海南岛陆缘扩张带形成及新生代岩石圈动力学机制:来自幔源包体的地球化学证据

海南岛陆缘扩张带蓬莱地区新生代玄武岩中捕获大量尖晶石二辉橄榄岩和方辉橄榄岩幔源包体。激光剥蚀等离子体质谱(LA-ICP-MS)分析结果表明,蓬莱地幔橄榄岩含有三种不同地球化学特征的单斜辉石(Cpx):(1)a类单斜辉石Mg~#=92.3~93.4,来自富集Cpx的二辉橄榄岩,具有极低的LREE和不相容元素含量,HREE平坦,Th、U、La、Sr正异常,经历了7%~10%的尖晶石相部分熔融,仅受到极低程度强不相容元素(Th、U、La、Sr)初期富集交代作用;(2)b类单斜辉石Mg~#=89.9~90.3,来自较富集Cpx的二辉橄榄岩,具有中等的LREE和LILE含量,HREE平坦,微量元素蛛网图上显示Th、U正异常,Rb、Ba、Nb、Ta、Sr、Ti负异常,经历4%~5%的尖晶石相部分熔融,可能受到了含LREE和Th、U等不相容元素的硅酸盐熔体交代;(3)c类单斜辉石Mg~#=91.4~92.8,来自贫Cpx的二辉橄榄岩和方辉橄榄岩,具有富集的LREE和LILE含量,HREE弱分异,微量元素蛛网图上显示Th、U正异常及强烈的Nb、Ta、Ti负异常,经历了8%~20%的尖晶石相部分熔融,其交代熔体可能是来自源区有石榴子石残留的碳酸盐熔体。全岩主、微量元素及模拟计算结果表明,这些幔源包体的主量元素主要受部分熔融程度影响,并且方辉橄榄岩经历的部分熔融程度大于二辉橄榄岩。地幔橄榄岩的Sr-Nd同位素组成表明该区具有MORB-OIB型亏损地幔特征。此外,蓬莱部分地幔橄榄岩包体显示正斜率的HREE分异特征((Gd/Yb)_N=0.4~0.7),暗示该区地幔经历了源自石榴子石稳定区的变压熔融,总体熔融程度为18%以上,指示了较高的地幔潜能温度。综合前人对海南岛新生代玄武岩最新研究成果,我们认为海南地幔柱可能为该区软流圈地幔置换古老岩石圈地幔提供了热源,导致了区域岩石圈地幔的破坏,从而引起包括地幔柱本身、软流圈和富集岩石圈的熔融。岩石圈地幔性质的改变和不均一性可能是海南岛陆缘扩张带新生代岩石圈减薄的主要动力学机制。     

江苏东海芝麻房预先导孔（CCSD-PPl）橄榄岩及其反映的上地幔亏损和交代事件

芝麻房预先导孔橄榄岩岩石类型有二辉橄榄岩、方辉橄榄岩、单辉橄榄岩和纯橄榄岩，其中前3种多数含有石榴子石。这些岩石在垂向上显示成层性，没有某种方向的韵律变化，它们之间呈渐变过渡关系。对样品进行主量元素、微量元素、稀土元素和Sr、Nd、O同位素分析表明，随着样品MgO含量的增高，Al2O3、CaO逐渐降低，呈良好的相关关系，并且大致上从石榴橄榄岩类到不含石榴子石的橄榄岩类主量元素的亏损程度增高，两者一般均比原始地幔主量元素亏损。稀土元素配分型式表现为右倾的轻稀土富集型，在微量元素蛛网图上，也表现为强不相容元素和微量元素到适度不相容元素的总体右倾型，与原始地幔相比，均富集强不相容元素。富集程度的不同与主量元素的亏损无对应关系，说明主量元素和微量元素各自的特点不是同一种事件造成的，而是先前地幔部分熔融作用之后又发生了富含不相容元素流体的交代作用。Sr、Nd同位素具有富集特点，变化范围较大，而且主量元素较亏损的岩石更富集Sr同位素，显示了交代富集的特点。稀土元素(Ce／Yb)x比值和εNd之间的负相关关系和Rb丰度与Sr同位素之间的正相关关系表明，富集Sr、Nd同位素的流体和富集不相容微量元素的流体均来自地幔。     

江苏东海芝麻房预先导孔(CCSD-PP1)橄榄岩及其反映的上地幔亏损和交代事件

芝麻房预先导孔橄榄岩岩石类型有二辉橄榄岩、方辉橄榄岩、单辉橄榄岩和纯橄榄岩,其中前3种多数含有石榴子石.这些岩石在垂向上显示成层性,没有某种方向的韵律变化,它们之间呈渐变过渡关系.对样品进行主量元素、微量元素、稀土元素和Sr、Nd、O同位素分析表明,随着样品MgO含量的增高,Al2O3、CaO逐渐降低,呈良好的相关关系,并且大致上从石榴橄榄岩类到不含石榴子石的橄榄岩类主量元素的亏损程度增高,两者一般均比原始地幔主量元素亏损.稀土元素配分型式表现为右倾的轻稀土富集型,在微量元素蛛网图上,也表现为强不相容元素和微量元素到适度不相容元素的总体右倾型,与原始地幔相比,均富集强不相容元素.富集程度的不同与主量元素的亏损无对应关系,说明主量元素和微量元素各自的特点不是同一种事件造成的,而是先前地幔部分熔融作用之后又发生了富含不相容元素流体的交代作用.Sr、Nd同位素具有富集特点,变化范围较大,而且主量元素较亏损的岩石更富集Sr同位素,显示了交代富集的特点.稀土元素(Ce/Yb)N比值和εNd之间的负相关关系和Rb丰度与Sr同位素之间的正相关关系表明,富集Sr、Nd同位素的流体和富集不相容微量元素的流体均来自地幔.     

岚皋早古生代碱质煌斑杂岩地球化学特征及成因探讨

岚皋碱质煌斑杂岩主量元素呈规律性演化，REE球粒陨石标准化配分型式呈强右倾型，富集不相容元素。岩石主元素和微量元素地球化学特征表明，煌斑岩浆在演化过程中存在2个平行的结晶分异演化趋势，岩浆演化过程中受地壳混染的程度很小。LREE、不相容元素的强烈富集及Dy／Yb、La／Yb、Zr／Sm、Ce／Y等微量元素的质量分数比值变化表明，煌斑杂岩不可能由尖晶石或石榴石二辉橄榄岩、含角闪石的尖晶石或石榴石二辉橄榄岩的部分熔融形成，它的源区应为含金云母和单斜辉石的交代地幔。Sr-Nd-Pb同位素结果显示，煌斑岩浆源区的地幔主要为HIMU端元组分。地幔柱的活动与煌斑岩浆的起源密切相关，并制约了其源区的地幔交代作用。     

赣东北蛇绿岩地幔橄榄岩岩石成因及其地质意义

赣东北蛇绿岩作为华南少有的前寒武纪蛇绿岩长期都是华南大地构造研究中的热点。该蛇绿岩地幔橄榄岩以方辉橄榄岩为主,稀土总量为0.83×10-6~2.62×10-6,远低于原始地幔的含量,而Mg O含量高于原始地幔。稀土元素表现为LREE相对富集,微量元素表现为大离子亲石元素(LILE)富集,高场强元素Nb明显亏损的特征,这表明赣东北地幔橄榄岩具有亏损地幔源区特征,同时也有不同程度的俯冲带流体的交代特征。HREE模拟的部分熔融程度为10%~30%;同时,赣东北蛇绿岩地幔橄榄岩中同时包含Cr#60和Cr#60两种不同成因的铬尖晶石,说明其经历了MOR(mid-ocean ridge)和SSZ(supra-subduction zone)两种构造环境。结合前人资料,笔者推测赣东北蛇绿岩早先可能形成于洋中脊环境(约1060 Ma),随后在洋内俯冲作用下(约970 Ma),位于俯冲带上部的地幔橄榄岩受到了来自俯冲带的流体/熔体的交代,经历了SSZ环境的改造。     

内蒙古集宁橄榄岩包体微区微量元素与Sr同位素特征及其岩石圈地幔演化的指示意义

为完整了解华北克拉通的破坏程度和机制,加深对其西部陆块岩石圈地幔的研究十分重要,而位于华北克拉通西部集宁新生代碱性玄武岩中的地幔橄榄岩包体,为研究人员认识该地区的岩石圈地幔的性质和演化起到指示作用.运用LA-ICP-MS和LA-MC-ICP-MS对集宁地区橄榄岩矿物进行原位微区测试,获得其主量、微量元素和Sr同位素成分的数据.根据矿物组成,可以将集宁地区的橄榄岩分为两类:第一类为贫单斜辉石橄榄岩 (单斜辉石体积分数小于8%),它们经历了高程度的部分熔融,可能是古老难熔岩石圈地幔的残留;第二类为二辉橄榄岩 (单斜辉石体积分数大于13%),其熔融程度低,代表了新生饱满的岩石圈地幔.第一类橄榄岩中单斜辉石REE含量整体偏低且轻微富集LREE,第二类橄榄岩中单斜辉石具有LREE富集和轻微亏损两种配分模式,大部分样品的核边有一定的强不相容元素及Sr同位素组成变化.这些微量元素和同位素特征都表明集宁橄榄岩包体经历过交代作用.(La/Yb)_N和Ti/Eu比值特征表明它们经历过多阶段的交代作用,交代介质有硅酸盐、碳酸盐熔/流体,这些交代介质可能为来源于古亚洲洋板块俯冲时释放的熔/流体.      

布什维尔德杂岩体Platreef矿床与金川铜镍硫化矿床微量元素地球化学特征对比及其意义

采用ICP-MS方法分析了布什维尔德杂岩体Platreef矿床含矿岩石的主量及微量元素含量,并与金川铜镍硫化矿床进行对比研究.Platreef矿床含矿岩石与金川二辉橄榄岩的∑REE接近,球粒陨石标准化后的稀土元素(REE)均表现为轻稀土元素(LREE)富集、重稀土元素(HREE)平坦的右倾型配分曲线,但Platreef含矿岩石总体表现为正Eu异常,金川二辉橄榄岩与矿石样品则大部分表现为较强的负Eu异常,二者的差异实质上反映了成岩矿物斜长石与斜方辉石比率细微的变动.Platreef矿床岩(矿)石富集和高度分异不相容元素,Nb、Ta亏损,显示地幔柱岩浆受到地壳混染后,产生与弧苦橄岩相似的微量元素地球化学特征.金川含矿岩石富集Fe和Ti以及强不相容亲石元素,显示铁质玄武岩微量元素分布特征,虽然也可能受到地壳物质混染,但总体表现出含矿岩体母岩浆则可能来自EMI型富集地幔源区.Platreef矿层原始地幔标准化后Pt、Pd、Ru富集,Ir、Rh相对亏损的PGE配分型式以及Cs、Rb、Th、U、LREE等不相容元素和亲铜元素Cu、Ni的富集.暗示布什维尔德杂岩体岩浆源区残留地幔橄榄岩遭受含高度不相容元素的富矿流体的渗透混染即地幔交代,导致幔源岩浆发生PGE分异.     

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

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。     

云南新平县双沟蛇绿岩稀土元素地球化学研究

双沟蛇绿岩中,变质橄榄岩的REE分布型式为LREE富集型,可能是富大离子亲石元素(LILE)组分交代造成的。地幔岩部分熔融生成的初始熔体LREE亏损,而辉绿岩和玄武岩则存在LREE亏损和富集两种型式,前者相当于N-MORB,后者相当于E-MORB。推测前者来源于亏损的软流圈地幔,而后者的地幔源区经历过一次富集LREE和LILE的事件。双沟蛇绿岩的REE特征暗示它可能形成于弧后盆地。     

中国大陆科学钻探工程主孔石榴石单辉橄榄岩的岩石化学研究

中国大陆科学钻探工程主孔石榴石橄榄岩产出于603.20～683.53m之间,主要为石榴石单辉橄榄岩,内部还见有巨晶状角闪石岩(曾经是岩脉)和榴辉岩与橄榄岩之间厘米级的方辉辉石岩.石榴石橄榄岩多数含有钛斜硅镁石.从其内部所夹的榴辉岩条带可知,橄榄岩发生过褶皱等构造变形作用.其围岩和内部夹层榴辉岩中的锆石中舍柯石英的事实可能表明岩体经历过超高压变质作用.石榴石橄榄岩的主要造岩矿物橄榄石的Fo端员低(0.82～0.86)、全岩岩石化学的低Mg#值(77.26～85.07)和高易熔元素以及低于典型阿尔卑斯型橄榄岩的(Mg Ni)/(Fe Mn)比值,意味着该石榴石橄榄岩体的原岩为岩浆固结成因.石榴石橄榄岩的稀土元素总量低,配分型式有LREE富集型((La/Lu)N比值为9～16.6)、LREE适度富集型((La/Lu)N比值为1.47～8.22)和LREE亏损型或平坦型((La/Lu)N比值为0.62、1.19～1.92)三种,并构成从亏损或平坦型到富集型的韵律,反映了结晶分异的特点.微量元素蛛网图上,石榴单辉橄榄岩微量元素的丰度总体上低于N-MOBB的丰度.Cu、Ni和MgO的判别图表明,石榴石橄榄岩的原岩岩浆的固结成岩时期主要为镁铁质矿物的分离结晶作用而硫化物不是重要的矿物相.Ti和P的剧烈变化表明了有磁铁矿和磷灰石的堆积作用.微量元素判别图表明岩体受到了明显的地壳物质混染作用.铂族元素丰度总量低,其丰度分布型式图表明原岩岩浆的部分熔融程度高,而且元素的判别图表明岩浆具有S不饱和特点.硫化物残留在地幔中而使得岩浆中PGE相对Cu、Ni亏损,在Ni-PGE-Cu图上呈总体上的凹形分布.同属PPGE的Pt比Pd所显示的更高的相容性也表明了岩浆中S的不饱和特点.而Ir的负异常可能说明原始岩浆在侵入前的分离结晶作用其间发生了Ir的结晶析出,或者是在地幔部分熔融时Ir残留在地幔残留体中.因此,主孔石榴石橄榄岩的原始岩浆是上地幔适当部分熔融程度的产物((Pd/Ir)N≥1),可能在源区有硫化物的残余而造成了PGE相对Cu、Ni亏损而在Ni-PGE-Cu图上呈总体上的凹形分布.后S不饱和的岩浆经历分离结晶作用而造成了Pt相时于Pd亏损的特点.     

西藏西南部休古嘎布蛇绿岩的成因:岩石学和地球化学证据

休古嘎布蛇绿岩块位于雅鲁藏布缝合带(YZSZ)西段,主要由地幔橄榄岩和侵入其中的基性岩墙所组成。基性岩墙具有弧后盆地地球化学亲缘性,其LREE亏损,(La/Yb)N为0.39~0.55;具有明显的Nb、Ti负异常及Sr、Ba正异常。Sr、Nd同位素特征表明它们起源于亏损地幔,并且受到了板片析出流体的影响。4个基性岩墙的Sm-Nd同位素样品获得内部等时线年龄为126.2±9.1Ma(MSWD=0.44)。地幔橄榄岩具有汤勺形和U形两组REE分布型式,显示出不同程度部分熔融和地幔交代作用的特征,具有弧-盆体系地球化学亲缘性。第一组橄榄岩的LREE弱富集或近于平坦,尖晶石的Cr#值低而且变化不大,部分熔融程度较低(15%~20%),可能形成于弧后扩张盆地;第二组橄榄岩的LREE明显富集,尖晶石的Cr#值高而且变化较大(0.4~0.77),部分熔融程度较高(25%~30%),并经历了强烈的交代富集作用,可能与洋内岛弧有关。     

Les xénolites ultramafiques du volcanisme alcalin quaternaire d'Oranie (Tell,Algérie occidentale), témoins d'une lithosphère cisaillée et enrichieUltramafic xenoliths from Quaternary alkali volcanism from Oranie (Tell,western Algeria): witnesses of a sheared and enriched lithosphere

Numerous ultramafic xenoliths occur within the A??n–Temouchent volcanic complex (Northwestern Oranie, Algeria). Most of them are type I mantle tectonites (lherzolites and harzburgites) and composite xenoliths (harzburgite/clinopyroxenite) are rare. Only a few samples of spinel lherzolites display relatively fertile compositions when the major part of type I xenoliths have refractory major element compositions but enriched LREE contents showing that they have been affected by mantle metasomatism. The composite xenoliths are witnesses of reactions of alkaline magmas with the upper mantle. An asthenospheric rising, in relation with the large strike slip fault affecting the North African plate margin at Trias time is proposed as a possible geodynamical setting. To cite this article: M. Zerka et al., C. R. Geoscience 334 (2002) 387–394.     

Rare earth element distribution in Plio-Quaternary volcanic rocks from southern Peru

Rare-earth element abundances of calc-alkaline andesitic rocks from southern Peru show that these rocks cannot be produceed by a single stage process. The high content of LILE, particularly LREE requires their derivation from a source already enriched in these elements and having a distinctly fractionated REE pattern. It is suggested that ascending hydrous fluids, released from the subducted oceanic lithosphere, enriched the upper mantle in LILE by zone refining. The partial melting of such an enriched upper mantle, followed by fractional crystallization, could produce andesitic rocks. REE data indicate that shoshonitic rocks from southern Peru can be derived from an unfractionated garnet-bearing peridotite by a low degree of partial melting.     

Geochemical variation in peridotite xenoliths and their constituent clinopyroxenes from Ray Pic (French Massif Central): implications for the composition of the shallow lithospheric mantle

Anhydrous mantle peridotite xenoliths from a single volcanic vent in the French Massif Central are compositionally varied, ranging from relatively fertile lherzolites to refractory harzburgites. Fertile lherzolites closely resemble previous estimates of undepleted mantle compositions but the average of the Ray Pic xenoliths is much less enriched in LILE and LREE than McDonough's (1990) average mantle [McDonough, W.F., 1990. Constraints on the composition of the continental lithospheric mantle. Earth Planet. Sci. Lett., 101, 1–18]. The wide geochemical variation in the bulk rocks reflects significant heterogeneities that can be attributed to two major processes within the shallow lithospheric mantle. The first process is depletion, related to variable degrees of partial melting and melt extraction from an originally near-chondritic mantle. This process has largely controlled the major elements and much of the trace element variation between fertile lherzolites and refractory peridotites. LREE-depleted compositions are also produced by this process. During partial melting, HREE behaved coherently with the major oxides and the moderately incompatible trace elements (Y, V and Sc). A subsequent process of enrichment is indicated by high concentrations of incompatible trace elements in many of the xenoliths. Sr, Ba, K, Th, U, Nb and LREE abundance are independent of major oxide variations and reflect enrichment related to infiltration by alkaline silicate melts/fluids. Both fertile and refractory mantle were enriched but harzburgites were particularly affected. Modal metasomatism occurred only rarely and is indicated by Cr-diopside-rich veins and patches in a few samples. Their chemistry suggests that they were also formed by migration of similar magmas/fluids from the asthenospheric mantle, although the presence of wehrlitic patches may indicate interaction with carbonate melts. In both depleted and enriched xenoliths, trace element patterns for separated clinopyroxenes closely reflect those of the bulk rock, except for Rb, Ba and Nb, which are probably hosted by other phases.     

山东昌乐新生代碱性玄武岩与地幔捕虏体中各类单斜辉石的地球化学特征及其意义

山东昌乐位于华北克拉通东部,郯庐断裂带中段,广泛出露挟裹幔源捕虏体的新生代碱性玄武岩.为深入了解华北克拉通演化,依据岩相学特征、矿物化学成分特征,把昌乐碱性玄武岩中单斜辉石主要分为三大类:第1类为幔源捕虏体中原生单斜辉石,其主、微量元素含量呈现趋势不一致的解耦现象,且LREE、HREE各异特征表明其经历了多期、不同程度的地幔富集交代和部分熔融作用（低于10%的尖晶石相部分熔融）,交代熔体包括地幔富碱（K）富Al硅酸盐熔/流体,可能有碳酸盐熔体的贡献;第2类为幔源捕虏体中筛状单斜辉石,是部分熔融和熔体交代作用的共同产物,从无筛孔部位到筛孔周围远离筛孔部位和紧挨着筛孔部位,呈现出受熔体作用逐渐加强趋势,离筛孔较远部位记录了玄武质熔浆作用之前的一次富碱（K）富Al熔体交代的特征,而紧挨着筛孔的部位受晚期玄武质熔浆作用的影响最强;第3类为幔源捕虏体中反应边单斜辉石及玄武岩中斑晶单斜辉石,两者的化学成分及形成条件相似,显示HFSE正异常,LILE负异常,受OIB特征寄主玄武岩浆强烈的影响.昌乐新生代碱性玄武岩地幔捕虏体中单斜辉石显示经历多期"熔体作用"和不同程度的部分熔融特征,是研究区岩石圈地幔不均一性的体现.      

Hydration and dehydration in the lower margin of a cold mantle wedge: implications for crust–mantle interactions and petrogeneses of arc magmas

Garnet orthopyroxenites from Maowu (Dabieshan orogen, eastern China) were formed from a refractory harzburgite/dunite protolith. They preserve mineralogical and geochemical evidence of hydration/metasomatism and dehydration at the lower edge of a cold mantle wedge. Abundant polyphase inclusions in the cores of garnet porphyroblasts record the earliest metamorphism and metasomatism in garnet orthopyroxenites. They are mainly composed of pargasitic amphibole, gedrite, chlorite, talc, phlogopite, and Cl-apatite, with minor anhydrous minerals such as orthopyroxene, sapphirine, spinel, and rutile. Most of these phases have high X_Mg, NiO, and Ni/Mg values, implying that they probably inherited the chemistry of pre-existing olivine. Trace element analyses indicate that polyphase inclusions are enriched in large ion lithophile elements (LILE), light rare earth elements (LREE), and high field strength elements (HFSE), with spikes of Ba, Pb, U, and high U/Th. Based on the P–T conditions of formation for the polyphase inclusions (?1.4 GPa, 720–850°C), we suggest that the protolith likely underwent significant hydration/metasomatism by slab-derived fluid under shallow–wet–cold mantle wedge corner conditions beneath the forearc. When the hydrated rocks were subducted into a deep–cold mantle wedge zone and underwent high-pressure–ultrahigh-pressure (HP–UHP) metamorphism, amphibole, talc, and chlorite dehydrated and garnet, orthopyroxene, Ti-chondrodite, and Ti-clinohumite formed during prograde metamorphism. The majority of LILE (e.g. Ba, U, Pb, Sr, and Th) and LREE were released into the fluid formed by dehydration reactions, whereas HFSE (e.g. Ti, Nb, and Ta) remained in the cold mantle wedge lower margin. Such fluid resembling the trace element characteristics of arc magmas evidently migrates into the overlying, internal, hotter part of the mantle wedge, thus resulting in a high degree of partial melting and the formation of arc magmas.     

Peridotites from the Mariana Trough: first look at the mantle beneath an active back-arc basin

Two dives of the DSV Shinkai 6500 in the Mariana Trough back-arc basin in the western Pacific sampled back-arc basin mantle exposures. Reports of peridotite exposures in back-arc basin setting are very limited and the lack of samples has hindered our understanding of this important aspect of lithospheric evolution. The Mariana Trough is a slow-spreading ridge, and ultramafic exposures with associated gabbro dykes or sills are located within a segment boundary. Petrological data suggest that the Mariana Trough peridotites are moderately depleted residues after partial melting of the upper mantle. Although some peridotite samples are affected by small-scale metasomatism, there is no evidence of pervasive post-melting metasomatism or melt-mantle interaction. Spinel compositions plot in the field for abyssal peridotites. Clinopyroxenes show depletions in Ti, Zr, and REE that are intermediate between those documented for peridotites from the Vulcan and Bouvet fracture zones (the American-Antarctic and Southwest Indian ridges, respectively). The open-system melting model indicates that the Mariana Trough peridotite compositions roughly correspond to theoretical residual compositions after ~7% near-fractional melting of a depleted MORB-type upper mantle with only little melt or fluid/mantle interactions. The low degree of melting is consistent with a low magma budget, resulting in ultramafic exposure. We infer that the mantle flow beneath the Mariana Trough Central Graben is episodic, resulting in varying magma supply rate at spreading segments.     

长江中下游中生代安山质火山岩记录的新元古代大洋板片-地幔相互作用

大陆弧安山岩的形成是大洋板片向大陆边缘之下俯冲的结果,但是在具体形成机制上存在很大争议.针对这个问题,对长江中下游地区中生代安山质火山岩及其伴生的玄武质和英安质火山岩进行了系统的岩石地球化学研究,结果对大陆弧安山质火成岩的成因提出了新的机制.分析表明,这些岩石形成于早白垩世,它们不仅表现出典型的岛弧型微量元素分布特征,而且具有高度富集的Sr-Nd-Hf同位素和高的放射成因Pb以及高的氧同位素组成.通过全岩和矿物地球化学成分变化检查发现,地壳混染和岩浆混合作用对其成分的富集特征贡献有限,而其岩浆源区含有丰富的俯冲地壳衍生物质才是其成分富集的根本原因.虽然这些火山岩的喷发年龄为中生代,但是其岩浆源区形成于新元古代早期的华夏洋壳俯冲对扬子克拉通边缘之下地幔楔的交代作用.大陆弧安山岩地幔源区中含有大量俯冲洋壳沉积物部分熔融产生的含水熔体,显著区别于大洋弧玄武岩的地幔源区,其中只含有少量俯冲洋壳来源的富水溶液和含水熔体.正是这些含水熔体交代上覆地幔楔橄榄岩,形成了不同程度富集的超镁铁质-镁铁质地幔源区.在早白垩纪时期,古太平洋俯冲过程的远弧后拉张导致中国东部岩石圈发生部分熔融,其中超镁铁质地幔源区熔融形成玄武质火山岩,镁铁质地幔源区则熔融形成安山质火山岩.因此,大陆弧安山岩成因与大洋弧玄武岩一样,可分为源区形成和源区熔融两个阶段,其中第一阶段对应于俯冲带壳幔相互作用.      

Geochronologic-petrochemical studies of the Hongshishan mafic–ultramafic intrusion,Beishan area,Xinjiang (NW China): petrogenesis and tectonic implications

The Hongshishan mafic–ultramafic intrusion (SIMS zircon U–Pb age 286.4 ± 2.8 Ma) consists of dunite, clinopyroxene peridotite, troctolite, and gabbro. Major elements display systematic correlations. Trace elements have identical distribution patterns, including flat rare-earth element (REE) patterns with positive Eu anomalies and enrichments in large ion lithophile elements (LILE) but depletions in Nb and Ta, indicating fractional crystallization as a key factor in magmatic evolution. Petrologic and geochemical variations in drill core samples demonstrate that minor assimilation and progressive magma injections were closely associated with Ni–Cu mineralization. Mass balance estimates and Sr–Nd isotopes reveal that the Hongshishan parental magmas were high-Mg and low-Ti tholeiitic basalts and were derived from a lithospheric mantle source that had been modified by subducted slab metasomatism before partial melting.

Southward subduction of the Palaeo-Tianshan–Junggar Ocean is further constrained by a compilation of inferred, subduction-induced modifications of mantle sources in mafic–ultramafic intrusions distributed in the eastern Tianshan–Beishan area. Integrating the regional positive ?_Nd(t) granites, high-Mg and low-Ti basaltic magmas (mafic–ultramafic intrusions), and slightly later high-Ti basalts in NW China suggests that their petrogenesis could be attributed to Permian mantle plume activities.     

Early Miocene adakite-like volcanism in the Balkuyumcu region, central Anatolia, Turkey: Petrology and geochemistry

The Balkuyumcu region, located in the southwestern part of Ankara in the Izmir-Ankara suture zone (central Anatolia, Turkey), consists of basic andesitic, andesitic, dacitic and rhyolitic rocks extruded during the Early Miocene (20–22 Ma) as a result of post-collisional volcanism. Balkuyumcu volcanic rocks can be divided into two groups on the basis of their mineralogy and composition: The basic andesitic (BA) and andesitic, dacitic and rhyolitic (ADR) groups. The ADR and BA group of rocks have adakite-like and calc-alkaline characteristics, respectively. The ADR group has higher SiO₂ content, Sr/Y and La/Yb ratios and low MgO, Mg#, Y and Yb contents than the BA group. Both groups have nearly the same Sr, Nd isotopic compositions and display similar normalized multi-element patterns with enrichments in LILE and LREE, depletions in Nb, Ti, Zr, P and a lack of Eu anomalies. Major, trace element and Sr, Nd isotopic data indicate that both groups of rocks were derived from the same source but affected by different magmatic processes during ascent. The adakite-like rocks may have been produced by partial melting of thickened lower continental crust. Fractional crystallization also played a major role in their formation. However, the BA group rocks were derived from partial melting of lower continental crust that was probably delaminated. These rocks appear to have had limited interaction with mantle peridodite during ascent to the surface.