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1.
Evidence for a major heating event accompanied by decompressionwas recently reported from crustal rocks drilled in the Alboranbasin. The metamorphic evolution recorded by these rocks impliescomplete removal of lithospheric mantle during the Cenozoic,a process that is confirmed by geophysical modelling indicatingthin lithosphere beneath the Alboran domain. In this region,the Ronda lherzolite massif (Betic Cordillera, southern Spain)provides a unique opportunity for the observation of mantleprocesses associated with lithospheric thinning. A strikingfeature of the Ronda peridotite is a narrow recrystallizationfront, which has been ascribed to kilometre-scale porous meltflow through the massif. The front separates the spinel tectonitedomain, interpreted as old, veined lithospheric mantle, fromthe granular domain where the lithospheric microstructures,mineralogical assemblages and geochemical signatures were obliteratedby grain growth coeval with pervasive infiltration of basalticmelts. On the basis of trace-element abundances in peridotitescollected over a distance of 12 km along the recrystallizationfront, our study confirms that the front is a relatively sharp(  相似文献   

2.
The western part of the Ronda peridotite massif (Southern Spain) consists mainly of highly foliated spinel-peridotite tectonites and undeformed granular peridotites that are separated by a recrystallization front. The spinel tectonites are interpreted as volumes of ancient subcontinental lithospheric mantle and the granular peridotites as a portion of subcontinental lithospheric mantle that underwent partial melting and pervasive percolation of basaltic melts induced by Cenozoic asthenospheric upwelling. The Re–Os isotopic signature of sulfides from the granular domain and the recrystallization front mostly coincides with that of grains in the spinel tectonites. This indicates that the Re–Os radiometric system in sulfides was highly resistant to partial melting and percolation of melts induced by Cenozoic lithospheric thermal erosion. The Re–Os isotopic systematics of sulfides in the Ronda peridotites thus mostly conserve the geochemical memory of ancient magmatic events in the subcontinental lithospheric mantle. Os model ages record two Proterozoic melting episodes at ~1.6 to 1.8 and 1.2–1.4 Ga, respectively. The emplacement of the massif into the subcontinental lithospheric mantle probably coincided with one of these depletion events. A later metasomatic episode caused the precipitation of a new generation of sulfides at ~0.7 to 0.9 Ga. These Proterozoic Os model ages are consistent with results obtained for several mantle suites in Central/Western Europe and Northern Africa as well as with the Nd model ages of the continental crust of these regions. This suggests that the events recorded in mantle sulfides of the Ronda peridotites reflect different stages of generation of the continental crust in the ancient Gondwana supercontinent.  相似文献   

3.
产于法国中央高原堡雷（Boree）的具镶嵌结构方辉橄榄岩包体被认为是大陆活动区碱性玄武岩捕获的、来源最深的尖晶石相上地幔样品。已有的岩浆堆积说和等物理化学环境中重结晶模式难于解释其矿物学、微量元素和Sr－Nd同位素地球化学特征,为此我们提出了热柱来源熔体渗滤岩石圈底部的新成因模式。渗滤熔体和岩石圈地幔之间的反应不仅导致了矿物含量的变化,而且形成了特征的微量元素配分型式和同位素组成。REE模拟计算表明,熔体／岩石比值的大小、熔体性质以及熔体-岩石反应机制的多样性是控制本区幔源包体地球化学及其岩石变形、结晶程度之间相关性的重要因素。  相似文献   

4.
Structural Petrology of the Ronda Peridotite, SW Spain: Deformation History   总被引：3，自引：3，他引：3  
Solid bodies of upper-mantle peridotite, emplaced in the Betic-Rifchains of SW Spain and North Morocco, show a variety of structuresdeveloped under different metamorphic conditions. These structuresand related metamorphism reflect tectonic processes in the WestMediterranean mantle during orogeny in the Betic-Rif realm.The largest of the peridotites, the Ronda massif, has preservedthree structural domains which are spatially associated withmetamorphic domains previously defined by Obata (Journal ofPetrology, 21,533–572, 1980). These structural domainsinclude: (1) porphyroclastic spinel peridotites (spinel tectonites)and mylonitic garnet-spinel peridotites (garnet-spinel mylonites),developed during progressive strain localization at ambientconditions changing from the Arigite subfacies to garnet peridotitefacies; (2) seemingly undeformed granular peridotites, developedduring a phase of extensive recrystallization affecting thespinel tectonites and garnet-spinel mylonites at Seiland subfaciesconditions, and separated from the spinel tectonites by a well-preservedrecrystallization front which forms a marked structural, metamorphicand possibly geochemical boundary probably unique to orogenicperidotites; (3) porphyroclastic plagioclase peridotites (plagioclasetectonites) developed at the expense of the granular peridotitesduring progressive shear localization allied to ductile emplacementof the Ronda massif into the crust. Our structural and microstructural data from the Ronda massifallow us to assess the relative ages of the different metamorphicfacies seen in the West Mediterranean peridotites. In orderof decreasing relative age, these are: Arigite-subfacies, garnetperidotite facies, Seiland subfacies and plagioclase peridotitefacies. In addition, the associated microstructures providesome insight into the microphysical conditions controlling thedevelopment of the different structures and, as a result, thestructural and chemical heterogeneity of the West Mediterraneanperidotites. KEY WORDS: structural geology; peridotite; Betic Cordillera; Ronda; recrystallization; strain localization *Corresponding author. Present address: Philips Electron Optics BV, Applications Laboratory, Building AAE, PO Box 218, 5600 MD Eindhoven, The Netherlands  相似文献   

5.
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   

6.
ABSTRACT

This study examines the major and trace elements of peridotites from the Yap Trench in the western Pacific to investigate mantle evolution beneath a subduction zone. Major element results show that the peridotites are low in Al2O3 (0.31–0.65 wt.%) and CaO (0.04–0.07 wt.%) contents and high in Mg# (Mg/(Mg+Fe)) (0.91–0.92) and have spinels with Cr# (Cr/(Cr+Al)) higher than 0.6 (0.61–0.73). Trace element results show that the peridotites have extremely low heavy rare earth element (HREE) contents compared with abyssal peridotites but have U-shaped chondrite-normalized rare earth element (REE) patterns. The degree of mantle melting estimated based on the major elements, HREEs, and spinel Cr# range from 19% to 25%, indicating that the Yap Trench peridotites may be residues of melting associated with the presence of water in the mantle source. In addition to light rare earth element (LREE) enrichment, the peridotites are characterized by high contents of highly incompatible elements, positive U and Sr anomalies, negative Ti anomalies, and high Zr/Hf ratios. The correlations between these elements and both the degree of serpentinization and high field strength element (HFSE) contents suggest that fluid alteration alone cannot account for the enrichment of the peridotites and that at least the enrichment of LREEs was likely caused by melt–mantle interaction. Comparison between the peridotites and the depletion trend defined by the primitive mantle (PM) and the depleted mantle (DM) suggests that the Yap Trench mantle was modified by subduction-related melt characterized by high contents of incompatible elements, high Zr/Hf ratios, and low HFSE contents. Hydrous melting may have been enhanced by tectonic erosion of the subducting Caroline Plate with complex tectonic morphostructures at the earliest stages of subduction initiation.  相似文献   

7.
The Ronda orogenic peridotite (southern Spain) contains a varietyof pyroxene-rich rocks ranging from high-pressure garnet granulitesand pyroxenites to low-pressure plagioclase–spinel websterites.The ‘asthenospherized’ part of the Ronda peridotitecontains abundant layered websterites (‘group C’pyroxenites), without significant deformation, that occur asswarms of layers showing gradual modal transitions towards theirhost peridotites. Previous studies have suggested that theselayered pyroxenites formed by the replacement of refractoryspinel peridotites. Here, we present a major- and trace-element,and numerical modelling study of a layered outcrop of groupC pyroxenite near the locality of Tolox aimed at constrainingthe origin of these pyroxenites after host peridotites by pervasivepyroxene-producing, refertilization melt–rock reactions.Mg-number [= Mg/(Mg + Fe) cationic ratio] numerical modellingshows that decreasing Mg-number with increasing pyroxene proportion,characteristic of Ronda group C pyroxenites, can be accountedfor by a melt-consuming reaction resulting in the formationof mildly evolved, relatively low Mg-number melts (0·65)provided that the melt fraction during reaction and the time-integratedmelt/rock ratio are high enough (>0·1 and > 1,respectively) to balance Mg–Fe buffering by peridotiteminerals. This implies strong melt focusing caused by melt channellingin high-porosity domains resulting from compaction processesin a partial melted lithospheric domain below a solidus isothermrepresented by the Ronda peridotite recrystallization front.The chondrite-normalized rare earth element (REE) patterns ofgroup C whole-rocks and clinopyroxenes are convex-upward. Numericalmodeling of REE variations in clinopyroxene produced by a pyroxene-forming,melt-consuming reaction results in curved trajectories in the(Ce/Nd)N vs (Sm/Yb)N diagram (where N indicates chondrite normalized).Based on (Ce/Nd)N values, two transient, enriched domains betweenthe light REE (LREE)-depleted composition of the initial peridotiteand that of the infiltrated melt may be distinguished in thereaction column: (1) a lower domain characterized by convex-upwardREE patterns similar to those observed in Ronda group C pyroxenite–peridotite;(2) an upper domain characterized by melts with strongly LREE-enrichedcompositions. The latter are probably volatile-rich, small-volumemelt fractions residual after the refertilization reactionsthat generated group C pyroxenites, which migrated throughoutthe massif—including the unmelted lithospheric spinel-tectonitedomain. The Ronda mantle domains affected by pyroxenite- anddunite- or harzburgite-forming reactions (the ‘layeredgranular’ subdomain and ‘plagioclase-tectonite’domain) are on average more fertile than the residual, ‘coarsegranular’ subdomain at the recrystallization front. Thisindicates that refertilization traces the moving boundariesof receding cooling of a thinned and partially melted subcontinentallithosphere. This refertilization process may be widespreadduring transient thinning and melting of depleted subcontinentallithospheric mantle above upwelling asthenospheric mantle. KEY WORDS: subcontinental mantle; refertilization; pyroxenite; peridotite; websterite; melt–rock reaction; plagioclase; trace elements  相似文献   

8.
休古嘎布蛇绿岩块位于雅鲁藏布缝合带(YZSZ)西段,主要由地幔橄榄岩和侵入其中的基性岩墙所组成。基性岩墙具有弧后盆地地球化学亲缘性,其LREE亏损,(La/Yb)N为0.39~0.55;具有明显的Nb、Ti负异常及Sr、Ba正异常。Sr、Nd同位素特征表明它们起源于亏损地幔,并且受到了板片析出流体的影响。4个基性岩墙的Sm-Nd同位素样品获得内部等时线年龄为126.2±9.1Ma(MSWD=0.44)。地幔橄榄岩具有汤勺形和U形两组REE分布型式,显示出不同程度部分熔融和地幔交代作用的特征,具有弧-盆体系地球化学亲缘性。第一组橄榄岩的LREE弱富集或近于平坦,尖晶石的Cr#值低而且变化不大,部分熔融程度较低(15%~20%),可能形成于弧后扩张盆地;第二组橄榄岩的LREE明显富集,尖晶石的Cr#值高而且变化较大(0.4~0.77),部分熔融程度较高(25%~30%),并经历了强烈的交代富集作用,可能与洋内岛弧有关。  相似文献   

9.
Spinel and plagioclase peridotites from the Mt.Maggiore (Corsica, France) ophiolitic massif record a composite asthenosphere–lithosphere history of partial melting and subsequent multi-stage melt–rock interaction. Cpx-poor spinel lherzolites are consistent with mantle residues after low-degree fractional melting (F = 5–10%). Opx + spinel symplectites at the rims of orthopyroxene porphyroclasts indicate post-melting lithospheric cooling (T = 970–1,100°C); this was followed by formation of olivine embayments within pyroxene porphyroclasts by melt–rock interaction. Enrichment in modal olivine (up to 85 wt%) at constant bulk Mg values, and variable absolute REE contents (at constant LREE/HREE) indicate olivine precipitation and pyroxene dissolution during reactive porous melt flow. This stage occurred at spinel-facies depths, after incorporation of the peridotites in the thermal lithosphere. Plagioclase-enriched peridotites show melt impregnation microtextures, like opx + plag intergrowths replacing exsolved cpx porphyroclasts and interstitial gabbronoritic veinlets. This second melt–rock interaction stage caused systematic chemical changes in clinopyroxene (e.g. Ti, REE, Zr, Y increase), related to the concomitant effects of local melt–rock interaction at decreasing melt mass, and crystallization of small (<3%) trapped melt fractions. LREE depletion in minerals of the gabbronoritic veinlets indicates that the impregnating melts were more depleted than normal MORB. Preserved microtextural evidence of previous melt–rock interaction in the impregnated peridotites suggests that they were progressively uplifted in response to lithosphere extension and thinning. Migrating melts were likely produced by mantle upwelling and melting related to extension; they were modified from olivine-saturated to opx-saturated compositions, and caused different styles of melt–rock interaction (reactive spinel harzburgites, vs. impregnated plagioclase peridotites) depending on the lithospheric depths at which interaction occurred. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

10.
西藏雅鲁藏布江蛇绿岩带罗布莎地幔橄榄岩的成因   总被引：17，自引：3，他引：14  
广泛分布在雅鲁藏布江蛇绿岩带的地幔橄榄岩是没有蛇纹石化的新鲜岩石,因而通过显微镜下观察和全岩化学分布探索其成因和地幔活动,是十分有利和简便的。最近几年来,利用岩石化学资料揭示远洋橄榄岩,认为它不是原始地幔岩经分或分离熔化作用萃取出玄武岩熔体后的直接残余物。罗布落蛇绿岩橄的镜下观察和岩石化学资料也证明,它不是原始地幔岩经局部熔化后的残余物,而且由来自更深层的地幔过渡带的硅酸盐超高压矿物底辟上升经熔取  相似文献   

11.
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.  相似文献   

12.
In this paper we present new data on the spatial variability of peridotite composition across a kilometer-scale mantle shear zone within the Lanzo massif (Western Alps, Italy). The shear zone separates the central from the northern part of the massif. Plagioclase peridotite shows gradually increasing deformation towards the shear zone, from porphyroclastic to mylonitic textures in the central body, while the northern body is composed of porphyroclastic rocks. The peridotite displays a large range of compositions, from fertile peridotite to refractory harzburgite and dunite. Deformed peridotites (proto-mylonite and mylonites) tend to be compositionally more homogeneous and fertile than weakly deformed peridotites. The composition of most plagioclase peridotites show rather high and constant (Ce/Yb) N ratios, and Yb N that cannot be explained by any simple melting model. Instead, refertilization modeling, consisting of melt increments from spinel peridotite sources, particularly with E-MORB melt, reasonably reproduces the plagioclase peridotite whole rock composition. Combined with constraints from Ce–Nb and Ce–Th systematics, we speculate that peridotites such as those from Lanzo record pervasive refertilization processes in the thermal boundary layer. In this scenario, mantle shear zones might act as important areas of melt focusing in the upper mantle that separates the thermal boundary layer from the conductively cooled mantle.  相似文献   

13.
Coarse-grained, granular spinel lherzolites xenoliths from the Premier kimberlite show evidence of melt extraction and metasomatic enrichment, documenting a complex history for the shallow mantle beneath the Bushveld complex. Compositions of orthopyroxene, clinopyroxene and spinel indicate equilibration within the spinel–peridotite facies of the upper mantle, at depths from 80 to 100 km and temperatures from 720 to 850 °C. Bulk compositions have lower Mg-number [atomic 100 Mg/(Mg + Fe*)] than previously studied spinel peridotites from Premier, and have higher Mg/Si than low-temperature coarse grained garnet lherzolites, suggesting shallower melting conditions or metasomatic enrichment. Clinopyroxene in one sample is highly LREE-depleted indicating very minor modification of a residue of 20% melt extraction, whereas the calculated REE pattern for a melt in equilibrium with a mildly LREE-depleted sample is similar to MORB or late Archean basalt, possibly related to the Bushveld Complex. Bulk and mineral compositions suggest minimal refertilization by silicate melts in four out of six samples, but REE patterns indicate introduction of a LIL-enriched component that may be related to kimberlite.  相似文献   

14.
The compositional variability of the lithospheric mantle at extensional settings is largely caused by the reactive percolation of uprising melts in the thermal boundary layer and in lithospheric environments.The Alpine-Apennine(A-A)ophiolites are predominantly constituted by mantle peridotites and are widely thought to represent analogs of the oceanic lithosphere formed at ocean/continent transition and slow-to ultraslow-spreading settings.Structural and geochemical studies on the A-A mantle peridotites have revealed that they preserve significant compositional and isotopic heterogeneity at variable scale,reflecting a long-lived multi-stage melt migration,intrusion and melt-rock interaction history,occurred at different lithospheric depths during progressive uplift.The A-A mantle peridotites thus constitute a unique window on mantle dynamics and lithosphere-asthenosphere interactions in very slow spreading environments.In this work,we review field,microstructural and chemical-isotopic evidence on the major stages of melt percolation and melt-rock interaction recorded by the A-A peridotites and discuss their consequences in creating chemical-isotopic heterogeneities at variable scales and enhancing weakening and deformation of the extending mantle.Focus will be on three most important stages:(i)old(pre-Jurassic)pyroxenite emplacement,and the significant isotopic modification induced in the host mantle by pyroxenite-derived melts,(ii)melt-peridotite interactions during Jurassic mantle exhumation,i.e.the open-system reactive porous flow at spinel facies depths causing bulk depletion(origin of reactive harzburgites and dunites),and the shallower melt impregnation which originated plagioclase-rich peridotites and an overall mantle refertilization.We infer that migrating melts largely originated as shallow,variably depleted,melt fractions,and acquired Si-rich composition by reactive dissolution of mantle pyroxenes during upward migration.Such melt-rock reaction processes share significant similarities with those documented in modern oceanic peridotites from slow-to ultraslow-spreading environments and track the progressive exhumation of large mantle sectors at shallow depths in oceanic settings where a thicker thermal boundary layer exists,as a consequence of slow-spreading rate.  相似文献   

15.
The Saramta peridotite massif is located within the Sharyzhalgai complex, SW margin of the Siberian craton. The Saramta massif was formed in the Archean and then juxtaposed with granulites of crystalline basement of the Siberian craton. The Saramta harzburgites are highly refractory in terms of lack of residual clinopyroxene, olivine Mg-number (up to 0.937), and spinel Cr-number (∼0.5), suggesting high degree of partial melting. Detailed study of their microstructures shows that they have extensively reacted with a SiO2-rich melt, leading to the crystallization of orthopyroxene, clinopyroxene, amphibole and spinel at the expense of olivine. The major element compositions of the least reacted harzburgites are similar to the residues of refractory peridotites produced by the fractional melting (initial melting pressures >3 GPa and melt fractions ∼40%). Moreover, non-residual clinopyroxenes are highly depleted in Yb, Zr and Ti, but highly enriched in LREE. A two-stage history is proposed for the Saramta peridotite: (1) primitive mantle underwent depletion in the garnet stability field followed by melting in the spinel stability field; (2) refractory harzburgites underwent refertilization by SiO2-rich melt in supra-subduction zone. Rare Saramta lherzolites probably formed from more refractory harzburgites as a result of such a melt–rock reaction. The Saramta peridotites are similar to low-T coarse-grained peridotites of subcratonic mantle. Processes of their formation, as reflected by textures and composition of minerals of the Saramta peridotites, are characteristic of the early stages of subcratonic mantle formation.  相似文献   

16.
IONOV  DMITRI 《Journal of Petrology》2004,45(2):343-367
Peridotite xenoliths in a Miocene picrite tuff from the Vitimvolcanic province east of Lake Baikal, Siberia, are samplesof the off-craton lithospheric mantle that span a depth rangefrom the spinel to garnet facies in a mainly fertile domain.Their major and trace element compositions show some scatter(unrelated to sampling or analytical problems), which is notconsistent with different degrees of partial melting or metasomatism.Some spinel peridotites and, to a lesser degree, garnet-bearingperidotites are depleted in heavy rare earth elements (HREE)relative to middle REE (MREE), whereas some garnet peridotitesare enriched in HREE relative to MREE, with Lu abundances muchhigher than in primitive mantle estimates. Clinopyroxenes fromseveral spinel peridotites have HREE-depleted patterns, whichare normally seen only in clinopyroxenes coexisting with garnet.Garnets in peridotites with similar modal and major elementcompositions have a broad range of Lu and Yb abundances. Overall,HREE are decoupled from MREE and Hf and are poorly correlatedwith partial melting indices. It appears that elements withhigh affinity to garnet were partially redistributed in theVitim peridotite series following partial melting, with feweffects for other elements. The Lu–Hf decoupling may disturbHf-isotope depletion ages and their correlations with meltingindices. KEY WORDS: garnet peridotite; lithospheric mantle; Lu–Hf isotope system; Siberia; trace elements  相似文献   

17.
Several spinel peridotite xenoliths from Spitsbergen have Sr–Ndisotopic compositions that plot to the right of the ‘mantlearray’ defined by oceanic basalts and the DM end-member(depleted mantle, with low 87Sr/86Sr and high 143Nd/144Nd).These xenoliths also show strong fractionation of elements withsimilar compatibility (e.g. high La/Ce), which cannot be producedby simple mixing of light rare earth element-depleted peridotiteswith ocean island basalt-type or other enriched mantle melts.Numerical simulations of porous melt flow in spinel peridotitesapplied to Sr–Nd isotope compositions indicate that thesefeatures of the Spitsbergen peridotites can be explained bychemical fractionation during metasomatism in the mantle. ‘Chromatographic’effects of melt percolation create a transient zone where thehost depleted peridotites have experienced enrichment in Sr(with a radiogenic isotope composition) but not in Nd, thusproducing Sr–Nd decoupling mainly controlled by partitioncoefficients and abundances of Sr and Nd in the melt and theperidotite. Therefore, Sr–Nd isotope decoupling, earlierreported for some other mantle peridotites worldwide, may bea signature of metasomatic processes rather than a source-relatedcharacteristic, contrary to models that invoke mixing with hypotheticalSr-rich fluids derived from subducted oceanic lithosphere. Pbisotope compositions of the Spitsbergen xenoliths do not appearto be consistently affected by the metasomatism. KEY WORDS: Spitsbergen; lithospheric mantle; metasomatism; radiogenic isotopes; theoretical modelling  相似文献   

18.
Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountainsvolcanic field (Montana, USA), derived from the lower lithosphereof the Wyoming craton, can be divided based on textural criteriainto tectonite and cumulate groups. The tectonites consist ofstrongly depleted spinel lherzolites, harzburgites and dunites.Although their mineralogical compositions are generally similarto those of spinel peridotites in off-craton settings, somecontain pyroxenes and spinels that have unusually low Al2O3contents more akin to those found in cratonic spinel peridotites.Furthermore, the tectonite peridotites have whole-rock majorelement compositions that tend to be significantly more depletedthan non-cratonic mantle spinel peridotites (high MgO, low CaO,Al2O3 and TiO2) and resemble those of cratonic mantle. Thesecompositions could have been generated by up to 30% partialmelting of an undepleted mantle source. Petrographic evidencesuggests that the mantle beneath the Wyoming craton was re-enrichedin three ways: (1) by silicate melts that formed mica websteriteand clinopyroxenite veins; (2) by growth of phlogopite fromK-rich hydrous fluids; (3) by interaction with aqueous fluidsto form orthopyroxene porphyroblasts and orthopyroxenite veins.In contrast to their depleted major element compositions, thetectonite peridotites are mostly light rare earth element (LREE)-enrichedand show enrichment in fluid-mobile elements such as Cs, Rb,U and Pb on mantle-normalized diagrams. Lack of enrichment inhigh field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf)suggests that the tectonite peridotites have been metasomatizedby a subduction-related fluid. Clinopyroxenes from the tectoniteperidotites have distinct U-shaped REE patterns with strongLREE enrichment. They have 143Nd/144Nd values that range from0·5121 (close to the host minette values) to 0·5107,similar to those of xenoliths from the nearby Highwood Mountains.Foliated mica websterites also have low 143Nd/144Nd values (0·5113)and extremely high 87Sr/86Sr ratios in their constituent phlogopite,indicating an ancient (probably mid-Proterozoic) enrichment.This enriched mantle lithosphere later contributed to the formationof the high-K Eocene host magmas. The cumulate group rangesfrom clinopyroxene-rich mica peridotites (including abundantmica wehrlites) to mica clinopyroxenites. Most contain >30%phlogopite. Their mineral compositions are similar to thoseof phenocrysts in the host minettes. Their whole-rock compositionsare generally poorer in MgO but richer in incompatible traceelements than those of the tectonite peridotites. Whole-rocktrace element patterns are enriched in large ion lithophileelements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb,Ta Zr and Hf) as in the host minettes, and their Sr–Ndisotopic compositions are also identical to those of the minettes.Their clinopyroxenes are LREE-enriched and formed in equilibriumwith a LREE-enriched melt closely resembling the minettes. Thecumulates therefore represent a much younger magmatic event,related to crystallization at mantle depths of minette magmasin Eocene times, that caused further metasomatic enrichmentof the lithosphere. KEY WORDS: ultramafic xenoliths; Montana; Wyoming craton; metasomatism; cumulates; minette  相似文献   

19.
粗粒与剪切结构橄榄岩捕虏体及其单斜辉石微量元素对比   总被引：1，自引：0，他引：1  
地山西栖霞具不同结构的“干”灾晶石相橄榄岩进行了全岩化学、微量元素,矿物成分和单斜辉石微量元素分析和对比。表明在橄榄岩从粗粒结构向剪切结构的转化中,随着变质变形作用的增强存在着复杂的熔／流体的加入富集和熔体的提取亏损作用;交代介质属具强渗透性的SiO2不饱和的硅酸盐碳酸岩熔体。同时发现不同结构橄榄岩中单斜辉石的REE与其全岩的REE程度有如下的关系;粗粒结构橄榄岩石中矿物与岩石的差别量大,但REE的配合分形可以反映全岩的情况;剪切结构橄榄岩中两者的差别较小。其它高度不相容微量元素可能主要赋存粒间组分或／和矿物流体包裹体中。  相似文献   

20.
东巧蛇绿岩位于班公湖—怒江缝合带中段,根据地理位置特征并以强玛镇为中心将东巧岩体划分为东西两个岩体。其中西岩体相对面积较大,由地幔橄榄岩、枕状玄武岩、辉长辉绿岩等组成;而东岩体面积较小,仅含地幔橄榄岩部分,各个不同单元之间呈断层接触关系。对东巧地幔橄榄岩开展岩石学、矿物学及地球化学研究发现:(1)东巧地幔橄榄岩以方辉橄榄岩为主,纯橄岩所占比例较小,约15%。豆荚状铬铁矿主要呈条带浸染状赋存在厚层且延伸较远的纯橄岩中。(2)东巧地幔橄榄岩中单斜辉石含量小于3%,矿物地球化学和全岩地球化学特征显示其来源于尖晶石相地幔源区的部分熔融,且部分熔融程度较高,估算在22%～28%,高于深海地幔橄榄岩的部分熔融程度(10%～22%)。(3)东巧地幔橄榄岩中的副矿物铬尖晶石Cr#值较高大于60,全岩具有U型球粒陨石标准化稀土元素配分模式,同时Rb、U、Zr和Sr相对富集,Hf和Nb相对亏损。全岩的地球化学特征指示了俯冲带之上的残余地幔与流体/熔体发生了反应,致使轻稀土元素以及部分微量元素选择性富集。综合东巧地幔橄榄岩的矿物化学组成成分以及全岩的地球化学特征,认为东巧地幔橄榄岩形成于大洋中脊的扩张环境中,后受到洋内俯冲作用的影响,导致俯冲带之上高度部分熔融的地幔橄榄岩与流体/熔体发生相互作用。  相似文献   

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