773 — — フィリピン海の海盆形成とマグマティズム 石 塚 治 * , ** 小 原 泰 彦 ** , *** 湯 浅 真 人 * Basin Genesis and Magmatism in the Philippine Sea Osamu ISHIZUKA * , ** , Yasuhiko OHARA ** , *** and Makoto YUASA * [Received 17 January, 2015; Accepted 30 April, 2015] Abstract Characterizating the igneous basement and reconstructing the volcanotectonic history of the Philippine Sea are the major targets of surveys for delineating Japan's continental shelves oper- ated by Ministry of Economy, Technology and Industry. Sampling using R/V Hakurei-maru No. 2 equipped with Deep-sea Boring Machine System (BMS) and arm dredges covers a wide area of the Philippine Sea. The results are summarized of 40 Ar/ 39 Ar dating and chemical and isotopic analyses on igneous rock samples from the Philippine Sea, particularly from the West Philippine Basin, and its surrounding area, and Kinan Seamount Chain in the Shikoku Basin. Both regions are characterized by Ocean Island Basalt (OIB) -like magmatism which formed oceanic plateaus (e.g., Urdaneta Plateau, Oki-Daito Rise) and seamounts (e.g., seamounts in Minami Daito Ba- sin, Kinan Seamount Chain) . However, the geochemical characteristics of OIB-like basalts and pattern of spatial and temporal variations of this type of magmatism indicate that sources and processes involved in magmatism in these two areas are clearly distinct. Key words:continental shelf survey, Philippine Sea, West Philippine Basin, Kinan Seamount Chain, OIB, back-arc basin キーワード:大陸棚調査,フィリピン海,西フィリピン海盆,紀南海山列,OIB,背弧海盆 I.はじめに 法的な定義に基づいて,大陸棚の限界を画定す る上でもっとも重要な点は,該当する海底が「領 土の自然延長」を構成するということである。こ れは地形的に明らかに連続していれば容易に認め られるが,そうでない場合は地質学的,地球物理 学的根拠に基づいて連続性を証明することも可能 である。そのため日本は,周辺海域において精密 海底地形調査・地殻構造探査・基盤岩採取を含む 科学的調査を実施した。日本のように島弧地殻が 発達している地域では,海底を構成する火山岩の 地球化学的特徴(例えばスラブ由来物質の寄与の 有無)や地震探査により求めた地殻の地震波速度 構造を用いて,島弧としての連続性を証明するこ とにより,延長大陸棚を獲得できる可能性もある。 本稿では,大陸棚限界画定調査の一環として行 われたフィリピン海地域での海底基盤岩採取に 地学雑誌 Journal of Geography (Chigaku Zasshi) 124 (5)773–786 2015 doi:10.5026/jgeography.124.773 * 産業技術総合研究所地質調査総合センター ** 海洋研究開発機構 *** 海上保安庁海洋情報部 * Geological Survey of Japan, AIST, Tsukuba, 305-8567, Japan ** Japan Agency for Marine-Earth Science and Technology, Yokosuka, 237-0061, Japan *** Hydrographic and Oceanographic Department, Japan Coast Guard, Tokyo, 135-0064, Japan
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773— —
フィリピン海の海盆形成とマグマティズム
石 塚 治*,** 小 原 泰 彦**,*** 湯 浅 真 人*
Basin Genesis and Magmatism in the Philippine Sea
Osamu ISHIZUKA*,**, Yasuhiko OHARA**,*** and Makoto YUASA*
[Received 17 January, 2015; Accepted 30 April, 2015]
Abstract Characterizating the igneous basement and reconstructing the volcanotectonic history of the Philippine Sea are the major targets of surveys for delineating Japan's continental shelves oper-ated by Ministry of Economy, Technology and Industry. Sampling using R/V Hakurei-maru No. 2 equipped with Deep-sea Boring Machine System (BMS) and arm dredges covers a wide area of the Philippine Sea. The results are summarized of 40Ar/39Ar dating and chemical and isotopic analyses on igneous rock samples from the Philippine Sea, particularly from the West Philippine Basin, and its surrounding area, and Kinan Seamount Chain in the Shikoku Basin. Both regions are characterized by Ocean Island Basalt (OIB)-like magmatism which formed oceanic plateaus (e.g., Urdaneta Plateau, Oki-Daito Rise) and seamounts (e.g., seamounts in Minami Daito Ba-sin, Kinan Seamount Chain). However, the geochemical characteristics of OIB-like basalts and pattern of spatial and temporal variations of this type of magmatism indicate that sources and processes involved in magmatism in these two areas are clearly distinct.
地学雑誌 Journal of Geography(Chigaku Zasshi) 124(5)773–786 2015 doi:10.5026/jgeography.124.773
* 産業技術総合研究所地質調査総合センター ** 海洋研究開発機構*** 海上保安庁海洋情報部 * Geological Survey of Japan, AIST, Tsukuba, 305-8567, Japan ** Japan Agency for Marine-Earth Science and Technology, Yokosuka, 237-0061, Japan*** Hydrographic and Oceanographic Department, Japan Coast Guard, Tokyo, 135-0064, Japan
図 1 フィリピン海地域の代表的な地質体とテクトニクス復元(Ishizuka et al., 2011b を改変). a)35 Ma以前,b)現在.
Fig. 1 Schematic cartoon depicting the volcanotectonic evolution of the Kyushu-Palau Ridge and the surrounding Philippine Sea Plate (modified after Ishizuka et al., 2011b). a) before 35 Ma, b)present.
Fig. 2 Bathymetric map of the NW Philippine Sea showing sample locations (symbols are as used in Figures 6, 7, and 9). Distribution of OIB mag-matism and remnant arc are indicated as shaded bathymetry. (Modified after Ishizuka et al., 2013). Bathymetric data are from Hy drographic and Oceanographic Department of Japan.
図 3 ベンハムライズと南大東海盆から報告されている OIB 類似の玄武岩の化学組成の中央海嶺玄武岩規格化パターン.データは Hickey-Vargas (1998a),Sun and McDonough (1989)による.
Fig. 3 Selected MORB-normalized whole rock com-positions of volcanics from Benham Rise and Minami-Daito Basin (DSDP Site 446). Data are normalized to N-MORB; Sun and McDonough (1989). Data source: Hickey-Vargas, 1998a; Sun and McDonough (1989).
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のと,LIL(large-ion lithophile)元素に富むがHFS(high field strength)元素に乏しいいわゆる島弧的な特徴をもつ 2種類が存在する。 さらに西フィリピン海盆については,おおむね
図 4 大陸棚限界画定調査で得られた西フィリピン海周辺の火山岩類の SiO2–アルカリプロット(データはIshizuka et al., 2013 による).火山岩組成区分は Le Maitre(1989)および Rickwood(1989)による.異なるシンボルは採取地点が異なっていることを示す.a)沖大東ライズおよびウルダネタ海台のデータ.b)西フィリピン海盆のデータ.
Fig. 4 A) SiO2–K2O plot of volcanic rocks from the area studied. Rock subdivision is adopted from Le Maitre (1989) and Rickwood (1989). Data are from Ishizuka et al. (2013). a) data from Oki-Daito Rise and Urdaneta Plateau; b) data from West Philipppine Basin.
図 5 大陸棚限界画定調査で得られた西フィリピン海周辺の火山岩類の中央海嶺玄武岩規格化パターン(データは Ishizuka et al., 2013による).
Fig. 5 MORB-normalized whole rock compositions of volcanics from the area studied. Data are from Ishizuka et al. (2013).
図 6 a)西フィリピン海周辺の火山岩類の Zr/Y–Nb/Y プロット.データは Ishizuka et al. (2013)による.フィリピン海 MORB のデータは,Hickey-Vargas(1998a),Ishizuka et al. (2009, 2010)による.2 本の実線は,Fitton et al.(1997)によるアイスランド溶岩における,MORB タイプの溶岩とプリュームの影響を受けた溶岩の組成範囲のリミット.b)206Pb/204Pb–ΔNb プロット.ΔNb= 1.74+ log(Nb/Y)-1.92 log(Zr/Y): Fitton et al. (1997).シンボルは図 2と同様.
Fig. 6 a) Zr/Y–Nb/Y relationship of studied samples. Philippine Sea MORB: Hickey-Vargas (1998a), Ishizuka et al. (2009, 2010). Two solid lines: reference lines mark the limits of Icelandic basalt data by Fitton et al. (1997), effectively showing OIB field. b) 206Pb/204Pb–ΔNb plot of studied samples. ΔNb= 1.74+ log (Nb/Y)-1.92 log(Zr/Y): Fitton et al., 1997). Symbols are as used in Figure 2.
図 7 a,b)西フィリピン海周辺の火山岩の 206Pb/204Pb–Δ208Pb(Hart, 1984),c)40Ar/39Ar age–Δ208Pbプロット.フィリピン海MORB,ベンハムライズ,南大東海盆DSDP Site 446,九州・パラオ海嶺,中生代古島弧のデータは,Hickey-Vargas(1991, 1998a); Savov et al. (2006); Ishizuka et al. (2009, 2010, 2011b)による.シンボルは図 2と同様.年代データのある試料のみプロットしている.
Fig. 7 a, b) 206Pb/204Pb–Δ208Pb (Hart, 1984), c) 40Ar/39Ar age–Δ208Pb plots for Oki-Daito Province and West Philippine Basin magmatism. Additional data from: Hickey-Vargas (1991, 1998a); Savov et al. (2006); Ishizuka et al. (2009, 2010, 2011b). Symbols are as used in Figure 2. Samples with age data are plotted in Fig. 7c.
図 8 西フィリピン海周辺の火山岩から得られた40Ar/39Ar年代(Ishizuka et al., 2013による).イタリックで示した年代値は OIB 類似でない玄武岩から得られた年代.
Fig. 8 Distribution of 40Ar/39Ar ages (in Ma) obtained for volcanic rocks from the area studied. Ages in italic indicate the data of OIB-unlike basalts.
図 9 OIB 類似玄武岩の年代とかつての西フィリピン海盆の拡大軸からの距離との関係(Ishizuka et al., 2013 に よる).シンボルは図 2と同様.DSDP Site 292および Site 446のデータは Hickey-Vargas(1998a)による.
Fig. 9 40Ar/39Ar age–distance relationship of Oki-Daito volcanism from extinct spreading center of West Philippine Basin (CBF Rift). Data from DSDP Site 292 at Benham Rise and Site 446 in Minami-Daito Basin are from Hickey-Vargas (1998a). Modified after Ishizuka et al. (2013). Symbols are as used in Figure 2.
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体を多数形成した。 西フィリピン海盆の成因については,大洋底がとり込まれたもの(trapped basin)とする説(例えば, Uyeda and Ben-Avraham, 1972; Jolivet
et al., 1989)と背弧海盆とする説(例えば, Lewis
et al., 1982)が提唱されたが,現在では海底地形と磁気異常パターンから背弧海盆起源であるとする考えが有力視されている(例えば, Okino and
Fujioka, 2003)。本研究の結果は,OIB類似マグマの活動が,時間的,空間的に西フィリピン海盆の拡大開始時期および開始地域にきわめて近いことを示しており,マントルプルームの上昇が背弧海盆拡大開始のきっかけとなった可能性が十分あると考える。このプルームの位置は,拡大軸の特定の位置に固定され,少なくとも約 35 Maまで活動し,海台の年代と噴出年代に正の相関がある海台(列)を形成した。 プルームの上昇とリフティングおよび海底拡大の関連は,大陸分裂と洪水玄武岩の関連において議論されてきた(例えば, Courtillot et al., 1999;
Buiter and Torsvik, 2014; Burov and Gerya,
2014)。Buiter and Torsvik(2014)は,マントルプルームの上昇が地殻のリフティングと玄武岩の噴出を引き起こす場合と,リフティングは他の要因で開始され,そこにプルームがトラップされる場合があるとした。西フィリピン海盆で
図 10 西フィリピン海盆周辺地域のテクトニクス復元(Ishizuka et al., 2013 による).西フィリピン海盆とその周辺の地質体の相対的な位置関係の復元であり,プレート全体の回転等の運動は含んでいない.
Fig. 10 Schematic tectonic history of the West Philippine Basin area (modified after Ishizuka et al., 2013). Rotation of Philippine Sea plate is not considered in this figure.
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ら鮮新世の島弧火山活動により形成されたことが明らかになっている(例えば, Ishizuka et al.,
図 11 四国海盆周辺の海底地形と紀南海山列から得られた 40Ar/39Ar 年代(Ishizuka et al., 2009による).地形データは海上保安庁海洋情報部による.
Fig. 11 Bathymetric map of Izu-Bonin back-arc and Shikoku Basin and distribution of 40Ar/39Ar ages (in Ma) obtained for volcanic rocks from Kinan Seamount Chain. Bathymetric data are from Hydrographic and Oceanographic Department of Japan. 40Ar/39Ar age: Ishizuka et al. (2009).
図 12 紀南海山列火山岩類の中央海嶺玄武岩規格化パターン(データは Ishizuka et al., 2009 による).四国海盆,伊豆弧背弧海山列のデータもあわせて示す.規格化に用いた N-MORB組成は,Sun and McDonough(1989)による.
Fig. 12 Selected MORB-normalized whole rock com-positions of volcanics from back-arc area of Izu-Bonin arc and Shikoku Basin area (normalized to N-MORB; Sun and McDonough, 1989). Data source: Ishizuka et al. (2009).
の変動は寄与したマントルの組成も変化していることを示している。時間とともにマントルの部分溶融度が低下し,それとともに融解しやすい単斜輝石に富む部分のマントルが選択的に溶融した,と考えることで説明可能である。 この現象は,四国海盆の主たる拡大がほぼ15 Maまでに停止し,その後拡大軸での熱の供給やマントルの対流,上昇が次第に衰えていった過程を示しているのではないか,と考えている(図 15)。背弧海盆における主要な海底拡大期の終了後の火山活動は,日本海,南シナ海でも報告されている(Kaneoka et al., 1990; Tu et al.,
図 13 紀南海山列火山岩類の同位体組成プロット(Ishizuka et al., 2009 を改変).a)Sr-Pb,b)Nd-Pb 同位体プロット.伊豆弧背弧海山列およびフィリピン海 MORBの組成をあわせて示した.c),d)紀南海山列の同位体組成とともに西フィリピン海盆周辺の OIB類似玄武岩の組成範囲をあわせて示した.
Fig. 13 Pb, Sr, and Nd isotope co-variation for Izu-Bonin back-arc area after Ishizuka et al. (2009). a) Sr-Pb, b)Nd-Pb isotopic plots. c), d) Isotopic composition of OIB-like magma from West Philippine Basin area are shown for comparison with basalts from Kinan Seamount Chain.
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