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931.
NEKVASIL H.; DONDOLINI A.; HORN J.; FILIBERTO J.; LONG H.; LINDSLEY D. H. 《Journal of Petrology》2004,45(4):693-721
Experimental simulation of incremental crystal fractionationof a hy-normative hawaiite indicates that the spectrum of compositionsfrom mildly alkalic hawaiite to sodic rhyolite found in silica-saturatedalkalic suites of the ocean islands and continental hotspotsand rifts can be produced by fractionation at 9·3 kbarwith bulk water contents > 相似文献
932.
The basement of Sardinia represents a nearly complete sectionof a segment of the Variscan belt that experienced a polyphasetectono-metamorphic evolution and Barrovian metamorphism. Thisbasement is well suited to investigate the relationship betweentectono-metamorphic evolution and argon isotope records in whitemica. Micaschists from the garnet zone (maximum T of up to 520560°C)contain two texturally and chemically resolvable generationsof white mica: (1) deformed celadonite-rich flakes, defininga relict S1 foliation preserved within the main S2 foliationor enclosed in rotated albite porphyroblasts; (2) celadonite-poorwhite micas aligned along the main S2 foliation. The S1 foliationdeveloped earlier and at a deeper crustal level with respectto that at which the thermal peak was reached. From the staurolitezone (T of up to 590625°C) to the sillimanite + K-feldsparzone, white mica is nearly uniform in composition (muscovite)and is predominantly aligned along the S2 foliation or is oflater crystallization. In situ 40Ar39Ar laser analysesof white mica yielded ages of 相似文献
933.
Mantle Xenoliths from the Southeastern Slave Craton: Evidence for Chemical Zonation in a Thick, Cold Lithosphere 总被引:7,自引:3,他引:7
We present the first data on the petrology of the mantle lithosphereof the Southeastern (SE) Slave craton, Canada. These are basedon petrographic, mineralogical and geochemical studies of mantlexenoliths in Pipe 5034 of the Cambrian Gahcho Kué kimberlitecluster. Major types of mantle xenoliths include altered eclogite,coarse garnet or spinel peridotite, and deformed garnet peridotite.The peridotites belong to the low-temperature suite and formedat T=6001300°C and P= 2580 kbar in a thick(at least 220250 km), cool lithosphere. The SE Slavemantle is cooler than the mantle of other Archaean cratons andthat below other terranes of the Slave craton. The thick lithosphereand the relatively cool thermal regime provide favourable conditionsfor formation and preservation of diamonds beneath the SE Slaveterrane. Similar to average Archaean mantle worldwide, the SESlave peridotite is depleted in magmaphile major elements andcontains olivine with forsterite content of 9193·5.With respect to olivine composition and mode, all terranes ofthe Slave mantle show broadly similar compositions and are relativelyorthopyroxene-poor compared with those of the Kaapvaal and Siberiancratons. The SE Slave spinel peridotite is poorer in Al, Caand Fe, and richer in Mg than deeper garnet peridotite. Thegreater chemical depletion of the shallow upper mantle is typicalof all terranes of the Slave craton and may be common for thesubcontinental lithospheric peridotitic mantle in general. Peridotiticxenoliths of the SE Slave craton were impregnated by kimberliticfluids that caused late-stage recrystallization of primary clinopyroxene,spinel, olivine and spinel-facies orthopyroxene, and formationof interstitial clinopyroxene. This kimberlite-related recrystallizationdepleted primary pyroxenes and spinel in Al. The kimberliticfluid was oxidizing, Ti-, Fe- and K-rich, and Na-poor, and introducedserpentine, chlorite, phlogopite and spinel into peridotitesat P < 35 kbar. KEY WORDS: kimberlite xenolith; lithosphere; mantle terrane; chemical zoning; thermobarometry; Slave craton 相似文献
934.
East African Rift System (EARS) Plume Structure: Insights from Quaternary Mafic Lavas of Turkana, Kenya 总被引:6,自引:2,他引:6
FURMAN TANYA; BRYCE JULIA G.; KARSON JEFFREY; IOTTI ANNAMARIA 《Journal of Petrology》2004,45(5):1069-1088
Quaternary mafic lavas from Lake Turkana (northern Kenya) provideinformation on processes operating beneath the East AfricanRift in an area of anomalous lithospheric and crustal thinning.Inferred depths of melting beneath Turkana (1520 km)are shallower than those recorded elsewhere along the rift,consistent with the anomalously thin crustal section. The maficlavas have elevated incompatible trace element contents whencompared with mid-ocean ridge basalts, requiring an enrichmentevent in the source region. Basalts with low Sr isotopic ratios( 相似文献
935.
Petrogenesis of Tertiary Continental Intra-plate Lavas from the Westerwald Region, Germany 总被引:2,自引:2,他引:2
HAASE KARSTEN M.; GOLDSCHMIDT BJORN; GARBE-SCHONBERG C.-DIETER 《Journal of Petrology》2004,45(5):883-905
Tertiary volcanic rocks from the Westerwald region range frombasanites and alkali basalts to trachytes, whereas lavas fromthe margin of the Vogelsberg volcanic field consist of morealkaline basanites and alkali basalts. Heavy rare earth elementfractionation indicates that the primitive Westerwald magmasprobably represent melts of garnet peridotite. The Vogelsbergmelts formed in the spinelgarnet peridotite transitionregion with residual amphibole for some magmas suggesting meltingof relatively cold mantle. Assimilation of lower-crustal rocksand fractional crystallization altered the composition of lavasfrom the Westerwald and Vogelsberg region significantly. Thecontaminating lower crust beneath the Rhenish Massif has a differentisotopic composition from the lower continental crust beneaththe Hessian Depression and Vogelsberg, implying a compositionalboundary between the two crustal domains. The mantle sourceof the lavas from the Rhenish Massif has higher 206Pb/204Pband 87Sr/86Sr than the mantle source beneath the Vogelsbergand Hessian Depression. The 3020 Ma volcanism of theWesterwald apparently had the same mantle source as the QuaternaryEifel lavas, suggesting that the magmas probably formed in apulsing mantle plume with a maximum excess temperature of 100°Cbeneath the Rhenish Massif. The relatively shallow melting ofamphibole-bearing peridotite beneath the Vogelsberg and HessianDepression may indicate an origin from a metasomatized portionof the thermal boundary layer. KEY WORDS: continental rift volcanism; basanites; trachytes; assimilation; fractional crystallization; partial melting 相似文献
936.
Polymetamorphism in the NE Shackleton Range, Antarctica: Constraints from Petrology and U-Pb, Sm-Nd, Rb-Sr TIMS and in situ U-Pb LA-PIMMS Dating 总被引:1,自引:0,他引:1
Metapelitic rock samples from the NE Shackleton Range, Antarctica,include garnet with contrasting zonation patterns and two agespectra. Garnet porphyroblasts in K-rich kyanitesillimanitestaurolitegarnetmuscovitebiotite schistsfrom Lord Nunatak show prograde growth zonation, and give SmNdgarnet, UPb monazite and RbSr muscovite ages of518 ± 5, 514 ± 1 and 499 ± 12 Ma, respectively.Geothermobarometry and PT pseudo-section calculationsin the model system CaONa2OK2O TiO2MnOFeOMgOAl2O3SiO2H2Oare consistent with garnet growth during prograde heating from540°C/7 kbar to 650°C/7·5 kbar, and partial resorptionduring a subsequent PT decrease to <650°C at <6kbar. All data indicate that rocks from Lord Nunatak were affectedby a single orogenic cycle. In contrast, garnet porphyroblastsin K-poor kyanitesillimanite staurolitegarnetcordieritebiotite-schistsfrom Meade Nunatak show two growth stages and diffusion-controlledzonation. Two distinct age groups were obtained. Laser ablationplasma ionization multicollector mass spectrometry in situ analysesof monazite, completely enclosed by a first garnet generation,yield ages of c. 1700 Ma, whereas monazite grains in open garnetfractures and in most matrix domains give c. 500 Ma. Both agegroups are also obtained by UPb thermal ionization massspectrometry analyses of matrix monazite and zircon, which fallon a discordia with lower and upper intercepts at 502 ±1 and 1686 ± 2 Ma, respectively. SmNd garnet datingyields an age of 1571 ± 40 Ma and RbSr biotiteanalyses give an age of 504 ± 1 Ma. Integrated geochronologicaland petrological data provide evidence that rocks from MeadeNunatak underwent a polymetamorphic Barrovian-type metamorphism:(1) garnet 1 growth and subsequent diffusive garnet annealingbetween 1700 and 1570 Ma; (2) garnet 2 growth during the RossOrogeny at c. 500 Ma. During the final orogenic event the rocksexperienced peak PT conditions of about 650°C/7·0kbar and a retrograde stage at c. 575°C/4·0 kbar. KEY WORDS: garnet microtexture; PT pseudosection; geochronology; polymetamorphism; Shackleton Range; Antarctica 相似文献
937.
Quantitative Constraints on Metamorphism in the Variscides of Southern Brittany--a Complementary Pseudosection Approach 总被引:1,自引:0,他引:1
Previous studies of metapelitic rocks from the core of the southernBrittany metamorphic belt suggest a complex clockwise PTevolution. We use pseudosections calculated for an average subaluminousmetapelite composition in the MnNCKFMASH system and averagePT calculations to investigate in more detail the metamorphicevolution of these rocks. For migmatites, sequential occurrenceof kyanite, kyanite + staurolite and sillimanite suggests thata prograde evolution to P > 8 kbar at T 相似文献
938.
KADIK A.; PINEAU F.; LITVIN Y.; JENDRZEJEWSKI N.; MARTINEZ I.; JAVOY M. 《Journal of Petrology》2004,45(7):1297-1310
Studies of iron-bearing silicate melt (ferrobasalt) + iron metallicphase + graphite + hydrogen equilibria show that carbon andhydrogen solubilities in melts are important for the evolutionof the upper mantle. In a series of experiments conducted at3·7 GPa and 15201600°C, we have characterizedthe nature (oxidized vs reduced) and quantified the abundancesof C- and H-compounds dissolved in iron-bearing silicate melts.Experiments were carried out in an anvil-with-hole apparatuspermitting the achievement of equal chemical potentials of H2in the inner Pt capsule and outer furnace assembly. The fO2for silicate meltiron equilibrium was 2·32 ±0·04 log units below ironwüstite (IW). Theferrobasalt used as starting material experienced a reductionof its iron oxides and silicate network. The counterpart wasa liberation of oxygen reacting with the hydrogen entering thecapsule. The amount of H2O dissolved in the glasses was measuredby ion microprobe and by step-heating and was found to be between1 and 2 wt %. The dissolved carbon content was found to be 1600ppm C by step-heating. The speciation of C and H componentswas determined by IR and Raman spectroscopy. It was establishedthat the main part of the liberated oxygen was used to formOH and to a much lesser extent H2O, and only traces ofH2, CO2 and 相似文献
939.
Complex Growth Textures in a Polymetamorphic Metabasite from the Kraubath Massif (Eastern Alps) 总被引:1,自引:0,他引:1
Zoned garnet and amphibole occur in metabasites of the KraubathMassif, Eastern Alps, that contain relic magmatic clinopyroxene.The amphibole composition gradually changes from core (XMg =0·83) to rim (XMg = 0·60·7). A numberof compositional varieties of garnet occur in the metabasite.An older porphyroblastic garnet (Py2327, Alm4143,Grs2933) has two different compositional domains, onerelatively rich in Mg (Py2730) and the other rich inCa (Grs3538) with a low Mg (Py2025) content. Theyoungest variety, which forms rims on, or microveins in, theporphyroblastic garnet, has high Ca and low Mg (Grs4057,Py27, Alm4651). The amphibole cores and garnetporphyroblasts are interpreted to represent minerals formedduring Variscan regional metamorphism under amphibolite-faciesconditions. Alpine metamorphism is represented by the most recentCa-rich and Mg-poor variety of garnet that coexists with theamphibole rims, epidote and chlorite. Fracturing in the porphyroblasticgarnet probably originated during retrogression of the Variscanamphibolite-facies assemblages. Textural relations suggest thatthe garnet in the microveins formed by dehydration of hydrousphases during an Alpine metamorphic overprint that reached PTconditions of 550583°C at 1·0 GPa. KEY WORDS: microveins; garnet; metabasites; Kraubath Massif; Eastern Alps 相似文献
940.
Sensitive high-resolution ion microprobe UPb dating showsthat a biotite orthogneiss from the Hercynian belt of westerncentral Iberia contains 1000300 Ma zircon. Older, 1000570Ma ages within this range represent inherited, detrital materialamong which four age components may be recognized: 相似文献