西藏日喀则地区德村-昂仁蛇绿岩内基性岩的元素与Sr-Nd-Pb同位素地球化学及其揭示的特提斯地幔域特征

系统研究了西藏雅鲁藏布江蛇绿岩带中部日喀则地区德村、吉丁和昂仁蛇绿岩中基性岩石的元素与 Sr-Nd-Pb 同位素地球化学特征。这些基性岩石,包括玄武岩、辉长岩和辉绿岩,属于低钾拉斑玄武岩系列,球粒陨石标准化稀土元素分配模式为轻稀土元素亏损的 N-MORB 型,(La/Yh)_N=0.31～0.65(除样品 DC993为1.17)。在原始地幔标准化微量元素图上,亏损高度不相容元素,与 N-MORB 配分模式一致。相对于 Th,无 Nb、Ta的亏损,显示样品不是产于 SSZ 环境。经构造环境图解判别,样品落入了 N-MORB 区域内;这些元素成分特征表明样品具有洋中脊环境或成熟的弧后盆地环境属性。Sr、Nd 和 Pb同位素组成特征表明特提斯地幔源区以 DM(亏损地幔)为主,同时存在少量 EMⅡ(富集地幔类型Ⅱ)、Sr,Nd 和 Pb 同位素组成特征还表明特提斯地幔域具有印度洋 MORB 型的 Sr-Nd-Pb 同位素组成特征。本文的结果进一步支持 Zhang et al.(2005)的研究结果,现今印度洋不仅在地理位置上占据了曾经是特提斯洋的大部分,而且它的地幔域还继承了曾经特提斯的地幔域的地球化学特征。     

大兴安岭吉峰科马提岩Sm-Nd等时线年龄测定

本文研究的科马提岩位于大兴安岭北段吉峰地区。地质 地球化学研究表明 ,科马提岩系列由超镁铁质科马提岩、玄武质科马提岩、拉斑玄武岩和辉长岩等组成 ,稀土配分型式为球粒陨石型。 8件科马提岩、辉长岩和拉斑玄武岩样品的Sm Nd同位素数据构成一条相关性较好的等时线 ,其Nd模式年龄多数为 15 89～ 1799Ma ,等时线年龄为 (172 7± 74 .7)Ma ,INd=0 .5 10 72 5± 0 .0 0 0 0 798,εNd(t) =6 .94± 1.5 6 ,表明该区科马提岩形成于中元古代早期 ,来源于亏损地幔源区。这一地壳增生事件可能与松嫩地块从西伯利亚地台南缘裂解有关。     

山东蓬莱,临朐新生代碱性玄武岩的钕,锶同位素组成

本文报道了鲁东和鲁西新生代碱性玄武岩１３个样品的Ｎｄ，Ｓｒ同位素组成，１４３Ｎｄ／１４４Ｎｄ＝０．５１２９６７－０．５１２７４４，８７Ｓｒ／８６Ｓｒ＝０．７０３４９－０．７０４５０。它们在地质剖面上呈现规律性变化，可能与其地幔源区同位素组成的层状分带有关。鲁西地幔源区具有较鲁东更加亏损的组份。两地地幔源区在演化中都曾发生过地幔交代（或富集）作用，根据玄武岩Ｎｄ同位素模式年龄估计地幔交代作用发生的时     

雅鲁藏布江蛇绿岩中地幔柱型岩浆作用——来自氦、氩同位素的证据

雅鲁藏布江蛇绿岩的氦、氩同位素组成分别是：蛇绿岩中变质橄榄岩的^3He/^4He比值为1．104-3．384Ra，平均为2．383Ra；玄武岩的He同位素组成比较均一，^3He/^4He平均值为5．359 Ra；辉绿岩的^3He／^4He比值变化较大，为1—5Ra。由于各岩石样品有不同程度的蚀变，造成放射性成因He的加入，因此大多数样品He同位素组成比低于亏损地幔大洋中脊玄武岩的值(MORB≈8Ra)，不能真实反映源区特征。而采自吉定的辉绿岩样品^3He/^4He比值平均高达31．57Ra，与夏威夷发现的热点地幔源区的样品比值接近；分步加热法进一步测试，其高比值的He是在低温区段释放的。这种高^3He/^4He比值He的捕获，表明雅鲁藏布江蛇绿岩形成时存在地幔柱型富集地幔岩浆作用。     

Re-Os同位素对峨眉山大火成岩省成因制约的探讨

峨眉山大火成岩省（ELIP）主要由玄武岩、玄武质火山碎屑岩及少量的苦橄岩（包括越南的科马提岩）、长英质岩石以及层状岩体和岩墙组成,其物质来源直接关系到其成因是否与地幔柱活动有关。Re-Os同位素体系是地核、地幔和地壳物质的最佳示踪剂。前人对ELIP内的Re-Os同位素研究表明,低Ti玄武岩的Os含量为0．006×10^-9-0．40010^-9,^187Os／^188Os初始值为0．1371～1．403,并提出其与地幔柱活动有关;而高Ti玄武岩的Os含量为0．00410^-9～0．56010^-9,^187Os／^188Os初始值为0．1271～5．19,认为起源于大陆岩石圈地幔或地幔柱上升过程中受到大量岩石圈地幔“混染”（xu JF et al．,2007）;科马提岩的0s含量为1．2410^-9～7．0010^-9,^187Os／^188Os初始值为0．1251～0．1261,苦橄岩的Os含量为0．3210^-9～2．32910^-9,^187Os／^188Os初始值为0．1233～0．1266,指示苦橄岩和科马提岩均来自亏损地幔源区（Hanski et al．,2004;陈雷等,2007）。本文利用Os含量最低、^187Os／^188Os最高的高Ti玄武岩作为地壳端员,用铁质陨石、原始上地幔（PUM）和亏损地幔（DMM）作为地核和各种地幔端员,分别做二元混合计算,结果显示绝大多数玄武岩和所有苦橄岩及科马提岩均落在地壳和DMM混合曲线附近,并且邻区特提斯洋地幔岩与DMM具有相近的Os含量和^187Os／^188Os组成,据此推测峨眉山火成岩的形成与特提斯洋的活动有关,主要受控于地壳和亏损地幔的相互作用。     

东太平洋CC区玄武岩岩石成因：39Ar/40Ar年代学与地球化学证据

玄武岩作为地幔岩浆的产物,其岩石地球化学特征在反映地幔组分和深部动力学作用方面有着极其重要的意义。本文对东太平洋CC区西部和魏源海山两个潜次(Dive64和Dive66)的9个玄武岩样品进行了年代学、全岩地球化学和Sr-Nd-Pb同位素研究。结果显示,Dive64与Dive66玄武岩样品³⁹Ar/⁴⁰Ar年龄分别为83.1±0.3 Ma、74.7±0.3 Ma,属于晚白垩世,主体分别为N-MORB和OIB。Dive64为亚碱性玄武岩,REE总体相对平坦,轻重稀土分馏较弱,无明显Eu负异常;Dive66玄武岩为碱性玄武岩,富集轻稀土元素,亏损重稀土元素,无明显Eu、 Ce异常,MgO含量与CaO/Al₂O₃比值呈反比,与Cr含量、 Sr含量呈略微正相关。Sr-Nd-Pb同位素表明两者的形成可能不止单一源区的贡献,Dive64玄武岩Sr-Nd-Pb同位素比值更靠近DM(亏损地幔)、HIMU(高U/Pb比值地幔)端元,Dive66玄武岩Sr-Nd-Pb同位素比值较接近EMⅠ(Ⅰ型富集地幔)、HIMU端元。...     

科马提岩揭示富水的太古宙地幔

正太古代科马提岩代表了地幔在极端熔融条件下的产物,其特殊的化学成分通常被认为与源区的极高温度有关。但是,由于目前对科马提岩岩浆中水含量的认识不足,源区温度的限定存存在不确定性。一种观点认为科马提岩基本上是"干"的,属于典型的高温地幔柱成因;而另一观点认为科马提岩含H2O量为百分之零点几至百分之几(重量份数),因此它的形成不需要类似核幔边界起源的地幔柱的高温,其成因可能与俯冲过程相关。因     

太古宙镁铁质和超镁铁质火山岩的地球化学特征:地幔组分和演化的意义

用一个 TiO_2/P_2O_5比值为10的恒量,来推测从38亿年到目前的原始地幔,表明地幔-地核的分异作用至少对上地幔来说,在38亿年以前就已完成。而且在早太古代(>38亿年)地幔-地壳和/或地幔内部的分异显著程度已由 Nd 和 Sr 同位素资料指示出来。球粒陨石比值对难熔的亲石元素来说(例如 Al、Zr、Y、REE)通常见到是介于非巴伯顿类型(Al-未亏损)的太古宙科马提岩和具有球粒陨石稀土模式的高镁质玄武岩之间。巴伯顿类型(Al-亏损)橄榄质科马提岩和分异岩产物(与同时代的拉斑玄武岩和非巴伯顿型科马提岩形成对照)常常发现35-38亿年的地体 Al/Ti 值仅具球粒陨石比值一半,而且重稀土和 Sc 为亏损的。这种差异可能是由于不同的岩浆产生过程(主要看法)或早太古代地幔的化学和矿物学的分层的结果。获得的 Hf 同位素资料不支持这个观点,即地幔内部的均匀性与出现在45亿年以前的地核-地幔分带有关。在主要元素和痕量元素丰度以及同位素比值方面,比较小范围的均匀性推测太古宙镁铁质和超镁铁质火山岩源类似于所推断的从全新世地幔派生的火山岩。许多太古宙镁铁质和超镁铁质火山岩显示的地球化学特征相似于典型的现代洋中脊玄武岩(轻稀土亏损),显示着主要元素和痕量元素丰度之间的内聚现象。然而,一些高镁质玄武岩以轻稀土富集和分为主要元素与某些常常不能共生的元素象 Ti、Zr、Nd、稀土及磷为特征,表明地幔富集过程与榴辉岩熔体有关。榴辉岩的出现可能与太古代俯冲或增生的与来自大陆底下未亏损的镁铁质-超镁铁质火成岩的沉没有关。太古宙大陆壳底下残留的橄榄岩造成的底侵作用和后来的地幔交代变质作用,可能在一定程度上改造壳下上地幔的痕量元素和同位素的组分。这些物质的活化能为元古宙到全新世大陆玄武岩提供主要的物源。因为化学和同位素的不均匀性是地幔内部的原始特征,为进一步了解现今地幔,需要更详细地了解早太古代和太古宙地幔作用。     

大兴安岭吉峰地区中元古代科马提岩及成因类型

大兴安岭吉峰地区的橄榄质科马提岩具有良好的显微鬣刺结构,在橄榄石和辉石晶体间隙中充填有基质物质,说明其为火山岩.通过研究,对其初步划分了冷凝带和鬣刺带,它们构成一个不完整的冷凝单元.吉峰科马提岩在岩石化上表现了良好的科马提岩属性,与南非及西澳大利亚橄榄质科马提岩一致.岩石地球化学研究表明,科马提岩及与其伴生的玄武岩具有紧密的成因联系,它们的演化符合科马提岩-拉斑玄武岩的演化趋势,具良好的正相关特点.在主量元素、微量元素及REE特征上,本区科马提岩具有较好的II类科马提岩属性,与南非巴伯顿、印度、西格陵兰II类科马提岩极为相似.吉峰科马提岩的ε_Nd（T）=+7.51,说明源于亏损地幔,并且以LREE轻度富集为特征,显示它们系亏损地幔经较小程度部分熔融产生的.吉峰拉马提岩的形成时代为中元古代,其形成环境可能是地壳拉伸减薄,上涌的科马提质岩浆在淬火快速冷却的条件下形成.这一岩石类型的发现为研究大兴安岭地区元古宙大地构造演化提供了一个新线索.     

日照胡家岭石榴单辉辉石岩体的地球化学特征及其成因

与原始地幔相比,苏鲁超高压变质带的日照胡家岭石榴单辉辉石岩体全岩化学成分易熔组分均较高,而难熔组分均较低,全岩的Si O2含量为44.04%~50.50%,高MgO(12.77%~18.73%),高CaO/Al2O3(1.6~8.0),低碱,Mg#的变化范围为74.82~86.13,成分特点类似铝亏损型科马提岩。稀土元素配分型式为典型的单斜辉石稀土元素配分型式,重稀土元素相对亏损,(Gd/Yb)N=3.51~5.87。铂族元素的总体丰度为19.18×10-9~59.44×10-9,变化范围较大,高于地幔橄榄岩的丰度,(Pd/Ir)N具有8.58~11.07的正斜率。这些特点表明形成石榴单辉辉石岩的岩浆为地幔源区残留石榴石、主要熔出单斜辉石的高程度部分熔融的产物,折返前可能由单斜辉石 石榴石相组成。铂族元素丰度高、正斜率的特点表明其分布受到了地幔高程度部分熔融的控制且没有硫化物的分离,说明熔浆中硫是不饱和的。     

Low- and high-alumina komatiites of Goiás, Central Brazil

Archean komatiites of Goiás, central Brazil, have experienced deformation and low-grade metamorphism, but several outcrops preserve primary volcanic features. Samples from less deformed komatiites of four out of five greenstone belts (Crixás, Guarinos, Pilar de Goiás, and Santa Rita) have been investigated for their geochemical properties. Komatiites from the Crixás greenstone belt have very low Al₂O₃/TiO₂, high CaO/Al₂O₃, and a hump-shaped rare earth element (REE) pattern. Those from the Guarinos and Pilar de Goiás belts have similar REE patterns, characterized by a slight enrichment in LREE coupled with almost flat HREE, but differ in their inter-incompatible element ratios. Compared with those from Pilar de Goiás and Guarinos, samples from the Santa Rita belt have fractionated REE patterns with LREE enrichment, as well as high Al₂O₃ contents, corresponding to Al-undepleted komatiites. Komatiites from Crixás have the lowest (La/Sm)_N, (La/Yb)_N, and Zr/Zr* ratios compared with their equivalents from the other belts, which suggests their source was relatively depleted in LREE and high field strength elements (HFSE), probably due to the retention of garnet in the residue. Komatiites from the Guarinos, Pilar de Goiás, and Santa Rita greenstone belts are enriched in incompatible elements, which can be attributed to either low-degree partial melting at high pressures or a source previously enriched in incompatible elements. Some of the studied komatiites belong to Al- and HREE-depleted and others to the Al- and HREE-undepleted types. The depleted komatiites probably derived by melting at depths greater than 200 km, the undepleted at less than 200 km. Therefore, the komatiites of the four belts may have been derived from either one single mantle plume with different melting depths or sources from distinct plumes.     

Mid-Proterozoic Plume-Related Thermal Event in Eastern Indian Craton: Evidence from Trace Elements,REE Geochemistry and Sr - Nd Isotope Systematics of Basic-Ultrabasic Intrusives from Dalma Volcanic Belt

Trace, REE, Sr and Nd isotopic studies have been carried out on gabbro-pyroxenite intrusives (Rb-Sr isochron age ∼ 1619±38 Ma; Sr_i ∼ 0.70552±0.00002) of the Dalma volcanic belt from eastern Indian craton. Primitive mantle-normalised trace element patterns show a general depletion of high field strength elements and LREE but more or less flat pattern in most compatible elements. Chondrite-normalised REE plots show depleted LREE-flat HREE patterns [(SLREE/SHREE)_N < 1, (Ce/Yb)_N < 1] strikingly similar to the komatiitic and tholeiitic lavas from this belt. Nd isotopic data with mean f_Sm/Nd ∼ +0.2704 and high e_Nd (mean +7.8) values indicate that the source of these rocks was depleted in LREE for considerably long time. When plotted on the global e_Nd evolution path for the upper mantle the Dalma intrusives fall exactly around the depleted MORB-type mantle at 1.6 Ga.Enrichment in some LILE like Rb, Ba, Th is found both in the tholeiitic lavas and the residues indicating them to be source characteristics. Positive DNb values of most of the mafic-ultramafic units (including komatiitic lavas) of this belt indicate that they originated from a mantle plume with thick envelope of hot upper mantle producing MORB-like depleted komatiites, tholeiites and intrusives. The mid-Proterozoic plume eventually rifted the continent above, forming a rapidly subsiding basin which was subsequently collapsed and compressed. The plume also caused widespread thermal events recorded in charnockitisation, migmatisation and granitisation around 1.6 Ga. This was possibly part of a global ∼1.6 Ga thermal anomaly which affected the pre-existing large landmass comprising atleast Antarctica, Australia and India (Mawson continent?).     

Low- and high-alumina komatiites from a Late Archaean sequence,Newton Township,Ontario

Late Archaean komatiitic lavas from Newton Township, Ontario, consist of 6 chemically distinct magma types: 3 komatiites and 3 komatiitic basalts. The succession is unusual in containing both Al- and HREE-depleted komatiites and Al- and HREE-undepleted komatiites. The two types form distinct stratigraphic units separated by komatiitic basalts. Two komatiite types are strongly LREE depleted, whilst the third and the associated komatiitic basalts range from mildly depleted to enriched. Of the six magma types, only the two strongly LREE depleted komatiites represent primary mantle melts. The other komatiite type and the komatiitic basalts were derived from the primary komatiite magmas by combinations of olivine (+chromite) fractionation, assimilation of continental crust, and magma mixing. The two primary magmas may have been derived from similar sources, their contrasting chemistry being due to differing degrees of garnet segregation during melting. A generally applicable conclusion is that a wide range of komatiitic magma types can be generated from a relatively homogeneous depleted mantle, under conditions likely to prevail during the eruption of late Archean greenstone belt sequences.     

Geochemical constraints on cumulate textured Ti-rich Al-depleted Komatiites from Chotanagpur Gneissic Complex,Eastern India

For the first time occurrence of Ti rich Al depleted ultramafic cumulates having komatiitic signature in the northwestern fringe of Chotanagpur Gneissic Complex is presented. These rocks exhibit intrusive relationship with metasedimentary rocks and metaultramafites. Geochemically they are characterized by Mg# 79.1–91.60, high TiO₂ (1.29–1.54 wt%), significantly low Al₂O₃/TiO₂ and (Gd/Yb)_n >1. Major oxides, trace and REE content suggest low degree of fractional crystallization and lesser degree of partial melting. These Al depleted komatiites are characterized by high concentration of incompatible elements than most suites of Barberton type komatiites. High Ti content suggests less degree of majorite garnet melting, leaving behind garnet in the restite. The rock shows higher Ti/Sc (190),Ti/V (22), Zr/Y (3), Zr/Sc (4), V/Sc (8), Zr/Sm (28) and Zr/Hf (47) ratios than primitive mantle and REE distribution pattern shows gentle slope from LREE to HREE in most samples pointing towards mantle metasomatism and crustal contamination during emplacement. The observed chemical characteristics indicate derivation of komatiite from an enriched mantle source and represent plume activity in an extensional tectonic regime of intracratonic setting.     

徐州地区辉绿岩地球化学特征

对徐州栏杆地区侵位于元古代地层中的辉绿岩进行了系统的地球化学研究。栏杆地区的辉绿岩属于板内玄武岩系列岩石,地球化学特征上略富集轻稀土元素(LREE),富集大离子亲石元素(LILE),略亏损高场强元素(HFSE),相对富集Cr和Ni;微量元素显示了EM-OIB(富集地幔-洋岛玄武岩)的特征,辉绿岩稀土元素特征显示在岩浆上侵过程中经历了分离结晶作用。结合新元古代全球Rodinia超大陆裂解事件及其岩浆活动与地幔柱的密切关系,认为徐州地区新元古代辉绿岩可能为地幔柱作用在华北陆块边缘的记录。     

Petrology and geochemistry of metamorphosed komatiites and basalts from the Sula Mountains greenstone belt, Sierra Leone

The Sula Mountains greenstone belt is the largest of the late-Archaean greenstone belts in the West African Craton. It comprises a thick (5 km) lower volcanic formation and a thinner (2 km) upper metasedimentary formation. Komatiites and basalts dominate the volcanic formation and komatiites form almost half of the succession. All the volcanic rocks are metamorphosed to amphibolite grade and have been significantly chemically altered. Two stages of alteration are recognised and are tentatively ascribed to hydrothermal alteration and later regional amphibolite facies metamorphism. Ratios of immobile trace elements and REE patterns preserve, for the most part, original igneous signatures and these are used to identify five magma types. These are: low-Ti komatiites – depleted in light REE; low-Ti komatiites – with flat REE patterns; high-Ti komatiitic basalts – with flat REE; low-Ti basalts – depleted in light REE; high-Ti basalts – with flat REE patterns. Much of the variation between the magma types can be explained in terms of different melt fractions of the mantle source, although there were two separate mantle sources one light REE depleted, the other not. The interleaving of the basalts and komatiites produced by this melting indicates that the two mantle sources were melted simultaneously. The simplest model with which to explain these complex melting processes is during melting within a rising mantle plume in which there were two different mantle compositions. The very high proportion of komatiites in the Sula Mountains relative to other greenstone belts suggests either extensive deep melting and/or the absence of a thick pre-existing crust which would have acted as a “filter” to komatiite eruption. Received: 10 February 1998 / Accepted: 28 July 1998     

Geochemistry of mafic–ultramafic magmatism in the Western Ghats belt (Kudremukh greenstone belt), western Dharwar Craton,India: implications for mantle sources and geodynamic setting

Western Ghats Belt of western Dharwar Craton is dominated by metavolcanic rocks (komatiites, high-magnesium basalts (HMBs), basalts, boninites) with occasional metagabbros. This rock-suite has undergone post-magmatic alteration processes corresponding to greenschist- to lower-amphibolite facies conditions. Komatiites are Al-depleted, characterized by lower Al₂O₃/TiO₂ and high CaO/Al₂O₃. Their trace element distribution patterns suggest most of the primary geochemical compositions are preserved with minor influence of post-magmatic alteration processes and negligible crustal contamination. Chemical characteristics of Al-depleted komatiites imply their derivation from deeper upper mantle with/without garnet involvement. HMBs and basalts are differentiated based on their magnesium content. Basalts and occasionally associated gabbroic sills have similar geochemical characteristics. HMB are characterized by light rare earth element (LREE) enrichment, with significant Nb–Ta and Zr negative anomalies. Basalts and associated gabbros display tholeiitic affinity, with LREE-enriched to slightly fractionated heavy rare earth element (HREE) patterns. Boninites are distinctive in conjunction of low abundances of incompatible elements with respect to the studied komatiites. Chondrite-normalized REE patterns of boninites show relative enrichment in LREE and HREE with respect to MREE. Prominent island arc signatures are evident in HMB, basalts, boninites, and gabbros in terms of their Nb–Ta and Zr–Hf negative anomalies, LREE enrichment and HFSE depletion. It is suggested that these HMB–basalts (associated gabbros)–boninites are the products of arc magmatism. Their REE chemistry attests to a gradual transition in melting depth varying between spinel and garnet stability field in an arc regime. The close spatial association but contrasting elemental characteristics of komatiites and HMB–basalts–boninites can be explained by a plume-arc model, in which the ~3.0 Ga komatiites are considered to be the products of plume volcanism in an oceanic setting, while the HMB, basalts, boninites, and associated gabbros were emplaced in a continental margin setting around 2.8–2.7 Ga.     

http://www.sciencedirect.com/science/article/pii/S1674987112001296

We present field, petrographic, major and trace element data for komatiites and komatiite basalts from Sargur Group Nagamangala greenstone belt, western Dharwar craton. Field evidences such as crude pillow structure indicate their eruption in a marine environment whilst spinifex texture reveals their komatiite nature. Petrographic data suggest that the primary mineralogy has been completely altered during post-magmatic processes associated with metamorphism corresponding to greenschist to lower amphibolite facies conditions. The studied komatiites contain serpentine, talc, tremolite, actinolite and chlorite whilst tremolite, actinolite with minor plagioclase in komatiitic basalts. Based on the published Sm-Nd whole rock isochron ages of adjoining Banasandra komatiites (northern extension of Nagamangala belt) and further northwest in Nuggihalli belt and Kalyadi belt we speculate ca. 3.2–3.15 Ga for komatiite eruption in Nagamangala belt. Trace element characteristics particularly HFSE and REE patterns suggest that most of the primary geochemical characteristics are preserved with minor influence of post-magmatic alteration and/or contamination. About 1/3 of studied komatiites show Al-depletion whilst remaining komatiites and komatiite basalts are Al-undepleted. Several samples despite high MgO, (Gd/Yb)_N ratios show low CaO/Al₂O₃ ratios. Such anomalous values could be related to removal of CaO from komatiites during fluid-driven hydrothermal alteration, thus lowering CaO/Al₂O₃ ratios. The elemental characteristics of Al-depleted komatiites such as higher (Gd/Yb)_N (>1.0), CaO/Al₂O₃ (>1.0), Al₂O₃/TiO₂ (<18) together with lower HREE, Y, Zr and Hf indicate their derivation from deeper upper mantle with minor garnet (majorite?) involvement in residue whereas lower (Gd/Yb)_N (<1.0), CaO/Al₂O₃ (<0.9), higher Al₂O₃/TiO₂ (>18) together with higher HREE, Y, Zr suggest their derivation from shallower upper mantle without garnet involvement in residue. The observed chemical characteristics (CaO/Al₂O₃, Al₂O₃/TiO₂, MgO, Ni, Cr, Nb, Zr, Y, Hf, and REE) indicate derivation of the komatiite and komatiite basalt magmas from heterogeneous mantle (depleted to primitive mantle) at different depths in hot spot environments possibly with a rising plume. The low content of incompatible elements in studied komatiites suggest existence of depleted mantle during ca. 3.2 Ga which in turn imply an earlier episode of mantle differentiation, greenstone volcanism and continental growth probably during ca. 3.6–3.3 Ga which is substantiated by Nd and Pb isotope data of gneisses and komatiites in western Dharwar craton (WDC).     

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。     

鲁西中、新生代镁铁质岩浆作用与地幔化学演化

镁铁质火成岩作为分布最为广泛的典型幔源岩石, 已成为探索地幔化学性状及示踪岩石圈深部过程的主要研究对象.通过对典型样品元素-同位素组成的系统测定, 并结合前人已有资料, 综合研究了鲁西中生代和新生代镁铁质岩石的地质与地球化学特征.研究结果表明, 中生代镁铁质火成岩总体具有富轻稀土和大离子亲石元素、贫高场强元素、I_Sr值变化范围大(0.70396~0.71247)、ε_Nd (t) 值显著偏低(-9.20~-21.21) 的地球化学特征, 但该区南部和北部的中生代镁铁质岩石在元素-同位素组成上仍存在一定差别, 主要表现在南部较之北部镁铁质岩石具有更高的稀土总量(ΣREE为325.52×10-6~555.75×10-6)和轻、重稀土比值(LREE/HREE=17.75~25.97), 以及更高的LILE/HFSE比值(如La/Nb=6.37~13.85, Th/Nb=0.52~1.53).南部镁铁质岩石较之北部镁铁质岩石也更富放射成因锶, I_Sr值分别为0.70844~0.71247和0.70396~0.70598.元素-同位素综合示踪指示鲁西中生代地幔总体具有因岩石圈大规模拆沉作用形成的EMⅠ型富集地幔特征, 但其南部叠加了因深俯冲而进入地幔的扬子陆壳的影响, 因而表现出EMⅠ和EMⅡ组分混合的富集地幔特征.新生代玄武岩具有类似于大洋玄武岩的地球化学特征, 其源区应为亏损的软流圈地幔, 但在部分熔融形成岩浆之前遭受了近期的交代作用.自中生代至新生代, 华北克拉通地幔具有由富集向亏损演变的趋势, 这一化学性状的演变最可能是中生代以来岩石圈大规模拆沉作用, 导致软流圈地幔上涌并对原有岩石圈地幔再改造所致.