鄧 斌, 冉 波, 葉玥豪, 王自劍, 王成善, 朱利東, 田 慶, 岳雅惠

(1.油氣藏地質(zhì)及開發(fā)工程國家重點(diǎn)實(shí)驗(yàn)室(成都理工大學(xué))，成都 610059; 2.生物地質(zhì)與環(huán)境地質(zhì)國家重點(diǎn)實(shí)驗(yàn)室(中國地質(zhì)大學(xué))，北京 100083; 3.大陸碰撞和青藏高原隆升重點(diǎn)開放實(shí)驗(yàn)室,中國科學(xué)院青藏高原研究所,北京 100101)

阿爾金斷裂東端酒西盆地古近系物源分析及意義

鄧 斌1, 冉 波1, 葉玥豪1, 王自劍1, 王成善2, 朱利東1, 田 慶1, 岳雅惠3

(1.油氣藏地質(zhì)及開發(fā)工程國家重點(diǎn)實(shí)驗(yàn)室(成都理工大學(xué))，成都 610059; 2.生物地質(zhì)與環(huán)境地質(zhì)國家重點(diǎn)實(shí)驗(yàn)室(中國地質(zhì)大學(xué))，北京 100083; 3.大陸碰撞和青藏高原隆升重點(diǎn)開放實(shí)驗(yàn)室,中國科學(xué)院青藏高原研究所,北京 100101)

探討酒西盆地古近紀(jì)沉積與阿爾金左行走滑斷裂的關(guān)系。通過對(duì)酒西盆地西部紅柳峽剖面古近系火燒溝組、白楊河組進(jìn)行系統(tǒng)的沉積學(xué)和年代學(xué)分析表明：火燒溝組的礫石成分主要以中-低級(jí)變質(zhì)巖和沉積巖為主，到了白楊河組則轉(zhuǎn)變?yōu)橐詭r漿巖為主；古水流方向主體來自西部-西北部；室內(nèi)砂巖碎屑鑒定結(jié)果顯示古近系砂巖的碎屑成分全部落入再旋回造山帶物源區(qū)；碎屑鋯石U-Pb年齡測(cè)定結(jié)果表現(xiàn)出新元古代和中元古代2個(gè)主要的年齡峰值。綜合對(duì)比酒西盆地周緣的物源區(qū)，初步認(rèn)為阿爾金地體為火燒溝組和白楊河組主要的物源供給區(qū)。結(jié)合前人對(duì)阿爾金左行走滑斷裂的研究，表明自漸新世以后阿爾金地體才離開酒西盆地，進(jìn)一步確定阿爾金斷裂帶最大左行走滑量>450 km。

阿爾金斷裂；酒西盆地；阿爾金地體；物源分析；走滑

作為青藏高原北緣邊界并分隔青藏高原與塔里木板塊的阿爾金斷裂是一條全長近1 600 km的左行走滑斷裂[1-3]，其構(gòu)造演化與走滑過程因新生代印度-歐亞板塊強(qiáng)烈碰撞所導(dǎo)致的遠(yuǎn)程變形效應(yīng)充滿爭(zhēng)議[1,2,4-6]。位于阿爾金斷裂中-東段的一系列小型新生代沉積盆地的沉積環(huán)境變化敏感地記錄了阿爾金斷裂的構(gòu)造運(yùn)動(dòng)和走滑過程[5,7-8]，使其可為反演斷裂的構(gòu)造歷史提供良好的研究素材[2,5,9-11]。前人主要針對(duì)阿爾金斷裂帶中段兩側(cè)新生代盆地的漸新世以來的陸相碎屑沉積物開展了系列研究[5,11]，大體認(rèn)為阿爾金斷裂直到漸新世以后才發(fā)生了大規(guī)模的走滑運(yùn)動(dòng)[2,12-13]。而前人在研究阿爾金斷裂東端的酒西盆地內(nèi)部沉積了始新世的陸相碎屑沉積物(火燒溝組)之后，得出了阿爾金斷裂的走滑活動(dòng)開始于始新世的觀點(diǎn)[3,9,14-16]。這些研究僅基于簡(jiǎn)單的地層剖面測(cè)制和礫石成分、古水流方向統(tǒng)計(jì)，而這種傳統(tǒng)的沉積學(xué)方法并未給出足夠充分的證據(jù)表明酒西盆地始新世沉積物與阿爾金斷裂帶的真實(shí)物源關(guān)系?；谒樾间喪腢-Pb年齡頻譜分析已在前人的古地理重建研究中對(duì)追蹤沉積巖物源區(qū)得到廣泛應(yīng)用[17-18]，本次研究專門挑選了最靠近阿爾金斷裂的紅柳峽剖面進(jìn)行系統(tǒng)的沉積物源分析，以期分析古近紀(jì)阿爾金斷裂與酒西盆地的耦合關(guān)系。

1 地質(zhì)概況

酒西盆地位于現(xiàn)今青藏高原最北緣的河西走廊地體西北部，與周緣的阿拉善地體、塔里木地體、祁連地體和敦煌地體相鄰(圖1-A)。北部的黑山-寬臺(tái)山斷裂分割河西走廊地體與阿拉善地體的斷裂帶[19]，該斷裂向東延伸轉(zhuǎn)變成龍首山斷裂帶[20]；南部的北祁連斷裂則分割了河西走廊地體與祁連地體[9]；西部的阿爾金斷裂則分割了河西走廊地體與敦煌地體[3]。酒西盆地新生代陸相沉積物缺失古新世沉積，從始新世開始保存完好的陸相沉積，古近紀(jì)的地層主要為始新世的火燒溝組(E3h)與漸新世的白楊河組[15,21-23]。盆地內(nèi)部火燒溝組主要沉積在盆地的西部和北部[9,15]，2條主要剖面分別位于西部靠近阿爾金斷裂帶的紅柳峽地區(qū)和北部靠近寬臺(tái)山-黑山斷裂帶的火燒溝地區(qū)(圖1-B)。酒西盆地火燒溝組的巖性主要為灰白色細(xì)礫巖、深棕紅色-紫紅色砂巖、淺紅色砂質(zhì)泥巖或泥巖，頂部發(fā)育鈣質(zhì)結(jié)核；白楊河組的巖性主要為橘紅色砂巖夾棕紅色泥巖，夾石膏層與天青色砂巖條帶[22]?；馃郎辖M與下伏下白堊統(tǒng)或志留系、上覆漸新世白楊河組均呈假整合或角度不整合接觸[14,16]。前人利用磁性地層、古生物共同約束了火燒溝組的沉積時(shí)間范圍為40.2～33.4 Ma B.P.[15]。

2 野外剖面和實(shí)驗(yàn)分析

2.1 野外剖面特征

紅柳峽剖面位于酒西盆地的西部，與阿爾金斷裂帶相鄰。剖面上火燒溝組出露完整，整體上可以分為3段，底部為雜色礫巖層，礫石雜亂分布，雜基支撐，分選差，最大礫石直徑>30 cm，形態(tài)以次棱角為主(圖3-A)，且與下伏志留系的石英砂巖呈角度不整合接觸(圖3-B)；中部的主要巖性為雜色礫巖(圖3-C)，夾含礫粗砂巖，礫石的排列成明顯的疊瓦狀(圖3-D)；上部以棕色粉砂巖為主，頂部可見不規(guī)則的鈣質(zhì)結(jié)核(圖3-E)，沉積環(huán)境為辮狀河沖積扇。白楊河組以棕色泥巖和粉砂巖為主，可見礫石組成的透鏡體(圖3-F)，沉積環(huán)境為河流相[14]?；馃郎辖M與白楊河組為假整合接觸。

圖1 研究區(qū)位置及地質(zhì)簡(jiǎn)圖Fig.1 Location and Geological map of the study area(A)青藏高原北緣新生代沉積盆地構(gòu)造分布(底圖來自GeoMapApp的數(shù)字高程模型，構(gòu)造框架修改自文獻(xiàn)[24-25]); (B)酒西盆地、酒東盆地與祁連地體的地質(zhì)圖(修改自文獻(xiàn)[5,9-10,26])1.第四系; 2.新近系; 3.古近系; 4.火燒溝組; 5.白堊系; 6.侏羅系; 7.三疊系; 8.二疊系; 9.石炭系; 10.志留系; 11.奧陶系; 12.寒武系; 13.元古宇; 14.花崗巖; 15.深成巖; 16.上沖斷裂; 17.隱伏斷裂; 18.走滑斷裂; 19.斷層(不確定性質(zhì)); 20.本次研究剖面; 21.沿阿爾金斷裂兩側(cè)的盆地分布: a.江尕勒薩依盆地, b.索爾庫里盆地, c.阿克塞盆地, d.肅北盆地, e.祁連盆地, f.酒西盆地, g.酒東盆地, h.張盆地, i.吐拉盆地。 JFS.金佛寺花崗巖體; ATF.阿爾金斷裂帶; HKF.黑山-寬臺(tái)山斷裂帶; NQF.北祁連斷裂帶; LSF.龍首山斷裂; MBF.廟北斷裂

圖2 阿爾金斷裂沿線(含北祁連山前)新生代沉積盆地的地層對(duì)比及相應(yīng)構(gòu)造事件[3,9,11,13,15,27-29]Fig.2 Correlation of Cenozoic sedimentary basins along the Altyn Tagh fault and North Qilian Mountain with corresponding tectonic events YMC.玉門礫巖; JQC.酒泉礫巖; ATF.阿爾金斷裂帶

圖3 酒西盆地紅柳峽剖面野外照片F(xiàn)ig.3 Representative field photographs of the Hongliuxia section in the Jiuxi basin(A)火燒溝組底部的礫巖層; (B)火燒溝組與下伏志留系石英砂巖(S)角度不整合接觸; (C)礫石層的宏觀特征; (D)礫石的疊瓦狀排列; (E)火燒溝組頂部的鈣質(zhì)結(jié)核; (F)白楊河組的河流沉積特征

在剖面測(cè)制過程中，挑選典型的疊瓦狀礫石進(jìn)行最大扁平面的產(chǎn)狀進(jìn)行統(tǒng)計(jì)，進(jìn)而分析古水流方向。本次研究一共在野外統(tǒng)計(jì)了310組的古水流方向。同時(shí)，在剖面垂向序列的不同部位，挑選出露較好的礫石層進(jìn)行成分統(tǒng)計(jì)。野外統(tǒng)計(jì)時(shí)，在1 m2的范圍內(nèi)，識(shí)別、統(tǒng)計(jì)所有礫石的成分，一共統(tǒng)計(jì)了2 037顆礫石。此外，在剖面上還采集了16塊火燒溝組薄片樣品和3個(gè)鋯石樣品，2塊白楊河組薄片樣品和1個(gè)鋯石樣品。此次鋯石選礦樣品主要采集新鮮的中-粗粒砂巖及礫巖中的砂巖透鏡體，每個(gè)樣品質(zhì)量m>2 kg，具體位置詳見圖4。

2.2 室內(nèi)分析

2.2.1 古近系砂巖的碎屑成分

圖4 酒西盆地紅柳峽剖面的巖性柱、采樣位置、古水流和礫巖成分Fig.4 Lithological column, sampling location, paleocurrent measurements, and clast compositions in the Hongliuxia section of the Jiuxi basin①～④代表碎屑鋯石的采樣位置，菱形代表薄片的采樣位置

本次研究從紅柳峽剖面上采集了16塊火燒溝組砂巖樣品，從白楊河組中挑選了2塊砂巖樣品。通過野外和顯微鏡下的鑒定，這18塊砂巖為巖屑砂巖、粗砂巖，主要為鈣質(zhì)膠結(jié)。為了確保統(tǒng)計(jì)結(jié)果的可靠性，在對(duì)每個(gè)砂巖薄片進(jìn)行統(tǒng)計(jì)的過程中需挑選超過300個(gè)碎屑顆粒。鏡下薄片觀察鑒定表明，砂巖主要為石英巖屑砂巖，基質(zhì)含量較少，碎屑組分多樣，主要包括單晶石英、多晶石英、長石、泥巖碎屑、砂巖碎屑、燧石、板巖碎屑、片巖碎屑等。

2.2.2 鋯石U-Pb年齡

本次在火燒溝組和白楊河組中挑選了4個(gè)砂巖樣品進(jìn)行鋯石年齡測(cè)定，樣品送到河北廊坊科大巖石礦物分選技術(shù)服務(wù)公司進(jìn)行系統(tǒng)分選。鋯石挑選的主要流程：在避免污染的條件下，將砂巖樣品粉碎至 <60目(<250 μm)，先用磁選和重液方法粗選鋯石，然后在雙目鏡下挑選鋯石顆粒，在每個(gè)樣品中挑選晶型較好的鋯石顆粒>120顆進(jìn)行制靶。

鋯石U-Pb年齡測(cè)定在中國科學(xué)院青藏高原研究所(北京)大陸碰撞與高原隆升重點(diǎn)實(shí)驗(yàn)室激光剝蝕電感耦合等離子體質(zhì)譜儀(LA-ICP-MS)上完成。首先將人工挑選出的鋯石顆粒粘貼在環(huán)氧脂表面制成樣品靶并拋光，選取鋯石U-Pb同位素年齡測(cè)定的打點(diǎn)部位和進(jìn)行后續(xù)數(shù)據(jù)分析解釋。實(shí)驗(yàn)采用激光剝蝕等離子質(zhì)譜體分析技術(shù)( LAICP-MS: Inductively Coupled Plasma Mass Spectrometry) ，其原理詳見文獻(xiàn)[30]。LA-ICP-MS激光剝蝕系統(tǒng)為美國NewWave公司生產(chǎn)的UP193FX型193 nm ArF準(zhǔn)分子系統(tǒng)。實(shí)驗(yàn)所使用激光器產(chǎn)自德國的ATL公司，ICP-MS為Agilent 7500a，其激光器波長為193 nm，束斑直徑為10/15/20/25/35/50/75/100/125 μm可調(diào)，脈沖寬度<4 ns，脈沖頻率1～200 Hz連續(xù)可調(diào)，所用的激光剝蝕物質(zhì)以氦氣為載氣。氦氣攜帶樣品氣溶膠在進(jìn)入ICP之前通過一個(gè)T型三通接頭與氬氣(載氣、等離子體氣和補(bǔ)償氣)混合，通過調(diào)節(jié)氦氣和氬氣氣流大小，以獲得NIST SRM 612(美國國家標(biāo)準(zhǔn)技術(shù)研究院研制的人工合成硅酸鹽玻璃標(biāo)準(zhǔn)參考物質(zhì))最佳信號(hào)為條件實(shí)現(xiàn)測(cè)試系統(tǒng)最優(yōu)化，利用動(dòng)態(tài)變焦擴(kuò)大色散，同時(shí)接收質(zhì)量數(shù)相差很大的U-Pb同位素，從而進(jìn)行鋯石U-Pb同位素原位測(cè)定。本次實(shí)驗(yàn)碎屑鋯石分析采用鋯石91500(91500 U/Pb standard zircon)作為外部鋯石年齡的標(biāo)樣，確保同位素辨別標(biāo)準(zhǔn)化；利用NIST612玻璃標(biāo)樣作為外標(biāo)準(zhǔn)，計(jì)算測(cè)試的鋯石樣品的U、Th、Pb含量，采用29Si作內(nèi)標(biāo)元素。同位素比值和元素濃度計(jì)算使用GLITTER(Version. 4.0)程序，詳細(xì)的數(shù)據(jù)處理及分析方法見參考文獻(xiàn)[31]以及《西北大學(xué)大陸動(dòng)力學(xué)國家重點(diǎn)實(shí)驗(yàn)室LA-ICP-MS 數(shù)據(jù)處理步驟》指導(dǎo)書。采用208Pb校正法對(duì)普通鉛進(jìn)行校正，詳細(xì)的實(shí)驗(yàn)流程見參考文獻(xiàn)[32]。鋯石年齡計(jì)算和諧和圖的繪制使用國際標(biāo)準(zhǔn)ISOPLOT(Version3.0)[33]完成。

3 實(shí)驗(yàn)結(jié)果

3.1 碎屑成分

通過鏡下的系統(tǒng)鑒定，對(duì)薄片中的主要礦物進(jìn)行了定量統(tǒng)計(jì)：(1)石英的平均質(zhì)量分?jǐn)?shù)(w)為38.7%，其中單晶石英的平均質(zhì)量分?jǐn)?shù)為33.9%；多晶石英(不包括花崗巖、糜棱巖及碎裂巖形成的多晶石英)含量較少，平均質(zhì)量分?jǐn)?shù)為4.8%。在大部分薄片中，石英顆粒具有顯著的波狀消光特征。(2)長石平均質(zhì)量分?jǐn)?shù)為1.2%。(3)沉積巖碎屑平均質(zhì)量分?jǐn)?shù)為29.7%，主要為泥巖、砂巖碎屑和燧石碎屑。(4)變質(zhì)巖碎屑平均質(zhì)量分?jǐn)?shù)為25.0%，主要為板巖碎屑、石英巖碎屑和碎裂巖碎屑。(5)巖漿巖碎屑平均質(zhì)量分?jǐn)?shù)為0.7%，僅為花崗巖碎屑；總巖屑平均質(zhì)量分?jǐn)?shù)為59.4%。統(tǒng)計(jì)結(jié)果投于Dickinson[34]和Ingersoll等[35]的Qm-F-Lt圖中，顯示火燒溝組和白楊河組砂巖的碎屑成分全部落入再旋回造山帶物源區(qū)中(圖5)。

3.2 鋯石U-Pb年齡

為了確保鋯石U-Pb年齡測(cè)定的可靠性，首先需排除具有明顯包裹體、裂紋的碎屑鋯石，然后再對(duì)挑選樣品的鋯石進(jìn)行LA-ICP-MS年齡測(cè)定。本次研究的3個(gè)火燒溝組砂巖和1個(gè)白楊河組砂巖樣品各分析90個(gè)點(diǎn)的數(shù)據(jù)。計(jì)算鋯石年齡諧和度時(shí)對(duì)于年齡>1 Ga的采用100×(207Pb/206Pb年齡)/(206Pb/238U年齡)；年齡<1 Ga的為100×(207Pb/235Pb年齡)/(206Pb/238U年齡)。將得出的數(shù)值挑選介于80～120之間的為諧和(諧和度≥80%)的，其余為不諧和的(諧和度<80%)，<1 Ga的用206Pb/238U和1σ對(duì)應(yīng)的年齡，>1 Ga的用207Pb/206Pb和1σ對(duì)應(yīng)的年齡(圖6)。最終火燒溝組的3個(gè)砂巖樣品：HLX-02-21G獲得33個(gè)有效數(shù)據(jù)，HLX-02-104G獲得41個(gè)有效數(shù)據(jù)，HLX-02-131G獲得54個(gè)有效數(shù)據(jù)；白楊河組砂巖樣品HLX-01-49G獲得了75個(gè)有效的鋯石年齡數(shù)據(jù)。4個(gè)砂巖樣品共獲得184個(gè)有效數(shù)據(jù)均符合標(biāo)準(zhǔn)的年齡分布統(tǒng)計(jì)要求[39]。

圖5 酒西盆地紅柳峽剖面火燒溝組和白楊河組砂巖成分的三元相圖Fig.5 Temary phase diagram of sandstone compositions of Huoshaogou and Baiyanghe Formations from the Hongliuxia section of the Jiuxi basinQm.單晶石英; F.長石; Lt.巖屑(據(jù)文獻(xiàn)[34-35])。不同盆地的碎屑組分?jǐn)?shù)據(jù)來自：(1)陳正樂等[28]; (2)方世虎等[36]; (3)本文; (4)Yin等[2]; (5)馬雪等[37]

圖6 酒西盆地白楊河組和火燒溝組砂巖中鋯石的LA-ICP-MS U-Pb諧和圖Fig.6 Zircon LA-ICP-MS U-Pb concordia diagram of metamorphic rocks from the sandstone of the Baiyanghe and Huoshaogou Formations in the Jiuxi basin

HLX-02-21G：該樣品根據(jù)從90個(gè)數(shù)據(jù)中僅獲得的33個(gè)有效數(shù)據(jù)進(jìn)行統(tǒng)計(jì)分析及碎屑鋯石年齡的頻率分布曲線圖，最小年齡為774 Ma，最大年齡為3 109 Ma，其中有一個(gè)800～950 Ma左右的峰值，最大峰值年齡為883 Ma。

HLX-02-104G：該樣品根據(jù)獲得的41個(gè)有效數(shù)據(jù)統(tǒng)計(jì)分析及碎屑鋯石年齡的頻率分布曲線圖，年齡范圍為750～2 739 Ma,有一個(gè)1 100～1 310 Ma左右的峰值，最大峰值年齡為1 308 Ma。

HLX-02-131G：該樣品根據(jù)獲得的54個(gè)有效數(shù)據(jù)統(tǒng)計(jì)分析及碎屑鋯石年齡的頻率分布曲線圖，年齡范圍為796～2 978 Ma，可見2個(gè)主要的峰值：(1)700～900 Ma左右的峰值，最大峰值年齡為828 Ma；(2)1.1～1.3 Ga左右的峰值，最大峰值年齡為1 227 Ma。

HLX-01-49G：該樣品根據(jù)獲得的75個(gè)有效數(shù)據(jù)統(tǒng)計(jì)分析及碎屑鋯石年齡的頻率分布曲線圖，最大峰值年齡為834 Ma，最小年齡為755 Ma，最大年齡為1 266 Ma。

整體而言，紅柳峽剖面火燒溝組3個(gè)砂巖樣品的碎屑鋯石年齡分布為0.7～1 Ga和1.1～1.3 Ga的2個(gè)主峰區(qū)間，樣品間各自有少量1.4～1.6 Ga，1.9～2.1 Ga，2.3～2.6 Ga，2.7～3.2 Ga等不同成分。白楊河組1個(gè)砂巖樣品的碎屑鋯石年齡分布為0.7～1 Ga的明顯主峰區(qū)間(圖7)。4個(gè)樣品的鋯石wTh/wU比值主體大于0.1，顯示為巖漿鋯石。

圖7 酒西盆地紅柳峽剖面火燒溝組與白楊河組碎屑鋯石年齡譜Fig.7 Detrital zircon age spectra of the Huoshaogou Formation and Baiyanghe Formation of the Hongliuxia section in the Jiuxi basin

4 討 論

古地理重建研究中碎屑鋯石的U-Pb年齡頻譜在追蹤沉積巖物源區(qū)得到廣泛應(yīng)用[17-18]。且前人對(duì)阿爾金斷裂沿線的鋯石U-Pb年齡頻譜分析已有一定進(jìn)展，主要針對(duì)2個(gè)不同的研究對(duì)象：盆地的碎屑沉積巖、地體的結(jié)晶基底。已研究的阿爾金斷裂帶沿線新生代沉積盆地主要包括：肅北盆地、酒西盆地、索爾庫里盆地、柴達(dá)木盆地[8,10,13,40]，而地體的結(jié)晶基底主要針對(duì)：敦煌地體、阿爾金地體、祁連地體、阿拉善地體和河西走廊地體(圖8)。通常地體的結(jié)晶基底被看作是盆地中碎屑沉積巖的鋯石主要來源，因此前人常將兩者的鋯石U-Pb年齡頻譜進(jìn)行大范圍的單一匹配對(duì)比[8,10,13,40]。同時(shí)，阿爾金斷裂帶的走滑作用使得同一個(gè)地體的結(jié)晶基底可為另一側(cè)不同盆地或兩側(cè)的盆地提供物源，也就是在不同盆地沉積物中呈現(xiàn)出相同的碎屑鋯石的U-Pb年齡頻譜[13]。本次研究特將阿爾金斷裂沿線盆地的新生代沉積物和結(jié)晶基底中碎屑鋯石的U-Pb年齡頻譜進(jìn)行了雙重約束，以匹配酒西盆地古近系碎屑沉積物的供給物源區(qū)。紅柳峽剖面火燒溝組碎屑鋯石表現(xiàn)出0.7～1 Ga和1.1～1.3 Ga兩個(gè)主要的U-Pb年齡頻譜，相對(duì)白楊河組碎屑鋯石卻僅表現(xiàn)出單一的700～900 Ma年齡頻譜。基于火燒溝組沉積時(shí)古水流方向的統(tǒng)計(jì)，剖面中下段的火燒溝組3個(gè)樣品主要受到來自西部古水流和次要的南部水流所攜帶的物源供給，白楊河組主體接受來自西部的物源(圖4)。

圖8 研究區(qū)及鄰區(qū)新生代沉積巖的碎屑鋯石年齡頻譜圖Fig.8 Detrital zircon age determinations from the Cenozoic sediments in the study area and neighboring areasAT.阿爾金地體結(jié)晶基底[25,42,46-51]; QL.北-中-南祁連地體結(jié)晶基底[52]; S-QL.南祁連地體結(jié)晶基底[25]; AL.阿拉善地體結(jié)晶基底[53-57]; DH.敦煌地體結(jié)晶基底[58-62]; HX.河西走廊地體結(jié)晶基底[30,41-43]

火燒溝組3個(gè)碎屑鋯石樣品的主峰值0.7～1 Ga年齡頻譜峰值與盆地周邊地體的結(jié)晶基底中鋯石U-Pb年齡頻譜進(jìn)行峰值對(duì)比表明：(1)0.7～1 Ga的鋯石U-Pb年齡頻譜主要與阿爾金地體、南祁連地體相似(圖8)。(2)1.1～1.3 Ga的鋯石U-Pb年齡頻譜與河西走廊地體的古生代沉積物中碎屑鋯石U-Pb年齡頻譜相似，對(duì)青藏高原北緣主要地體的結(jié)晶基底的鋯石U-Pb年齡頻譜統(tǒng)計(jì)并未發(fā)現(xiàn)有類似1.1～1.3 Ga的峰值，僅河西走廊地體的古生代沉積物中碎屑鋯石U-Pb年齡頻譜表現(xiàn)出一個(gè)1～1.3 Ga的峰值[41-43]。白楊河組1個(gè)碎屑鋯石樣品展示出單一的700～900 Ma的U-Pb年齡峰值可受2個(gè)地體的物源支撐：阿爾金地體、南祁連地體[23]。

對(duì)于火燒溝組和白楊河組砂巖碎屑鋯石中同時(shí)出現(xiàn)的0.7～1 Ga的鋯石U-Pb年齡頻譜，來自西部的古水流方向無法直接區(qū)分現(xiàn)今同時(shí)位于酒西盆地南部的阿爾金地體和南祁連地體，但砂巖的碎屑組分統(tǒng)計(jì)給出了一定的判別信息。先假定來自南祁連地體的物源供給，由于南祁連地體的物源需跨越中、北祁連山才能到達(dá)沉積區(qū)，水流即便是路過島弧火山巖為主的中-北祁連地區(qū)[2,44]，在中祁連地區(qū)肅北盆地漸新世的砂巖中表現(xiàn)出中祁連地區(qū)典型的島弧火山巖碎屑物質(zhì)[2]和400～500 Ma的碎屑鋯石U-Pb年齡頻譜[8]，河流環(huán)境下的古水流不攜帶相應(yīng)的島弧火山碎屑物質(zhì)到達(dá)酒西盆地的沉積區(qū)，而僅完整地保留南祁連地體的碎屑組分的可能性較低。相對(duì)而言，火燒溝組和白楊河組砂巖的碎屑組分表現(xiàn)出的再旋回造山帶特征與阿爾金地體西南緣的江尕勒薩依盆地新生代沉積物一致(圖5)[28]，同時(shí)其碎屑鋯石U-Pb年齡頻譜與阿爾金地體內(nèi)部的索爾庫里盆地中漸新世砂巖的碎屑鋯石U-Pb年齡頻譜表現(xiàn)出驚人的一致性，除了0.7～1 Ga這個(gè)年齡頻譜段，僅有極少量更老的碎屑鋯石U-Pb值(圖8)。當(dāng)然，基于現(xiàn)今的阿爾金地體的地理位置來看，物源區(qū)與沉積區(qū)仍相距較遠(yuǎn)(圖1)。但考慮到阿爾金斷裂的左行走滑作用[45]，阿爾金地體比南祁連地體具有更大的優(yōu)勢(shì)可以作為白楊河組的物源供給區(qū)。由于白楊河組主體是河流沉積環(huán)境，那就需要物源區(qū)比沉積區(qū)具有相應(yīng)的海拔高程支撐，基于低溫?zé)崮甏鷮W(xué)的數(shù)據(jù)表明漸新世中-北祁連地體處于低地狀態(tài)[3]，相反阿爾金地體北部在漸新世已隆升到現(xiàn)今的高度[29]。故本文認(rèn)為阿爾金地體應(yīng)為酒西盆地紅柳峽剖面火燒溝組和白楊河組的物源供給區(qū)。

上面討論了古近紀(jì)阿爾金地體與酒西盆地物源輸入的密切關(guān)系，這里就提出了另一個(gè)問題：阿爾金地體如果在始新世位于酒西盆地西部，那后期阿爾金斷裂則將走滑近450 km。這一推斷的走滑量可得到其他區(qū)域資料的支持。李海兵等[6]研究表明青藏高原北部阿爾金斷裂帶北側(cè)的北阿爾金山藍(lán)片巖-榴輝巖高壓變質(zhì)帶和南阿爾金山榴輝巖超高壓變質(zhì)帶可與南側(cè)的北祁連山藍(lán)片巖-榴輝巖高壓變質(zhì)帶和柴北緣榴輝巖超高壓變質(zhì)帶作很好的對(duì)比。也有研究認(rèn)為阿爾金地體從始新世就一直位于柴達(dá)木盆地的北緣，可惜柴達(dá)木盆地西緣的新生代沉積巖中碎屑鋯石的U-Pb年齡頻譜[40]和砂巖碎屑物質(zhì)[37]卻表現(xiàn)出不同與阿爾金地體的沉積面貌(圖5、圖8)。因此，本次研究更支持阿爾金斷裂在漸新世以后發(fā)生了>450 km的大規(guī)模左行走滑運(yùn)動(dòng)[1,13,66]。

5 結(jié) 論

a.基于野外觀察，火燒溝組的沉積相分為2部分：下段以沖積扇為主，上段則轉(zhuǎn)變?yōu)楹恿飨?；白楊河組則以單一的河流相為主。其礫石成分也出現(xiàn)了明顯的轉(zhuǎn)換：火燒溝組礫石主要以中-低級(jí)變質(zhì)巖和沉積巖為主；白楊河組則轉(zhuǎn)變?yōu)閹r漿巖為主。

b.火燒溝組砂巖的主要成分包括具有明顯波狀消光的石英、長石和變質(zhì)巖-沉積巖碎屑，屬于再旋回造山帶的物源供給。

c.酒西盆地的古近系砂巖中碎屑鋯石的U-Pb年齡測(cè)定結(jié)果表現(xiàn)出新元古代和中元古代2個(gè)主要的年齡峰值，結(jié)合其來自西部-西北部的古水流輸入，對(duì)比青藏高原北緣主要地體的基底年齡，進(jìn)一步表明現(xiàn)今距酒西盆地450 km以外、且位于阿爾金斷裂西側(cè)的阿爾金地體應(yīng)是古近紀(jì)酒西盆地的主要物源輸入?yún)^(qū)，另外還有部分來自河西走廊地體的物源供給。這一結(jié)果表明阿爾金斷裂在漸新世后的走滑量>450 km。

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Provenance analysis of Paleogene in the Jiuxi basin on eastern Altyn Tagh fault and its implication

DENG Bin1, RAN Bo1, YE Yuehao1, WANG Zijian1, WANG Chengshan2,ZHU Lidong1, TIAN Qing1, YUE Yahui3

1.StateKeyLaboratoryofOilandGasReservoirGeologyandExploitation,ChengduUniversityofTechnology,Chengdu610059,China; 2.StateKeyLaboratoryofBiogeologyandEnvironmentalGeology,SchoolofEarthSciencesandResources,ChinaUniversityofGeosciences,Beijing100083,China; 3.KeyLaboratoryofContinentalCollisionandPlateauUplift,InstituteofTibetanPlateauResearch,ChineseAcademyofSciences,Beijing100101,China

Sedimentology and geochronology of Paleogene sequences (including the Huoshaogou and Baiyanghe Formations) in the Hongliuxia section of Jiuxi basin are studied so as to explore the relation between the Paleogene sediments with left strike slip Altyn Tagh fault. It shows that the conglomerates in the formations are mainly composed of low-middle grade metamorphic rocks and sedimentary rocks in the Huoshaogou Formation, and of magmatic rocks in the Baiyanghe Formation. Study of these conglomerates reveals that the main paleocurrents are west and northwest direction. The sandstones from the Huoshaogou and Baiyanghe Formations belong to the recycled orogeny environment based on the Q-F-L diagram. Detrital zircons of the four Eocene sandstones obtained two Precambrian characteristic age populations, Neoproterozoic and Mesoproterozoic. Contrasting with periphery provenance sources, it is showed that the provenance of Huoshaogou Formation was mainly from the Altyn Tagh terrane. On the basis of geochronology data, along with the regional stratigraphic and paleocurrent studies, it is considered that the Altyn Tagh terrane is departed from Jiuxi basin after Oligocene, and the maximum left strike slip for Altyn Tagh fault is over 450 km.

Altyn Tagh fault; Jiuxi basin; Altyn Tagh terrane; provenance analysis; strike-slip

10.3969/j.issn.1671-9727.2017.03.04

1671-9727(2017)03-0305-13

2017-01-13。

國家自然科學(xué)基金項(xiàng)目(41102064, 41230313); 教育部新教師基金項(xiàng)目(20105122120013); 四川省教育廳重點(diǎn)項(xiàng)目(09ZA005); The 111 Project of China Grant(Project B07011)。

鄧斌(1957-)，男，講師，研究方向：構(gòu)造地質(zhì)， E-mail: 740640938@qq.com。

P588.21

A