排序方式： 共有20条查询结果，搜索用时 15 毫秒
1.
L. G. Medaris Jr. E. D. Ghent H. F. Wang J. H. Fournelle E. Jelínek 《Mineralogy and Petrology》2006,86(3-4):203-220
Summary In the Kutná Hora Complex, the Běstvina Formation, which is similar to Gf?hl granulite, contains eclogite that has escaped
widespread retrograde recrystallization. The eclogite assemblage, garnet + omphacite + quartz + rutile ± plagioclase, yields
an estimate for peak metamorphic conditions of 18–20 kbar and 835–935 °C, which is comparable to that determined from felsic
granulite, 14–20 kbar and 900–1000 °C. Garnet in eclogite exhibits both prograde and retrograde compositional zoning, from
which constraints on thermal history of the Gf?hl terrane can be derived by diffusion modelling. At 900 °C, a garnet grain
of 800–1000 μm radius would homogenize in 7.5–11.7 million years, but the existence of compositional gradients on a length
scale of 100–200 μm suggests that the duration of peak metamorphism may have been limited to ∼500,000 years. Diffusion modelling
of retrograde zoning in garnet yields a cooling rate of 150–100 °C/m.y. for a radius of 800–1000 μm and initial temperature
of 900 °C. The relatively brief duration of high-pressure/high-temperature metamorphism and rapid cooling and exhumation of
the Gf?hl terrane may be a consequence of lithospheric delamination during Early Carboniferous collision of Bohemia (Teplá-Barrandia)
and Moldanubia (Franke, 2000). 相似文献
2.
M. Z. Stout M. L. Crawford E. D. Ghent 《Contributions to Mineralogy and Petrology》1986,92(2):236-247
Metamorphosed pelitic rocks from Mica Creek, British Columbia contain sillimanite, kyanite with minor fibrolite and andalusite-bearing quartz pods. Mineral equilibria were used to infer peak P-T conditions and fluid compositions in equilibrium with the solid phases. Fluid inclusions in three schist samples appear to be good indicators of conditions affecting those rocks during and after peak metamorphic conditions. In samples from two localities, fluid inclusions from schist and quartz-rich segregations have densities appropriate to the peak metamorphic conditions. The observed compositions for these fluids (low salinity with 12 mole % dissolved CO2) agree with calculated
values of 0.84 to 0.85, based upon paragonite-quartz-albite-Al2SiO5 equilibria. The fluids unmixed as the schists were uplifted and cooled; fluid inclusions trapped during this stage outline a solvus in the CO2-H2O-NaCl system. A later influx of fluids containing CH4 and N2 accompanied formation of andalusite-bearing plagioclaserich segregations. The restricted association of andalusite-bearing pods and low density fluids suggest a localized but pervasive fluid influx during uplift. Preservation of high density fluid inclusions during uplift and erosion, coupled with evidence for unmixing of H2O- and CO2-rich fluids on the solvus, provide constraints on the P-T uplift path. 相似文献
3.
The biotite zone assemblage: calcite-quartz-plagioclase (An25)-phengite-paragonite-chlorite-graphite, is developed at the contact between a carbonate and a pelite from British Columbia. Thermochemical data for the equilibrium paragonite+calcite+2 quartz=albite+ anorthite+CO2+H2O yields: $$\log f{\text{H}}_{\text{2}} {\text{O}} + \log f{\text{CO}}_{\text{2}} = 5.76 + 0.117 \times 10^{ - 3} (P - 1)$$ for a temperature of 700°K and a plagioclase composition of An25. By combining this equation with equations describing equilibria between graphite and gas species in the system C-H-O, the following partial pressures: \(P{\text{H}}_2 {\text{O}} = 2572{\text{b, }}P{\text{CO}}_2 = 3162{\text{b, }}P{\text{H}}_2 = 2.5{\text{b, }}P{\text{CH}}_4 = 52.5{\text{b, }}P{\text{CO}} = 11.0{\text{b}}\) are obtained for \(f{\text{O}}_2 = 10^{ - 26}\) . If total pressure equals fluid pressure, then the total pressure during metamorphism was approximately 6 kb. The total fluid pressure calculated is extremely sensitive to the value of \(f{\text{O}}_2\) chosen. 相似文献
4.
Vitrinite reflectance data for two sections of the Kootenay Formation are different, but increase with depth. Illite crystallinities (XRD) are irregular with depth but detailed S.E.M. examination shows that crystal size and morphology increase with depth.At Elk Valley (low reflectance) S.E.M. examination shows kaolinite + dolomite as the authigenic assemblage. At Mt Allan (45 km north, higher reflectance) kaolinite disappears at 265 m and chlorite + calcite appears implying the equilibrium 5 CaMg(CO3)2 + Al2Si2O5(OH)4 + SiO2 + H2OagMg5Al2Si3O10(OH)8 + 5CaCO3 + 5CO2. (1)Equilibrium (1) involves CO2 and H2O which form an immiscible fluid at diagenetic conditions. Thermochemical data, activity coefficients for the fluid mixture (Margules solution model), and fugacity coefficients allow the position of intersection of the miscibility surface and equilibria similar to (1) to be calculated. Maximum temperatures (180–250°C) in water-rich fluids are in accord with geothermal gradients of 33°C/km and temperatures from vitrinite reflectance (220°C). It is suggested that produced CO2 may be responsible for the development of secondary porosity during deep burial. 相似文献
5.
We investigate the distinctive distribution and pattern of subkilometer-sized cones in Isidis Planitia, and their relationship to the material that fills the basin. We observe that: (1) the cones show uniform size, spacing, and morphology across the entire basin; (2) they have large central depressions (relative to cone basal diameters) that are generally filled in and commonly show layering; (3) cone chains form highly organized spatial patterns; cones are also found in isolation and in dense fields; (4) many adjacent chains are parallel, and evenly spaced, creating a pattern that mimics lava flows that have experienced compressional folding; (5) no flows are associated with the cones, even where summit crater rims are breached; (6) the cones are at least temporally related to basin fill; (7) the basin fill material is largely fine-grained, but with locally indurated layers at shallow depth; blocks ejected by small craters from these locally indurated layers are commonly dark in color; and (8) relationships between cones along a chain show no systematic temporal formation sequence along the chain. We conclude that the basin was likely inundated by one or more hot, partially welded pyroclastic flows; devolatilization of these flows remobilized volcaniclastic material to form the cones without associated lava flows. The volume of gas required to account for the observed number of cones is low and does not require the presence of ground water or ice. 相似文献
6.
Bruce A. Campbell Lynn M. Carter Michael Nolan Rebecca R. Ghent Ross F. Anderson 《Icarus》2010,208(2):565-181
We present results of a campaign to map much of the Moon’s near side using the 12.6-cm radar transmitter at Arecibo Observatory and receivers at the Green Bank Telescope. These data have a single-look spatial resolution of about 40 m, with final maps averaged to an 80-m, four-look product to reduce image speckle. Focused processing is used to obtain this high spatial resolution over the entire region illuminated by the Arecibo beam. The transmitted signal is circularly polarized, and we receive reflections in both senses of circular polarization; measurements of receiver thermal noise during periods with no lunar echoes allow well-calibrated estimates of the circular polarization ratio (CPR) and the four-element Stokes vector. Radiometric calibration to values of the backscatter coefficient is ongoing. Radar backscatter data for the Moon provide information on regolith dielectric and physical properties, with particular sensitivity to ilmenite content and surface or buried rocks with diameter of about one-tenth the radar wavelength and larger.Average 12.6-cm circular polarization ratio (CPR) values for low- to moderate-TiO2 mare basalt deposits are similar to those of rough terrestrial lava flows. We attribute these high values to abundant few-centimeter diameter rocks from small impacts and a significant component of subsurface volume scattering. An outflow deposit, inferred to be impact melt, from Glushko crater has CPR values near unity at 12.6-cm and 70-cm wavelengths and thus a very rugged near-surface structure at the decimeter to meter scale. This deposit does not show radar-brightness variations consistent with levees or channels, and appears to nearly overtop a massif, suggesting very rapid emplacement. Deposits of similar morphology and/or radar brightness are noted for craters such as Pythagoras, Rutherfurd, Theophilus, and Aristillus. Images of the north pole show that, despite recording the deposition of Orientale material, Byrd and Peary craters do not have dense patterns of radar-bright ejecta from small craters on their floors. Such patterns in Amundsen crater, near the south pole, were interpreted as diagnostic of abundant impact melt, so the fraction of Orientale-derived melt in the north polar smooth plains, 1000 km farther from the basin center, is inferred to be much lower. 相似文献
7.
Edward D. Ghent Philip S. Simony Clifford C. Knitter 《Contributions to Mineralogy and Petrology》1980,74(1):67-73
Late Precambrian and Lower Paleozoic metapelitic rocks near Mica Creek, British Columbia, range in metamorphic grade from biotite zone to sillimanite zone. The kyanite-sillimanite isograd was established after phase 2 folding but was deformed by phase 3 folding. Topographic relief of about 2 km, combined with phase 3 folding, permits reasonably precise determination of the geometry of the isogradic surfaces. The effects of phase 3 folding have been accounted for and the isobaric surfaces at the time of metamorphism are inferred to have been dipping gently. Using the kyanite-sillimanite experimental phase diagram, intersection of the isobars and the isograd permits estimates of differences in temperature and pressure along a cross section. These estimates are about 400 bars and 20° C. Mineral geothermometry (garnet-biotite) and geobarometry (garnetplagioclase-Al 2SiO5-quartz) does not provide a fine enough resolution to detect these predicted differences. 相似文献
8.
Conditions and timing of high-pressure Variscan metamorphism in the South Carpathians, Romania 总被引：2，自引：0，他引：2
High-pressure (HP) metamorphic rocks, including garnet peridotite, eclogite, HP granulite, and HP amphibolite, are important constituents of several tectonostratigraphic units in the pre-Alpine nappe stack of the Getic–Supragetic (GS) basement in the South Carpathians. A Variscan age for HP metamorphism is firmly established by Sm–Nd mineral–whole-rock isochrons for garnet amphibolite, 358±10 Ma, two samples of eclogite, 341±8 and 344±7 Ma, and garnet peridotite, 316±4 Ma.
A prograde history for many HP metamorphic rocks is documented by the presence of lower pressure mineral inclusions and compositional zoning in garnet. Application of commonly accepted thermobarometers to eclogite (grt+cpx±ky±phn±pg±zo) yields a range in “peak” pressures and temperatures of 10.8–22.3 kbar and 545–745 °C, depending on tectonostratigraphic unit and locality. Zoisite equilibria indicate that activity of H2O in some samples was substantially reduced, ca. 0.1–0.4. HP granulite (grt+cpx+hb+pl) and HP amphibolite (grt+hbl+pl) may have formed by retrogression of eclogites during high-temperature decompression. Two types of garnet peridotite have been recognized, one forming from spinel peridotite at ca. 1150–1300 °C, 25.8–29.0 kbar, and another from plagioclase peridotite at 560 °C, 16.1 kbar.
The Variscan evolution of the pre-Mesozoic basement in the South Carpathians is similar to that in other segments of the European Variscides, including widespread HP metamorphism, in which P–T–t characteristics are specific to individual tectonostratigraphic units, the presence of diverse types of garnet peridotite, diachronous subduction and accretion, nappe assembly in pre-Westphalian time due to collision of Laurussia, Gondwana, and amalgamated terranes, and finally, rapid exhumation, cooling, and deposition of eroded debris in Westphalian to Permian sedimentary basins. 相似文献
9.
10.