印尼北蘇拉威西中部～10Ma閃長巖的巖石成因及對西里伯斯海向南俯沖的啟示

盧向紅, 錢 鑫, 2*, 甘成勢, 2, 張玉芝, 2, 吳賽男, 王岳軍, 2

印尼北蘇拉威西中部～10Ma閃長巖的巖石成因及對西里伯斯海向南俯沖的啟示

盧向紅1, 錢 鑫1, 2*, 甘成勢1, 2, 張玉芝1, 2, 吳賽男1, 王岳軍1, 2

(1. 中山大學 地球科學與工程學院, 廣東省地球動力作用與地質(zhì)災(zāi)害重點實驗室, 廣東 珠海 519082; 2. 南方海洋科學與工程廣東省實驗室(珠海), 廣東 珠海 519082)

蘇拉威西島位于歐亞、太平洋和印度?澳大利亞板塊的交匯處, 經(jīng)歷了復(fù)雜的構(gòu)造演化歷史, 保存了大量中?新生代巖漿記錄。本研究對北蘇拉威西中部閃長巖開展了鋯石U-Pb年代學和Lu-Hf同位素、全巖元素?同位素地球化學研究。研究顯示, 閃長巖鋯石U-Pb年齡為10.32±0.44 Ma和9.87±0.11 Ma, 形成于晚中新世。樣品SiO2含量為53.81%～59.90%, MgO為3.27%～4.27%, K2O為0.48%～1.22%, A/CNK=0.86～0.92, Mg#為47～49, 屬高鎂中鉀鈣堿性閃長巖。該閃長巖以富集大離子親石元素和輕稀土元素、虧損高場強元素為特征, 具明顯Nb-Ta和Ti負異常, 輕微Eu負異常(Eu/Eu*=0.78～0.96)。閃長巖(87Sr/86Sr)i值變化于0.70498～0.70509之間,Nd()值變化于+5.6～+5.7之間, 鋯石原位Hf()值為+11.2～+16.9。地球化學特征顯示印尼北蘇拉威西中部閃長巖源區(qū)受到了俯沖組分的交代改造, 可能是虧損地幔源區(qū)輝石巖的部分熔融產(chǎn)物。結(jié)合區(qū)域地質(zhì)研究, 認為北蘇拉威西晚中新世閃長巖形成于島弧背景, 受控于晚中新世西里伯斯海的向南俯沖。

鋯石U-Pb年代學; 晚中新世; 高鎂閃長巖; 西里伯斯海俯沖; 北蘇拉威西

0 引 言

東南亞地區(qū)分布有一系列邊緣海盆地(如蘇祿海、西里伯斯海、馬魯古海、班達海和爪哇海等)和島鏈(如菲律賓、巴拉望、北蘇拉威西和爪哇等), 它們的形成大多與新生代印度?澳大利亞板塊與歐亞板塊的匯聚、太平洋板塊的俯沖和南海的擴張消亡等構(gòu)造過程密切相關(guān)(圖1a; Hall, 2002, 2012; Advokaat et al., 2018)。蘇拉威西島呈K字型展布于印尼東部, 由多個地質(zhì)單元構(gòu)成(圖1b; Katili, 1978; Hamilton 1979; Polvé et al., 1997; White et al., 2014, 2017)。自中生代以來, 該地區(qū)經(jīng)歷了陸塊裂離、板片俯沖和增生碰撞等復(fù)雜演化歷史, 已成為研究俯沖帶巖漿作用的天然實驗室(Priadi et al., 1994; Polvé et al., 1997; Hall, 2012; Hennig et al., 2016; Van Leeuwen et al., 2016; Maunala et al., 2016, 2019; Zhang et al., 2020)。

北蘇拉威西主要發(fā)育有新生代島弧巖漿巖和相應(yīng)沉積作用(圖1c), 該區(qū)已發(fā)表的巖漿巖年代學資料多為K-Ar年齡數(shù)據(jù), 主要分布于22～0.9 Ma, 被認為是多期巖漿活動的產(chǎn)物(Kavalieris et al., 1992; Surmont et al., 1994; Polvé et al., 1997; Elburg and Foden, 1998; Hanyu et al., 2012)。但時至今日, 對北蘇拉威西巖漿巖的精細年代學限定還相對薄弱, 對它們的地球化學特征及其巖石成因缺乏深入認知(圖1a; Kopp et al., 1999; Hanyu et al., 2012; Leo et al., 2013)。現(xiàn)有資料表明, 西里伯斯海形成于始新世, 發(fā)育多期俯沖巖漿作用(Silver et al., 1983a; Hutchison, 1989), 而馬魯古海俯沖作用以發(fā)育大量晚中新世?上新世弧巖漿巖為特征(Hanyu et al., 2012)。目前北蘇拉威西地區(qū)的巖漿作用與其西北側(cè)西里伯斯?；驏|南側(cè)馬魯古海的俯沖之間存在何種關(guān)聯(lián)仍不清晰。值得關(guān)注的是, 西里伯斯海海盆具與南海和蘇祿海近于平行展布的磁異常條帶特征(Silver et al., 1983b), 因此, 在位于南海以南、西里伯斯海南緣的北蘇拉威西地區(qū)開展詳細的巖漿作用研究, 可以更好地理解西里伯斯海的形成演化, 乃至了解南海的擴張與俯沖消減提供重要啟示(Silver and Rangin, 1991; Spadea et al., 1996; 李家彪等, 2011; 王鵬程等, 2017)。

GF. 哥倫打洛斷層; PKF. 帕魯?科羅斷層; WF. 瓦倫奈斷層; MF. 馬塔諾斷層; LF. 勞諾波斷層; KF. 科拉卡斷層。

1 地質(zhì)背景及巖相學特征

蘇拉威西島發(fā)育的主要斷裂包括哥倫打洛斷裂、帕魯?科羅斷裂、馬塔諾斷裂、勞諾波斷裂、科拉卡斷裂和瓦倫奈斷裂等(圖1b)。根據(jù)時代和演化的不同, 蘇拉威西島又分為西蘇拉威西、北蘇拉威西、中蘇拉威西、東蘇拉威西、邦蓋?蘇拉微陸塊和布頓?圖康伯西微陸塊(圖1b)。其中西蘇拉威西除零星出露具岡瓦納大陸親緣性的Malino、Palu、Latimojong、Bantimala和Barru等變質(zhì)雜巖體以外, 其余地區(qū)均被新生代火山?沉積地層所覆蓋(Hutchison, 1989; Metcalfe, 1990; Maunala et al., 2013, 2016)。中蘇拉威西大規(guī)模分布的變質(zhì)帶主要由變沉積巖和蛇綠巖組成, 其變質(zhì)作用主要發(fā)生在早白堊世或漸新世?中新世(Parkinson, 1991; Monnier et al., 1994, 1995; Cornée et al., 1995; Kadarusman et al., 2004)。東蘇拉威西主體由白堊紀或始新世蛇綠混雜巖及上覆新生代沉積巖構(gòu)成, 有人認為其具澳大利亞親緣性, 也有學者認為其起源于西里伯斯海(Simandjuntak, 1986; Monnier et al., 1994; Mubroto et al., 1994; Hall, 1996; Sidimantjuk and Barber, 1996; Elburg and Foden, 1999)。邦蓋?蘇拉微陸塊發(fā)育有島內(nèi)最古老的古生代?中生代花崗質(zhì)巖石, 其主體被新生代被動大陸邊緣型沉積所覆蓋(Kundig, 1956; Simandjuntak, 1986; Rangin et al., 1990; Villeneuve et al., 1992; Cornée et al., 1995)。布頓?圖康伯西微陸塊由布頓島、穆納島和周圍島嶼組成, 具中生代基底巖石, 上覆白堊紀?漸新世沉積地層(Davidson, 1991; Smith and Silver, 1991; Hall and Wilson, 2000)。

一般認為, 北蘇拉威西不具古老大陸基底, 以洋內(nèi)弧屬性為特征, 主體被新生代火山?沉積地層所覆蓋(Silver et al., 1983b; Kavalieris et al., 1992; Polvé et al., 1997; Hanyu et al., 2012)。古近紀巖漿巖主要分布在北蘇拉威西的西部和中部, 而新近紀以來的巖漿巖則廣布于整個北蘇拉威西(圖1c; Kavalieris et al., 1992; Polvé et al., 1997; Elburg and Foden, 1998; Elburg et al., 2003; Maunala et al., 2016)。在北蘇拉威西馬里薩、博羅科和哥倫打洛等地區(qū)發(fā)育有大量中性?長英質(zhì)侵入體(圖1c; 高小衛(wèi)等, 2015)。本文研究的對象為北蘇拉威西中部哥倫打洛地區(qū)的閃長巖巖體, 該巖體侵位于古近紀火山?沉積序列中, 并被晚中新世?上新世火山?沉積單元所覆蓋(圖1c, 2a、b)。文中采集的8件閃長巖樣品均具相似礦物組合, 主要由斜長石(50%～60%)、角閃石(15%～20%)、石英(5%～10%)、斜方輝石(8%～12%)、黑云母(3%～5%)和不透明金屬礦物(2%～3%)組成, 可見少量鋯石和磷灰石等副礦物(圖2c～f)。斜長石呈板條狀, 自形程度較好, 部分具有聚片雙晶、環(huán)帶結(jié)構(gòu); 磷灰石呈針狀分布于斜長石和石英等礦物中(圖2c～f)。

2 分析方法

本次研究對8件閃長巖樣品開展了元素?同位素地球化學分析, 并對其中19SU-32-1和19SU-46-1兩件樣品開展了鋯石U-Pb年代學及Lu-Hf同位素分析。上述分析均在中山大學地球科學與工程學院和廣東省地球動力作用與地質(zhì)災(zāi)害重點實驗室完成。

通過人工重砂法和電磁選技術(shù)從閃長巖樣品中分選出足量鋯石, 制靶后用 Carl Zeiss ∑igmaTM場發(fā)射掃描電子顯微鏡拍攝陰極發(fā)光圖像。用激光剝蝕系統(tǒng)和iCAP RQ型電感耦合等離子體質(zhì)譜儀(ICP-MS)聯(lián)用開展鋯石U-Pb年代學測試, 詳細的儀器介紹和測試方法參考Wang et al. (2020)。實驗監(jiān)測和分餾校正的鋯石標樣為91500和Ple?ovice, 用GLITTER軟件(Griffin et al., 2008)和ISOPLOT軟件(Ludwig, 2003)對原始數(shù)據(jù)和加權(quán)平均年齡等進行處理。鋯石原位Lu-Hf同位素分析中采用Geolas HD 193 nm ArF準分子激光剝蝕系統(tǒng)和Neptune Plus多接收器電感耦合等離子體質(zhì)譜儀(MC-ICP-MS), 詳細的儀器參數(shù)和操作程序見Hu et al. (2012)。將新鮮巖石樣品碎至小于200目的粉末后開展主量、微量元素和同位素組成測試。主量元素分析采用熔片法制備待測樣品, 通過ARL-Perform’X4200型X射線熒光光譜分析儀(XRF)測試完成。微量元素分析前用硝酸和氫氟酸對樣品進行初步溶解后, 經(jīng)高溫溶解和冷卻, 加入稀硝酸和內(nèi)標溶液制備為待測溶液, 分析測試通過iCAP RQ型ICP-MS儀器完成。Sr和Nd同位素測試通過Neptune Plus型MC-ICP-MS進行,86Sr/88Sr=0.1194和146Nd/144Nd=0.7219用于質(zhì)量分餾校正, 詳細測試過程及參數(shù)設(shè)置參考Wang et al. (2020)。

3 結(jié) 果

3.1 鋯石U-Pb年代學和原位Lu-Hf同位素

北蘇拉威西中部閃長巖的鋯石U-Pb年代學和Lu-Hf同位素測試結(jié)果分別見表1和表2。樣品19SU-32-1和19SU-46-1的鋯石粒徑約150～500 μm, 自形?半自形, 大多呈粒狀或長柱狀, 長寬比介于1︰1與4︰1之間, 發(fā)育振蕩環(huán)帶, Th/U值變化于0.51～1.49之間, 為典型的巖漿成因鋯石(圖3a、b)。樣品19SU-32-1的14顆鋯石的206Pb/238U加權(quán)平均年齡為10.32±0.44 Ma(MSWD=0.03)(圖3a), 對應(yīng)的鋯石176Hf/177Hf值變化于0.283128～0.283243之間(圖3c),Hf()=+12.8～+16.9, 一階段模式年齡DM1為10～173 Ma。樣品19SU-46-1中的29顆鋯石給出206Pb/238U加權(quán)平均年齡為9.87±0.11 Ma(MSWD=0.47)(圖3b), 對應(yīng)的鋯石176Hf/177Hf值變化于0.283082～0.283193之間,Hf()=+11.2～+15.1,DM1=84～241 Ma(圖3c)。兩件樣品具相似的加權(quán)平均年齡, 代表了該閃長巖體的結(jié)晶年齡為～10 Ma, 形成于晚中新世。

礦物代號: Pl. 斜長石; Amp. 角閃石; Opx. 斜方輝石; Qtz. 石英; Bi. 黑云母; Zrn. 鋯石; Ap. 磷灰石。

表1 北蘇拉威西中部閃長巖LA-ICP-MS鋯石U-Pb定年結(jié)果

續(xù)表1:

表2 北蘇拉威西中部閃長巖鋯石原位Lu-Hf同位素分析結(jié)果

3.2 全巖主量、微量元素地球化學及Sr-Nd同位素特征

北蘇拉威西中部閃長巖全巖元素和Sr-Nd同位素分析結(jié)果見表3。所分析的8件樣品燒失量LOI變化范圍為0.05%～0.47%, 無水標準化后樣品的SiO2變化于53.66%～60.15%, Fe2O3T變化于6.70%～ 8.01%, CaO含量為6.72%～8.48%, Al2O3含量為17.08%～19.18%, TiO2含量為0.75%～0.88%, 具有較高的Na2O含量(3.16%～3.84%)和較低K2O含量(0.48%～1.22%), (K2O+Na2O)為4.13%～4.57%。在TAS圖解中, 閃長巖樣品點落入輝長閃長巖?閃長巖區(qū)域(圖4a)。在K2O-SiO2圖解中, 樣品落入(低鉀)拉斑?中鉀鈣堿性系列區(qū)域(圖4b)。與正常島弧火山巖相比, 本研究中的閃長巖具有更高MgO含量(3.28%～4.26%)和Mg#值(47～49), 類似加里曼丹島東北部沙巴地區(qū)中新世安山巖, 均落在高鎂安山巖/閃長巖區(qū)域(圖4c; Bergman et al., 2000; Baharuddin, 2011)。

閃長巖樣品稀土元素總量(ΣREE)為64.2×10?6～ 79.8×10?6, 球粒隕石標準化稀土元素配分曲線呈富集輕稀土元素、虧損重稀土元素的右傾型(圖5a), (La/Yb)N、(La/Sm)N、(Gd/Yb)N值分別為2.67～3.42, 1.59～1.95和1.14～1.42, Eu負異常不明顯(Eu/Eu*=0.78～0.96)。在原始地幔標準化微量元素蛛網(wǎng)圖中(圖5b)中, 樣品以富集大離子親石元素、虧損高場強元素為特征, 具較低的Cr(12.7×10?6～38.8×10?6)、Ni(11.2×10?6～ 17.7×10?6)含量, 具有明顯的Nb-Ta和Ti負異常, 其特征類似沙巴地區(qū)中新世安山巖(圖5; Bergman et al., 2000; Baharuddin, 2011)。

沙巴基性巖數(shù)據(jù)來自Tsikouras et al. (2021)。

表3 北蘇拉威西中部閃長巖全巖主量(%)、微量(×10–6)和Sr-Nd同位素分析結(jié)果

續(xù)表3:

沙巴中新世安山巖數(shù)據(jù)來自Bergman et al. (2000)和Baharuddin (2011)。

標準化值數(shù)據(jù)Sun and McDonough (1989); 沙巴中新世安山巖數(shù)據(jù)來自Bergman et al. (2000)和Baharuddin (2011)。

數(shù)據(jù)來源: 南海新生代玄武巖、DMM、OIB和I-MORB數(shù)據(jù)引自Lai et al. (2021); 西里伯斯?；仔鋷r數(shù)據(jù)引自Elburg and Foden (1998), 哥倫打洛花崗巖數(shù)據(jù)引自Maunala et al. (2016)。

閃長巖樣品的Sr-Nd同位素分析表明,87Sr/86Sr值為0.705014～0.705148,143Nd/144Nd值為0.512922～ 0.512929, 回算得到 (87Sr/86Sr)i變化于0.704981～ 0.705094之間,Nd()值為+5.60～+5.74, 其同位素組成與南海玄武巖和哥倫打洛晚中新世?上新世花崗巖相似(圖6; Maunala et al., 2016; Lai et al., 2021)。

4 討 論

4.1 巖石成因

北蘇拉威西中部閃長巖樣品新鮮且燒失量較低(LOI=0.05%～0.47%), 鏡下未發(fā)現(xiàn)蝕變礦物, 表明樣品未遭受明顯的后期蝕變影響。盡管閃長巖樣品SiO2含量變化較大但樣品19SU-46-4的SiO2含量最低為53.66%, 說明閃長巖不可能是地殼巖石直接部分熔融的產(chǎn)物。一般遭受顯著地殼混染的巖石具較高的(La/Sm)N值(>4.5)(張永明等, 2019), 而本次研究的閃長巖樣品具較低的(La/Sm)N值(1.59～1.95)。此外, 閃長巖樣品SiO2含量與Nb/La值未見明顯負相關(guān)關(guān)系。同時, 這些樣品結(jié)晶年齡一致、Mg#值(47～49)變化范圍較窄、具有高的正Nd()值(+5.6～ +5.7)和鋯石Hf()值(+11.2～+16.9), 表明閃長巖樣品在侵位過程中沒有遭受明顯的地殼混染。相對于正常島弧火山巖,北蘇拉威西中部閃長巖樣品具更高的MgO、Na2O含量及更低的K2O含量, 為高鎂中鉀鈣堿性閃長巖(圖4a～c), 其可能的成因包括:①拆沉榴輝巖質(zhì)下地殼部分熔融(Rapp et al., 1991, 1999; Kelemen et al., 1998; Xu et al., 2002); ②殼源和幔源熔體混合(Kawabata and Shuto, 2005; Guo et al., 2007; Streck et al., 2007); ③年輕俯沖板片部分熔融(Rapp et al., 1991, 1999; Sen and Dunn, 1994); ④俯沖板片熔體/流體交代的地幔楔部分熔融(Tatsumi, 1981; Kelemen, 1995; Qian et al., 2017)。

拆沉榴輝巖質(zhì)下地殼部分熔融形成的高鎂巖石常具較低重稀土元素含量與較高SiO2、Cr、Ni、Sr含量和La/Yb、Sr/Y值(Kelemen et al., 1998)。本研究中, 閃長巖樣品鏡下未見金紅石, 其地球化學特征也明顯有別于上述特征。研究區(qū)地質(zhì)背景為初始未成熟島弧環(huán)境(Silver et al., 1983b; Rangin et al., 1997),難以發(fā)育地殼加厚拆沉事件, 故區(qū)域上也不具備形成榴輝質(zhì)下地殼拆沉的條件。閃長巖樣品的La/Yb值隨著Yb含量增加變化微弱, 表現(xiàn)出以分離結(jié)晶作用為主的演化趨勢(圖7a)。相似的礦物組成, 相對均一的全巖Sr-Nd、鋯石Lu-Hf同位素組成, 以及相似的微量和稀土元素配分模式(圖2、3c、5、6)表明閃長巖樣品不是殼幔熔體混合產(chǎn)物。年輕俯沖板片部分熔融與地幔橄欖巖反應(yīng)可產(chǎn)生高鎂巖石但此條件下形成的巖石常具有較高SiO2、Sr(>400×10?6)含量和Sr/Y(>20)值, 低Y(<18×10?6)和Yb(<1.9×10?6)含量, 表現(xiàn)出埃達克質(zhì)巖石的地球化學特征(Defant and Drummond, 1990)。而本研究中, 閃長巖樣品表現(xiàn)出較低的SiO2含量, Y>20.5×10?6, Sr<381×10?6, Sr/Y值為10.3～18.6, 明顯不同于埃達克質(zhì)巖的特征(Kay, 1978)。

北蘇拉威西中部閃長巖顯示出強烈富集大離子親石元素, 虧損高場強元素的特征和顯著的Nb、Ta、Ti負異常(圖5), 類似于島弧巖漿巖。低Nb/La (0.30～0.39)、Nb/U(3.42～9.44)和Ce/Pb(0.22～3.34)值反映閃長巖樣品源自受俯沖熔體/流體交代的地幔楔源區(qū)(Tatsumi et al., 1986)。在Rb/Y-Nb/Y和Nb/Y-Ba圖解上(圖7b、c), 樣品表現(xiàn)出顯著的以俯沖流體交代為主的特征(Zhang et al., 2012; Qian et al., 2017)。通常洋殼來源的熔體/流體常具較低Al2O3和Th含量, 其全巖Sr-Nd同位素與鋯石Hf同位素組成類似大洋板片(Wang et al., 2013; Gou et al., 2014)。而俯沖沉積物來源的熔體/流體可具較高Al2O3、Th和LREEs含量及更富集的Sr-Nd同位素組成(Gan et al., 2020)。已有研究表明, 俯沖沉積物流體在交代過程中, 因為Nd相對Hf更具活動性, 可使地幔楔熔體中沉積物Nd-Hf同位素組成發(fā)生解耦(You et al., 1996; Gou et al., 2014)。北蘇拉威西中部閃長巖具有較高的Al2O3含量、相對虧損地幔富集的Sr-Nd同位素組成, 但鋯石Hf()值很高, 類似虧損地幔, 表明源區(qū)可能存在俯沖沉積物的交代改造, 且源區(qū)在交代過程中可能發(fā)生了Nd-Hf同位素的輕度解耦。北蘇拉威西中新世(22～14 Ma)鈣堿性火成巖研究也表明其源區(qū)經(jīng)歷了俯沖流體交代作用(Polvé et al., 1997)。

北蘇拉威西中部閃長巖以斜長石、斜方輝石和角閃石作為其主要礦物, 鏡下未觀察到單斜輝石。通常斜方輝石作為巖漿早期結(jié)晶礦物, 表明初始巖漿為硅飽和或過飽和熔體。同時閃長巖樣品的高鎂(Mg#=47～49), 低Cr (12.7×10?6～38.8×10?6)、Ni(11.2× 10?6～17.7×10?6)和Co(18.7×10?6～23.7×10?6)含量特征, 反映了北蘇拉威西中部閃長巖的源區(qū)可能以輝石巖為主。因此, 北蘇拉威西中部閃長巖是受俯沖沉積物流體交代的地幔楔源區(qū)輝石巖部分熔融產(chǎn)物。

4.2 構(gòu)造意義

北蘇拉威西廣泛出露新生代巖漿巖, 根據(jù)已發(fā)表的一些K-Ar和極少量Ar-Ar年齡數(shù)據(jù), 有學者提出北蘇拉威西中新世巖漿活動以22～13 Ma和<9.5 Ma的活躍期、及晚中新世(～10 Ma)的寧靜期為主要特征(Bellon and Rangin, 1991; Kavalieris et al., 1992; Priadi, 1993; Polvé et al., 2001; Elburg et al., 2003; Van Leeuwen et al., 2007; Hanyu et al., 2012)。但Polvé et al. (1997)和Maunala et al. (2016)報道了11～8.3 Ma的北蘇拉威西火山巖, 認為巖漿活動自始新世中期一直持續(xù)到上新世。結(jié)合本研究年代學結(jié)果, 北蘇拉威西中部閃長巖形成于～10 Ma, 證實晚中新世(～10 Ma)該地區(qū)存在巖漿活動, 并不是一個明顯的寧靜期。

圖7 北蘇拉威西中部閃長巖La/Yb-Yb(a; 據(jù)Wang et al., 2017)、Rb/Y-Nb/Y(b; 據(jù)Kepezhinskas et al., 1997)和Nb/Y-Ba (c; 據(jù)Zhang et al., 2012)圖解

西里伯斯海盆地的擴張大致始于始新世, 發(fā)生了多期俯沖事件(Spadea et al., 1996)。北蘇拉威西和東北婆羅洲沙巴地區(qū)作為西里伯斯海南北兩側(cè)新生代巖漿巖的典型出露地區(qū), 記錄了西里伯斯海的形成與演化(Spadea et al., 1996; Rangin et al., 1997; Kopp et al., 1999; Hanyu et al., 2012)。在沙巴地區(qū)仙本那半島和丹特半島發(fā)育與西里伯斯海俯沖有關(guān)的中新世島弧成因安山巖(Hutchison, 1992; Bergman et al., 2000; Baharuddin, 2011)。Maunala et al. (2016)提出北蘇拉威西哥倫打洛地區(qū)花崗巖的形成是西里伯斯海俯沖的結(jié)果。另外, 西里伯斯海呈現(xiàn)出不對稱磁異常條帶分布特征, 蘇祿弧方向的北側(cè)磁異常條帶保存相對完整, 而南側(cè)面向北蘇拉威西方向磁條帶保存較少, 這可能是由于中新世以來印度?澳大利亞板塊向北擠壓、西里伯斯海洋殼向南俯沖消減于北蘇拉威西之下的結(jié)果(Silver et al., 1983b; Rangin et al., 1997, 1999)。如前所述, 北蘇拉威西中部晚中新世閃長巖主量、微量元素地球化學特征、鋯石Lu-Hf同位素組成與沙巴地區(qū)中新世安山巖類似(圖4、5; Bergman et al., 2000; Baharuddin, 2011), 并具有與南海新生代玄武巖及西里伯斯?；仔鋷r相似的Sr-Nd同位素組成, 是受俯沖沉積物流體交代的地幔楔源區(qū)輝石巖部分熔融產(chǎn)物。在Th/Yb-Nb/Yb和Hf/3-Th-Ta構(gòu)造判別圖解中, 北蘇拉威西閃長巖樣品落入島弧巖漿巖區(qū)域(圖8)。因此, 中?晚中新世北蘇拉威西屬俯沖島弧背景。

南海作為東南亞最大的邊緣海, 位于三大板塊的交匯部位(Metcalfe, 2009, 2013; Hall, 2012; Maunala et al., 2016, 2019), 其擴張作用開始于～33 Ma,終止于～15 Ma(Tapponnier et al., 1982; Xu et al., 2012;孫衛(wèi)東等, 2018)。西里伯斯海、蘇祿海和南海近平行展布的大洋磁條帶表明, 三者的形成演化具有密切聯(lián)系(Silver et al., 1983b), 西里伯斯海與蘇祿海自漸新世–中新世以來可能作為南海俯沖的弧后盆地或邊緣海盆持續(xù)演化(Tapponnier et al., 1982; Rangin and Silver, 1991; Xu et al., 2012; 孫衛(wèi)東等, 2018)。因此, 蘇祿海和西里伯斯海周緣約22～14 Ma廣泛的巖漿作用可能代表了其弧后擴張的產(chǎn)物(Shyu et al., 1991; Spadea et al., 1996)。以往的研究認為, ～7.5 Ma以來大規(guī)模島弧巖漿活動暗示了南海邊緣海盆俯沖作用的再次啟動(Bellon and Rangin, 1991; Polvé et al., 1997)。而本研究的綜合對比表明, 西里伯斯海、蘇祿海和南海南部的俯沖消減的啟動或再次啟動很可能發(fā)生在10 Ma之前, 而不是7.5 Ma或之后(Polvé et al., 2001; Lai et al., 2021), 晚中新世～10 Ma北蘇拉威西或已處于西里伯斯海向南俯沖的構(gòu)造背景, 俯沖沉積物派生流體交代上覆地幔, 從而形成了北蘇拉威西中部閃長巖的源區(qū)。

圖8 北蘇拉威西中部閃長巖Th/Yb-Nb/Yb(a; 據(jù)Pearce and Peate, 1995)和Hf/3-Th-Ta(b; 據(jù)Wood et al., 1980)圖解

5 主要結(jié)論

(1) 印尼北蘇拉威西中部閃長巖的 LA-ICP-MS 鋯石U-Pb 年齡為～10 Ma, 形成于晚中新世。

(2) 北蘇拉威西中部閃長巖主要屬高鎂中鉀鈣堿性系列, 具有島弧微量元素的地球化學特征和虧損的全巖Sr-Nd和鋯石Hf同位素組成。

(3) 北蘇拉威西中部閃長巖源自受俯沖派生流體交代改造的地幔輝石巖源區(qū), 其形成受控于晚中新世西里伯斯海的南向俯沖。

致謝:本研究樣品野外采集和實驗分析得到了印尼Jony, 中山大學余小清、王玉琨、楊雪和徐暢博士等的幫助, 兩位匿名審稿專家提出了建設(shè)性意見, 在此一并表示衷心的感謝。

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Petrogenesis of ～10 Ma Diorites from Central North Sulawesi, Indonesia: Implications for the Subduction of the Celebes Sea

LU Xianghong1, QIAN Xin1, 2*, GAN Chengshi1, 2, ZHANG Yuzhi1, 2,WU Sainan1, WANG Yuejun1, 2

(1. Guangdong Provincial Key Lab of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat-sen University, Zhuhai 519082, Guangdong, China; 2. Southern Marine Science and Engineering Guangdong Laboratory, Zhuhai 519082, Guangdong, China)

The Sulawesi Island, located at the junction region of Eurasia, Pacific, and Indo-Australian plates, has undergone complex tectonic evolution andpreserved abundant Meso-Cenozoic igneous rocks. This paper presents zircon U-Pb geochronological, Lu-Hf isotopic, and whole-rock geochemical results for a dioritic pluton at Central North Sulawesi. The pluton consists of gabbro and diorite. Two representative diorite samples yield zircon U-Pb ages of 10.32±0.44 Ma and 9.87±0.11 Ma respectively.Eight diorite samples have SiO2, MgO, and K2O contents in ranges 53.81% to 59.90%, 3.27% to 4.27% and 0.48% to 1.22%, with Mg#=47–49and A/CNK=0.86–0.92, respectively. Classified as high-Mg and medium-K calc-alkaline series, they are enriched in LILEs and LREEs, and depleted in HFSEs, showing obvious Nb, Ta, and Ti negative anomaliesand slightly Eu negative anomalies (Eu/Eu*=0.78–0.96). They have low (87Sr/86Sr)ivalues of 0.704981–0.705094 and positiveNd() values of +5.6–+5.7. Their in-situ zirconHf() values are in range of +11.2 to +16.9. These geochemical characteristics indicate that the diorites were derived from a depleted mantle that metasomatized by sediment-released fluids. In combination with the available observations, it is concluded that the late Miocene diorites in North Sulawesi were formed in an arc setting in response to the southward subduction of the Celebes Sea.

zircon U-Pb dating; Late Miocene; diorite; Celebes Sea slab subduction; North Sulawesi

10.16539/j.ddgzyckx.2022.03.011

2021-12-10;

2022-02-25

國家自然科學基金項目(U1701641、41830211、42072256)和廣東省基礎(chǔ)與應(yīng)用基礎(chǔ)研究基金項目(2018B030312007)聯(lián)合資助。

盧向紅(1997–), 女, 博士研究生, 地球化學專業(yè)。E-mail: luxh9@mail2.sysu.edu.cn

錢鑫(1988–), 男, 副教授, 從事東南亞大地構(gòu)造及巖石地球化學研究。E-mail: qianx3@mail.sysu.edu.cn

P581; P597; P595

A

1001-1552(2022)03-0569-016