余小清, 錢 鑫*, 張玉芝, 徐 暢, 王岳軍

蘇門答臘中部Panti早始新世S型花崗巖地球化學(xué)特征及其新特提斯構(gòu)造意義

余小清1, 2, 錢 鑫1, 2*, 張玉芝1, 2, 徐 暢1, 2, 王岳軍1, 2

(1.廣東省地球動(dòng)力作用與地質(zhì)災(zāi)害重點(diǎn)實(shí)驗(yàn)室, 中山大學(xué) 地球科學(xué)與工程學(xué)院, 廣東 廣州 510275; 2.南方海洋科學(xué)與工程廣東省實(shí)驗(yàn)室, 廣東 珠海 519082)

蘇門答臘地處特提斯構(gòu)造域與印度洋島弧系統(tǒng)之內(nèi), 區(qū)內(nèi)廣泛發(fā)育中?新生代火成巖。這些火成巖對(duì)特提斯構(gòu)造帶的延伸及界定具有重要意義, 但目前對(duì)于蘇門答臘島的研究程度較低。本文選取蘇門答臘島中部Panti地區(qū)的花崗巖進(jìn)行了巖相學(xué)、鋯石年代學(xué)、鋯石原位Hf同位素和全巖地球化學(xué)研究。年代學(xué)研究表明, Panti花崗巖鋯石U-Pb年齡為55.5±0.3 Ma。該套花崗巖具有高SiO2(76.61%～78.37%)和高堿(Na2O+K2O=7.47%～8.21%)的特征, 其分異指數(shù)(DI)為95～96, A/CNK為1.08～1.12, 為高分異S型花崗巖。稀土配分模式圖及微量元素蛛網(wǎng)圖顯示其具有強(qiáng)烈的Eu負(fù)異常(Eu/Eu*=0.04～0.05), 并虧損Nb、Ta、Sr、P和Ti的特征。且該套花崗巖還具有高Rb/Sr和低CaO/Na2O 值, 同時(shí)顯示富集的鋯石Hf同位素組成(Hf()=?9.1～?2.6)和較老的Hf二階模式年齡(DM2=1.29～1.70 Ga), 這些特征均指示該套花崗巖來(lái)源于古老的變泥質(zhì)巖的部分熔融。對(duì)比緬甸?滇西?藏南一帶同時(shí)期的巖漿事件, 我們認(rèn)為Panti早始新世高分異S型花崗巖可與滇西騰梁和東盈江同時(shí)期花崗巖進(jìn)行對(duì)比, 指示了新特提斯構(gòu)造巖漿帶南延至蘇門答臘中部。

蘇門答臘中部; 早始新世; 高分異S型花崗巖; 鋯石U-Pb年代學(xué); 地球化學(xué); 新特提斯

0 引 言

早白堊世開(kāi)始, 隨著新特提斯洋的俯沖, 印度板塊與歐亞板塊碰撞, 形成了全球范圍內(nèi)最為壯觀的喜馬拉雅造山帶(Yin and Harrison, 2000)。喜馬拉雅造山帶構(gòu)造地質(zhì)學(xué)、年代學(xué)及巖漿巖巖石學(xué)的研究對(duì)理解新特提斯洋的俯沖閉合過(guò)程及不同塊體的碰撞拼合演化歷史具有重要意義(Xu et al., 2012)。目前, 大量研究表明位于班公?怒江與雅魯藏布江之間的岡底斯造山帶(即拉薩地塊南緣)與新特提斯洋的俯沖閉合相關(guān)(Chung et al., 2005; Chu et al., 2006; Wen et al., 2008; 朱弟成等, 2008; Zhu et al., 2011)。岡底斯造山帶向東可以延伸到藏東南波密?察隅地區(qū)(Liang et al., 2008), 之后向南轉(zhuǎn)入滇西的高黎貢?騰沖?梁河?盈江一帶(Xu et al., 2012; 馬莉燕, 2013; 馬莉燕等, 2013; Ma et al., 2014; Wang et al., 2014), 經(jīng)緬甸的抹谷變質(zhì)帶(Barley et al., 2003; Searle et al., 2007; Mitchell et al., 2007; Mitchell et al., 2012)南延至蘇門答臘島(Mitchell, 1993; Barley et al., 2003)。也有學(xué)者認(rèn)為西緬甸的白堊紀(jì)?始新世巖漿弧是岡底斯帶的延伸, 為新特提斯巖漿帶在東南亞的延伸(Wang et al., 2014b)。

東南亞地區(qū)是晚古生代或早中生代期間由岡瓦納北緣分離出來(lái)的陸塊, 隨古特提斯洋俯沖消減拼貼至歐亞大陸東南緣形成的(Metcalfe, 1996; Wang et al., 2018)。其中蘇門答臘島呈NW-SE向狹長(zhǎng)分布于東南亞西南岬角, 并被劃分為東蘇門答臘、西蘇門答臘和Woyla地體, 三者分別沿中央構(gòu)造帶和Woyla 縫合線拼貼(圖1a、b)?，F(xiàn)有研究認(rèn)為西蘇門答臘在中?晚泥盆世期間, 隨著古特提斯洋打開(kāi), 與西緬甸和華夏陸塊從岡瓦納古陸北緣向北裂離。早二疊世期間, 受古特提斯洋的俯沖作用, 西蘇門答臘發(fā)育安第斯型巖漿弧(Barber and Crow, 2003)。而有的學(xué)者認(rèn)為, 隨著東古特提斯洋的俯沖, 東蘇門答臘地塊于中三疊世拼貼至印支地塊, 構(gòu)成了滇緬?cǎi)R蘇(Sibumas)地塊的一部分(Barber and Crow, 2003, 2009; Wang et al., 2018)。晚中生代, Woyla地體形成, 隨著特提斯洋俯沖, 其與蘇門答臘主體拼貼(Barber and Crow, 2003)。最近研究表明蘇門答臘島中部地區(qū)保存有與新特提斯洋演化有關(guān)的始新世碎屑鋯石, 證明了新特提斯構(gòu)造帶可以延伸至蘇門答臘島內(nèi)(Zhang et al., 2019)。此外, 蘇門答臘島的古地磁數(shù)據(jù)也顯示蘇門答臘島Woyla帶是藏南地區(qū)與新特提斯洋俯沖相關(guān)的Incertus洋內(nèi)弧的南延(Hall, 2012)。但上述研究均集中于沉積巖及古地磁方面, 缺少對(duì)相關(guān)巖漿巖年代學(xué)和地球化學(xué)的研究, 因此限制了我們對(duì)新特提斯構(gòu)造帶延伸及演化的認(rèn)識(shí)。近期本課題組在蘇門答臘島進(jìn)行的野外地質(zhì)調(diào)查中, 在中部Panti地區(qū)識(shí)別出一套始新世花崗巖。本次研究將以這套花崗巖為研究對(duì)象, 系統(tǒng)開(kāi)展巖相學(xué)、鋯石年代學(xué)、鋯石原位Hf同位素及全巖地球化學(xué)研究, 闡明其巖石成因及構(gòu)造背景, 進(jìn)而與區(qū)域相關(guān)火成巖進(jìn)行對(duì)比, 為新特提斯巖漿帶的延伸提供新的證據(jù)。

1 地質(zhì)背景及巖相學(xué)特征

蘇門答臘的大陸基底被認(rèn)為由結(jié)晶片巖、前石炭紀(jì)的沉積單元及時(shí)代相當(dāng)?shù)那秩塍w組成(Hamilton, 1979)。自古近紀(jì)中期起, 印度洋沿巺他海溝斜向俯沖, 在蘇門答臘西海岸形成從北西至南東的巨型巴里桑山火山弧, 該火山弧被NW-SE右型走滑斷層體系(巺他斷層區(qū))切分成多個(gè)狹長(zhǎng)地塊(圖1b; 如Mccourt et al., 1996; Barber et al., 2005)。蘇門答臘島前新生代地層主要?jiǎng)澐譃槭肯?二疊系Tapanuli群、二疊系?三疊系Peusangan群和侏羅系?白堊系Woyla群(Cameron et al., 1980), 且大部分地區(qū)被新生代沉積巖和火山巖覆蓋。其中西蘇門答臘分布有石炭紀(jì)地層, 并發(fā)育親華夏體系的低緯度暖水動(dòng)植物群?熱帶維憲期珊瑚藻動(dòng)植物群(Barber and Crow, 2009)。而東蘇門答臘分布有二疊紀(jì)地層, 發(fā)育高緯度的冷水動(dòng)物群, 且分布有與錫礦相關(guān)的花崗巖(Cobbing, 2005; Barber and Crow, 2009)。至少在晚三疊世前, 西蘇門答臘和西緬甸沿中央構(gòu)造帶拼貼至東蘇門答臘(滇緬?cǎi)R蘇地塊西部), 并在中央構(gòu)造帶發(fā)育一系列的ES向地塹構(gòu)造, 且形成盆地(Barber and Crow, 2003)。關(guān)于Woyla地體的形成, 有人認(rèn)為其是晚侏羅世特提斯洋俯沖形成的洋內(nèi)島弧和增生雜巖體, 并隨著白堊紀(jì)特提斯洋閉合拼貼至西蘇門答臘(Wajzer et al., 1991; Barber, 2000; Barber and Crow, 2003; Barber et al., 2005), 包含北部的西庫(kù)萊(Sikuleh)和中部的納塔爾(Natal)2個(gè)古老微陸塊(Barber, 2000; Barber et al., 2005); 也有學(xué)者認(rèn)為這些微陸塊是在晚侏羅世由澳大利亞裂離而來(lái)的(Metcalfe, 1993)。此外Woyla地體還包括了新生代的火山弧巖漿巖(玄武?安山質(zhì)火山巖和火山碎屑巖等)、海洋增生雜巖(枕狀玄武巖和紅色頁(yè)巖等)和中侏羅世?早白堊世的雜巖(石英鈣質(zhì)砂頁(yè)巖)等(Advokaat et al., 2018)。中新世, 隨著蘇門答臘北部安得曼海的打開(kāi), 西蘇門答臘和西緬甸逐漸分離(Barber et al., 2005; Barber and Crow, 2008, 2009; Metcalfe, 2011, 2013)。

圖1 蘇門答臘區(qū)域構(gòu)造圖(a、b)和研究區(qū)地質(zhì)簡(jiǎn)圖(c)(據(jù)Barley et al., 2003; Advokaat et al., 2018)

Panti花崗巖位于蘇門答臘島中部, 處于西蘇門答臘與Woyla地體交界處。研究區(qū)還出露有大量新生代沉積巖和火山巖, 其中沉積巖包括新近紀(jì)石英砂巖、碳質(zhì)頁(yè)巖、海綠石頁(yè)巖、粉砂巖、礫巖和第四紀(jì)砂巖; 火山巖主要為中新世層狀火山巖、第四紀(jì)熔巖及火山碎屑巖為主的沖積層。區(qū)內(nèi)新生代花崗巖普遍侵入二疊紀(jì)的板巖、變石英砂礫巖、變石英巖和千枚巖等變沉積巖中, 部分侵入至新生代的變火山巖、變雜砂巖、綠片巖、千枚巖和板巖中(圖1c)。

本次研究的花崗巖樣品采自Panti東邊約6 km處的TANDUNGK UMBANG 花崗巖巖體(坐標(biāo)為: 00°22′57.25″S、100°07′04.02″E)。Panti花崗巖主要礦物為石英(40%～50%)、斜長(zhǎng)石(20%～25%)和堿性長(zhǎng)石(20%～30%), 次要礦物有黑云母(5%～8%), 可見(jiàn)少量鋯石副礦物(1%～2%), 其中堿性長(zhǎng)石中含較多呈條紋結(jié)構(gòu)的條紋長(zhǎng)石, 斜長(zhǎng)石發(fā)育聚片雙晶(圖2)。

2 分析方法

2.1 LA-ICP-MS鋯石U-Pb定年及原位Hf同位素測(cè)定

鋯石U-Pb同位素定年和原位微區(qū)微量元素含量分析在中山大學(xué)地球動(dòng)力作用與地質(zhì)災(zāi)害省重點(diǎn)實(shí)驗(yàn)室利用激光剝蝕系統(tǒng)與電感耦合等離子體質(zhì)譜儀聯(lián)用(LA-ICP-MS)完成。GeolasHD激光剝蝕系統(tǒng)由COMPexPro 102 ArF 193 nm準(zhǔn)分子激光器和MicroLas光學(xué)系統(tǒng)組成, ICP-MS型號(hào)為iCAP RQ。分析采用的激光束斑為32 μm, 頻率為5 Hz。U-Pb同位素定年采用鋯石標(biāo)樣91500和Ple?ovice來(lái)進(jìn)行同位素分餾校正, 采用玻璃標(biāo)準(zhǔn)物質(zhì)NIST610進(jìn)行微量元素校正。對(duì)分析數(shù)據(jù)的處理采用軟件GLITTER(Griffin et al., 2008)進(jìn)行。

鋯石Hf同位素原位分析是在中山大學(xué)地球動(dòng)力作用與地質(zhì)災(zāi)害省重點(diǎn)實(shí)驗(yàn)室利用Geolas HD準(zhǔn)分子ArF激光剝蝕系統(tǒng)通過(guò)Neptune Plus多接收器電感耦合等離子體質(zhì)譜儀(MC-ICP-MS)完成。采用44 μm的激光斑束直徑和8 Hz的激光頻率剝蝕鋯石, 詳細(xì)的儀器參數(shù)和程序見(jiàn)Hu et al. (2012)。以91500和Ple?ovice作為標(biāo)樣監(jiān)測(cè)數(shù)據(jù)質(zhì)量。Hf的初始同位素值是使用1.867×10?11a?1的176Lu衰變常數(shù)計(jì)算的(S?derlund et al., 2004)。使用176Hf/177Hf=0.283250和176Lu/177Hf=0.0384(Griffin et al., 2000)計(jì)算相對(duì)于虧損地幔的Hf模式年齡(DM), 并以大陸地殼平均值176Lu/177Hf=0.015(Griffin et al., 2002)計(jì)算Hf的二階模式年齡(DM2)。

2.2 主量、微量元素分析

選用新鮮樣品碎至200目用于全巖主量、微量測(cè)試。全巖主量、微量元素分析均在中山大學(xué)地球動(dòng)力作用與地質(zhì)災(zāi)害省重點(diǎn)實(shí)驗(yàn)室完成。主量元素分析在ARL-Perform’X4200型X射線熒光光譜分析儀(XRF)上測(cè)試完成。樣品制備采用熔片法, 將烘干后的樣品與硼酸鋰混合熔劑稱量至坩堝中, 加入飽和碘化銨(NH4I)溶液, 在鉑金坩堝中加熱至1050 ℃共熔制成熔片, 熔片方法參考Claisse et al. (2006), 分析精度優(yōu)于1%。微量元素分析利用iCAP RQ型ICP-MS完成。儀器分析精度一般優(yōu)于5%, 用高純度HNO3和HF對(duì)樣品進(jìn)行初步溶解后再高溫溶解, 待樣品溶液冷卻后加入HNO3和內(nèi)標(biāo)溶液進(jìn)行測(cè)試。

礦物代號(hào): Qtz. 石英; Pl. 斜長(zhǎng)石; Bt. 黑云母; Kfs. 鉀長(zhǎng)石; Pth.條紋長(zhǎng)石; Zrn. 鋯石。

3 分析結(jié)果

3.1 鋯石U-Pb年齡及Hf同位素特征

樣品18SM-40-1的30顆鋯石U-Pb測(cè)試結(jié)果見(jiàn)表1。CL圖像表明鋯石具有弱的振蕩環(huán)帶(圖3), 鋯石的Th/U值在0.19～0.34之間, 平均為0.25。所測(cè)鋯石點(diǎn)諧和度較好, 樣品點(diǎn)均落在諧和線上, 其206Pb/238U加權(quán)平均年齡為55.5±0.3 Ma(2σ, MSWD= 0.87,=30; 圖4)。在鋯石U-Pb測(cè)試的相同位置點(diǎn)上進(jìn)行了原位的Hf同位素分析(表2), 所獲得的176Lu/177Hf 值為0.003195～0.006121,176Hf/177Hf 值為0.282486～0.282671, 對(duì)應(yīng)的Hf()值為?9.1～?2.6, 二階模式年齡(DM2)為1.29～1.70 Ga。樣品落入滇西東盈江和南緬甸始新世花崗巖的Hf同位素組成區(qū)域內(nèi)(圖4c; Xu et al., 2012; Li et al., 2019)。

3.2 巖石地球化學(xué)特征

Panti花崗巖樣品主量、微量元素測(cè)試結(jié)果見(jiàn)表3。樣品燒失量(LOI)均小于0.6%(0.34%～0.54%), 具有高SiO2(76.61%～78.37%)含量, 其CaO(0.26%～0.39%), Fe2O3(1.21%～1.54%), MgO(0.03%～0.05%), TiO2(0.08%～ 0.1%)和P2O5(0.01%～0.02%)含量均較低。樣品CIPW標(biāo)準(zhǔn)化礦物含石英(37%～44%), 正長(zhǎng)石(26%～33%), 鈉長(zhǎng)石(20%～32%), 鈣長(zhǎng)石(1%～2%)和剛玉(1%～1.3%)。通過(guò)標(biāo)準(zhǔn)礦物在QAP三角圖解投點(diǎn)(圖5a), 樣品均落在二長(zhǎng)花崗巖區(qū)域。其鋁飽和指數(shù)A/CNK值為1.08～1.12, A/NK值為1.14～1.18。在A/NK-A/CNK圖解中, 均顯示過(guò)鋁質(zhì)的特征(圖5b)。樣品全堿含量較高(K2O+Na2O=7.47%～8.21%), 并具高K2O/Na2O (1.18～2.21)值, 在K2O-SiO2圖解中, 落入高鉀鈣堿性系列(圖5c)。總體上樣品主量元素特征與滇西和南緬甸始新世花崗巖類似(圖5; 趙少偉等, 2017; Li et al., 2019),分異指數(shù)DI(DI=Q+Or+Ab+Ne+Lc+Kp)值較高, 為95～96, 表明Panti花崗巖具有高分異的特征。

表1 Panti花崗巖(18SM-40-1) LA-ICP-MS鋯石測(cè)試分析結(jié)果

圖3 Panti花崗巖樣品代表性鋯石顆粒的陰極發(fā)光圖像

Panti花崗巖稀土元素總量(ΣREE)為160×10?6～342×10?6。在球粒隕石標(biāo)準(zhǔn)化稀土元素配分圖解上(圖6a)顯示輕稀土元素富集, 重稀土元素虧損的右傾型模式, (La/Yb)N為6.05～20.79, (Gd/Yb)N為1.46～2.51, 且具強(qiáng)烈的Eu負(fù)異常(Eu/Eu*=0.04～0.05)。在原始地幔標(biāo)準(zhǔn)化的微量元素蛛網(wǎng)圖中(圖6b), 樣品顯示富集大離子親石元素Rb、U和K, 而虧損高場(chǎng)強(qiáng)元素Nb、Ta和Ti的特征, 同時(shí)可見(jiàn)明顯的Sr負(fù)異常, 且樣品顯示高Rb/Sr和低K/Rb值的高分異特征。此外, 樣品Nb/Ta值為10～13(<5); 稀土元素四分組強(qiáng)度TE1, 3為0.97～1.02(<1.1), 應(yīng)該是無(wú)流體相的單一體系下花崗質(zhì)巖漿正常分異結(jié)晶形成的高分異花崗巖(陶繼華等, 2013; Ballouard et al., 2016)。稀土元素配分模式和微量元素蛛網(wǎng)圖顯示Panti花崗巖與滇西和南緬甸始新世花崗巖特征類似(趙少偉等, 2017; Li et al., 2019)。

圖c中數(shù)據(jù)來(lái)源: 蘇門答臘據(jù)Zhang et al. (2019); 滇西東盈江據(jù)Xu et al. (2012); 南緬甸據(jù)Li et al. (2019); 岡底斯帶親緣、北巖漿帶親緣據(jù)Zhang et al. (2017)。

表2 Panti花崗巖(18SM-40-1)鋯石Hf同位素分析結(jié)果

表3 Panti花崗巖主量(%)和微量元素(×10?6)組成

注: A/CNK=Al2O3/(CaO+Na2O+K2O); A/NK= Al2O3/(Na2O+K2O)。

圖5 Panti花崗巖的QAP (a)、A/NK-A/CNK (b)和K2O-SiO2 (c)圖解

圖6 Panti花崗巖球粒隕石標(biāo)準(zhǔn)化稀土元素配分曲線(a)和原始地幔標(biāo)準(zhǔn)化微量元素蛛網(wǎng)圖(b)(球粒隕石和原始地幔標(biāo)準(zhǔn)化數(shù)據(jù)引自Sun and McDonough, 1989)

4 討 論

4.1 巖石成因

Panti花崗巖具有較低的Ga、Zr、Nb等高場(chǎng)強(qiáng)元素組成, 其10000×Ga/Al為1.87～2.44, Zr含量為142×10?6～166×10?6, Zr+Nb+Ce+Y平均含量為327×10?6, 均低于Whalen et al. (1987)提出的A型花崗巖的元素含量(10000×Ga/Al>2.6; Zr>250×10?6; Zr+Nb+Ce+Y>350×10?6; Whalen et al., 1987), 而落入I、M、S型花崗巖區(qū)內(nèi)(圖7a、b)。通過(guò)全巖主量元素及Zr含量, 計(jì)算得到樣品的鋯石飽和溫度為746～764 ℃(Boehnke et al., 2013), 均低于A型和I型花崗巖的平均鋯石飽和溫度(A型為839 ℃, I型為781 ℃; 據(jù)King et al., 1997)。研究認(rèn)為在準(zhǔn)鋁質(zhì)到弱過(guò)鋁質(zhì)巖漿中, 磷灰石的溶解度很低, 并在巖漿分異過(guò)程中隨 SiO2的增加而降低; 而在強(qiáng)過(guò)鋁質(zhì)巖漿中, 磷灰石溶解度變化趨勢(shì)與此相反(Wolf and London, 1994)。此外, 由于沉積巖因地表風(fēng)化而使得Ca含量降低, 這也會(huì)抑制磷灰石的結(jié)晶, 所以有研究者認(rèn)為隨結(jié)晶分異作用進(jìn)行, P2O5-SiO2相關(guān)關(guān)系依舊是判別I型和S型花崗巖的重要指標(biāo)(陶繼華等, 2013)。Panti花崗巖顯示弱?強(qiáng)過(guò)鋁質(zhì)的特征, 其P2O5隨著SiO2含量增加而遞增(圖7c), 該特征與S型花崗巖的演化趨勢(shì)相類似。結(jié)合其高分異的特征, 認(rèn)為Panti花崗巖為高分異的S型花崗巖。

在稀土元素配分圖中(圖6a), Panti花崗巖樣品顯示明顯的Eu負(fù)異常, Rb/Ba值與Eu/Eu*呈明顯的負(fù)相關(guān)關(guān)系(圖8a), 指示其形成過(guò)程中存在斜長(zhǎng)石的分離結(jié)晶作用。Panti花崗巖具有明顯Ba和Eu的虧損(圖6), 結(jié)合Ba-Sr和Rb-Sr圖解(圖8b、c), 顯示該套花崗巖巖漿在演化過(guò)程中存在斜長(zhǎng)石和鉀長(zhǎng)石的分離結(jié)晶作用。此外, 微量元素蛛網(wǎng)圖中樣品具有明顯P和Ti負(fù)異常(圖6b), 這可能與磷灰石及含鈦礦物的分離結(jié)晶有關(guān)。

一般認(rèn)為強(qiáng)過(guò)鋁質(zhì)花崗巖主要由變沉積巖部分熔融形成(Koester et al., 2002; Jiang et al., 2011)。Panti花崗巖樣品具有低Sr(2.15×10?6～7.17×10?6)、Ba(8.05×10?6～14.1×10?6)和高Rb(259×10?6～432×10?6)含量的特征, 表明其源區(qū)可能為富云母的變沉積巖(Zen, 1986)。Zeng et al. (2011)認(rèn)為變泥質(zhì)巖的云母脫水熔融可以產(chǎn)生具有低Sr、Ba和高Rb/Sr值的花崗質(zhì)巖漿(Zeng et al., 2011)。變泥質(zhì)巖派生的花崗巖熔體比變砂屑巖派生的花崗巖熔體具有更低的CaO/Na2O值(<0.3; Sylvester, 1998; Jung and Pf?nder, 2007)。且樣品CaO/Na2O值很低(0.08～0.13), 且具高的Al2O3/TiO2和Al2O3/(MgO+FeOt)值特征(圖9), 與變泥質(zhì)巖派生的熔體一致。由于泥質(zhì)巖白云母部分熔融程度較低, 在蒸氣相條件下白云母部分熔融形成的熔體具有Rb和Sr富集的特征; 無(wú)蒸氣相條件下白云母部分熔融程度更低, 且源區(qū)會(huì)有大量長(zhǎng)石殘留, 這與Panti花崗巖樣品Rb/Ba值與Eu/Eu*呈明顯負(fù)相關(guān)的特征不符, 所以推測(cè)Panti花崗巖不是泥質(zhì)巖的低程度部分熔融形成。在蒸氣相條件下, 泥質(zhì)巖黑云母部分熔融, 需要高熔點(diǎn)且部分熔融程度很高(F>0.6); 無(wú)蒸氣相條件下, 泥質(zhì)巖中白云母和黑云母均發(fā)生部分熔融時(shí), 部分熔融程度增大, 達(dá)=0.28, 形成Rb富集, Sr和Ba含量降低的熔體(Harris and Ingcr, 1991)。且隨著黑云母的熔融, 熔體的Rb含量更加富集, 這與本次研究樣品富集Rb、虧損Sr和Ba特征一致。另外該花崗巖鋯石Hf()為?9.1～?2.6, 類似滇西東盈江和南緬甸地區(qū)的S型花崗巖特征(圖4c), 說(shuō)明其源區(qū)主要為變沉積物, 且幔源物質(zhì)的參與不明顯。綜合以上分析, Panti花崗巖是由富泥質(zhì)的變沉積巖較高程度的部分熔融后, 經(jīng)斜長(zhǎng)石和鉀長(zhǎng)石的分離結(jié)晶作用而形成的。

4.2 區(qū)域巖漿對(duì)比及構(gòu)造背景探討

中生代, 新特提斯洋的俯沖及隨后印度板塊與歐亞板塊碰撞, 在藏南、滇西、緬甸等地區(qū)形成了巨型的特提斯構(gòu)造巖漿巖帶, 該巖漿巖帶上廣泛分布有始新世花崗巖。特提斯巖漿帶被認(rèn)為可以從藏南拉薩地塊(Ji et al., 2012; Lee et al., 2012)延伸至藏東南波密?察隅地區(qū)(Lin et al., 2012), 經(jīng)滇西騰梁?盈江地區(qū)(楊啟軍等, 2009; Ma et al., 2014), 向南延伸至緬甸的抹谷變質(zhì)帶(Bertrand et al., 1999, 2001; Barley et al., 2003; Mitchell et al., 2007; Searle et al., 2007; Mitchell et al., 2012)。前人研究認(rèn)為白堊紀(jì)?早始新世(120～50 Ma), 自巴基斯坦、印度、中國(guó)西藏和尼泊爾的喜馬拉雅巖基經(jīng)緬甸至蘇門答臘島發(fā)育有近200 km寬、與俯沖相關(guān)的大型花崗巖巖漿帶(Mitchell, 1993; Barley et al., 2003)。最近Zhang et al. (2019)通過(guò)碎屑鋯石年齡對(duì)比認(rèn)為從藏南至蘇門答臘島存在約6000 km的新特提斯弧系統(tǒng), 其中蘇門答臘島發(fā)育有～52 Ma的弧巖漿期次, 為新特提斯洋俯沖的產(chǎn)物。此外Katili (1973)通過(guò)K/Ar定年發(fā)現(xiàn)Panti東部及其西邊的Sontang地區(qū)分別出露42.7 Ma和47.7 Ma花崗閃長(zhǎng)巖, 認(rèn)為該區(qū)也存在始新世巖漿事件。結(jié)合區(qū)域已報(bào)道的與新特提斯洋有關(guān)的巖漿巖年齡數(shù)據(jù)(圖10), 我們認(rèn)為蘇門答臘島中部Panti花崗巖可以與藏南岡底斯、滇西和緬甸的火成巖年齡進(jìn)行對(duì)比, 證明特提斯巖漿帶從藏南經(jīng)云南西部及緬甸, 向南可以延伸至蘇門答臘島內(nèi)。

圖7 Panti花崗巖(K2O+Na2O)/CaO-(Zr+Nb+Ce+Y)(a)、Zr-10000×Ga/Al(b)和P2O5-SiO2(c)判別圖解(圖a、b據(jù)Whalen et al., 1987)

圖8 Panti花崗巖Eu/Eu*-Rb/Ba(a)、Ba-Sr(b)和Rb-Sr(c)圖解

圖9 Panti花崗巖CaO/Na2O-Al2O3/TiO2(a)和Al2O3/(MgO+FeOt)-CaO/(MgO+FeOt)(b)巖漿源區(qū)來(lái)源判別圖(a和b底圖據(jù)Ma et al., 2014)

對(duì)于印度板塊與歐亞板塊碰撞的時(shí)間從晚白堊世(～70 Ma; Yin and Harrison, 2000; ～65 Ma; Ding et al., 2005)到晚漸新世(～34 Ma; Aitchison et al., 2007)尚未有定論, 但目前主流觀點(diǎn)認(rèn)為印度板塊與歐亞板塊主碰撞時(shí)間約為50～55 Ma(Klootwijk et al., 1992; 朱弟成等, 2004; Leech et al., 2005; Garzanti, 2008; Wu et al., 2008; Najman et al., 2010; Wang et al., 2011; Zhu et al., 2015)。在藏南拉薩地塊包括岡底斯巖基和林子宗群上分布41～65 Ma I型花崗巖, 被認(rèn)為是下地殼底侵重熔的產(chǎn)物, 并有幔源物質(zhì)的貢獻(xiàn)(Ji et al., 2009, 2012; 李洪梁等, 2019), 即該區(qū)存在巖漿底侵和巖漿混合作用(黃玉等, 2010; 阮冰等, 2019)。并且藏南地區(qū)至少?gòu)?1 Ma起, 特提斯洋消減后, 發(fā)生新生玄武巖底侵地殼加厚作用, 并隨著俯沖作用的進(jìn)行, 從活動(dòng)大陸邊緣向陸內(nèi)地殼物質(zhì)貢獻(xiàn)逐漸增多(Ji et al., 2012)。在滇西地區(qū), Wang et al. (2014)和Ma et al. (2014)則認(rèn)為約50～55 Ma是新特提斯洋俯沖向印度板塊與歐亞板塊陸陸碰撞的轉(zhuǎn)換時(shí)間, 并隨著遠(yuǎn)離俯沖帶, 從西向東幔源物質(zhì)貢獻(xiàn)越來(lái)越少, 西部表現(xiàn)為有新生地殼參與的I型花崗巖為主(Ma et al., 2014); 而東部騰梁及東盈江地區(qū)則以殼源物質(zhì)部分熔融具負(fù)Hf()值的S型花崗巖為主; 而在緬甸地區(qū), 西側(cè)靠近俯沖帶的西緬甸始新世花崗巖是具有正Hf()值(1.3～16.5)的I型花崗巖, 被認(rèn)為是新生地殼物質(zhì)和古老俯沖沉積物混合的弧巖漿巖(Zhang et al., 2017)。東部南緬甸包括抹谷變質(zhì)帶, 始新世花崗巖是以負(fù)Hf()和Nd()為特征的S型花崗巖為主(Zaw, 1990; Mitchell et al., 2012; Jiang et al., 2017), 被認(rèn)為是由于新特提斯洋俯沖, 古老的滇緬?cǎi)R蘇地殼衍生而來(lái)(Li et al., 2019)。

一直以來(lái)對(duì)于地處在特提斯構(gòu)造域的蘇門答臘島的研究相對(duì)較薄弱。最近Advokaat et al. (2018)通過(guò)古地磁數(shù)據(jù)認(rèn)為, 特提斯洋閉合記錄可能保存在Woyla地體向北延伸的西緬甸(Liu et al., 2016)和安得曼群島的蛇綠巖中(Sarma et al., 2010); 周蒂等(2003)通過(guò)區(qū)域地層對(duì)比, 認(rèn)為Woyla地體是藏南拉薩地塊班公?怒江特提斯帶向南的延伸; Zhang et al., (2019)通過(guò)對(duì)蘇門答臘島碎屑鋯石年齡譜系及Hf同位素區(qū)域?qū)Ρ? 認(rèn)為該區(qū)始新世巖漿巖是藏南岡底斯巖基的南延, 均為新特提斯洋俯沖的產(chǎn)物, 并源于新生地殼。本次研究表明, Panti始新世花崗巖是由富泥質(zhì)變沉積巖的部分熔融而形成的, 與其北延的滇西騰梁(趙少偉等, 2017)、盈江東部(Xu et al., 2012)及南緬甸(Li et al., 2019)地區(qū)同時(shí)期與俯沖相關(guān)的S型花崗巖相似。此外, 該花崗巖與新特提斯構(gòu)造演化有關(guān)的東盈江和南緬甸始新世花崗巖、蘇門答臘新特提斯弧系統(tǒng)的始新世巖漿鋯石的Hf同位素組成相似(圖4c; Xu et al., 2012; Li et al., 2019; Zhang et al., 2019)。Khan et al. (2017)認(rèn)為自晚中生代開(kāi)始, 蘇門答臘沿巺他海溝一直受新特提斯洋的俯沖作用, 并在晚始新世至早漸新世期間, 發(fā)育相應(yīng)的弧后擴(kuò)張盆地(Barber et al., 2005)。McCourt et al. (1996)認(rèn)為60～50 Ma, 蘇門答臘大陸邊緣存在一個(gè)島弧體系, 并形成廣布的火山弧型花崗巖, 其中57～52 Ma I型和S型花崗巖可延伸至緬甸境內(nèi)(Cobbing et al., 1992; Mitchell, 1993)。此外, 在Rb-Hf-Ta和Ta-Yb的構(gòu)造判別圖解中, Panti花崗巖樣品和新特提斯巖漿帶上的騰梁、盈江和緬甸地區(qū)同時(shí)期的花崗巖均落在火山弧花崗巖中(圖11; Ma et al., 2014; 趙少偉等, 2017; Li et al., 2019)。因此, 我們認(rèn)為Panti花崗巖應(yīng)該是俯沖構(gòu)造背景下形成的。綜合研究表明Panti花崗巖與騰梁和東盈江花崗巖特征相似,為新特提斯洋俯沖過(guò)程中的產(chǎn)物, 證明新特提斯構(gòu)造巖漿帶可從藏南經(jīng)滇西南延至蘇門答臘中部。結(jié)合該區(qū)新特提斯洋俯沖形成的巖漿期次(Zhang et al., 2019), 表明～55 Ma蘇門答臘島地區(qū)新特提斯洋尚未關(guān)閉。

數(shù)據(jù)來(lái)源: 藏南: (1) Wen et al. (2008); (2) Ji et al. (2009); 滇西: (3) Liang et al. (2008); (4) Xu et al. (2012); (5) Ma et al. (2014); (6) Wang et al. (2014); 緬甸: (7) Li et al. (2019); (8) Crow et al. (2017); 蘇門答臘: (9) Cobbing (2005); (10) McCourt et al. (1996); 紅色為本文樣品。

圖11 Panti花崗巖構(gòu)造背景判別圖解(a據(jù)Harris et al., 1986; b據(jù)Pearce et al., 1984)

5 結(jié) 論

(1) 蘇門答臘島中部Panti地區(qū)識(shí)別出了早始新世花崗巖, 其鋯石U-Pb年齡為55.5±0.3 Ma(2σ, MSWD=0.87,=30)。

(2) Panti花崗巖為過(guò)鋁質(zhì)的高分異S型花崗巖, 其Hf()為?9.1～?2.6, 二階模式年齡為1.29～1.70 Ga, 地球化學(xué)研究表明該套花崗巖為富泥質(zhì)變沉積巖部分熔融的產(chǎn)物。

(3) Panti花崗巖可以與藏南岡底斯、滇西和緬甸地區(qū)的同期花崗巖對(duì)比和聯(lián)系, 證明了新特提斯洋構(gòu)造巖漿帶可以向南延伸至蘇門答臘島Panti地區(qū)。

致謝:野外樣品采集過(guò)程中得到了廣州海洋地質(zhì)調(diào)查局張立敏博士的幫助, 實(shí)驗(yàn)分析中得到了中山大學(xué)甘成勢(shì)、王玉琨和楊雪博士的幫助, 在此一并致以衷心的感謝。感謝中國(guó)地質(zhì)大學(xué)(武漢)徐亞軍教授和另一位匿名審稿專家對(duì)該論文提出寶貴的修改意見(jiàn)。

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Geochemical Characteristics of the Early Eocene S-type Granites in Panti, Central Sumatra and its Neotethyan Tectonic Implications

YU Xiaoqing1, 2, QIAN Xin1, 2*, ZHANG Yuzhi1, 2, XU Chang1, 2and WANG Yuejun1, 2

(1. Guangdong Key Lab of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, China; 2. Southern Marine Science and Engineering Guangdong Laboratory, Zhuhai 519082, Guangdong, China)

The Sumatra Island, which is located in the tectonic junction of the Tethyan domain and the Indian Ocean subduction zone, has widespread outcrops of Meso-Cenozoic igneous rocks. These igneous rocks are important to constrain the extension of the Tethys tectonic belt in Sumatra, but remain poorly studied. In this paper, we present a set of petrographic, zircon U-Pb geochronological, zirconHf isotopic, and whole-rock geochemical data for the granites from the Panti area in the central Sumatra. LA-ICP-MS zircon U-Pb dating showed that the Panti granites were crystallized at 55.5±0.3 Ma. The Panti granites have high SiO2(76.61%–78.37%), high alkali (Na2O+K2O=7.47%–8.21%), differentiation index (DI) (95–96), and A/CNK (1.08–1.12) values, indicative of highly fractionated S-type granite. The granites are characterized by significant negative Eu anomalies (Eu/Eu*=0.04–0.05) and depletion of Nb, Ta, Sr, P, and Ti. The high Rb/Sr and low CaO/Na2O ratios, along with the negativeHf() values (?9.1 to ?2.6) and oldDM2ages (1.29–1.70 Ga) indicate that the granites were derived from partial melting of ancient pelitic-rich metasediments. The early Eocene granites in the Panti area have geochemical features similar to those of the Tengliang and Yingjiang areas in Western Yunnan. Therefore, it is suggested that the Neotethyan belt southerly extend to the central Sumatra.

central Sumatra; early Eocene; highly fractionated S-type granite; zircon U-Pb dating; geochemistry; Neotethys

2020-01-16;

2020-05-04

國(guó)家自然科學(xué)基金項(xiàng)目(41830211、U1701641、42072256)、國(guó)家重點(diǎn)研發(fā)計(jì)劃項(xiàng)目(2016YFC0600303)和廣東省基礎(chǔ)與應(yīng)用基礎(chǔ)研究基金項(xiàng)目(2018B030312007、2019B1515120019)聯(lián)合資助。

余小清(1994–), 女, 碩士研究生, 地球化學(xué)專業(yè)。Email: yuxq9@mail2.sysu.edu.cn

錢鑫(1988–), 男, 副教授, 從事東南亞大地構(gòu)造研究。Email: qianx3@mail.sysu.edu.cn

P597

A

1001-1552(2021)03-0570-016

10.16539/j.ddgzyckx.2021.03.007