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1.
Crystallization and Breakdown of Metasomatic Phases in Graphite-bearing Peridotite Xenoliths from Marsabit (Kenya) 总被引:1,自引:0,他引:1
Mantle-derived xenoliths from the Marsabit shield volcano (easternflank of the Kenya rift) include porphyroclastic spinel peridotitescharacterized by variable styles of metasomatism. The petrographyof the xenoliths indicates a transition from primary clinopyroxene-bearingcryptically metasomatized harzburgite (light rare earth element,U, and Th enrichment in clinopyroxene) to modally metasomatizedclinopyroxene-free harzburgite and dunite. The metasomatic phasesinclude amphibole (low-Ti Mg-katophorite), Na-rich phlogopite,apatite, graphite and metasomatic low-Al orthopyroxene. Transitionalsamples show that metasomatism led to replacement of clinopyroxeneby amphibole. In all modally metasomatized xenoliths melt pockets(silicate glass containing silicate and oxide micro-phenocrysts,carbonates and empty vugs) occur in close textural relationshipwith the earlier metasomatic phases. The petrography, majorand trace element data, together with constraints from thermobarometryand fO2 calculations, indicate that the cryptic and modal metasomatismare the result of a single event of interaction between peridotiteand an orthopyroxene-saturated volatile-rich silicate melt.The unusual style of metasomatism (composition of amphibole,presence of graphite, formation of orthopyroxene) reflects lowP –T conditions (850–1000°C at < 1·5GPa) in the wall-rocks during impregnation and locally low oxygenfugacities. The latter allowed the precipitation of graphitefrom CO2. The inferred melt was possibly derived from alkalinebasic melts by melt–rock reaction during the developmentof the Tertiary–Quaternary Kenya rift. Glass-bearing meltpockets formed at the expense of the early phases, mainly throughincongruent melting of amphibole and orthopyroxene, triggeredby infiltration of a CO2-rich fluid and heating related to themagmatic activity that ultimately sampled and transported thexenoliths to the surface. KEY WORDS: graphite; peridotite xenoliths; Kenya Rift; modal metasomatism; silicate glass 相似文献
2.
GARCIA-CASCO ANTONIO; HAISSEN FAOUZIYIA; CASTRO ANTONIO; EL-HMIDI HASSAN; TORRES-ROLDAN RAFAEL LUIS; MILLAN GUILLERMO 《Journal of Petrology》2003,44(10):1727-1757
We document experiments on a natural metapelite in the range650775°C, 614 kbar, 10 wt % of added water,and 700850°C, 410 kbar, no added water. Staurolitesystematically formed in the fluid-present melting experimentsabove 675°C, but formed only sporadically in the fluid-absentmelting experiments. The analysis of textures, phase assemblages,and variation of phase composition and FeMg partitioningwith P and T suggests that supersolidus staurolite formed at(near-) equilibrium during fluid-present melting reactions.The experimental results are used to work out the phase relationsin the system K2ONa2OFeOMgOAl2O3SiO2H2Oappropriate for initial melting of metapelites at the upperamphibolite facies. The PT grid developed predicts theexistence of a stable PT field for supersolidus staurolitethat should be encountered by aluminous Fe-rich metapelitesduring fluid-present melting at relatively low temperature andintermediate pressures (675700°C, 610 kbarfor XH2O = 1, in the KNFMASH system), but not during fluid-absentmelting. The implications of these findings for the scarcityof staurolite in migmatites are discussed. KEY WORDS: metapelites; migmatites; partial melting; PT grid; staurolite 相似文献
3.
Somnath Dasgupta Jürgen Ehl Michael M. Raith Pulak Sengupta Pranesh Sengupta 《Contributions to Mineralogy and Petrology》1997,129(2-3):182-197
Pelitic rocks were thermally metamorphosed at the contact of the Chimakurthy mafic-ultramafic igneous complex, Eastern Ghats
Belt, India. The rocks show progressive change in mineralogy from biotite-sillimanite-quartz-garnet-K-feldspar (association
I, 150 m from the intrusive contact) to garnet-spinel-cordierite-K-feldspar-sillimanite (association II, 20–30 m from the
intrusive contact) to cordierite-K-feldspar-(cordierite-orthopyroxene-K-feldspar symplectite after osumilite)-spinel-FeTiAl
oxides with/without garnet (associations III and IV, 5 m from the intrusive contact), and finally to spinel-orthopyroxene-cordierite-K-feldspar
(association V, xenoliths). Oxide mineral clots in associations III and IV resemble emery-type rocks. Initial mineral reactions
involved biotite-dehydration melting with partial segregation of the melt. Down-temperature mineral reactions were largely
diffusion controlled and preservation of symplectitic and coronitic textures in microdomains is common. Interpretation of
reaction textures in relevant petrogenetic grids for the sytems KFMASH and FMAS and combined with geothermobarometry suggest
that the pelitic rocks were thermally metamorphosed at c. 6 kbar pressure along a heating-cooling trajectory within the temperature interval between c. 750 °C and c. 1000 °C.
Received: 20 October 1996 / Accepted: 17 June 1997 相似文献
4.
喜马拉雅造山带东部错那县麻玛沟地区发育多种类型的花岗片麻岩和淡色花岗岩。锆石SHRIMP U-Pb地质年代学研究结果表明:花岗片麻岩(MM15)原岩结晶年龄为500.7±4.5Ma,含石榴子石淡色花岗岩中携带的继承性核部锆石年龄为498.6±3.4Ma,表明该地区经历了早古生代的岩浆作用事件。淡色花岗岩的结晶年龄区间为15.7~25.1Ma之间,为白云母脱水熔融的产物,可能是晚元古代-早古生代花岗质岩石发生低程度部分熔融的结果。锆石形态学表明该区的花岗片麻岩和淡色花岗岩均为过铝质花岗岩,并相对富集Cs、Rb、U、Pb,亏损Zr、Hf和低Nb/Ta比值,属于造山型花岗岩,支持该区域古生代岩浆作用事件与俯冲-碰撞造山作用相关,不是被动大陆边缘构造背景。结合前人数据推断:(1)从晚元古代末期开始,原特提斯洋向印度大陆的初始俯冲为自东向西的俯冲扩展模式;和(2)喜马拉雅造山带中新世淡色花岗岩为白云母脱水熔融和水致白云母熔融共同作用的结果,岩浆活动至少存在五个相。 相似文献
5.
Kun Zhou Yi‐Xiang Chen Yong‐Fei Zheng Li‐Juan Xu 《Journal of Metamorphic Geology》2019,37(8):1099-1127
Partial melting of ultrahigh‐pressure (UHP) metamorphic rocks is common during collisional orogenesis and post‐collisional reworking, indicating that determining the timing and processes involved in this partial melting can provide insights into the tectonic evolution of collisional orogens. This study presents the results of a combined whole‐rock geochemical and zirconological study of migmatites from the Sulu orogen in eastern China. These data provide evidence of multiple episodes of crustal anatexis and geochemical differentiation within the UHP metamorphic rocks. The leucosomes contain higher concentrations of Ba and K and lower concentrations of the rare earth elements (REE), Th and Y, than associated melanosomes and granitic gneisses. The leucosomes also have homogenous Sr–Nd–O isotopic compositions that are similar to proximal (i.e. within the same outcrop) melanosomes, suggesting that the anatectic melts were generated by the partial melting of source rocks that are located within individual outcrops. The migmatites contain zircons with six different types of domains that can be categorized using differences in structures, trace element compositions, and U–Pb ages. Group I domains are relict magmatic zircons that yield middle Neoproterozoic U–Pb ages and contain high REE concentrations. Group II domains represent newly grown metamorphic zircons that formed at 230 ± 1 Ma during the collisional orogenesis. Groups III, IV, V, and VI zircons are newly grown anatectic zircons that formed at 222 ± 2 Ma, 215 ± 1 Ma, 177 ± 2 Ma, and 152 ± 2 Ma, respectively. The metamorphic zircons have higher Th/U and lower (Yb/Gd)N values, flat heavy REE (HREE) patterns with no significantly negative Eu anomalies relative to the anatectic zircons, which are characterized by low Th/U ratios, steep HREE patterns, and negative Eu anomalies. The first two episodes of crustal anatexis occurred during the Late Triassic at c. 222 Ma and c. 215 Ma as a result of phengite breakdown. The other two episodes of anatexis occurred during the Jurassic period at c. 177 Ma and c. 152 Ma and were associated with extensional collapse of the collision‐thickened orogen. The majority of Triassic anatectic zircons and all of the Jurassic zircons are located within the leucosomes, whereas the melanosomes are dominated by Triassic metamorphic zircons, suggesting that the leucosomes within the migmatites record more episodes of crustal anatexis. Both metamorphic and anatectic zircons have elevated εHf(t) values compared with relict magmatic zircon cores, suggesting that these zircons contain non‐zircon Hf derived from material with more radiogenic Hf isotope compositions. Therefore, the Sulu and Dabie orogens experienced different episodes of reworking during the exhumation and post‐collisional stages. 相似文献
6.
Garnet Granulite Xenoliths from the Northern Baltic Shield--the Underplated Lower Crust of a Palaeoproterozoic Large Igneous Province? 总被引:3,自引:0,他引:3
KEMPTON P. D.; DOWNES H.; NEYMARK L. A.; WARTHO J. A.; ZARTMAN R. E.; SHARKOV E. V. 《Journal of Petrology》2001,42(4):731-763
Garnet granulite facies xenoliths hosted in Devonian lamprophyresfrom the Kola Peninsula are interpreted to represent the high-grademetamorphic equivalents of continental flood tholeiites, emplacedinto the Baltic Shield Archaean lower crust in early Proterozoictime. Geochronological data and similarities in major and traceelement geochemistry suggest that the xenoliths formed duringthe same plume-related magmatic event that created a widespreadPalaeoproterozoic large igneous province (LIP) at 2·42·5Ga. They are, thus, the first samples of the lower crust ofa Palaeoproterozoic LIP to be studied in petrological detail.The suite includes mafic granulites (gar + cpx + rutile ±plag ± opx ± phlog ± amph), felsic granulites(plag + gar + cpx + rutile ± qtz ± Kspar ±phlog ± amph) and pyroxenites (± phlog ±amph), but mafic garnet granulites predominate. Although somesamples are restites, there is no evidence for a predominanceof magmatic cumulates, as is common for Phanerozoic lower-crustalxenolith suites. Metasediments are also absent. Phlogopite and/oramphibole occur in xenoliths of all types and are interpretedto be metasomatic in origin. The K-rich metasomatic event occurredat 相似文献
7.
Upper mantle xenoliths from the southern Rio Grande rift axis (Potrillo and Elephant Butte) and flank (Adam’s Diggings) have
been investigated to determine chemical depletion and enrichment processes. The variation of modal, whole rock, and mineral
compositions reflect melt extraction. Fractional melting is the likely process. Fractional melting calculations show that
most spinel peridotites from rift axis locations have undergone <5% melting versus 7–14% melting for xenoliths from the rift
shoulder, although the total range of fractional melting overlaps at all three locations. In the rift axis, deformed (equigranular
and porphyroclastic texture) spinel peridotites are generally characterized by significantly less fractional melting (2–5%)
than undeformed (protogranular) xenoliths (up to 16%). This difference may reflect undeformed xenoliths being derived from
greater depths and higher temperatures than deformed rocks. Spinel peridotites from the axis and shoulder of the Rio Grande
rift have undergone mantle metasomatism subsequent to melt extraction. Under the rift shoulder spinel peridotites have undergone
both cryptic and patent (modal) metasomatism, possibly during separate events, whereas the upper mantle under the rift axis
has undergone only cryptic metasomatism by alkali basaltic magma. 相似文献
8.
Xenoliths hosted by Quaternary basanites and alkali basaltsfrom Marsabit (northern Kenya) represent fragments of Proterozoiclithospheric mantle thinned and chemically modified during riftingin the Mesozoic (Anza Graben) and in the TertiaryQuaternary(Kenya rift). Four types of peridotite xenoliths were investigatedto constrain the thermal and chemical evolution of the lithosphericmantle. Group I, III and IV peridotites provide evidence ofa cold, highly deformed and heterogeneous upper mantle. Textures,thermobarometry and trace element characteristics of mineralsindicate that low temperatures in the spinel stability field(750800°C at <1·5 GPa) were attained bydecompression and cooling from initially high pressures andtemperatures in the garnet stability field (9701080°Cat 2·32·9 GPa). Cooling, decompressionand penetrative deformation are consistent with lithosphericthinning, probably related to the development of the Mesozoicto Paleogene Anza Graben. Re-equilibrated and recrystallizedperidotite xenoliths (Group II) record heating (from 800°Cto 1100°C). Mineral trace element signatures indicate enrichmentby mafic silicate melts, parental to the Quaternary host basanitesand alkali basalts. Relationships between mineral textures,PT conditions of equilibration, and geochemistry canbe explained by metasomatism and heating of the lithosphererelated to the formation of the Kenya rift, above a zone ofhot upwelling mantle. KEY WORDS: East African Rift System; Anza Graben; in situ LA-ICPMS; peridotite xenoliths; thermobarometry 相似文献
9.
Internal morphology, habit and U-Th-Pb microanalysis of amphibolite-to-granulite facies zircons: geochronology of the Ivrea Zone (Southern Alps) 总被引:52,自引:2,他引:50
Gerhard Vavra Rolf Schmid Dieter Gebauer 《Contributions to Mineralogy and Petrology》1999,134(4):380-404
Several types of growth morphologies and alteration mechanisms of zircon crystals in the high-grade metamorphic Ivrea Zone
(IZ) are distinguished and attributed to magmatic, metamorphic and fluid-related events. Anatexis of pelitic metasediments
in the IZ produced prograde zircon overgrowths on detrital cores in the restites and new crystallization of magmatic zircons
in the associated leucosomes. The primary morphology and Th-U chemistry of the zircon overgrowth in the restites show a systematic
variation apparently corresponding to the metamorphic grade: prismatic (prism-blocked) low-Th/U types in the upper amphibolite
facies, stubby (fir-tree zoned) medium-Th/U types in the transitional facies and isometric (roundly zoned) high-Th/U types
in the granulite facies. The primary crystallization ages of prograde zircons in the restites and magmatic zircons in the
leucosomes cannot be resolved from each other, indicating that anatexis in large parts of the IZ was a single and short lived
event at 299 ± 5 Ma (95% c. l.). Identical U/Pb ages of magmatic zircons from a metagabbro (293 ± 6 Ma) and a metaperidotite
(300 ± 6 Ma) from the Mafic Formation confirm the genetic context of magmatic underplating and granulite facies anatexis in
the IZ. The U-Pb age of 299 ± 5 Ma from prograde zircon overgrowths in the metasediments also shows that high-grade metamorphic
(anatectic) conditions in the IZ did not start earlier than 20 Ma after the Variscan amphibolite facies metamorphism in the
adjacent Strona–Ceneri Zone (SCZ). This makes it clear that the SCZ cannot represent the middle to upper crustal continuation
of the IZ. Most parts of zircon crystals that have grown during the granulite facies metamorphism became affected by alteration
and Pb-loss. Two types of alteration and Pb-loss mechanisms can be distinguished by cathodoluminescence imaging: zoning-controlled
alteration (ZCA) and surface-controlled alteration (SCA). The ZCA is attributed to thermal and/or decompression pulses during
extensional unroofing in the Permian, at or earlier than 249 ± 7 Ma. The SCA is attributed to the ingression of fluids at
210 ± 12 Ma, related to hydrothermal activity during the breakup of the Pangaea supercontinent in the Upper Triassic/Lower
Jurassic.
Received: 7 July 1998 / Accepted: 4 November 1998 相似文献
10.
Summary Abundant upper mantle and rare lower crustal xenoliths have been found in the Plio-Pleistocene alkali basalts of the Nógrád-G?m?r
Volcanic Field, situated in the northern Pannonian Basin, on the border between northern Hungary and southern Slovakia. A
few lower crustal granulite xenoliths have been found in a small basaltic pyroclastic cone at Baglyaskő. The mafic granulite
xenoliths are plagioclase-bearing hornblende clinopyroxenites, plagioclase-bearing clinopyroxene hornblendites and plagioclase-bearing
clinopyroxenites. They contain unusual symplectites, composed of spinel feldspar and clinopyroxene. These symplectites are
interpreted as the product of garnet breakdown. Following the breakdown reaction, the symplectite underwent in situ partial melting. Mineral constituents of these granulite xenoliths have chemical compositions similar to those of other granulite
xenoliths worldwide. However, a distinctive positive Pb and Ce anomaly in mineral constituents of these granulites is characteristic.
Granulite xenoliths from the Nógrád-G?m?r Volcanic Field must have experienced granulite facies metamorphism at pressures
that correspond to the ‘original’ thickness of the crust (>1.1 GPa; >∼30 km), whereas the breakdown reaction of garnet and
subsequent melting and recrystallization of clinopyroxenes in the symplectites happened at shallower depths close to the present-day
MOHO (0.6–0.7 GPa; ∼16–19 km).
Present address: Research School of Earth Sciences, Australian National University, Australia 相似文献
11.
The meta-igneous granulite xenoliths from Kerguelen Archipelago: evidence of a continent nucleation in an oceanic setting 总被引:1,自引:1,他引:0
Michel Grégoire Jean Yves Cottin André Giret Nadine Mattielli Dominique Weis 《Contributions to Mineralogy and Petrology》1998,133(3):259-283
Xenoliths of meta-igneous origin occur as one of the two main types of ultramafic and mafic xenoliths entrained by alkaline
lavas of the Kerguelen islands. These are designated type II xenoliths and are subdivided into three mineralogical groups.
Subtype IIa and IIc xenoliths are interpreted as crystallisation products of basaltic melts that were emplaced near the crust-mantle
boundary during the early tholeiitic–transitional magmatic activity of the Kerguelen islands. Younger magmatism became more
alkaline and subtype IIb xenoliths were formed as high-pressure alkaline cumulates related to the last alkaline volcanic stage.
Subsequently, the plagioclase-bearing type II rocks have been re-equilibrated under granulite facies conditions. This addition
of mafic material around the crust-mantle boundary is consistent with seismic evidence for crustal thickening to 14–20 km.
Calculated compressional seismic velocities (Vp) for the basic granulites are consistent with the range of observed Vp in
the low-velocity region underlying the oceanic crust. Such growth in the thickness of the oceanic crust may be caused by intrusion
of basalts at different levels in the lithosphere and may provide the heat responsible for granulitic metamorphism in the
oceanic setting. This study suggests that basic granulites can account for the observed seismic characteristics of oceanic
plateaux and can be important components of Kerguelen oceanic lithosphere where there has been large-scale magma production.
Moreover we speculate that the Kerguelen islands and perhaps the surrounding plateau represent a continental nucleation process.
Received: 30 September 1997 / Accepted: 17 June 1998 相似文献
12.
Dmitri A. Ionov Antoine Bénard Pavel Y. Plechov 《Contributions to Mineralogy and Petrology》2011,162(6):1159-1174
Glass-bearing inclusions hosted by Cr-spinel in harzburgite xenoliths from Avacha are grouped based on homogenization temperatures
and daughter minerals into high-T (1,200°C; opx + cpx), intermediate (900–1,100°C; cpx ± amph), and low-T (900°C; amph) and
are commonly accompanied by larger “melt pockets”. Unlike previous work on unheated inclusions and interstitial glass in xenoliths
from Kamchatka, the homogenized glass compositions in this study are not affected by low-pressure melt fractionation during
transport and cooling or by interaction with host magma. Primary melt compositions constrained for each inclusion type differ
in major and trace element abundances and were formed by different events, but all are silica saturated, Ca-rich, and K-poor,
with enrichments in LREE, Sr, Rb, and Ba and negative Nb anomalies. These melts are inferred to have been formed with participation
of fluids produced by dehydration of slab materials. The high-T inclusions trapped liquids produced by ancient high-degree,
fluid-induced melting in the mantle wedge. The low-T inclusions are related to percolation of low-T melts or hydrous fluids
in arc mantle lithosphere. Melt pockets arise from localized heating and fluid-assisted melting induced by rising magmas shortly
before the entrapment of the xenoliths. The “high-T” melt inclusions in Avacha xenoliths are unique in preserving evidence
of ancient, high-T melting events in arc mantle, whereas the published data appear to characterize pre-eruption enrichment
events. 相似文献
13.
Formation of cordierite-bearing lavas during anatexis in the lower crust beneath Lipari Island (Aeolian arc,Italy) 总被引:2,自引:1,他引:1
Corrado Di Martino Francesca Forni Maria Luce Frezzotti Rosaria Palmeri James D. Webster Robert A. Ayuso Federico Lucchi Claudio A. Tranne 《Contributions to Mineralogy and Petrology》2011,162(5):1011-1030
Cordierite-bearing lavas (CBL;~105 ka) erupted from the Mt. S. Angelo volcano at Lipari (Aeolian arc, Italy) are high-K andesites,
displaying a range in the geochemical and isotopic compositions that reflect heterogeneity in the source and/or processes.
CBL consist of megacrysts of Ca-plagioclase and clinopyroxene, euhedral crystals of cordierite and garnet, microphenocrysts
of orthopyroxene and plagioclase, set in a heterogeneous rhyodacitic-rhyolitic groundmass containing abundant metamorphic
and gabbroic xenoliths. New petrographic, chemical and isotopic data indicate formation of CBL by mixing of basaltic-andesitic
magmas and high-K peraluminous rhyolitic magmas of anatectic origin and characterize partial melting processes in the lower
continental crust of Lipari. Crustal anatectic melts generated through two main dehydration-melting peritectic reactions of
metasedimentary rocks: (1) Biotite + Aluminosilicate + Quartz + Albite = Garnet + Cordierite + K-feldspar + Melt; (2) Biotite + Garnet + Quartz = Orthopyroxene + Cordierite + K-feldspar + Melt.
Their position into the petrogenetic grid suggests that heating and consequent melting of metasedimentary rocks occurred at
temperatures of 725 < T < 900°C and pressures of 0.4–0.45 GPa. Anatexis in the lower crust of Lipari was induced by protracted emplacement of basic
magmas in the lower crust (~130 Ky). Crustal melting of the lower crust at 105 ka affected the volcano evolution, impeding
frequent mafic-magma eruptions, and promoting magma stagnation and fractional crystallization processes. 相似文献
14.
Tatjana Rehfeldt Stephen F. Foley Richard W. Carlson 《Geochimica et cosmochimica acta》2008,72(23):5722-5756
Dunite, wehrlite and websterite are rare members of the mantle xenolith suite in the Kimberley kimberlites of the Kaapvaal Craton in southern Africa. All three types were originally residues of extensive melt extraction and experienced varying amounts and types of melt re-enrichment. The melt depletion event, dated by Re-Os isotope systematics at 2.9 Ga or older, is evidenced by the high Mg# (Mg/(Mg + Fe)) of silicate minerals (olivine (0.89-0.93); pyroxene (0.88-0.93); garnet (0.72-0.85)), high Cr# (Cr/(Cr + Al)) of spinel (0.53-0.84) and mostly low whole-rock SiO2, CaO and Al2O3 contents. Shortly after melt depletion, websterites were formed by reaction between depleted peridotites and silica-rich melt (>60 wt% SiO2) derived by partial melting of eclogite before or during cratonization. The melt-peridotite interaction converted olivine into orthopyroxene.All three xenolith types have secondary metasomatic clinopyroxene and garnet, which occur along olivine grain boundaries and have an amoeboid texture. As indicated by the preservation of oxygen isotope disequilibrium in the minerals and trace-element concentrations in clinopyroxene and garnet, this metasomatic event is probably of Mesozoic age and was caused by percolating alkaline basaltic melts. This melt metasomatism enriched the xenoliths in CaO, Al2O3, FeO and high-field-strength-elements, and might correspond to the Karoo magmatism at 200 Ma. The websterite xenoliths experienced both the orthoyproxene-enrichment and clinopyroxene-garnet metasomatic events, whereas dunite and wehrlite xenoliths only saw the later basaltic melt event, and may have been situated further away from the source of melt migration channels. 相似文献
15.
Nine pieces of gabbroic xenoliths from Hannuoba were examined for their major and trace elements and Nd,Sr and Pb isotopes.The results show that the gab-broic xenoliths are of more mafic basaltic composition .Their abundances show narrow variations in major elements.The trace element contents are highly variable in contrast with those of host basalts and lherzolite xenoliths.The gabbroic xenoliths are rich in Nd(0.51159-0.51249),Sr(0.70491-0.70768) and low in radiogenic Pb(16.283-17.046, 15.191-15.381 and 36.999-37.476),significantly different from basalts and lherzolites in isotopic space.The calculated Nd and Pb model ages are about 3.0-3.5 Ga.The rocks have relatively low equilibrium T(-850℃) and P(0.8-0.9 Gpa).They could be inter-preted to be the product of upper mantle melting at the boundary between the lower crust and the upper mantle.Their chemical and isotopic variations can be ascribed to different degrees of melting,segregation and long-term evolution. 相似文献
16.
Summary Mantle-derived xenoliths from Baarley in the Quaternary West Eifel volcanic field contain six distinct varieties of glass
in veins, selvages and pools. 1) Silica-undersaturated glass rich in zoned clinopyroxene microlites that forms jackets around
and veins within the xenoliths. This glass is compositionally similar to groundmass glass in the host basanite. 2) Silica-undersaturated
alkaline glass that contains microlites of Cr-diopside, olivine and spinel associated with amphibole in peridotites. This
glass locally contains corroded primary spinel and phlogopite. 3) Silica-undersaturated glass associated with diopside, spinel ± olivine
and rh?nite microlites in partly to completely broken down amphibole grains in clinopyroxenites. 4) Silica-undersaturated
to silica-saturated, potassic glass in microlite-rich fringes around phlogopite grains in peridotite. 5) Silica-undersaturated
potassic glass in glimmerite xenoliths. 6) Silica-rich glass around partly dissolved orthopyroxene crystals in peridotites.
Geothermometry of orthopyroxene–clinopyroxene pairs (P = 1.5 GPa) gives temperatures of ∼ 850 °C for unveined xenoliths to
950–1020 °C for veined xenoliths. Clinopyroxene – melt thermobarometry shows that Cr-diopside – type 2 glass pairs in harzburgite
formed at 1.4 to 1.1 GPa and ∼ 1250 °C whereas Cr-diopside – type 2 glass pairs in wehrlite formed at 0.9 to 0.7 GPa and 1120–1200 °C.
This bimodal distribution in pressure and temperature suggests that harzburgite xenoliths may have been entrained at greater
depth than wehrlite xenoliths.
Glass in the Baarley xenoliths has three different origins: infiltration of an early host melt different in composition from
the erupted host basanite; partial melting of amphibole; reaction of either of these melts with xenolith minerals. The composition
of type 1 glass suggests that jackets are accumulations of relatively evolved host magma. Mass balance modelling of the type
2 glass and its microlites indicates that it results from breakdown of disseminated amphibole and reaction of the melt with
the surrounding xenolith minerals. Type 3 glass in clinopyroxenite xenoliths is the result of breakdown of amphibole at low
pressure. Type 4 and 5 glass formed by reaction between phlogopite and type 2 melt or jacket melt. Type 6 glass associated
with orthopyroxene is due to the incongruent dissolution of orthopyroxene by any of the above mentioned melts.
Compositional gradients in xenolith olivine adjacent to type 2 glass pools and jacket glass can be modelled as Fe–Mg interdiffusion
profiles that indicate melt – olivine contact times between 0.5 and 58 days. Together with the clinopyroxene – melt thermobarometry
calculations these data suggest that the glass (melt) formed over a short time due to decompression melting of amphibole and
infiltration of evolved host melt. None of the glass in these xenoliths can be directly related to metasomatism or any other
process that occurred insitu in the mantle.
Received November 23, 1999; revised version accepted September 5, 2001 相似文献
17.
Michel Faure Alain Cocherie Eugne B Mzme Nicolas Charles Philippe Rossi 《Gondwana Research》2010,18(4):369
In France, the Devonian–Carboniferous Variscan orogeny developed at the expense of continental crust belonging to the northern margin of Gondwana. A Visean–Serpukhovian crustal melting has been recently documented in several massifs. However, in the Montagne Noire of the Variscan French Massif Central, which is the largest area involved in this partial melting episode, the age of migmatization was not clearly settled. Eleven U–Th–Pbtot. ages on monazite and three U–Pb ages on associated zircon are reported from migmatites (La Salvetat, Ourtigas), anatectic granitoids (Laouzas, Montalet) and post-migmatitic granites (Anglès, Vialais, Soulié) from the Montagne Noire Axial Zone are presented here for the first time. Migmatization and emplacement of anatectic granitoids took place around 333–326 Ma (Visean) and late granitoids emplaced around 325–318 Ma (Serpukhovian). Inherited zircons and monazite date the orthogneiss source rock of the Late Visean melts between 560 Ma and 480 Ma. In migmatites and anatectic granites, inherited crystals dominate the zircon populations. The migmatitization is the middle crust expression of a pervasive Visean crustal melting event also represented by the “Tufs anthracifères” volcanism in the northern Massif Central. This crustal melting is widespread in the French Variscan belt, though it is restricted to the upper plate of the collision belt. A mantle input appears as a likely mechanism to release the heat necessary to trigger the melting of the Variscan middle crust at a continental scale. 相似文献
18.
Xenolith evidence for lithospheric melting above anomalously hot mantle under the northern Canadian Cordillera 总被引:2,自引:0,他引:2
Lang Shi Don Francis John Ludden Andrew Frederiksen Michael Bostock 《Contributions to Mineralogy and Petrology》1998,131(1):39-53
A comparison of mantle xenolith suites along the northern Canadian Cordillera reveals that the xenoliths from three suites
exhibit bimodal populations whereas the xenoliths from the other four suites display unimodal populations. The bimodal suites
contain both fertile lherzolite and refractory harzburgite, while the unimodal suites are dominated by fertile lherzolite
xenoliths. The location of the three bimodal xenolith suites correlates with a newly discovered P-wave slowness anomaly in
the upper mantle that is 200 km in width and extends to depths of 400–500 km (Frederiksen AW, Bostock MG, Van Decar JC, Cassidy
J, submitted to Tectonophysics). This correlation suggests that the bimodal xenolith suites may either contain fragments of
the anomalously hot asthenospheric mantle or that the lithospheric upper mantle has been affected by the anomalously hot mantle.
The lherzolite xenoliths in the bimodal suites display similar major element compositions and trace element patterns to the
lherzolite xenoliths in the unimodal suites, suggesting that the lherzolites represent the regional lithospheric upper mantle.
In contrast, the harzburgite xenoliths are highly depleted in terms of major element composition, but their clinopyroxenes
[Cpx] have much higher incompatible trace element contents than those in the lherzolite xenoliths. The major element and mildly
incompatible trace element systematics of the harzburgite and lherzolite xenoliths indicate that they could be related by
a partial melting process. The lack of textural and geochemical evidence for the former existence of garnet argues against
the harzburgite xenoliths representing actual fragments of the deeper anomalous asthenospheric mantle. Furthermore, the calculated
P-wave velocity difference between harzburgite and lherzolite end-members is only 0.8%, with the harzburgites having higher
P-wave velocities. Therefore the 3% P-wave velocity difference detected teleseismically cannot be produced by the compositional
difference between the lherzolite and harzburgite xenoliths. If temperature is responsible for the observed 3% P-wave velocity
perturbation, the anomalous mantle is likely to be at least 200 °C higher than the surrounding mantle. Taken together these
data indicate that the refractory harzburgite xenoliths represent the residue of 20–25% partial melting of a lherzolite lithospheric
mantle. The incompatible trace element enrichment of the harzburgites suggests that this melting was accompanied by the ingress
of fluids. The association of the bimodal xenolith suites with the mantle anomaly detected teleseismically suggests that anomalously
hot asthenospheric mantle provided both the heat and volatiles responsible for the localized melting and enrichment of the
lithospheric mantle.
Received: 16 May 1997 / Accepted: 25 October 1997 相似文献
19.
Synplutonic mafic dykes from late Archaean granitoids in the Eastern Dharwar Craton,southern India 总被引:2,自引:0,他引:2
M. Jayananda T. Miyazaki R. V. Gireesh N. Mahesha T. Kano 《Journal of the Geological Society of India》2009,73(1):117-130
We present a first overview of the synplutonic mafic dykes (mafic injections) from the 2.56–2.52 Ga calcalkaline to potassic
plutons in the Eastern Dharwar Craton (EDC). The host plutons comprise voluminous intrusive facies (dark grey clinopyroxene-amphibole
rich monzodiorite and quartz monzonite, pinkish grey porphyritic monzogranite and grey granodiorite) located in the central
part of individual pluton, whilst subordinate anatectic facies (light grey and pink granite) confined to the periphery. The
enclaves found in the plutons include highly angular screens of xenoliths of the basement, rounded to pillowed mafic magmatic
enclaves (MME) and most spectacular synplutonic mafic dykes. The similar textures of MME and adjoining synplutonic mafic dykes
together with their spatial association and occasional transition of MME to dismembered synplutonic mafic dykes imply a genetic
link between them. The synplutonic dykes occur in varying dimension ranging from a few centimeter width upto 200 meters width
and are generally dismembered or disrupted and rarely continuous. Necking of dyke along its length and back veining of more
leucocratic variant of the host is common feature. They show lobate as well as sharp contacts with chilled margins suggesting
their injection during different stages of crystallization of host plutons in magma chamber. Local interaction, mixing and
mingling processes are documented in all the studied crustal corridors in the EDC. The observed mixing, mingling, partial
hybridization, MME and emplacement of synplutonic mafic dykes can be explained by four stage processes: (1) Mafic magma injected
during very early stage of crystallization of host felsic magma, mixing of mafic and felsic host magma results in hybridization
with occasional MME; (2) Mafic magma introduced slightly later, the viscosities of two magmas may be different and permit
only mingling where by each component retain their identity; (3) When mafic magma injected into crystallizing granitic host
magma with significant crystal content, the mafic magma is channeled into early fractures and form dismembered synplutonic
mafic dykes and (4) Mafic injections enter into largely crystallized (>80% crystals) granitic host results in continuous dykes
with sharp contacts. The origin of mafic magmas may be related to development of fractures to mantle depth during crystallization
of host magmas which results in the decompression melting of mantle source. The resultant hot mafic melts with low viscosity
rise rapidly into the crystallizing host magma chamber where they interact depending upon the crystallinity and viscosity
of the host. These hot mafic injections locally cause reversal of crystallization of the felsic host and induce melting and
resultant melts in turn penetrate the crystallizing mafic body as back veining. Field chronology indicates injection of mafic
magmas is synchronous with emplacement of anatectic melts and slightly predates the 2.5 Ga metamorphic event which affected
the whole Archaean crust. The injection of mafic magmas into the crystallizing host plutons forms the terminal Archaean magmatic
event and spatially associated with reworking and cratonization of Archaean crust in the EDC. 相似文献
20.
Jean-Louis Paquette Juliette Mergoil-Daniel 《Contributions to Mineralogy and Petrology》2009,158(2):245-262
Zircon-bearing xenoliths in continental basalts are often interpreted as witnesses of the continental basement uplifted during
volcanic eruptions. Nevertheless, their origin is still debated. The Devès basaltic plateau belongs to the alkaline volcanic
province of the French Massif Central. In few outcrops, zircon-bearing nepheline syenite xenoliths were preserved. U–Pb dating
of the zircon crystals define an age of 956 ± 11 kyr constraining the crystallisation time of the zircons and consequently
of the host xenoliths. This age, together with mineral chemistry arguments lead us to conclude that these minerals do not
derive from a continental protolith. Rather, they likely result from the crystallisation of a liquid characterised by a nepheline–felspar
composition and produced by the differentiation of a basaltic magma or, alternatively, by the low degree partial melting of
a metasomatised lithospheric mantle. Such alkaline sialic rock and xenoliths may occur in large volumes at depth and generate
the large amounts of zircon megacrysts discovered worldwide in secondary deposits within continental basaltic provinces. 相似文献