黃惠明, 李鵬飛, 胡萬萬, 令佳琪

西天山伊犁地塊早古生代拼貼: 來自碎屑鋯石U-Pb年代學(xué)和Hf同位素的制約

黃惠明1, 2, 3, 李鵬飛1, 2*, 胡萬萬1, 2, 3, 令佳琪1, 2, 3

(1.中國科學(xué)院 廣州地球化學(xué)研究所, 同位素地球化學(xué)國家重點(diǎn)實(shí)驗(yàn)室, 廣東 廣州 510640; 2.中國科學(xué)院深地科學(xué)卓越創(chuàng)新中心, 廣東 廣州 510640; 3.中國科學(xué)院大學(xué), 北京 100049)

西天山伊犁地塊記錄著中亞造山帶構(gòu)造演化方面的關(guān)鍵信息, 前人多認(rèn)為伊犁地塊是一個單獨(dú)塊體, 但其西延境外部分對應(yīng)著多個塊體。這些塊體在奧陶紀(jì)末期完成拼貼, 它們之間的縫合帶在西天山的延伸部分被晚古生代?中新生代地層覆蓋, 但通過境內(nèi)外地質(zhì)對比, 認(rèn)為其很可能沿著伊犁地塊中部向東延伸, 并將伊犁地塊劃分為南、北伊犁兩個塊體。目前, 對于南、北伊犁地塊的拼貼時限缺少精確限定, 其是否類似于境外部分于奧陶紀(jì)末期完成拼貼仍不清楚。為此, 本項(xiàng)研究選取北伊犁地塊志留系庫茹爾組為研究對象, 通過開展碎屑鋯石U-Pb年代學(xué)和Hf同位素分析, 約束南、北伊犁兩個塊體的拼貼時限。結(jié)果顯示, 庫茹爾組碎屑鋯石U-Pb年齡主要集中在早古生代(422～519 Ma), 其最年輕峰值432 Ma約束了庫茹爾組的最大沉積時代。在對比南北伊犁地塊巖漿活動的基礎(chǔ)上, 發(fā)現(xiàn)庫茹爾組沉積地層同時接收到了南北伊犁地塊的物源供給, 表明南北伊犁地塊已在庫茹爾組沉積前完成拼貼。通過綜合解析碎屑鋯石及區(qū)域巖漿巖鋯石Hf同位素數(shù)據(jù), 發(fā)現(xiàn)其在奧陶紀(jì)末期同時向富集和虧損的方向演化, 與顯生宙以來東亞多塊體拼貼過程中的Hf同位素演化趨勢相似, 因而我們將這種Hf()值扇形演化樣式歸因于南北伊犁地塊的碰撞拼貼, 進(jìn)而約束伊犁地塊的拼貼時限在奧陶紀(jì)末期。

中亞造山帶; 哈薩克斯坦微陸塊; 西天山; 伊犁地塊; 碎屑鋯石

0 引 言

中亞造山帶是全球典型的增生型造山帶, 其形成演化涉及到一系列微陸塊、巖漿弧的碰撞拼貼(Windley et al., 2007; Wilhem et al., 2012; Xiao et al., 2015)。伊犁地塊(Yili block)位于中亞造山帶西南部的西天山地區(qū), 并向西延伸至境外(圖1a)。一些學(xué)者認(rèn)為, 伊犁地塊自早古生代起即是一個統(tǒng)一的整體, 并把伊犁地塊中具有弧特征的早古生代巖漿巖歸因于其南北兩側(cè)大洋的俯沖作用(胡靄琴等, 2008; Gao et al., 2009; Qian et al., 2009; Long et al., 2011; Huang et al., 2013; Xu et al., 2013; Gou and Zhang, 2016)。然而也有學(xué)者發(fā)現(xiàn), 伊犁地塊在境外的延伸部分, 對應(yīng)多個前寒武紀(jì)地塊(圖1a): 其北部為阿克套?準(zhǔn)噶爾地塊(AJ), 南部為科克切塔夫?北天山地塊(KNTB), 二者之間產(chǎn)出另一個楚?伊犁地塊(CY)(Windley et al., 2007; Wang et al., 2012, 2014a, 2014b; Rojas-Agramonte et al., 2014)。目前, 基于古地磁、區(qū)域巖漿活動和地層不整合覆蓋縫合帶等證據(jù), 學(xué)術(shù)界認(rèn)為境外這些地塊于奧陶紀(jì)末期拼貼在一起, 構(gòu)成哈薩克斯坦微陸塊(Kazakhstan microcontinent),并形成了數(shù)條早古生代縫合帶(Biske and Seltmann, 2010; Alexeiev et al., 2011, 2016; Bazhenov et al., 2012)。這些縫合帶向東延伸進(jìn)入我國西天山伊犁地塊內(nèi)部, 被晚古生代?中新生代地層覆蓋?；诖? Wang et al. (2012, 2014a, 2014b)將伊犁地塊劃分為南北兩個塊體(圖1b), 但二者拼貼的時間仍不清楚, 其是否如境外延伸部分在奧陶紀(jì)末期拼貼仍未限定。

碎屑鋯石U-Pb年代學(xué)是示蹤沉積物源、約束地層時代的有效手段(Bruguier et al., 1997; Cawood and Nemchin, 2000; Cawood et al., 2012), 結(jié)合鋯石Hf同位素可以有效約束造山帶的構(gòu)造演化過程(Kemp et al., 2009; Gehrels and Pecha, 2014; Han et al., 2016; Huang et al., 2016; Zhang et al., 2016; Li et al., 2020)。本文擬利用該方法, 通過研究北伊犁地塊志留系庫茹爾組碎屑鋯石年齡譜及Hf同位素特征, 以期限定南北伊犁地塊的拼貼時限。該地層在北伊犁地塊中分布廣泛, 出露面積及地層厚度在志留系各組地層中占70%以上(劉曉宇, 2009), 是判別伊犁地塊是否在志留紀(jì)之前完成拼貼的理想對象。目前, 針對伊犁地塊開展的碎屑鋯石年代學(xué)研究工作, 主要集中在晚古生代地層(Yang et al., 2012; 白建科等, 2015; Huang et al., 2017; 石鑫等, 2018; Wang et al., 2018a, 2018b; 陳有炘等, 2019), 對于早古生代地層, 馮博等(2019)報道了北伊犁地塊志留系地層的碎屑鋯石年齡譜, 但對其在伊犁地塊拼貼中的指示意義未進(jìn)行深入討論。

1 地質(zhì)背景

1.1 哈薩克斯坦微陸塊

哈薩克斯坦微陸塊位于中亞造山帶西南部(圖1a), 由數(shù)個前寒武紀(jì)地塊組成, 包括伊希姆?中天山地塊(IMT)、阿克套?準(zhǔn)噶爾地塊(AJ)、楚?伊犁地塊(CY)和科克切塔夫?北天山地塊(KNTB)(Windley et al., 2007; Alexeiev et al., 2011)。這些地塊沿著數(shù)條早古生代縫合帶(伊諾曼套?伊犁縫合帶(EY)、扎里爾?奈曼縫合帶(DN)和吉爾吉斯?帖爾斯克依縫合帶(KT)), 于奧陶紀(jì)末期拼貼在一起(Bakirov and Maksumova, 2001; Windley et al., 2007; Biske and Seltmann, 2010; Alexeiev et al., 2011, 2015, 2016; Bazhenov et al., 2012; Degtyarev et al., 2013)。

1.2 西天山(伊犁地塊及其鄰區(qū))

西天山位于天山造山帶西段, 夾于準(zhǔn)噶爾盆地和塔里木克拉通之間(圖1a)。其大地構(gòu)造格架由北到南可以劃分為: 北天山構(gòu)造帶、伊犁地塊、中天山地塊和南天山構(gòu)造帶(圖1b; Gao et al., 1998; Charvet et al., 2011; Xiao et al., 2013)。

北天山構(gòu)造帶呈NWW走向展布, 與南側(cè)的伊犁地塊以北天山斷層為界(圖1b)。其主要包括晚古生代深海相濁積巖及蛇綠混雜巖(Wang et al., 2006; Xiao et al., 2013), 向北呈疊瓦式逆沖推覆到中新生代準(zhǔn)噶爾盆地地層之上(Gao et al., 1998; Xiao et al., 2013)。

伊犁地塊呈三角形向東尖滅, 是哈薩克斯坦微陸塊向東的延伸部分。前寒武基底主要在伊犁地塊北緣(溫泉地區(qū); 圖1b)和南緣(沿北那拉提斷層; 圖1b)出露, 晚古生代?中新生代沉積地層覆蓋于伊犁地塊中部(Gao et al., 1998; Wang et al., 2006)。伊犁地塊內(nèi)分布有大量鈣堿性的中酸性巖漿巖, 并主要沿其南北緣出露, 巖漿活動持續(xù)時間主要為497～260 Ma (Xu et al., 2013; Han et al., 2016; Huang et al., 2017; Tang et al., 2017; Han and Zhao, 2018)。伊犁地塊由南北兩個塊體拼貼而成(圖1b), 南伊犁地塊對應(yīng)阿克套?準(zhǔn)噶爾地塊(AJ), 北伊犁地塊對應(yīng)科克切塔夫?北天山地塊(KNTB), 其縫合位置可能在伊犁地塊中部, 并被晚古生代至中新生代地層所覆蓋(圖1b) (Windley et al., 2007; Wang et al., 2012, 2014a, 2014b; Rojas-Agramonte et al., 2014)。

中天山地塊呈狹窄細(xì)長形向東延伸至巴倫臺地區(qū), 與伊犁地塊以北那拉提斷層為界(Gao et al., 2009; Qian et al., 2009)。中天山地塊產(chǎn)出元古代基底, 上覆古生代火山沉積巖, 缺失泥盆系(Xiao et al., 2013; Xia et al., 2014; Cao et al., 2017), 并被奧陶紀(jì)?二疊紀(jì)花崗巖侵入(Dong et al., 2011; Gou and Zhang, 2016)。

南天山構(gòu)造帶主要包括志留系?石炭系海相火山沉積巖, 與蛇綠混雜巖一起被強(qiáng)烈褶皺, 呈疊瓦狀向南逆沖推覆到塔里木克拉通北緣之上(Gao et al., 1998, 2009; Xiao et al., 2013), 并被二疊紀(jì)火山碎屑巖和中生代陸相碎屑巖覆蓋(Han et al., 2016; Huang et al., 2017)。南天山構(gòu)造帶與中天山地塊以南那拉提斷層為界, 沿著該斷層產(chǎn)出有超高壓變質(zhì)巖, 最新的鋯石U-Pb和石榴石Lu-Hf年代學(xué)研究, 約束其變質(zhì)峰期年齡為324～312 Ma(Klemd et al., 2011; Li et al., 2011; Su et al., 2011; Yang et al., 2013; Liu et al., 2014)。

2 樣品描述和分析方法

2.1 樣品描述

沿著北伊犁地塊北緣, 早古生代地層出露齊全, 主要包括寒武系磷礦溝組(?1)、肯薩依組(?2)、果子溝組(?3), 奧陶系新二臺組(O1)、奈楞格勒達(dá)坂組(O2)、呼獨(dú)克達(dá)坂組(O3)和志留系尼勒克河組(S1)、基夫克組(S2)、庫茹爾組(S3)、博羅霍洛山組(S3)(蔡土賜等, 1999; 馮博等, 2019)。樣品H19WT51采于北伊犁地塊志留系庫茹爾組(圖2), 該組主要為一套淺海相陸源細(xì)碎屑巖建造, 可見微波狀層理構(gòu)造(劉曉宇, 2009)。其沉積序列自下而上以海水逐漸變淺為特征, 并在尼勒克河中游剖面的上部層位中產(chǎn)出龜裂構(gòu)造, 指示該套地層曾暴露于水面之上(張少鵬, 2010)。庫茹爾組按巖性分為上下兩部分: 下部以灰色?灰綠色條帶狀粉砂巖、白云質(zhì)粉砂巖和鈣質(zhì)粉砂質(zhì)泥巖為主, 夾凝灰粉砂巖、硅質(zhì)泥巖及少量礫巖; 上部主要為灰綠色、灰紫色鈣質(zhì)細(xì)粒石英砂巖、凝灰質(zhì)粉砂巖及鈣質(zhì)?鐵質(zhì)粉砂巖(新疆維吾爾自治區(qū)地質(zhì)礦產(chǎn)局, 1988)。

采樣點(diǎn)位于尼勒克縣以北20 km處(43°59′13.65″N, 82°37′3.83″E, 圖2), 樣品野外露頭為灰綠色, 巖性為細(xì)砂巖, 發(fā)育波痕層面構(gòu)造(圖3a)。經(jīng)顯微鏡下鑒定, 碎屑以石英為主, 約占60%, 長石、方解石等其他礦物約占15%～20%, 雜基約占20%～25%。碎屑粒度較為均一, 磨圓度較差(圖3b)。

2.2 分析方法

沉積巖樣品H19WT51應(yīng)用傳統(tǒng)重液和磁選方法分選出鋯石, 并用環(huán)氧樹脂填充固結(jié)制成靶, 拋光至顯露鋯石內(nèi)部結(jié)構(gòu)。在顯微鏡下進(jìn)行鋯石透/反射光圖像拍攝, 隨后在中國科學(xué)院廣州地球化學(xué)研究所(同位素地球化學(xué)國家重點(diǎn)實(shí)驗(yàn)室)的掃描電鏡(JXA-8100, JEOL)下拍攝鋯石陰極發(fā)光圖像(CL), 以此揭示鋯石的內(nèi)部結(jié)構(gòu)。在選取鋯石U-Pb年齡和Hf同位素測試點(diǎn)時, 充分避開鋯石的裂縫、包裹體等區(qū)域。

碎屑鋯石U-Pb年齡測試在中國科學(xué)院廣州地球化學(xué)研究所礦物學(xué)與成礦學(xué)重點(diǎn)實(shí)驗(yàn)室, 利用激光剝蝕?電感耦合?等離子質(zhì)譜分析儀(LA-ICP-MS)完成。該儀器配有RESOlution S-155激光剝蝕系統(tǒng), 激光束斑直徑為29 μm, 頻率6 Hz。分析過程中使用He作為剝蝕物質(zhì)載氣, 以提高分析的靈敏度。采用91500和Plesovice鋯石標(biāo)樣進(jìn)行同位素分餾校正(Sláma et al., 2008; Liu et al., 2010), 詳細(xì)分析流程見Liu et al. (2010)。隨后使用ICPMSDataCal10.8軟件處理數(shù)據(jù), 所獲得的碎屑鋯石U-Pb年齡諧和圖和年齡頻譜圖由ISOPLOT4.15軟件繪制而成(Ludwig, 2003)。

圖2 樣品采集區(qū)地質(zhì)簡圖(據(jù)新疆維吾爾自治區(qū)地質(zhì)礦產(chǎn)局, 1980, 1988)

圖3 樣品H19WT51的野外露頭(a, 可見波痕層面構(gòu)造)和顯微鏡下特征(b, 碎屑以石英(Qz)、長石(Fsp)、方解石(Cal)為主, 粒度較為均一)

碎屑鋯石Hf同位素測試在中國科學(xué)院廣州地球化學(xué)研究所同位素地球化學(xué)國家重點(diǎn)實(shí)驗(yàn)室, 利用激光剝蝕?電感耦合?等離子質(zhì)譜分析儀(LA-ICP-MS)完成。該儀器配有RESOlution M-50 ArF 193 nm激光剝蝕系統(tǒng), 激光束斑直徑為45 μm, 頻率6 Hz。采用Penglai鋯石標(biāo)樣與測試樣品進(jìn)行交叉分析校正(Li et al., 2010)。用于校正的176Yb/173Yb和176Lu/175Lu值分別為0.79381 (Segal et al., 2003)和0.02656 (Wu et al., 2006),176Hf/177Hf的質(zhì)量歧視校正通過179Hf/177Hf=0.7325的指數(shù)法則進(jìn)行。用于計(jì)算初始176Hf/177Hf值的176Lu衰變常數(shù)選用(1.865 ± 0.015)×10?11a?1(Scherer et al., 2001), 用于計(jì)算Hf()值的球粒隕石標(biāo)準(zhǔn)值選用(176Lu/177Hf)CHUR=0.0332±2和(176Hf/177Hf)CHUR, 0=0.282772±29 (Blichert-Toft and Albarède, 1997)。虧損地幔線計(jì)算選用當(dāng)今的176Lu/177Hf=0.0384和176Hf/177Hf=0.28325, 計(jì)算鋯石二階段模式年齡(DM2)時選用平均大陸地殼值176Lu/177Hf=0.015 (Griffin et al., 2000)。

3 分析結(jié)果

細(xì)砂巖樣品H19WT51中碎屑鋯石U-Pb年齡和Hf同位素分析結(jié)果見表1和表2。代表性鋯石陰極發(fā)光(CL)圖像見圖4。本次研究中, 大部分鋯石具有明顯的振蕩環(huán)帶, 顏色偏暗, Th/U值范圍為0.15～1.48 (表1), 指示其為典型的巖漿成因鋯石(Hanchar and Rudnick, 1995; Hoskin and Schaltegger, 2003)。鋯石形態(tài)上, 具有一定磨圓(圖4), 指示其經(jīng)歷了一定距離的搬運(yùn)過程。

對樣品H19WT51中的65顆鋯石開展了U-Pb年代學(xué)分析, 對于表觀年齡小于1000 Ma的數(shù)據(jù), 采用206Pb/238U年齡, 而表觀年齡大于1000 Ma的數(shù)據(jù), 則選取207Pb/206Pb年齡。63個測試點(diǎn)的年齡誤差橢圓落在諧和曲線上(圖5a), 可以視為諧和年齡(諧和度大于90%)。其中, 51顆鋯石具有早古生代年齡(422～519 Ma), 年齡峰值為432 Ma和447 Ma(圖5b), 其余12顆前寒武紀(jì)鋯石年齡較為分散(903～1755 Ma)。

從上述63顆鋯石中選取40顆早古生代年齡的鋯石進(jìn)行Hf同位素分析測試, 獲得的Hf()值范圍為?7.2～+13.4, 單階段模式年齡(DM)為0.57～1.33 Ga, 二階段模式年齡(DM2)為0.61～1.87 Ga(表2, 圖6)。

4 討 論

4.1 庫茹爾組的沉積時代

庫茹爾組(S3)主要分布在西天山博羅霍洛山南坡, 北伊犁地塊內(nèi)部(圖2、7), 上與博羅霍洛山組(S3)呈整合接觸, 下與基夫克組(S2)呈不整合接觸, 且含少量腕足類和珊瑚類化石(新疆維吾爾自治區(qū)地質(zhì)礦產(chǎn)局, 1988)。李永鐵(1993)根據(jù)庫茹爾組中發(fā)現(xiàn)的角石sp.,cf.和三葉蟲sp.等化石, 限定庫茹爾組的沉積時代為志留紀(jì)。

目前, 碎屑鋯石年齡被廣泛應(yīng)用于約束地層的沉積時限, 其最年輕年齡可以代表地層的最大沉積時限(Nelson, 2001; Williams, 2001; Fedo et al., 2003)。Dickinson and Gehrels (2009)介紹了3種確定碎屑鋯石最年輕年齡的方法, 包括: (1)最年輕單顆粒年齡; (2)年齡頻譜圖中最年輕峰值年齡; (3)多顆最年輕鋯石的年齡加權(quán)平均值(誤差在1σ或2σ內(nèi))?？紤]到由Pb丟失引起的最年輕碎屑鋯石不具有代表性, 因此本文采用碎屑鋯石U-Pb年齡頻譜圖中最年輕峰值, 約束樣品所在地層的最大沉積年齡。這種方法得到的年齡上限可能會略老于真實(shí)值(Wang et al., 2018b), 但卻是最保守可靠的。樣品H19WT51碎屑鋯石U-Pb年齡頻譜圖(圖5b), 顯示出最年輕峰值為432 Ma, 因而約束庫茹爾組的最大沉積年齡為志留紀(jì)中期。馮博等(2019)也對庫茹爾組砂巖開展了碎屑鋯石U-Pb年代學(xué)研究, 獲得最年輕的鋯石年齡為425 Ma, 與本研究得出的432 Ma較為接近(圖8c、d)。

表1 樣品H19WT51碎屑鋯石U-Pb年齡數(shù)據(jù)

續(xù)表1:

注: 標(biāo)注*的測點(diǎn)是未用于統(tǒng)計(jì)的不諧和年齡數(shù)據(jù)。

表2 樣品H19WT51碎屑鋯石Lu-Hf同位素測試結(jié)果

續(xù)表2:

圖4 樣品H19WT51代表性碎屑鋯石陰極發(fā)光(CL)圖(實(shí)線圈為U-Pb測試位置, 直徑為29 μm; 虛線圈為Hf測試位置, 直徑為45 μm)

圖5 樣品H19WT51碎屑鋯石U-Pb年齡諧和圖(a)和年齡頻譜圖(b)

圖6 樣品H19WT51碎屑鋯石Hf同位素特征圖

綜上所述, 碎屑鋯石年齡數(shù)據(jù)支持前人通過古生物化石限定庫茹爾組為志留系沉積地層的結(jié)論。

4.2 沉積物源分析

本次研究在庫茹爾組砂巖樣品(H19WT51)中獲得51顆碎屑鋯石年齡集中在早古生代(422～519 Ma), 其余12顆前寒武紀(jì)的鋯石年齡較為分散(903～1755 Ma) (圖5b)。目前, 在北伊犁地塊報道有前寒武紀(jì)巖漿巖產(chǎn)出, 年齡主要集中在777～1069 Ma(陳義兵等, 1999;李繼磊等, 2009; 胡靄琴等, 2010; Wang et al., 2014a, 2014b), 其可能是庫茹爾組新元古代碎屑鋯石的主要來源。而北伊犁地塊溫泉地區(qū)(圖1b)新元古代變質(zhì)沉積巖廣泛產(chǎn)出中元古代碎屑鋯石(Huang et al., 2016), 其年齡與庫茹爾組中的中元古代碎屑鋯石年齡相似, 因而其可能是后者中元古代碎屑鋯石的主要來源。此外, 通過系統(tǒng)收集整理前人關(guān)于伊犁地塊早古生代巖漿巖年齡數(shù)據(jù)(圖7, 表3), 發(fā)現(xiàn)北伊犁地塊溫泉地區(qū)產(chǎn)出的早古生代巖漿巖, 年齡主要集中在447～466 Ma(胡靄琴等, 2008; Wang et al., 2012; Huang et al., 2013); 而在南伊犁地塊, 早古生代巖漿巖空間上沿北那拉提斷層帶展布, 年齡跨度為426～516 Ma(韓寶福等, 2004; 朱志新等, 2006; Gao et al., 2009; Qian et al., 2009; 徐學(xué)義等, 2010; Long et al., 2011; 朱永峰, 2012; Xu et al., 2013; Gou and Zhang, 2016)。地球化學(xué)研究顯示部分早古生代巖漿巖富集Rb、Sr、Ba、K等大離子親石元素, 虧損Nb、Ta、Ti等高場強(qiáng)元素, 指示其可能產(chǎn)出于俯沖環(huán)境(Gao et al., 2009; Qian et al., 2009; Wang et al., 2012; Huang et al., 2013; Xu et al., 2013; Gou and Zhang, 2016)。北伊犁地塊和南伊犁地塊的早古生代巖漿巖, 與庫茹爾組砂巖樣品H19WT51中的早古生代碎屑鋯石年齡一致(圖8a～d), 因而可能是早古生代碎屑鋯石的主要來源。

圖7 西天山伊犁地塊早古生代巖漿巖分布圖(據(jù)王洪亮等, 2007; 巖漿巖年齡的參考文獻(xiàn)信息見表3)

表3 西天山伊犁地塊早古生代巖漿巖鋯石年齡數(shù)據(jù)

續(xù)表3:

巖漿巖年齡數(shù)據(jù)來源見表3。

4.3 伊犁地塊早古生代構(gòu)造演化

如前文所述, 伊犁地塊西延境外部分對應(yīng)多個前寒武紀(jì)塊體, 其縫合帶很可能沿著伊犁地塊中部向東延伸, 將伊犁地塊分為南、北伊犁兩個塊體(圖1a、b)(Windley et al., 2007; Wang et al., 2012, 2014a, 2014b; Rojas-Agramonte et al., 2014)。其中, 北伊犁地塊早古生代巖漿巖主要集中在447～466 Ma(圖8a), 相比于庫茹爾組中423～519 Ma的碎屑鋯石, 缺少423～447 Ma和466～519 Ma的年齡, 顯示北伊犁地塊不能為其內(nèi)部的庫茹爾組提供全部物源。而南伊犁地塊產(chǎn)出小于447 Ma和大于466 Ma的早古生代巖漿巖(圖8b), 可為北伊犁地塊中庫茹爾組提供該時期物源。由此可見, 南北伊犁地塊均為庫茹爾組提供了早古生代物源, 這指示南北伊犁地塊于志留系庫茹爾組沉積前已完成拼貼。

碎屑鋯石Hf同位素在奧陶紀(jì)末期(約450 Ma)逐漸變得相對富集(圖9c), 指示源區(qū)地殼混染程度加大, 可能與地殼加厚有關(guān)。但本次研究僅有40個鋯石Hf同位素數(shù)據(jù), 為彌補(bǔ)數(shù)據(jù)量不足的問題, 綜合了西天山伊犁地塊早古生代巖漿巖的Hf同位素數(shù)據(jù)(圖9a、b), 并把巖漿巖中的鋯石和樣品H19WT51的碎屑鋯石Hf同位素特征進(jìn)行綜合投圖(圖9d), 系統(tǒng)約束伊犁地塊早古生代巖漿Hf同位素演化趨勢。結(jié)果顯示, 鋯石Hf()值在奧陶紀(jì)末期呈扇形演化樣式, 其巖漿源區(qū)向富集和虧損方向的演化程度明顯增強(qiáng)(圖9d灰色箭頭)。Collins et al. (2011)通過統(tǒng)計(jì)全球顯生宙造山帶中的鋯石Hf同位素數(shù)據(jù), 發(fā)現(xiàn)在環(huán)太平洋增生造山體系中, 隨著俯沖的持續(xù)進(jìn)行,年輕洋殼物質(zhì)不斷底墊到上覆板片之下, 導(dǎo)致巖漿巖鋯石的Hf()值向著正值方向演化, 指示其源區(qū)趨向于虧損。而在顯生宙亞洲多塊體碰撞造山過程中, 隨著微陸塊拼貼底墊、巖石圈拆沉、造山帶垮塌及俯沖翻轉(zhuǎn)等動力學(xué)過程,Hf()值同時向著正和負(fù)值方向演化, 并在中新生代Hf()值范圍達(dá)到了?33～+20(圖9e), 這種Hf()扇形演化樣式指示其源區(qū)同時趨向于富集和虧損，與伊犁地塊早古生代Hf同位素演化樣式類似。由圖9所見, 大約在450 Ma之后, 伊犁地塊中Hf()值演化到了更寬的范圍(?9～+14, 圖9d), 指示南北伊犁地塊可能在奧陶紀(jì)末期碰撞拼貼, 從而導(dǎo)致了Hf()的扇形演化樣式。

巖漿巖中鋯石Hf同位素數(shù)據(jù)來源見表3。

伊犁地塊的境外延伸部分大致于奧陶紀(jì)末期拼貼形成哈薩克斯坦微陸塊, 主要證據(jù)包括: (1)中?上奧陶統(tǒng)沉積地層不整合覆蓋在這些塊體之間的縫合帶之上(Degtyarev, 2003; Alexeiev et al., 2011, 2016); (2)侵入吉爾吉斯?帖爾斯克依縫合帶(KT)的釘合巖體鋯石U-Pb年齡約為460 Ma (Degtyarev et al., 2013); (3)區(qū)域內(nèi)的花崗質(zhì)巖漿活動在450～435 Ma達(dá)到峰期, 可能與哈薩克斯坦微陸塊的形成有關(guān)(Alexeiev et al., 2016); (4)古地磁及古生物數(shù)據(jù)指示這些地塊在奧陶紀(jì)末期?志留紀(jì)早期具有相似的古緯度(Bazhenov et al., 2012)。本研究得出的伊犁地塊拼貼時間(奧陶紀(jì)末期), 與境外對應(yīng)塊體的拼貼時間大致吻合, 因此伊犁地塊的拼合可能對應(yīng)著中亞西部哈薩克斯坦微陸塊的形成。

綜合本研究獲得的碎屑鋯石數(shù)據(jù)和區(qū)域地質(zhì)資料, 我們推測在早中奧陶世, 南北伊犁地塊之間可能存在著一個古洋盆(圖10a), 并在奧陶紀(jì)末期, 洋盆閉合, 南北伊犁地塊發(fā)生碰撞拼貼, 構(gòu)成統(tǒng)一的伊犁地塊(圖10b)。隨后在早中泥盆世, 受到準(zhǔn)噶爾洋的俯沖作用, 在哈薩克斯坦微陸塊之上形成了泥盆紀(jì)巖漿巖帶(DVB, 圖1a、10c)(Wang et al., 2006; Windley et al., 2007; Xiao et al., 2013)。前人把北伊犁地塊中具有弧特征的早古生代巖漿巖歸因于伊犁地塊北側(cè)洋盆的南向俯沖(胡靄琴等, 2008; Wang et al., 2012; Huang et al., 2013), 把南伊犁地塊中具有弧特征的早古生代巖漿巖歸因于伊犁地塊南側(cè)洋盆的北向俯沖(朱志新等, 2006; Gao et al., 2009; Qian et al., 2009; 徐學(xué)義等, 2010; Long et al., 2011; 朱永峰, 2012; Xu et al., 2013; Gou and Zhang, 2016)。這些解釋均基于伊犁地塊為一個僅受到南北兩側(cè)大洋俯沖作用影響的單獨(dú)塊體, 但伊犁地塊可能由兩個塊體拼貼而成, 二者之間洋盆的俯沖消亡, 很可能導(dǎo)致了早古生代弧巖漿巖的產(chǎn)出(圖10a)。

(a) 早中奧陶世, 南北伊犁地塊之間存在一個古洋盆, 其俯沖形成早古生代巖漿巖; (b) 奧陶紀(jì)末期, 洋盆閉合, 南北伊犁地塊發(fā)生碰撞拼貼; (c)早中泥盆世, 伊犁地塊北側(cè)的準(zhǔn)噶爾洋南向俯沖于伊犁地塊之下, 形成泥盆紀(jì)巖漿巖帶(DVB: Devonian Volcanic Belt; 見圖1a)。注: 北伊犁地塊可能對應(yīng)于阿克套?準(zhǔn)噶爾地塊(AJ), 南伊犁地塊可能對應(yīng)于科克切塔夫?北天山地塊(KNTB)。

5 結(jié) 論

(1) 北伊犁地塊中庫茹爾組碎屑鋯石U-Pb年齡主要集中在早古生代(422～519 Ma), 其Hf()值為?7.2～+13.4, 少量分散的前寒武紀(jì)年齡在903～1755 Ma之間。綜合碎屑鋯石最年輕峰值432 Ma及前人的古生物化石資料, 約束庫茹爾組的沉積時代為志留紀(jì)。

(2) 伊犁地塊可能由南北伊犁兩個塊體拼貼而成, 通過對比南北伊犁地塊巖漿活動和碎屑鋯石年齡產(chǎn)出特征, 發(fā)現(xiàn)北伊犁地塊庫茹爾組同時接收了南北伊犁地塊的物源供給, 顯示其在伊犁地塊拼貼后沉積。綜合分析碎屑鋯石和區(qū)域巖漿巖中鋯石的Hf同位素特征, 發(fā)現(xiàn)鋯石Hf()值在奧陶紀(jì)末期呈扇形演化, 其源區(qū)同時趨向于虧損和富集, 指示南北伊犁地塊可能在這一時期發(fā)生碰撞拼貼, 形成統(tǒng)一的伊犁地塊。

致謝:實(shí)驗(yàn)測試過程中得到了中國科學(xué)院廣州地球化學(xué)研究所吳丹工程師和張樂工程師的技術(shù)指導(dǎo), 2名匿名審稿人提出了寶貴的修改意見, 在此深表謝意！

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Early Paleozoic Amalgamation of the Yili Block (Chinese West Tianshan): Insight from Detrital Zircon U-Pb Geochronology and Hf Isotopes

HUANG Huiming1, 2, 3, LI Pengfei1, 2*, HU Wanwan1, 2, 3and LING Jiaqi1, 2, 3

(1.State Key Laboratory of Isotope Geochemistry, 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. University of Chinese Academy of Sciences, Beijing 100049, China)

The Yili Block in the Chinese West Tianshan records key information for the tectonic evolution of the southwestern Central Asian Orogenic Belt (CAOB). Traditionally, the Yili Block is considered as a discrete block. It extends farther west into Kazakhstan/Kyrgyzstan, and is correlated to multiple blocks bounded by a series of sutures. The spatial extension of these sutures within the Yili Block in the Chinese West Tianshan is concealed by late Paleozoic to Cenozoic sedimentary and volcanic rocks. Based on the regional geological correlation, these sutures are considered to merge in the China/Kazakhstan border, and extend eastward along the central Yili Block, and separate the Yili Block into two subunits of northern and southern Yili blocks. In this contribution, we report detrital zircon U-Pb and Hf isotope results of sedimentary rocks of the Silurian Kuru’er Formation within the northern Yili block, with an aim to constrain the amalgamation time of the Yili Block. 51 detrital zircon grains yield early Paleozoic ages of 422 to 519 Ma, and the rest of 12 detrital zircon grains have ages of 903 to 1755 Ma. The youngest age peak of 432 Ma provides a maximum age constraint for the deposition of the Kuru’er Formation, which together with available fossils suggest a Silurian deposition age for this formation.Compared with the early Paleozoic magmatism in the Yili Block, we conclude that the Kuru’er Formation receives detrital zircon from both northern and southern Yili blocks, suggesting that the amalgamation of the Yili Block occurred prior to the deposition of the Kuru’er Formation in the Silurian. Furthermore, a compilation of Hf isotope data of new detrital and magmatic zircon grains from the early Paleozoic igneous rocks shows a fanning array for the εHf() values in the late Ordovician to Silurian, which was interpreted to result from the amalgamation of the Yili Block. Altogether, we consider that the Yili Block may have been amalgamated since the late Ordovician, which is consistent with the amalgamation of the Kazakhstan microcontinent in the western CAOB.

Central Asian Orogenic Belt; Kazakhstan microcontinent; West Tianshan; Yili block; Detrital zircon

2021-01-05;

2021-03-12

中國科學(xué)院國際合作項(xiàng)目(132744KYSB20200001)、國家自然科學(xué)基金(41872222)和廣東省項(xiàng)目(2019QN01H101)聯(lián)合資助。

黃惠明(1995–), 男, 碩士研究生, 構(gòu)造地質(zhì)學(xué)專業(yè)。Email: huanghuiming@gig.ac.cn

李鵬飛(1983–), 男, 研究員, 主要從事造山帶大地構(gòu)造學(xué)研究。Email: pengfeili@gig.ac.cn

P597; P542

A

1001-1552(2021)04-0786-019

10.16539/j.ddgzyckx.2021.04.007