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

Hiroi Yoshikuni

Hokada Tomokazu

Horie Kenji

Kazuyuki Shiraishi

Motoyoshi Yoichi

Takehara Mami

堀江 憲路

外田 智千

廣井 美邦

本吉 洋一

白石 和行

竹原 真美

Shiraishi Kazuyuki

National Institute of Polar Research Repository

  東南極レイナー岩体西部地域における U–Pb 年代測定  堀江憲路 1、外田智千 1、本吉洋一 1、白石和行 1，廣井美邦 2 1国立極地研究所 2千葉大学理学部  U–Pb geochronology in western part of the Rayner Complex, East Antarctica  Kenji Horie1, Tomokazu Hokada1, Yoichi Motoyoshi1, Kazuyuki Shiraishi1, Yoshikuni Hiroi 2 1NIPR 2Chiba University  The Rayner Complex in East Antarctica was initially defined by Kamenev (1972) to include coastal outcrops and minor inland nunataks in western Enderby Land and Kemp Land adjacent to the Napier Complex and further east into MacRobertson Land where amphibolite-granulite-facies metamorphism occurred at ~900 Ma (e.g., Sheraton et al., 1987).  Shiraishi et al. (1997) reported that western coastal region of this Complex contains younger zircons (537–522 Ma) and monazites (~500Ma) than inland region (~1320–760 Ma), and this western coastal region is later defined as Western Rayner Complex (Shiraishi et al., 2008).  However, the boundary and the mutual relationship between the main Rayner Complex and the Western Rayner Complex is still remained ambiguity.   U–Pb isotopic analyses of zircon were performed for Mt. Yuzhnaya, Condon Hills, and Mt. Lira using a Sensitive High Resolution Ion Microprobe (SHRIMP II) at National Institute of Polar Research.  The studied samples were collected during the field work at the 2004-2005 Japanese Antarctic Research Expedition.  3 garnet-biotite gneisses were collected from Mt. Yuzhnaya and Mt. Lira, respectively.  2 garnet-biotite gneisses and quartzite were collected from Condon Hills. U–Pb age data of three gneiss samples from Mt. Yuzhnaya are scattered from 572 to 2462 Ma.  The gneiss samples show continuous age population ranging from 860–1030 Ma and older inheritances centered at ca. 1940 Ma and ca. 2181 Ma.  Two gneiss samples are characterized by young age population at around 580 Ma and lower Th/U ratios.  The other gneiss sample does not contain zircons of ca. 580 Ma, and shows the youngest age peak of 890 Ma.  Three gneiss samples from Condon Hills contain inheritance older than 2500 Ma.  The oldest zircon ages are over 3600 Ma.  Main age population of Condon Hills samples are centered at ca. 2073 Ma, ca. 1934 Ma, and ca. 1878 Ma.  Overgrowth rim with low Th/U ratio yields a weighted mean 206Pb/238U age of 894 ± 2Ma (95% confidence). Based on these newly obtained zircon data, we could suggest the following points: (1) The ca.1000~860 Ma Rayner metamorphic event is commonly recorded in these areas. (2) The presence of common magmatic 2200–1940 Ma zircons in all analyzed samples suggests that the area share the common history after the supply of sediments with these age materials.  (3) Neoproterozoic ~580 Ma age event, which is relatively older than the previously reported 530–520 Ma ages, is obtained only in two samples from Mt. Yuzhnaya and is typically lacked in Condon Hills samples.   (4) Archean inherited (>2500 Ma) zircon is only found from Condon Hills. These lines of geochronological evidences combined with the petrographical data can constrain the geologic evolution of Proterozoic-Cambrian boundary region of this part of Antarctica.                                                         References Kamenev, E.N. Geological structure of Enderby Land. In: Antarctic geology and geophysics. International Union of Geological Sciences (IUGS), Oslo, 579–583, 1972. Sheraton, J.W., Tingey, R.J., Black, L.P., Offe, L.A. and Ellis, D.J. Geology of an unusual Precambrian high-grade metamorphic terrane—Enderby Land and western Kemp Land, Antarctica. Australian Bureau of Mineral Resources Bulletin 223, p 51, 1987. Shiraishi, K., Ellis, D.J., Fanning, C.M., Hiroi, Y., Kagami, H. and Motoyoshi, Y. , Re-examination of the metamorphic and protolith ages of the Rayner Complex, Antarctica: evidence for the Cambrian (Pan-African) regional metamorphic event, The Antarctic Region: Geological Evolution and Processes, 79–88, 1997. Shiraishi, K., Dunkley, D.J., Hokada, T., Fanning, C.M., Kagami, H. and Hamamoto, T. Geochronological constraints on the Late Proterozoic to Cambrian crustal evolution of eastern Dronning Maud Land, East Antarctica: a synthesis of SHRIMP U–  Pb age and Nd model age data, In: Geodynamic Evolution of East Antarctica: A Key to the East-West Gondwana Connection. Geological Society Special Publication No. 308, 21–67, 2008. 

U-Pb geochronology in western part of the Rayner Complex, East Antarctica

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

  東南極レイナー岩体西部地域における U–Pb ジルコン年代測定  堀江憲路 1,2、外田智千 1,2、本吉洋一 1,2、白石和行 1,2、廣井美邦 3、竹原真美 1 1国立極地研究所 2総合研究大学院大学 3千葉大学理学部  U–Pb zircon geochronology in western part of the Rayner Complex, East Antarctica  Kenji Horie1, 2, Tomokazu Hokada1, 2, Yoichi Motoyoshi1, 2, Kazuyuki Shiraishi1, 2, Yoshikuni Hiroi3, and Mami Takehara1 1National Institute of Polar Research, 10-3, Midori-cho, Tachikawa 190-8518, Japan 2Department of Polar Science, The Graduate University for Advanced Studies (SOKENDAI), Midori-cho 10-3, Tachikawa, 190-8518, Japan 3Department of Earth Sciences, Graduate School of Science, Chiba University, 1-33, Yayoi-cho, Inage-ku, 263-8522, Japan  The East Antarctic Shield consists of various Archean and Proterozoic to Cambrian high-grade metamorphic terranes which have distinct crustal histories and were amalgamated at various times from Precambrian to Cambrian.  The Rayner Complex, initially defined by Kamenev (1972), includes coastal outcrops and minor inland nunataks in western Enderby Land and Kemp Land adjacent to the Napier Complex and further east into MacRobertson Land, and is considered to belong to the Proterozoic mobile belt.  The rocks were affected by amphibolite-granulite-facies metamorphism at ~900 Ma (Harley, 2003, and references therein), whereas the Napier Complex is characterized by Neoarchean ultrahigh temperature (UHT) granulite-facies metamorphism (Harley and Motoyoshi, 2000, and references therein).  Transition zone between the Rayner Complex and the Napier Complex is obscured in western Enderby Land, but most of the Rayner Complex has been thought to represent reworked Napier Complex rocks (Sheraton and Black, 1983).  However, previously reported zircon inheritance and depleted mantle model ages of Sm–Nd system indicate that there is minor contribution of Archean crust to the Rayner Complex (Black et al., 1987). Recent geochronological works in the Rayner Complex using secondary ion mass spectrometry (SIMS) and an electron microprobe (EMP) revealed that western coastal region adjacent to Lützow-Holm Complex contains younger zircons (537-522 Ma; Shiraishi et al., 1997) and younger monazites (528-517Ma; Motoyoshi et al., 2006, 533-523Ma; Asami et al., 2005) than those of inland region (~1320-760 Ma; Shiraishi et al., 1997), which suggests that the western coastal region was overprinted by high grade metamorphism during Cambrian (Pan-African).  Therefore, the Rayner Complex is probably subdivided into two terranes having different thermal history and the western coastal region and was defined as the western Rayner Complex (Shiraishi et al., 2008).  However, the boundary and the mutual relationship between the Rayner Complex and the western Rayner Complex is still remained ambiguity. In this study, the U–Pb zircon geochronology using a sensitive high resolution ion microprobe (SHRIMP-IIe) was applied to the inland region of the Rayner Complex, in fact Mt. Yuzhnaya, Condon Hills, and Mt. Lira.  Condon Hills and Mt. Lira revealed metamorphic ages of 894 ± 1 Ma and 934 ± 2 Ma, respectively.  These metamorphic ages are consistent with previously reported timing of the metamorphism in association with the Rayner Structural Episode.  Zircon grains in Condon Hills and Mt. Lira do not show any signatures of the Pan-African reworking.  Previous works for Rb–Sr whole rock isochron and isotope dilution thermal ionization mass spectrometry (ID–TIMS) U–Pb zircon dating of granite in Condon Hills suggested the Pan-African reworking, but U–Pb zircon dating carried out in this study does not provide the evidence of the Pan-African reworking.  Mt. Yuzhnaya samples contain detrital zircons from Paleoproterozoic (2464 Ma) to late Neoproterozoic, and the Mt. Yuzhnaya area did not record the Rayner episode at c.900 Ma, and was probably affected by the Pan-African reworking.  Therefore, the western Rayner Complex undergoing the Cambrian metamorphism can be extended up to Mt. Yuzhnaya.  The only Condon Hills samples include signatures of the Archean crusts and the age peaks from 3850 Ma to 2491 Ma are the oldest components in the Rayner Complex of western Enderby Land.  There is no evidence of reworked Napier Complex rocks in the studied Rayner samples.  References Asami, M., Suzuki, K. and Grew, E.S., Monazite and zircon dating by the chemical Th–U–total Pb isochron method (CHIME) from Alasheyev Bight to the Sør Rondane Mountains, East Antarctica: a reconnaissance study of the Mozambique Suture in eastern Queen Maud Land, The Journal of geology, 113, 59–82, 2005.    Black, L.P., Harley, S.L., Sun, S.S. and McCulloch, M.T., The Rayner Complex of East Antarctica: complex isotopic systematics within a Proterozoic mobile belt, Journal of Metamorphic Geology, 5, 1–26, 1987 Harley, S.L., Archaean-Cambrian crustal development of East Antarctica: metamorphic characteristics and tectonic implications. In: Proterozoic East Gondwana: Supercontinent Assembly and Breakup (Yoshida, M., Windley, B.F. and Dasgupta, S. Eds.). pp. 480, Geological Society Special Publication 206, Geological Society PublishingHouse, UK, 203–230, 2003. Harley, S.L. and Motoyoshi, Y., Al zoning in orthopyroxene in a sapphirine quartzite: evidence for > 1120 °C UHT metamorphism in the Napier Complex, Antarctica, and implications for the entropy of sapphirine, Contributions to Mineralogy and Petrology, 138, 293–307, 2000. Kamenev, E.N., Geological structure of Enderby Land. In: Antarctic geology and geophysics (Adie, R.J. Ed.). Universitetsforlaget, Oslo, 579–583, 1972. Motoyoshi, Y., Hokada, T. and Shiraishi, K., Electron microprobe (EMP) dating on monazite from Forefinger Point granulites, east Antarctica: implication for pan-African overprint. In Antarctica (Fütterer, D.K., Damaske, D., Kleinschmidt, G., Miller, H. and Tessensohn, F. Eds.). pp. 477, Springer Berlin Heidelberg, New York, 63–68, 2006. Sheraton, J.W. and Black, L.P., Geochemistry of Precambrian gneisses: relevance for the evolution of the East Antarctic Shield. Lithos, 16, 273–296, 1983. Shiraishi, K., Ellis, D.J., Fanning, C.M., Hiroi, Y., Kagami, H. and Motoyoshi, Y., Re-examination of the metamorphic and protolith ages of the Rayner Complex, Antarctica: evidence for the Cambrian (Pan-African) regional metamorphic event. In The Antarctic Region: Geological Evolution and Processes (Ricci, C.A. Ed.). pp. 1206, Siena: Terra Antarctica Publication, Italy, 79–88, 1997. Shiraishi, K., Dunkley, D.J., Hokada, T., Fanning, C.M., Kagami, H. and Hamamoto, T., Geochronological constraints on the Late Proterozoic to Cambrian crustal evolution of eastern Dronning Maud Land, East Antarctica: a synthesis of SHRIMP U–Pb age and Nd model age data. Geological Society, London, Special Publications, 308, 21–67, 2008. 

U-Pb zircon geochronology in western part of the Rayner Complex, East Antarctica

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） 国立極地研究所 ３階ラウン

  東南極レイナー岩体西部地域における U–Pb 年代測定  堀江憲路 1、外田智千 1、本吉洋一 1、白石和行 1，廣井美邦 2 1国立極地研究所 2千葉大学理学部  U–Pb geochronology in western part of the Rayner Complex, East Antarctica  Kenji Horie1, Tomokazu Hokada1, Yoichi Motoyoshi1, Kazuyuki Shiraishi1, Yoshikuni Hiroi 2 1NIPR 2Chiba University  The Rayner Complex in East Antarctica was initially defined by Kamenev (1972) to include coastal outcrops and minor inland nunataks in western Enderby Land and Kemp Land adjacent to the Napier Complex and further east into MacRobertson Land where amphibolite-granulite-facies metamorphism occurred at ~900 Ma (e.g., Sheraton et al., 1987).  Shiraishi et al. (1997) and Motoyoshi et al. (2006) reported that western coastal region of this Complex contains younger zircons (537–522 Ma) and monazites (~500Ma) than inland region (~1320–760 Ma), and this western coastal region is later defined as Western Rayner Complex (Shiraishi et al., 2008).  However, the boundary and the mutual relationship between the main Rayner Complex and the Western Rayner Complex is still remained ambiguity.   U–Pb isotopic analyses of zircon were performed for Mt. Yuzhnaya, Condon Hills, and Mt. Lira using a sensitive high resolution ion microprobe (SHRIMP II) at National Institute of Polar Research.  The studied samples were collected during the field work at the 2004-2005 Japanese Antarctic Research Expedition.  3 garnet-biotite gneisses were collected from Mt. Yuzhnaya and Mt. Lira, respectively.  2 garnet-biotite gneisses and quartzite were collected from Condon Hills. U–Pb age data of three gneiss samples from Mt. Yuzhnaya are scattered from 572 to 2462 Ma.  The gneiss samples show continuous age population ranging from 860–1030 Ma and older inheritances centered at ca. 1940 Ma and ca. 2181 Ma.  Two gneiss samples are characterized by young age population at around 580 Ma and lower Th/U ratios.  The other gneiss sample does not contain zircons of ca. 580 Ma, and shows the youngest age peak of 890 Ma.  Three gneiss samples from Condon Hills contain inheritance older than 2500 Ma.  The oldest zircon ages are over 3600 Ma.  Main age population of Condon Hills samples are centered at ca. 2073 Ma, ca. 1934 Ma, and ca. 1878 Ma.  Overgrowth rim with low Th/U ratio yields a weighted mean 206Pb/238U age of 894 ± 2 Ma (95% confidence).  All samples collected from Mt. Lira yielded constant U–Pb age of 934.0 ± 1.7 Ma (95% confidence). Based on these newly obtained zircon data, we could suggest the following points: (1) The ca.1000~860 Ma metamorphic event is commonly recorded in these areas and seems to be consistent with the Rayner event (~930–900 Ma). (2) Neoproterozoic ~580 Ma age event, which is relatively older than the previously reported 530–520 Ma ages, is obtained only in two samples from Mt. Yuzhnaya and is typically lacked in samples from Condon Hills and Mt. Lira. (3) ~ 2.2–2.1 Ga and ~ 1.9 Ga are new protolith ages in the Rayner Complex, which suggests that the protolith was deposited between 1.9 Ga and ~ 1.0–0.9Ga. (4) The presence of common magmatic 2200–1940 Ma zircons suggests that the area share the common history after the supply of sediments with these age materials.  (5) There is no evidence of metamorphism at 2550–2480 Ma and reworking in the Napier Complex. These lines of geochronological evidences combined with the petrographical data can constrain the geologic evolution of Proterozoic-Cambrian boundary region of this part of Antarctica.                                                         References Kamenev, E.N. Geological structure of Enderby Land. In: Antarctic geology and geophysics. International Union of Geological Sciences (IUGS), Oslo, 579–583, 1972. Motoyoshi, Y., Hokada, T. and Shiraishi, K. Electron microprobe (EMP) dating on monazite from Forefinger Point granulites, east Antarctica: implication for pan-African overprint. In: Antarctica: contributions to global earth science. Berlin, Springer, p. 63–68, 2006.   Sheraton, J.W., Tingey, R.J., Black, L.P., Offe, L.A. and Ellis, D.J. Geology of an unusual Precambrian high-grade metamorphic terrane—Enderby Land and western Kemp Land, Antarctica. Australian Bureau of Mineral Resources Bulletin 223, p 51, 1987. Shiraishi, K., Ellis, D.J., Fanning, C.M., Hiroi, Y., Kagami, H. and Motoyoshi, Y. , Re-examination of the metamorphic and protolith ages of the Rayner Complex, Antarctica: evidence for the Cambrian (Pan-African) regional metamorphic event, The Antarctic Region: Geological Evolution and Processes, 79–88, 1997. Shiraishi, K., Dunkley, D.J., Hokada, T., Fanning, C.M., Kagami, H. and Hamamoto, T. Geochronological constraints on the Late Proterozoic to Cambrian crustal evolution of eastern Dronning Maud Land, East Antarctica: a synthesis of SHRIMP U–Pb age and Nd model age data, In: Geodynamic Evolution of East Antarctica: A Key to the East-West Gondwana Connection. Geological Society Special Publication No. 308, 21–67, 2008. 

U–Pb geochronology in western part of the Rayner Complex, East Antarctica

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U-Pb geochronology in western part of the Rayner Complex, East Antarctica

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