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洋壳俯冲过程中的地幔楔上升对流：来自芝麻坊石榴子石二辉橄榄岩早期变质的证据 总被引：1，自引：1，他引：0
苏鲁超高压变质地体中发现了大量包裹在超高压（UHP）变质片麻岩和混合岩中的造山带石榴橄榄岩。根据它们的野外产出特征和全岩地球化学成分,其中一部分石榴橄榄岩的原岩来自于亏损地幔,后来被卷入俯冲陆壳并经受过俯冲陆壳产生的熔/流体的交代。但是,对这些岩石早期的亏损过程尚缺乏清晰的认识。本文报道了东海芝麻坊石榴子石二辉橄榄岩早期变质演化的新证据。根据详细的变质反应结构观察和矿物成分研究,芝麻坊石榴子石二辉橄榄岩在经历高压低温俯冲带型超高压变质之前经历了至少两期变质演化。其原岩矿物组合由石榴子石变斑晶的高Ca-Cr核部及其中包裹的高Mg单斜辉石、高Al-Cr斜方辉石和高Mg-Ni橄榄石所记录;指示芝麻坊石榴子石二辉橄榄岩的原岩为高温-高压的富集石榴子石二辉橄榄岩。第二期矿物组合为包裹在低Cr变斑晶石榴子石幔部和细粒新生石榴子石核部的大量富Al铬铁矿和高Mg低Ni橄榄石以及少量高Mg斜方辉石。该期组合未发现单斜辉石,表明岩石随后被转变为高温低压的难熔尖晶石方辉橄榄岩或尖晶石纯橄岩。芝麻坊石榴子石二辉橄榄岩的早期变质演化记录了它们被卷入大陆板片俯冲带之前的地幔楔上升对流过程。笔者认为芝麻坊石榴子石二辉橄榄岩的原岩来源于早期俯冲大洋板片之上的深部高温富集地幔楔,洋壳俯冲过程中的地幔楔对流导致其上升到弧后或岛弧之下的地幔楔浅部,减压部分熔融使原本富集的石榴子石二辉橄榄岩转化为难熔的尖晶石方辉橄榄岩或尖晶石纯橄岩。 相似文献
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西准噶尔蛇绿混杂岩中的白云石大理岩和石榴角闪岩：早古生代残余洋壳深俯冲的证据 总被引：17，自引：11，他引：6
本文在描述达拉布特和克拉玛依蛇绿岩中四个典型剖面(达拉布特、萨尔托海、白碱滩和百口泉)的基础上,通过研究变白云石大理岩和石榴角闪岩的形成和演化过程,展示西准噶尔蛇绿混杂岩的复杂性。白碱滩白云石大理岩是尖晶石二辉橄榄岩变质反应的产物。变质反应导致单斜辉石转变为白云石+石英+磁铁矿,橄榄石和斜方辉石先转变为蛇纹石+磁铁矿,蛇纹石再转变为白云石+石英+磁铁矿。透辉石+CO2=白云石+石英+磁铁矿以及蛇纹石+CaCO3=白云石+石英 +磁铁矿+H2O这两个变质反应基本同时发生,他们以及石英—柯石英相变关系所限定的P-T空间(660~670℃,26.5~27.5 kbar)代表白云石大理岩的形成温度和压力。白云石大理岩与尖晶石二辉橄榄岩在一个不大的区域共存,说明循环到地球深部的尖晶石二辉橄榄岩(即：白云石大理岩的原岩)折返到地壳浅部后又与蛇绿岩中未俯冲的尖晶石二辉橄榄岩混杂在一起。因此,克拉玛依蛇绿混杂岩代表古俯冲带的位置。变辉长岩和尖晶石二辉橄榄岩中的单斜辉石均发育出溶结构(出溶尖晶石和斜方辉石)。这些出溶结构是蛇绿岩快速上升就位时由于温度和压力突然降低辉石发生分解反应的产物。因此,西准噶尔蛇绿混杂岩中的岩石组合记录着蛇绿岩形成时期、以及之后部分岩块通过俯冲带循环到地幔深处(最大深度~70km)并折返到地壳浅部的全过程。西准噶尔蛇绿混杂岩带是一个多时期、多阶段和多成因岩石的混杂体。 相似文献
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加拿大Slave克拉通Jericho金伯利岩筒携带的橄榄岩包体提供了研究大陆岩石圈地幔物质组成和热结构的窗口。文章总结了地幔岩矿物温压计的研究进展,测量了Jericho金伯利岩携带的9个新鲜橄榄岩包体的矿物主量元素和微量元素,并使用不同的矿物温压计估算了平衡温度和压力。结果表明Nickel 和 Green（1985）的石榴子石—斜方辉石压力计可以较好地估算含石榴子石橄榄岩形成时的压力,Taylor（1998）二辉石温度计和Nimis 和 Taylor（2000）单斜辉石温度计的计算结果一致。具有粗粒变晶结构的尖晶石—石榴子石橄榄岩和石榴子石橄榄岩样品的平衡温度为575~843℃,压力为2.4~3.6 GPa,表明Slave克拉通岩石圈地幔温度较低。而残斑结构尖晶石—石榴子石二辉橄榄岩的平衡温度1109℃,压力为5.0 GPa,来源深度为~156 km,可能被早期金伯利岩浆携带到岩石圈地幔中部冷却,然后再被侏罗纪喷发的Jericho金伯利岩筒带到地表。使用石榴子石—单斜辉石稀土元素温压计获得的平衡温度高于主量元素温度计的结果,表明Slave克拉通岩石圈地幔经历了逐渐冷却的过程。此外,Slave克拉通浅部的尖晶石橄榄岩保留了强烈亏损的早期岩石圈地幔特征,而下部的岩石圈地幔经历了金伯利岩熔体和硅酸盐熔体的交代作用。 相似文献
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西准噶尔白碱滩二辉橄榄岩中两种辉石的出溶结构及其地质意义 总被引：3，自引：11，他引：3
克拉玛依白碱滩尖晶石二辉橄榄岩主要由橄榄石、单斜辉石、斜方辉石和尖晶石组成,橄榄石和斜方辉石均发生程度不等的蛇纹石化。单斜辉石一般很新鲜。单斜辉石和斜方辉石均发育出溶结构,出溶条纹或者平直或者发生舒缓的弯曲变形(即便是在发生弯曲的情况下也是完全平行的)。透辉石-普通辉石出溶体一般呈针状(直径一般为1μm,长度>150μm),顽火辉石出溶条纹直径一般为1～3μm(长度>300μm)。斜方辉石主晶属于顽火辉石-易变辉石,单斜辉石主晶为透辉石(成分很均一)。地质温度压力估算表明,白碱滩二辉橄榄岩中辉石出溶结构发生的温度为700℃～1000℃、压力为2.0～2.7GPa,它们代表辉石出溶结构形成的最低PT条件。白碱滩二辉橄榄岩至少经历了三个演化阶段:原始辉石与尖晶石和橄榄石平衡共生(阶段Ⅰ,>94km);随着地幔上隆,原始辉石结构不稳定,分解并形成出溶结构(阶段Ⅱ,700℃～1000℃),斜方辉石开始分解的深度为94km,单斜辉石开始分解的深度为78km;之后,蛇绿岩经历的侵位事件导致辉石发生塑性变形(阶段Ⅲ)。蛇绿岩侵位之前,地幔岩曾发生了>50km的隆升,而且,在隆升过程中地幔岩没有发生明显部分熔融(地幔岩因此没有经历明显的岩浆抽提过程)。 相似文献
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两类蛇绿岩中变质橄榄岩的矿物表现出的共同特征是:从二辉橄榄岩→斜辉橄榄岩→斜辉辉橄岩→纯橄岩→铬铁矿石,其橄榄石、斜方辉石和单斜辉石的Mg/(Mg+Fe)(矿物牌号)依次升高,表现出富Mg贫Fe的演化趋势;铬尖晶石的Cr/(Cr+Al)亦同时升高,表现为富Cr贫Al的演化特征。 变质橄榄岩中的矿物均是原始上地幔岩部分熔融的残余物。在部分熔融过程中,橄榄石与铬尖晶石是生成相矿物,而斜方与单斜辉石则是消失相矿物,正是通过两种辉石的不断消失,岩石才从二辉橄榄岩依次转化为纯橄岩,并造成纯橄岩与铬铁矿的紧密伴生。在此过程中,矿物成分时刻都在变化,造岩矿物向富Mg贫Fe,金属矿物向富Cr贫Al方向调整,这与实际测定结果是一致的。 相似文献
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中国东部某些地区碱性玄武岩中包体的温度、压力的计算 总被引：13，自引：0，他引：13
一、概述中国东部新生代碱性玄武岩系列的火山岩中广泛分布着超镁铁岩包体。这些包体是我们直接能观察到的该地区上地幔的标本。它对于了解该地区上地幔物质组成和地温分布具有重要的意义。现仅就收集到的包体资料,侧重对其温度压力条件进行估计,尝试着对这些地区上地幔的组成和地温分布进行一些讨论。超镁铁岩包体主要是尖晶石二辉橄榄岩。在浙江和福建等地还分布有石榴石二辉橄榄岩。此外,还有少量的纯橄榄岩、斜方辉石岩、单斜辉石岩、二辉石岩,含角闪石的尖晶石二辉橄榄岩 相似文献
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昌乐方山新生代玄武岩中携带有大量深源捕虏岩,其岩石类型以二辉辉石岩、二辉橄榄岩为主,方辉橄榄岩、单斜辉石岩、单辉橄榄岩少见,未见纯橄岩。二辉辉石岩、二辉橄榄岩、方辉橄榄岩的矿物岩石、地球化学特征和平衡温度压力计算显示为地幔来源,采用地质温度压力计进行计算,获取三者都源于岩石圈地幔,形成的深度范围为41~66.7 km。根据单辉橄榄岩和单斜辉石岩的矿物化学分析,单斜辉石岩和单辉橄榄岩形成深度要低于前三者,且与前三者有密切成因联系。"熔体—岩石"反应在各类深源捕虏岩中广泛存在,除方辉橄榄岩样品不存在明显的"熔体—岩石"反应外,二辉辉石岩、单斜辉石岩、二辉橄榄岩中不同的矿物边缘与玄武质熔体接触部位有着不同类型的反应边和反应矿物出现:橄榄石边部发生(Mg,Fe)_2SiO_4(镁橄榄石)+熔体(原始岩浆)=(Mg,Fe)_2SiO_4(贵橄榄石)+熔体(演化岩浆)反应,使得边部富铁贫镁;单斜辉石与玄武质熔体反应,多具有粉红色反应边,具有富Ti、Al、Fe,贫Si、Mg,较玄武岩基质中单斜辉石斑晶更低Mg~#的特征。斜方辉石与玄武质熔体反应生成复杂的反应带,存在斜方辉石+熔体=橄榄石+SiO_2(熔体)+单斜辉石的反应,反应带内新生成的矿物由内向外具有明显趋向玄武质熔体成分的变化;尖晶石与玄武质熔体反应边缘具有富Fe、Ti,贫Mg、Al,以及更高的Cr~#,趋向于形成钛铁矿的变化特征。二辉橄榄岩中熔体囊和单辉橄榄岩中筛状单斜辉石以及粉红色的反应边都暗示了二者至少经历了两个期次的熔体作用。随着岩石圈地幔的演化,熔体的再富集作用使得难熔的方辉橄榄岩逐渐转变为相对富集的二辉橄榄岩,后期二辉橄榄岩又与玄武质熔体反应,生成更加富集的单辉橄榄岩,而方山方辉橄榄岩也受到了熔体的轻度改造,已不具有难熔古老岩石圈地幔岩石的显著特征,鲁西新生代玄武岩中深源捕虏体与熔体的反应广泛存在,熔体成分以玄武质为主,这可能是华北克拉通岩石圈置换的重要方式之一,导致古老岩石圈地幔的消失。 相似文献
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胶东—苏北石榴石二辉橄榄岩形成条件及年代 总被引：7，自引：0，他引：7
在胶东—苏北地区,由石榴石二辉橄榄岩、尖晶石二辉橄榄岩、石榴石辉石岩、榴辉岩、斜辉辉橄岩和纯橄岩组成的杂岩成群和成带分布,延长590km。该岩带越过郯庐断裂带延伸至大别山,全长约1200km。岩带内石榴石二辉橄榄岩与尖晶石橄榄岩伴生,组成较大的岩块,并且与榴辉岩伴生,曾被作者确定为一种有层序的岩套。石榴石二辉橄榄岩的Sm-Nd同位素年龄为925±53Ma,矿物地质温度计和压力计表明其形成于115—160km深部,温度为800—1371℃。该石榴石二辉橄榄岩平衡温度和压力符合大陆地温线,而不同于中国东部沿海地区玄武岩中包体石榴石二辉橄榄岩,后者的平衡温度压力符合于大洋地温线。 相似文献
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内蒙古阿巴嘎旗新生代玄武岩中超镁铁岩包体的特征 总被引：2，自引：4，他引：2
内蒙古东部阿巴嘎旗及其以北地区第四纪更新世钠质碱性火山岩中发现的超镁铁岩包体,主要为尖晶石二辉橄榄岩、方辉橄榄岩、纯橄榄岩、橄榄二辉岩、尖晶石单斜辉石岩和单斜辉石巨晶以及辉长岩、花岗闪长岩、石英长石片麻岩等壳源包体。超镁铁岩包体和巨晶的岩相学特征显示它们不是寄主玄武岩岩浆分离结晶堆积体;其主要元素、稀土元素特征与阿尔卑斯型橄榄岩相似,其组成接近于原始地幔岩值;同位素地球化学特征表明它们与寄主岩-玄武岩浆不是同源的;尖晶石二辉橄榄岩包体形成的平衡温度和压力范围分别为1056.9℃～1103.1℃,压力为1.50～1.84GPa,形成相当深度为49.5～60.72km。辉石岩包体形成温度为887.1℃～958.6℃,压力为1.25～1.4GPa,形成相当深度为41.25～46.2km。寄主玄武岩形成温度1390℃～1450℃,压力1.75～2.20GPa,形成相当深度为60～70km。可以认为,研究区在新生代发生了软流圈地幔上涌,导致岩石圈减薄以及地壳的拉张,大量岩浆物质沿构造通道溢出地表。其中超镁铁岩包体与寄主玄武岩浆之间没有熔体与残余物这种成因联系,它们是寄主玄武岩浆在上升过程中捕获的岩石圈地幔碎块。 相似文献
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Porphyroclastic enstatite in a garnet lherzolite xenolith from the Monastery Mine kimberlite, South Africa, has exsolved pyrope garnet, Cr-diopside and Al-chromite, and the specimen is interpreted as representing a transition from fertile harzburgite, (containing high Ca-Al-Cr enstatite) to granular garnet lherzolite. Although the exsolved phases occur in morphologically different forms (fine and coarse lamellae; equant, ripened grains), indicating textural disequilibrium, the exsolved grains are very constant in composition, indicating chemical equilibrium. Theoretically, the exsolution could have been due to a fall in temperature, but the close association of exsolution and deformation of the host enstatite suggests that exsolution was also aided by straining of the enstatite lattice. The phase compositions can be broadly matched with those in other mantle peridotites, except that all phases are characterised by a virtual absence of Ti. In the garnet and diopside Ti, Co, Zr and most of the REE are lower than in published analyses of garnet and diopside in both granular and sheared garnet lherzolites from Southern African kimberlites, and diopside/garnet partitioning for Sr and the REE is higher. Comparison with the trace element chemistry of an enstatite from a fertile harzburgite indicates that, except for Nb, the trace element content and distribution found in the Monastery phases could arise by isochemical exsolution from such an enstatite. On the assumption that (a) the Monastery specimen represents a transition from harzburgite to garnet lherzolite, and (b) many garnet lherzolites are of exsolution origin (as suggested by their modal compositions), the inference is that most garnet lherzolites, and not just the sheared variety, have been subject to varying degrees of Ti, Zr, Sr and REE metasomatism. 相似文献
12.
安徽女山地幔橄榄岩捕虏体的同位素组成:中国东部新生代岩石圈地幔时代制约 总被引：3，自引：2，他引：1
位于安徽省境内的女山新生代碱性玄武岩中含有大量而且类型丰富的地幔橄榄岩包体,主要类型有尖晶石相、石榴石相、尖晶石-石榴子石过渡相二辉橄榄岩以及少量的方辉橄榄岩,其中部分尖晶石二辉橄榄岩样品中出现富含挥发分的角闪石、金云母和磷灰石。本文选择该区的尖晶石二辉橄榄岩和方辉橄榄岩包体进行了较为详细的岩石学、矿物学、地球化学研究工作。结果显示,除2个方辉橄榄岩表现难熔特征外,其它25件尖晶石相二辉橄榄岩均具有饱满的主量元素组成。二辉橄榄岩样品的Sr-Nd-Hf同位素均表现为亏损地幔的性质,不同于古老克拉通型难熔、富集的岩石圈地幔。富含挥发份交代矿物的出现以及轻稀土元素不同程度的富集,表明女山岩石圈地幔经历了较为强烈的交代作用,然而Re-Os同位素及PGE分析结果表明交代作用并没有显著改变Os同位素组成。二辉橄榄岩样品均具有较高的Os同位素组成,结合其饱满的主量元素组成,亏损的同位素特征,表明女山地区岩石圈地幔整体为新生岩石圈地幔。但1个方辉橄榄岩样品给出了较低的Os同位素比值0.1184,其Re亏损年龄为1.5Ga,它可能来自于软流圈中残留的古老难熔地幔。 相似文献
13.
14.
Martin Menzies 《Contributions to Mineralogy and Petrology》1975,51(4):303-309
Spinels from cumulus and non-cumulus members of the Othris ophiolite display a considerable variation in composition. Cumulus picrites and gabbros contain either a primary chromite and/or a reaction spinel formed by reaction with co-existing silicates (Cr-Al varia tion) or intercumulus liquid (Cr-Fe variation). Non-cumulus peridotites contain spinels which vary along a Cr-Al trend. Harzburgites contain a Cr-spinel and lherzolites a more aluminous spinel. The occurence of gabbroic segregations within the host lherzolite appears to affect the spinel chemistry. Spinels adjacent to these plagioclase—diopside veinlets are richer in aluminium than the spinels scattered within the depleted lherzolite surrounding the veinlet. [Protoclastic harzburgites contain a highly aluminous spinel phase either as an exsolution phase within pyroxenes or as a groundmass spinel.] The Cr-Al variation of the peridotites is believed to have resulted from interaction with interstitial aluminous liquid—in situ basaltic melt from a fused peridotite? 相似文献
15.
A. A. Krasnobaev A. I. Rusin I. A. Rusin S. V. Busharina 《Geochemistry International》2011,49(5):482-497
The paper reports the results of mineralogical and isotope-geochronological study of zircons from the Uzyansky Kraka (UK) Massif, which represents the part of the large (more than 900 km2) lherzolite allochthon thrusted onto the Paleozoic sequences of the East European margin. The massif shows a distinct stratification (from the top downward): spinel lherzolites, garnet pyroxenites, and dunites. The formation of stratified section is considered to be related to the decompression uplift of mantle lherzolite block. Zircons from the massif rocks were dated using SHRIMP-II ion microprobe. The oldest relict datings characterizing endogenous transformations of protolith were established in the zircons from the lherzolites (2037 ± 20 and 1132 ± 6 Ma), garnet pyroxenites (953 ± 11 Ma), and dunites (632 ± 11 Ma). All rock associations contain zircons with ages within 590–550 and 445–390 Ma, which mark the stages of mantle stratification of lherzolite block into complementary series and their emplacement at the upper crustal level. Age values within 299–196 Ma were found only in the dunites and date the influence of the Paleozoic strike-slipping. Our studies led us to conclude that the modern structure of the Ural collision orogen contains the fragments of subcontinental lithosphere, which were previously described only for the massifs of the root zones of the Western and Central Europe. Some general petrogenetic questions of lherzolite massifs from orogenic regions are discussed. 相似文献
16.
D.A. Carswell 《Lithos》1980,13(2):121-138
Occurrences, petrography and major element chemistry of lherzolite nodules are reviewed. Spinel-garnet stability relationships in these nodules are discussed and are shown to be controlled by chemical as well as physical (P/T) considerations. On the basis of a survey of spinel compositions it is proposed that three classes of spinel bearing lherzolite nodules should be recognised: namely Al-spinel lherzolites [spinel 100 Cr/(Cr + Al)<25], Cr-spinel lherzolites [spinel 100 Cr/(Cr + Al) 25–65] and chromite lherzolites [spinel 100 Cr/(Cr + Al)>65].All lherzolite nodules yield sub-solidus P/T equilibration estimates and are interpreted as fragments of upper mantle wall rocks incorporated during the volatile charged eruptions. Depths of derivation increase from < 60 kms for most nodules in alkali basaltic magmas to > 150 kms for some nodules in kimberlites. The fact that Al-spinel lherzolites are the most common nodule type in magmas of the alkali basalt suite whilst garnet lherzolites are dominant in kimberlites is attributed to the combined effects of a typically steeper geotherm in the mantle beneath areas of alkali basaltic volcanism and a shallower depth of origin for this type of magmatism. Al-spinel lherzolites do, however, occur in the kimberlites of the western U.S.A. and south-west Greenland in contrast to their apparent absence in the kimberlites of southern Africa and Yakutia, U.S.S.R. This suggests that the uppermost mantle beneath these latter regions (as represented by nodules of chromite lherzolites and chromite or Cr-spinel harzburgites) has a more refractory residual type composition with a higher Cr/(Cr + Al) ratio—although the evidence indicates an overall decrease in the level of depletion in ‘basaltic’ magma yielding constituents with depth. Lherzolite nodules generally have chemical compositions which are depleted in such constituents relative to the pyrolite model compositions for primitive or pristine mantle; nevertheless their composition range is thought to encompass both highly ‘depleted’ and essentially ‘undepleted’ upper mantle compositions.The fact that consistent temperature estimates can be obtained from the various calibrated element exchange reaction geothermometers for some lherzolite nodules but not for others (notably those with strikingly porphyroclastic textures) may indicate that some lherzolite nodules comprise mantle derived rocks which failed to totally re-equilibrate following the diapiric movements which immediately preceded their incorporation and rapid transportation to the surface. 相似文献
17.
A. A. Krasnobaev A. I. Rusin V. N. Anfilogov P. M. Valizer S. V. Busharina E. V. Medvedeva 《Doklady Earth Sciences》2017,474(2):662-667
The age trend (SHRIMP U/Pb) of the evolution of zircon is obtained for the first time in lherzolites of the Nurali Massif. Zircons are subdivided into groups by the crystallomorphological and geochemical features. These specific features in zircon development are confirmed by the age dates. Precambrian dates (no younger than 1190 Ma) correspond to mantle sources of the lherzolite block. The Early Silurian (445–448 and 439–440 Ma) wass the time of lherzolite magmatism of 10–15 Ma in duration. The Middle Devonian (382.9 ± 8.7 Ma) corresponded to postmagmatic processes related to the effect of gabbro–diorite intrusions crowning in the Nurali Massif. 相似文献
18.
西藏班公湖—怒江缝合带中段蓬湖蛇绿岩中的洋脊型二辉橄榄岩 总被引：1，自引：0，他引：1
蓬湖蛇绿岩产于西藏藏北湖区的蓬湖西侧,属班公湖-怒江缝合带中段白拉拉弄-依拉山亚带。该蛇绿岩主要由地幔橄榄岩、堆晶岩和辉绿岩等组成。其中地幔橄榄岩由方辉橄榄岩和二辉橄榄岩组成。蓬湖二辉橄榄岩的橄榄石Fo值介于88.85～90.33之间、斜方辉石的Al2O3含量范围在4.26%～6.60%。与原始地幔相比,蓬湖二辉橄榄岩岩石有较高的MgO含量和较低的Al2O3、CaO和TiO2等易熔组分含量;稀土元素总量介于1.11×10-6～1.53×10-6之间,明显低于原始地幔值,配分模式为轻稀土轻微亏损。在原始地幔微量元素蛛网图中,蓬湖二辉橄榄岩显示Rb、Zr亏损,U、Ta、Sr强烈富集特征。蓬湖二辉橄榄岩的铂族元素总量介于22.9×10-9～27×10-9之间,PGEs球粒陨石标准化图解显示其为接近原始地幔的"平坦型"。以上特征与深海橄榄岩相似,指示它们可能形成于大洋中脊环境。定量模拟估算表明,蓬湖二辉橄榄岩可能来源于地幔中尖晶石相二辉橄榄岩源区,系经历了约5%～10%的部分熔融残余。蓬湖堆晶岩矿物结晶顺序为橄榄石-单斜辉石-斜长石,其中异剥橄榄岩中的单斜辉石Mg#值介于86.92～89.93之间、橄榄石Fo平均值为84.45,明显不同于MOR型蛇绿岩堆晶岩。蓬湖堆晶岩的矿物组成、岩浆结晶顺序和矿物成分均与俯冲带上SSZ型蛇绿岩形成的堆晶岩类似。以上结果表明,蓬湖二辉橄榄岩形成于大洋脊环境,为尖晶石二辉橄榄岩源区经历了不超过10%部分熔融的残余,后期由于洋内俯冲作用经历了岩石-熔体反应,形成了SSZ型堆晶岩和含较高Cr#值尖晶石的方辉橄榄岩。 相似文献
19.
Harry Becker 《Contributions to Mineralogy and Petrology》1997,128(2-3):272-286
High-temperature peridotite massifs occur as lensoid bodies with high-pressure granulites in the southern Bohemian massif.
In lower Austria the peridotites comprise garnet lherzolites lacking primary spinel, rare garnet and garnet-spinel harzburgites,
and harzburgites containing Cr-rich primary spinel instead of garnet. These phase assemblages suggest initial high-pressure
equilibration and are consistent with results from garnet-orthopyroxene geobarometry indicating equilibration at around 3–3.5 GPa.
Maximum temperature estimates obtained on core compositions of coexisting minerals from the peridotites are not higher than
ca. 1100 °C. In contrast, pyroxene megacryst compositions, garnet exsolution textures in the garnet pyroxenites, and results
from geothermometry indicate much higher original equilibration temperatures in most of the pyroxenites (up to 1400 °C). High
temperatures, modal zoning, the occasional presence of Mg-rich garnetites and chemical evidence suggest that the pyroxenites
are cumulates which crystallized from low-degree melts derived from the sub-lithospheric mantle. Isothermal interpolation
of the high temperatures to an upper mantle adiabat suggests that the melts were derived from a minimum depth of 180–200 km.
The formation of small garnet II grains and garnet exsolution lamellae in the pyroxenites and pyroxene megacrysts may reflect
isobaric cooling of the cumulates from temperatures above 1400 °C to ca. 1100–1200 °C (at 3–3.5 GPa) to approach the ambient
lithospheric isotherm. This model differs from other models in which the formation of garnet II was explained by an increase
in pressure during cooling in a subduction zone. Isobaric cooling was followed by near-isothermal decompression from 3–3.5 GPa
to 1.5–2 GPa at 1000–1200 °C, as indicated by the increase of Al in pyroxenes near garnet. Further cooling in the spinel lherzolite
stability field is indicated by spinel exsolution lamellae in pyroxenes from lherzolites. The formation of symplectites and
kelyphites indicate sub-millimetre scale re-equilibration during exhumation in the course of the Carboniferous collision in
the Bohemian massif. The peridotite massifs represent fragments of normal (non-cratonic) lithospheric mantle from a Paleozoic
convergent plate margin.
Received: 22 July 1996 / Accepted 28 February 1997 相似文献
20.
The abundance and distribution of Au, Pd, Ir, Cu, Co and Cr has been determined in mantle-derived spinel lherzolite xenoliths in basanites from Mt Porndon (Victoria, Australia) and Kilbourne Hole (New Mexico, U.S.A.) and in garnet lherzolites from the Matsoku and Thaba Putsoa kimberlites (Lesotho). Minerals in the lherzolites concentrate Au, Pd and Ir in the following sequence of increasing platinum group element (PGE) content; garnet, olivine, orthopyroxene, clinopyroxene, spinel. and demonstrate that there exists a real crystallochemical control on the distribution of PGE. Whole rock PGE abundances calculated from the modal mineralogy are less than actually determined and indicate that the bulk of the PGE (60–80%) occur in a sulphide-rich intergranular component. A metasomatic origin for this component is considered to be unlikely and it is proposed that it represents an immiscible sulphide melt which has been retained in the mantle after extraction of a sulphur saturated basic partial melt. This component may in the case of garnet lherzolites have been modified by metasomatic events in the mantle leading to Au depletion and rare earth element addition. Spinel lherzolites are relatively homogeneous at a given locality but differ in their PGE content regionally. The weighted average abundances of PGE in a spinel lherzolite upper mantle are 0.6 ppb. Au, 4.0 ppb Pd. 3.6 ppb Ir. Garnet lherzolites are very heterogeneous and insufficient data is available to allow calculation of geochemically meaningful averages. Spinel lherzolite-basalt based pyrolite contains 0.9 ppb Au, 4.3 ppb Pd, 3.0 ppb Ir, and indicates that the mantle contains an apparent excess of Au over a calculated abundance based upon the siderophilic equilibrium distribution of Au between core and mantle. This excess is considered to be due to failure to consider the chalcophilic nature of Au in the mantle and not to the addition of a meteoritic component to a mantle equilibrated with the core. 相似文献