阿拉善地塊前中生代構(gòu)造歸屬的新認識

楊振宇,袁 偉 ,仝亞博,劉 靜,王 艷

1)中國地質(zhì)科學(xué)院地質(zhì)力學(xué)研究所,國土資源部古地磁與古構(gòu)造重建重點實驗室,北京 100081;2)中國石化石油勘探開發(fā)研究院無錫石油地質(zhì)研究所,江蘇無錫 214126;3)同濟大學(xué)海洋與地球科學(xué)學(xué)院,上海 200092;4)江蘇省有色金屬華東地質(zhì)勘查局,江蘇南京 210007

通常地質(zhì)學(xué)家認為橫貫中國中部的昆侖—秦嶺—大別造山帶(或中央造山帶)將中國大陸分為中國北方地塊和南方地塊(群),南、北方地塊展示了不同的地層組合(王澤九等,2014)。這些地塊群組成一個復(fù)合大陸(任紀(jì)舜等,1994,1995),北方地塊又通常被認為由中朝(—阿拉善)—塔里木板塊構(gòu)成(李春昱等,1982;王鴻禎,1985),阿拉善地塊作為“地腰”連接著華北和塔里木地塊。許效松等(1996)通過對中國主要陸塊的古地理研究,則將界于古亞洲洋以南與岡瓦納陸塊群之間的特提斯洋盆系統(tǒng)中具特提斯暖水動物群和華夏植物群的地塊稱之為“泛華夏陸塊群”,該群又被分之為“北方陸塊群”包括華北、朝鮮、阿拉善等地塊,而南方型“泛揚子陸塊群”包括揚子、華夏、塔里木、柴達木等地塊,也有稱之為“西域地塊”(葛肖虹等,1996),這些地塊的顯著特點是具有晉寧構(gòu)造旋回和揚子型南華—震旦系蓋層。與前人的認識基本一致,他們認為阿拉善地塊一直是華北的一部分,早古生代至早泥盆世初塔里木陸塊和柴達木陸塊分別與華北(—阿拉善)陸塊及其所圍繞的微陸塊聚合,形成“北方陸塊群”;Xiao等(2009)總結(jié)了阿拉善地塊與祁連山造山帶的構(gòu)造演化,也提出晚泥盆世阿拉善地塊與祁連山造山帶拼合,構(gòu)成柴達木—祁連山—阿拉善構(gòu)造體。似乎晚泥盆世華北通過柴達木—祁連山—阿拉善構(gòu)造體與塔里木連接構(gòu)成北方陸塊群。顯然,泥盆紀(jì)后阿拉善地塊似乎構(gòu)成了塔里木—柴達木地塊的一部分,這些地質(zhì)方面的認識還存在差異,使我們對古生代時期阿拉善地塊究竟是華北地塊的一部分？還是與塔里木地塊親緣產(chǎn)生疑問,因為古地磁的研究表明,晚二疊世時塔里木地塊已與西伯利亞—哈薩克斯坦板塊碰撞、拼合(Zhao et al.,1990;Enkin et al.,1992),而華北與蒙古碰撞并拼合形成了華北—蒙古聯(lián)合地塊,兩者則遠離西伯利亞板塊,直至晚侏羅世華北—蒙古聯(lián)合地塊和西伯利亞板塊的古地磁極才趨向一致,形成統(tǒng)一整體(朱日祥等,1998)。

阿拉善地塊前寒武紀(jì)地質(zhì)研究表明,其基底由新太古代迭布斯格巖群下部依肯烏蘇巖組(2700 Ma)和古元古代的巴彥烏拉山巖群(2500—2264 Ma)組成(耿元生等,2007;Dan et al.,2012),并經(jīng)歷了早古生代(480—446 Ma)和晚古生代(281—266 Ma)兩次構(gòu)造熱事件。近年來,在阿拉善北部發(fā)現(xiàn)的多條蛇綠巖帶,恩格爾烏蘇蛇綠巖帶的發(fā)現(xiàn)表明塔里木陸塊和華北陸塊在早古生代也是分離的(王廷印等,1992),詳細的年代學(xué)研究表明,蛇綠巖的年齡可晚至晚二 疊 世 ((274±3) Ma (MSWD=0.35)和(262±5) Ma(MSWD=1.2),Feng et al.,2013)。阿拉善地塊與華北及塔里木地塊的構(gòu)造關(guān)系長期受到關(guān)注,但由于騰格里沙漠的覆蓋,阿拉善地塊的構(gòu)造屬性和構(gòu)造演化一直懸而未決,對其深入研究應(yīng)是理解中國北方大陸構(gòu)造演化的一個重要課題。

圖1 中國大陸(A)及阿拉善地塊東南緣(B)地質(zhì)簡圖Fig.1 Simplified geological map of China’s mainland(A) and southeastern Alxa Block(B)

1 中—晚泥盆世砂巖碎屑鋯石 U-Pb年齡及Hf同位素結(jié)果

本次研究對河西走廊帶—阿拉善地塊東部牛首山地區(qū)中—晚泥盆世砂巖中碎屑鋯石開展了 U-Pb年代學(xué)與Hf同位素的詳細研究(Yuan et al.,2014a)(圖1),鋯石U-Pb年齡譜主要集中在0.4—0.7 Ga、1.0—1.3 Ga、1.5—1.8 Ga、2.4—2.7 Ga 和＞3.0 Ga五個年齡區(qū)間(圖2d,g)。研究表明華北板塊西部前寒武紀(jì)—晚中生代地層中碎屑鋯石 U-Pb年齡譜,主要集中在1.8—2.1 Ga和2.4—2.7 Ga(圖2e)。而本次研究中獲得牛首山地區(qū)明顯有別于華北的鋯石年齡譜,其年齡譜中并未出現(xiàn)華北板塊典型的 1.85—2.1 Ga峰值。鄂爾多斯盆地西北部桌子山地區(qū)的新元古代—早古生代沉積巖中碎屑鋯石 U-Pb年齡譜存在1.85—2.1 Ga和2.4—2.6 Ga兩個明顯峰值(圖2f),它具有典型的華北板塊特征。桌子山(華北板塊西緣)與牛首山(阿拉善地塊南緣)相距很近,卻具有截然不同的碎屑物質(zhì)來源,這也暗示著二者的古地理位置可能是彼此分開。

目前 Gondwana大陸各主要板塊均有碎屑鋯石U-Pb年齡峰值為～2.5 Ga的報道,但是單從U-Pb年齡的峰值已經(jīng)很難判斷碎屑物質(zhì)的來源。而鋯石Hf同位素數(shù)據(jù)則被認為是判斷碎屑物源的一個很好的輔助工具(Hoskin et al.,2000;Fedo et al.,2003)。阿拉善地塊東部牛首山地區(qū)中—晚泥盆世砂巖中的～2.5 Ga碎屑鋯石Hf同位素研究結(jié)果表明,其εHf(t)值在+4.9 ～ –23.7之間,對應(yīng)的地殼模式年齡分布在2689—4501 Ma,其峰值為3010 Ma。然而,目前報道的華北板塊峰值在～2.5 Ga的鋯石對應(yīng)的Hf同位素模式年齡峰值為2.75 Ga(Li et al.,2007;Jiang et al.,2010;Diwu et al.,2011;Wan et al.,2011a,b;Wang et al.,2011),二者存在明顯的差別。此外,牛首山地區(qū)＞3.0 Ga碎屑鋯石的 Hf同位素特征也明顯區(qū)別于華北板塊同時代碎屑鋯石,這些特征說明牛首山地區(qū)中—上泥盆統(tǒng)的碎屑物源很可能不是來自華北地塊,這些結(jié)果暗示河西走廊—阿拉善地塊與華北板塊的拼合時間可能是在晚泥盆世以后。本次研究中牛首山中—上泥盆統(tǒng)出現(xiàn)的典型的格林威爾期碎屑物質(zhì),其U-Pb峰值和Hf同位素特征與華夏地塊古生代地層中碎屑鋯石十分相似(圖2c,d)。古生物研究表明祁連山地區(qū)寒武紀(jì)地層中所產(chǎn)的三葉蟲Galahetes,Sanduspis,Huzhuia,Jialaopsis,Szechuanella,Acrocephalina等均為塔里木揚子生物地理區(qū)的特有屬種(段吉業(yè)等,2005),與我國湘西、黔東過渡相區(qū)及澳大利亞昆士南西北的三葉蟲動物群有比較密切的關(guān)系(周志強等,1996)。牛首山地區(qū)中寒武統(tǒng)碎屑物質(zhì)來源與澳大利亞也有一定的親緣性(袁偉等,2012b)。而在本次采樣的牛首山地區(qū),上泥盆統(tǒng)中寧組中發(fā)現(xiàn)的淡水魚類胴甲魚,在華南以及澳大利亞的新南威爾士同時期地層中均有出現(xiàn)(潘江等,1987;Jia et al.,2010)。并且,中寧組下段地層中發(fā)現(xiàn)了中華豆石介族(介形類),該化石以下垂“V”字型肌痕為特征與世界其它地區(qū)地層同期的豆石介族相區(qū)別,其主要產(chǎn)于華南的泥盆紀(jì)地層中(王尚啟等,1995)。這些古生物學(xué)證據(jù)表明寒武紀(jì)—泥盆紀(jì)期間阿拉善地塊與華南同屬一個生物地理區(qū),它們的地理位置應(yīng)該比較接近。本研究還在牛首山上泥盆系地層中發(fā)現(xiàn)了一顆 U-Pb年齡為(4022±17) Ma的碎屑鋯石,它與西澳的Jack Hills、華南、西藏等地冥古代鋯石沉積物源區(qū)可能具有成因聯(lián)系(袁偉等,2012a)。另外,0.4～0.7 Ga碎屑鋯石的Hf同位素研究顯示,對于＞550 Ma鋯石的εHf(t)在0.5 ～ –26.0之間(只有一個正值),代表地殼物質(zhì)的再循環(huán),而＜550 Ma的鋯石εHf(t)開始出現(xiàn)了正值,表明開始出現(xiàn)新生地殼物質(zhì),標(biāo)志北祁連洋洋殼在 550 Ma開始形成,與北祁連造山帶中發(fā)現(xiàn)的蛇綠巖的年代分布(490—550 Ma)相一致(袁偉等,2012b)。

表1 河西走廊—阿拉善地塊與華北板塊晚古生代以來古地磁極Table 1 Late Paleozoic paleomagnetic data from the Hexi Corridor–Alxa Block and North China Block

2 石炭紀(jì)—二疊紀(jì)古地磁學(xué)研究結(jié)果

本次研究對河西走廊—阿拉善地塊東部的早石炭世、晚二疊世和早中三疊世地層開展詳細的古地磁研究(Yuan et al.,2014b)。巖石磁學(xué)研究結(jié)果表明,早石炭世灰?guī)r的載磁礦物主要為磁鐵礦和赤鐵礦,可能含有鈦磁鐵礦,磁性礦物多為假單疇顆粒;晚二疊世和早—中三疊世紫紅色長石石英砂巖主要載磁礦物為赤鐵礦。

早石炭世樣品系統(tǒng)熱退磁共設(shè)12～17步,300℃以下以 50℃為間隔,340～420℃以 30～40℃為間隔,420℃以上以10～20℃為間隔。熱退磁過程中出現(xiàn)兩個分量,第一個分量在 200～300℃時解阻,為低溫分量;第二個分量到達 460℃時剩磁達到最小,但是該分量未趨向于原點,應(yīng)為中溫分量。460℃以后樣品剩磁過于微弱,超出了儀器測試范圍,未能獲得高溫分量。運用主向量分析法對所有樣品退磁結(jié)果進行分析(Kirschvink,1980),采點平均采用通用的 Fisher統(tǒng)計平均法(Fisher,1953),獲得低溫分量A和中溫分量M。其中,低溫分量未能通過褶皺檢驗(McFadden,1990),其方向與現(xiàn)代地磁場一致,很可能是現(xiàn)代地磁場的粘滯剩磁。中溫分量的形成可能與造山熱液流體活動有關(guān)(Addison et al.,1985)。研究區(qū)緊鄰祁連造山帶,并且上泥盆世—中石炭世的巖層間斷裂中賦存有熱液型金、銅礦,中石炭世地層局部受到熱液活動影響,經(jīng)受不同程度的變質(zhì)作用,變質(zhì)熱液的溫度在 250～300℃(邵潔漣等,1988;宋新華等,2009)。因此,該中溫分量M很可能是熱液流體作用后重磁化的結(jié)果。中溫分量M在 95%置信度下通過了 Mcfadden褶皺檢驗(McFadden,1990),表明該分量為中侏羅世褶皺前獲得。但是其極位置又明顯有別于河西走廊—阿拉善地區(qū)二疊紀(jì)以后的古地磁極,因此我們推測早石炭世中溫 M 分量重磁化時間可能在中—晚石炭世,其對應(yīng)的古地磁極為 λ=12.1°N,Ф=10.7°E,A95=4.2°,古緯度在 3.6±4.2°N。

晚二疊世和早—中三疊世樣品系統(tǒng)熱退磁共設(shè) 14～16步,500℃以下以 80～100℃為間隔,500～600℃以 20～30℃為間隔,600℃以上以 5～20℃為間隔。運用主向量分析法對所有樣品退磁結(jié)果進行分析(Kirschvink,1980),采點平均采用通用的Fisher統(tǒng)計平均法(Fisher,1953)。兩個時代的樣品均獲得了低溫和高溫分量,其中低溫分量為現(xiàn)代地磁場的粘滯剩磁;高溫分量均趨向于原點,并且通過了褶皺檢驗和區(qū)域一致性檢驗,代表了原生特征剩磁。晚二疊世對應(yīng)的古地磁極為λ=26.2°N,Ф=18.0°E,A95=7.5°,古緯度為 17.4±7.5°N;早—中三疊世古地磁極為λ=37.5°N,Ф=14.4°E,A95=3.7°,古緯度為 21.6°N(Liu et al.,2010)。

圖3 阿拉善地塊(藍色)與華北板塊(紅色)石炭紀(jì)—三疊紀(jì)視極移曲線(據(jù)Yuan et al.,2014b修改)Fig.3 Equal-area projection of Carboniferous-Triassic apparent polar wander paths for the Alxa Block (blue) and North China Block (red) (modified after Yuan et al.,2014b)

圖4 晚二疊世亞洲主要板塊古地理重建圖(據(jù)Yuan et al.,2014b修改)Fig.4 Paleomagnetic-based paleogeographic reconstruction of major Asian blocks in the Late Permian(modified after Yuan et al.,2014b)

通過系統(tǒng)的古地磁學(xué)研究,我們獲得了阿拉善地塊東部中—晚石炭世、晚二疊世和早—中三疊世可靠的古地磁極,這些數(shù)據(jù)與華北地塊同期的古地磁極有明顯差別(圖3,表1)。河西走廊—阿拉善地塊中東部新獲得的中—晚石炭世、晚二疊世和早—中三疊世構(gòu)成的視極移曲線可以與華北地塊同時期極移曲線可以進行擬合對比,若以44°N,84°E為參考歐拉極,將阿拉善地塊的視極移曲線旋轉(zhuǎn) 32°,該曲線與華北地塊同時期極移曲線重合(圖 3),這說明了在中三疊世后,特別是印支運動使阿拉善地塊相對于華北發(fā)生了 32°的逆時針旋轉(zhuǎn),正是這一旋轉(zhuǎn)運動使河西走廊—阿拉善地塊最終與華北地塊拼合形成了統(tǒng)一地塊,該邊界很可能位于賀蘭山與桌子山之間(圖4;中國地質(zhì)科學(xué)院,2014)。最新的野外地質(zhì)研究表明巴彥烏拉山—狼山存在一條中生代早期線理近水平的韌性剪切帶,很可能代表華北和阿拉善地塊的拼合界線(Zhang et al.,2013)。

3 總結(jié)

阿拉善南緣牛首山地區(qū)的中—晚泥盆世沉積巖中碎屑鋯石的U-Pb年代學(xué)和Hf同位素研究結(jié)果表明,其碎屑物質(zhì)來源明顯區(qū)別于華北板塊,表明在晚泥盆世時二者并未相連,彼此的古地理位置還相距較遠。此時,阿拉善地塊與華南、澳大利亞以及特提斯—喜馬拉雅在碎屑物質(zhì)來源上更具親緣性(圖2a,b,c,d)。同時,對阿拉善地塊南緣石炭紀(jì)—三疊紀(jì)沉積巖開展系統(tǒng)的古地磁研究結(jié)果表明,阿拉善地塊與華北板塊拼合時間是在早—中三疊世之后,二者的分界線可能位于賀蘭山與桌子山之間的南北走向的延長線之上。華北地塊在晚三疊世—中侏羅世發(fā)生了～42°的逆時針旋轉(zhuǎn),而揚子地塊則發(fā)生了～35°順時針旋轉(zhuǎn);這些相對旋轉(zhuǎn)作用使得揚子地塊與華北地塊完成拼合(Yang et al.,1992)。秦嶺—大別造山帶南緣的勉略蛇綠混雜巖帶,很可能代表了華南地塊與華北地塊最后拼合的結(jié)合帶,勉略洋盆泥盆紀(jì)時打開,石炭紀(jì)—二疊紀(jì)是洋盆主體擴張期,至三疊紀(jì)逐漸閉合(賴紹聰?shù)?2003;Lai et al.,2004);晚三疊世由于華北地塊與揚子地塊發(fā)生碰撞造山,形成大量印支期鈣堿性同碰撞花崗巖(張國偉等,2001)。至晚侏羅世,西伯利亞、華北—蒙古聯(lián)合地塊、揚子地塊完全拼合,形成了統(tǒng)一的大陸(Enkin et al.,1992;朱日祥等,1998)。

致謝:本研究是《中國西部大陸新生代構(gòu)造格局與演化》(編號:SinoProbe08-01;201011074)的部分成果,吉林大學(xué)葛肖虹教授審閱了初稿并提出寶貴的修改意見;野外工作得到南京大學(xué)李軍鵬、秦永鵬和張海峰等大力協(xié)助,在此深表感謝！

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