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1.
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 206Pb/238U 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, Al2O3, K2O, FeO, MgO, Na2O and H2O. 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   

2.
Low-pressure granulite facies metasedimentary gneisses exposed in MacRobertson Land, east Antarctica, include hercynitic spinel-bearing metapelitic gneisses. Peak metamorphic mineral assemblages include spinel + rutile + ilmenite + sillimanite + garnet, spinel + ilmenite + sillimanite + garnet + cordierite, ortho-pyroxene + magnetite + ilmenite + garnet, spinel + cordierite + biotite + ilmenite and orthopyroxene + cordierite + biotite, each with quartz, K-feldspar and melt. The presence of garnet + biotite- and cordierite + orthopyroxene-bearing assemblages implies crossing tie-lines in AFM projection for the K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH) system. This apparent contradiction, and the presence of spinel, rutile and ilmenite in the assemblages, is acounted for by using the KFMASH-TiO2-O2 system, i.e. AFM + TiO2+ Fe2O3. We derive a petrogenetic grid for this system, applicable to low-pressure granulite facies metamorphic conditions. Retrograde assemblages are interpreted from corona textures on hercynitic spinel and Fe-Ti oxides. The relative positions of the peak and retrograde metamorphic assemblages on the petrogenetic grid suggest that corona development occurred during essentially isobaric cooling.  相似文献   

3.
Calculated results using thermocalc for melting of pelitic compositions are compared with the results of experimental melting of pelite from Morton Pass, Wyoming, USA. The experiments were carried out at 1, 2 and 3.5 kbar, dominantly at 2 kbar. For experimental charges with ('wet') and without ('dry') added H2O, the agreement is good. This is true for the compositions without added H2O, in which liquid first appears with the production of orthopyroxene at ∼800 °C, and for the runs with added H2O, in which melting begins at ∼700 °C, and continues, with the appearance of orthopyroxene between 780 and 800 °C at 2 kbar. The compositions of melts are also compared: the comparison is generally good, except in the modelling of wet runs below ∼780 °C, where the calculated values for FeO and MgO are about one-tenth of the already low analytical values, and in somewhat low calculated values of Al2O3 compared to the analytical data. A quantitative model to illustrate melting of pelite at Morton Pass is calculated, giving T – X (H2O) conditions for the observed sets of natural assemblages, along with the reactions at and near the beginning of melting.  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% Al2O3)–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% Al2O3), 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% Al2O3)–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   

5.
Results of electron microprobe and microthermometric studies of samples collected from the Bouvet Triple Junction Region (BTJR) during a joint Russian-Italian geological expedition on the R/V Academician Nikolaj Strakhov (1994) have revealed new data on the composition of basaltic magmas and oceanic hydrothermal fluids connected with magmatic processes. Detailed analysis of basaltic glasses shows that the modem Mid-Atlantic Ridge (MAR) rift valley is composed of normal mid-ocean ridge basalts with low concentrations of K2 O and TiOz (N-MORB), while its flanks are more enriched with these components approaching E-MORB. A marked influence of the Bouvet hot spot volcanism on magma generation on the South-West Indian Ridge (SWIR) near Bouvet Island is observed. Basaltic melts in this area belong to alkalic and transitional series and have maximum contents of K2O, TiO2, H2O.
Microthermometric analyses of fluid inclusions in the samples from the BTJR have revealed major differences in the oceanic hydrothermal fluid systems on the MAR and near SWIR, which depends on the peculiarities of magma. In the area of the MAR (with dry melts) only H2O solution inclusions in quartz were found; thus, seawater is probably the only primary source of hydrothermal fluids (NaCl + MgCl2+ H2O; T = 170–200°C). In the SWIR area (with the high content of water in melts) syngenetic liquid CO2 and H2O solution inclusions in quartz indicate the influence of the magmatic fluid component on the ore-forming water/carbon dioxide solutions (NaCl + CaC12+ H2O + CO2; T = 200–310 °C; P = 900–1700 bar).  相似文献   

6.
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, Al2O3, MnO, MgO, CaO, Na2O, K2O, 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   

7.
Abstract Chloritoid–hornblende occurs in quartz–muscovite pelitic schist derived from sediment in a volcaniclastic sequence of the Grenville Supergroup and from reworked sedimentary and regolithic material above the unconformity at the base of the Flinton Group. Comparison of these samples with other pelitic rocks on triangular composition diagrams and in the ACNF and ACFM tetrahedra indicates that the presence of hornblende cannot be explained by unusually high CaO content. The rare assemblage is attributed to a combination of relatively low Al2O3 and high K2O with high CaO/(CaO+Na2O) and FeO/(FeO+MgO).
On two qualitative reaction grids derived from AFM diagrams projected through CaO and plagioclase, respectively, the P–T stability field of chloritoid–hornblende overlaps the first appearance of staurolite–biotite in normal pelitic rocks in the kyanite field. Staurolite–hornblende overlaps chloritoid–hornblende and extends to the higher temperatures and pressures of the kyanite–hornblende field.
The phase relations in these rocks provide a link between the conventional hornblende-absent grids for pelitic rocks and those for K2O-poor (muscovite-absent) pelitic and mafic amphibolitic rocks.  相似文献   

8.
The Kontiagarh placer deposit in the Ganjam district, Orissa, India extends in northeast direction having a width of 700–1000 m. A total of 187 samples were collected meterwise from 55 bore holes in a grid pattern from beach, frontal, intermediate and back dunes covering an area of approximately 1 km2. Light minerals decrease in size from the beach to the back dunes, whereas the size distribution of heavy minerals in the beach and dunes is more or less uniform. The average heavy mineral content in the beach and dunes vary from 9.38% to 24.20%. The heavy minerals are ilmenite, garnet, sillimanite, rutile, monazite, and zircon with trace amounts of magnetite, hornblende, diopside, sphene, tourmaline, and epidote. Heavy minerals are mostly less than 350 µm in size, with a peak distribution in the range between 180 and 125 µm. Ilmenite shows exsolution intergrowth with hematite. Mineral chemistry of ilmenite, hematite, leucoxene, magnetite, monazite and sillimanite are examined by EPMA. Leucoxene is lower in Fe and higher in Ti, Al, Cr and V than ilmenite. The litho‐units of the Precambrian Eastern Ghats Mobile Belt, comprising primarily khondalite, charnockite, calc‐silicate granulite and gneiss, are the source of heavy minerals for this deposit. The bulk sample has 7.30% ilmenite, 5.24% sillimanite, 9.16% garnet, 0.18% rutile, 0.14% monazite, 0.06% zircon and 0.52% other heavy minerals. The deposit has good potential for economic exploitation of ilmenite, rutile, sillimanite, monazite, zircon and garnet.  相似文献   

9.
Abstract In granulite facies metapelitic rocks in the Musgrave Complex, central Australia, reaction between S1 garnet and sillimanite involves the development in S2 of both garnet + cordierite + hercynitic spinel + biotite and hercynitic spinel + cordierite + sillimanite + biotite. The S2 assemblages occur either in coronas and symplectites, mainly around garnet, or, in rocks in which S2 is more strongly developed, as recrystallized assemblages. Ignoring the presence of biotite and ilmenite, the mineral textures can be accounted for qualitatively by a consideration of the model system FeO-MgO-Al2O3-SiO2 (FMAS); the textural relationships accord with decompression accompanying the change from S1 to S2. However, since biotite and ilmenite are involved in the assemblages, the parageneses are better accounted for in terms of equilibria in the expanded model system K2O-FeO-MgO-Al2O3-SiO2-H2-TiO2-Fe2O3 (KFMASHTO), i.e. AFM + TiO2+ Fe2O3. The coronas reflect the tectonic unroofing of at least part of the Musgrave Complex from peak S1 conditions of about 8 kbar to S2 conditions of about 4 kbar.  相似文献   

10.
Abstract Late Archaean orthogneisses and aluminous and iron-rich metasedimentary rocks intruded by anorthosite and a ferrodiorite-granite suite were completely recrystallized during Proterozoic granulite facies metamorphism. Geobarometry and geothermometry indicate P-T conditions of around 7.5kbar. 700°C, with a CO2-rich fluid phase and logfO2 at or below -16. A two-stage high-grade history of near isochemical corona growth is preserved in metasediments with the reaction cycle opx + plag + H2O → hbl+gar+SiO2→ opx+plag+H2O. End product compositions resemble those of the initial phases, and the only mobile components were SiO2 and/or H2O. The coronas reflect shortlived fluctuations in chemical activity at essentially constant P and T, contrary to simple progressive change in equilibrium parameters recorded by most corona-bearing textures.  相似文献   

11.
A new method has been developed for the determination of platinum and palladium based on separation and preconcentration with a microcolumn packed with nanometric TiO2 immobilised on silica gel (immobilised nanometric TiO2) prior to their determination by inductively coupled plasma-atomic emission spectrometry. The optimum experimental parameters for the preconcentration of Pt and Pd, such as the pH of the sample solution, its flow rate and volume, the type and concentration of eluent and interfering ions, have been investigated. Platinum and Pd could be quantitatively retained by immobilised nanometric TiO2 in the pH range 6–8, then eluted completely with 2.0 ml of 3% m/v thiourea in 1.0 mol l−1 HNO3. The detection limits of this method for Pt and Pd were 12 and 7. 6 ng l−1 with an enrichment factor of 100, and the relative standard deviations were 4.7% and 3.3% at the 10 ng ml−1 level. The method has been applied for the determination of Pt and Pd in geological samples with satisfactory results.  相似文献   

12.
Abstract. The Erdenet porphyry copper deposit is one of the major mineral deposits in Mongolia. The geochemical data of granitoids acquired for the Erdenet and its surrounding areas are re-examined. The granitoids of the Erdenet deposit with hypogene type mineralization show the lowest TiO2 content. Although Ti was possibly lost through the pyritization, it is also possible that the hypogene type mineralization occurred accompanied with the most differentiated granitoids. The variation of the element contents related to the mineralization of the Erdenet deposit shows the decrease of MgO and CaO contents, rather constant K2O content, rather constant to decrease of Na2O content, with respect to the Cu contents. The rather constant Na2O in the mineralized zone is owing to the residual albite against the sericitic alteration. The granitoids of the Erdenet area show an increase of Na2O content and a decrease of K2O content with an increase of SiO2 content. This trend makes clear contrast to granitoids in the surrounding areas. The granitoids of the Erdenet area might have the adakitic nature based on the Sr and Y contents.  相似文献   

13.
The chemical analysis by EMPEDS of 140 samples in cores from six Red Sea troughs and basins has led to the discovery in the Nereus Deep of high vanadium concentrations (up to 1.3% V2O3) in several bulk samples, and to the isolation of a major magnetite phase. This vanado-magnetite, remarkable for its high content of vanadium (mean = 1.45% V2O3) and its lack of titanium, is frequently zoned, the V-content decreasing toward the outer zones. Oxides with up to 42% V2O3, have also been found. Since the origin of the magnetite is clearly authigenic, its high vanadium and low titanium concentrations are traced back to the differential hydrothermal leaching of Fe—Ti-oxides which occur profusely in basic eruptive rock clasts, actually present in some of the overlying seams.  相似文献   

14.
Eighteen Chinese geologic reference samples (stream sediments GSD 9–12, soils GSS 1–8, and rocks GSR 1–6) were analyzed by wavelength-dispersive X-ray fluorescence spectrometry (XRFS) for major elements Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P, and by energy-dispersive XRFS for trace elements Ba, Ce, Cr, Cu, La, Nb, Ni, Rb, Sr, Y, Zn, and Zr. Major element analysis followed gravimetric determination of loss on ignition, and samples were prepared by fusion with Li2B4O7. A loose-powder sample preparation was used for trace element analysis. The results reported in this study are, generally, in good agreement with concentrations compiled by X. Xie of the Geochemical Standard Reference Group and the Institute of Geophysical and Geochemical Exploration (People's Republic of China).  相似文献   

15.
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:1.1Al2O3 spinel assembly deformed at 1173K, 800 MPa in a Griggs apparatus; (Mg,Fe)3(Al,Mg,Si)2Si3O12 majorite – (Mg,Fe)2SiO4 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   

16.
Abstract Scapolite, wollastonite, calcite, diopside, grossular-andradite garnet and sphene occur in calc-silicate rocks in the granulite terrain of the Arunta Block, central Australia. This assemblage buffers the CO2 activity at a low value, so that any coexisting fluid phase must be H2O rich and CO2 poor ( X co2 = 0.2-0.3). In contrast, the H2O activity in the surrounding felsic and mafic granulites was low. Thus fluid activities during granulite facies metamorphism were locally buffered in various rock units and fluid flow appears to have been restricted or fluid may have been absent. Late retrograde rims of garnet and garnet-quartz separate phases formed in the high-grade stage. Formation of these rims would have required either an influx of water-rich fluid or a decrease in pressure. Evidence from the surrounding granulites shows that in one locality, the calc-silicate rocks had undergone late isobaric hydration; in another locality, minor uplift had occurred soon after peak P-T conditions. In both, scapolite had partly broken down to plagioclase-calite. A calc silicate rock from the granulite terrain of Enderby Land, Antarctica, contains scapolite, wollastonite, calcite, diopside, quartz and sphene; this assemblage also indicates low CO2 activities. In this rock, wollastonite has broken down to calcite-quartz, to indicate isobaric cooling without influx of hydrous fluid.  相似文献   

17.
The Higo metamorphic unit in west-central Kyushu island, southwest Japan is an imbricated crustal section in which a sequence of units with increasing metamorphic grade from high (northern part) to low (southern part) structural levels is exposed. The basal part of the metamorphic sequence representing an original depth of 23–24  km consists mainly of garnet–cordierite–biotite gneiss, garnet–orthopyroxene gneiss, orthopyroxene-bearing amphibolite and orthopyroxene-bearing S-type tonalite. These metamorphic rocks underwent high amphibolite-facies up to granulite facies metamorphism with peak P – T  conditions of 720  MPa, 870  °C. In addition sapphirine-bearing granulites and related high-temperature metamorphic rocks also occur as tectonic blocks in a metamorphosed peridotite intrusion. The sapphirine-bearing granulites and their related high-temperature metamorphic rocks can be subdivided into five types of mineral assemblages reflecting their bulk chemical compositions as follows: (1) sapphirine–corundum–spinel–cordierite (2) corundum–spinel–cordierite (3) garnet–corundum–spinel–cordierite (4) garnet–spinel–gedrite–corundum, and (5) orthopyroxene–spinel–gedrite. These metamorphic rocks are characterized by unusually high Al2O3 and low SiO2 contents, which could represent a restitic nature remaining after partial melting of pelitic granulite under the ultra high-temperature contact metamorphism at the peak metamorphic event of the Higo metamorphic unit. The metamorphic conditions are estimated to be about 800  MPa and above 950  °C which took place at about 250  Ma as a result of the thermal effect of the regional gabbroic rock intrusions.  相似文献   

18.
Calc-silicate granulites from the Bolingen Islands, Prydz Bay, East Antarctica, exhibit a sequence of reaction textures that have been used to elucidate their retrograde P–T path. The highest temperature recorded in the calc-silicates is represented by the wollastonite- and scapolite-bearing assemblages which yield at least 760°C at 6 kbar based on experimental results. The calc-silicates have partially re-equilibrated at lower temperatures (down to 450°C) as evidenced by the successive reactions: (1) wollastonite + scapolite + calcite = garnet + CO2, (2) wollastonite + CO2= calcite + quartz, (3) wollastonite + plagioclase = garnet + quartz, (4) scapolite = plagioclase + calcite + quartz, (5) garnet + CO2+ H2O = epidote + calcite + quartz, and (6) clinopyroxene + CO2+ H2O = tremolite + calcite + quartz.
The reaction sequence observed indicates that a CO2 was relatively low in the wollastonite-bearing rocks during peak metamorphic conditions, and may have been further lowered by local infiltration of H2O from the surrounding migmatitic gneisses on cooling. Fluid activities in the Bolingen calc-silicates were probably locally variable during the granulite facies metamorphism, and large-scale CO2 advection did not occur.
A retrograde P–T path, from the sillimanite stability field ( c. 760°C at 6 kbar) into the andalusite stability field ( c. 450°C at <3 kbar), is suggested by the occurrence of secondary andalusite in an adjacent cordierite–sillimanite gneiss in which sillimanite occurs as inclusions in cordierite.  相似文献   

19.
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, Al2O3 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 Al2O3, 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 Al2O3. 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   

20.
笏山金矿床为陡崖断裂带南段内最大的金矿床,为了揭示其矿床成因,在矿床地质研究的基础上开展了矿体围岩元素地球化学和流体包裹体研究。研究结果表明：笏山金矿床沿NE向断裂发育大规模的热液蚀变带,地表出露的断裂带形成一个庞大的面形蚀变带,矿化较弱,分带不明显,钻孔显示金矿体两侧依次发育黄铁绢英岩、绢英岩化花岗岩蚀变带和二长花岗岩,金矿体主要产于陡崖断裂带下盘黄铁绢英岩蚀变带内。质量平衡计算表明：绢英岩化过程中,主量元素SiO2、Al2O3、FeO、MnO、Na2O、K2O、P2O5和部分稀土元素从二长花岗岩中迁出,而MgO、CaO、Fe2O3,成矿元素Au、Ag,亲硫元素Cu、Pb及亲铁元素V、Cr、Co、Ni迁入二长花岗岩;黄铁绢英岩化过程中,主量元素FeO、Fe2O3、MnO、MgO、CaO,成矿元素Au、Ag,亲硫元素Cu及亲铁元素V、Cr、Co、Ni迁入,SiO2、Al2O3、Na2O、K2O和P2O5迁出。岩相学观察、显微测温以及单个包裹体成分激光拉曼分析提示其流体包裹体类型主要为气液两相包裹体和含CO2三相包裹体,具有低盐度（w（NaCl）为5.33%~13.29%）和中温（260~300℃）CO2-NaCl-H2O体系,在成矿过程中,含矿流体经历了流体不混溶作用。结合矿床地质特征及实验结果分析,确定该矿床为受陡崖断裂带控制的中温蚀变岩型金矿床。  相似文献   

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