西藏班公湖-怒江縫合帶中段侏羅紀(jì)高鎂安山質(zhì)巖石對(duì)中特提斯洋演化的制約*

唐躍 翟慶國(guó) 胡培遠(yuǎn) 肖序常 王海濤 王偉 朱志才 吳昊

自然資源部深地動(dòng)力學(xué)重點(diǎn)實(shí)驗(yàn)室，中國(guó)地質(zhì)科學(xué)院地質(zhì)研究所，北京 100037

班公湖-怒江縫合帶在構(gòu)造上夾持于拉薩和南羌塘地塊之間，是中特提斯洋(或新特提斯洋北支)消減閉合的遺跡，它對(duì)研究中特提斯洋構(gòu)造演化和青藏高原中生代陸塊聚合過(guò)程具有重要的地質(zhì)意義(Metcalfe, 1996, 2013; Yin and Harrison, 2000; Panetal., 2012; Zhuetal., 2013)。通常而言，班公湖-怒江縫合帶可進(jìn)一步劃分為西段(班公湖-改則)、中段(班戈-那曲)和東段(丁青-怒江)。班公湖-怒江縫合帶中段的構(gòu)造演化最為復(fù)雜，由自北向南呈面狀分布的三條分支縫合帶組成(東巧-安多、北拉-拉弄和永珠-納木錯(cuò))，分布范圍南北寬度超過(guò)200km，是研究該縫合帶最為關(guān)鍵的區(qū)域(圖1b)。

島弧巖漿活動(dòng)是大洋板塊俯沖消減作用的最直接記錄，對(duì)反演古大洋俯沖及其動(dòng)力學(xué)過(guò)程具有至關(guān)重要的作用(Stern, 2002; Duceaetal., 2015)。已有研究顯示，在班公湖-怒江縫合帶西段的羌塘南緣地區(qū)發(fā)育一條侏羅-白堊紀(jì)的島弧型巖漿巖帶(170～110Ma)，它是該地區(qū)中特提斯洋洋盆北向俯沖的結(jié)果(Kappetal., 2005; Lietal., 2014, 2018a, b; Zhuetal., 2016; Haoetal., 2018)。然而在班公湖-怒江縫合帶的其它地區(qū)，尤其是中段一帶的侏羅紀(jì)島弧巖漿作用仍缺乏系統(tǒng)研究(李小波等, 2015; Zengetal., 2016b)，對(duì)其起源和成因仍不清楚。此外，前人對(duì)班公湖-怒江縫合帶侏羅紀(jì)島弧型巖漿巖的研究多集中于西段地區(qū)，對(duì)于中段地區(qū)的侏羅紀(jì)巖漿巖及其與各分支縫合帶之間的關(guān)系以及對(duì)應(yīng)的俯沖消減動(dòng)力學(xué)過(guò)程尚缺乏有效約束。

本文對(duì)班公湖-怒江縫合帶中段新近厘定的中晚侏羅世高鎂安山巖和閃長(zhǎng)巖開(kāi)展了系統(tǒng)的野外、巖石地球化學(xué)和鋯石U-Pb年代學(xué)研究，在此基礎(chǔ)上探討其成因及其對(duì)班公湖-怒江中特提斯洋洋盆俯沖消減動(dòng)力學(xué)過(guò)程的約束，為進(jìn)一步厘清中特提斯洋洋盆的演化提供關(guān)鍵證據(jù)。

1 區(qū)域地質(zhì)概況

班公湖-怒江縫合帶橫貫于青藏高原中部，東西向延伸近2000km(圖1a)。沿縫合帶斷續(xù)分布了侏羅紀(jì)蛇綠巖、俯沖增生雜巖、侏羅-白堊紀(jì)中酸性巖漿巖和洋島型巖石，它們保留了較為完整的中特提斯洋從擴(kuò)張到俯沖消減、直至閉合的地質(zhì)記錄。前人的研究結(jié)果表明，班公湖-怒江縫合帶蛇綠巖主要形成于侏羅紀(jì)，且以中侏羅世為主(Wangetal., 2016)。這些蛇綠巖大都顯示出SSZ型蛇綠巖的特征，形成于大洋俯沖過(guò)程(Wangetal., 2016)，并在晚侏羅世到早白堊世構(gòu)造侵位于大陸邊緣之上。侏羅紀(jì)巖漿巖主要分布在班公湖-怒江縫合帶內(nèi)及以北的南羌塘地塊南緣，又以縫合帶西段最為發(fā)育。這些巖漿巖顯示出典型島弧巖漿巖特征，是班公湖-怒江洋北向俯沖的直接地質(zhì)記錄。白堊紀(jì)巖漿巖沿著縫合帶及兩側(cè)地塊邊緣廣泛發(fā)育，通常被認(rèn)為與班公湖-怒江洋的消減閉合及伴隨的板片斷離、地殼拆沉作用有關(guān)(Zhuetal., 2011, 2016; Huetal., 2017)。

班公湖-怒江縫合帶中段，也稱(chēng)藏北湖區(qū)，是整條縫合帶內(nèi)最寬廣，也是蛇綠巖出露范圍最廣的地區(qū)(王希斌等, 1987)。自北向南，中段地區(qū)的蛇綠巖可劃分為三條亞帶，分別為東巧-安多、北拉-拉弄和永珠-納木錯(cuò)蛇綠巖亞帶，它們代表了班公湖-怒江洋中段不同分支洋盆或小洋盆閉合的遺跡(圖1b)。已有研究資料顯示，東巧和安多蛇綠巖形成于早侏羅世(188～181Ma)的弧前或弧后環(huán)境(Liuetal., 2016; Wangetal., 2016)。變質(zhì)底板角閃石K-Ar和40Ar/39Ar年齡約為175～179Ma，暗示這些蛇綠巖形成不久即發(fā)生仰沖就位(王希斌等, 1987; Zhouetal., 1997)。北拉-拉弄和永珠-納木錯(cuò)蛇綠巖主要形成于中-晚侏羅世(172～148Ma)，是班公湖-怒江洋南側(cè)分支洋盆演化的產(chǎn)物(Zhongetal., 2015, 2017; Tangetal., 2018a; Zengetal., 2018)。安多微陸塊位于班公湖-怒江縫合帶內(nèi)，夾持于東巧-安多和北拉-拉弄蛇綠巖亞帶之間，是縫合帶內(nèi)部一獨(dú)立的微陸塊，主要由前寒武變質(zhì)基底、侏羅-白堊紀(jì)花崗巖和中新生代沉積蓋層組成(Guynnetal., 2006, 2013; Zhangetal., 2014)。在安多微陸塊中北部發(fā)育一套高壓麻粒巖，峰期變質(zhì)時(shí)代大致為180～175Ma，折返冷卻年齡大致為～165Ma，并在早白堊世完全剝露地表(Guynnetal., 2006; Zhangetal., 2014)。這套高壓麻粒巖記錄了東巧-安多北側(cè)分支洋盆關(guān)閉后，安多微陸塊向南羌塘地塊發(fā)生大陸俯沖并折返的全過(guò)程(Guynnetal., 2006)。

圖1 青藏高原大地構(gòu)造簡(jiǎn)圖(a,據(jù)Zhai et al., 2016修改)、班公湖-怒江縫合帶中段地質(zhì)簡(jiǎn)圖(b)和佳瓊地區(qū)地質(zhì)簡(jiǎn)圖(c) KSZ-昆侖縫合帶；JSZ-金沙江縫合帶：LSSZ-龍木錯(cuò)-雙湖縫合帶；BNSZ-班公湖-怒江縫合帶；IYZSZ-印度-雅魯藏布江縫合帶Fig.1 Tectonic framework of the Tibet Plateau (a, modified after Zhai et al., 2016), simplified geological map of the middle Bangong-Nujiang suture zone (b) and geological sketch map of the Jiaqiong area (c)

本次研究區(qū)地處藏北班戈縣佳瓊鎮(zhèn)(圖1c)，大地構(gòu)造位置位于東巧-安多和北拉-拉弄蛇綠巖亞帶之間。區(qū)內(nèi)出露地層主要包括中上泥盆統(tǒng)查果羅瑪組(D2-3c)、下二疊統(tǒng)下拉組(P1x)、上三疊統(tǒng)確哈拉群(T3Q)、上三疊-下侏羅統(tǒng)木嘎崗日群(MQ)、中上侏羅統(tǒng)接奴群(J2-3j)和下白堊統(tǒng)去申拉組(K1q)。古生代地層主要由灰?guī)r組成，并夾厚度不等的砂巖，它們與區(qū)內(nèi)其他地層均呈斷層接觸。確哈拉群主要為一套淺變質(zhì)巖系，以板巖、片巖為主，與中生代地層呈斷層接觸或被侏羅紀(jì)火山巖不整合覆蓋。木嘎崗日群為一套復(fù)理石沉積，以硅質(zhì)巖、砂巖、泥頁(yè)巖為主，并夾有灰?guī)r團(tuán)塊。接奴群主要由板巖、砂巖和泥頁(yè)巖組成，并含有玄武質(zhì)和安山質(zhì)火山巖夾層。一般認(rèn)為，木嘎崗日群和接奴群代表了一套弧前俯沖增生雜巖，與中特提斯洋北向俯沖消減有關(guān)，并在洋盆消減閉合過(guò)程中發(fā)生不同程度的剪切破碎(Zengetal., 2016a; Maetal., 2018)。蛇綠巖主要分布在達(dá)如錯(cuò)東部地區(qū)，以橄欖巖和堆晶雜巖為主，并見(jiàn)有少量玄武巖，它們主要構(gòu)造就位于木嘎崗日群之中，并被后期早白堊世花崗巖侵入(唐躍等, 2015)。去申拉組主要為一套中酸性火山巖、火山角礫巖、礫巖和砂巖等，不整合覆蓋于接奴群之上。

本項(xiàng)研究涉及的侏羅紀(jì)巖漿巖以安山巖為主，并有少量閃長(zhǎng)巖呈脈體侵入于蛇綠巖的橄欖巖之中。安山巖主要出露于佳瓊鎮(zhèn)周邊和達(dá)如錯(cuò)東岸，佳瓊鎮(zhèn)西側(cè)安山巖呈角度不整合覆蓋在確哈拉群之上，而南側(cè)以斷層與接奴群接觸。達(dá)如錯(cuò)東地區(qū)安山巖主要與蛇綠巖和木嘎崗日群呈斷層接觸(圖1c)，并在局部呈不規(guī)則脈體侵入到木嘎崗日群增生雜巖之中，或呈不規(guī)則構(gòu)造巖塊混雜產(chǎn)出于木嘎崗日巖群(圖2a)。閃長(zhǎng)巖主要呈巖脈產(chǎn)出，脈寬一般在20～100cm，長(zhǎng)5～8m。野外和鏡下觀(guān)察顯示，安山巖和閃長(zhǎng)巖均經(jīng)歷了不同程度的后期蝕變(綠泥石化和絹云母化)(圖2)。安山巖呈深灰色-灰綠色，斑狀結(jié)構(gòu)，斑晶主要為角閃石(2%～8%)、輝石(1%～5%)和斜長(zhǎng)石(3%～10%)，基質(zhì)主要由斜長(zhǎng)石微晶和隱晶質(zhì)組成(圖2b, d)。閃長(zhǎng)巖比安山巖經(jīng)歷了更強(qiáng)的蝕變，主要礦物組成為斜長(zhǎng)石(50%～55%)、角閃石(30%～40%)和石英(2%～8%)，并可見(jiàn)少量輝石、黑云母(<5%)(圖2f)。斜長(zhǎng)石發(fā)生不同程度絹云母化，部分角閃石發(fā)生綠泥石化。

圖2 西藏班戈佳瓊地區(qū)高鎂安山巖和閃長(zhǎng)巖野外和顯微照片 Amp-角閃石;Pl-斜長(zhǎng)石;Qz-石英Fig.2 Field photographs and microphotographs of the Jiaqiong high-Mg andesite and diorite in the Bange County, Tibet

2 分析方法

2.1 鋯石U-Pb分析

鋯石分選在河北省區(qū)域地質(zhì)調(diào)查研究所完成，采用傳統(tǒng)的重液和磁選方法分選。鋯石透、反射光照片和陰極發(fā)光圖像拍攝在中國(guó)地質(zhì)科學(xué)院地質(zhì)研究所完成，基于此進(jìn)一步觀(guān)察鋯石內(nèi)部結(jié)構(gòu)。鋯石U-Pb定年在北京科薈測(cè)試技術(shù)有限公司完成，利用LA-ICP-MS系統(tǒng)對(duì)鋯石開(kāi)展U-Th-Pb同位素分析。激光剝蝕系統(tǒng)為ESI NWR 193nm，ICP-MS為Analytikjena PlasmaQuantmS Elite ICP-MS。具體的分析過(guò)程參見(jiàn)侯可軍等(2009)。分析過(guò)程中采用氦氣作載氣，氬氣為補(bǔ)償氣以調(diào)節(jié)靈敏度，分析束斑為25μm，激光的能量為6.25J/cm2，脈沖頻率為8Hz。U-Pb同位素定年中采用鋯石標(biāo)準(zhǔn)GJ-1作外標(biāo)進(jìn)行同位素分餾校正，每分析5～10個(gè)樣品點(diǎn)，分析2次GJ-1。對(duì)于與分析時(shí)間有關(guān)的U-Th-Pb同位素比值漂移、元素含量校正及U-Th-Pb同位素比值和年齡計(jì)算，均采用軟件ICPMSDataCal 8.0(Liuetal., 2010)完成。鋯石樣品的U-Pb年齡諧和圖繪制和加權(quán)平均年齡計(jì)算采用Isoplot 3.0完成(Ludwig, 2003)。

圖3 西藏班戈佳瓊高鎂安山巖和閃長(zhǎng)巖鋯石U-Pb年齡諧和圖和代表性鋯石陰極發(fā)光(CL)圖像Fig.3 Zircon U-Pb concordia diagrams and representative CL images of the Jiaqiong high-Mg andesite and diorite in the Bange County, Tibet

2.2 鋯石Lu-Hf同位素分析

鋯石Lu-Hf同位素測(cè)試在中國(guó)科學(xué)院地質(zhì)與地球物理研究所多接收等離子體質(zhì)譜實(shí)驗(yàn)室完成，測(cè)試儀器為T(mén)hermo Fisher公司生產(chǎn)的Neptune Plus多接收電感耦合等離子體質(zhì)譜儀與193nm ArF準(zhǔn)分子激光剝蝕系統(tǒng)聯(lián)用構(gòu)成的LA-MC-ICP-MS。鋯石Lu-Hf同位素分析點(diǎn)位與U-Pb測(cè)年點(diǎn)位重合。詳細(xì)的分析過(guò)程見(jiàn)Wuetal. (2006) 和Xieetal. (2008)。實(shí)驗(yàn)過(guò)程中采用的束斑直徑約為44μm，剝蝕能量為10J/cm2，剝蝕時(shí)間為26s，脈沖頻率為8Hz。標(biāo)準(zhǔn)鋯石Mud Tank和GJ-1作為雙重外標(biāo)，監(jiān)測(cè)儀器穩(wěn)定程度。本次分析過(guò)程中獲得的Mud Tank和GJ-1標(biāo)樣測(cè)試值分別為0.282501±5(2SD,n=10)和0.281997±8(2SD,n=19)，這與推薦值在誤差范圍內(nèi)一致(Moreletal., 2008; Woodhead and Hergt, 2005)。

2.3 全巖主量和微量元素分析

全巖主量和微量元素分析在中國(guó)地質(zhì)科學(xué)院國(guó)家地質(zhì)實(shí)驗(yàn)測(cè)試中心完成。主量元素測(cè)定采用熔片X熒光光譜法(XRF)，使用的儀器為日本島津(SHIMADZU)公司生產(chǎn)的MXF-2100 型X射線(xiàn)熒光光譜儀。樣品燒失量(LOI)的測(cè)定采用馬弗爐加熱燒失法。稱(chēng)取0.5g預(yù)先干燥的粉末樣品置于坩堝中，然后放入馬弗爐加熱至1000℃并恒溫90min，最后將樣品冷卻，稱(chēng)取燒失后的重量。燒失前后質(zhì)量差與樣品重量百分比即為燒失量。主量元素的分析精度優(yōu)于5%。

全巖微量元素分析利用PE300D等離子體質(zhì)譜儀(ICP-MS)完成，執(zhí)行標(biāo)準(zhǔn)為DZ/T0223-2001。分析過(guò)程首先將樣品粉末烘干，準(zhǔn)確稱(chēng)取約50mg加入HF和HNO3溶解，然后再密封到高壓釜中，轉(zhuǎn)移至烘箱內(nèi)加熱48h，然后開(kāi)蓋蒸干。蒸干后的樣品加入6N HNO3再次密封到高壓釜中，轉(zhuǎn)移至烘箱內(nèi)加熱烘干。冷卻后，用3N HNO3充分溶解樣品，再用MQ水將樣品稀釋2000倍，搖勻后送至ICP-MS測(cè)定微量元素。分析檢測(cè)下限為(0.n～n)×10-9，分析誤差一般小于5%。

表1西藏班戈佳瓊高鎂安山巖和閃長(zhǎng)巖鋯石U-Pb定年結(jié)果

Table 1 Zircon U-Pb age data of the Jiaqiong high-Mg andesite and diorite in the Bange County, Tibet

測(cè)點(diǎn)號(hào)Pb?ThU(×10-6)Th/U同位素比值年齡(Ma)207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ17T046-安山巖-013615439821.570.0494560.0015740.1735230.0056130.0253820.000269168.671.3162.54.9161.61.7-02422559330.270.0505220.0014840.2909500.0085940.0416860.000401220.468.5259.36.8263.32.5-03288748561.020.0488950.0018750.1704500.0063720.0252740.000256142.790.7159.85.5160.91.6-04511804340.410.0605170.0015170.8759860.0216780.1050520.000923620.453.7638.811.7643.95.4-05361724600.370.0560840.0014180.5492120.0136340.0708480.000585457.555.6444.58.9441.33.5-06206026270.960.0495280.0021140.1726880.0074110.0253100.000482172.3127.8161.76.4161.13.0-07441525870.260.0566260.0023030.5764110.0222670.0735180.001053476.090.7462.214.3457.36.3-08206286350.990.0496710.0028160.1683860.0069540.0250450.000410189.0131.5158.06.0159.52.6-09215386930.780.0497300.0024510.1742180.0101950.0251450.000725189.0114.8163.18.8160.14.6-1060110420430.540.0502300.0023070.1755930.0081020.0252430.000504205.6107.4164.37.0160.73.2-11174145550.750.0503530.0025400.1778490.0095630.0254220.000419213.0112.0166.28.2161.82.6-1261265515431.720.0501940.0016750.1776020.0082730.0253850.000793211.277.8166.07.1161.65.017T056-安山巖-0194242421.760.0500940.0035440.1789150.0121440.0261250.000452198.2166.6167.110.5166.32.8-02104242951.440.0492000.0031740.1787430.0103070.0259520.000387166.8135.2167.08.9165.22.4-03154934431.110.0513200.0022640.1833250.0082370.0258160.000335253.8101.8170.97.1164.32.1-04103142931.070.0507630.0023710.1831230.0086730.0261470.000348231.6113.9170.77.4166.42.2-05167124141.720.0502130.0022180.1814180.0080170.0260200.000336205.6103.7169.36.9165.62.1-06103232941.100.0504280.0038840.1803530.0139940.0257630.000557213.0175.0168.412.0164.03.5-07199034651.940.0483670.0044820.1757860.0169530.0261080.000423116.8207.4164.414.6166.12.7-08103122881.080.0497310.0024030.1776950.0081950.0259390.000299189.0117.6166.17.1165.11.9-09114492831.590.0494260.0024720.1764500.0088030.0257580.000370168.6112.0165.07.6163.92.3-10156274231.480.0505620.0026050.1809920.0095650.0257840.000427220.4123.1168.98.2164.12.7-1193092591.200.0489560.0033430.1733850.0108180.0257860.000399146.4151.8162.49.4164.12.5-12114753031.570.0507450.0049990.1784170.0176020.0258080.000512227.8214.8166.715.2164.33.2-13207535531.360.0504390.0019120.1804680.0069380.0258700.000420216.788.9168.56.0164.62.6-1493412431.400.0494760.0037330.1750440.0126420.0258760.000698172.3166.6163.810.9164.74.4-15144524331.040.0490530.0027010.1752140.0095380.0258480.000345150.1129.6163.98.2164.52.2-16166904391.570.0500490.0025040.1775370.0084900.0259380.000365198.2116.7165.97.3165.12.3-17196555481.200.0480840.0019910.1720120.0067970.0258780.000300101.9-100.9161.25.9164.71.9-18238246611.250.0502390.0020850.1809030.0078020.0258730.000361205.696.3168.86.7164.72.317T063-閃長(zhǎng)巖-0192252950.760.0500950.0025080.1746580.0079880.0255080.000380198.2121.3163.56.9162.42.4-0282182610.830.0498140.0036670.1722370.0116100.0255820.000523187.1-25.0161.410.1162.83.3-0382262560.880.0493820.0022280.1737080.0078360.0255500.000415164.9137.9162.66.8162.62.6-0481922400.800.0500790.0027200.1731250.0086220.0255000.000408198.2121.3162.17.5162.32.6

續(xù)表1

Continued Table 1

測(cè)點(diǎn)號(hào)Pb?ThU(×10-6)Th/U同位素比值年齡(Ma)207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ-0571622060.780.0487210.0029440.1731050.0106760.0257990.000391200.1133.3162.19.2164.22.5-0692232870.780.0480170.0021140.1685390.0072630.0255140.000367101.9109.2158.26.3162.42.3-07195985801.030.0518590.0021840.1828210.0077280.0255100.000344279.791.7170.56.6162.42.2-08133963961.000.0503040.0024360.1746540.0079120.0254100.000352209.3108.3163.46.8161.82.2-09124063461.170.0512320.0031740.1778440.0096100.0255510.000447250.1142.6166.28.3162.62.8-1092342950.790.0483100.0027130.1737600.0093430.0255960.000541122.3120.4162.78.1162.93.4-11102533040.830.0501940.0020160.1773720.0072120.0256150.000372211.292.6165.86.2163.02.3-1271912340.820.0498500.0025670.1746030.0082870.0254900.000390187.1118.5163.47.2162.32.5-13113223300.980.0482600.0022340.1703570.0075380.0256140.000476122.394.4159.76.5163.03.0-14163995120.780.0499890.0016970.1765760.0061430.0255970.000356194.575.0165.15.3162.92.2-1592442890.840.0501530.0022910.1768550.0082860.0255620.000375211.2102.8165.47.1162.72.4-16102453070.800.0500110.0030810.1761240.0109120.0255350.000417194.5137.9164.79.4162.52.6-1771762360.750.0491110.0027230.1719170.0093380.0254420.000368153.8131.5161.18.1162.02.3-1881902470.770.0484860.0025640.1706260.0087930.0256540.000343124.2118.5160.07.6163.32.2-19164404750.930.0485350.0025180.1713110.0094220.0253280.000302124.2118.5160.68.2161.21.9-20113363430.980.0504310.0026530.1759480.0086790.0255160.000378216.7122.2164.67.5162.42.4-2171712350.720.0503610.0031920.1769640.0113910.0255480.000395213.0148.1165.49.8162.62.5-22102583250.790.0501660.0036110.1734210.0115820.0254360.000572211.2163.9162.410.0161.93.617T081-安山巖-0182012530.800.0497960.0030640.1736530.0110290.0252120.000348187.1144.4162.69.5160.52.2-0292352690.870.0465810.0030630.1635980.0105230.0254570.00045027.9161.1153.89.2162.12.8-03165074591.110.0483810.0018320.1679320.0064010.0251760.000284116.886.1157.65.6160.31.8-04103093071.010.0504340.0022770.1749420.0076060.0253210.000296216.7105.5163.76.6161.21.9-0592922821.030.0505780.0022500.1746450.0083000.0250160.000404220.499.1163.47.2159.32.5-0651291600.800.0502560.0041700.1722070.0130580.0252050.000664205.6-6.5161.311.3160.54.2-0761901811.050.0517150.0051260.1787750.0168800.0253560.000625272.3229.6167.014.5161.43.9-08229265771.600.0484030.0016800.1686220.0055280.0253430.000347120.581.5158.24.8161.32.2-09102422980.810.0502070.0025600.1772610.0100330.0254630.000404211.2118.5165.78.7162.12.5-1061941791.090.0480480.0024910.1677900.0087630.0254380.000348101.9118.5157.57.6161.92.2-11133453760.920.0549990.0020270.1900300.0064110.0251740.000328413.086.1176.75.5160.32.1-12197505091.480.0491560.0023370.1719250.0084380.0253320.000353153.8111.1161.17.3161.32.2-13133843641.060.0492970.0021140.1724090.0073400.0254330.000334161.2100.0161.56.4161.92.1-1471792160.830.0495440.0034610.1713970.0125620.0249810.000438172.3-36.1160.610.9159.12.8-15102962811.050.0499750.0033460.1726910.0120120.0249960.000457194.5155.5161.810.4159.22.9-16187114671.520.0490420.0019520.1723870.0073630.0254080.000330150.192.6161.56.4161.72.1-17144744051.170.0511950.0017530.1773000.0055990.0252840.000288250.179.6165.74.8161.01.8-1892372630.900.0498490.0020630.1735380.0067400.0254210.000351187.196.3162.55.8161.82.2

3 測(cè)試結(jié)果

3.1 鋯石U-Pb年齡

本次研究對(duì)4件樣品(3件安山巖和1件閃長(zhǎng)巖)進(jìn)行了鋯石U-Pb定年，分析測(cè)試結(jié)果見(jiàn)圖3和表1。透反射和陰極發(fā)光圖像表明，所有鋯石均具有相似的晶體形態(tài)和內(nèi)部結(jié)構(gòu)，未見(jiàn)繼承核與增生邊。鋯石為短柱狀至長(zhǎng)柱狀、自形-半自形晶體，長(zhǎng)約150～300μm，長(zhǎng)寬比1:1到3:1，并具有密集的巖漿振蕩環(huán)帶(圖3)。安山巖和閃長(zhǎng)巖鋯石具有相似的Th和U含量，其變化范圍分別為152×10-6～2655×10-6和160×10-6～2043×10-6，Th/U比值為0.26～1.94。這些特征表明這些鋯石均為典型的巖漿成因鋯石(Hoskin and Schaltegger, 2003)。

3件安山巖樣品獲得了相似的206Pb/238U加權(quán)平均年齡，分別為161.0±1.7Ma (17T046;n=8, MSWD=0.09)、164.9±1.1Ma (17T056;n=18, MSWD=0.10)和161.0±1.1Ma (17T081;n=18, MSWD=0.16)(圖3)。樣品17T046中4顆鋯石具有較老的206Pb/238U年齡，分別為263Ma、441Ma、457Ma和643Ma(表1)，可能代表了繼承或捕獲鋯石的年齡。閃長(zhǎng)巖樣品(17T063)采自侵入到橄欖巖中的巖脈，獲得鋯石206Pb/238U加權(quán)平均年齡為162.5±1.0Ma (n=22, MSWD=0.07)，這與安山巖樣品測(cè)年結(jié)果基本一致，表明安山巖和閃長(zhǎng)巖是同期巖漿作用的產(chǎn)物。

表2西藏班戈佳瓊高鎂安山巖和閃長(zhǎng)巖鋯石Hf同位素分析結(jié)果

Table 2 Zircon Hf isotopic compositions of the Jiaqiong high-Mg andesite and diorite in the Bange County, Tibet

測(cè)點(diǎn)號(hào)176Yb/177Hf176Lu/177Hf176Hf/177Hf2σ176Hf/177HfiεHf(t)2σtDM (Ma)tDM2 (Ma)fLu/Hf17T046-010.0653300.0019570.2824710.0000210.282465-7.30.711341672-0.94-030.0832510.0028910.2824510.0000230.282443-8.10.811931723-0.91-060.0728240.0020810.2824540.0000200.282447-7.90.711631712-0.94-080.0462240.0013280.2824790.0000210.282475-7.00.811041651-0.96-090.0614710.0021060.2825670.0000360.282561-3.91.310001458-0.94-100.0648500.0022740.2824880.0000280.282481-6.81.011191637-0.93-110.0687470.0019150.2824630.0000190.282457-7.60.711441689-0.9417T056-010.0481730.0014320.2824550.0000230.282451-7.80.811411703-0.96-020.0294100.0008750.2824020.0000210.282399-9.60.711991818-0.97-030.0572330.0016800.2824320.0000220.282427-8.60.811811756-0.95-040.0676740.0019180.2824800.0000210.282474-6.90.811201650-0.94-050.0713990.0020160.2824550.0000210.282449-7.80.811591706-0.94-060.0772290.0020690.2825080.0000280.282501-6.01.010851589-0.94-070.0354950.0010450.2824970.0000200.282494-6.20.710701606-0.97-080.0920840.0027470.2824630.0000260.282454-7.60.911711694-0.92-090.0977960.0028570.2825780.0000260.282569-3.50.910041436-0.91-100.0510510.0014460.2824430.0000210.282439-8.20.811581729-0.96-110.0862390.0024240.2825680.0000250.282560-3.90.910071456-0.93-120.0423630.0011810.2825310.0000210.282528-5.00.710261530-0.96-130.0615280.0017680.2824760.0000220.282471-7.00.811211657-0.95-140.0714230.0020470.2824180.0000220.282412-9.10.812131789-0.94-150.0731110.0020840.2825250.0000220.282518-5.40.810601551-0.94-160.0507090.0015170.2824810.0000220.282476-6.80.811071645-0.9517T063-010.0300450.0011610.2824360.0000220.282433-8.40.811591744-0.97-020.0317970.0012550.2824120.0000220.282408-9.30.811961800-0.96-030.0268790.0010230.2823980.0000190.282395-9.80.712091830-0.97-040.0349010.0011510.2824230.0000150.282419-8.90.511781775-0.97-050.0277340.0010010.2824520.0000190.282449-7.90.711321708-0.97-060.0276100.0009530.2824140.0000160.282411-9.20.611841792-0.97-070.0329240.0011580.2824050.0000190.282402-9.60.712031814-0.97-080.0320180.0010990.2823980.0000260.282395-9.80.912111830-0.97-090.0345520.0012270.2824410.0000200.282438-8.30.711541734-0.96-100.0255570.0008750.2824040.0000170.282402-9.60.611951814-0.97-110.0302420.0010060.2824510.0000170.282448-7.90.611341711-0.97-120.0340380.0011240.2824010.0000160.282397-9.70.612081824-0.97-130.0305110.0010410.2824200.0000190.282417-9.00.711781780-0.97-140.0376470.0012570.2823990.0000190.282395-9.80.712151828-0.96-150.0338540.0012590.2824350.0000200.282431-8.50.711631747-0.96-160.0310470.0009990.2823770.0000180.282374-10.50.612371876-0.97-170.0409970.0014050.2824320.0000200.282428-8.60.711731756-0.9617T081-010.0208750.0009960.2824840.0000140.282481-6.70.510861636-0.97

續(xù)表2

Continued Table 2

測(cè)點(diǎn)號(hào)176Yb/177Hf176Lu/177Hf176Hf/177Hf2σ176Hf/177HfiεHf(t)2σtDM (Ma)tDM2 (Ma)fLu/Hf-020.0161460.0007650.2824760.0000170.282474-7.00.610911653-0.98-030.0283120.0013300.2824420.0000170.282438-8.30.611561733-0.96-040.0165690.0008020.2825020.0000160.282500-6.10.610561595-0.98-050.0117090.0005770.2824410.0000170.282439-8.20.611351731-0.98-060.0199080.0009580.2824350.0000180.282432-8.50.611551747-0.97-070.0379330.0017360.2824650.0000160.282460-7.50.611351683-0.95-080.0233660.0010530.2824030.0000140.282400-9.60.512021819-0.97-090.0208690.0009740.2824480.0000160.282445-8.00.611371717-0.97-100.0182430.0008570.2824560.0000140.282454-7.70.511221699-0.97-110.0267410.0012480.2824630.0000140.282459-7.50.511241686-0.96-120.0100570.0004820.2824840.0000160.282483-6.70.610721634-0.99-130.0124240.0005990.2824510.0000150.282449-7.90.511221709-0.98-140.0219050.0010340.2824850.0000170.282482-6.70.610861634-0.97-150.0196660.0009320.2824260.0000180.282423-8.80.611671767-0.97-160.0117000.0005580.2824380.0000160.282437-8.30.611381736-0.98

圖4 西藏班戈佳瓊高鎂安山巖和閃長(zhǎng)巖鋯石U-Pb年齡-εHf(t)值圖解 達(dá)如錯(cuò)安山巖數(shù)據(jù)據(jù)李小波等 (2015), Zeng et al. (2016b)；安多花崗巖數(shù)據(jù)據(jù)劉敏等 (2011)Fig.4 Plots of U-Pb ages vs. εHf(t) values for Jiaqiong high-Mg andesite and diorite in the Bange County, Tibet

3.2 鋯石Lu-Hf同位素

對(duì)4件鋯石樣品開(kāi)展Lu-Hf同位素分析，結(jié)果見(jiàn)表2。3件安山巖樣品鋯石具有相似的(176Hf/177Hf)i比值(0.282399～0.282569)和相對(duì)一致的εHf(t)值(-9.6～-3.5)，對(duì)應(yīng)的一階段(tDM)和二階段(tDM2)鋯石Hf同位素模式年齡分別為1000～1213Ma和1436～1819Ma(圖4)。閃長(zhǎng)巖樣品鋯石表現(xiàn)出相對(duì)均一的(176Hf/177Hf)i比值(0.282374～0.282449)和εHf(t)值(-10.5～-7.9)，對(duì)應(yīng)的一階段(tDM)和二階段(tDM2)鋯石Hf同位素模式年齡分別為1132～1237Ma和1708～1876Ma(圖4)。因此，安山巖和閃長(zhǎng)巖具有相似的鋯石Hf同位素組成，這與相鄰地區(qū)以及安多微陸塊內(nèi)已報(bào)道的近同期中酸性巖漿巖具有相似的鋯石Hf同位素組成(圖4; 劉敏等, 2011; 李小波等, 2015; Zengetal., 2016b)。

3.3 全巖主量和微量元素

安山巖和閃長(zhǎng)巖全巖地球化學(xué)測(cè)試結(jié)果見(jiàn)表3。測(cè)試樣品具有相對(duì)高的燒失量(1.00%～6.68%)，表明樣品均受到不同程度的蝕變，這與野外和鏡下觀(guān)察到巖石經(jīng)歷不同程度的綠泥石化和絹云母化相一致。通常而言，SiO2、MgO、FeO、TiO2、Al2O3等主量元素和稀土(REE)、高場(chǎng)強(qiáng)元素(HFSE)，以及Ba、Th、U等元素在蝕變過(guò)程中相對(duì)不活動(dòng)(Pearce, 2014)。因此，本文主要基于這些相對(duì)不活動(dòng)元素進(jìn)行巖石的類(lèi)型和成因討論。在Mg#-元素圖解上(圖5)，除K2O和Na2O外，Mg#與其他元素基本呈現(xiàn)較好的線(xiàn)性關(guān)系。

安山巖SiO2含量為52.03%～55.90%，Al2O3為15.74%～17.35%，MgO為4.14%～5.68%、FeOT為6.71%～7.81%，對(duì)應(yīng)的Mg#值介于50.9～58.0之間(表3)。在Zr/TiO2-SiO2圖解中，所有樣品均落入安山巖的區(qū)域內(nèi)(圖6a)。在Th-Co圖解上(圖6b)， 它們均呈現(xiàn)出高鉀鈣堿性巖漿巖的特征(Hastieetal., 2007)。在給定的SiO2條件下，安山巖樣品具有相對(duì)高的MgO含量，大部分樣品均落入高鎂安山巖的范疇(Kelemen, 1995; Kelemenetal., 2007; 圖6c)。同時(shí)，在SiO2-FeOT/MgO圖解上，安山巖相對(duì)于贊岐巖具有相對(duì)低的SiO2含量和高的FeOT/MgO比值，暗示其可能并非由贊岐巖直接分異而來(lái)(Tatsumi, 2006)(圖6d)。微量元素組成上，安山巖相對(duì)富集輕稀土元素(LREE; (La/Yb)N>8)，同時(shí)顯示出Eu的負(fù)異常(Eu/Eu*=0.70～0.87)(圖7a)。原始地幔標(biāo)準(zhǔn)化蛛網(wǎng)圖上，所有樣品均顯示出了一致的配分曲線(xiàn)，富集Rb、Th、U、Pb等元素，虧損Ba、Nb、Ta和Ti等元素(圖7b)。這些特征與前人報(bào)道的達(dá)如錯(cuò)高鎂安山巖相一致(李小波等, 2015; Zengetal., 2016b)。

表3西藏班戈佳瓊高鎂安山巖和閃長(zhǎng)巖全巖主量(wt%)和微量(×10-6)元素組成

Table 3 Whole-rock major (wt%) and trace (×10-6) element compositions of the Jiaqiong high-Mg andesite and diorite in the Bange County, Tibet

樣品號(hào)17T04517T04617T04717T04817T04917T05017T05217T05317T05417T05617T057巖性安山巖SiO254.4755.1654.2254.5455.9055.4955.4654.7654.7455.5653.79Al2O316.7216.5016.6616.7417.3117.3515.7416.6217.2616.6416.75MgO4.765.174.894.994.144.165.684.825.044.765.01Na2O3.273.473.743.344.594.353.093.333.113.622.14K2O2.341.102.152.271.401.451.842.331.062.752.51P2O50.170.160.170.170.180.180.140.170.150.170.15TiO20.800.830.790.790.810.810.700.800.860.780.77CaO6.937.606.536.526.046.347.626.748.435.847.86FeOT7.597.817.707.707.097.177.447.637.226.717.25MnO0.110.100.090.090.080.080.150.090.110.130.15LOI2.061.342.582.122.171.791.351.921.003.602.50Mg#52.854.153.153.651.050.957.652.955.455.855.2Sc252624.925.721.924.22924.825.319.525.5Ti49535038494451514977519245625013517246434731V200192203211193206213202198175198Cr52.176.24949.223.228.113348.644.331.450.5Co27.12625.928.424.426.930.825.423.621.629.9Ni36.328.125.325.346.846.659.223.917.320.227.9Ga18.418.317.918.918.319.116.918.218.617.617.8Ge1.431.451.591.581.371.431.421.431.481.31.39Rb10756.997.310969.177.891.510866.6117183Sr358312325355365393337363324354357Y22.223.421.3222221.919.221.624.12220.4Zr141144130137144149100137135143108Nb7.127.877.077.47.467.335.417.438.137.516.58Cs2.63.282.182.841.942.383.472.964.981.717.2Ba476176455396231259301403183717386La33.329.931.230.637.734.223.330.729.930.728.1Ce63.557.960.258.371.864.744.959.257.457.954.3Pr7.186.536.756.727.837.075.156.646.496.596.21Nd26.22525.62629.527.420.524.925.225.723.9Sm5.235.275.125.145.515.24.154.975.174.984.76Eu1.271.291.231.241.251.241.111.241.351.161.14Gd4.674.944.484.34.554.443.684.424.754.474.07Tb0.710.760.690.670.730.710.580.690.760.690.66Dy3.974.424.034.124.24.213.594.084.494.113.84Ho0.840.910.820.840.860.850.730.820.90.850.78Er2.442.622.432.392.482.482.132.442.672.512.29Tm0.350.380.340.340.350.350.310.350.380.360.33Yb2.282.522.242.252.392.352.042.252.552.392.17Lu0.340.390.350.340.370.360.320.350.390.360.33Hf3.843.993.683.783.994.233.263.763.6743.37Ta0.560.630.590.590.610.620.540.590.660.610.55Pb14.55.915.114.213.61428.113.24.38.599.33Th13.512.912.91313.714.210.513.111.513.611.4U2.6822.062.062.162.21.492.161.482.572.25Eu/Eu?0.790.770.790.810.760.790.870.810.830.750.79

續(xù)表3

Continued Table 3

樣品號(hào)17T05817T05917T06017T06117T06317T06417T06517T06617T06717T06817T069巖性安山巖閃長(zhǎng)巖SiO253.8453.0653.9752.0361.2059.6460.6961.2759.9757.2161.37Al2O316.7216.1616.7015.9015.5815.4515.4515.3615.6515.5715.44MgO5.075.354.944.893.944.414.514.144.165.284.58Na2O2.112.612.044.214.233.514.134.154.182.784.38K2O2.502.892.622.611.801.842.001.862.242.981.52P2O50.150.160.150.140.130.130.130.120.130.160.13TiO20.770.740.770.670.570.580.560.560.590.720.58CaO8.056.237.425.284.694.683.504.394.315.314.37FeOT7.476.897.476.934.884.994.864.665.326.705.35MnO0.130.130.130.150.090.080.070.080.090.150.09LOI2.356.222.976.683.004.674.093.333.392.692.97Mg#54.758.054.155.759.061.262.361.358.258.460.4Sc26.322.625.622.317.217.516.817.216.822.517.4Ti48744672479543673530357334913394350344603473V198178199175120117122109123154115Cr50.938.85441.994.495.198.594.992.87696.3Co29.526.329.822.916.618.217.316.917.524.716.4Ni27.322.427.722.82424.924.423.824.227.623.7Ga18.216.517.815.616.615.215.71515.816.314.9Ge1.471.281.431.241.271.211.161.111.161.431.17Rb18712519798.456.386.978.553.270.116652.3Sr383344372371356199250280318260290Y2120.32119.317.817.716.115.81621.413.4Zr10912611699.2177135143140129152127Nb6.767.066.746.128.177.978.087.657.978.797.79Cs22.82.99201.840.61.60.960.40.731.780.59Ba4064998001738337238300301375427222La28.629.828.726.828.328.426.227.128.531.128.5Ce55.657.155.651.351.95349.250.952.258.852.6Pr6.266.436.265.815.565.595.195.335.496.615.51Nd23.125.423.52220.420.418.819.820.125.320.1Sm4.814.954.824.354.023.923.633.813.924.953.66Eu1.141.181.130.951.010.990.890.9411.130.96Gd4.214.24.284.013.73.523.233.313.484.372.97Tb0.680.670.660.620.570.560.510.530.560.680.5Dy3.973.993.853.553.363.293.093.163.284.112.85Ho0.80.830.790.750.680.690.610.640.670.830.58Er2.342.352.392.171.981.981.751.891.932.471.64Tm0.330.330.340.310.290.280.250.270.270.340.24Yb2.22.262.232.041.941.881.621.761.822.321.68Lu0.340.340.330.320.290.280.250.270.280.360.26Hf3.463.583.493.24.63.723.743.923.734.113.71Ta0.550.570.550.480.710.70.680.70.740.750.71Pb11.210.812.38.8716.310.712.313.516.912.215.8Th11.512.111.810.715.612.712.212.61314.812.9U2.272.362.322.413.392.412.32.352.582.92.61Eu/Eu?0.770.790.760.700.800.810.790.810.830.740.89

圖5 西藏班戈佳瓊高鎂安山巖和閃長(zhǎng)巖SiO2、Al2O3、K2O+Na2O、Fe2O3T、Cr、Sr、Y、La、Yb對(duì)Mg#關(guān)系圖解 達(dá)如錯(cuò)安山巖數(shù)據(jù)據(jù)李小波等(2015)和Zeng et al. (2016b)，后同F(xiàn)ig.5 Elements of SiO2, Al2O3, K2O+Na2O, Fe2O3T, Cr, Sr, Y, La and Yb plotted against Mg# of the high-Mg Jiaqiong andesite and diorite in the Bange County, Tibet

圖6 西藏班戈佳瓊高鎂安山巖和閃長(zhǎng)巖SiO2-Zr/TiO2 (a,據(jù)Winchester and Floyd, 1977)、Th-Co (b,據(jù)Hastie et al., 2007)、SiO2-MgO (c,據(jù)McCarron and Smellie, 1998)和FeOT/MgO-SiO2 (d,據(jù)Tatsumi, 2006)圖解Fig.6 SiO2 vs. Zr/TiO2 (a, after Winchester and Floyd, 1977), Th vs. Co (b, after Hastie et al., 2007), SiO2 vs. MgO (c, after McCarron and Smellie, 1998) and FeOT/MgO vs. SiO2 (d, after Tatsumi, 2006) diagrams of the Jiaqiong high-Mg andesite and diorite in the Bange County, Tibet

圖7 西藏班戈佳瓊高鎂安山巖和閃長(zhǎng)巖球粒隕石標(biāo)準(zhǔn)化稀土元素配分圖(a、c)和原始地幔標(biāo)準(zhǔn)化微量元素蛛網(wǎng)圖(b、d)(標(biāo)準(zhǔn)化值據(jù)Sun and McDonough, 1989)Fig.7 Chondrite-normalized REE diagrams (a, c) and primitive-normalized multi-element spider diagrams (b, d) of the Jiaqiong high-Mg andesite and diorite in the Bange County, Tibet (normalization values are from Sun and McDonough, 1989)

閃長(zhǎng)巖相對(duì)于安山巖具有高的SiO2(57.21%～61.37%)含量和相對(duì)低的Al2O3(15.36%～15.65%)、FeOT(4.66%～6.70%)含量。同時(shí)，與安山巖類(lèi)似，也具有較高的MgO(3.94%～5.28%)含量和Mg#值(58.2～62.3)(表3)。在Th-Co圖解上(圖6b)，所有樣品均顯示出高鉀鈣堿性巖漿巖的特征。在SiO2-MgO(圖6c, d)，閃長(zhǎng)巖顯示出與安山巖類(lèi)似的地球化學(xué)特征，暗示二者可能為同源巖漿的產(chǎn)物。而在SiO2-FeOT/MgO圖解上，閃長(zhǎng)巖顯示出與分異的贊岐巖類(lèi)似的地球化學(xué)組成。閃長(zhǎng)巖與安山巖的稀土和微量元素配分曲線(xiàn)基本一致，富集LREE，具負(fù)Eu異常(Eu/Eu*=0.74～0.89)(圖7c)。在原始地幔標(biāo)準(zhǔn)化蛛網(wǎng)圖上，所有樣品也顯示出富集Rb、Th、U、Pb，虧損HFSE(Nb、Ta和Ti)的特征(圖7d)。

4 討論

4.1 侏羅紀(jì)巖漿巖

班公湖-怒江縫合帶中段佳瓊地區(qū)高鎂安山巖和閃長(zhǎng)巖的鋯石均具有典型的巖漿成因特征，如巖漿振蕩環(huán)帶和高的Th/U比值，因此，它們的年齡代表了巖漿冷凝結(jié)晶的時(shí)代。本項(xiàng)研究共獲得了4件較好的鋯石U-Pb諧和年齡，主要集中于165～161Ma(圖3)，指示安山巖和閃長(zhǎng)巖形成于侏羅紀(jì)中晚期，這與前人在達(dá)如錯(cuò)東部所報(bào)道的高鎂安山巖的時(shí)代一致(李小波等, 2015; Zengetal., 2016b)。此外，近同期的中酸性巖漿巖在東北部的安多微陸塊中也有報(bào)道(～170Ma)(劉敏等, 2011; Yanetal., 2016; 李小波等, 2017)。因此，在班公湖-怒江縫合帶中段北緣的侏羅紀(jì)中酸性巖漿作用主要發(fā)育在夾持于東巧-安多和北拉-拉弄蛇綠巖亞帶之間的區(qū)域內(nèi)，而在縫合帶中段緊鄰的南羌塘地塊內(nèi)部(東巧-安多以北地區(qū))基本不發(fā)育(Guynnetal., 2006)(圖1b)。近年來(lái)，在班公湖-怒江縫合帶西段以及南羌塘地塊西南緣也已報(bào)道了多處侏羅紀(jì)島弧型巖漿巖，并認(rèn)為它們與中特提斯洋北向俯沖有關(guān)(Kappetal., 2005; Lietal., 2014, 2018a, b; Haoetal., 2018)。總體而言，這些侏羅紀(jì)的巖漿巖沿班公湖-怒江縫合帶呈近東西向帶狀分布，它們形成時(shí)代一致，并具有相似的巖石組合和地球化學(xué)特征。因此，它們的形成可能具有相似的地球動(dòng)力學(xué)背景。

4.2 巖石成因

佳瓊地區(qū)高鎂安山巖和閃長(zhǎng)巖的形成時(shí)代一致，并且具有相似的地球化學(xué)特征和鋯石Hf同位素組成。所有樣品均具有鉀鈣堿性火山巖的特征，輕重稀土分異明顯，富集LREE和LILE，虧損HFSE(圖6、圖7)，這與典型島弧巖漿巖的特征相一致，是大洋俯沖消減背景下的產(chǎn)物(Kelemenetal., 2007)。此外，安山巖和閃長(zhǎng)巖均顯示出高鎂安山質(zhì)巖石的特征，在Mg#-元素圖解上表現(xiàn)出成分的連續(xù)變化(圖5)。這些特征表明安山巖和閃長(zhǎng)巖可能源自相似的源區(qū)，具有相同的成因機(jī)制，且均與班公湖-怒江中特提斯洋的俯沖消減作用有關(guān)。

一般而言，與俯沖相關(guān)的(高鎂)安山質(zhì)巖石有以下幾種形成機(jī)制：(1)基性巖漿與殼源巖漿混合(Strecketal., 2007)；(2)含水地幔橄欖巖部分熔融(Wood and Turner, 2009)；(3)俯沖板片熔體與地幔楔相互反應(yīng)(Tatsumi, 2001; Wangetal., 2011)。巖漿混合過(guò)程中殼源長(zhǎng)英質(zhì)巖漿的注入，會(huì)顯著降低巖漿的MgO含量和Mg#，同時(shí)導(dǎo)致變化較大的鋯石Hf同位素組成(通常εHf(t)變化范圍>10; Griffinetal., 2002; Shaw and Flood, 2009)，這與佳瓊高鎂安山巖和閃長(zhǎng)巖的特征相矛盾。此外，野外也未見(jiàn)安山巖和閃長(zhǎng)巖中具有巖漿混合的現(xiàn)象(如基性包體)。因此，巖漿混合模式可以排除。就第二種模式而言，含水地幔楔的部分熔融，產(chǎn)生的巖漿主要為基性和少量中性巖漿，這與研究區(qū)附近同期主要出露安山質(zhì)和長(zhǎng)英質(zhì)巖石的地質(zhì)事實(shí)不符(李小波等, 2015)。另外，含水地幔橄欖巖部分熔融產(chǎn)生的安山巖應(yīng)繼承源區(qū)虧損的同位素特征，這與佳瓊安山質(zhì)巖石具有明顯負(fù)的鋯石εHf(t)值的特征不符(圖4)。因此，含水地幔橄欖巖的部分熔融模式也不足以形成研究區(qū)內(nèi)的高鎂安山質(zhì)巖石。

俯沖板片熔體與地幔楔相互反應(yīng)可能是形成佳瓊高鎂安山巖和閃長(zhǎng)巖的主要機(jī)制。高鎂安山巖和閃長(zhǎng)巖樣品均顯示出高Y和低Sr的特征，明顯區(qū)別于埃達(dá)克質(zhì)巖石，與單純俯沖洋殼熔融產(chǎn)生的熔體特征相矛盾(Defant and Drummond, 1990; Castillo, 2012)。安山巖和閃長(zhǎng)巖樣品具有較高的(La/Sm)N和Th含量，顯示出明顯俯沖沉積物組分加入的趨勢(shì)(圖8; Plank and Langmuir, 1998; Kelemenetal., 2007)。此外，明顯富集的鋯石Hf同位素組成，也進(jìn)一步支持其巖漿源區(qū)加入了殼源組分(俯沖沉積物)。以上這些特征表明佳瓊地區(qū)高鎂安山巖和閃長(zhǎng)巖的巖漿可能主要源于一個(gè)俯沖沉積物為主的源區(qū)，這與部分安山質(zhì)巖石出露在俯沖增生雜巖之中的地質(zhì)事實(shí)相吻合。在板片俯沖過(guò)程中，俯沖沉積物發(fā)生部分熔融，產(chǎn)生的熔體和地幔相互作用可能是產(chǎn)生這套安山質(zhì)巖石的主要過(guò)程。但是，這套巖漿巖地球化學(xué)特征又有別于典型的贊岐巖，區(qū)內(nèi)還同時(shí)發(fā)育更高SiO2含量的酸性火山巖(李小波等, 2015)，暗示俯沖沉積物熔體可能與地幔楔并未達(dá)到平衡(Tatsumi, 2001, 2006)。

圖8 西藏班戈佳瓊高鎂安山巖和閃長(zhǎng)巖Ba/Th-(La/Sm)N (a,據(jù)Labanieh et al., 2012)和 Th/La-Th (b,據(jù)Plank and Langmuir, 1998)圖解 深海和全球俯沖沉積物(GLOSS)數(shù)據(jù)據(jù)Plank and Langmuir (1998)Fig.8 Ba/Th vs. (La/Sm)N (a, after Labanieh et al., 2012) and Th/La vs. Th (b, after Plank and Langmuir, 1998) diagrams of the Jiaqiong high-Mg andesite and diorite in the Bange County, Tibet

4.3 構(gòu)造環(huán)境

島弧巖漿作用是大洋板片俯沖最直接的記錄，對(duì)于深入理解大洋俯沖消減過(guò)程具有重要的科學(xué)意義(Stern, 2002)。佳瓊地區(qū)中晚侏羅世高鎂安山巖和閃長(zhǎng)巖具有典型的島弧巖漿巖特征，它們的形成與大洋板片俯沖消減作用有關(guān)。安山巖和閃長(zhǎng)巖與蛇綠巖和弧前增生雜巖(木嘎崗日群、接奴群)共生，并且局部見(jiàn)閃長(zhǎng)巖侵入于蛇綠巖及木嘎崗日群之中，暗示它們產(chǎn)出于靠近海溝的大陸邊緣環(huán)境(Tatsumi, 2006)。本項(xiàng)研究高鎂安山巖和閃長(zhǎng)巖具有高Y和低Sr/Y的特征，且研究區(qū)內(nèi)也未有同期埃達(dá)克質(zhì)巖石的報(bào)道，指示巖漿應(yīng)源自淺部源區(qū)，并未達(dá)到石榴石相，即熔融發(fā)生在俯沖帶淺部異常高熱的環(huán)境(Tatsumi, 2001, 2006; 唐功建和王強(qiáng), 2010)，這與達(dá)如錯(cuò)東部高鎂安山巖的研究結(jié)果相一致(Zengetal., 2016b)。海溝邊緣淺部高熱的巖漿形成，一般對(duì)應(yīng)兩種構(gòu)造模式：洋脊俯沖或者初始俯沖(DeLongetal., 1979; Sunetal., 2010; Hall, 2012)。洋脊俯沖一般發(fā)生于大洋演化的末期，常伴隨著埃達(dá)克巖、MORB-OIB類(lèi)巖石和其他高溫長(zhǎng)英質(zhì)巖石的產(chǎn)出(Sunetal., 2010; Windley and Xiao, 2018)。此外，洋脊俯沖過(guò)程中常伴隨著軟流圈物質(zhì)的上涌，而形成具有虧損同位素特征的巖石(Lietal., 2016)。這些特征與佳瓊地區(qū)的高鎂安山巖和閃長(zhǎng)巖的特征不符，也與安多微陸塊及鄰區(qū)已報(bào)道的近同時(shí)代巖漿巖的類(lèi)型和特征不一致(劉敏等, 2011; Zengetal., 2016b; 李小波等, 2017)。另外，研究區(qū)以南的北拉-拉弄和永珠-納木錯(cuò)蛇綠巖主要形成于中晚侏羅世(170～150Ma)(Zhongetal., 2015; Tangetal., 2018a; Zengetal., 2018)，也從側(cè)面表明大洋中脊在～165Ma的時(shí)候尚在擴(kuò)張。因此，佳瓊地區(qū)165～161Ma的高鎂安山質(zhì)巖石很可能是大洋板塊初始俯沖的產(chǎn)物(Zengetal., 2016b)。

圖9 西藏班戈佳瓊地區(qū)侏羅紀(jì)島弧巖漿形成的構(gòu)造模式簡(jiǎn)圖Fig.9 Tectonic model for the formation of Jurassic arc magmatic rocks in Jiaqiong area in the Bange County, Tibet

已有研究資料顯示，東巧-安多蛇綠巖具有SSZ型蛇綠巖的特征，時(shí)代為190～180Ma，它們的形成與班公湖-怒江中特提斯洋洋盆的北側(cè)分支北向俯沖消減有關(guān)(Liuetal., 2016; Wangetal., 2016)。一般認(rèn)為它們?cè)?80～170Ma伴隨著安多微陸塊拼貼到羌塘南緣而就位(Zhouetal., 1997; Guynnetal., 2006, 2013; Zhangetal., 2014)。北拉-拉弄蛇綠巖發(fā)育一套MORB-IAT-玻安質(zhì)巖石組合，表現(xiàn)出弧前蛇綠巖的特征(Dilek and Furnes, 2014)，主要形成于172～148Ma的弧前環(huán)境(徐力峰等, 2010; Zhongetal., 2017; Tangetal., 2018a)。這與佳瓊地區(qū)以及安多微陸塊中的侏羅紀(jì)島弧型巖漿作用的時(shí)代和構(gòu)造背景基本吻合。據(jù)此我們認(rèn)為佳瓊地區(qū)165～161Ma高鎂安山巖和閃長(zhǎng)巖可能形成于北拉-拉弄分支洋盆北向初始俯沖背景。這一初始俯沖作用可能與同時(shí)期(或稍早)安多微陸塊拼貼到羌塘南緣所導(dǎo)致的俯沖躍遷有關(guān)(圖9; Stern, 2004; Stern and Gerya, 2018)。此外，佳瓊地區(qū)和安多微陸塊內(nèi)部的侏羅紀(jì)到早白堊世巖漿作用總體表現(xiàn)出相似的同位素特征，它們均具有較老的鋯石Hf模式年齡(1.4～1.8Ga)，反應(yīng)了其巖漿來(lái)源于相似的古老地殼基底物質(zhì)(劉敏等, 2011; 唐躍等, 2015; Zengetal., 2016b; Huetal., 2017; Yanetal., 2016)。因此，盡管佳瓊地區(qū)及其鄰區(qū)主要被中晚侏羅-早白堊世沉積地層所覆蓋，它可能也屬于安多微陸塊西延部分。

4.4 班公湖-怒江中特提斯洋的演化

佳瓊地區(qū)高鎂安山質(zhì)巖石為進(jìn)一步約束班公湖-怒江中特提斯洋的演化提供了新的證據(jù)。已有研究顯示，中特提斯洋在晚三疊世之前就已經(jīng)打開(kāi)(Yin and Harrison, 2000; Zhuetal., 2013; Zengetal., 2016a; Fanetal., 2018)，安多微陸塊則作為獨(dú)立的塊體位于該大洋之中(Guynnetal., 2006; Zhangetal., 2014)。東巧-安多蛇綠巖亞帶早侏羅世(190～180Ma)SSZ蛇綠巖的存在，指示中特提斯洋北側(cè)分支洋盆此時(shí)已開(kāi)始向北俯沖消減(Liuetal., 2016; Wangetal., 2016)，而南側(cè)的北拉-拉弄分支洋盆仍在持續(xù)擴(kuò)張(圖9)。早侏羅世晚期，進(jìn)一步的俯沖消減以及安多微陸塊和南羌塘地塊的拼合，導(dǎo)致東巧-安多分支洋盆關(guān)閉，從而形成安多微陸塊內(nèi)高壓變質(zhì)巖(Guynnetal., 2006; Zhangetal., 2014)。與此同時(shí)，北側(cè)分支洋盆的閉合以及陸陸碰撞導(dǎo)致的俯沖躍遷，在南側(cè)的北拉-拉弄形成新的北向俯沖，并在此基礎(chǔ)上逐步形成了安多微陸塊內(nèi)廣泛分布的中晚侏羅世島弧巖漿巖(圖9)。值得注意的是，班公湖-怒江縫合帶西段羌塘南緣地區(qū)所報(bào)道的島弧巖漿巖時(shí)代也主要起始于中侏羅世(～170Ma)(Lietal., 2016, 2018b)。同時(shí)，班公湖-怒江縫合帶自班公湖-改則，向東到北拉-拉弄，再到丁青地區(qū)的蛇綠巖，主體形成于一個(gè)非常窄的時(shí)間范圍，主要集中于175～160Ma(史仁燈等, 2007; Wangetal., 2016; Huangetal., 2017; Zhongetal., 2017; Tangetal., 2018a, b)。這些蛇綠巖均顯示出明顯的SSZ親緣性，指示了與俯沖相關(guān)的構(gòu)造環(huán)境。因此，島弧巖漿巖和蛇綠巖共同指示整個(gè)中特提斯洋的北向俯沖時(shí)間可能起始于中侏羅世，并隨著晚侏羅-早白堊世拉薩和南羌塘地塊逐步碰撞而結(jié)束(Zhuetal., 2016; Fanetal., 2018)。

5 結(jié)論

(1)佳瓊地區(qū)安山巖和閃長(zhǎng)巖形成于中晚侏羅世(165～161Ma)；

(2)安山巖和閃長(zhǎng)巖均顯示出高鎂安山質(zhì)巖石的特征，是俯沖過(guò)程中俯沖板片上覆沉積物部分熔融的熔體與地幔楔交代的產(chǎn)物；

(3)佳瓊及鄰區(qū)中晚侏羅世高鎂安山質(zhì)巖石是中特提斯洋北拉-拉弄分支洋盆北向初始俯沖的產(chǎn)物，這一俯沖過(guò)程可能與安多微陸塊與南羌塘地塊碰撞導(dǎo)致的俯沖南向躍遷有關(guān)。

致謝鋯石陰極發(fā)光圖片拍攝得到了中國(guó)地質(zhì)科學(xué)院地質(zhì)研究所施斌的幫助；鋯石U-Pb定年得到了中國(guó)地質(zhì)科學(xué)院礦產(chǎn)資源研究所侯可軍老師以及北京科薈測(cè)試技術(shù)有限公司肇創(chuàng)的協(xié)助；中國(guó)科學(xué)院地質(zhì)與地球物理研究所楊岳衡研究員、張維麒博士等在鋯石Hf同位素分析實(shí)驗(yàn)中提供了幫助。中國(guó)地質(zhì)科學(xué)院地質(zhì)研究所劉建峰研究員和李舢副研究員詳細(xì)審閱了本文并提出寶貴修改意見(jiàn)。在此一并表示感謝。

謹(jǐn)以此文祝賀肖序常院士90壽辰，感謝先生一直以來(lái)的指導(dǎo)與幫助，祝先生健康長(zhǎng)壽。