著者

Tetsuo KAWAKAMI Simon L. HARLEY

出版者

Japan Association of Mineralogical Sciences

雑誌

Journal of Mineralogical and Petrological Sciences (ISSN:13456296)

巻号頁・発行日

pp.230131b, (Released:2023-04-25)

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Boron isotope compositions were measured in kornerupine and tourmaline from lenses consisting primarily of kornerupine, plagioclase and corundum. The lenses occur within hornblende-gneiss or along the boundary between this gneiss and an amphibolite lens at Akarui Point in the Lützow-Holm Complex, Prince Olav Coast, East Antarctica. The peak metamorphic conditions have been estimated to be ~ 800-900 oC and ~ 8-11 kbar. The δ11B compositions of kornerupine, which is interpreted to have been a stable phase at the metamorphic peak, are -11.6 ± 0.4 to -7.8 ± 0.5 ‰ and -9.8 ± 0.3 to -6.1 ± 0.2 ‰ in two different samples. Grains of prograde tourmaline included in kornerupine and corundum yielded δ11B = -2.1 ± 0.3 to +0.6 ± 0.3 ‰, and the secondary tourmaline replacing kornerupine yielded δ11B = -4.6 ± 0.2 to -3.7 ± 0.2 ‰. Therefore, the isotopic fractionation between kornerupine and tourmaline, △11BTur–Krn (= δ11BTur - δ11BKrn), of the average prograde tourmaline and average host kornerupine is +6.7 ± 1.5 ‰, which is interpreted to indicate isotopic equilibrium at the metamorphic peak on the basis of previous studies of isotope fractionation between tourmaline and minerals of the kornerupine-prismatine series. The δ11B values obtained on prograde tourmaline are between whole rock δ11B of MORB and mantle rocks and of some sedimentary rocks, and are similar to the δ11B of blackwall tourmalines that crystallized during the decompression stage following high-pressure metamorphism. We infer that the syn-metamorphic B-bearing fluid present in the kornerupine-plagioclase-corundum lens is likely sourced from a mixture of sedimentary, mafic and ultramafic lithologies in a subduction setting. The metabasic and meta-ultramafic lenses found in Akarui Point could be interpreted as the remnant of mixing zone of Ediacaran to Cambrian subduction channel.

著者

Fumiko Higashino Tetsuo Kawakami Tatsuro Adachi Masaoki Uno

出版者

Japan Association of Mineralogical Sciences

雑誌

Journal of Mineralogical and Petrological Sciences (ISSN:13456296)

巻号頁・発行日

pp.230131a, (Released:2023-04-21)

被引用文献数

1

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This paper reports multiple fluid infiltration events during retrograde metamorphism in the Sør Rondane Mountains, East Antarctica. Pelitic gneisses from southern part of Perlebandet have cordierite-biotite intergrowth rimming garnet, implying that garnet breakdown occurred by fluid infiltration. Using the Raman peak of CO2 in cordierite and Cl-bearing composition in biotite, this study revealed that the cordierite-biotite intergrowth was formed in equilibrium with one-phase CO2-Cl-H2O fluid. The intergrowth texture is cut by thin selvages composed of Cl-bearing biotite, suggesting Cl-bearing fluid infiltration. Since andalusite is exclusively observed in the selvage, near isobaric cooling path is presumed for the pressure-temperature (P-T) path of these post-peak fluid-related reactions. The inconsistence with counter-clockwise P-T path reported from northern Perlebandet is probably due to the granodiorite/leucocratic granite bodies beneath the studied metamorphic rocks. In order to understand the tectonic evolution at the final stage of Gondwana amalgamation, therefore, effect of hidden igneous rocks needs to be taken into consideration.

著者

Fumiko HIGASHINO Tetsuo KAWAKAMI

出版者

Japan Association of Mineralogical Sciences

雑誌

Journal of Mineralogical and Petrological Sciences (ISSN:13456296)

巻号頁・発行日

vol.117, no.1, pp.220325, 2022 (Released:2022-10-08)

参考文献数

59

被引用文献数

6

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This paper reports the first outcrop occurrence of an ultrahigh–temperature (UHT) metamorphic rock from the Sør Rondane Mountains (SRM), East Antarctica. A pelitic gneiss from Balchenfjella, eastern SRM, contains mesoperthite that gave UHT condition (>900 °C) by ternary feldspar thermometry. The UHT mesoperthite is present both in the matrix and as an inclusion in garnet. The garnet also has nanogranitoid inclusions next to the mesoperthite, which are interpreted to be an UHT melt. The re–integrated nanogranitoid composition is plotted in the primary phase region of quartz and classified as granite. Even crystallized nanogranitoids can provide appropriate original melt composition in the An–Ab–Or and Qz–Ab–Or spaces, whereas Mg concentration is enriched due to local retrograde Fe–Mg exchange reaction between the nanogranitoid inclusions and the host garnet. Although metamorphic rocks in the SRM are highly retrogressed, this study revealed that the microstructural evidence of UHT condition is partially preserved. Further investigation of timing and areal extent of UHT metamorphism helps us to understand the tectonic model of the SRM.

著者

Kota SUZUKI Tetsuo KAWAKAMI

出版者

Japan Association of Mineralogical Sciences

雑誌

Journal of Mineralogical and Petrological Sciences (ISSN:13456296)

巻号頁・発行日

vol.114, no.6, pp.267-279, 2019 (Released:2020-01-22)

参考文献数

32

被引用文献数

12

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The metamorphic pressure–temperature (P–T) conditions of high–grade pelitic gneisses from Akarui Point, Skarvsnes, Skallen, and Rundvågshetta in the Lützow–Holm Complex (LHC), East Antarctica are re–examined by applying the Zr–in–rutile geothermometer to rutile inclusions in garnet enclosing Al2SiO5 minerals and to matrix rutile grains. By utilizing the P zoning of garnet to indicate isochronous surface, samples from Akarui Point, Skarvsnes, and Skallen were shown to have experienced almost the same P–T conditions around the kyanite/sillimanite transition boundary (~ 830–850 °C/~ 11 kbar). From Rundvågshetta, higher–T condition (850 ± 15 °C/0.1 kbar to 927 ± 16 °C/12.5 kbar) was confirmed from rutile inclusions in garnet enclosing sillimanite. Matrix rutile yielded similar temperature as inclusion rutile for Akarui Point, Skarvsnes, and Skallen samples. Therefore, the traditional metamorphic zone mapping based on matrix mineral assemblages, which classified Akarui Point as belonging to the transitional zone between the upper–amphibolite and the granulite facies zones, does not reflect the highest metamorphic conditions attained. The P–T–t evolution of the LHC needs to be re–evaluated utilizing detailed petrochronological approaches.