Electron microprobe technique for U-Th-Pb and REE chemistry of monazite, and its implications for pre-, peak- and post- metamorphic events of the Lutzow-Holm Complex and the Napier Complex, East Antarctica

Monazites in high-grade metapelites from the Lutzow-Holm Complex and Napier Complex have been examined in terms of U, Th, Pb and rare earth element (REE) chemistry using an electron microprobe. The studied samples include four granulite-facies garnet-biotite-bearing metapelites from Skallen within the Lutzow-Holm Complex, and a re-hydrated garnet-sillimanite gneiss from the Mt. Riiser-Larsen area within the UHT zone of the Napier Complex. Two out of four garnet-bearing metapelitic samples from Skallen gave simple 560-500Ma monazite U-Th-Pb ages, whereas the other two samples yielded two age populations, i.e., 560-500Ma and 650-580Ma. The younger age group is consistent with the 550-520Ma metamorphic ages reported by SHRIMP. The older>580Ma monazites are relatively enriched in Nd, Sm, Gd, Dy (MREE) and depleted in Si (Ca and Th) compared with the younger (560-500Ma) ones. These older monazites possibly formed through M-HREE-enriched conditions such as garnet-free conditions, suggesting that the growth of these monazites pre-dated the peak metamorphism. Garnet-sillimanite gneiss from the Mt. Riiser-Larsen area shows various post-UHT re-hydration textures such as biotite-sillimanite aggregates, and fine-grained biotite flakes around or intracrystalline fractures within garnet porphyroblasts. Monazites enclosed within garnet cores have 2480-2440Ma U-Th-Pb ages consistent with the reported zircon and monazite SHRIMP dates. On the other hand, those associated with re-hydrated zones gave fluctuating 2200-700Ma ages. These younger ages are thought to reflect the incomplete chemical disturbance during the post-UHT crustal processes

TiO2 solubility in garnet coexisting with orthopyroxene, quartz and rutile: Ti-in-garnet thermometer for ultrahigh-temperature granulites

第6回極域科学シンポジウム[OG] 地圏11月16日（月）　国立極地研究所１階交流アトリウ

Basement geological research of East Antarctica

The Tenth Symposium on Polar Science/Special session: [S] Future plan of Antarctic research: Towards phase X of the Japanese Antarctic Research Project (2022-2028) and beyond, Tue. 3 Dec. / Entrance Hall (1st floor) at National Institute of Polar Research (NIPR

Geology of Byobu Rock and Gobanme Rock, Prince Olav Coast, East Antarctica

We report here the geology of Byobu Rock and Gobanme Rock outcrops hitherto unmapped in the Prince Olav Coast, East Antarctica. Both these outcrops expose high-grade metamorphic and igneous rocks. The metamorphic rock units comprise mainly of gneisses migmatized to variable extent and amphibolites, whereas igneous rocks comprise of granites and pegmatites. Preliminary structural data obtained from the outcrops identified the regional, steep to moderately dipping foliation trend in the WNW-ESE at Byobu Rock, whereas they trend in the NW-SE direction at Gobanme Rock. Two generations of folding were identified, an earlier tight isoclinal intrafolial folds and a late regional open fold. The three deformational events identified are comparable and consistent with those reported in the neighboring outcrops in the Prince Olav Coast. Metamorphic P-T conditions estimated based on various geothermobarometries indicate that the rocks have experienced granulite grade conditions during peak metamorphism (770-880°C and 6-9kbar). The finding of orthopyroxene at Byobu Rock in this study extends the orthopyroxene-in isograd in the progressive metamorphic zone in the Lutzow-Holm Complex further eastward beyond Tenmondai Rock. Inclusions of kyanite within garnet in metapelitic rocks suggest a clockwise P-T path, consistent with the P-T paths suggested for the Lutzow-Holm Complex. Preliminary bulk rock geochemical investigations indicate that granitic rocks and quartzo-feldspathic gneiss mostly belong to volcanic arc granite suite, though their origin remains indistinct. Electron microprobe dating of monazites from representative rocks gave Pan-African ages (557±33Ma), consistent with the regional metamorphic ages reported earlier. Thus, based on the similarities on structural, petrological, geochemical and geochronological data, the Byobu Rock and Gobanme Rock are considered to be integral part of the Pan-African Lutzow-Holm Complex with analogous geological history

Mineral ages for multi isotope system in phlogopite-bearing pyroxene granulite and felsic gneiss, the Howard Hills, Enderby Land, East Antarctica: Possible Proterozoic tectonothermal events in the Napier Complex

Large mafic to ultramafic blocks in felsic gneisses on the northern part of the Howard Hills, Napier Complex, East Antarctica are accompanied by phlogopite-bearing pyroxene granulite occurring in the margins of the block. In order to understand the crustal evolution of the Napier Complex, especially regarding the thermal history after peak metamorphism, Rb-Sr, Sm-Nd and U-Pb isotopic analyses have been carried out on different minerals from the phlogopite-bearing pyroxene granulite and adjacent orthopyroxene felsic gneiss. Zircon grains from the orthopyroxene felsic gneiss yielded near concordant U-Pb isotopic ages of about 2.5Ga by conventional isotope dilution methods and defined a discordia with 2.44±0.02Ga lower intercept age. This age shows the waning stage of UHT granulite facies metamorphism in the Howard Hills. Rutile fractions from pyroxene granulite yielded a near concordant U-Pb isotopic age of about 1.5Ga. This age is interpreted as the final thermal episode, excepting local igneous intrusions, in the Howard Hills region. Fluorophlogopite fractions from pyroxene granulite yielded Rb-Sr model ages of about 1.85Ga, although evidence of retrograde metamorphism with fluid activity or deformation were poor in the pyroxene granulite. An internal Sm-Nd isochron of whole rock and orthopyroxene and feldspar separated from the same sample shows 1.85±0.15Ga. The Rb-Sr phlogopite model age, along with the Sm-Nd internal isochron age, records the time when the rocks of the Howard Hills underwent medium to high grade metamorphism at temperatures well above the currently accepted closure temperature of biotite (about 300-350°C )

‘Okunen’ in a long, hot orogen: a timeline of tectonometamorphic activity in southern LützowHolm Bay

第3回極域科学シンポジウム/第32回極域地学シンポジウム 11月30日（金） 統計数理研究所 ３階セミナー

ケイコウXセン ブンセキ ソウチ ニヨル ガンセキ ノ テイリョウ カガク ブンセキ チョウ エンキセイ イワ ノ シュヨウ ビリョウ ゲンソ

国立極地研究所の蛍光X線分析装置(RIX3000理学電機工業製: Rh/WデュアルターゲットX線管球)による超塩基性岩試料中の主要元素(SiO_2, TiO_2, Al_2O_3, Fe_2O_3, MnO, MgO, CaO, Na_2O, K_2O, P_2O_5)と微量元素(Ba, Co, Cr, Cu, Nb, Ni, Rb, Sr, V, Y, Zn, Zr)の定量化学分析法をまとめた.超塩基性岩類は,一般にSiO_2含有量が低く,一方MgOやFeOに富んでいる.また,Cr, Ni, Coなどは突出して含有量が高い.そのため,地球化学的標準試料の混合試料と化学試薬に原子吸光分析用標準溶液を加えた調合試料を調製して超塩基性岩の化学組成範囲をカバーする検量線を新たに作成した.測定は粉末試料と融剤(Spectroflux No. 100B; 80% LiBO_2+20% Li_2B_4O_7)の希釈率を1:2(重量比)のガラスビードにして行った.検量線の補正方法として,主要元素は装置に組み込まれているマトリックス補正方法を,微量元素はpeak-over-back方法を用いた.希釈率1: 2のガラスビードでの主要及び微量元素含有量の同時測定が可能となった.Analytical procedures for major and trace elements (SiO_2, TiO_2, Al_2O_3, Fe_2O_3, MnO, MgO, CaO, Na_2O, K_2O, P_2O_5, Ba, Co, Cr, Cu, Nb, Ni, Rb, Sr, V, Y, Zn, Zr) in ultrabasic rocks by means of an X-ray fluorescnece analyzer RIGAKU RIX3000 at the National Institute of Polar Research are summarized. Because ultrabasic rocks are generally characterized by relatively low SiO_2, high MgO and FeO, and extremely high concentration of Cr, Ni and Co, we newly draw calibration curves by synthesizing standard samples. These standards were prepared by mixing preexisting geochemical standards, chemical reagents and standard solutions for atomic absorption analysis in order to cover the possible chemical compositions of natural ultrabasic rocks. For the elemental analyses, glass beads were prepared from powder samples that were diluted two times by Spectroflux No. 100B (80% LiBO_2+20% Li_2B_4O_7). The matrix correction method and the peak-over-back method were adopted for the measurements of major elements and trace elements, respectively

Granulites from Cape Hinode in the amphibolite-facies eastern part of Prince Olav Coast, East Antarctica: New evidence for allochthonous block in the Lutzow-Holm Complex

High-grade metamorphic rocks occurring along the Prince Harald, Soya, and Prince Olav Coasts make up the Latest Proterozoic-Early Paleozoic Lutzow-Holm Complex, which is the youngest orogenic belt in the East Antarctic Shield. A systematic increase in metamorphic grade from east to west, ranging from upper amphibolite facies on the eastern Prince Olav Coast to upper granulite facies at the head of Lutzow-Holm Bay, has been well-established in the complex. However, granulites are newly found to occur as blocks sitting within meta-tonalites at Cape Hinode located on the amphibolite-facies eastern Prince Olav Coast. In addition, it is newly revealed that kyanite occurs rather commonly in meta-tonalites which contain hornblende with or without clinopyroxene. The modes of occurrence in the field, petrographical features, and major element bulk rock compositions of the granulites and related rocks are given in some detail in this study. These, along with the previously presented geochronological, geochemical and petrographical data, would indicate that the rocks in the Cape Hinode area as a whole make up a Mesoproterozoic allochthonous block in the Latest Proterozoic-Early Paleozoic Lutzow-Holm Complex