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1.
The evolution of fluorine-rich felsic magmas: source dichotomy, magmatic convergence and the origins of topaz granite 总被引:3,自引:0,他引:3
Topaz granite is alkali-feldspar granite that contains essential albite, quartz, K-feldspar, lithium-mica, and topaz. As a group topaz granites are characterized by their extreme enrichment in F (up to 3 wt%) and a wide variety of lithophile elements. They can be subdivided into a 'low-P2 O5 subtype' (P2 O5 < 0.1 wt%, Al2 O3 < 14.5 wt%, SiO2 > 73 wt%) and a 'high-P2 O5 subtype' (P2 O5 > 0.4 wt%, Al2 O3 > 14.5 wt%, SiO2 < 73 wt%), the δ18 O values of which indicate a dichotomy of source rock: the low-P2 O5 subtype (δ18 O < 10‰) having a meta-igneous protolith and the high-P2 O5 subtype (δ18 O > 10 ‰) a source with a significant component of pelitic material. The unusually high F contents enhance the efficacy of melt segregation and crystal-melt fractionation and so facilitate extreme differentiation in topaz granite magmas. Very low melt volumes restrict the bulk composition of the partial melts regardless of the nature of the source; and extreme fractionation forces them along a path of magmatic convergence, to produce a group of granitic rocks with near-minimum compositions so enriched in a variety of lithophile elements (Li, Nb, Ta, Sn) that economic mineralization often results. 相似文献
2.
Mo-related Adakitic Granitoids from Non-island-arc Setting: Jecheon Pluton of South Korea 总被引:2,自引:0,他引:2
Abstract. The Jecheon granitoids, having an elongated shape of NE-SW 27 km and NW-SE 13 km (190 km2 ), are composed mostly of magnetite-series hornblende-biotite granodiorite and biotite granite, which intrude into the Neoproterozoic metamor-phic and Paleozoic sedimentary rocks of the Ogcheon Belt. The granitoids have Triassic-Jurassic age of 202.7 ±1.9 Ma with very high 87 Sr/86 Sr initial ratio of 0.7140. The granodiorite has 63–69 % SiO2 , 15.1–17.3 % Al2 O3 , <1.6 % MgO, 6–15 ppm Y and Sr/Y ratios of 24–76, and is depleted in HREE. Biotite granite together, the Jecheon pluton has adakitic characteristics, which are unique in a continental tectonic setting. The granitoids may have been generated by partial melting of an older adakitic granitoid of I-type basement, or by separation of early crystallized garnet and hornblende from an anatectic melt. 相似文献
3.
In order to better identify the mineral phase which controls the rheology of the transition zone (between 410 and 660 km depth) transmission electron microscopy observations were made on several coexisting spinel-garnet assemblies: alkremite xenolith; pyrope-rich – MgO:1.1Al2 O3 spinel assembly deformed at 1173K, 800 MPa in a Griggs apparatus; (Mg,Fe)3 (Al,Mg,Si)2 Si3 O12 majorite – (Mg,Fe)2 SiO4 spinel assembly synthesized in a laser heated diamond anvil cell. It was found that garnet crystals systematically remain undeformed while spinel crystals are plastically deformed. These results are in accord with the assumption that the rheology of majorite is stronger than the rheology of spinel, in the conditions of the transition zone. 相似文献
4.
Magnesian metapelites of probable Archaean age from Forefinger Point, SW Enderby Land, East Antarctica, contain very-high-temperature granulite facies mineral assemblages, which include orthopyroxene (8–9.5 wt% Al2 O3 )–sillimanite ± garnet ± quartz ± K-feldspar, that formed at 10 ± 1.5 kbar and 950 ± 50°C. These assemblages are overprinted by symplectite and corona reaction textures involving sapphirine, orthopyroxene (6–7 wt% Al2 O3 ), cordierite and sometimes spinel at the expense of porphyroblastic garnet or earlier orthopyroxene–sillimanite. These textures mainly pre-date the development of coarse biotite at the expense of initial mesoperthite, and the subsequent formation of orthopyroxene (4–6 wt% Al2 O3 )–cordierite–plagioclase rinds on late biotite.
The early reaction textures indicate a period of near-isothermal decompression at temperatures above 900°C. Decompression from 10 ± 1.5 kbar to 7–8 kbar was succeeded by biotite formation at significantly lower temperatures (800–850°C) and further decompression to 4.5 ± 1 kbar at 700–800°C.
The later parts of this P–T evolution can be ascribed to the overprinting and reworking of the Forefinger Point granulites by the Late-Proterozoic ( c . 1000 Ma) Rayner Complex metamorphism, but the age and timing of the early high-temperature decompression is not known. It is speculated that this initial decompression is of Archaean age and therefore records thinning of the crust of the Napier Complex following crustal thickening by tectonic or magmatic mechanisms and preceding the generally wellpreserved post-deformational near-isobaric cooling history of this terrain. 相似文献
The early reaction textures indicate a period of near-isothermal decompression at temperatures above 900°C. Decompression from 10 ± 1.5 kbar to 7–8 kbar was succeeded by biotite formation at significantly lower temperatures (800–850°C) and further decompression to 4.5 ± 1 kbar at 700–800°C.
The later parts of this P–T evolution can be ascribed to the overprinting and reworking of the Forefinger Point granulites by the Late-Proterozoic ( c . 1000 Ma) Rayner Complex metamorphism, but the age and timing of the early high-temperature decompression is not known. It is speculated that this initial decompression is of Archaean age and therefore records thinning of the crust of the Napier Complex following crustal thickening by tectonic or magmatic mechanisms and preceding the generally wellpreserved post-deformational near-isobaric cooling history of this terrain. 相似文献
5.
Yasushi Watanabe 《Resource Geology》2003,53(3):221-226
Abstract. 40 Ar-39 Ar analyses of two alunite samples from phreatic craters in the Pliocene Muine volcano in southwest Hokkaido, Japan, were carried out. The alunite with 17.4 permil δ34 SV_CDT value in hydrothermal breccia from the Nagaoyama crater and that with 14.3 permil δ34 SV_CDT value in silicified andesite from the Konuma crater give total fusion ages of 1.40 ± 0.04 Ma (la uncertainty) and 1.24 ± 0.08 Ma, respectively. However, the spectra of these samples indicate they have been effected by thermal overprinting and/or the existence of excess argon. These preliminary 40 Ar-39 A analyses suggest that the alunite underwent multiple hydrothermal activity by magmatic gas and vapor subsequent to the main hydrothermal activity. 相似文献
6.
Viorica Morogan 《地学学报》1994,6(2):166-176
Ijolite-carbonatite complexes are ubiquitously surrounded of an aureole of metasomatically altered rocks. The process of alteration is termed fenitization and is generally caused by peralkaline fluids emanating from cooling alkaline, i.e. ijolite and carbonatite magmas. Ijolites and carbonatites normally occur together and attempts to determine the source of the fenitizing fluids may therefore lead to controversial, if not erroneous, conclusions.
Mineralogical and chemical data of fenites from Oldoinyo Lengai (Tanzania), Fen (Norway), and Alnö (Sweden) are reviewed in the present paper in order to reveal the main factors controlling the fenitization around ijolite and carbonatite. Despite the overall alkaline nature of the process, variables such as XCO2 of the fluid, activity gradients of SiO2 , Al2 O3 and CaO, FeO/MgO ratio, f O2 and temperature gradients may differ, producing distinctive patterns of fenitization around the two magmatic sources. The ijolitic-type fluid has low XCO2 , high activities of alkalies, SiO2 and Al2 O3 , and low activity of CaO. The f O2 evolves along the hm-mt buffer conditions and the temperature falls gradually with distance from the magmatic source. The carbonatitic-type fluid has high XCO2 , high activities of alkalies and CaO, and low activities of SiO2 and Al2 O3 . Temperatures and f O2 are initially high, but decrease sharply with distance from the source. Moreover, the CO2 -rich fluid may complex and transport the REE. 相似文献
Mineralogical and chemical data of fenites from Oldoinyo Lengai (Tanzania), Fen (Norway), and Alnö (Sweden) are reviewed in the present paper in order to reveal the main factors controlling the fenitization around ijolite and carbonatite. Despite the overall alkaline nature of the process, variables such as XCO
7.
Abstract: The Karlapat bauxite deposit occurs in the Eastern Ghat Group of rocks in Orissa and has developed in the khondalites. Mineralogical and physical observations on bore hole samples reflect the presence of a maximum of six weathered zones from top to bottom. These zones are termed as topsoil, siliceous laterite, ferruginous laterite, bauxite, lithomarge and altered khondalite. Four-dimensional trend surface models are developed for the data of 45 and 36 bore holes from north and south blocks, respectively, on Al2 O3 and SiO2 to delineate the zones of metallurgical grade bauxite (SiO2 5 % and Al2 O3 40 %). The results indicate about 15 m thickness of bauxite in the north block while it could be up to 20 m thick in the south block, leaving about 10 m of lat-erites at the top of each block. High grade bauxite (>47 % Al2 O3 ) is also encountered at specific locations. 相似文献
8.
The results are given for all the determinations made by the co-operating laboratories of major elements in the six NIMROC rock samples (granite, syenite, lujavrite, norite, pyroxenite and dunite) prepared by the National Institute for Metallurgy in 1966. Relevant statistical data are given for the sets of results for each major constituent, and recommended values for all constituents except Al2 03 , Na2 0, K2 0and CO2 in dunite, Fe2 O3 , MgO and CO2 in the granite, Fe2 3 and CO2 in the norite and CO2 in the pyroxenite.
This report and that on the trace and minor elements issued in 1978 complete the revision of the recommended values. It is suggested that analysts should concentrate rather on those constituents for which the results have shown such a wide scatter that they can be of no usc for reference purposes, than on those for which the ualues are fairly well established. 相似文献
This report and that on the trace and minor elements issued in 1978 complete the revision of the recommended values. It is suggested that analysts should concentrate rather on those constituents for which the results have shown such a wide scatter that they can be of no usc for reference purposes, than on those for which the ualues are fairly well established. 相似文献
9.
Z. M. ZHANG H.-P. SCHERTL J. L. WANG K. SHEN J. G. LIOU 《Journal of Metamorphic Geology》2009,27(4):317-333
The 5-km deep Chinese Continental Scientific Drilling Main Hole penetrated a sequence of ultrahigh pressure (UHP)-metamorphic rocks consisting mainly of eclogite, gneiss and garnet-peridotite with minor schist and quartzite. Zircon separates taken from thin layers of schist and gneiss within eclogite were investigated. Cathodoluminescence images of zircon grains show that they have oscillatory zoned magmatic cores and unzoned to patchy zoned metamorphic rims. Zircon rims contain rare coesite and calcite inclusions whereas cores contain inclusions of both low- P minerals (e.g. feldspar, biotite and quartz) and coesite and other eclogite-facies minerals such as phengite and jadeite. The zircon cores give highly variable 206 Pb/238 U ages ranging from 760 to 431 Ma for schist and from 698 to 285 Ma for gneiss, and relatively high but variable Th/U ratios (0.16–1.91). We suggest that the coesite and other eclogite facies mineral inclusions in zircon cores were not magmatic but formed through metasomatic processes caused by fluids during UHP metamorphism, and that the fluids contain components of SiO2 , Al2 O3 , K2 O, FeO, MgO, Na2 O and H2 O. Metasomatism of the Sulu UHP rocks during continental subduction to mantle depths has partly altered magmatic zircon cores and reset isotopic systems. This study provides key evidence that mineral inclusions within magmatic zircon domains are not unequivocal indicators of the formation conditions of the respective domain. This finding leads us to conclude that the routine procedure for dating of metamorphic events solely based on the occurrence of mineral inclusions in zoned zircon could be misleading and the data should be treated with caution. 相似文献
10.
Bhaskar Chandra ACHARYA Prasanta Kumar PANIGRAHY Binod Bihari NAYAK Rama Krishna SAHOO 《Resource Geology》1998,48(2):125-136
Abstract: Ilmenite, hematite, garnet, monazite, zircon, rutile, magnetite, sillimanite, pyroxene and amphibole from the beach sands of Ekakula, Gahiramatha coast, Orissa, India are reported here for the first time. Their total concentration varies from 26. 4 to 100%. Ilmenite, monazite and zircon are between 100 and 300 um in size and are well rounded in shape. Ilmenite-hematite intergrowth is common. Ilmenite has 50. 02–54. 73% TiO2 , 42. 42–46. 90% FeO (total Fe) and small amounts of Al, Mn, Mg, Ca, Ba, Si, V, Cr, and Zn. The bulk samples contain 10. 63–41. 42 % TiO2 , 6. 15–26. 07 % FeO, 5. 86–16. 75 % Fe2 O3 , 7. 41–61. 74 % SiO2 , 1. 39–12. 83% A12 O3 , 0. 32–4. 97% CaO, 0. 53–4. 24% P2 O5 , 0. 17–3. 27% MgO, 0. 15–2. 97% Na2 O, 0. 07–2. 34% K2 O, and 0. 05–0. 71% V2 O5 together with appreciable amounts of La, Ce, Pr, Nd, Sm, Eu, Y, U, Th, Zr, and trace amounts of Pb, Zn, Cu, Ni, Co, and Cr. Khondalite, charnockite, calc-silicate granulite, leptynite, migmatite, gneiss, basic granulite and pegmatite of the Eastern Ghats appear to be the major source for the above heavy mineral assemblages. The samples are amenable to gravity and magnetic methods of beneficiation. 相似文献
11.
Diffusion-controlled corona reaction and overstepping of equilibrium in a garnet granulite, Yenisey Ridge, Siberia 总被引:2,自引:1,他引:1
J. R. ASHWORTH V. S. SHEPLEV N. A. BRYXINA V. Yu. KOLOBOV & V. V. REVERDATTO 《Journal of Metamorphic Geology》1998,16(2):231-246
Diffusion modelling is applied to layered garnet–pyroxene–quartz coronas, formed by a pressure-induced reaction between plagioclase and primary pyroxene in a metabasic granulite. The reconstructed reaction involves some change in composition of reactant minerals. The distribution of minerals between layers is satisfactorily explained by diffusion-controlled reaction with local equilibrium, in which the diffusion coefficient for Al was smaller than those for Fe, Mg and Ca by a factor of approximately four. Diffusion of Mg towards plagioclase implies a chemical-potential gradient for MgO component in a direction opposite to the changing Mg content of garnet; this is explained by the influence of Al2 O3 on the chemical potential of the pyrope end-member. Grain-boundary diffusion is suggested to have operated, possibly with composition gradients different from those in the bulk minerals. Chemical-potential differences across the corona are estimated from the variation in garnet composition, enabling affinity (the free energy change driving the reaction) to be estimated as 6.9±1.8 kJ per 24-oxygen mole of garnet produced. This implies that the pressure for equilibrium among the minerals was overstepped by 1.4±0.4 kbar. The probable P–T conditions of reaction were in the range 650–790 °C, 8–10 kbar. Assuming a timescale of reaction between 106 and 108 years, estimated diffusion coefficients for Fe, Mg and Ca are in the range 9×10−23 to 5×10−20 m2 s−1 . These are consistent with experimental values in the literature for solid-state diffusion, including grain-boundary diffusion. 相似文献
12.
Abstract: Major and rare earth element contents are reported for Late Archean banded iron formations (BIFs) in the Bababudan Group of the Dharwar Craton, South India. The BIFs are mostly composed of SiO2 (average1ρ = 54.88.1 wt%) and Fe2 O3 * (44.38.2 wt%). The Al2 O3 and TiO2 contents are remarkably low, suggesting that detrital components were starved during the BIF deposition. The BIFs have a LREE-enriched pattern with a relatively high (La/Yb)N (6.644.07). Total REE concentrations (RE) vary from 5.2 to 65.3 ppm. The REE patterns are characterized by the presence of a very large negative Ce anomaly (Ce/Ce*: 0.13-0.83) and a positive Eu anomaly (Eu/Eu*: 0.96-2.45). The Eu/Eu* decreases and (La/Yb)N increases with a increase of RE. These correlations of REE indices are similar to those of modern hydrothermal iron-rich sediments near a mid-ocean ridge (MOR). Greenstones associated with the BIFs have MORB-like geochemical features. These geochemical and geological lines of evidence indicate that the depositional site of the BIFs was remote from a continent and/or island arc and that the BIFs were in situ hydrothermal sediments near a MOR. A striking negative Ce anomaly in the BIFs indicates that oxygenated deep-sea environments emerged at 2.9-2.7 Ga. The existence of contemporaneous Mn deposits in the Dharwar Craton supports this assertion. Our scenario of oxygen in the Earth's surface of the Late Archean is different from long-held notion that the atmosphere and ocean were persistently anoxic throughout the Archean. 相似文献
13.
On the Occurrence of Silician Magnetites 总被引:2,自引:0,他引:2
Hidehiko Shimazaki 《Resource Geology》1998,48(1):23-29
Abstract: About 120 specimens of magnetite from various localities are examined by an electron microprobe analyzer. Magnetites containing more than one weight percent of silica but lack of any other components than ferrous and ferric iron, called silician magnetites in the present paper, are recognized in 23 skarn, one vein and one thermally metamorphosed massive sulfide deposits. Thus it is confirmed that this mineral occurs in nature much more frequently than so far expected. Besides silician magnetites, magnetites with appreciable amounts of Al2 O3 , CaO, MgO and other components along with silica, are also recognized in some skarn deposits. Magnetites with such unusual compositions are found only in hydrothermal environments, and it is suggested that precipitation mechanisms seem to be responsible for their formation.
In silician magnetites, excess electric charge brought by the replacement of ferric iron in tetrahedral site by silicon, could be compensated by the replacement of ferric iron in octahedral site by ferrous iron, known as γ–Fe2 SiO4 component. The natural occurrence of silician magnetites, however, gives no positive support to the existence of this component at crustal pressures. Instead a preliminary Mössbauer experiment demonstrates that one silician magnetite has a maghemite –like structure by the omission of ferrous iron from octahedral site. 相似文献
In silician magnetites, excess electric charge brought by the replacement of ferric iron in tetrahedral site by silicon, could be compensated by the replacement of ferric iron in octahedral site by ferrous iron, known as γ–Fe
14.
J.G. SEN GUPTA 《Geostandards and Geoanalytical Research》1977,1(2):149-154
Experimental results for most rare-earth elements, yttrium and thorium in several international geological reference samples are presented and compared with other published values, where available. The analytical method used to obtain these results involved a preliminary concentration of the rare-earths On milligram quantities of iron as carrier for atomic-absorption, flame-emission and spectrophotometric determinations, or on milligram quantities of Fe2 O3 , Al2 O3 and SiO2 as carriers for optical-emission spectrometric determination. 相似文献
15.
Phosphorus-rich alkali feldspars were found in three peraluminous highly differentiated albite-topaz-Li-mica granites in the W and S parts of the Bohemian Massif. The average contents of P2 O5 in K-feldspars (Křížovy kámen 0.57 wt%, Homolka 0.77 wt%, and Podlesí 0.83 wt%) are higher than the average contents of P2 O5 in albites (Křížovy kámen 0.23 wt%, Homolka 0.23 wt%, and Podlesí 0.39 wt%). The analyses of feldspars indicate that partition coefficient of phosphorus between K-feldspar and albite range from 1.5 to 2.5. Measured data in K-feldspars suggest a statistically significant difference from the Al3+ +P5+ =2 Si4+ substitution mechanism for higher phosphorus concentration. The P-content in K-feldspars from the Podlesí-granite represent the highest P-content in natural feldspar known to date (up to 2.5 wt% of P2 O5 )- It is suggested that all studied granites retained nearly all phosphorus of granitic melts and that their alkali feldspars represent major reservoirs of phosphorus. 相似文献
16.
Proterozoic melting in the northern peridotite Massif, Zabargad Island: Os isotopic evidence 总被引:1,自引:0,他引:1
The geodynamic history of the three Zabargad peridotite bodies is the key to their significance as samples of the upper mantle. Currently, there are two main hypotheses that differ greatly in their implications for the origin and age of the peridotite complex as a whole. In the first, the Zabargad peridotite bodies all represent young asthenospheric mantle that was juxtaposed with ancient crustal gneisses during the opening of the Red Sea. In the second, the complex may represent a single package of residual mantle and lower crust of Pan-African age that was uplifted during the Miocene. In order to distinguish between these two models, we have analysed five samples of Zabargad peridotite and two pyroxenites for Os isotopes. The 187 Os/188 Os results range from 0.1198 to 0.1320 and correlate with Al2 O3 , allowing an age inference for the northern body, based on the Os isotopic evolution curve for the bulk Earth, of 1.4±0.3 Gyr. This is at least as old as the previously inferred Pan-African age and may be older, and agrees with previous suggestions that the northern and central peridotite bodies probably represent ancient continental lithospheric mantle, while the southern body represents recent asthenospheric mantle. Our results from both central and southern bodies are consistent with either crustal age hypothesis. 相似文献
17.
Shigeru Terashima Kohei Sato Masahiro Taniguchi Takashi Okai Noboru Imai 《Geostandards and Geoanalytical Research》2003,27(3):259-271
Two new geochemical reference materials, copper ore JCu-1 and zinc ore JZn-1 have been prepared by the Geological Survey of Japan (GSJ) for the determination of major and minor elements and isotopic compositions. JCu-1 is a sample of Cu-bearing sulfide ore typical of the Kamaishi mine in Iwate Prefecture, Japan, and is composed mainly of hedenbergite, chalcopyrite, quartz and calcite. Pyrrhotite, magnetite and actinolitic amphibole were also commonly found. The Zn-rich ore, JZn-1 is a crude ore from the Kamioka Pb-Zn mine in Gifu Prefecture, Japan. The sample consists of hedenbergite, quartz, calcite, sphalerite and epidote as main crystalline phase. Homogeneity test results showed that all studied constituents including ore elements such as Cu, Pb and Zn can be considered to be homogeneously distributed. Provisional collaborative analyses were carried out in ten laboratories, and the data were evaluated using a robust statistical method using z-scores. Recommended values for a number of major elements including TiO2 , Al2 O3 , MnO, MgO, CaO, Na2 O, K2 O, Fe (total), Zn, Cu and Pb were established. In addition, information values for eighteen major, minor and trace elements are presented to support future collaborative analyses. 相似文献
18.
Determinations of the absolute age of cleavage formation can provide fundamental information about the evolution of orogenic belts. However, when applied to cleavages in slates and phyllites, conventional dating methods are complicated by problems related to mineral separation and the presence of multiple cleavage generations. In situ high-spatial-resolution 40 Ar/39 Ar laser microprobe geochronology and microstructural observations indicate that the age of cleavage formation in slates and phyllites can be constrained by analysing zones of tightly packed cleavage domains. Three regionally developed cleavages (S2 , S3 , and S4 ) are present in the northern Taconic Allochthon of Vermont and New York. Representative samples were studied from a variety of localities where these cleavages, which are defined by white micas, are well developed. In the suite of samples, only S3 and S4 are expressed as domains that are sufficiently wide and spatially isolated in thin section to permit quantitative 40 Ar/39 Ar geochronology. Mean 40 Ar/39 Ar laser microprobe ages for these domains are 370.7 ± 1.0 Myr for S3 and 345.5 ± 1.7 Myr for S4 . Because estimates of the Ar closure temperature for white micas are substantially higher than the inferred growth temperatures of the micas defining S3 and S4 , these values are interpreted as periods since cleavage formation. This interpretation is consistent with independent geochronological constraints on the age of the Acadian orogeny in the region. 相似文献
19.
ABSTRACT A cave in granitic rocks was studied in Mezesse, South Cameroon. Coralloid speleothems, draperies and dissolution traces on the cave walls attest to its truly karstic nature. The speleothems consist of microlayers of opal and taranakite (K,NH4 )Al3 (PO4 )3 (OH).9H2 O. They indicate a significant mobilization of silica, Al and K from granite during the formation of the cave. Identification of silicified bacteria in the speleothems layers suggests a possible role of these micro-organisms in silica deposition. The presence of taranakite and of silicified organic remains within the speleothems lead to a better understanding of the genesis of the cave. 相似文献
20.
J.C. de WIT C.M. van der STRAATEN W.G. MOOK 《Geostandards and Geoanalytical Research》1980,4(1):33-36
By mixing 1 H2 O and 2 H2 O, both with accurately known purity, samples were prepared with 2 H/1 H ratios close to those of the international isotopic water standards: V-SMOW and SLAP. A mass spectrometrical comparison of these calibration samples with the actual water standards revealed:
2 H/1 H of V-SMOW = (155.95 ± 0.08) × 10−6
2 H/1 H of SLAP = (89.12 ± 0.07) × 10−6
δ2 HV-SMOW (SLAP) =−428.5 ± 0.5 % 相似文献
δ