來志慶， 韓宗珠， 李三忠， 部雪嬌， 劉 博

(中國海洋大學(xué)海洋地球科學(xué)學(xué)院 海底科學(xué)與探測技術(shù)教育部重點(diǎn)實(shí)驗(yàn)室，山東 青島 266100)

揚(yáng)子地塊東南緣新元古代下江群地層白云母40Ar-39Ar年齡及其地質(zhì)意義?

來志慶， 韓宗珠??， 李三忠， 部雪嬌， 劉 博

(中國海洋大學(xué)海洋地球科學(xué)學(xué)院 海底科學(xué)與探測技術(shù)教育部重點(diǎn)實(shí)驗(yàn)室，山東 青島 266100)

古太平洋板塊在燕山期的俯沖后撤致使華南板塊的大地構(gòu)造背景由造山增厚轉(zhuǎn)換為伸展減薄，但目前對于俯沖后撤的起始時間仍有爭議，且針對揚(yáng)子地塊東南緣甚少有年齡數(shù)據(jù)來制約此次中生代構(gòu)造屬性的轉(zhuǎn)換。本文對揚(yáng)子地塊東南緣下江群地層的變質(zhì)礦物白云母開展40Ar-39Ar同位素測年研究，并分別獲得(103±2)Ma和(102±2)Ma的坪年齡。結(jié)合已有研究數(shù)據(jù)，上述年齡表明揚(yáng)子地塊東南緣在早白堊世晚期(～100Ma)已完全轉(zhuǎn)變?yōu)樘窖髽?gòu)造域，并由陸內(nèi)造山轉(zhuǎn)換為伸展擴(kuò)張構(gòu)造階段。此時古太平洋板塊的俯沖后撤已啟動和逐漸東移，并最終導(dǎo)致華夏地塊東部晚白堊世(～86Ma)大量花崗巖和雙峰式火山巖的形成。

揚(yáng)子地塊；下江群；太平洋板塊；俯沖后撤

揚(yáng)子地塊作為華南板塊的主要組成構(gòu)造單元，位于特提斯與古太平洋兩大構(gòu)造域的接觸帶。受太平洋板塊俯沖與后撤的影響，揚(yáng)子地塊乃至整個東亞的大地構(gòu)造在中生代發(fā)生了重要轉(zhuǎn)折[1-2]。燕山期陸內(nèi)構(gòu)造作用導(dǎo)致?lián)P子地塊乃至整個華南板塊普遍發(fā)生強(qiáng)烈褶皺和沖斷變形[3-6]，形成一系列NE—SW走向的斷陷盆地并伴隨著巨量巖漿侵入和火山活動[7-8]。有學(xué)者強(qiáng)調(diào)華南褶皺構(gòu)造主要形成于印支期[9]。華南晚中生代斷陷盆地的形成與古太平洋俯沖無關(guān)，其白堊紀(jì)巖漿活動是巖石圈伸展減薄造成減壓熔融的結(jié)果[8]。但也有研究者認(rèn)為，古太平洋板塊的西向俯沖后撤導(dǎo)致NE—SW向白堊紀(jì)伸展盆地群的形成，太平洋板塊的遠(yuǎn)距離平板俯沖造成華南板塊在1200km范圍內(nèi)廣泛分布火成巖和構(gòu)造帶[10]，而板塊俯沖逐漸后退變陡導(dǎo)致華南地區(qū)中晚侏羅世以來巖漿活動由內(nèi)陸向海岸線方向的遷移[11-12]。古太平洋板塊自二疊紀(jì)中期向華南板塊之下俯沖[13]，在歐亞大陸東部形成造山帶[14]。其大陸邊緣在中生代(250～90Ma)屬于安第斯型[15-16]，同時造成華南板塊大量造山期火成巖的形成，其侏羅紀(jì)巖漿活動的峰期為170～140Ma[17-18]；由于古太平洋板塊的俯沖后撤，其在晚白堊世(約90Ma)后轉(zhuǎn)變?yōu)槲魈窖笮?，同時造成華夏地塊形成大量晚白堊世A型花崗巖和雙峰式火山巖，峰期主要為107～86Ma[6,19-22]。目前古太平洋板塊俯沖后撤的起始時間仍有爭議，也有學(xué)者認(rèn)為俯沖后撤始于早白堊世甚至晚侏羅紀(jì)(早于120Ma)[23-24]。因此，華南中生代大地構(gòu)造演化歷史是極其復(fù)雜和存在爭議的，也一直是地學(xué)界研究的熱點(diǎn)。但前人研究主要通過火成巖地球化學(xué)和年代學(xué)證據(jù)制約古太平洋板塊俯沖后撤和構(gòu)造屬性的轉(zhuǎn)換，且主要集中于華夏地塊，關(guān)于揚(yáng)子地塊的大地構(gòu)造轉(zhuǎn)換事件的研究甚少。揚(yáng)子地塊東南緣存在大量的華南典型變質(zhì)沉積巖地層-下江群及其相應(yīng)地層，下江群變質(zhì)變形明顯，巖石內(nèi)變質(zhì)礦物白云母變形及定向排列顯著，且含量較多。本文以下江群變質(zhì)地層為研究對象，利用白云母礦物開展40Ar-39Ar定年研究，旨在制約揚(yáng)子地塊東南緣燕山期構(gòu)造屬性轉(zhuǎn)換事件，并進(jìn)一步討論太平洋板塊俯沖后撤對華南板塊的影響。

1 區(qū)域地質(zhì)概況及樣品巖石學(xué)特征

1.1 區(qū)域地質(zhì)概況

揚(yáng)子地塊西鄰印度板塊，北鄰華北板塊，東南與華夏地塊相鄰[25]。其與華夏地塊之間為雪峰山陸內(nèi)復(fù)合構(gòu)造系統(tǒng)，是兩大陸塊長期相互運(yùn)動和作用過程中形成的陸內(nèi)變形帶(見圖1a)。揚(yáng)子地塊存在太古代-古元古代結(jié)晶基底(崆嶺群等)，基底周圍為中元古代-新元古代褶皺帶，褶皺帶與震旦系蓋層不整合接觸[26-27]。

(a:華南板塊構(gòu)造簡圖[34]，b：研究區(qū)域地質(zhì)圖及采樣位置，c：上白堊統(tǒng)與新元古界地層不整合接觸關(guān)系。a:Tectonic map of Yangtze Block[34];b: Sampling sites and location of the study area in Yangtze Block;c: Unconformity between upper Cretaceous and Neoproterozoic stratas.)

本文研究區(qū)位于揚(yáng)子地塊東南緣，其基底地層為新元古代綠片巖相火山沉積地層(970～825Ma)，與上覆新元古代地層(815～750Ma)以及震旦紀(jì)蓋層(<750Ma)不整合接觸，并有過鋁質(zhì)S型花崗巖(825～815Ma)侵入[26-29]。早新元古代變質(zhì)火山沉積地層(970～825Ma)以梵凈山群(貴州)、四堡群(廣西)、冷家溪群(湖南)、雙橋山群(江西)、上溪群(安徽)和雙溪塢群(浙江)為代表[27,30-31]。晚新元古代變沉積地層(815～760Ma)主要有下江群(貴州)、丹洲群(廣西)、板溪群(湖南)、登山群(安徽)和歷口群(浙江)等[26]。侵入梵凈山群及相應(yīng)地層的新元古代花崗侵入體(825～815Ma)主要有三防、本洞、摩天嶺和元寶山等巖體[32-33]。

梵凈山群及相應(yīng)地層主要分布于黔東南及桂北九萬大山、元寶山一帶，與上覆下江群呈平行不整合至角度不整合接觸關(guān)系，主要是由灰色、灰綠色變質(zhì)細(xì)砂巖、 變質(zhì)粉砂巖及泥質(zhì)粉砂巖組成的海相碎屑巖系。

下江群及相應(yīng)地層是一套沉積序列和沉積構(gòu)造背景特殊的地層單元，主要為一套陸源碎屑夾大量晶屑凝灰?guī)r和沉凝灰?guī)r夾層沉積地層，包括變質(zhì)砂巖、板巖、千枚巖、片巖、大理巖等[35-36]。其沉積厚度大, 沉積速率高, 成熟度低。作為揚(yáng)子地塊基底的下江群及對應(yīng)地層經(jīng)歷綠片巖相變質(zhì)作用，并發(fā)育大量的直立褶皺和開闊褶皺。

區(qū)域內(nèi)還出露南華-震旦系、寒武系、奧陶系、泥盆系、石炭系、二疊系、侏羅系、白堊系等地層(見圖1b)。南華-震旦系與丹洲群為整合接觸關(guān)系，主要巖性為一套輕變質(zhì)的含礫砂泥巖、砂巖、泥巖等。石炭系為碳酸鹽臺和碎屑巖沉積。白堊系為一套紅層沉積，主要巖性有礫巖，雜砂巖和粉砂巖等，與下伏地層為不整合接觸關(guān)系(見圖1c)。

1.2 樣品巖石學(xué)特征

本次研究在基于野外詳細(xì)地質(zhì)調(diào)查的基礎(chǔ)上，采集下江群變沉積巖地層樣品兩件并進(jìn)行40Ar-39Ar測年。測試樣品巖石學(xué)特征主要有：

樣品GX2007-140為黑色千枚巖，變晶結(jié)構(gòu)，千枚狀構(gòu)造，主要礦物有白云母(50%)+石英(25%)+磁鐵礦(25%)。白云母含量高，但礦物顆粒較小，變形定向排列明顯；石英破碎并定向排列，沿片理分布，最大者200μm，多數(shù)小于100μm。斜長石少見，且碳酸鹽化和云母化明顯(見圖2a)。

(a:樣品GX2007-140；b：樣品GX2007-152。a:Sample GX2007-140;b:Sample GX2007-152.)

樣品GX2007-152為青灰色千枚巖，變晶結(jié)構(gòu)，千枚狀構(gòu)造，變質(zhì)程度低于樣品GX2007-140。主要礦物有石英(50%)+白云母(30%)。鏡下石英呈條帶狀，最大者小于100μm；白云母顆粒較小，定向排列明顯(見圖2b)。

2 40Ar-39Ar年代學(xué)實(shí)驗(yàn)方法及結(jié)果

樣品經(jīng)清洗、室溫風(fēng)干后逐步破碎至0.30～0.45mm，在雙目鏡下分別挑選出無蝕變白云母單礦物，純度在99%以上。樣品用鋁箔包裹，鎘箔屏蔽，與參考標(biāo)準(zhǔn)樣品(黑云母ZBH- 2506，年齡為132.5Ma)一起放于核反應(yīng)堆照射。每個樣品分多階段，50W CO2激光器輸出功率從4.0%開始加熱，最高加熱至18.0%。所有的數(shù)據(jù)在回歸到時間零點(diǎn)值后再進(jìn)行質(zhì)量歧視校正、本底校正和干擾元素同位素校正。Ca、K產(chǎn)生的干擾同位素校正因子為：(36Ar/37Ar)(Ca)= 0.00026726，(39Ar/37Ar)(Ca)= 0.0008984，(40Ar/39Ar)K=0.00597，38Ar/36Ar (a)=0.1869，38Ar/36Ar (a)=5.543×1010a-1，J=0.008196。樣品測定由中國科學(xué)院同位素年代學(xué)和地球化學(xué)重點(diǎn)實(shí)驗(yàn)室完成，測試方法參照文獻(xiàn)[37]，同位素數(shù)據(jù)處理和年齡計算采用ArArCALC軟件[38]，測試結(jié)果如下：

樣品GX2007-140獲得坪年齡為(103±2)Ma， 主坪的39Ar釋放量達(dá)到77%，等時線年齡為(107±1)Ma(MSWD=1.43)。樣品GX2007-152獲得坪年齡為(102±2)Ma，主坪的39Ar析出占總量的99%。樣品的等時線年齡為(99±5)Ma(MSWD=1.43)，擬合很好(見圖3)。

圖3 下江群變沉積巖白云母40Ar/39Ar同位素坪年齡和等時線年齡

3 討論

礦物坪年齡是礦物冷卻至其相應(yīng)封閉溫度時記錄的封閉溫度年齡，因此，礦物坪年齡實(shí)際上是構(gòu)造熱事件的冷卻年齡[39]。本研究樣品白云母均一的“主坪”揭示其受后期改造并不強(qiáng)烈，說明白云母形成過程中保持了較理想的同位素封閉體系，受外界因素干擾較少，樣品40Ar-39Ar寬闊坪年齡的39Ar釋放率為70%～99%，年齡數(shù)據(jù)可靠，與相似的等時線年齡數(shù)據(jù)對照能夠代表變形的冷卻年齡。

目前，對于揚(yáng)子地塊和華夏地塊的碰撞形成江南-雪峰造山帶的時間仍有爭議。有學(xué)者認(rèn)為碰撞始于古生代[40-41]，也有學(xué)者認(rèn)為兩地塊在新元古代就已發(fā)生碰撞并拼合成一體[31-33]。但華南板塊在中生代之前完成拼合是已得到共識的，且最新研究成果已將碰撞時間界定至825～815 Ma(見圖4a)[42]。

晚三疊世，伴隨著古特提斯洋的消減閉合，華南板塊北緣與華北板塊發(fā)生碰撞并拼合到一起(見圖4b)，此次構(gòu)造事件使得揚(yáng)子地塊邊緣受到的強(qiáng)烈的碰撞與擠壓作用。華北板塊和華南板塊的碰撞是中生代最重要的一次地質(zhì)事件，兩板塊約在早中侏羅世結(jié)束碰撞并造成東亞的最終形成[43]。

燕山期(侏羅-白堊紀(jì))，華南板塊主要處于太平洋板塊俯沖導(dǎo)致的大地構(gòu)造轉(zhuǎn)換階段。古太平洋板塊NW向俯沖于華南板塊之下[44]，有效彈性厚度與熱流值以及地震學(xué)的綜合分析說明古太平洋板塊已俯沖至揚(yáng)子克拉通(四川盆地)下方[49]。華南板塊廣泛發(fā)育向內(nèi)陸不斷遷移的造山帶和山前前陸盆地,多層逆沖推覆系統(tǒng)也說明揚(yáng)子地塊內(nèi)部的造山事件是自侏羅紀(jì)-白堊紀(jì)的一次由東南向西北方向漸進(jìn)的變形[32, 50]。磷灰石裂變徑跡年齡揭示，自湘鄂西向川東華鎣山構(gòu)造變形發(fā)展的時代從 165～95Ma具有遞進(jìn)變新的趨勢[51]。中侏羅世巖漿巖展布方位和太平洋俯沖帶邊界平行[52]。另外，雪峰造山帶加里東-印支-燕山運(yùn)動自南東向北西不斷穿時拓展說明其力來源于南東方向[54]。所以揚(yáng)子地塊在中侏羅世后已完全受古太平洋板塊活動的影響。

(YB：揚(yáng)子地塊；CB：華夏地塊；IN：印度地塊；IC：印度支那地塊；AN：南極洲地塊；AU：澳大利亞地塊；LA：勞倫古大陸；NCB：華北地塊；SCB：華南地塊；Q：羌塘地塊；S：滇緬馬地塊；SA：南美陸塊；AF：非洲陸塊; MA:馬達(dá)加斯加陸塊; TB：塔里木盆地；QB：柴達(dá)木盆地；QT：羌塘地體；LT：拉薩地體；SGT：松潘-甘孜地體；SB：四川盆地；QDOB：秦嶺大別造山帶. YB: Yangtze block; CB: Cathaysia Block; IN: Indian Block; IC: Indochina Block; An: Antarctica Block; Au: Australia Block; LA: laurentia; NCB: North China block; SCB: South China Block; Q = Qiangtang Block; S =Sibumasu Block; SA: South American plate; AF:African plate; MA:Madagascar landmass; TB: Tarim basin; QB: Qaidam basin; QT: Qiangtang terrane; LT= Lhasa terrane; SGT:Songpan-Ganzi terrane; SB: Sichuan basin;QDOB: Qinling-Dabie orogeny.)

由于古太平洋板塊NW向俯沖，燕山期構(gòu)造熱事件分布于自海岸線向內(nèi)陸延伸的1300km范圍內(nèi)，而大范圍的變形帶分布于近2000km的范圍內(nèi)，遠(yuǎn)至四川盆地[54]。華夏地塊和揚(yáng)子地塊形成大量的造山期火成巖，其侏羅紀(jì)巖漿作用的高峰期為170～140Ma[17-18]。后由于古太平洋板塊的俯沖后撤致使華夏地塊與揚(yáng)子地塊形成大量伸展擴(kuò)張性質(zhì)的火成巖，其主要巖漿峰期為107～86Ma[19,21-22,56-57]。但是，目前對于華南板塊在140～100Ma之間的構(gòu)造屬性仍存在造山壓縮和伸展擴(kuò)張的異議[21-24]。部分學(xué)者認(rèn)為140～100Ma之間出現(xiàn)的A型花崗巖具有造山期后的特征，并認(rèn)為華南板塊在100Ma之前已經(jīng)改變?yōu)樯煺箶U(kuò)張構(gòu)造環(huán)境[21]。也有學(xué)者認(rèn)為晚白堊世(約98Ma)I型花崗巖出現(xiàn)說明在華夏地塊在100Ma之前為俯沖造山環(huán)境[22]。因此，不妨認(rèn)為華南板塊區(qū)域構(gòu)造環(huán)境由擠壓到伸展并非一蹴而就，兩者之間可能存在一個過渡期。140～100Ma，華南板塊可能處于陸殼造山增厚與伸展減薄的過渡期，古太平洋板塊NW向持續(xù)俯沖過程中，由于俯沖距離和板片密度不斷加大，最終導(dǎo)致俯沖板片的斷離和拆沉，由此引起軟流圈上涌并導(dǎo)致巖石圈地幔和地殼部分熔融而形成A型與I型花崗巖以及雙峰式火山巖。

野外觀察研究區(qū)晚白堊地層不整合于晚元古代地層之上(見圖1c)。揚(yáng)子地塊西部的四川盆地晚侏羅-早白堊沉積巖存在大量的NE向向斜和背斜，盆地內(nèi)晚白堊地層與早白堊地層不整合接觸，晚白堊與新第三紀(jì)河流湖泊相沉積并沒有發(fā)生褶皺變形。中國東部的擴(kuò)張和裂谷形成大量的NE—SW向裂谷盆地，在湘鄂西地區(qū)出露少量擴(kuò)張期的碎屑巖說明揚(yáng)子地塊在晚白堊世經(jīng)歷一次伸展擴(kuò)張事件[55]。晚白堊世伸展構(gòu)造性質(zhì)火成巖在空間上呈NE—SW向展布，且年齡較老的多在內(nèi)陸發(fā)育(如廣西錫田花崗巖)[56-57]，年齡較年輕的巖漿巖(如臺灣90～86Ma的A型花崗巖)則主要集中于東南沿海和臺灣地區(qū)，說明晚白堊世巖漿活動存在由內(nèi)陸向沿海逐漸遷移的規(guī)律[6,8-9]。另外，揚(yáng)子地塊磷灰石裂變徑跡年齡也存在自西至東由約90Ma逐漸變小的規(guī)律[58]。因此，作者認(rèn)為本文所得40Ar-39Ar年齡(103±2)Ma和(102±2)Ma代表了古太平洋板塊俯沖后撤的起始時間，此時揚(yáng)子地塊已由陸內(nèi)造山完全轉(zhuǎn)換為伸展構(gòu)造環(huán)境，并形成大量的北東向盆山構(gòu)造。古太平洋俯沖板片的后撤以及后續(xù)俯沖角度的增大使得俯沖作用對于揚(yáng)子地塊乃至整個華南板塊的影響逐漸東移，并在華夏地塊東部和下?lián)P子地區(qū)形成大量的晚白堊世花崗巖和雙峰式火成巖。

此外，印度-歐亞板塊的陸陸碰撞始于60Ma。晚始新世(約40Ma)藏南最高海相層可能是印度-亞洲大陸碰撞完成的標(biāo)志[59-60]。最新研究認(rèn)為印度板塊和歐亞板塊的碰撞始于55Ma，板塊邊緣的弧巖漿作用主要集中于80～40Ma，并自南向北遷移，并在70～43Ma再次回遷至西藏南部[61]。 因此，印度板塊和歐亞板塊的陸陸碰撞構(gòu)造事件在下江群地層中并未有地質(zhì)記錄。

4 結(jié)論

(1)下江群變沉積巖樣品的白云母40Ar/39Ar定年結(jié)果分別為(103±2)Ma和(102±2)Ma，指示揚(yáng)子地塊東南緣在早白堊世晚期經(jīng)歷一次構(gòu)造熱事件。

(2)早白堊世晚期(～100Ma)，古太平洋板塊的俯沖后撤使得華南板塊由陸內(nèi)造山轉(zhuǎn)變?yōu)樯煺箻?gòu)造環(huán)境，且由于后續(xù)俯角度的增大導(dǎo)致華夏地塊東部巨量晚白堊世(～86Ma)花崗巖和雙峰式火山巖的形成。

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責(zé)任編輯 徐 環(huán)

40Ar-39Ar Dating for Muscovite in Neoproterozoic Meta-sedimentary Rocks of Xiajiang Group in Southeastern Yangtze Block and Its Geological Significance

LAI Zhi-Qing， HAN Zong-Zhu， LI San-Zhong， BU Xue-Jiao， LIU Bo

(The Key Lab of Submarine Geosciences and Prospecting Techniques，Ministry of Education； College of Marine Geosciences，Ocean University of China，Qingdao 266100，China)

During Yanshanian, tectonic of southern Yangtze Block converted due to rollback of Pacific plate. nevertheless，there was no accurate geochronology to constrain the conversion and onset of rollback. Therefore, muscovite40Ar-39Ar ages were measured, and samples GX2007-140 and GX2007-152 collected from Xiajiang Group yielded plateau ages (103±2)Ma and (102±2)Ma, respectively.40Ar-39Ar thermochronology indicates that rollback of Pacific plate had also began and migrated eastward since early Cretaceous(～100 Ma), and tectonics had change from intracontinental orogeny into extension in southeastern margin of Yangtze Block. At this time, Huge amounts of cretaceous granites and bimodal volcanic rocks (～86 Ma) in eastern margin of Eurasian Plate were eventually formed.

Yangtze block；Xiajiang group；Pacific Ocean plate；rollback

中石化總公司重大科技攻關(guān)項目(G0800-06-ZS-281)；國家自然科學(xué)基金項目(41376053)資助 Supported by the Key Proggram of Petro China(G0800-06-ZS-281); The National Natruanal Science Foundation of China (41376053)

2015-11-10；

2015-12-14

來志慶(1983-)，男，博士生，主要從事巖石地球化學(xué)和海洋地質(zhì)學(xué)研究。E-mail:zqlai@ouc.edu.cn

?? 通訊作者： E-mail：hanzongzhu@ouc.edu.cn

P597

A

1672-5174(2016)05-094-07

10.16441/j.cnki.hdxb.20150391

來志慶， 韓宗珠， 李三忠, 等. 揚(yáng)子地塊東南緣新元古代下江群地層白云母40Ar-39年齡及其地質(zhì)意義[J]. 中國海洋大學(xué)學(xué)報(自然科學(xué)版), 2017, 47(5): 94-100.

LAI Zhi-Qing， HAN Zong-Zhu， LI San-Zhong， et al.40Ar-39Ar Dating for muscovite in neoproterozoic meta-sedimentary rocks of Xiajiang group in Southeastern Yangtze block and its geological significance [J]. Periodical of Ocean University of China, 2017, 47(5): 94-100.