祁連山玉石溝橄欖巖巖漿作用的記錄和鉻鐵礦的成因

胡振興，牛耀齡，劉益，孫文禮，陳碩，李繼永，張國瑞

(1.蘭州大學(xué)地質(zhì)科學(xué)與礦產(chǎn)資源學(xué)院，甘肅 蘭州 730000；2.Department of Earth Sciences，Durham University，Durham DH1 3LE，UK；3.中國科學(xué)院海洋研究所，山東 青島 266071；4.中國地質(zhì)大學(xué)地球科學(xué)與資源學(xué)院，北京 100083)

祁連山玉石溝橄欖巖巖漿作用的記錄和鉻鐵礦的成因

胡振興1，牛耀齡2,3,4，劉益4，孫文禮1，陳碩3，李繼永3，張國瑞1

(1.蘭州大學(xué)地質(zhì)科學(xué)與礦產(chǎn)資源學(xué)院，甘肅 蘭州 730000；2.Department of Earth Sciences，Durham University，Durham DH1 3LE，UK；3.中國科學(xué)院海洋研究所，山東 青島 266071；4.中國地質(zhì)大學(xué)地球科學(xué)與資源學(xué)院，北京 100083)

祁連山玉石溝蛇綠巖型鉻鐵礦是典型的高鉻型鉻鐵礦床。通過對玉石溝鉻鐵礦和橄欖巖圍巖進行原位定距離采樣研究表明：全巖高的MgO和低的SiO2含量指示這些蛇紋石化橄欖巖不是簡單的部分熔融殘余，而是經(jīng)歷了后期的熔體再富集作用。玉石溝近礦橄欖巖尖晶石(Cr#>65)和鉻鐵礦尖晶石的礦物包裹體化學(xué)特征反映礦體和圍巖形成于俯沖帶上覆巖石圈地幔。距離礦體由近及遠，賦礦圍巖尖晶石化學(xué)成分(Cr#=43.9～77.2，TiO2=0.06%～0.34%)無規(guī)律變化。這說明高鉻型鉻鐵礦體成礦所需鉻并非來源于圍巖，而是與玻安質(zhì)熔體滲濾有關(guān)。尖晶石含水礦物包裹體僅賦存在玉石溝鉻鐵礦尖晶石與硅酸鹽礦物接觸部位附近可能揭示了玉石溝鉻鐵礦體的成因：成礦鉻尖晶石在上侵的含水熔體與周圍方輝橄欖巖的反應(yīng)過程中形成聚集。

豆莢狀鉻鐵礦；包裹體；玻安質(zhì)熔體；水；玉石溝

蛇綠巖型鉻鐵礦因礦體形似豆莢又稱豆莢狀鉻鐵礦，礦體外圍常被一層薄薄的純橄巖包圍。這類純橄巖常被解釋為熔-巖反應(yīng)的產(chǎn)物(Büchl et al., 2004；Zhou et al., 2005)。近年來,通過對太平洋、大西洋和印度洋洋中脊的直接深海鉆探(Leg147，Leg209等)，發(fā)現(xiàn)了一些小的豆莢狀礦石，厚度約1～4 cm。一些學(xué)者將其作為洋中脊能夠提供形成豆莢狀鉻鐵礦環(huán)境的直接證據(jù)(Matsukage et al., 1998；Abe, 2011；Payot et al., 2013)。此外，羅布莎等豆莢狀鉻鐵礦中原位金剛石包裹體的存在指示豆莢狀鉻鐵礦可能來源于深部地幔(Yang et al., 2014)。熔-巖反應(yīng)、熔體不混溶、含水流體相與硅酸鹽相的分離是目前用來解釋豆莢狀鉻鐵礦成因的流行假說(González-Jiménez et al., 2014)，但并未解答鉻鐵礦為什么會在一定部位集中結(jié)晶這一關(guān)鍵問題(胡振興等，2014)。

祁連玉石溝蛇綠巖套賦存著典型的高鉻型豆莢狀鉻鐵礦床。認為玉石溝地幔橄欖巖經(jīng)歷了熔體再富集作用(Song et al., 2009；饒萬祥等，2012)；玉石溝鉻鐵礦與地幔橄欖巖的高程度部分熔融有關(guān)(周會武等，1995；童海奎等，2012)。通過對玉石溝鉻鐵礦及其橄欖巖圍巖的巖相學(xué)和地球化學(xué)特征研究，提出一個鉻鐵礦成因假設(shè)；并獲得對此類鉻鐵礦體賦存圍巖的進一步認識，這有助于在實際找礦工作中對礦體進行更準確的定位。

1 區(qū)域地質(zhì)背景

玉石溝蛇綠巖體位于青海省祁連縣野牛溝鄉(xiāng)，認為其代表了早古生代祁連洋洋殼的殘體(圖1a；肖序常等，1978；Song et al., 2013)。玉石溝蛇綠巖巖石組合發(fā)育較完整，有變質(zhì)橄欖巖、堆晶巖、枕狀熔巖和硅質(zhì)巖等。南北均以斷裂為界(馮益民等，1996)。前人對該巖體周圍地層與古生物化石及枕狀熔巖全巖Rb-Sr(肖序常等，1978)、輝長巖鋯石U-Pb同位素年代學(xué)研究(史仁燈等，2004；Song et al., 2013)指出玉石溝蛇綠巖年齡約550～521 Ma。

2 礦體地質(zhì)及巖石學(xué)特征

玉石溝蛇綠巖超基性巖體群主要由北、中、小、南4個巖體組成(圖1b)，較大的鉻鐵礦體多集中在南巖體。鉻鐵礦礦體多呈雁行式排列，以透鏡體狀賦存在蛇紋石化純橄巖中(姚培慧，1996)。所研究的鉻鐵礦礦體位于南巖體中，礦體長軸方向與水平面夾角約45°。礦體與圍巖界線明顯(圖2a)；礦石主要為致密塊狀礦石，圍巖為蛇紋石化橄欖巖。距離礦體約20 m處可見一稀疏浸染狀鉻鐵礦脈。

對玉石溝鉻鐵礦體及其圍巖進行了原位采樣：致密塊狀礦石、稠密浸染狀礦石、稀疏浸染狀礦石共12個，礦石直接圍巖(圖2a)、與礦體距離小于1 m的近礦圍巖(沿礦體長軸方向)、按距離礦體由近及遠一定間隔(5～10 m)采得的遠礦圍巖共21個。圍巖橄欖巖中的礦物已嚴重蛇紋石化，尖晶石與蛇紋石等蝕變礦物接觸截然(圖2b)。鉻鐵礦和橄欖巖尖晶石都有橄欖石、單斜輝石等硅酸鹽礦物包裹體，其中鉻鐵礦尖晶石中的硅酸鹽包裹體以單斜輝石為主，這些包裹體礦物粒徑為2～100 um不等(圖2c，圖2d)。

3 樣品分析方法

全巖主量元素在中國地質(zhì)大學(xué)(武漢)地質(zhì)過程與礦產(chǎn)資源國家重點實驗室用XRF方法測定(Ma et al., 2012)。

單礦物電子探針測試在長安大學(xué)國土資源部成礦作用及其動力學(xué)實驗室用JXA-8100完成。儀器工作條件: 加速電壓15 kV，探針束流20 nA，束斑直徑1 um。

(b)1.中寒武世砂板巖夾灰?guī)r；2.震旦紀白云巖和石英片巖等；3.奧陶紀灰?guī)r板巖及中基性火山巖；4.滑石菱鎂片巖；5.蛇紋巖；6.純橄巖；7.方輝橄欖巖；8.輝長巖；9.斷層；10.地質(zhì)界線圖1 (a)玉石溝蛇綠巖體大地構(gòu)造位置圖(據(jù)Xu et al., 2010 and Song et al.2013修改)；(b)玉石溝超基性巖塊分布圖(據(jù)史仁燈等，2004修改)Fig.1 (a)Geological map of Yushigou Ophiolite (After Xu et al., 2010 and Song et al.2013);(b)Distribution map of Yushigou Ophiolite(After Shi et al., 2004)

a.塊狀礦石和橄欖巖圍巖接觸界線明顯；b.礦體圍巖橄欖巖嚴重蛇紋石(Serp)化；c.稠密浸染狀鉻鐵礦尖晶石(Sp)含單斜輝石(Cpx)包裹體；d.橄欖巖尖晶石含橄欖石(Ol)包裹體圖2 玉石溝鉻鐵礦及橄欖巖巖相學(xué)照片F(xiàn)ig.2 Photomicrographs of the Yushigou chromitites andserpentinized peridotites

4 地球化學(xué)特征

4.1 橄欖巖全巖主量元素化學(xué)特征

采得的玉石溝鉻鐵礦體周圍橄欖巖已高度蛇紋石化，對其全巖主量元素含量(表1)進行無水校正至100%。全巖高的MgO(46.23%～51.10%，圖3a)、MgO/SiO2值(1.06～1.24，圖3b)，低的Al2O3(0.01%～2.02%)、CaO(0.09%～0.56%)等指示這些蛇紋石化橄欖巖過度虧損，明顯不符合“全球陣列”趨勢(圖3b)。礦體周圍橄欖巖Mg’為90.17～96.93(Mg’=MgO/[MgO+TFeOtot]×100)。

4.2 尖晶石礦物化學(xué)特征

玉石溝致密鉻鐵礦礦體的近礦圍巖(距離礦體<1m)存在不同成分尖晶石(Cr#=43.9～77.2，Mg#=38.1～63.7，TiO2=0.09%～0.34%，表2)。這與目前研究發(fā)現(xiàn)的俯沖帶上覆巖石圈(“上俯沖帶”)地幔橄欖巖特征類似(在Izu～Bonin海溝處采樣發(fā)現(xiàn)不同鉻值(Cr#=81.8，53.5～55.0)的純橄巖緊密共生；Morishita et al., 2011a)。稀疏浸染狀鉻鐵礦(Cr#=77.1，Mg#=52.9，TiO2=0.10%)距離致密鉻鐵礦礦體約20 m，其周圍橄欖巖(遠礦圍巖，與致密鉻鐵礦礦體距離>5 m)尖晶石鉻值為45.0～55.6(圖4a)。

距離玉石溝致密鉻鐵礦礦體由近及遠，圍巖尖晶石的Mg#與Cr#無規(guī)律變化(表2)。致密鉻鐵礦尖晶石的TiO2含量(0.04%～0.19%，103個單礦物數(shù)據(jù)平均值為0.11%：AVE103=0.11%)比圍巖尖晶石(0.06%～0.34%，AVE64=0.18%)小(圖4b)。距離致密鉻鐵礦礦體由近及遠，圍巖尖晶石TiO2含量不規(guī)則波動，無逐漸增大趨勢(表2)。

表1 玉石溝鉻鐵礦礦體周圍橄欖巖的全巖主量元素數(shù)據(jù)表(%)Tab.1 Major elements data ofhost peridotites(%)

對鉻鐵礦尖晶石母熔體化學(xué)成分模擬計算顯示：母熔體TiO2含量為0.13%～0.30%，Al2O3含量為13.76%～15.16%。玉石溝鉻鐵礦尖晶石母熔體具有類似的化學(xué)成分 (圖4c)。

4.3 尖晶石包裹體特征

玉石溝鉻鐵礦和圍巖尖晶石都有橄欖石、單斜輝石等硅酸鹽礦物包裹體(圖5)，但化學(xué)成分有明顯差別(表4)。后期包裹體礦物與尖晶石發(fā)生亞固相線的Mg-Fe交換(Melcher et al., 1997)，導(dǎo)致橄欖石、單斜輝石、斜方輝石包裹體的Fo值高達93.0～97.4。相對于玉石溝新鮮橄欖巖相應(yīng)單礦物(Song et al., 2009)和鉻鐵礦圍巖尖晶石的礦物包裹體中鉻鐵礦尖晶石的礦物包裹體：橄欖石具有高的NiO(0.40%～1.03%，圖6a)，輝石具有低的Al2O3(Cpx=0.52%～1.52%，圖6b；Opx=0.31%～0.57%，圖6c)，這與全球蛇綠巖型高鉻值鉻鐵礦和橄欖巖尖晶石硅酸鹽礦物包裹體化學(xué)特征一致(Mcelduff et al., 1991；Ahmed et al., 2001；Zhou et al., 2014)。

值得指出的是玉石溝鉻鐵礦尖晶石還有角閃石、云母含水礦物包裹體(圖7)。角閃石為韭閃石(Na2O含量為1.14%～2.01%，K2O含量為0.09%～0.20%，TiO2含量為0.17%～0.33%；圖6d)。云母為金云母(Al2O3含量為11.53%～13.24%，MgO含量為21.99%～26.61%，F(xiàn)eO含量為0.59%～1.18%)。

表2 玉石溝鉻鐵礦及其周圍橄欖巖代表性尖晶石化學(xué)成分電子探針分析結(jié)果(%)Tab.2 Representative analyses of spinel fromchromitites and host peridotites by electron microprobe(%)

注：Fe2O3和FeO含量根據(jù)化合價平衡計算；Mg#=Mg/(Mg+Fe2+)×100, Cr#=Cr/(Cr+Al)×100。

圖3 (a)玉石溝蛇綠巖橄欖巖全巖MgO-SiO2；(b)Al2O3/SiO2-MgO/SiO2；(c)SiO2-MgO/SiO2投圖Fig.3 Bulk rock analyses for serpentinized peridotites from Yushigou ophiolite in spaces of MgO-SiO2(a); Al2O3/SiO2-MgO/SiO2(b); SiO2-MgO/SiO2 (c)注：原始地幔(PM)數(shù)值見Niu, 1997附錄B。玉石溝新鮮橄欖巖(Y)數(shù)據(jù)來源于Song et al., 2009。全球深海橄欖巖(ABP)數(shù)據(jù)引自Niu, 2004。俯沖帶上覆巖石圈地幔橄欖巖(SSZ)數(shù)據(jù)引自Parkinson et al., 1998；Pearce et al., 2000。部分熔融曲線參考文獻已在圖中列出。全球陣列來自Jagoutz et al., 1979；Hart et al., 1986。所有數(shù)據(jù)統(tǒng)一標準化到無水總量100%

圖4 (a)玉石溝鉻鐵礦及橄欖巖尖晶石Cr#-Mg#；(b)Cr#-TiO2(wt%)；(c)Al2O3melt-TiO2melt投圖Fig.4 Spinel compositions in spaces of (a)Cr#-Mg#; (b)Cr#-TiO2(%); (c)Al2O3melt-TiO2melt of chromi-tites and peridotites from Yushigou ophiolite注：Mg#=Mg/(Mg + Fe2+)，Cr#= Cr/(Cr + Al)。玉石溝新鮮橄欖巖尖晶石(Y)數(shù)據(jù)來源于Song et al., 2009。圖4c中島弧中基性巖尖晶石(ARC)和洋中脊玄武巖尖晶石(MORB)范圍來源于Kamenetsky et al., 2001；鉻鐵礦尖晶石母熔體Al2O3和TiO2含量計算公式為Al2O3 melt = 5.225 3 ln(Al2O3 spinel) + 1.123 2, Zaccarini et al., 2011；TiO2 melt=1.089 7 TiO2 spinel + 0.089 2(Kamenetsky et al., 2001)。全球深海橄欖巖尖晶石(ABP)和俯沖帶上覆巖石圈地幔橄欖巖尖晶石(SSZ)以及Izu-Bonin和Tonga海溝處玻安巖尖晶石(BON)數(shù)據(jù)來源見表3

表3 全球深海橄欖巖和俯沖帶上覆巖石圈地幔橄欖巖尖晶石數(shù)據(jù)來源統(tǒng)計表Tab.3 Synthesis of the spinel datas of abyssal peridotites (ABP) and supra-subduction peridotites (SSZ) in the world and key for references used in Fig.4

5 討論

玉石溝鉻鐵礦體圍巖全巖高的MgO/SiO2值指示這些橄欖巖不是簡單的部分熔融殘余(圖3)，而是經(jīng)歷了熔體再富集作用(Niu, 2004)。前人研究認為，此富集過程(熔-巖反應(yīng))能夠解釋豆莢狀鉻鐵礦體的成因(Zhou et al., 1994；Arai, 2013)，但礦體頂?shù)讓拥募冮蠋r層相對于礦體厚度非常薄(Proenza et al., 1999；Shi et al., 2012)。距致密鉻鐵礦體由近及遠，玉石溝橄欖巖圍巖(礦石直接圍巖、近礦圍巖、遠礦圍巖)尖晶石的Cr#、Mg#、TiO2含量無規(guī)律變化(表2)。

這可能說明礦體圍巖的部分熔融或熔-巖反應(yīng)都不能解釋礦體的成因。

相對于深海橄欖巖(Mg’=85.96～92.59；Niu, 2004)和“上俯沖帶”橄欖巖(Mg’=88.36～92.28；Parkinson et al., 1998；Pearce et al., 2000)，玉石溝鉻鐵礦體礦石直接圍巖和近礦圍巖具有較高的Mg’值(90.83～96.93)。這是由于鉻鐵礦礦體與圍巖發(fā)生了高溫下的Mg-Fe交換。對這一特征反應(yīng)的重視研究將有助于實際找礦工作中對礦體進行更準確的定位。

a.橄欖石(Ol)和單斜輝石(Cpx)包裹體賦存在同一尖晶石中；b.斜方輝石(Opx)和單斜輝石包裹體(Cpx)賦存在同一尖晶石中，發(fā)育輝石出熔現(xiàn)象；c.斜方輝石(Opx)和單斜輝石包裹體(Cpx)緊密共生(出熔)；d.橄欖石(Ol)和單斜輝石包裹體(Cpx)緊密共生圖5 玉石溝鉻鐵礦尖晶石硅酸鹽礦物包裹體背散射圖像Fig.5 Back-scattered scanning electron microscope images of silicate mineral inclusions in spinel of Yushigou chromitites

5.1 鉻鐵礦形成的構(gòu)造位置

研究表明深海橄欖巖尖晶石Cr#值小于65(圖4a；Niu et al., 1997)。高鉻值尖晶石的近礦圍巖(Cr#=65.6～78.9)和礦石直接圍巖(Cr#=74.0～78.4)指示玉石溝鉻鐵礦體可能形成于“上俯沖帶”環(huán)境。尖晶石中的礦物包裹體代表了尖晶石結(jié)晶時的伴生熔體成分。盡管后期包裹體與尖晶石發(fā)生亞固相線的物質(zhì)交換，但這種反應(yīng)并未改變包裹體的主要化學(xué)特征。鉻鐵礦和圍巖尖晶石(圖4a、圖4b)及其礦物包裹體(圖6a、圖6c)化學(xué)成分的差別可能反映了不同期次的熔-巖反應(yīng)過程。區(qū)別于橄欖巖尖晶石包裹體，玉石溝鉻鐵礦尖晶石包裹體斜方輝石低的Al2O3，單斜輝石低的Al2O3以及角閃石低的TiO2指示該礦體形成于“上俯沖帶”而不是洋中脊。

5.2 鉻鐵礦成礦物質(zhì)來源

玉石溝礦體圍巖的尖晶石化學(xué)成分在空間上無規(guī)律變化(表2)指示成礦所需鉻并非來源于礦體圍巖。礦體圍巖的全巖主量元素成分(圖3)指示這些橄欖巖經(jīng)歷了與富MgO、SiO2熔體的反應(yīng)過程。致密鉻鐵礦礦體周圍賦存著含高鉻值尖晶石的橄欖巖。鉻鐵礦尖晶石母熔體組成(TiO2melt=0.13%～0.30%，Al2O3melt=13.76%～15.16%)與“上俯沖帶”的玻安巖尖晶石類似(TiO2=0.07%～0.69%，Al2O3=4.78%～16.13%；圖4c)。這可能說明鉻鐵礦體的形成與玻安質(zhì)熔體有關(guān)，即鉻鐵礦尖晶石從含玻安質(zhì)熔體的巖漿中大量結(jié)晶。

圖6 (a)玉石溝鉻鐵礦和橄欖巖尖晶石Cr#-橄欖石包裹體NiO(%)；(b)尖晶石Cr#-斜方輝石包裹體Al2O3 (%)；(c)尖晶石Cr#-單斜輝石包裹體Al2O3t%)；(d)角閃石包裹體Na2O-TiO2(%)投圖Fig.6 Tectonic discrimination diagrams using spinels and silicate mineral inclusions of chromitites and peridotites from Yushigou ophiolite注：玉石溝新鮮橄欖巖單礦物(Y)數(shù)據(jù)來源于Song et al., 2009。全球深海橄欖巖尖晶石包裹體(ABP)數(shù)據(jù)來源于Matsukage et al., 1998；Tamura et al., 2008,2014；Morishita et al., 2011a。Izu-Bonin海溝處橄欖巖尖晶石包裹體(SSZ)數(shù)據(jù)來源于Morishita et al., 2011a

5.3 鉻鐵礦包裹體對成礦過程的啟示

玉石溝鉻鐵礦尖晶石礦物包裹體主要為單斜輝石。斜方輝石或橄欖石與單斜輝石賦存在同一鉻鐵礦尖晶石中(圖5a，圖5b)。橄欖石和單斜輝石共存于一鉻鐵礦尖晶石包裹體中(圖5d)，即微觀上的異剝橄欖巖礦物組合反映了成礦巖漿中水的存在(Niu, 2005)。

含水礦物包裹體角閃石、云母出現(xiàn)在蛇綠巖型鉻鐵礦(Talkington et al., 1984；Akmaz et al., 2014)及其圍巖(Morishita et al., 2011b；Payot et al., 2013)、“上俯沖帶”橄欖巖(Morishita et al., 2011a)、深海橄欖巖(Matsukage et al., 1998；Tamura et al., 2014)尖晶石中，這多被解釋為熔-巖反應(yīng)的產(chǎn)物(Arai et al., 1997；Boudier et al., 2014)。然筆者發(fā)現(xiàn)角閃石、云母包裹體僅賦存在玉石溝鉻鐵礦尖晶石與硅酸鹽礦物接觸部位附近(圖7)。有學(xué)者認為此現(xiàn)象可能說明這些含水流體的富集晚于鉻鐵礦尖晶石的結(jié)晶(Leblanc et al., 1992；Borisova et al., 2012)。

流體(水)相的存在能夠促進尖晶石的結(jié)晶(Edwards et al., 2000；Matveev et al., 2002)。玉石溝鉻鐵礦尖晶石從含水巖漿中結(jié)晶。玻安質(zhì)熔體的加入能夠促進鉻鐵礦尖晶石的結(jié)晶(形成一條稀疏浸染狀礦脈)，但并不是形成致密鉻鐵礦礦體的充分條件。對含水礦物僅賦存在致密鉻鐵礦尖晶石與硅酸鹽礦物接觸部位附近此現(xiàn)象更合理的解釋是：島弧巖漿在上升過程中接觸到了一含水環(huán)境，含水流體觸發(fā)了島弧巖漿在此部位更快速集中結(jié)晶成礦。

表4 玉石溝鉻鐵礦及其周圍橄欖巖尖晶石中的代表性硅酸鹽礦物包裹體化學(xué)成分電子探針分析結(jié)果(%)Tab.4 Representative analyses of silicate mineral inclusions in spinel from chromitites and host peridotites by electron microprobe(%)

注：Fo=Mg/(Mg+TFe2+)×100。

a.角閃石(Amp)和單斜輝石(Cpx)包裹體賦存在同一鉻鐵礦尖晶石中；b.云母(Phl)和單斜輝石包裹體(Cpx)賦存在同一鉻鐵礦尖晶石中；c.角閃石賦存在鉻鐵礦尖晶石和硅酸鹽礦物接觸部位附近；d.云母與單斜輝石緊密共生，賦存在鉻鐵礦尖晶石和硅酸鹽礦物接觸部位附近圖7 玉石溝鉻鐵礦尖晶石角閃石、云母含水礦物包裹體背散射圖像Fig.7 Back-scattered scanning electron microscope images of hydrous silicate mineral inclusions in spinel of Yushigou chromitites

6 結(jié)論

(1)玉石溝橄欖巖全巖主量元素成分特征指示其經(jīng)歷了熔體再富集過程。

(2)玉石溝鉻鐵礦尖晶石和賦礦橄欖巖尖晶石及其硅酸鹽礦物包裹體的礦物化學(xué)特征指示鉻鐵礦體形成于俯沖帶上覆巖石圈地幔。

(3)玉石溝鉻鐵礦礦體圍巖的部分熔融和熔-巖反應(yīng)過程不是鉻尖晶石成礦過程。鉻鐵礦尖晶石從含玻安質(zhì)熔體的巖漿中大量結(jié)晶。

(4) 對鉻鐵礦-純橄巖這一具有巨大化學(xué)成分差異界面的詳細研究，尤其是流體的作用，將有助于進一步揭示鉻鐵礦成礦過程，確定礦體成礦位置，指導(dǎo)實際找礦。

致謝：本工作受中國地質(zhì)調(diào)查局地質(zhì)調(diào)查項目(1212011121092、1212011220928)資助。樣品測試工作得到了中國地質(zhì)大學(xué)(武漢)李璽瑤、魏穎博士，長安大學(xué)劉明武教授等的大力幫助，在此一并表示感謝。

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The Magmatic Record in the Peridotites from Yushigou, Qilian Orogen and the Petrogenesis of the Ophiolite-Type Chromitites

HU Zhenxing1,NIU Yaoling2,3,4,LIU Yi4,SUN Wenli1,CHEN Suo3,LI Jiyong3, ZHANG Guorui1

(1.School of Earth Sciences, Lanzhou University, Lanzhou 730000, Gansu, China; 2.Department of Earth Sciences, Durham University, Durham DH1 3LE, UK; 3.Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, Shandong, China; 4.China University of Geosciences, Beijing 10083, Beijing,China)

The origin of ophiolite-type chromites remain poorly understood despite the great effort over the years.We have sampled podiform chromites and its host peridotites at certain intervals in Yushigou ophiolite, which is well-known for its high-Cr chromites in the Early Paleozoic Qilian suture zone.The very high MgO and low SiO2content of these serpentinized peridotites reflect that they are too depleted to be residues of partial melting.Both the chromites and peridotites may have undergone multi-processes of melt refertilization.Compared with global abyssal peridotites, their spinels have very high Cr#(>65).Furthermore, the uniform chemical characteristics of chromite-hosted silicate mineral inclusions suggest that the orebody may have formed in the suprasubduction zones setting.With irregular changes of spatial distribution in Cr#from 77.2 to 43.9 and TiO2from 0.34% to 0.06%, the spinels in host peridotites show that formation of the high-Cr chromites have a boninitic melt affinity instead of originating from the host peridotites.The hydrous mineral inclusions only occur near the outer edge of chromian spinels and silicate minerals may provide evidence for our hypothesis: chromite ore formation results from interaction of hydrous melt with harzburgitic ambience during ascent.

podiform chromite; inclusions; boninitic melt; water; Yushigou

2014-09-10；

2015-02-09

中國地質(zhì)調(diào)查局地質(zhì)調(diào)查項目“新疆北部晚古生代大規(guī)模巖漿作用與成礦藕合關(guān)系研究”(1212011121092)，中國大型-超大型礦床成礦地球動力學(xué)背景、過程和定量評價綜合研究(1212011220928)

胡振興(1989-)，男，碩士，礦物、巖石、礦床學(xué)專業(yè)。Email：huzhx12@lzu.cn

P511.4

A

1009-6248(2015)01-0001-15