李建華,張?jiān)罉?,徐先兵,李海龍,董樹文,李廷?/p>
1.中國(guó)地質(zhì)科學(xué)院地質(zhì)力學(xué)研究所,北京 100081 2.中國(guó)地質(zhì)大學(xué)地球科學(xué)學(xué)院, 武漢 430074 3.中國(guó)地質(zhì)科學(xué)院,北京 100037
湖南白馬山龍?zhí)冻瑔卧?、瓦屋塘花崗巖鋯石SHRIMP U-Pb年齡及其地質(zhì)意義
李建華1,張?jiān)罉?,徐先兵2,李海龍1,董樹文3,李廷棟3
1.中國(guó)地質(zhì)科學(xué)院地質(zhì)力學(xué)研究所,北京 100081 2.中國(guó)地質(zhì)大學(xué)地球科學(xué)學(xué)院, 武漢 430074 3.中國(guó)地質(zhì)科學(xué)院,北京 100037
運(yùn)用陰極發(fā)光技術(shù),對(duì)湖南白馬山龍?zhí)冻瑔卧?個(gè)樣品和瓦屋塘花崗巖1個(gè)樣品的鋯石進(jìn)行了內(nèi)部結(jié)構(gòu)分析,在此基礎(chǔ)上利用鋯石SHRIMP U-Pb定年方法進(jìn)行了同位素年代學(xué)測(cè)定。其中,白馬山龍?zhí)冻瑔卧?個(gè)黑云母二長(zhǎng)花崗巖樣品分別給出了(215.9±1.9) Ma和(212.2±2.1) Ma的主體諧和年齡。同時(shí)還測(cè)得了一組較年輕的諧和年齡((201.0±2.8) Ma)和一組較老的鋯石核部年齡(230.3~227.0 Ma),表明研究區(qū)印支晚期存在多期花崗質(zhì)巖漿的侵入活動(dòng)。瓦屋塘巖體黑云母二長(zhǎng)花崗巖1個(gè)樣品給出了(217.7±1.8) Ma的諧和年齡。這2個(gè)巖體的形成進(jìn)一步佐證了華南大陸印支晚期巖漿活動(dòng)于210~225 Ma,達(dá)到巖漿活動(dòng)的峰期。地球化學(xué)測(cè)試結(jié)果顯示,白馬山和瓦屋塘巖體均為弱過鋁-強(qiáng)過鋁質(zhì)花崗巖,具殼源型花崗巖的特征,形成于后碰撞期或碰撞晚期的構(gòu)造環(huán)境,源于早元古代變質(zhì)雜砂巖的部分熔融。結(jié)合區(qū)域大地構(gòu)造背景認(rèn)為,這2個(gè)巖體形成于秦嶺-大別和松馬2條印支期縫合帶碰撞結(jié)束后的印支晚期伸展構(gòu)造背景下,為熱-應(yīng)力松弛階段,板內(nèi)擠壓加厚的地殼減壓熔融作用的產(chǎn)物。
白馬山;瓦屋塘;鋯石SHRIMP U-Pb定年;印支期花崗巖;地殼伸展;湖南
華南陸塊是由揚(yáng)子地塊和華夏地塊拼合形成的大陸板塊,夾于秦嶺-大別-蘇魯碰撞造山帶和紅河斷裂帶之間,東南緣瀕臨西太平洋板塊,位于特提斯構(gòu)造域和濱太平洋構(gòu)造域的中間部位[1]。中生代以來,受這兩大構(gòu)造域構(gòu)造活動(dòng)的影響,華南陸塊地質(zhì)和大地構(gòu)造演化極其復(fù)雜,特別是早中生代印支期構(gòu)造變形-巖漿與沉積作用響應(yīng)在全球中生代構(gòu)造中獨(dú)具特色[2]。印支運(yùn)動(dòng)的提出,始于20世紀(jì)初[3],原指發(fā)生于越南三疊紀(jì)Pre-Norian和Pre-Rhaetian間的構(gòu)造不整合事件,并認(rèn)為與印支和華南陸塊之間的大陸碰撞有關(guān)[4-5]。這期強(qiáng)烈的構(gòu)造事件引發(fā)了華南陸塊廣泛的陸內(nèi)變形及地殼增厚,同時(shí)誘發(fā)了大規(guī)模的巖漿活動(dòng)及伴生的成礦作用。這些獨(dú)具特色的地質(zhì)現(xiàn)象吸引了國(guó)內(nèi)外地質(zhì)學(xué)者的廣泛關(guān)注,而其大地構(gòu)造過程及其發(fā)生的深部動(dòng)力學(xué)背景則成為爭(zhēng)議的焦點(diǎn)問題之一。一些學(xué)者認(rèn)為華南大陸是印支期陸內(nèi)碰撞帶,并提出華南大陸三疊紀(jì)阿爾卑斯造山模式[6-7],但研究證實(shí)華南大陸內(nèi)部并不存在早中生代洋盆或洋陸俯沖事件[8-9]。其他學(xué)者則試圖利用西太平洋板塊的俯沖作用來解釋華南大陸印支期構(gòu)造-巖漿-沉積響應(yīng)[10-11];但印支期花崗巖分布較分散,整體呈面狀分布,且缺乏共生的火山巖的特征,暗示其成因應(yīng)與俯沖消減作用無直接關(guān)聯(lián)[12]。深入研究華南大陸內(nèi)部印支期巖漿作用,對(duì)于理解華南大陸中生代構(gòu)造演化具有重要意義[13]。然而,由于印支期花崗巖分布較局限且成礦作用較弱,相對(duì)于燕山期花崗巖而言,其研究程度較為薄弱[14],已取得的認(rèn)識(shí)尚存在爭(zhēng)議:王岳軍等[15]構(gòu)建的地質(zhì)-物理模型認(rèn)為印支期花崗巖為陸殼疊置加厚作用的產(chǎn)物;而周新民等[12]認(rèn)為印支早期花崗巖與陸塊碰撞引發(fā)的地殼疊置熔融作用有關(guān),印支晚期花崗巖則與碰撞峰期后的應(yīng)力松弛階段的減壓熔融有關(guān),這一觀點(diǎn)也在南嶺地區(qū)得到了進(jìn)一步證實(shí)[16]。
白馬山和瓦屋塘巖體位于江南隆起南緣,為具典型代表性的印支期花崗巖,是研究華南大陸印支期巖漿作用的關(guān)鍵地區(qū)。其中,白馬山巖體為多期巖漿侵入形成的復(fù)式巖體[17],目前已獲得的花崗巖及包體LA-ICP-MS鋯石U-Pb年齡集中在以下區(qū)間:243 Ma,221~227 Ma,204~209 Ma,203~205 Ma,177 Ma[13, 18-19]。關(guān)于瓦屋塘巖體的巖石學(xué)特征及形成年代,目前暫無報(bào)道。筆者選擇這2個(gè)巖體開展了精細(xì)的鋯石U-Pb同位素年代學(xué)研究,厘定了巖體形成的精確年代,并結(jié)合已發(fā)表的全巖主、微量元素和Sr-Nd同位素地球化學(xué)資料,討論了其成因的深部動(dòng)力學(xué)背景,為重建華南陸塊印支期花崗巖的時(shí)序格架及早中生代大地構(gòu)造演化過程提供年代學(xué)依據(jù)。
圖1 華南陸塊印支期花崗巖分布圖Fig. 1 Distribution of Indosinian granites in the South China
印支期花崗巖呈面狀廣泛展布于華南大陸廣大地區(qū),夾于政和-大浦?jǐn)嗔褞Ш途缚h-溆浦?jǐn)嗔褞еg(圖1),出露面積約3 260 km2。地球化學(xué)和同位素年代學(xué)資料證實(shí)[12],印支期花崗巖具有早、晚兩期,無論從巖性,還是從形成的大地構(gòu)造背景來看,二者均存在明顯的差異。筆者對(duì)近年來報(bào)道的華南陸塊印支期巖體的鋯石U-Pb年齡進(jìn)行了詳細(xì)統(tǒng)計(jì)(表1),結(jié)果(圖1)顯示,這2類花崗巖可以共生,也可獨(dú)立產(chǎn)出,它們呈面狀廣泛分布在湘、桂、粵、贛、瓊等省區(qū),如:江西大富足[20]、大吉山[21-22]、龍?jiān)磯蝃23]、富城[24]、隘高[11]、三標(biāo)[11]和柯樹嶺[25]花崗巖;湖南歇馬[26]、溈山[13-14, 26]、錫田[27]、關(guān)帝廟[13]、白馬山[11, 13, 18-19]、陽(yáng)明山[11, 28]、淋陽(yáng)花崗巖和道縣輝長(zhǎng)巖包體[2];廣東那蓬[26]、下莊[16, 29]、西淋[30]、共和[30]、長(zhǎng)沙[16]、河臺(tái)[31]、貴東和孟冬花崗巖[11];廣西大容山、浦北和舊州花崗巖[32];福建洋紡、鐵山[33]和小陶花崗巖[34]。同時(shí),筆者繪制了華南大陸印支期巖體鋯石U-Pb年齡分布直方圖(圖1角圖)。從直方圖上可見,早期花崗巖形成于230~259 Ma,主要為強(qiáng)過鋁質(zhì)淺色花崗巖,含白云母、石榴石、電氣石等高鋁礦物,屬S型花崗巖,以發(fā)育片麻理為主要特征,顯微鏡下可見明顯的擠壓變形構(gòu)造,其形成與華南大陸陸緣俯沖/碰撞造山作用引發(fā)的陸內(nèi)地殼物質(zhì)疊置加厚作用有關(guān)[12-13, 35]。晚期花崗巖形成于200~230 Ma,約占印支期花崗巖的90%[35-36],巖體主要為弱過鋁或準(zhǔn)鋁花崗巖,包含泥質(zhì)、玄武質(zhì)巖石以及明顯的幔源巖漿組分,以中粒結(jié)構(gòu)、塊狀構(gòu)造為特征,其形成與碰撞造山結(jié)束后地殼伸展-減薄背景下引發(fā)的減壓熔融作用有關(guān)[12, 14, 18, 28]。
白馬山和瓦屋塘花崗巖體位于雪峰山東緣的湘西南地區(qū)(圖2)。巖體周緣出露地層主要為新元古界-中生界。新元古界主要為板溪群,由一套層次分明的雜砂巖-板巖-片巖組成,沉積時(shí)代為760~820 Ma[37-38]。古生界主要為淺海相碳酸鹽巖,包括震旦系白云巖和冰磧巖,寒武系板狀頁(yè)巖、砂巖和灰?guī)r,奧陶系灰?guī)r,志留系砂頁(yè)巖,泥盆系-石炭系-二疊系灰?guī)r和白云巖。中生界主要由淺海相碳酸鹽巖和陸相碎屑巖組成,包括中-下三疊統(tǒng)灰?guī)r,上三疊統(tǒng)-侏羅系砂巖和礫巖,白堊系紅色砂礫巖和泥巖[17]。靖縣-溆浦?jǐn)嗔押统遣?安化斷裂為研究區(qū)內(nèi)重要的區(qū)域性一級(jí)斷裂,具有多期復(fù)雜的活動(dòng)歷史[39-41]。中三疊世-早侏羅世,這2條斷裂均以左行走滑剪切變形為主,40Ar/39Ar定年結(jié)果證實(shí)剪切變形發(fā)生于244~195 Ma[10]。早白堊世,斷裂活動(dòng)以伸展作用為主,它們共同控制溆浦地塹白堊紀(jì)的沉積及斷陷作用。另外,城步-安化斷裂還表現(xiàn)出明顯的右行走滑特征,其切割白馬山和瓦屋塘巖體,并導(dǎo)致巖體發(fā)生右行錯(cuò)斷(圖2),這期活動(dòng)可能與新生代印度-歐亞板塊碰撞作用有關(guān)[42]。
表1 華南大陸印支期花崗巖鋯石U-Pb年齡統(tǒng)計(jì)
表1(續(xù))
圖2 白馬山和瓦屋塘巖體構(gòu)造綱要圖與采樣位置Fig.2 Simplified structural map of the Baimashan and Wawutang granites and sample locations
白馬山巖體位于湘西溆浦縣南約50 km,呈E-W向串珠狀展布,由黑云母二長(zhǎng)花崗巖和花崗閃長(zhǎng)巖組成,侵位于新元古代-早古生代地層中(圖2)。結(jié)合巖石的礦物組合特征及年代學(xué)資料,其可分為水車、龍?zhí)丁⑿∩辰妄埐貫?個(gè)超單元[43]。其中:龍?zhí)?、小沙江超單元形成于印支期,其長(zhǎng)軸沿近E-W復(fù)背斜褶皺軸展布,證實(shí)其形成受到早期褶皺構(gòu)造的控制(圖2);水車、龍藏灣超單元?jiǎng)t分別形成于海西-加里東期、燕山期。本次研究的采樣位置主要位于白馬山巖體西緣的龍?zhí)冻瑔卧?Ym132,Ym134),樣品巖性為黑云母二長(zhǎng)花崗巖,由鉀長(zhǎng)石(20%~24%),石英(25%~28%),黑云母(9%~10%),斜長(zhǎng)石(An=30~42,37%~50%)等礦物組成。
瓦屋塘巖體位于綏寧縣境內(nèi),由細(xì)-粗粒黑云母二長(zhǎng)花崗巖和角閃石黑云母二長(zhǎng)花崗巖構(gòu)成。巖體侵入早古生代到泥盆紀(jì)地層中,下白堊統(tǒng)紅色碎屑巖覆蓋其上(圖2)。重力異常測(cè)量資料顯示該巖體在深部可能與北側(cè)的崇陽(yáng)坪巖體相連[17]。巖石具細(xì)-中粒結(jié)構(gòu),部分具似斑狀結(jié)構(gòu),斑晶主要為鉀長(zhǎng)石、斜長(zhǎng)石,長(zhǎng)徑達(dá)3~5 cm。巖石局部發(fā)生弱蝕變作用。本次研究的樣品采于巖體東緣(Ym135),樣品巖性為黑云母二長(zhǎng)花崗巖,主要由鉀長(zhǎng)石(23%~29%),斜長(zhǎng)石(An=7~37,<25%),石英(26%~ 31%),黑云母(3%~19%),電氣石(1%~10%)等礦物組成。
樣品經(jīng)人工破碎后,按照常規(guī)重力和磁選分選出鋯石,在雙目鏡下挑選出晶形和透明度較好的鋯石顆粒,將其和標(biāo)準(zhǔn)鋯石TEM(年齡417 Ma)在玻璃板上用環(huán)氧樹脂固定、拋光,然后進(jìn)行反射光、投射光照相和陰極發(fā)光掃描電鏡圖像分析。鋯石的陰極發(fā)光圖像在中國(guó)地質(zhì)科學(xué)院礦產(chǎn)資源研究所電子探針室完成。鋯石SHRIMP U-Pb分析在中國(guó)地質(zhì)科學(xué)院地質(zhì)研究所北京離子探針中心的SHRIMP-Ⅱ上完成,采用標(biāo)準(zhǔn)流程進(jìn)行測(cè)試,詳細(xì)的實(shí)驗(yàn)流程和原理見參考文獻(xiàn)[44-45]。分析時(shí)一次離子為4.5 nA,10 kV的O-2,束斑25~30 μm。數(shù)據(jù)處理采用Ludwig的SQUID1.02及ISOPLOT程序[46]。普通鉛根據(jù)實(shí)驗(yàn)測(cè)的204Pb進(jìn)行校正,同位素比值誤差為1σ。年齡計(jì)算采用IUGS(1997)推薦值。
具有復(fù)雜地質(zhì)演化的地區(qū), 巖石中鋯石往往具有復(fù)雜的內(nèi)部結(jié)構(gòu)特征[47],記載著復(fù)雜的演化歷史[48]。因此,對(duì)鋯石內(nèi)部結(jié)構(gòu)分析是合理解釋所測(cè)年齡的依據(jù)。目前, 陰極發(fā)光(CL)圖像是揭示鋯石內(nèi)部結(jié)構(gòu)的有效手段[49]。由于235U和238U的半衰期及豐度存在差異,導(dǎo)致在放射性成因組分積累較少的年輕鋯石中,放射性成因207Pb的豐度比放射性成因206Pb的豐度約低一個(gè)數(shù)量級(jí)[50],對(duì)于年齡較輕(<1 Ga)的鋯石,采用206Pb/238U年齡更加準(zhǔn)確[51-52]。本文所測(cè)定的花崗巖主要形成于中生代,因此,采用206Pb/238U年齡進(jìn)行加權(quán)平均值計(jì)算。
樣品Ym132中的鋯石多為淺黃色或無色,透明-半透明,柱狀或長(zhǎng)柱狀,晶體自形程度較好。顆粒較大,長(zhǎng)度多為100~300 μm,寬為80~150 μm,長(zhǎng)寬比為1.5∶1~4.5∶1。陰極發(fā)光電子(CL)圖像顯示,大部分鋯石為無核具韻律震蕩環(huán)帶結(jié)構(gòu)(圖3a),反映被測(cè)鋯石為典型的巖漿結(jié)晶鋯石[18],且沒有發(fā)生顯著的Pb丟失[53]。少數(shù)鋯石(如測(cè)點(diǎn)2.1,3.1)具有弱核-邊結(jié)構(gòu),核部呈現(xiàn)明顯的港灣狀溶蝕結(jié)構(gòu)(圖3a),顏色較深,顯示弱CL特征,其可能是來自圍巖的捕虜晶或原巖中的殘留鋯石,邊部具清晰的韻律環(huán)帶結(jié)構(gòu),揭示巖漿成因特征。選擇具韻律環(huán)帶的巖漿鋯石,進(jìn)行了14個(gè)點(diǎn)的定年分析,這些點(diǎn)w(Th)為(327~1 084)×10-6,w(U)為(633~1 742)×10-6,Th/U值為0.41~0.71(表2)。大部分?jǐn)?shù)據(jù)點(diǎn)都位于諧和線上或附近(圖3b),獲得的諧和年齡值集中在210~223 Ma,對(duì)其進(jìn)行加權(quán)平均值計(jì)算,獲得加權(quán)平均年齡為(215.9±1.9) Ma(MSWD=1.4),屬于印支晚期,代表了巖體的形成年齡。
樣品Ym134中的鋯石多為淺黃色或無色,透明-半透明,自形或半自形長(zhǎng)柱狀。顆粒長(zhǎng)度多為100~250 μm,寬為50~150 μm,長(zhǎng)寬比為2∶1~4.5∶1。陰極發(fā)光電子(CL)圖像(圖3c)顯示,該樣品的鋯石顯示3種不同的結(jié)構(gòu)特征:無核結(jié)構(gòu),核-邊雙層結(jié)構(gòu)和核-幔-邊3層結(jié)構(gòu)。前2類鋯石邊部均顯示強(qiáng)烈振蕩韻律環(huán)帶的結(jié)構(gòu)面貌,揭示典型巖漿成因鋯石特征。具核-幔-邊結(jié)構(gòu)鋯石以被測(cè)點(diǎn)13.1最為典型,鋯石邊部表現(xiàn)出類似巖漿鋯石的CL特征,幔部CL較弱,顏色較深,核部具明顯的港灣狀溶蝕結(jié)構(gòu),并顯示出強(qiáng)CL特征,可能為捕虜晶鋯石在巖漿作用時(shí)進(jìn)一步生長(zhǎng),并在以后的地質(zhì)作用中發(fā)生重結(jié)晶作用[13, 54],這與后面該鋯石有較大的年齡結(jié)果一致。幔部弱CL帶代表了重結(jié)晶作用的前鋒帶[49]。對(duì)這個(gè)樣品進(jìn)行了16個(gè)點(diǎn)的定年分析,這些點(diǎn)的w(Th)為(72~1 348)×10-6,w(U)為(200~5 948)×10-6,Th/U值為0.12~1.12(表2)。除2個(gè)樣品點(diǎn)(Ym134-10.1, 13.1)表現(xiàn)為801.0 Ma,471.6 Ma外,大部分?jǐn)?shù)據(jù)點(diǎn)都位于諧和線上或附近(圖3d),獲得的諧和年齡值集中在197.6~230.3 Ma。對(duì)照鋯石的CL圖像,樣品點(diǎn)Ym134-10.1,13.1均剛好位于捕虜鋯石的核部,801.0 Ma和471.6 Ma反映的是捕虜鋯石的年齡。其余14點(diǎn)的諧和年齡主要集中在3個(gè)時(shí)間段:測(cè)自具核-邊雙層結(jié)構(gòu)鋯石核部的2個(gè)年齡值最老,分別為230.3 Ma和227.0 Ma;具核-邊雙層結(jié)構(gòu)鋯石邊部及無核結(jié)構(gòu)的鋯石年齡明顯集中在197.6~203.8 Ma和208.5~218.3 Ma 2個(gè)區(qū)間,加權(quán)平均年齡分別為(201.0±2.8) Ma(n=4, MSWD=0.96)和(212.2±2.1) Ma(n=8, MSWD=0.45),均屬印支晚期。從圖3c來看,這2類鋯石具有類似的CL圖像特征而難以區(qū)分,但這2組年齡應(yīng)該也不屬于同一巖漿過程不同結(jié)晶階段的產(chǎn)物,因?yàn)閹r漿的冷凝凝固過程時(shí)間一般小于1.0 Ma[25]。這2類鋯石可能分別代表了印支晚期不同幕次的巖漿活動(dòng)。
圖3 白馬山和瓦屋塘花崗巖被測(cè)鋯石陰極發(fā)光(CL)圖像和鋯石SHRIMP U-Pb年齡諧和圖Fig.3 CL diagrams and zircon U-Pb Concordia diagrams of the Baimashan and Wawutang granites
樣品Ym135中的鋯石多為淺黃色或無色,透明-半透明,自形或半自形長(zhǎng)柱狀。顆粒長(zhǎng)度多為100~450 μm,寬為80~150 μm,長(zhǎng)寬比為1.5∶1~6∶1。陰極發(fā)光電子(CL)圖像顯示,該樣品鋯石主要為無核具強(qiáng)烈韻律震蕩環(huán)帶結(jié)構(gòu)(圖3e),反映被測(cè)鋯石為典型的巖漿結(jié)晶鋯石。選擇具韻律環(huán)帶的巖漿鋯石,進(jìn)行了14個(gè)點(diǎn)的定年分析,這些點(diǎn)的w(Th)為(99~512)×10-6,w(U)為(631~4 191)×10-6,Th/U值為0.07~0.26,獲得的諧和年齡值集中在210.4~222.4 Ma(表2)。12個(gè)數(shù)據(jù)點(diǎn)都位于諧和線上或附近(圖3f),2個(gè)數(shù)據(jù)點(diǎn)(6.1和11.1)水平地偏離諧和曲線,但其分布形式明顯不同于Pb丟失引起的不諧和,且相關(guān)的鋯石CL圖像也顯示清晰的韻律振蕩環(huán)帶結(jié)構(gòu)(圖3e),表明鋯石并沒有發(fā)生明顯的Pb丟失。這樣數(shù)據(jù)的分布可能與207Pb的測(cè)定有關(guān)。幸運(yùn)的是,207Pb的測(cè)定結(jié)果并不影響206Pb/238U的值[14]。對(duì)14個(gè)數(shù)據(jù)點(diǎn)進(jìn)行加權(quán)平均值計(jì)算,獲得加權(quán)平均年齡為 (217.7±1.8) Ma(MSWD=1.05),屬于印支晚期,代表了巖體的形成年齡。
關(guān)于白馬山和瓦屋塘巖體的巖石地球化學(xué)特征,前人已做了詳細(xì)的研究,積累了一批高質(zhì)量的測(cè)試結(jié)果[13, 17-18]。綜合前人測(cè)試結(jié)果[13, 17-18],這2個(gè)巖體花崗巖的主量元素質(zhì)量分?jǐn)?shù)如下:w(SiO2)=66.43%~71.65%,w(Al2O3)=13.87%~16.18%,w(Na2O+K2O)=5.92%~6.99%,w(P2O5)=0.08%~0.17%,w(TiO2)=0.25%~0.58%,w(MgO)=0.64%~2.12%,w(TFeO)=1.92%~3.89%,w(CaO)=1.22%~3.66%。鋁飽和指數(shù)A/CNK值為0.99~1.53,CIPW標(biāo)準(zhǔn)礦物C(剛玉)質(zhì)量分?jǐn)?shù)為0.21%~6.11%,顯示了弱過鋁-強(qiáng)過鋁質(zhì)花崗巖的特征。在NK/A(堿質(zhì)指數(shù))-A/CNK圖解(圖4a)上,數(shù)據(jù)點(diǎn)投影在亞堿過鋁質(zhì)花崗巖區(qū);在ACF圖解(圖4b)上,數(shù)據(jù)點(diǎn)位于黑云母-斜長(zhǎng)石-堇青石組合內(nèi),與殼源型過鋁質(zhì)花崗巖類似[55]。在微量元素組成上,這2個(gè)巖體的花崗巖顯示富Rb,K,U,Th,Pb,Zr和Hf,而貧Sr,Ba,Nb,P的特征(圖5a),為低Ba-Sr花崗巖,與南嶺東段強(qiáng)過鋁質(zhì)花崗巖相似[16]。Nb/Ta值為4.82~9.31,明顯低于地殼平均值(12.22)[28]。低Nb/Ta值為地殼部分熔融作用的結(jié)果[56],暗示花崗巖具殼源型巖漿的特征。稀土元素配分模式(圖5b)上,w(∑REE)為(111.59~197.96)×10-6,負(fù)Eu異常顯著(δEu=0.45~0.74),輕稀土富集(La/Yb)N=13.26~33.97,與殼源花崗巖的稀土元素特征一致[57-58]。全巖Sr-Nd同位素測(cè)試結(jié)果顯示[18],巖體的(87Sr/86Sr)i值為0.718 306~0.727 348,εNd(t)值為-11.44~-10.73,與南嶺東段強(qiáng)過鋁質(zhì)花崗巖類似(εNd(t)=-11.1~-10.6)[16],這些測(cè)試結(jié)果進(jìn)一步佐證了殼源的特征[24]。Nd模式年齡T2DM為1.9~2.0 Ga,與華夏地塊古老的沉積變質(zhì)基底的Nd模式年齡一致(1.8~2.2 Ga)[59],暗示這2個(gè)巖體起源于華夏地塊早元古代地殼物質(zhì)的部分熔融。
5.1 白馬山和瓦屋塘花崗巖形成年代及地質(zhì)意義
白馬山巖體4個(gè)超單元分別侵位于元古界-下古生界不同地層中。以往的研究主要采用K-Ar、鋯石U-Pb等測(cè)年方法對(duì)龍?zhí)?、小沙江超單元的形成時(shí)代進(jìn)行了測(cè)定,獲得的年齡主要集中在243~221 Ma,209~203 Ma,177 Ma等年齡區(qū)間[11, 13, 17, 19, 28, 40]。筆者采用高精度SHRIMP 鋯石U-Pb方法對(duì)龍?zhí)冻瑔卧M(jìn)行了重新定年,所獲得的巖體主體年齡為(215.9±1.9) Ma和(212.2±2.1) Ma,與前人獲得的結(jié)晶鋯石年齡(216.2±1.6) Ma (ICP-MS鋯石U-Pb法,羅志高等[19]; (217±2) Ma,SHRIMP鋯石U-Pb法,Li等[11])基本吻合。具核-邊雙層結(jié)構(gòu)鋯石核部的年齡為230.3~227.0 Ma,與陳衛(wèi)鋒等[28]獲得的白馬山黑云母二長(zhǎng)花崗巖及包體的核部年齡226.5~221.4 Ma基本一致;同時(shí),筆者還獲得了(201.0±2.8) Ma的巖漿鋯石年齡,與陳衛(wèi)鋒等[28]獲得的白馬山花崗閃長(zhǎng)巖及暗色包體的年齡209.2~203.2 Ma在誤差范圍內(nèi)一致。綜合前人[11, 13, 17, 19, 28, 40]及本次測(cè)試結(jié)果可知,白馬山巖體印支晚期存在多期次幕式花崗質(zhì)巖漿的侵入活動(dòng)。瓦屋塘巖體形成年齡為(217.7±1.8) Ma,與白馬山巖體龍?zhí)冻瑔卧黧w年齡基本一致,進(jìn)一步反映了約215 Ma巖漿活動(dòng)在華南大陸的廣泛性。從形成年代來看,白馬山和瓦屋塘巖體均為典型的印支晚期花崗巖。該期花崗巖在華南大陸呈面狀廣泛展布,如江西大富足巖體(217~226 Ma)[20],龍?jiān)磯螏r體(211 Ma)[22],湖南歇馬巖體(214 Ma)[26],溈山巖體(210~218 Ma)[14, 26],陽(yáng)明山巖體(217~219 Ma)[28],廣東下莊、長(zhǎng)沙巖體(225~228 Ma)[16],福建小陶巖體(222 Ma)[34]等,它們的形成證實(shí)華南大陸印支晚期巖漿活動(dòng)于210~225 Ma,達(dá)到巖漿活動(dòng)的峰期[14]。
5.2 白馬山和瓦屋塘花崗巖的源區(qū)性質(zhì)及地球動(dòng)力學(xué)背景探討
現(xiàn)有的主、微量、稀土和Sr-Nd同位素等地球化學(xué)資料為探討白馬山和瓦屋塘花崗巖的源區(qū)性質(zhì)提供了有力約束[13, 17-18]。前已述及,微量元素和Sr-Nd同位素特征顯示這2個(gè)花崗巖體具殼源花崗巖的特征,表明其源巖為富鋁的地殼。據(jù)陳衛(wèi)峰等[18]地球化學(xué)測(cè)試結(jié)果,這2個(gè)花崗巖體的CaO/Na2O值(質(zhì)量分?jǐn)?shù)之比)為0.38~1.24,與砂屑質(zhì)巖的值接近(CaO/Na2O>0.3)[60],暗示它們的源巖為成熟度較低的砂屑質(zhì)巖。結(jié)合Al2O3/(MgO+TFeO)-CaO/(MgO+TFeO)圖解及Nd模式年齡(1.9~2.0 Ga)[18],可推測(cè)它們主要源于早元古代變質(zhì)雜砂巖的部分熔融。另外,在花崗巖構(gòu)造環(huán)境判別圖解中,這2個(gè)花崗巖體的投影點(diǎn)大多聚集在碰撞晚期或后碰撞花崗巖區(qū)[18],反映它們?yōu)榈湫偷呐鲎餐砥诨蚝笈鲎不◢弾r。關(guān)于后碰撞花崗巖成因的地球動(dòng)力學(xué)背景的判別,還應(yīng)綜合考慮華南大陸中生代大地構(gòu)造過程及印支運(yùn)動(dòng)的影響。
圖4 白馬山和瓦屋塘花崗巖的主要元素圖解Fig.4 Diagrams showing the major elements of the Baimashan and Wawutang granites
圖5 白馬山花崗巖微量元素(a)和稀土元素(b)配分圖解Fig. 5 Trace element spider diagram (a) and chondrite-normalized REE patterns(b) for the Baimashan granite
區(qū)域構(gòu)造位置上,華南大陸地處秦嶺-大別和松馬2條印支期縫合帶之間,印支運(yùn)動(dòng)對(duì)其中生代大地構(gòu)造演化影響深刻。研究表明,華南大陸印支運(yùn)動(dòng)形成的褶皺構(gòu)造線方向?yàn)榻黈-E向[1, 61-62],指示印支期主擠壓應(yīng)力方向?yàn)榻媳毕颉=Y(jié)合區(qū)域大地構(gòu)造背景分析,筆者認(rèn)為華南大陸印支期構(gòu)造變形的動(dòng)力源于南北陸緣強(qiáng)烈的俯沖/碰撞造山作用,這一觀點(diǎn)可從近幾年的研究成果得到證實(shí)。華南大陸北緣揚(yáng)子陸塊與秦嶺-大別-蘇魯造山帶于印支早期(240~220 Ma)發(fā)生深俯沖/碰撞作用,導(dǎo)致大別-蘇魯超高壓變質(zhì)帶形成[63-65]。南緣印支陸塊于258~243 Ma在越南北部與華南陸塊發(fā)生拼合碰撞,這一過程在越南北部Song Chay Massif、Day Nui Con Voi地塊,中部的Truong Son帶、Da Nang-Khe Sanh和Kontum地塊均有記錄[4, 66-69]。南北陸緣碰撞峰期時(shí)代存在先后,北緣(240~220 Ma)滯后南緣(258~243 Ma)為18~23 Ma,可能與印支板塊與華南大陸碰撞后引發(fā)的應(yīng)力由南向北逐步傳播有關(guān)[14, 18, 28]。陸緣碰撞造山作用造成應(yīng)力在華南內(nèi)陸不斷聚集,誘發(fā)了華南大陸強(qiáng)烈的陸內(nèi)造山作用[1],導(dǎo)致蓋層廣泛褶皺或沖斷及基底強(qiáng)烈韌性剪切變形,從而形成糜棱巖帶或韌性剪切帶,如浙江龍泉糜棱巖帶(230~237 Ma,白云母和金云母40Ar/39Ar)[70],海南公愛、戈枕和沖卒嶺韌性剪切帶(227~250 Ma,白云母40Ar/39Ar)[71],廣東泗輪、分界南和永定橋糜棱巖帶(229~255 Ma,白云母40Ar/39Ar)[72],信宜和高州糜棱巖帶(222~230 Ma,黑云母40Ar/39Ar)[73],長(zhǎng)江以北滁縣三界藍(lán)片巖帶(245 Ma,白云母40Ar/39Ar)[74]。這些構(gòu)造帶的剪切變形年齡將華南內(nèi)陸印支運(yùn)動(dòng)主造山-變形變質(zhì)作用的時(shí)代進(jìn)一步限定為255~220 Ma。受南北陸緣擠壓作用的影響,華南大陸地殼明顯縮短加厚,局部地區(qū)(南嶺)地殼加厚至≤50 km[12, 16]。王岳軍等[15]通過構(gòu)建的地質(zhì)模型,證實(shí)陸殼疊置加厚作用為控制湖南印支期構(gòu)造-巖漿作用形成的主導(dǎo)因素,這一理論合理解釋了華南大陸印支早期(259~230 Ma)強(qiáng)過鋁質(zhì)花崗巖的形成。同時(shí),值得注意的是,地殼在加厚10~20 Ma的時(shí)間間隔內(nèi)發(fā)生熱-應(yīng)力松弛作用,從而導(dǎo)致地殼伸展減薄、減壓熔融,形成花崗質(zhì)巖漿[75]。十萬大山盆地的研究證實(shí),華南大陸在早-中三疊世處于擠壓構(gòu)造背景,而在晚三疊世則進(jìn)入應(yīng)力伸展階段[76],這一轉(zhuǎn)換進(jìn)一步佐證了碰撞造山后的地殼熱-應(yīng)力松弛作用的存在。在華南大陸,發(fā)生在印支運(yùn)動(dòng)碰撞結(jié)束后的熱-應(yīng)力松弛作用,導(dǎo)致早-中元古代沉積變質(zhì)巖系部分熔融,從而形成了遍布的以弱過鋁質(zhì)或準(zhǔn)鋁質(zhì)為典型特征的印支晚期(230~200 Ma)后碰撞花崗巖(表1)。另外,湖南境內(nèi)存在小規(guī)模印支晚期基性巖體,如桃江巖體、道縣輝長(zhǎng)巖包體[14, 77],其代表了熱-應(yīng)力松弛階段局部發(fā)生的玄武質(zhì)巖漿底侵作用。白馬山及瓦屋塘黑云母二長(zhǎng)花崗巖形成時(shí)代為230~201 Ma,滯后于華南大陸印支運(yùn)動(dòng)主造山-變形變質(zhì)時(shí)代(255~220 Ma)約20 Ma。從這個(gè)意義上,它們形成于印支運(yùn)動(dòng)主碰撞造山結(jié)束后大陸地殼伸展-減薄構(gòu)造背景下,為應(yīng)力松弛階段加厚的地殼減壓熔融作用的產(chǎn)物。
1)湖南白馬山龍?zhí)冻瑔卧屯呶萏翈r體均由黑云母二長(zhǎng)花崗巖構(gòu)成。鋯石的CL圖像分析表明,大部分鋯石內(nèi)部結(jié)構(gòu)較為簡(jiǎn)單,呈無核具強(qiáng)烈韻律震蕩環(huán)帶結(jié)構(gòu),為巖漿成因鋯石,少部分鋯石具核-邊或核-幔-邊結(jié)構(gòu)。
2)鋯石SHRIMP U-Pb定年結(jié)果顯示,龍?zhí)冻瑔卧?個(gè)黑云母二長(zhǎng)花崗巖樣品的年齡值為(215.9±1.9) Ma和(212.2±2.1) Ma,獲得的鋯石核部諧和年齡為230.3~227.0 Ma,均屬印支晚期,表明白馬山巖體印支晚期存在多期次花崗質(zhì)巖漿活動(dòng)。瓦屋塘黑云母二長(zhǎng)花崗巖形成于(217.7±1.8) Ma。這2個(gè)巖體的形成反映了約215 Ma巖漿活動(dòng)在華南大陸的廣泛性,同時(shí)進(jìn)一步佐證了印支晚期巖漿活動(dòng)于210~225 Ma達(dá)到巖漿活動(dòng)的峰期。
3)地球化學(xué)特征顯示它們?yōu)槿踹^鋁-強(qiáng)過鋁質(zhì)花崗巖,且具殼源型后碰撞花崗巖的特征,源于后碰撞或碰撞晚期早元古代變質(zhì)雜砂巖的部分熔融。綜合考慮華南大陸中生代大地構(gòu)造過程及印支運(yùn)動(dòng)的影響,白馬山和瓦屋塘巖體形成于印支運(yùn)動(dòng)主碰撞造山(255~220 Ma)結(jié)束后約20 Ma的熱-應(yīng)力松弛階段,為陸殼伸展-減薄構(gòu)造背景下加厚的地殼減壓熔融作用的產(chǎn)物。
[1] 張?jiān)罉?,徐先兵,賈東,等. 華南早中生代從印支期碰撞構(gòu)造體系向燕山期俯沖構(gòu)造體系轉(zhuǎn)換的形變記錄[J]. 地學(xué)前緣,2008,15(6):1-14. Zhang Yueqiao, Xu Xianbing, Jia Dong, et al. Deformation Record of the Change from Indosinian Collision-Related Tectonic System to Yanshanian Subduction-Related Tectonic System in South China During the Early Mesozoic[J]. Earth Science Frontiers, 2008, 15(6):1-14.
[2] 范蔚茗,王岳軍,郭鋒,等. 湘贛地區(qū)中生代鎂鐵質(zhì)巖漿作用與巖石圈伸展[J]. 地學(xué)前緣,2003,10(3): 159-169. Fan Weiming, Wang Yuejun, Guo Feng, et al. Mesozoic Mafic Magmatism in Hunan-Jiangxi Provinces and the Lithospheric Extension[J]. Earth Science Frontiers, 2003, 10(3): 159-169.
[3] Deprat J. Etude Des Plissements et Des Zones Décra-sement de Lamoyenne et de la Basse Rivière Noire[J]. Mèmoire du Service Géologique Indochine, 1914, 3: 59.
[4] Carter A,Roques D,Bristow C.Understanding Me-sozoic Accretion in Southeast Asia: Significance of Triassic Thermotectonism (Indosinian Orogeny) in Vietnam[J]. Geology, 2001, 29: 211-214.
[5] Meng Qingren, Zhang Guowei. Geologic Framework and Tectonic Evolution of Qinling Orogen, Central China[J]. Tectonophysics, 2000, 323:183-196.
[6] 許靖華,孫樞,李繼亮. 是華南造山帶而不是華南地臺(tái)[J]. 中國(guó)科學(xué):B 輯,1987(10): 1107-1115. Hsu K J, Sun Shu, Li Jiliang. Huanan Orogen, Not South China Platform[J]. Science in China: Series B, 1987(10): 1107-1115.
[7] Hsu K J, Li J L, Chen H H, et al. Tectonics of South China: Key to Understanding West Pacific Geology[J]. Tectonophysics, 1990, 183: 9-39.
[8] Gilder S A, Gill J, Coe R S, et al. Isotopic and Paleomagnetic Constraints on the Mesozoic Tectonic Evolution of South China[J]. Journal of Geophysical Research, 1996, 107(B7): 16137-16154.
[9] Rowley D B, Ziegler A M, Nie G. Comment on “Mesozoic Over-Thrust Tectonics in South China”[J]. Geology, 1997, 17: 384-386.
[10] Wang Y J, Zhang Y H, Fan W M, et al. Structural Signatures and40Ar-39Ar Geochronology of the Indosinian Xuefengshan Tectonic Belt, South China Block[J]. Journal of Structual Geology, 2005, 27: 985-998.
[11] Li Z X, Li X H. Formation of the 1 300 km-Wide Intracontinental Orogen and Postorogenic Magmatic Province in Mesozoic South China: A Flat-Slab Subduction Model[J]. Geology, 2007, 35: 179-182.
[12] 周新民. 對(duì)華南花崗巖研究的若干思考[J].高校地質(zhì)學(xué)報(bào),2003,9(4): 556-565. Zhou Xinmin. My Thinking About Granite Geneses of South China[J]. Geological Journal of China Universities, 2003, 9(4): 556-565.
[13] 王岳軍,范蔚茗,梁新權(quán),等. 湖南印支期花崗巖SHRIMP鋯石U-Pb年齡及其成因啟示[J]. 科學(xué)通報(bào),2005,50(12): 1259-1266. Wang Yuejun, Fan Weiming, Liang Xinquan, et al. SHRIMP Zircon U-Pb Geochronology of Indosinian Granites in Hunan Province and Its Petrogenetic Implications[J]. Chinese Science Bulletin, 2005, 50(13): 1395-1403.
[14] 丁興,陳培榮,陳衛(wèi)鋒,等. 湖南溈山花崗巖中鋯石LA-ICPMS U-Pb定年: 成巖啟示和意義[J].中國(guó)科學(xué):D輯,2005,35(7): 606-616. Ding Xing, Chen Peirong, Chen Weifeng, et al. LA-ICPMS Zircon Dating of Weishan Granitic Plutons in Hunan Province: Petrogenesis and Tectonic Implications[J]. Science in China:Series D, 2005, 35(7):606-616.
[15] 王岳軍,Zhang Y H,范蔚茗,等. 湖南印支期過鋁質(zhì)花崗巖的形成: 巖漿底侵與地殼加厚熱效應(yīng)的數(shù)值模擬[J]. 中國(guó)科學(xué):D輯,2002,32(6): 491-499. Wang Yuejun, Zhang Y H, Fan Weiming, et al. Numerical Modeling for Generation of Indo-Sinian Peraluminous Granitoids Hunan Province: Basaltic Underplating vs Tectonic Thickening[J]. Science in China:Series D, 2002, 45(11): 1042-1056.
[16] 孫濤,周新民,陳培榮,等. 南嶺東段中生代強(qiáng)過鋁花崗巖成因及其大地構(gòu)造意義[J]. 中國(guó)科學(xué):D輯,2003,33(12): 1209-1218. Sun Tao, Zhou Xinmin, Chen Peirong, et al. Strongly Peraluminous Granites of Mesozoic in Eastern Nanling Range, Southern China: Petrogenesis and Implications for Tectonics[J]. Science in China:Series D, 2005, 48(2): 165-174.
[17] 湖南省地質(zhì)礦產(chǎn)局. 湖南省區(qū)域地質(zhì)志[M]. 北京: 地質(zhì)出版社,1988: 380-460. Hunan Bureau of Geology and Mineral Resource. Regional Geology of Hunan province[M]. Beijing: Geological Publishing House, 1988: 380-460.
[18] 陳衛(wèi)鋒,陳培榮,黃宏業(yè),等. 湖南白馬山巖體花崗巖及其包體的年代學(xué)和地球化學(xué)研究[J].中國(guó)科學(xué):D輯,2007,37(7): 873-893. Chen Weifeng, Chen Peirong, Huang Hongye, et al. The Geochronology and Geochemistry Studies on the Baimashan Granites, Hunan[J]. Science in China:Series D, 2007, 37(7): 873-893.
[19] 羅志高,王岳軍,張菲菲,等. 金灘和白馬山印支期花崗巖體LA-ICPMS鋯石U-Pb定年及其成巖啟示[J]. 大地構(gòu)造與成礦學(xué),2010,34(2): 282-290. Luo Zhigao, Wang Yuejun, Zhang Feifei, et al. LA-ICPMS Zircon U-Pb Dating for Baimashan and Jintan Indosinian Granitic Plutons and Its Petrogenetic Implications[J]. Geotectonica et Metallogenia, 2010, 34(2): 282-290.
[20] 張萬良. 贛南大富足巖體巖石地球化學(xué)特征及其構(gòu)造環(huán)境判別[J]. 大地構(gòu)造與成礦學(xué),2006,30(1): 98-107. Zhang Wanliang. Petrogeochemistry and Tectonic Environment of Dafuzu Rock Mass in Southern Jiangxi Province[J]. Geotectonica et Metallogenia, 2006, 30(1): 98-107.
[21] 張文蘭,華仁民,王汝成,等. 江西大吉山五里亭花崗巖單顆粒鋯石U-Pb同位素年齡及其地質(zhì)意義探討[J]. 地質(zhì)學(xué)報(bào),2004,78(3): 352-258. Zhang Wenlan, Hua Renmin, Wang Rucheng, et al. Single Zircon U-Pb Isotopic Age of the Wuliting Granite in Dajishan Area of Jiangxi, and Its Geological Implication[J]. Acta Geologica Sinica, 2004, 78(3): 352-258.
[22] 邱檢生,Mclnnes B I A,徐夕生,等. 贛南大吉山五里亭巖體的鋯石ELA-ICP-MS定年及其與鎢成礦關(guān)系的新認(rèn)識(shí)[J]. 地質(zhì)論評(píng),2004,50(2): 125-133. Qiu Jiansheng, Mclnnes B I A, Xu Xisheng, et al. Zircon ELA-ICP-MS Dating for Wuliting Pluton at Dajishan, Southern Jiangxi and New Recognition About Its Relation to Tungsten Mineralization[J]. Geological Review, 2004, 50(2): 125-133.
[23] 張敏,陳培榮,黃國(guó)龍,等.南嶺東段龍?jiān)磯螐?fù)式巖體La-ICP-MS鋯石U-Pb年齡及其地質(zhì)意義[J]. 地質(zhì)學(xué)報(bào),2006,80(7): 984-994. Zhang Min, Chen Peirong, Huang Guolong, et al. Single-Zircon La-ICP-MS Ages of the Longyuanba Pluton in the Eastern Nanling Region and Geological Implication[J]. Acta Geologica Sinica, 2006, 80(7): 984-994.
[24] 于津海,王麗娟,王孝磊,等. 贛東南富城雜巖體的地球化學(xué)和年代學(xué)研究[J]. 巖石學(xué)報(bào),2007,23(6),1441-1456. Yu Jinhai, Wang Lijuan, Wang Xiaolei, et al. Geochemistry and Geochronology of the Fucheng Complex in the Southeastern Jiangxi Province, China[J]. Acta Petrologica Sinica, 2007, 23(6), 1441-1456.
[25] 郭春麗,陳毓川,藺志永,等. 贛南印支期柯樹嶺花崗巖體SHRIMP鋯石U-Pb年齡、地球化學(xué)、鋯石Hf同位素特征及成因探討[J]. 巖石礦物學(xué)雜志,2011,30(4): 567-580. Guo Chunli, Chen Yuchuan, Lin Zhiyong, et al. SHRIMP Zircon U-Pb Dating, Geochemistry and Zircon Hf Isotopic Characteristics of Granitoids in Keshuling Granites, Jiangxi Province and Their Genetic Analysis[J]. Acta Perologica et Mineralogica, 2011, 30(4): 567-580.
[26] Peng B X, Wang Y J, Fan W M, et al. LA-ICPMS Zircon U-Pb Dating for Three Indosinian Granitic Plutons from Central Hunan and Western Guangdong Provinces and Its Petrogenetic Implications[J]. Acta Geologica Sinica, 2006, 80(5): 660-669.
[27] 馬鐵球,柏道遠(yuǎn),鄺軍,等. 湘東南茶陵地區(qū)錫田巖體鋯石SHRIMP定年及其地質(zhì)意義[J]. 地質(zhì)通報(bào),2005,24(5): 415-419. Ma Tieqiu, Bai Daoyuan, Kuang Jun, et al. Zircon SHRIMP Dating of the Xitian Granite Pluton, Chaling, Southeastern Hunan, and Its Geological Significance[J]. Geological Bulletin of China, 2005, 24(5): 415-419.
[28] 陳衛(wèi)鋒,陳培榮,周新民,等. 湖南陽(yáng)明山巖體的LA-ICP-MS鋯石U-Pb定年及成因研究[J]. 地質(zhì)學(xué)報(bào),2006,80(7): 1066-1077. Chen Weifeng, Chen Peirong, Zhou Xinmin, et al. Single-Zircon LA-ICP-MS U-Pb Dating of the Yangmingshan Granitic Pluton in Hunan, South China and Its Petrogenetic Study[J]. Acta Geologica Sinica, 2006, 80(7): 1066-1077.
[29] 徐夕生,鄧平,Reilly S Y O,等. 華南貴東雜巖體單顆粒鋯石激光探針I(yè)CPMS U-Pb定年及其成巖意義[J]. 科學(xué)通報(bào),2003,48(12): 1328-1334. Xu Xisheng, Deng Ping, Reilly S Y O, et al. Single Zircon LA-ICPMS U-Pb Dating of Guidong Complex (SE China) and Its Petrogenetic Significance[J]. Chinese Science Bulletin, 2003, 48(17): 1892-1899.
[30] 莊文明,黃友義,陳紹前.粵中印支期花崗巖類的基本特征與成巖構(gòu)造環(huán)境[J]. 廣東地質(zhì),2000,15(3): 33-39. Zhuang Wenming, Huang Youyi, Chen Shaoqian.Basic Features and Tectonic Setting of Indosinian Granitoid in Central Guangdong[J]. Guangdong Geology, 2000, 15(3): 33-39.
[31] 翟偉,李兆麟,孫曉明,等.粵西河臺(tái)金礦鋯石SHRIMP年齡及其地質(zhì)意義[J]. 地質(zhì)論評(píng),2006,52(5): 690-699. Zhai Wei, Li Zhaolin, Sun Xiaoming, et al. U-Pb Isotope Age of Zircons in Gold-Bearing Quartz Veins from the Hetai Gold Deposit, Western Guangdong, China: Constraints on the Timing of Gold Metallogenesis[J]. Geological Review, 2006, 52(5): 690-699.
[32] 鄧希光,陳志剛,李獻(xiàn)華,等. 桂東南地區(qū)大容山-十萬大山花崗巖帶SHRIMP 鋯石U-Pb定年[J]. 地質(zhì)論評(píng),2004,50(4): 426-432. Deng Xiguang, Chen Zhigang, Li Xianhua, et al. SHRIMP U-Pb Zircon Dating of the Darongshan-Shiwandashan Granitoid Belt in Southeastern Guangxi, China[J]. Geological Review, 2004, 50(4): 426-432.
[33] 王強(qiáng),趙振華,簡(jiǎn)平,等. 武夷山洋坊霓輝石正長(zhǎng)巖的鋯石SHRIMP U-Pb年齡及其構(gòu)造意義[J]. 科學(xué)通報(bào),2003,48(14): 1582-1588. Wang Qiang, Zhao Zhenhua, Jian Ping, et al. The Zircon SHRIMP U-Pb Dating of the Simaites in the Yangfang and Its Tectonic Significance, Wuyishan[J]. Chinese Science Bulletin, 2003, 48(14): 1582-1588.
[34] 王麗娟,于津海,徐夕生,等. 閩西南古田-小陶花崗質(zhì)雜巖體的形成時(shí)代和成因[J]. 巖石學(xué)報(bào),2007,23(6): 1470-1484. Wang Lijuan, Yu Jinhai, Xu Xisheng, et al. Formation Age and Origin of the Gutian-Xiaotao Granitic Complex in the Southwestern Fujian Province, China[J]. Acta Petrologica Sinica, 2007, 23(6): 1470-1484.
[35] Zhou X M, Sun T, Shen W Z, et al. Petrogenesis of Mesozoic Granitoids and Volcanic Rocks in South China: A Response to Tectonic Evolution[J]. Episodes, 2006, 29: 26-33.
[36] 徐先兵,張?jiān)罉?,賈東,等.華南早中生代大地構(gòu)造過程[J]. 中國(guó)地質(zhì),2009,36 (3): 573-593. Xu Xianbing, Zhang Yueqiao, Jia Dong, et al. Early Mesozoic Geotectonic Processes in South China[J]. Geology in China, 2009, 36(3): 573-593.
[37] Wang J, Li Z X. History of Neoproterozoic Rift Basins in South China: Implications for Rodinia Break-Up[J]. Precambrian Research, 2003, 122(1/2/3/4): 141-158.
[38] Yin C Y, Liu D Y, Gao L Z, et al. Lower Boundary Age of the Nanhua System and the Gucheng Glacial Stage: Evidence from SHRIMP Ⅱ Dating[J]. Chinese Science Bulletin, 2003, 48 (16): 1657-1662.
[39] 郭令智,施央申,馬瑞士. 華南大地構(gòu)造格架和地殼演化[C]//國(guó)際交流地質(zhì)學(xué)術(shù)論文集.北京:地質(zhì)出版社,1980: 109-116. Guo Lingzhi, Shi Yangshen, Ma Ruishi. Tectonic Fragment of South China and Crustal Evolution[C]//International Exchanges of Geology. Beijing: Geological Publishing House, 1980: 109-116.
[40] 賈寶華. 雪峰山區(qū)韌性剪切構(gòu)造帶[J]. 湖南地質(zhì),1992,11(3): 203-208. Jia Baohua. The Ductile Shear Tectonic Zones in Xuefengshan Region[J]. Hunan Geology, 1992, 11(3): 203-208.
[41] 楊奎鋒,楊坤光,馬昌前.雪峰山安化-溆浦?jǐn)嗔褞ё冃翁卣骷癊SR定年[J]. 大地構(gòu)造與成礦學(xué),2004,28(3): 263-269. Yang Kuifeng, Yang Kunguang, Ma Changqian. Deformation Characteristics and ESR Dating of Anhua-Xupu Fault Belt in the Xuefeng Mountains, Hunan[J]. Geotectonica et Metallogenia, 2004, 28(3): 263-269.
[42] Li J H, Zhang Y Q, Dong S W, et al. Late Mesozoic-Early Cenozoic Deformation History of the Yuanma Basin, Central South China[J]. Tectonophysics, 2012,570/571: 163-183.
[43] 鄭基儉. 花崗巖單元-超單元填圖的理論基礎(chǔ)劃分標(biāo)志與歸并原則:以湖南省花崗巖劃分成果和桂東試驗(yàn)區(qū)為例[J]. 湖南地質(zhì),1995,14(4): 200-204. Zheng Jijian. Theoritical Basis, Distinguishing Criteria and Merging Principles of the Mapping of Granite Units:An Example from the Achievement of Diversion of Granites in Hunan and Guidong Test Area[J]. Hunan Geology, 1995, 14(4): 200-204.
[44] Compston W, Williams I S, Mcyer C. U-Pb Geochronology of Zircons from Lunar Breccia 73217 Using a Sensitive High Massresolution Microprobe[J]. Journal of Geophysical Research, 1984, 89(Sup.): 325-534.
[45] 簡(jiǎn)平,劉敦一,孫曉猛. 滇川西部金沙江石炭紀(jì)蛇綠巖SHRIMP測(cè)年:古特提斯洋殼演化的同位素年代學(xué)制約[J]. 地質(zhì)學(xué)報(bào),2003,77(2): 217-228. Jian Ping, Liu Dunyi, Sun Xiaomeng. SHRIMP Dating of Carboniferous Jinshajiang Ophiolite in Western Yunnan and Sichuan: Geochronological Constraints on the Evolution of the Paleo-Tethys Oceanic Crust[J]. Acta Geologica Sinica, 2003, 77(2): 217-228.
[46] Ludwig K R. Sqiud 1.02: A User Manual[M]. Berkeley:Geochronological Center Special Publication, 2001:1-219.
[47] Vavra G, Gebauer D, Schmid R. Multiple Zircon Growth and Recrystallization During Polyphase Late Carboniferous to Triassic Metamorphism in Granulites of the Ivrea Zone (Southern Alps): Anion Microprobe (SHRIMP) Study[J]. Contribution to Mineral Petrology, 1996, 122: 337-358.
[48] 梁細(xì)榮,李獻(xiàn)華,劉永康,等. 激光探針等離子體質(zhì)譜同時(shí)測(cè)定鋯石微區(qū)鈾-鉛年齡及微量元素[J]. 巖礦測(cè)試,1999,18(4): 253-258. Liang Xirong, Li Xianhua, Liu Yongkang, et al. Simultaneous Determination of U-Pb Ages and Trace Elements in Single Zircon by Using LA-ICPMS[J]. Rock and Mineral Analysis, 1999, 18(4): 253-258.
[49] 吳元保,鄭永飛.鋯石成因礦物學(xué)及其對(duì)U-Pb年齡解釋的制約[J]. 科學(xué)通報(bào),2004,49(16): 1589-1604. Wu Yuanbao, Zheng Yongfei. The Origin of the Zircons and Its Constraints for the U-Pb Age[J]. Chinese Science Bulletin, 2004, 49(16): 1589-1604.
[50] 邱檢生,劉亮,李真. 浙江黃巖望海崗石英正長(zhǎng)巖的鋯石U-Pb年代學(xué)與Sr-Nd-Hf同位素地球化學(xué)及其對(duì)巖石成因的制約[J]. 巖石學(xué)報(bào),2011,27(6): 1557-1572. Qiu Jiansheng, Liu Liang, Li Zhen. Zircon U-Pb Geochronology and Sr-Nd-Hf Isotopic Geochemistry of Quartz Syenite from Wanghaigang Pluton in Huangyan County, Zhejiang Province and Their Implications for Petrogenesis[J]. Acta Petrologica Sinica, 2011, 27(6): 1557-1572.
[51] Sircombe K N. Tracing Provenance Through the Isotope Ages of Littoral and Sedimentary Detrital Zircon, Eastern Australia[J]. Sedimentary Geology, 1999, 124: 47-67.
[52] Griffin W L, Belousova E A, Shee S R, et al. Archean Crustal Evolution in the Northern Yilgarn Craton: U-Pb and Hf-Isotope Evidence from Detrital Zircons[J]. Precambrian Research, 2004, 131: 231-282.
[53] Connelly J N. Degree of Preservation of Igneous Zonation in Zircon as a Signpost for Concordancy in U/Pb Geochronology[J]. Chemical Geology, 2000, 172: 25-39.
[54] 吳元保,陳道公,夏群科,等. 大別山雙河地區(qū)花崗質(zhì)片麻巖鋯石的離子探針定年[J]. 礦物巖石地球化學(xué)通報(bào),2001,20(4): 298-301. Wu Yuanbao, Chen Daogong, Xia Qunke, et al. SIMS U-Pb Dating of Zircons from Shuanghe Orthgneiss, Dabie Area[J]. Bulletin of Mineralogy, Petrology and Geochemistry, 2001, 20(4): 298-301.
[55] White A J R, Chappell B W. Ultrametamorphism and Granitoid Genesis[J]. Tectonophysics, 1977, 43: 7-22.
[56] 陳小明,王汝成,劉昌實(shí),等. 廣東從化佛岡(主體)黑云母花崗巖定年和成因[J]. 高校地質(zhì)學(xué)報(bào),2002,8(3): 293-307. Chen Xiaoming, Wang Rucheng, Liu Changshi, et al. Isotopic Dating and Genesis for Fogang Biotite Granties of Conghua Area, Guangdong Province[J]. Geological Journal of China Universities, 2002, 8(3): 293-307.
[57] Visona D, Lombardo B. Two-Mica and Tourmaline Leucogranites from the Everest-Makalu Region (Nepal-Tibet). Himalayan Leucogranite Genesis by Isobaric Heating?[J].Lithos, 2002, 62: 125-150.
[58] Harris N B W, Igner S. Trace Element Modeling of Pelite-Derived Granites[J]. Contributions to Mineralogy and Petrology, 1992, 110: 46-56.
[59] 陳江峰,江博明. Nd,Sr,Pb同位素示蹤和中國(guó)東南大陸地殼演化[C]//鄭永飛.化學(xué)地球動(dòng)力學(xué). 北京: 科學(xué)出版社,1999: 262-287. Chen Jiangfeng, Jahn Borming. Nd, Sr, Pb Isotope Tracer and Continental Crust Evolution in SE China[C]//Zheng Yongfei.Chemical Geodynamics. Beijing: Science Press, 1999: 262-287.
[60] Sylvester P J. Post-Collisional Strongly Peraluminous Granites[J]. Lithos, 1998, 45: 29-44.
[61] 萬天豐. 中國(guó)大地構(gòu)造學(xué)綱要[M]. 北京:地質(zhì)出版社,2004:1-387. Wan Tianfeng. Tectonic Outline of China[M]. Beijing:Geological Publishing House, 2004:1-387.
[62] 萬天豐. 中國(guó)東南六省元古代-侏羅紀(jì)構(gòu)造演化[J]. 地球科學(xué):中國(guó)地質(zhì)大學(xué)學(xué)報(bào),1989,14(1): 45-50. Wan Tianfeng. Tectonic Evolution of Proterozoic-Jurassic in Six Provinces of Southeastern China[J]. Earth Science Journal of China University of Geosciences, 1989, 14(1): 45-50.
[63] Li S G, Xiao Y L, Liou D L, et al. Collision of the North China and Yangtze Blocks and Formation of Coesite-Bearing Eclogites: Timing and Processes[J]. Chemical Geology, 1993, 109: 89-111.
[64] Zhang K J. North and South China Collision Along the Eastern and Southern North China Margins[J]. Tectonophysics, 1997, 270: 145-156.
[65] 劉福來,許志琴,楊經(jīng)綏,等. 中國(guó)大陸科學(xué)鉆探工程主孔及周邊地區(qū)花崗質(zhì)片麻巖的地球化學(xué)性質(zhì)和超高壓變質(zhì)作用標(biāo)志的識(shí)別[J]. 巖石學(xué)報(bào),2004,20(1): 9-26. Liu Fulai, Xu Zhiqin, Yang Jingsui, et al. Geochemical Characteristics and Genetic Mechanism of Orthgneiss and Paragneiss in the Depth Intervals of 2 000-3 000 m from Main Drill Hole of Chinese Continental Scientific Drilling Project[J]. Acta Petrologica Sinica, 2004, 20(1): 9-26.
[66] Lepvrier C, Maluski H, Van Vuong N, et al. Indosinian NW-Trending Shear Zones Within the Truong Son Belt (Vietnam):40Ar-39Ar Triassic Ages and Cretaceous to Cenozoic Overprints[J]. Tectonophysics, 1997, 283: 105-128.
[67] Nam Tran Ngoc. Thermotectonic Events from Early Proterozoic to Miocene in the Indochina Craton: Implication of K-Ar Ages in Vietnam[J]. Journal of Asian Earth Sciences, 1998, 16: 475-484.
[68] Lan C Y, Chung S L, Chen C H, et al. Geochemical and Sr-Nd Isotopic Characteristics of Granitic Rocks from Northern Vietnam[J]. Journal of Asian Earth Sciences, 2000, 18: 267-280.
[69] Maluski H, Lepvrier C, Jolivet L, et al. Ar-Ar and Fission-Track Ages in the Song Chay Massif: Early Triassic and Cenozoic Tectonics in Northern Vietnam[J]. Journal of Asian Earth Sciences, 2001, 19: 233-248.
[70] 朱炳泉,王一先,王慧芬,等.黃山-溫州地球化學(xué)剖面及廊區(qū)解析[J]. 地球化學(xué),1997,26(2): 1-13. Zhu Bingquan, Wang Yixian, Wang Huifen, et al. Huangshan-Wenzhou Geochemical Section and Its Corridor Area of Lithosphere in Southeastern China[J]. Geochimica, 1997, 26(2 ): 1-13.
[71] 陳新躍,王岳軍,韋牧,等.海南公愛NW向韌性剪切帶構(gòu)造特征及其40Ar-39Ar年代學(xué)約束[J].大地構(gòu)造與成礦學(xué),2006,30(3): 312-319. Chen Xinyue, Wang Yuejun, Wei Mu, et al. Microstructure Characteristics of NW Trend Ductile Shear Zones of Gongai, Hainan: Constraints from40Ar-39Ar Geochronology[J]. Geotectonica et Metallogenia, 2006, 30(3): 312-319.
[72] 邵建國(guó),彭少梅,彭松柏. 云開地塊周邊斷裂帶40Ar/36Ar-39Ar/36Ar等時(shí)線定年[J]. 廣東地質(zhì),1995,10(2): 34-40. Shao Jianguo, Peng Shaomei, Peng Songbai.40Ar/36Ar-39Ar/36Ar Isotope Dating of Faults in Yunkai Area[J]. Guangdong Geology, 1995, 10(2): 34-40.
[73] Wang Y J, Fan W M, Cawood P A, et al. Indosinian High-Strain Deformation for the Yunkaidashan Tectonic Belt, South China: Kinematics and40Ar/39Ar Geochronological Constraints[J]. Tectonics, 2007,26(6): TC 6008, doi: 1029/2007TC002099.
[74] 李曙光,劉德良,陳移之,等. 中國(guó)中部藍(lán)片巖的形成時(shí)代[J]. 地質(zhì)科學(xué),1993,28(1):21-27. Li Shuguang, Liu Deliang, Chen Yizhi, et al. Time of the Blueshicst Belt Formation in Central China[J]. Scientia Geologica Sinica, 1993, 28(1): 21-27.
[75] Patino A E, Humphreys E D, Johnston A D. Ana-texis and Metamorphism in Tectonically Thickened Continental Crust Exemplified by the Sevier Hinterland, Western North America[J]. Earth and Planetary Science Letters, 1990, 97: 290-315.
[76] 梁新權(quán),李獻(xiàn)華,丘元禧,等. 華南印支期碰撞造山:十萬大山盆地構(gòu)造和沉積學(xué)證據(jù)[J]. 大地構(gòu)造與成礦學(xué),2005,29(1): 99-112. Liang Xinquan, Li Xianhua, Qiu Yuanxi, et al. Indosinian Collisional Orogeny: Evidence from Structural and Sedimentary Geology in Shiwandashan Basin, South China[J]. Geotectonica et Metallogenia, 2005, 29(1): 99-112.
[77] 郭峰,范蔚茗,林舸,等. 湘南道縣輝長(zhǎng)巖包體的年代學(xué)研究及成因探討[J]. 科學(xué)通報(bào),1997,42(15): 1661-1663. Guo Feng, Fan Weiming, Lin Ge, et al. Chronology Study and Genesis Investigation of Gabbro Xenolith in Daoxian Country, Hunan Province[J]. Chinese Science Bulletin, 1997, 42(15): 1661-1663.
SHRIMP U-Pb Dating of Zircons from the Baimashan Longtan Super-Unit and Wawutang Granites in Hunan Province and Its Geological Implication
Li Jianhua1, Zhang Yueqiao1, Xu Xianbing2, Li Hailong1, Dong Shuwen3, Li Tingdong3
1.InstituteofGeomechanics,ChineseAcademyofGeologicalSciences,Beijing100081,China2.FacultyofEarthSciences,ChinaUniversityofGeosciences,Wuhan430074,China3.ChineseAcademyofGeologicalSciences,Beijing100037,China
In this study, internal structures of zircons from the Baimashan and Wawutang biotite monzogranite plutons are analyzed by the CL technology, and reliable ages of zircons are dated by the SHRIMP U-Pb method. Two samples from the Baimashan Longtan super-unit yield weighted mean206Pb/238U ages of (215.9±1.9) Ma and (212.2±2.1) Ma, respectively. Moreover, several zircons exhibit core-rim textures, in which the zircon cores yield older ages of 230.3-227.0 Ma, and the zircon rims yield younger mean206Pb/238U age of (201.0±2.8)Ma. These data suggest that the Baibashan biotite monzogranite was generated by multi-phase magmatic intrusions. One biotite monzogranite sample from the Wawutang granite yields a weighted mean206Pb/238U age of (217.7±1.8) Ma. These geochronological data provide new evidences for the conclusion that there occurred a peak magmatism (210-225 Ma) in South China during the late stage of the Indosinian orogeny. Geochemical studies indicate that the Baimashan and Wawutang plutons are typical post-orogenic crust-sourced peraluminous granites, resulted from partial melting of the Paleoproterozic metamorphic greywacke. These data allow us to infer that the two plutons were formed after the Early Triassic collision along the Songma and Qingling-Dabie sutures, associated with the parting melting of the thickened crust in the post orogenic extensional setting.
Baimashan; Wawutang; SHRIMP U-Pb dating of zircon; Indosinian granite; crustal extension; Hunan Province
10.13278/j.cnki.jjuese.201401113.
2013-08-12
財(cái)政部“深部探測(cè)技術(shù)與實(shí)驗(yàn)研究”專項(xiàng)之子課題(Sinoprobe-08-01);國(guó)家自然科學(xué)基金項(xiàng)目(41172184)
李建華(1985-),男,博士,主要從事構(gòu)造地質(zhì)學(xué)及地質(zhì)年代學(xué)研究,E-mail:lijianhua0301@126.com。
10.13278/j.cnki.jjuese.201401113
P597.1
A
李建華,張?jiān)罉?,徐先兵,?湖南白馬山龍?zhí)冻瑔卧?、瓦屋塘花崗巖鋯石SHRIMP U-Pb年齡及其地質(zhì)意義.吉林大學(xué)學(xué)報(bào):地球科學(xué)版,2014,44(1):158-175.
Li Jianhua, Zhang Yueqiao, Xu Xianbing,et al.SHRIMP U-Pb Dating of Zircons from the Baimashan Longtan Super-Unit and Wawutang Granites in Hunan Province and Its Geological Implication.Journal of Jilin University:Earth Science Edition,2014,44(1):158-175.doi:10.13278/j.cnki.jjuese.201401113.