華南二疊系卡匹敦階高分辨率浮點年代標(biāo)尺

薛武強, 李波， 顏佳新*, Brooks B. Ellwood,Jonathan H. Tomkin, 王艷, 朱宗敏

1 中國地質(zhì)大學(xué)(武漢)，地球科學(xué)學(xué)院，生物地質(zhì)與環(huán)境地質(zhì)國家重點實驗室， 武漢 430074 2 國土資源部海底礦產(chǎn)資源重點實驗室，廣州海洋地質(zhì)調(diào)查局， 廣州 510075 3 Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA 4 School of Earth, Society, and Environment, University of Illinois, Urbana, IL 61801, USA 5 廣東省有色地質(zhì)勘查院， 廣州 510080

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華南二疊系卡匹敦階高分辨率浮點年代標(biāo)尺

薛武強1, 李波2， 顏佳新1*, Brooks B. Ellwood3,Jonathan H. Tomkin4, 王艷5, 朱宗敏1

1 中國地質(zhì)大學(xué)(武漢)，地球科學(xué)學(xué)院，生物地質(zhì)與環(huán)境地質(zhì)國家重點實驗室， 武漢 430074 2 國土資源部海底礦產(chǎn)資源重點實驗室，廣州海洋地質(zhì)調(diào)查局， 廣州 510075 3 Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA 4 School of Earth, Society, and Environment, University of Illinois, Urbana, IL 61801, USA 5 廣東省有色地質(zhì)勘查院， 廣州 510080

基于詳細的生物地層學(xué)研究，以磁化率為古氣候替代指標(biāo)，對廣西來賓鐵橋剖面卡匹敦階地層開展時間序列分析，建立高分辨率浮點年代標(biāo)尺(FPTS).結(jié)果表明，磁化率記錄了鐵橋剖面中二疊世晚期沉積序列中的米蘭科維奇旋回，卡匹敦階上部磁化率突然增加與峨眉山玄武巖噴發(fā)和卡匹敦晚期全球性海退有關(guān)，這些事件導(dǎo)致同期沉積物中碎屑物質(zhì)增加.鐵橋剖面瓜德魯普—樂平統(tǒng)界線附近磁化率和蓬萊灘剖面(樂平統(tǒng)底界GSSP)表現(xiàn)出一致的變化趨勢，具可對比性.利用多窗譜法(MTM)和傅里葉變換(FT)從磁化率序列中識別出五個米蘭科維奇周期：長偏心率周期(E2，405 ka)、短偏心率周期(E1，100 ka)、長地軸斜率周期(O2，44.1 ka)、長歲差周期(P2，20.95 ka)和短歲差周期(P1，17.7 ka).對比基于E2周期建立的磁性地層磁化率(MSS)帶和標(biāo)準參考帶(SRZ)，建立整個沉積序列的高分辨率(200 ka)FPTS，提出卡匹敦階的時限為3.85 Ma(存在+0～0.28 Ma誤差)，整段沉積序列的平均沉積速率為2.91 cm·ka-1.同時計算出卡匹敦階內(nèi)部七個牙形石帶的時限，從最短26.6 ka到最長2.3 Ma.另外，估算出峨眉山大火成巖省噴發(fā)啟動時間為262.67 Ma，位于瓜德魯普—樂平統(tǒng)界線之下1.42 Ma.

磁化率； 牙形石； 時間序列分析； 卡匹敦階； 峨眉山大火成巖省； 浮點年代標(biāo)尺

1 引言

在整個顯生宙建立高分辨率地質(zhì)年代標(biāo)尺是地層學(xué)的一項重要工作.現(xiàn)今，地質(zhì)年代標(biāo)尺的建立主要基于生物地層框架、放射性同位素定年和磁性地層，如二疊—三疊系界線年齡251.902±0.024 Ma，來源于界線層火山灰的鋯石U/Pb年齡(Burgess et al., 2014).然而，許多界線附近缺少火山灰層或難以從火山灰鋯石中獲得準確年齡，限制了地質(zhì)年代標(biāo)尺的精度，即便獲得了放射性同位素年齡，其誤差通常也是百萬年級別.米蘭科維奇理論的提出與證實(Hays et al., 1976)為建立高分辨率地質(zhì)年代標(biāo)尺提供了一種新方法(龔一鳴等, 2008；黃春菊，2014；吳懷春等, 2011).天文軌道的周期性變化導(dǎo)致地球氣候周期性變化，而沉積物記錄了周期性氣候，從沉積物中提取出天文軌道周期，將其調(diào)諧到理論的天文周期曲線上，即可確定該段沉積物沉積持續(xù)的時間，據(jù)此可建立浮動的地質(zhì)年代標(biāo)尺，其分辨率可達萬年級別，極大提高了地質(zhì)年代標(biāo)尺的精度.目前，天文軌道調(diào)諧定年已廣泛應(yīng)用于中生代以來的地層中(Gradstein et al., 2012)，但在古生代或更老的地層中應(yīng)用相對較少.

中二疊世卡匹敦期是峨眉山大火成巖省猛烈噴發(fā)的時期，也是古—中生代之交雙幕式生物大滅絕事件啟動的時間，是研究生物和生態(tài)響應(yīng)環(huán)境變化的極佳時期，引起眾多學(xué)者關(guān)注(Ali et al., 2002；Bond et al., 2010a, b； He et al., 2007；Sun et al., 2010；Wignall, 2001；Wignall et al., 2009a, b, 2012；Zhou et al., 2002；李波等，2015).然而，卡匹敦階的時限至今仍缺乏精確約束，歸因于頂?shù)捉缇€精確放射性同位素年齡缺乏，唯一精確的鋯石年齡來源于卡匹敦階底界之下37.2 m的火山灰層，為卡匹敦階底界提供一個最大年齡約束(Bowring et al., 1998).國際地質(zhì)年代表中給出的卡匹敦階參考時限為5.4 Ma(Gradstein et al., 2004；Ogg et al., 2008)或5.3 Ma(Gradstein et al., 2012)，誤差為±0.8～1.4 Ma，其精度遠低于早二疊世和晚二疊世，嚴重影響對生物大滅絕和峨眉山玄武巖噴發(fā)啟動時間和持續(xù)時間的了解.

磁化率是用來表征物體在外加磁場中被磁化難易程度的量(Borradaile, 1988).海相沉積巖磁化率的大小反映巖石中亞鐵磁性礦物和順磁性礦物的含量(Ellwood et al., 2000)，這些磁性礦物主要來自于陸源碎屑，通過河流或風(fēng)力搬運到沉積盆地中.由于周期性氣候變化控制陸源物質(zhì)侵蝕與搬運，繼而控制了沉積巖的磁化率(Crick et al., 1997；Ellwood et al., 1999).磁化率測量簡單快捷、無損，已作為可靠的古氣候替代指標(biāo)廣泛應(yīng)用于沉積地層的旋回地層學(xué)研究中(Ellwood et al., 2012；Guo et al., 2008；Huang et al., 2011；Jovane et al., 2006；Wu et al., 2012, 2013a, b, c).因此，本文利用磁化率數(shù)據(jù)對廣西來賓鐵橋剖面(樂平統(tǒng)底界GSSP輔助剖面)茅口組開展高精度旋回地層學(xué)研究，試圖確定米蘭科維奇沉積旋回，結(jié)合生物地層資料建立卡匹敦階高分辨率浮點年代標(biāo)尺(Float Point Time Scale, FPTS)，并探討峨眉山大火成巖省噴發(fā)的啟動時間和持續(xù)時間.

2 地質(zhì)背景

廣西來賓地區(qū)位于來賓—合山孤立碳酸鹽臺地東南緣(圖1)，在二疊紀晚期被深水盆地包圍(鄭和榮和胡宗全, 2010).來賓地區(qū)周邊的剖面在中二疊世大部分時期為盆地相沉積，主要發(fā)育放射蟲硅質(zhì)巖，但在中二疊世晚期發(fā)育比較典型的淺水孤立臺地沉積(Qiu et al., 2014；李波等，2015；沙慶安等, 1990；姚堯等, 2012)，這個沉積相的轉(zhuǎn)變通常被認為是海平面下降(Mei et al., 1998)或東吳運動抬升造成(Shen et al., 2007；Wang and Jin, 2000).

圖1 研究區(qū)中二疊世晚期古地理圖(鄭和榮和胡宗全, 2010)及剖面位置圖(Jin et al., 1998)Fig.1 Paleogeographic map (revised from Zheng and Hu, 2010) and geological map with locations (revised from Jin et al. ,1998) of the studying sections in Laibin, South China

圖2 廣西來賓鐵橋剖面沃德—吳家坪階巖性柱和牙形石帶茅口組層號參考沙慶安等(1990)，段(Member)采用Shen等(2007)方案，圖中樣品位置為牙形石樣品采樣位置.Fig.2 Lithological column with conodont zones for the Wordian-Wuchiapingian period at Tieqiao section, Laibin, South ChinaBed numbers of Maokou Formation are referring to subdivision in Sha et al. (1990). Member numbers are from Shen et al. (2007). Sampling locations for conodont biostratigraphy are marked with short bars on the right side of the lithological column.

鐵橋剖面位于紅水河北岸(23° 42.733′ N, 109° 13.533′ E)，距離來賓市區(qū)約5 km.鐵橋剖面與蓬萊灘剖面(瓜德魯普—樂平統(tǒng)界線GSSP)分別位于來賓向斜兩翼，兩者相距十幾公里(Jin et al., 1998).二疊系棲霞組、茅口組、吳家坪組和大隆組依次沿著紅水河河岸出露.中—上二疊統(tǒng)茅口組大部分層位是深水盆地相碳酸鹽巖和硅質(zhì)巖，但在頂部發(fā)育一套厚層灰?guī)r(來賓灰?guī)r)(沙慶安等, 1990).茅口組之上為吳家坪組放射蟲硅質(zhì)巖，兩者呈整合接觸，卡匹敦階位于茅口組上部(Mei et al., 1998).前人在鐵橋剖面開展了大量牙形石生物地層工作，但這些生物地層工作大多集中在瓜德魯普—樂平統(tǒng)(G-L)界線(Henderson et al., 2002; Jin et al.,1998,2001, 2006；Mei et al.,1998; Shen et al.,2007).由于卡匹敦階中部和下部硅質(zhì)巖較多，前人對該層位的生物地層工作研究精度相對較低.

3 采樣與實驗方法

3.1 牙形石采樣和處理

本文對整個卡匹敦階進行了較為完整的生物地層工作，以期獲得更精細的牙形石帶延限.從卡匹敦階底界之下31.2 m到卡匹敦—吳家坪階(C-W)界線之上2.6 m，大致以3 m間距采集牙形石樣品45個(圖2)，每個樣品重量大于3.5 kg.按江海水等(2004)的方法對樣品進行處理：室內(nèi)先將樣品破碎到1～2 cm大小，然后浸泡于10%冰醋酸水溶液中進行酸解處理，待反應(yīng)完全后進行濕篩篩洗.先用20目篩洗去除較大顆粒，再用160目篩洗去除泥質(zhì)及細小顆粒，充分清洗后自然風(fēng)干，采用三溴甲烷和丙酮配制密度為2.80～2.81 g·mL-1的重液進行分選，最后在雙目體視鏡下人工挑樣.

3.2 磁化率采樣與測試

以0.1 m間距對卡匹敦階底界之下31.2 m至C-W界線之上2.6 m的地層采樣(圖2)，共采集無定向樣品1456個.低場體積磁化率測試在卡帕橋KLY-3S 型磁力儀上完成，稱重后換算成質(zhì)量磁化率()(Ellwood et al., 1988)，測試在中國地質(zhì)大學(xué)(武漢)地空學(xué)院巖石磁學(xué)實驗室完成.為了判斷樣品中磁性礦物類型，選擇代表性樣品進行熱磁實驗(κ-T曲線).測試時，粉末狀樣品在氬氣環(huán)境中以11 ℃/min的速度從室溫加熱到700 ℃，隨后以相同速率冷卻至室溫.熱磁實驗在中國地質(zhì)大學(xué)(北京)古地磁和環(huán)境磁學(xué)實驗室完成，所用儀器為卡帕橋KLY-4S/CS3 型磁力儀/溫控系統(tǒng).

3.3 時間序列分析

為方便數(shù)據(jù)展示和對比，在圖形中采用Bar-Log格式(Ellwood et al., 2011；García-Alcalde et al., 2011)，類似于磁性地層學(xué)中磁極性帶劃分.利用樣條函數(shù)對原始質(zhì)量磁化率曲線進行平滑擬合，平滑擬合類似于帶通濾波，提取出目標(biāo)頻率信號，當(dāng)需要提取長周期時(如偏心率)，增加系數(shù)λ，曲線更平滑，長周期顯現(xiàn)出來.根據(jù)需要平滑曲線后，如同磁性地層學(xué)中建立極性帶一樣，選擇最高點和最低點之間的中點定義為磁性地層磁化率(Magnetostratigraphic Susceptibility, MSS)帶界線，如果的變化趨勢(升高或降低)由兩個或以上的點組成，認為這個變化是一個有效周期(MSS帶)，將高值部分充填成黑色，低值部分留為空白(Ellwood et al., 2012).

4 結(jié)果

4.1 巖性與生物地層

鐵橋剖面卡匹敦階的巖性和牙形石延限帶見圖2.將沃德—卡匹敦階(W-C)界線定在H116層下部，由牙形石分子Jinogondolellapostserrata首現(xiàn)確定.由于H116層下部硅化嚴重，牙形石采樣精度有限(首現(xiàn)牙形石分子J.postserrata樣品與前一個樣品之間的間距為11.8 m，圖2)，可能導(dǎo)致W-C界線存在不確定性(實際界線可能會下移0～11.8 m).基于牙形石分子Clarkinapostbitteripostbitteri的首現(xiàn)(Jin et al., 2006)，卡匹敦—吳家坪階(C-W)界線位于H119層頂部，6j和7a—7b之間.在卡匹敦階內(nèi)部共確定了7個牙形石帶(圖2)，從底到頂依次為J.postserrata,J.shannoni,J.altudaensis,J.prexuanhanensis,J.xuanhanensis,J.granti和C.postbitterihongshuiensis(圖3)，與前人劃分方案一致(Jin et al., 2001, 2006; Mei et al., 1998).H115—H118層由灰?guī)r與硅質(zhì)巖互層組成，其中灰?guī)r為灰色中薄層，主要是灰泥巖—粒泥巖，含有孔蟲、海百合莖和介形蟲；硅質(zhì)巖或硅化灰?guī)r主要為暗紅色，含有大量放射蟲和海綿骨針.濁流沉積在灰?guī)r層中非常發(fā)育，通常厚3～6 cm，由向上變細的粒序?qū)咏M成，代表不完整鮑馬序列.濁流沉積的結(jié)構(gòu)和生物組成與背景沉積存在較大差異，濁流沉積通常由生屑泥粒巖組成，含有豐富的有孔蟲、海百合莖、苔蘚蟲及管殼石碎片，生屑強烈侵蝕，從鄰近陸架邊緣或臺地搬運而來，采樣時避開這些異地沉積.上述特征表明，H115—H118層形成于斜坡—盆地環(huán)境.H119層(來賓灰?guī)r)主要由灰白色中厚層灰?guī)r組成，化石豐富，主要可分為7個單元，對應(yīng)于Jin 等(2001, 2006)中的Unit 2—8，與下伏的硅質(zhì)巖—灰?guī)r互層(H115—H118)截然不同.H113—H118層到H119層沉積相的突然變化記錄了卡匹敦晚期全球性的海退事件(Chen et al., 2009；Jin et al., 2001, 2006；Qiu et al., 2014；Wignall et al., 2009b)，這一海平面下降事件導(dǎo)致華南大部分地區(qū)茅口組上部地層缺失.

來賓灰?guī)r底部(Unit 2)由塊狀灰?guī)r組成，下部含燧石透鏡體，發(fā)育向上變細的正粒序?qū)?燧石中放射蟲豐富，塊狀灰?guī)r中富含海綿和有孔蟲，為斜坡—盆地相沉積.上覆Unit 3由薄層灰?guī)r組成，發(fā)育重力滑塌作用形成的軟沉積變形構(gòu)造，指示斜坡沉積環(huán)境.Unit 4和5主要由塊狀淺粉紅色灰?guī)r組成，富含苔蘚蟲、珊瑚、海百合莖和有孔蟲，縫合線大量發(fā)育.此外，在Unit 5上部發(fā)育一層平行于層面的腕足殼體，指示風(fēng)暴作用.Unit 4和5為典型的淺水碳酸鹽臺地相沉積.Unit 6為灰色灰?guī)r，層面見極豐富的海百合莖，大小混雜，莖板保存完好，顯示原地埋藏特征，解釋為海百合莖灘相沉積.Unit 7—8轉(zhuǎn)變成暗灰色中薄層灰?guī)r，該段指示海侵開始(李波等，2015).

茅口組上覆的吳家坪組由暗色薄層硅質(zhì)巖和硅化灰?guī)r組成，富含放射蟲、海綿骨針，缺乏底棲生物，指示吳家坪早期海侵后低能斜坡或盆地相沉積環(huán)境.綜上所述，H113—H118層和吳家坪組底部沉積于相對較深的斜坡—盆地相環(huán)境，而來賓灰?guī)r(H119)主要沉積于淺海環(huán)境，因此，可認為來賓灰?guī)r是硅質(zhì)巖中的灰?guī)r“內(nèi)序列”(intersequence)，形成于最大低水位期(Jin et al., 2001, 2006).

圖3 廣西來賓鐵橋剖面關(guān)鍵牙形石(a)口視圖；(b)側(cè)視圖；(c)反口視圖.圖中標(biāo)尺長度為100 μm.1 Jinogondolella shannoni (Wardlaw 1994), SP_012; 2 Jinogondolella granti (Mei and Wardlaw 1994), 06-70_027b; 3 Jinogondolella altudaensis (Kozur 1992), S3_033; 4 Jinogondolella xuanhanensis (Mei and Wardlaw 1994), S3_055; 5 Clarkina postbitteri postbitteri (Mei and Wardlaw 1994), S6_054.Fig.3 Key conodonts from the Tieqiao section, Laibin, South China(a) for upper view, (b) for lateral view and (c) for lower view. Scale bar for 100 μm.

圖4 廣西來賓鐵橋剖面沃德—吳家坪階磁化率()剖面陰影部分為峨眉山大火成巖省噴發(fā)的時間范圍(Ali et al., 2002; Sun et al., 2010)，巖性圖例見圖2.Fig.4 Raw MS profile across the Wordian-Wuchiapingian period at Tieqiao section, Laibin, South China Time duration of the Emeishan LIP emplacement event is marked with a grey shaded box (Ali et al., 2001; Sun et al., 2010). See lithology legend in Fig.2.

圖5 廣西來賓鐵橋剖面沃德—吳家坪階磁化率和MSS帶虛線為原始數(shù)據(jù)(剔除異常點，用相鄰點插值替代)，實線由原始經(jīng)樣條函數(shù)擬合得出，右側(cè)曲線是磁化率剖面中-下部(520～630 m)放大后的曲線.Fig.5 MS data and MSS zonation for the Wordian-Wuchiapingian period at Tieqiao section, Laibin, South ChinaDashed lines represent raw MS data with anomalous data points removed and substituted by adjacent data points via linear interpolation. Solid lines are smoothed using splines. The close-up of the middle-lower interval (520～630 m) of MS profile is on the right side of the figure.

圖6 廣西來賓鐵橋剖面沃德—吳家坪階代表性樣品的κ -T曲線Fig.6 κ -T curves for selected samples of the Wordian-Wuchiapingian period at Tieqiao section, Laibin, South China

圖7 廣西來賓鐵橋剖面沃德—吳家坪階數(shù)據(jù)功率譜圖Fig.7 Analysis result of spectral power with Multi-Taper (MTM) and Fourier Transform (FT) methods for the raw (unsmoothed) MS data sets from Tieqiao section, Laibin, South China

4.3 磁化率-溫度(κ-T)曲線

代表性樣品的κ-T曲線特征見圖6，所有加熱曲線均表現(xiàn)出相似的變化趨勢.在低溫段(20～200 ℃)磁化率曲線變化不大，當(dāng)溫度超過350 ℃時，磁化率迅速增加，在500 ℃時達到峰值，然后迅速降低，在580 ℃時降為零，顯示出磁鐵礦的居里溫度(Dunlop and ?zdemir, 1997).冷卻曲線同樣反映磁鐵礦存在，冷卻至室溫時，磁化率比初始磁化率高出很多，表明350 ℃時發(fā)生了相轉(zhuǎn)變，生成了新的強磁性磁鐵礦，這些新生成的磁鐵礦可能是由含鐵硅酸鹽礦物或粘土礦物(如伊利石)、含鐵水合物、針鐵礦中的鐵受熱轉(zhuǎn)化而成(Dunlop and ?zdemir, 1997；Ellwood et al., 2007).

4.4 時間序列分析

5 討論

鐵橋剖面樣品磁化率普遍較低(大部分為負值)，表明其主要由抗磁性方解石或石英組成，順磁性和亞鐵磁性礦物含量極低，反映來賓地區(qū)在卡匹敦期較孤立，遠離陸源.從全球角度來說，卡匹敦期，華南板塊位于古特提斯洋中，遠離聯(lián)合古陸(Kasuya et al., 2012；Ziegler et al., 1997)，而來賓地區(qū)處于贛湘桂盆地內(nèi)部，遠離揚子板塊內(nèi)部的華夏古陸和康滇古陸(圖1)，陸源物質(zhì)來源匱乏導(dǎo)致輸入到沉積盆地中的碎屑物質(zhì)較少，樣品中自生方解石和石英占主導(dǎo)，磁化率低.

卡匹敦階上部(630 m之上，J.altudaensis帶頂部)突然增加，表明輸入到海洋中的陸源碎屑物質(zhì)通量增加.雖然卡匹敦期晚期的海退作用對突然增加有所貢獻，但明顯的海退始于來賓灰?guī)r段(656.5 m)(Jin et al., 2001；Jin et al., 2006)，對應(yīng)于J.granti帶中增加，晚于J.altudaensis帶中增加，因此不能把卡匹敦晚期突然增加全部歸因于海退.本文認為卡匹敦晚期突然增加與峨眉山大火成巖省噴發(fā)有關(guān)，主要原因如下：(1)峨眉山玄武巖的大規(guī)模噴發(fā)(體積0.3×106～0.5×106km3，Ali et al., 2005)以及隨后的風(fēng)化剝蝕作用一定會導(dǎo)致大量碎屑物質(zhì)輸入到海洋系統(tǒng)，同時期或后期的沉積物勢必會記錄這一事件；(2)詳細的生物地層工作表明，峨眉山大火成巖省噴發(fā)的啟動位于J.altudaensis帶中，噴發(fā)規(guī)模隨后在J.xuanhanensis帶中增大(Ali et al., 2002；Sun et al., 2010)，這與鐵橋剖面卡匹敦階上部J.altudaensis帶頂部增加一致(圖4)；(3)在鐵橋剖面和蓬萊灘剖面卡匹敦晚期地層中均發(fā)現(xiàn)了砂?；鹕剿樾?，由暗色燒渣子顆粒組成，包含微小的斜長石斑晶，這些顆粒通常具有尖銳的棱角，氣泡腔被打碎形成破碎邊緣，表明它們并沒有經(jīng)歷河流的搬運和磨蝕作用，應(yīng)該是風(fēng)成成因(Wignall et al., 2009b)，證明火山噴發(fā)帶來的碎屑物質(zhì)確實進入到沉積盆地中.所有這些證據(jù)均表明，在卡匹敦晚期峨眉山玄武巖噴發(fā)導(dǎo)致卡匹敦晚期信號增強.而且，峨眉山玄武巖噴發(fā)對整個生物界均有較大影響，例如在J.altudaensis帶上部，蟲筳類和鈣藻大量滅絕(Bond et al., 2010b；Wignall et al., 2009a).如果卡匹敦階上部突然增加是峨眉山大火成巖省噴發(fā)引起這一假設(shè)成立，卡匹敦階頂部增加的起點即是峨眉山玄武巖噴發(fā)開始，利用文中建立的FPTS即可對峨眉山玄武巖噴發(fā)的啟動時間精確定年.

圖8 廣西來賓地區(qū)鐵橋剖面和蓬萊灘剖面(G-L GSSP)卡匹敦—吳家坪階界線對比蓬萊灘剖面的磁化率數(shù)據(jù)和牙形石帶來自于Clark (2012).帶點虛線為原始數(shù)據(jù)，實線為平滑后的數(shù)據(jù)，平滑數(shù)據(jù)由JMP統(tǒng)計軟件中的樣條函數(shù)計算.Fig.8 The profile across the Capitanian-Wuchiapingian (C-W) boundary at Tieqiao section in comparision with GSSP of Guadalupian-Lopingian (G-L) at Penglaitan section, Laibin, China The MS () data set and conodont zones of the G-L GSSP are modified from Clark (2012). Raw MS data are represented as dashed curves with solid circles. Solid curves are smoothed using splines (splines are calculated with JMP statistical software package by SAS Institute Inc.).

圖9 廣西來賓鐵橋剖面沃德—吳家坪階MSS帶與SRZ帶的圖形對比Fig.9 Graphic comparison of the MSS zonation from Tieqiao section to a standardized reference zonation (SRZ) model for the Wordian-Wuchiapingian period

5.3 浮點年代標(biāo)尺(FPTS)

以半E2周期(200 ka)為單位建立一個持續(xù)時間為5.07 Ma的標(biāo)準參考帶(Standard Reference Zonation, SRZ)(圖9)，包含25個200 ka等間距周期(WorU—Cap20).圖5中的MSS帶同樣是以半E2周期為單位建立，但由于沉積速率存在差異，每個MSS帶并非等間距.將MSS帶與SRZ進行圖形對比，以SRZ氣候模型為橫坐標(biāo)，以MSS帶為縱坐標(biāo)，將SRZ和MSS帶中對應(yīng)的每半個E2周期的頂?shù)淄队暗阶鴺?biāo)軸中，然后對這些點進行線性擬合，得到一系列最佳相關(guān)線(Lines of Correlation, LOC)，這與Shaw(1964)應(yīng)用在生物地層數(shù)據(jù)中的圖形對比方法類似.

利用這個方法可在卡匹敦階建立高分辨率的地質(zhì)年代標(biāo)尺，分辨率為200 ka.這個地質(zhì)年代標(biāo)尺獨立于放射性同位素年齡，可用來計算生物帶的時限，估算峨眉山大火成巖省噴發(fā)的啟動時間.對于這個地質(zhì)年代標(biāo)尺絕對年齡(“錨點”)的確定，本文選擇265.1 Ma(Gradstein et al., 2012)作為卡匹敦階底界年齡.如果其他學(xué)者的后續(xù)研究賦予卡匹敦階底界新的更為精確的絕對年齡，或在某個MSS帶中獲得精確的放射性同位素年齡，即可將這個新年齡標(biāo)記到對應(yīng)的SRZ氣候模型坐標(biāo)上，然后重新調(diào)整SRZ氣候模型上每個刻度的絕對年齡.因此，本文建立的地質(zhì)年代標(biāo)尺可隨時更新，是一個浮點年代標(biāo)尺.

在卡匹敦階共識別出19個200 ka 的半E2旋回，因此卡匹敦階的時限為3.85 Ma，由于W-C底界不確定，因此卡匹敦階時限可能存在+0～0.28 Ma的誤差(圖9).這個時限比其他學(xué)者給出的估計值要短，例如，2004年與2008年國際地質(zhì)年代表給出的參考值為5.4 Ma(Gradstein et al., 2004；Ogg et al., 2008)，2012年國際地質(zhì)年代表給出的參考值為5.3 Ma(Gradstein et al., 2012)，但存在±0.8～1.4 Ma的誤差.

鐵橋剖面MSS帶與SRZ帶之間的圖形對比，產(chǎn)生了五個LOC段(圖9中的A—E段)，代表不同時期不同的沉積速率(SAR).這些沉積速率的差異降低了MTM和FT數(shù)據(jù)中功譜峰值的置信水平(圖7).A段(1.77 cm·ka-1)和C段(1.27 cm·ka-1)沉積速率相對最低，D段(3.10 cm·ka-1)和E段(2.66 cm·ka-1)沉積速率中等，B段(4.76 cm·ka-1)沉積速率最大.沉積速率差異在MSS帶中非常明顯，Wor U—Wor Y、Cap5—Cap7非常短，沉積速率快；Wor Z—Cap 4非常長，該段沉積速率慢；而Cap 8—Cap20厚度中等，沉積速率中等(圖5).

5.4 牙形石帶時限和峨眉山大火成巖省噴發(fā)的時間

基于SRZ FPTS模型，估算出卡匹敦階內(nèi)各個牙形石帶的時限(表1)，延限最短的C.postbitterihongshuiensis僅僅26600年，延限最長的J.altudaensis持續(xù)了2.3 Ma.峨眉山玄武巖在瓜德魯普晚期噴發(fā)，雖然前人對其開展了大量地球化學(xué)研究，但噴發(fā)啟動的絕對年齡和噴發(fā)持續(xù)時間仍不確定，因為很難在噴發(fā)開始或結(jié)束時獲得精確的鋯石年齡(Liu and Zhu, 2009)，因此，精確的絕對年齡約束仍需進一步研究.生物地層對比提供一種確定噴發(fā)啟動時間的折中辦法，前人通過詳細的生物地層對比工作將峨眉山大火成巖省噴發(fā)啟動時間精確約束在有孔蟲(He et al., 2003；He et al., 2006)或牙形石帶(Sun et al., 2010)內(nèi)，為估算峨眉山大火成巖省噴發(fā)的啟動時間奠定了基礎(chǔ).峨眉山玄武巖噴發(fā)始于J.altudaensis帶頂部，對應(yīng)于突然增加，將這一點投影到FPTS上，計算出噴發(fā)始于262.67 Ma，在G-L界線之下1.42 Ma.在261.86 Ma時，即在J.xuanhanensis帶中，玄武巖噴發(fā)范圍和規(guī)模增大(Sun et al., 2010).研究表明峨眉山玄武巖主階段的噴發(fā)可能在G-L界線之下(或上)結(jié)束(Ali et al., 2002; Sun et al., 2010)，基于本文建立的FPTS，計算出峨眉山玄武巖大規(guī)模噴發(fā)可能持續(xù)了1.42 Ma(圖9).

表1 鐵橋剖面卡匹敦階牙形石帶時限Table 1 Duration of conodont zones of Capitanian in Tieqiao section

峨眉山玄武巖大規(guī)模噴發(fā)被認為是導(dǎo)致中二疊世晚期生物大滅絕的直接原因(Bond et al., 2010a；Wignall et al., 2009a)，即峨眉山玄武巖噴發(fā)和生物滅絕啟動具有同步性，因此，峨眉山玄武巖噴發(fā)的啟動時間即是中二疊世晚期生物大滅絕的開始時間，為262.67 Ma，位于G-L界線之下1.42 Ma.

6 結(jié)論

通過詳細的生物地層學(xué)工作，在卡匹敦階識別出7個牙形石帶，從底到頂依次為J.postserrata,J.shannoni,J.altudaensis,J.prexuanhanensis,J.xuanhanensis,J.granti和C.postbitterihongshuiensis.

質(zhì)量磁化率作為古氣候替代指標(biāo)，記錄了廣西來賓鐵橋剖面中二疊世晚期沉積序列中的米蘭科維奇旋回.大部分為負值(抗磁性)，表明來賓地區(qū)在該段沉積期孤立于陸源.卡匹敦階上部突然增加與峨眉山玄武巖噴發(fā)和卡匹敦晚期全球性海退有關(guān)，這些事件導(dǎo)致同期沉積物中的碎屑物質(zhì)增加.磁化率在鐵橋剖面G-L界線附近和蓬萊灘剖面(G-L界線GSSP)表現(xiàn)出一致的變化趨勢，具可對比性.κ-T曲線測試結(jié)果表明樣品中主要攜磁礦物為磁鐵礦.

利用MTM和FT法對原始質(zhì)量磁化率數(shù)據(jù)進行頻譜分析，提取出五個米蘭科維奇周期，分別是位于0.08 cycles·m-1的E2周期(405 ka)、位于0.35 cycles·m-1的E1周期(100 ka)、位于0.78 cycles·m-1的O2周期(44.1 ka)、位于1.69 cycles·m-1的P2周期(20.95 ka)和位于1.94 cycles·m-1的P1周期(17.7 ka)，其中四個周期的置信水平超過99%.基于MTM和FT分析提取的E2周期(405 ka)，對進行平滑處理，劃分出25個MSS帶.對比MSS帶和SRZ帶，對整個沉積序列建立了高分辨率(200 ka)浮點年代標(biāo)尺.同時計算出卡匹敦階的時限為3.85 Ma(可能存在+0～0.28 Ma誤差)，整個沉積序列(包括沃德階上部、整個卡匹敦階和吳家坪階下部)的平均沉積速率為2.91 cm·ka-1.計算出卡匹敦階內(nèi)部七個牙形石帶的時限，從最短的26.6 ka到最長的2.3 Ma.另外，估算出峨眉山大火成巖省噴發(fā)的啟動時間為262.67 Ma，位于G-L界線之下1.42 Ma，這也是古-中生代之交雙幕式生物大滅絕事件的啟動時間.致謝 中國地質(zhì)大學(xué)(武漢)孫亞東博士在牙形石樣品處理和鑒定中起到了決定性作用，實驗室成員嚴雅娟、李傲竹、姚堯、胡宗杰等在樣品采集和處理過程中提供了幫助，中國地質(zhì)大學(xué)李永濤和張世紅教授在實驗測試時提供了幫助，兩位匿名審稿人提出了寶貴意見，在此一并致謝！

AliJ R, Thompson G M, Song X Y, et al. 2002. Emeishan Basalts (SW China) and the ‘end-Guadalupian’ crisis: magnetobiostratigraphic constraints.JournaloftheGeologicalSociety, 159(1): 21-29, doi: 10.1144/0016-764901086.

Ali J R, Thompson G M, Zhou M F, et al. 2005. Emeishan large igneous province, SW China.Lithos, 79(3-4): 475-489, doi: 10.1016/j.lithos.2004.09.013.

Bond D P G, Hilton J, Wignall P B, et al. 2010a. The Middle Permian (Capitanian) mass extinction on land and in the oceans.Earth-ScienceReviews, 102(1-2): 100-116.

Bond D P G, Wignall P B, Wang W, et al. 2010b. The mid-Capitanian (Middle Permian) mass extinction and carbon isotope record of South China.Palaeogeography,Palaeoclimatology,Palaeoecology, 292(1-2): 282-294.Borradaile G J. 1988. Magnetic susceptibility, petrofabrics and strain.Tectonophysics, 156(1-2): 1-20.

Bowring S A, Erwin D H, Jin Y G, et al. 1998. U/Pb zircon geochronology and tempo of the End-Permian mass extinction.Science, 280 (5366): 1039-1045.

Burgess S D, Bowring S, Shen S Z. 2014. High-precision timeline for Earth's most severe extinction.ProceedingsoftheNationalAcademyofSciencesoftheUnitedStatesofAmerica, 111(9): 3316-3321.

Chen Z Q, George A D, Yang W R. 2009. Effects of Middle-Late Permian sea-level changes and mass extinction on the formation of the Tieqiao skeletal mound in the Laibin area, South China.AustralianJournalofEarthSciences, 56(6): 745-763.

Clark M A. 2012. Magnetostratigraphy susceptibility correlations for the Guadalupian-Lopingian boundary and the placement of the North American Ochoan series: Texas (USA) and South China [Master′s thesis]. Baton Rouge, LA: Louisiana State University.Crick R E, Ellwood B B, El Hassani A, et al. 1997. Magnetosusceptibility event and cyclostratigraphy (MSEC) of the Eifelian-Givetian GSSP and associated boundary sequences in North Africa and Europe.Episodes, 20(3): 167-175.

Dettinger M D, Ghil M, Strong C M, et al. 1995. Software expedites singular-spectrum analysis of noisy time series.EOSTransactionsAmericanGeophysicalUnion, 76(2): 12-21.

Dunlop D J, ?zdemir ?. 1997. Rock Magnetism: Fundamentals and Frontiers. Cambridge: Cambridge University Press.

Ellwood B B, Hrouda F, Wagner J J. 1988. Symposia on magnetic fabrics: introductory comments.PhysicsoftheEarthandPlanetaryInteriors, 51(4): 249-252.

Ellwood B B, Crick R E, El Hassani A. 1999. The magneto-susceptibility event and cyclostratigraphy (MSEC) method used in geological correlation of Devonian rocks from Anti-Atlas Morocco.AAPGBulletin, 83(7): 1119-1134.

Ellwood B B, Crick R E, El Hassani A, et al. 2000. Magnetosusceptibility event and cyclostratigraphy method applied to marine rocks: detrital input versus carbonate productivity.Geology, 28(12): 1135-1138.Ellwood B B, Brett C E, Macdonald W D. 2007. Magnetostratigraphy susceptibility of the Upper Ordovician Kope Formation, Northern Kentucky.Palaeogeography,Palaeoclimatology,Palaeoecology, 243(1-2): 42-54.

Ellwood B B, Tomkin J H, El Hassani A, et al. 2011. A climate-driven model and development of a floating point time scale for the entire Middle Devonian Givetian Stage: a test using magnetostratigraphy susceptibility as a climate proxy.Palaeogeography,Palaeoclimatology,Palaeoecology, 304(1-2): 85-95.Ellwood B B, Lambert L L, Tomkin J H, et al. 2012. Magnetostratigraphy susceptibility for the Guadalupian series GSSPs (Middle Permian) in Guadalupe Mountains National Park and adjacent areas in West Texas. ∥ Jovane L, Herrero-Bervera, E, Hinnov L A, et al., eds. Magnetic Methods and the Timing of Geological Processes. Geological Society, London: Special Publications, 373, doi: 10.1144/SP373.1.

García-Alcalde J L, Ellwood B B, Soto F, et al. 2011. Precise timing of the Upper Taghanic Biocrisis, Geneseo Bioevent, in the Middle—Upper Givetian (Middle Devonian) boundary in Northern Spain using biostratigraphic and magnetic susceptibility data sets.Palaeogeography,Palaeoclimatology,Palaeoecology, 313-314: 26-40, doi: 10.1016/j.palaeo.2011.10.006.

Ghil M, Allen M R, Dettinger M D, et al. 2002. Advanced spectral methods for climatic time series.ReviewsofGeophysics, 40(1): 3-1-3-41, doi: 10.1029/2001RG000092.

Gong Y M, Du Y S, Tong J N, et al. 2008. Cyclostratigraphy: the third milestone of stratigraphy in understanding time.EarthScience-JournalofChinaUniversityofGeosciences(in Chinese), 33(4): 443-457.Gradstein F M, Ogg J G, Smith A G. 2004. A Geologic Time Scale 2004. Cambridge: Cambridge University Press.

Gradstein F M, Ogg J G, Schmitz M, et al. 2012. The Geologic Time Scale. Boston: Elsevier.

Guo G, Tong J N, Zhang S H, et al. 2008. Cyclostratigraphy of the Induan (Early Triassic) in West Pingdingshan Section, Chaohu, Anhui Province.ScienceinChinaSeriesD:EarthSciences, 51(1): 22-29.Hays J D, Imbrie J, Shackleton N J. 1976. Variations in the Earth′s orbit: pacemaker of the ice ages.Science, 194(4270): 1121-1132.He B, Xu Y G, Chung S L, et al. 2003. Sedimentary evidence for a rapid, kilometer-scale crustal doming prior to the eruption of the Emeishan flood basalts.EarthandPlanetaryScienceLetters, 213(3-4): 391-405.

He B, Xu Y G, Wang Y M, et al. 2006. Sedimentation and lithofacies paleogeography in southwestern China before and after the Emeishan flood volcanism: new insights into surface response to mantle plume activity.TheJournalofGeology, 114(1): 117-132.

He B, Xu Y G, Huang X L, et al. 2007. Age and duration of the Emeishan flood volcanism, SW China: geochemistry and SHRIMP zircon U-Pb dating of silicic ignimbrites, post-volcanic Xuanwei Formation and clay tuff at the Chaotian section.EarthandPlanetaryScienceLetters, 255(3-4): 306-323.

Henderson C M., Mei S L, Wardlaw B R. 2002. New conodont definitions at the Guadalupian-Lopingian boundary. ∥Hills L V, Henderson C M, Bamber E W Eds. Carboniferous and Permian of the World. Calgary: Canadian Society of Petroleum Geologists, Memoir 19, 725-735.

Huang C J, Tong J N, Hinnov L, et al. 2011. Did the great dying of life take 700 ky? evidence from global astronomical correlation of the Permian-Triassic boundary interval.Geology, 39(8): 779-782.Huang C J. 2014. The current status of cyclostratigraphy and astrochronology in the Mesozoic.EarthScienceFrontiers(in Chinese), 21(2): 48-66.

Jenkins G M, Watts D G. 1968. Spectral Analysis and Its Applications. San Francisco: Holden-Day.

Jiang H S, Luo G M, Lai X L. 2004. Summary of approaches for conodont separation.GeologicalScienceandTechnologyInformation(in Chinese), 23(4): 109-112.

Jin Y G, Henderson C M, Wardlaw B R, et al. 2001. Proposal for the Global Stratotype Section and Point (GSSP) for the Guadalupian-Lopingian boundary.Permophiles, 39(3): 32-42.

Jin Y G, Mei S L, Wang W, et al. 1998. On the Lopingian series of the Permian system.Palaeoworld, 9: 1-18.

Jin Y G, Shen S Z, Henderson C M, et al. 2006. The Global Stratotype Section and Point (GSSP) for the boundary between the Capitanian and Wuchiapingian Stage (Permian).Episodes, 29(4): 253-262.

Jovane L, Florindo F, Sprovieri M, et al. 2006. Astronomic calibration of the late Eocene/early Oligocene Massignano section (central Italy).Geochemistry,Geophysics,Geosystems, 7: Q07012, doi: 10.1029/2005GC001195.Kasuya A, Isozaki Y, Igo H. 2012. Constraining paleo-latitude of a biogeographic boundary in Mid-Panthalassa: fusuline province shift on the Late Guadalupian(Permian) migrating seamount.GondwanaResearch, 21(2-3): 611-623.

Li B, Xue W Q, Yan J X, et al. 2015. Magnetic properties of the Middle-Late Permian carbonates in South China and their environmental significances.EarthScience-JournalofChinaUniversityofGeosciences(in Chinese), 40(7): 1226-1236.

Liu C Y, Zhu R X. 2009. Geodynamic significances of the Emeishan Basalts.EarthScienceFrontiers, 16(2): 52-69.

Mei S L, Jin Y G, Wardlaw B R. 1998. Conodont succession of the Guadalupian-Lopingian boundary strata in Laibin of Guangxi, China and West Texas, USA.Palaeoworld, 9: 53-57.

Ogg J G, Ogg G, Gradstein F M. 2008. The Concise Geologic Time Scale. Cambridge: Cambridge University Press.

Qiu Z, Wang Q C, Zou C N, et al. 2014. Transgressive-regressive sequences on the slope of an isolated carbonate platform (Middle-Late Permian, Laibin, South China).Facies, 60(1): 327-345.

Sha Q A, Wu W S, Fu J M. 1990. An Integrated Investigation on the Permian System of Qin-Gui Areas, with Discussion on the Hydrocarbon Potential (in Chinese). Beijing: Science Press.

Shaw A B. 1964. Time in Stratigraphy. New York: McGraw-Hill.

Shen S Z, Wang Y, Henderson C M, et al. 2007. Biostratigraphy and lithofacies of the Permian System in the Laibin-Heshan area of Guangxi, South China.Palaeoworld, 16(1-3): 120-139.

Sun Y D, Lai X L, Wignall P B, et al. 2010. Dating the onset and nature of the Middle Permian Emeishan large igneous province eruptions in SW China using conodont biostratigraphy and its bearing on mantle plume uplift models.Lithos, 119(1-2): 20-33.

Wang Y, Jin Y G. 2000. Permian palaeogeographic evolution of the Jiangnan Basin, South China.Palaeogeography,Palaeoclimatology,Palaeoecology, 160(1-2): 35-44.Weedon G P. 2003. Time-series Analysis and Cyclostratigraphy: examining Stratigraphic Records of Environmental Cycles. Cambridge: Cambridge University Press.

Wignall P B. 2001. Large igneous provinces and mass extinctions.Earth-ScienceReviews, 53(1-2): 1-33.

Wignall P B, Sun Y D, Bond D P G, et al. 2009a. Volcanism, mass extinction, and carbon isotope fluctuations in the Middle Permian of China.Science, 324(5931): 1179-1182.

Wignall P B, Védrine S, Bond D P G, et al. 2009b. Facies analysis and sea-level change at the Guadalupian-Lopingian Global Stratotype (Laibin, South China), and its bearing on the end-Guadalupian mass extinction.JournaloftheGeologicalSociety, 166(4): 655-666.Wignall P B, Bond D P G, Haas J, et al. 2012. Capitanian (Middle Permian) mass extinction and recovery in western Tethys: a fossil, facies, and δ13C study from Hungary and Hydra island (Greece).Palaios, 27(2): 78-89.

Wu H C, Zhang S H, Feng Q L, et al. 2011. Theoretical basis, research advancement and prospects of cyclostratigraphy.EarthScience-JournalofChinaUniversityofGeosciences(in Chinese), 36(3): 409-428.Wu H C, Zhang S H, Feng Q L, et al. 2012. Milankovitch and sub-Milankovitch cycles of the early Triassic Daye Formation, South China and their geochronological and paleoclimatic implications.GondwanaResearch, 22(2): 748-759.

Wu H C, Zhang S H, Hinnov L A, et al. 2013a. Time-calibrated Milankovitch cycles for the late Permian.NatureCommunications, 4: 2452, doi: 10.1038/ncomms3452.Wu H C, Zhang S H, Jiang G Q, et al. 2013b. Astrochronology for the Early Cretaceous Jehol Biota in northeastern China.Palaeogeography,Palaeoclimatology,Palaeoecology, 385: 221-228, doi: 10.1016/j.palaeo.2013.05.017.

Wu H C, Zhang S H, Jiang G Q, et al. 2013c. Astrochronology of the Early Turonian-Early Campanian terrestrial succession in the Songliao Basin, northeastern China and its implication for long-period behavior of the Solar System.Palaeogeography,Palaeoclimatology,Palaeoecology, 385: 55-70, doi: 10.1016/j.palaeo.2012.09.004.

Yao Y, Yan J X, Li A Z. 2012. Sedimentary features and evolution of Mid-Permian carbonates from Laibin of Guangxi.EarthScience-JournalofChinaUniversityofGeosciences(in Chinese), 37(S2): 184-194.

Zheng H R, Hu Z Q. 2010. Chinese pre-Mesozoic Tectonic: Atlas of Lithofacies and Paleogeography (in Chinese). Beijing: Geologic Publishing House.

Zhou M F, Malpas J, Song X Y, et al. 2002. A temporal link between the Emeishan large igneous province (SW China) and the end-Guadalupian mass extinction.EarthandPlanetaryScienceLetters, 196(3-4): 113-122.

Ziegler A M, Hulver M L, Rowley D B. 1997. Permian world topography and climate. ∥Martini I P ed. Late Glacial and Postglacial Environmental Changes-Quaternary, Carboniferous-Permian, and Proterozoic. New York: Oxford University Press, 111-146.

附中文參考文獻

龔一鳴, 杜遠生, 童金南等. 2008. 旋回地層學(xué): 地層學(xué)解讀時間的第三里程碑. 地球科學(xué)—中國地質(zhì)大學(xué)學(xué)報, 33(4): 443-457.

黃春菊. 2014. 旋回地層學(xué)和天文年代學(xué)及其在中生代的研究現(xiàn)狀. 地學(xué)前緣, 21(2): 48-66.

江海水, 羅根明, 賴旭龍. 2004. 牙形石的分離方法簡介. 地質(zhì)科技情報, 23(4): 109-112.

李波，薛武強，顏佳新等. 2015. 華南中—晚二疊世之交碳酸鹽巖磁學(xué)特征及環(huán)境意義. 地球科學(xué)—中國地質(zhì)大學(xué)學(xué)報, 40(7): 1226-1236.

沙慶安, 吳望始, 傅家謨. 1990. 黔桂地區(qū)二疊系綜合研究—兼論含油氣性. 北京: 科學(xué)出版社.

吳懷春, 張世紅, 馮慶來等. 2011. 旋回地層學(xué)理論基礎(chǔ), 研究進展和展望. 地球科學(xué)—中國地質(zhì)大學(xué)學(xué)報, 36(3): 409-428.

姚堯, 顏佳新, 李傲竹. 2012. 廣西來賓中二疊世碳酸鹽巖沉積特征與孤立臺地演化. 地球科學(xué)—中國地質(zhì)大學(xué)學(xué)報, 37(增刊2): 184-194.

鄭和榮, 胡宗全. 2010. 中國前中生代構(gòu)造—巖相古地理圖集. 北京: 地質(zhì)出版社.

(本文編輯 胡素芳)

High-resolution floating point time scale (FPTS) of Permian Capitanian Stage in South China

XUE Wu-Qiang1, LI Bo2, YAN Jia-Xin1*, Brooks B. Ellwood3, Jonathan H. Tomkin4, WANG Yan5, ZHU Zong-Min1

1StateKeyLaboratoryofBiogeologyandEnvironmentalGeology,SchoolofEarthSciences,ChinaUniversityofGeosciences,Wuhan430074,China2KeyLaboratoryofMarineMineralResources,GuangzhouMarineGeologicalSurvey,MinistryofLandandResources,Guangzhou510075,China3DepartmentofGeologyandGeophysics,LouisianaStateUniversity,BatonRouge,LA70803,USA4SchoolofEarth,Society,andEnvironment,UniversityofIllinois,Urbana,IL61801,USA5GuangdongNonferrousMetalsGeologicalExplorationInstitution,Guangzhou510080,China

The Capitanian stage is a critical time interval when fierce eruption of the Emeishan large igneous province (LIP) occurred with onset of a double-episodic Paleozoic-Mesozoic bio-crisis. However, the time duration of the Capitanian stage is geochronologically poorly-constrained due to a lack of absolute age data with radioactive dating method constraining both the basal and the top boundary of the Capitanian stage. Cyclostratigraphy can provide a high-resolution astronomical time scale by tuning the cyclic stratigraphic records to astronomical solutions. In this study, a floating point time scale (FPTS) for the Capitanian stage was developed based on the result of time-series analysis of magnetic susceptibility (MS) and additional biostratigraphic data sets. In addition, we estimate the age and time duration of the Emeishan LIP eruption.1456 unoriented samples (sampling interval ～0.1 m) for MS measurement and 45 samples for biostratigraphic study were collected from outcrops with stratigraphic interval of ～147.5 m. The bottom horizon of sampling range is located at ～31.2 m below the Wordian-Capitanian (W-C) boundary. We conducted continuous sampling with the top horizon at 2.6 m above the Capitanian-Wuchiapingian (C-W) boundary at Tieqiao section, Laibin in South China, as a supplementary reference section for the Guadalupian-Lopingian (Middle-Upper Permian) Global Boundary Stratotype Section and Point (GSSP). The stratigraphic succession consists of alternations between thin-bedded chert and micritic limestone recognized as typical slope-basin facies with carbonate cap on shallow-water platform. Representative samples were selected to be carried out thermomagnetic susceptibility measurements after analysis of low-field bulk susceptibility for all samples. Then spectral (time-series) analysis of the MS data was performed by means of Multi Taper Method (MTM) and Fourier Transform (FT) analysis. Samples for biostratigraphic study were processed using acetic acid digestion in order to obtain conodont fossils.Seven conodont zones are identified within the Capitanian stage, includingJinogondolellapostserrata,J.shannoni,J.altudaensis,J.prexuanhanensis,J.xuanhanensis,J.granti, andClarkinapostbitterihongshuiensisfrom the base to the top horizons of the Capitanian stage. The MS data sets are mostly negative (diamagnetic), suggesting that the Laibin area was isolated from terrigenous source during most period of the Capitanian stage. An abrupt increase in MS signal immediately below the Guadalupian-Lopingian (G-L) boundary is recorded. Although regression at the end of Capitanian might account for the increase, the coincidence of the increase with the onset of the Emeishan Large Igneous Province (LIP) eruption is intensively concerned for volcanic role in controlling the MS at the section. The results of MS and thermomagnetic susceptibility measurements indicate that the dominant magnetic constituents in this study are diamagnetic calcite and chert with subordinate paramagnetic and ferrimagnetic minerals producing the cyclicity observed in the data set. The variation of MS near the G-L boundary at Tieqiao section is essentially as same as previously published results for MS at Penglaitan Section (G-L boundary GSSP).Six Milankovitch bands are reported with eccentricity peaks E2 (～405 ka) at 0.08 cycles·m-1and E1 (～100 ka) at 0.35 cycles·m-1, obliquity peaks, O2 (～44.1 ka) at 0.78 cycles·m-1and O1 (～35.0 ka), and precessional peaks P2 (～20.95 ka) at 1.69 cycles·m-1, and P1 (～17.7 ka) at 1.94 cycles·m-1. Four peaks exceed 99% confidence levels. In terms of the results of magnetostratigraphic susceptibility (MSS) zones developed from smoothed data to conform to the 405 ka (E2) eccentricity band identified in the time-series data set, a high-resolution FPTS was developed for the entire Tieqiao succession reported here. This FPTS allows a temporal resolution of ～200 ka for the studying section because each MSS zone represents half of the E2 eccentricity cycle. The results indicate that the time duration of the Capitanian Stage is estimated as ～3.85 Ma (with +0～0.28 Ma error). The mean sediment accumulation rate (SAR) for the succession is ～2.91 cm·ka-1. Moreover, conodont zones within the Capitanian stage is estimated to spanning the shortest time interval of ～26.6 ka and the longest duration of ～2.3 Ma. In addition, the onset of the Emeishan LIP eruptions is estimated at ～262.67 Ma, 1.42 Ma earlier than the Guadalupian-Lopingian boundary.

Magnetic susceptibility (MS)； Conodont； Time-series analysis； Capitanian； Emeishan large igneous province； Floating point time scale (FPTS)

10.6038/cjg20151023.

Xue W Q, Li B, Yan J X, et al. High-resolution floating point time scale (FPTS) of Permian Capitanian Stage in South China.ChineseJ.Geophys. (in Chinese),58(10):3719-3734,doi:10.6038/cjg20151023.

國家自然科學(xué)基金項目(41472087、41072078)，科技部973項目(2011CB808804)，國土資源部海底礦產(chǎn)資源重點實驗室開放基金課題(KLMMR-2014-A-12)，美國國家科學(xué)基金會(EAR-0745393003)聯(lián)合資助.

薛武強，男，1971年生，博士研究生，主要從事沉積地質(zhì)學(xué)的研究工作.E-mail：xuewq_718888@sina.com

*通訊作者 顏佳新，男，1962年生，教授、博導(dǎo)，主要從事沉積地質(zhì)學(xué)研究. E-mail：jaxy2008@163.com

10.6038/cjg20151023

P318

2015-03-13，2015-09-14收修定稿

≤≥? ?薛武強, 李波， 顏佳新等. 華南二疊系卡匹敦階高分辨率浮點年代標(biāo)尺.地球物理學(xué)報，58(10):3719-3734,