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

Rare Earth Element analysis of mafic metamorphic rocks in the Lutzow-Holm Complex, East Antarctica, using X-ray Fluorence Spectrometer

第2回極域科学シンポジウム/第31回極域地学シンポジウム 11月17日（木） 国立極地研究所　2階大会議室前フロ

Calc-silicate rocks and marbles from Lutzow-Holm Complex,East Antarctica, with special reference to the mineralogy and geochemical characteristics of calc-silicate mega-boudins from Rundvagshetta

We report here the mode of occurrence of calc-silicate rocks and marbles from the Lutzow-Holm Complex, East Antarctica, and a worked example from Rundvagshetta. Calc-silicate boudins were observed in Cape Hinode, Akarui Point, Byobu Rock, Skarvsnes, Skallevikshalsen and Rundvagshetta, whereas they were reported earlier from Sinnan Rock, Cape Ryugu, Akebono Rock, Cape Hinode, Niban Rock, Kasumi Rock, Daruma Rock, Cape Omega, Langhovde, Ytrehovdeholmen and Skarvsnes. They vary in size from decimeters to few meters and are commonly enclosed within pelitic or psammitic gneisses. In addition, extensive layers of marbles and calc-silicate rocks are distributed in Skallevikshalsen. The calc-silicate mega-boudins within the layered pyroxene-gneiss from Rundvagshetta, up to 5m long and 2m thick, comprises of coarse to medium grained assemblage of scapolite+anorthite+garnet+clinopyroxene+calcite+quartz+titanite±wollastonite. Co-existing scapolite and plagioclase suggest a "minimum" estimate of peak metamorphic temperature of ～830°C . Peak metamorphic mineral assemblages equilibrated at moderate to high X_(CO2) conditions (0.3-0.7) and temperatures between 850 and 1000°C , consistent with the ultrahigh temperature metamorphic conditions reported in the region. Multistage garnet corona formation preserved in the calc-silicate assemblage suggests a local increase in hydrous fluid activity during retrogression. Preliminary bulk rock geochemistry of different mineralogical zones in the boudin shows chemical potential gradients in some major elements, especially SiO_2, Al_2O_3 and CaO, possibly controlled by the compositional variations in the protolith. Altogether, these results suggest that calc-silicate rocks preserve information on the metamorphic evolution and help us in deducing the geodynamic evolution of high-grade terrains

Geochemistry of mafic metamorphic rocks in the Lutzow-Holm Complex, East Antarctica: Implications for tectonic evolution

Mafic metamorphic rocks are widely distributed through the Lutzow-Holm Complex (LHC) of East Antarctica, as layers between or enclaves within metasedimentary and metaigneous lithologies. It has been inferred that the peak metamorphic grade of the LHC progressively increases in a southwestern direction from amphibolite-facies to granulite-facies conditions, with mineral assemblages in the mafic metamorphic rocks changing from hornblende (magnesiohornblende)±biotite+plagioclase to orthopyroxene+clinopyroxene±hornblende (pargasite, magnesiohasting-site and tschermakite)±biotite±garnet+plagioclase. Field relationships suggest that amphibolite-grade mafic metamorphic rocks derive from mafic magma intruded into metasedimentary units, whereas granulite-facies mafic metamorphic rocks are a mixture of detrital blocks and mafic sill or intrusions. Major and trace element compositions of mafic metamorphic rocks are similar to those of igneous rocks of tholeiite affinity, and can be divided into volcanic-arc basalt (VAB)-type or mid-ocean ridge basalt (MORB)-type compositions. On a regional scale, VAB-type lithologies are predominant in amphibolite-facies areas, and MORB-type lithologies predominate in granulite-facies areas. On the basis of HFSE concentrations and Nb/Y ratios, MORB-type lithologies have T-type and E-type MORB compositions with oceanic plateau basalt and back-arc basin basalt affinities, and are occasionally found in the field intercalated with metasedimentary layers, characteristic of magmatism and sedimentation cycles in a marginal sea basin setting. Such field relationships provide information on the tectonic environment of protolith formation in the LHC. Various crustal components have been amalgamated into a relatively narrow mobile belt, which was subjected to high-grade metamorphism during the final closure of oceanic basins as a result of continent-continent collision

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

ケイコウXセン ブンセキ ソウチ ヲ モチイタ テイキシャク ガラスビードホウ ニヨル ケイサンエン ガンセキ チュウ ノ シュヨウ ビリョウ キドルイ ゲンソ ノ テイリョウ ブンセキ

国立極地研究所設置の蛍光X線分析装置を用いた珪酸塩岩の定量分析法を更新した.測定元素にScとThを加え,Nbの測定にThの重なり補正を加えた.希土類元素とSiO_2の検量線に,カナダの標準試料(SY-3)と日本の標準試料(JCh-1)を加えた.また,分析値と同時にその誤差を算出するルーチンを確立させた.このことにより,得られた分析値の誤差を考慮した上での地球化学的解析への適用が可能となった.さらに,ロット番号の異なる融剤を用い分析を行ったところ,ある融剤から明らかなLaの混染が確認された.新たな融剤を使用する際は,必ず測定元素が分析誤差の範囲内で一定であることを確かめる必要がある.This paper reports on updates to the system used for quantitative analyses of major, trace, and rare earth elements in silicate rocks by X-ray fluorescence spectrometry (RIX3000) at the National Institute of Polar Research. We added Sc and Th to the list of measured elements, and the overlap coefficient of Th is used for the measurement of Nb. The geochemical standard samples SY-3 (Canada) and JCh-1 (Japan) have been added to the existing set of standard samples to improve the precision of analyses of SiO_2 and rare earth elements. Furthermore, we established a new method of evaluating the analytical error, making it possible to use analytical data for geochemical evaluation while considering the precision of the data. We also found that the analytical error may increase as a consequence of using a different lot number of the flux used in preparing the fused glass beads. It is therefore necessary to assess the background of the flux as well as the target elements, in terms of whether they are within the analytical error, before using a new flux

ダイ46ジ ナンキョク チイキ カンソクタイ リュツォ・ホルムワン , プリンスオラフ カイガン オヨビ ニシ エンダビーランド チシツ・コチジキ カキ ヤガイ チョウサ ホウコク 2004-2005

第46次南極地域観測隊は,リュツォ・ホルム湾,プリンスオラフ海岸及び西エンダビーランドの露岩域を対象に,2004年12月17日から2005年2月17日にかけて地質・古地磁気野外調査活動を行った.この観測は,南極地域観測第VI期計画のプロジェクト研究観測「南極域から探る地球史」の中の「東南極リソスフェアの構造と進化の研究II」の一環として計画立案されたものである.第46次夏期観測では野外観測支援のための小型ヘリコプターが導入され,「しらせ」搭載ヘリコプターとともに野外調査の支援作業にあたった.ここでは,観測計画を実施するための,主に設営面での計画,準備,そして活動経過と問題点について報告する.The 46th Japanese Antarctic Research Expedition (JARE-46) conducted field operations on geological and paleomagnatic studies in the regions of Lutzow-Holm Bay, Prince Olav Coast and western Enderby Land during December 17, 2004 through February 17, 2005, under a part of the SEAL II (Structure and Evolution of East Antarctic Lithosphere II) project. A small helicopter was used to support field parties during the summer operations conducted by JARE-46. This report summarizes preparations for field operations and activities of the geological and paleo-magnetic team, and points out some problems for the future planning

X-ray fluorescence analysis of major, trace, and rare earth elements in silicate rocks using the low-dilution glass bead method

This paper reports on updates to the system used for quantitative analyses of major, trace, and rare earth elements in silicate rocks by X-ray fluorescence spectrometry (RIX3000) at the National Institute of Polar Research. We added Sc and Th to the list of measured elements, and the overlap coefficient of Th is used for the measurement of Nb. The geochemical standard samples SY-3 (Canada) and JCh-1 (Japan) have been added to the existing set of standard samples to improve the precision of analyses of SiO_2 and rare earth elements. Furthermore, we established a new method of evaluating the analytical error, making it possible to use analytical data for geochemical evaluation while considering the precision of the data. We also found that the analytical error may increase as a consequence of using a different lot number of the flux used in preparing the fused glass beads. It is therefore necessary to assess the background of the flux as well as the target elements, in terms of whether they are within the analytical error, before using a new flux