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21.
The existence of Archaean komatiites with eruption temperatures greater than 1650°C requires that the mantle be vertically differentiated by the time of komatiite eruption. If in the unlikely event that undifferentiated mantle had survived primordial planetary differentiation and had been hot enough to deliver 1650°C komatiite, it would have been extensively molten to depths of ～250 km, resulting in rapid, profound, vertical differentiation anyway. During primordial differentiation (or Archaean komatiite petrogenesis) the high density and compressibility of ultrabasic melt allowed storage of a global melt layer beneath a buoyant residue of dunite and/or harzburgite. This refractory cap segregated by extraction of melt both upwards and downwards from the depth at which the density contrast between crystals and liquid vanishes. Eruption of komatiite from the melt layer by corrosion of the cap was the Archaean earth's principal means of dissipating excess heat. This subterranean magma ocean precluded vertical homogenization of the Archaean mantle by convection but effectively absorbed lateral mantle heterogeneities and imposed the relative uniformity of maximum eruption temperature and MgO contents (～32%) seen in primitive Archaean komatiites on all continents.Verification of the postulated density relations of liquids and crystals to 100 kbar becomes a pressing concern in view of the expected consequences these relations may have had. 相似文献
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Relocation of well observed, intermediate depth earthquakes in the Fiordland region by the method of joint hypocentre determination has revealed some fine structure in the Benioff zone. The earthquakes occur in three groups. The central group is the largest and occupies a planar volume less than 15 km thick striking N40°E and dipping at 80°. The deepest events in the region, at depths of 150 km, occur at the northeast end of this group. The two smaller groups lie to the northeast and to the south of the main group. The focal mechanism of the majority of the main group is that of thrust faulting. We suggest that the main group lies within a section of Indian plate lithosphere which has been broken off and rotated into its observed position and that the northern edge of the unbroken subducted Indian plate is indicated by the southern group. We suggest that the small northeastern group has quite a different tectonic origin and is similar to a group of earthquakes further north which are at a similar distance from, and presumably related to, the Alpine Fault.Use has also been made of the travel-time information which is a by-product of the joint hypocentre method to construct upper mantle velocity models for P and S waves in the South Island. The features of this model are a high-velocity region in the vicinity of the Benioff zone, and a subcrustal zone of high seismic velocities running east-west across the center of the South Island in an otherwise normal mantle. 相似文献
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The Silurian rocks and fossils of the Midland Valley of Scotland, and its extension across the northern parts of Ireland, have fascinated and challenged geologists for over 150 years. The spectacular diversity of facies and faunas, displayed in a series of inliers, is matched by contrasts in scenery from the rugged Atlantic coast of western Ireland to the undulating hills and heather moors on the northern margin of the Southern Uplands. Here, tectonic events near the climax of the Caledonian orogenic cycle served to fashion a mosaic of dynamic sedimentary environments on the Laurentian margin, at an exciting time in Earth history, as life established its foothold on the land. 相似文献
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Reversed Pn velocities have been determined from regional earthquake sources for the Wairarapa region in the southeast North Island, New Zealand. Values were 8.64 (±0.13) km/s in the easternmost part of the region and 8.90 (±0.13) km/s in the central Wairarapa, which are consistent with subducted Pacific plate lying at shallow depth under the region. A value of 8.22 km/s between Wellington and the Wairarapa region is interpreted as a possible difference in the velocity structure above the refractor between the two localities. 相似文献
26.
Sm---Nd isotopic systems have been studied in: (1) a sequence of coronitic olivine gabbros showing different degrees of transition to eclogite; (2) coronitic leuco-gabbro norite; and (3) country-rock eclogite, all from the Nordøyane-Brattvåg area, western Norway. Isochrons defined by combinations of whole-rock samples and relict igneous phases give ages of 1198 ± 56 and 1289 ± 48 Ma (olivine gabbro) and 926 ± 70 Ma (leuco-gabbro norite) and have been interpreted in terms of two different episodes of igneous intrusion.
In gabbro with advanced corona formation, relict augite tends to retain its original Nd isotopic composition, while plagioclase may show selective disturbance related to recrystallization and partial replacement by garnet. In completely eclogitized samples, there is a correlation between the degree of isotopic equilibrium and microstructural equilibrium. Whole-rock eclogites occurring within gabbro are characterized by pseudomorphs after igneous phases, and preserve original igneous Nd isotopic composition. Results for eclogite minerals suggest Caledonian ages, but these phases have not achieved complete isotopic equilibrium. In contrast, Nd isotopic systems in texturally equilibrated and strongly deformed eclogite within adjacent gneisses have been disturbed on a whole-rock scale. Minerals from an external eclogite define a Sm---Nd isochron with an age of 400 ± 16 Ma which is interpreted to date synkinematic eclogite equilibration at high-P and -T conditions. 相似文献
27.
Sm---Nd ages for norwegian garnet peridotite 总被引：13，自引：0，他引：13
Euan W. Mearns 《Lithos》1986,19(3-4):269-278
Sm---Nd results for ga-WR-cpx from garnet Iherzolite in Almklovdalen, western Norway, define an isochron with age of 1703 ± 29 Ma and /get = + 4.7 ± 0.7 (2/gs) which falls on a depleted mantle curve. Garnet websterite from the same peridotite body falls close to the Iherzolite isochron. Minerals from the websterite, however, define a younger age of 1040 ± 30 Ma.
The age of 1703 Ma is interpreted to date formation of the peridotite body and crystallisation of the garnet-bearing assemblage. The younger age from the websterite may record an uplift stage in the history, or alternatively may only reflect secondary disturbance of the isotopic systems in this sample. 相似文献
28.
It is shown that, using cross-sectional data, there are several important relationships between four manufacturing characteristics and both the size of an urban centre and its position in the national city system. The manufacturing characteristics are the proportion of the total work-force in manufacturing, the degree of specialisation (diversification) within the manufacturing sector, the plant size distribution and the extent to which different stages of the manufacturing process are represented within the city. The relationships shown in this exploratory study appear to be sufficiently strong to justify a detailed examination of the way they operate in a dynamic context and to encourage an investigation into the causal relationships underlying the spatial associations which are identified. 相似文献
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The freshwater Zippel Bay (Minnesota, USA), from its headwaters to its outlet on Lake of the Woods, a larger lake, was identified
as a degraded system because of augmented nutrient and sediment loads, and for low oxygen affecting aquatic life. Anthropogenic
impacts were purportedly from stressors including agriculture, urban development and hydrologic modifications, so paleolimnology
using multiple indicators was employed in the bay and its two primary tributaries to characterize long-term trends since European
settlement of the region. Isotope analyses of the sediment cores indicated notable shifts in sedimentation rates in response
to human activities, and the application of a new diatom-based nutrient model to fossil diatom assemblages indicated that
phosphorus levels have approximately doubled over the last ∼150 years. While Zippel Bay appears to be naturally eutrophic,
increased nutrient loads have resulted in significant algal blooms, particularly during late summer. The multiple-core approach
revealed that anthropogenic activities upgradient of the two tributaries varied in their contributions to aquatic degradation
in Zippel Bay, and that despite the close proximity of the cores, there has been notable spatial variation in water quality.
This research also further validates the application of traditional freshwater paleolimnological methods to lotic systems
with sufficient sedimentary regimes. This study has important implications for Zippel Bay as it quantifies the extent of limnological
impacts and underlines the need for remediation. 相似文献