喻順，陳文*，張斌，孫敬博，李超，袁霞，沈澤，楊莉，馬勛

1 中國地質(zhì)科學(xué)院地質(zhì)研究所同位素?zé)崮甏鷮W(xué)實(shí)驗(yàn)室，北京　100037 2 中國地質(zhì)科學(xué)院Re-Os同位素地球化學(xué)重點(diǎn)實(shí)驗(yàn)室，北京　100037 3 中原油田分公司采油五廠，河南 濮陽　457001

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中天山科克蘇河地區(qū)隆升剝蝕歷史
——來自(U-Th)/He年齡的制約

喻順1，陳文1*，張斌1，孫敬博1，李超2，袁霞1，沈澤1，楊莉1，馬勛3

1 中國地質(zhì)科學(xué)院地質(zhì)研究所同位素?zé)崮甏鷮W(xué)實(shí)驗(yàn)室，北京100037 2 中國地質(zhì)科學(xué)院Re-Os同位素地球化學(xué)重點(diǎn)實(shí)驗(yàn)室，北京100037 3 中原油田分公司采油五廠，河南 濮陽457001

天山是中亞造山帶重要組成部分，其中-新生代構(gòu)造熱演化及隆升剝露史研究是認(rèn)識(shí)中亞造山帶構(gòu)造變形過程與機(jī)制的關(guān)鍵.本文應(yīng)用磷灰石(U-Th)/He技術(shù)重建中天山南緣科克蘇河地區(qū)中-新生代構(gòu)造熱演化及隆升剝蝕過程.磷灰石(U-Th)/He數(shù)據(jù)綜合解釋及熱演化史模擬表明該地區(qū)至少存在晚白堊世、早中新世、晚中新世3期快速隆升剝蝕事件，起始時(shí)間分別為～90 Ma、～13 Ma及～5 Ma，且這3期隆升剝蝕事件在整個(gè)天山地區(qū)具有廣泛的可對(duì)比性.相對(duì)于磷灰石裂變徑跡，磷灰石 (U-Th)/He年齡記錄了中天山南緣地質(zhì)演化史中更新和更近的熱信息，即中天山在晚中新世(～5 Ma)快速隆升剝蝕，其剝蝕速率為～0.47 mm·a-1，剝蝕厚度為～2300 m.總體上，中天山科克蘇地區(qū)隆升剝蝕起始時(shí)間從天山造山帶向昭蘇盆地(由南向北)逐漸變老，表明了中天山南緣隆升剝蝕存在不均一性，并發(fā)生了多期揭頂剝蝕事件.

中天山；科克蘇；隆升剝蝕；(U-Th)/He；低溫?zé)崮甏鷮W(xué)

1　引言

天山造山帶東西綿延約2500 km，是中亞造山帶重要的組成部分(圖1a).天山造山帶形成于古生代，在中-新生代經(jīng)歷了多次構(gòu)造疊加和改造過程.現(xiàn)今天山造山帶是受新生代印度—?dú)W亞大陸板塊碰撞作用的影響再次褶皺隆升形成(Avouac et al.,1993；Tapponnier and Molnar,1979).中-新生代以來天山復(fù)雜的構(gòu)造演化過程導(dǎo)致了天山地區(qū)存在多階段、多期次的隆升剝蝕事件.前人研究天山中-新生代隆升剝蝕歷史，取得了一系列重要的成果：① 根據(jù)不整合面的發(fā)育推測天山經(jīng)歷了三疊紀(jì)-晚侏羅世夷平、晚侏羅世-早白堊世隆升及晚白堊世夷平3個(gè)階段(張良臣和吳乃元，1985)；② 裂變徑跡技術(shù)研究表明天山在早白堊世、晚白堊世期間都發(fā)生了明顯的快速隆升(楊庚和錢祥麟，1995；楊樹鋒等，2003；杜治利和王清晨，2007；王彥斌等，2001；賈承造等，2003；馬前等，2006；陳正樂等，2006)，而新生代天山的快速隆升起始時(shí)間有始新世(杜治利和王清晨，2007；Wang et al.,2009)、漸新世(Hendrix et al.,1994;Sobel and Dumitru,1997;Dumitru et al.,2001；De Grave et al.,2013)及中新世(11～17 Ma)( Bullen et al.,2001)等3種觀點(diǎn)；③ 根據(jù)天山周緣盆地的沉積速率、磁性地層學(xué)及構(gòu)造特征等反演了天山山脈的隆升時(shí)間(Windley et al.,1990；Yin et al.,1998 ;Sun et al.,2004,2009;Charreau et al.,2006;Huang et al.,2006)；④ 根據(jù)山前沉積盆地沉積特征、沉降中心變化、沉積物成分及重礦物含量特征等揭示天山隆升剝蝕特征(李忠等,2003；李雙建等，2007)；⑤利用GPS對(duì)天山大地測量推斷天山中新世快速抬升(Abdrakhmatov et al.,1996).上述成果促進(jìn)了我們對(duì)天山造山帶中-新生代剝蝕歷史的認(rèn)識(shí)，同時(shí)也表明天山中-新生代隆升剝蝕歷史仍存在爭議，這可能是由于研究方法的不同引起的，也可能是隆升剝蝕的時(shí)空分布差異性引起的.以往的研究主要是根據(jù)天山山前盆地資料反演天山隆升剝蝕史，而利用熱年代學(xué)技術(shù)研究中天山南緣巖體隆升剝蝕主要集中在境外中天山(吉爾吉斯段)(Glorie et al.,2010；De Grave et al.,2011，2013).另外，天山地區(qū)晚中新世快速隆升剝蝕已被山前盆地內(nèi)磁性地層、巖石磁及生長地層等多種證據(jù)證實(shí)(Huang et al.,2006；Sun et al.,2004,2009;Zhang et al.,2014;Yu et al.,2014;喻順等，2014)，然而在天山造山帶少有該期隆升剝蝕事件的年代學(xué)證據(jù).

近年來，國際上新發(fā)展的磷灰石(U-Th)/He定年技術(shù)具有對(duì)低溫條件的敏感性(40～85 ℃)和較低的封閉溫度(Wolf et al.,1998)，能有效記錄地質(zhì)體經(jīng)歷較低溫度范圍的時(shí)代和溫度信息，即地質(zhì)演化史中最新和最近的信息(地表下1～3 km 深度)，已成為研究造山帶抬升剝露作用的有效手段(Ehlers and Farley,2003;Reiners et al.,2002).本文利用磷灰石(U-Th)/He技術(shù)恢復(fù)中天山南緣科克蘇河地區(qū)中-新生代構(gòu)造熱演化史，揭示該區(qū)中-新生代隆升剝蝕過程，為天山隆升剝蝕事件研究提供年代學(xué)證據(jù)，同時(shí)結(jié)合前人資料探討天山造山帶隆升剝蝕特征，這有助于研究天山地區(qū)構(gòu)造演化機(jī)制、過程及建立構(gòu)造熱演化事件對(duì)比框架，對(duì)于深入認(rèn)識(shí)中亞造山帶構(gòu)造變形過程與機(jī)理具有重要意義.

2　地質(zhì)概況

在古生代晚泥盆世-早石炭世、晚石炭世-早二疊世等期間，多個(gè)地塊之間碰撞、增生融合逐步形成了天山造山帶(Windley et al.,1990;Avouac et al.,1993；Yin et al.,1998；Gao et al.,1998；Xiao et al.,2004).晚二疊世天山抬升作用加速，在天山地區(qū)廣泛發(fā)育陸相磨拉石(Gao et al.,1998；Chen et al.,1999)，中生代早期(三疊紀(jì)-晚侏羅世)天山在地貌上處于準(zhǔn)平原化狀態(tài)，晚侏羅世-早白堊世進(jìn)入隆升作用階段，中生代末期(晚白堊世)再次進(jìn)入?yún)^(qū)域剝蝕夷平狀態(tài)(張良臣和吳乃元，1985；Allen et al.,1993;Bullen et al.,2001；Jolivet et al.,2010).新生代以來，印度—?dú)W亞大陸板塊碰撞作用的遠(yuǎn)程效應(yīng)使天山再次活躍，天山遭受了強(qiáng)烈的擠壓和隆升變形作用而形成陸內(nèi)造山帶(Molnar and Tapponnier,1975;Allen et al.,1993；Hendrix et al.,1994)，并在其兩側(cè)發(fā)育了與陸內(nèi)造山帶相關(guān)的再生前陸盆地(Lu et al.,1994).

圖1　天山地區(qū)地質(zhì)簡圖(a)中亞造山帶構(gòu)造簡圖；(b)天山造山帶地貌圖，圖中斷裂據(jù)Klemd等(2015)修改；(c)中天山南緣科克蘇地區(qū)地質(zhì)簡圖(據(jù)Wang等(2010)修改).Fig.1　Geological and structural sketch map of southwestern Chinese Tianshan(a)Structural sketch map of the Central Asian Orogenic Belt；(b)Present geomorphological map of Tianshan orogeny；(c)Geological map of Kekesu area in the south of Center Tianshan.

天山造山帶發(fā)育多條不同時(shí)代的板塊縫合線，其中北天山縫合帶和中天山南緣縫合帶(圖1b)(Windley et al.,1990；Allen et al.,1993；Gao et al.,1998)將天山造山帶自北向南劃分為北天山、中天山和南天山.中天山南緣縫合帶以北的伊犁—中天山板塊南緣出露元古代變質(zhì)巖系，主要分布在那拉提北緣斷裂與中天山南緣縫合帶之間.高壓變質(zhì)巖帶寬約20 km，主要由綠片巖、藍(lán)片巖、榴輝巖和少量大理巖(Gao et al.,1995,1998)組成(圖1c).由于南天山洋向北俯沖，伊犁-中天山板塊南緣發(fā)育了早古生代晚期至晚古生代的島弧型火山巖、花崗巖，這些島弧型巖漿巖及高壓變質(zhì)巖的形成時(shí)代主要是早志留世-早石炭世(Gao et al.,1995,1998;Klemd et al.,2005;Zhu et al.,2005；朱志新等，2006).在那拉提北緣斷裂與那拉提南緣斷裂(或中天山南緣斷裂)之間，高壓變質(zhì)帶的北側(cè)發(fā)育一條寬近8 km的大型韌性剪切帶，其中心部位遭后期花崗巖侵入、破壞(高俊等，1995；Wang et al.,2007；Xia et al.,2014).

本文的研究區(qū)為新疆特克斯縣南科克蘇河谷剖面，該剖面位于伊犁-中天山地塊南緣至哈爾克山北緣(圖2)，跨越多個(gè)不同的巖石構(gòu)造單元，是研究天山構(gòu)造演化的重要窗口.科克蘇河剖面從特克斯縣向南到韌性剪切帶廣泛出露石炭系大哈拉軍山組和阿克沙克組火山巖及陸源碎屑巖.火山巖主要由玄武巖、玄武安山巖、安山巖、粗面巖、流紋巖、凝灰?guī)r等組成(Wang et al.,2007)，并伴有大量同時(shí)代侵入巖(王博等，2007).侵入巖主要為輝長巖、花崗閃長巖、英云閃長巖、鉀長石花崗巖和花崗質(zhì)巖脈.沉積巖則主要為凝灰?guī)r、砂礫巖、薄層灰?guī)r和泥質(zhì)巖.近年來，很多高精度同位素年代學(xué)研究成果陸續(xù)發(fā)表，其中大哈拉軍山組輝長玢巖中輝石39Ar-40Ar年齡為326.85±15 Ma(劉友梅等，1994)，火山巖鋯石SHRIMP U-Pb年齡分布在354±5 Ma～313±4 Ma(Zhu et al.,2005).角閃花崗巖和鉀長花崗巖鋯石U-Pb年齡分布在352±6 Ma～338±8 Ma(王博等，2007；Gao et al.,2009)，花崗閃長巖鋯石U-Pb年齡為313±4 Ma，黑云母坪39Ar-40Ar年齡為263.4±0.6 Ma(王博等，2007)，被認(rèn)為可能與巖體經(jīng)歷的后期熱事件改造有關(guān)；閃長巖鋯石U-Pb年齡為398～433 Ma(Gao et al.,2009).那拉提二長花崗巖的鋯石SHRIMP U-Pb 年齡為436 Ma和370～366 Ma(朱志新等，2006).

3　樣品和實(shí)驗(yàn)

3.1樣品采集及制備

科克蘇一帶發(fā)育大量的海西期花崗巖，部分巖體侵入到高壓變質(zhì)雜巖帶中.在高壓變質(zhì)雜巖帶北側(cè)，二疊紀(jì)發(fā)生過大規(guī)模的區(qū)域韌性走滑事件.剖面南部那拉提斷裂區(qū)正長巖侵入中-新元古代黑云石英片巖、石榴石斜長麻粒巖、片麻巖、斜長角閃巖及古生代綠片巖(圖2，圖3).剖面北部出露一個(gè)復(fù)合閃長巖體，主要由閃長巖、角閃花崗巖及二長閃長巖組成，該巖體侵入中-新元古代變質(zhì)巖中，同時(shí)該巖體被黑云母花崗巖侵入.樣品TS1370及TS1371位于那拉提韌性剪切帶中(圖2)，構(gòu)造上位于那拉提北緣斷裂與中天山南緣縫合帶之間，TS1370為黑云母石英片巖，分布在前寒武紀(jì)地層中；TS1371為閃長巖，礦物顆粒較粗，該巖體鋯石U-Pb年齡為398±1 Ma(Gao et al.,2009).TS1372黑云母花崗巖與TS1373角閃黑云母花崗巖位于那拉提北緣斷裂的北部，黑云母花崗巖鋯石U-Pb年齡為352±6 Ma(Gao et al.,2009)；TS1373角閃黑云母花崗巖鋯石 U-Pb年齡為349±6 Ma(Gao et al.,2009).樣品TS1374石英正長巖位于科克蘇河剖面最北端，即昭蘇盆地南緣斷裂南側(cè)，這些巖體及地層自中-新生代以來發(fā)生了強(qiáng)烈的抬升剝蝕(Glorie et al.,2010).

圖2　天山科克蘇河地區(qū)地質(zhì)圖(改自Gao等(2009)及Wang等(2010))(圖中鋯石U-Pb年齡引自Gao 等(2009))Fig.2　Geological and structural sketch map of Kekesu section in Chinese Tianshan (modified from Gao et al. (2009)and Wang et al.(2010))

圖3　新疆天山科克蘇河剖面示意圖(剖面位置見圖2，圖中(U-Th)/He年齡見表1)Fig.3　Schematic regional cross section along the Kekesu River (location shown in figure 2)

磷灰石單礦物的分選及制備采用Donelick等(2005)描述的程序.樣品TS1370磷灰石晶體形態(tài)較好，晶體直徑主要分布在60～100 μm；TS1371磷灰石直徑以大于70 μm為主，礦物表面較為粗糙，晶體形態(tài)較完整.TS1372樣品磷灰石顆粒形態(tài)較差，礦物直徑較小.TS1373磷灰石形態(tài)較好、表面光滑、透明均一、無包裹體，然而該樣品磷灰石直徑較小(主要分布在40～60 μm)，僅少量磷灰石直徑大于80 μm.TS1374磷灰石晶體形態(tài)較好，磷灰石直徑多數(shù)大于70 μm、透明均一、無包裹體.磷灰石(U-Th)/He實(shí)驗(yàn)顯微鏡下礦物挑選原則為：挑選晶形較好(自形程度高)、不含包裹體(盡可能少)、無裂縫、干凈透明，且直徑大于70 μm的磷灰石顆粒(減少Ft校正誤差及增加單顆粒He含量)，并在顯微鏡下測量礦物顆粒的尺寸，用于計(jì)算α校正因子(Ft)(Farley et al.,1996).上述每個(gè)樣品在顯微鏡下精選5～10粒磷灰石，測量磷灰石顆粒的大小并照相，再進(jìn)行綜合對(duì)比分析，最終挑選出3～5粒磷灰石用于(U-Th)/He測試(圖4).

3.2(U-Th)/He年齡測試

磷灰石(U-Th)/He年齡測試在中國地質(zhì)科學(xué)院地質(zhì)研究所同位素?zé)崮甏鷮W(xué)實(shí)驗(yàn)室進(jìn)行.磷灰石(U-Th)/He年齡是根據(jù)礦物中U、Th及He含量計(jì)算獲得，其實(shí)驗(yàn)流程如下.

利用Alphachron II四極桿質(zhì)譜系統(tǒng)完成磷灰石He測量.將已完成尺寸測量磷灰石顆粒放入Pt包中，加載到Alphachron II儀器激光室.利用970 nm半導(dǎo)體二極管激光加熱兩次，加熱溫度約為900～1000 ℃，持續(xù)5 min，完全提取磷灰石4He.之后，將4He與同位素稀釋劑3He充分混合，利用Pfeiffer Prisma四極桿質(zhì)譜測量He同位素比值；根據(jù)標(biāo)準(zhǔn)罐氣體4He校正同位素稀釋劑3He量；依據(jù)同位素稀釋劑3He量計(jì)算磷灰石4He量，總體上He的測量精確度小于1%.

磷灰石U、Th含量測定是根據(jù)同位素稀釋電感耦合等離子體質(zhì)譜法(ID ICP MS).在測完He的磷灰石樣品中加入25 μL濃度為50% HNO3(體積比，大約7 mol/L)的稀釋劑溶液(包含約15ppb235U 和5 ppb230Th)，然后置于超聲波清洗槽15 min，隨后樣品在酸性稀釋劑溶液中溶解12 h.向25 μL標(biāo)準(zhǔn)溶液(25ppb U和25 ppb Th)中加入25 μL稀釋劑溶液.在標(biāo)準(zhǔn)溶液和樣品溶液中各加入250 μL 蒸餾水.利用Thermofisher-X2(中國科學(xué)院地質(zhì)與地球物理研究所)電感耦合等離子體質(zhì)譜儀(ICPMS)測量化學(xué)處理后溶液的U、Th同位素比值.U、Th同位素比值測量精確度(相對(duì)標(biāo)準(zhǔn)偏差)小于2%.樣品中U、Th同位素含量依據(jù)標(biāo)準(zhǔn)溶液U、Th含量計(jì)算獲得.磷灰石(U-Th)/He實(shí)驗(yàn)測量精確度依據(jù)磷灰石標(biāo)準(zhǔn)樣品年齡的測量結(jié)果確定，中國地質(zhì)科學(xué)院地質(zhì)研究所同位素?zé)崮甏鷮W(xué)實(shí)驗(yàn)室測試21粒Durango磷灰石(U-Th)/He年齡為31.5±1.4(1σ)Ma(參考年齡為31.02±1.01 Ma(McDowell et al.,2005))，磷灰石(U-Th)/He年齡測試精確度(相對(duì)標(biāo)準(zhǔn)偏差)小于2%.

圖4　科克蘇河磷灰石鏡下測量照片F(xiàn)ig.4　Images of apatite grains analyzed in this study

4　構(gòu)造熱演化史及地質(zhì)意義

4.1(U-Th)/He測年結(jié)果判讀

科克蘇河剖面樣品磷灰石(U-Th)/He年齡測試分析結(jié)果見表1.(U-Th)/He未校正年齡分布在2～160 Ma，經(jīng)過Ft校正的年齡分布在2.7～226 Ma.本次采集樣品(TS1371-TS1374)的巖性為閃長巖、花崗巖及石英正長巖，這些巖石在該地區(qū)廣泛發(fā)育，前人研究表明它們形成于二疊紀(jì)及以前(王博等，2007；Gao et al.,2009；Wang et al.,2010)，且測試的磷灰石(U-Th)/He年齡遠(yuǎn)小于巖體的形成年齡，表明磷灰石(U-Th)/He年齡為巖體中-新生代冷卻年齡；另外一個(gè)樣品TS1370巖性為新元古代片巖，其磷灰石(U-Th)/He年齡也代表了該樣品中-新生代冷卻年齡.這些樣品冷卻年齡與該地區(qū)后期發(fā)生的構(gòu)造事件有關(guān)，記錄了樣品抬升或逆沖引起的剝蝕信息.

科克蘇河剖面部分樣品磷灰石 (U-Th)/He年齡較為分散，如樣品TS1373和TS1374.磷灰石(U-Th)/He年齡分散原因?yàn)椋孩?未識(shí)別的富含U、Th的包裹體(Farley,2002;Lippolt et al.,1994)的影響.通過在顯微鏡下精細(xì)地挑選磷灰石，可以將此種影響因素減小到最低.另外，在測量He時(shí)第二次加熱去氣結(jié)果也可以作為一個(gè)輔助參數(shù)，如第二次加熱釋放的He量與熱空白接近，表明磷灰石含有包裹體可能性小，本次實(shí)驗(yàn)中第二次所測量的磷灰石He量與熱空白較為接近，表明了礦物包裹體可能性較小.② 磷灰石顆粒大小變化導(dǎo)致磷灰石He封閉溫度的差異(Farley,2000).Wolf 等 (1998)建立磷灰石熱擴(kuò)散模型，表明了如果磷灰石長時(shí)間停留在He部分保留區(qū)，磷灰石顆粒半徑的差異可引起(U-Th)/He年齡分散，本次測量的樣品磷灰石顆粒半徑與(U-Th)/He年齡關(guān)系見圖5，TS1374磷灰石(U-Th)/He年齡與顆粒半徑正相關(guān)，表明其年齡分散可能與半徑有關(guān).③ U和Th的植入效應(yīng)(Farley,2002；Spiegel et al.,2009).當(dāng)磷灰石顆粒內(nèi)U和Th含量遠(yuǎn)低于其周圍巖層U和Th含量，特別是在磷灰石顆粒內(nèi)eU濃度(eU=U+0.235Th)小于5 ppm時(shí)，U和Th的植入效應(yīng)影響明顯(Spiegel et al.,2009)，科克蘇河地區(qū)樣品僅TS1370-3磷灰石eU濃度較小(表1)，僅在7.5 ppm左右，然而樣品TS1370各顆粒磷灰石(U-Th)/He年齡較集中，表明了年齡受此影響因素不明顯.④ 增強(qiáng)He保留效應(yīng)(輻射損傷)(Fitzgerald et al.,2006;Flowers et al.,2009;Gautheron et al.,2009;Green and Duddy,2006;Shuster et al.,2006;Shuster and Farley,2009)導(dǎo)致(U-Th)/He年齡偏大.Shuster 等 (2006)和Flowers 等(2007)研究成果表明，如磷灰石(U-Th)/He年齡與eU濃度和He濃度呈正相關(guān)，說明它們受輻射損傷影響明顯.圖5表明科克蘇河地區(qū)樣品僅TS1374磷灰石(U-Th)/He年齡與eU濃度正相關(guān)，表明TS1374年齡的分散可能與增強(qiáng)He保留效應(yīng)有關(guān).另外，樣品TS1372-1磷灰石(U-Th)/He年齡與其他顆粒年齡差異較大，分析原因可能是該磷灰石顆粒直徑53 μm(圖4)，(U-Th)/He年齡校正系數(shù)Ft(0.43)較小，影響了測年結(jié)果的準(zhǔn)確性.因此，該顆粒不參與樣品的平均(U-Th)/He年齡計(jì)算.圖2顯示了自南向北樣品磷灰石(U-Th)/He平均年齡總體上增大的特征.

表1　科克蘇河磷灰石(U-Th)/He年齡測試結(jié)果

注：rad 顆粒等效半徑；Unc.age 未校正年齡；Cor.Age 校正年齡；eU=U+0.235Th(Shuster et al.,2006)；Ft為α粒子射出效應(yīng)的校正參數(shù)(Farley et al.,1996)；* 表示該磷灰石顆粒半徑較小(見圖4)，F(xiàn)t校正系數(shù)較大，未參與平均年齡計(jì)算；黑體數(shù)據(jù)為樣品單顆粒年齡的平均.

圖5　科克蘇河磷灰石半徑和eU與(U-Th)/He年齡關(guān)系圖Fig.5　Apatite (U-Th)/He ages versus effective U concentration (eU=U+0.235Th)and radius for the analyzed samples in Kekesu section

4.2構(gòu)造熱演化史模擬原理

低溫?zé)嵫莼瘹v史可以利用數(shù)值模擬軟件進(jìn)行模擬，如QTQt(Gallagher，2012)軟件等.QTQt 軟件模擬原理是以Bayesian(貝葉斯)統(tǒng)計(jì)方法為基礎(chǔ)，采用Markov Chain Monte Carlo(馬爾可夫鏈蒙特卡爾)理論模擬計(jì)算，獲取樣品一定范圍內(nèi)可能的熱演化模型，作為樣品經(jīng)歷熱演化的時(shí)間和溫度先驗(yàn)信息，利用Bayesian方法獲取適合數(shù)據(jù)的簡單熱史模型，優(yōu)選概率值較大的熱史模型，定量化可接受模型的概率分布(后驗(yàn)概率分布模型).上述軟件模擬時(shí)也考慮了顆粒尺寸、輻射損傷及冷卻速率等對(duì)熱史的影響.Flowers等(2007，2009)認(rèn)為磷灰石(U-Th)/He年齡與eU濃度和He濃度的相關(guān)性對(duì)樣品所經(jīng)歷的熱演化史敏感，當(dāng)樣品經(jīng)歷一樣的熱史，如磷灰石(U-Th)/He年齡與eU濃度呈非線性正相關(guān)，認(rèn)為磷灰石(U-Th)/He體系受輻射損傷退火影響明顯，并建立了磷灰石(U-Th)/He輻射損傷退火模型(RDAAM模型)處理這類樣品.圖5顯示樣品TS1374磷灰石(U-Th)/He年齡與eU濃度和He濃度的呈正相關(guān)，在運(yùn)用QTQt軟件對(duì)研究區(qū)(U-Th)/He數(shù)據(jù)模擬時(shí)，樣品TS1374選擇輻射損傷退火模型進(jìn)行定量模擬.

對(duì)于用于模擬的同一樣品，所有的磷灰石顆粒都經(jīng)歷了相同的地質(zhì)演化過程，其(U-Th)/He數(shù)據(jù)是樣品所經(jīng)歷的熱演化過程反映，模擬時(shí)根據(jù)基礎(chǔ)地質(zhì)演化史輸入溫度-年齡等約束條件.將已知的現(xiàn)今地表溫度(15±5 ℃)輸入軟件作為邊界條件.科克蘇地區(qū)發(fā)育大量的海西期花崗巖，部分巖體侵入到高壓變質(zhì)雜巖體中.在高壓變質(zhì)帶的北側(cè)，二疊紀(jì)發(fā)生了大型的區(qū)域韌性走滑事件，北側(cè)花崗閃長巖巖體黑云母39Ar-40Ar坪年齡為263.4±0.6 Ma(王博等，2007)，樣品TS1371(閃長巖)黑云母39Ar-40Ar坪年齡為294.8±1.6 Ma(另文發(fā)表)，這些年齡可能與巖體經(jīng)歷了此期構(gòu)造熱事件有關(guān).因此，根據(jù)以上地質(zhì)條件假設(shè)樣品的模擬起始時(shí)間為260±20 Ma，起始溫度為350±50 ℃.圖6顯示了距今最近且最相關(guān)的熱演化史.熱模擬模型對(duì)樣品早期的熱演化路徑約束較差，但對(duì)距今最近的冷卻和剝蝕路徑給予了較好的約束，即使這些熱模擬模型不能給出精確的剝蝕路徑，但是它們能提供樣品隨時(shí)間-溫度演化的窗口，顯示樣品開始冷卻及通過磷灰石He部分保留區(qū)的時(shí)間，因而為樣品經(jīng)歷低溫階段冷卻歷史提供了有力的信息.

4.3構(gòu)造熱演化史

樣品TS1370和TS1371位于那拉提韌性剪切帶中，其中樣品TS1370為前寒武紀(jì)黑云母石英片巖；TS1371閃長巖鋯石U-Pb年齡為398±1 Ma(Gao et al.,2009).TS1370磷灰石(U-Th)/He年齡分布在4.5～8.5 Ma，平均年齡為6.0 Ma；TS1371磷灰石(U-Th)/He年齡分布在2.7～4.4 Ma，平均年齡為3.5 Ma，因此，將剪切帶內(nèi)上述兩個(gè)樣品 (U-Th)/He年齡解釋為冷卻年齡，它們記錄了巖體經(jīng)過磷灰石He部分保留區(qū)間的時(shí)間.熱模擬(圖6)結(jié)果顯示韌性剪切帶內(nèi)樣品在新生代存在一期快速冷卻事件，快速冷卻的起始時(shí)間為～5 Ma，即樣品～5 Ma開始快速冷卻通過磷灰石He部分保留區(qū)，表明了該地區(qū)新生代快速隆升剝蝕事件起始時(shí)間為～5 Ma.假設(shè)地溫梯度為30 ℃/km(馮昌格等，2009；王良書等，2003)，地表溫度為15 ℃，磷灰石(U-Th)/He封閉溫度為85 ℃(喻順等，2014；Qiu et al.,2012)，計(jì)算新生代晚中新世該地區(qū)隆升剝蝕厚度為～2300 m，剝蝕速率為～0.47 mm·a-1.

樣品TS1372為黑云母花崗巖，鋯石U-Pb年齡為352±6 Ma(Gao et al.,2009)，該樣品測試了3粒磷灰石，其(U-Th)/He年齡分別為138.6 Ma、98.6 Ma及87.7 Ma，其中年齡為138.6 Ma的磷灰石顆粒直徑較小，未用于熱史模擬，另外兩粒磷灰石(U-Th)/He年齡較年輕，記錄了該區(qū)黑云母花崗巖熱演化信息，這兩粒磷灰石(U-Th)/He數(shù)據(jù)模擬結(jié)果見圖6.模擬結(jié)果揭示了該樣品經(jīng)歷了3階段的冷卻，第一階段起始于溫度大于120 ℃，接著從90 Ma到85 Ma迅速的冷卻通過磷灰石He半保留區(qū)，最后緩慢抬升冷卻至地表.

圖6　科克蘇地區(qū)樣品QTQt軟件熱史模擬圖左：樣品熱演化史模擬(頻率分布圖)；右：樣品期望熱史模擬.Max.Like Model 最大似然模型；Max.Post.Model 最大后驗(yàn)?zāi)Ｐ?；Expected Model 預(yù)期模型；Max.Mode Model 最大模態(tài)模型(模型詳細(xì)解釋見Gallagher(2012)).Fig.6　Time-temperature paths of AHe ages modeled using QTQt softwareLeft：Thermal history modeling of the samples；right：Thermal history of the Expected model.

樣品TS1373角閃黑云母花崗巖鋯石U-Pb年齡為349±6 Ma(Gao et al.,2009).該樣品測試了5粒磷灰石，其磷灰石(U-Th)/He年齡較為分散，分別為～48 Ma,35.8 Ma,70.7 Ma,59.7 Ma 和28.3 Ma，平均年齡為48.5 Ma.熱史模擬結(jié)果顯示該樣品經(jīng)歷了2個(gè)階段的冷卻史，第一階段為大于13 Ma，樣品緩慢冷卻通過磷灰石He半保留區(qū)，之后樣品快速抬升剝蝕至地表.由圖6可見該樣品長期停留于磷灰石He半保留區(qū)(40～85 ℃)(Wolf et al.,1998)，這可能是引起磷灰石(U-Th)/He年齡分散的原因之一.

TS1374石英正長巖鋯石(U-Th)/He年齡約為320 Ma(另文發(fā)表).該區(qū)花崗閃長巖巖體黑云母坪39Ar-40Ar年齡為263.4±0.6 Ma(王博等，2007)，4粒磷灰石(U-Th)/He年齡分別為226.7 Ma、191.3 Ma、176.8 Ma及207.8 Ma(平均年齡為201 Ma)，小于該區(qū)鋯石(U-Th)/He年齡及黑云母39Ar-40Ar年齡.熱史模擬結(jié)果見圖6，熱史可分為兩個(gè)階段，即～220 Ma之前的快速冷卻階段和～220 Ma之后的緩慢冷卻階段，表明了該巖體自～220 Ma以來處于緩慢隆升剝蝕階段，這可能與該巖體處于盆地邊緣相對(duì)較低的部位(低海拔)及受后期構(gòu)造運(yùn)動(dòng)事件影響較小有關(guān).

5　討論

天山造山帶經(jīng)歷了古生代、中生代及新生代多旋回、復(fù)雜的構(gòu)造和巖漿作用疊加、改造過程.磷灰石(U-Th)/He年齡數(shù)據(jù)熱演化史模擬表明該區(qū)至少存在3期快速隆升剝蝕事件，分別起始于90 Ma、13 Ma及5 Ma，這3期隆升剝蝕事件在區(qū)域上具有廣泛的可對(duì)比性.中天山拉爾墩黑云母花崗巖磷灰石裂變徑跡年齡89 Ma(王彥斌等，2001)記錄了中天山晚白堊世隆升剝蝕事件，庫車盆地花崗巖裂變徑跡分析表明該地區(qū)晚白堊世隆升剝蝕事件發(fā)生在89 Ma前(賈承造等，2003).De Grave等(2013)及Glorie等(2010)利用磷灰石裂變徑跡及(U-Th)/He技術(shù)研究吉爾吉斯段天山巖體獲得了110～90 Ma冷卻年齡，天山那拉提山脈東部也獲得相似磷灰石裂變徑跡及(U-Th)/He年齡(Jolivet et al.,2010;Dumitru et al.,2001).庫車盆地北緣碎屑巖磷灰石裂變徑跡研究表明天山存在～90 Ma晚白堊世快速隆升剝蝕事件(Hendrix et al.,1994；羅夢等，2014；杜治利和王清晨，2007).柴窩堡盆地、庫車盆地及巴倫臺(tái)裂變徑跡數(shù)據(jù)表明天山在～96 Ma發(fā)生了較快的抬升剝蝕(杜治利和王清晨，2007).吉爾吉斯境內(nèi)的西天山磷灰石裂變徑跡研究表明該地區(qū)早白堊世-古近紀(jì)經(jīng)歷過一次快速抬升作用(Dobrestsov et al.,1996).陳正樂等(2006)根據(jù)西天山察汗烏蘇山石炭紀(jì)火山巖的裂變徑跡數(shù)據(jù)分析揭示該地區(qū)存在110～80 Ma的快速隆升剝蝕事件.吐魯番—哈密盆地南部造山帶的磷灰石裂變徑跡年齡數(shù)據(jù)表明構(gòu)造抬升發(fā)生在 88～97 Ma(郭召杰等,2002).這些研究成果與科克蘇河磷灰石(U-Th)/He記錄的年齡一致.另一方面天山山脈中普遍缺乏了白堊系的沉積，這與晚白堊世區(qū)域性隆升剝蝕有關(guān)，如庫車坳陷乃至整個(gè)塔里木盆地東部地區(qū)缺失上白堊統(tǒng)，古近系直接覆蓋在下白堊統(tǒng)之上；柴窩堡盆地普遍缺失上白堊統(tǒng)(曹守連和何登發(fā)，1997)；吐哈盆地上、下白堊統(tǒng)之間的角度不整合，也揭示在白堊紀(jì)時(shí)期發(fā)生過大面積的隆升剝露作用.上述研究表明整個(gè)天山山脈在晚白堊世存在一期強(qiáng)烈的抬升剝蝕事件，該期構(gòu)造事件可能是青藏高原地區(qū)Kohistan-Dras島弧與拉薩地體碰撞遠(yuǎn)距離效應(yīng)的結(jié)果.

QTQt軟件及磷灰石(U-Th)/He數(shù)據(jù)模擬揭示了科克蘇河角閃黑云母花崗巖快速抬升剝蝕發(fā)生在～13 Ma.在天山其他地區(qū)，關(guān)于～13～10 Ma的快速隆升剝蝕時(shí)間已有相關(guān)報(bào)道.磷灰石裂變徑跡記錄了西天山山前帶快速逆沖抬升剝蝕的時(shí)間為13.6±2.2(1σ)(Sobel and Dumitru,1997)；根據(jù)磁性地層及地質(zhì)年代學(xué)數(shù)據(jù)研究表明吉爾吉斯西段的天山快速隆起開始于～11 Ma(Bullen et al.,2001;Sobel et al.,2006).另外，根據(jù)磁性地層和巖石磁學(xué)研究奎屯河沉積剖面表明北天山在～10 Ma快速抬升(Charreau et al.,2005).Abdrakhmatov等(1996)和 Reigber 等(2001)通過GPS測量現(xiàn)今地殼縮短率推算天山新生代重新活動(dòng)起始于～10 Ma.Charreau等(2006)通過對(duì)克拉蘇—依奇克里克構(gòu)造帶南部牙哈剖面詳細(xì)調(diào)查，認(rèn)為中新世沉積地層沉積速率快速增大，指示了天山地區(qū)抬升剝蝕時(shí)間為～11 Ma.劉志宏等(2000)利用生長斷層相關(guān)褶皺理論模型研究克拉蘇背斜，認(rèn)為變形時(shí)間起始于新近系康村組(13.5～5.9 Ma)沉積期，即約為13.5 Ma.庫車盆地構(gòu)造地質(zhì)調(diào)查、地震地層分析及古地磁定年研究表明天山在～13 Ma開始加速變形(Zhang et al.,2014).Yu等(2014)根據(jù)磷灰石(U-Th)/He數(shù)據(jù)模擬揭示庫車盆地克拉蘇—依奇克里克構(gòu)造帶北緣邊界斷層上盤快速抬升剝蝕發(fā)生在～10 Ma.這些研究都表明在中新世(～13～10 Ma)天山地區(qū)(包括南天山、中天山及北天山)整體抬升，且抬升范圍和強(qiáng)度有增大的趨勢，同時(shí)這次抬升事件導(dǎo)致了科克蘇河地區(qū)快速隆升剝蝕事件.

磷灰石(U-Th)/He年齡及熱史模擬研究表明科克蘇河韌性剪切帶晚中新世快速隆升剝蝕的起始時(shí)間為～5 Ma，這一重要的隆升剝蝕事件在天山南北緣有地層學(xué)、古地磁學(xué)及熱年代學(xué)記錄.磷灰石(U-Th)/He數(shù)據(jù)研究庫車盆地坎亞肯背斜及吐孜洛克背斜表明～5～6 Ma存在一期快速隆升剝蝕事件(喻順等，2014；Yu et al.,2014)；利用塔北隆起磷灰石(U-Th)/He數(shù)據(jù)模擬反演表明其物源區(qū)天山快速隆升剝蝕發(fā)生在～8～5 Ma(Qiu et al.,2012).庫車盆地克拉蘇—依奇克里克構(gòu)造帶一些鉆井(如吐孜2，克拉204)缺乏了庫車組及其上覆地層，這表明該構(gòu)造帶隆升剝蝕起始時(shí)間約為5.9 Ma.Huang等(2006)及Zhang等(2014)研究庫車盆地磁性地層、巖石磁及平衡剖面表明盆地沉積速率在～7～6.5 Ma顯著增加，并認(rèn)為南天山地區(qū)在～7～6.5 Ma快速隆升剝蝕.天山南北緣山前盆地古地磁學(xué)、生長地層及巖層幾何形態(tài)等相結(jié)合研究顯示天山在7～6 Ma快速隆升剝蝕(Sun et al.,2004,2009;Sun and Zhang,2009).

然而，這期隆升剝蝕事件并沒有被天山地區(qū)磷灰石裂變徑跡數(shù)據(jù)記錄(Dumitru et al.,2001;Hendrix et al.,1994;Sobel and Dumitru,1997;杜治利等,2007;羅夢等,2012;楊庚和錢祥麟,1995;賈承造等,2003;馬前等,2006)，其原因可能是相對(duì)于天山新生代晚期的快速抬升，磷灰石裂變徑跡退火溫度顯得“過高”，當(dāng)構(gòu)造抬升至較淺位置后，磷灰石裂變徑跡已不能記錄“更低溫度”的抬升歷史(圖7a)，地表樣品的磷灰石裂變徑跡未能記錄該區(qū)域中新世隆升剝蝕事件；對(duì)沉積盆地樣品而言，相對(duì)于磷灰石裂變徑跡退火溫度(60～120 ℃)(Green,1988)，磷灰石(U-Th)/He體系具有更低敏感溫度區(qū)間(40～85 ℃)，當(dāng)沉積地層埋藏深度小于裂變徑跡退火深度而大于磷灰石(U-Th)/He封閉溫度(深度)時(shí)，磷灰石(U-Th)/He體系可記錄快速隆升剝蝕事件(圖 7b)，這也是天山山前盆地大量裂變徑跡數(shù)據(jù)只記錄了物源區(qū)侏羅紀(jì)-白堊紀(jì)及漸新世等隆升剝蝕事件，而磷灰石(U-Th)/He年齡卻能記錄盆地原位晚中新世隆升剝蝕事件的原因(Yu et al.,2014).

圖7　磷灰石裂變徑跡及(U-Th)/He對(duì)晚新生代熱演化史的約束示意圖AHePRZ指磷灰石He部分保留區(qū)，APAZ表示磷灰石裂變徑跡部分退火帶.Fig.7　Sketch map of thermal history during late Cenozoic based on AFT and AHe data

總體上，科克蘇河地區(qū)磷灰石(U-Th)/He年齡被解釋為冷卻年齡，QTQt軟件模擬表明中天山南緣科克蘇河地區(qū)自造山帶向盆地方向隆升剝蝕起始時(shí)間變老，即韌性剪切帶(靠近變質(zhì)帶區(qū))在～5 Ma快速隆升剝蝕，向北擴(kuò)展存在～13 Ma及～90 Ma大范圍隆升剝蝕事件，而在昭蘇盆地邊緣區(qū)中-新生代緩慢抬升剝蝕，這不僅體現(xiàn)了科克蘇地區(qū)隆升剝蝕的不均一性，也表明在該地區(qū)存在多期揭頂剝蝕事件.

6　結(jié)論

(1)中天山南緣科克蘇河地區(qū)磷灰石(U-Th)/He年齡有效地記錄了該地區(qū)中-新生代抬升剝蝕時(shí)間及溫度演化信息.磷灰石 (U-Th)/He數(shù)據(jù)綜合解釋及熱演化史模擬表明晚白堊世以來中天山南緣至少存在3期快速隆升剝蝕事件，起始時(shí)間分別為～90 Ma、～13 Ma及～5 Ma，且這3期隆升剝蝕事件在整個(gè)天山區(qū)域上具有廣泛的可對(duì)比性，如中天山南緣在晚白堊世及中新世發(fā)生了強(qiáng)烈的快速隆升剝蝕，在南天山及北天山地區(qū)均有相對(duì)應(yīng)的熱年代學(xué)及地層學(xué)證據(jù)，這表明了天山地區(qū)存在多階段整體抬升剝蝕.

(2)相對(duì)于磷灰石裂變徑跡數(shù)據(jù)，磷灰石 (U-Th)/He記錄了中天山南緣地質(zhì)演化中最新和最近的信息，即中天山南緣在晚中新世快速隆升剝蝕起始時(shí)間為～5 Ma，隆升剝蝕厚度為～2300 m，剝蝕速率為～0.47 mm·a-1，這期隆升剝蝕事件得到了天山兩側(cè)沉積盆地古地磁學(xué)、生長地層及地層不整合等研究成果的證實(shí).

(3)中天山南緣隆升剝蝕起始時(shí)間從天山造山帶向昭蘇盆地(由南向北)逐漸變老，體現(xiàn)了科克蘇地區(qū)隆升剝蝕具有強(qiáng)烈的不均一性，也表明在該地區(qū)存在多期揭頂剝蝕事件.

致謝磷灰石單礦物分選工作在河北省區(qū)域地質(zhì)礦產(chǎn)調(diào)查所實(shí)驗(yàn)室完成，磷灰石化學(xué)處理工作得到了張巧大研究員的幫助，U、Th元素質(zhì)譜分析測試得到了中國科學(xué)院地質(zhì)與地球物理研究所王非研究員、吳林博士、張煒斌等人的幫助，蘇榕、徐志華等參與了野外地質(zhì)調(diào)查工作，在此一并表示衷心的感謝!

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(本文編輯胡素芳)

Mesozoic and Cenozoic uplift and exhumation history of the Kekesu section in the Center Tianshan:constrained from (U-Th)/He thermochronometry

YU Shun1,CHEN Wen1*,ZHANG Bin1,SUN Jing-Bo1,LI Chao2,YUAN Xia1,SHEN Ze1,YANG Li1,MA Xun3

1 Laboratory of Isotope Thermochronology,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China 2 Key Laboratory of Re-Os Isotope Geochemistry,Chinese Academy of Geological Sciences,Beijing 100037,China 3 NO.5 Oil Production Plant,Zhongyuan Oil Field Company,SINOPEC,Puyang Henan 457001,China

The Tianshan is the main part of the Central Asian Orogenic Belt.Knowledge of Mesozoic-Cenozoic tectonothermal evolution and exhumational history is critical to better understand the deformed process and mechanism of the Central Asian Orogenic Belt.Apatite (U-Th)/He thermochronometry was applied to compile a low temperature,multi-stage thermal history of tectonic uplift and exhumation of the Kekesu section in the South of Center Tianshan.New apatite (U-Th)/He ages and modeling indicated that the Center Tianshan experienced at least three-stage events of uplift and exhumation during Mesozoic and Cenozoic,which initiated at ～90 Ma,～13 Ma and ～5 Ma,respectively.The three-stage events of uplift and exhumation were also discovered in the South and North Tianshan.Compared with apatite fission track ages,apatite (U-Th)/He ages recorded the younger event (～5 Ma)of uplift and exhumation since the Miocene,which means in the last ～5 Ma,～2300 m of overburden was removed during Cenozoic folding and thrusting with a corresponding denudation rate of ～0.47 mm·a-1.It was suggested the initial timing of uplift and exhumation gradually became younger from the South to the North by QTQt modeling of the AHe ages in the Kekesu section during the Mesozoic-Cenozoic,which is more likely that there has been differential uplift/exhumation and multistage unroofing in the South of Center Tianshan.

Center Tianshan；Kekesu；Uplift and exhumation；(U-Th)/He ages；Low-temperature thermochronology

喻順，陳文，張斌等.2016.中天山科克蘇河地區(qū)隆升剝蝕歷史——來自(U-Th)/He年齡的制約.地球物理學(xué)報(bào)，59(8):2922-2936,

10.6038/cjg20160817.

Yu S,Chen W,Zhang B,et al.2016.Mesozoic and Cenozoic uplift and exhumation history of the Kekesu section in the Center Tianshan:constrained from (U-Th)/He thermochronometry.Chinese J.Geophys.(in Chinese),59(8):2922-2936,doi:10.6038/cjg20160817.

國家自然科學(xué)基金(41503058，41473053，41503057)，公益性行業(yè)專項(xiàng)經(jīng)費(fèi)(201511064-2)，地質(zhì)礦產(chǎn)調(diào)查評(píng)價(jià)項(xiàng)目(12120113015600,DD20160123-02)和中國地質(zhì)科學(xué)院基本科研業(yè)務(wù)費(fèi)項(xiàng)目(J1625)資助.

喻順，男，1982年生，博士，主要從事同位素?zé)崮甏鷮W(xué)研究．E-mail:yushun0722@163.com

陳文，男，1962年生，博士，研究員，博士生導(dǎo)師，從事同位素地質(zhì)年代學(xué)研究.E-mail:chenwenf@vip.sina.com

10.6038/cjg20160817

P314,P597

2016-01-17，2016-04-01收修定稿