徐 扛, 舒 坦, 孔令竹, 李智勇, 汪 晟, 蔣映德

中國阿爾泰造山帶南緣晚古生代花崗巖脈年代學(xué)特征及構(gòu)造意義

徐 扛1, 2, 舒 坦1, 2, 孔令竹1, 2, 李智勇1, 2, 汪 晟1, 2, 蔣映德1*

(1.中國科學(xué)院 廣州地球化學(xué)研究所, 同位素地球化學(xué)國家重點(diǎn)實(shí)驗(yàn)室, 廣東 廣州 510640; 2.中國科學(xué)院大學(xué), 北京 100049)

中國阿爾泰造山帶南緣晚古生代A型花崗巖體被認(rèn)為是該區(qū)域處于伸展構(gòu)造的重要巖石學(xué)證據(jù), 但是現(xiàn)有工作多集中在利用其地球化學(xué)特征對(duì)大地構(gòu)造背景的追索, 而對(duì)于其構(gòu)造變形特征卻鮮有深入研究。為探索阿爾泰南緣晚古生代花崗巖體產(chǎn)出的大地構(gòu)造背景, 本文對(duì)瑪因鄂博巖體、喀拉蘇巖體及其圍巖開展了詳細(xì)的野外地質(zhì)觀測(cè)和年代學(xué)研究。結(jié)果顯示研究區(qū)普遍經(jīng)歷了一期近S-N向地殼縮短事件(D2), 區(qū)域上形成平行于造山帶走向的緊閉褶皺(F2)和近直立的軸面劈理(S2)?，斠蚨醪r體和喀拉蘇巖體在野外露頭雖沒有展示出明顯的變形特征, 但是從其衍生的花崗巖脈體展示出與D2同構(gòu)造變形特征。對(duì)瑪因鄂博巖體的兩條同構(gòu)造變形花崗巖脈進(jìn)行了獨(dú)居石U-Pb測(cè)年, 獲得其年齡分別為273±3 Ma、266±2 Ma; 對(duì)喀拉蘇巖體的一條同構(gòu)造變形花崗巖脈進(jìn)行了鋯石U-Pb測(cè)年, 獲得其年齡為282±10 Ma, 均代表了同構(gòu)造花崗巖脈的形成時(shí)代。結(jié)合構(gòu)造觀測(cè)和定年結(jié)果, 表明青河地區(qū)和喀拉蘇地區(qū)在二疊紀(jì)早期處于強(qiáng)烈的構(gòu)造擠壓環(huán)境。因此, 中國阿爾泰造山帶南緣在早?中二疊世仍處于擠壓而非伸展的大地構(gòu)造環(huán)境。

中國阿爾泰; 二疊紀(jì); 同構(gòu)造花崗巖脈; U-Pb年代學(xué); 大地構(gòu)造環(huán)境

0 引 言

增生型造山帶是區(qū)別于碰撞型造山帶的另一種造山作用類型, 它記錄了匯聚板塊邊緣長(zhǎng)期且復(fù)雜的地體拼貼過程(?eng?r et al., 1993; Cawood et al., 2009; 肖文交等, 2019)。匯聚于造山帶內(nèi)部的地質(zhì)單元實(shí)為經(jīng)歷成百上千公里運(yùn)移的島弧、海山、洋底高原和大陸碎片等不同地體, 這也是增生型造山帶最顯著的特征之一(Coney et al., 1980; Schermer et al., 1984)。由于缺乏陸?陸碰撞及深俯沖過程的直接證據(jù), 所以揭示地體匯聚或拼貼過程是研究增生型造山帶造山演化中所面臨的重要難題(李繼亮, 2004; 肖文交等, 2008)。其中, 如何準(zhǔn)確限定增生造山作用時(shí)限已成為增生型造山帶研究最棘手難題之一(肖文交等, 2019)。在傳統(tǒng)的造山作用研究中, A型花崗巖的出現(xiàn)多被厘定為造山后(post-orogenic)伸展過程的重要巖石學(xué)記錄(Loiselle and Wones, 1979), 因此其形成時(shí)代被認(rèn)為是厘定造山后時(shí)限關(guān)鍵的參考指標(biāo)之一。特別是, 當(dāng)A型花崗巖侵入拼貼或者在增生地體邊界及相鄰地體之中時(shí), 它們更是被賦予了特殊的構(gòu)造學(xué)意義, 有學(xué)者將其稱為釘合巖體(stitching pluton; Jones et al., 1983)。一般來說, A型花崗巖往往形成直徑數(shù)公里未變形的橢球狀、圓形侵入體, 顯著區(qū)別于同造山期強(qiáng)烈變形的侵入體。近30年以來, 具A型花崗巖特征的釘合巖體在限定造山帶碰撞或增生時(shí)限方面發(fā)揮了重要的作用。例如Dickinson(2008)和Reese et al. (2000)通過對(duì)釘合巖體的研究有效地限定了北美科迪勒拉增生造山事件的時(shí)間上限。韓寶福等(2010)通過對(duì)北疆主要縫合帶之中蛇綠巖及兩側(cè)釘合巖體的系統(tǒng)分析, 對(duì)該區(qū)主要匯聚及增生事件的時(shí)限提出了新的認(rèn)識(shí)。同樣, 在秦嶺?大別造山帶(Wang et al., 2015)及江南造山帶(Wang et al., 2006a)也有類似研究被廣泛報(bào)道, 這為限定區(qū)域性的拼貼、增生和碰撞事件時(shí)間提供了至關(guān)重要的制約。

中亞造山帶(CAOB)涵蓋了西起烏拉爾山, 東至西太平洋, 北起西伯利亞克拉通, 南至華北和塔里木克拉通的廣袤區(qū)域, 是地球上最大的顯生宙增生型造山帶, 同時(shí)也是同時(shí)期全球陸殼生長(zhǎng)量最大的地區(qū)(?eng?r et al., 1993; ?eng?r and Natal’In, 1996; Jahn et al., 2000; Windley et al., 2007)。有關(guān)該造山帶中不同地體之間的匯聚過程及整體造山帶最終匯聚及碰撞的動(dòng)力學(xué)過程, 至今尚存在較大爭(zhēng)議。特別是, 中亞造山帶南緣造山作用晚期不同地體的匯聚方式及時(shí)間等問題更是關(guān)注的焦點(diǎn), 致使中亞造山帶由南天山高壓變質(zhì)帶(Gao et al., 1995; Gao and Klemd, 2003; Klemd et al., 2015)至索倫庫爾蛇綠巖縫合帶(Xiao et al., 2003)自西向東數(shù)千公里區(qū)域成為2013年度國際地球科學(xué)領(lǐng)域研究前沿之首(King and Pendlebury, 2013)。作為中亞造山帶重要組成部分, 新疆北部的額爾齊斯構(gòu)造帶見證了其北部阿爾泰早古生代活動(dòng)陸緣增生體系與南部準(zhǔn)噶爾中?晚古生代島弧體系的匯聚過程(曲國勝和張進(jìn)江, 1991; Li et al., 2017; Jiang et al., 2019), 是中亞造山帶內(nèi)部最受關(guān)注的重要構(gòu)造帶之一。

近年來, 針對(duì)額爾齊斯構(gòu)造帶兩側(cè)的中國阿爾泰及準(zhǔn)噶爾地區(qū)構(gòu)造屬性及巖漿?變質(zhì)作用差異等方面研究已取得了諸多進(jìn)展, 主要觀點(diǎn)有, 中國阿爾泰造山帶為蒙古微大陸活動(dòng)大陸邊緣龐大增生體系的一部分, 為早古生代增生楔(Long et al., 2007; Sun et al., 2008; Xiao et al., 2009; Cai et al., 2011b; Jiang et al., 2017), 且這一巨型增生楔在泥盆紀(jì)經(jīng)歷了強(qiáng)烈的造山作用改造, 形成了大范圍的中泥盆世高溫變質(zhì)作用, 并伴隨混合巖化、廣泛的花崗巖發(fā)育及顯著的褶皺變形(Wei et al., 2007; Jiang et al., 2010, 2016, 2019; Li et al., 2010, 2016; Cai et al., 2011b; Zhang et al., 2015; Liu et al., 2020)。而也有學(xué)者主張準(zhǔn)噶爾地區(qū)主要為一套泥盆紀(jì)?石炭紀(jì)島弧拼貼體系, 并有少量奧陶紀(jì)洋殼殘片(簡(jiǎn)平等, 2003; 肖文交等, 2006; 趙恒樂等, 2012; 劉亞然等, 2016)。張弛和黃萱(1992)依據(jù)殘留洋殼中堆晶巖巖石組合和地球化學(xué)性質(zhì)將西準(zhǔn)噶爾地區(qū)的蛇綠巖分為兩類(唐巴勒亞類和洪古勒楞亞類), 并主張?jiān)绻派鷼埩粞髿ば纬捎跀U(kuò)張洋中脊附近, 而晚古生代蛇綠巖則形成于島弧和弧后盆地環(huán)境中, 代表不成熟的洋殼, 這一時(shí)空分布特征指示準(zhǔn)噶爾地區(qū)由早古生代擴(kuò)張洋中脊環(huán)境到晚古生代陸緣島弧環(huán)境的演化歷程(張弛和黃萱, 1992; Wang et al., 2003)。這一看法也得到了相關(guān)基性和安山質(zhì)島弧巖漿時(shí)空分布的佐證(Wang et al., 2003), 指示了早古生代?晚古生代持續(xù)的俯沖增生過程(Wang et al., 2003; Long et al., 2012)。

近年來, 阿爾泰與準(zhǔn)噶爾的匯聚過程成為學(xué)術(shù)界關(guān)注的焦點(diǎn), 但在兩者匯聚的動(dòng)力學(xué)過程及時(shí)限方面仍未能達(dá)成共識(shí)。一方面, 大量晚石炭世?中二疊世具有A型花崗巖地球化學(xué)特征的巖體在阿爾泰南緣及額爾齊斯帶被識(shí)別出來(王濤等, 2005; Liu et al., 2013, 2018b; Tong et al., 2014b; 董增產(chǎn)等, 2019; Zheng et al., 2019), 這些巖體均呈巖株?duì)町a(chǎn)出, 近圓形, 出露面積小, 無明顯變形特征。一些學(xué)者認(rèn)為這些巖體是阿爾泰與準(zhǔn)噶爾匯聚過程中形成的釘合巖體, 即阿爾泰與準(zhǔn)噶爾兩者之間的拼合發(fā)生在這些巖體侵位之前(Liu et al., 2018a; Zheng et al., 2019)。除此之外, 在阿爾泰南緣還發(fā)育290～270 Ma不連續(xù)帶狀的高溫(超高溫)低壓變質(zhì)作用及混合巖化作用(Wang et al., 2009, 2014; 仝來喜等, 2014; Li et al., 2014; Tong et al., 2014a; Broussolle et al., 2018; Liu et al., 2020), 及相伴生的290～270 Ma小規(guī)模、不連續(xù)的基性巖體或者巖脈(Han et al., 2004; 陳漢林等, 2006; Zhang et al., 2014; Yang et al., 2015; Cai et al., 2016; Broussolle et al., 2018)。這些特征均被認(rèn)為是代表了中國阿爾泰南緣晚古生代一期重要的伸展事件, 因此, 諸多學(xué)者主張這一事件可能反映了阿爾泰與準(zhǔn)噶爾兩者匯聚之后的造山后伸展過程(Li et al., 2014; Tong et al., 2014a; Wang et al., 2014; Cai et al., 2016)。然而, 這一觀點(diǎn)難以與該區(qū)域地質(zhì)特征相協(xié)調(diào)。例如, 在額爾齊斯構(gòu)造帶及阿爾泰南緣均顯示出平行造山帶走向的緊閉褶皺和軸面劈理明顯發(fā)育的二疊紀(jì)構(gòu)造特征, 以及在部分區(qū)域展示出向南逆沖的變形特征(Briggs et al., 2007, 2009; Li et al., 2015, 2016, 2017; 汪晟等, 2018), 而那些被認(rèn)為是造山后伸展產(chǎn)物的高溫低壓變質(zhì)地體也呈構(gòu)造擠出特征(Li et al., 2016; Broussolle et al., 2018; Jiang et al., 2019)。這些證據(jù)無一例外地說明, 阿爾泰南緣與準(zhǔn)噶爾的匯聚過程并非那么單一。因此, 該區(qū)域晚古生代構(gòu)造環(huán)境的研究仍有待深入, 其中, 解譯該地區(qū)晚古生代構(gòu)造變形的應(yīng)力學(xué)狀態(tài)則顯得至關(guān)重要。

本次研究通過詳細(xì)的野外地質(zhì)填圖及構(gòu)造觀測(cè), 限定阿爾泰南緣典型的A型花崗巖巖株(也被認(rèn)為是釘合巖體)及其圍巖的變形特征, 輔以高精度鋯石及獨(dú)居石U-Pb定年方法, 綜合限定該巖體就位時(shí)間?應(yīng)變關(guān)系, 并結(jié)合區(qū)域地質(zhì)資料, 探討阿爾泰?準(zhǔn)噶爾匯聚的動(dòng)力學(xué)演化過程。

1 區(qū)域地質(zhì)背景

中國阿爾泰造山帶位于中國新疆北部, 是阿爾泰造山帶在中國境內(nèi)的延伸部分(圖1)。造山帶北部主要出露晚泥盆世?早石炭世低綠片巖相變火山巖和變沉積巖(莊育勛, 1994; Windley et al., 2002)。造山帶中部主要為奧陶系哈巴河群濁流沉積巖和火山碎屑巖, 總體為綠片巖相到角閃巖相, 局部達(dá)到麻粒巖相(莊育勛, 1994; Wei et al., 2007)。哈巴河群曾被認(rèn)為是被動(dòng)陸緣沉積物, 但是最新研究表明其為中奧陶世活動(dòng)陸緣沉積物(Long et al., 2007, 2008), 代表蒙古巨型增生體系外緣早?中古生代增生楔的重要組成部分(Jiang et al., 2017)。造山帶南部主要由弱變質(zhì)的泥盆紀(jì)片巖和片麻巖組成, 原巖為火山碎屑巖夾少量的濁流沉積巖和硅質(zhì)巖(Windley et al., 2002; Long et al., 2007), 其中火山碎屑巖具島弧相關(guān)的地球化學(xué)特征(Chai et al., 2009)。造山帶南緣則出露強(qiáng)烈變形及變質(zhì)的片麻巖、糜棱巖(Briggs et al., 2007; Zhang et al., 2012; Li et al., 2015, 2016)。傳統(tǒng)觀點(diǎn)認(rèn)為中國阿爾泰造山帶自北向南可以分成數(shù)個(gè)大致平行造山帶走向展布的地體單元(Windley et al., 2002; Cai et al., 2011b)。然而, 最新的研究表明先前所劃分的不同地體只是代表了不同深度造山帶地殼物質(zhì)的剝露, 這種差異性并不足作為區(qū)別不同地體的標(biāo)準(zhǔn)(Broussolle et al., 2019)。

圖1 中國阿爾泰區(qū)域地質(zhì)簡(jiǎn)圖(改自Windley et al., 2002)

巨量的巖漿巖活動(dòng)是中國阿爾泰造山帶的另一顯著特征(Zou et al., 1989)。近年來, 高精度鋯石U-Pb定年研究揭示中國阿爾泰地區(qū)主要有420～ 380 Ma和280～260 Ma兩期強(qiáng)烈的巖漿活動(dòng)(Yuan et al., 2007; Sun et al., 2008; Cai et al., 2011a; 董連慧等, 2012; Tong et al., 2014b; Wang et al., 2017)。早期志留紀(jì)?泥盆紀(jì)花崗巖幾乎遍布整體阿爾泰造山帶, 它們多具有高鉀鈣堿性、準(zhǔn)鋁質(zhì)至弱過鋁質(zhì)的I型花崗巖特點(diǎn)(Yuan et al., 2007; 童英等, 2007; Cai et al., 2011a; Tong et al., 2012; 宋鵬等, 2017)。對(duì)于這期I型花崗巖的成因仍存有爭(zhēng)議, 部分學(xué)者認(rèn)為是島弧巖漿的產(chǎn)物(Wang et al., 2006b; Yuan et al., 2007; Cai et al., 2011a), 而其他學(xué)者則認(rèn)為該區(qū)增生楔雜巖重熔也可以產(chǎn)生在地球化學(xué)性質(zhì)上與之相匹配的巖漿(Jiang et al., 2016; Huang et al., 2020)。晚期二疊紀(jì)花崗巖主要出露在造山帶的南緣, 它們同樣具有高鉀鈣堿性的地球化學(xué)特征, 屬于I型、A型或者I-A過渡型花崗巖(童英等, 2006; 周剛等, 2009; 高福平等, 2010; Tong et al., 2014b; 董增產(chǎn)等, 2019)。相較而言, 早期的花崗巖占據(jù)了中國阿爾泰近一半的地表填圖面積; 而晚期花崗巖則呈圓珠狀產(chǎn)出, 出露面積要比早期花崗巖少一個(gè)數(shù)量級(jí), 且這些巖體變形較弱, 展示出與釘合巖體相似的特征(韓寶福等, 2010), 加之其類似A型花崗巖的地球化學(xué)特征, 因此常被認(rèn)為形成于碰撞造山后的拉張構(gòu)造環(huán)境(孫桂華等, 2009; 王濤等, 2010; 董增產(chǎn)等, 2019)。

伴隨著上述兩期主要巖漿作用, 中國阿爾泰還發(fā)育了同期的變質(zhì)作用?，F(xiàn)有研究表明, 早期變質(zhì)作用為始于與埋深及地殼加厚有關(guān)的早泥盆世巴羅型變質(zhì)作用(Wei et al., 2007; Jiang et al., 2015, 2019), 緊隨其后是中泥盆世與地殼伸展減薄相關(guān)的高溫低壓變質(zhì)作用。變質(zhì)地溫梯度的顯著變化, 暗示著異常高的熱量涌入, 這一現(xiàn)象被解釋為洋中脊俯沖(Sun et al., 2009; Cai et al., 2010, 2011a; Jiang et al., 2010)或者巖石圈地幔減薄(Jiang et al., 2019)導(dǎo)致熱的軟流圈上涌所致。晚期變質(zhì)作用主要是早二疊世高溫低壓變質(zhì)作用, 局部區(qū)域可以達(dá)麻粒巖相; 巖石學(xué)觀察結(jié)果顯示其典型的礦物組合為石榴石+斜方輝石+矽線石+堇青石+尖晶石+黑云母+斜長(zhǎng)石+石英, 相對(duì)應(yīng)的溫壓條件為960 ℃/0.80 GPa(Tong et al., 2014a)。

2 研究區(qū)地質(zhì)特征

本次研究主要對(duì)距青河縣城南東30 km的瑪因鄂博花崗巖巖體、阿勒泰縣城南東20 km處的喀拉蘇花崗巖巖體及其圍巖的變形特征開展了詳細(xì)的野外地質(zhì)觀測(cè)和分析。主要地質(zhì)特征描述如下。

2.1 瑪因鄂博巖體

瑪因鄂博花崗巖體位于青河縣城東南克孜賽村附近(圖1)。該區(qū)域北部主要出露奧陶系哈巴河群, 南部則為泥盆系瑪因鄂博組(圖2)。奧陶系哈巴河群主要由變質(zhì)程度較高的片麻巖組成。早期片麻理S1發(fā)生強(qiáng)烈褶皺變形, 褶皺軸跡往南東方向傾伏, 角度平緩至中等。伴隨早期面理的褶皺變形, 新近直立的透入性面理S2廣泛發(fā)育, 并與第二期褶皺軸面劈理相平行(圖2b、3a); 局部保留的早期面理受到晚期構(gòu)造改造形成水平褶皺(圖3b)。南部泥盆系瑪因鄂博組發(fā)育一組平行于原始沉積層理(S0)的近水平變質(zhì)面理(S1), 該期變質(zhì)面理經(jīng)歷了近S-N向構(gòu)造擠壓, 發(fā)育一系列樞紐近水平、走向NW-SE的直立褶皺(圖3c), 局部發(fā)育近直立的軸面劈理(圖3d)。即, 受到晚期(D2)擠壓作用的影響, 早期S1面理分別在強(qiáng)、弱變形域發(fā)育緊閉和寬緩褶皺F2。

花崗巖在該區(qū)域廣泛出露, 呈圓柱狀產(chǎn)出, 形成時(shí)代為305～260 Ma(周剛等, 2007; Song et al., 2019; 圖4a)。地球化學(xué)研究表明, 花崗巖為低硅、高鉀、高鈉, 普遍具A型花崗巖屬性, 屬于典型的后碰撞花崗巖(童英等, 2006; 周剛等, 2009; 董增產(chǎn)等, 2019)。野外露頭上, 這些花崗巖均未表現(xiàn)出明顯變形特征(圖4b), 但從這些巖體中衍生出眾多長(zhǎng)達(dá)數(shù)百米至上千米的花崗巖脈體(圖4c)切穿圍巖中主要面理, 并受擠壓作用發(fā)生了顯著的褶皺變形; 顯微巖相觀測(cè)發(fā)現(xiàn)脈體中的石英具顯著波狀消光特征(圖4d、e)。通過構(gòu)造恢復(fù)可知, 這些巖脈在變形之前均為近S-N向展布, 與區(qū)域性的晚期面理近乎呈正交關(guān)系, 暗示了它們就位于與晚期近S-N向擠壓相關(guān)的張性裂隙之中, 隨后在晚期變形中進(jìn)一步褶皺彎曲。

圖2 青河地區(qū)地質(zhì)簡(jiǎn)圖以及花崗巖脈變形特征(改自新疆維吾爾自治區(qū)地質(zhì)礦產(chǎn)局, 1993)

2.2 喀拉蘇巖體

喀拉蘇花崗巖體位于阿勒泰市東南喀拉蘇村一帶(圖1)。該區(qū)域北部為哈巴河群變沉積地層, 并發(fā)育有大量泥盆紀(jì)花崗巖, 其形成年齡在410～396 Ma之間(Yang et al., 2011; Broussolle et al., 2018)。該區(qū)域南部主要出露早泥盆世島弧火山沉積以及中泥盆世的海相碎屑沉積巖序列, 局部夾有哈巴河群高級(jí)變質(zhì)巖石(Cai et al., 2011a; Broussolle et al., 2018)(圖5)。

泥盆紀(jì)地層變質(zhì)程度從低綠片巖相至麻粒巖相不等。在麻粒巖相變質(zhì)巖中, 既有年代約為 390 Ma變質(zhì)鋯石(Long et al., 2007; Jiang et al., 2010), 也有292～271 Ma變質(zhì)鋯石報(bào)道(Wang et al., 2009; Tong et al., 2014a), 指示該區(qū)可能經(jīng)歷了多期變質(zhì)事件的疊加(Broussolle et al., 2018)。相應(yīng)的, 構(gòu)造變形也展示出多期次的特征。野外可見普遍發(fā)育的早期高角度變質(zhì)面理(S1), 且多平行于成分層理(S1//S0)。前人研究表明, 早期S1變質(zhì)面理包含典型的巴羅型變質(zhì)礦物, 可能記錄了該區(qū)早期地殼不斷加厚的過程(Broussolle et al., 2018), 且這期變質(zhì)面理受晚期近S-N向D2構(gòu)造擠壓不均勻改造, 形成強(qiáng)弱變形域。因此, 在強(qiáng)變形域內(nèi)發(fā)育緊閉褶皺及近E-W走向的新一期近直立面理(S2; 圖6a、b), 而在弱變形域則以寬緩褶皺為特征, 且保留了早期面理的痕跡(圖6c、d)。

(a) 奧陶紀(jì)片麻巖中透入性的S2面理; (b) 奧陶紀(jì)片麻巖中保存的早期面理(S1)發(fā)生褶皺; (c) 泥盆紀(jì)低級(jí)變質(zhì)巖中D2弱變形域發(fā)育樞紐近水平的F2寬緩褶皺(近水平的S1面理在F2褶皺翼部被旋轉(zhuǎn)到近直立位置, 并與區(qū)域S2面理近平行); (d) 泥盆紀(jì)地層中D2強(qiáng)變形域發(fā)育的近直立的NW-SE走向的S2面理。

早古生代花崗巖體經(jīng)歷了強(qiáng)烈的剪切變形而形成花崗質(zhì)糜棱巖, 而晚古生代花崗巖則很少變形。晚古生代喀拉蘇花崗巖體主要就位于南部泥盆紀(jì)變質(zhì)地層之中, 巖體呈近圓珠形產(chǎn)出, 直徑大約4 km (圖7a), 且在露頭尺度上, 基本沒有展示出任何變形特征(圖 7b)。相反, 從巖體中所衍生出的花崗巖脈體則切穿了圍巖中主要的面理, 并與之成正交關(guān)系, 隨后發(fā)生褶皺變形(圖 7c), 應(yīng)變特征與圍巖中晚期變形特征相一致。這些變形特征也說明了花崗巖脈很可能是在第二期變形(D2)的某個(gè)階段侵位于平行擠壓方向的張性裂隙之中, 并進(jìn)一步經(jīng)受了晚期構(gòu)造變形的改造, 即, 這些花崗巖脈為同構(gòu)造擠壓背景下的產(chǎn)物。

3 分析測(cè)試方法

為了限定第二期變形(D2)的時(shí)代, 我們采集變形花崗巖巖脈樣品進(jìn)行鋯石和獨(dú)居石U-Pb同位素年代學(xué)分析。樣品經(jīng)擊碎、磨細(xì)、過篩后, 以重液集中重礦物等一系列工序后, 分選出獨(dú)居石和鋯石。然后將獨(dú)居石和鋯石制成環(huán)氧樹脂靶, 并進(jìn)行打磨和拋光, 其后進(jìn)行反射光、透射光、背散射(BSE)和陰極發(fā)光(CL)拍照。最后, 根據(jù)顯微照片、背散射以及陰極發(fā)光圖像選擇適合U-Pb定年位置。

(a) 瑪因鄂博巖體以及樣品位置(衛(wèi)星圖); (b) 顯微鏡下, 巖體中礦物顆粒未變形(正交偏光, ×5物鏡); (c) 花崗巖脈體發(fā)生褶皺變形(衛(wèi)星圖); (d) 花崗巖脈體發(fā)生褶皺變形; (e) 顯微鏡下, 花崗巖脈體中石英因發(fā)生變形而產(chǎn)生波狀消光(單偏光, ×5物鏡)。礦物代號(hào): Q. 石英; Pl. 斜長(zhǎng)石; Ser. 絹云母; Bi. 黑云母。

獨(dú)居石和鋯石U-Pb同位素定年在武漢上譜分析科技有限責(zé)任公司利用LA-ICP-MS分析完成。該套實(shí)驗(yàn)裝置配置有GeoLas 2005激光剝蝕系統(tǒng)(LA)并與Agilent 7500a質(zhì)譜儀(ICP-MS)聯(lián)用。獨(dú)居石剝蝕實(shí)驗(yàn)選取的激光束斑大小均為16 μm; 鋯石剝蝕實(shí)驗(yàn)選取的激光束斑大小均為32 μm。分析過程中, 鋯石樣品靶每個(gè)樣點(diǎn)的背景采集時(shí)間為20 s, 信號(hào)采集時(shí)間為65 s。每測(cè)定5次未知樣品點(diǎn), 測(cè)定2 次鋯石標(biāo)樣91500, 用來校正U-Pb、Th-Pb 同位素分餾和儀器質(zhì)量歧視。獨(dú)居石樣品測(cè)試流程與之相同, 但是采用標(biāo)樣為獨(dú)居石44069(Aleinikoff et al., 2006)。原始數(shù)據(jù)處理采用ICPMSDataCal 9.0軟件(Liu et al., 2009), 年齡諧和圖的繪制采用ISOPLOT 3.2(Ludwig, 2001)軟件。測(cè)試結(jié)果見表1。

4 結(jié)果和討論

4.1 測(cè)試結(jié)果

4.1.1 瑪因鄂博巖體變形花崗巖脈

進(jìn)行獨(dú)居石定年的樣品18QH93和19CA08分別采自瑪因鄂博巖體中所衍生出的兩條變形花崗巖脈體(采樣位置見圖4)。樣品18QH93中的獨(dú)居石呈灰棕色, 透明, 自形粒狀, 粒度在60～120 μm之間, 背散射BSE圖像顯示出環(huán)帶特征(圖 8a)。該樣品中20個(gè)獨(dú)居石顆粒U-Pb定年得到206Pb/238U年齡范圍在291～265 Ma之間, 加權(quán)平均年齡為 273±3 Ma (MSWD=2.0; 圖 8a)。

樣品19CA08中的獨(dú)居石顆粒略小, 直徑在30～60 μm之間。其形態(tài)特征與樣品18QH93中獨(dú)居石顆粒相似, 背散射圖像同樣呈現(xiàn)出環(huán)帶特征, 個(gè)別顆粒包含有較多的包裹體(圖8b)。該樣品中包裹體較少的獨(dú)居石定年中19個(gè)有效點(diǎn)給出的206Pb/238U 年齡為274～256 Ma, 加權(quán)平均年齡為266±2 Ma(MSWD=3.4; 圖 8b)。

4.1.2 喀拉蘇巖體變形花崗巖脈

樣品19CA62采自喀拉蘇巖體中的脈體, 該樣品中鋯石大多是透明的, 自形粒狀, 大小大多在50～100 μm之間, 長(zhǎng)寬比為1∶1～2∶1。陰極發(fā)光圖像顯示這些鋯石具有典型的巖漿鋯石的環(huán)帶特征(圖8c)。該樣品獲得的16個(gè)有效數(shù)據(jù)點(diǎn)中, 給出了早中泥盆世的年齡(419～377 Ma), 加權(quán)平均值為397±7 Ma(MSWD=7.8; 圖8c), 可能代表了源區(qū)繼承鋯石的年齡; 另5顆鋯石給出了相近的晚古生代的年齡(288～261 Ma), 加權(quán)平均值為282±10 Ma (MSWD=6.2; 圖8c), 代表了巖體的結(jié)晶年齡; 與Linet al., (2019)研究結(jié)果一致。綜上, 喀拉蘇變形脈體的形成時(shí)代可以限定為～282 Ma。

4.2 討 論

由于花崗巖記錄了大陸形成演化的各種信息而成為研究大陸形成演化的重要載體(翟明國等, 2016), 在多年研究的基礎(chǔ)上, 發(fā)展形成的花崗巖大地構(gòu)造更是將花崗巖形成和侵位的物理特性、花崗巖體變形改造、花崗巖物源、巨型花崗巖帶發(fā)育等與大地構(gòu)造演化過程之間的相互耦合關(guān)系作為研究的主要內(nèi)容(王濤等, 2017)。在巖體、變形和大地構(gòu)造演化相關(guān)聯(lián)的重要的研究實(shí)例中, 釘合巖體曾被用來限定縫合帶最終形成時(shí)代(韓寶福等, 2010)。

圖5 喀拉蘇地區(qū)地質(zhì)簡(jiǎn)圖以及花崗巖脈變形特征(改自新疆維吾爾自治區(qū)地質(zhì)礦產(chǎn)局, 1993)

(a) 在強(qiáng)變形區(qū)域發(fā)育的透入性S2高角度面理; (b) 區(qū)域性NW-SE走向的面理(S2)投影; (c) 泥盆紀(jì)低級(jí)變質(zhì)巖中弱變形域保留早期面理S1(S1//S0); (d) 奧陶紀(jì)片麻巖中高傾角的S1面理發(fā)生寬緩褶皺。

在中國阿爾泰造山帶南緣, 發(fā)育了單個(gè)面積較小、數(shù)量眾多、具有一定規(guī)律排列的圓形/卵形花崗巖株。這些巖體普遍具有高SiO2、Na2O+K2O、Fe/Mg、Ga/Al、Zr、Nb、Ga、Y、Ce含量以及低的CaO、Sr含量, 與典型意義上A型花崗巖(Collins et al., 1982; Whalen et al., 1987)類似。雖然此類巖石多分布于阿爾泰南緣的額爾齊斯構(gòu)造帶, 但是在與之彼鄰的準(zhǔn)噶爾地體北側(cè)也偶有出現(xiàn), 因此, 它們多被認(rèn)為是阿爾泰與準(zhǔn)噶爾拼貼后侵位的釘合巖體(Liu et al., 2018a; Zheng et al., 2019)?；贏型花崗巖的地球化學(xué)特征, 學(xué)者們主張其產(chǎn)出于碰撞后板內(nèi)伸展環(huán)境(王濤等, 2005; 童英等, 2006; 孫桂華等, 2009; Liu et al., 2018b; 董增產(chǎn)等, 2019)。然而, 這種依靠花崗巖地球化學(xué)特性來限定大地構(gòu)造背景的方法仍面臨挑戰(zhàn)。例如, 在阿爾泰南緣同時(shí)期侵入巖類卻又展示出與俯沖帶島弧相關(guān)的地球化學(xué)特征(Wan et al., 2013), 暗示大洋俯沖仍在繼續(xù)。

研究區(qū)內(nèi), 兩個(gè)不同巖體的圍巖都經(jīng)歷了強(qiáng)烈變形, 并伴隨形成了一期NWW-SEE走向?yàn)橹鞯慕绷⑤S面劈理S2, 指示晚期D2變形主要受控于SSW-NNE向構(gòu)造擠壓作用。雖然巖體本身沒有展示出明顯的變形特征, 但是由其衍生出來的巖脈卻經(jīng)歷了一定程度的褶皺變形, 且變形特征與圍巖中晚期變形特征相一致, 揭示了其亦可能受到SSW- NNE方向的構(gòu)造擠壓。此外, 根據(jù)這些巖脈的變形特征, 不難得出其就位時(shí)近正交切穿區(qū)域中晚期NWW-SEE走向面理, 其延伸方向幾乎平行于晚期擠壓時(shí)的最大應(yīng)力方向, 所以這些巖脈很可能是以被動(dòng)就位方式侵入于該期變形形成的張性裂隙之中。也就是, 這些巖脈不僅經(jīng)歷了晚期變形的疊加, 且與該期擠壓構(gòu)造同期就位。而其所對(duì)應(yīng)的巖體沒有明顯的變形特征, 有可能暗示這些巖體就位深度非常淺(相應(yīng)的變形溫度比較低), 其就位機(jī)制類似于受浮力驅(qū)動(dòng)的主動(dòng)就位方式(Hut D和劉永江, 1991), 以至于在同等應(yīng)力下, 它們與巖脈展示出完全不同的應(yīng)變特征。因此, 這些圓形、卵形巖體可能是浮力驅(qū)動(dòng)下的主動(dòng)就位, 而脈體則是在區(qū)域性擠壓作用與巖體主動(dòng)就位過程中產(chǎn)生裂隙時(shí)侵入的, 屬于被動(dòng)就位方式。綜上所述, 這些巖脈的年齡(282～ 266 Ma)可以用來制約晚期構(gòu)造擠壓的時(shí)代, 即, 上述兩個(gè)研究區(qū)在此期間能很可能仍受近S-N向構(gòu)造擠壓作用。

(a) 喀拉蘇巖體以及樣品位置(衛(wèi)星圖); (b) 喀拉蘇花崗巖體未發(fā)生變形; (c) 花崗巖脈體發(fā)生顯著褶皺變形。

表1 獨(dú)居石和鋯石的LA-ICP-MS年齡數(shù)據(jù)

續(xù)表1:

續(xù)表1:

圖8 獨(dú)居石和鋯石U-Pb諧和曲線圖及背散射(BSE)和陰極發(fā)光(CL)照片

縱觀中國阿爾泰南緣, 與擠壓構(gòu)造相伴生的緊閉褶皺及南向逆沖等特征沿造山帶走向方向廣泛發(fā)育(圖9)。而那些被普遍認(rèn)為是碰撞后伸展的高溫變質(zhì)體, 亦展示出受近S-N向構(gòu)造擠壓而發(fā)生強(qiáng)烈構(gòu)造擠出的特征, 在局部地區(qū)甚至形成約280 Ma的混合巖?花崗巖穹窿(Broussolle et al., 2018)。近期的研究表明, 這期構(gòu)造變形與阿爾泰?準(zhǔn)噶爾近S-N向匯聚密切相關(guān)(Li et al., 2017; Jiang et al., 2019)。因此, 這些地質(zhì)事實(shí)不約而同地指示中國阿爾泰造山帶南部在早?中二疊世仍處于強(qiáng)烈的構(gòu)造擠壓環(huán)境, 也暗示著阿爾泰與準(zhǔn)噶爾的匯聚過程至少在該時(shí)期仍未演化到碰撞后伸展的階段。

圖9 中國阿爾泰南緣變形特征(圖例見圖1)

5 結(jié) 論

(1) 晚古生代中國阿爾泰造山帶南緣青河地區(qū)和喀拉蘇地區(qū)普遍經(jīng)歷了顯著的近S-N向構(gòu)造擠壓作用, 進(jìn)而形成平行于造山帶走向的緊閉褶皺和近直立軸面劈理。

(2) 獨(dú)居石和鋯石U-Pb年代學(xué)結(jié)果顯示瑪因鄂博和喀拉蘇釘合巖體的同構(gòu)造變形脈體形成時(shí)代為282～266 Ma。

(3) 中國阿爾泰造山帶南緣早?中二疊世仍處于擠壓構(gòu)造環(huán)境, 暗示著中國阿爾泰與其南緣的準(zhǔn)噶爾地體匯聚過程仍在繼續(xù)。

致謝:在稿件評(píng)審過程中, 中國科學(xué)院新疆生態(tài)與地理研究所肖文交院士和中山大學(xué)張健教授對(duì)稿件進(jìn)行了仔細(xì)的評(píng)審, 并且提出了詳細(xì)的修改意見和建議, 為文章的高質(zhì)量發(fā)表提供了極大的幫助, 在此表示衷心的感謝！同時(shí)也感謝李志泰、王連峰和寧俊在文章寫作過程中提出的意見和幫助！

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Late Paleozoic Granite Veins in the Southern Chinese Altai: Structural Characteristics, Geochronology and Tectonic Implications

XU Kang1, 2, SHU Tan1, 2, KONG Lingzhu1, 2, LI Zhiyong1, 2, WANG Sheng1, 2and JIANG Yingde1*

(1. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China)

TheLate Paleozoic A-type granites in the southern Chinese Altai have been traditionally taken as an indicator for a post-orogenic extensional environment after the amalgamation between the Chinese Altai and the southern Junggar arc domain. However, this conception was established merely based on the geochemical characteristics of the granites, and deformational patterns of these granites have rarely been investigated by using modern petro-structural methods. Moreover, this notion is at odds with the notion that the southern Chinese Altai is featured by contractional structures. In order to clarifying the regional tectonic setting during the emplacement of the late Paleozoic granites in the southern Chinese Altai, deformational characteristics as well as zircon and monazite U-Pb geochronology of the granite veins in the Qinghe (Mayinebo pluton) and Kalasu (Kalasu pluton) areas were investigated. Field observations indicate that these two areas are characterized by a major NNE-SSE-direction compression that resulted in the formation of tight folds and penetrative NWW-SEE foliation S2. Oval-shape granite intrusions in these regions lack apparent deformation. In contrast, abundant granite veins evolved from these intrusions are orthogonal to the regional main foliation S2and followed by their tight folding, indicating their syntectonic emplacement. Monazite U-Pb dating yielded 273±3 Ma and 266±2 Ma for two deformed granite veins from the Mayinebo area. Zircon U-Pb dating yielded 282±10 Ma for a deformed granite vein from the Kalasu area. Collectively, these features suggest that these Early to Middle Permian granite veins were emplaced under a contraction environment. Combined with the available regional data, we suggest that the southern Chinese Altai was under a compressional tectonic setting which was probably responsible for the on-going amalgamation between the Chinese Altai and the Junggar arc domain during Early-to-Middle Permian.

Chinese Altai; Permian; syn-tectonic granite; U-Pb geochronology; tectonic setting

2020-01-17;

2020-03-20;

2020-12-09

國家自然科學(xué)基金項(xiàng)目(41672056)和中國科學(xué)院國際伙伴計(jì)劃項(xiàng)目(132744KYSB20190039)聯(lián)合資助。

徐扛(1991–), 男, 博士研究生, 構(gòu)造地質(zhì)學(xué)專業(yè)。Email: xukang@gig.ac.cn

蔣映德(1982–), 男, 研究員, 主要從事造山帶構(gòu)造變形與變質(zhì)演化研究。Email: jiangyd@gig.ac.cn

P597; P542

A

1001-1552(2021)03-0444-019

10.16539/j.ddgzyckx.2020.05.018