碎屑電氣石的LA-MC-ICPMS硼同位素原位微區(qū)分析及其源區(qū)示蹤: 以哀牢山構(gòu)造帶為例

聶小松, 夏小平, 張 樂, 任鐘元, 李 玲

(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)

0 引 言

硼有兩個(gè)穩(wěn)定同位素10B和11B, 是自然界中同位素相對質(zhì)量差最大的元素之一(Δm/m≈10%), 在自然過程中同位素分餾效應(yīng)十分顯著。目前文獻(xiàn)報(bào)道的自然界中不同儲庫的δ11B值變化范圍為–37‰ ～+58‰[1–4], 其中較負(fù)的δ11B值常見于非海相蒸發(fā)硼酸鹽礦物和某些電氣石, 而較正的δ11B值則常見于某些鹽湖鹵水和蒸發(fā)海水[4–5]。硼同位素已廣泛應(yīng)用于地?zé)崤c水環(huán)境地球化學(xué)[6–9]、殼-幔演化作用[10–12]、礦床成礦環(huán)境及物質(zhì)來源[13–18]、海陸相形成環(huán)境[19–21]、重建古海洋環(huán)境與古氣候演變[22–25]以及天體地球化學(xué)[1,24]研究之中。

目前最廣泛使用的硼同位素分析方法為表面熱電離質(zhì)譜(TIMS)法, 該方法具有較高的分析精度但化學(xué)分離提純過程繁瑣, 而且該過程可能產(chǎn)生硼同位素分餾[26–27]。近些年硼同位素微區(qū)原位分析方法取得了很大的發(fā)展, 用離子探針或激光剝蝕多接收電感耦合等離子體質(zhì)譜(LA-MC-ICPMS)直接對地質(zhì)樣品進(jìn)行原位硼同位素比值測定可以取得很好的分析結(jié)果[28–32]。原位微區(qū)分析方法不僅避免了常規(guī)熱電離質(zhì)譜法繁雜的化學(xué)分離純化流程, 提高了工作效率, 而且可以對礦物的環(huán)帶和微層等進(jìn)行原位分析, 揭示礦物形成的多期次精細(xì)過程和不同的形成條件[31–32]。

電氣石是常見的副礦物之一, 可以在近地表到上地幔的溫壓范圍內(nèi)保持穩(wěn)定, 它硬度大, 物理化學(xué)性質(zhì)非常穩(wěn)定, 能在巖石風(fēng)化、沉積物搬運(yùn)、成巖過程中保持穩(wěn)定, 是沉積巖中常見的重礦物之一,與鋯石、金紅石合稱為沉積巖中的三大“超穩(wěn)定的礦物”。這種優(yōu)良的物理化學(xué)穩(wěn)定性使得碎屑電氣石能夠準(zhǔn)確地指示源區(qū)組成, 有效地保留原巖信息[33–35],是物源分析的理想指針礦物[36–37]。巖漿成因和變質(zhì)成因的電氣石具有不同的 Mg、Ca、Fe、Al[34–35]和F、Li[34]組成。這一特征已廣泛應(yīng)用于沉積巖物源研究之中[37–39]。電氣石是自然界中硼的主要載體之一,硼含量高(～3%), 因此也是一個(gè)硼同位素分析和示蹤的理想礦物。

電氣石的硼同位素在礦床成因和成巖物質(zhì)來源[13,15,33,40,41]等方面的研究顯示它能很好地保存其形成時(shí)的地球化學(xué)和同位素信息, 對其寄主巖組成具有良好指示作用[33–35]。結(jié)合電氣石穩(wěn)定的物理化學(xué)性質(zhì), 電氣石的硼同位素可以用來對沉積巖進(jìn)行源區(qū)分析, 對沉積巖的源區(qū)分析提供新的工具。因此本文利用中國科學(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)室建立的LA-MC-ICPMS電氣石硼同位素原位微區(qū)分析方法, 以哀牢山構(gòu)造帶縫合線兩側(cè)碎屑巖為例, 對兩側(cè)志留系-泥盆系沉積巖中的碎屑電氣石進(jìn)行原位硼同位素測定, 結(jié)合已有的碎屑鋯石年代學(xué)數(shù)據(jù), 嘗試反演其物源信息, 為哀牢山構(gòu)造帶的古特提斯演化提供新的制約,探討碎屑電氣石的硼同位素對碎屑巖源區(qū)示蹤的意義。

1 區(qū)域地質(zhì)背景及樣品描述

哀牢山構(gòu)造帶夾持于思茅-印支地塊與揚(yáng)子陸塊之間[42–43], 經(jīng)歷了哀牢山古特提斯洋或者弧后盆地的打開與閉合, 是理解古特提斯演化最關(guān)鍵的地區(qū)之一。該構(gòu)造帶總體呈北西-南東向展布, 北西窄,南東寬, 在云南省中西部延伸上千千米[42,44–47]。哀牢山構(gòu)造帶是一個(gè)由不同時(shí)代地質(zhì)單元組合構(gòu)成的復(fù)合構(gòu)造帶, 其組成多樣、結(jié)構(gòu)復(fù)雜, 經(jīng)歷了長時(shí)期、多期次的地塊拼貼和改造。該構(gòu)造帶可沿哀牢山斷裂分為東西兩個(gè)部分, 西部主要為奧陶系-早三疊沉積地層, 而東部為元古代哀牢山深變質(zhì)巖系,而整個(gè)區(qū)域都發(fā)育有大量不同時(shí)期的火山-沉積巖、基性-超基性和中-酸性巖(圖1)[43]。

哀牢山構(gòu)造帶鄰區(qū)主要地質(zhì)單元和斷裂邊界可依次自西向東劃分為: 蘭坪-思茅盆地、李仙江-阿墨江斷裂、哀牢山構(gòu)造帶、紅河斷裂、揚(yáng)子板塊(圖1)。揚(yáng)子板塊具有太古代-古元古代的結(jié)晶基底, 靠近哀牢山構(gòu)造帶的揚(yáng)子板塊西緣結(jié)晶基底僅在大紅山、東川等地方出露, 大部分區(qū)域被新元古代火山-沉積巖、未變質(zhì)-弱變質(zhì)古生代地層和二疊紀(jì)峨眉山溢流玄武巖所覆蓋[43,48]。

圖1 哀牢山構(gòu)造帶及鄰區(qū)區(qū)域地質(zhì)簡圖Fig.1 Simplified geological map of the Ailaoshan belt and its adjacent region據(jù)1990年云南區(qū)域地質(zhì)圖[48]及Wang et al.[49]改編

此次的研究地區(qū)位于綠春墨江地區(qū)和相鄰揚(yáng)子西緣的建水地區(qū)。綠春-墨江地區(qū)志留系-泥盆系地層出露較完整, 巖性上主要為中細(xì)粒砂巖及粉砂巖夾少量頁巖(圖1, 圖2); 建水地區(qū)志留系-泥盆系主要為一套中細(xì)粒砂巖及粉砂巖夾少量頁巖(圖1, 圖2)。在綠春-墨江地區(qū)和建水地區(qū)分別采集了樣品CLX55、CLX61、CLX63和 CLX04、CLX05、CLX19,所有樣品均為中細(xì)粒砂巖, 其具體采樣層位及位置見圖1和圖2。

2 實(shí)驗(yàn)方法

野外采集的新鮮巖石樣品送河北廊坊區(qū)調(diào)研究所實(shí)驗(yàn)室通過機(jī)械破碎, 淘洗、 磁選和重液分選后,每個(gè)樣品分離出約1000粒碎屑電氣石。然后在雙目鏡下每個(gè)樣品隨機(jī)挑選 200粒, 固定在玻璃板上,以環(huán)氧樹脂充填固結(jié)制成靶, 將其進(jìn)行拋光。隨后將樣品靶進(jìn)行透/反射光以及BSE照相, 獲得其內(nèi)部結(jié)構(gòu), 以便分析測試時(shí)避開破裂或含有包裹體的位置, 并區(qū)分可能不同期次形成的環(huán)帶結(jié)構(gòu), 篩選出最佳的分析點(diǎn)位(圖3)。

電氣石的LA-MC-ICPMS微區(qū)原位硼同位素分析測定在中國科學(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)室完成。分析所使用的儀器為Thermo Scientific公司生產(chǎn)的Neptune Plus多接收等離子體質(zhì)譜儀及與之相連接的美國 Resonetics LLC公司生產(chǎn)的RESOlution M-50激光剝蝕系統(tǒng)。激光剝蝕條件為: 束斑直徑45 μm, 剝蝕頻率5 Hz, 激光輸出能量100 mJ, 經(jīng)過50%的衰減后作用于樣品表面。剝蝕產(chǎn)生的氣溶膠以He氣作為載氣帶出, 通過三通與Ar氣混合載入MC-ICPMS進(jìn)行離子化。10B和11B分別以法拉第杯L3和H3同時(shí)靜態(tài)接收。正式測定之前先以線掃描國際原子能機(jī)構(gòu)的電氣石硼同位素標(biāo)樣IAEA B4[26–27]對儀器參數(shù)進(jìn)行調(diào)試, 使之達(dá)到最佳狀態(tài)。數(shù)據(jù)采集所用的積分時(shí)間為0.131 s,共采集400組數(shù)據(jù), 包括200組氣體空白測試(不開激光), 共耗時(shí)約為54 s。每個(gè)點(diǎn)分析完成后等待30 s清洗時(shí)間再開始下一個(gè)點(diǎn)的分析。10B和11B的信號強(qiáng)度在0.5 V和2.3 V左右, 背景信號強(qiáng)度分別小于0.003 V和0.01 V。在分析過程中采用每10個(gè)未知樣品點(diǎn)前后分別分析 2個(gè)標(biāo)樣點(diǎn), 以 4個(gè)標(biāo)樣點(diǎn)的平均值校正未知樣品的方法, 來校正儀器質(zhì)量歧視和同位素分餾。以 IAEA B4 ((δ11B＝(–8.71 ± 0.18)‰)為校正標(biāo)準(zhǔn), 以中國地質(zhì)科學(xué)院礦產(chǎn)資源研究所電氣石標(biāo)樣 IMR RB1作為監(jiān)控標(biāo)樣, 本實(shí)驗(yàn)測試中25個(gè)IMR RB1分析點(diǎn)給出的δ11B結(jié)果(表1)加權(quán)平均值為(–13.34±0.20)‰ (1σ, 圖 4), 跟侯可軍等[32]報(bào)道的(–12.96±0.49)‰ (1σ)在誤差范圍內(nèi)一致。

圖3 樣品碎屑電氣石BSE圖像及對應(yīng)的δ11B值Fig.3 BSE images of some detrital tourmalines and corresponding δ11B valuesδ11B (‰) = [(11B/10B)樣品/(11B/10B)NIST SRM 951-1]×1000, 紅色圓圈代表分析點(diǎn)位, 紅色數(shù)值為 δ11B 值The red circles indicate the analytical spots. Numbers near the analytical spots are the δ11B values

表1 電氣石標(biāo)樣IMR RB1硼同位素測試結(jié)果Table 1 Analytical results of the tourmaline standard IMR RB1

圖4 電氣石標(biāo)樣IMR RB1 δ11B分析結(jié)果Fig.4 Analytical results of the tourmaline standard IMR RB1

3 分析結(jié)果

樣品中的碎屑電氣石多呈長柱狀, 自形到半自形, 長 80～200 μm, 長寬比約為 1∶1 到 2∶1。反射光圖像下顏色較為均一, 可見部分包裹體及裂縫。大部分顆粒磨圓較好, 呈次棱角狀到次圓狀, 顯示其在水動(dòng)力搬運(yùn)過程中經(jīng)歷了較強(qiáng)的磨蝕作用(圖3)。

依據(jù)透反射光及BSE圖像, 在每個(gè)碎屑巖樣品中隨機(jī)挑選了約 60顆碎屑電氣石進(jìn)行原位微區(qū)硼同位素分析測試, 其詳細(xì)結(jié)果見表 2, 并利用ISOPLOT version 4.7軟件[50]對每個(gè)樣品的碎屑電氣石硼同位素繪制概率密度圖(圖5)。

來自哀牢山構(gòu)造帶內(nèi)的 3個(gè)樣品(CLX55,CLX61, CLX63)中的碎屑電氣石硼同位素δ11B值均較為分散(圖 5a, 圖 5b, 圖 5c), 大多數(shù)電氣石δ11B值集中在–21‰ ～ –1‰之間。其中下泥盆統(tǒng)樣品CLX63共測定了 60個(gè)數(shù)據(jù)點(diǎn), 計(jì)算的δ11B值介于–24.91‰ ～ 8.73‰之間, 在概率密度圖上呈多峰狀,主要峰值有–13.51‰和–16.68‰(圖 5a); 中志留統(tǒng)樣品CLX61共測試了60個(gè)數(shù)據(jù)點(diǎn), 計(jì)算的δ11B值介于–20.04‰～ 9.09‰之間, 在概率密度圖上呈多峰狀,主要峰值有–12.54‰、–14.80‰和–16.84‰(圖 5b);下志留統(tǒng)樣品 CXL55共測試了 60個(gè)數(shù)據(jù)點(diǎn), 計(jì)算的δ11B 值在–19.4‰～4.16‰之間, 在概率密度圖上呈多峰狀, 主要峰值有–13.72‰和–17.64‰ (圖 5c)。

來自揚(yáng)子西緣建水地區(qū)的 3個(gè)樣品(CLX04,CLX05, CLX19)碎屑電氣石硼同位素δ11B值均較為集中(圖 5d, 圖 5e, 圖 5f), 大多數(shù)電氣石δ11B值集中在–14‰ ～ –4‰之間。其中中泥盆統(tǒng)樣品CXL04共測試了 60個(gè)數(shù)據(jù)點(diǎn), 計(jì)算的δ11B值在–17.41‰～14.65‰之間, 在概率密度圖上呈 1個(gè)峰值為–11.38‰主峰和幾個(gè)較小的峰(圖 5d); 中志留統(tǒng)樣品CLX05共分析了56個(gè)數(shù)據(jù)點(diǎn), 計(jì)算的δ11B值介于–17.20‰ ～ 0.91‰之間, 在概率密度圖上呈1個(gè)峰值為–11.44‰的主峰和幾個(gè)較小峰值(圖5e); 下泥盆統(tǒng)樣品 CLX19共測定了 60個(gè)數(shù)據(jù)點(diǎn), 計(jì)算的δ11B值介于–18.55‰ ～ –4.01‰之間, 在概率密度圖上呈一個(gè)峰值為峰值為–12.69‰主峰(圖5f)。

4 討 論

哀牢山構(gòu)造帶內(nèi) 3個(gè)不同時(shí)代的樣品(CLX55,CLX61, CLX63)的碎屑電氣石δ11B值主要集中在–21‰ ～ –1‰, 均呈現(xiàn)多個(gè)峰值(圖 5a, 圖 5b, 圖 5c和圖 6a), 說明哀牢山構(gòu)造帶內(nèi)下志留統(tǒng)-下泥盆系碎屑巖可能接受大致相同物源區(qū)的剝蝕供給, 而δ11B值分布范圍廣, 且呈多峰狀則可能指示了物源

區(qū)組成較為復(fù)雜多樣。樣品中有37%～42%(平均39%)的碎屑電氣石δ11B值與來自殼源花崗巖的原生電氣石的δ11B 值(–14‰ ～ –10‰)一致[4,13,41,51–53], 反映這些電氣石可能來源于殼源花崗巖[4,40]。此前的研究表明 ,結(jié) 晶 于 變 質(zhì) 流 體 中 的 電 氣 石 (–16.0‰ ～–17.1‰)[4,40]、經(jīng)歷強(qiáng)烈去氣作用演化晚期巖漿中結(jié)晶的電氣石 (–23‰ ～ –13.9‰)[4,54,55]以及與非海相蒸發(fā)巖相關(guān)的電氣石[56–57]具有輕的硼同位素組成, 因此樣品中 30%～44% (平均 38%)具有較輕硼同位素組成(小于–14‰)的碎屑電氣石可能來自這些環(huán)境。

表2 碎屑電氣石硼同位素測試結(jié)果Table 2 LA-MC-ICPMS in-situ boron isotopic analyses of detrital tourmalines

(續(xù)表 2)

(續(xù)表 2)

圖5 哀牢山構(gòu)造帶與揚(yáng)子西緣志留系-泥盆系碎屑巖樣品碎屑電氣石硼同位素概率密度圖Fig.5 Detrital tourmalines δ11B probability histograms for the analyzed samples from the Ailaoshan belt and the western margin of the Yangtze Block

圖6 哀牢山構(gòu)造帶與揚(yáng)子西緣志留-泥盆系樣品碎屑鋯石年齡(Nie et al., in review)及碎屑電氣石硼同位素概率密度圖Fig.6 Summary of detrital tourmalines δ11B distributions for this study and detrital zircon age distributions for previous study of sedimentary rocks from the Ailaoshan belt and the western margin of the Yangtze Block

而揚(yáng)子板塊西緣 3個(gè)樣品(CLX04, CLX05,CLX19)的碎屑電氣石硼同位素概率密度圖分布較為集中,δ11B 主要集中在–16‰ ～ –1‰, 概率密度圖中呈現(xiàn)一個(gè)顯著主峰, 峰值大約在–12‰(圖5d,圖5e,圖5f, 圖6b), 可能指示主要物源較為穩(wěn)定。3個(gè)樣品中有 47%～78%(平均 58%)的碎屑電氣石具有的δ11B值在來自殼源花崗巖的原生電氣石的δ11B(–14‰ ～ –10‰)范圍內(nèi), 接近大陸地殼值[4], 反映其物源可能主要為殼源花崗巖來源[40]; 揚(yáng)子西緣樣品中小于–14‰的碎屑電氣石比例為 10%～16%(平均13%), 明顯低于哀牢山構(gòu)造帶內(nèi)樣品, 可能說明這些電氣石的寄主巖對物源貢獻(xiàn)較低; 而這些樣品中較高比例(平均27%)的電氣石具有較重硼同位素(大于–10%), 可能其源區(qū)受到來自俯沖板片流體[58–60]或者海相碳酸鹽巖、蒸發(fā)巖的影響更為明顯[4,5,61]。

對比兩側(cè)碎屑電氣石硼同位素?cái)?shù)據(jù)可以發(fā)現(xiàn),哀牢山構(gòu)造帶內(nèi)樣品所具有的特征與揚(yáng)子西緣樣品顯著不同, 哀牢山構(gòu)造內(nèi)樣品δ11B值較為分散, 相對較輕, 而揚(yáng)子西緣樣品的δ11B值較為集中, 相對較重, 指示兩者物源存在明顯差異(圖5,圖6)。這與利用碎屑鋯石方法得出的結(jié)論較為一致, 即認(rèn)為特提斯縫合線哀牢山藤條河斷裂兩側(cè)古生代沉積巖中碎屑鋯石年齡譜存在顯著差異(圖6c,圖6d), 縫合線以西思茅一側(cè)志留系-泥盆系碎屑鋯石年齡譜極為相似, 年齡主要集中在400～500 Ma和900～1000 Ma,指示碎屑巖物源較為穩(wěn)定, 但是源區(qū)組成較為復(fù)雜,可能來自印支地塊及岡瓦那大陸北緣[49,62,63]; 而縫合線以東揚(yáng)子西緣志留系-泥盆系碎屑鋯石年齡主要集中在 730～1000 Ma, 指示碎屑巖物源較為穩(wěn)定且單一, 揚(yáng)子西緣新元古代漢南-攀西-元江新元古代巖漿弧[45,63–66]可能為其主要源區(qū)[63]。

5 結(jié) 論

(1) 測定標(biāo)樣 IMR RB1δ11B 值為(–13.34±0.20)‰ (1σ), 與前人報(bào)道在誤差范圍內(nèi), 證明我們在中國科學(xué)院廣州地球化學(xué)研究所開發(fā)的電氣石LA-MC-ICPMS硼同位素分析方法準(zhǔn)確可靠。

(2) 哀牢山構(gòu)造帶內(nèi)古特提斯縫合線以西思茅側(cè)志留系-泥盆系碎屑巖中碎屑電氣石硼同位素組成偏輕, 較為分散, 指示源區(qū)組成較為復(fù)雜。

(3) 古特提斯縫合線以東揚(yáng)子西緣建水地區(qū)志留系-泥盆系碎屑巖中碎屑電氣石硼同位素組成偏重, 較為集中, 指示源區(qū)較為單一, 且源區(qū)可能受到了來自俯沖板片流體或者海相碳酸鹽巖、蒸發(fā)巖的影響。

(4) 哀牢山構(gòu)造帶古特提斯縫合線兩側(cè)碎屑電氣石硼同位素特征顯著不同, 顯示兩者物源有明顯差異, 哀牢山構(gòu)造帶內(nèi)的志留系-泥盆系碎屑巖并不是前人所認(rèn)為的揚(yáng)子被動(dòng)大陸邊緣的斜坡沉積。

本研究受國家自然科學(xué)基金項(xiàng)目(41173007)資助。中國地質(zhì)科學(xué)院礦產(chǎn)資源研究所侯可軍博士提供了本次實(shí)驗(yàn)中使用的標(biāo)準(zhǔn)電氣石樣品IAEA B4和IMR RB1; 野外和實(shí)驗(yàn)過程中得到了龍曉平研究員、蔡永峰博士的支持和幫助, 在此一并表示感謝。

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