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131.
John P. Armstrong Mark Wilson Robert L. Barnett Tom Nowicki Bruce A. Kjarsgaard 《Lithos》2004,76(1-4):415-433
A suite of fresh, Late Cretaceous to Eocene hypabyssal kimberlites from the Lac de Gras field were studied in order to understand better carbonate, silicate and oxide paragenesis. The samples have excellent preservation of textures and primary mineralogy and are archetypal or Group 1 kimberlite. Five kimberlite localities are identified as calcite-bearing based on the presence of high Sr–Ba calcite as phenocrysts, microphenocrysts and in segregations. Three kimberlite localities are identified as dolomite-bearing based on the presence of mixed calcite–dolomite segregations containing oscillatory and banded textures of calcite–dolomite solid solution and dolomite (±magnesite). Sr–Ba calcite are characterized by high XCa (>0.95) and are enriched in Sr (4900–11,100 ppm) and Ba (3200–14,200 ppm). The calcite–dolomite and dolomite–magnesite solid solution compositions span the XCa range from 0.42 to 0.95, and typically have Sr and Ba contents in the range of 1000–4000 ppm. The carbonate, silicate and oxide mineral compositions suggest that the origin of the calcite-bearing versus dolomite-bearing kimberlites studied is related to subtle differences in parent magma composition, in particular, the CO2/H2O ratio. Formation of the carbonates reflects the latter part of a protracted magmatic crystallization sequence, in which Sr–Ba calcite precipitates from an evolved kimberlite melt. Subsequently, calcite–dolomite solid solution and dolomite is precipitated from localized, Mg-rich carbonate fluids at relatively high temperatures (higher than serpentine stability). 相似文献
132.
R.A. Armstrong W. Compston M.J. de Wit I.S. Williams 《Earth and Planetary Science Letters》1990,101(1)
Recent field and geochemical studies indicate a need to test the stratigraphy of the ca. 3.5 Ga Barberton Greenstone Belt as it is presently adopted [1,2]. This work uses the ion microprobe SHRIMP, to attempt such a test. Results show that: (1) Volcaniclastic sediments of the Theespruit Formation (< 3453 ± 6Ma) could be younger than the (structurally) overlying mafic and ultramafic volcanics of the Komati Formation (3482 ± 5Ma). A major structural discontinuity may therefore exist between the two formations. (2) An age of 3538 ± 6Ma established for a tectonic wedge of tonalitic gneiss within the Theespruit Formation confirms the presence of a sialic basement and deformed unconformity below that unit. The tonalitic gneiss is the oldest unit yet recorded within the greenstone belt, equal in age to the older components of the adjacent Ancient Gneiss Complex. (3) The interpreted ages of felsic volcanic rocks from both the Hooggenoeg (3445 ± 8Ma) and Theespruit Formations and the nearby Theespruit Pluton (3437 ± 6Ma) are essentially the same, and corroborate field and geochemical evidence that the felsic units were probably cogenetic and emplaced simultaneously as high-level equivalents of trondjhemite-tonalite plutons that intrude the greenstone belt at its southwestern extremity. (4) Felsic-intermediate volcanic-volcaniclastic rocks locally separating the two major groups (the Fig Tree and Moodies Groups) which overlie the Onverwacht Group record a second major peak of tonalitic magmatism in the Barberton terrain at about 3250 Ma. This is close to the age of the Kaap Valley tonalite pluton which intrudes the Barberton Greenstone Belt at ca. 3226 Ma along its northwestern margin. The present results indicate the Barberton Greenstone Belt is part of an allochthonous sequence containing major tectonic and stratigraphic breaks, with a protracted history; of which the last 200 million years, at least, evolved within a tectonically active convergent environment. 相似文献
133.
Donald R. Gunderson David A. Armstrong Yun-Bing Shi Robert A. McConnaughey 《Estuaries and Coasts》1990,13(1):59-71
Extensive trawl surveys were conducted in two large estuaries (Grays Harbor and Willapa Bay) on the Washington coast during 1983–1987, and in adjacent areas of the open coast. These surveys have shown that both English sole and Dungeness crab rely heavily on these estuaries as nursery areas, although the pattern of utilization differs substantially. Juvenile migration patterns can show substantial interannual variability and can only be delineated by concurrent surveys in both coastal and estuarine areas, conducted over a period of several years. English sole eggs and Dungeness crab larvae are released in coastal waters. Larvae of both species transform to the benthic stage in both coastal and estuarine areas, but most English sole eventually migrate into the estuaries during the first year of life, even if initial settlement is along the open coast. By the time English sole have attained a length of 55 mm (TL), most of them are found in estuaries. English sole begin emigrating from the estuaries at about 75 mm, and few remain there during the second year of life. In contrast, Dungeness crab appear to remain in the area of initial settlement throughout the first year of life. Growth is substantially faster in estuaries where 0+ crab reach a mean size of about 40 mm carapace width (CW) by September, with those off the coast are only about 14 mm CW. Juveniles remain in the area of settlement over their first winter but, in contrast to English sole, most coastal 1+ crab immigrate to estuaries to join siblings that settled there the previous year. By September of the second year, crab at about 100 mm CW emigrate to the open coast where they reach maturity. Advantages to juvenile stages that reside in estuaries are discussed in terms of accelerated growth at higher temperatures and potentially greater food supplies than found nearshore along the coast. 相似文献
134.
Adrian C. Armstrong 《地球表面变化过程与地形》1987,12(1):17-30
Slope profile sequences are as much influenced by their boundary conditions as they are by the processes operating. The development of a simple slope under a single process can adopt a variety of forms depending on initial and boundary conditions, and a given form can be derived from several different processes. For example, convexo-concave forms develop under soil creep with basal accumulation, under wash with an initial step form, and under mixed wash and creep processes. 相似文献
135.
A. R. McBirney H. P. Taylor R. L. Armstrong 《Contributions to Mineralogy and Petrology》1987,95(1):4-20
Following its birth on the 20th of February 1943, the Mexican volcano Paricutin discharged a total of 1.38 km3 of basaltic andesite and andesite before the eruption came to an end in 1952. Until 1947, when 75% of the volume had been erupted, the lavas varied little in chemical or isotopic composition. All were basaltic andesites with 55 to 56% SiO2,
18O of +6.9 to 7.0, and 87Sr/86Sr ratios close to 0.7038. Subsequent lavas were hypersthene andesites with silica contents reaching 60%, 18O values up to +7.6, and 87Sr/86Sr of 0.7040 to 0.7043. The later lavas were enriched in Ba, Rb, Li, and K2O and depleted in MgO, Cu, Zn, Cr, Ni, Sr, and Co. The isotopic and other chemical changes, which appeared abruptly over a few months in 1947, are interpreted as the result of tapping a sharply zoned and density stratified magma chamber. Xenoliths of partially fused felsic basement rocks in the lavas have silica contents greater than 70%,
18O of +5.6 to 9.9 and 87Sr/86Sr between 0.7043 and 0.7101. In many respects they resemble samples of basement rocks collected from nearby outcrops. Three analysed samples of the latter have silica contents of 65 to 67%,
18O of +7.7 to 8.6, and 87Sr/86Sr between 0.7047 and 0.7056.These new data provide strong support for the original interpretations of Wilcox (1954), who explained the chemical variations by a combination of fractional crystallization and concurrent assimilation of up to 20 weight % continental crust. Except for a few trace elements, particularly Ba, Sr, and Zr, the chemical and isotopic compositions of the xenoliths and basement rocks that crop out nearby match the type of contaminant required to explain the late-stage lavas. Some of the discrepancies may be explained by postulating a contaminant that was older and richer in Ba, Sr, and Zr than those represented by the analysed xenoliths. Others can be attributed to chemical changes accompanying disequilibrium partial melting, contact metamorphism, and meteoric-hydrothermal alteration of the country rock. Many of the xenoliths show evidence of having been affected by such processes.The lavas were erupted from a zoned magma chamber that had differentiated by liquid fractionation prior to the eruption. The order of appearance of the lavas can be explained in terms of withdrawal of stratified liquids of differing densities and viscosities. 相似文献
136.
A detailed mineralogic and chemical study of Willy, a very large (150 μm diameter) Fremdling from the Allende CAI 5241, was performed and compared to other Fremdlinge from Allende CAI's 5241 and TS-34 in an attempt to understand the nature and mode of formation of these exotic and complex objects. Willy is composed primarily of V-rich magnetite, V-rich fassaite, and Ni-Fe metal containing Co and Pt. Minor phases include an Fe-Ni-sulfide, V-rich spinel, scheelite (the first reported occurrence in a meteorite), and Cl-apatite. Phases found in trace amounts include nuggets of Os-Ru-Re metal, molybdenite, an unidentified Fe-, Mg-molybdate, and diopside. The Fremdling is concentrically zoned and contains a complex porous core of magnetite, metal, sulfide, scheelite, and other minor phases surrounded by a compact mantle of magnetite with minor apatite. The mantle is surrounded by a dense rim composed of fassaite with minor spinel that appears to be a typical occurrence around oxide-containing Fremdlinge. At the boundary between the fassaite-rim and the magnetite-mantle of Willy is a thin zone (<20 μm wide) of an apparent reaction assemblage consisting of V-rich MgAl2O4, FeAl2O4, and a third V-rich spinel, possibly FeV2O4, in intimate intergrowth.From the observed chemistry and texture, a multistage sequence of formation of Willy, possibly occurring in the solar nebula and involving major changes in T and , can be deduced. The first phases that may have formed in the interior are magnetite and an Fe or Ca tungstate. Refractory metal nuggets and sulfide were introduced after this stage followed by two stages of Ni-Fe formation during which Pt was dissolved in the metal. This was followed by formation of the magnetite mantle, introduction of apatite and possible alteration of ferberite to scheelite. Finally, the V-rich fassaite rim formed, accompanied or followed by reaction forming the complex Fe-, V-rich spinels. All of these steps preceded introduction of the Fremdling into the CAI precursor which in turn appears to have occurred prior to formation of spinel and the major silicate phases in the CAI.The concentric mineral zoning and texture of Willy may indicate that it is one of the few Fremdlinge that was not substantially recrystallized after having been captured in the CAI. It thus may represent the precursor material for many of the other Fremdlinge, since numerous Fremdlinge studied exhibit many of the mineral and chemical features observed in Willy. The co-existence of magnetite, sulfide, and Ni-Fe metal in the interior of Willy and the co-existence of metal, hercynite spinel and magnetite at the rimmantle boundary suggests that the maximum temperature at which Willy could have existed for long periods of time in its present state was about 500–600°C. Similarly, the existence of highly heterogeneous V-, Ti-rich fassaite rims around Willy and other Fremdlinge implies rapid cooling rates for the CAI's. 相似文献
137.
Robert B. Lawrence Richard Lee Armstrong Robert G. Berman 《Journal of Volcanology and Geothermal Research》1984,21(3-4)
The Salal Creek area, at the north end of the main group of vents for the Quaternary Garibaldi (Cascade) Volcanic Belt, southwestern British Columbia, was the site of several small eruptions of mafic lava during the past 1 Ma. In contrast to the calc-alkaline character of all other parts of the Garibaldi Belt and the geographically nearly coincident Miocene and older Pemberton Volcanic Belt, the Salal Creek area Quaternary lavas are predominantly alkaline basalt and hawaiite with typical alkaline volcanic petrography, chemistry, and fractionation trends. Trace elements Ti-Zr-Y show within-plate character for the suite. As for other Garibaldi Belt volcanic rocks, Rb is low, Rb/Sr very low, and 87Sr/86Sr ratio is low, averaging 0.7032. The oxygen isotopic composition average, 18O = 5.9, is normal for mantle-derived volcanic rocks.This distinct change in magma type at the end of a volcanic are may be the consequence of a smaller degree of melting, melting at a slightly greater depth than calc-alkaline magma production, or a descending-plate edge effect.Ponded flows and pillow-palagonite accumulations indicate that several Salal Creek area eruptions occurred in proximity to ice which filled major valleys during pre-Wisconsin glacial periods. 相似文献
138.
Geoffrey R. Hosack Brett R. Dumbauld Jennifer L. Ruesink David A. Armstrong 《Estuaries and Coasts》2006,29(6):1150-1160
The complexity of habitat structure created by aquatic vegetation is an important factor determining the diversity and composition
of soft-sediment coastal communities. The introduction of estuarine organisms, such as oysters or other forms of aquaculture,
that compete with existing forms of habitat structure, such as seagrass, may affect the availability of important habitat
refugia and foraging resources for mobile estuarine fish and decapods. Fish and invertebrate communities were compared between
adjacent patches of native seagrass (Zostera marina), nonnative cultured oyster (Crassostrea gigas), and unvegetated mudflat within a northeastern Pacific estuary. The composition of epibenthic meiofauna and small macrofaunal
organisms, including known prey of fish and decapods, was significantly related to habitat type. Densities of these epifauna
were significantly higher in structured habitat compared to unstructured mudflat. Benthic invertebrate densities were highest
in seagrass. Since oyster aquaculture may provide a structural substitute for seagrass being associated with increased density
and altered composition of fish and decapod prey resources relative to mudflat, it was hypothesized that this habitat might
also alter habitat preferences of foraging fish and decapods. The species composition of fish and decapods was more strongly
related to location within the estuary than to habitat, and fish and decapod species composition responded on a larger landscape
scale than invertebrate assemblages. Fish and decapod species richness and the size of ecologically and commercially important
species, such as Dungeness crab (Cancer magister), English sole (Parophrys vetulus), or lingcod (Ophiodon elongatus), were not significantly related to habitat type. 相似文献
139.
F. Zeb Page Lora S. Armstrong Eric J. Essene Samuel B. Mukasa 《Contributions to Mineralogy and Petrology》2007,153(5):533-555
Quantitative thermobarometry of inclusions in zoned garnet from a Franciscan eclogite block record a counter-clockwise P–T path from blueschist to eclogite and back. Garnet retains prograde zoning from inclusion-rich Alm52Grs30Pyp6Sps12 cores to inclusion-poor Alm62Grs25Pyp12Sps1 mantles, with overgrowths of highly variable composition. Barometry using the Waters–Martin version of the garnet–phengite–omphacite
thermobarometer yields conditions of 7–15 kbar, 400–500°C (garnet cores), 18–22 kbar, ∼550°C (mantles), and 10–14 kbar, 350–450°C
(overgrowths), in agreement with clinozoisite–sphene–rutile–garnet–quartz barometry. These pressures are ∼10–15 kbar less
than those obtained using more recent, fully thermodynamic calibrations of the phengite–omphacite–garnet thermobarometer.
Low early temperatures suggest that the block was subducted in a thermally mature subduction zone and not at the inception
of subduction when prograde temperature is expected to be higher. Franciscan high-grade blocks likely represent crust subducted
throughout the history of this convergent margin, rather than only at the inception of the subduction zone. 相似文献
140.
The Nsuze Group is the lower, dominantly volcanic, division of the Pongola Supergroup that accumulated on a sialic basement between 3.1 and 2.9 Ga. The Nsuze Group is subdivided into a lower sedimentary unit (800 m thick), a middle volcanic unit (± 7500 m thick) and an upper volcaniclastic-sedimentary unit (5–600 m thick). The predominant sediments in the lower unit are immature, medium- to very coarse-grained quartz wackes with thin intercalated lenses of quartz and arkosic arenites, and minor conglomeratic wackes. These sediments were deposited in a distal braided stream environment.There followed a major period of volcanism during which lavas showing a continuous spectrum of compositions from basalt to rhyolite were extruded subaerially. Flows of both different and similar compositions are complexly interfingered on both regional and local scales. As volcanism waned, pyroclastic and sedimentary rocks became dominant in the upper unit. The Nsuze Group is gently dipping and is metamorphosed to low greenschist grade.The Nsuze Group is significant in that it provides evidence for the existence of high-standing continental sialic crust in the southeastern part of the Kaapvaal province at ca. 3.0 Ga. Volcanism and sedimentation in the Pongola Supergroup are more typical of Proterozoic basins than of Archaean environments, despite their age. Komatiitic and high-Mg basalts were, however, being extruded in Zimbabwe contemporaneously with the Nsuze lavas. 相似文献