付勇，夏鵬，龍珍，張恭境，譙文浪，郭川，楊鎮(zhèn)

1) 貴州大學(xué)資源與環(huán)境工程學(xué)院，貴陽，550025； 2) 喀斯特地質(zhì)資源與環(huán)境教育部重點(diǎn)實(shí)驗(yàn)室，貴陽，550025； 3) 中國科學(xué)院地質(zhì)與地球物理研究所新生代地質(zhì)與環(huán)境院重點(diǎn)實(shí)驗(yàn)室，北京，100029； 4) 貴州省地礦局102地質(zhì)大隊(duì)，遵義，563003； 5) 貴州民族大學(xué)生態(tài)環(huán)境工程學(xué)院，貴陽，550025

內(nèi)容提要: 通過調(diào)研揚(yáng)子地區(qū)震旦紀(jì)(埃迪卡拉紀(jì))—寒武紀(jì)(E—C)轉(zhuǎn)折期大陸風(fēng)化、海洋環(huán)境和有機(jī)質(zhì)富集特征研究。結(jié)果顯示，揚(yáng)子地區(qū)E—C轉(zhuǎn)折期遭受了較強(qiáng)烈的化學(xué)風(fēng)化作用，且在這一時(shí)期區(qū)內(nèi)兼具淺水碳酸鹽臺(tái)地和深水盆地環(huán)境。在臺(tái)地相區(qū)，下寒武統(tǒng)牛蹄塘組黑色巖系不整合接觸于震旦系燈影組白云巖之上，該不整合與E—C轉(zhuǎn)折期全球“大不整合”具有緊密的關(guān)系，盆地區(qū)發(fā)育的完整的沉積地層記錄了臺(tái)地區(qū)被風(fēng)化地層的痕跡?，F(xiàn)有大陸風(fēng)化的地球化學(xué)證據(jù)集中于n(87Sr)/ n(86Sr)、δ13C、CIA和εNd(t)等，同時(shí)這些地球化學(xué)數(shù)據(jù)也局限在少數(shù)的剖面和層段，因此迫切需要更多的地球化學(xué)參數(shù)來反映該風(fēng)化作用的影響范圍和演化特征。準(zhǔn)確認(rèn)識(shí)揚(yáng)子地區(qū)E—C轉(zhuǎn)折期大陸風(fēng)化作用與海洋環(huán)境演變間的耦合關(guān)系，是揭示古生物演化、有機(jī)質(zhì)富集機(jī)制的重要環(huán)節(jié)。

震旦紀(jì)(埃迪卡拉紀(jì))—寒武紀(jì)(E—C)轉(zhuǎn)折期同時(shí)發(fā)生了超大陸的裂解(Rodinia)與聚合(Gondwana)(Li Da et al., 2013; Yao Weihua et al., 2014)、海洋化學(xué)與生物化學(xué)的動(dòng)蕩變化(Hoffman et al., 1998; Fike et al., 2006; McFadden et al., 2008; Och et al., 2013; Sahoo et al., 2012; Li Da et al., 2013)、生命的幕式更替(張文堂，1997; Zhang Xingliang et al., 2014; 朱茂炎等，2019)等重大地質(zhì)事件。其中，最引人注目的是新元古代末期埃迪卡拉紀(jì)動(dòng)物群的消失，以及隨后從早寒武紀(jì)開始的骨骼化動(dòng)物爆發(fā)輻射(即“寒武紀(jì)生命大爆發(fā)”)。普遍認(rèn)為，E—C轉(zhuǎn)折期大氣氧濃度的持續(xù)增加(圖1)以及海水氧化還原條件的動(dòng)態(tài)變化在早期生命演化過程中起著顯著的作用(Li Chao et al., 2010; Och et al., 2013; Wen Hanjie et al., 2015; Li Weiping et al., 2017; 朱茂炎等，2010，2019)。然而，目前對(duì)這一時(shí)期大氣氧濃度和海洋氧化還原條件變化的驅(qū)動(dòng)力仍然沒有定論。

圖1 新元古代以來構(gòu)造、地球化學(xué)特征及大氣氧氣濃度(大氣氧含量參考Kump et al., 2007; Lyons et al., 2014；海水環(huán)境參考Canfield et al., 2008；全球n(87Sr)/n(86Sr)、εNd數(shù)據(jù)及構(gòu)造活動(dòng)等參考Hoffman and Li, 2009; Peter and Gaines, 2012；華南E—C轉(zhuǎn)折期n(87Sr)/n(86Sr)、εNd參考Wei Guangyi et sl., 2019; Li Meng et al., 2020)Fig. 1 Characteristics of tectonic movements, geochemistry, and atmospheric oxygen concentration (oxygen concentration is from Kump et al., 2007, Lyons et al., 2014; Marine environment is from Camfield et al., 2008; Globaln(87Sr)/n(86Sr), εNd, and tectonic movements are from Hoffman and Li, 2009, Peter and Gaines, 2012; n(87Sr)/n(86Sr) and εNd in South China are Wei Guangyi et al., 2019, Li Meng et al., 2020)

前寒武紀(jì)末期，地球氣候曾發(fā)生劇烈的波動(dòng)，并出現(xiàn)四次影響深遠(yuǎn)的冰期氣候(圖1)(Hoffman and Li, 2009; Zhou Chuanming et al., 2019; Li Minglong et al., 2019；李明龍等，2021)。在冰期氣候之后，冰川的迅速消融使地球進(jìn)入溫室氣候，導(dǎo)致前寒武紀(jì)末期化學(xué)風(fēng)化強(qiáng)度迅速增強(qiáng)(Hoffman et al., 1998; Peter and Gaines, 2012)。隨后在早寒武世期間廣泛的海侵和基底改造，使寒武系不整合沉積于前寒武紀(jì)風(fēng)化地層之上，導(dǎo)致E—C轉(zhuǎn)折期“大不整合(Great unconformity)”的形成(Peter and Gaines, 2012; Shahkarami et al., 2020)?！按蟛徽稀本哂腥蛞?guī)模，在E—C轉(zhuǎn)折期各主要大陸均有分布(圖2)，并且該不整合面具有穿時(shí)特征。例如，在西伯利亞、外蒙古、挪威等地發(fā)育在埃迪卡拉系上統(tǒng)內(nèi)部，以碎屑角礫巖層不整合覆蓋于碳酸鹽巖層之上為特征(Nielsen and Schovsbo, 2011; Smith et al., 2015; Markov et al., 2019)；在加拿大麥肯錫、美國加利福尼亞等地區(qū)表現(xiàn)為黑色細(xì)粒沉積巖系與下伏碳酸鹽巖或角礫巖之間的不整合接觸(Peter and Gaines, 2012; Smith et al., 2016)；在澳大利亞弗林德斯，該不整合面為埃迪卡拉系與寒武系的界線，寒武系含礫砂巖不整合覆蓋于埃迪卡拉系碳酸鹽巖之上(Mapstone and Mcllry, 2006)；我國揚(yáng)子地區(qū)，該不整合面同樣是埃迪卡拉系與寒武系的界線，但不整合面之上為下寒武統(tǒng)黑色細(xì)粒巖系，有機(jī)質(zhì)豐富(Zhu Maoyan et al., 2010, 2019; Li Chao et al., 2020)；在納米比亞，該不整合面位于埃迪卡拉系與寒武系的界線以上，為寒武系底礫巖與下伏埃迪卡拉系碳酸鹽巖的不整合接觸(Linnemann et al., 2019)。E—C轉(zhuǎn)折期“大不整合”的出現(xiàn)與這一時(shí)期海水環(huán)境和大氣氧濃度變化存在較好的對(duì)應(yīng)關(guān)系(圖1)，暗示“大不整合”形成過程中產(chǎn)生的大量風(fēng)化產(chǎn)物輸入海洋，驅(qū)動(dòng)早寒武世海洋環(huán)境變化，觸發(fā)“寒武紀(jì)生命大爆發(fā)”(Peter and Gaines, 2012)。然而，“大不整合”的分布特征(圖2)反映E—C轉(zhuǎn)折期不同地區(qū)大陸風(fēng)化特征存在明顯差異。

本文以揚(yáng)子地區(qū)E—C轉(zhuǎn)折期為例，學(xué)習(xí)和總結(jié)了關(guān)于這一時(shí)期古大陸風(fēng)化、古海洋環(huán)境演化以及有機(jī)質(zhì)富集特征方面的研究進(jìn)展，討論了大陸風(fēng)化與古海洋環(huán)境的協(xié)同演化關(guān)系及相關(guān)問題。

2 大陸風(fēng)化作用強(qiáng)度研究方法

2.1 化學(xué)蝕變指數(shù)

上地殼遭受化學(xué)風(fēng)化的過程中，K+、Na+、Ca2+等離子活性強(qiáng)，容易隨地表流體大量流失，然而，Al3+、Ti4+等離子較穩(wěn)定，容易保存在風(fēng)化殘留物中，導(dǎo)致風(fēng)化殘留物中主成分Al2O3所占的比重隨風(fēng)化作用強(qiáng)度而不斷變化。據(jù)此，Nesbitt和Young(1982，1989)將“化學(xué)蝕變指數(shù)”(chemical index of alteration,CIA)作為評(píng)價(jià)物源區(qū)風(fēng)化強(qiáng)度和氣候條件的指標(biāo)，即：

其中n(CaO*)為硅酸鹽礦物中的CaO。高的CIA值反映較強(qiáng)烈的化學(xué)風(fēng)化作用，對(duì)應(yīng)溫暖潮濕的氣候條件，K+、Na+、Ca2+等易遷移陽離子大量去除，Al3+、Ti4+等難遷移陽離子逐漸在風(fēng)化殘余物中富集。低CIA值反映化學(xué)風(fēng)化作用較弱甚至不存在，對(duì)應(yīng)寒冷干燥的氣候條件。未經(jīng)風(fēng)化的沉積巖CIA標(biāo)準(zhǔn)值為48(Rudnick and Gao Shan, 2014)，遭受風(fēng)化后的沉積物CIA值高于該標(biāo)準(zhǔn)值，并且隨風(fēng)化強(qiáng)烈程度增加而不斷升高(Nesbitt and Young, 1982, 1989)。

基于上述理論基礎(chǔ)，CIA在化學(xué)風(fēng)化強(qiáng)度研究方面得到了廣泛應(yīng)用，例如，我國南方新元古代古城冰期和南沱冰期細(xì)碎屑巖CIA值較低，分布在56.5～64.6，均值約59.8，反映物源區(qū)經(jīng)歷了輕微的化學(xué)風(fēng)化作用(馮連君等，2003；熊晨，2019；李明龍等，2021)，與冰期寒冷干燥的氣候相對(duì)應(yīng)；晚奧陶世五峰組—早志留世龍馬溪組碎屑巖CIA值為75～90，反映物源區(qū)經(jīng)歷了較強(qiáng)烈的化學(xué)風(fēng)化(Yan Detian et al., 2010)。然而，CIA是基于化學(xué)組成相對(duì)均一的巖石建立的(Ohta et al., 2007)，受原巖成分影響大，鉀交代作用、變質(zhì)作用、古老沉積巖的再循環(huán)沉積等會(huì)改變?cè)瓗r成分，進(jìn)而制約CIA判斷風(fēng)化強(qiáng)度的準(zhǔn)確性。因此在應(yīng)用CIA指標(biāo)時(shí)不僅對(duì)樣品的選取有嚴(yán)格的要求，還需結(jié)合校正公式去除鉀交代的影響(Panahi et al., 2000; Zhai Lina et al., 2018)，以及結(jié)合ICV(成分變異指數(shù)，index of compositional variability)和Th/Sc—Zr/Sc、A—CN—K等圖解評(píng)價(jià)沉積分選及再循環(huán)作用(馮連君等，2003；Yan Detian et al., 2010；吳蓓娟等，2016)。結(jié)合這些參數(shù)能夠在一定程度上強(qiáng)化CIA判斷化學(xué)風(fēng)化的準(zhǔn)確性，但在評(píng)價(jià)化學(xué)組成極不均一的黑色頁巖時(shí)仍然存在較大分歧，以我國南方主要頁巖氣儲(chǔ)層—龍馬溪組黑色頁巖—為例，經(jīng)過鉀交代校正后CIA值可達(dá)90，反映極強(qiáng)烈的化學(xué)風(fēng)化作用(Yan Detian et al., 2010)，但近期的研究顯示該黑色頁巖受成巖及交代作用影響較小，CIA平均值為66，風(fēng)化作用強(qiáng)度較小(張茜等，2020)，這可能是受樣品較高的碳酸鹽礦物含量的影響，同時(shí)黑色頁巖的強(qiáng)非均質(zhì)性也是可能的影響因素。針對(duì)湘中下寒武統(tǒng)黑色頁巖，吳蓓娟等(2016)成功構(gòu)建了WB指數(shù)評(píng)價(jià)其風(fēng)化程度，但該指數(shù)是基于現(xiàn)今黑色頁巖風(fēng)化特征的評(píng)價(jià)，對(duì)其評(píng)價(jià)地質(zhì)歷史中黑色頁巖風(fēng)化強(qiáng)度的適用性還需進(jìn)一步證實(shí)?？梢?，采用CIA單一指標(biāo)評(píng)價(jià)風(fēng)化強(qiáng)度很難得到可靠的結(jié)果，通常將CIA與同位素地球化學(xué)指標(biāo)相結(jié)合判斷大陸風(fēng)化作用。

2.2 同位素地球化學(xué)指標(biāo)2.2.1 Sr同位素

海水中的Sr主要有大陸風(fēng)化輸入和海底熱液兩種來源，其中，大陸風(fēng)化作用提供的Sr相對(duì)富87Sr [具放射性，n(87Sr)/n(86Sr)=0.7119] (Palmer and Edmond, 1989)，海底熱液活動(dòng)提供的Sr相對(duì)富86Sr(n(87Sr)/n(86Sr)=0.703)(Holland, 1984)。溶解Sr在海水中停留時(shí)間(2～3 Ma)遠(yuǎn)長于海水混合時(shí)間(1～1.5 ka)，所以海水的Sr同位素比值n(87Sr)/n(86Sr)在任何時(shí)間均被認(rèn)為是均一的(McArthur et al., 2012)，那么原生海底沉積物和自生礦物中Sr同位素比值，反映的是大陸風(fēng)化作用來源Sr和熱液活動(dòng)來源Sr的動(dòng)態(tài)平衡。因此，海水n(87Sr)/n(86Sr)值能夠反映風(fēng)化作用強(qiáng)度(或造山作用)與洋殼增生速度(或火山—熱液活動(dòng)強(qiáng)度)，并對(duì)構(gòu)造演化和氣候變化(或P(CO2))進(jìn)行約束(Jenkyns et al., 2002; Wang Wei et al., 2007；王文倩等，2014)。

如圖1，新元古代至寒武紀(jì)，古海洋n(87Sr)/n(86Sr)持續(xù)升高，這一時(shí)期正好對(duì)應(yīng)Rodinia大陸的裂解、剝蝕，形成“大不整合”(Peters and Gaines, 2012)，顯示了Sr同位素變化對(duì)大陸風(fēng)化作用的良好響應(yīng)。在華南、北美、蒙古、西伯利亞、阿拉伯半島和南非等地均存在E—C轉(zhuǎn)折期高n(87Sr)/n(86Sr)值(0.707～0.711)(Sawaki et al., 2008; Li Da et al., 2013; Wei Guangyi et al., 2019)，表明在E—C轉(zhuǎn)折期，強(qiáng)烈的化學(xué)風(fēng)化作用遍及全球大部分地區(qū)，形成的“大不整合”具有全球規(guī)模(Shahkarami et al., 2020)(圖2)。盡管海水Sr同位素組成在示蹤大陸風(fēng)化方面取得了較多成功的運(yùn)用，但Sr同位素易受原巖(主要是碳酸鹽巖)性質(zhì)的影響，因此通過海洋沉積的Sr同位素示蹤大陸風(fēng)化過程存在多解性。

2.2.2 Os同位素

海水中Os元素主要有三種來源：① 大陸地殼中Os經(jīng)河流帶入；② 洋中脊熱液蝕變來源；③ 宇宙塵埃來源。大陸地殼Os同位素富集放射性187Os，n(187Os)/n(188Os)值較高，現(xiàn)今陸源輸入n(187Os)/n(188Os)平均值為1.54(Levasseur et al., 1999; Cohen et al., 2004)(圖3)。洋中脊熱液蝕變來源和宇宙塵埃來源的Os為非放射性成因，并且兩種來源Os具有相近的n(187Os)/n(188Os)值，約為0.126，遠(yuǎn)低于大陸地殼風(fēng)化來源Os的n(187Os)/n(188Os)值。海水中Os元素的組成特征主要是這三種來源的綜合結(jié)果，其中約80%來自陸源輸入，僅20%來源于海底熱液蝕變和宇宙塵埃(Sharma and Wasserburg, 1997)，因此可以通過海水Os同位素的變化來約束大陸風(fēng)化強(qiáng)度和海底熱液噴發(fā)等過程(Cohen, 2004; Zhu Bi et al., 2013; Tripathy et al., 2018)。

圖3 前寒武紀(jì)海水初始n(187Os)/n(188Os)值(數(shù)據(jù)引自Kendall et al., 2009; Rooney et al., 2010, 2011; Zhu Bi et al., 2013)Fig. 3 Initialn(187Os)/n(188Os) of marine water of Precambrian (data are from Kendall et al., 2009; Rooney et al., 2010, 2011; Zhu Bi et al., 2013)

強(qiáng)烈的化學(xué)風(fēng)化作用對(duì)應(yīng)高的n(187Os)/n(188Os)值，例如，近50 Ma以來，海水n(187Os)/n(188Os)值逐漸升高，與海水的Sr同位素組成升高的趨勢(shì)一致，反映這一時(shí)期喜馬拉雅抬升運(yùn)動(dòng)造成的大陸風(fēng)化強(qiáng)度逐漸增強(qiáng)(Pegram et al., 1992)。英格蘭Yorkshire早侏羅世經(jīng)歷了溫暖濕潤的古氣候和強(qiáng)烈的化學(xué)風(fēng)化，對(duì)應(yīng)的含礫石黑色頁巖段n(187Os)/n(188Os)值為0.8～1.0，n(87Sr)/n(86Sr)值由0.70706迅速升至0.70720(Cohen et al., 2004)。新元古代晚期，蘇格蘭、愛爾蘭、毛里塔尼亞以及中國等均顯示海水初始n(187Os)/n(188Os)值迅速升高(圖3)，反映這一時(shí)期存在強(qiáng)烈的化學(xué)風(fēng)化作用(Zhu Bi et al., 2013)，與“大不整合”的時(shí)間相當(dāng)(Peters and Gaines, et al., 2012; Li Meng et al., 2020; Shahkarami et al., 2020)，說明海洋n(187Os)/n(188Os)值的變化對(duì)“大不整合”有較好的響應(yīng)，能夠反映地質(zhì)時(shí)期的化學(xué)風(fēng)化強(qiáng)度。然而，黑色巖系(主要是黑色頁巖)富集有機(jī)質(zhì)和硫化物會(huì)大量吸附海水中Os，導(dǎo)致黑色巖系中Os異常富集，因此Os同位素示蹤黑色巖系的大陸風(fēng)化過程存在多解性。

2.2.3 Li同位素

Li同位素在示蹤大陸硅酸巖風(fēng)化方面具有以下優(yōu)勢(shì)：① 化合價(jià)單一，不受氧化還原狀態(tài)影響；② 大陸硅酸巖地殼具有相對(duì)較高的Li含量(李東永等，2019)，且在風(fēng)化過程中可以產(chǎn)生極大分餾(Rudnick et al., 2004; Tomascak., 2004)；③ 不受生物過程影響(Rudnick et al., 2004; Penniston-Dorland et al., 2017)。因此，所有地質(zhì)過程中Li同位素的分餾發(fā)生在大陸風(fēng)化作用過程中(Rudnick et al., 2004; Henchiri et al., 2014)。目前已經(jīng)基本獲得自然儲(chǔ)庫中Li的豐度和δ7Li值(圖4)，為Li同位素示蹤大陸風(fēng)化研究奠定了基礎(chǔ)。由于Li是水溶性元素，受淋溶作用易遷移至溶液中，經(jīng)搬運(yùn)注入海洋。Li的同位素分餾主要發(fā)生在搬運(yùn)過程中，在淋溶過程中僅僅發(fā)生微弱的同位素分餾(Wimpenny et al., 2010a, 2010b; Verney-Carron et al., 2011)。搬運(yùn)過程中，6Li優(yōu)先在次生黏土礦物中富集，造成黏土礦物中Li含量高(平均約為80 μg/g)，δ7Li值相對(duì)較低(1.6‰～5.0‰)；大部分7Li跟隨流體注入海洋，并在俯沖過程中被帶到下地殼或者地幔，導(dǎo)致上地殼富集6Li(Marschall et al., 2007; Steinhoefel et al., 2021)。

圖4 自然儲(chǔ)庫中Li同位素(茍龍飛等，2017)和Mg 同位素(Huang Jinxiang et al., 2016)的分布Fig. 4 Lithium(Gou Longfei et al., 2017&) and magnesium (Huang Jinxiang et al., 2016) isotopic composition in natural reservoir

Li同位素已被成功運(yùn)用到示蹤大陸風(fēng)化過程的多項(xiàng)研究中：太古宙3.0～2.9 Ga期間，快速的大陸風(fēng)化作用導(dǎo)致海水δ7Li值降低，遠(yuǎn)低于現(xiàn)代海水(付露露等，2020)；在445 Ma的Hirnantian冰期，大陸風(fēng)化強(qiáng)度降低，海水δ7Li值明顯升高(von Strandmann et al., 2017)；晚白堊世生物大滅絕及大洋缺氧事件(OAE2)前夕，大陸風(fēng)化強(qiáng)度增加，海水δ7Li值顯著降低(von Strandmann et al., 2013; Sun He et al., 2018)。然而，也有研究表明細(xì)粒級(jí)(<63 μm)沉積物中δ7Li對(duì)氣候變化不敏感，因此δ7Li示蹤大陸風(fēng)化的可靠性還有待進(jìn)一步證實(shí)。

2.2.4 Mg同位素

Mg有三個(gè)穩(wěn)定同位素，即24Mg(78.99%)、25Mg(10.00%)和26Mg(11.01%)，它們之間存在較大的相對(duì)質(zhì)量差，如24Mg與26Mg之間相對(duì)質(zhì)量差約達(dá)8%，在地質(zhì)作用過程中可以發(fā)生顯著的Mg同位素質(zhì)量分餾(Catanzaro et al., 1966；朱祥坤等，2013)。同時(shí)，Mg不僅是地殼和地幔中的主量元素，也是主要的流體活動(dòng)性元素，這決定了它在化學(xué)風(fēng)化過程中會(huì)伴隨明顯的同位素分餾。已有研究表明，Mg同位素在化學(xué)風(fēng)化過程中會(huì)產(chǎn)生高達(dá)2‰的分餾，其中輕同位素24Mg、25Mg更易隨流體遷移，而重同位素26Mg不易被溶解遷移，保留在風(fēng)化殘余物中，導(dǎo)致風(fēng)化殘余物中具有高δ26Mg值(圖4)，搬運(yùn)介質(zhì)中具有低δ26Mg值(Wimpenny et al., 2010a; Teng Fangzhen et al., 2010; von Strandmann et al., 2012; Liu Xiaoming et al., 2014)，因此風(fēng)化殘余物中更高的δ26Mg值指示了溫暖濕潤的氣候條件，更低的δ26Mg值則指示了寒冷干燥的氣候條件(Huang Jinxiang et al., 2016)。

黏土礦物對(duì)不同同位素吸附能力差異可能也是導(dǎo)致風(fēng)化過程中Mg同位素質(zhì)量分餾的原因。Huang Kangjun 等 (2012)、Liu Xiaoming 等 (2014)對(duì)玄武巖風(fēng)化剖面的研究顯示高嶺石、三水鋁石優(yōu)先吸附26Mg，造成26Mg在風(fēng)化殘余物中的富集。Wimpenny 等 (2014)對(duì)黏土礦物中不同賦存形態(tài)Mg同位素的測(cè)量結(jié)果表明，黏土礦物晶體結(jié)構(gòu)中Mg同位素組成較重，黏土礦物表面和層間的Mg同位素組成較輕，黏土礦物吸附過程并不產(chǎn)生同位素分餾?？梢?，目前對(duì)風(fēng)化作用過程中Mg同位素的分餾機(jī)制還不完全清楚，同時(shí)現(xiàn)有Mg同位素示蹤大陸風(fēng)化的研究集中于玄武巖、安山巖、花崗巖、碳酸鹽巖等巖石風(fēng)化剖面，還有待更深入和更廣泛的研究。

2.2.5 K同位素

K是地表河流和地殼中的主量元素，約90%的河流溶解K來自于硅酸鹽的風(fēng)化(Meybeck, 1987; Berner and Berner, 2012)，因此K穩(wěn)定同位素(39K和41K)可以示蹤大陸硅酸鹽風(fēng)化。在硅酸鹽風(fēng)化過程中，輕K同位素優(yōu)先遷移到水溶液中，河流溶解負(fù)荷δ41K值降低，風(fēng)化殘余物具有較高的δ41K值，河流溶解K同位素與風(fēng)化強(qiáng)度負(fù)相關(guān)(圖5)(Hu Yan et al., 2020)。重K同位素隨河流輸入海洋造成海水δ41K值的變化，因此，利用古海水δ41K記錄可以從地球歷史的角度推斷大陸風(fēng)化強(qiáng)度(Hille et al., 2019; Teng Fangzhen et al., 2020)。

圖5 自然儲(chǔ)庫δ41K(引自Hu Yan et al., 2020; Teng Fangzhen et al., 2020)以及河流沉積物中δ41K 與CIA關(guān)系(引自Hu Yan et al., 2020)Fig. 5 Potassium isotopic composition in natural reservoir (Hu Yan et al., 2020; Teng Fangzhen et al., 2020), and the plot of δ41K vs. CIA (from Hu Yan et al., 2020)

2.2.6 Cu同位素和Zn同位素

Cu和Zn均屬于過渡金屬元素。大陸風(fēng)化作用是海洋Cu和Zn地球化學(xué)循環(huán)主要的“源”，風(fēng)化過程中Cu和Zn分餾的可能原因包括: ① 主要造巖礦物的溶解過程發(fā)生同位素分餾； ② 溶解態(tài)以及次生礦物吸附態(tài)的同位素分餾；③ 大氣浮沉的輸入；④ 植物的吸收作用(Moynier et al., 2017)。

巖石氧化淋濾過程中，重Cu同位素被優(yōu)先釋放進(jìn)入河流體系，進(jìn)而注入海洋，造成河水和海水中相對(duì)較重的Cu同位素組成(圖6)。氧化淋濾過程對(duì)Zn同位素的分餾程度較低，水體中Zn同位素組成和原巖幾乎一致(圖6)，不會(huì)超過0.1‰～0.3‰(Weiss et al., 2014)。Cu、Zn同位素在示蹤大陸風(fēng)化強(qiáng)度方面的應(yīng)用還較匱乏，關(guān)于大陸風(fēng)化對(duì)Cu和Zn同位素分餾影響機(jī)制還不清楚。但是已有的研究(呂逸文，2018)證實(shí)在黑色頁巖和碳酸鹽巖的風(fēng)化過程中，存在Cu和Zn同位素分餾現(xiàn)象，值得再做更加深入和廣泛的研究。

圖6 表生環(huán)境自然樣品Cu、Zn同位素組成(引自Moynier et al., 2017；呂逸文，2018)Fig. 6 Copper and zinc isotopic composition in natural reservoir under supergene environment (from Moynier et al., 2017; Yiwen et al., 2018&)

上述研究表明，目前沒有任何地球化學(xué)參數(shù)能夠完全準(zhǔn)確地示蹤大陸風(fēng)化強(qiáng)度，需要多參數(shù)結(jié)合，互相驗(yàn)證，才能保證解釋結(jié)果的準(zhǔn)確性。有研究顯示Tethyan南部黏土礦物的分布與氣候分帶之間相關(guān)性非常明顯，揭示黏土礦物的分布能夠反映古氣候和大陸風(fēng)化特征(Chenot et al., 2018)。因此，礦物學(xué)與地球化學(xué)參數(shù)的有效結(jié)合，有望為大陸風(fēng)化強(qiáng)度研究提供更可靠的手段。

3 揚(yáng)子地區(qū)E—C轉(zhuǎn)折期大陸 風(fēng)化作用

“雪球地球”理論認(rèn)為在新元古代成冰紀(jì)結(jié)束后，距今652.5 Ma左右(鄧俊等，2020，及該文中相關(guān)引用文獻(xiàn))，地球迅速轉(zhuǎn)入溫室氣候(Hoffman et al., 1998)，E—C轉(zhuǎn)折期全球化學(xué)風(fēng)化強(qiáng)烈(Shield, 2005)。然而有研究發(fā)現(xiàn)在埃迪卡拉紀(jì)晚期亦有冰期沉積物的存在，如我國華北的正目觀組、羅圈組和鳳臺(tái)組(Le Heron et al., 2018；岳亮等，2020)，以及西北的漢格爾喬克組和紅鐵溝組(Xiao Shuhai et al., 2004; Shen Bing et al., 2010)。在高緯度地區(qū)的愛沙尼亞(位于波羅的大陸，60°S)，有研究發(fā)現(xiàn)其寒武紀(jì)早期黑色頁巖樣品的CIA值低至59，指示寒冷干燥氣候下較弱的化學(xué)風(fēng)化作用(Tosca et al., 2010)。因此埃迪卡拉紀(jì)晚期—寒武紀(jì)早期可能并非長期處于穩(wěn)定的超級(jí)溫室氣候環(huán)境中(Shen Bing et al., 2010；岳亮等，2020)。

對(duì)于我國華南揚(yáng)子地區(qū)而言，其在埃迪卡拉紀(jì)晚期發(fā)育臺(tái)內(nèi)凹陷和大規(guī)模開放盆地，臺(tái)地區(qū)發(fā)育燈影組白云巖，盆地區(qū)發(fā)育老堡組硅質(zhì)巖(圖7a)；早寒武世早期，全球海平面上升，碳酸鹽臺(tái)地遭到廣泛淹沒，到牛蹄塘沉積期形成以黑色細(xì)粒沉積為主的陸架環(huán)境(戴傳固等，2013；Yeasmin et al., 2017)?？焖俸Ｇ趾蟊４媪酥暗牡匦蔚孛玻谏珟r系在臺(tái)地區(qū)直接不整合沉積于白云巖之上(圖7b)，如上揚(yáng)子臺(tái)地區(qū)域燈影組頂部廣泛發(fā)育有不整合面或巖溶面(朱東亞等，2014；楊雨等，2014；劉宏等，2015；丁一，2018)，在盆地區(qū)則與下伏硅質(zhì)巖呈整合接觸(圖7c)?？梢?，揚(yáng)子地區(qū)在E—C轉(zhuǎn)折期經(jīng)歷了風(fēng)化作用和快速海侵。近年來，我國揚(yáng)子地區(qū)大陸風(fēng)化強(qiáng)度研究已積累了一定的基礎(chǔ)。前人通過對(duì)三峽地區(qū)與云南東部E—C地層序列的研究，發(fā)現(xiàn)在埃迪卡拉紀(jì)末期n(87Sr)/n(86Sr)顯著增高并于E—C轉(zhuǎn)折期附近達(dá)到最大值，而在進(jìn)入寒武紀(jì)之后，n(87Sr)/n(86Sr)又逐漸減小(Sawaki et al., 2008, 2014; Li Da et al., 2013)。最近，Stammeier et al.(2019)統(tǒng)計(jì)了全球各地E—C時(shí)期的n(87Sr)/n(86Sr)數(shù)據(jù)，同樣發(fā)現(xiàn)了在E—C轉(zhuǎn)折期n(87Sr)/n(86Sr)迅速增高，進(jìn)入寒武紀(jì)又明顯降低的變化規(guī)律。說明在E—C轉(zhuǎn)折期大陸風(fēng)化作用較為強(qiáng)烈，但可能存在明顯的波動(dòng)。Chen Can 等(2020)通過對(duì)三峽地區(qū)多個(gè)剖面的陡山沱組地層開展研究，系統(tǒng)地重建了埃迪卡拉紀(jì)末期CIA變化曲線，并識(shí)別出了三次CIA降低階段，結(jié)合巖石礦物學(xué)、地球化學(xué)(n(87Sr)/n(86Sr)，δ18O)指標(biāo)指出這三次CIA的降低對(duì)應(yīng)三次氣候變冷事件。貴州銅仁道坨剖面埃迪卡拉系陡山沱組CIA值總體較高，位于70～85之間(圖8)，指示其源區(qū)氣候溫暖濕潤，化學(xué)風(fēng)化程度較強(qiáng)，寒武系九門沖組下部(黑色頁巖段)CIA值降至55～70，反映源區(qū)氣候轉(zhuǎn)為寒冷干燥，風(fēng)化作用以物理風(fēng)化為主，而九門沖組上部CIA值再次升高，表明E—C轉(zhuǎn)折期風(fēng)化作用經(jīng)歷了強(qiáng)—弱—強(qiáng)的波動(dòng)(Zhai Lina et al., 2018)。另外有研究發(fā)現(xiàn)在道坨和壩黃剖面的牛蹄塘組底部，Ti/Al值增加且高于平均值，表明此時(shí)風(fēng)塵輸入增強(qiáng)，氣候變得相對(duì)干燥(Yeasmin et al., 2017; Zhai Lina et al., 2018)。廣西省三江縣石門剖面和泗里口剖面清溪組頁巖CIA值顯示穩(wěn)定的高值(76～84)，指示寒武紀(jì)早期其源區(qū)中等至較強(qiáng)程度的化學(xué)風(fēng)化作用，源區(qū)古氣候條件以溫暖濕潤為主，與揚(yáng)子陸塊中部上斜坡(貴州東北部)源區(qū)存在顯著差異，與同時(shí)期位于赤道附近的阿曼地區(qū)化學(xué)風(fēng)化程度和古氣候一致(張子虎，2018；熊晨，2019)。E—C轉(zhuǎn)折期碳酸鹽巖Nd元素豐度和εNd(t)值均顯著降低，表明陸相風(fēng)化物質(zhì)向陸架海水的輸出逐漸加強(qiáng)(Wei Guangyi et al., 2019)?？紤]到不同剖面距離揚(yáng)子或華夏板塊的位置，Li Chao 等(2020)推測(cè)石門剖面和泗里口剖面主要記錄了風(fēng)化作用強(qiáng)烈的華夏板塊的源巖信息，而靠近揚(yáng)子一側(cè)的道坨、硝灘、龍鼻嘴等剖面則反映了在E—C時(shí)期揚(yáng)子區(qū)域的化學(xué)風(fēng)化作用相對(duì)較弱，但具體造成化學(xué)風(fēng)化強(qiáng)度不同的原因還不明晰，有待進(jìn)一步探究。

圖7 (a)揚(yáng)子地區(qū)E—C轉(zhuǎn)折期地層劃分對(duì)比(改自陳建書等，2020；年齡數(shù)據(jù)來自朱日祥等，2009； Xu Lingang et al., 2011; Wang Xinqiang et al.,2012; Zhu Bi et al., 2013; Chen Daizhao et al., 2016; Fu Yong et al., 2016; Zhou Chuanming et al., 2020)；(b)燈影組與牛蹄塘組不整合接觸，遵義松林；(c)老堡組與牛蹄塘組整合接觸，銅仁Fig. 7 (a) Stratigraphical division of E—C strata in Yangtze area (revised from Chen Jianshu et al., 2020&, and age data are from Zhu Rixiang et al., 2009&; Xu Lingang et al., 2011; Wang Xinqiang et al.,2012; Zhu Bi et al., 2013; Chen Daizhao et al., 2016; Fu Yong et al., 2016; Zhou Chuanming et al., 2020)

現(xiàn)有示蹤揚(yáng)子地區(qū)E—C轉(zhuǎn)折期大陸風(fēng)化的地球化學(xué)證據(jù)不足，集中在n(87Sr)/n(86Sr)、δ13C、CIA和εNd(t)等，需要更多的地球化學(xué)參數(shù)來反映該風(fēng)化作用的影響范圍和演化特征，同時(shí)n(87Sr)/n(86Sr)、δ13C、CIA和εNd(t)等數(shù)據(jù)也局限在少數(shù)剖面和層段，如εNd(t)僅涉及晚埃迪卡拉紀(jì)碳酸鹽巖，缺少對(duì)早寒武紀(jì)黑色頁巖的分析。風(fēng)化作用與揚(yáng)子地區(qū)古海洋環(huán)境、古生物演化和有機(jī)質(zhì)富集等有什么影響？

圖9 風(fēng)化作用對(duì)海洋環(huán)境及有機(jī)質(zhì)富集影響模式示意圖(注：OM為有機(jī)質(zhì)；DIC為溶解無機(jī)碳)Fig. 9 Affecting pattern of continental weathering onmarine environment and organic matter enrichment (note: OM is organic matter; DIC is dissolved inorganic carbon)

揚(yáng)子地區(qū)E—C轉(zhuǎn)折期大陸風(fēng)化對(duì)古海洋環(huán)境、有機(jī)質(zhì)富集有何影響？準(zhǔn)確認(rèn)識(shí)該問題有助于揭示古海洋環(huán)境演變和古生物演化規(guī)律，能為區(qū)內(nèi)油氣資源的勘探開發(fā)提供新的思路。

4 結(jié)論與展望

筆者等學(xué)習(xí)和總結(jié)了前人對(duì)揚(yáng)子地區(qū)震旦紀(jì)(埃迪卡拉紀(jì))—寒武紀(jì)(E—C)轉(zhuǎn)折期大陸風(fēng)化作用的研究成果，取得以下主要認(rèn)識(shí)：

(1)揚(yáng)子地區(qū)震旦紀(jì)(埃迪卡拉紀(jì))—寒武紀(jì)(E—C)轉(zhuǎn)折期遭受了較強(qiáng)烈的化學(xué)風(fēng)化作用，在臺(tái)地相區(qū)，牛蹄塘組黑色巖系不整合接觸于燈影組白云巖之上，斜坡—盆地相區(qū)，該黑色巖系與下伏老堡組硅質(zhì)巖呈整合接觸。臺(tái)地區(qū)不整合與E—C轉(zhuǎn)折期全球“大不整合”具有緊密的關(guān)系，而盆地區(qū)發(fā)育的完整的沉積地層則記錄了臺(tái)地區(qū)被風(fēng)化地層的痕跡。揚(yáng)子地區(qū)E—C轉(zhuǎn)折期兼具淺水臺(tái)地和深水盆地環(huán)境，是研究這一時(shí)期全球大陸風(fēng)化作用的重要窗口。

(2)現(xiàn)有揚(yáng)子地區(qū)E—C轉(zhuǎn)折期大陸風(fēng)化的地球化學(xué)證據(jù)集中于n(87Sr)/n(86Sr)、δ13C、CIA和εNd(t)等，同時(shí)n(87Sr)/n(86Sr)、δ13C、CIA和εNd(t)等數(shù)據(jù)也局限在少數(shù)剖面和層段。因此迫切需要更多的地球化學(xué)參數(shù)來反映該風(fēng)化作用的影響范圍和演化特征。

(3)揚(yáng)子地區(qū)E—C轉(zhuǎn)折期大陸風(fēng)化作用與海洋環(huán)境演變存在耦合關(guān)系，準(zhǔn)確認(rèn)識(shí)這種關(guān)系有助于揭示古生物演化、有機(jī)質(zhì)富集的機(jī)制，能夠?yàn)閾P(yáng)子地區(qū)油氣資源勘探開發(fā)提供新思路。

致謝：感謝評(píng)審專家和責(zé)任編輯對(duì)本文修改提出的寶貴建議。

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