峨眉山大火成巖省平川苦橄巖地幔源區(qū)性質(zhì)——來自橄欖石微量元素的制約

張 磊, 任鐘元, 張 樂

峨眉山大火成巖省平川苦橄巖地幔源區(qū)性質(zhì)——來自橄欖石微量元素的制約

張 磊1, 2, 3, 任鐘元1, 2*, 張 樂1, 2

(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)國家重點實驗室, 廣東 廣州 510640; 2. 中國科學(xué)院深地科學(xué)卓越創(chuàng)新中心, 廣東 廣州 510640; 3. 中國科學(xué)院大學(xué), 北京 100049)

橄欖石是基性巖漿中最早期結(jié)晶的硅酸鹽礦物之一, 其主量、微量元素特征可以反映出巖漿演化環(huán)境、巖漿源區(qū)巖性和再循環(huán)組分性質(zhì)等重要信息。本次研究通過對峨眉山大火成巖省平川苦橄巖中橄欖石主量和微量元素分析, 以及橄欖石內(nèi)尖晶石包裹體分析, 并與大理苦橄巖中橄欖石和尖晶石成分進(jìn)行對比, 來探討不同苦橄巖母巖漿氧逸度及源區(qū)性質(zhì)的異同。橄欖石?尖晶石?斜方輝石氧逸度計計算結(jié)果表明, 平川苦橄巖母巖漿比大理苦橄巖母巖漿具有更高的氧逸度(分別為ΔQFM+0.24和ΔQFM?0.51), 這與平川橄欖石較大理橄欖石具有更高的V/Sc值相一致。平川橄欖石具有較高Ni/Co值, 暗示苦橄巖源區(qū)中存在再循環(huán)物質(zhì)的參與, 橄欖石較低的Li含量則進(jìn)一步表明這種再循環(huán)物質(zhì)更可能是洋殼而不是陸殼物質(zhì)。在地幔源區(qū)判別圖中, 平川橄欖石主要位于橄欖巖起源熔體結(jié)晶的橄欖石成分區(qū)域。然而, 平川橄欖石比大理橄欖石具有更高Ni、Co含量和Zn/Fe、Fe/Mn值, 低Ca、Mn、Sc含量和Mn/Zn值, 暗示平川苦橄巖源區(qū)中存在輝石巖組分。少量的再循環(huán)洋殼與橄欖巖地?；旌? 會形成富橄欖石的混合源區(qū), 且這種地幔產(chǎn)生的熔體結(jié)晶的橄欖石與平川橄欖石一樣, 成分仍近似于橄欖巖熔體中結(jié)晶的橄欖石。但平川橄欖石主量、微量元素特征表明, 其成分受源區(qū)中輝石巖組分的控制, 結(jié)合平川橄欖石較大的成分變化范圍, 進(jìn)一步表明平川苦橄巖地幔源區(qū)中是含有輝石巖組分的, 而不是均一的橄欖巖源區(qū)。

峨眉山大火成巖省; 苦橄巖; 橄欖石; 輝石巖; 氧逸度

0 引 言

大火成巖省(LIP)代表了地球歷史上短時間存在的大規(guī)模巖漿事件(Coffin and Eldholm, 1992; Bryan and Ernst, 2008), 這些事件產(chǎn)生了巨量的基性火山巖和相應(yīng)的侵入體, 廣泛分布在現(xiàn)今大陸和大洋盆地之上。許多LIP被認(rèn)為與地幔柱有關(guān), 因此對研究地球地幔性質(zhì)具重要意義。峨眉山大火成巖省位于揚(yáng)子板塊西緣, 是我國境內(nèi)典型的LIP之一(Chung and Jahn, 1995; 圖1)。已有研究表明峨眉山LIP形成與地幔柱活動有關(guān), 且可能是導(dǎo)致瓜德普魯統(tǒng)生物滅絕的因素之一(Zhang et al., 2013; Yang et al., 2015)。峨眉山LIP的巖性以基性巖為主, 伴有少量中?酸性巖(Xu et al., 2001, 2010)。這些基性巖絕大多數(shù)為拉斑玄武巖, 較演化(MgO<8%), 并普遍遭受了強(qiáng)烈的蝕變作用(Li et al., 2010; Ren et al., 2017)。在地球化學(xué)組成上, Xu et al. (2001)提出以Ti/Y=500為界, 把峨眉山大火成巖省中基性巖分為高鈦和低鈦兩類玄武巖。隨著研究的不斷深入, 人們逐漸發(fā)現(xiàn)峨眉山玄武巖Ti/Y值是連續(xù)變化的(Kamenetsky et al., 2012; Ren et al., 2017)。

對于峨眉山大火成巖省高鈦?低鈦巖漿的成因以及地幔源區(qū)性質(zhì), 一直是地學(xué)界研究的熱點問題。一些研究者認(rèn)為, 高鈦巖漿與地幔柱直接相關(guān), 而低鈦巖漿則來源于大陸巖石圈地幔的高程度熔融, 或受到地殼物質(zhì)的明顯混染(Xu et al., 2001; Xiao et al., 2004; Wang et al., 2007; Fan et al., 2008; Zhou et al., 2008)。Xu et al. (2007)通過Re-Os同位素研究認(rèn)為, 低鈦玄武巖來源于地幔柱源區(qū), 而高鈦玄武巖來源于大陸巖石圈地幔, 或是地幔柱有關(guān)的巖漿在上升過程中有大量巖石圈地幔物質(zhì)的混染; Shellnutt and Jahn (2011)提出, 高鈦和低鈦玄武質(zhì)巖漿可能來源于相同的源區(qū), 它們的成分差異可能是由于部分熔融程度不同和地殼混染造成的; Hou et al. (2011)認(rèn)為巖漿中Ti/Y值差異并不是源區(qū)不同造成的, 而主要是受控于鐵鈦氧化物的分離結(jié)晶。巖漿源區(qū)性質(zhì)方面, 前人普遍認(rèn)為峨眉山玄武巖的源區(qū)以橄欖巖為主(Xu et al., 2001; Xiao et al., 2004; Zhang et al., 2006; Lai et al., 2012; Huang et al., 2014)。隨著研究的深入, 一些學(xué)者認(rèn)為輝石巖可能存在于峨眉山玄武巖源區(qū)(Kamenetsky et al., 2012; Ren et al., 2017; Liu et al., 2017; Yu et al., 2017)。Kamenetsky et al. (2012)提出高鈦巖漿來源于輝石巖源區(qū), 低鈦巖漿來源于橄欖巖源區(qū); Ren et al. (2017)根據(jù)峨眉山苦橄巖中的橄欖石成分, 以及大理苦橄巖較均一的熔體包裹體Pb同位素特征, 認(rèn)為高鈦巖漿和低鈦巖漿均來源于較均一的輝石巖源區(qū)。

橄欖石是基性巖漿中早期結(jié)晶的硅酸鹽礦物。橄欖石微量元素(如Ni、Ca、Fe、Mn等)廣泛用于區(qū)分不同的地幔源區(qū)組分(Ren et al., 2017; Liu et al., 2017; Yu et al., 2017; Yao et al., 2019; Xu et al., 2020)。在組成上地幔的礦物中, Ni主要存在于橄欖石中, Ca、Mn主要存在于輝石和石榴子石中, 且只有橄欖石的Fe/Mn>1(Humayun et al., 2004)。因此, 輝石巖源區(qū)起源的熔體被認(rèn)為比橄欖巖源區(qū)熔體具更高Ni含量和Fe/Mn值, 以及低Ca、Mn含量, 這種特征也同樣會反映在這些熔體結(jié)晶的橄欖石中(Sobolev et al., 2005, 2007; Herzberg, 2011)。不過也有學(xué)者提出, 這些判斷指標(biāo)并不是絕對的, 主要由于橄欖石中微量元素含量和比值會受源區(qū)成分、形成時溫壓條件以及熔體成分影響(Yang et al., 2016; Heinonen and Fusswinkel, 2017; Matzen et al., 2017)。Matzen et al. (2017)指出, 橄欖巖源區(qū)在不同壓力下部分熔融對Ni和Mn的分配有很強(qiáng)影響。Heinonen and Fusswinkel (2017)提出Karoo大火成巖省麥美奇巖中橄欖石的高Ni、低Mn特征是由于深部較高溫壓的熔融條件所導(dǎo)致, 因此不需要引入輝石巖組分來解釋鎂鐵?超鎂鐵質(zhì)巖石中富Ni和貧Mn橄欖石的成分特征。在峨眉山大火成巖省研究中, Yao et al. (2019)指出峨眉山苦橄巖中橄欖石的Mn/Zn、10000×Zn/Fe值與橄欖巖源區(qū)熔體中結(jié)晶的橄欖石成分一致; Xu et al. (2020)提出結(jié)晶/熔融壓力變化可用于解釋峨眉山大火成巖省苦橄巖中橄欖石的成分范圍, 并認(rèn)為峨眉山大火成巖省的源區(qū)中沒有輝石巖組分的參與; Yao et al. (2019)和Xu et al. (2020)均發(fā)現(xiàn)在Ni-Mn/Zn和100×Mn/Fe-10000×Zn/Fe圖解中, 峨眉山大火成巖省樣品主要落在橄欖巖區(qū)域。因此, 峨眉山大火成巖省源區(qū)中是否含有輝石巖至今仍然存在爭議。

橄欖石中的第一排過渡元素(FRTE)(如Sc、Zn、Co)特征可反映源區(qū)巖性信息(Le Roux et al., 2010, 2011; Davis et al., 2013; Foley et al., 2013; S?ager et al., 2015), 通過對橄欖石成分研究也可以獲得巖漿氧逸度、橄欖石結(jié)晶溫度和巖漿中H2O含量等信息(Ballhaus et al., 1991; Wan et al., 2008; Mallmann and O’Neill, 2013; Coogan et al., 2014; Gavrilenko et al., 2016; Nikolaev et al., 2016)。峨眉山苦橄巖中, 普遍存在高Fo橄欖石(比如本研究中的平川苦橄巖中的橄欖石Fo可達(dá)91.6), 表明其是從原始的巖漿結(jié)晶而來。本文以平川苦橄巖中的橄欖石為研究對象, 選取含有尖晶石包裹體的橄欖石, 進(jìn)行橄欖石主、微量元素以及尖晶石的主量元素成分分析, 從而估算平川苦橄巖的巖漿氧逸度, 討論橄欖石成分特征與源區(qū)礦物組成以及與再循環(huán)物質(zhì)之間的關(guān)系, 希望對峨眉山大火成巖省源區(qū)組成及性質(zhì)有所啟示。

1 地質(zhì)背景與樣品介紹

峨眉山大火成巖省位于揚(yáng)子板塊西緣, 在越南、中國西藏羌塘地區(qū)和廣西西部也有少量峨眉山玄武巖及苦橄巖分布(Chung et al., 1998; Hanski et al., 2004; Wang et al., 2007; Fan et al., 2008; Anh et al., 2011; Liu et al., 2016), 大火成巖省西南部在新生代印度板塊和亞歐板塊的碰撞中被侵蝕(Ali et al., 2004; Zhou et al., 2006)。峨眉山大火成巖省分布面積大于2.5×105km2。噴出巖巖性由亞堿性及偏堿性的基性火山熔巖及火山碎屑巖組成, 多為無斑隱晶質(zhì); 酸性巖主要為流紋巖和花崗巖, 主要與巖漿活動晚期的玄武巖共存(Xu et al., 2010); 苦橄巖較少, 呈小露頭發(fā)現(xiàn)于麗江、大理、賓川、永勝、平川、松達(dá)、玉門等地(Zhang et al., 2006, 2008; Wang et al., 2007; Hanski et al., 2010; Kamenetsky et al., 2012; Tao et al., 2015; Ren et al., 2017; Yu et al., 2017, 2020; Wu et al., 2018; Yao et al., 2019)。峨眉山大火成巖省中產(chǎn)有釩鈦磁鐵礦(如攀枝花、紅格、白馬、新街)和銅鎳硫化物礦床(如金寶山、力馬河、朱布)(Wang et al., 2005, 2007, 2008, 2010; Tao et al., 2007, 2008, 2010; Hou et al., 2012; Tang et al., 2013; Song et al., 2013; Wang and Zhou, 2013; 宋謝炎等, 2018; Bai et al., 2019; Cao et al., 2019), 其中釩鈦磁鐵礦礦床賦存于多個鎂鐵質(zhì)?超鎂鐵質(zhì)層狀巖體中, 組成了攀西地區(qū)超大型Fe-Ti氧化物礦集區(qū)(王焰等, 2017)。峨眉山大火成巖省巖漿事件的主噴發(fā)階段在260 Ma (Zhong et al., 2014, 2020; Li et al., 2017), 噴發(fā)持續(xù)時間至少為6 Ma(Shellnutt et al., 2020), 這些火山噴發(fā)可能導(dǎo)致了瓜德普魯期生物大滅絕(Wignall et al., 2009; Zhang et al., 2013; Yang et al., 2015; Xu et al., 2018)。根據(jù)峨眉山玄武巖下伏茅口組灰?guī)r的剝蝕程度差異, 峨眉山大火成巖省可劃分為內(nèi)帶、中帶、外帶三個區(qū)域(圖1)(He et al., 2003; Xu et al., 2004)。

本次研究樣品采自四川省鹽源縣平川鐵礦區(qū), 位于峨眉山大火成巖省內(nèi)帶北緣。礦區(qū)內(nèi)可見苦橄質(zhì)巖脈呈巖墻產(chǎn)出, 厚度約10 m, 巖脈侵入到二疊系茅口組灰?guī)r中, 且與圍巖接觸界線清晰?？嚅蠋r樣品呈斑狀結(jié)構(gòu), 斑晶主要由橄欖石(30%～40%)以及極少量(<5%)單斜輝石組成。橄欖石斑晶多呈自形?它形產(chǎn)出, 粒度多介于0.5～3 mm之間, 沿裂隙和邊部發(fā)生蛇紋石化和綠泥石化, 剩下橄欖石殘晶(圖3); 橄欖石中多包裹有自型尖晶石, 其粒度通常小于20 μm?；|(zhì)主要由細(xì)粒的橄欖石、單斜輝石、斜長石以及Fe-Ti氧化物組成。平川苦橄巖具有較高M(jìn)gO含量(17.6%～24.8%), 較低CaO(8.0%～9.6%)和Al2O3(8.0%～9.6%)含量, 屬于拉斑玄武巖; 全巖Ti/Y=472～507(吳亞東, 2018)。大理苦橄巖(全巖Ti/Y=358～408)是峨眉山大火成巖省中研究程度較高的苦橄巖(Zhang et al., 2013, 2019a; Ren et al., 2017; Wu et al., 2018; 張樂, 2019), 本次也對大理苦橄巖的橄欖石及尖晶石包裹體進(jìn)行分析, 并與平川橄欖石進(jìn)行對比。

圖1 峨眉山大火成巖省玄武巖和鎂鐵質(zhì)?超鎂鐵質(zhì)侵入體分布圖(據(jù)Kamenetsky et al., 2012修改)

Karoo苦橄巖中的橄欖石來自具有高比例殼源再循環(huán)物質(zhì)的無橄欖石輝石巖地幔源區(qū)(Harris et al., 2015; Kamenetsky et al., 2017, Howarth and Harris, 2017); Etendeka苦橄巖中的橄欖石來自于橄欖巖源區(qū)(Thompson et al., 2001, Howarth and Harris, 2017); Southern Payenia玄武巖中的橄欖石代表橄欖巖?輝石巖混合源區(qū)熔體中產(chǎn)生的橄欖石(S?ager et al. 2015, Howarth and Harris, 2017); Iceland橄欖石來自于含有約10%輝石巖的地幔源區(qū)(Shorttle et al., 2014, Neave et al., 2018)。數(shù)據(jù)來源: Karoo苦橄巖和Etendeka苦橄巖中橄欖石據(jù)Howarth and Harris (2017); Southern Payenia橄欖石據(jù)S?ager et al. (2015); Iceland橄欖石據(jù)Neave et al. (2018)。

2 分析方法

首先切除樣品的風(fēng)化表面, 將樣品破碎成1 cm大小的小塊, 之后使用SELFRAG高壓脈沖破碎儀對樣品進(jìn)行進(jìn)一步的破碎。該儀器可以使樣品盡可能沿著顆粒邊界裂解開, 從而保持礦物顆粒的相對完整。使用雙目顯微鏡從粉碎的礦物顆粒中手工挑取新鮮的橄欖石顆粒, 將挑取的橄欖石鑲嵌在環(huán)氧樹脂中, 仔細(xì)拋光使包裹在橄欖石中的尖晶石暴露至靶表面用于后續(xù)分析(Ren et al., 2005, 2017)。所有分析均在中國科學(xué)院廣州地球化學(xué)研究所同位素地球化學(xué)國家重點實驗室進(jìn)行。

橄欖石的主量元素分析在JEOL JXA-8210上進(jìn)行。測試條件為: 加速電壓15 kV, 電流20 nA, 束斑直徑3 μm。重復(fù)分析橄欖石標(biāo)樣MongOL(Batanova et al., 2019)顯示, SiO2、MgO和FeO的分析精度(2RSD)均優(yōu)于1.4%。尖晶石的主量元素分析在Cameca SXFiveFE場發(fā)射電子探針上完成, 具體條件為: 加速電壓20 kV, 電流20 nA, 束斑直徑2 μm。每測試5個點測試一次Chromite-1玻璃標(biāo)樣, 其中Cr2O3、Al2O3和MgO的分析精度(2RSD)優(yōu)于1%, FeO、TiO2的分析精度(2RSD)優(yōu)于2.4%。

采用激光剝蝕高分辨扇形磁場電感耦合等離子體質(zhì)譜儀(LA-SF-ICP-MS)進(jìn)行橄欖石微量元素分析(表1)。配有193 nm準(zhǔn)分子激光器(COMPexPro)的激光剝蝕系統(tǒng)(RESOlution M-50)用于原位燒蝕樣品, 分析使用的激光直徑為45 μm, 頻率6 Hz, 能量密度為約4 J/cm2。氦氣被用作載氣, 將剝蝕產(chǎn)生的氣溶膠運載到質(zhì)譜中。測試使用的質(zhì)譜儀為Thermo Fisher Scientific的 ELEMENT XR電感耦合等離子質(zhì)譜儀, 該儀器的靈敏度高于傳統(tǒng)四級桿ICP-MS, 并可以在低、中、高質(zhì)量分辨率模式下工作。該檢測器由一個二次電子倍增器和一個法拉第杯組成, 可提供從0到1012次計數(shù)/秒(cps)的線性動態(tài)范圍。質(zhì)譜的接口部分采用標(biāo)準(zhǔn)的Ni采樣錐和Ni截取錐, 相比X截取錐和Jet樣品錐可以降低分子氧化物的產(chǎn)率。在測試每個樣品前, 激光剝蝕3下來清除樣品表面可能的污染。每測試一個點包括20 s的背景采集和30 s的激光剝蝕。在分析過程中使用USGS標(biāo)樣BCR-2G、BHVO-2G和GSD-1G作為校正標(biāo)樣, 使用TB-1G作為監(jiān)控標(biāo)樣。TB-1G的測試結(jié)果顯示大部分元素的測試誤差(2RSD)小于10%。詳細(xì)的實驗過程見Zhang et al. (2019b)。

表1 平川苦橄巖中橄欖石的主量元素和微量元素組成

續(xù)表1:

續(xù)表1:

注: 主量元素單位為%; Fo為mol%; 微量元素中Ga、Y和Zr的單位為ng/g, 其他微量元素單位為μg/g。部分橄欖石微量低于檢出限, 故未顯示。

3 分析結(jié)果

平川苦橄巖中橄欖石主量、微量元素組成, 以及尖晶石成分見表1和表2。結(jié)果顯示, 平川苦橄巖中橄欖石CaO和Al2O3含量分別為0.38%～0.58%和0.037%～0.10%, 高于典型的地幔橄欖石(CaO<0.1%, Al2O3< 0.03%; Foley et al., 2013)。結(jié)合橄欖石中多含有尖晶石包裹體以及熔體包裹體, 表明這些橄欖石是巖漿成因。在全巖Mg#與橄欖石Fo的圖解中, 橄欖石的最高Fo與寄主全巖的Mg#平衡, 無明顯橄欖石堆晶(圖4)。橄欖石中Ni與Fo呈正相關(guān), Ca、Mn、Sc、Co與Fo呈負(fù)相關(guān), 符合橄欖石的分離結(jié)晶趨勢(圖5)。平川橄欖石Ni含量為2429～4519 μg/g, Zn含量為62.3～161 μg/g, Co含量為158～232 μg/g, Sc含量為5.11～11.3 μg/g, Fe/Mn值為62.9～76.3, Mn/Zn值高于11.8, 100×Mn/Fe值高于1.31。與大理橄欖石相比, 平川橄欖石整體上具有較高Ni、Co含量, 較低Ca、Mn、Sc含量, 以及較高Fe/Mn和Zn/Fe值(圖5)。在Ni-Mn/Zn和100×Mn/Fe-10000×Zn/Fe源區(qū)判別圖解中, 平川苦橄巖中橄欖石成分整體上與橄欖巖源區(qū)熔體中結(jié)晶的橄欖石成分相似, 但呈現(xiàn)出較高的Zn/Fe值和較低的Mn/Fe值, 其成分特征更趨向于橄欖巖源區(qū)和輝石巖源區(qū)的混合(圖2)。

平川苦橄巖中Cr尖晶石的Mg#值具有較大變化范圍(Mg#=0.25～0.56), Cr2O3含量為41.4%～48.0%, Cr#為0.43～0.72。與大理苦橄巖相比, 平川苦橄巖中尖晶石具有較低Mg#值, 其Mg#值更接近于高鈦苦橄巖(永勝苦橄巖; Kamenetsky et al., 2012)中的尖晶石(圖6)。

4 討 論

4.1 巖漿的氧逸度

橄欖石(Ol)?尖晶石(Sp)?斜方輝石(Opx)氧逸度計是估算地幔超基性巖氧逸度常用的方法(Ballhaus et al., 1991)。Nikolaev et al. (2016)結(jié)合不同氧化還原條件下的天然樣品和合成樣品的礦物?熔體平衡實驗的最新數(shù)據(jù), 重新評估、并改進(jìn)了該公式, 并將這種方法擴(kuò)展至不含斜方輝石(Opx)的礦物組合, 因此該氧逸度計也適用于基性巖漿氧逸度的估算。需要注意的是, 利用Nikolaev et al. (2016)新公式得到的峨眉山巖漿氧逸度比利用Ballhaus et al. (1991)公式得到的氧逸度(logo2)要低約1個log unit。Nikolaev et al. (2016)認(rèn)為Ballbaus et al. (1991)的公式系統(tǒng)高估了氧逸度環(huán)境。Nikolaev et al. (2016)實驗數(shù)據(jù)的參數(shù)范圍為: 溫度: 1015～1500 ℃,氧化還原條件: IW?3～NNO+1, Cr含量: 0～16%, 壓力范圍0.0001～2.7 GPa, 并且包括了含有斜方輝石和不含斜方輝石的實驗礦物組合。該方法的誤差約為±0.3 log units(Erdmann et al., 2019), 其公式如下:

表2 平川苦橄巖橄欖石中尖晶石包裹體成分(%)

Mg#=100×Mg2+/(Mg2++Fe2+), 假定Fe2+/FeT=0.9。

其中,的單位為K,的單位為GPa,Ol和Sp是橄欖石和尖晶石中的各離子摩爾分?jǐn)?shù)。

將平川橄欖石和尖晶石包裹體的成分帶入該公式。其中, 壓力假定為1.3 GPa(Tao et al., 2015), Fe3+和Fe2+的摩爾分?jǐn)?shù)是根據(jù)電價平衡原理計算獲得, 橄欖石?尖晶石的共結(jié)溫度可由Coogan et al. (2014)提出的橄欖石?尖晶石Al溫度計獲得。Nikolaev et al. (2016)指出, 當(dāng)Fe3+的含量大于2mol%時, 估算尖晶石中Fe3+比例所產(chǎn)生的誤差小于氧逸度計本身的誤差; 當(dāng)Fe3+的含量大于10mol%時, 由估算Fe3+導(dǎo)致的氧逸度偏差可以忽略不計。平川尖晶石的Fe3+摩爾分?jǐn)?shù)為1.5mol%～9.5mol%, 絕大多數(shù)大于4mol%, 由估算Fe3+引起的誤差較小。通過此公式, 我們獲得平川巖漿的氧逸度(logo2)范圍為ΔQFM–0.5～ ΔQFM+0.6, 這個氧逸度范圍與洋島玄武巖相似(ΔQFM=?0.6～+0.7; Hong et al., 2019), 低于島弧巖漿的氧逸度(ΔQFM=+0.03～+2.5; Hong et al., 2019)。由于在巖漿分離結(jié)晶過程中, 橄欖石和尖晶石的分離結(jié)晶分別會導(dǎo)致巖漿氧逸度升高和降低, 因此通過高Fo橄欖石計算獲得的氧逸度更能反映初始熔體的氧逸度。本研究中, 平川苦橄巖和大理苦橄巖最高Fo橄欖石具有相似的Fo, 計算獲得的平川和大理苦橄巖中Fo>88的橄欖石氧逸度平均值分別為ΔQFM=+0.24和ΔQFM=–0.51。

此外, 橄欖石V/Sc值也可以指示巖漿氧逸度。V和Sc在橄欖石中具有相似的分配系數(shù), 但是在氧化環(huán)境時, V5+的相對含量會比其他價態(tài)(V2+, V3+, V4+)高(Li and Lee, 2004; Lee et al., 2005)。V5+的增加會導(dǎo)致V在橄欖石?熔體間的分配系數(shù)降低, 導(dǎo)致V在氧化條件下更不相容(Canil, 1997), 所以高氧逸度環(huán)境下結(jié)晶的橄欖石通常具有較低的V/Sc值(Foley et al., 2013)。平川橄欖石V/Sc值整體上與橄欖石?尖晶石氧逸度計得出的氧逸度呈負(fù)相關(guān)關(guān)系(圖7), 表明平川苦橄巖母巖漿比大理苦橄巖母巖漿具有更高的氧逸度。影響巖漿氧逸度的因素較多, 部分熔融程度、部分熔融方式、壓力(深度)以及交代作用等均可造成巖漿氧逸度的差異(Wood et al., 1990; Kress and Carmichael, 1991; Dixon et al., 1997; Brounce et al., 2014; Gaetani, 2016; 柏中杰等, 2019)。峨眉山大火成巖省中, 高鈦巖漿熔融程度低于低鈦巖漿(Shellnut and Jahn, 2011; Ren et al., 2017; Zhang et al., 2019a; Xu et al., 2020)。平川苦橄巖較大理苦橄巖具有更高Ti/Y值, 表明前者的熔融程度相對較低。前人研究發(fā)現(xiàn)低熔融程度的巖漿通常具有更高的氧逸度(Dixon et al., 1997; Mungall et al., 2006)。由于Fe3+在部分熔融過程中是不相容的, 低程度部分熔融的熔體傾向于具有更高的Fe3+/FeT值(Herzberg and Asimow, 2008; Gaetani, 2016), 因此它們也應(yīng)具有較高的氧逸度。然而最近對尖晶石橄欖巖部分熔體的研究表明, 源區(qū)殘留的尖晶石會緩沖巖漿中Fe3+/FeT值, 因此對于尖晶石橄欖巖源區(qū), 部分熔融程度差異對巖漿氧逸度的改變很有限(Davis and Cottrell, 2018; Sorbadere et al., 2018)。但是峨眉山大火成巖省的研究表明部分熔融發(fā)生在石榴石相(Zhang et al., 2019a), 因此本研究初步認(rèn)為部分熔融程度的差異可能是導(dǎo)致平川苦橄巖母巖漿和大理苦橄巖母巖漿氧逸度差異的主要因素之一。

圖5 平川和大理苦橄巖中橄欖石Ni、Ca、Mn、Fe/Mn、10000×Zn/Fe、Sc、Sc/Y和Co與橄欖石Fo的相關(guān)關(guān)系圖(a～d中底圖來自Herzberg, 2011; 數(shù)據(jù)來源同圖2)

4.2 平川苦橄巖的源區(qū)巖性

除了傳統(tǒng)觀點認(rèn)為的輝石巖源區(qū)熔體中結(jié)晶的橄欖石比橄欖巖源區(qū)熔體中結(jié)晶的橄欖石具有高Ni含量、Fe/Mn值, 低Ca、Mn含量特征外(Sobolev et al., 2007; Herzberg, 2011), 也可以通過橄欖石微量元素特征對源區(qū)組分進(jìn)行識別。比如, 輝石巖相對于橄欖巖具有低Zn和Co的全巖?熔體分配系數(shù)(Le Roux et al., 2011; Davis et al., 2013), 因此, 與橄欖巖熔體相比, 輝石巖熔體會具有更高的Zn和Co含量。而且在組成地幔的主要礦物中, Sc更相容于單斜輝石, 因此輝石巖熔體相比橄欖巖熔體也會呈現(xiàn)出較低Sc含量和Sc/Y值。此外, 橄欖石中Zn/Fe值也可以用來識別源區(qū)特征(Le Roux et al., 2011; Foley et al., 2013; Lee, 2014)。Zn/Fe值在橄欖石、斜方輝石與熔體間的分配系數(shù)為1, Zn/Fe值在橄欖巖熔融時基本不會發(fā)生分餾; 而單斜輝石和石榴子石相比橄欖石、斜方輝石和熔體之間具有低的Zn/Fe分配系數(shù), 因此富含石榴子輝石巖的源區(qū)熔融, 產(chǎn)生的熔體相比其源區(qū)會具有高Zn/Fe值(Lee, 2014)。而且氧逸度也會影響巖漿中Zn/Fe值, 高氧逸度環(huán)境下, 熔體具有更高Fe3+/Fe2+值。Fe3+比Fe2+更不相容, 因此熔體會具有較高的Fe含量, 導(dǎo)致熔體中Zn/Fe值較低(Le Roux et al., 2010)。由于平川巖漿的氧逸度高于大理巖漿(圖7), 若主要是氧逸度造成平川苦橄巖和大理苦橄巖中橄欖石Zn/Fe值差異, 則平川橄欖石應(yīng)比大理橄欖石具有更低Zn/Fe值, 而本次研究結(jié)果顯示平川苦橄巖中橄欖石Zn/Fe值高于大理苦橄巖中的橄欖石(圖5), 因此平川苦橄巖和大理苦橄巖中橄欖石Zn/Fe值差異不是氧逸度引起的。綜合平川橄欖石比大理橄欖石顯示出的更高Ni、Co含量和Zn/Fe、Fe/Mn值, 低Ca、Mn、Sc含量和Mn/Zn值等特征(圖2、5), 指示了平川苦橄巖源區(qū)中可能存在輝石巖組分。

賓川苦橄巖(Ti/Y=297)和永勝苦橄巖(Ti/Ymean=790)中的尖晶石數(shù)據(jù)來自Kamenetsky et al. (2012)。大理尖晶石文獻(xiàn)數(shù)據(jù)來自Kamenetsky et al. (2012) 和 Liu et al. (2017)。(Cr#=Cr/(Cr+Al); Mg#=Mg/(Mg+Fe2+), molar)。

橄欖石?尖晶石共結(jié)時熔體的氧逸度(ΔQFM)是利用Nikolaev et al. (2016)的橄欖石?尖晶石氧逸度公式計算的。氧逸度(ΔQFM)和橄欖石V/Sc整體具有負(fù)相關(guān)性。

在橄欖巖?輝石巖源區(qū)的判別圖中, 平川苦橄巖中橄欖石樣品點普遍落在橄欖巖源區(qū)范圍內(nèi), 并不具有明顯輝石巖源區(qū)熔體中結(jié)晶的橄欖石的特征(圖2)。無橄欖石輝石巖部分熔融產(chǎn)生的熔體成分受源區(qū)中輝石礦物組分的控制明顯, 其結(jié)晶的橄欖石會具有明顯的高Ni、Co含量, 高Fe/Mn、Zn/Fe值, 低Mn、Ca含量, 以及低Sc/Y、Mn/Zn值特征。但峨眉山大火成巖省的地幔源區(qū)是含有橄欖石的而不是無橄欖石輝石巖。無橄欖石輝石巖產(chǎn)生的熔體MgO含量較低(MgO<15%; Yang et al., 2016), 而峨眉山大火成巖省初始熔體的MgO可達(dá)23.2%(Zhang et al., 2006; Hanski et al., 2010; Ren et al., 2017)。橄欖巖和輝石巖成分是過渡變化的, 主要區(qū)別在于源區(qū)礦物中橄欖石的比例。根據(jù)Le Ma?tre et al. (2002)的定義, 由橄欖石、輝石、石榴子石為主要成分, 其中橄欖石含量小于40%的地幔巖為輝石巖。而進(jìn)入地幔的再循洋殼會影響地幔源區(qū)中的礦物組成, 即隨著進(jìn)入地幔橄欖巖的再循環(huán)洋殼比例增加, 橄欖巖與再循環(huán)物質(zhì)產(chǎn)生的熔體反應(yīng), 逐漸消耗橄欖石, 轉(zhuǎn)變?yōu)檩x石巖。再循環(huán)洋殼與虧損地?；旌系谋壤煌? 形成地幔源區(qū)中的橄欖石所占的比例也不同。根據(jù)Yang et al. (2016)的模型, 若10%再循環(huán)洋殼加入到虧損地幔中, 形成的源區(qū)含有約60%的橄欖石; 若30%再循環(huán)洋殼加入到虧損地幔中時, 源區(qū)中的橄欖石含量降低到約40%, 根據(jù)定義這種地幔源區(qū)可以稱之為輝石巖, 如KG2輝石巖(實驗輝石巖, 成分相當(dāng)于67%富集的尖晶石二輝橄欖KLB-1與33%平均 MORB的混合物; Kogiso et al., 1998); 若70%再循環(huán)洋殼加入虧損地幔時, 則形成無橄欖石輝石巖, 類似于MIX1G(近似于典型地幔輝石巖, 為兩塊來自Balmuccia地塊天然輝石巖的混合物; Hirschmann et al., 2003)。

Ni/Co>20指示源區(qū)中存在再循環(huán)殼源物質(zhì)(Sobolev et al., 2007; Foley et al., 2013)。Howarth and Harris (2017)認(rèn)為Ni/Co>20也可以指示地幔源區(qū)中輝石巖組分的存在。

地中海鉀鎂煌斑巖中橄欖石成分來自Prelevic et al. (2013); 典型大陸溢流玄武巖橄欖石成分范圍來自Foley et al. (2013); 加格達(dá)奇早白堊世苦橄巖中的橄欖石來自含有再循環(huán)陸殼的源區(qū)(羅清晨等, 2020)。

Yang and Liu (2019)、Zhang et al. (2019a, 2021)通過麗江苦橄巖Mg同位素, 麗江、大理苦橄巖熔體包裹體微量元素及同位素研究, 估算出峨眉山大火成巖省源區(qū)組分中含有約15%(0～20%)的再循環(huán)洋殼。根據(jù)Yang et al. (2016)的模型, 15%的再循環(huán)洋殼與虧損地?；旌蠒纬珊蠙焓幕旌显磪^(qū)(類似KG2輝石巖, 但具有更高的橄欖石/輝石比例), 這種地幔源區(qū)產(chǎn)生的熔體中結(jié)晶的橄欖石成分特征可能與平川橄欖石類似, 仍近似于橄欖巖熔體中結(jié)晶的橄欖石成分, 但同時橄欖石成分也顯示出源區(qū)中存在輝石巖組分的控制, 具有較大的變化范圍。這或許也是平川和大理橄欖石樣品均主要落在橄欖巖區(qū)域, 但成分上又有明顯差異的原因。在Howarth and Harris (2017)的判別圖中, Iceland玄武巖橄欖石成分也出現(xiàn)類似情況(圖2、5), Iceland地幔源區(qū)被認(rèn)為含有約10%富集的輝石巖組分(Shorttle et al., 2014; Neave et al., 2018; Hole and Natland, 2019), 但I(xiàn)celand橄欖石成分也基本落在橄欖巖區(qū)域。而且前人研究也發(fā)現(xiàn), 由于輝石巖源區(qū)成分以及礦物組成極不均一, 其產(chǎn)生的巖漿也是多樣的, 可能產(chǎn)生成分類似于橄欖巖源區(qū)的巖漿, 尤其貧Si含橄欖石輝石巖熔體成分與橄欖巖并沒有多大區(qū)別(Lambart et al., 2016; Lambart, 2017; Hole, 2018)。因此, 橄欖石成分在判別圖解中與典型橄欖石區(qū)域一致的, 其源區(qū)不一定是不含有輝石巖的。

5 結(jié) 論

(1) 平川苦橄巖巖漿氧逸度高于大理苦橄巖, 指示了峨眉山大火成巖省中高鈦巖漿具有更高的氧逸度。

(2) 平川苦橄巖中橄欖石主量元素以及微量元素成分特征支持其源區(qū)可能存在輝石巖組分的控制, 區(qū)別于純的橄欖巖源區(qū)。平川橄欖石成分符合少量再循環(huán)洋殼進(jìn)入橄欖巖源區(qū)形成的混合地幔源區(qū)熔體中結(jié)晶的橄欖石成分特征。由于峨眉山大火成巖省地幔源區(qū)中再循環(huán)洋殼的平均含量較低, 導(dǎo)致源區(qū)礦物組成中整體上具有較低的單斜輝石比例, 因此橄欖石成分在橄欖巖?輝石巖判別圖中整體投在了橄欖巖范圍。

致謝:電子探針測試得到中國科學(xué)院廣州地球化學(xué)研究所陳林麗工程師、賀鵬麗工程師和楊帆博士后的大力幫助, 兩位匿名審稿人對本文提出了寶貴意見, 在此一并表示感謝。

柏中杰, 鐘宏, 朱維光. 2019. 幔源巖漿氧化還原狀態(tài)及對巖漿礦床成礦的制約. 巖石學(xué)報, 35(1): 204–214.

羅清晨, 任鐘元, 張樂, 徐曉波. 2020 大興安嶺中生代玄武巖成因及深部動力學(xué)機(jī)制. 地球化學(xué), 49(2): 168– 192.

宋謝炎, 陳列錳, 于宋月, 陶琰, 佘宇偉, 欒燕, 張曉琪, 何海龍. 2018. 峨眉大火成巖省釩鈦磁鐵礦礦床地質(zhì)特征及成因. 礦物巖石地球化學(xué)通報, 37(6): 1003– 1018.

王焰, 王坤, 邢長明, 魏博, 董歡, 曹永華. 2017. 二疊紀(jì)峨眉山地幔柱巖漿成礦作用的多樣性. 礦物巖石地球化學(xué)通報, 36(3): 404–417.

吳亞東. 2018. 峨眉山大火成巖省源區(qū)性質(zhì)和巖漿演化過程: 來自于苦橄巖熔體包裹體和礦物的制約. 廣州: 中國科學(xué)院大學(xué)博士學(xué)位論文: 65–68.

張樂. 2019. 峨眉山大火成巖省中高鈦和低鈦鎂鐵質(zhì)巖漿成因?熔體包裹體和微量元素模擬的研究. 廣州: 中國科學(xué)院大學(xué)博士學(xué)位論文: 45–78.

Ali J R, Lo C H, Thompson G M, Song X Y. 2004. Emeishan Basalt Ar-Ar overprint ages define several tectonic events that affected the western Yangtze platform in the Mesozoic and Cenozoic., 23(2): 163–178.

Anh T V, Pang K N, Chung S L, Lin H M, Hoa T T, Anh T T, Yang H J. 2011. The Song Da magmatic suite revisited: A petrologic, geochemical and Sr-Nd isotopic study on picrites, flood basalts and silicic volcanic rocks., 42(6): 1341–1355.

Bai Z J, Zhong H, Hu R Z, Zhu W G, Hu W J. 2019. Composition of the chilled marginal rocks of the Panzhihua layered intrusion, Emeishan Large Igneous Province, SW China: Implications for parental magma compositions, sulfide saturation history, and Fe-Ti oxide mineralization., 60(3): 619–648.

Bai Z J, Zhong H, Li C S, Zhu W G, He D F, Qi L. 2014. Contrasting parental magma compositions for the Honggeand Panzhihua magmatic Fe-Ti-V oxide deposits, Emeishan Large Igneous Province, SW China., 109(6): 1763–1785.

Ballhaus C, Berry R F, Green D H. 1991. High pressure experimental calibration of the olivine-orthopyroxene- spinel oxygen geobarometer: Implications for the oxidation state of the upper mantle., 107(1): 27–40.

Batanova V G, Thompson J M, Danyushevsky L V, Portnyagin M V, Garbe-Sch?nberg D, Hauri E, Kimura J I, Chang Q, Senda R, Goemann K, Chauvel C, Campillo S, Ionov D A, Sobolev A V. 2019. New olivine reference material formicroanalysis., 43(3): 453–473.

Brounce M N, Kelley K A, Cottrell E. 2014. Variations in Fe3+/∑Fe of Mariana arc basalts and mantle wedgeo2., 55(12): 2513–2536.

Bryan S E, Ernst R E. 2008. Revised definition of large igneous provinces (LIPs)., 86(1–4): 175–202.

Canil D. 1997. Vanadium partitioning and the oxidation state of Archaean komatiite magmas., 389(6653): 842–845.

Cao Y H, Wang C Y, Huang F, Zhang Z F. 2019. Iron isotope systematics of the Panzhihua mafic layered intrusion associated with giant Fe-Ti oxide deposit in the Emeishan Large Igneous Province, SW China.:, 124(1), 358–375.

Chung S L, Jahn B M. 1995. Plume-lithosphere interactionin generation of the Emeishan flood basalts at the Permian- Triassic boundary., 23(10): 889–892.

Chung S L, Jahn B M, Genyao W, Lo C H, Bolin C. 1998. The Emeishan flood basalt in SW China: A mantle plume initiation model and its connection with continental breakup and mass extinction at the Permian-Triassic boundary // Flower M F J, Chung S L, Lo C H, Lee T Y. Mantle Dynamics and Plate Interactions in East Asia. Washington, D C: American Geophysical Union: 47– 58.

Coffin M F, Eldholm O. 1992. Volcanism and continental break-up: A global compilation of large igneous provinces.,,, 68(1): 17–30.

Coogan L, Saunders A, Wilson R. 2014. Aluminum-in- olivine thermometry of primitive basalts: Evidence of an anomalously hot mantle source for large igneous provinces., 368: 1–10.

Davis F A, Cottrell E. 2018. Experimental investigation of basalt and peridotite oxybarometers: Implications for spinel thermodynamic models and Fe3+compatibility during generation of upper mantle melts., 103(7): 1056–1067.

Davis F A, Humayun M, Hirschmann M M, Cooper R S. 2013. Experimentally determined mineral/melt partitioning of first-row transition elements (FRTE) during partial melting of peridotite at 3 GPa., 104: 232–260.

Delavault H, Chauvel C, Sobolev A, Batanova V. 2015. Combined petrological, geochemical and isotopic modeling of a plume source: Example of Gambier Island, Pitcairn chain., 426: 23–35.

Dixon J E, Clague D A, Wallace P, Poreda R. 1997. Volatiles in alkalic basalts form the North arch volcanic field, Hawaii: Extensive degassing of deep submarine- eruptedalkalic series lavas., 38(7): 911–939.

Erdmann S, Chen L H, Liu J Q, Xue X Q, Wang X J. 2019. Hot, volatile-poor, and oxidized magmatism above the stagnant Pacific plate in Eastern China in the Cenozoic.,,, 20(11): 4849– 4868.

Fan W M, Zhang C H, Wang Y J, Guo F, Peng T P. 2008. Geochronology and geochemistry of Permian basalts in western Guangxi Province, Southwest China: Evidence for plume-lithosphere interaction., 102(1–2).

Foley S F, Prelevic D, Rehfeldt T, Jacob D E. 2013. Minor and trace elements in olivines as probes into early igneousand mantle melting processes., 363: 181–191.

Frey F A, Huang S C, Xu G P, Jochum K P. 2016. The geochemical components that distinguish Loa-and Kea-trend Hawaiian shield lavas., 185: 160–181.

Gaetani G A. 2016. The behavior of Fe3+/∑Fe during partial melting of spinel lherzolite., 185: 64–77.

Gavrilenko M, Herzberg C, Vidito C, Carr M J, Tenner T, Ozerov A. 2016. A calcium-in-olivine geohygrometer and its application to subduction zone magmatism., 57(9): 1811–1832.

Hanski E, Kamenetsky V S, Luo Z Y, Xu Y G, Kuzmin D V. 2010. Primitive magmas in the Emeishan large igneous province, southwestern China and northern Vietnam., 119(1–2): 75–90.

Hanski E, Walker R J, Huhma H, Polyakov G V, Balykin P A, Hoa T T, Phuong N T. 2004. Origin of the Permian- Triassic komatiites, northwestern Vietnam., 147(4): 453–469.

Harris C, le Roux P, Cochrane R, Martin L, Duncan A R, Marsh J S, le Roex A P, Class C. 2015. The oxygen isotope composition of Karoo and Etendeka picrites: High δ18O mantle or crustal contamination?, 170(1): 1–24.

He B, Xu Y G, Chung S L, Xiao L, Wang Y M. 2003. Sedimentary evidence for a rapid, kilometer-scale crustal doming prior to the eruption of the Emeishan flood basalts., 213(3–4): 391–405.

Heinonen J S, Fusswinkel T. 2017. High Ni and low Mn/Fe in olivine phenocrysts of the Karoo meimechites do not reflect pyroxenitic mantle sources., 467: 134–142.

Herzberg C. 2011. Identification of source lithology in the Hawaiian and Canary islands: Implications for origins., 52: 113–146.

Herzberg C, Asimow P D. 2008. Petrology of some oceanic island basalts: PRIMELT2. XLS software for primary magma calculation.,,, 9(9): Q09001.

Hirschmann M M, Kogiso T, Baker M B, Stolper E M. 2003. Alkalic magmas generated by partial melting of garnet pyroxenite., 31(6): 481–484.

Hole M J. 2018. Mineralogical and geochemical evidence for polybaric fractional crystallization of continental flood basalts and implications for identification of peridotite and pyroxenite source lithologies., 176: 51–67.

Hole M J, Natland J H. 2019. Magmatism in the North Atlantic Igneous Province: Mantle temperatures, rifting and geodynamics., 206, 102794.

Hong L B, Xu Y G, Zhang L, Liu Z, Xia X P, Kuang Y S. 2019. Oxidized Late Mesozoic subcontinental lithospheric mantle beneath the eastern North China Craton: A clue to understanding cratonic destruction., 81: 230–239.

Hou T, Zhang Z C, Encarnacion J, Santosh M, Sun Y L. 2013. The role of recycled oceanic crust in magmatism and metallogeny: Os-Sr-Nd isotopes, U-Pb geochronology and geochemistry of picritic dykes in the Panzhihua giant Fe-Ti oxide deposit, central Emeishan large igneous province, SW China., 165(4): 805–822.

Hou T, Zhang Z C, Kusky T, Du Y S, Liu J L, Zhao Z D. 2011. A reappraisal of the high-Ti and low-Ti classification of basalts and petrogenetic linkage between basalts and mafic-ultramafic intrusions in the Emeishan Large Igneous Province, SW China., 41(1): 133–143.

Hou T, Zhang Z C, Pirajno F. 2012. A new metallogenic model of the Panzhihua giant V-Ti-iron oxide deposit (Emeishan Large Igneous Province) based on high-Mg olivine-bearing wehrlite and new field evidence., 54: 1721–1745.

Howarth G H, Harris C. 2017. Discriminating between pyroxenite and peridotite sources for continental flood basalts (CFB) in southern Africa using olivine chemistry., 475: 143–151.

Huang H, Du Y S, Yang J H, Zhou L, Hu L S, Huang H W, Huang Z Q. 2014. Origin of Permian basalts and clastic rocks in Napo, Southwest China: Implications for the erosion and eruption of the Emeishan large igneous province., 208: 324–338.

Huang S C, Zheng Y F. 2017. Mantle geochemistry: Insights from ocean island basalts., 60(11): 1976–2000.

Humayun M, Qin L P, Norman M D. 2004. Geochemical evidence for excess iron in the mantle beneath Hawaii., 306(5693): 91–94.

Kamenetsky V S, Chung S L, Kamenetsky M B, Kuzmin D V. 2012. Picrites from the Emeishan Large Igneous Province, SW China: A compositional continuum in primitive magmas and their respective mantle sources., 53(10): 2095–2113.

Kamenetsky V S, Maas R, Kamenetsky M B, Yaxley G M, Ehrig K, Zellmer G F, Bindeman I N, Sobolev A V, Kuzmin D V, Ivanov A V, Woodhead J, Schilling J G. 2017. Multiple mantle sources of continental magmatism: Insights from “high-Ti” picrites of Karoo and other large igneous provinces., 455: 22–31.

Kogiso T, Hirose K, Takahashi E. 1998. Melting experiments on homogeneous mixtures of peridotite and basalt: Application to the genesis of ocean island basalts., 162(1–4): 45–61.

Kress V C, Carmichael I S E. 1991. The compressibility of silicate liquids containing Fe2O3and the effect of composition, temperature, oxygen fugacity and pressure on their redox states., 108(1–2): 82–92.

Lai S C, Qin J F, Li Y F, Li S Z, Santosh M. 2012. Permian high Ti/Y basalts from the eastern part of the Emeishan Large Igneous Province, southwestern China: Petrogenesis and tectonic implications., 47: 216–230.

Lambart S, Baker M B, Stolper E M. 2016. The role of pyroxenite in basalt genesis: Melt-PX, a melting parameterization for mantle pyroxenites between 0.9 and 5 GPa.:, 121(8): 5708–5735.

Lambart S. 2017. No direct contribution of recycled crust in Icelandic basalts., 4: 7–12.

Le Ma?tre R W. Streckeisen A, Zanettin B, Le Bas M J, Bonin B, Bateman P, Bellieni G, Dudek A, Efremova S, Keller J, Lameyre J, Sabine P A, Schmid R, Sorensen H, Woolley A R. 2002. Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcom-mission on the Systematics of Igneous Rocks. Cambridge: Cambridge University Press: 236.

Le Roux V, Dasgupta R, Lee C T A. 2011. Mineralogical heterogeneities in the Earth’s mantle: Constraints from Mn, Co, Ni and Zn partitioning during partial melting., 307(3–4): 395–408.

Le Roux V, Lee C T A, Turner S J. 2010. Zn/Fe systematics in mafic and ultramafic systems: Implications for detecting major element heterogeneities in the Earth’s mantle., 74(9): 2779–2796.

Lee C T A. 2014. Physics and chemistry of deep continental crust recycling // Holland H D, Turekian K K. Treatise on Geochemistry. Amsterdam: Elsevier: 423–456.

Lee C T A, Leeman W P, Canil D, Li Z X A. 2005. Similar V/Sc systematics in MORB and arc basalts: Implications for the oxygen fugacities of their mantle source regions., 46: 2313–2336.

Li J, Xu J F, Suzuki K, He B, Xu Y G, Ren Z Y. 2010. Os, Nd and Sr isotope and trace element geochemistry of the Muli picrites: Insights into the mantle source of the Emeishan Large Igneous Province., 119(1–2): 108–122.

Li Y J, He H Y, Ivanov A V, Demonterova E I, Pan Y X, Deng C L, Zheng D W, Zhu R X. 2017.40Ar/39Ar age of the onset of high-Ti phase of the Emeishan volcanism strengthens the link with the end-Guadalupian mass extinction., 60(15): 1906–1917.

Li Z X A, Lee C T A. 2004. The constancy of upper mantleo2through time inferred from V/Sc ratios in basalts., 228: 483–493.

Liu J, Xia Q K, Kuritani T, Hanski E, Yu H R. 2017. Mantle hydration and the role of water in the generation of large igneous provinces., 8(1): 1824.

Liu X J, Liang Q D, Li L Z, Castillo P R, Shi Y, Xu J F, Huang X L, Liao S, Huang W L, Wu W N. 2016. Origin of Permian extremely high Ti/Y mafic lavas and dykes from western Guangxi, SW China: Implications for the Emeishan mantle plume magmatism., 141: 97–111.

Mallmann G, O’Neill H S C. 2013. Calibration of an empirical thermometer and oxybarometer based on the partitioning of Sc, Y and V between olivine and silicate melt., 54(5): 933–949.

Matzen A K, Wood B J, Baker M B, Stolper E M. 2017. The roles of pyroxenite and peridotite in the mantle sources of oceanic basalts., 10(7): 530–535.

Mungall J E, Hanley J J, Arndt N T, Debecdelievre A. 2006. Evidence from meimechites and other low-degree mantle melts for redox controls on mantle-crust fractio-nation of platinum-group elements., 103(34): 12695–12700.

Neave D A, Shorttle O, Oeser M, Weyer S, Kobayashi K. 2018. Mantle-derived trace element variability in olivines and their melt inclusions., 483: 90–104.

Nikogosyan I K, Sobolev A V. 1997. Ion-Microprobe analysis of melt inclusions in olivine: Experience in estimating the olivine-melt partition coefficients of trace elements., 35(2): 155–157.

Nikolaev G S, Ariskin A A, Barmina G S, Nazarov M A, Almeev R R. 2016. Test of the Ballhaus-Berry-Green Ol-Opx-Sp oxybarometer and calibration of a new equation for estimating the redox state of melts saturated with olivine and spinel., 54(4): 301–320.

Prelevic D, Jacob D E, Foley S F. 2013. Recycled continental crust is an essential ingredient of Mediterraneanorogenic mantle lithosphere., 362: 187–197.

Putirka K D, Perfit M, Ryerson F J, Jackson M G. 2007. Ambient and excess mantle temperatures, olivine thermo-metry, and activepassive upwelling., 241(3–4): 177–206.

Ren Z Y, Ingle S, Takahashi E, Hirano N, Hirata T. 2005. The chemical structure of the Hawaiian mantle plume., 436: 837–840.

Ren Z Y, Wu Y D, Zhang L, Nichols A R, Hong L B, Zhang Y H, Zhang Y, Liu J Q, Xu Y G. 2017. Primary magmas and mantle sources of Emeishan basalts constrained frommajor element, trace element and Pb isotope compositions of olivine-hosted melt inclusions, 208: 63–85.

Rudnick R L, Gao S. 2003. Composition of the continental crust // Holland H D, Turekian K K. Treatise on Geochemistry. Amsterdam: Elsevier: 659.

Shellnutt J G, Jahn B M. 2011. Origin of Late Permian Emeishan basaltic rocks from the Panxi region (SW China): Implications for the Ti-classification and spatial-compositional distribution of the Emeishan flood basalts., 199(1–2): 85–95.

Shellnutt J G, Pham T T, Denyszyn S W, Yeh M W, Tran T A. 2020. Magmatic duration of the Emeishan large igneous province: Insight from northern Vietnam., 48(5): 457–461.

Shorttle O, Maclennan J, Lambart S. 2014. Quantifying lithological variability in the mantle., 395: 24–40.

S?ager N, Portnyagin M, Hoernle K, Holm P M, Hauff F, Garbe-Sch?nberg D. 2015. Olivine major and trace element compositions in southern Payenia basalts, Argentina: Evidence for pyroxenite-peridotite melt mixing in a back-arc setting., 56(8): 1495–1518.

Sobolev A V, Hofmann A W, Kuzmin D V, Yaxley G M, Arndt N T, Chung S L, Danyushevsky L V, Elliott T, Frey F A, Garcia M O, Gurenko A A, Kamenetsky V S, Kerr A C, Krivolutskaya N A, Matvienkov V V, Nikogosian I K, Rocholl A, Sigurdsson I A, Sushchevskaya N M, Teklay M. 2007. The amount of recycled crust in sources of mantle-derived melts., 316: 412–417.

Sobolev A V, Hofmann A W, Sobolev S V, Nikogosian I K. 2005. An olivine-free mantle source of Hawaiian shield basalts., 434(7033): 590–597.

Song X Y, Hua W Q, Hu R Z, Chen L M, Yu S Y. 2013. Formation of thick stratiform Fe-Ti oxide layers in layered intrusion and frequent replenishment of fractionated mafic magma: Evidence from the Panzhihua intrusion, SW China.,,, 14: 712–732.

Sorbadere F, Laurenz V, Frost D J, Wenz M, Rosenthal A, McCammon C, Rivard C. 2018. The behaviour of ferric iron during partial melting of peridotite., 239: 235–254.

Tang Q Y, Ma Y S, Zhang M J, Li C S, Zhu D, Tao Y. 2013. Origin of Ni-Cu-PGE sulfide mineralization in the margin of the Zhubu mafic-ultramafic intrusion in the Emeishan Large Igneous Province, Southwestern China., 108: 1889–1901.

Tao Y, Li C S, Hu R Z, Qi L, Qu W J, Du A D. 2010. Re-Os isotopic constraints on the genesis of the Limahe Ni-Cu deposit in the Emeishan large igneous province, SW China., 119: 137–146.

Tao Y, Li C S, Hu R Z, Ripley E M, Du A D, Zhong H. 2007. Petrogenesis of the Pt-Pd mineralized Jinbaoshan ultramafic intrusion in the Permian Emeishan large igneous province, SW China., 153(3): 321–337.

Tao Y, Li C S, Song X Y, Ripley E M. 2008. Mineralogical, petrological, and geochemical studies of the Limahe mafic-ultramatic intrusion and associated Ni-Cu sulfide ores, SW China., 43(8): 849–872.

Tao Y, Putirka K, Hu R Z, Li C S. 2015. The magma plumbing system of the Emeishan large igneous province and its role in basaltic magma differentiation in a continental setting., 100(11–12): 2509–2517.

Thompson R N, Gibson S A, Dickin A P, Smith P M. 2001. Early Cretaceous basalt and picrite dykes of the southern Etendeka region, NW Namibia: Windows into the role of the Tristan mantle plume in Paraná-Etendeka magmatism., 42(11): 2049–2081.

Wan Z, Coogan L A, Canil D. 2008. Experimental calibration of aluminum partitioning between olivine and spinel as a geothermometer., 93(7): 1142– 1147.

Wang C Y, Zhou M F. 2013. New textural and mineralogical constraints on the origin of the Hongge Fe-Ti-V oxide deposit, SW China., 48(6): 787–798.

Wang C Y, Zhou M F, Qi L. 2007. Permian flood basalts and mafic intrusions in the Jinping (SW China)-Song Da (northern Vietnam) district: Mantle sources, crustal contamination and sulfide segregation., 243(3–4): 317–343.

Wang C Y, Zhou M F, Qi L.2010. Origin of extremely PGE-rich mafic magma system: An example from the Jinbaoshan ultramafic sill, Emeishan Large Igneous Province, SW China., 119: 147–161.

Wang C Y, Zhou M F, Zhao Z D. 2005. Mineral chemistryof chromite from the Permian Jinbaoshan Pt-Pd-sulphide- bearing ultramafic intrusion in SW China with petro-genetic implications., 83(1): 47–66.

Wang C Y, Zhou M F, Zhao Z D. 2008. Fe-Ti-Cr oxides from the Permian Xinjie mafic-ultramafic layered intrusion in the Emeishan large igneous province, SW China: Crystallization from Fe- and Ti-rich basaltic magmas., 102: 198–207.

Wignall P B, Sun Y D, Bond D P G, Izon G, Newton R J, Védrine S, Widdowson M, Ali R J, Lai X L, Jiang H S, Cope H, Bottrell S H. 2009. Volcanism, mass extinction, and carbon isotope fluctuations in the Middle Permian of China., 324(5931): 1179–1182.

Wood B J, Bryndzia L T, Johnson K E. 1990. Mantle oxidation state and its relationship to tectonic environment and fluid speciation., 248(4953): 337–345.

Wu Y D, Ren Z Y, Handler M R, Zhang L, Qian S P, Xu Y G, Wang C Y, Wang Y, Chen L L. 2018. Melt diversity and magmatic evolution in the Dali picrites, Emeishan Large Igneous Province.:, 123(11): 9635–9657.

Xiao L, Xu Y G, Mei H J, Zheng Y F, He B, Pirajno F. 2004. Distinct mantle sources of low-Ti and high-Ti basalts from the western Emeishan large igneous province, SW China: Implications for plume-lithosphere interaction., 228(3): 525–546.

Xu J F, Suzuki K, Xu Y G, Mei H J, Li J. 2007. Os, Pb, and Nd isotope geochemistry of the Permian Emeishan continental flood basalts: Insights into the source of a large igneous province., 71(8): 2104–2119.

Xu R, Liu Y S, Lambart S. 2020. Melting of a hydrous peridotite mantle source under the Emeishan large igneous province., 207, 103253.

Xu Y C, Yang Z Y, Tong Y B, Jing X Q. 2018. Paleo-magnetic secular variation constraints on the rapid eruption of the Emeishan continental flood basalts in southwestern China and northern Vietnam.:, 123(4): 2597–2617.

Xu Y G, Chung S L, Jahn B M, Wu G Y. 2001. Petrologic and geochemical constraints on the petrogenesis of Permian-Triassic Emeishan flood basalts in southwe-stern China., 58: 145–168.

Xu Y G, Chung S L, Shao H, He B. 2010. Silicic magmas from the Emeishan large igneous province, Southwest China: Petrogenesis and their link with the end-Guadalupian biological crisis., 119(1–2): 47–60.

Xu Y G, He B, Chung S L, Menzies M A, Frey F A. 2004. Geologic, geochemical, and geophysical consequences of plume involvement in the Emeishan flood-basalt province., 32(10): 917–920.

Yang C, Liu S A. 2019. Zinc isotope constraints on recycled oceanic crust in the mantle sources of the Emeishan large igneous province.:, 124(22), 12537–12555.

Yang J, Cawood P A, Du Y S. 2015. Voluminous silicic eruptions during late Permian Emeishan igneous provinceand link to climate cooling., 432: 166–175.

Yang Z F, Li J, Liang W F, Luo Z H. 2016. On the chemical markers of pyroxenite contributions in continentalbasalts in Eastern China: Implications for source lithology and the origin of basalts., 157: 18–31.

Yao J H, Zhu W G, Li C, Zhong H, Yu S, Ripley E M, Bai Z J. 2019. Olivine O isotope and trace element constraints on source variation of picrites in the Emeishan flood basalt province, SW China., 338: 87–98.

Yu S Y, Chen L M, Lan J B, He Y S, Chen Q, Song X Y. 2020. Controls of mantle source and condition of melt extraction on generation of the picritic lavas from the Emeishan large igneous province, SW China., 203, 104534.

Yu S Y, Shen N P, Song X Y, Ripley E M, Li C, Chen L M. 2017. An integrated chemical and oxygen isotopic study of primitive olivine grains in picrites from the Emeishan Large Igneous Province, SW China: Evidence for oxygen isotope heterogeneity in mantle sources., 215: 263–276.

Yu S Y, Song X Y, Chen L M, Li X B. 2014. Postdated melting of subcontinental lithospheric mantle by the Emeishan mantle plume: Evidence from the Anyi intrusion, Yunnan, SW China., 57: 560–573.

Zhang L, Ren Z Y, Handler M R, Wu Y D, Zhang L, Qian S P, Xia X P, Yang Q, Xu Y G. 2019a. The origins of high-Ti and low-Ti magmas in large igneous provinces, insights from melt inclusion trace elements and Sr-Pb isotopes in the Emeishan large Igneous Province., 344: 122–133.

Zhang L, Ren Z Y, Xia X P, Yang Q, Hong L B, Wu D. 2019b.determination of trace elements in melt inclusions using laser ablation-inductively coupled plasma- sector field-mass spectrometry., 33(4): 361–370.

Zhang L, Ren Z Y, Zhang L, Wu Y D, Qian S P, Xia X P, Xu Y G. 2021. Nature of the mantle plume under the Emeishan large igneous province: Constraints from olivine-hosted melt inclusions of the Lijiang picrites.:, 126(5), e2020JB021022.

Zhang Y, Ren Z Y, Xu Y G. 2013. Sulfur in olivine-hosted melt inclusions from the Emeishan picrites: Implications for S degassing and its impact on environment.:, 118(18): 4063– 4070.

Zhang Z C, Mahoney J J, Mao J W, Wang F S. 2006.Geochemistry of picritic and associated basalt flows of the western Emeishan flood basalt province, China., 47(10): 1997–2019.

Zhang Z C, Zhi X C, Chen L, Saunders A D, Reichow M K. 2008. Re-Os isotopic compositions of picrites from the Emeishan flood basalt province, China., 276(1–2): 30–39.

Zhong H, Qi L, Hu R Z, Zhou M F, Gou T Z, Zhu W G, Liu B G, Zhu Y C. 2011. Rhenium-osmium isotope and platinum-group elements in the Xinjie layered intrusion, SW China: Implications for source mantle composition, mantle evolution, PGE fractionation and mineralization., 75(6): 1621–1641.

Zhong Y T, He B, Mundil R, Xu Y G. 2014. CA-TIMS zircon U-Pb dating of felsic ignimbrite from the Binchuan section: Implications for the termination age of Emeishan large igneous province., 204: 14–19.

Zhong Y T, Mundil R, Chen J, Yuan D X, Denyszyn S W, Jost A B, Payne J L, He B, Shen S J, Xu Y G. 2020. Geochemical, biostratigraphic, and high-resolution geochronological constraints on the waning stage of Emeishan Large Igneous Province., 132(9–10): 1969–1986.

Zhou M F, Arndt N T, Malpas J, Wang C Y, Kennedy A K. 2008. Two magma series and associated ore deposit types in the Permian Emeishan large igneous province, SW China., 103(3–4): 352–368.

Zhou M F, Zhao J H, Qi L, Su W C, Hu R Z. 2006. Zircon U-Pb geochronology and elemental and Sr-Nd isotope geochemistry of Permian mafic rocks in the Funing area, SW China., 151(1): 1–19.

The Nature of Mantle Source of the Pingchuan Picrites in the Emeishan Large Igneous Province — Constrains from the Trace Element Composition of Olivines

ZHANG Lei1, 2, 3, REN Zhongyuan1, 2*, ZHANG Le1, 2

(1. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China; 2. CAS Center for Excellence in Deep Earth Science, Guangzhou 510640, Guangdong, China; 3. University of Chinese Academy of Sciences, Beijing 100049, China)

Olivine is one of the earliest crystallized silicates from basaltic magmas. The major and trace element compositions of olivine phenocrysts can provide important information about the thermodynamic condition of the magma, source lithology, and source component, including the nature of the recycled component. The major and trace element compositions of the olivines and Cr-spinel inclusions in the olivines from the Pingchuan picrites in the Emeishan large igneous province were analyzed in this study, and compared with the compositions of the olivines and spinels in the Dali picrites, and the differences of source component and oxygen fugacity of parental magmas of different picrites were discussed. The Pingchuan olivines have higher V/Sc ratio than the Dali olivines, which indicates a more oxidized condition for the Pingchuan magma. The olivine-spinel oxybarometer shows that the logo2values of the Pingchuan and the Dali magmas, which are saturated with olivine and spinel, are ΔQFM+0.24 and ΔQFM?0.51, respectively. The high Fo olivines in the Pingchuan picrites have Ni/Co>20, which implies the presence of recycled component in the source of the Pingchuan picrites. The lower Li content of the Pingchuan olivines indicates that the component that recycled into the source is oceanic crust. In the mantle source discrimination diagrams of olivine NiMn/Zn and 100×Mn/Fe10000×Zn/Fe, the Pingchuan olivines are mainly plotted within the fields of peridotite sources. However, the Pingchuan olivines exhibit higher Ni, Co, Zn/Fe, Fe/Mn, and lower Ca, Mn, Sc, Mn/Zn than the Dali olivines, which suggest the presence of pyroxenite component in the source of the Pingchuan picrites. A small amount of recycled oceanic crust adding into the mantle peridotite would produce an olivine relatively enriched sources. The olivines crystallized from melts derived from such a mantle source would have characteristics similar to the olivines from the Pingchuan picrites, which still have compositions similar to the olivines crystallized from a peridotite partial melting melts. However, the compositions of the olivines in the Pingchuan picrites are also controlled by the pyroxenite component in the source. Combined with the wide compositional ranges of the olivines, we suggest that the Pingchuan picrites were derived from a source containing pyroxenite components rather than a homogeneous peridotite source.

Emeishan large igneous province; picrite; olivine; pyroxenite; oxygen fugacity

2020-12-04;

2021-03-09

中國科學(xué)院戰(zhàn)略性先導(dǎo)科技專項(B類)(XDB18000000)資助。

張磊(1992–), 男, 博士研究生, 礦物學(xué)、巖石學(xué)、礦床學(xué)專業(yè)。E-mail: zhanglei415@mails.ucas.edu.cn

任鐘元(1962–), 男, 研究員, 從事巖石學(xué)和地球化學(xué)方向研究。E-mail: zyren@gig.ac.cn

P584; P595

A

1001-1552(2022)01-0112-020