暴宏天, 王 焰, 曹勇華, 魏 博, 3

內(nèi)蒙古中部二疊紀(jì)額布圖巖體的橄欖石成分特征及其對(duì)輝石巖地幔源區(qū)的指示

暴宏天1, 2, 王 焰1, 3*, 曹勇華1, 魏 博1, 3

(1.中國(guó)科學(xué)院 廣州地球化學(xué)研究所, 礦物學(xué)與成礦學(xué)重點(diǎn)實(shí)驗(yàn)室, 廣東 廣州 510640; 2.中國(guó)科學(xué)院大學(xué), 北京 100049; 3.廣東省礦物物理與材料研究開(kāi)發(fā)重點(diǎn)實(shí)驗(yàn)室, 廣東 廣州 510640)

內(nèi)蒙古中部的額布圖超鎂鐵質(zhì)巖體位于中亞造山帶與華北克拉通北緣的碰撞拼貼帶內(nèi), 發(fā)育銅鎳硫化物礦化。該巖體主要由橄欖斜方輝石巖和斜方輝石巖組成, 橄欖斜方輝石巖中橄欖石含量～15%, 斜方輝石巖中橄欖石含量<5%, 兩種巖石中橄欖石的Fo值介于83～87之間。在Fo值相同時(shí), 與橄欖巖地幔部分熔融熔體中結(jié)晶的橄欖石相比, 額布圖巖體中的橄欖石具有較低的Mn含量, 指示其母巖漿可能不是橄欖巖地幔的熔融產(chǎn)物。同時(shí), 該巖體的橄欖石具有較低的Mn/Zn和Mn/Fe值、較高的Zn/Fe值, 與來(lái)源于典型輝石巖地幔的Mwenezi和Tuli玄武巖中橄欖石斑晶的相應(yīng)比值一致。因此, 額布圖巖體的地幔源區(qū)可能以輝石巖為主。同時(shí), 該巖體中橄欖石的Ca含量較低、Li含量較高指示其源區(qū)可能經(jīng)歷了俯沖流體的改造。因此, 額布圖巖體地幔源區(qū)中的輝石巖組分可能是當(dāng)古亞洲洋板片向南俯沖至華北克拉通之下時(shí), 來(lái)自俯沖板片的流體/揮發(fā)分進(jìn)入上覆地幔楔, 造成先存于地幔中的古老下地殼組分發(fā)生熔融并交代周圍的地幔橄欖巖形成的。

橄欖石; 輝石巖地幔; 額布圖超鎂鐵質(zhì)巖體; 中亞造山帶

0 引 言

隨著賦含銅鎳硫化物礦床(化)的鎂鐵?超鎂鐵質(zhì)巖體在匯聚板塊邊緣不斷被發(fā)現(xiàn), 與俯沖作用有關(guān)的含礦鎂鐵?超鎂鐵質(zhì)巖體的成因受到了廣泛關(guān)注。中亞造山帶自古生代以來(lái)發(fā)育了一系列賦含銅鎳硫化物礦床(化)的巖體, 構(gòu)成了世界上最長(zhǎng)的匯聚板塊邊緣銅鎳硫化物成礦帶。在我國(guó)境內(nèi)的中亞造山帶發(fā)育大量含礦巖體, 包括菁布拉克、喀拉通克、圖拉爾根、黑山、黃山東、黃山西、黃山南、香山、坡北、白石泉、葫蘆、天宇、紅旗嶺7號(hào)、漂河川4號(hào)和額布圖巖體等。這些巖體中巖石的微量元素組成普遍具有大離子親石元素富集及高場(chǎng)強(qiáng)元素虧損的島弧巖漿特征, 可能與其地幔源區(qū)受到來(lái)自俯沖板片的流體/揮發(fā)分交代作用有關(guān)(Zhou et al., 2004; Song et al., 2011, 2013; Li et al., 2012; Xie et al., 2012)。

有學(xué)者認(rèn)為, 俯沖板片釋放的流體/揮發(fā)分進(jìn)入地幔楔后, 有可能導(dǎo)致地幔中的再循環(huán)地殼組分發(fā)生熔融并交代其周圍的橄欖巖地幔, 從而產(chǎn)生輝石巖地幔組分(Gao et al., 2004, 2008; Liu et al., 2005)。而地幔中的再循環(huán)地殼組分多以榴輝巖形式存在(Stracke, 2012; Hofmann, 2014)。相比地幔橄欖巖, 榴輝巖的固相線溫度較低(Yaxley and Green, 1998; Rapp et al., 1999; Kogiso et al., 2003)。當(dāng)軟流圈地幔巖漿上涌或有水加入時(shí), 均會(huì)造成榴輝巖組分熔融, 如果其與周圍的地幔橄欖巖反應(yīng), 則形成相對(duì)富硅的地幔輝石巖(Hirschmann et al., 2003; Gao et al., 2004, 2008; Liu et al., 2005; Sobolev et al., 2005)。

橄欖石是幔源基性巖漿中較早結(jié)晶的礦物, 其成分可能記錄了初始巖漿的特征。Ni和Mn在橄欖石與熔體之間的分配系數(shù)分別為7.37～11.9(Wang and Gaetani, 2008)和0.89(Foley et al., 2013), 它們?cè)谳x石與熔體中的分配系數(shù)分別為2.44～3.78(Le Roux et al., 2011)和1 (Foley et al., 2013)。因此, 相比橄欖巖地幔, 在同等部分熔融程度條件下, 輝石巖地幔部分熔融產(chǎn)生的熔體會(huì)具有較高的Si和Ni含量, 以及較低的Mn含量, 從輝石巖地幔熔體中結(jié)晶的橄欖石通常也具有高Ni、低Mn的特征(Sobolev et al., 2005, 2007; Herzberg and Asimow, 2008; De Hoog et al., 2010; Herzberg, 2011; Foley et al., 2013; Howarth and Harris, 2017)。另外, 在橄欖石?硅酸鹽熔體之間分配系數(shù)相似的元素對(duì), 如Mn-Fe、Ni-Mg、Zn-Fe、Mn-Zn等, 其在橄欖石中的比值基本不受巖漿分離結(jié)晶作用的影響, 可以近似代表其在硅酸鹽熔體中的初始比值(Sobolev et al., 2007; Le Roux et al., 2010; Herzberg, 2011; Mallik and Dasgupta, 2012; S?ager et al., 2015), 因此橄欖石中的這些元素比值可用來(lái)探討鎂鐵?超鎂鐵質(zhì)巖體的地幔源區(qū)特征。與橄欖巖地幔部分熔融熔體中結(jié)晶的橄欖石相比, 從輝石巖地幔部分熔融產(chǎn)生的熔體中結(jié)晶的橄欖石可能具有較高的Ni/Mg和Zn/Fe值, 以及較低的Mn/Fe和Mn/Zn值(Sobolev et al., 2007; Le Roux et al., 2010; Howarth and Harris, 2017)。因此, 鎂鐵?超鎂鐵質(zhì)巖體中橄欖石的成分特征可為探討其地幔源區(qū)組成提供重要依據(jù)。

額布圖巖體位于內(nèi)蒙古中部索倫縫合帶(圖1), 具銅鎳硫化物礦化, 礦石的Ni品位為0.3%～2%, Ni金屬量約1 Mt(Peng et al., 2013)。巖體的形成與古亞洲洋俯沖作用有關(guān), 其母巖漿具高M(jìn)g和高Si的特征, 與玻安質(zhì)巖漿的成分類似(Peng et al., 2013)。其地幔源區(qū)是否存在輝石巖組分、輝石巖組分對(duì)鎳礦化是否有貢獻(xiàn), 是兩個(gè)值得探討的問(wèn)題。本文通過(guò)分析額布圖巖體中橄欖石的成分特征, 探討其地幔源區(qū)中輝石巖組分的貢獻(xiàn), 以及輝石巖地幔組分的存在與銅鎳硫化物礦化的關(guān)系。

圖1 中亞造山帶的構(gòu)造位置(a, 據(jù)Jahn, 2004)和內(nèi)蒙古中部區(qū)域地質(zhì)簡(jiǎn)圖(b, 據(jù)Jian et al., 2008)

1 地質(zhì)背景和巖體特征

中亞造山帶位于西伯利亞板塊、華北板塊和塔里木板塊之間, 其東西向延伸超過(guò)7000 km, 是全球范圍內(nèi)顯生宙地殼增生和改造最顯著的地區(qū), 同時(shí)也是全球礦產(chǎn)資源潛力最大的地區(qū)之一(Xiao et al., 2008)。中亞造山帶是古亞洲洋在新元古代至晚古生代期間經(jīng)歷了打開(kāi)、消減和閉合形成的巨型縫合帶, 該帶由一系列微陸塊、島弧、蛇綠巖和火山巖組成(Jahn, 2004; Windley et al., 2007; Xiao et al., 2009)。自古生代以來(lái), 在中亞造山帶我國(guó)境內(nèi)部分發(fā)育了大量的鎂鐵?超鎂鐵質(zhì)巖體, 其中賦含銅鎳硫化物礦化的巖體主要形成于早二疊世?晚三疊世, 且主要集中于中亞造山帶東段和西段, 其中大型含礦巖體主要包括紅旗嶺7號(hào)、黃山西、黃山東和喀拉通克巖體(Song and Li, 2009; Gao and Zhou, 2013; Sun et al., 2013; Wei et al., 2013)(圖1a), 中?小型含礦巖體主要包括葫蘆、白石泉、天宇和紅旗嶺1號(hào)巖體等(孫濤等, 2010; Tang et al., 2011, 2013; 呂林素等, 2012)。在中亞造山帶中段的內(nèi)蒙古中部地區(qū), 鎂鐵?超鎂鐵質(zhì)巖體呈帶狀分布, 包括小南山、黃花灘、克布、溫根、特頗格日?qǐng)D、達(dá)布遜、額布圖巖體等(江思宏等, 2003; 李鵬等, 2013; 黨智財(cái)?shù)? 2014; 李志丹等, 2015; 趙澤霖等, 2016)。這些巖體大多規(guī)模較小、且礦化程度低, 相對(duì)而言, 額布圖巖體是其中礦化程度較高的一個(gè)。

內(nèi)蒙古中部地區(qū)自北向南由索倫縫合帶、溫都爾廟俯沖?增生雜巖體和白乃廟島弧巖漿巖帶三個(gè)東西向延伸的構(gòu)造單元組成, 白乃廟島弧巖漿巖帶南部與華北克拉通北緣相接(Xiao et al., 2003)。區(qū)內(nèi)主要出露晚二疊世火山?沉積巖, 石炭紀(jì)侵入巖零星出露在索倫縫合帶內(nèi), 二疊紀(jì)侵入巖在索倫縫合帶和白乃廟島弧巖漿巖帶內(nèi)較發(fā)育, 零星發(fā)育于溫都爾廟俯沖?增生雜巖體內(nèi), 早古生代侵入巖主要發(fā)育在白乃廟島弧巖漿巖帶中。索倫縫合帶中還發(fā)育大量蛇綠巖, 普遍認(rèn)為其可能代表了古亞洲洋最終閉合的位置(Xiao et al., 2003; Jian et al., 2008; Chen et al., 2009; Eizenh?fer et al., 2014)(圖1b)。

額布圖巖體位于白乃廟巖漿巖帶西側(cè), 平面上近似呈紡錘狀, 長(zhǎng)約200 m, 寬約100 m (圖2a)。鉆孔數(shù)據(jù)顯示, 巖體在縱剖面上呈碗狀, 向下延伸約200 m(圖2b), 直接圍巖為中元古界寶音圖組變火山?沉積巖。巖體上部以斜方輝石巖為主, 下部以橄欖斜方輝石巖為主。上部的斜方輝石巖平均厚度約60 m,其頂部5～30 m已風(fēng)化為氧化帶, 橄欖斜方輝石巖的平均厚度約100 m。斜方輝石巖的鋯石U-Pb年齡為294.2±2.7 Ma(Peng et al., 2013)。兩種巖性均發(fā)育浸染狀礦化(圖3a), 硫化物主要分布在斜方輝石和橄欖石之間的粒間相中(圖3b、c)。

2 巖相學(xué)特征

斜方輝石巖主要由斜方輝石(～80%)、單斜輝石(<10%)、橄欖石(<5%)和少量角閃石組成。斜方輝石呈半自形短柱狀或板狀, 顆粒大小介于0.4～2 mm之間, 在大顆粒的內(nèi)部裂隙或邊部可見(jiàn)輕微蝕變。單斜輝石呈半自形?它形, 粒徑多介于0.5～0.6 mm之間, 邊部多輕微蝕變。橄欖石為半自形渾圓狀, 粒徑一般<0.4 mm, 大多被斜方輝石包裹, 呈包含結(jié)構(gòu)。沿裂隙蛇紋石化較發(fā)育。角閃石呈它形, 主要位于斜方輝石和橄欖石粒間(圖4a)。

圖2 額布圖超鎂鐵質(zhì)巖體及圍巖地質(zhì)簡(jiǎn)圖(a)和額布圖超鎂鐵質(zhì)巖體剖面圖(b, 據(jù)Peng et al., 2013)

礦物代號(hào): Ol. 橄欖石; Opx. 斜方輝石; Cpy. 黃銅礦; Pn. 鎳黃鐵礦; Po. 磁黃鐵礦; Sul. 硫化物。

橄欖斜方輝石巖主要由斜方輝石(～75%)、橄欖石(～15%)、單斜輝石(<10%)和少量角閃石組成。斜方輝石粒徑可達(dá)2～3 mm。橄欖石呈粒狀或渾圓狀, 粒度介于0.2～1 mm之間。被斜方輝石和單斜輝石包裹的橄欖石的粒度多小于0.3 mm。沿橄欖石的裂理發(fā)育強(qiáng)烈蛇紋石化。單斜輝石呈它形粒狀, 粒徑介于0.4～0.5 mm之間, 邊部具輕微蝕變。角閃石呈它形, 充填于兩種輝石和橄欖石粒間(圖4b)。

3 分析方法

選取斜方輝石巖和橄欖斜方輝石巖樣品進(jìn)行光薄片磨制和觀察, 并挑選相對(duì)新鮮的巖石樣品進(jìn)行碎樣, 選用剛玉破碎機(jī)對(duì)樣品進(jìn)行粗碎, 粗碎后用瑪瑙球狀研磨機(jī)磨至200目左右, 以供全巖分析。

3.1 全巖主量元素分析

全巖主量元素分析在澳實(shí)分析檢測(cè)(廣州)有限公司利用X射線熒光光譜法(XRF)完成。儀器型號(hào)為PANalytical Axios型XRF光譜儀, 分析精度優(yōu)于5%。分析時(shí)所采用的標(biāo)樣為GBM306-12、GBM315-13、NCSDC73303和OREAS 195。

3.2 橄欖石成分分析

選擇樣品中相對(duì)新鮮的橄欖石顆粒進(jìn)行主、微量元素分析, 測(cè)試分析在中國(guó)科學(xué)院礦物學(xué)與成礦學(xué)重點(diǎn)實(shí)驗(yàn)室完成。主量元素分析采用JEOL JXA-8230型電子探針(EPMA), 束斑直徑1 μm, 加速電壓15 kV, 電流20 nA。分析Mg、Fe和Si時(shí), 分析時(shí)間為20 s, 背景分析時(shí)間為10 s; 分析Ca時(shí), 分析時(shí)間為40 s, 背景分析時(shí)間為20 s; 分析Ni和Mn時(shí), 分析時(shí)間為60 s, 背景分析時(shí)間為30 s。分析Mg、Si采用TAP晶體; 分析Fe、Mn和Ni采用 LIF晶體; 分析Ca采用PET晶體。使用的標(biāo)樣包括橄欖石(Si、Mg)、磁鐵礦(Fe)、透輝石(Ca)、菱錳礦(Mn)和鎳金屬(Ni)。分析元素的檢出限普遍<100 μg/g, 不需要波峰重疊校正。分析結(jié)果利用ZAF校正程序進(jìn)行了修正。

橄欖石微量元素分析采用Resonetic RESOlution S-155激光器結(jié)合 Agilent 7900質(zhì)譜聯(lián)機(jī)的激光剝蝕電感耦合等離子體質(zhì)譜儀(LA-ICP-MS)。激光剝蝕束斑直徑為43 μm, 頻率6 Hz, 以氦氣為載氣, 氬氣為補(bǔ)償氣來(lái)調(diào)節(jié)靈敏度, 背景分析時(shí)間為20 s, 數(shù)據(jù)采集時(shí)間為50 s。分別采用NIST SRM 610和GOR-132為外標(biāo)和監(jiān)控標(biāo)樣。最后以電子探針得到的橄欖石的Si含量為內(nèi)標(biāo)利用ICPMS-DataCal 軟件(Liu et al., 2008)對(duì)原始數(shù)據(jù)進(jìn)行校正。大多數(shù)微量元素檢出限小于 0.1 μg/g, 分析精度優(yōu)于10%。

4 分析結(jié)果

4.1 全巖主量元素組成

額布圖巖體全巖主量元素組成見(jiàn)表1。橄欖斜方輝石巖樣品的燒失量(LOI)均>4.0%, 因此, 全巖成分均被校正至100%硅酸鹽總量。在全巖的MgO與其他主量元素的二元圖解上, 橄欖斜方輝石巖相比斜方輝石巖, 具有較高的MgO和FeOT(FeOT= Fe2O3T′0.89)含量、較低的SiO2和Al2O3含量, 以及相似的CaO含量(圖5)。

(a) 斜方輝石巖由大小不一的斜方輝石堆晶組成, 含少量橄欖石、單斜輝和角閃石; (b) 橄欖斜方輝石巖主要由斜方輝石和橄欖石組成, 巖石具包橄結(jié)構(gòu)即橄欖石被包裹在斜方輝石。礦物代號(hào): Ol. 橄欖石; Opx. 斜方輝石; Cpx. 單斜輝石; Hb. 角閃石。

表1 額布圖巖體的巖石主量元素組成(%)

兩種巖石都表現(xiàn)出隨MgO含量降低, FeOT含量逐漸降低, 而SiO2和Al2O3含量逐漸升高的趨勢(shì), CaO含量變化不明顯。樣品的全巖氧化物含量多落在斜方輝石端元附近, 表明額布圖巖體兩種巖性巖石的礦物組成主要為斜方輝石, 其次為橄欖石和單斜輝石。Peng et al. (2013)樣品的全巖FeOT含量明顯高于本次研究的樣品, 樣品的Cu含量(117～1249 μg/g)也遠(yuǎn)高于本次樣品的Cu含量(36～304 μg/g, 課題組未發(fā)表數(shù)據(jù))。考慮到Cu與S之間的正相關(guān)關(guān)系(圖6), 表明Peng et al. (2013)的樣品含有更多的硫化物, 而硫化物可以提供一定量的Fe2+。因此, 樣品中FeOT含量的差別可能與其中硫化物的含量不同有關(guān)。

4.2 橄欖石的主量和微量元素組成

額布圖巖體橄欖石的Fo值介于83～87之間(表2), 其Fo值與Ni含量之間無(wú)明顯相關(guān)性。Fo<85的橄欖石與Fo>85的橄欖石具有明顯不同的Ni含量: Fo>85橄欖石的Ni含量與洋中脊玄武巖(MORB)中橄欖石斑晶的Ni含量范圍基本一致, 而Fo<85橄欖石的Ni含量則明顯偏高, 與夏威夷Koolau玄武巖中橄欖石斑晶的Ni含量類似(圖7a)。Fo>85橄欖石的Co含量與Fo值之間呈正相關(guān)關(guān)系, 而Fo<85橄欖石的Co含量與Fo值之間呈負(fù)相關(guān)關(guān)系(圖7b)。橄欖石的Mn和Zn含量總體與Fo值之間呈明顯的負(fù)相關(guān)關(guān)系(圖7c、d), 與巖漿結(jié)晶分異的趨勢(shì)一致, 但其Mn含量明顯低于MORB和Koolau玄武巖中橄欖石斑晶的Mn含量。橄欖石的Ca含量普遍<1000 μg/g,與Duke Island雜巖體中橄欖石的Ca含量類似(Li et al., 2011), 遠(yuǎn)低于MORB及洋島玄武巖(OIB)中橄欖石斑晶的Ca含量(>1500 μg/g, Foley et al., 2013)(圖7e)。而額布圖巖體中橄欖石的Ca含量大致與Fo值呈負(fù)相關(guān)關(guān)系(圖6e), 這可能與巖體形成過(guò)程中僅有少量的單斜輝石結(jié)晶分異有關(guān)。額布圖巖體中橄欖石的Li含量普遍高于5 μg/g, 略高于板內(nèi)巖漿巖中橄欖石的Li含量(≤ 5 μg/g, Foley et al., 2013) (圖7f)。

圖中橄欖石(Ol)、斜方輝石(Opx)和單斜輝石(Cpx)的成分范圍是礦物的電子探針?lè)治鼋Y(jié)果。

5 討 論

5.1 橄欖石Ni和Co含量對(duì)巖漿體系中硫化物飽和過(guò)程的指示

Ni和Co同為親鐵元素, 在橄欖石中均為相容元素, 橄欖石?硅酸鹽熔體之間的Ni分配系數(shù)(NiOl-Liq)為7.37～11.9(Wang and Gaetani, 2008), Co分配系數(shù)(CoOl-Liq)為2.48±0.68(Laubier et al., 2014)。二者又同為親銅元素, 極易進(jìn)入硫化物中, 硫化物?硅酸鹽熔體之間Ni的分配系數(shù)(NiSul-Liq)為300～1000(Patten et al., 2013), Co分配系數(shù)(CoSul-Liq)為20～580(Li and Audétat, 2012; Kiseeva and Wood, 2015), 均遠(yuǎn)遠(yuǎn)高于其在橄欖石?硅酸鹽熔體間的分配系數(shù)。在硫不飽和的巖漿體系中, 隨著橄欖石結(jié)晶, 巖漿的Ni和Co含量均逐漸降低, 同時(shí),NiOl-Liq及CoOl-Liq均逐漸升高, 但由于NiOl-Liq>CoOl-Liq,巖漿中Ni含量的下降幅度高于Co。因此, 隨著巖漿分離結(jié)晶作用的進(jìn)行, 從硫不飽和巖漿中結(jié)晶的橄欖石, 其Ni含量與Fo值呈正相關(guān)關(guān)系, 但Co含量與Fo值則可能呈負(fù)相關(guān)關(guān)系(Papike et al., 1999; Herd et al., 2009)。當(dāng)巖漿體系達(dá)到硫化物飽和時(shí), Ni和Co則優(yōu)先進(jìn)入硫化物熔體, 導(dǎo)致與硫化物熔體同時(shí)結(jié)晶的橄欖石的Ni和Co含量明顯降低。在亞固相階段, 硫化物熔體與橄欖石之間還可能發(fā)生Fe-Ni交換, 造成橄欖石的Ni含量與Fe含量呈正相關(guān), 而與Fo值之間呈負(fù)相關(guān)關(guān)系(Barnes and Naldrett, 1985; Li et al., 2007)。因此, 橄欖石的Ni和Co含量與Fo值的相關(guān)性可作為判別體系是否達(dá)到硫化物飽和的指標(biāo)。

圖6 額布圖巖體的全巖S與Cu含量二元圖解

表2 額布圖巖體斜方輝石巖和橄欖斜方輝石巖中的橄欖石平均成分

續(xù)表2:

MORB. 全球范圍內(nèi)洋中脊玄武巖中的橄欖石斑晶成分(據(jù)Sobolev et al., 2007); Koolau. 夏威夷Koolau洋島玄武巖中的橄欖石斑晶成分(據(jù)Sobolev et al., 2005); FC-1. 地幔橄欖巖來(lái)源熔體在3 GPa、1515 ℃和QFM條件下, 20%分離結(jié)晶過(guò)程中橄欖石成分的變化趨勢(shì)(據(jù)Sobolev et al., 2007)。MORB+OIB橄欖石的Ca含量和板內(nèi)巖漿巖中橄欖石的Li含量據(jù)Foley et al. (2013); Duke island 雜巖體中橄欖石的成分據(jù)Li et al. (2011)。

額布圖巖體Fo>85橄欖石的Ni含量明顯低于Fo<85橄欖石的Ni含量, 在橄欖石Fo>85時(shí)其Co含量與Fo值呈正相關(guān)關(guān)系(圖7a、b), 表明該巖漿體系在早期已達(dá)到了硫化物飽和, 即Ni和Co優(yōu)先進(jìn)入硫化物熔體中, 導(dǎo)致Fo>85橄欖石的Ni和Co含量明顯降低。另一方面, Fo<85橄欖石的Ni含量與Fo值之間呈負(fù)相關(guān)關(guān)系, 表明橄欖石與硫化物熔體之間發(fā)生了Fe-Ni交換(Barnes and Naldrett, 1985; Li et al., 2007), 說(shuō)明體系發(fā)生了硫化物熔離。因此, 額布圖巖體中橄欖石的Ni含量受到了硫化物熔離的影響, 其Ni/Mg值不能用來(lái)探討地幔源區(qū)的組成特征。而橄欖石的Mn和Zn含量主要受控于巖漿結(jié)晶分異過(guò)程, 基本不受體系硫化物飽和的影響, 因此可利用橄欖石的Mn、Zn含量和Mn/Zn值等對(duì)其源區(qū)性質(zhì)進(jìn)行判別。

5.2 橄欖石微量元素比值對(duì)源區(qū)輝石巖組分的甄別

前人模擬計(jì)算結(jié)果顯示, 在Fo值相同時(shí), MORB中橄欖石斑晶與從橄欖巖地幔3 GPa壓力下部分熔融熔體中結(jié)晶的橄欖石具有類似的Mn含量, 均高于輝石巖地幔部分熔融熔體中結(jié)晶橄欖石的Mn含量(Sobolev et al., 2007; Herzberg, 2011)。額布圖巖體中橄欖石的Mn含量明顯低于MORB中橄欖石, 但接近于輝石巖地幔來(lái)源的Koolau玄武巖中橄欖石(圖7c)。這表明, 額布圖巖體的母巖漿可能不是橄欖巖地幔部分熔融的產(chǎn)物, 而更可能與輝石巖地幔部分熔融作用有關(guān)。

額布圖巖體橄欖石的Mn/Zn值隨Fo值降低基本保持不變, 說(shuō)明該比值基本未受分離結(jié)晶作用的影響。該Mn/Zn值<13, 遠(yuǎn)低于來(lái)源于橄欖巖地幔的Baffin苦橄巖和Etendeka玄武巖中橄欖石斑晶的Mn/Zn值, 而與來(lái)源于輝石巖地幔的Mwenezi和Tuli玄武巖中橄欖石斑晶的Mn/Zn值一致(圖8)。

橄欖石的Mn/Fe和Zn/Fe值可以近似代表其幔源巖漿的相應(yīng)比值, 可用來(lái)判斷其地幔源區(qū)的性質(zhì)(Le Roux et al., 2010; Howarth and Harris, 2017)。額布圖巖體中橄欖石的Mn/Fe和Zn/Fe值與Mwenezi和Tuli玄武巖中橄欖石斑晶的相應(yīng)比值一致, 均位于從輝石巖地幔熔融產(chǎn)生的熔體中結(jié)晶橄欖石的相應(yīng)比值區(qū)域(圖9b), 也指示其地幔源區(qū)可能存在輝石巖組分。

選取額布圖巖體中Fo值最高的橄欖石(Fo=86.6), 利用Sobolev et al. (2007)提出的公式計(jì)算了來(lái)自輝石巖地幔的熔體對(duì)額布圖巖體母巖漿成分的貢獻(xiàn)。

px=3.48?2.071×(100×Mn/Fe)

式中:px表示輝石巖地幔來(lái)源熔體的比例; Mn和Fe分別為橄欖石中兩種元素的含量。橄欖巖地幔對(duì)應(yīng)的px值為?0.2～0.2, 輝石巖地幔對(duì)應(yīng)的px值為0.8～1.2(Sobolev et al., 2009)。計(jì)算結(jié)果為1.06, 表明額布圖巖體的母巖漿可能幾乎全部是輝石巖地幔部分熔融的產(chǎn)物。

5.3 橄欖石的Ca和Li含量對(duì)地幔交代過(guò)程的指示

通常認(rèn)為, 橄欖石的Ca含量主要受溫度、壓力和熔體成分的控制(Jurewicz and Watson, 1988; O’Reilly et al., 1997; Libourel, 1999; De Hoog et al., 2010), 但這些因素不能完全解釋MORB、OIB和大陸溢流玄武巖(CFB)中橄欖石斑晶的Ca含量范圍相互重疊的現(xiàn)象(Gavrilenko and Herzberg, 2016)。越來(lái)越多的證據(jù)表明, 橄欖石的Ca含量可能還受到巖漿體系中水含量的控制, 巖漿的含水量越高, Ca則越不容易進(jìn)入橄欖石中。例如, 從富水的島弧巖漿中結(jié)晶的橄欖石相比從相對(duì)貧水的板內(nèi)巖漿中結(jié)晶的橄欖石, 具有明顯更低的Ca含量(Feig et al., 2006; Kamenetsky et al., 2006; Gavrilenko and Herzberg, 2016)。因此, 橄欖石的Ca含量可用來(lái)表征幔源巖漿的含水程度。額布圖巖體中橄欖石的Ca含量遠(yuǎn)低于MORB和OIB中橄欖石斑晶的Ca含量, 與島弧環(huán)境形成的Duke island 雜巖體中橄欖石的Ca含量接近(圖7e), 表明額布圖巖體的母巖漿含水量較高, 這與巖石中普遍發(fā)育含水礦物(如角閃石)(圖4)的特征一致。

Etandeka玄武巖和Baffin苦橄巖來(lái)源于橄欖巖地幔, Mwenezi和Tuli玄武巖來(lái)源于輝石巖地幔。數(shù)據(jù)來(lái)源: Tuli, Mwenezi, Etendeka玄武巖中的橄欖石斑晶成分據(jù)Howarth and Harris (2017); Baffin苦橄巖中的橄欖石斑晶成分據(jù)De Hoog et al. (2010)。

橄欖巖地幔和輝石巖地幔參考范圍分別引自Sobolev et al. (2007)和Howarth and Harris (2017)。

橄欖石?硅酸鹽熔體之間Li的分配行為基本不受溫度控制, 橄欖石的Li含量主要與熔體成分有關(guān)(Brenan et al., 1998)。地幔橄欖巖的Li含量為1～2 μg/g (Seitz and Woodland, 2000), MORB的Li含量一般<4 μg/g(Tomascak et al., 2008; Marschall et al., 2017), 板內(nèi)玄武巖中橄欖石斑晶的Li含量一般<5 μg/g (Jeffcoate et al., 2007; Chan et al., 2009; Foley et al., 2013)。但是, 在俯沖過(guò)程中, 板片脫水可將蝕變洋殼及大陸沉積物中的Li遷移至地幔楔, 造成地幔楔部分熔融產(chǎn)生的熔體中具有較高的Li含量(Chan and Kastner, 2000; Chan et al., 2002; Tomascak et al., 2002; Bouman et al., 2004; Tang et al., 2014)。額布圖巖體橄欖石的Li含量明顯高于板內(nèi)巖漿巖中橄欖石的Li含量(圖7f), 表明其地幔源區(qū)可能有俯沖板片組分的加入。

額布圖巖體橄欖石總體具有低Ca、高Li的特征, 表明其地幔源區(qū)可能經(jīng)歷了俯沖板片來(lái)源熔體/流體的交代, 與Peng et al. (2013)認(rèn)為該巖體形成于俯沖階段的認(rèn)識(shí)一致。而中亞造山帶其他含礦巖體則主要形成在碰撞后伸展階段, 初始巖漿中可能還具有來(lái)自軟流圈地幔熔體的組分(Li et al., 2012; Mao et al., 2014)。

5.4 輝石巖地幔組分的成因

額布圖巖體中鋯石的Hf()值介于–8.5～–4.2之間(Peng et al., 2013), 全巖Nd()值介于–8～–5之間(課題組未發(fā)表數(shù)據(jù)), 其地幔源區(qū)具富集地幔特征, 與華北克拉通西部大陸下巖石圈地幔的特征基本一致(趙磊等, 2011; Guo et al., 2014; Pang et al., 2017)。前人通過(guò)對(duì)華北克拉通西部地幔包體和中?新生代巖漿巖的地球化學(xué)研究, 證實(shí)華北克拉通西部大陸下巖石圈地幔曾遭受了明顯的交代作用(Guo et al., 2014; Dai et al., 2018; Dai and Zheng, 2019)、并存在古老的下地殼組分(榴輝巖)(Guo et al., 2014)。本次研究認(rèn)為, 當(dāng)古亞洲洋向南俯沖至華北克拉通大陸下巖石圈地幔時(shí), 俯沖板片釋放的流體交代了上覆含有榴輝巖組分的地幔楔, 由于榴輝巖的固相線溫度較低, 其優(yōu)先熔融后產(chǎn)生的熔體與周圍的地幔橄欖巖反應(yīng)形成了輝石巖地幔組分。輝石巖地幔部分熔融產(chǎn)生的熔體形成了額布圖巖體的母巖漿, 因此, 該巖體的巖石微量元素和同位素組成具有來(lái)自富集地幔島弧巖漿的地球化學(xué)特征。

5.5 輝石巖地幔組分對(duì)銅鎳硫化物礦床形成的貢獻(xiàn)

幔源巖漿中的成礦金屬元素含量(如Ni、Cu和PGE等)是能否形成巖漿銅鎳硫化物礦床的重要因素之一, 其主要與地幔的部分熔融程度有關(guān)(Naldrett, 1989; Barnes and Lightfoot, 2005)。地幔高程度部分熔融不僅可以提高巖漿中成礦元素的含量, 而且可形成較大體積的巖漿量, 大量巖漿流經(jīng)穿越地殼的巖漿通道/巖漿房時(shí), 如果發(fā)生地殼混染, 則更有利于造成巖漿的硫化物飽和(Arndt et al., 2005; Barnes and Lightfoot, 2005)。因此, 絕大多數(shù)世界級(jí)銅鎳硫化物礦床都與地幔柱引發(fā)的大規(guī)模巖漿作用有關(guān)(Pirajno, 2000; Ernst, 2007; Pirajno et al., 2009; Begg et al., 2010), 如Noril’sk, Bushveld, Voisey’s Bay, Jinchuan, Pechenga等(Barnes et al., 2001; Barnes and Lightfoot, 2005; Li et al., 2005; Naldrett, 2010)。然而, 額布圖巖體形成于匯聚板塊邊緣環(huán)境, 其形成與俯沖作用有關(guān)。在俯沖環(huán)境產(chǎn)生的巖漿量無(wú)法與板內(nèi)地幔柱活動(dòng)或裂谷伸展環(huán)境產(chǎn)生的巖漿量相比, 但額布圖巖體的地幔源區(qū)中含有大量的輝石巖組分。相比橄欖巖地幔, 輝石巖地幔部分熔融產(chǎn)生的熔體具有較高的Ni含量(Sobolev et al., 2005; Straub et al., 2008; Herzberg, 2011)。因此, 額布圖巖體中發(fā)育的銅鎳硫化物礦化說(shuō)明, 在匯聚板塊邊緣發(fā)育的鎂鐵?超鎂鐵質(zhì)巖體, 如果其地幔源區(qū)含有輝石巖組分, 則地幔源區(qū)部分熔融產(chǎn)生的巖漿成礦元素含量有可能比較高, 有利于對(duì)銅鎳硫化物礦化。

6 結(jié) 論

額布圖巖體形成在匯聚板塊邊緣環(huán)境, 其地幔源區(qū)可能經(jīng)歷過(guò)俯沖板片來(lái)源流體的交代作用, 巖石中的橄欖石具有低Ca、高Li的特征。巖體的母巖漿可能是輝石巖地幔的部分熔融產(chǎn)物, 造成橄欖石具有較低的Mn含量和Mn/Fe值, 以及較高的Zn/Fe值。古亞洲洋板塊向南俯沖至華北克拉通之下, 來(lái)自俯沖板塊的流體和揮發(fā)分進(jìn)入上覆地幔楔, 導(dǎo)致華北克拉通大陸下巖石圈地幔中的古老下地殼組分(榴輝巖)發(fā)生熔融、并與周圍的地幔橄欖巖反應(yīng), 從而造成額布圖巖體的地幔源區(qū)中含有大量的輝石巖組分。由于輝石巖地幔部分熔融產(chǎn)生的熔體具有較高的Ni含量, 造成了額布圖巖體具有一定的銅鎳硫化物礦化潛力。因此, 輝石巖地幔組分的存在, 對(duì)于匯聚板塊邊緣環(huán)境中鎂鐵?超鎂鐵質(zhì)巖漿的銅鎳硫化物成礦作用是十分有利的。

中國(guó)科學(xué)院礦物學(xué)與成礦學(xué)重點(diǎn)實(shí)驗(yàn)室邢長(zhǎng)明副研究員和吳丹老師在電子探針和LA-ICP-MS測(cè)試方面給予了指導(dǎo)和幫助; 中國(guó)科學(xué)院廣州地球化學(xué)研究所黃小龍研究員和匿名審稿人對(duì)本文提出的寶貴意見(jiàn)和建議, 一并表示感謝。

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Compositions of Olivine of Permian Erbutu Ultramafic Intrusion in the Central Asian Orogenic Belt (Inner Mongolia): Insights for the Pyroxenite Mantle Source

BAO Hongtian1, 2, WANG Christina Yan1, 3*, CAO Yonghua1and WEI Bo1, 3

(1.510640,; 2.100049,; 3.510640,)

The Erbutu ultramafic intrusion is located in the suture zone between the Central Asian Orogenic Belt and the northern margin of the North China Craton (NCC). The intrusion contains Ni-Cu sulfide mineralization. The intrusion is mainly composed of orthopyroxenite and olivine orthopyroxenite, both of which contain olivine that has Fo contents ranging from 83 to 87. At given Fo contents, the olivine in the rocks of the Erbutu intrusion has much lower Mn than that of the olivine crystallized from the magma derived from the peridotite mantle, indicating that the parental magma of the Erbutu intrusion was unlikely derived from a peridotite mantle. The olivine in the rocks of the Erbutu intrusion has Mn/Zn, Mn/Fe and Zn/Fe ratios similar to those of the olivine in the Mwenezi and Tuli basalts that were derived from typical pyroxenite mantle. Therefore, the Erbutu intrusion is likely derived from a pyroxenite dominated mantle source. The olivine in the rocks of the Erbutu intrusion contains lower Ca but higher Li than that of the olivine in igneous rocks in intra-plate settings, indicating that the mantle source of the intrusion may have been metasomatized by slab-derived fluids. It is proposed that during the subduction of the paleo-Asian oceanic slab beneath the northern margin of the NCC, slab-derived fluids were released and added to the mantle wedge and triggered melting of ancient lower crustal materials (., eclogite) in the mantle, the melt of the eclogite was then reacted with the ambient mantle peridotite and produced the pyroxenite component in the mantle.

olivine; pyroxenite mantle; Erbutu ultramafic intrusion; Central Asian Orogenic Belt

P595

A

1001-1552(2021)06-1185-017

10.16539/j.ddgzyckx.2021.06.005

2020-08-29;

2020-10-14

國(guó)家自然科學(xué)基金項(xiàng)目(41730423、41902077)資助。

暴宏天(1994–), 男, 博士研究生, 礦物學(xué)巖石學(xué)礦床學(xué)專業(yè)。Email: hongtian_b@outlook.com

王焰(1969–), 女, 研究員, 從事巖漿作用與成礦研究。Email: wang_yan@gig.ac.cn