共查询到19条相似文献，搜索用时 697 毫秒
1.
泽当蛇绿岩位于雅鲁藏布江缝合带东段,由地幔橄榄岩、辉长辉绿岩、火山岩等组成。地幔橄榄岩主要为方辉橄榄岩和二辉橄榄岩,有少量的铬铁矿化方辉橄榄岩和透镜状纯橄岩。地幔橄榄岩中橄榄石的Fo值为89.6~91.8,属镁橄榄石。斜方辉石为顽火辉石,En端员组分变化于87.8~90.3。单斜辉石En组分变化于44.1~50.0,主要为顽透辉石和透辉石。二辉橄榄岩与方辉橄榄岩铬尖晶石的Cr#为17.0~31.8,为富铝型尖晶石。泽当地幔橄榄岩PGE总量为16.67×10-9~32.59×10-9,与原始地幔相似。矿物化学特征显示泽当二辉橄榄岩属于深海型地幔橄榄岩,方辉橄榄岩属于弧前地幔橄榄岩。尖晶石Cr#、橄榄石Mg#的变化以及高Os含量(3.50×10-9~7.75×10-9)表明泽当地幔橄榄岩经历了部分熔融过程;正斜率的PGE配分模式以及较高的Pd/Ir值(1.09~3.94)表明泽当地幔橄榄岩受到了俯冲带环境下地幔交代作用的改造。泽当地幔橄榄岩矿物学特征与铂族元素地球化学特征显示其形成于MOR环境,后受到SSZ环境的改造。 相似文献
2.
雅鲁藏布江缝合带西段普兰蛇绿岩中地幔橄榄岩的岩石学研究 总被引：14，自引：10，他引：4
西藏雅鲁藏布江缝合带西段普兰蛇绿岩以出现面积约600余平方千米的特大型地幔橄榄岩体而引人注目.该地幔橄榄岩以方辉橄榄岩为主体,含有少量的二辉橄榄岩和纯橄榄岩,岩体中另有一些橄榄单斜辉石岩、辉长岩和辉绿岩等侵入体.地幔橄榄岩的主要造岩矿物橄榄石的Fo 90～93,其中呈包裹体的橄榄石的Fo略高,斜方辉石为顽火辉石(En 88～90),单斜辉石主要为顽透辉石和透辉石,以低铝(0.48％～3.96％)和高Mg#(91～96)为特征,铬尖晶石的Cr#值为18～69,其中方辉橄榄岩和二辉橄榄岩中的铬尖晶石属富铝型尖晶石,而纯橄岩中为富铬型尖晶石.橄榄单斜辉石岩的橄榄石Fo值一致较低,平均为88.4,斜方辉石En平均87,单斜辉石以透辉石为主,铬尖晶石的Cr#值为45～69.普兰地幔橄榄岩及橄榄单斜辉石岩都具有相似的稀土元素和微量元素配分模式,表现为LREE相对富集,Eu亏损不明显,微量元素中大离子亲石元素含量较低,部分样品高场强元素亏损,另一些则相对富集,显示地幔橄榄岩具有亏损地幔源区特征,但也具有俯冲带流体的交代特征,表明普兰岩体可能经历了MOR和SSZ两种构造环境,该特征与雅鲁藏布江缝合带东段的罗布莎地幔橄榄岩的特征可以对比. 相似文献
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西藏雅鲁藏布江缝合带西段巴尔地幔橄榄岩成因及构造意义 总被引：10，自引：7，他引：3
巴尔蛇绿岩属于雅鲁藏布江缝合带的西延部分,距拉萨约1200km,主要由地幔橄榄岩、少量的橄长堆晶岩和玄武岩组成.地幔橄榄岩主体为合单辉方辉橄榄岩,少量为二辉橄榄岩.根据巴尔蛇绿岩地幔橄榄岩的结构构造特征,将矿物组合划分为3个世代,第一世代残余地幔矿物组合:橄榄石+斜方辉石+单斜辉石;第二世代部分熔融及熔-岩反应矿物组合:橄榄石+斜方辉石+单斜辉石+尖晶石;第三世代地幔交代作用矿物组合,主要为含水矿物角闪石.将3个世代的矿物组合归并为2个演化阶段:第一阶段,包括第一世代和第二世代矿物组合,形成于MOR(mid-ocean ridge)构造环境下的洋脊扩张阶段;第二阶段,为第三世代矿物角闪石,形成于SSZ(super-subduction zone)环境下的俯冲阶段.对比雅鲁藏布江缝合带不同区段蛇绿岩中地幔橄榄岩的特征,发现雅鲁藏布江缝合带存在MOR和SSZ两种类型的蛇绿岩,其中中段的蛇绿岩主要以典型的SSZ型地幔橄榄岩为主,而东、西段则以受到不同程度SSZ环境改造的MOR型地幔橄榄岩为主,认为雅鲁藏布江缝合带蛇绿岩地幔橄榄岩演化分段性的特征,与新特提斯洋沿弧方向上板块活动的动力学机制的不均一有关. 相似文献
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西藏雅鲁藏布江缝合带东段泽当地幔橄榄岩特征及其意义 总被引：1，自引：1，他引：0
泽当岩体位于雅鲁藏布江缝合带东段,主要由地幔橄榄岩、辉长辉绿岩和基性火山岩等组成。地幔橄榄岩主要为方辉橄榄岩和二辉橄榄岩,有少量透镜状纯橄岩。地幔橄榄岩经历了强烈的塑性变形作用。地幔橄榄岩中橄榄石的Fo值为89.6~91.8,属镁橄榄石;斜方辉石为顽火辉石,En 87.8~90.3;单斜辉石En 44.1~50.0,主要为顽透辉石和透辉石。铬尖晶石的Cr#值(=100×Cr/(Cr+Al))为17.0~93.6,其中,二辉橄榄岩和方辉橄榄岩中的铬尖晶石为富铝型尖晶石,纯橄岩中的铬尖晶石Cr#最高,为富铬型尖晶石。地幔橄榄岩的部分熔融程度为17%~34%,表明泽当地幔橄榄岩可能经历了多阶段的过程。亏损的主量元素组成和低于原始地幔的稀土元素含量(0.15×10-6~0.61×10-6)指示泽当地幔橄榄岩为经历过部分熔融和熔体抽取的亏损残余地幔岩石。REE配分型式为中稀土亏损的"V"型或"U"型,原始地幔标准化元素比值(La/Sm)N为0.5~8.0,表明泽当地幔橄榄岩经历过交代作用。矿物化学与地球化学数据表明泽当地幔橄榄岩形成于MOR环境,后受到SSZ环境的改造。 相似文献
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哀牢山南段蛇绿岩中变质橄榄石单元,由方辉橄榄石和二辉橄榄岩组成,前者已强烈蚀变,后者的矿物组成有橄榄石,斜方辉石,单斜辉石及少量尖晶石等,对这些矿物进行了较详细的化学成分分析,橄榄石,斜方辉石的化学分显示,二辉橄榄石为较弱亏损的地幔岩,橄榄石,尖晶石的矿物化学反应,变质橄榄石的原始岩石具有深海橄榄石的特征,单斜辉石,尖晶石的化学特征显示,二辉橄榄石经历过程熔融作用,因此,二辉橄榄石为部分熔融较弱亏 相似文献
6.
达拉布特蛇绿岩中地幔橄榄岩的主体为方辉橄榄岩,含少量纯橄岩和二辉橄榄岩,岩石遭受强烈蚀变。方辉橄榄岩单斜辉石、斜方辉石、橄榄石和尖晶石的主量元素特征均显示从深海地幔橄榄岩向SSZ地幔橄榄岩过渡的特征,与斜方辉石原位LA-ICP-MS微量元素特征一致,二辉橄榄岩具有深海地幔岩的性质。采用尖晶石-橄榄石平衡氧逸度计算方法,得出方辉橄榄岩的Δlog(fo2)FMQ在-0.14至+0.96log FMQ之间,具有MOR地幔橄榄岩向SSZ地幔橄榄岩过渡的特点或弧后盆地至岛弧过渡的特征。尖晶石Ga-Ti-Fe3+#图解显示纯橄岩成因可能和地幔橄榄岩与岛弧拉斑玄武岩的反应有关,而方辉橄榄岩可能为地幔橄榄岩与MOR熔体反应以及SSZ环境中含水熔体反应后的残余。纯橄岩和方辉橄榄岩∑REE都低于球粒陨石,且具有LREE富集的U型稀土元素配分模式,暗示了岩石和流体/熔体之间的相互作用。综合以上研究表明,达拉布特蛇绿岩形成于弧后扩张脊并受俯冲流体/熔体影响。 相似文献
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西藏阿里地区的南公珠错蛇绿岩产在公珠错的南侧,空间上属于雅鲁藏布江缝合带西段之南亚带蛇绿岩。该蛇绿岩主要由地幔橄榄岩和辉长岩等基性岩类组成。地幔橄榄岩中约80%为方辉橄榄岩,20%为二辉橄榄岩,纯橄岩较少。南公珠错地幔橄榄岩矿物化学特征表现为橄榄石具有较低的Fo(89.3~91.4)值、辉石具有较高的Al_2O_3含量(1.89%~6.06%)、尖晶石具有较低的Cr~#(12.7~28.3)值。与原始地幔相比南公珠错地幔橄榄岩的全岩地球化学特征具有较高的MgO含量和较低的Al_2O_3、CaO和TiO_2等易熔元素含量;方辉橄榄岩和二辉橄榄岩的稀土元素总含量分别介于0.66×10-6~1.10×10-6和0.90×10~(-6)~3.78×10~(-6)之间,明显低于原始地幔值,其稀土元素配分模式为轻稀土元素轻微富集型;在原始地幔标准化微量元素蜘蛛图中,南公珠错地幔橄榄岩显示出强烈的U正异常、Nd轻微正异常和强不相容元素Zr的负异常;方辉橄榄岩和二辉橄榄岩的铂族元素总量分别介于15.26×10~(-9)~25.23×10~(-9)和18.74×10~(-9)~26.86×10~(-9)之间,二者含量的变化较小,南公珠错地幔橄榄岩PGEs球粒陨石标准化图解显示其为接近于原始地幔的"平坦型"。南公珠错地幔橄榄岩的矿物化学和全岩地球化学特征与深海橄榄岩相似,指示它们可能形成于大洋扩张脊环境。定量模拟估算表明,南公珠错地幔橄榄岩可能来源于地幔中的尖晶石相二辉橄榄岩源区,系经历了至多16%部分熔融的残余。LREE的微富集和较高的Pd/Ir、Rh/Ir比值指示它们还经历了岩石-熔体反应作用。初步结论认为南公珠错地幔橄榄岩形成于大洋脊环境,为尖晶石相二辉橄榄岩地幔源区较低程度部分熔融的残余,但经历了后期岩石-熔体反应作用。 相似文献
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东波超镁铁岩体:西藏雅鲁藏布江缝合带西段一个甚具铬铁矿前景的地幔橄榄岩体 总被引：11，自引：5，他引：6
东波超镁铁岩体产在雅鲁藏布江缝合带的西段,与周边白垩纪沉积岩地层和火山岩以断层接触.航磁资料显示该岩体约400km2规模,地表出露连续,地下有一定延深.超镁铁岩体由亏损的地幔橄榄岩组成,主要有高镁的方辉橄榄岩、纯橄岩和少量二辉橄榄岩.方辉橄榄岩和二辉橄榄岩中橄榄石和斜方辉石属高镁型,分别为Fo=89.5～91.5和Mg#=90～91.5.但二辉橄榄岩中的Al2O3和CaO含量明显高于方辉橄榄岩.方辉橄榄岩中单斜辉石Mg#=92～95,二辉橄榄岩的Mg#=92～93,两者的值也重叠.二辉橄榄岩中的Al2O3和CaO含量要明显高于方辉橄榄岩.这些均为阿尔卑斯型地幔橄榄岩的典型特征.纯橄岩中的橄榄石Fo=92～93.2,其斜方辉石和单斜辉石的Mg#=～93,但Al2O3和CaO的含量比方辉橄榄岩和二辉橄榄岩的低.三种岩石的成分变化规律,反映了地幔部分熔融程度的差异.二辉橄榄岩铬尖晶石的Cr#值20～30,反映为典型深海橄榄岩特征,指示MOR环境.与其不同的是,方辉橄榄岩的铬尖晶石的Cr#=20～75,指示MOR和SSZ两者兼有环境.岩石的原始地幔标准化的REE和微量元素蛛网图模式支持了上述的认识.东波地幔橄榄岩中的岩石学特征与产有大型铬铁矿床的罗布莎地幔橄榄岩可对比,岩体中已多处发现块状铬铁矿石,其铬铁矿的Cr2O3含量56％～59％,表明东波是寻找铬铁矿大矿和富矿甚具前景的一个超镁铁岩体. 相似文献
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西藏罗布莎蛇绿岩中不同产出的纯橄岩及成因探讨 总被引：2，自引：2，他引：0
罗布莎蛇绿岩中的纯橄岩有三种产出情况,除了与豆荚状铬铁矿伴生的薄壳状纯橄岩外,还有产在方辉橄榄岩底部被认为是堆晶岩的厚层状纯橄岩和方辉橄榄岩中的透镜状纯橄岩。厚层状纯橄岩约700～1000m厚,以橄榄石富镁(Fo93～95),单斜辉石低铝富镁(Al2O30.47%～0.85%,Mg#95～97),铬尖晶石高铬低镁(Cr#值平均77,Mg#平均51)为特征。该纯橄岩中的浸染状铬铁矿也是高铬低镁型,但Mg#值(平均59)高于厚层状纯橄岩的副矿物铬尖晶石。薄壳状纯橄岩与厚层状纯橄岩成分相近,其橄榄石Fo92～94,单斜辉石Al2O3<1%和Mg#95～97;铬尖晶石的Cr#值平均71,Mg#值平均52。与薄壳状纯橄岩伴生的块状铬铁矿为高镁高铬型,但Mg#值(平均68)相对更高些,Cr#值平均79。透镜状纯橄岩的特征是橄榄石Fo(91～92)和铬尖晶石Cr#(60左右)均低于前两类纯橄岩,但单斜辉石的Al2O3(1.41%～1.71%)则高于前两者。透镜状纯橄岩的矿物成分与方辉橄榄岩重叠,两者为渐变过渡关系。研究对比表明,罗布莎厚层状纯橄岩不同于经典的蛇绿岩的超镁铁质堆晶岩,认为将其成因解释为拉斑玄武质熔体与地幔橄榄岩的反应较为合理。透镜状纯橄岩与方辉橄榄岩存在成生联系,可能是地幔橄榄岩高度部分熔融的产物,或熔体和方辉橄榄岩在原位发生反应的产物;薄壳状纯橄岩成因与厚层状纯橄岩相同,但与其相伴的块状铬铁矿是否由拉斑玄武质熔体与方辉橄榄岩反应形成,值得商榷。 相似文献
10.
达拉布特蛇绿岩位于中亚造山带西南缘,是古亚洲洋的扩张、俯冲、消减和闭合过程的产物,保留了洋盆形成及构造演化信息。前人对达拉布特蛇绿岩的形成大地构造背景始终未取得统一的认识。为探讨蛇绿岩所代表的构造演化过程,笔者以达拉布特蛇绿岩中的地幔橄榄岩为研究对象,通过详细的矿物地球化学及其氧同位素研究,对达拉布特地幔橄榄岩成因及构造背景提出新的制约。达拉布特方辉橄榄岩中橄榄石Ni/Co值为21~22,Ni/Mn值为3.0~7.8具有部分熔融残余的特征,此外,橄榄石中不相容元素相对于正常地幔橄榄石亏损,表明方辉橄榄岩为部分熔融的残余组分。方辉橄榄岩中尖晶石Cr# 为47~52、TiO2含量0.01%~0.04%,橄榄石Fo为90.34%~90.98%指示方辉橄榄岩经历>20%的部分熔融。方辉橄榄岩中橄榄石δ18 Oolivine值+5.1~+6.2‰、单斜辉石δ18Ocpx值+5.6~+6.9‰,其变化范围较大且整体高于正常地幔中橄榄石和单斜辉石的δ18 O值,矿物间氧同位素分馏系数Δ18OOpx—olivine平均-0.3‰,Δ18OOpx—cpx平均-0.7‰,显著区别于正常地幔中平衡的氧同位素分馏系数,具有明显的交代作用特征。结合方辉橄榄岩橄榄石中亏损的微量元素特征,认为方辉橄榄岩可能受到流体交代作用的影响,俯冲壳源物质脱水形成的高δ18O流体交代地幔橄榄岩导致了矿物与矿物间不平衡的氧同位素特征。通过尖晶石与单斜辉石成分判别,尖晶石与单斜辉石的主量元素具有介于弧前与深海地幔之间的过渡型特征,与俯冲初始阶段形成的地幔橄榄岩相似。综合矿物地球化学与氧同位素特征,笔者认为达拉布特地幔橄榄岩为形成于弧前初始俯冲环境。板块俯冲导致弧前扩张形成新洋壳,上涌的软流圈MORB like熔体与俯冲壳源物质熔融形成熔/流体与地幔橄榄岩相互作用,形成达拉布特地幔橄榄岩。 相似文献
11.
《International Geology Review》2012,54(11):1395-1412
The Neyriz ophiolite along the northeast flank of the Zagros fold-thrust belt in southern Iran is an excellent example of a Late Cretaceous supra-subduction zone (SSZ)-related ophiolite on the north side of the Neotethys. The ophiolite comprises a mantle sequence including lherzolite, harzburgite, diabasic dikes, and cumulate to mylonitic gabbro lenses, and a crustal sequence comprising a sheeted dike complex and pillow lavas associated with pelagic limestone and radiolarite. Mantle harzburgites contain less CaO and Al2O3, are depleted in rare earth elements, and contain spinels that are more Cr-rich than lherzolites. Mineral compositions of peridotites are similar to those of both abyssal and SSZ- peridotites. Neyriz gabbroic rocks show boninitic (SSZ-related) affinities, while crustal rocks are similar to early arc tholeiites. Mineral compositions of gabbroic rocks resemble those of SSZ-related cumulates such as high forsterite olivine, anorthite-rich plagioclase, and high-Mg# clinopyroxene. Initial εNd(t) values range from +7.9 to +9.3 for the Neyriz magmatic rocks. Samples with radiogenic Nd overlap with least radiogenic mid-ocean ridge basalts and with Semail and other Late Cretaceous Tethyan ophiolitic rocks. Initial 87Sr/86Sr ranges from 0.7033 to 0.7044, suggesting modification due to seafloor alteration. Most Neyriz magmatic rocks are characterized by less radiogenic 207Pb/204Pb (near the northern hemisphere reference line), suggesting less involvement of sediments in their mantle source. Our results for Neyriz ophiolite and the similarity to other Iranian Zagros ophiolites support a subduction initiation setting for its generation. 相似文献
12.
Petrology and Geochemistry of the Dangqiong Ophiolite,Western Yarlung-Zangbo Suture Zone,Tibet,China
The Dangqiong ophiolite, the largest in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)ophiolite belt in southern Tibet, consists of discontinuous mantle peridotite and intrusive mafic rocks. The former is composed dominantly of harzburgite, with minor dunite, locally lherzolite and some dunite containing lenses and veins of chromitite. The latter, mafic dykes(gabbro and diabase dykes), occur mainly in the southern part. This study carried out geochemical analysis on both rocks. The results show that the mantle peridotite has Fo values in olivine from 89.92 to 91.63 and is characterized by low aluminum contents(1.5–4.66 wt%) and high Mg# values(91.06–94.53) of clinopyroxene. Most spinels in the Dangqiong peridotites have typical Mg# values ranging from 61.07 to 72.52, with corresponding Cr# values ranging from 17.67 to 31.66, and have TiO2 contents from 0 to 0.09%, indicating only a low degree of partial melting(10–15%). The olivine-spinel equilibrium and spinel chemistry of the Dangqiong peridotites suggest that they originated deeper mantle(20 kbar). The gabbro dykes show N-MORB-type patterns of REE and trace elements. The presence of amphibole in the Dangqiong gabbro suggests the late-stage alteration of subduction-derived fluids. All the lherzolites and harzburgites in Dangqiong have similar distribution patterns of REE and trace elements, the mineral chemistry in the harzburgites and lherzolites indicates compositions similar to those of abyssal and forearc peridotites, suggesting that the ophiolite in Dangqiong formed in a MOR environment and then was modified by late-stage melts and fluids in a suprasubduction zone(SSZ) setting. This formation process is consistent with that of the Luobusa ophiolite in the eastern Yarlung-Zangbo Suture Zone and Purang ophiolite in the western Yarlung-Zangbo Suture Zone. 相似文献
13.
内蒙古柯单山蛇绿岩地幔橄榄岩铂族元素(PGEs)的分布特征及分异机制探讨 总被引：1，自引：0，他引：1
采用Carius管结合MC-ICPMS法分析了内蒙古柯单山蛇绿岩地幔橄榄岩中Ir、Ru、 Pt 和Pd 的含量,与典型的地幔橄榄岩进行对比研究,发现柯单山地幔橄榄岩中Ir和Ru明显亏损,Pt和Pd强烈富集,具有极高的Pd/Ir值,PGEs地幔标准化配分模式具有较陡的正斜率,明显不同于通常观测到的代表部分熔融残留相中铂族元素配分模式(负斜率或平坦型)。柯单山地幔橄榄岩的Ir和Ru与MgO呈正相关关系,表明Ir和Ru的亏损可能与部分熔融过程中硫化物的消耗程度有关,而与PGEs在硫化物/硅酸盐间的能斯特分配系数没有直接关系; Pt、Pd的富集表明本区的地幔橄榄岩不仅仅是经历过部分熔融的残余,而与来自深海的橄榄岩和大陆岩石圈地幔(SCLM)中的方辉橄榄岩相似,因此推测,本区地幔橄榄岩在部分熔融后又经历了富Pd的熔/流体交代,而熔/流体的来源可能是在岩浆分异演化过程中"熔离"出来的硫化物。 相似文献
14.
ABUNDANCE AND DISTRIBUTION OF PLATINUM-GROUP ELEMENTS (PGE) IN PERIDOTITE FROM THE DAGZHUKA OPHIOLITE IN TIBET:IMPLICATIONS FOR MANTLE METASOMATISM 相似文献
15.
İbrahim Uysal A. Dündar Şen E. Yalçın Ersoy Yildirim Dilek Samet Saka Federica Zaccarini Monica Escayola Orhan Karslı 《Mineralogy and Petrology》2014,108(1):49-69
We present the whole-rock and the mineral chemical data for upper mantle peridotites from the Harmanc?k region in NW Turkey and discuss their petrogenetic–tectonic origin. These peridotites are part of a Tethyan ophiolite belt occurring along the ?zmir-Ankara-Ercincan suture zone in northern Turkey, and include depleted lherzolites and refractory harzburgites. The Al2O3 contents in orthopyroxene and clinopyroxene from the depleted lherzolite are high, and the Cr-number in the coexisting spinel is low falling within the abyssal field. However, the orthopyroxene and clinopyroxene in the harzburgites have lower Al2O3 contents for a given Cr-number of spinel, and plot within the lower end of the abyssal field. The whole-rock geochemical and the mineral chemistry data imply that the Harmanc?k peridotites formed by different degrees of partial melting (~%10–27) of the mantle. The depleted lherzolite samples have higher MREE and HREE abundances than the harzburgitic peridotites, showing convex-downward patterns. These peridotites represent up to ~16 % melting residue that formed during the initial seafloor spreading stage of the Northern Neotethys. On the other hand, the more refractory harzburgites represent residues after ~4–11 % hydrous partial melting of the previously depleted MOR mantle, which was metasomatized by slab-derived fluids during the early stages of subduction. The Harmanc?k peridotites, hence, represent the fragments of upper mantle rocks that formed during different stages of the tectonic evolution of the Tethyan oceanic lithosphere in Northern Neotethys. We infer that the multi-stage melting history of the Harmanc?k peridotites reflect the geochemically heterogeneous character of the Tethyan oceanic lithosphere currently exposed along the ?zmir-Ankara-Erzincan suture zone. 相似文献
16.
The ophiolites of Othris, northern Greece, and Troodos, Cyprus, are of mixed lherzolite-harzburgite and harzburgite sub-type respectively. Within both ophiolites an entire spectrum of harzburgite, plagioclase harzburgite, lherzolite and plagioclase lherzolite interpreted respectively as residual and highly, moderately and slightly modified upper mantle has been recognised. Plagioclase lherzolite frequently contains gabbroic segregations generated by partial melting and incomplete extraction. Othris shows the full range of mantle peridotites while Troodos is dominated by harzburgite with very minor occurrences of plagioclase harzburgite and lherzolite. Chemically, the plagioclase lherzolites have low contents of the basaltophillic minor elements, and a composition slightly more residual than postulated upper mantle compositions, suggestive of a preceding phase of minor depletion. The Othris and Troodos ophiolites seem to have formed under fundamentally different environments — Othris as a marginal ophiolite at the inception on rifting of continental crust, and Troodos later in such an event when spreading was well established. 相似文献
17.
The Zedang and Luobusa ophiolites are located in the eastern section of the Yalung Zangbo ophiolite belt,and they share similar geological tectonic setting and age.Thus,an understanding of their origins is very important for discussion of the evolution of the Eastern Tethys Ocean.There is no complete ophiolite assemblage in the Zedang ophiolite.The Zedang ophiolite is mainly composed of mantle peridotite and a suite of volcanic rocks as well as siliceous rocks,with some blocks of olivinepyroxenite.The mantle peridotite mainly consists of Cpx-harzburgite,harzburgite,some lherzolite,and some dunite.A suite of volcanic rocks is mainly composed of caic-aikaline pyroclastic rocks and secondly of tholeiitic pillow lavas,basaltic andesites,and some boninitic rocks with a lower TiO2 content （TiO2 ＜ 0.6％）.The pyroclastic rocks have a LREE-enriched REE pattern and a LILE-enriched （compared to HFSE） spider diagram,demonstrating an island-arc origin.The tholeiitic volcanic rock has a LREE-depleted REE pattern and a LILE-depleted （compared to HFSE） spider diagram,indicative of an origin from MORB.The boninitic rock was generated from fore-arc extension.The Luobusa ophiolite consists of mantle peridotite and mafic-ultramaflc cumulate units,without dike swarms and volcanic rocks.The mantle peridotite mainly consists of dunite,harzburgite with low-Opx （Opx ＜ 25％）,and harzburgite （Opx ＞ 25％）,which can be divided into two facies belts.The upper is a dunite-harzburgite （Opx ＜ 25％） belt,containing many dunite lenses and a large-scale chromite deposit with high Cr203; the lower is a harzburgite （Opx ＞25％） belt with small amounts of dunite and lherzolite.The Luobusa mantle peridotite exhibits a distinctive vertical zonation of partial melting with high melting in the upper unit and low melting in the lower.Many mantle peridotites are highly depleted,with a characteristic U-shaped REE pattern peculiar to fore-arc peridotite.The Luobusa cumulates are composed of wehrlite and olivine-pyroxenite,of the P-P-G ophiolite series.This study indicates that the Luobusa ophiolite was formed in a fore-arc basin environment on the basis of the occurrence of highly depleted mantle peridotite,a high-Cr2O3 chromite deposit,and cumulates of the P-P-G ophiolite series.We conclude that the evolution of the Eastern Tethys Ocean involved three stages：the initial ocean stage （formation of MORB volcanic rock and dikes）,the forearc extension stage （formation of high-Cr203 chromite deposits and P-P-G cumulates）,and the islandarc stage （formation of caic-alkaline pyroclastic rocks）. 相似文献
18.
Behavior of Siderophile and Chalcophile Elements in the Subcontinental Lithospheric Mantle beneath the Changbaishan Volcano,NE China 总被引：1，自引：0，他引：1
The mantle xenoliths in the Quaternary ChangbaishanVolcano in southern Jilin Province contain spinel-facies lherzolites. The equilibration temperatures for these samples range from 902oC to 1064oC based on the two-pyroxene thermometer of Brey and K?hler (1990), and using the oxybarometry of Nell and Wood (1991), the oxidation state was estimated from FMQ-1.32 to -0.38 with an average value of FMQ-0.81 (n?=?8), which is comparable to that of abyssal peridotites and the asthenospheric mantle. The fO2 values of peridotites, together with their bulk rock compositions (e.g., Mg#, Al2O3, CaO, Ni, Co, Cr) and mineral compositions (e.g., Mg# of olivine and pyroxene, Cr# [=Cr/[Cr+Al]] and Mg# [=Mg/[Mg+Fe2+] of spinel), suggest that the present-day subcontinental lithospheric mantle (SCLM) beneath the Changbaishan Volcano most likely formed from an upwelling asthenosphere at some time after the late Mesozoic and has undergone a low degree of partial melting. The studied lherzolite xenoliths show low concentrations of S, Cu, and platinum group elements (PGE), which plot a flat pattern on primitive-mantle normalized diagram. Very low concentrations in our samples suggest that PGEs occur as alloys or hosted by silicate and oxide minerals. The compositions of the studied samples are similar to those of peridotite xenoliths in the Longgang volcanic field (LVF) in their mineralogy and bulk rock compositions including the abundance of chalcophile and siderophile elements. However, they are distinctly different from those of peridotite xenoliths in other areas of the North China Craton (NCC) in terms of Cu, S and PGE. Our data suggest that the SCLM underlying the northeastern part of the NCC may represent a distinct unit of the newly formed lithospheric mantle. 相似文献
19.
攀西地区大板山岩体的年代学、元素地球化学及其对铜镍硫化物矿床成因的约束 总被引：2，自引：0，他引：2
大板山镁铁-超镁铁质杂岩体位于攀西地区西北部,受金河-菁河断裂带控制,主要岩石类型有辉长岩、苏长辉长岩和二辉橄榄岩。本次研究集中于苏长辉长岩和二辉橄榄岩,其中,二辉橄榄岩具有明显的堆晶结构,橄榄石Fo值最高可达81.4。二辉橄榄岩SiO2含量为42.93%~44.18%,苏长辉长岩SiO2含量为44.89%~52.76%,稀土总量(∑REE)相对较低(23.2×10-6~61.7×10-6),并均呈现出近平行的、相对平缓的右倾稀土元素配分模式,说明其为同源岩浆演化的关系。此外,大部分样品相对富集大离子亲石元素(LILE;如Rb、Sr、Ba等),适度亏损高场强元素(HFSE;如Nb、Ta、Ti等)。结合其较低的SiO2含量和微量元素比值(如Th/Ta和La/Nb等),表明岩浆未遭受大量的地壳混染。岩相学和岩石地球化学特征均显示了非常明显的分离结晶作用。根据(Tb/Yb)PM-(Yb/Sm)PM图解,大板山岩体的原始岩浆主要来源于岩石圈地幔轻度富集不相容元素的尖晶石相二辉橄榄岩。二辉橄榄岩样品中橄榄石Fo-NiO图解中多数点落在硫化物未熔离的范围,且样品全岩Cu/Zr比值为7.04~103,而苏长辉长岩中Cu/Zr比值为0.88~5.56,反映了二辉橄榄岩-苏长辉长岩可能经历了由S不饱和到过饱和的过程。推测硫化物的熔离可能是岩浆在上地壳岩浆房中发生了以橄榄石和辉石为主的分离结晶作用,造成硫化物达到过饱和。锆石LA-ICP-MS测年表明,岩体年龄为259.69±0.61Ma,与峨眉山大火成岩省其它镁铁-超镁铁质岩体的形成时间基本一致。 相似文献