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 207 毫秒
1.
Garnet‐bearing peridotite lenses are minor but significant components of most metamorphic terranes characterized by high‐temperature eclogite facies assemblages. Most peridotite intrudes when slabs of continental crust are subducted deeply (60–120 km) into the mantle, usually by following oceanic lithosphere down an established subduction zone. Peridotite is transferred from the resulting mantle wedge into the crustal footwall through brittle and/or ductile mechanisms. These ‘mantle’ peridotites vary petrographically, chemically, isotopically, chronologically and thermobarometrically from orogen to orogen, within orogens and even within individual terranes. The variations reflect: (1) derivation from different mantle sources (oceanic or continental lithosphere, asthenosphere); (2) perturbations while the mantle wedges were above subducting oceanic lithosphere; and (3) changes within the host crustal slabs during intrusion, subduction and exhumation. Peridotite caught within mantle wedges above oceanic subduction zones will tend to recrystallize and be contaminated by fluids derived from the subducting oceanic crust. These ‘subduction zone peridotites’ intrude during the subsequent subduction of continental crust. Low‐pressure protoliths introduced at shallow (serpentinite, plagioclase peridotite) and intermediate (spinel peridotite) mantle depths (20–50 km) may be carried to deeper levels within the host slab and undergo high‐pressure metamorphism along with the enclosing rocks. If subducted deeply enough, the peridotites will develop garnet‐bearing assemblages that are isofacial with, and give the same recrystallization ages as, the eclogite facies country rocks. Peridotites introduced at deeper levels (50–120 km) may already contain garnet when they intrude and will not necessarily be isofacial or isochronous with the enclosing crustal rocks. Some garnet peridotites recrystallize from spinel peridotite precursors at very high temperatures (c. 1200 °C) and may derive ultimately from the asthenosphere. Other peridotites are from old (>1 Ga), cold (c. 850 °C), subcontinental mantle (‘relict peridotites’) and seem to require the development of major intra‐cratonic faults to effect their intrusion.  相似文献   

2.
论述了大陆俯冲碰撞带中地幔橄榄岩的基本特征和成岩类型,并重点讨论柴北缘超高压变质带中不同性质的橄榄岩及其成因。根据岩石学特征,我们确定柴北缘超高压带中发育有两种类型的橄榄岩:(1)石榴橄榄岩,岩石类型包括石榴二辉橄榄岩、石榴方辉橄榄岩、纯橄岩和石榴辉石岩,是大陆型俯冲带的标志性岩石。金刚石包裹体、石榴石和橄榄石的出溶结构、温压计算等均反映其来源深度大于200km。地球化学特征表明该橄榄岩的原岩是岛弧环境下高镁岩浆在地幔环境下堆晶的产物。(2)大洋蛇绿岩型地幔橄榄岩,与变质的堆晶杂岩(包括石榴辉石岩、蓝晶石榴辉岩)和具有大洋玄武岩特征的榴辉岩构成典型的蛇绿岩剖面,代表大洋岩石圈残片。这两类橄榄岩的确定对了解柴北缘超高压变质带的性质和构造演化过程有重要意义。  相似文献   

3.
The main hole (MH), and pre-pilot holes PP1, and PP3 of the Chinese Continental Scientific Drilling Project (CCSD) penetrated three different garnet peridotite bodies in the Sulu ultrahigh pressure (UHP) metamorphic belt, which are 80 m, 120 m, and 430 m thick, respectively. The bodies occur as tectonic blocks hosted in eclogite (MH peridotite) and gneisses (PP1 and PP3 peridotites). The peridotites in the MH are garnet wehrlites, whose protoliths were ultramafic cumulates based on olivine compositions (Fo79-89) and other geochemical features. Zoned garnet and omphacite (with 4-5 wt.% Na2O) are typical metamorphic minerals in these rocks, and, along with P-T estimates based on mineral pairs, suggest that the rocks have undergone UHP metamorphism. SHRIMP U-Pb isotope dating of zircon from the garnet wehrlite yielded a Paleozoic protolith age (ca. 346-461 Ma), and a Mesozoic UHP metamorphic age (ca. 220-240 Ma). The peridotites in PP1 consist of interlayered garnet (Grt)-bearing and garnet-free (GF) peridotite. Both types of peridotite have depleted mantle compositions (Mg# = 90-92) and they display transitional geochemical features. The intercalated layers probably reflect variations in partial melting rather than pressure variations during metamorphism, and the garnets may have been formed by exsolution from orthopyroxene during exhumation. These peridotites were probably part of the mantle wedge above the subduction zone that produced the UHP metamorphism and thus belonged to the North China Block before its tectonic emplacement. The exhumation of the subducted Yangtze Block brought these mantle fragments to shallow crustal levels. The ultramafic rocks in PP3 are dominantly dunite with minor garnet dunite. Their high Mg# (92-93) and relatively uniform chemical compositions indicate that they are part of a depleted mantle sequence. The presence of garnet replacing spinel and enclosing pre-metamorphic minerals such as olivine, clinopyroxene and spinel suggests that these rocks have undergone progressive metamorphism. SHRIMP U-Pb isotope dating of zircon from these rocks yielded two age groups: 726 ± 56 Ma for relic magmatic zircon grains and 240 ± 2.7 Ma for the newly formed metamorphic zircon. The older group is similar in age to granitic intrusions within the Dabie-Sulu belt, suggesting that the PP3 garnet peridotite may record the early emplacement of the peridotite into the crust. The younger dates coincide with the age of UHP metamorphism during continent-continent collision between the Yangtze and North China Blocks, suggesting that these peridotites were subducted to depths equivalent to the coesite facies and later exhumed. Thus, the garnet peridotites in the CCSD cores include both ultramafic rocks that existed originally in the subducted plate and rocks from the mantle wedge above the subducted plate, i.e., part of the North China Block.  相似文献   

4.
宋衍茹  叶凯  续海金 《岩石学报》2009,25(1):147-158
苏鲁超高压变质地体中发现了大量包裹在超高压（UHP）变质片麻岩和混合岩中的造山带石榴橄榄岩。根据它们的野外产出特征和全岩地球化学成分,其中一部分石榴橄榄岩的原岩来自于亏损地幔,后来被卷入俯冲陆壳并经受过俯冲陆壳产生的熔/流体的交代。但是,对这些岩石早期的亏损过程尚缺乏清晰的认识。本文报道了东海芝麻坊石榴子石二辉橄榄岩早期变质演化的新证据。根据详细的变质反应结构观察和矿物成分研究,芝麻坊石榴子石二辉橄榄岩在经历高压低温俯冲带型超高压变质之前经历了至少两期变质演化。其原岩矿物组合由石榴子石变斑晶的高Ca-Cr核部及其中包裹的高Mg单斜辉石、高Al-Cr斜方辉石和高Mg-Ni橄榄石所记录;指示芝麻坊石榴子石二辉橄榄岩的原岩为高温-高压的富集石榴子石二辉橄榄岩。第二期矿物组合为包裹在低Cr变斑晶石榴子石幔部和细粒新生石榴子石核部的大量富Al铬铁矿和高Mg低Ni橄榄石以及少量高Mg斜方辉石。该期组合未发现单斜辉石,表明岩石随后被转变为高温低压的难熔尖晶石方辉橄榄岩或尖晶石纯橄岩。芝麻坊石榴子石二辉橄榄岩的早期变质演化记录了它们被卷入大陆板片俯冲带之前的地幔楔上升对流过程。笔者认为芝麻坊石榴子石二辉橄榄岩的原岩来源于早期俯冲大洋板片之上的深部高温富集地幔楔,洋壳俯冲过程中的地幔楔对流导致其上升到弧后或岛弧之下的地幔楔浅部,减压部分熔融使原本富集的石榴子石二辉橄榄岩转化为难熔的尖晶石方辉橄榄岩或尖晶石纯橄岩。  相似文献   

5.
Garnet peridotites occur as lenses, blocks or layers within granulite–amphibolite facies gneiss in the Dabie-Sulu ultra-high-pressure (UHP) terrane and contain coesite-bearing eclogite. Two distinct types of garnet peridotite were identified based on mode of occurrence and petrochemical characteristics. Type A mantle-derived peridotites originated from either: (1) the mantle wedge above a subduction zone, (2) the footwall mantle of the subducted slab, or (3) were ancient mantle fragments emplaced at crustal depths prior to UHP metamorphism, whereas type B crustal peridotite and pyroxenite are a portion of mafic–ultramafic complexes that were intruded into the continental crust as magmas prior to subduction. Most type A peridotites were derived from a depleted mantle and exhibit petrochemical characteristics of mantle rocks; however, Sr and Nd isotope compositions of some peridotites have been modified by crustal contamination during subduction and/or exhumation. Type B peridotite and pyroxenite show cumulate structure, and some have experienced crustal metasomatism and contamination documented by high 87Sr/86Sr ratios (0.707–0.708), low εNd( t ) values (−6 to −9) and low δ18O values of minerals (+2.92 to +4.52). Garnet peridotites of both types experienced multi-stage recrystallization; some of them record prograde histories. High- P–T  estimates (760–970 °C and 4.0–6.5±0.2 GPa) of peak metamorphism indicate that both mantle-derived and crustal ultramafic rocks were subducted to profound depths >100 km (the deepest may be ≥180–200 km) and experienced UHP metamorphism in a subduction zone with an extremely low geothermal gradient of <5 °C km−1.  相似文献   

6.
New evidence for ultrahigh‐pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet‐bearing ultramafic rocks from the Pohorje Mountains in Slovenia. The garnet peridotites are closely associated with UHP kyanite eclogites. These rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. On the basis of petrochemical data, garnet peridotites could have been derived from depleted mantle rocks that were subsequently metasomatized by melts and/or fluids either in the plagioclase‐peridotite or the spinel‐peridotite field. At least four stages of recrystallization have been identified in the garnet peridotites based on an analysis of reaction textures and mineral compositions. Stage I was most probably a spinel peridotite stage, as inferred from the presence of chromian spinel and aluminous pyroxenes. Stage II is a UHPM stage defined by the assemblage garnet + olivine + low‐Al orthopyroxene + clinopyroxene + Cr‐spinel. Garnet formed as exsolutions from clinopyroxene, coronas around Cr‐spinel, and porphyroblasts. Stage III is a decompression stage, manifested by the formation of kelyphitic rims of high‐Al orthopyroxene, aluminous spinel, diopside and pargasitic hornblende replacing garnet. Stage IV is represented by the formation of tremolitic amphibole, chlorite, serpentine and talc. Geothermobarometric calculations using (i) garnet‐olivine and garnet‐orthopyroxene Fe‐Mg exchange thermometers and (ii) the Al‐in‐orthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at conditions of around 900 °C and 4 GPa. These results suggest that garnet peridotites in the Pohorje Mountains experienced UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of continental crust, which incorporated mantle peridotites from the upper plate, in an intracontinental subduction zone. Sinking of the overlying mantle and lower crustal wedge into the asthenosphere (slab extraction) caused the main stage of unroofing of the UHP rocks during the Upper Cretaceous. Final exhumation was achieved by Miocene extensional core complex formation.  相似文献   

7.
The Jiangzhuang ultrahigh‐pressure (UHP) metamorphic peridotite from south Sulu, eastern China occurs as a layer within gneiss with eclogite blocks, and consists of coarse‐grained garnet porphyroblasts and a fine‐grained matrix assemblage of garnet + forsterite + enstatite + diopside ± phlogopite ± Ti‐clinohumite ± magnesite. Both types of garnet are characterized by high MgO content and depletion of light rare earth element (LREE) and enrichment of heavy rare earth element, but the matrix garnet has lower MgO, TiO2 and higher Cr2O3 and REE contents. Diopside displays LREE enrichment, and has low but variable large‐ion lithophile element (LILE) contents. Phlogopite is a major carrier of LILE. Ti‐clinohumite contains high Nb, Ta, Cr, Ni, V and Co contents. The P–T conditions of 4.5–6.0 GPa and 850–950 °C were estimated for matrix mineral assemblages. Most peridotites are depleted in Al2O3, CaO and TiO2, and enriched in SiO2, K2O, REE and LILE. In contrast to phlogopite‐free peridotites, the phlogopite‐bearing peridotites have higher K2O, Zr, REE and LILE contents. Zircon occurs only in the phlogopite‐bearing peridotites, shows no zoning, with low REE contents and Th/U ratios, and yields tight U–Pb ages of 225–220 Ma, indicating the peridotites experienced consistent Triassic UHP metamorphism with subducted supercrustal rocks. These data demonstrate that the Jiangzhuang peridotites were derived from the depleted mantle wedge of the North China Craton, and experienced various degrees of metasomatism. The phlogopite‐free peridotites may have been subjected to an early cryptic metasomatism at UHP conditions of the mantle wedge, whereas the phlogopite‐bearing peridotites were subjected to a subsequent strong metasomatism, characterized by distinctly enrichment in LILE, LREE, Zr and K as well as the growth of zircon and volatile‐bearing minerals at UHP subduction conditions. The related metasomatism may have resulted from the filtration of fluids sourced mainly from deeply subducted supracrustal rocks.  相似文献   

8.
Garnet peridotites and pyroxenites have been reported from 11 of the 15 or so high-pressure/ultrahigh-pressure (HP/UHP) terranes in Eurasia. Most of these ultramafic rocks are Mg-Cr types, derived from depleted upper mantle, but some are more Fe-rich and originated by crystallization in ultramafic-mafic igneous complexes. The peridotites are polymetamorphic, with HP/UHP garnet-bearing assemblages being followed by a succession of retrograde assemblages related to exhumation and cooling; some also contain evidence for a pre-HP/UHP stage, such as spinel inclusions in garnet or the presence of Ti-clinohumite. Equilibration conditions have been calculated from all available analyses of garnetiferous assemblages, by application of the olivine-garnet Fe-Mg exchange thermometer and the Alin-orthopyroxene barometer, resulting in two distinct P-T regimes for garnet peridotites—one at high P/T in the coesite and diamond fields, and another at low P/T in the vicinity of the spinel-to-garnet transition.

Garnet peridotites are thought to have evolved in at least four different tectonothermal settings, including: (1) emplacement of peridotites into oceanic or continental crust, followed by transport of peridotites and associated crust to UHP conditions by a subducting plate; (2) transfer of peridotites from a mantle wedge to the crust of an underlying, subducting plate; (3) origination from upwelling asthenosphere that passed through a high-temperature spinel peridotite stage, followed by cooling into the garnet peridotite field; and (4) extraction of garnet peridotites from ancient subcontinental lithosphere, perhaps by deep-seated faulting within a continental plate.  相似文献   

9.
The Dabie-Sulu ultrahigh-pressure (UHP) terrane is the largest in the world. Mafic-ultramafic rocks occur as ubiquitous minor components in it, and preserve UHP index minerals such as coesite and microdiamond. Eclogites and garnet peridotites together with their country rocks including quartzofeldspathic, pelitic, psammatic, and carbonate rocks were subjected to coeval UHP meta-morphism during the Triassic collision between the Sino-Korean and Yangtze cratons. This review summarizes petrological and geochemical characteristics of eclogites, garnet peridotites, and UHP metasediments from published data and ongoing research in the Dabie-Sulu belt of east-central China.

Parageneses of minerals from coesite-bearing eclogites encased in gneiss, garnet peridotite, and marble delineate clockwise P-T paths characterized by nearly isothermal decompression. Many eclogites have been overprinted by Barrovian-zone amphibolite- to granulite-facies assemblages, whereas others preserve primary igneous minerals and textures in the cores of coesite-bearing eclogitic blocks. In addition, eclogites contain UHP hydrous and carbonate phases including talc, epidote, zoisite, magnesite, and dolomite; these together with Ti-clinohumite, phlogopite, amphibole, chlorite, and possible talc in garnet peridotites and OH-topaz in kyanite quartzites document the role of hydrous mineral transport to the deep mantle in fluid-deficient UHP metamorphic regions. Both crustal- and mantle-derived garnet peridotites from the Dabie-Sulu region were recrystallized within the diamond stability field, with the Earth's lowest recorded geothermal gradient. <5°C/km, suggesting that UHP metamorphic recrystallization took place in a previously unrecognized, forbidden P-T region.

Geochemical and isotopic data indicate that Dabie-Sulu mafic-ultramafic rocks have diverse origins; their compositions in some cases have been complicated by metamorphic recrystallization, crustal contamination, and fluid metasomatism. Nevertheless, REE geochemical and Nd isotope data clearly indicate that they have “continental” affinities and cannot represent a subducted Tethyan oceanic slab. Garnet peridotites and their enclosing eclogites display variable isotopic compositions; mantle-derived fragments preserve a mantle signature, whereas crust-hosted mafic-ultramafics display distinct crustal contamination and metasomatism. Among the many outstanding projects remaining to be investigated, geochemical and isotopic constraints of mantle-derived garnet peridotites and eclogites should provide an additional window to our understanding of mantle heterogeneity, metasomatism, slab/mantle interactions, and lithospheric evolution of the Sino-Korean craton.  相似文献   

10.
Garnet-bearing mantle peridotites, occurring as either xenoliths in volcanic rocks or lenses/massifs in high-pressure and ultrahigh-pressure terrenes within orogens, preserve a record of deep lithospheric mantle processes. The garnet peridotite xenoliths record chemical equilibrium conditions of garnet-bearing mineral assemblage at temperatures (T) ranging from ~700 to 1,400°C and pressures (P) > 1.6–8.9 GPa, corresponding to depths of ~52–270 km. A characteristic mineral paragenesis includes Cr-bearing pyropic garnet (64–86 mol% pyrope; 0–10 wt% Cr2O3), Cr-rich diopside (0.5–3.5 wt% Cr2O3), Al-poor orthopyroxene (0–5 wt% Al2O3), high-Cr spinel (Cr/(Cr + Al) × 100 atomic ratio = 2–86) and olivine (88–94 mol% forsterite). In some cases, partial melting, re-equilibration involving garnet-breakdown, deformation, and mantle metasomatism by kimberlitic and/or carbonatitic melt percolations are documented. Isotope model ages of Archean and Proterozoic are ubiquitous, but Phanerozoic model ages are less common. In contrast, the orogenic peridotites were subjected to ultrahigh-pressure (UHP) metamorphism at temperature ranging from ~700 to 950°C and pressure >3.5–5.0 GPa, corresponding to depths of >110–150 km. The petrologic comparisons between 231 garnet peridotite xenoliths and 198 orogenic garnet peridotites revealed that (1) bulk-rock REE (rare earth element) concentrations in xenoliths are relatively high, (2) clinopyroxene and garnet in orogenic garnet peridotites show a highly fractionated REE pattern and Ce-negative anomaly, respectively, (3) Fo contents of olivines for off-cratonic xenolith are in turn lower than those of orogenic garnet and cratonic xenolith but mg-number of garnet for orogenic is less than that of off-cratonic and on-cratonic xenolith, (4) Al2O3, Cr2O3, CaO and Cr# of pyroxenes and chemical compositions of whole rocks are very different between these garnet peridotites, (5) orogenic garnet peridotites are characterized by low T and high P, off-cratonic by high T and low P, and cratonic by medium T and high P and (6) garnet peridotite xenoliths are of Archean or Proterozoic origin, whereas most of orogenic garnet peridotites are of Phanerozoic origin. Taking account of tectonic settings, a new orogenic garnet peridotite exhumation model, crust-mantle material mixing process, is proposed. The composition of lithospheric mantle is additionally constrained by comparisons and compiling of the off-cratonic, on-cratonic and orogenic garnet peridotite.  相似文献   

11.
Two Rongcheng eclogite‐bearing peridotite bodies (Chijiadian and Macaokuang) occur as lenses within the country rock gneiss of the northern Sulu terrane. The Chijiadian ultramafic body consists of garnet lherzolite, whereas the Macaokuang body is mainly meta‐dunite. Both ultramafics are characterized by high MgO contents, low fertile element concentrations and total REE contents, which suggests that they were derived from depleted, residual mantle. High FeO contents, an LREE‐enriched pattern and trace‐element contents indicate that the bulk‐rock compositions of these ultramafic rocks were modified by metasomatism. Oxygen‐isotope compositions of analysed garnet, olivine, clinopyroxene and orthopyroxene from these two ultramafic bodies are between +5.2‰ and +6.2‰ (δ18O), in the range of typical mantle values (+5.1 to +6.6‰). The eclogite enclosed within the Chijiadian lherzolite shows an LREE‐enriched pattern and was formed by melts derived from variable degrees (0.005–0.05) of partial melting of peridotite. It has higher δ18O values (+7.6‰ for garnet and +7.7‰ for omphacite) than those of lherzolite. Small O‐isotope fractionations (ΔCpx‐Ol: 0.4‰, ΔCpx‐Grt: 0.1‰, ΔGrt‐Ol: 0.3–0.4‰) in both eclogite and ultramafic rocks suggest isotopic equilibrium at high temperature. The P–T estimates suggest that these rocks experienced subduction‐zone ultrahigh‐pressure (UHP) metamorphism at ～700–800 °C, 5 GPa, with a low geothermal gradient. Zircon from the Macaokuang eclogite contains inclusions of garnet and diopside. The 225 ± 2 Ma U/Pb age obtained from these zircon may date either the prograde conditions just before peak metamorphism or the UHP metamorphic event, and therefore constrains the timing of subduction‐related UHP metamorphism for the Rongcheng mafic–ultramafic bodies.  相似文献   

12.
中国东部上地幔岩石相转变及其意义   总被引：2，自引：0，他引：2  
樊祺诚  隋建立 《地球科学》2009,34(3):387-391
中国东部新生代玄武岩和大别-苏鲁超高压变质带中的含石榴石相橄榄岩, 带来了发生在上地幔的尖晶石→石榴石相转变和铝辉石→贫铝辉石+石榴石的重要信息, 为中国东部上地幔岩石结构分层奠定了重要基础.通过岩石学与实验岩石学的研究, 推导出发生相转变的P-T条件, 为建立中国东部大陆上地幔岩石分层结构提供了重要约束.尖晶石二辉橄榄岩向石榴石二辉橄榄岩相转变发生在55~70km, 随着深度增加, 石榴石二辉橄榄岩从富铝石榴石二辉橄榄岩(70~120km) 转变为贫铝石榴石二辉橄榄岩(> 120~150km).   相似文献   

13.
Garnet peridotites from the southern Su‐Lu ultra‐high‐pressure metamorphic (UHPM) terrane, eastern China, contain porphyroblastic garnet with aligned inclusions comprising a low‐P–T mineral assemblage (chlorite, hornblende, Na‐gedrite, Na‐phlogopite, talc, spinel and pyrite). Orthopyroxene porphyroblasts show fine exsolution lamellae of clinopyroxene and minor chromite. A clinopyroxene inclusion in garnet shows some orthopyroxene exsolution lamellae. Both the rims of porphyroblastic pyroxene and garnet and the matrix pyroxene and garnet crystallized at the expense of olivine. This is interpreted as a result of metasomatism of the peridotites by an SiO2‐rich melt at UHP conditions. A chromian garnet further overgrew on the rims of the garnet. The XMg values (Mg/(Mg+Fe)) of porphyroblastic garnet decrease from core to rim and vary in different peridotite samples, while the compositions of both the porphyroblastic and the matrix pyroxene are similar in terms of Ca–Mg–Fe. The Mg‐rich cores of porphyroblastic garnet and orthopyroxene record high temperatures and pressures (c. 1000 °C, ≥5.1 GPa), whereas the matrix minerals, including the rims of porphyroblasts, record much lower P–T (c. 4.2 GPa, c. 760 °C). Sm–Nd data give apparent isochron ages of c. 380 Ma and negative εNd(0) values (c.?9). These dates are considered meaningless due to isotopic disequilibrium between garnet cores and the rest of the rocks. The isotopic disequilibrium was probably caused by metasomatism of the peridotites by melt/fluids derived from the coevally subducted crustal materials. On the other hand, the Rb–Sr isotopic systems of phlogopite and clinopyroxene appear to have reached equilibrium and record a cooling age of c. 205 Ma. It is suggested that the garnet peridotites were originally emplaced into a low‐P–T environment prior to the c. 220 Ma continental collision, during which they were subducted together with crustal rocks to mantle depth and subjected to UHP metamorphism. An important corollary is that at least some of the coevally subducted crustal rocks in the Su‐Lu terrane have been subjected to peak metamorphism at P–T conditions much higher than presently estimated (≥2.7 GPa, ≤800 °C).  相似文献   

14.
Strain patterns within mantle rocks and surrounding coarse‐grained felsic granulites from the Kutná Hora Crystalline Complex in the Variscan Bohemian Massif have been studied in order to assess their strain coupling. The studied rock association occurs within low‐strain domains surrounded by fine‐grained granulite and migmatite. The Doubrava peridotite contains closely spaced and steeply dipping layers of garnet clinopyroxenite, which are parallel to the NE–SW‐striking, high‐temperature foliation in nearby granulites, while the Úhrov peridotite lacks such layering. The Spa?ice eclogite is not associated with peridotite and shows upright folds of alternating coarse‐ and fine‐grained varieties bearing NE–SW‐striking axial planes. Electron back‐scattered diffraction measurements revealed full strain coupling between clinopyroxenites and coarse‐grained granulites in the S1 fabric that is superposed on the S0 fabric preserved in peridotites. The B‐type olivine lattice preferred orientation (LPO) characterizes the S0 fabric in peridotites and its reworking is strongly controlled by the presence of macroscopic clinopyroxenite layering. The S1 in clinopyroxenites and coarse‐grained granulites is associated with the LS‐type clinopyroxene LPO and prism <c> slip in quartz respectively. While the S1 fabric in these rock types is accompanied invariably by a sub‐vertical stretching lineation, the S1 fabric developed in reworked Úhrov peridotite is associated with strongly planar axial (010) type of olivine LPO. The peridotites with the S0 fabric are interpreted to be relicts of a fore‐arc mantle wedge hydrated to a various extent above the Saxothuringian subduction zone. The prograde metamorphism recorded in peridotites and eclogites occurred presumably during mantle wedge flow and was reaching UHP conditions. Strain coupling in the S1 fabric between clinopyroxenites and granulites at Doubrava and upright folding of eclogites at Spa?ice document a link between tectonic and magmatic processes during orogenic thickening, coeval with intrusions of the arc‐related calcalkaline suites of the Central Bohemian Plutonic Complex (c. 360–345 Ma). Juxtaposition of peridotites and granulites could be explained by a rheological heterogeneity connected to the development of clinopyroxenite layering in the upper mantle and a previously published model of a lithospheric‐scale transpressional arc system. It invokes vertical shearing along NE–SW trending, sub‐vertical foliations in the upper mantle that could have led to an emplacement of mantle bodies into the granulitized, orogenic root in the sub‐arc region. Clearly, such a transpressional arc system could represent an important pathway for an emplacement of deep‐seated rocks in the orogenic lower crust.  相似文献   

15.
Equilibrium pressure–temperature (P–T) conditions were estimated for kyanite‐bearing eclogite from Nové Dvory, Czech Republic, by using garnet–clinopyroxene thermometry and garnet–clinopyroxene–kyanite–coesite (or quartz) barometry. The estimated P–T conditions are 1050–1150 °C, 4.5–4.9 GPa, which are mostly the same as previously estimated values for garnet peridotite from Nové Dvory (～1100–1250 °C, 5–6 GPa). Such very high‐P conditions, which correspond to about 150‐km depth, have been obtained for some garnet peridotites in the Gföhl Unit of the Bohemian Massif, but pressure conditions of eclogites associated with the garnet peridotites have not been so well constrained. This is the first substantial finding of eclogite that gives such very high‐P conditions in the Gföhl Unit of the Bohemian Massif. The Gföhl Unit mainly consists of felsic granulite or migmatitic gneiss, but these rock types do not display high‐P (>2.5 GPa) evidence. It is unclear whether both the peridotite body and surrounding felsic rocks in the Gföhl Unit were buried to very deep levels, but at least some garnet peridotites and associated eclogites in the Gföhl Unit have ascended from about 150‐km depth.  相似文献   

16.
This paper presents monomineral and multiphase inclusions in garnet from eclogites and clinopyroxenites, which form layers and boudins in garnet peridotites from two areas in the Moldanubian zone of the Bohemian Massif. The garnet peridotites occur in felsic granulites and reached UHP conditions prior to their granulite facies overprint. In addition to complex compositional zoning, garnets from hosting eclogites and clinopyroxenites preserve inclusions of hydrous phases and alkali silicate minerals including: amphiboles, chlorites, micas and feldspars. Amphibole, biotite and apatite inclusions in garnet have a high concentration of halogens; CO2 and sulfur are involved in carbonates and sulfide inclusions, respectively. The inclusion patterns and compositional zoning in garnet in combination with textural relations among minerals, suggest that the ultramafic and mafic bodies are derived from lithospheric mantle above the subduction zone and were transformed into garnet pyroxenites and eclogites in the subduction zone. Based on compositional, mineral and textural relations, all of these rocks along with the surrounding crustal material were overprinted by granulite facies metamorphism during their exhumation.  相似文献   

17.
Five domains (microplates) have been recognized by seismic anisotropy in the mantle lithosphere of the Bohemian Massif. The mantle domains correspond to major crustal units and each of the domains bears a consistent fossil olivine fabric formed before their Variscan assembly. The present-day mantle fabric indicates that this process consisted of at least three oceanic subductions, each followed by an underthrusting of the continental lithosphere. The seismic anisotropy does not detect remnants of the oceanic subductions, but it can trace boundaries of the preserved continental domains subsequently underthrust along the paths of previous oceanic subductions. The most robust continent–continent collision was followed by westward underthrusting of the Brunovistulian mantle lithosphere, still detectable by seismic anisotropy more than 100 km beneath the Moldanubian mantle lithosphere. Major occurrences of the high-pressure/ultra high-pressure (HP–UHP) rocks follow the ENE and NNE oriented sutures and boundaries of the mantle–lithosphere domains mapped from three-dimensional modeling of body-wave anisotropy. The HP–UHP rocks are products of oceanic subductions and the following underthrusting of the continental crust and mantle lithosphere exhumed along the mantle boundaries. The close relation of the mantle sutures and occurrences of the HP–UHP rocks near the paleosubductions testifies for models interpreting the granulite–garnet peridotite association by oceanic/continental subduction/underthrusting followed by the exhumation of deep-seated rocks. Our findings support the bivergent subduction model of tectonic development of the central part of the Bohemian Massif. The inferences from seismic anisotropy image the Bohemian Massif as a mosaic of microplates with a rigid mantle lithosphere preserving a fossil olivine fabric. The collisional mantle boundaries, blurred by tectonometamorphic processes in easily deformed overlying crust, served as major exhumation channels of the HP–UHP rocks.  相似文献   

18.
The Ronda high temperature peridotite: Geochemistry and petrogenesis   总被引：2，自引：0，他引：2  
The Ronda peridotite in southern Spain is a large (~300 km2) exposure of upper mantle which provides direct information about mantle processes on a scale much larger than that provided by mantle xenoliths in basalt. Ronda peridotites range from harzburgite to lherzolite, and vary considerably in major element content, e.g., Al2O3 from 0.9 to 4.8%, and trace element abundances, e.g., Sr, Zr and La abundances vary by factors of 20 to 40. These compositional variations are systematic and correlate with (pyroxene + garnet)/olivine ratios and olivine compositions. The data are consistent with formation of residual peridotites by variable degrees of melting (~0 to 30%) of a compositionally homogeneous peridotite. None of the peridotites have geochemical characteristics of residues formed by extensive (?5%) fractional melting and the data can be explained by equilibrium (batch) melting, possibly with incomplete melt segregation in some samples. Based on compositional differences between Ronda peridotites, the segregated melts were picritic (12–22% MgO) with relative rare earth element abundances similar to mid-ocean ridge basalt (MORB). Prior to the melting event the Ronda peridotite body was a suitable source for MORB. The compositional characteristics of Ronda peridotites are consistent with diapiric rise of a fertile mantle peridotite with relatively small degrees of melting near the diapir-wall rock interface yielding residues of garnet iherzolite, and larger degrees of melting in the diapir interior yielding residues of garnet-free peridotite. Subsequently these residual rocks were recrystallized at sub-solidus conditions (Obata, 1980), and emplaced in the crust by thrusting (Lundeen, 1978).  相似文献   

19.
Garnet peridotites occur in quartzofeldspathic gneisses in the Northern Qaidam Mountains, western China. They are rich in Mg and Cr, with mineral compositions similar to those in mantle peridotites found in other orogenic belts and as xenoliths in kimberlite. Garnet‐bearing lherzolites interlayered with dunite display oriented ilmenite and chromite lamellae in olivine and pyroxene lamellae in garnet that have been interpreted to indicate pressures in excess of 6 GPa. However, some garnet porphyroblasts include hornblende, chlorite and spinel + orthopyroxene symplectite after garnet; some clinopyroxene porphyroblasts include abundant actinolite/edenite, calcite and lizardite in the lherzolite; some olivine porphyroblasts (Fo92) include an earlier generation Mg‐rich olivine (Fo95–99), F‐rich clinohumite, pyroxene, chromite, anthophyllite/cummingtonite, Cl‐rich lizardite, carbonates and a new type of brittle mica, here termed ‘Ca‐phlogopite’, in the associated dunite. The pyrope content of garnet increases from core to rim, reaching the pyrope content (72 mol.%) of garnet typically found in the xenoliths in kimberlite. The simplest interpretation of these observations is that the rock association was formerly mantle peridotite emplaced into the oceanic crust that was subjected to serpentinization by seawater‐derived fluids near the sea floor. Dehydration during subduction to 3.0–3.5 GPa and 700 °C transformed these serpentinites into garnet lherzolite and dunite, depending on their Al and Ca contents. Pseudosection modelling using thermocalc shows that dehydration of the serpentinites is progressive, and involved three stages for Al‐rich and two stages for Al‐poor serpentinites, corresponding to the breakdown of the key hydrous minerals. Static burial and exhumation make olivine a pressure vessel for the pre‐subduction mineral inclusions during ultrahigh‐pressure (UHP) metamorphism. The time span of the UHP event is constrained by the clear interface between the two generations of olivine to be very short, implying rapid subduction and exhumation.  相似文献   

20.
The peridotite bodies of the Ulten Zone (Upper Austroalpine, Italian Eastern Alps) are enclosed in Variscan migmatites and derive from a mantle wedge environment. They display the progressive transformation of porphyroclastic spinel peridotites (T=1,200°C; P=1.5 GPa) into fine-grained garnet–amphibole peridotites (T=850°C; P=3 GPa). Detailed bulk-rock and mineral trace element analyses of a sample suite documenting the entire metamorphic evolution of the peridotites revealed several stages of metasomatism. The spinel peridotites derive from a depleted mantle that became enriched in some large ion lithophile element (LILE) and light rare earth elements (LREE). The same signature pertains to clinopyroxene and orthopyroxene, indicating that this metasomatic signature was acquired at the recorded temperature of 1,200°C. Such a temperature is considerably above the wet peridotite solidus and hence the metasomatic agent must have been a hydrous melt. Moreover, the Li-enrichment of the spinel-facies pyroxenes (up to 24 ppm Li) reflects disequilibrium distribution after exchange with a presumably mafic melt. cpx/opx D Li=3–7 and cpx/ol D Li=2.7–8 indicate that the spinel-facies clinopyroxene hosts higher Li amounts than the coexisting minerals. LREE fractionation, variable LREE enrichment, LILE enrichment with respect to HFSE (average clinopyroxene Pb N /Nb N =16–90) in spinel lherzolites can be related to chromatographic effects of porous melt flow. The significant enrichment of pyroxenes from the spinel lherzolites in Pb, U and Li indicates that the metasomatic melt was subduction-related. All these features suggest that the spinel lherzolites formed a mantle wedge layer percolated by melts carrying recycled crustal components and rising from a deeper source of subduction magmas. The garnet + amphibole peridotites equilibrated at temperatures well below the wet solidus in the presence of an aqueous fluid. Bulk-rock trace element patterns display pronounced positive anomalies in Cs, Ba, Pb and U and moderate enrichment in Li, indicating addition of a crustal component to the mantle rocks. Amphibole hosts most of these trace elements. Clinopyroxene displays high LILE/HFSE (Pb N /Nb N =300–600), low Ce/Pb (1.4–2.7 in garnet-facies clinopyroxene compared with 2.6–24.5 in the spinel-facies one) and variable LILE and LREE enrichments. The coupled increase of modal amphibole, Sr and Pb, together with positive Pb–Sr and Pb–U correlations, further indicate that incompatible element influx in these samples was fluid-mediated. In the garnet-facies samples, amphibole and, interestingly, olivine have similarly high Li concentrations as clinopyroxene, leading to cpx/amph D Li=0.7 and cpx/ol D Li=0.7–0.8, the latter being up to ten times lower than in the spinel-facies rocks. Due to its high modal abundance, olivine is the main host of Li in the garnet–amphibole peridotites. The observed metasomatic features provide evidence for the infiltration of an aqueous fluid in the mantle wedge above a subducting slab. This fluid most likely derived from subducted crustal rocks that underwent partial melting. Successive retrograde re-equilibration during exhumation of the garnet peridotite is accompanied by garnet and clinopyroxene breakdown and amphibole formation. This process produced minor changes, such as an increase of HREE and Li in amphibole, and an increase of Li in olivine. The general trace element signature remains essentially unchanged during retrogression and further hydration, indicating that fluids with a similar composition to the one present at the garnet–amphibole peridotite formation, were responsible for increased amphibole formation. The combined evidence from the metamorphic and metasomatic evolution indicates that the peridotites experienced first corner flow in a mantle wedge, followed by subduction and finally entrapment and exhumation within a crustal slab. During their entire history the Ulten peridotites were percolated first by melts and then by aqueous fluids, which added recycled crustal components to the mantle wedge.  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