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1.
Experiments on MORB?+?4 wt% H2O at 0.8–2.8 GPa and 700–950 °C (Liu in High pressure phase equilibria involving the amphibolite–eclogite transformation. PhD dissertation, Stanford University, Stanford, California, 1997; Liu et al. in Earth Planet Sci Lett 143:161–171, 1996) were reexamined for their major and trace element melt compositions and melting relations. Degree of melting diminishes at greater pressures, with corresponding evolution of melt from andesitic at the lowest pressures and hottest temperatures to high-silica rhyolitic at the greatest pressure and coolest temperature. Quartz contributes greatly to the production of near-solidus melts of basaltic eclogite, with the result that melt productivity falls markedly following quartz exhaustion. This limits the extent of melting attainable in the basaltic eclogite portions of sub-arc subducting plates to no more than ~?2?×?the modal wt% quartz in the mafic eclogite protolith. Synthesized residual mineral assemblages lack an epidote-series mineral at temperatures?>?750 °C, and as a result, melts from the rutile eclogite and rutile-amphibole eclogite facies have elevated concentrations of light rare earth elements, U, Th, have elevated Ba, K, and Sr, high Sr/Y, and are strongly depleted in Nb, Y, and the heavy rare earth elements. Models of eclogite partial melt reacting with peridotite of the mantle wedge reproduce major and trace element characteristics of parental arc magmas so long as the proportions of infiltrating melt to peridotite are relatively high, consistent with channelized ascent. Melt mass is estimated to increase roughly three- to ten-fold, consistent with H2O concentrations of 3–7 wt% in the magmas produced by reaction. Partial melts of subducting basaltic eclogite are predicted to have positive Sr concentration anomalies, relative to Ce and Nd, that persist through melt-peridotite reactions. Primitive arc magmas commonly have positive Sr anomalies, whereas such anomalies are smaller in estimates of the bulk continental crust. Overall, Sr anomalies diminish passing from primitive to more evolved arc volcanic rocks, consistent with extensive mineral-melt differentiation (crystallization, partial remelting) involving plagioclase. On the order of 50 wt% differentiation would be necessary to eliminate Sr positive anomalies, based on geochemical variations in the Cascade and western Aleutian magmatic arcs. Loss to the mantle of cumulates and restites with high Sr anomalies, in abundances broadly equal to the mass of the preserved crust, would be required to form the continents via processes similar to present-day subduction magmatism.  相似文献   

2.
Orthopyroxene‐free garnet + clinopyroxene + plagioclase ± quartz‐bearing mineral assemblages represent the paragenetic link between plagioclase‐free eclogite facies metabasites and orthopyroxene‐bearing granulite facies metabasites. Although these assemblages are most commonly developed under P–T conditions consistent with high pressure granulite facies, they sometimes occur at lower grade in the amphibolite facies. Thus, these assemblages are characteristic but not definitive of high pressure granulite facies. Compositional factors favouring their development at amphibolite grade include Fe‐rich mineral compositions, Ca‐rich garnet and plagioclase, and Ti‐poor hornblende. The generalized reaction that accounts for the prograde development of garnet + clinopyroxene + plagioclase ± quartz from a hornblende + plagioclase + quartz‐bearing (amphibolite) precursor is Hbl + Pl + Qtz=Grt + Cpx + liquid or vapour, depending on whether the reaction occurs above or below the solidus. There are significant discrepancies between experimental and natural constraints on the P–T conditions of orthopyroxene‐free garnet + clinopyroxene + plagioclase ± quartz‐bearing mineral assemblages and therefore on the P–T position of this reaction. Semi‐quantitative thermodynamic modelling of this reaction is hampered by the lack of a melt model and gives results that are only moderately successful in rationalizing the natural and experimental data.  相似文献   

3.
The sub-solidus fields of crystallization of a spectrum of synthetic aluminous basic compositions (high-alumina basalt, anorthite-enriched high-alumina basalt, kyanite eclogite, grosspydite and gabbroic anorthosite) have been investigated at pressures of up to 36 kb. At low pressures the assemblages are characterized by abundant plagioclase, clinopyroxene and possibly minor olivine and orthopyroxene. These correspond to natural gabbroic and pyroxene granulite assemblages. As pressure is increased garnet appears and increases gradually in amount at the expense of other ferromagnesian minerals and plagioclase, until finally at pressures of >23 kb at 1,100° C, plagioclase disappears and high pressure clinopyroxene+garnet+kyanite±quartz assemblages equivalent to eclogite are obtained. In the eclogite stability field, with further rise in pressure, the ratio ga/cpx and the grossular content of the garnet increase.In the high-alumina basalt composition the transitional garnet granulite assemblage (clinopyroxene+plagioclase+garnet±quartz) is spread over a pressure interval of 11 kb at 1,100° C. This is a greater interval than observed for other basalt compositions and is important in considering the hypothesis that the Mohorovicic Discontinuity is a phase change from basalt to eclogite. It indicates that the change in V p would be spread over a significant depth range, and no sharp seismic velocity discontinuity could result.The first experimental synthesis of kyanite eclogite from both high-alumina basalt and kyanite eclogite compositions has been obtained, as well as synthesis of unusual grossular-clinopyroxene-kyanite assemblages (grosspydite) from grosspydite and gabbroic anorthosite compositions. The pressures needed to synthesize these assemblages are somewhat greater than the pressures needed to synthesize eclogite from basic compositions of lower alumina content at the same temperature. Experimental confirmation of the observation that there is a direct relation between Gross/Alm + Py ratio of garnet and the Jd/Di ratio of co-existing pyroxene in grosspydite and kyanite eclogite assemblages found in kimberlite pipes has also been obtained.  相似文献   

4.
中国大陆科学钻探主孔0-4500米的岩心主要由榴辉岩、斜长角闪岩、副片麻岩、正片麻岩以及少量的超基性岩所组成。岩相学研究结果表明,榴辉岩的围岩普遍经历了强烈角闪岩相退变质作用的改造,峰期超高压变质的矿物组合已完全被后期退变质过程中角闪岩相矿物组合所替代。采用激光拉曼技术,配备电子探针和阴极发光测试,发现主孔224件岩心中有121件(包括榴辉岩、斜长角闪岩、副片麻岩和正片麻岩)样品的锆石中普遍隐藏以柯石英为代表的超高压矿物包体,且不同岩石类型锆石中所保存的超高压矿物包体组合存在明显差异。(含多硅白云母)金红石石英榴辉岩锆石中保存的典型超高压包体矿物组合为柯石英 石榴石、柯石英 石榴石 绿辉石 金红石和柯石英 多硅白云母 磷灰石。黑云绿帘斜长角闪岩锆石中保存的超高压矿物组合为柯石英 石榴石 绿辉石、柯石英 石榴石 多硅白云母和柯石英 绿辉石 金红石,与榴辉岩所保存的超高压矿物组合十分相似,表明该类斜长角闪岩是由超高压榴辉岩在构造折返过程中退变质而成。在副片麻岩类岩石,如石榴绿帘黑云二长片麻岩锆石中,代表性的超高压包体矿物组合为柯石英 多硅白云母和柯石英 石榴石等;而在石榴黑云角闪钠长片麻岩锆石中,则保存柯石英 硬玉 石榴石 磷灰石、柯石英 硬玉 多硅白云母 磷灰石和柯石英 石榴石 磷灰石等超高压矿物包体。在正片麻岩锆石中,标志性的超高压矿物包体为柯石英、柯石英 多硅白云母、柯石英 蓝晶石 磷灰石和柯石英 蓝晶石 榍石等。此外,在南苏鲁东海至临沭一带的地表露头以及一系列卫星孔岩心的锆石中,也普遍发现以柯石英为代表的标志性超高压矿物包体,表明在南苏鲁地区由榴辉岩及其围岩的原岩所组成的巨量陆壳物质(方圆>5000km2,厚度超过4.5km)曾整体发生深俯冲,并经历了超高压变质作用。该项研究对于重塑苏鲁-大别超高压变质带俯冲-折返的动力学模式有着重要的科学意义。  相似文献   

5.
富铌玄武岩：板片熔体交代的地幔楔橄榄岩部分熔融产物   总被引：4，自引：0，他引：4  
富铌玄武岩是一类具有特殊地球化学特征的岛弧玄武岩。与正常岛弧玄武岩相比，它具有硅饱和并富钠的特征；同时具有相对高的Nb（一般>7×10-6）、TiO2（1%～2%）和P含量，以及低的LILE/HFSE和HREE/HFSE比值，并富集高场强元素；它的原始地幔标准化微量元素图显示了弱的Nb、Ta负异常（有时出现弱的正异常），原始地幔标准化La/Nb比值小于2（但很少小于0.7），它是由受埃达克质熔体交代过的地幔橄榄岩部分熔融形成的。由于富铌玄武岩与埃达克岩是大洋板片俯冲作用的直接产物，因此，通过对该岩石组合及与俯冲作用有关的流体和熔体的研究，不仅可以查明洋壳俯冲作用过程中的岩浆活动特征，还可以阐明洋壳俯冲及壳幔相互作用，具十分重要的地质意义。  相似文献   

6.
Rare earth elements, Rb, Sr, Ba and K have been determined in tonalite, trondhjemite, dacite, tholeiite and graywacke from the 2700 m.y. old Early Precambrian greenstone-granite terrane of northeastern Minnesota-northwestern Ontario, and also in trondhjemite from the 3550 m.y. old Morton Gneiss, southwestern Minnesota; and the Mesozoic Craggy Peak Pluton, Klamath Mountains, California.The Early Precambrian tholeiites have trace element compositions similar to modern oceanic tholeiites, while the quartz dioritic rocks, regardless of age, have total rare earth contents lower than that of tholeiitic basalts, with near chondritic heavy rare earth contents. Rb, Sr, Ba and K contents of the quartz diorites are about five times that of oceanic tholeiites, with similar alkali and alkaline earth ratios. The Early Precambrian graywacke has a rare earth content intermediate between greenstone and quartz diorite, reflecting its provenance.It is proposed that the analyzed quartz dioritic rocks, whether plutonic tonalite, dacite porphyry, gneissic or plutonic trondhjemite, or trondhjemite dikes had similar modes of origin, and were derived by partial melting of amphibolite or eclogite of basaltic or gabbroic composition at depths greater than thirty kilometers, leaving a residue consisting predominantly of garnet and clinopyroxene.  相似文献   

7.
Olivine is abundant in Earth’s upper mantle and ubiquitous in basaltic lavas, but rarely occurs in eclogite. Partial melts of eclogite are, therefore, not in equilibrium with olivine, and will react with peridotite as they migrate through the upper mantle. If such melts erupt at Earth’s surface, their compositions will be highly modified and they may be olivine-saturated. We investigated experimentally the reaction between olivine and siliceous eclogite partial melt, and determined element partitioning between olivine and the melt produced by this reaction. Our results demonstrate that mixing of reacted eclogite partial melt with primitive basalt is capable of producing the positive correlation between melt SiO2 content and olivine Ni content observed in some Hawaiian lavas. Experiments were carried out by equilibrating eclogite partial melt or basalt with San Carlos olivine at 1 bar and 1,201–1,350°C. Our results show that eclogite partial melts equilibrated with mantle olivine retain their high SiO2, low FeO and MgO characteristics. Further, olivine-melt partition coefficients for Ni measured in these experiments are significantly larger than for basalt. Mixing of these melts with primitive Hawaiian tholeiitic lavas results in crystallization of high-Ni olivines similar to those in Makapuu-stage Koolau lavas, even though the mixed magmas have only moderate Ni contents. This results from a hyperbolic increase of the Ni partition coefficient with increasing polymerization of the mixed melt. Note that while eclogite partial melt in contact with peridotite will equilibrate with pyroxene as well as olivine, this will have the effect of buffering the activity of SiO2 in the reacted melt at a higher level. Therefore, an eclogite partial melt equilibrated with harzburgite will have higher SiO2 than one equilibrated with dunite, enhancing the effects observed in our experiments. Our results demonstrate that an olivine-free “hybrid” pyroxenite source is not required to explain the presence of high-Ni olivines in Hawaiian lavas and, therefore, indicate that the proportion of eclogite in the Hawaiian plume is less than has been estimated in recent studies.  相似文献   

8.
The Shuanghe ultrahigh-pressure (UHP) slab in the Dabie Mountains consists of layered coesite-bearing eclogite, jadeite quartzite, marble and biotite gneiss, and is fault bounded against hosting orthogneiss. Representative assemblages of eclogite are Grt+Omp+Coe+Rt±Ky±Phn±Mgs; it formed at P>27 kbar and 680–720±50 °C. During exhumation, these UHP rocks experienced multistage retrograde metamorphism. Coesite was overprinted by quartz aggregates, phengite by biotite±muscovite and rutile by titanite. Garnet was successively replaced by a thin rim of Amp, Amp+Pl, and Amp+Ep±Bt+Pl (minor). Omphacite and kyanite were replaced by Amp+Pl±Cpx (or ±Bt) and by Zo+Pl+Ms±Mrg±Bt, respectively. Secondary calcite occurs as irregular pockets in some layers. An outcrop near the UHP slab border is composed of 20 thin, concordant layers of foliated eclogites, amphibolite and gneissic rocks of variable bulk composition. These layers exhibit mineral assemblages and textures transitional from less altered through extensively retrograded eclogite to gneissic rock of low-amphibolite facies through hydration, metasomatism and recrystallization. Retrograde metamorphism has caused oxygen and hydrogen isotope disequilibria between some of the minerals, but the fluid for retrograde reactions was internally buffered in the stable isotope compositions. Retrograde metamorphism of variable extent may be attributed to selective infiltration of retrograde fluids of CO2-rich and low-salinity aqueous, intensity of deformation and mineral resistance to alteration. The fluid phase for retrogression may have occurred either as discontinuous flow along grain boundaries in completely retrograded eclogites, and/or as isolated pockets in extensive or less altered eclogite layers.  相似文献   

9.
Most models of high-alumina arc basalt petrogenesis rely heavilyon the supposition that the abundances of certain trace elements,in particular the relatively unfractionated Rare Earth Element(REE) patterns and the unusually high concentrations of K, Rb,Sr, and Ba are incompatible with a garnet-bearing subductedoceanic crustal (quartz eclogite) source rock. We have carefullyexamined this apparently unequivocal evidence in light of recentprogress on the physics of melt extraction and the heat transferand mechanics of magma ascent. The weakest element of all traceelement models involving a quartz eclogite source is the assumptionthat the element concentrations are fixed at the source andonly later modified in the near-surface environment. We expandon such models by monitoring the concentrations of REE and majorand trace elements during initial melting, ascent, and extractionof magma. This is done by combining calculated cooling curvesfor ascending magmatic bodies with high pressure phase equilibria.The amount that each phase contributes to the melt is monitoredalong with the composition of the melt and residual solids.With quartz eclogite, initial melting initiates gravitationalinstability of the entire source material (melt plus solids)before melt extraction can occur. During ascent of this mush,melting increases until the solids can be repacked to free themelt. This extraction takes place some 15–20 km abovethe slab, after about 50 per cent melting, at which point themelt has a pattern of REE and other trace element concentrationsalmost identical to those observed in high-alumina arc basalts,assuming an initial composition equivalent to altered oceaniccrust plus 5 per cent pelagic sediment. Sr abundances are theonly ones which are not well-matched by this process. The majorelement concentrations of the extracted melt also closely matchthose of high-alumina arc basalt. A similar, but less detailedevaluation of both fertile and depleted peridotite source rocksyields good agreement for the REE and other trace element concentrationsassuming a LREE-enriched source rock strongly enriched in K,Rb, Sr, and Ba. Ni, Cr, and Co abundances are satisfied onlythrough substantial low pressure fractionation of mafic phases,in particular olivine. Though not rigorously tested, such aprocess may be compatible with the observed major element concentrationsof high-alumina basalt. However, the experimentally verifiedfact that high-alumina basalts could never have been in equilibriumwith either an olivine-bearing magma or source rock eliminatesthis possibility altogether. Thus, the simultaneous considerationof the mechanics of ascent and melt extraction along with phaseequilibria clearly shows that partial melting of quartz eclogitebest satisfies the chemical constraints of major, trace, andREE characteristics of high-alumina arc basalts.  相似文献   

10.
Natural basalts and eclogites were investigated experimentallyat a series of temperatures in the pressure range 1 atm to 40kb and with water pressures 1 to 10 kb. Some runs were alsomade on related synthetic systems at 10 and 33 kb. The two principal magma types recognized by field investigators—tholeiiteand alkali basalt types-appear to be separated by equilibriumthermal divides at 1 atm. The principal divides were found byexperiment at elevated pressures to give way to a new set ofequilibrium thermal divides resulting from a new mineralogy.The change of the equilibrium thermal divides with pressureleads to the derivation of the two principal magma trends fromthe same bulk composition. The melting behavior of basalts and eclogites indicates thatboth are the partial melting products of a more primitive rock(e.g. garnet peridotite). In the region of magma generation(below 60 km) the parental material, presumed to be garnet peridotite,yields an eclogitic magma and its fractionation depends on thegarnet and omphacite of the eclogite, not on plagioclase andclinopyroxene of a basaltic magma. Increase of the garnet constituentsin the magma at high pressure by effective removal of omphaciteor shift of the garnet-omphacite boundary ‘surface’will give rise to a tholeiite-type magma at low pressure. Similarly,increase of the omphacite constituents in the magma at highpressure by physical or physicochemical means will give riseto an alkali basalt-type magma at low pressure. In general,alkali basalt-type magmas are to be expected to be generatedat greater depths than tholeiite-type magmas from the same primarysource rock. Establishment of the two major basalt series takes place inthe region of generation; additional minor diversification ofeach series may come about after emplacement in or on the crustby crystal settling, oxidation or reduction, gas fluxing, contamination,and other processes. The derivative magmas are greatly restrictedby the course of liquid thermal descent imposed at generation. Pressure-temperature limits established experimentally suggestthat the basalt-eclogite transformation may be responsible forthe Mohorovii discontinuity under the continents, but not underthe oceans. The field of stability of basalt is drastically reduced in thepresence of water, and amphibolite is produced. The meltingof amphibolite takes place over a much greater range of temperaturethan basalt. At 10 kb water pressure the beginning of meltingof amphibolite closely approaches that of granite. Partial meltingof amphibolite may yield anorthositic liquids having a relativelylow anorthite content at exceptionally low temperatures. Eclogiteitself is not stable in the presence of water and gives placeto amphibolite or pyroxene hornblendite. Magmas which crystallizeto basalt, gabbro, or eclogite must have had a low water-contentat the time of crystallization. Fifteen rock and twenty-three mineral analyses as well as numerouspartial chemical analyses of experimental products were madeby J. H. Scoon in the course of the investigation. These chemicalanalyses bear on many mineralogical and petrological problems.  相似文献   

11.
We present melt and mineral compositions from nominally anhydrouspartial melting experiments at 2–3 GPa on a quartz eclogitecomposition (G2) similar to average oceanic crust. Near-soliduspartial melts at 3 GPa, determined with melt traps of vitreouscarbon spheres, have 55–57 wt % SiO2, rather less silicathan the dacitic compositions that are generally assumed fornear-solidus eclogite partial melts. At 2 GPa, equivalent near-soliduspartial melts are less silicic (  相似文献   

12.
George W. DeVore 《Lithos》1983,16(4):255-263
The hydrated altered basalts of the oceanic crust, if subducted, could release large volumes of water during their phase transformations and partial melting reactions. If heat from shear strain recrystallizations largely heats the altered basaltic layer, only a 2 km thick layer could be heated significantly before the slab reaches 80–125 km depth. Most of the released waters should be driven into the deeper but cooler regions at the slab and andesitic-tonalitic partial melts should form in the heated altered basalt layer. Large volumes of water initially at the surface could be added to the mantle and tonalitic partial melts could be lost from the oceanic crust and added to the continental plate and could account for continental growth.  相似文献   

13.
Although a 1972 dredging by USNS Eltanin from the submarine Naturaliste Plateau was reported to yield rocks of continental origin, a re‐examination of the dredge haul shows that the rock clasts are in fact altered tholeiitic basalts. They have affinities both with MOR basalts and, especially, within‐plate basalts. Petrographically they correlate most closely with the Bunbury Tholeiitic Suite on the Australian mainland to the east. The basalts are reworked cobbles in a manganiferous Quaternary slump mass, which contains a quartz‐rich, felsic, detrital mineral suite with a granite‐gneiss provenance. The basalt cobbles were part of a basal conglomerate, which covered large areas of the Plateau. It is suggested that this was laid down from nearby elevated volcanic structures formed during the inception of seafloor spreading and the separation of Greater India from Australia at about 122 Ma BP.  相似文献   

14.
Geochemical differences between island arc basalts (IAB) andocean-floor basalts (mid-ocean ridge basalts; MORB) suggestthat the large-ion lithophile elements (LILE) K, Ba, Rb andCs are probably mobilized in subduction zone fluids and melts.This study documents LILE enrichment of eclogite, amphibolite,and epidote ± garnet blueschist tectonic blocks and relatedrocks from melanges of two subduction complexes. The samplesare from six localities of the Franciscan Complex, California,and related terranes of Oregon and Baja California, and fromthe Samana Metamorphic Complex, Samana Peninsula, DominicanRepublic. Most Franciscan blocks are MORB-like in their contentsof rare earth elements (REE) and high field strength elements(HFSE); in contrast, most Samana blocks show an IAB signatureof these elements. The whole-rock K2O contents of both groupsrange from 1 to 3 wt %; K, Ba, Rb, and Cs are all strongly intercorrelated.Many blocks display K/Ba similar to metasomatized transitionzones and rinds at their outer margins. Some transition zonesand rinds are enriched in LILE compared with host blocks; othersare relatively depleted in these elements. Some LILE-rich blockscontain ‘early’ coarse-grained muscovite that isaligned in the foliation defined by coarse-grained omphaciteor amphibole grains. Others display ‘late’ muscovitein veins and as a partial replacement of garnet; many containboth textural types. The muscovite is phengite that contains3.25–3.55 Si per 11 oxygens, and 0.25–0.50 Mg per11 oxygens. Lower-Si phengite has a significant paragonite component:Na per 11 oxygens ranges to 0.12. Ba contents of phengite rangeto over 1 wt % (0.027 per 11 oxygens). Ba in phengite does notcovary strongly with either Na or K. Ba contents of phengiteincrease from some blocks to their transition zones or rinds,or from blocks to their veins. Averaged K/Ba ratios for phengiteand host samples define an array which describes other subsamplesof the block and other analyzed blocks. Phengite carries essentiallyall of the LILE in otherwise mafic eclogite, amphibolite, andgarnet blueschist blocks that are enriched in these elementscompared with MORB. It evidently tracks a distinctive type ofLILE metasomatism that attends both high-T and retrograde subductionzone metamorphism. An obvious source for the LILE is a fluidin equilibrium with metasedimentary rocks. High-grade semipeliticschists from subduction complexes and subductable sediment displayLILE values that resemble those seen in the most LILE-rich blocks.Modeling of Ba and Ti suggests that 1–40 wt % of phengiteadded to MORB can produce their observed LILE enrichment. Thus,the release of LILE from such rocks to fluids or melts in veryhigh-T and -P parts of subduction zones probably depends criticallyon the stability and solubility relations of phengite, whichis thought to be stable at pressures as high as 95–110kbar at T=750–1050°C. KEY WORDS: geochemistry; LT eclogite; mineral chemistry; metasomatism; phengite  相似文献   

15.
ABSTRACT The early metamorphic history of high-grade exotic blocks in the Franciscan Complex may be more complicated than previously supposed. The different assemblages of high-grade glaucophane schist, eclogite, amphibolite and hornblende schist are commonly considered to have formed at the same time from essentially unmetamorphosed oceanic crust. However, new textural and mineralogical data presented here suggest that high-grade glaucophane schist and eclogite have replaced an earlier epidote-amphibolite facies assemblage that is identical to the primary assemblages in many of the hornblende-rich blocks. At least some of the hornblende-rich blocks may therefore be well-preserved remnants of the earlier metamorphism. Comparison of the mineral assemblages and element partitioning in the mixed-assemblage blocks suggests that the glaucophane schist and eclogite metamorphism took place at slightly lower temperatures but at the same or higher pressures than the earlier, hornblende-forming stage.  相似文献   

16.
Minor trachyte and dacite temporally associated with, but spatially separated from, arc-trench type volcanoes in Papua New Guinea have distinctive REE abundances similar to experimentally produced and theoretically predicted partial melts of eclogite. However, modelling based on small amounts of equilibrium partial melting indicates that only fractionation involving a garnet-dominated residuum can account for the observed REE patterns if the source rock was subducted oceanic basalt. If the source was geochemically evolved, other mineral phases (e.g., amphibole) are possible in the residuum, and there is no necessity to postulate that the downgoing slab was the source for these magmas. The REE fractionated trachytes and dacites appear to be a part of the nearby late Cenozoic volcanic provinces, and possibly represent minor partial melts which only in unusual tectonic situations arrive unmodified at the Earth's surface.  相似文献   

17.
西藏工布江达县龙崖松多榴辉岩的发现及意义   总被引：1，自引：0，他引：1  
唐加-松多榴辉岩高压变质带位于冈底斯板块中部,对研究古特提斯构造演化具有至关重要的意义,但是榴辉岩的出露范围和规模尚不清楚。在龙崖松多地区新发现榴辉岩的出露点,通过详细的野外地质调查、岩石学及矿物学研究,发现其岩性为金红石榴辉岩和石英榴辉岩,以透镜体的形式产出,围岩以低绿片岩相岩石为主,少部分为石英白云母片岩。龙崖松多榴辉岩的主要矿物组合为石榴子石、绿辉石、多硅白云母、角闪石、金红石、石英及少量榍石、钛铁矿等,石榴子石的电子探针分析结果显示,从核部到边部,镁铝榴石和铁铝榴石含量逐渐增加,锰铝榴石含量显著减少,具有典型的进变质环带特征,同时榴辉岩边部石榴子石角闪岩的发现,说明该露头的榴辉岩可能经历了后期退变质作用改造。该榴辉岩的发现对探讨唐加-松多蛇绿混杂带的构造演化具有重要意义。  相似文献   

18.
在滇西鲁甸地区金沙江结合带新发现退变榴辉岩,其在野外呈透镜体状产于石榴子石白云母石英片岩中.利用电子探针及激光拉曼分析发现石榴子石和锆石中残留绿辉石包体.石榴子石及基质中的白云母为多硅白云母(Si(p.f.u)=3.27~3.53),指示岩石经历了高压变质作用过程.石榴子石发育进变质生长成分环带.岩相学及矿物化学特征显示,退变榴辉岩大致经历了进变质角闪岩相、峰期榴辉岩相、早期退变质以及晚期强退变这4个世代矿物组合,各阶段典型的矿物组合依次为Grt+AmpI+Qtz、Grt+Omp+Rt+Qtz+Phe、Pl+Di+AmpⅡ+Ilm+Spn+Qtz、AmpⅢ+Pl+Czo+Ilm+Qtz.该新发现对金沙江结合带复杂的变质演化P-T-t轨迹样式及年代格架、以金沙江洋为代表的整个西南三江地区古特提斯洋-陆俯冲-碰撞-造山的复杂构造演化历史以及微陆块的拼贴机制等关键科学问题的解决提供了极为重要的素材和更多的约束,具有重要的科学意义.   相似文献   

19.
Based on published data, we reappraise the classification of high-Ti and low-Ti basalt from the Emeishan large igneous province (ELIP) and the correlations between basalts and mafic–ultramafic intrusions. Because of the lack of clear spatial and temporal variations of different types of basalts, we suggest that the basalts in the ELIP cannot be classified into high-Ti and low-Ti groups, by TiO2 contents and/or Ti/Y ratios. The distinctive characteristics of these high-Ti and low-Ti lavas probably result largely from the different fractionating assemblages. Whether or not fractional crystallization of the Fe–Ti oxides occurred probably is the key factor that controls the Ti abundances and Ti/Y ratios in the residual melts, e.g., lavas, although the nature of the mantle sources, variable degrees of partial melting of mantle and crustal contamination also influence the geochemical signatures of the lavas. Therefore, neither Ti abundance nor Ti/Y ratios in basalts can reflect the nature of their mantle source. Moreover, the different types of mafic–ultramafic intrusions in the ELIP cannot simply be attributed to be genetically related special types of basalts, either high-Ti or low-Ti basalts. It is likely that they are merely the cumulus phases, i.e. chamber or conduit of the basaltic lavas. Hence, caution should be exercised in the use of high-Ti or low-Ti basalts as prospecting vectors for ore deposits in the region. Potential implications are proposed that both the Fe–V–Ti oxide and Cu–Ni–(PGE) sulfide mineralization in the ELIP intrusions is largely due to the variable differentiation and crustal contamination during magmatic processes.  相似文献   

20.
Extremely low-K basaltic andesite to andesite lavas at Nekomavolcano, situated in the frontal volcanic zone of the NE Honshuarc, were produced from melts that originated in the lower crust.Multiple incompatible trace element model calculations suggestthat extremely low-K basalt found in the same arc is a naturalanalog for the source composition. However, fractional crystallization,magma mixing, and crustal contamination models of primary low-Kbasalt cannot reproduce the Nekoma chemical composition. Derivationof melts from an extremely low-K amphibolitic lower-crustalrock with the residual mineral phases hornblende, olivine, pyroxenes,plagioclase, and magnetite is plausible. Major element compositionsof Nekoma lavas are very similar to those of experimental meltsof amphibolite dehydration melting, which further support theproposal. Light rare earth elements are slightly enriched, buttotal rare earth element abundances are relatively low, suggestinga high degree of partial melting of the source. Ba/Th ratiosare low for frontal arc lavas, reflecting modification of theratio during partial melting. Zr/Hf and Nb/Ta ratios are significantlygreater than is usual for arc lavas, suggesting an anomaloussource composition. Markedly low K, Rb, Cs contents in the extremelylow-K lavas are attributed to an extremely low-K source. Underplatingof an extremely low-K basalt originating from a hydrous depletedmantle wedge could form such an amphibolite. In contrast, Ndand Sr isotope ratios fall close to Bulk Earth values, indicatingan isotopically enriched source. Hornblende-bearing rocks maypredominate in the lower crust of the NE Honshu arc, based onthe observation of crustal xenoliths. The presence of largelow-Vp regions at lower-crustal depths beneath the frontal arcis suggested by geophysical observations. These observationsfurther support lower-crustal melting beneath Nekoma as theorigin of the intermediate low-K lavas. KEY WORDS: amphibolite source; crustal melting; low-K andesite; Sr–Nd isotopes; trace element  相似文献   

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