长白山新生代玄武岩中橄榄岩包体所揭示的岩石圈地幔特征

长白山地区位于华北克拉通东北部,广泛出露富含地幔橄榄岩包体的新生代玄武岩,为研究岩石圈地幔的性质和演化提供了优越条件。本文对长白山地区天池和龙岗新生代火山岩群中尖晶石相橄榄岩包体进行了岩石学、全岩主微量元素、矿物主量元素、单斜辉石微量元素和Sr-Nd-Hf同位素分析。研究结果表明,尖晶石相橄榄岩包体由二辉橄榄岩和少量的方辉橄榄岩组成,Mg#值为87.4～91.2,表现出新生饱满的特征,平衡温度为900～1100℃。橄榄石的Mg#值(%Fo)为85.6～91.3。单斜辉石包括四种类型:(1)轻稀土元素严重亏损型;(2)轻稀土元素亏损型;(3)向右微倾型和(4)"勺型"。单斜辉石表现出Sr同位素(87Sr/86Sr=0.702749～0.707276)整体亏损,部分样品富集的特征,单斜辉石的Nd-Hf同位素呈现出亏损特征(143Nd/144Nd=0.512886～0.51333、εHf=+17.7～+49.8)。长白山地区二辉和方辉橄榄岩分别经历了小于10%和略大于10%的部分熔融作用,并受到富水硅酸盐熔体的地幔交代作用。太平洋板块向西俯冲作用使得软流圈上涌并携带大量壳源物质进入地幔深部,与岩石圈地幔发生橄榄岩-熔体反应,形成了长白山地区不均一的岩石圈地幔,以新增生饱满地幔为主,夹有少量古老难熔岩石圈地幔碎片。     

吉林蛟河地幔橄榄岩捕虏体的Sr-Nd-Hf-Os同位素特征与岩石圈地幔时代

吉林省蛟河市境内大石河新生代玄武岩中含有丰富的地幔橄榄岩包体,详细的岩石学与矿物学研究显示,这些包体的主要岩石类型为尖晶石二辉橄榄岩-方辉橄榄岩,未发现石榴石橄榄岩。岩相学及地球化学资料显示它们都是经历过熔体抽取而形成的岩石圈地幔残留。矿物平衡温度计算发现,本区的这些地幔橄榄岩包体来自地下40～60km 深度,且下部以二辉橄榄岩为主,而上部以贫单斜辉石的二辉橄榄岩和方辉橄榄岩为主,显示明显的岩石圈地幔分层现象。Sr-Nd-Hf 同位素资料反映这些地幔包体均表现为亏损性质,而 Re-Os 同位素资料确定上述岩石圈地幔形成于中元古代,明显老于上覆地壳的新元古宙时代,反映壳幔年龄上的解耦。因此我们推测,该区曾经历过华北克拉通类似的早期岩石圈地幔的整体丢失事件,然后形成于其它地区的中元古宙岩石圈地幔在本区增生。     

江苏盘石山幔源橄榄岩包体成因的地球化学

根据主要元素、微量元素，若干矿物相主要元素和单斜辉石微量元素测定认为：盘石山幔源橄榄岩包体是由接近原始地幔源区的地幔经不同比例部分熔融后的残余二辉橄榄岩和方辉橄榄岩组成，基本未受其它过程干扰。由主要元素和ＭｇＯ相关性的回归方程，根据二元混合规律估算了原始熔体的组成范围。计算了包体的平衡温度，认为较亏损的方辉橄榄岩可能位于弱亏损的二辉橄榄岩之上，全岩和单斜辉石中不相容元素含量有可能作为地幔部分熔融和     

南海北缘新生代构造演化的深部制约(I):幔源包体

对南海北缘新生代玄武岩中幔源包体的研究揭示了该地区上地幔的不均一性和复杂的地幔过程。部分橄榄岩中单斜辉石(Cpx)具有极低的Ti含量(<160μg/g),这与这些样品中较高的Cpx含量(8%～12%)不匹配。主元素成分变化趋势以及Cpx中HREE分异现象,暗示该区上地幔经历了变压熔融。地幔熔融始于深部的石榴子石稳定区并延续到浅部的尖晶石稳定区(总熔融程度达23%)。这些熔融残余受到了硅酸盐和含水流体的交代。橄榄岩的微量元素组成指示南海北缘地幔具有大陆裂谷型地幔的特征,而与俯冲带之上的地幔楔或受俯冲作用影响的地幔有很大的差别。包体的平衡温度和橄榄石中Fo之间的负相关关系暗示岩石圈地幔具成分分层结构。该地幔的下部由大洋型橄榄岩组成,而地幔顶部为类似于太古宙-元古宙地幔的富斜方辉石方辉橄榄岩。这一岩石圈结构与该地区岩石圈的减薄和软流圈对老岩石圈的置换有关。深源岩石记录的信息支持南海海盆是陆缘扩张引起的主动盆地的观点。     

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.      

内蒙古阿尔山——柴河地区陆下岩石圈地幔性质研究——第四纪碱性玄武岩中橄榄岩捕掳体中单斜辉石微量元素证据

阿尔山—柴河第四纪碱性玄武岩中地幔捕掳体为尖晶石相的二辉橄榄岩和方辉橄榄岩,方辉橄榄岩数量略多于二辉橄榄岩。采用激光剥蚀等离子体质谱(LA--ICP--MS)对研究区地幔橄榄岩中的单斜辉石和橄榄石等矿物进行了成分分析,结合橄榄岩包体的岩相学、岩石化学的特征,重点探讨了研究区所经历的部分熔融作用和地幔交代作用。结果显示,少数样品的熔融程度5%,大多数样品熔融程度范围为10%~20%,研究区陆下岩石圈地幔性质以难熔、亏损为主要特征。同时也经历了复杂的交代作用改造,交代介质为富挥发组分的硅酸盐熔/流体。与华北克拉通东北缘陆下岩石圈地幔比较,推测研究区遭受破坏和改造的程度较小,并保留有相当量的古老地幔残余。     

华北中、新生代岩石圈地幔的交代作用：含金云母地幔岩提供的证据

本文对华北克拉通三个不同地区(河北汉诺坝、内蒙古集宁三义堂、河南鹤壁)新发现的含金云母尖晶石二辉橄榄岩和尖晶石橄榄单斜辉石岩捕虏体进行了详细的矿物组成、单斜辉石的微量元素和 Sr-Nd 同位素研究。通过与相同地区不含金云母尖晶石二辉橄榄岩捕虏体的系统对比发现通常含金云母的地幔橄榄岩比不含金云母的地幔橄榄岩岩富 Al_2O_3、CaO、NaO、K_2O、TiO_2,但相对贫镁;其单斜辉石的 LREE 更为富集,但 Sr、Nd 同位素组成则相对亏损。这说明地幔交代作用不仅能够造成地幔橄榄岩的玄武质组分和稀土元素的富集,而且亦能够造成全岩和橄榄石 Mg~#的降低和同位素组成的相对亏损。捕虏体的 Rb-Sr 等时线年龄暗示地幔交代作用发生在中、新生代;其交代熔体来源于软流圈。同时说明华北新生代岩石圈地慢普遍存在的主、微量元素和同位素组成类似于"大洋型"岩石圈地幔的特征很可能是岩石圈地幔橄榄岩与软流圈来源的熔体的大规模反应的结果,而非真正意义上的新增生的岩石圈地幔。     

汉诺坝辉石岩包体中单斜辉石的微区微量元素组成特征及其地球动力学意义

本文对来自汉诺坝玄武岩中的三类辉石岩包体的单斜辉石（Cpx）利用LA—ICP—MS进行了原位微区微量元素分析研究。金云母单斜辉石岩中的单斜辉石具有低Cr,Co和Ni，高CaO、Zr、Hf、Ta和LREE特征，并且具有较低的Mg^#，反映金云母单斜辉石岩可能是经受了后期交代作用的地慢岩浆堆晶产物尖晶石二辉橄榄岩中的石榴辉石岩脉体中的Cpx具有高Na2O低CaO特征，其Ti、Zr、Hf、Y含量明显高于其它石榴辉石岩Cpx中的含量，反映了石榴辉石岩脉体可能是富Na硅酸盐熔体与地幔尖晶石二辉橄榄岩反应的成因，部分石榴辉石岩包体的Cpx具有特征的La富集或微量元素（如Zr、Hf、Ta、Li、P、REE等）环状对称分布特征，暗示华北克拉通北缘可能存在多期次地幔交代富集作用．通过对同一Cpx颗粒中的流体包裹体密集区和干净区域的对比研究，发现Cpx中的流体包襄体不仅富集LILE和LREE，而且HFSE中的Nb、Ta、Zr和P也具有明显的富集特征，LREE和HFSE未发生明显分异，这可能暗示地幔交代流体中含有少量的络阴离子（如F^-和Cl^-等）．     

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。     

内蒙古宁城早中生代地幔捕虏体及熔/流体交代作用

内蒙古宁城地区发现的尖晶石二辉橄榄岩捕虏体,其寄主岩为早中生代堆晶成因的辉石岩。尖晶石二辉橄榄岩在矿物组成和Sr、Nd、Pb同位素特征等方面区别于寄主辉石岩,具有地幔岩特征。相对原始地幔岩,尖晶石二辉橄榄岩具有Fe高,Mg、Al低,富集K、Na、Ca、LREE和Rb、Sr、Ba、Th等不相容元素的特征,据此可以认为捕虏体来源于交代的富集地幔。对熔/流体交代反应形成的结构及其交代矿物金云母、韭闪石、白云石、方解石的进一步研究,揭示富K、Al、Ca、LREE和不相容元素的硅酸质和碳酸质熔/流体的交代作用致使地幔岩向不断饱满和富集的趋势演化,导致地幔岩Mg#值、Fo值的降低,Al和其他不相容元素的增高。单斜辉石环带原位微量元素测定也证实交代作用的存在。剪切结构的发育可能与软流圈底辟体上涌引发的塑性流变有关。联系华北古老岩石圈地幔多次的地质事件,笔者认为,早中生代地幔的特征与华北克拉通长期以来自身的深部演化有关。     

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.     

Depletion and enrichment processes in the lithospheric mantle beneath the Kola Peninsula (Russia): Evidence from spinel lherzolite and wehrlite xenoliths

A suite of spinel lherzolite and wehrlite xenoliths from a Devonian kimberlite dyke near Kandalaksha, Kola Peninsula, Russia, has been studied to determine the nature of the lithospheric mantle beneath the northern Baltic Shield. Olivine modal estimates and Fo content in the spinel lherzolite xenoliths reveal that the lithosphere beneath the Archaean–Proterozoic crust has some similarities to Phanerozoic lithospheric mantle elsewhere. Modal metasomatism is indicated by the presence of Ti-rich and Ti-poor phlogopite, pargasite, apatite and picroilmenite in the xenoliths. Wehrlite xenoliths are considered to represent localised high-pressure cumulates from mafic–ultramafic melts trapped within the mantle as veins or lenses. Equilibration temperatures range from 775 to 969 °C for the spinel lherzolite xenoliths and from 817 to 904 °C for the wehrlites.

Laser ablation ICP-MS data for incompatible trace elements in primary clinopyroxenes and metasomatic amphiboles from the spinel lherzolites show moderate levels of LREE enrichment. Replacement clinopyroxenes in the wehrlites are less enriched in LREE but richer in TiO₂. Fractional melt modelling for Y and Yb concentrations in clinopyroxenes from the spinel lherzolites indicates 7–8% partial melting of a primitive source. Such a volume of partial melt could be related to the 2.4–2.5 Ga intrusion of basaltic magmas (now metamorphosed to garnet granulites) in the lower crust of the northern Baltic Shield. The lithosphere beneath the Kola Peninsula has undergone several episodes of metasomatism. Both the spinel lherzolites and wehrlites were subjected to an incomplete carbonatitic metasomatic event, probably related to an early carbonatitic phase associated with the 360–380 Ma Devonian alkaline magmatism. This resulted in crystallisation of secondary clinopyroxene rims at the expense of primary orthopyroxenes, with development of secondary forsteritic olivine and apatite. Two separate metasomatic events resulted in the crystallisation of the Ti–Fe-rich amphibole, phlogopite and ilmenite in the wehrlites and the low Ti–Fe amphibole and phlogopite in the spinel lherzolites. Alternatively, a single metasomatic event with a chemically evolving melt may have produced the significant compositional differences seen in the amphibole and phlogopite between the spinel lherzolites and wehrlites. The calculated REE pattern of a melt in equilibrium with clinopyroxenes from a cpx-rich pocket is identical to that of the kimberlite host, indicating a close petrological relationship.     

Nature and Evolution of Cenozoic Lithospheric Mantle beneath Shandong Peninsula,Sino-Korean Craton,Eastern China

The Shanwang and Qixia basalts lie within the North China block and were erupted in Miocene to Pliocene time (18.1 to 4.3 Ma) and Pliocene time (6.4 to 5.9 Ma), respectively. The Shanwang area lies astride the Tancheng-Lujiang (Tanlu) fault zone, a major lithospheric fault, whereas the Qixia area lies east of the fault zone. The basaltic rocks (alkali olivine basalts, basanites, nephelinites) carry abundant deep-seated xenoliths including spinel lherzolite (dominant), dunite, and pyroxenite, and a megacryst suite including augite, anorthoclase, phlogopite, ilmenite, and garnet. Xenoliths with coarse-grained microstructures are common in the Qixia xenolith suite, but are absent in Shanwang. Reconstructed bulk compositions of the lherzolites range from relatively depleted (<3% modal diopside) to fertile (>12% modal diopside). Equilibration temperatures of 850° to 1020°C indicate entrainment of these lherzolites from depths ≤45 km, within the lithosphere; the geotherm may have been higher beneath Shanwang. The Shanwang suite contains less-depleted lherzolites, and more pyroxenites, than the Qixia suite. The chondrite-normalized REE patterns in clinopyroxenes of the Shandong xenoliths vary from LREE depleted, through concave shaped, to LREE enriched; spidergrams for the clinopyroxenes can be divided into depleted, fertile, and metasomatic types. Progressive depletion in Na and Al is accompanied by depletion in moderately incompatible elements such as Y, Yb, and Zr, and an increase in Mg#. Ti and Zr in clinopyroxenes have not been affected by the metasomatic process, and MREE have been little disturbed, whereas the light rare-earth elements, Nb, and Sr have been strongly enriched during metasomatism; this suggests that carbonate-rich fluids/melts were the metasomatic agent. The mantle beneath the Shandong Peninsula sampled by these basalts is dominantly Phanerozoic in character rather than Archean or Proterozoic lithospheric mantle. This mantle probably represents a mixture of older lithospheric mantle and newly accreted material that replaced the Archean lithospheric keel through extension, thermal erosion, and fluid/melt metasomatism. The differences in micro-structures, chemistry, temperature, and fluid/melt activity between Shanwang and Qixia are ascribed to their spatial relationships to the Tanlu fault, which is a major translithospheric suture that hasplayed an important role in the Cenozoic replacement of the pre-existing Archean lithospheric mantle.     

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.      

Xenolith evidence for polybaric melting and stratification of the upper mantle beneath South China

Mantle xenoliths from Hainan and Qilin, South China have been studied to constrain the nature of the upper mantle and mantle processes beneath a continental margin. The extremely low Ti (160–245 ppm) contents in clinopyroxenes from some spinel lherzolites, indicative of high degrees of partial melting are inconsistent with the relatively high clinopyroxene modes (7.4–13%) in these samples. This inconsistency could be due to polybaric melting that started in the garnet stability field, then, after the breakdown of garnet to pyroxene and spinel, continued in the spinel stability field. Polybaric melting, due to adiabatic decompression of upwelling mantle, would leave a residual mantle in which the degree of depletion decreases with depth. The predicted stratified lithospheric mantle is evidenced by the negative correlation between the forsterite content in olivine and the equilibration temperature, proportional to the depth in the lithosphere from which the xenolith was derived. The lower part of the lithospheric mantle beneath South China consists predominantly of fertile and moderately depleted peridotites, which are either devoid of LREE enrichment, or show the trace element signature of incipient metasomatism, and plot within the Phanerozoic mantle domain. In contrast, the upper part of the mantle contains harzburgite and cpx-poor lherzolite, which are strongly affected by metasomatism of melt/fluid of highly variable composition. The anomalously high orthopyroxene mode (up to 47%) makes some of these refractory samples compositionally similar to the Proterozoic/Archean mantle. Their low equilibrium temperature (800–900 °C) points to the presence of old lithospheric relicts in the uppermost mantle beneath South China. Such lithosphere architecture may have resulted from partial replacement of Archean–Proterozoic lithosphere by asthenosphere that rose adiabatically subsequent to lithospheric thinning during the Cenozoic.     

吉林省辽源晚中生代碱性玄武岩成因及岩石圈地幔性质

报道了吉林辽源晚中生代玄武岩的全岩K-Ar定年结果和岩石地球化学资料, 以及其中幔源捕虏体的矿物化学分析结果, 探讨了晚中生代碱性玄武岩的成因及岩石圈地幔的性质.全岩K-Ar定年结果显示, 辽源玄武岩形成于晚白垩世(91.7±1.7 Ma、82.5±1.9 Ma、99.3±1.6 Ma、82.2±2.4 Ma).该玄武岩的SiO₂含量介于45.1%~45.9%之间, 全碱(Na₂O+K₂O)含量介于4.50%~6.72%之间, Mg#[100×Mg/(Mg+Fe2+)]值介于60~63之间, 属于碱性玄武岩系列.该玄武岩相对富集轻稀土元素(LREEs)、亏损重稀土元素(HREEs), (La/Yb)_N值介于12.0~14.0之间, 无Eu异常, 具有洋岛玄武岩(OIB)型微量元素配分型式.辽源玄武岩中的幔源捕虏体以尖晶石二辉橄榄岩为主, 捕虏体中橄榄石的Mg#值较低(87.1~91.2), 结合捕虏体中其他矿物的主量和微量元素地球化学特征, 认为辽源晚白垩世岩石圈地幔以新增生的岩石圈地幔为主.捕虏体中单斜辉石的LREEs具有从富集到亏损的变化特征, 暗示岩石圈地幔经历了部分熔融和交代作用的改造, 地幔交代类型以硅酸盐熔体交代为主, 同时存在碳酸盐熔体交代作用.辽源晚白垩世玄武岩来源于软流圈地幔的部分熔融, 结合区域研究成果, 认为辽源晚白垩世玄武岩形成于古太平洋板块俯冲背景下的弧后伸展环境.      

Garnet Lherzolites from the Kaapvaal Craton (South Africa): Trace Element Evidence for a Metasomatic History

Kimberlites from the Kaapvaal craton have sampled numerous mantlegarnet lherzolites in addition to garnet harzburgites. Traceelement characteristics of constituent clinopyroxenes allowtwo groups of garnet lherzolites to be distinguished. Traceelement compositions of all clinopyroxenes are characterizedby enrichment in light rare earth elements (LREE) and largeion lithophile elements and by a relative depletion in Ti, Nb,Ta, and to a lesser extent Zr and Hf. However, the LREE enrichmentand the depletion in Nb and Zr (Hf) are less in the Type 1 clinopyroxenesthan in the Type 2 clinopyroxenes. Our study suggests that thetwo melts responsible for the metasomatic imprints observedin the two garnet lherzolite groups are highly alkaline maficsilicate melts. Type 1 clinopyroxenes that have trace elementsimilarities to those of PIC (Phlogopite–Ilmenite–Clinopyroxene)rocks appear to have crystallized from, or been completely equilibratedwith, the same melt related to Group I kimberlite magma. TheType 2 clinopyroxenes have trace element similarities to thoseof MARID (Mica– Amphibole–Rutile–Ilmenite–Diopside)rocks and are therefore probably linked to melt related to GroupII kimberlite magma. KEY WORDS: garnet lherzolites; Kaapvaal craton; mantle xenoliths; mantle metasomatism; trace elements     

The effect of the Fernando de Noronha plume on the mantle lithosphere in north-eastern Brazil

New xenolith occurrences in the Cenozoic alkali basalts of north-eastern Brazil have been studied in order to constrain the possible imprint on the continental mantle lithosphere of its passage over the Fernando de Noronha plume and the regional mantle processes. Texturally, the lherzolite and harzburgite xenoliths define three groups: group 1, porphyroclastic; group 2, protogranular; group 3, transitional between groups 1 and 2. Equilibrium temperatures are highest for group 1 and lowest for group 2. Clinopyroxenes from group 1 peridotites have Primitive Mantle (PM)-normalised REE patterns varying from L-MREE-enriched convex-upward, typical of phases in equilibrium with alkaline melts, to LREE-enriched, spoon-shaped, to LREE-enriched, steadily fractionated in a wehrlite. Group 2 clinopyroxenes show patterns slightly depleted in LREE to nearly flat. The M-HREE are at 3–5 ×PM concentration level, as typical in fertile lithospheric lherzolites. Most of group 3 clinopyroxenes show LREE-depleted patterns similar to the group 2 ones, but in two samples the clinopyroxenes are characterised by LREE-enriched, spoon-shaped profiles. Sr and Nd isotopes of the group 1 clinopyroxenes form an array between DM and EMI-like components, both of them are also present in the host basalts. Melts estimated to be in equilibrium with the group 1 clinopyroxenes having L-MREE-enriched, convex-upward patterns are similar to the Cenozoic alkaline magmas. The groups 2 and 3 clinopyroxenes define two distinct compositional fields at higher ¹⁴³Nd/¹⁴⁴Nd values, correlated with their LREE composition. The isotopes of the groups 2 and 3 LREE-depleted clinopyroxenes form an array from DM towards the isotopic composition of Mesozoic tholeiitic basalts from north-eastern Brazil. Melts in equilibrium with these clinopyroxenes are similar to these basalts, thus suggesting that such xenoliths record geochemical imprint from older melt-related processes.

The LREE-enriched spoon-shaped group 3 clinopyroxenes are characterised by the highest ¹⁴³Nd/¹⁴⁴Nd values at any given ⁸⁷Sr/⁸⁶Sr composition. These results are interpreted in terms of a lithospheric mantle section which underwent thermo-chemical and mechanical erosion by infiltration of asthenospheric alkali basalts having EMI-like isotope characteristics during Cenozoic time. At that time, the lithospheric mantle consisted of fertile lherzolites and harzburgites recording the geochemical imprint of Mesozoic mantle processes. The onset of the interaction between lithospheric peridotites and alkaline melts was characterised by the porous flow percolation of small melt volumes that induced chromatographic enrichments in highly incompatible elements and the isotope signature of the spoon-shaped, group 3 clinopyroxenes. Group 1 peridotites represent the base of the lithospheric column eroded by the ascending alkaline melts, whereas the group 2 documents the shallower lithospheric section, with group 3 being the transition. The similarity of processes and isotope components in the protogranular xenoliths from Fernando de Noronha area and north-eastern Brazil supports the hypothesis that the lithosphere beneath Fernando de Noronha is a detached portion of the continental one. Furthermore, the similarity in terms of textural and geochemical features documented by the mantle samples coming from the two different regions seems to confirm the interference of the two regions with the same plume.     

The Nature of Mantle Rocks in Ophiolites of the Polar Urals

This work presents the results of geochemical (LA-ICP-MS) study of minerals of peridotites from ophiolite complexes of the Polar Urals to clarify the nature of these formations. The distribution of trace and rare earth elements in clinopyroxenes testifies that there were three types of the mantle substratum, which formed in different geodynamic settings. Two types of primary peridotites were formed upon partial melting of the mantle at different-depth levels in the subduction zone. The first type is represented by lherzolites and diopside harzburgites, formed at partial melting under the spinel facies conditions; the second type is represented by diopside harzburgites, formed under polybaric partial melting under the garnet and spinel facies conditions. In the suprasubduction zone, peridotites experienced fluid-induced partial melting that resulted in crystallization of harzburgites. All types of harzburgites were transformed by ascending melts and fluids (refertilization) and high-temperature hydration with the formation of amphibole. These processes are recorded in variations in the REE spectra of minerals.     

Metasomatism in the subcontinental mantle beneath the Eastern Carpathians (Romania): new evidence from trace element geochemistry

A wide range of trace elements have been analysed in mantle xenoliths (whole rocks, clinopyroxene and amphibole separates) from alkaline lavas in the Eastern Carpathians (Romania), in order to understand the process of metasomatism in the subcontinental mantle of the Carpatho-Pannonian region. The xenoliths include spinel lherzolites, harzburgites and websterites, clinopyroxenites, amphibole veins and amphibole clinopyroxenites. Textures vary from porphyroclastic to granoblastic, or equigranular. Grain size increases with increasing equilibrium temperature of mineralogical assemblages and results from grain boundary migration. In peridotites, interstitial clinopyroxenes (cpx) and amphiboles resulted from impregnation and metasomatism of harzburgites or cpx-poor lherzolites by small quantities of a melt I with a melilitite composition. Clinopyroxenites, amphibole veins and amphibole clinopyroxenites are also formed by metasomatism as a result of percolation through fracture systems of large quantities of a melt II with a melanephelinite composition. These metasomatic events are marked by whole-rock enrichments, relative to the primitive mantle (PM), in Rb, Th and U associated in some granoblastic lherzolites and in clinopyroxene and amphibole veins with enrichments in LREE, Ta and Nb. Correlations between major element whole-rock contents in peridotites demonstrate that the formation of interstitial amphibole and clinopyroxene induced only a slight but variable increase of the Ca/Al ratio without apparent modifications of the initial mantle composition. Metasomatism is also traced by enrichments in the most incompatible elements and the LREE. The Ta, Nb, MREE and HREE contents remained unchanged and confirm the depleted state of the initial but heterogeneous mantle. Major and trace element signature of clinopyroxene suggests that amphibole clinopyroxenites and some granoblastic lherzolites have been metasomatized successively by melts I and II. Both melts I and II were Ca-rich and Si-poor, somewhat alkaline (Na > K). Melt I differed from melt II in having higher Mg and Cr contents offset by lower Ti, Al, Fe and K contents. Both were highly enriched in all incompatible trace elements relative to primitive mantle, showing positive anomalies in Rb, Ba, Th, Sr and Zr. They contrasted by their Ta, Nb and LREE contents, lower in melt I than in melt II. Melts I and II originate during a two-stage melting event from the same source at high pressure and under increasing temperature. The source assemblage could be that of a metasomatized carbonated mantle but was more likely that of an eclogite of crustal affinity. Genetic relationships between calc-alkaline and alkaline lavas from Eastern Carpathians and these melts are thought to be only indirect, the former originating from partial melting of mantle sources respectively metasomatized by the melts I and II. Received: 17 March 1997 / Accepted: 14 July 1997