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991.
WOLFF J. A.; ROWE M. C.; TEASDALE R.; GARDNER J. N.; RAMOS F. C.; HEIKOOP C. E. 《Journal of Petrology》2005,46(2):407-439
The MioceneQuaternary Jemez Mountains volcanic field(JMVF), the site of the Valles caldera, lies at the intersectionof the Jemez lineament, a Proterozoic suture, and the CenozoicRio Grande rift. Parental magmas are of two types: K-depletedsilica-undersaturated, derived from the partial melting of lithosphericmantle with residual amphibole, and tholeiitic, derived fromeither asthenospheric or lithospheric mantle. Variability insilica-undersaturated basalts reflects contributions of meltsderived from lherzolitic and pyroxenitic mantle, representingheterogeneous lithosphere associated with the suture. The Kdepletion is inherited by fractionated, crustally contaminatedderivatives (hawaiites and mugearites), leading to distinctiveincompatible trace element signatures, with Th/(Nb,Ta) and La/(Nb,Ta)greater than, but K/(Nb,Ta) similar to, Bulk Silicate Earth.These compositions dominate the mafic and intermediate lavas,and the JMVF is therefore derived largely, and perhaps entirely,from melting of fertile continental Jemez lineament lithosphereduring rift-related extension. Significant variations in Pband Nd isotope ratios (206Pb/204Pb = 17·2018·93;143Nd/144Nd = 0·512440·51272) result fromcrustal contamination, whereas 87Sr/86Sr is low and relativelyuniform (0·70400·7048). We compare theeffects of contamination by low-87Sr/86Sr crust with assimilationof high-87Sr/86Sr granitoid by partial melting, with Sr retainedin a feldspathic residue. Both models satisfactorily reproducethe isotopic features of the rocks, but the lack of a measurableEu anomaly in most JMVF mafic lavas is difficult to reconcilewith a major role for residual plagioclase during petrogenesis. KEY WORDS: Jemez Mountains volcanic field; Rio Grande rift; lithospheric mantle; crustal contamination; trace elements; radiogenic isotopes 相似文献
992.
RAMPONE E.; ROMAIRONE A.; ABOUCHAMI W.; PICCARDO G. B.; HOFMANN A. W. 《Journal of Petrology》2005,46(4):799-827
Mantle peridotites from the ErroTobbio (ET) ophioliticunit (Voltri Massif, Ligurian Alps) record a tectono-metamorphicdecompressional evolution, indicated by re-equilibration fromspinel- to plagioclase- to amphibole-facies conditions, andprogressive deformation from granular to tectonite to mylonitefabrics. The peridotites are considered to represent subcontinentallithospheric mantle that was tectonically denuded during riftingand opening of the Jurassic Ligurian Tethys ocean, similar tothe Northern Apennine (External Ligurides) ophiolitic peridotites.We performed chemical and isotopic investigations on selectedgranular and tectonite spinel peridotites and plagioclase tectonitesand mylonites, with the aim of defining the nature of the mantleprotoliths, and to date the onset of exhumation of the ET peridotites.Spinel- and plagioclase-bearing tectonites and mylonites exhibitheterogeneous bulk-rock major and trace element composition,despite rather homogeneous mineral chemistry, thus indicatingthat the ET mantle protoliths record a composite history ofpartial melting and melt migration by reactive porous flow.The lack of correlation between the observed geochemical heterogeneityand the structural type (granular, tectonite, mylonite) indicatesthat the inferred reactive porous flow event preceded the exhumation-relatedlithospheric history of the ErroTobbio mantle. The tectono-metamorphicevolution caused systematic chemical changes in minerals: (1)Al decrease in orthopyroxene; (2) Al decrease, and Cr and Tiincrease in spinels; (3) Al and Sr decrease, Cr, Ti, Zr, Sc,V and middle to heavy rare earth element increase and developmentof a negative Eu anomaly in clinopyroxene. The studied sampleshave Nd isotope compositions consistent with a mid-ocean ridgebasalt mantle reservoir. Sm/Nd isotope data on plagioclase andclinopyroxene separates (and corresponding whole rocks) fromtwo plagioclase peridotites, representative of the plagioclase-bearingmylonitic extensional shear zone, have yielded ages of 273 ±16 Ma and 313 ± 16 Ma, for the plagioclase-facies recrystallizationstage, significantly older than the expected Jurassic age. Thisindicates that the ErroTobbio peridotites represent subcontinentallithospheric mantle that was tectonically exhumed from spinel-faciesdepths to shallower lithospheric levels during Late CarboniferousPermiantimes. Our results are consistent with the previously documentedevidence for an extensional regime in the EuropeAdrialithosphere during Late Palaeozoic time, and they representthe first record that extensional mechanisms were also activeat lithospheric mantle levels. KEY WORDS: plagioclase-bearing peridotites; subcontinental lithospheric mantle; mantle exhumation; Sm/Nd dating 相似文献
993.
Mantle Preconditioning by Melt Extraction during Flow: Theory and Petrogenetic Implications 总被引:2,自引:0,他引:2
Mantle preconditioning may be defined as the extraction of smallmelt fractions from mantle asthenosphere during its flow tothe site of magma generation. Equations may be written for mantlepreconditioning, assuming that the mantle comprises enrichedplums in a depleted matrix. The equations takeinto account variations in mass fraction of plums, the relativerate of melting of plums and matrix, the temperature and pressureof melt extraction, the mass fraction of melt extracted, theextent of chemical exchange between plums and matrix, and theefficiency of melt extraction. Monitoring mineralogical changesand variations in partition coefficients along the inferredPTt path of the mantle asthenosphere allows theequations to be correctly applied to the conditions under whichmelt extraction takes place. Numerical experiments demonstratethe influence of petrogenetic variables on the shape of meltextraction trajectories and provide new criteria for distinguishingbetween melt extraction and mixing as the cause of regionalgeochemical gradients. Representative examples of arcback-arcsystems (Scotia), continental break-up (Afar) and plumeridgeinteraction (Azores) indicate that the compositions of the mantlesources of mid-ocean ridge basalts and island arc basalts maybe determined, at least in part, by the melt extraction historiesof their asthenospheric sources. KEY WORDS: geochemical modelling; mantle flow; isotope ratios; trace elements 相似文献
994.
Lamprophyresarewidespreadinthe“Sanjiang”(tri river)alkali richintrusiverockbelt(Ailaoshan Jinshajiangalkali richintrusiverockbelt)insouth westernChinaandhaveaclosetemporalandspatialre lationwithAu,Cumineralization(HuYunzhonget al.,1995;HuangZhilongetal.,1999).Therefore,thereiscrucialsignificanceinsuchaspectsastectonic environment,mantlemetasomatism,crust mantlein teraction,magmaticevolutionandassociatedminerali zation.TheBaimazhainickeldepositislocatedinthe southernsegmentofthe“Sanjiang… 相似文献
995.
文章介绍了地幔柱理论研究的一系列进展,诸如地幔柱特征、动力学模式及其地质意义。在重力分异过程中,随着地球质量不断向地心集中,地球自转动能也不断向地核集中,从而产生圈层分化和差异旋转。地核相对地壳、地幔高速旋转,具有巨大动能,旋转阻力不断将其转化为热能,在核幔边界聚积起来,为液核对流和热幔柱提供足够的能量。核幔边界是产生热幔柱的位置。阐明了地幔柱构造与板块构造的关系。地幔柱理论涉及地幔深部物质垂直运动的机制,对了解地球深部动力学机制有重大意义。 相似文献
996.
Sr, Nd and Pb isotopic compositions of the Cenozoic basalts were analyzed from Baengnyeongdo Island, Jeongok, Ganseong, and Jejudo Island of Korea. They reveal relatively enriched Sr and Nd isotopic compositions (87Sr/86Sr = 0.703300.70555, 143Nd/144Nd = 0.512980.51256) compared with MORB.207Pb/204Pb and 208Pb/204Pb values of all the analyzed Korean basalts lie above the Northern Hemisphere Reference Line (NHRL) defined by Hart (1984). Pb isotopic compositions of basalts from Jejudo Islands (206Pb/204Pb = 18.6119.12, 207Pb/204Pb = 15.5415.69, 208Pb/204Pb = 38.9839.72) are significantly more radiogenic than the rest (206Pb/204Pb = 17.7218.03, 207Pb/204Pb = 15.4415.58, 208Pb/204Pb = 37.7738.64). The Cenozoic Korean basalts thus can be divided into two groups based on their Sr, Nd and Pb isotopic compositions. The north group reveals mixing between DMM and EM1 while the south group displays DMM-EM2 mixing. Such a distribution is the same as Chinese Cenozoic basalts and it can be interpreted that the subcontinental lithospheric mantle under Korea represents simple lateral continuation of the South and North China Blocks. We suggest that Korean continental collision zone cross the Korean Peninsula through the region between the north and south basalt groups of Korea. 相似文献
997.
998.
999.
1000.
A three-dimensional radially anisotropic model of shear velocity in the whole mantle 总被引:4,自引:0,他引:4
We present a 3-D radially anisotropic S velocity model of the whole mantle (SAW642AN), obtained using a large three component surface and body waveform data set and an iterative inversion for structure and source parameters based on Non-linear Asymptotic Coupling Theory (NACT). The model is parametrized in level 4 spherical splines, which have a spacing of ∼ 8°. The model shows a link between mantle flow and anisotropy in a variety of depth ranges. In the uppermost mantle, we confirm observations of regions with VSH > VSV starting at ∼80 km under oceanic regions and ∼200 km under stable continental lithosphere, suggesting horizontal flow beneath the lithosphere. We also observe a VSV > VSH signature at ∼150–300 km depth beneath major ridge systems with amplitude correlated with spreading rate for fast-spreading segments. In the transition zone (400–700 km depth), regions of subducted slab material are associated with VSV > VSH , while the ridge signal decreases. While the mid-mantle has lower amplitude anisotropy (<1 per cent), we also confirm the observation of radially symmetric VSH > VSV in the lowermost 300 km, which appears to be a robust conclusion, despite an error in our previous paper which has been corrected here. The 3-D deviations from this signature are associated with the large-scale low-velocity superplumes under the central Pacific and Africa, suggesting that VSH > VSV is generated in the predominant horizontal flow of a mechanical boundary layer, with a change in signature related to transition to upwelling at the superplumes. 相似文献