祁连山玉石沟橄榄岩岩浆作用的记录和铬铁矿的成因

胡振兴，牛耀龄，刘益，孙文礼，陈硕，李继永，张国瑞

(1.兰州大学地质科学与矿产资源学院，甘肃 兰州 730000；2.Department of Earth Sciences，Durham University，Durham DH1 3LE，UK；3.中国科学院海洋研究所，山东 青岛 266071；4.中国地质大学地球科学与资源学院，北京 100083)

祁连山玉石沟橄榄岩岩浆作用的记录和铬铁矿的成因

胡振兴1，牛耀龄2,3,4，刘益4，孙文礼1，陈硕3，李继永3，张国瑞1

(1.兰州大学地质科学与矿产资源学院，甘肃 兰州 730000；2.Department of Earth Sciences，Durham University，Durham DH1 3LE，UK；3.中国科学院海洋研究所，山东 青岛 266071；4.中国地质大学地球科学与资源学院，北京 100083)

祁连山玉石沟蛇绿岩型铬铁矿是典型的高铬型铬铁矿床。通过对玉石沟铬铁矿和橄榄岩围岩进行原位定距离采样研究表明：全岩高的MgO和低的SiO2含量指示这些蛇纹石化橄榄岩不是简单的部分熔融残余，而是经历了后期的熔体再富集作用。玉石沟近矿橄榄岩尖晶石(Cr#>65)和铬铁矿尖晶石的矿物包裹体化学特征反映矿体和围岩形成于俯冲带上覆岩石圈地幔。距离矿体由近及远，赋矿围岩尖晶石化学成分(Cr#=43.9～77.2，TiO2=0.06%～0.34%)无规律变化。这说明高铬型铬铁矿体成矿所需铬并非来源于围岩，而是与玻安质熔体渗滤有关。尖晶石含水矿物包裹体仅赋存在玉石沟铬铁矿尖晶石与硅酸盐矿物接触部位附近可能揭示了玉石沟铬铁矿体的成因：成矿铬尖晶石在上侵的含水熔体与周围方辉橄榄岩的反应过程中形成聚集。

豆荚状铬铁矿；包裹体；玻安质熔体；水；玉石沟

蛇绿岩型铬铁矿因矿体形似豆荚又称豆荚状铬铁矿，矿体外围常被一层薄薄的纯橄岩包围。这类纯橄岩常被解释为熔-岩反应的产物(Büchl et al., 2004；Zhou et al., 2005)。近年来,通过对太平洋、大西洋和印度洋洋中脊的直接深海钻探(Leg147，Leg209等)，发现了一些小的豆荚状矿石，厚度约1～4 cm。一些学者将其作为洋中脊能够提供形成豆荚状铬铁矿环境的直接证据(Matsukage et al., 1998；Abe, 2011；Payot et al., 2013)。此外，罗布莎等豆荚状铬铁矿中原位金刚石包裹体的存在指示豆荚状铬铁矿可能来源于深部地幔(Yang et al., 2014)。熔-岩反应、熔体不混溶、含水流体相与硅酸盐相的分离是目前用来解释豆荚状铬铁矿成因的流行假说(González-Jiménez et al., 2014)，但并未解答铬铁矿为什么会在一定部位集中结晶这一关键问题(胡振兴等，2014)。

祁连玉石沟蛇绿岩套赋存着典型的高铬型豆荚状铬铁矿床。认为玉石沟地幔橄榄岩经历了熔体再富集作用(Song et al., 2009；饶万祥等，2012)；玉石沟铬铁矿与地幔橄榄岩的高程度部分熔融有关(周会武等，1995；童海奎等，2012)。通过对玉石沟铬铁矿及其橄榄岩围岩的岩相学和地球化学特征研究，提出一个铬铁矿成因假设；并获得对此类铬铁矿体赋存围岩的进一步认识，这有助于在实际找矿工作中对矿体进行更准确的定位。

1 区域地质背景

玉石沟蛇绿岩体位于青海省祁连县野牛沟乡，认为其代表了早古生代祁连洋洋壳的残体(图1a；肖序常等，1978；Song et al., 2013)。玉石沟蛇绿岩岩石组合发育较完整，有变质橄榄岩、堆晶岩、枕状熔岩和硅质岩等。南北均以断裂为界(冯益民等，1996)。前人对该岩体周围地层与古生物化石及枕状熔岩全岩Rb-Sr(肖序常等，1978)、辉长岩锆石U-Pb同位素年代学研究(史仁灯等，2004；Song et al., 2013)指出玉石沟蛇绿岩年龄约550～521 Ma。

2 矿体地质及岩石学特征

玉石沟蛇绿岩超基性岩体群主要由北、中、小、南4个岩体组成(图1b)，较大的铬铁矿体多集中在南岩体。铬铁矿矿体多呈雁行式排列，以透镜体状赋存在蛇纹石化纯橄岩中(姚培慧，1996)。所研究的铬铁矿矿体位于南岩体中，矿体长轴方向与水平面夹角约45°。矿体与围岩界线明显(图2a)；矿石主要为致密块状矿石，围岩为蛇纹石化橄榄岩。距离矿体约20 m处可见一稀疏浸染状铬铁矿脉。

对玉石沟铬铁矿体及其围岩进行了原位采样：致密块状矿石、稠密浸染状矿石、稀疏浸染状矿石共12个，矿石直接围岩(图2a)、与矿体距离小于1 m的近矿围岩(沿矿体长轴方向)、按距离矿体由近及远一定间隔(5～10 m)采得的远矿围岩共21个。围岩橄榄岩中的矿物已严重蛇纹石化，尖晶石与蛇纹石等蚀变矿物接触截然(图2b)。铬铁矿和橄榄岩尖晶石都有橄榄石、单斜辉石等硅酸盐矿物包裹体，其中铬铁矿尖晶石中的硅酸盐包裹体以单斜辉石为主，这些包裹体矿物粒径为2～100 um不等(图2c，图2d)。

3 样品分析方法

全岩主量元素在中国地质大学(武汉)地质过程与矿产资源国家重点实验室用XRF方法测定(Ma et al., 2012)。

单矿物电子探针测试在长安大学国土资源部成矿作用及其动力学实验室用JXA-8100完成。仪器工作条件: 加速电压15 kV，探针束流20 nA，束斑直径1 um。

(b)1.中寒武世砂板岩夹灰岩；2.震旦纪白云岩和石英片岩等；3.奥陶纪灰岩板岩及中基性火山岩；4.滑石菱镁片岩；5.蛇纹岩；6.纯橄岩；7.方辉橄榄岩；8.辉长岩；9.断层；10.地质界线图1 (a)玉石沟蛇绿岩体大地构造位置图(据Xu et al., 2010 and Song et al.2013修改)；(b)玉石沟超基性岩块分布图(据史仁灯等，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.块状矿石和橄榄岩围岩接触界线明显；b.矿体围岩橄榄岩严重蛇纹石(Serp)化；c.稠密浸染状铬铁矿尖晶石(Sp)含单斜辉石(Cpx)包裹体；d.橄榄岩尖晶石含橄榄石(Ol)包裹体图2 玉石沟铬铁矿及橄榄岩岩相学照片Fig.2 Photomicrographs of the Yushigou chromitites andserpentinized peridotites

4 地球化学特征

4.1 橄榄岩全岩主量元素化学特征

采得的玉石沟铬铁矿体周围橄榄岩已高度蛇纹石化，对其全岩主量元素含量(表1)进行无水校正至100%。全岩高的MgO(46.23%～51.10%，图3a)、MgO/SiO2值(1.06～1.24，图3b)，低的Al2O3(0.01%～2.02%)、CaO(0.09%～0.56%)等指示这些蛇纹石化橄榄岩过度亏损，明显不符合“全球阵列”趋势(图3b)。矿体周围橄榄岩Mg’为90.17～96.93(Mg’=MgO/[MgO+TFeOtot]×100)。

4.2 尖晶石矿物化学特征

玉石沟致密铬铁矿矿体的近矿围岩(距离矿体<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)。这与目前研究发现的俯冲带上覆岩石圈(“上俯冲带”)地幔橄榄岩特征类似(在Izu～Bonin海沟处采样发现不同铬值(Cr#=81.8，53.5～55.0)的纯橄岩紧密共生；Morishita et al., 2011a)。稀疏浸染状铬铁矿(Cr#=77.1，Mg#=52.9，TiO2=0.10%)距离致密铬铁矿矿体约20 m，其周围橄榄岩(远矿围岩，与致密铬铁矿矿体距离>5 m)尖晶石铬值为45.0～55.6(图4a)。

距离玉石沟致密铬铁矿矿体由近及远，围岩尖晶石的Mg#与Cr#无规律变化(表2)。致密铬铁矿尖晶石的TiO2含量(0.04%～0.19%，103个单矿物数据平均值为0.11%：AVE103=0.11%)比围岩尖晶石(0.06%～0.34%，AVE64=0.18%)小(图4b)。距离致密铬铁矿矿体由近及远，围岩尖晶石TiO2含量不规则波动，无逐渐增大趋势(表2)。

表1 玉石沟铬铁矿矿体周围橄榄岩的全岩主量元素数据表(%)Tab.1 Major elements data ofhost peridotites(%)

对铬铁矿尖晶石母熔体化学成分模拟计算显示：母熔体TiO2含量为0.13%～0.30%，Al2O3含量为13.76%～15.16%。玉石沟铬铁矿尖晶石母熔体具有类似的化学成分 (图4c)。

4.3 尖晶石包裹体特征

玉石沟铬铁矿和围岩尖晶石都有橄榄石、单斜辉石等硅酸盐矿物包裹体(图5)，但化学成分有明显差别(表4)。后期包裹体矿物与尖晶石发生亚固相线的Mg-Fe交换(Melcher et al., 1997)，导致橄榄石、单斜辉石、斜方辉石包裹体的Fo值高达93.0～97.4。相对于玉石沟新鲜橄榄岩相应单矿物(Song et al., 2009)和铬铁矿围岩尖晶石的矿物包裹体中铬铁矿尖晶石的矿物包裹体：橄榄石具有高的NiO(0.40%～1.03%，图6a)，辉石具有低的Al2O3(Cpx=0.52%～1.52%，图6b；Opx=0.31%～0.57%，图6c)，这与全球蛇绿岩型高铬值铬铁矿和橄榄岩尖晶石硅酸盐矿物包裹体化学特征一致(Mcelduff et al., 1991；Ahmed et al., 2001；Zhou et al., 2014)。

值得指出的是玉石沟铬铁矿尖晶石还有角闪石、云母含水矿物包裹体(图7)。角闪石为韭闪石(Na2O含量为1.14%～2.01%，K2O含量为0.09%～0.20%，TiO2含量为0.17%～0.33%；图6d)。云母为金云母(Al2O3含量为11.53%～13.24%，MgO含量为21.99%～26.61%，FeO含量为0.59%～1.18%)。

表2 玉石沟铬铁矿及其周围橄榄岩代表性尖晶石化学成分电子探针分析结果(%)Tab.2 Representative analyses of spinel fromchromitites and host peridotites by electron microprobe(%)

注：Fe2O3和FeO含量根据化合价平衡计算；Mg#=Mg/(Mg+Fe2+)×100, Cr#=Cr/(Cr+Al)×100。

图3 (a)玉石沟蛇绿岩橄榄岩全岩MgO-SiO2；(b)Al2O3/SiO2-MgO/SiO2；(c)SiO2-MgO/SiO2投图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)数值见Niu, 1997附录B。玉石沟新鲜橄榄岩(Y)数据来源于Song et al., 2009。全球深海橄榄岩(ABP)数据引自Niu, 2004。俯冲带上覆岩石圈地幔橄榄岩(SSZ)数据引自Parkinson et al., 1998；Pearce et al., 2000。部分熔融曲线参考文献已在图中列出。全球阵列来自Jagoutz et al., 1979；Hart et al., 1986。所有数据统一标准化到无水总量100%

图4 (a)玉石沟铬铁矿及橄榄岩尖晶石Cr#-Mg#；(b)Cr#-TiO2(wt%)；(c)Al2O3melt-TiO2melt投图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)。玉石沟新鲜橄榄岩尖晶石(Y)数据来源于Song et al., 2009。图4c中岛弧中基性岩尖晶石(ARC)和洋中脊玄武岩尖晶石(MORB)范围来源于Kamenetsky et al., 2001；铬铁矿尖晶石母熔体Al2O3和TiO2含量计算公式为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)。全球深海橄榄岩尖晶石(ABP)和俯冲带上覆岩石圈地幔橄榄岩尖晶石(SSZ)以及Izu-Bonin和Tonga海沟处玻安岩尖晶石(BON)数据来源见表3

表3 全球深海橄榄岩和俯冲带上覆岩石圈地幔橄榄岩尖晶石数据来源统计表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 讨论

玉石沟铬铁矿体围岩全岩高的MgO/SiO2值指示这些橄榄岩不是简单的部分熔融残余(图3)，而是经历了熔体再富集作用(Niu, 2004)。前人研究认为，此富集过程(熔-岩反应)能够解释豆荚状铬铁矿体的成因(Zhou et al., 1994；Arai, 2013)，但矿体顶底层的纯橄岩层相对于矿体厚度非常薄(Proenza et al., 1999；Shi et al., 2012)。距致密铬铁矿体由近及远，玉石沟橄榄岩围岩(矿石直接围岩、近矿围岩、远矿围岩)尖晶石的Cr#、Mg#、TiO2含量无规律变化(表2)。

这可能说明矿体围岩的部分熔融或熔-岩反应都不能解释矿体的成因。

相对于深海橄榄岩(Mg’=85.96～92.59；Niu, 2004)和“上俯冲带”橄榄岩(Mg’=88.36～92.28；Parkinson et al., 1998；Pearce et al., 2000)，玉石沟铬铁矿体矿石直接围岩和近矿围岩具有较高的Mg’值(90.83～96.93)。这是由于铬铁矿矿体与围岩发生了高温下的Mg-Fe交换。对这一特征反应的重视研究将有助于实际找矿工作中对矿体进行更准确的定位。

a.橄榄石(Ol)和单斜辉石(Cpx)包裹体赋存在同一尖晶石中；b.斜方辉石(Opx)和单斜辉石包裹体(Cpx)赋存在同一尖晶石中，发育辉石出熔现象；c.斜方辉石(Opx)和单斜辉石包裹体(Cpx)紧密共生(出熔)；d.橄榄石(Ol)和单斜辉石包裹体(Cpx)紧密共生图5 玉石沟铬铁矿尖晶石硅酸盐矿物包裹体背散射图像Fig.5 Back-scattered scanning electron microscope images of silicate mineral inclusions in spinel of Yushigou chromitites

5.1 铬铁矿形成的构造位置

研究表明深海橄榄岩尖晶石Cr#值小于65(图4a；Niu et al., 1997)。高铬值尖晶石的近矿围岩(Cr#=65.6～78.9)和矿石直接围岩(Cr#=74.0～78.4)指示玉石沟铬铁矿体可能形成于“上俯冲带”环境。尖晶石中的矿物包裹体代表了尖晶石结晶时的伴生熔体成分。尽管后期包裹体与尖晶石发生亚固相线的物质交换，但这种反应并未改变包裹体的主要化学特征。铬铁矿和围岩尖晶石(图4a、图4b)及其矿物包裹体(图6a、图6c)化学成分的差别可能反映了不同期次的熔-岩反应过程。区别于橄榄岩尖晶石包裹体，玉石沟铬铁矿尖晶石包裹体斜方辉石低的Al2O3，单斜辉石低的Al2O3以及角闪石低的TiO2指示该矿体形成于“上俯冲带”而不是洋中脊。

5.2 铬铁矿成矿物质来源

玉石沟矿体围岩的尖晶石化学成分在空间上无规律变化(表2)指示成矿所需铬并非来源于矿体围岩。矿体围岩的全岩主量元素成分(图3)指示这些橄榄岩经历了与富MgO、SiO2熔体的反应过程。致密铬铁矿矿体周围赋存着含高铬值尖晶石的橄榄岩。铬铁矿尖晶石母熔体组成(TiO2melt=0.13%～0.30%，Al2O3melt=13.76%～15.16%)与“上俯冲带”的玻安岩尖晶石类似(TiO2=0.07%～0.69%，Al2O3=4.78%～16.13%；图4c)。这可能说明铬铁矿体的形成与玻安质熔体有关，即铬铁矿尖晶石从含玻安质熔体的岩浆中大量结晶。

图6 (a)玉石沟铬铁矿和橄榄岩尖晶石Cr#-橄榄石包裹体NiO(%)；(b)尖晶石Cr#-斜方辉石包裹体Al2O3 (%)；(c)尖晶石Cr#-单斜辉石包裹体Al2O3t%)；(d)角闪石包裹体Na2O-TiO2(%)投图Fig.6 Tectonic discrimination diagrams using spinels and silicate mineral inclusions of chromitites and peridotites from Yushigou ophiolite注：玉石沟新鲜橄榄岩单矿物(Y)数据来源于Song et al., 2009。全球深海橄榄岩尖晶石包裹体(ABP)数据来源于Matsukage et al., 1998；Tamura et al., 2008,2014；Morishita et al., 2011a。Izu-Bonin海沟处橄榄岩尖晶石包裹体(SSZ)数据来源于Morishita et al., 2011a

5.3 铬铁矿包裹体对成矿过程的启示

玉石沟铬铁矿尖晶石矿物包裹体主要为单斜辉石。斜方辉石或橄榄石与单斜辉石赋存在同一铬铁矿尖晶石中(图5a，图5b)。橄榄石和单斜辉石共存于一铬铁矿尖晶石包裹体中(图5d)，即微观上的异剥橄榄岩矿物组合反映了成矿岩浆中水的存在(Niu, 2005)。

含水矿物包裹体角闪石、云母出现在蛇绿岩型铬铁矿(Talkington et al., 1984；Akmaz et al., 2014)及其围岩(Morishita et al., 2011b；Payot et al., 2013)、“上俯冲带”橄榄岩(Morishita et al., 2011a)、深海橄榄岩(Matsukage et al., 1998；Tamura et al., 2014)尖晶石中，这多被解释为熔-岩反应的产物(Arai et al., 1997；Boudier et al., 2014)。然笔者发现角闪石、云母包裹体仅赋存在玉石沟铬铁矿尖晶石与硅酸盐矿物接触部位附近(图7)。有学者认为此现象可能说明这些含水流体的富集晚于铬铁矿尖晶石的结晶(Leblanc et al., 1992；Borisova et al., 2012)。

流体(水)相的存在能够促进尖晶石的结晶(Edwards et al., 2000；Matveev et al., 2002)。玉石沟铬铁矿尖晶石从含水岩浆中结晶。玻安质熔体的加入能够促进铬铁矿尖晶石的结晶(形成一条稀疏浸染状矿脉)，但并不是形成致密铬铁矿矿体的充分条件。对含水矿物仅赋存在致密铬铁矿尖晶石与硅酸盐矿物接触部位附近此现象更合理的解释是：岛弧岩浆在上升过程中接触到了一含水环境，含水流体触发了岛弧岩浆在此部位更快速集中结晶成矿。

表4 玉石沟铬铁矿及其周围橄榄岩尖晶石中的代表性硅酸盐矿物包裹体化学成分电子探针分析结果(%)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.角闪石(Amp)和单斜辉石(Cpx)包裹体赋存在同一铬铁矿尖晶石中；b.云母(Phl)和单斜辉石包裹体(Cpx)赋存在同一铬铁矿尖晶石中；c.角闪石赋存在铬铁矿尖晶石和硅酸盐矿物接触部位附近；d.云母与单斜辉石紧密共生，赋存在铬铁矿尖晶石和硅酸盐矿物接触部位附近图7 玉石沟铬铁矿尖晶石角闪石、云母含水矿物包裹体背散射图像Fig.7 Back-scattered scanning electron microscope images of hydrous silicate mineral inclusions in spinel of Yushigou chromitites

6 结论

(1)玉石沟橄榄岩全岩主量元素成分特征指示其经历了熔体再富集过程。

(2)玉石沟铬铁矿尖晶石和赋矿橄榄岩尖晶石及其硅酸盐矿物包裹体的矿物化学特征指示铬铁矿体形成于俯冲带上覆岩石圈地幔。

(3)玉石沟铬铁矿矿体围岩的部分熔融和熔-岩反应过程不是铬尖晶石成矿过程。铬铁矿尖晶石从含玻安质熔体的岩浆中大量结晶。

(4) 对铬铁矿-纯橄岩这一具有巨大化学成分差异界面的详细研究，尤其是流体的作用，将有助于进一步揭示铬铁矿成矿过程，确定矿体成矿位置，指导实际找矿。

致谢：本工作受中国地质调查局地质调查项目(1212011121092、1212011220928)资助。样品测试工作得到了中国地质大学(武汉)李玺瑶、魏颖博士，长安大学刘明武教授等的大力帮助，在此一并表示感谢。

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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

中国地质调查局地质调查项目“新疆北部晚古生代大规模岩浆作用与成矿藕合关系研究”(1212011121092)，中国大型-超大型矿床成矿地球动力学背景、过程和定量评价综合研究(1212011220928)

胡振兴(1989-)，男，硕士，矿物、岩石、矿床学专业。Email：huzhx12@lzu.cn

P511.4

A

1009-6248(2015)01-0001-15