黃小龍, 于 洋

華南早古生代穿時(shí)巖漿活動(dòng)成因與驅(qū)動(dòng)機(jī)制

黃小龍1, 2, 3, 于 洋1, 2, 3

(1. 中國(guó)科學(xué)院 廣州地球化學(xué)研究所, 同位素地球化學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室, 廣東 廣州 510640; 2. 中國(guó)科學(xué)院 深地科學(xué)卓越研究中心, 廣東 廣州 510640; 3. 南方海洋科學(xué)與工程廣東省實(shí)驗(yàn)室(廣州), 廣東 廣州 511458)

華南地區(qū)廣泛分布早古生代巖漿巖, 被認(rèn)為是陸內(nèi)造山作用的產(chǎn)物, 但其動(dòng)力學(xué)機(jī)制存在爭(zhēng)論。本文收集了近年發(fā)表的華南地區(qū)早古生代巖漿巖的年代學(xué)和巖石地球化學(xué)資料, 以揭示其巖漿活動(dòng)的時(shí)空分布規(guī)律及動(dòng)力機(jī)制。華南早古生代巖漿巖以S型花崗巖(片麻狀和塊狀)為主, 少量為I型花崗巖與基性巖漿巖(包括玄武巖、輝長(zhǎng)巖、輝長(zhǎng)閃長(zhǎng)巖和鎂鐵質(zhì)微粒包體等)。S型花崗巖廣泛分布于東揚(yáng)子板塊和西華夏板塊, 但存在穿時(shí)性。鄰近于東揚(yáng)子?西華夏板塊古縫合帶的云開(kāi)地塊和武功地塊(云開(kāi)?武夷造山帶內(nèi)部帶)的片麻狀S型花崗巖形成于約470～410 Ma, 峰期年齡為～442 Ma; 位于西華夏板塊東緣?武夷地塊(云開(kāi)?武夷造山帶東緣)的片麻狀S型花崗巖持續(xù)時(shí)間較短(約455～415 Ma), 峰期年齡略年輕(～435 Ma)。I型花崗巖與基性巖主要分布于東揚(yáng)子?西華夏板塊古縫合帶或西華夏板塊東緣地區(qū), 最早形成于約455～450 Ma, 明顯晚于內(nèi)部帶的片麻狀S型花崗巖, 但與西華夏板塊東緣的片麻狀S型花崗巖相近。西華夏板塊與岡瓦納北緣的微陸塊在早古生代發(fā)生俯沖碰撞, 西華夏板塊東緣之下的板片斷離, 軟流圈上涌減壓熔融, 發(fā)生玄武質(zhì)巖漿底侵, 導(dǎo)致下地殼增厚, 形成基性巖及埃達(dá)克質(zhì)巖石。西華夏板塊東緣俯沖碰撞產(chǎn)生的擠壓應(yīng)力遠(yuǎn)程傳入板塊內(nèi)部, 導(dǎo)致該時(shí)期的板內(nèi)構(gòu)造薄弱帶即東揚(yáng)子?西華夏板塊古縫合帶活化、地殼抬升并增厚, 發(fā)生地殼深熔作用, 形成片麻狀花崗巖。因此, 西華夏板塊東緣的俯沖碰撞遠(yuǎn)程效應(yīng)是華南早古生代陸內(nèi)造山作用的因素。陸內(nèi)造山帶垮塌過(guò)程中, 軟流圈沿東揚(yáng)子?西華夏板塊古縫合帶上涌, 巖石圈活化并發(fā)生強(qiáng)烈的巖漿作用。板內(nèi)古縫合帶作為構(gòu)造薄弱帶, 為軟流圈上涌及玄武質(zhì)巖漿底侵提供了通道。

花崗巖; 基性巖; 陸內(nèi)造山帶; 構(gòu)造薄弱帶; 早古生代; 華南

造山作用是大陸地殼形成演化的重要階段, 造山帶通常分為增生型、碰撞型與陸內(nèi)型三類(lèi)(Cawood et al., 2009)。增生型與碰撞型造山帶發(fā)育在匯聚板塊邊緣, 并伴隨著大規(guī)模的鎂鐵質(zhì)和長(zhǎng)英質(zhì)巖漿作用(Atherton and Petford, 1996; Jahn et al., 2000; Greene et al., 2006; Mo et al., 2008; Jagoutz, 2010; Jagoutz and Kelemen, 2015), 因而被認(rèn)為是大陸地殼生長(zhǎng)與演化的主要場(chǎng)所(Taylor and McLennan, 1985; Arndt and Goldstein, 1989; Kay and Kay, 1993; Rudnick and Gao, 2003; Jagoutz, 2010; Niu et al., 2013; Jagoutz and Kelemen, 2015)。陸內(nèi)造山帶發(fā)育在板塊內(nèi)部, 遠(yuǎn)離板塊邊緣(Raimondo et al., 2014), 其形成過(guò)程一般與遠(yuǎn)程擠壓作用有關(guān)。典型的陸內(nèi)造山帶包括中亞地區(qū)(天山和阿爾泰)新生代造山帶、澳大利亞中部的新元古代Peterman造山帶和早古生代Alice Springs造山帶, 以及澳大利亞南部的新生代Sprigg’s造山帶(Coblentz et al., 1995; Dyksterhuis and Müller, 2008), 它們?nèi)鄙賻r漿作用, 并主要發(fā)育中低級(jí)變質(zhì)作用, 因此一般被認(rèn)為對(duì)大陸地殼生長(zhǎng)與演化的作用較小。

華南地區(qū)在早古生代發(fā)生了一次大規(guī)模的構(gòu)造?巖漿作用事件, 此次事件被認(rèn)為是一次陸內(nèi)造山運(yùn)動(dòng)(Li et al., 1999, 2010; Wang et al., 2011; Charvet, 2013), 并形成了早古生代陸內(nèi)造山帶。該陸內(nèi)造山作用與歐洲加里東造山運(yùn)動(dòng)同期, 因而曾被稱(chēng)為華南加里東造山帶或褶皺帶, 后來(lái)稱(chēng)之為廣西造山運(yùn)動(dòng)(Wang et al., 2011)或武夷?云開(kāi)造山帶(Li et al., 2010); 其影響范圍主要為揚(yáng)子板塊東緣與西華夏板塊, 但其地質(zhì)記錄在武夷和云開(kāi)地區(qū)最為顯著(圖1), 因此本文沿用了武夷?云開(kāi)造山帶這一稱(chēng)謂。有別于典型的陸內(nèi)造山帶, 武夷?云開(kāi)造山帶伴隨有大規(guī)模的長(zhǎng)英質(zhì)巖漿作用和高級(jí)變質(zhì)作用(圖1; 陳斌和莊育勛, 1994; Wang et al., 2007, 2011, 2013b; Li et al., 2010; Chen et al., 2012; Zhang et al., 2012; Huang et al., 2013; Zhong et al., 2013; Yu et al., 2016, 2018b), 因而兼具陸內(nèi)型和碰撞型造山帶的雙重特征。盡管前人對(duì)華南早古生代武夷?云開(kāi)造山帶進(jìn)行了數(shù)十年的研究, 并取得了一系列的重要成果(Li et al., 2010; Wang et al., 2011; Shu et al., 2014), 但對(duì)其兼具陸內(nèi)型和碰撞型造山帶雙重特征的成因機(jī)制仍不清楚。認(rèn)識(shí)武夷?云開(kāi)造山作用的過(guò)程與性質(zhì)對(duì)于理解華南板塊與其他大陸的相互作用乃至東亞地區(qū)的構(gòu)造演化具有重要意義。同時(shí), 系統(tǒng)研究華南地區(qū)早古生代巖漿巖分布特征將有助于理解陸內(nèi)造山作用過(guò)程中的巖漿活動(dòng)、基底性質(zhì)及熱?化學(xué)條件的時(shí)空耦合關(guān)系(Li et al., 2010; Wang et al., 2013b)。因此, 本文廣泛收集近年來(lái)發(fā)表的有關(guān)華南地區(qū)早古生代巖漿巖的年代學(xué)(表1)和巖石地球化學(xué)資料, 以系統(tǒng)分析該時(shí)期巖漿活動(dòng)的時(shí)空分布規(guī)律, 揭示其巖石成因機(jī)制及演化特征, 從而探討華南早古生代造山帶形成演化的構(gòu)造控制因素。

(a) 華南板塊構(gòu)造框架, 包括揚(yáng)子板塊和華夏地塊(據(jù)Li et al., 2010); (b) 華南板塊東部早古生代花崗巖分布圖。華南早古生代巖漿巖年齡數(shù)據(jù)見(jiàn)表1。

1 華南早古生代造山帶及其巖漿活動(dòng)類(lèi)型

華南板塊由東南部的華夏板塊與西北部的揚(yáng)子板塊在新元古代沿江南造山帶碰撞拼貼而成。前人研究通常將呈NE-NEE走向展布的紹興?江山?萍鄉(xiāng)斷裂帶作為揚(yáng)子板塊與華夏板塊新元古代的構(gòu)造縫合帶(圖1; Wang et al., 2003a; Yu et al., 2016), 對(duì)于該縫合帶的南延段目前并沒(méi)有統(tǒng)一認(rèn)識(shí)(Zhao and Cawood, 2012; Yu et al., 2016)。但是, 地球物理研究認(rèn)為郴州?臨武斷裂帶也是揚(yáng)子與華夏板塊構(gòu)造縫合帶的一部分(Zhang et al., 2013), 從郴州?臨武斷裂帶向西南延伸則進(jìn)入云開(kāi)地區(qū)早古生代造山帶核部(圖1)。華夏板塊則大致以政和?大浦?jǐn)嗔褞榻? 分為具有不同前寒武紀(jì)基底的西華夏板塊和東華夏板塊(圖1; Xu et al., 2007; Yu et al., 2009, 2010; Lin et al., 2018)。

新元古代?早古生代早期, 華南板塊受地幔柱構(gòu)造?巖漿作用以及造山帶垮塌而形成陸內(nèi)裂谷, 即南華裂谷(Li et al., 1999; Zheng et al., 2008; Shu et al., 2011), 裂谷中發(fā)育有10 km厚的沉積物(Wang and Li, 2003), 但迄今為止并未發(fā)現(xiàn)可代表洋殼殘片的蛇綠混雜巖, 推斷當(dāng)時(shí)華南板塊并未發(fā)生完全裂解(舒良樹(shù), 2006; 舒良樹(shù)等, 2008)。因此, 武夷?云開(kāi)早古生代造山運(yùn)動(dòng)是華南板塊自新元古代羅迪尼亞超大陸裂解后首次發(fā)生的廣泛的構(gòu)造?熱事件(Li et al., 1999, 2006, 2010; Wang et al., 2007, 2011; Huang et al., 2013)。此次造山運(yùn)動(dòng)導(dǎo)致華夏板塊顯著抬升, 造成泥盆系與前志留紀(jì)地層之間大規(guī)模的不整合, 產(chǎn)生強(qiáng)烈的長(zhǎng)英質(zhì)巖漿作用(圖1), 并在部分地區(qū)發(fā)育中、高級(jí)變質(zhì)作用(陳斌和莊育勛, 1994; Wang et al., 2007, 2011, 2013b; Li et al., 2010; Chen et al., 2012; Zhang et al., 2012; Huang et al., 2013; Zhong et al., 2013; Yu et al., 2016, 2018b)。對(duì)于武夷?云開(kāi)造山帶的構(gòu)造環(huán)境及造山作用的驅(qū)動(dòng)機(jī)制一直存在爭(zhēng)議(Ren, 1964, 1991; Huang et al., 1980; Guo et al., 1989; Hsü et al., 1990; Hsü, 1994; Li et al., 2010; Wang et al., 2011, 2013a, 2013b)。部分學(xué)者認(rèn)為, 華夏板塊與揚(yáng)子板塊之間的華南殘余洋盆閉合形成了早古生代俯沖?碰撞造山帶(Guo et al., 1989; Hsü et al., 1990; Zhang et al., 2015a; Liu et al., 2018), 或者是與兩板塊間的未知陸塊的俯沖?碰撞形成了造山帶(Zhao and Cawood, 2012; Zhang et al., 2015a; Lin et al., 2018)。但是, 由于該造山帶缺乏早古生代俯沖相關(guān)的蛇綠巖、島弧火山巖及高壓變質(zhì)巖(Li et al., 2010; Wang et al., 2011; Shu et al., 2014), 目前越來(lái)越多的研究認(rèn)為其為發(fā)育于華南板塊內(nèi)部的陸內(nèi)造山帶(Ren, 1991; 舒良樹(shù), 2006; Wang et al., 2007, 2011, 2013b; Li et al., 2010; Shu et al., 2011, 2014; Yao et al., 2012; Huang et al., 2013; Yu et al., 2016, 2018b, 2022), 并推測(cè)導(dǎo)致陸內(nèi)造山作用的驅(qū)動(dòng)力受控于遠(yuǎn)程效應(yīng), 如來(lái)自印支板塊、澳大利亞?印支板塊或者未知大陸的擠壓作用(Wang et al., 2007, 2010; Li et al., 2010; Zhao and Cawood, 2012; Charvet, 2013; Xu and Xu, 2015), 或陸內(nèi)俯沖(Faure et al., 2009; Charvet et al., 2010; Charvet, 2013)。

武夷?云開(kāi)造山帶巖漿巖分布在揚(yáng)子板塊東緣與華夏板塊的政和?大浦?jǐn)嗔岩晕鞯貐^(qū)(即西華夏板塊), 主要包括S型花崗巖和少量的I型花崗巖與鎂鐵質(zhì)巖石(圖1; Wang et al., 2007, 2011, 2013b; Yao et al., 2012; Huang et al., 2013; Zhong et al., 2013; Yu et al., 2016, 2018b), 廣泛分布的早古生代長(zhǎng)英質(zhì)巖漿巖反映了強(qiáng)烈的地殼活化特征。

S型花崗巖主要包括黑云母花崗巖、白云母花崗巖和二云母花崗巖, 按結(jié)構(gòu)可分為塊狀和片麻狀。塊狀S型花崗巖出露面積超過(guò)2×104km2, 呈面狀分布在安化?羅成斷裂帶與政和?大浦?jǐn)嗔褞еg; 而片麻狀S型花崗巖主要出露在造山帶核部, 包括武夷山地區(qū)、云開(kāi)地區(qū)、武功山地區(qū)和萬(wàn)洋山?諸廣山地區(qū)(圖1b), 典型的巖體包括松旺巖體、詩(shī)洞巖體等。

鎂鐵質(zhì)巖石包括了火山巖(玄武巖、安山巖)、侵入巖(輝長(zhǎng)巖、閃長(zhǎng)巖)以及花崗巖中的暗色微粒包體等, 主要沿?fù)P子?華夏板塊邊界和西華夏板塊東緣呈帶狀分布。大部分鎂鐵質(zhì)巖石具有富集大離子親石元素(LILE)和輕稀土元素(LREE), 虧損高場(chǎng)強(qiáng)元素(HFSE)和重稀土元素(HREE), 以及富集的Sr-Nd同位素特征, 指示其為富集巖石圈地幔熔融的產(chǎn)物(Wang et al., 2013c; Yao et al., 2012; Zhang et al., 2015b)。另外, 武夷?云開(kāi)造山帶中還發(fā)育有高M(jìn)gO、Ba、Sr特征的安山巖和閃長(zhǎng)巖(Liu et al., 2018; Yu et al., 2018b, 2021), 與典型的高鎂安山巖類(lèi)似。最近的研究還發(fā)現(xiàn)Nb-Ta-Ti負(fù)異常不明顯、Sr-Nd同位素相對(duì)虧損的輝長(zhǎng)巖, 應(yīng)來(lái)源于虧損的軟流圈地幔(Yu et al., 2022)。

表1 武夷?云開(kāi)造山帶早古生代巖漿巖年齡數(shù)據(jù)

續(xù)表1:

早古生代I型花崗巖富含角閃石, 主要為花崗閃長(zhǎng)巖, 出露于揚(yáng)子?華夏板塊邊界和西華夏板塊東緣, 具有相對(duì)富集的Sr-Nd-Hf同位素特征, 被認(rèn)為是中?下地殼變火成巖基底部分熔融的產(chǎn)物(Wang et al., 2011; Huang et al., 2013; Yu et al., 2016, 2018b)。

2 華南早古生代造山帶巖漿活動(dòng)時(shí)空分布特征

華南早古生代造山作用影響范圍涉及揚(yáng)子板塊東部和華夏板塊的大部分地區(qū), 造山帶的變質(zhì)核心主要分布在武夷、云開(kāi)及武功山等地區(qū), 與片麻狀S型花崗巖的露頭分布基本一致(圖1; Li et al., 2010)?？傮w上, 華夏板塊西部(武夷?云開(kāi)造山帶內(nèi)部帶)的武功山和云開(kāi)地區(qū)的片麻狀S型花崗巖的年齡分布范圍相似, 分別為456～424 Ma和466～413 Ma, 年齡峰值也較為接近, 分別為～445 Ma和～441 Ma(圖2a)。而位于西華夏地塊東緣(武夷?云開(kāi)造山帶東緣)的武夷地塊的片麻狀S型花崗巖的年齡分布總體較晚, 為451～410 Ma, 其年齡峰值為～435 Ma(圖2a)。不同于片麻狀S型花崗巖的年齡分布特征, 武夷?云開(kāi)造山帶內(nèi)部帶塊狀S型花崗巖的形成時(shí)間(445～405 Ma, 峰值430 Ma)卻明顯晚于造山帶東緣的塊狀S型花崗巖(447～422 Ma, 峰值443 Ma; 圖2b)。

武夷?云開(kāi)造山帶內(nèi)部帶和東緣的I型花崗巖形成年齡范圍基本一致, 總體上與塊狀S型花崗巖的年齡分布范圍相似, 但顯示出早、晚兩期的特征(圖2c)。另外, 內(nèi)部帶的I型花崗巖的早期年齡峰值(～440 Ma)接近于片麻狀S型花崗巖, 但明顯早于塊狀S型花崗巖; 晚期年齡峰值(～418 Ma)則明顯晚于塊狀S型花崗巖。相對(duì)而言, 造山帶東緣的I型花崗巖的早期年齡峰值(～441 Ma)略晚于塊狀S型花崗巖, 但早于片麻狀S型花崗巖; 晚期的年齡峰值(～411 Ma)則明顯地晚于兩類(lèi)S型花崗巖。

武夷?云開(kāi)造山帶的基性巖漿活動(dòng)時(shí)間除了個(gè)別分析結(jié)果外(Liu et al., 2018), 總體較為集中(448～420 Ma), 無(wú)論是造山帶內(nèi)部帶還是造山帶東緣, 均接近于I型花崗巖的早期年齡峰值。

總體上, 武夷?云開(kāi)造山帶在內(nèi)部帶和東緣的巖漿活動(dòng)過(guò)程并不一致, 各類(lèi)花崗質(zhì)巖漿活動(dòng)的持續(xù)時(shí)間較長(zhǎng), 存在明顯的時(shí)空重疊, 反映出巖漿活動(dòng)的穿時(shí)性。根據(jù)峰期年齡值, 內(nèi)部帶各類(lèi)巖漿活動(dòng)高峰期的出現(xiàn)順序?yàn)槠闋頢型花崗巖→早期I型花崗巖→基性巖→塊狀S型花崗巖→晚期I型花崗巖; 而東緣各類(lèi)巖漿活動(dòng)高峰期的出現(xiàn)順序?yàn)榛詭r、早期I型花崗巖與塊狀S型花崗巖→片麻狀S型花崗巖→晚期I型花崗巖(圖2)。

3 武夷?云開(kāi)造山帶巖漿巖地球化學(xué)變化特征

華南地區(qū)早古生代武夷?云開(kāi)造山帶內(nèi)部帶及東緣巖漿巖類(lèi)型相同, 均發(fā)育基性巖、片麻狀與塊狀S型花崗巖以及兩期I型花崗巖。內(nèi)部帶與東緣的巖漿巖成分變化較大, 除個(gè)別樣品外, 它們的總堿含量不高, 主要為鈣堿性或高鉀鈣堿性系列(圖3), 但各類(lèi)巖漿巖在地球化學(xué)成分變化特征上有所差異。

武夷?云開(kāi)造山帶內(nèi)部帶和東緣的片麻狀S型花崗巖總體上具有相似的地球化學(xué)組成特征(圖3), 其SiO2、Na2O、K2O含量相近, 稀土元素配分曲線均顯示LREE富集、HREE平坦、Eu負(fù)異常的特征, 微量元素蛛網(wǎng)圖具有明顯的Ba、Sr、Ti負(fù)異常和Pb正異常的特征(圖4); 但其Sr-Nd同位素組成略有差別。內(nèi)部帶片麻狀S型花崗巖Nd()值均為負(fù)值, 并且變化范圍較大; 而東緣片麻狀S型花崗巖Nd()值相對(duì)更低, 變化范圍較小, (87Sr/86Sr)i值變化較大(圖5)。

武夷?云開(kāi)造山帶內(nèi)部帶和東緣塊狀S型花崗巖成分變化范圍略有差別?？傮w上, 內(nèi)部帶塊狀S型花崗巖的K2O含量變化較大(2.08%～6.34%), Na2O含量變化不大(1.84%～4.76%); 而東緣塊狀S型花崗巖則為Na2O含量變化大(0.11%～3.09%), K2O含量變化較小(3.19%～5.32%)。它們的稀土元素配分曲線和微量元素蛛網(wǎng)圖特征相似(圖4), 但相對(duì)而言, 東緣塊狀S型花崗巖具有更強(qiáng)的Sr負(fù)異常、較高Th/U值與較低Nb/Ta值, 且(87Sr/86Sr)i明顯偏高,Nd()值偏低(圖5)。

武夷?云開(kāi)造山帶早期I型花崗巖中, 內(nèi)部帶和東緣樣品成分變化范圍都較大, 但它們的微量元素蛛網(wǎng)圖特征相似, 均顯示出明顯的Ba、Sr、Nb、Ta、Ti負(fù)異常和Pb正異常(圖4); 總體上, 內(nèi)部帶樣品比東緣樣品相對(duì)貧K2O、富Na2O, LREE、HREE分異更明顯, 部分樣品具有較低的HREE和Y含量, 顯示出較高的La/Yb、Sr/Y值(圖6)和相對(duì)高的Nd()值(圖5)。相對(duì)于早期I型花崗巖, 造山帶內(nèi)部帶晚期I型花崗巖SiO2、HREE、Y平均含量較高, La/Yb、Sr/Y值總體較低(圖6a、b), Sr-Nd同位素組成總體較為富集(圖5a); 造山帶東緣晚期I型花崗巖SiO2偏高, 但具有明顯較低的HREE和Y含量、明顯較高的Sr/Y和La/Yb值(圖6c、d)以及相對(duì)更虧損的Sr-Nd同位素組成(圖5b)。

武夷?云開(kāi)造山帶內(nèi)部帶和東緣基性巖成分有較明顯的差別。內(nèi)部帶樣品以富Na2O為主(K2O/Na2O=0.02～2.00), 少量樣品富K2O, 其稀土元素配分曲線總體上較為平坦, LREE略為分異, 而HREE平坦, REE含量總體較低; 微量元素蛛網(wǎng)圖上Nb-Ta-Ti負(fù)異常相對(duì)較弱, 但具有明顯的Pb正異常和富集Rb、Ba、Th、U的特征; Sr-Nd同位素相對(duì)于花崗巖樣品更虧損, 但絕大部分樣品仍顯示為負(fù)Nd()值(圖5a)。相對(duì)而言, 造山帶東緣樣品主要為富K2O(K2O/Na2O=0.60～3.80), 僅少數(shù)樣品相對(duì)富Na2O。其中富K2O樣品顯示出較為分異的稀土元素配分曲線, 總體上類(lèi)似于該地區(qū)S型花崗巖及早期I型花崗巖; 在微量元素蛛網(wǎng)圖上, 也顯示出明顯的Nb-Ta-Ti負(fù)異常(圖4d); 且Sr-Nd同位素相對(duì)富集, 總體上較S型花崗巖和早期I型花崗巖虧損, 但比晚期I型花崗巖富集(圖5b)。富Na2O樣品則具有平坦的稀土元素配分曲線和微量元素蛛網(wǎng)圖, Nb-Ta-Ti負(fù)異常并不明顯(圖4d), 并具有更虧損的Sr-Nd同位素, 均為正Nd()值(圖5b), 應(yīng)為虧損軟流圈地幔熔融的產(chǎn)物(Yu et al., 2021)。

圖2 華南早古生代巖漿巖年齡分布頻譜圖(數(shù)據(jù)來(lái)源同表1)

數(shù)據(jù)來(lái)源: 彭松柏等, 2006; 沈渭洲等, 2008; 張芳榮等, 2009, 2010; Li et al., 2010; Wan et al., 2010; Wang et al., 2011, 2013c; Yao et al., 2012; Zhang et al., 2012, 2015b; Zhong et al., 2013; Huang et al., 2013; Guan et al., 2014; Xia et al., 2014; Xu and Xu, 2015; Peng et al., 2015; Song et al., 2015; Zhao et al., 2015, 2019; Yu et al., 2016, 2018b, 2021, 2022; Liu et al., 2018, 2020, 2021; Xie et al., 2020; Tang et al., 2021; Yang et al., 2021; Kong et al., 2021。

4 華南早古生代板內(nèi)構(gòu)造薄弱帶與巖漿活動(dòng)時(shí)空不一致性

板塊內(nèi)部古老板塊邊界屬于板內(nèi)構(gòu)造薄弱帶, 在遠(yuǎn)程擠壓應(yīng)力的作用下, 通常更易于受到影響而發(fā)生活化。例如, 中亞地區(qū)古生代陸緣增生?碰撞造山帶在新生代受到了印度?歐亞大陸碰撞產(chǎn)生的遠(yuǎn)程擠壓應(yīng)力作用而活化隆升, 形成了天山和阿爾泰等造山帶(Avouac et al., 1993; Yin et al., 1998)。新元古代, 華夏板塊與揚(yáng)子板塊碰撞拼貼形成華南板塊, 兩板塊之間的新元古代構(gòu)造縫合帶從紹興?江山?萍鄉(xiāng)斷裂帶向南延伸至云開(kāi)地塊以西(Wang et al., 2003; Yu et al., 2016); 早古生代該縫合帶已屬于華南板塊內(nèi)部的古縫合帶, 為板內(nèi)構(gòu)造薄弱帶, 在遠(yuǎn)程作用力的影響下將易于活化形成陸內(nèi)造山帶(Li et al., 2010; Yao et al., 2012; Huang et al., 2013; Yu et al., 2016)。江山?紹興斷裂帶附近的陳蔡雜巖體在早古生代經(jīng)歷了順時(shí)針變質(zhì)作用過(guò)程(Li et al., 2010), 亦證明了該時(shí)期板內(nèi)構(gòu)造薄弱帶的地殼活化?？傮w上, 華南早古生代變形主要發(fā)生在華夏板塊的邊緣地區(qū), 如發(fā)育片麻狀S型花崗巖的武夷地塊位于西華夏板塊東緣, 武功地塊和云開(kāi)地塊則位于華夏?揚(yáng)子板塊古縫合帶附近(圖1)。而華夏板塊內(nèi)部的變形較弱, 如粵西南恩平和臺(tái)山地區(qū)早古生代花崗巖體(圖1)的圍巖僅發(fā)生了低綠片巖相變質(zhì)作用(邱元禧, 1992), 表明其地殼并未經(jīng)歷強(qiáng)烈的變形和變質(zhì)作用。因此, 武夷?云開(kāi)造山帶的形成演化過(guò)程可能類(lèi)似于天山和阿爾泰造山帶的新生代隆升, 主要沿板內(nèi)構(gòu)造薄弱帶發(fā)生顯著的地殼變形變質(zhì)作用。

圖4 華南早古生代巖漿巖(平均值)稀土元素配分曲線和微量元素蛛網(wǎng)圖(數(shù)據(jù)來(lái)源同圖3)

數(shù)據(jù)來(lái)源: 沈渭洲等, 2008; 張芳榮等, 2009, 2010; Li et al., 2010; Wang et al., 2011, 2013c; Zhang et al., 2012, 2015b; Zhong et al., 2013; Huang et al., 2013; Guan et al., 2014; Xia et al., 2014; Xu and Xu, 2015; Peng et al., 2015; Zhao et al., 2015; Yu et al., 2016, 2018b, 2021, 2022; Xie et al., 2020; Tang et al., 2021; Yang et al., 2021; Kong et al., 2021。

圖6 華南早古生代巖漿巖La/Yb-Yb與Sr/Y-Y協(xié)變圖(數(shù)據(jù)來(lái)源同圖3)

華南早古生代武夷?云開(kāi)造山帶巖漿活動(dòng)存在明顯的時(shí)空不一致性, 反映了造山作用過(guò)程的時(shí)空差異。Yu et al. (2018b)曾提出華夏板塊西緣地殼活化可能比武夷地塊更早。由于片麻狀S型花崗巖是淺部地殼巖漿活動(dòng)過(guò)程中疊加了構(gòu)造擠壓應(yīng)力的結(jié)果(Rosenberg and Berger, 2001), 因此其活動(dòng)時(shí)間可以代表造山帶淺部地殼經(jīng)受構(gòu)造擠壓作用的時(shí)間?？傮w上, 武夷?云開(kāi)造山帶內(nèi)部帶片麻狀S型花崗巖的年齡峰值(約450～440 Ma)及其巖漿活動(dòng)啟動(dòng)時(shí)間(～470 Ma)都明顯早于東緣(年齡峰值和巖漿啟動(dòng)時(shí)間分別為約440～430 Ma、～450 Ma)(圖2), 表明造山帶內(nèi)部帶地殼活化時(shí)間比造山帶東緣更早(圖7a)。除武夷地塊、云開(kāi)地塊及武功地塊等造山帶變質(zhì)核心區(qū)域外, 揚(yáng)子板塊東緣帽兒山和海洋山巖體中也發(fā)育片麻狀S型花崗巖(圖1), 說(shuō)明揚(yáng)子板塊東緣因靠近揚(yáng)子?華夏板塊古縫合帶, 同樣受到了遠(yuǎn)程擠壓應(yīng)力的影響。因此, 片麻狀S型花崗巖的時(shí)空分布特征表明, 揚(yáng)子?華夏板塊古縫合帶的地殼活化主要發(fā)生于約450～440 Ma, 造山帶東緣的地殼活化則主要發(fā)生于約440～430 Ma(圖2)。另外, 內(nèi)部帶與東緣片麻狀S型花崗巖巖漿活動(dòng)停止時(shí)間相近, 暗示了華南早古生代造山運(yùn)動(dòng)同時(shí)結(jié)束。

5 華南早古生代造山帶構(gòu)造演化框架與驅(qū)動(dòng)機(jī)制

5.1 武夷?云開(kāi)造山帶內(nèi)部帶巖漿?構(gòu)造演化序列

武夷?云開(kāi)造山帶內(nèi)部帶片麻狀S型花崗巖的巖漿活動(dòng)持續(xù)時(shí)間最長(zhǎng)(466～413 Ma), 其啟動(dòng)時(shí)間明顯早于其他類(lèi)型巖漿活動(dòng), 但結(jié)束時(shí)間與其他巖漿活動(dòng)相近(～410 Ma), 表明淺部地殼在造山帶演化過(guò)程中持續(xù)受到了擠壓應(yīng)力作用。內(nèi)部帶早期I型花崗巖與基性巖不但巖漿活動(dòng)開(kāi)始時(shí)間明顯晚于片麻狀S型花崗巖(圖2), 它們的峰期年齡(～440 Ma, ～438 Ma)也相對(duì)略晚于片麻狀S型花崗巖(～443 Ma), 很可能反映了內(nèi)部帶巖石圈整體上從擠壓到拉張的演化過(guò)程。這一過(guò)程造成了武夷?云開(kāi)造山帶內(nèi)部帶巖石圈從根部開(kāi)始垮塌, 從而促進(jìn)軟流圈上涌和強(qiáng)烈的玄武質(zhì)巖漿底侵(Xu et al., 2017; Yu et al., 2018b)。內(nèi)部帶基性巖的峰期年齡與早期I型花崗巖的巖漿活動(dòng)時(shí)間相近, 亦可能反映了兩者的成因聯(lián)系。值得注意的是, 發(fā)育在揚(yáng)子?華夏板塊古縫合帶附近的早古生代I型花崗巖普遍含有鎂鐵質(zhì)微粒包體, 表現(xiàn)出幔源巖漿和殼源巖漿混合的特征(Zhang et al., 2012; Guan et al., 2014; Yu et al., 2016), 典型的研究實(shí)例為武功地塊西部張家坊早古生代I型花崗巖體及其鎂鐵質(zhì)微粒包體(Yu et al., 2016)。張家坊I型花崗巖具有很低的全巖Nd()值(?11～?9)和鋯石Hf()值(?14～?4), 類(lèi)似于華夏板塊的基底組成, 鎂鐵質(zhì)微粒包體也具有明顯的Nb-Ta-Ti負(fù)異常, 其全巖Nd()值(?9～?7)和鋯石Hf()值(?9～?4)較低, 但總體上高于寄主花崗巖(Yu et al., 2016), 反映了地幔來(lái)源熔體與地殼巖漿的混合作用。相對(duì)而言, 華夏板塊內(nèi)部早古生代I型花崗巖形成過(guò)程一般缺少地幔巖漿參與的信息, 典型的研究實(shí)例為粵西南地區(qū)恩平巖體(Yu et al., 2018b)和臺(tái)山巖體(Huang et al., 2013)。恩平花崗巖和臺(tái)山花崗巖均具有較低的全巖Nd()值, 表明其源區(qū)缺少新生物質(zhì)的加入(Huang et al., 2013; Yu et al., 2016); 其中臺(tái)山花崗巖顯示出相對(duì)一致但又很低的Nd()值(?11～?9), 并且明顯低于華南地區(qū)早古生代S型花崗巖及其副片麻巖, 應(yīng)為古老中?下地殼物質(zhì)在較高地溫梯度(>35 °C/km)條件下部分熔融的產(chǎn)物(Huang et al., 2013)。因此, 在華夏板塊內(nèi)部, 陸內(nèi)造山帶垮塌可能主要為地殼活化提供了熱量, 并不存在明顯的幔源物質(zhì)加入(Huang et al., 2013)。總體上, 華南早古生代基性巖石和含鎂鐵質(zhì)微粒包體的I型花崗巖主要沿江山?紹興?萍鄉(xiāng)斷裂帶分布, 表明造山帶內(nèi)部帶垮塌主要發(fā)生在板塊內(nèi)部的構(gòu)造薄弱帶; I型花崗巖與基性巖漿活動(dòng)的峰期年齡記錄了造山帶內(nèi)部帶垮塌過(guò)程的高峰期為～440 Ma。造山帶垮塌過(guò)程伴隨著巖石圈拆沉和玄武質(zhì)巖漿底侵作用(Arndt and Goldstein, 1989; Kay and Kay, 1993; Lustrino, 2005), 位于華南早古生代古板塊邊界的古老基底在造山帶垮塌階段應(yīng)被幔源巖漿廣泛改造(Yu et al., 2016, 2021)。實(shí)際上, 內(nèi)部帶早期I型花崗巖部分樣品具有低HREE、Y含量和高Sr/Y、La/Yb值, 顯示出埃達(dá)克巖的特征(圖6a、b), 應(yīng)為造山帶垮塌階段加厚下地殼(>30 km)部分熔融產(chǎn)物。內(nèi)部帶早期I型花崗巖全巖Nd()值變化較大, 少量樣品Nd()值接近于基性巖(圖5a), 也反映出中?下地殼源區(qū)在造山帶垮塌過(guò)程中已被幔源巖漿所改造。晚期I型花崗巖Sr/Y值總體較低(圖6a、b), 全巖Nd()值較低但變化較大(圖5a), 說(shuō)明其巖漿源區(qū)很可能為造山帶垮塌階段被幔源巖漿廣泛改造過(guò)的中?下地殼。

圖7 華南早古生代巖漿活動(dòng)的構(gòu)造演化模型

武夷?云開(kāi)造山帶內(nèi)部帶塊狀S型花崗巖的巖漿活動(dòng)開(kāi)始時(shí)間與早期I型花崗巖相近, 雖然峰期年齡介于早、晚兩期I型花崗巖之間, 但與I型花崗巖巖漿活動(dòng)總體持續(xù)時(shí)間相似(圖2b、c), 表明內(nèi)部帶局部區(qū)域淺部地殼巖漿活動(dòng)與深部地殼、地幔巖漿活動(dòng)具有成因聯(lián)系。武夷?云開(kāi)造山帶早古生代S型花崗巖可能是低壓沉積巖源區(qū)(即淺部地殼)部分熔融的產(chǎn)物(Wang et al., 2011), 而沉積巖源區(qū)中白云母和黑云母的低壓分解需要較高溫度(一般白云母為～650 ℃, 黑云母為～750 ℃; Le Breton and Thompson, 1988; Patin? Douce and Beard, 1995)。造山帶垮塌過(guò)程中, 軟流圈上涌和玄武質(zhì)巖漿底侵導(dǎo)致淺部地殼也具有較高的地溫梯度, 從而易于部分熔融形成S型花崗巖。因此, 塊狀S型花崗巖亦主要發(fā)育于揚(yáng)子?華夏板塊古縫合帶及其鄰區(qū)(圖1)。另外, 塊狀S型花崗巖的峰期年齡明顯晚于片麻狀S型花崗巖(圖2a、b), 反映了淺部地殼構(gòu)造擠壓應(yīng)力作用影響逐漸變?nèi)醯内厔?shì)。

5.2 武夷?云開(kāi)造山帶東緣巖漿?構(gòu)造演化序列

武夷?云開(kāi)造山帶東緣富鈉基性巖(如橋頭輝長(zhǎng)巖; Yu et al., 2022)具有虧損的Sr-Nd同位素特征(圖5b), 其微量元素蛛網(wǎng)圖顯示Nb-Ta-Ti正異?；蛘哓?fù)異常不明顯(圖4c、d), 表明造山帶東緣演化過(guò)程中存在虧損軟流圈地幔物質(zhì)的參與。造山帶主要有兩種機(jī)制導(dǎo)致軟流圈地幔的上涌: 加厚地殼拆沉(Arndt and Goldstein, 1989; Kay and Kay, 1993; Lustrino, 2005)和俯沖板片的斷離(Davies and von Blanckenburg, 1995; Garzanti et al., 2018)。地殼拆沉需要地殼的顯著增厚, 導(dǎo)致地殼根部形成比地幔物質(zhì)密度更大的榴輝巖(Kay and Kay, 1993)。在造山帶東緣, 與橋頭輝長(zhǎng)巖同期的早期I型花崗巖, 包括新四、墩頭、下灣、大康、桂陽(yáng)、四堡及和平巖體等(Li et al., 2010; Xia et al., 2014; Zhao et al., 2015; Xie et al., 2020), 均具有較高的HREE含量(圖4)以及較低的Sr/Y、La/Yb值(圖6), 說(shuō)明該時(shí)期地殼并沒(méi)有顯著增厚。因此, 武夷?云開(kāi)造山帶東緣軟流圈上涌的主要機(jī)制并非地殼拆沉。在陸?陸碰撞過(guò)程中, 俯沖洋殼與陸殼的連接部位在拉應(yīng)力作用下會(huì)發(fā)生斷裂, 產(chǎn)生板片窗(Davies and von Blanckenburg, 1995; Garzanti et al., 2018)。洋殼的斷裂促進(jìn)軟流圈通過(guò)板片窗口上涌(Mahéo et al., 2002; Chung et al., 2005; Garzanti et al., 2018), 從而將大量的熱量傳遞到巖石圈中, 引起巖石圈地幔和地殼的活化。西華夏地塊與北部岡瓦納地塊之間的小洋盆在早古生代完全俯沖, 之后兩地塊發(fā)生了陸?陸碰撞作用(Lin et al., 2018)。陳蔡雜巖中部分角閃巖(496～436 Ma)具有N-MORB化學(xué)特征, 可能代表了早古生代洋殼殘留(Zhao et al., 2015)。造山帶東緣早古生代I型花崗巖中廣泛發(fā)育鎂鐵質(zhì)微粒包體, 反映了殼源和幔源熔體之間強(qiáng)烈的巖漿混合作用(Li et al., 2010; Xia et al., 2014; Zhao et al., 2015; Xie et al., 2020), 這與碰撞造山帶板塊斷離過(guò)程中強(qiáng)烈的殼幔相互作用相符。早期I型花崗巖形成時(shí)間與基性巖相近, 其全巖Nd()值變化較大, 總體上介于S型花崗巖和基性巖之間, 亦反映與幔源巖漿活動(dòng)的成因聯(lián)系。塊狀S型花崗巖巖漿活動(dòng)時(shí)間也與基性巖及早期I型花崗巖相近(圖2b～d), 其形成很可能與地幔巖漿活動(dòng)導(dǎo)致的淺部地殼地溫梯度升高有關(guān)。

武夷?云開(kāi)造山帶東緣各類(lèi)巖漿活動(dòng)在時(shí)間上并不同步。其中, S型花崗巖巖漿活動(dòng)開(kāi)始和結(jié)束時(shí)間都略早于早期I型花崗巖(圖2)。片麻狀S型花崗巖與塊狀S型花崗巖持續(xù)時(shí)間相似, 但是兩類(lèi)巖漿活動(dòng)強(qiáng)度不一致。塊狀S型花崗巖主峰期年齡在～445 Ma, 次要峰期年齡為～435 Ma; 而片麻狀S型花崗巖主峰期年齡為～435 Ma, 但也出現(xiàn)～450 Ma的次峰期年齡, 顯示出兩類(lèi)S型花崗巖此消彼長(zhǎng)特征。片麻狀S型花崗巖增多反映其巖漿活動(dòng)過(guò)程中淺部地殼的構(gòu)造擠壓應(yīng)力逐漸加強(qiáng), 明顯不同于造山帶內(nèi)部帶。造山帶東緣基性巖漿活動(dòng)年齡較為集中, 峰期年齡介于片麻狀S型花崗巖兩個(gè)峰期年齡之間(圖2), 很可能反映了造山帶東緣在持續(xù)的構(gòu)造擠壓環(huán)境下瞬間松弛。實(shí)際上, 造山過(guò)程中板塊斷離將導(dǎo)致上覆大陸巖石圈的伸展(Davies and von Blanckenburg, 1995; Williams et al., 2004; Garzanti et al., 2018), 同時(shí)產(chǎn)生板片窗促進(jìn)軟流圈物質(zhì)上涌減壓熔融, 并引起巖石圈的活化, 深部地殼變火成巖部分熔融形成I型花崗巖, 而變沉積巖部分熔融則形成S型花崗巖。因此, 造山帶東緣基性巖、早期I型花崗巖以及塊狀和片麻狀S型花崗巖巖漿活動(dòng)時(shí)序與造山帶板塊斷離過(guò)程相符。

另外, 目前在武夷?云開(kāi)造山帶東緣仍未發(fā)現(xiàn)與晚期I型花崗巖同期的基性巖漿活動(dòng), 很可能反映了晚期I型花崗巖具有不同的成因機(jī)制。值得注意的是, 造山帶東緣晚期I型花崗巖具有明顯較低的HREE、Y含量和明顯較高的Sr/Y、La/Yb值(圖6c、d), 顯示出埃達(dá)克巖的地球化學(xué)特征, 并且其SiO2含量相對(duì)較高, 總體富鉀(圖3c), 很可能為加厚下地殼部分熔融的產(chǎn)物; 它們具有較為虧損的Sr-Nd同位素(圖5), 全巖Nd()值介于富鈉基性巖與富鉀基性巖之間, 因此, 造山帶東緣下地殼加厚除了與持續(xù)的構(gòu)造擠壓有關(guān), 還與板片斷離過(guò)程中的幔源巖漿底侵作用相關(guān)。造山帶東緣地殼加厚過(guò)程很可能持續(xù)到華南早古生代造山運(yùn)動(dòng)末期, 由于整體的構(gòu)造松弛, 加厚下地殼發(fā)生部分熔融產(chǎn)生埃達(dá)克巖。因此, 造山帶東緣晚期I型花崗巖出現(xiàn)在S型花崗巖巖漿活動(dòng)已基本停止階段(圖2a～c)。

5.3 華南早古生代陸內(nèi)造山作用驅(qū)動(dòng)機(jī)制

有關(guān)華南地區(qū)武夷?云開(kāi)造山帶的構(gòu)造環(huán)境及其形成動(dòng)力學(xué)機(jī)制長(zhǎng)期以來(lái)一直存在爭(zhēng)議, 近期研究逐漸認(rèn)為其可能代表了陸內(nèi)造山事件(Li et al., 2010; Wang et al., 2011; Shu et al., 2014; Yu et al., 2018b, 2021)。但武夷?云開(kāi)造山帶發(fā)育大規(guī)模巖漿作用和高級(jí)變質(zhì)作用, 具有碰撞型造山帶的特征, 明顯區(qū)別于典型陸內(nèi)造山帶, 現(xiàn)有構(gòu)造演化模型難以合理解釋武夷?云開(kāi)造山帶兼具陸內(nèi)型與碰撞型造山帶雙重特征。

巖漿作用、變質(zhì)作用、古生物學(xué)、沉積物源和古地磁等方面大量證據(jù)表明, 華南板塊在早古生代應(yīng)位于岡瓦納大陸北緣附近, 與澳大利亞和Sibumasu板塊相鄰(圖7a; Cawood et al., 2018; Li et al., 2018; Zhao and Xiao, 2018)。因此, 武夷?云開(kāi)造山帶的隆起可能是早古生代(470～460 Ma)華夏板塊與西澳大利亞?Mawson板塊碰撞造成的(Cawood et al., 2018; Li et al., 2018)。華夏板塊可以分為西、東兩部分, 它們具有不同的前寒武紀(jì)基底(Xu et al., 2007; Yu et al., 2009, 2010; Cawood et al., 2013; Wang et al., 2014; Lin et al., 2018)。目前研究表明, 早古生代武夷?云開(kāi)造山帶在東華夏地塊影響較小(圖1), 而東、西華夏板塊均經(jīng)歷了中生代印支運(yùn)動(dòng), 因此二者的拼貼可能發(fā)生在中生代(Lin et al., 2018)。早古生代, 西華夏板塊與岡瓦納大陸北緣微陸塊發(fā)生俯沖碰撞, 產(chǎn)生的擠壓應(yīng)力傳導(dǎo)至板塊內(nèi)部, 并集中在板內(nèi)構(gòu)造薄弱帶, 導(dǎo)致地殼擠壓加厚及地殼沉積巖的深部熔融, 形成片麻狀花崗巖(圖7a)。武夷?云開(kāi)造山帶東緣新四花崗閃長(zhǎng)巖(441±2 Ma)和橋頭輝長(zhǎng)巖(448±8 Ma)形成于碰撞造山板片的斷離階段(Li et al., 2010; Wang et al., 2011; Yu et al., 2022), 洋殼斷裂造成上覆巖石圈短暫松弛(Davies and von Blanckenburg, 1995; Williams et al., 2004; Garzanti et al., 2018)。軟流圈通過(guò)板片窗上涌, 將大量熱量傳遞到巖石圈中, 從而導(dǎo)致西華夏板塊東緣巖石圈地幔和地殼的活化, 形成塊狀S型花崗巖、I型花崗巖及鎂鐵質(zhì)巖漿巖(圖7b)。另一方面, 西華夏板塊東緣巖石圈的松弛減弱了擠壓力的遠(yuǎn)程效應(yīng), 進(jìn)而誘發(fā)了武夷?云開(kāi)造山帶內(nèi)部的垮塌(圖7b; Yu et al., 2018b)。造山帶內(nèi)部帶由擠壓轉(zhuǎn)為持續(xù)伸展, 造成片麻狀S型花崗巖巖漿作用的減弱(圖2)。軟流圈物質(zhì)沿板內(nèi)構(gòu)造薄弱帶上涌導(dǎo)致了巖石圈地幔和地殼的活化, 形成塊狀S型花崗巖、早期I型花崗巖及鎂鐵質(zhì)巖漿巖(圖7b)。后期I型花崗巖相較于早期I型花崗巖缺少埃達(dá)克質(zhì)巖特征, 表明造山帶內(nèi)部的持續(xù)伸展可能造成了地殼減薄(圖6)。

西華夏板塊東緣巖石圈在經(jīng)歷了板片斷離引起的短暫松弛后, 與岡瓦納大陸北緣微陸塊持續(xù)碰撞造成了地殼被進(jìn)一步擠壓抬升(圖7c), 導(dǎo)致片麻狀S型花崗巖的大規(guī)模發(fā)育, 以及塊狀S型花崗巖、早期I型花崗巖及鎂鐵質(zhì)巖漿巖等巖漿作用的減弱(圖2)。造山帶東緣晚期I型花崗巖具有埃達(dá)克巖的特征(圖6), 反映下地殼發(fā)生顯著加厚, 導(dǎo)致地殼根部可能發(fā)生榴輝巖質(zhì)變質(zhì)作用, 并具有比地幔物質(zhì)更大的密度, 從而發(fā)生地殼拆沉, 引起造山帶垮塌(Kay and Kay, 1993)。武夷?云開(kāi)造山帶東緣垮塌作用可能發(fā)生在425 Ma之后(圖7c)。晚期I型花崗巖巖漿活動(dòng)較弱, 暗示西華夏板塊東緣可能主要與岡瓦納大陸北緣的微陸塊發(fā)生碰撞。造山帶東緣的垮塌使得西華夏板塊整體進(jìn)入伸展?fàn)顟B(tài), 進(jìn)一步促進(jìn)了造山帶內(nèi)部沿構(gòu)造薄弱帶的地殼活化(圖7c)。武夷?云開(kāi)造山帶內(nèi)部帶與東緣的巖漿作用均持續(xù)到～410 Ma(圖2), 表明造山帶垮塌在內(nèi)部與東緣可能在泥盆紀(jì)早期同時(shí)結(jié)束。

綜上所述, 華南早古生代武夷?云開(kāi)造山帶是由板塊邊緣碰撞引起板內(nèi)地殼擠壓抬升形成的陸內(nèi)?碰撞復(fù)合型造山帶。造山帶中花崗巖形成過(guò)程是以古老地殼物質(zhì)的再循環(huán)作用為主, 表明造山作用過(guò)程中并沒(méi)有顯著的地殼增生。先存板塊邊界對(duì)陸內(nèi)造山帶隆升及造山后伸展垮塌并發(fā)生軟流圈上涌和玄武質(zhì)巖漿底侵作用具有顯著的促進(jìn)作用。

6 存在問(wèn)題及未來(lái)研究方向

華南地區(qū)早古生代武夷?云開(kāi)造山運(yùn)動(dòng)作為華南板塊形成后第一次大規(guī)模的構(gòu)造?巖漿作用事件, 奠定了華南大陸地殼物質(zhì)組成與基本構(gòu)造框架。本文研究表明, 武夷?云開(kāi)造山帶中巖漿作用與先存的構(gòu)造薄弱帶再活化具有密切的成因聯(lián)系。片麻狀S型花崗巖、I型花崗巖與基性巖呈帶狀分布, 集中發(fā)育在揚(yáng)子?華夏板塊邊界與西華夏板塊東緣, 分別對(duì)應(yīng)了武夷?云開(kāi)造山帶中陸內(nèi)造山帶部分和碰撞造山帶部分(Yu et al., 2018b, 2022), 對(duì)比分析這兩條構(gòu)造帶上巖漿與變質(zhì)變形作用的差異將為理解武夷?云開(kāi)造山運(yùn)動(dòng)演化機(jī)制提供新的思路和認(rèn)識(shí)。

現(xiàn)有年代學(xué)數(shù)據(jù)表明, 武夷?云開(kāi)造山帶內(nèi)部帶與東緣均發(fā)育兩期I型花崗巖, 其地球化學(xué)特征不同反映了這兩期I型花崗巖源區(qū)組成和熔融過(guò)程的差異性。對(duì)比研究?jī)善贗型花崗巖的巖石學(xué)成因?yàn)榻沂驹焐綆?nèi)部帶和東緣在垮塌過(guò)程中巖石圈地幔與地殼演化機(jī)制提供新的認(rèn)識(shí)。武夷?云開(kāi)造山帶中發(fā)育少量鎂鐵質(zhì)巖石, 現(xiàn)有數(shù)據(jù)表明軟流圈物質(zhì)直接參與了造山帶東緣的碰撞造山過(guò)程, 并為巖石圈活化提供了大量的熱。造山帶內(nèi)部基性巖主要為巖石圈地幔部分熔融的產(chǎn)物(Wang et al., 2013c; Yu et al., 2018b, 2021)。在內(nèi)部帶, 軟流圈物質(zhì)是否直接參與造山帶垮塌過(guò)程或者僅為巖石圈活化提供了熱量仍不清楚。對(duì)造山帶內(nèi)部帶基性巖的研究可以為制約軟流圈在陸內(nèi)造山過(guò)程中的作用提供新的理論依據(jù)。整體而言, 未來(lái)仍需對(duì)武夷?云開(kāi)造山帶中不同類(lèi)型的巖漿巖開(kāi)展更為系統(tǒng)的年代學(xué)與巖石地球化學(xué)研究, 以更詳細(xì)深入地認(rèn)識(shí)華南早古生代造山運(yùn)動(dòng)的構(gòu)造?巖漿作用的時(shí)空演化過(guò)程。

致謝:本文撰寫(xiě)過(guò)程中得到了很多專(zhuān)家的指點(diǎn), 中國(guó)地質(zhì)大學(xué)(武漢)鄭建平教授、云南大學(xué)王選策教授和中國(guó)科學(xué)院廣州地球化學(xué)研究所郭鋒研究員均對(duì)本文初稿提出了寶貴的修改意見(jiàn), 謹(jǐn)致謝忱！

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Geodynamics and Origin of the Early Paleozoic Diachronous Magmatism in South China

HUANG Xiaolong1, 2, 3, YU Yang1, 2, 3

(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. Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou 511458, Guangdong, China)

The early Paleozoic igneous rocks are widely distributed in South China, which record the first extensive tectono-thermal event in the South China Block (SCB) since the breakup of the Neoproterozoic Rodinia supercontinent. These rocks have been considered to be the products of intracontinental orogeny, but the geodynamics and origin of magmatism remain controversial. In this paper, we compiled the published geochronological and geochemical data of the early Paleozoic igneous rocks in the SCB, and discussed the temporal-spatial distribution of magmatism in this orogenic belt. The early Paleozoic igneous rocks in the SCB mainly consist of S-type granites (including gneissic and massive), a small amount of I-type granites, and mafic magmatic rocks (including basalt, gabbro, gabbroic diorite, and mafic micro-granular enclaves). The diachronous S-type granites are widely distributed in the eastern Yangtze Block (EYB) and Western Cathaysia Block (WCB). Gneissic S-type granites in the Yunkai and Wugong areas nearby the ancient suture zone between the EYB and WCB (the inner belt) were formed at470–410 Ma with a peak at ～442 Ma, while those in the Wuyi area in the eastern margin of the WCB were generated in a shorter period (455–415 Ma) with a younger peak (～435 Ma). I-type granites and mafic rocks are mainly distributed along the ancient suture zone between the EYB and WCB or the eastern margin of the WCB. They were initially formed at455–450 Ma, much later than the gneissic S-type granites in the inner belt but mostly coeval with those in the eastern margin of the WCB. The collision between the WCB and a microcontinent in the northern margin of the Gondwana might have occurred in the early Paleozoic. The subducting slab beneath the eastern margin of the WCB might break off when the microcontinent collided with the WCB. This process would result in mafic magmatism in the eastern margin of the WCB because of the decompression melting of the upwelling asthenosphere. The lower crust in the eastern margin of the WCB would have been thickened due to the slab subduction and consequent continental collision and magma underplating. The far-field stress transmission into the continental interiors from active plate margins in the eastern margin of the WCB would reactivate the ancient suture zone between the EYB and WCB, which was a tectonically weak zone within the plate during the early Paleozoic. This resulted in uplift, thickening, and anataxis of the crust at the ancient plate margins, causing the intracontinental orogenesis. During the collapse of the intraplate orogenic belt, the upwelling of asthenosphere along the pre-existing suture zone between the EYB and WCB might trigger the reactivation of lithosphere and intensive magmatism. As a tectonically weak zone, the ancient suture zone within the block may have provided channels for the upwelling of asthenosphere and the emplacement of mantle-derived mafic magmas.

granite; mafic rock; intracontinental orogenic belt; tectonically weak zone; early Paleozoic; South China

2021-12-10;

2022-02-21

國(guó)家自然科學(xué)基金項(xiàng)目(U1701641、41625007、41902056)和南方海洋科學(xué)與工程廣東省實(shí)驗(yàn)室(廣州)人才團(tuán)隊(duì)引進(jìn)重大專(zhuān)項(xiàng)(GML2019ZD0202)聯(lián)合資助。

黃小龍(1972–), 男, 研究員, 博士生導(dǎo)師, 從事巖石地球化學(xué)研究。E-mail: xlhuang@gig.ac.cn

P581; P511.4

A

1001-1552(2022)03-0435-020

10.16539/j.ddgzyckx.2022.03.003