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1.
The compositions of minerals and whole rocks of the Luobusa ophiolite in South Tibet, a fragment of Neo‐Tethyan forearc lithosphere, is used to investigate the magmatic evolution of nascent mantle wedges in newly‐initiated subduction zones. Clinopyroxenes in the Luobusa peridotites all have diopsidic compositions, and their Al2O3 contents vary from ～ 2% in the dunites and refractory harzburgites to 2‐4% in the cpx‐bearing harzburgites. The REE of clinopyroxenes in the harzburgites have left‐sloping patterns with contents comparable to those in abyssal peridotites that have experienced 5‐15% partial melting. Chromites in the Luobusa chromitites have the highest Cr#s (～ 80) and TiO2 contents (0.1‐0.2%), and those in the cpx‐bearing harzburgites have the lowest Cr#s (20‐60) and TiO2 contents (0‐0.1%), whereas those in refractory harzburgites and dunites have intermediate compositions. Cpx‐bearing and refractory harzburgites show spoon‐and U‐shaped REE patterns, respectively, and their HREE distribution patterns suggest at least 15%‐ 20% partial melting. The REE patterns of dunites and high‐Cr chromitites vary from spoon‐ to U‐shaped and require 15‐30% partial melting in their mantle sources to produce their parental melts. Our dataset reveals that the nascent Luobusa mantle wedge was first infiltrated by slab‐derived fluids and later refertilized by transitional lava‐like melts, resulting in cpx‐bearing harzburgites. Partial melting in the deeper cpx‐bearing mantle generated high‐Ca boninitic to arc picritic melts, which interacted with the peridotites in the uppermost mantle to generate high‐Cr chromitites, dunites and some refractory harzburgites. Lithological variation from cpx‐bearing to refractory harzburgites in forearc ophiolites is the result of multi‐stage melt events rather than increasing degrees of partial melting. Intermittent slab rollback during subduction initiation induces asthenospheric upwelling and high heat flux in nascent mantle wedges. Elevated geothermal gradients play a more important role than slab dehydration in triggering Mg‐rich magmatism in newly‐initiated subduction zones.  相似文献   

2.
This study focuses on the origin of the Os isotope heterogeneities and the behaviour of Os and Re during melt percolation and partial melting processes in the mantle sequence of the Troodos Ophiolite Complex. The sequence has been divided into an eastern (Unit 1) and a western part (Unit 2) (Batanova and Sobolev, 2000). Unit 1 consists mainly of spinel-lherzolites and a minor amount of dunites, which are surrounded by cpx-bearing harzburgites. Unit 2 consists of harzburgites, dunites, and contains chromitite deposits.Unit 1 (187Os/188Os: 0.1169 to 0.1366) and Unit 2 (187Os/188Os 0.1235 to 0.1546) peridotites both show large ranges in their Os isotopic composition. Most of the 187Os/188Os ratios of Unit 1 lherzolites and harzburgites are chondritic to subchondritic, and this can be explained by Re depletion during ancient partial melting and melt percolation events. The old Os isotope model ages (>800 Ma) of some peridotites in a young ophiolitic mantle show that ancient Os isotopic heterogeneities can survive in the Earth upper mantle. Most harzburgites and dunites of Unit 2 have suprachondritic 187Os/188Os ratios. This is the result of the addition of radiogenic Os during a younger major melt percolation event, which probably occurred during the formation of the Troodos crust 90 Ma ago.Osmium concentrations tend to decrease from spinel-lherzolites (4.35 ± 0.2 ng/g) to harzburgites (Unit 1: 4.06 ± 1.12 ng/g; Unit 2: 3.46 ± 1.38 ng/g) and dunites (Unit 1: 2.71 ± 0.84 ng/g; Unit 2: 1.85 ± 1.20 ng/g). Therefore, this element does not behave compatibly during melt percolation as it is observed during partial melting, but becomes dissolved and mobilized by the percolating melt. The Os contents and Re/Os ratios in the mantle peridotites can be explained if they represent mixing products of old depleted mantle with cpx- and opx-veins, which are crystallization products of the percolating melt. This mixing occurred during the melting of a continuously fluxed mantle in a supra-subduction zone environment.This study shows that Unit 1 and Unit 2 of the Troodos mantle section have a complex and different evolution. However, the Os isotopic characteristics are consistent with a model where the harzburgites and dunites of both units belong to the same melting regime producing the Troodos oceanic crust.  相似文献   

3.
The Qinling Orogenic Belt was formed by subduction and collision between the North and South China Blocks along the Shangdan suture. The Songshugou ultramafic massif located on the northern side of the Shangdan suture provides essential insights into the mantle origin and evolutionary processes during spreading and subduction of the Shangdan oceanic lithosphere. The ultramafic massif comprises harzburgite, coarse- and fine-grained dunites. The spinels from harzburgite exhibit low Cr# and high Mg# numbers, suggesting a mid-ocean ridge peridotite origin, whereas spinels from both coarse- and fine-grained dunites are indicated as resulted from melt-rock reaction due to their systematic higher Cr# and low Mg# numbers. This melt-rock reaction in the dunites is also indicated by the low TiO2 (mostly <0.4 wt%) in the spinel and high Fo (90–92) in olivines. Due to its relatively homogeneous nature in the mantle, oxygen isotopic composition is a sensitive indicator for the petrogenesis and tectonic setting of the Songshugou ultramafic rocks. Based on in-situ oxygen isotope analyses of olivines from twenty-six rock samples, most harzburgites from the Songshugou ultramafic massif show low δ18O values of 4.54–5.30‰, suggesting the olivines are equilibrium with N-MORB magmas and originally formed in a mid-ocean ridge setting. The coarse- and fine-grained dunites exhibit slightly higher olivine δ18O values of 4.69–6.00‰ and 5.00–6.11‰, respectively, suggesting they may have been modified by subduction-related boninitic melt-rock reaction. The δ18O values of olivines systematically increasing from the harzburgites, to coarse-grained dunites and fine-grained dunites may suggest enhancing of melt-rock reaction. The decreasing of Os concentration, 187Re/188Os and 187Os/188Os ratios from harzburgite to dunite suggest an 187Os-enriched, subduction zone melt was responsible for creating the melt channel for melt-rock reactions. Together with the high-temperature ductile deformation microstructures, these isotopic and mineral geochemical features suggest that the harzburgites represent mantle residues after partial melting at mid-ocean ridge or supra-subduction zone, while the dunites were probably resulted from reactions between boninitic melt and harzburgites in a supra-subduction zone. Re-Os geochronology yields a maximum Re depletion model age (TRD) of 805 Ma, constraining the minimum formation age of the harzburgites derived from oceanic mantle. Eight samples of whole rock and chromite yield a Re-Os isochron age of 500 ± 120 Ma, constraining the timing of melt-rock reactions. Combined with the regional geology and our previous investigations, the Songshugou ultramafic rocks favors a mantle origin at mid-ocean ridge before 805 Ma, and were modified by boninitic melt percolations in a SSZ setting at ca. 500 Ma. This long-term tectonic process from spreading to subduction might imply a huge Pan-Tethyan ocean between the Laurasia (e.g., North China Block) and Gondwana (e.g., South China Block) and/or a one-side subduction.  相似文献   

4.
Podiform chromitites are diagnostic but rare features of Phanerozoic ophiolites, and often contain the most pristine textural, chemical and isotopic record of convective upper mantle conditions extant during ophiolite genesis. Ophiolitic podiform chromitites, owing to their high Os concentrations and low Re/Os ratios provide the best evidence for the Os-isotopic evolution of oceanic mantle, but established records of ophiolitic chromites from bona fide Archean ophiolites are still lacking. We report Re–Os isotopic compositions of the world's oldest known ophiolitic podiform chromites from the 2.5 billion year old Dongwanzi–Zunhua ophiolite, North China. This provides the oldest Os isotope composition for the convective upper mantle yet obtained from ophiolitic podiform chromitites, and reveals a chondritic Os isotopic composition of the Archean convective upper mantle.  相似文献   

5.
We present the whole-rock and the mineral chemical data for upper mantle peridotites from the Harmanc?k region in NW Turkey and discuss their petrogenetic–tectonic origin. These peridotites are part of a Tethyan ophiolite belt occurring along the ?zmir-Ankara-Ercincan suture zone in northern Turkey, and include depleted lherzolites and refractory harzburgites. The Al2O3 contents in orthopyroxene and clinopyroxene from the depleted lherzolite are high, and the Cr-number in the coexisting spinel is low falling within the abyssal field. However, the orthopyroxene and clinopyroxene in the harzburgites have lower Al2O3 contents for a given Cr-number of spinel, and plot within the lower end of the abyssal field. The whole-rock geochemical and the mineral chemistry data imply that the Harmanc?k peridotites formed by different degrees of partial melting (~%10–27) of the mantle. The depleted lherzolite samples have higher MREE and HREE abundances than the harzburgitic peridotites, showing convex-downward patterns. These peridotites represent up to ~16 % melting residue that formed during the initial seafloor spreading stage of the Northern Neotethys. On the other hand, the more refractory harzburgites represent residues after ~4–11 % hydrous partial melting of the previously depleted MOR mantle, which was metasomatized by slab-derived fluids during the early stages of subduction. The Harmanc?k peridotites, hence, represent the fragments of upper mantle rocks that formed during different stages of the tectonic evolution of the Tethyan oceanic lithosphere in Northern Neotethys. We infer that the multi-stage melting history of the Harmanc?k peridotites reflect the geochemically heterogeneous character of the Tethyan oceanic lithosphere currently exposed along the ?zmir-Ankara-Erzincan suture zone.  相似文献   

6.
Here we present Sm-Nd, Re-Os, and Pb isotopic data of carefully screened, least altered samples of boninite-like metabasalts from the Isua Supracrustal Belt (ISB, W Greenland)that characterize their mantle source at the time of their formation. The principal observations of this study are that by 3.7-3.8 Ga melt source regions existed in the upper mantle with complicated enrichment/depletion histories. Sm-Nd isotopic data define a correlation line with a slope corresponding to an age of 3.69 ± 0.18 Gy and an initial εNd value of +2.0 ± 4.7. This Sm-Nd age is consistent with indirect (but more precise) U-Pb geochronological estimates for their formation between 3.69-3.71 Ga. Relying on the maximum formation age of 3.71 Gy defined by the external age constraints, we calculate an average εNd [T = 3.71 Ga] value of +2.2 ± 0.9 (n = 18, 1σ) for these samples, which is indicative of a strongly depleted mantle source. This is consistent with the high Os concentrations, falling in the range between 1.9-3.4 ppb, which is similar to the estimated Os concentration for the primitive upper mantle. Re-Os isotopic data (excluding three outliers) yield an isochron defining an age of 3.76 ± 0.09 Gy (with an initial γOs value of 3.9 ± 1.2), within error consistent with the Sm-Nd age and the indirect U-Pb age estimates. An average initial γOs [T = 3.71 Ga] value of + 4.4 ± 1.2 (n = 8; 2σ) is indicative of enrichment of their source region during, or prior to, its melting. Thus, this study provides the first observation of an early Archean upper mantle domain with a distinctly radiogenic Os isotopic signature. This requires a mixing component characterized by time-integrated suprachondritic Re/Os evolution and a Os concentration high enough to strongly affect the Os budget of the mantle source; modern sediments, recycled basaltic crust, or the outer core do not constitute suitable candidates. At this point, the nature of the mantle or crustal component responsible for the radiogenic Os isotopic signature is not known.Compared with the Sm-Nd and Re-Os isotope systems, the Pb isotope systematics show evidence for substantial perturbation by postformational hydrothermal-metasomatic alteration processes accompanying an early Archean metamorphic event at 3510 ± 65 Ma and indicate that the U-Th-Pb system was partially opened to Pb-loss on a whole rock scale. Single stage mantle evolution models fail to provide a solution to the Pb isotopic data, which requires that a high-μ component was mixed with the depleted mantle component before or during the extrusion of the basalts. Relatively high 207Pb/204Pb ratios (compared to contemporaneous mantle), support the hypothesis that erosion products of the ancient terrestrial protocrust existed for several hundred My before recycling into the mantle before ∼3.7 Ga.Our results are broadly consistent with models favoring a time-integrated Hadean history of mantle depletion and with the existence of an early Hadean protocrust, the complement to the Hadean depleted mantle, which after establishment of subduction-like processes was, at least locally, recycled into the upper mantle before 3.7 Ga. Thus, already in the Hadean, the upper mantle seems to be characterized by geochemical heterogeneity on a range of length scales; one property that is shared with the modern upper mantle. However, a simple two component mixing scenario between depleted mantle and an enriched-, crustal component with a modern analogue can not account for the complicated and contradictory geochemical properties of this particular Hadean upper mantle source.  相似文献   

7.
Ophiolites exposed across the western Tauride Belt in SW Turkey represent tectonically emplaced fragments of oceanic lithosphere incorporated into continental margin following the closure of the Neotethys Ocean during the Late Cretaceous. The mantle sections of the ophiolites contain peridotites with diverse suites of geochemical signatures indicative of residual origin by melt depletion in both mid‐ocean ridge (MOR) and supra‐subduction zone (SSZ) settings. This study uses a laser‐ablation inductively‐coupled plasma‐mass spectrometry (LA‐ICP‐MS) for in situ measurements of trace elements in primary mantle phases in order to identify the upper mantle petrogenetic processes effective during variable stage of melt extraction in these discrete tectonic settings and to discriminate between the effects of reaction with chemically distinct mantle melts migrating through the solid residues. Trace element signatures in pyroxenes suggest small‐length scales of compositional variations which may be interpreted to be a result of post‐melting petrogenetic processes. Relative distribution of rare earth elements and Li between coexisting orthopyroxene‐clinopyroxene pairs in the peridotites suggests compositional disequilibrium in sub‐solidus conditions, which possibly reflects differential effects of diffusive exchange during melting and melt transport or interaction with subduction melts/fluids. On the basis of Ga abundances and Ga–Ti–Fe+3# [Fe+3/(Fe+3 + Cr + Al)] relationships of chrome‐spinels it is documented that the peridotites have experienced the combined effects of partial melting and variable extent of melt‐solid interaction. The MOR peridotites have spinels with geochemical signatures indicative of melt‐depleted residual origin with subsequent incompatible element enrichment through melt impregnation, while the Ga–Ti–Fe+3# relationships of chrome‐spinels in SSZ peridotites indicate that these highly depleted peridotites are not simple melt residues, but have been subject to significant compositional modification by interaction with subduction related melts/fluids. The observed compositional variations, which are related to long‐term tectonic reorganisation of oceanic lithosphere, provide evidence for a time integrated evolution from a mid‐ocean ridge to a supra‐subduction zone setting and may be a possible analogue to explain the coexistence of geochemically diverse MOR–SSZ suites in other Tethyan ophiolites. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

8.
Mesozoic ophiolites crop out discontinuously in the Indo‐Myanmar Ranges in NE India and Myanmar, and represent the remnants of the Neotethyan oceanic lithosphere. These ophiolites in the Indo‐Myanmar Ranges are the southern continuation of the Neotethyan ophiolites occurring along the Yarlung Zangbo Suture Zone in southern Tibet farther northwes, as indicated by their coeval crystallization ages and geochemical compositions. The Kalaymyo ophiolite is located in the central part of the Indo‐Myanmar Ranges (Myanmar). The Kalaymyo ophiolite are composed of olivine (Fo = 89.8–90.5), orthopyroxene (En86‐91Wo1‐4Fs8‐10; Mg#=89.6–91.9), clinopyroxene (En46‐49Wo47‐50Fs3‐5; Mg# = 90.9–93.6) and spinel (Mg# = 67.1–78.9; Cr# = 13.5–31.5), and have relatively homogeneous whole‐rock compositions with Mg# of 90.1–90.8 and SiO2 (41.5–43.65 wt.%), Al2O3 (1.66–2.66 wt.%) and CaO (1.45–2.67 wt.%) contents. They display Light Rare Earth Element (LREE)‐depleted chondrite‐normalized REE patterns and show a slight enrichment from Pr to La. The Kalaymyo peridotites are characterized by Pd‐enriched chondrite‐normalized PGE patterns with superchondritic (Pd/Ir)CN ratios (1.15–2.36). Their calculated oxygen fugacities range between QFM–0.57 and QFM+0.90. These features collectively suggest that the Kalaymyo peridotites represent residual upper mantle rocks after low to moderate degrees (5–15%) of partial melting at a mid‐ocean‐ridge environment. The observed enrichment in LREE and Pd was a result of their reactions with enriched MORB‐like melts, percolating through these already depleted, residual peridotites. The Kalaymyo and other ophiolites in the Indo‐Myanmar Ranges hence represent mid‐ocean ridge–type Tethyan oceanic lithosphere derived from a downgoing plate and accreted into a westward migrating subduction–accretion system along the eastern margin of India.  相似文献   

9.
西藏北部班公湖MOR型蛇绿岩主要由角砾状的地幔橄榄岩和玄武岩组成,其中地幔橄榄岩主要是低Cr#尖晶石相含单斜辉石(Cpx)方辉橄榄岩和少量不含Cpx的方辉橄榄岩,玄武岩具有MORB地球化学特点。岩石地球化学特征和二元混合模拟计算表明,含Cpx方辉橄榄岩是由较为亏损的方辉橄榄岩与玄武质熔体发生反应再富集形成的,玄武质熔体和方辉橄榄岩的混合比例为1∶9至1∶4。9个含Cpx方辉橄榄岩样品(含5个重复测试样)的Re和Os含量分别为0.19×10-9~1.49×10-9和2.91×10-9~5.40×10-9,187Re/188Os变化范围为0.169±0.009(2σ)~1.833±0.183(2σ),187Os/188Os变化范围相对较小,介于0.121 13±0.000 44~0.128 53±0.000 36(2σ)之间。含Cpx方辉橄榄岩的Re-Os参考等时线年龄为254±28 Ma。由于不同比例熔体的加入造成橄榄岩具有不同的Re/Os比值,因而不同含Cpx方辉橄榄岩样品具有不同187Os/188Os比值。样品的Re含量与烧失量中的H2O没有相关性,说明蛇纹石化对样品Re-Os体系的影响可以忽略,Re-Os同位素体系在低温地质作用下能够保持相对封闭。参考等时线年龄可能代表亏损方辉橄榄岩与玄武质熔体发生反应的时间,即含Cpx方辉橄榄岩的形成年龄,它表明在该时期特提斯洋经历了一次构造热事件。这一构造岩浆热事件的时间与早期定义的班公湖-怒江特提斯洋的裂解时间晚二叠世至早三叠世较为一致,推测本文MOR型蛇绿岩地幔橄榄岩的Re-Os同位素年龄可能代表班公湖-怒江特提斯洋开始裂解的时间。  相似文献   

10.
The Dangqiong ophiolite, the largest in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)ophiolite belt in southern Tibet, consists of discontinuous mantle peridotite and intrusive mafic rocks. The former is composed dominantly of harzburgite, with minor dunite, locally lherzolite and some dunite containing lenses and veins of chromitite. The latter, mafic dykes(gabbro and diabase dykes), occur mainly in the southern part. This study carried out geochemical analysis on both rocks. The results show that the mantle peridotite has Fo values in olivine from 89.92 to 91.63 and is characterized by low aluminum contents(1.5–4.66 wt%) and high Mg# values(91.06–94.53) of clinopyroxene. Most spinels in the Dangqiong peridotites have typical Mg# values ranging from 61.07 to 72.52, with corresponding Cr# values ranging from 17.67 to 31.66, and have TiO2 contents from 0 to 0.09%, indicating only a low degree of partial melting(10–15%). The olivine-spinel equilibrium and spinel chemistry of the Dangqiong peridotites suggest that they originated deeper mantle(20 kbar). The gabbro dykes show N-MORB-type patterns of REE and trace elements. The presence of amphibole in the Dangqiong gabbro suggests the late-stage alteration of subduction-derived fluids. All the lherzolites and harzburgites in Dangqiong have similar distribution patterns of REE and trace elements, the mineral chemistry in the harzburgites and lherzolites indicates compositions similar to those of abyssal and forearc peridotites, suggesting that the ophiolite in Dangqiong formed in a MOR environment and then was modified by late-stage melts and fluids in a suprasubduction zone(SSZ) setting. This formation process is consistent with that of the Luobusa ophiolite in the eastern Yarlung-Zangbo Suture Zone and Purang ophiolite in the western Yarlung-Zangbo Suture Zone.  相似文献   

11.
Tectonically emplaced peridotites from North Hebei Province, North China Craton, have retained an original harzburgite mineral assemblage of olivine(54%–58%) + orthopyroxene(40%–46%)+minor clinopyroxene(1%)+spinel. Samples with boninite-like chemical compositions also coexist with these peridotites. The spinels within the peridotites have high-Al end-members with Al_2O_3 content of 30 wt % –50 wt %, typical of mantle spinels. When compared with experimentally determined melt extraction trajectories, the harzburgites display a high degree of melting and enrichment of SiO_2, which is typical of cratonic mantle peridotites. The peridotites display variably enriched light rare earth elements(REEs), relatively depleted middle REEs and weakly fractionated heavy REEs, which suggest a melt extraction of over 25% in the spinel stability field. The occurrence of arc-and SSZ-type chromian spinels in the peridotites suggests that melt extraction and metasomatism occurred mostly in a subduction-related setting. This is also supported by the geochemical data of the coexisting boninite-like samples. The peridotites have ~(187)Os/~(188)Os ratios ranging from 0.113–0.122, which is typical of cratonic lithospheric mantle. These ~(187)Os/~(188)Os ratios yield model melt extraction ages(TRD) ranging from 981 Ma to 2054 Ma, which may represent the minimum estimation of the melt extraction age. The Al_2O_3-~(187)Os/~(188)Os-proxy isochron ages of 2.4 Ga–2.7 Ga suggest a mantle melt depletion age between the Late Achaean and Early Paleoproterozoic. Both the peridotites and boninite-like rocks are therefore interpreted as tectonically exhumed continental lithospheric mantle of the North China Craton, which has experienced mantle melt depletion and subduction-related mantle metasomatism during the Neoarchean-Paleoproterozoic.  相似文献   

12.
《Gondwana Research》2015,28(4):1560-1573
We used Os isotopic systematics to assess the geochemical relationship between the lithospheric mantle beneath the Balkans (Mediterranean), ophiolitic peridotites and lavas derived from the lithospheric mantle. In our holistic approach we studied samples of Tertiary post-collisional ultrapotassic lavas sourced within the lithospheric mantle, placer Pt alloys from Vardar ophiolites, peridotites from nearby Othris ophiolites, as well as four mantle xenoliths representative for the composition of the local mantle lithosphere. Our ultimate aim was to monitor lithospheric mantle evolution under the Balkan part of the Alpine-Himalayan belt. The observations made on Os isotope and highly siderophile element (HSE) distributions were complemented with major and trace element data from whole rocks as well as minerals of representative samples. Our starting hypothesis was that the parts of the lithospheric mantle under the Balkans originated by accretion and transformation of oceanic lithosphere similar to ophiolites that crop out at the surface.Both ophiolitic peridotites and lithospheric mantle of the Balkan sector of Alpine-Himalayan belt indicate a presence of a highly depleted mantle component. In the ophiolites and the mantle xenoliths, this component is fingerprinted by the low clinopyroxene (Cpx) contents, low Al2O3 in major mantle minerals, together with a high Cr content in cogenetic Cr-spinel. Lithospheric mantle-derived ultrapotassic melts have high-Fo olivine and Cr-rich spinel that also indicate an ultra-depleted component in their mantle source. Further resemblance is seen in the Os isotopic variation observed in ophiolites and in the Serbian lithospheric mantle. In both mantle types we observed an unusual increase of Os abundances with increase in radiogenic Os that we interpreted as fluid-induced enrichment of a depleted Proterozoic/Archaean precursor. The enriched component had suprachondritic Os isotopic composition and its ultimate source is attributed to the subducting oceanic slab. On the other hand, a source–melt kinship is established between heterogeneously metasomatised lithospheric mantle and lamproitic lavas through a complex vein + wall rock melting relationship, in which the phlogopite-bearing pyroxenitic metasomes with high 187Re/188Os and extremely radiogenic 187Os/188Os > 0.3 are produced by recycling of a component ultimately derived from the continental crust.We tentatively propose a two-stage process connecting lithospheric mantle with ophiolites and lamproites in a geologically reasonable scenario: i) ancient depleted mantle “rafts” representing fragments of lithospheric mantle “recycled” within the convecting mantle during the early stages of the opening of the Tethys ocean and further refertilized, were enriched by a component with suprachondritic Os isotopic compositions in a supra-subduction oceanic environment, probably during subduction initiation that induced ophiolite emplacement in Jurassic times. Fluid-induced partial melts or fluids derived from oceanic crust enriched these peridotites in radiogenic Os; ii) the second stage represents recycling of the melange material that hosts above mantle blocks, but also a continental crust-derived terrigenous component accreted to the mantle wedge, that will later react with each other, producing heterogeneously distributed metasomes; final activation of these metasomes in Tertiary connects the veined lithospheric mantle and lamproites by vein + wall rock partial melting to generate lamproitic melts. Our data are permissive of the view that the part of the lithospheric mantle under the Balkans was formed in an oceanic environment.  相似文献   

13.
西藏泽当—罗布莎蛇绿岩的地球化学特征及其构造意义   总被引：3，自引：0，他引：3  
西藏泽当—罗布莎蛇绿岩是雅鲁藏布江蛇绿岩带的重要组成部分,受控于板块碰撞结合带。该蛇绿岩主要由变质橄榄岩、辉长辉绿岩及玄武岩等组成。变质橄榄岩具有低的A l2O3和CaO含量,以富Mg,贫Ti、∑REE为特征,与世界上典型蛇绿岩中方辉橄榄岩的特征值一致。辉长岩和玄武岩的主量元素、微量元素特征显示其含有洋脊拉斑玄武岩和大洋岛弧拉斑玄武岩的双重成分,其中高场强元素Nb、Ta、Zr、H f等亏损,大离子亲石元素Rb、Sr、Ba等相对富集,具有大洋岛弧玄武岩的特点。辉长岩在球粒陨石标准化稀土元素配分模式图上为LREE亏损的平坦型,无负Eu异常,与洋中脊玄武岩类似。根据上述特征并结合区域地质构造特征,认为泽当?罗布莎蛇绿岩可能形成于边缘海盆地的海底慢速扩张环境。  相似文献   

14.
The microstructures, major‐ and trace‐element compositions of minerals and electron backscattered diffraction (EBSD) maps of high‐ and low‐Cr# [spinel Cr# = Cr3+/(Cr3++Al3+)] chromitites and dunites from the Zedang ophiolite in the Yarlung Zangbo Suture (South Tibet) have been used to reveal their genesis and the related geodynamic processes in the Neo‐Tethyan Ocean. The high‐Cr# (0.77‐0.80) chromitites (with or without diopside exsolution) have chromite compositions consistent with initial crystallization by interaction between boninitic magmas, harzburgite and reaction‐produced magmas in a shallow, mature mantle wedge. Some high‐Cr# chromitites show crystal‐plastic deformation and grain growth on previous chromite relics that have exsolved needles of diopside. These features are similar to those of the Luobusa high‐Cr# chromitites, possibly recycled from the deep upper mantle in a mature subduction system. In contrast, mineralogical, chemical and EBSD features of the Zedang low‐Cr# (0.49‐0.67) chromitites and dunites and the silicate inclusions in chromite indicate that they formed by rapid interaction between forearc basaltic magmas (MORB‐like but with rare subduction input) and the Zedang harzburgites in a dynamically extended, incipient forearc lithosphere. The evidence implies that the high‐Cr# chromitites were produced or emplaced in an earlier mature arc (possibly Jurassic), while the low‐Cr# associations formed in an incipient forearc during the initiation of a new episode of Neo‐Tethyan subduction at ～130‐120 Ma. This two‐episode subduction model can provide a new explanation for the coexistence of high‐ and low‐Cr# chromitites in the same volume of ophiolitic mantle.  相似文献   

15.
错不扎蛇绿岩位于雅鲁藏布江缝合带西段北亚带,岩体呈北西-南东走向带状产出,主要由地幔橄榄岩和辉长岩脉组成。地幔橄榄岩主体为方辉橄榄岩,详细的矿物学及岩石地球化学研究表明,错不扎方辉橄榄岩中橄榄石为镁橄榄石,斜方辉石主要为顽火辉石,而单斜辉石主要为顽透辉石和透辉石,铬尖晶石具有高Al和高Mg(Mg#=60~70)特征。稀土配分图解显示其具有轻稀土亏损而重稀土富集的左倾型亏损地幔源区特征,(La/Yb)N=0.11~0.60,模拟结果显示其为经历了15%~20%部分熔融后的残余,与快速扩张大洋中脊环境下形成的深海橄榄岩的熔融程度(10%~22%)较为一致。此外,错不扎方辉橄榄岩轻稀土含量明显高于部分熔融模型中LREE的含量,而且,在微量元素原始地幔标准化图解中富集大离子亲石元素Rb、Sr和高场强元素Ta、Hf和Ti,这一特征指示错不扎方辉橄榄岩在大洋中脊环境形成后又受到后期俯冲带熔/流体的改造。结合南北两带不同蛇绿岩体构造环境的对比,笔者认为雅鲁藏布江西段南北两带蛇绿岩体具有相似的形成环境,两者在地理位置以及产状方面的差别可能是受到构造侵位的影响。  相似文献   

16.
The Tasmanian dolerites, part of the Ferrar Province of Australia and Antarctica, have some trace‐element and isotopic compositions that suggest continental contamination of mantle‐derived magmas. The debate has centred on whether the contamination occurred during intrusion into the crust, or if the mantle source itself was contaminated. The behaviour of Sr and O isotopes suggests that the mantle source had a δ18O composition of +6‰ and an initial 87Sr/86Sr ratio of 0.709, which supports the latter contention. Recently published Re–Os data likewise dismiss upper crustal contamination: Re–Os isotopic compositions of magnetite‐rich mineral separates from seven Tasmanian dolerites yield an isochron that gives the same age, within uncertainties, as other dating techniques, namely 175 ± 5 Ma. Moreover, Re–Os data from a study of Antarctic Ferrar Province samples lie on the same isochron and the data together give an age of 177.3 ± 3.5 Ma. The initial 187Os/188Os of 0.125 ± 0.033 is the calculated mantle composition at the time. These results support previous models that attribute chemical features of the Ferrar magmas to re‐enrichment of a depleted mantle source region rather than processes involving assimilation of crust by basaltic magma.  相似文献   

17.
The mantle section of Al'Ays ophiolite consists of heterogeneously depleted harzburgites, dunites and large-sized chromitite pods. Two chromitite-bearing sites (Site1 and Site2), about 10 km apart horizontally from one another, were examined for their upper mantle rocks. Cr-spinels from the two sites have different chemistry; Cr-rich in Site1 and Al-rich in Site2. The average Cr-ratio = (Cr/(Cr + Al) atomic ratio) of Cr-spinels in harzburgites, dunites and chromitites is remarkably high 0.78, 0.77 and 0.87, respectively, in Site1, compared with those of Site2 which have intermediate ratio averages 0.5, 0.56 and 0.6, respectively. The platinum-group elements (PGE) in chromitites also show contrasting patterns from Site1 to Site2; having elevated IPGE (Os, Ir, Ru) and strongly depleted in PPGE (Rh, Pt, Pd) with steep negative slopes in the former, and gentle negative slopes in the latter. The oxygen fugacity (Δlog fO2) values deduced from harzburgites and dunites of Site1 show a wide variation under reducing conditions, mostly below the FMQ buffer. The Site2 harzburgites and dunites, on the other hand are mostly above the FMQ buffer. Two magmatic stages are suggested for the lithospheric evolution of Al'Ays ophiolite in response to a switch of tectonic setting. The first stage produced a peridotites–chromitites suite with Al-rich Cr-spinels, possibly beneath a mid-ocean ridge setting, or most likely in back-arc rift of a supra-subduction zone setting. The second stage involved higher degrees of partial melting, produced a peridotites–chromitites suite with Cr-rich Cr-spinels, possibly in a fore-arc setting. The coexistence of compositionally different mantle suites with different melting histories in a restricted area of an ophiolite complex may be attributable to a mechanically juxtaposed by mantle convection during recycling. The mantle harzburgites and dunites are apt to be compositionally modified during recycling process; being highly depleted (Site1 case) than their original composition (Site2 case).  相似文献   

18.
New major- and trace-element data of bulk-rocks and constituent minerals, and whole-rock Re-Os isotopic compositions of samples from the Lherz Massif, French Pyrenees, reveal complex petrological relationships between the dominant lithologies of lherzolite ± olivine-websterite and harzburgite. The Lherz peridotite body contains elongate, foliation parallel, lithological strips of harzburgite, lherzolite, and olivine-websterite cross-cut by later veins of hornblende-bearing pyroxenites. Peridotite lithologies are markedly bimodal, with a clear compositional gap between harzburgites and lherzolites ± olivine-websterite. Bulk-rock and mineral major-element oxide (Mg-Fe-Si-Cr) compositions show that harzburgites are highly-depleted and result from ∼20-25 wt.% melt extraction at pressures <2 GPa. Incompatible and moderately-compatible trace-element abundances of hornblendite-free harzburgites are analogous to some mantle-wedge peridotites. In contrast, lherzolites ± olivine-websterite overlap estimates of primitive mantle composition, yet these materials are composite samples that represent physical mixtures of residual lherzolites and clinopyroxene dominated cumulates equilibrated with a LREE-enriched tholeiitic melt. Trace-element compositions of harzburgite, and some lherzolite bulk-rocks and pyroxenes have been modified by; (1) wide-spread interaction with a low-volume LREE-enriched melt +/− fluid that has disturbed highly-incompatible elements (e.g., LREEs, Zr) without enrichment of alkali- and Ti-contents; and (2) intrusion of relatively recent, small-volume, hornblendite-forming, basanitic melts linked to modal and cryptic metasomatism resulting in whole-rock and pyroxene Ti, Na and MREE enrichment.Rhenium-Os isotope systematics of Lherz samples are also compositionally bimodal; lherzolites ± olivine-websterite have chondritc to suprachondritic 187Os/188Os and 187Re/188Os values that overlap the range reported for Earth’s primitive upper mantle, whereas harzburgites have sub-chondritic 187Os/188Os and 187Re/188Os values. Various Os-model age calculations indicate that harzburgites, lherzolites, and olivine-websterites have been isolated from convective homogenisation since the Meso-Proterozoic and this broadly coincides with the time of melt extraction controlled by harzburgite Os-isotope compositions. The association between harzburgites resulting from melting in mantle-wedge environments and Os-rich trace-phases (laurite-erlichmanite sulphides and Pt-Os-Ir-alloys) suggests that a significant portion of persistent refractory anomalies in the present-day convecting mantle of Earth may be linked to ancient large-scale melting events related to wide-spread subduction-zone processing.  相似文献   

19.
The Zedong ophiolites in the eastern Yarlung–Zangbo suture zone of Tibet represent a mantle slice of more than 45 km~2. This massif consists mainly of mantle peridotites, with lesser gabbros, diabases and volcanic rocks. The mantle peridotites are mostly harzburgite, lherzolite; a few dike-like bodies of dunite are also present. Mineral structures show that the peridotites experienced plastic deformation and partial melting. Olivine(Fo89.7–91.2), orthopyroxene(En_(88–92)), clinopyroxene(En_(45–49) Wo_(47–51) Fs_(2–4)) and spinel [Mg~#=100×Mg/(Mg+Fe)]=49.1–70.7; Cr~#=(100×Cr/(Cr+Al)=18.8–76.5] are the major minerals. The degree of partial melting of mantle peridotites is 10%–40%, indicating that the Zedong mantle peridotites may experience a multi–stage process. The peridotites are characterized by depleted major element compositions and low REE content(0.08–0.62 ppm). Their "spoon–shaped" primitive–mantle normalized REE patterns with(La/Sm)_N being 0.50–6.00 indicate that the Zedong ultramafic rocks belong to depleted residual mantle rocks. The PGE content of Zedong peridotites(18.19–50.74 ppb) is similar with primary mantle with Pd/Ir being 0.54–0.60 and Pt/Pd being 1.09–1.66. The Zedong peridotites have variable, unradiogenic Os isotopic compositions with ~(187)Os/~(188)Os=0.1228 to 0.1282. A corollary to this interpretation is that the convecting upper mantle is heterogeneous in Os isotopes. All data of the Zedong peridotites suggest that they formed originally at a mid-ocean ridge(MOR) and were later modified in supra–subduction zone(SSZ) environment.  相似文献   

20.
熊发挥  杨经绥  高健  来盛民  陈艳虹  张岚 《岩石学报》2016,32(12):3635-3648
泽当蛇绿岩位于雅鲁藏布江缝合带东段,岩体由地幔橄榄岩、辉长辉绿岩、辉石岩、火山岩等组成。地幔橄榄岩主要为方辉橄榄岩、纯橄岩和二辉橄榄岩。在方辉橄榄岩中发现7处豆荚状铬铁矿,矿石类型主要有致密块状和浸染状。出露地表的长度0.5~3m,厚0.2~1m。矿体的延伸方向为北西向,与岩体展布的方向一致,铬铁矿的Cr~#=67.9~88.5,属于高铬型铬铁矿。泽当地幔橄榄岩岩相学特征以及矿物组合、矿物化学成份及岩相学特征,显示岩体至少存在两次的部分熔融过程,即为早期的MOR构造背景,以及后期SSZ环境的改造。铬铁矿的铂族元素(PGE)以富集Os、Rh、Pd,亏损Ir、Ru、Pt的负斜率分布模式,表明其形成过程中经受后期熔体/流体的改造。对比罗布莎岩体的矿物组合,矿物化学和地球化学等特征,显示泽当豆荚状铬铁矿矿体与典型高铬型具相似性,存在较大的找矿空间。  相似文献   

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