following melting reaction [2] ;
Bt + Sil (And) + Qtz + Pl - Grt + Kfs + L [2]
Reactions [1] and L2] are prograde reactions and both L2 and L3 are considered to be products of prograde metamorphism. Available experimental data suggest that prograde pressure was about >4 kbar. Lack of Opx in pelitic gneiss suggests that peak temperature weLS Iower than that of the beginning of biotite dehydration melting to Opx + K-feldspar (T<820℃).
Garnet in pelitic gneiss was subsequently affected by retrograde breakdown reaction to biotite+sillimanite (andalusite) (reaction [3])
Grt + Kfs + H2O - Bt + Sil (And) + Qtz [3]
Petrographic and geothermobarometric data indicate that the reaction [3] took place around the boundary between sillimanite and andalusite fields. Our data therefore suggest that pelitic gneiss from Oki-Dogo Island suffered crustal thickening toward mid-crustal level followed by rapid uplifting and subsequent cooling.
Keywords : Metamorphic rock, Petrography, P-T history, Geothermobarometry, Oki-Dogo
Journal Title
Memoirs of the Faculty of Education, Shimane University. Natural science
Volume
30
Start Page
53
End Page
72
ISSN
05869943
Published Date
1996-12-27
NCID
AN00107941
Publisher
島根大学教育学部
Publisher Aalternative
The Faculty of Education Shimane University
NII Type
Departmental Bulletin Paper
OAI-PMH Set
Faculty of Education
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