首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Our knowledge of the lithosphere beneath the Carpathian–Pannonian Region (CPR) has been greatly improved through petrologic, geochemical and isotopic studies of upper mantle xenoliths hosted by Neogene–Quaternary alkali basalts. These basalts occur at the edge of the Intra-Carpathian Basin System (Styrian Basin, Nógrád-Gömör and Eastern Transylvanian Basin) and its central portion (Little Hungarian Plain, Bakony-Balaton Highland).The xenoliths are mostly spinel lherzolites, accompanied by subordinate pyroxenites, websterites, wehrlites, harzburgites and dunites. The peridotites represent residual mantle material showing textural and geochemical evidence for a complex history of melting and recrystallization, irrespective of location within the region. The lithospheric mantle is more deformed in the center of the studied area than towards the edges. The deformation may be attributed to a combination of extension and asthenospheric upwelling in the late Tertiary, which strongly affected the central part of CPR subcontinental lithosphere.The peridotite xenoliths studied show bulk compositions in the following range: 35–48 wt.% MgO, 0.5–4.0 wt.% CaO and 0.2–4.5 wt.% Al2O3 with no significant differences in regard to their geographical location. On the other hand, mineral compositions, particularly of clinopyroxene, vary according to xenolith texture. Clinopyroxenes from less deformed xenoliths show higher contents of ‘basaltic’ major elements compared to the more deformed xenoliths. However, clinopyroxenes in more deformed xenoliths are relatively enriched in strongly incompatible trace elements such as light rare earth elements (LREE).Modal metasomatic products occur as both hydrous phases, including pargasitic and kearsutitic amphiboles and minor phlogopitic micas, and anhydrous phases — mostly clinopyroxene and orthopyroxene. Vein material is dominated by the two latter phases but may also include amphibole. Amphibole mostly occurs as interstitial phases, however, and is more common than phlogopite. Most metasomatized peridotites show chemical and (sometimes) textural evidence for re-equilibration between metasomatic and non-metasomatic phases. However, amphiboles in pyroxenites are sometimes enriched in K, Fe and LREE. The presence of partially crystallized melt pockets (related to amphiboles and clinopyroxenes) in both peridotites and pyroxenites is an indication of decompression melting and, rarely, incipient partial melting triggered by migrating hydrous melts or fluids. Metasomatic contaminants may be ascribed to contemporaneous subduction beneath the Carpathian–Pannonian Region between the Eocene and Miocene.Sulfide inclusions are more abundant in protogranular and porphyroclastic xenoliths relative to equigranular types. In mantle lithologies, sulfide bleb compositions vary between pentlandite and pyrrhotite correlating with the chemistry and texture of the host xenoliths. While sulfides in peridotites are relatively rich in Ni, those in clinopyroxene-rich xenoliths are notably Fe-rich.  相似文献   

2.
Anhydrous and amphibole-bearing mantle peridotite xenoliths from Kapfenstein (Styrian Basin) have been studied with the aim of understanding both the processes responsible for amphibole formation and the nature of metasomatizing agents which affected this portion of lithosphere. This area of the Pannonian Basin underwent a subduction event which was followed after about 15 Ma, by alkaline intraplate magmatism. Primary clinopyroxene (cpx1) in four-phase lherzolite xenoliths is characterized by LREE-depleted to slightly LREE-enriched patterns. LREE-depleted cpx1 have low Th and U contents and Zr (and Hf) anomalies varying from slightly negative to positive. LREE-enriched cpx have high Th and U contents and remarkable positive anomalies of Zr and Hf. Primary clinopyroxenes in amphibole-bearing lherzolites present a comparable compositional variation from LREE (and Th, U, Zr, Hf)-depleted type to LREE (and Th, U, Zr, Hf)-enriched type. LREE-depleted cpx1, with strong negative Zr and Ti anomalies, are also recognized in the peridotite matrix of a composite sample cut by a large amphibole vein. Textural and geochemical evidence indicates that amphibole disseminated within the matrix grew at the expense of primary spinel and clinopyroxene, mimicking the trace element patterns of the latter. As a consequence, the geochemical features of amphibole vary in relation to those of clinopyroxene, from enriched to depleted. On the other hand, the composition of vein amphibole in the composite xenolith compares well with amphibole megacrysts and microphenocrysts, suggesting that it represents a fractionation product of alkaline melt that passed through the lithosphere. Two kinds of metasomatism, superimposed on a slightly depleted lithospheric mantle, were identified. A slab-derived melt (proto-adakite?) metasomatic agent was responsible for the first enrichment in Th, U, Zr and Hf observed in clinopyroxene, whereas an alkaline within-plate metasomatic agent caused the formation of the Nb (and Ta)- rich disseminated amphibole. The final process was the alkaline magmatism, which was responsible for the formation of the large amphibole vein and megacrysts. It is proposed that the Nb-poor and Nb-rich amphiboles record the transition between the suprasubduction slab melt-related and the intraplate alkaline metasomatism.

These geochemical features are consistent with a lithospheric portion enriched in slab melt components which was subsequently metasomatized by alkaline melt. Alternatively an asthenospheric uprising could have scavenged a previously slab melt-enriched region of the lithosphere.  相似文献   


3.
Two spinel lherzolite xenoliths from Hungary that contain pyroxene–spinel symplectites have been studied using EPMA, Laser ablation ICP-MS and universal stage. Based on their geochemical and structural characteristics, the xenoliths represent two different domains of the shallow subcontinental lithospheric mantle beneath the Pannonian Basin. The occurrence of symplectites is attributed to the former presence and subsequent breakdown of garnets due to significant pressure decrease related to lithospheric thinning. This implies that both mantle domains were once part of the garnet lherzolitic upper mantle and had a similar history during the major extension that formed the Pannonian Basin.

Garnet breakdown resulted in distinct geochemical characteristics in the adjacent clinopyroxene crystals in both xenoliths. This is manifested by enrichment in HREE, Y, Zr and Hf towards the clinopyroxene porphyroclast rims and also in the neoblasts with respect to porphyroclast core compositions. This geochemical feature, together with the development and preservation of the texturally very sensitive symplectites, enables us to determine the relative timing of mantle processes. Our results indicate that garnets had been metastable in the spinel lherzolite environment and their breakdown to pyroxene and spinel is one of the latest processes that took place within the upper mantle before the xenoliths were brought to the surface.  相似文献   


4.
The Carpathian–Pannonian Region contains Neogene to Quaternary magmatic rocks of highly diverse composition (calc-alkaline, shoshonitic and mafic alkalic) that were generated in response to complex microplate tectonics including subduction followed by roll-back, collision, subducted slab break-off, rotations and extension. Major element, trace element and isotopic geochemical data of representative parental lavas and mantle xenoliths suggests that subduction components were preserved in the mantle following the cessation of subduction, and were reactivated by asthenosphere uprise via subduction roll-back, slab detachment, slab-break-off or slab-tearing. Changes in the composition of the mantle through time are evident in the geochemistry, supporting established geodynamic models.Magmatism occurred in a back-arc setting in the Western Carpathians and Pannonian Basin (Western Segment), producing felsic volcaniclastic rocks between 21 to 18 Ma ago, followed by younger felsic and intermediate calc-alkaline lavas (18–8 Ma) and finished with alkalic-mafic basaltic volcanism (10–0.1 Ma). Volcanic rocks become younger in this segment towards the north. Geochemical data for the felsic and calc-alkaline rocks suggest a decrease in the subduction component through time and a change in source from a crustal one, through a mixed crustal/mantle source to a mantle source. Block rotation, subducted roll-back and continental collision triggered partial melting by either delamination and/or asthenosphere upwelling that also generated the younger alkalic-mafic magmatism.In the westernmost East Carpathians (Central Segment) calc-alkaline volcanism was simultaneously spread across ca. 100 km in several lineaments, parallel or perpendicular to the plane of continental collision, from 15 to 9 Ma. Geochemical studies indicate a heterogeneous mantle toward the back-arc with a larger degree of fluid-induced metasomatism, source enrichment and assimilation on moving north-eastward toward the presumed trench. Subduction-related roll-back may have triggered melting, although there may have been a role for back-arc extension and asthenosphere uprise related to slab break-off.Calc-alkaline and adakite-like magmas were erupted in the Apuseni Mountains volcanic area (Interior Segment) from15–9 Ma, without any apparent relationship with the coeval roll-back processes in the front of the orogen. Magmatic activity ended with OIB-like alkali basaltic (2.5 Ma) and shoshonitic magmatism (1.6 Ma). Lithosphere breakup may have been an important process during extreme block rotations (60°) between 14 and 12 Ma, leading to decompressional melting of the lithospheric and asthenospheric sources. Eruption of alkali basalts suggests decompressional melting of an OIB-source asthenosphere. Mixing of asthenospheric melts with melts from the metasomatized lithosphere along an east–west reactivated fault-system could be responsible for the generation of shoshonitic magmas during transtension and attenuation of the lithosphere.Voluminous calc-alkaline magmatism occurred in the Cãlimani-Gurghiu-Harghita volcanic area (South-eastern Segment) between 10 and 3.5 Ma. Activity continued south-eastwards into the South Harghita area, in which activity started (ca. 3.0–0.03 Ma, with contemporaneous eruption of calc-alkaline (some with adakite-like characteristics), shoshonitic and alkali basaltic magmas from 2 to 0.3 Ma. Along arc magma generation was related to progressive break-off of the subducted slab and asthenosphere uprise. For South Harghita, decompressional melting of an OIB-like asthenospheric mantle (producing alkali basalt magmas) coupled with fluid-dominated melting close to the subducted slab (generating adakite-like magmas) and mixing between slab-derived melts and asthenospheric melts (generating shoshonites) is suggested. Break-off and tearing of the subducted slab at shallow levels required explaining this situation.  相似文献   

5.
Twenty spinel peridotite xenoliths from Pliocene alkali basaltic tuffs and lavas of the western Pannonian Basin (Hungary) have been analysed for bulk rock major and trace elements, electron probe mineral compositions, and REE and Sr, Nd isotopes on separated and leached clinopyroxenes. The xenoliths are texturally diverse, including protogranular, porphyroclastic, equigranular and poikilitic textures which can generally be correlated with geochemical features. Protogranular xenoliths are relatively undepleted in Ca, Al, Ti and Na, whereas poikilitic xenoliths are more refractory. LREE-depleted patterns. and MORB-like Nd and Sr values are associated with protogranular peridotites. In contrast, xenoliths with complex textures are generally LREE-enriched. Much of the isotopic variation in the suite (Sr=–20.4 to +10.4, +Nd=+1.8 to +13.7) can be related to interaction between protogranular mantle and melts resembling the host alkali basalts, but a third (high Sr) component may be due to Miocene subduction beneath the region.  相似文献   

6.
ULTRAMAFIC XENOLITHS FROM A KAMAFUGITE LAVA IN CENOZOIC VOLCANIC FIELD OF WEST QINLING, CHINA AND ITS GEOLOGICAL IMPLICATION  相似文献   

7.
《地学前缘(英文版)》2020,11(3):943-964
Clinopyroxene-enriched upper mantle xenoliths classified as wehrlites are common(～20% of all xenoliths) in the central part of the Nograd-G(o| ")m(o|")r Volcanic Field(NGVF),situated in the northern margin of the Pannonian Basin in northern Hungary and southern Slovakia.In this study,we thoroughly investigated 12 wehrlite xenoliths,two from each wehrlite-bearing occurrence,to determine the conditions of their formation.Specific textural features,including clinopyroxene-rich patches in an olivine-rich lithology,orthopyroxene remnants in the cores of newlyformed clinopyroxenes and vermicular spinel forms all suggest that wehrlites were formed as a result of intensive interaction between a metasomatic agent and the peridotite wall rock.Based on the major and trace element geochemistry of the rock-forming minerals,significant enrichment in basaltic(Fe,Mn,Ti) and high field strength elements(Nb,Ta,Hf,Zr) was observed,compared to compositions of common lherzolite xenoliths.The presence of orthopyroxene remnants and geochemical trends in rock-forming minerals suggest that the metasomatic process ceased before complete wehrlitization was achieved.The composition of the metasomatic agent is interpreted to be a mafic silicate melt,which was further confirmed by numerical modelling of trace elements using the plate model.The model results also show that the melt/rock ratio played a key role in the degree of petrographic and geochemical transformation.The lack of equilibrium and the conclusions drawn by using variable lherzolitic precursors in the model both suggest that wehrlitization was the last event that occurred shortly before xenolith entrainment in the host mafic melt.We suggest that the wehrlitization and the Plio-Pleistocene basaltic volcanism are related to the same magmatic event.  相似文献   

8.
秦岭显生宙地幔组成及其演化   总被引：2，自引：1，他引：2  
通过对秦岭造山带及扬子克拉通北缘显生宙时期 3个含地幔捕虏体的煌斑岩、钾镁煌斑岩、碱性玄武岩以及 11个不含捕虏体的辉石岩、辉长岩、玄武岩出露点的岩石地球化学对比研究 ,揭示出研究区地幔演化经历了自古生代的OIB亏损地幔到中生代的高度富集地幔再到中生代末期 -新生代的OIB MORB的亏损地幔的两次明显变更。制约这种变更的主要因素是熔融岩浆时源区发生的层圈相互作用类型。鉴于大陆岩石圈软流层体系的特征 ,有必要划分出岩石圈 /软流层相互作用带(过渡带 ) ,它是大陆岩浆作用的重要源区。  相似文献   

9.
Xenoliths from the upper mantle and lower crust are abundant in Plio–Pleistocene alkali basalts of the Nógrád-Gömör Volcanic Field (NGVF; northern Pannonian Basin, northern Hungary/southern Slovakia), representing a valuable ‘probe’ of lithospheric structures and processes. Ultramafic xenoliths have been divided into two groups: (1) Type-I, composed mostly of olivine with subsidiary orthopyroxene, clinopyroxene and spinel, and (2) Type-II, containing mostly Al- and Ti-rich clinopyroxene with subordinate olivine, spinel and plagioclase. Both types often contain amphibole and, to a lesser extent, mica. The refractory character of Type-I xenoliths suggests they represent mantle depleted by prior episodes of partial melting. In contrast, Type-II series (wehrlites, olivine clinopyroxenites, clinopyroxenites and plagioclase-bearing ultramafic lithologies), on the basis of their textural features, thermobarometric histories and major and trace element variation, appear to have formed as magmatic cumulates. Petrologic and geochemical studies of Type-II xenoliths from Nógrád-Gömör suggest they crystallized from basaltic melts emplaced within the lithospheric mantle and lower crust, prior to the onset of Plio–Pleistocene volcanic activity. After their consolidation, metasomatic agents reacted with the anhydrous cumulate phases producing amphiboles and micas at the expense of olivine and clinopyroxene. The metasomatic agents appear to have been adakitic rather than basaltic in composition, possibly linked to a retreating arc–forearc system. Large-scale contamination of the lithospheric mantle can therefore be attributed to fluid and melt fractions related to subduction beneath the outer Carpathian arc.  相似文献   

10.
Five spinel lherzolite xenoliths hosted in Neogene alkali basalts from the marginal parts of the Pannonian Basin (Styrian Basin in Austria and Persani Mts. in the Eastern Transylvanian Basin, Romania) contain orthopyroxene–clinopyroxene–spinel clusters, which are believed to represent former garnet in lherzolitic mantle material. 'Palaeo' equilibrium pressure of this former garnet lherzolite was estimated to be equivalent to depths of 90–120 km using calculated garnet compositions and measured orthopyroxene compositions from the clusters. 'Neo' equilibrium pressures of the xenoliths indicate depths of 55–65 km, suggesting c.  50–60 km uprise of the mantle section represented by these xenoliths. This petrological result confirms the observations from previous geophysical studies that significant mantle uplift has occurred beneath the Pannonian Basin.  相似文献   

11.
闫峻  陈江峰 《地质论评》2005,51(1):16-26
本文综合运用不同时代幔源包体平衡温压对比、玄武岩地球化学性质对岩石圈厚度的反演以及不同时代岩石圈地幔地球化学性质的对比的方法，把华北地块东部岩石圈的减薄时间限定在晚中生代至新生代之间。减薄的机制可能是华北东部地区晚白垩世以来大陆岩石圈的拉张作用。由于机械性拉薄和热、机械和化学侵蚀作用，岩石圈厚度最终减薄到70km以下。但古老的岩石圈地幔并没有完全因减薄而消失，残留部分受到了来自软流圈物质的强烈改造，使其Sr、Nd同位素组成类似于软流圈，但Os同位素没有受到明显的改变。改造后的岩石圈地幔成为华北地块东部新生代岩石圈地幔的主体。在时空上，岩石圈的减薄具有不均一的性质。  相似文献   

12.
韩江伟  熊小林  朱照宇 《岩石学报》2009,25(12):3208-3220
对雷琼地区21个晚新生代玄武岩样品的主量、微量元素和Sr、Nd、Pb同位素分别用湿化学法、ICP-MS和MC-ICPMS进行了测定.这些玄武岩主要为石英拉斑玄武岩,其次为橄榄拉斑玄武岩和碱性玄武岩.大多数样品的微量元素和同位素成分与洋岛玄武岩(OIBs)相似,而且随着SiO_2不饱和度增加,不相容元素含量也增加.除R4-1可能受到地壳混染外,其他样品相对均一的Nd同位素(ε_(Nd)=2.5-6.0)以及变化明显但范围有限的Sr同位素(0.703106～0.704481),可能继承了地幔源区的特征.~(87)Sr/~(86)Sr与~(206)Pb/~(204)Pb的正相关和~(143)Nd/~(144)Nd与~(206)Pb/~(204)Pb的负相关特征暗示DM(软流圈地幔)与EM2(岩石圈地幔)的混合.地幔捕虏体的同位素特征暗示EM2成分不可能存在于尖晶石橄榄岩地幔,而La/Yb和Sm/Yb系统表明岩浆由石榴石橄榄岩部分熔融产生,这意味着EM2成分可能存在于石榴石橄榄岩地幔.雷琼地区玄武岩的地球化学变化可以用软流圈地幔为主的熔体加入不同比例石榴石橄榄岩地幔不同程度熔融产生的熔体来解释:碱性玄武岩和橄榄拉斑玄武岩是软流圈熔体与石榴石橄榄岩地幔较低程度(7%～9%)熔融体混合,而石英拉斑玄武岩是软流圈熔体与石榴石橄榄岩地幔较高程度(10%～20%)熔融体的混合.  相似文献   

13.
本文对华北克拉通三个不同地区(河北汉诺坝、内蒙古集宁三义堂、河南鹤壁)新发现的含金云母尖晶石二辉橄榄岩和尖晶石橄榄单斜辉石岩捕虏体进行了详细的矿物组成、单斜辉石的微量元素和 Sr-Nd 同位素研究。通过与相同地区不含金云母尖晶石二辉橄榄岩捕虏体的系统对比发现通常含金云母的地幔橄榄岩比不含金云母的地幔橄榄岩岩富 Al_2O_3、CaO、NaO、K_2O、TiO_2,但相对贫镁;其单斜辉石的 LREE 更为富集,但 Sr、Nd 同位素组成则相对亏损。这说明地幔交代作用不仅能够造成地幔橄榄岩的玄武质组分和稀土元素的富集,而且亦能够造成全岩和橄榄石 Mg~#的降低和同位素组成的相对亏损。捕虏体的 Rb-Sr 等时线年龄暗示地幔交代作用发生在中、新生代;其交代熔体来源于软流圈。同时说明华北新生代岩石圈地慢普遍存在的主、微量元素和同位素组成类似于"大洋型"岩石圈地幔的特征很可能是岩石圈地幔橄榄岩与软流圈来源的熔体的大规模反应的结果,而非真正意义上的新增生的岩石圈地幔。  相似文献   

14.
报道了吉林辽源晚中生代玄武岩的全岩K-Ar定年结果和岩石地球化学资料, 以及其中幔源捕虏体的矿物化学分析结果, 探讨了晚中生代碱性玄武岩的成因及岩石圈地幔的性质.全岩K-Ar定年结果显示, 辽源玄武岩形成于晚白垩世(91.7±1.7 Ma、82.5±1.9 Ma、99.3±1.6 Ma、82.2±2.4 Ma).该玄武岩的SiO2含量介于45.1%~45.9%之间, 全碱(Na2O+K2O)含量介于4.50%~6.72%之间, Mg#[100×Mg/(Mg+Fe2+)]值介于60~63之间, 属于碱性玄武岩系列.该玄武岩相对富集轻稀土元素(LREEs)、亏损重稀土元素(HREEs), (La/Yb)N值介于12.0~14.0之间, 无Eu异常, 具有洋岛玄武岩(OIB)型微量元素配分型式.辽源玄武岩中的幔源捕虏体以尖晶石二辉橄榄岩为主, 捕虏体中橄榄石的Mg#值较低(87.1~91.2), 结合捕虏体中其他矿物的主量和微量元素地球化学特征, 认为辽源晚白垩世岩石圈地幔以新增生的岩石圈地幔为主.捕虏体中单斜辉石的LREEs具有从富集到亏损的变化特征, 暗示岩石圈地幔经历了部分熔融和交代作用的改造, 地幔交代类型以硅酸盐熔体交代为主, 同时存在碳酸盐熔体交代作用.辽源晚白垩世玄武岩来源于软流圈地幔的部分熔融, 结合区域研究成果, 认为辽源晚白垩世玄武岩形成于古太平洋板块俯冲背景下的弧后伸展环境.   相似文献   

15.
Mantle xenoliths and xenocrysts were retrieved from three of the 88–86 Ma Buffalo Hills kimberlites (K6, K11, K14) for a reconnaissance study of the subcontinental lithospheric mantle (SCLM) beneath the Buffalo Head Terrane (Alberta, Canada). The xenoliths include spinel lherzolites, one garnet spinel lherzolite, garnet harzburgites, one sheared garnet lherzolite and pyroxenites. Pyroxenitic and wehrlitic garnet xenocrysts are derived primarily from the shallow mantle and lherzolitic garnet xenocrysts from the deep mantle. Harzburgite with Ca-saturated garnets is concentrated in a layer between 135–165 km depth. Garnet xenocrysts define a model conductive paleogeotherm corresponding to a heat flow of 38–39 mW/m2. The sheared garnet lherzolite lies on an inflection of this geotherm and may constrain the depth of the lithosphere–asthenosphere boundary (LAB) beneath this region to ca 180 km depth.

A loss of >20% partial melt is recorded by spinel lherzolites and up to 60% by the garnet harzburgites, which may be related to lithosphere formation. The mantle was subsequently modified during at least two metasomatic events. An older metasomatic event is evident in incompatible-element enrichments in homogeneous equilibrated garnet and clinopyroxene. Silicate melt metasomatism predominated in the deep lithosphere and led to enrichments in the HFSE with minor enrichments in LREE. Metasomatism by small-volume volatile-rich melts, such as carbonatite, appears to have been more important in the shallow lithosphere and led to enrichments in LREE with minor enrichments in HFSE. An intermediate metasomatic style, possibly a signature of volatile-rich silicate melts, is also recognised. These metasomatic styles may be related through modification of a single melt during progressive interaction with the mantle. This metasomatism is suggested to have occurred during Paleoproterozoic rifting of the Buffalo Head Terrane from the neighbouring Rae Province and may be responsible for the evolution of some samples toward unradiogenic Nd and Hf isotopic compositions.

Disturbed Re–Os isotope systematics, evident in implausible model ages, were obtained in situ for sulfides in several spinel lherzolites and suggest that many sulfides are secondary (metasomatic) or mixtures of primary and secondary sulfides. Sulfide in one peridotite has unradiogenic 187Os/188Os and gives a model age of 1.89±0.38 Ga. This age coincides with the inferred emplacement of mafic sheets in the crust and suggests that the melts parental to the intrusions interacted with the lithospheric mantle.

A younger metasomatic event is indicated by the occurrence of sulfide-rich melt patches, unequilibrated mineral compositions and overgrowths on spinel that are Ti-, Cr- and Fe-rich but Zn-poor. Subsequent cooling is recorded by fine exsolution lamellae in the pyroxenes and by arrested mineral reactions.

If the lithosphere beneath the Buffalo Head Terrane was formed in the Archaean, any unambiguous signatures of this ancient origin may have been obliterated during these multiple events.  相似文献   


16.
Magmatism-related localized deformation in the mantle: a case study   总被引：2，自引：0，他引：2  
A deformed composite peridotite-pyroxenite xenolith in Pliocene alkali basalts from the Pannonian Basin (Szentbékkálla, Bakony—Balaton Highland Volcanic Field) has been studied in detail. A narrow shear zone of intense deformation marked by porphyroclast elongation and recrystallization runs along the peridotite-pyroxenite contact. The xenolith contains a large volume of euhedral olivine neoblasts and tablet grains of olivine away from the shear zone interpreted as products of annealing and recrystallization in the presence of grain boundary fluid. Estimates of the time required for growth of recrystallized olivine grains suggest that the annealing took place in situ in the mantle and not during transport of the xenolith in the basalt magma. The grain boundary fluid present during recrystallization was a vapor rich silicate-melt different from the host basaltic melt that entrained the xenolith. This study demonstrates that high-stress deformation zones and associated fluid-assisted recrystallization, which are common features in kimberlite mantle xenoliths, also occur in some mantle xenoliths from alkali basalts. The suggested high-stress deformation zones in the mantle beneath the Pannonian Basin may be produced by paleoseismic events in the lithosphere associated with faulting related to the ascent of basalt magma.Editorial responsibility: J. Hoefs
Csaba SzabóEmail: Phone: +36-1-2090555Fax: +36-1-3812108
  相似文献   

17.
In situ trace-element and isotopic (87Sr/86Sr) data and whole-rock Sr–Nd–Hf data on 12 gabbro xenoliths from the Hyblean Plateau (south-eastern Sicily) illustrate the complex petrogenetic evolution of this lithospheric segment. The gabbros formed by precipitation of plagioclase + clinopyroxene from a HIMU-type alkaline melt, then were cryptically metasomatized by a low-Rb, high-87Sr/86Sr fluid, and finally infiltrated by an exotic, late Fe–Ti-rich melt with 87Sr/86Sr ~ 0.7055, carrying high concentrations of Sr, Rb and HFSE. The geochemical and isotopic features of both the metasomatizing fluid and the Fe–Ti-rich melt are compatible with their common derivation by the progressive melting of an amphibole–phlogopite–ilmenite metasomatic domain (MARID-type?) that probably resided within the subcontinental lithospheric mantle. Therefore, both the astenosphere and the lithosphere underneath the Hyblean Plateau contributed to the petrogenesis of the gabbros. Sm–Nd dating yields an age of 253 ± 60 Ma for the cumulitic pile, roughly coinciding with a hydrothermal event recorded by crustal zircons in the area. We suggest that the Hyblean Plateau suffered a thermal event—probably related to lithospheric thinning and upwelling and melting of the asthenosphere—in Permo-Triassic time (the opening of the Ionian Basin?). The induced perturbation in the lithosphere caused consequent melting of some previously metasomatised portions.  相似文献   

18.
Harrat Al-Birk volcanics are products of the Red Sea rift in southwest Saudi Arabia that started in the Tertiary and reached its climax at ~5 Ma. This volcanic field is almost monotonous and is dominated by basalts that include mafic–ultramafic mantle xenoliths (gabbro, websterite, and garnet-clinopyroxenite). The present work presents the first detailed petrographic and geochemical notes about the basalts. They comprise vesicular basalt, porphyritic basalt, and flow-textured basalt, in addition to red and black scoria. Geochemically, the volcanic rock varieties of the Harrat Al-Birk are low- to medium-Ti, sodic-alkaline olivine basalts with an enriched oceanic island signature but extruded in a within-plate environment. There is evidence of formation by partial melting with a sort of crystal fractionation dominated by clinopyroxene and Fe–Ti oxides. The latter have abundant titanomagnetite and lesser ilmenite. There is a remarkable enrichment of light rare earth elements and depletion in Ba, Th and K, Ta, and Ti. The geochemical data in this work suggest Harrat Al-Birk basalts represent products of water-saturated melt that was silica undersaturated. This melt was brought to the surface through partial melting of asthenospheric upper mantle that produced enriched oceanic island basalts. Such partial melting is the result of subducted continental mantle lithosphere with considerable mantle metasomatism of subducted oceanic lithosphere that might contain hydrous phases in its peridotites. The fractional crystallization process was controlled by significant separation of clinopyroxene followed by amphiboles and Fe–Ti oxides, particularly ilmenite. Accordingly, the Harrat Al-Birk alkali basalts underwent crystal fractionation that is completely absent in the exotic mantle xenoliths (e.g. Nemeth et al. in The Pleistocene Jabal Akwa Al Yamaniah maar/tuff ring-scoria cone complex as an analogy for future phreatomagmatic to magmatic explosive eruption scenarios in the Jizan Region, SW Saudi Arabia 2014).  相似文献   

19.
Clinopyroxene is a major host for lithophile elements in the mantle lithosphere, and therefore it is critical whether we are to understand the constraints that this mineral puts on mantle evolution and melt generation. This study presents a detailed in situ trace element and Sr isotope study of clinopyroxene, amphibole and melt from two spinel lherzolites from the Middle Atlas Mountains, Morocco. The results show that there is limited, but discernable, Sr isotopic variation between clinopyroxene crystals within these xenoliths [87Sr/86Sr ranging from 0.703416 (±11 2SE) to 0.703681 (±12 2SE)]. Trace element patterns show similar interelement fractionation with LREE enrichment, but there is a considerable range in terms of elemental concentration (e.g. over 100 ppm in Sr concentrations). Observed modal clinopyroxene is far more abundant than that predicted from estimates of melt depletion. This along with isotope and trace element variability found in these xenoliths supports a multistage metasomatic process in which clinopyroxene and amphibole are recent secondary additions to the lithospheric mantle. Elemental systematics indicate that the metasomatic mineral assemblage has most recently equilibrated with a carbonatitic melt prior to inclusion in the host basalt. The clinopyroxene from this study is typical of global off-craton clinopyroxene in terms of Sr isotope composition, suggesting that the majority of clinopyroxene in off-craton settings may have a recent metasomatic origin. These findings indicate that caution is required when using peridotite xenoliths to estimate the degree of elemental enrichment in the subcontinental lithosphere.  相似文献   

20.
Lithospheric thinning beneath the North China Craton is widely recognized, but whether the Yangtze block has undergone the same process is a controversial issue. Based on a detailed petrographic study, a suite of xenoliths from the Lianshan Cenozoic basalts have been analyzed for the compositions of minerals and whole rocks, and their Sr–Nd isotopes to probe the nature and evolution of the subcontinental lithospheric mantle beneath the lower Yangtze block. The Lianshan xenoliths can be subdivided into two Types: the main Type 1 xenoliths (9–15% clinopyroxene and olivine-Mg# < 90) and minor Type 2 peridotites (1.8–6.2% clinopyroxene and olivine-Mg# > 90). Type 1 peridotites are characterized by low MgO, high levels of basaltic components (i.e., Al2O3, CaO and TiO2), LREE-depleted patterns in clinopyroxenes and whole rocks, and relatively high 143Nd/144Nd (0.513219–0.513331) and low 86Sr/87Sr (0.702279–0.702789). These features suggest that Type 1 peridotites represent fragments of the newly accreted fertile lithospheric mantle that have undergone ~ 1% of fractional partial melting and later weak silicate–melt metasomatism, similar to Phanerozoic lithospheric mantle beneath the eastern North China Craton. Type 2 peridotites may be shallow relics of the older lithospheric mantle depleted in basaltic components, with LREE-enriched and HREE-depleted patterns, relatively low 143Nd/144Nd (0.512499–0.512956) and high 86Sr/87Sr (0.703275–0.703997), which can be produced by 9–14% partial melting and subsequent carbonatite–melt metasomatism. Neither type shows a correlation between equilibration temperatures and Mg# in olivine, indicating that the lithospheric mantle is not compositionally stratified, but both types coexist at similar depths. This coexistence suggests that the residual refractory lithospheric mantle (i.e., Type 2 peridotites) may be irregularly eroded by upwelling asthenosphere materials along weak zones and eventually replaced to create a new and fertile lithosphere mantle (i.e., Type 1 xenoliths) as the asthenosphere cooled. Therefore, the subcontinental lithospheric mantle beneath the lower Yangtze block shared a common evolutional dynamic environment with that beneath the eastern North China Craton during late Mesozoic–Cenozoic time.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号