華南陸塊東部晚中生代巖漿作用的深部動力學過程

郭 鋒, 趙 亮, 張曉兵, 吳揚名, 張 博, 張 峰, 4

華南陸塊東部晚中生代巖漿作用的深部動力學過程

郭 鋒1, 2, 趙 亮1, 2, 張曉兵1, 2, 吳揚名3, 張 博1, 張 峰1, 4

(1. 中國科學院 廣州地球化學研究所, 同位素地球化學國家重點實驗室, 廣東 廣州 510640; 2. 中國科學院 深地科學卓越研究中心, 廣東 廣州 510640; 3. 中山大學 地球科學與工程學院, 廣東 珠海 519082; 4. 中國科學院大學 地球與行星科學學院, 北京 100049)

本文總結和回顧了過去幾年來本研究團隊在華南陸塊東部(主要是東南沿海地區)開展的火成巖巖石地球化學研究進展, 并重點探討了晚中生代(白堊紀)巖漿作用的深部動力學過程, 取得了以下主要認識: ①華南晚中生代鎂鐵質巖漿作用記錄了古太平洋板塊從俯沖到后撤?撕裂的深部動力學過程, 其地幔源區的富集組分從進俯沖時期的板片上覆沉積物逐漸過渡到后撤?撕裂階段的下部鎂鐵質洋殼; ②長英質火山巖的地殼源區從俯沖階段的低溫(700～810 ℃)含水下地殼轉變為后撤?撕裂階段的高溫(790～860 ℃)貧水陸殼; ③東南沿海地區晚中生代經歷了強烈的弧地殼增生和置換作用, 形成了具有“等同位素效應”的雙峰式侵入雜巖體。我們提出的板片俯沖?后撤?撕裂模式同樣適用于解釋華南陸塊東部早中生代構造?巖漿演化的深部的動力學機制。

板片俯沖?后撤?撕裂作用; 地殼演化; 古太平洋; 晚中生代巖漿作用; 華南陸塊

0 引 言

大洋板塊俯沖作用、弧下地幔楔富集改造和俯沖板片的歸宿是探索地球層圈相互作用和物質再循環過程的重要內容, 也是當今板塊構造學說的奠基石之一。目前地球大約有長達43000 km的俯沖帶, 每年有大量的大洋巖石圈和上覆沉積物被帶到弧下地幔中, 其中俯沖沉積物通量每年在2.5～3.0 km3之間(von Huene and Scholl, 1991; Clift et al., 2009; Scholl and Von Huene, 2010)。如此巨量的再循環地殼物質如何改造地幔楔？這些被俯沖的大洋巖石圈歸宿在哪里？俯沖帶與板內巖漿存在何種成因聯系？這些問題都是地球演化研究的主要內容(Hofmann, 1997; Van der Lee and Nolet, 1997; Wu et al., 2009)。

早中生代以來古太平洋板塊經歷了初期的洋底擴張、增生、俯沖到最后消亡的全過程, 不同時期其與周邊板塊的相互作用存在異同(Sun et al., 2007; Seton et al., 2012; Müller et al., 2016)。中生代期間, 中國東部為古太平洋俯沖作用下形成的活動大陸邊緣(Engebretson et al., 1985; Maruyama et al., 1989; Faure and Natal’in, 1992; Zheng et al., 2013; Guo et al., 2015; 郭鋒, 2016; 唐杰等, 2018; 朱日祥和徐義剛, 2019; Ma and Xu, 2021), 記錄了古太平洋板塊俯沖、地殼增生、俯沖板片再循環等深部過程。華南地區位于東亞大陸邊緣的東南部, 記錄了古太平洋板塊從俯沖、后撤到撕裂的完整過程(Jahn et al., 1990; Charvet et al., 1994; Lapierre et al., 1997; 董傳萬等, 1997; Chen and Jahn, 1998; Xu et al., 1999; Zhou and Li, 2000; Zhou et al., 2006; Li and Li, 2007; Li et al., 2007, 2019; 李獻華等, 2007; 張國偉等, 2013; Wang et al., 2013; Lin et al., 2018; Guo et al., 2021; Shu et al., 2021; Mao et al., 2021)。在國家自然科學基金委員會?廣東省聯合基金等項目的資助下, 我們針對華南陸塊東部尤其是中國東南沿海地區晚中生代巖漿作用開展了系統的巖石學、年代學、元素?同位素地球化學和數值模擬等綜合研究。結合前人的研究基礎, 本文將從大洋板塊俯沖、后撤、撕裂的動力學過程來理解深部巖石圈地幔演變、地殼熱結構與成分的變化, 從而為活動大陸邊緣鎂鐵質?長英質巖漿作用形成機制提供新視角。

1 華南陸塊東部晚中生代巖漿作用的時空格架

華南陸塊尤其是東部地區廣泛分布了中生代火成巖(圖1a), 這些火成巖巖漿作用的時空格架一直備受關注。Charvet et al. (1994)將中國東南沿海地區火山作用分為兩個階段: 晚侏羅世?早白堊世以酸性火山巖噴發為主的第一階段, 晚白堊世以拉斑玄武巖及酸性火山巖為主的第二階段。Zhou et al. (2006)將東南沿海地區火山作用也劃分為兩個主要階段: 燕山早期和燕山晚期, 或稱之為上、下火山巖系。Guo et al. (2012)將粵東?福建東南部中生代火成巖劃分為三個階段: 168～145 Ma、143～130 Ma和104～95 Ma。Wang et al. (2013)則把華南地區巖漿作用與構造變形結合起來, 將華南地區中生代巖漿作用細分為六個峰期: 240 Ma、220 Ma、175 Ma、158 Ma、125 Ma和93 Ma, 其中前面兩個峰期與印支期變質?變形年齡相吻合, 集中在華南內部, 空間上對應于印支期花崗巖的分布范圍; 最后一個峰期與燕山晚期的變形年齡基本一致, 主要分布在東南沿海地區。Cao et al. (2021b)根據華南地區780多個侏羅紀?白堊紀火成巖年代學數據, 將巖漿作用分為四個主要階段: 190～175 Ma、165～155 Ma、145～125 Ma和105～95 Ma, 對應的巖漿活動峰值時間為160 Ma、130 Ma和100 Ma(圖1b), 其中前面兩個階段主要為侏羅紀侵入巖, 集中分布于華南內部, 與古海溝的距離在550～1200 km之間; 后兩個階段主要為白堊紀火山巖, 集中分布在東南沿海地區, 與古海溝的距離為400～800 km。

本次研究統計了長英質火山巖和花崗巖鋯石Hf同位素組成, 結果顯示, 無論是噴出巖還是侵入巖, 鋯石Hf同位素組成與其形成年齡呈現出一定的負相關關系, 反映了隨著時間推移, 巖漿源區中虧損幔源組分不斷增加(圖1c、d), 而且巖漿溫度隨巖石年齡變年輕而逐漸增高(圖1e)。長英質火山巖的Hf(t)變化趨勢略優于花崗質巖石。總體上, 華南地區中生代巖漿作用呈現出多期次幕式特點, 尤其是晚中生代火成巖的分布與古太平洋板塊俯沖作用之間存在密切聯系(Zhou et al., 2006)。

2 鎂鐵質巖漿作用記錄的俯沖?后撤?撕裂過程

華南陸塊東部地區廣泛發育白堊紀弧巖漿和板內鎂鐵質巖漿作用(圖2; 董傳萬等, 1997; Xu et al., 1999; Wang et al., 2003, 2008; Zhao et al., 2007; Meng et al., 2012; Li et al., 2014; Zhang et al., 2019, 2020a; 秦社彩等, 2019; Wu et al., 2020)。毫無疑問, 這兩類巖漿作用與當時的古太平洋板塊俯沖作用存在著成因聯系(Charvet et al., 1994; Lapierre et al., 1997; Xu et al., 1999; Zhou and Li, 2000), 但是它們之間存在何種聯系, 仍不清晰。為此, 我們對該區白堊紀(120～70 Ma)鎂鐵質火成巖進行全面的地球化學數據匯編和重新分類, 并結合俯沖板片熔融的二維數值模擬, 構建了古太平洋俯沖、后退和撕裂作用的統一構造動力學模型, 并以此為基礎來闡述俯沖帶和板內鎂鐵質巖漿作用的內在成因聯系(Guo et al., 2021)。

根據Nb含量以及Nb/Y、Nb/U、Ba/Nb和Zr/Nb值等, 華南陸塊東部白堊紀(120～77 Ma)鎂鐵質巖石可分為拉斑?鈣堿性島弧型玄武巖(Nb<20′10?6)、低Nb(Nb<40′10?6)的弱堿性玄武巖(OIB型)和高Nb(Nb>50′10?6)堿性玄武巖(OIB型)(圖3a)。隨著Nb含量變化, 白堊紀鎂鐵質巖石不僅在巖石類型上變化明顯, 同時在微量元素地球化學特征方面也有所反映: 從島弧型玄武巖Nb-Ta相對虧損, 到低Nb玄武巖無高場強元素(Nb、Ta、Zr、Hf)虧損, 到高Nb玄武巖Nb-Ta正異常; 相應地, Pb從正異常逐漸向無異常到負異常轉變(圖4), 反映了再循環陸殼物質對巖漿成因的貢獻逐漸降低(Sun and McDonough, 1989; Rudnick and Gao, 2014)。

同位素組成上(圖5), 島弧型玄武巖顯示出相對富集的Sr-Nd-Pb-Hf同位素組成: (87Sr/86Sr)i>0.705,Nd()<0,Hf()<+4, (206Pb/204Pb)i>18.10, (207Pb/204Pb)i> 15.57, (208Pb/204Pb)i>38.20。高Nb玄武巖則顯示出最虧損的Sr-Nd-Hf同位素組成, (87Sr/86Sr)i<0.704,Nd()>+4,Hf()>+8。而低Nb玄武巖介于二者之間, 同位素組成變化較大, 如Nd()變化在?1.7～+6.8之間, (87Sr/86Sr)i的變化范圍為0.7036～0.7075。但在Pb同位素組成上, 三類巖石則較相似。

根據Lu/Hf對Th/La、Th/Yb判別圖解(圖略), 華南陸塊東部弧巖漿具低Lu/Hf值、高Th/La和Th/Yb值, 反映它們形成于活動大陸邊緣或者陸緣弧(Zhao et al., 2019), 其源區主要為俯沖陸源沉積物熔體改造的地幔楔, 與現代小安德烈斯和西南日本俯沖帶相似(White and Dupré, 1986; Hanyu et al., 2006; Labanieh et al., 2010), 為相對較熱的俯沖帶(Plank et al., 2009; Zhang et al., 2019及其中參考文獻)。

鋯石U-Pb年齡數據來源: Cao et al., 2021b及其中參考文獻。鋯石Hf同位素數據來源: Guo et al., 2012; Liu et al., 2012, 2014, 2016a, 2016b, 2018; Huang et al., 2015; Jiang et al., 2015; Zhao et al., 2015, 2016a, 2016b, 2021; Wang et al., 2015; Li et al., 2016a, 2016b, 2017; Xia et al., 2016; Yan et al., 2016。巖漿溫度為全巖鋯飽和溫度數據來源: Lapierre et al., 1997; 余明剛等, 2008; He et al., 2009, 2012; Guo et al., 2012; Jiang et al., 2013, 2015; Li et al., 2013, 2016a, 2016b, 2017; Liu et al., 2014, 2016a, 2018; Song et al., 2016; Wang et al., 2016a; Yan et al., 2016; Zhang et al., 2018。

早、晚白堊世A型花崗巖分布據Peng et al. (2021)。

匯編的詳細數據見Guo et al. (2021)附表1。

圖b中鎂鐵質侵入巖母巖漿的微量元素含量根據Guo et al. (2015, 2016)的方法進行了重新計算。原始地幔標準化數據來自Sun and McDonough (1989)。

LILE/Nb與REE/Nb、Zr/Nb等協變關系可以用來約束再循環地殼組分的性質。協變關系圖解顯示, 華南陸塊東部高Nb玄武巖源區再循環地殼組分主要為脫水的洋殼, 而低Nb玄武巖的源區中則包含了一定量的海洋沉積物(圖6)。此外, Wu et al. (2020)研究發現江西吉安地區螺絲山晚白堊世玄武巖中橄欖石斑晶具有相對地幔值更低的O同位素組成, 反映其再循環洋殼經歷了高溫水?巖反應(Eiler, 2001)。而且華南陸塊東部高Nb玄武巖總體上具一定的Eu、Sr正異常和高Ba/Th值, 以及初始巖漿貧水等特征(Wang et al., 2003, 2008; Wu et al., 2020), 說明再循環物質很可能來自輝長巖等堆晶巖組成的下部洋殼(Bach et al., 2001; Kelley et al., 2003), 并在俯沖過程中經歷了強烈脫水作用, 因而呈現出強烈的Pb負異常(圖4c)。部分低Nb玄武巖表現出低Ca同位素組成, 也反映了俯沖沉積物改造的地幔源區(Zhang et al., 2020a)。

華南陸塊東部白堊紀鎂鐵質巖石地球化學特征和成因解釋揭示, 區域地幔的改造介質從洋殼上部的海洋沉積物轉變到洋殼下部的基性地殼部分, 這些鎂鐵質洋殼在熔融之前經歷了強烈的俯沖脫水過程(Zhang et al., 2019, 2020a; Wu et al., 2020)。因此無論是島弧玄武巖還是板內OIB型玄武巖, 其源區都包含了來自俯沖的古太平洋板片組分, 只是代表了不同的洋殼部分。

根據華南陸塊東部白堊紀鎂鐵質巖石的地球化學變化趨勢, 采納有限元計算方法(Gerya and Yuen, 2003), 對脫水洋殼熔融過程進行二維熱力學模擬研究。模擬結果顯示, 當俯沖脫水板片在軟流圈下沉過程中, 未減薄或者撕裂的完整脫水板片不會發生熔融; 只有當俯沖板片發生撕裂、破碎或者碎片化, 脫水的洋殼才能沿著俯沖巖石圈的撕裂面發生部分熔融作用, 其熔融的程度與板片的寬度、厚度呈負相關關系(圖7; Guo et al., 2021)。

綜合華南陸塊東部白堊紀鎂鐵質巖石地球化學變化趨勢、時空分布特征和熱力學數值模擬結果, 我們提出了古太平洋板塊俯沖到后撤?撕裂過程與鎂鐵質巖石成因模式: 俯沖沉積物熔體交代的地幔楔為弧巖漿的源區; 不均一的板片后撤很可能導致了板片撕裂和脫水板片的部分熔融, 熔體與軟流圈反應形成OIB型板內玄武巖的地幔源區(圖8; Guo et al., 2021)。

最近, 華南東南部也發現了早白堊世OIB型基性脈巖(Yan et al., 2021)和同時期的A型花崗巖(Peng et al., 2021), 被認為與俯沖板片的后撤?撕裂作用相關。Peng et al. (2021)對白堊紀A型花崗巖進行了統計, 識別出兩條A型花崗巖帶, 其中早白堊世A型花崗巖帶從西南向東北方向侵位時代逐漸變年輕, 可能與古太平洋板片由西南向東北后撤過程有關; 晚白堊世A型花崗巖則主要分布在東南沿海地區, 與古太平洋板片向東后撤過程相關(圖2)。這與Sun et al. (2007)認為古太平洋板塊向歐亞板塊運動方向在125 Ma左右發生了轉變相吻合。

3 白堊紀長英質火山作用記錄的板片俯沖?后撤過程

相對于鎂鐵質巖漿, 中國東部發育更為廣泛的中生代長英質火山巖和花崗質巖石(圖9), 盡管這些長英質巖漿活動普遍被認為與古太平洋板塊俯沖作用相關(Xu et al., 1999; Zhou and Li, 2000; Zhou et al., 2006), 但古太平洋板塊的俯沖方式(如平板俯沖、洋脊俯沖以及周期性的俯沖與后撤)及其對長英質巖漿作用成因的影響目前還存在爭議(Li and Li, 2007; Li et al., 2007; Sun et al., 2007; Guo et al., 2012; Liu et al., 2012, 2014, 2016b)。前人研究表明, 早白堊世晚期?晚白堊世, 中國東南部經歷了兩期火山作用, 這兩期火山巖表現出明顯的地球化學成分變化, 暗示其可能來自于不同的構造環境(Guo et al., 2012; Liu et al., 2012, 2014, 2016; Li et al., 2014)。通過對浙江東南部白堊紀長英質火山巖詳細的年代學和巖石地球化學研究, 我們識別出古太平洋板塊俯沖與后撤過程在地殼演化中的記錄, 建立了長英質巖漿作用與古太平洋板塊俯沖之間的成因聯系(Zhao et al., 2021)。

日本西南部數據來自Hanyu et al., 2006。

高Nb玄武巖地幔源區的改造組分主要是脫水洋殼, 低Nb玄武巖地幔源區的改造組分中包括脫水洋殼和較大比例的沉積物。模擬參數來源: Zhang et al., 2020a; Guo et al., 2021。模擬參數中元素含量單位: ×10?6。

兩期火山巖均表現出高SiO2、富K2O和中等?強過鋁質的特征, 并且具有相似的Nd和Hf同位素組成(早期火山巖:Nd()=?9.5～?7.5,Hf()=?8.5～?0.7; 晚期火山巖:Nd()=?7.1～?6.1,Hf()=?7.8～?2.2; Zhao et al., 2021)。Sr-Nd同位素模擬計算結果顯示, 其來源于華夏地塊古老基底巖石和中生代新生地殼的混合源區(Chen and Jahn, 1998; Zhang et al., 2019)。

由于云母類礦物富集Rb而具高Rb/Sr值, 因此地殼熔融過程中, 水致熔融(云母穩定而在源區殘留)和脫水熔融(云母不穩定發生分解進入熔體)(Pati?o Douce and Beard, 1995; Gao et al., 2017)會使熔體中Rb含量和Rb/Sr值產生差異。兩期火山巖雖然具有相似的同位素組成, 但早期火山巖具有高Sr、Ba含量和低Rb/Sr值特征, 暗示其為含水條件下的熔融產物, 而黑云母在源區殘留; 晚期火山巖則表現出低Sr、Ba含量和高Rb/Sr值特征, 反映其為相對貧水狀態下黑云母脫水熔融的產物(圖10a、b; Gao et al., 2017)。同時, 全巖鋯飽和溫度計算顯示, 早期火山巖顯示較低的熔融溫度(700～810 ℃); 而晚期火山巖具有較高的熔融溫度(790～860 ℃; 圖10c)。此外, 早期火山巖顯示較高的Sr/Y和(La/Yb)CN值, 暗示其來源于較深的熔融源區; 而晚期火山巖具有較低的Sr/Y和(La/Yb)CN值, 暗示其來源于較淺的地殼深度(圖 10c、d; Profeta et al., 2015)。

板片越薄, 越有利于脫水洋殼熔融; 板片越寬, 下沉變慢, 加熱充分, 熔融范圍增大。

(a) 相對較熱古太平洋板塊俯沖導致了沉積物大量熔融, 其熔體交代上覆地幔楔, 被沉積物熔體交代地幔楔熔融形成了區域上弧巖漿; (b) 俯沖板塊回卷后撤導致海溝后退和板片撕裂, 被撕裂的脫水板片熔融形成的熔體交代軟流圈地幔形成OIB型玄武巖的地幔源區。

晚白堊世A型花崗巖年代學數據來源: 邱檢生等, 1999; 肖娥等, 2007; 林清茶等, 2011; Chen et al., 2013, 2019。

因此, 白堊紀兩期火山巖礦物學和地球化學組成特征差異可能是由其源區不同的熔融條件造成的, 即早期火山巖來源于較深、相對濕?冷的地殼源區; 而晚期火山巖來源于較淺、相對干?熱的地殼源區。熔融源區--H2O條件改變暗示, 華南陸塊東部地殼在早白堊世晚期(約110～100 Ma)經歷了從俯沖擠壓環境向板內伸展環境的轉變(圖11), 這與古太平洋板塊從俯沖到板片后撤的動力學過程相吻合。根據區域鎂鐵質和長英質巖漿的時空演變趨勢, 華南陸塊東部地區從俯沖到后撤?撕裂的轉折時間大體發生在110～100 Ma之間, 與區域上開始出現A型花崗巖和OIB型鎂鐵質巖漿作用的時間大體一致。

綜上所述, 中國東南部早白堊世晚期?晚白堊世經歷了兩期地殼熔融。117～113 Ma, 古太平洋板塊俯沖在華南陸塊東部形成活動大陸邊緣(Zhang et al., 2019), 俯沖板片脫水引起地殼發生熔融, 形成早期相對富水、冷的長英質火山巖; 同時俯沖沉積物熔體交代地幔楔, 形成同期的弧鎂鐵質巖漿。110～93 Ma期間, 俯沖的古太平洋板片發生后撤和撕裂, 引起弧后伸展和軟流圈物質上涌, 大量來自軟流圈的熱能引起下地殼物質的熔融, 形成晚期相對貧水、熱的長英質火山巖以及同期的A型花崗巖。

(a) Rb/Sr-Ba; (b) Sr-Rb(據Gao et al., 2017); (c) (La/Yb)CN-TZr(據Profeta et al., 2015); (d) Sr/Y-La/Yb(據Wang et al., 2016b)。

WPG. 板內花崗巖; VAG. 火山弧花崗巖; syn-COLG. 同碰撞花崗巖; ORG. 洋中脊花崗巖。浙東南114～113 Ma和100～93 Ma火山巖數據來源: Zhao et al., 2021。東南沿海114～90 Ma火山巖數據來源: Lapierre et al., 1997; 余明剛等, 2008; He et al., 2009; Guo et al., 2012; Jiang et al., 2013, 2015; Yan et al., 2016。

4 “等同位素效應”鎂鐵質?長英質侵入雜巖記錄的地殼演化過程

在中國東南沿海地區, 經常可以看到一些鎂鐵質?長英質侵入雜巖中鎂鐵質巖石與長英質巖石具有相似的同位素組成, 被稱為“等同位素效應”(薛懷民等, 1996; 邢光福和陶奎元, 1998; Xing et al., 2004), 如平潭和泉州侵入雜巖(圖12)。平潭和泉州的花崗巖都具有高分異I型花崗巖特點, 而伴生的鎂鐵質侵入巖為角閃石輝長巖, 二者在Hf和Nd同位素組成上非常相似(圖13)。與此同時, 花崗巖中鋯石具有類似地幔的O同位素組成(圖14), 反映其來源于幔源的鎂鐵質原巖, 且很少有變沉積巖組分的參與。關于這些花崗巖成因, 主要有以下三種可能性。

(1) 花崗巖來自鎂鐵質巖漿的分異作用。由于泉州和平潭花崗巖與輝長巖具有幾乎一致的形成時間(～115 Ma), 二者可能為同源巖漿不同階段分異的產物。在東南沿海地區, 鎂鐵質巖漿的分布規模相對于長英質巖石小很多, 如果二者之間是分異關系, 那么應該存在更多的鎂鐵質巖石。另外, 鎂鐵質與長英質巖石之間表現出雙峰式特點, 幾乎沒有中性過渡成分巖石, 因此也很難用分異作用來解釋。

圖12 平潭和泉州侵入雜巖體地質圖(據張博等, 2020)

圖13 平潭和泉州的花崗巖與輝長巖Nd(a)和Hf(b)同位素組成對比(據Zhang et al., 2019; 張博等, 2020)

其他東南沿海的鋯石Hf-O同位素數據來自Chen et al. (2013, 2019)及其中參考文獻。地幔鋯石的O同位素組成據Valley (2003)。

(2) 花崗巖為鎂鐵質巖漿與殼源巖漿混合的產物。Griffin et al. (2002)根據平潭花崗巖中鋯石較大的Hf同位素變化范圍, 認為其為殼?幔源巖漿混合作用的結果。但是我們最近的研究結果顯示無論是平潭花崗巖還是泉州花崗巖, 其鋯石Hf同位素組成的變化范圍較小, 不能用源區的不均一性來解釋(張博等, 2020)。平潭雜巖體中可觀察到基性微粒包體(MME), 反映了局部巖漿混合作用。對這些巖體不同巖性(輝長巖、花崗閃長巖和花崗巖)的磷灰石進行詳細地球化學研究, 結果顯示它們三者之間并不存在巖漿混合關系, 而是各自母巖漿分異與流體作用或者脫氣作用的結果(Zhang et al., 2020b, 2021)。

(3) 同期底侵鎂鐵質巖石的熔融作用。在成分上, 花崗巖具高鉀鈣堿性、強烈的Sr-Eu負異常以及偏鋁到過鋁質特征, 反映了其殼源成因特點(Sisson et al., 2005)。但在同位素組成上, 花崗巖與輝長巖呈現出一致性, 反映了其同源性。且二者在空間上的密切共生, 也反映了相互之間緊密的成因聯系。

通常, 鎂鐵質巖石相對變沉積巖具有高得多的熔融溫度, 因此只要地殼源區中存在這些變沉積巖組分, 它們會首先發生部分熔融, 形成過鋁質的長英質巖漿以及類似華夏基底非常富集的Nd-Hf同位素組成(Chen and Jahn, 1998; Yu et al., 2009, 2010)。這顯然與花崗巖較華夏地塊基底高得多的Nd-Hf同位素組成相矛盾。花崗巖中鋯石具類似地幔的O同位素組成特征, 從另一個側面也說明花崗巖的熔融源區幾乎沒有古老華夏基底巖石的貢獻。

近些年來, 陸續報道華南陸塊東部存在古老華夏基底(于津海等, 2006; Xu et al., 2007; Yu et al., 2009, 2010及其中參考文獻)。那么一種可能性是, 這些基底巖石被加厚、榴輝巖化而拆沉到地幔中, 但是區域上幾乎沒有出現強烈的擠壓變形、超高壓變質巖和埃達克質巖漿作用等白堊紀地殼加厚的地質學和巖石學記錄。另一種可能就是, 早先存在的地殼巖石被底侵的幔源巖漿破壞、稀釋和置換(Guo et al., 2019), 變成了新增生的弧地殼; 這與地球物理觀察到區域中下地殼存在4～5 km的低速層相吻合(Zhang et al., 2008)。

綜合分析表明, 區域“等同位素效應”雙峰式侵入雜巖體的成因: 古太平洋板塊俯沖背景下, 幔源巖漿不斷底侵到中、下地殼, 破壞并置換了原有的華夏古老地殼, 部分幔源巖漿上升侵位于上地殼, 形成了雜巖的鎂鐵質組分(圖15)。固結的幔源巖漿在中、下地殼發生部分熔融作用形成長英質巖漿, 這些巖漿發生不同程度的分異、自混合作用和脫氣過程形成了花崗巖和花崗閃長巖(張博等, 2020; Xu et al., 2021; Cao et al., 2021a; Zhang et al., 2021)。由于長英質巖漿與鎂鐵質巖漿是同源的, 因此在同位素組成上具有一致性。這種新生弧地殼對古老地殼的置換在活動大陸邊緣廣泛存在, 如中國東北、俄羅斯遠東、澳大利亞塔斯馬尼亞造山帶等地(Guo et al., 2019), 并形成了眾多的“等同位素效應”火成雜巖。

5 總結與展望

(1) 根據Nb含量, 華南陸塊東部白堊紀鎂鐵質巖石可以劃分為島弧型玄武巖、低Nb玄武巖和高Nb玄武巖, 其地幔源區記錄了從俯沖洋殼上覆沉積物到下洋殼蝕變輝長巖的熔體改造過程, 反映了古太平洋板片從俯沖到后撤?撕裂的深部動力學過程, 轉折的時間主要發生在110～100 Ma之間。

(2) 古太平洋板片從俯沖到后撤?撕裂過程改變了區域地殼的熱?化學結構, 尤其是在東南沿海地區, 長英質巖漿的地殼源區從俯沖階段的富水低溫狀態轉變到后撤?撕裂階段的貧水高溫狀態。

(3) 古太平洋板塊俯沖作用在華南陸塊東部形成了大量的新生弧地殼, 它們破壞和置換了早先存在的華夏地塊古老地殼, 新生弧殼在幔源巖漿的不斷底侵過程中發生熔融形成了廣泛的中酸性巖漿, 它們與上侵的鎂鐵質巖漿形成了區域上的“等同位素效應”雙峰式火成雜巖。

圖15 東南沿海早白堊世“等同位素效應”雙峰式雜巖的形成模式(據張博等, 2020; Xu et al., 2021; Cao et al., 2021a修改)

華南陸塊東部地區中生代巖漿作用期次非常多, 成分復雜, 巖石組合多樣, 前人曾經提出了多種深部動力學成因模式。基于我們提出的板塊俯沖?后撤?撕裂模型, 可以將區域上廣泛發育的中生代早期(比如印支期和中?晚侏羅世)殼源巖漿作用理解為平板俯沖作用導致地殼縮短加厚和深熔作用的產物, 而OIB型玄武巖或其對應的鎂鐵質侵入巖和A型花崗巖組合(早侏羅世、早白堊世和晚白堊世)則可以視作板片后撤?撕裂或斷離作用的巖石學記錄。然而, 除了早白堊世古太平洋板塊俯沖作用有清晰的地質學和巖石學記錄外, 有關中生代其他各個時期的俯沖帶位置和俯沖方式仍然存在較大的爭議。理論上, 古太平洋板塊的平板俯沖作用勢必導致地殼的縮短加厚, 并促進板片的高壓熔融, 從而產生類似于南美安第斯造山帶的埃達克質巖漿, 但是這類巖漿在華南陸塊東部地區中生代火成巖中罕見, 因此需要更詳細的巖石學和地球化學研究。綜上所述, 關于古太平洋與華南大陸之間的相互作用動力學過程仍需要更多的來自古地磁學、古板塊恢復、地質學、巖石學和地球化學甚至是動力學模擬等多學科的交叉和集成研究, 以期取得更為可靠的研究成果。

致謝:成文過程中得益于與中山大學王岳軍教授和中國科學院廣州地球化學研究所黃小龍研究員的討論, 中國地質大學(武漢)鄭建平教授和云南大學王選策教授在評審過程中提出了寶貴意見和建議, 在此一并致以誠摯的謝意。

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Geodynamics of Late Mesozoic Magmatism in the Eastern South China Block: An Overview

GUO Feng1, 2, ZHAO Liang1, 2, ZHANG Xiaobing1, 2, WU Yangming3, ZHANG Bo1, ZHANG Feng1, 4

(1. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China; 2. Center of Excellence of Deep Earth Sciences, Guangzhou 510640, Guangdong, China; 3. School of Earth Sciences and Engineering, Sun Yat-sen University, Zhuhai 519082, Guangdong, China; 4. College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China)

In this paper, we review the petrological and geochemical studies of the late Mesozoic igneous rocks in the eastern part of South China (mainly the southeast coastal area) conducted by our group during the past five years, focusing mainly on the geodynamic process of the Cretaceous magmatism. The main advancements include: (1) The late Mesozoic mafic magmatism in South China recorded the geodynamic processes from subduction to rollback and tearing of the paleo-Pacific slab. During these processes, the mantle sources for the mafic magmas were enriched by the subducted sediments of the upper oceanic crust during the advanced subduction and were then metasomatized by the lower oceanic crust in response to the slab retreat and tearing. (2) The crustal sources for felsic volcanic rocks changed from the low-temperature (700–810 ℃) water-rich crust in the advanced subduction stage to the high-temperature (790–860 ℃) water-poor continental crust in the rollback-tearing stage. (3) The late Mesozoic crust in the southeast coastal area experienced extensive crustal accretion and subduction-induced replacement, forming the ‘equal-isotope’ and petrochemically bimodal intrusive complexes. We propose that the slab subduction-rollback-tearing model may also be applicable to the geodynamics of the early Mesozoic tectonic-magmatic evolution in the eastern South China Block.

slab subduction and rollback-tearing; crustal evolution; the paleo-Pacific; late Mesozoic magmatism; South China Block

2021-12-10;

2022-02-25

國家自然科學基金項目(U1701641、41525006、42073032、42021002)資助。

郭鋒(1971–), 男, 研究員, 主要從事巖石學和大地構造學研究。E-mail: guofengt@263.net; fengguo@gig.ac.cn

P581; P511.4

A

1001-1552(2022)03-0416-019

10.16539/j.ddgzyckx.2022.03.002