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1.
The Haji‐Abad ophiolite in SW Iran (Outer Zagros Ophiolite Belt) is a remnant of the Late Cretaceous supra‐subduction zone ophiolites along the Bitlis–Zagros suture zone of southern Tethys. These ophiolites are coeval in age with the Late Cretaceous peri‐Arabian ophiolite belt including the Troodos (Cyprus), Kizildag (Turkey), Baer‐Bassit (Syria) and Semail (Oman) in the eastern Mediterranean region, as well as other Late Cretaceous Zagros ophiolites. Mantle tectonites constitute the main lithology of the Haji‐Abad ophiolite and are mostly lherzolites, depleted harzburgite with widespread residual and foliated/discordant dunite lenses. Podiform chromitites are common and are typically enveloped by thin dunitic haloes. Harzburgitic spinels are geochemically characterized by low and/or high Cr number, showing tendency to plot both in depleted abyssal and fore‐arc peridotites fields. Lherzolites are less refractory with slightly higher bulk REE contents and characterized by 7–12% partial melting of a spinel lherzolitic source whereas depleted harzburgites have very low abundances of REE and represented by more than 17% partial melting. The Haji‐Abad ophiolite crustal sequences are characterized by ultramafic cumulates and volcanic rocks. The volcanic rocks comprise pillow lavas and massive lava flows with basaltic to more‐evolved dacitic composition. The geochemistry and petrology of the Haji‐Abad volcanic rocks show a magmatic progression from early‐erupted E‐MORB‐type pillow lavas to late‐stages boninitic lavas. The E‐MORB‐type lavas have LREE‐enriched patterns without (or with slight) depletion in Nb–Ta. Boninitic lavas are highly depleted in bulk REEs and are represented by strong LREE‐depleted patterns and Nb–Ta negative anomalies. Tonalitic and plagiogranitic intrusions of small size, with calc‐alkaline signature, are common in the ophiolite complex. The Late Cretaceous Tethyan ophiolites like those at the Troodos, eastern Mediterranean, Oman and Zagros show similar ages and geochemical signatures, suggesting widespread supra‐subduction zone magmatism in all Neotethyan ophiolites during the Late Cretaceous. The geochemical patterns of the Haji‐Abad ophiolites as well as those of other Late Cretaceous Tethyan ophiolites, reflect a fore‐arc tectonic setting for the generation of the magmatic rocks in the southern branch of Neotethys during the Late Cretaceous. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

2.
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   

3.
Petrochemical studies indicate that the Yanghou metamorphie ultramafic rocks are composed of metamorphosed harzburgite and ultramafic cumulate.Trace element geochemistry and mineral chemistry of the metamorphic harzburgite indicate that they are relicts of depleted mantle.Systematic petrochemical,mineral chemical and geochronological studies led to such a conclusion that the Yanghou metamorphic ultramafic rocks may be the components of Late Sinian-Early Paleozoic ophiolite in South China.  相似文献   

4.
新疆天山地区榆树沟-铜花山蛇绿岩特征和构造背景   总被引：5，自引：3，他引：2  
榆树沟-铜花山蛇绿岩出露于南天山北缘,属哈萨克斯坦-伊犁板块和塔里木板块之间的缝合带。蛇绿岩已被构造肢解,主要由超镁铁岩、堆晶岩、熔岩类组成。超镁铁岩以方辉橄榄岩为主,显示典型的亏损地幔岩特征,橄榄石为富镁型,Fo为90;斜方辉石为顽火辉石,En为90,单斜辉石含量少;副矿物铬尖晶石属低铬型,Cr#值(＝Cr/(Cr+Al)×100)16~28,Mg#值(=Mg/(Mg+Fe)×100)63~75,反映了深海橄榄岩特征。归一化后榆树沟超镁铁岩MgO含量38.84%~44.53%,Al2O3为1.51%~3.63%,CaO为0.42%~5.77%,成分近于大洋二辉橄榄岩;铜花山的超镁铁岩叠加碳酸岩化,LREE强烈富集,可能经历了俯冲洋壳流体改造。熔岩类在榆树沟和铜花山均有较大规模产出,其中榆树沟玄武岩为主,铜花山安山岩和英安岩较多。熔岩多已遭受绿片岩相海底热液蚀变。榆树沟玄武岩的REE含量总体比铜花山的低,稀土配分模式均为轻稀土富集型;玄武岩的微量元素特征表明其源区可能遭受过流体作用影响。铜花山三个高镁火山岩化学成份具有SiO2(32%~36%)含量很低、MgO(20.12%~28.50%)含量高、K2O+Na2O(0.06%~0.46%)含量小于2%的特征,综合分析可归为苦橄岩类。堆晶岩通常以构造小岩块产在超镁铁岩块中或基性熔岩中,以辉长岩为主,有少量橄榄辉石岩等,岩石蚀变、构造变形强烈。堆晶岩的成分接近玄武岩,存在LREE富集和略亏损平坦两种类型。岩石地球化学特征表明榆树沟和铜花山的蛇绿岩形成于MOR构造环境,但具有受俯冲带流体改造特征,并伴生有岛弧火山岩。  相似文献   

5.
Serpentinized ultramafic rocks occur in two separate basement complexes in the South Arm of Sulawesi, the Bantimala and Barru Blocks. We present petrographic, mineral chemical and geochemical data for these rocks, and interpret them in terms of petrogenesis and tectonic setting. The rocks of both blocks show strong serpentinization of original anhydrous silicates. The Bantimala ultramafics consist mainly of peridotite (harzburgite and dunite) and clinopyroxenite, with lenses of podiform chromitite. Metamorphism is evidenced by the occurrence of amphibolite-facies tremolite schist. In contrast, the Barru ultramafics consist of harzburgite peridotite and podiform chromitite, which also show an amphibolite-facies overprint that in this case may be related to intrusion by a large dacite/granodiorite body. Whole-rock trace element analyses and spinel compositions show that the Barru harzburgite is depleted relative to primitive mantle, and has had some melt extracted. In contrast, the Bantimala dunite, harzburgite and clinopyroxenite are cumulates. Both are derived from a supra-subduction zone environment, and were obducted during the closure of small back-arc basins. If there has been no rotation of the blocks, then the Bantimala ultramafics were emplaced from an ENE direction, while the Barru ultramafics were emplaced from the WNW. The ultramafic suites from these two blocks are juxtaposed with metamorphic assemblages, which were later intruded by younger volcanics, particularly in the Barru Block.  相似文献   

6.
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   

7.
The Hegenshan ophiolite in Inner Mongolia is a remnant of oceanic lithosphere of probable Devonian age. The ophiolite consists of several blocks composed chiefly of serpentinized ultramafic rocks with lesser amounts of troctolite and gabbro, and sparse lavas and dikes. The ultramafic rocks consist chiefly of depleted harzburgite and minor dunite and are interpreted as mantle tectonites. In the Hegenshan block dunite is relatively abundant and is typically associated with podiform chromitite. Both the chromite ore and the residual chromites in this body are relatively aluminous with average Cr numbers of 44–54. A few small chromite bodies and some of the residual chromites have much higher Cr numbers (72–76). Several blocks have well-layered cumulate sequences of gabbro and troctolite. Sheeted dikes are absent but small mafic dikes are common in some of the ultramafic sections. Most of the mafic dikes have flat chondrite-normalized REE patterns and are strongly depleted in incompatible elements, similar to depleted tholeiites from immature island arcs. The basaltic lavas of the Hegenshan ophiolite have two distinctly different chemical signatures—one similar to the mafic dikes and one similar to ocean island basalts. The entire complex was probably formed within an island arc–marginal basin system that was later accreted to the southern margin of the Siberian Altaids.  相似文献   

8.
岩石化学研究表明，洋后变质超镁铁岩是由变质方辉橄榄岩和变质超镁铁堆积岩组成。变质方辉橄榄岩的微量元素地球化学及矿物化学特征表明，应为亏损的残余地幔产物。通过系统的岩石地球化学、矿物化学及年代学研究，认为洋后变质超镁铁岩可能为华南晚震旦－早古生代的蛇绿岩组成部分。  相似文献   

9.
Up to 3 km of dunitic rocks occur below crustal gabbro in the Blow Me Down massif (Bay of Islands Ophiolite, Newfoundland). Analyses of dunite- and gabbro-hosted clinopyroxene grains (cpx) for rare earth elements (REE), Zr, and Ti reveal three types of chondrite-normalized patterns: N-group patterns are similar to cpx grains as they would form by fractionation from a range of mid ocean ridge basalts (MORB). They are typical for a few higher level dunitic samples as well as mafic cumulates. F-group patterns show light REE depletion, very strong middle REE fractionation and a positive Zr anomaly and occur in dunites only. R-group patterns are severely depleted in both light and heavy REEs relative to MORB-like cpx and two samples of the group display a positive Ti anomaly. They are also restricted to dunitic rocks. The patterns are explained in a two stage model in which an established dunite sequence, dominated by MORB-type cumulate signatures (N-group), was infiltrated by extremely refractory melts. During infiltration of the refractory melt chromatographic fractionation occurred, transforming N-group dunites into F-group and R-group dunites. The F-group patterns are composite patterns: heavy REE, Ti ± Zr reflect the original MORB-like cumulate dunite host, light REEs indicate equilibrium with the infiltrating, refractory melts. Steep slopes in the middle REEs reflect the position of the chromatographic front. For more intense percolation of refractory melts, R-group patterns with a positive Ti anomaly will form by the same process. The rest of the R-group patterns displaying no positive Ti anomaly may represent either the most intensely reacted host rocks or these dunites derive directly as cumulates from refractory melts. Only small volumes of refractory melt (a 5 m column) are required to imprint the observed trace element pattern on the thick original dunite sequence. One of several possible origins for the refractory melts is transformation of original MORB-type melts by way of chromatographic fractionation within the highly depleted, residual uppermost mantle. In the framework of an oceanic spreading centre, the migrating, refractory liquids are considered a late event following the main constructive stage dominated by aggregated melts. The study demonstrates that highly refractory melts can exist under oceanic spreading centres dominated by a MORB-like cumulate and volcanic sequence. Received: 2 September 1996 / Accepted: 20 November 1997  相似文献   

10.
This paper re-describes the characteristics of pre-Ordovician (Pt3) metamorphic volcanic rocks in the Huimin-Manlai region of Yunnan Province from the aspects of petrographic characteristics, rock assemblage, petrochemistry, REE, trace elements, lead isotopes and geotectonic setting. The metamorphic volcanic rocks maintain blasto-intergranular and blasto-andesitic textures; the volcanic rocks are characterized by a basalt-andesite-dacite assemblage; the volcanic rocks are basic-intermediate-intermediate-acid in chemical composition, belonging to semi-alkaline rocks, with calc-alkaline series and tholeiite series coexisting, and they are characterized by low TiO2 contents; their REE distribution patterns are of the LREE-enrichment right-inclined type; the volcanic rocks are enriched in large cation elements and commonly enriched in Th and partly depleted in Ti, Cr and P, belonging to the Gondwana type as viewed from their Pb isotopic composition; petrochemically the data points fall mostly within the field of island-arc volcanic rocks. All these characteristics provided new evidence for the existence of original Tethysan island-arc volcanic rocks in the region studied.  相似文献   

11.
The ophiolite complex of Chamrousse (Belledonne Massif, Alps), consists of mafic to ultramafic cumulates and non-cumulates metamorphosed to amphibolite facies grade. The non-cumulitic rocks are similar in chemical composition to recent ocean-floor olivine tholeiites (both N-type and enriched P-type). The distribution of lithophile elements shows that the non-cumulitic rocks represent several magmas of different parentage. The character of the magmas varies according to the time of emplacement.Geological and geochemical data suggest that the Chamrousse complex was formed at a spreading oceanic ridge. The dynamic partial melting of an upper mantle diapir generated tholeiitic melt which decreased in amount and in REE contents. The first melt, enriched in light REE, was generated along the axis of the ridge while the second batch of melt, of lesser quantity and slightly depleted in light REE, was emplaced on the flank of the ridge. The third melt formed cross-cutting dikes with REE abundances typical of N-type (strongly light REE depleted) mid-ocean ridge basalts.  相似文献   

12.
鄂西黄陵背斜南部元古宙庙湾蛇绿岩的发现及其构造意义   总被引：2，自引：1，他引：1  
对鄂西黄陵背斜南部宜昌太平溪、邓村一带崆岭岩群中的元古宙庙湾岩组强烈变形变质超镁铁—镁铁质岩的研究表明,镁铁质岩主要为似层状细粒斜长角闪岩,变辉长岩岩体、岩脉及辉绿岩岩脉,超镁铁质岩则主要为蛇纹石化纯橄榄岩、方辉橄榄岩,呈构造岩片、岩块分布于斜长角闪岩之中。细粒斜长角闪岩TiO2=1.14%～1.48%,稀土元素配分型式为略亏损—平坦型,无明显的Eu异常,(La/Yb)N=0.87～1.12,La/Nb、Ce/Zr、Zr/Nb、Zr/Y、Ti/Y平均值分别为1.04、0.15、18.78、2.53、290.51,Nb/Th平均为9.88,显示为大洋中脊构造环境形成的N-MORB型拉斑玄武岩;变辉长岩具典型的堆晶结构特征,稀土元素配分型式为平坦型,具明显的Eu正异常;蛇纹石化纯橄榄岩的稀土元素配分型式具中稀土元素略亏损的U形特征,显示为LREE略富集的地幔岩。上述特征表明,黄陵背斜南部崆岭岩群中的元古宙庙湾岩组实际上是一套混杂堆积的古大洋蛇绿岩残片。元古宙庙湾蛇绿岩的发现为华南扬子克拉通存在中元古代洋盆和哥伦比亚超大陆聚合、裂解构造事件提供了重要的证据。  相似文献   

13.
南澜沧江带南段原特提斯岛弧火山岩新证据   总被引：1，自引：0，他引：1  
从岩相学特征、岩石组合、岩石化学、稀土元素、微量元素、铅同位素、大地构造环境等诸方面对云南惠民-曼来地区前泥盆纪(Pt3)变质火山岩的特征进行研究,结果表明:该地区变质火山岩保留有变余间粒及变余安山结构;火山岩组合为玄武岩-安山岩-英安岩;火山岩的化学成分:基性—中性—中酸性,属亚碱性,钙碱性系列与拉斑系列并存,以低TiO2为特征;稀土模式为轻稀土富集右倾斜型;微量元素大阳离子元素富集,且普遍富Th,亏损Ti,部分亏损Cr;铅同位素组成属于冈瓦纳型;岩石化学投点绝大部分落在岛弧火山岩区,为该区原特提斯岛弧火山岩的存在提供了新证据。  相似文献   

14.
在西藏冈底斯山以北、唐古拉山以南地区,分布着侏罗纪到白垩纪的基性和中性火山岩。它们与本区的超基性岩空间上接近,或者呈断层接触。对这些岩石组合的成因有不同的解释。有人认为是大陆碰撞缝合带的蛇绿岩(常承法与郑锡澜,1974),有人认为是岛弧与边缘海的产物。近来,张旗(1983)在丁青发现了比较连续的蛇绿岩组合。要正确地对这些岩石的构造环境进行解释,必须进行详细的岩石学、岩石化学和微量元素地球化学研究。本文对采自拉弄沟、罗布中(图1)两地的玄武岩和采自切里湖的安山岩的岩石化学和微量元素地球化学做了初步研究,这里将初步结果及讨论作一报道。  相似文献   

15.
The ultramafic rocks of the Kan block, East Sayan, are confined mainly to the Idar greenstone belt. In terms of formational affiliation, they are subdivided into two groups: magmatic (Kingash Complex) and residual (Idar Complex) ones. The magmatic ultramafic rocks compose hypabyssal and subvolcanic bodies, which are represented by rocks of dunite-wehrlite-picrite association with cumulate textures. Uninterrupted chemical variations of the magmatic ultramafic rocks indicate subsequent magmatic differentiation of parental picritic melt in the intermediate deep-seated chambers and emplacement of its derivatives in the crystallization site. Differentiation leads to proportional increase of all rare-earth and other incompatible elements. The residual ultramafics occur as boudined dunite-harzburgite bodies showing metamorphic granoblastic textures. They have more homogenous chemical composition close to those of ophiolite complexes, which represent strongly depleted mantle rocks brought to the upper lithospheric levels via deep-seated thrusts. Residual ultramafics differ from magmatic rocks in notably lower contents of some trace and rare earth elements.  相似文献   

16.
The Red Hills peridotite in the Dun Mountain ophiolite of SouthIsland, New Zealand, is assumed to have been produced in a paleo-mid-oceanridge tectonic setting. The peridotite is composed mostly ofharzburgite and dunite, which represent residual mantle andthe Moho transition zone (MTZ), respectively. Dunite channelswithin harzburgite blocks of various scales represent the MTZcomponent. Plagioclase- and clinopyroxene-bearing dunites occursporadically within common dunites. These dunites representproducts of melt–wall-rock interaction. Chondrite-normalizedrare earth element (REE) patterns of MTZ clinopyroxenes showa wide compositional range. Clinopyroxenes in plagioclase dunitesare extremely depleted in light REE (LREE) ([Lu/La]N >100),and are comparable with clinopyroxenes in abyssal peridotitesfrom normal mid-ocean ridges. Interstitial clinopyroxenes inthe common dunite have flatter patterns ([Lu/La]N 2) comparablewith those for dunite in the Oman ophiolite. Clinopyroxenesin the lower part of the residual mantle harzburgites are evenmore strongly depleted in LREE ([Lu/La]N = 100–1000) thanare mid-ocean ridge peridotites, and rival the most depletedabyssal clinopyroxenes reported from the Bouvet hotspot. Incontrast, those in the uppermost residual mantle harzburgiteand harzburgite blocks in the MTZ are less LREE depleted ([Lu/La]N= 10–100), and are similar to those in plagioclase dunite.Clinopyroxenes in the clinopyroxene dunite in the MTZ are similarto those reported from mid-ocean ridge basalt (MORB) cumulates,and clinopyroxenes in the gabbroic rocks have compositions similarto those reported from MORB. Strong LREE and middle REE (MREE)depletion in clinopyroxenes in the harzburgite suggests thatthe harzburgites are residues of two-stage fractional melting,which operated initially in the garnet field, and subsequentlycontinued in the spinel lherzolite field. The early stage meltingproduced the depleted harzburgite. The later stage melting wasresponsible for the gabbroic rocks and dunite. Strongly LREE–MREE-depletedclinopyroxene in the lower harzburgite and HREE-enriched clinopyroxenein the upper harzburgite and plagioclase dunite were formedby later reactive melt migration occurring in the harzburgite. KEY WORDS: clinopyroxene REE geochemistry; Dun Mountain ophiolite; Moho transition zone; orogenic peridotite; Red Hills  相似文献   

17.
Summary Polymetamorphic ultramafic rocks in orogenic terranes rarely preserve relic structures or minerals from their former mantle stages. The determination of their protoliths and their tectonic evolution by chemical discrimination methods is often difficult due to possible metasomatic processes. Ultramafics of the pre-Variscan Helvetic basement (Central Alps, Switzerland) have been investigated geochemically to address these problems. These ultramafics are partially to completely serpentinised. According to field observations several ultramafic lenses were part of an ophiolite suite, but distinct cumulate ultramafic lenses were also recognized. CIPW norms indicate that large parts of the ultramafics are harzburgites, but metasomatic CaO depletion may have produced an overestimation of the importance of the harzburgite protoliths. Major element distributions suggest a depleted mantle protolith. Close to chondritic or slightly depleted REE patterns are characteristics of the studied samples. The REE normalized patterns confirm the presence of harzburgites, lherzolites und cumulates. In some samples light REE enrichment processes have occurred. The noble metal concentrations are both affected (Pt-Pd-Au) und unaffected (Ir-Os-Ru) by melt infiltration processes. They suggest the presence of undepleted or slightly enriched harzburgites und more differentiated, probably cumulate ultramafics. Information obtained by different chemical elements leads to contrasting results. REE and noble metals show enrichment inconsistent with the major element depletion. Refertilization of depleted ultramafics is proposed.
Geochemie von polymetamorphen Ultramafiten (Haupt-, Spuren-Elemente, Edelmetalle und Seltene Erden): Ein Beispiel vom Helvetischen Grundgebirge (Zentralalpen, Schweiz)
Zusarnmenfassung Ultramafische Gesteine aus polymetamorphen orogenetischen Terrains enthalten selten Strukturen und Mineralien als Relikte aus ihren Mantelstadien. Bestimmung ihrer Protolithe und deren tektonischen Entwicklung durch geochemische Parameter wird durch metasomatische Prozesse erschwert. Ultramafite aus dem prävariszischen Sockel der Schweizer Zentralalpen wurden geochemisch untersucht, um die Aussagekraft dieser Parameter zu bestimmen. Die untersuchten Gesteine sind teilweise bis völlig serpentinisiert. Aufgrund der Felduntersuchungen wurden die Ultramafite als Teile einer ophiolithischen Suite klassifiziert, aber ultramafische Kumulate anderer Entstehungsgeschichte konnten unterschieden wurden. CIPW-Norm Berechnungen zeigen, daß Harzburgite den größten Anteil der Proben ausmachen, aber metasomatische Abreicherung an Caokönnte die Bedeutung der Harzburgite als Protolithe überbewerten. Haupt- und Spurenelemente weisen auf einen abgereicherten Mantel als Ausgangsgesteinhin. Fast chondritische bzw. leicht abgereicherte Seltene Erden verteilungsmuster sind charakteristisch. Sic belegen auch die harzburgitische, lherzolithische bzw. kumulative Natur der untersuchten Gesteine. Die Edelmetallkonzentrationen sind teilweise primär (Ir, Os, Ru) sowie beeinflußt durch Schmelzinfiltrationen (Pr, Pd und Au). Die Verteilungsmuster der Edelmetalle bezeugen die Natur des abgereicherten Mantels sowie die Gegenwart von höher differenzierten—vermutlich kumulativen—Gesteinen. Die Hinweise aus den geochemischen Untersuchungen führen zu konsistenten Ergebnissen. Die Anreicherung der Seltenen Erden und Edelmetalle ist mit der verarmten Signatur der Hauptelemente nicht vereinbar, daher wird eine sekundäre Elementanreicherung durch Schmelzinfiltration der Ultramafite vorgeschlagen.
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18.
Petrographic and geochemical studies of peridotites and melagabbros from the Maures massif (SE France) provide new constraints on the Early Palaeozoic evolution of the continental lithosphere in Western Europe. Peridotites occur as lenses along a unit rooted in the main Variscan suture zone. They are dominantly spinel peridotites and minor garnet–spinel peridotites. Spinel peridotites represent both residual mantle and ultramafic cumulates. Mantle-related dunites and harzburgites display high temperature textures, with olivine (Mg#0.90), orthopyroxene (Mg#0.90) and spinel (TiO2 < 0.2%; Cr#0.64–0.83) compositions typical of fore-arc upper mantle. Ultramafic cumulates are dunite adcumulates, harzburgite heteradcumulates and mesocumulates, melagabbro heteradcumulates and amphibole peridotites, with olivine (Mg#0.85–0.89), orthopyroxene (Mg#0.86–0.89) and Cr-spinel (TiO2 = 0.5–3.3%; Cr#0.7–0.98) compositions typical of ultramafic cumulates. Cr-spinel compositions of both spinel peridotite types suggest their genesis in a supra-subduction zone lithosphere. Core to rim zoning in spinel is related to the incomplete influence of regional metamorphism and serpentinisation. The covariation of major and minor elements with Al2O3 for cumulates is consistent with igneous processes involving crystal accumulation. Both mantle and cumulate dunites and harzburgites have U-shaped REE patterns and extremely low trace element contents, similar to peridotites from modern fore-arc peridotites (South Atlantic) and from ophiolites related to supra-subduction zones (Semail, Cyclops, Pindos, Troodos). Melagabbros also have U-shaped REE patterns similar to xenoliths from the Philippine island arc, but also similar to intrusive ultramafic cumulates from the Semail nappe of Oman related to a proto-subduction setting. A wehrlite has a REE pattern similar to that of amphibole peridotites reflecting metasomatism of clinopyroxene-bearing peridotites due to subduction-related fluids. The Maures spinel peridotites and melagabbros are therefore interpreted as the lowermost parts of a crustal sequence and minor residual mantle of lithosphere generated in a supra-subduction zone during Early Palaeozoic time. Garnet–spinel peridotites are chemically close to melagabbros, but have recorded high pressure metamorphism before their retrogression similar to spinel peridotites into amphibolites to greenschists facies metamorphism. They indicate burial to mantle depths of the margin of the supra-subduction lithosphere during the Early Palaeozoic continental subduction. Both peridotite types were exhumed during the Upper Palaeozoic continental collision. Comparable observations from other Variscan-related peridotites, in particular of the Speik complex of the Autroalpine basement, and a common age for the subduction stage allow extension of these regional conclusions to a broad area sharing the Cambrian suture zone, extending from the Ossa-Morena to the Bohemian massif.  相似文献   

19.
The western flank of the Paleoproterozoic Imandra-Varzuga rift zone consists of three volcanogenic-sedimentary series and layered mafic-ultramafic intrusions of different age (2.50–2.45 Ga). The earliest Monchegorsk and Monche Tundra layered massifs were formed about 2.50 Ga during the prerift stage of the evolution of the Imandra-Varzuga zone. The early rift stage (～2.45 Ga) produced layered intrusions of the Imandra complex and volcanic rocks of the Strelna Group, consisting of the Kuksha and Seidorechka formations. In terms of chemical composition, the volcanic rocks of the Seidorechka Formation belong to a single basalt-rhyolite series, mostly of normal alkalinity and both tholeiitic and calc-alkaline affinity. The rocks of the Imandra Complex are characterized by moderate LREE enrichment, relatively flat HREE patterns, and a positive Eu anomaly. Similar REE distribution patterns were observed in the volcanic rocks of the Seidorechka Formation, which show a gradual increase in REE content with increasing SiO2. The upper part of the Seidorechka Formation in the southern Khibiny region is composed of metarhyodacites. They terminate the sequence of the Strelna Group and have a U-Pb zircon age of 2448 ± 8 Ma. This age presumably reflects the upper age boundary of the rocks of the Seidorechka Formation and the end of the early stage of the evolution of the Imandra-Varzuga zone. Xenogenic zircon from the same sample yielded a U-Pb zircon age of 2715 ± 42 Ma. A U-Pb age of 2202 ± 17 Ma was obtained for titanite and rutile and interpreted as the metamorphic age of the Seidorechka Formation. The metavolcanic rocks of the Seidorechka Formation have negative ?Nd (T) varying from ?2.84 to ?2.32, and ISr values of 0.7041–0.7038, which are higher than those of the depleted mantle and suggest their derivation from an enriched mantle reservoir (EM1). The spatial association of the volcanic rocks of the Seidorechka Formation and the rocks of the Imandra Complex, similarity in the behavior of most major elements, similar REE distribution patterns, and close formation ages and isotope signatures give grounds to combine them in a single volcanoplutonic association.  相似文献   

20.
Kazuhito Ozawa 《Lithos》1983,16(1):1-16
The Miyamori ultramafic complex forms the basal ultramafic portion of an ophiolite. The complex consists of a tectonic member which is composed dominantly of harzburgite and dunite, and a cumulate member which is composed of interlayered wehrlite, dunite and clinopyroxenite. The tectonite member is overlain by the cumulate member and characterized by tabular granular or porphyroclastic textures, a strong lineation and magnesian olivine (Mg/Mg + Fe = 0.88–0.93). In contrast, the cumulate member exhibits igneous textures and shows no evidence of a penetrative deformation. The olivine is less magnesian than that of the tectonite member (Mg/Mg + Fe = 0.82–0.89). At the boundary of the two members, harzburgite xenoliths have been found in wehrlite of the cumulate member. The minerals at the core of a few large harzburgite xenoliths preserve the compositional characteristics of typical harzburgites in the tectonic member. The occurrence of the harzburgite xenolith in vehrlite and the structural and textural features of the two members indicate that the tectonite member had already been deformed before a magma intruded into the tectonite member and formed a magma chamber in which cumulates were deposited together with harzburgite fragments on the floor of the tectonite. The xenoliths show a fine grained mosaic texture, which may be attributed by the heat of the intruded magma. This hiatus implies that the magma which made the cumulate member did not originate directly from the underlying harzburgite.  相似文献   

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