朱作飛, 閆 義, 趙 奇

古南海俯沖過程: 婆羅洲晚白堊世?漸新世地層沉積記錄

朱作飛1, 2, 3, 閆 義1, 2, 4, 5*, 趙 奇1, 2, 4, 5

(1. 中國科學院 廣州地球化學研究所邊緣海與大洋地質重點實驗室, 廣東 廣州 510640; 2. 中國科學院 深地科學卓越創(chuàng)新中心, 廣東 廣州 510640; 3. 中國科學院大學 地球與行星科學學院, 北京 100049; 4. 南方海洋科學與工程廣東省實驗室(廣州), 廣東 廣州 511458; 5. 中國科學院南海生態(tài)環(huán)境工程創(chuàng)新研究院, 廣東 廣州 510301)

古南海的俯沖消亡是深入揭示南海擴張機制和重塑東南亞中新生代構造演化的關鍵, 然而目前對于古南海的俯沖過程仍存在諸多爭議。馬來西亞婆羅洲出露完整的晚白堊世?漸新世沉積地層, 是研究古南海構造演化的重要窗口。本文通過碎屑礦物組成、元素地球化學及Nd同位素分析, 對婆羅洲晚白堊世?漸新世地層沉積物來源進行示蹤, 反演區(qū)域古地理格局及構造演化。結果顯示, 晚白堊世?古新世Rajang群沉積物主要來源于古太平洋俯沖形成的巖漿巖帶, 馬來半島與印支陸塊南緣對古新世?晚漸新世地層沉積貢獻明顯增加, 暗示古太平洋板塊俯沖的影響持續(xù)到早古新世(～60 Ma)。晚始新世, 隨著澳大利亞板塊持續(xù)向北漂移, 婆羅洲逆時針旋轉引起殘余海盆剪刀式閉合?！?7 Ma, 曾母陸塊與婆羅洲碰撞, Rajang群抬升剝蝕。漸新世, 古南海在婆羅洲東北部沙巴開始俯沖, 對應于南海的打開。古南海自西向東斜向俯沖消亡, 婆羅洲的逆時針旋轉與沿盧帕爾線的走滑使Rajang群與Kuching超級群疊置。

古南海; 古太平洋; 婆羅洲; 物源; 構造演化

0 引 言

“古南?！边@一概念最早是由Hinz et al. (1991)依據前人研究資料提出, 用來描述位于華南大陸南緣至加里曼丹的晚中生代古海洋。而Taylor and Hayes (1983)及Hall and Breitfeld (2017)認為古南海是西太平洋的一個殘留海灣, 古南海的消亡與南海的打開幾乎同步(Holloway, 1982; Taylor et al., 1983), 因此古南海的演化與南海的擴張息息相關。理清古南海消亡過程不僅可以揭示南海擴張機制, 而且有助于重塑東南亞中新生代構造演化歷史。

三疊紀?晚白堊世, 古太平洋板塊沿華南與巽他大陸邊緣俯沖, 形成了一條北到日本、南至蘇門答臘延伸數千千米的晚中生代巖漿巖帶(Hutchison, 2010; Xu et al., 2016; Li et al., 2018; Breitfeld et al., 2020a; Wang et al., 2021b)。晚白堊世, 由于古太平洋板塊俯沖后撤(孫衛(wèi)東等, 2008; Li et al., 2014; Wang et al., 2021c), 華南和巽他大陸邊緣經歷了明顯的拉張, 發(fā)育一系列裂陷盆地(Li et al., 2014)。漸新世, 南海開始擴張, 推動古南海向南俯沖消亡于婆羅洲?巴拉望一帶(Li et al., 2014)。古太平洋俯沖與古南海俯沖在時間上是否連續(xù)？空間上是否重疊？這一系列問題至今還存在諸多爭議: Hutchison (1996, 2005)認為古南海俯沖始于晚白堊世, 由大陸島弧的施瓦納山(Schwaner Mountains)(Hutchison, 2005)、弧前盆地的Kuching超級群(Williams et al., 1988)以及古俯沖帶和增生楔的盧帕爾線(Lupar Line)和Rajang群(Hutchison, 2005)構成了完整的溝弧盆系統(tǒng)。在該模式中, Rajang群晚始新世由深海沉積轉變成陸相沉積標志著俯沖的結束, 拉讓不整合(Rajang Unconformity)代表著曾母陸塊(Luconia block)與婆羅洲的碰撞(Hutchison, 1996, 2005)。然而, 這一模式遭到了不少研究者質疑, 施瓦納山巖漿活動僅持續(xù)到約72～80 Ma (Moss, 1998; Davies et al., 2014; Breitfeld et al., 2020a), 且Hennig-Breitfeld et al. (2017)指出施瓦納山巖漿巖與西婆羅洲同時期巖漿巖地球化學特征相似, 應與古太平洋俯沖有關。鋯石及重礦物特征也顯示Kuching超級群并非弧前盆地(Breitfel and Hall, 2018; Breitfeld et al., 2018), Rajang群只是被盧帕爾走滑斷層與Kuching超級群分割的被動大陸邊緣沉積(Galin et al., 2017; Breitfeld and Hall, 2018; Breitfeld et al., 2018)。因此, Hall and Breitfeld (2017)認為古太平洋板塊在婆羅洲的俯沖止于晚白堊世。澳大利亞板塊北移引起的區(qū)域板塊重組事件導致Rajang群在晚始新世抬升形成拉讓不整合。也有研究者認為古太平洋俯沖結束于古新世(Moss, 1998; Fyhn et al., 2010; Hutchison, 2010; Madon et al., 2013; Wang et al., 2016)。Zhao et al. (2021a)通過Lubok Antu混雜巖和Lupar組自生伊利石定年, 獲得～60 Ma和36 Ma的變形年齡, 分別對應俯沖結束時間和拉讓不整合的形成時間。晚始新世, 古南海在婆羅洲北部西巴蘭姆線(West Baram Line)以東的沙巴?卡加延地區(qū)開始俯沖。地球物理手段顯示沙巴?菲律賓地塊下方的P波高速異常體可能為古南海俯沖板片殘留(Hall and Spakman, 2015; Wu and Suppe, 2018)。中?晚始新世沙巴地區(qū)沉積環(huán)境由深水向陸相轉變, 且漸新世地層中大量火山碎屑對應于古南海的俯沖(Hall and Breitfeld, 2017)。中中新世, 南沙陸塊與婆羅洲碰撞(Hall, 2013)。

南海南部婆羅洲出露完整的晚白堊世?中新世沉積地層(Liechi et al., 1960; Haile, 1974; Hutchison, 2005), 是研究古南海演化歷史絕佳的窗口。晚白堊世?始新世沉積地層自婆羅洲西北部的古晉帶、西布帶呈弓形延伸至東北部的沙巴地區(qū), 分別是古晉帶的Kuching超級群、西布帶的Rajang群以及沙巴地區(qū)的Crocker群(圖1)。有研究者也將西布帶的Rajang群與沙巴同時期地層合稱為“Rajang-Crocker群”(圖2;van Hattum et al., 2006, 2013)。對于巨厚的Rajang-Crocker群沉積物來源還存在很大爭議, 支持遠源觀點的研究者認為該時期巨厚沉積物來源于印支半島(Hamilton, 1979)或是華南(Moss, 1998; Moss and Chambers, 1999),通過流經華南以及印支的主要河流, 如古湄公河, 將較老陸塊的沉積物搬運至巽他陸架(Hutchison, 1996; Hall, 1996; Métivier et al., 1999)。然而Hennig-Breitfeld et al. (2018)通過對越南南部地層碎屑鋯石分析認為古湄公河流向為自南向北, 與現今具有較大差異。碎屑鋯石以及重礦物結果指示沉積物搬運距離較近, 源區(qū)可能為婆羅洲西南部的施瓦納山和馬來半島(van Hattum et al., 2006, 2013; Galin et al., 2017; Breitfeld et al., 2018)。除物源爭議外, Rajang-Crocker群的構造屬性也不清楚。Hutchison (1996, 2005)認為整個Rajang群具有增生楔屬性, 為古南海晚白堊世?晚始新世俯沖形成。Moss (1998)提出晚白堊世?古新世Rajang群為俯沖增生的產物, Rajang群上部地層為殘余海盆沉積。而Hall and Breitfeld (2017)則認為Rajang群與Kuching超級群均為被動大陸邊緣的正常沉積地層。漸新世?早中新世地層主要分布在米里帶(Miri)和沙巴地區(qū), 與下伏Rajang-Crocker群呈不整合接觸。該不整合代表沙撈越(Sarawak)地區(qū)區(qū)域隆升事件, Hutchison (2005)稱之為曾母陸塊與婆羅洲碰撞引起的“沙撈越造山運動”; 也有研究者認為這一事件代表著古南海俯沖的開始(Hall and Breitfeld, 2017; Hennig-Breitfeld et al., 2019)。本文通過對Rajang-Crocker群進行詳細的野外觀察、鏡下礦物鑒定、元素地球化學和Nd同位素分析, 結合區(qū)域巖漿及構造演化研究成果, 重建婆羅洲晚白堊世?漸新世的構造演化過程。

1 區(qū)域地質概況

婆羅洲由眾多微陸塊拼貼而成, 可分為西南婆羅洲(SW Borneo)、古晉帶、西布帶、米里帶和東婆羅洲(East Borneo)五個部分(圖1)。西南婆羅洲于晚侏羅世從岡瓦納大陸裂離, 并在早白堊世拼貼于巽他大陸東南緣(Metcalfe, 2009, 2011; Hennig- Breitfeld et al., 2017)。施瓦納山北部為白堊紀變質巖(Davies et al., 2014; Breitfeld et al., 2020a); 南部為侏羅紀板內花崗巖、85～135 Ma I型花崗巖(Setiawan et al., 2013; Davies et al., 2014; Hennig et al., 2017; Breitfeld et al., 2020a), 以及72～85 Ma碰撞后花崗巖。施瓦納山以北為古晉帶, 古晉帶西部屬于巽他大陸基底, 出露有三疊紀巖漿巖, 同時也有零星的三疊系Sadong組、Kuching組以及白堊系Pedawan組沉積地層出露。東部主要為晚白堊世?晚始新世巨厚陸相沉積地層——Kuching超級群(圖1; Breitfeld et al., 2018; Breitfeld and Hall, 2018), 分為Kayan群和Ketungau群, 主要由砂巖、泥巖和礫巖構成, 部分地層含火山碎屑。沿古晉帶北部邊界, 盧帕爾線分布有Lubok Antu混雜巖以及Sambas-Mangkaliat花崗巖帶(圖1; Tan, 1982; William et al., 1988; Hutchison, 1996; Amiruddin, 2009)。碎屑鋯石U-Pb年齡顯示, Lubok Antu混雜巖最大沉積年齡為105～115 Ma (Zhao et al., 2021a)。Sambas-Mangkaliat巖漿巖年齡為74.9～80.6 Ma, 被認為是古南海俯沖后撤導致巖漿巖帶北移的結果(Amiruddin, 2009)。盧帕爾線以北為西布帶(圖1), 被巨厚的Rajang群深海復理石沉積覆蓋, 基底沒有出露, 性質不清。有限的微體古生物化石顯示區(qū)內地層時代為晚白堊世?晚始新世, 且由南向北逐漸變年輕(Liechti et al., 1960; Hutchison, 2005)。Rajang群分為Lupar組和Belaga組, 其中Belaga組又分為五個段, 分別是Layar段、Kapit段、Pelagus段、Metah段以及Bawang段。Rajang群主要由深海濁積砂巖、粉砂巖、泥巖以及頁巖組成的韻律層, 可觀察到鮑瑪序列的Tc～Te段(圖3a); 地層傾角大甚至直立, 滑塌、褶皺現象普遍(圖3b、e、f、g), 其中Layar段和Kapit段經歷一定程度的變質作用(圖3c、d)。Galin et al. (2017)和Hennig-Breitfeld et al. (2019)根據碎屑鋯石和重礦物特征將Rajang群分為四個單元, 其中Lupar組、Layar段以及下Kapit段歸為一單元, 上Kapit段、Pelagus段為二單元, Metah段與出露在米里帶的Bawang段分別為三單元和四單元(圖2)。米里帶由武吉?米辛線(Bukit-Mersing Line)與西布帶分隔(圖1), 區(qū)內大部分被漸新世?中新世陸相地層覆蓋, 出露少量古新世地層。漸新世Tatau組不整合覆蓋在Rajang群之上, 地層傾角較緩且變形較小。Tatau組底部為礫巖, 向上為砂巖, 砂巖中含煤層, 指示沉積環(huán)境已由深海相轉變?yōu)檎訚上?圖3h)。米里帶西南部Nyalau組為河流?三角洲沉積相, 厚度巨大; 東北部Satap Shale組為深海黑色泥頁巖。沙巴位于米里帶以及東婆羅洲北部(圖1), 發(fā)育較為完整的白堊紀?新生代沉積地層?；壮雎队诨{巴盧山(Kinabalu Mountains)以及南部達衛(wèi)灣(Darvel Bay), 為角閃石片巖、片麻巖以及超鎂鐵質巖組成的蛇綠巖, 時代為中侏羅世?早白堊世(Hutchison, 1989; Rangin et al., 1990; Macpherson et al., 2010)。上白堊統(tǒng)世?中始新統(tǒng)Trusmadi組和Sapulut組主要為一套深海相濁積巖, 不整合覆蓋于基底之上, 對應于沙撈越地區(qū)的Rajang群(Hutchison, 1996)。濁積巖局部可見交錯層理、爬升層理以及鮑瑪序列(圖3i), 滑塌、褶皺等構造現象普遍(圖3j), 巖層中常見蟲洞以及生物擾動痕跡(圖3k)。Hutchison (1996)稱Trusmadi組和Sapulut組為“下Crocker群”。“上Crocker群”為Crocker組(圖3l), 沉積時代為始新世?早中新世, 厚度超過10 km。早中新世前, 沙巴地區(qū)一直處于深海沉積環(huán)境, Trusmadi組和Crocker組向西逐漸變年輕(van Hattum et al., 2013)。東婆羅洲南端出露白堊紀基性巖和深海相地層(圖1), 被認為與蘇拉維西海向婆羅洲的俯沖有關, 其他大部分區(qū)域被中新世?第四紀沉積物覆蓋。

圖1 婆羅洲地質簡圖(據Haile, 1974; Hutchison, 2010; Wang et al., 2016修改)

圖2 婆羅洲晚白堊世?漸新世地層柱狀圖(據van Hattum et al., 2013; Galin et al., 2017; Breitfeld and Hall, 2018; Breitfeld et al., 2018; Hennig-Breitfeld et al., 2019修改)

(a) Lupar組濁積砂巖, 顯示鮑瑪序列Tc～Te段以及包卷層理; (b) Lupar組滑塌和褶皺構造; (c) Layar段千枚巖; (d) Kapit段輕微變質形成鉛筆構造; (e) 砂巖滑塌進入泥巖層中; (f) Metah段板巖; (g) Bawang段塊狀砂巖滑塌進入泥巖中; (h) Bawang段與Tatau組不整合接觸; (i) Sapulut組濁積砂巖, 顯示鮑瑪序列Tb和Te段; (j) Trusmadi組砂巖因滑塌形成的褶皺; (k) Crocker組砂巖表面生物擾動痕跡; (l) Crocker組砂泥巖互層。

2 婆羅洲沉積物來源

作為世界上最大的海底沉積扇之一, 婆羅洲深水扇沉積規(guī)模類似于現今的孟加拉扇(Moss, 1998)。位于沙撈越中部的西布帶Rajang群寬度可達200 km,晚白堊世?中新世沉積地層厚度至少達上萬米(表1)。Zhu et al. (2021)估算婆羅洲晚白堊世?晚始新世沉積量約達10.9×105～19.5×105km3。

2.1 碎屑礦物組成

選取Rajang群和Tatau組14個新鮮的中?粗砂巖樣品進行鏡下礦物鑒定及碎屑組分統(tǒng)計, 每個樣品統(tǒng)計顆粒為300～500顆。與Kuching超級群和Crocker群相似(van Hattum et al., 2006, 2013; Ferdous and Farazi, 2016; Galin et al., 2017; Breitfeld and Hall, 2018), Rajang群碎屑礦物組成以石英為主, 含有大量燧石以及少量巖屑與長石。重礦物主要有鋯石、金紅石、石榴石、電氣石、綠簾石、尖晶石等。Lupar組和Layar段具有相似的特征, 以隱晶質?微晶長英質及黏土為基質, 礦物碎屑整體顆粒較小, 分選性差且磨圓度低。石英大多為單晶石英, 占70%～80%, 多晶石英占4%～10%, 長石含量很少, 燧石較為常見。單晶石英偶爾可見火山巖成因的港灣狀構造(圖4a)。部分薄片可見石英、長石等礦物受剪切作用定向排列, 形成眼球狀、云母魚等構造形態(tài)(圖4b)。Kapit段和Pelagus段整體粒度較為均一, 礦物顆粒磨圓度較差。多晶石英含量增加, 約為7%～13%, 長石含量為2%～6%, 且燧石、長石等絹云母化嚴重(圖4c)。重礦物自型程度高, 具很高的正突起, 礦物的邊緣粗而黑(圖4d)。火山巖屑主要由交織的板條狀斜長石與基質組成, 斜長石微晶定向排列。沉積巖巖屑多為粉砂巖巖屑, 填隙物以黏土礦物為主(圖4e)。Metah段多晶石英以及長石含量明顯增加, 多晶石英含量高達20%, 長石占總體含量的8%, 以具卡氏雙晶的斜長石為主(圖4f), 偶爾可見具格子雙晶的微斜長石。石英晶體大多被壓扁拉長, 具齒狀變晶結構, 為片狀石英巖巖屑, 也有少量粒狀變晶結構的變質石英巖巖屑。重礦物(鋯石)邊角被磨圓呈圓弧狀(圖4g), 具環(huán)帶結構。含少量碳質板巖巖屑, 主要由碳質以及呈定向排列的絹云母等組成, 具板狀構造。拉讓不整合上下Bawang段與Tatau組砂巖薄片呈現截然不同的特征, Bawang段砂巖顯微鏡下特征與Kapit段和Pelagus段相似, 但顆粒磨圓度更高; 而Tatau組砂巖礦物顆粒具有很好的磨圓度且顆粒較大(圖4h), 均在200～400 μm, 部分石英顆?？捎^察到自生加大邊現象(圖4i), 表明為沉積再循環(huán)的產物。

Q-F-L圖顯示Rajang群和Tatau組樣品均位于再旋回造山區(qū); Qm-F-Lt圖中, 除一個Bawang段和一個Tatau組樣品落入混合區(qū)外, 其余樣品落入石英質再旋回和過渡型再旋回區(qū)域(圖5a)。該結果與前人對婆羅洲晚白堊世?漸新世地層碎屑組成統(tǒng)計結果一致(van Hattum et al., 2013; Galin et al., 2017; Breitfeld and Hall, 2018; Hennig-Breitfeld et al., 2019)。Rajang群石英?長石?巖屑以及多晶石英?單晶石英含量顯示一定的規(guī)律性: Pelagus段和Metah段長石含量較下伏地層明顯增加, 巖屑隨著地層沉積年代的減小而略微增多(圖5b)。由于婆羅洲地處熱帶, 伴隨著強烈的化學風化作用, 輝石、角閃石等基性礦物以及長石等不穩(wěn)定礦物容易在后期風化作用中溶解, 可能導致統(tǒng)計結果產生偏差。抗風化能力較強的石英類礦物統(tǒng)計結果顯示, 單晶石英隨著地層變年輕占比逐漸減少; 而多晶石英則相反, 表明變質碎屑輸入增加(圖5c)。

2.2 主量、微量元素特征

晚白堊世?漸新世沉積物稀土元素配分模式呈現右傾特點, 伴隨著Eu元素的虧損; 微量元素也顯示出極強的Sr負異常(圖6a、b; Ferdous and Farazi, 2016; Ahmed et al., 2020; Ramasamy et al., 2021; Baioumy et al., 2021; Zhao et al., 2021b; Zhu et al., 2021)。TiO2-Zr物源判別圖中, Lubok Antu混雜巖和Rajang群沉積物落入長英質以及中性物質源區(qū), 其中Lubok Antu混雜巖樣品與Rajang群下部Lupar組和Layar段樣品均更靠近中性物質區(qū)域, 而上部地層沉積巖樣品大部分落于長英質區(qū)域內(圖6c)。TiO2-Al2O3物源判別圖也顯示Lubok Antu混雜巖與Rajang群下部地層集中落于花崗閃長巖和花崗巖二者交界處, 而Rajang群上部地層樣品數據點則零散分布在花崗閃長巖和花崗巖區(qū)域范圍內(圖6d)。Rajang群樣品地球化學特征隨時間變化, 表現為晚白堊世?古新世地層(Lupar組、Layar段和下Kapit段)相比上覆地層含有更多的中?基性物質, 隨著地層逐漸年輕, 酸性物質含量逐漸增加。Kuching超級群和Rajang群均來自于花崗閃長巖以及花崗巖。Kuching超級群中的Kayan群、Ketungau群與Rajang群地球化學特征相似, 物源判別圖均位于長英質花崗巖?花崗閃長巖區(qū)域, 但相較于Rajang群具有更低的TiO2和Al2O3值(圖6d)。

表1 婆羅洲晚白堊世?中新世沉積地層厚度統(tǒng)計

(a) Lupar組單晶石英、燧石, 石英具溶蝕港灣狀邊緣表明來自噴出巖; (b) Layar段單晶石英, 礦物定向排列; (c) Kapit段多晶石英、燧石, 燧石絹云母化; (d) Pelagus段單晶石英、多晶石英、鋯石; (e) Pelagus段沉積巖屑; (f) Metah段斜長石、燧石; (g) Metah燧石以及次圓狀鋯石; (h) Bawang段單晶石英、多晶石英; (i) Tatau組單晶石英、多晶石英、燧石, 石英加大邊顯示再旋回特征。礦物代號: Qm. 單晶石英; Qp. 多晶石英; Ch. 燧石; Kfs. 鉀長石; Plg. 斜長石; Lm. 變質巖屑; Ls. 沉積巖屑。

2.3 Nd同位素地球化學特征

Lubok Antu混雜巖143Nd/144Nd值為0.512306～ 0.512498, 對應Nd值為?6.5～?2.7。Rajang群下部的Lupar組和Layar段143Nd/144Nd值為0.512346～0.512457, 對應的Nd值高達?5.7～?3.5, 上部古新世?晚始新世地層Nd值減小到?9.7～?5.7。拉讓不整合之上Tatau組地層Nd值則穩(wěn)定在?7.9～?7.7之間(圖7)。沉積物中Nd的相對高值表明有年輕的地殼物質加入(Li et al., 2003; Yan et al., 2007)。相較于南?，F代沉積物Nd值而言, Lubok Antu混雜巖與Rajang群下部樣品具有異常高Nd值(圖8), 說明同時期近源存在大面積火山噴發(fā)或是年輕巖漿巖巖體出露和剝蝕。由于婆羅洲晚白堊世?古新世火山活動較少, 且在鏡下未觀察到大量火山碎屑, 因此認為近源出露的大面積巖漿巖是晚白堊世?古新世Rajang群沉積物的主要物質來源。古新世后, 沉積物中Nd值開始逐漸減小到?9.7～?5.7, 這一變化表明物源由年輕的巖漿巖轉變?yōu)楣爬系拇箨懳镔|。

2.4 婆羅洲沉積物源演化過程

前人通過古水流、碎屑鋯石和重礦物研究認為Rajang群沉積物主要來自施瓦納山和馬來半島(Tan, 1982; Hutchison, 2005; van Hattum et al., 2006, 2013; Galin et al., 2017; Breitfeld and Hall, 2018; Breitfeld et al., 2018), 且Galin et al. (2017)以及Breitfeld and Hall (2018)認為此期間Rajang群與Kuching超級群經歷了兩次物源轉變, 晚白堊世?古新世沉積物主要來源于施瓦納山和馬來半島, 古新世?早/中始新世沉積物幾乎全部來自于施瓦納山, 中始新世?晚始新世物源則受馬來半島以及施瓦納山共同影響。但是Zhu et al. (2021)認為施瓦納山和馬來半島的剝蝕量遠小于Rajang-Crocker群巨大的沉積量。從礦物組成、主量和微量元素、Nd同位素數據來看, 古新世前后Rajang群物源發(fā)生明顯變化。晚白堊世?古新世Rajang群沉積物具有近源特征, 且由大面積年輕的中?酸性花崗巖剝蝕沉積, 分布于越南南部至婆羅洲西南部中生代巖漿巖帶可能是晚白堊世?古新世Rajang群的主要物源區(qū)。地表露頭與鉆孔等資料顯示環(huán)太平洋花崗巖具有相似的年齡以及地球化學特征(圖8; Katili, 1973; Li and Li, 2007; Hutchison, 2010; Shellnutt et al., 2013; Hennig et al., 2017; Li et al., 2018; Breitfeld et al., 2020a; Hennig-Breitfeld et al., 2021), 且經歷了晚白堊世?古新世快速風化剝蝕(Areshev et al., 1992; 周蒂等, 2005; Cuong and Warren, 2009)。越南南部Dalat花崗巖Nd值為?2.8～+0.6 (Shellnutt et al., 2013), 分布于Kuching帶的白堊紀火山巖及花崗巖Nd值同樣高達+0.9～+3.6(Wang et al., 2021b)。這一花崗巖帶的剝蝕使Rajang群晚白堊世?古新世沉積物具有相對較高的Nd同位素組成(Shellnutt et al., 2013; Nong et al., 2021; Waight et al., 2021; Hennig-Breitfeld et al., 2021; Wang et al., 2021b)。古新世后, 馬來半島及印支陸塊逐漸成為主要物源區(qū), 具較低Nd值(?10.8～?5.6; Wang et al., 2021a)的酸性物質逐漸成為Rajang群的物源。古新世?早始新世, Kapit段、Pelagus段沉積物地球化學特征反映的物源區(qū)與Galin et al. (2017)研究的差異, 可能是由于采樣位置以及測試手段引起。進行沉積物地球化學分析的細粒粉砂巖?泥巖樣品主要位于沙撈越西北部, 受馬來半島影響較大, 而中部砂巖中碎屑鋯石更可能受施瓦納山影響(Zhu et al., 2021)。Rajang群物源區(qū)由晚中生代巖漿巖帶向馬來半島的轉變, 可能與巖漿巖帶的裂解(Shellnut et al., 2013)以及巽他大陸的整體抬升(Morley, 2012; Cottam et al., 2013)有關。漸新世, Tatau組樣品Nd同位素值穩(wěn)定在?7.9～?7.7之間, 強烈的再循環(huán)特征表明Tatau組是由Rajang群抬升剝蝕沉積。

(a) Rajang群砂巖Q-F-L和Qm-F-Lt圖解, Trusmadi組引自van Hattum et al. (2013), Kuching超級群引自Ferdous and Farazi (2016); Breitfeld and Hall (2018), Rajang群引自Galin et al. (2017), Tatau組引自Hennig-Breitfeld et al. (2019); (b) 石英?長石?巖屑組分變化圖; (c) 單晶石英?多晶石英占碎屑組分比值變化圖。礦物代號: Q. 石英顆粒; F. 長石; L. 巖屑; Qm. 單晶石英; Qp. 多晶石英。

(a) 球粒隕石標準化稀土元素配分圖(標準化值據Sun and McDonough, 1989); (b) 上地殼標準化微量元素蛛網圖(標準化值據Rudnick and Gao, 2003); (c) TiO2-Zr二元判別圖(底圖據McLennan et al., 1980); (d) TiO2-Al2O3二元判別圖(底圖據McLennan et al., 1980)。

圖8 婆羅洲周緣Nd同位素及年齡分布圖(據Wei et al., 2012; Breitfeld et al., 2020a修改)

3 婆羅洲晚中生代構造演化

在Th-Zr/10-Co構造背景判別圖中, Rajang群底部Lupar組、Layar段位于大陸島弧構造背景, 其余樣品大多落入主動大陸邊緣區(qū)域內, Tatau組樣品則在主動大陸邊緣和被動大陸邊緣之間的區(qū)域內(圖9a)。log(K2O/Na2O)-SiO2二元構造判別圖中, 西布帶Rajang群樣品都落入主動大陸邊緣和被動大陸邊緣區(qū)域, Tatau組樣品則完全落于被動大陸邊緣范圍(圖9b)。同時我們搜集Kuching超級群、Lubok Antu混雜巖以及沙巴始新世Trusmadi組沉積地層地球化學數據(Burgan et al., 2008; Ferdous and Farazi, 2016; Khan et al., 2017; Zhu et al., 2021; Zhao et al., 2021b)并進行對比, 發(fā)現古晉帶、西布帶、米里帶以及沙巴樣品反映的構造背景自西向東呈現出時空差異。古晉帶上白堊統(tǒng)?上始新統(tǒng)Kayan群、Ketungau群樣品完全處于被動大陸邊緣, 同沉積期的Rajang群則大多處于主動大陸邊緣和被動大陸邊緣之間(圖9a、b), 而位于沙巴地區(qū)的中/上始新統(tǒng)Trusmadi組則完全處于主動大陸邊緣背景(圖9b)。

三疊紀?晚白堊世, 古太平洋沿華南?婆羅洲俯沖形成安第斯型島弧。古晉帶白堊系Pedawan組作為弧前盆地沉積接受來自周緣巖漿弧的物質供給(Breitfeld et al., 2017), 古晉帶沉積環(huán)境由深海相Pedawan組向陸相Kayan組轉變, 形成Pedawan不整合(Morley, 1998; Breitfeld et al., 2017), 標志著古太平洋板塊在古晉帶的俯沖作用于90 Ma左右停止(Breitfeld et al., 2018; 趙帥等, 2019)。西沙撈越地區(qū)古晉帶上白堊統(tǒng)?上始新統(tǒng)Kayan群、Ketungau群構造背景相對穩(wěn)定。然而, 沿Lupar線出露的Lubok Antu俯沖混雜巖以及西布帶出露的Rajang群底部的Lupar組、Layar段具有較高的Nd同位素比值, 主量、微量元素特征顯示二者含有更高的中?基性物質, 構造背景相對活躍, 說明古太平洋板塊于婆羅洲的俯沖可能持續(xù)到古新世。晚白堊紀?始新世時期, 地球化學特征顯示古晉帶穩(wěn)定的構造背景與西布帶較為活躍的構造背景之間的差異, 可能是由于古太平洋在沙撈越地區(qū)的俯沖后撤造成。當古太平洋沿盧帕爾線俯沖停止并向東后撤時, 古晉帶遠離俯沖帶而靠近內陸地區(qū), 構造背景相對穩(wěn)定, 由深海沉積環(huán)境轉變?yōu)殛懴喑练e環(huán)境, Kuching超級群開始沉積。而西布帶深海相Rajang群作為增生楔沉積了同時期的Lupar組與Layar段, 具有明顯的俯沖背景信號。古晉帶內發(fā)育的77～80 Ma具有火山弧花崗巖性質的Pueh和Gading侵入體(Hennig et al., 2017)也可能對應俯沖帶的后撤。～60 Ma, 古太平洋板塊在西北婆羅洲的俯沖停止(Zhao et al., 2021a)。另外, 俯沖后撤引起區(qū)域擠壓應力向區(qū)域伸展應力轉變, 發(fā)育一系列區(qū)域性伸展構造(Shellnutt et al., 2013; Liu et al., 2016), 位于巽他陸架的巖漿巖帶坍塌裂解, 使得Rajang群源區(qū)由近源巖漿巖帶向馬來半島遷移, 大量具有較低Nd同位素組成且更具酸性的沉積物為Rajang群提供物質來源?！?7 Ma, 曾母陸塊與婆羅洲碰撞(Fuller et al., 1999; Madon et al., 2013; Advokaat et al., 2018), Rajang群變形抬升(Hutchison, 2010; Zhao et al., 2021a), 且由于后期板塊邊界重組及婆羅洲逆時針旋轉, 最初具有縫合線性質的盧帕爾線轉變?yōu)樽呋瑪鄬?。婆羅洲的逆時針旋轉及沿盧帕爾線的走滑使Rajang群與Kuching超級群疊置。漸新世, 深海相Rajang群被陸相Tatau組和Nyalau組地層不整合覆蓋, Tatau組Nd同位素比值穩(wěn)定且位于Rajang群Nd同位素比值范圍內, 主量、微量元素反映的此時構造背景由活躍轉變?yōu)榉€(wěn)定, 且碎屑礦物形態(tài)證明沉積物來源于早期Rajang群的物質循環(huán)(圖4); Nyalau組碎屑鋯石及礦物形態(tài)也反映出一致的循環(huán)特征(Breitfeld et al., 2020b)。同時古南海在西巴蘭姆線以東的沙巴地區(qū)開始俯沖, Trusmadi組具有強烈的主動大陸邊緣沉積地球化學信號(圖9b), 沙巴南部中新世砂巖中發(fā)現少量的始新世?早中新世巖漿鋯石(22～48 Ma), 暗示沙巴地區(qū)存在與古南海俯沖相關的火山活動(Suggate, 2011), 這與沙巴南部漸新世砂巖中含有大量火山巖屑一致(van Hattum et al., 2013)。此外, 古南海向南俯沖形成卡加延火山島弧(Holloway, 1982; 金康辰, 1989; Hinz et al., 1991; Rangin and Silver, 1991; Spadea et al., 1996; Keenan et al., 2016; Hall and Breitfeld, 2017), 卡加延弧鉆井樣品定年結果為14～26 Ma, 甚至更老, 表明其很可能向西南連接至沙巴山打根(Sandakan)附近, 共同代表與古南海俯沖相關的巖漿活動事件(Kudrass et al., 1990; Rangin and Silver, 1991)。而漸新世?中新世深水Crocker組更被認為是古南海俯沖于沙巴?巴拉望過程中的產物(Taylor and Hayes, 1983; Hall, 1996, 2013; Hutchison et al., 2000)。

(a) Th-Zr/10-Co構造背景判別圖(底圖據Bhatia and Crook, 1986); (b) Log(K2O/Na2O)-SiO2構造背景判別圖(底圖據Roser abd Korsch, 1986)。AM. 主動大陸邊緣; PM. 被動大陸邊緣; OIA. 大洋島弧; CIA. 大陸島弧。

4 結 論

(1) Rajang群沉積物來源在古新世前后經歷了一次較為明顯的變化。晚白堊世?早古新世Rajang群沉積物具有近源特征, Nd同位素比值相對較高, 其物源主要來自巽他大陸邊緣晚中生代巖漿巖巖帶。早始新世?晚始新世物源區(qū)向馬來半島遷移。

(2) 晚白堊世, 位于古晉帶的俯沖停止, 然而西部帶的俯沖持續(xù)到～60 Ma。Rajang群底部的Lupar組和Layar段與Lubok Antu俯沖混雜巖為古南海俯沖增生產物。古新世后, 盧帕爾線由縫合線轉變?yōu)樽呋瑪鄬? 婆羅洲的逆時針旋轉及沿盧帕爾線的走滑使Rajang群與Kuching超級群疊置。

(3) ～37 Ma, 曾母陸塊與婆羅洲碰撞, Rajang群抬升剝蝕。漸新世, 古南海在婆羅洲東北部沙巴地區(qū)開始俯沖, 對應于南海的打開。

致謝:兩位匿名審稿專家對本文提出的寶貴建議及意見, 在此表示衷心的感謝。

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Subduction Processes of the Proto-South China Sea: Evidence from the Late Cretaceous-Oligocene Stratigraphic Record in Borneo

ZHU Zuofei1, 2, 3, YAN Yi1, 2, 4, 5*, ZHAO Qi1, 2, 4, 5

(1. CASKey Laboratory of Ocean and Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China; 2. Center for Excellence in Deep Earth Science, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China; 3. College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China; 4. Southern Marine Science and Engineering Guangdong Laboratory, Guangzhou 511458, Guangdong, China; 5. Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences, Guangzhou 510301, Guangdong, China)

The subduction and extinction of the Proto-South China Sea is the key to reveal the expansion mechanism of the South China Sea and to reconstruct the Meso-Cenozoic tectonic evolution of Southeast Asia. However, there are still many controversies about the subduction process of the Proto-South China Sea. The overall Late Cretaceous-Oligocene sedimentary strata are outcropped in Borneo, Malaysia, which are important window for studying the tectonic evolution of the Proto-South China Sea. Detrital mineral composition, sedimentary geochemistry and Nd isotope analysis have been used to trace the origin of sediments from the Late Cretaceous-Oligocene strata in Borneo, and reconstruct the regional palaeogeographic pattern. The results show that the magmatic rock belt formed by subduction of the Paleo-Pacific Plate was the main source area for the Late Cretaceous-Paleocene sediments of the Rajang Group, and the sedimentary contribution of the Malay Peninsula and Southern margin of the Indochina were increased during Paleocene-Late Eocene, indicating that the influence of the Paleo-Pacific Plate subduction continued into the Early Paleocene (. 60 Ma). During the Late Eocene, the counterclockwise rotation of Borneo caused the scissor-like closure of the residual basin as the Australian Plate continued to drift northward. About 37 Ma, the Luconia block collided with Borneo, leading the uplift and erosion of the Rajang Group. During the Oligocene, the Proto-South China Sea started to subduct under Sabah, which located in Northeast Borneo, corresponding to the opening of the South China Sea. The Proto-South China Sea subducted obliquely from west to east, the rotation of Borneo and the strike-slip along the Lupar Line superimposed the Rajang Group over the Kuching Supergroup.

Proto-South China Sea; Paleo-Pacific Ocean; Borneo; provenance; tectonic evolution

2021-12-10;

2022-02-14

國家自然科學基金委(NSFC)–廣東聯(lián)合基金項目(U1701641)、南方海洋科學與工程廣東省實驗室(廣州)人才團隊引進重大專項 (GML2019ZD0205)聯(lián)合資助。

朱作飛(1994–), 男, 博士研究生, 構造地質學專業(yè)。E-mail: zhuzuofei94@163.com

閆義(1973–), 男, 研究員, 從事海洋地質方面研究。E-mail: yanyi@gig.ac.c

P588.21

A

1001-1552(2022)03-0552-017

10.16539/j.ddgzyckx.2022.03.010