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141.
Field relations, petrographic features, major and selected trace element compositions, and Sr- and Pb- isotope characteristics indicate that the Coire Uaigneich Granophyre (CUG) was formed by the mixing of two magmas. One of these, a liquid of high 87Sr/86Sr ratio (ca. 0.731) formed by the anatexis of late Precambrian Torridonian sediments, contained relict quartz and zircon. The other liquid was an acid differentiate of basaltic magma and was enriched in incompatible elements such as Zr and Y. The two magmas mixed in the proportions of approximately two parts anatectic melt to one part acid differentiate. Hydrothermal metamorphism had no significant effect on the bulk chemistry of the CUG. 相似文献
142.
Alan P. Heward 《Earth》1981,17(3):223-276
Barrier islands, beaches, wave-dominated deltas and chenier plains probably form more than one-fifth of present coasts, and constitute an even larger proportion of the subsiding lowland coasts likely to be preserved in the stratigraphic record. The geometry, vertical sequences and composition of their deposits are the result of: (1) shoreline behaviour, whether transgressive, regressive or stable; (2) physical processes, particularly the relative intensities of waves and tides; (3) climate; (4) global tectonic setting; and (5) topography of underlying deposits.Published interpretations indicate that equivalent deposits are abundant in the geological record. Sandstones are mostly quartzose, cemented by carbonates and quartz, and have sheet or linear geometries. Ancient deposits can provide evidence of palaeogeography and some are economically significant, particularly as hydrocarbon reservoir rocks.Four end-member sedimentation models are proposed for the interpretation of ancient deposits: (1) transgressive sheet sands associated with deltas; (2) non-deltaic transgressive sheet sands; (3) regressive sheet sands; and (4) linear sand bodies. Probable depositional settings, geometries, characteristics of deposits and palaeocurrent patterns are discussed. 相似文献
143.
Subsolidus and melting reactions involving calcic plagioclase in pelitic assemblages in the K-Na-Ca model system occur at higher temperatures than their K-Na counterparts. For the most calcic plagioclase compositions observed in high-grade pelitic rocks (An25-An40) the equilibria are rarely extended by more than 30 ° C above those in KA1O2-NaAlO2-Al2O3-SiO2-H2O, although all discontinuities in facies inferred for the K-Na system are continuously displaced when they involve Ca-bearing plagioclase. The maximum pressure-temperature overlap between muscovite dehydration and initial melting reactions occurs in the pressure range of 4–6 kbar between about 640 ° and 720 ° C. This provides optimum conditions for anatectic melt generation in felsic rocks of the appropriate compositions progressively metamorphosed in kyanite-sillimanite facies series. Progressive regional metamorphism at pressures of 2–4 kbar, corresponding to andalusite-sillimanite facies series, shows little overlap between muscovite dehydration and initial melting reactions. Consequently anatectic melt generation in andalusite-sillimanite facies series would require the participation of biotite in dehydration-melting reactions. Felsic intrusive rock in andalusite-sillimanite terranes could have risen upward from their anatectic sites in high grade kyanite-sillimanite facies series at depth. Many andalusite-sillimanite facies series terranes culminating in migmatites could represent upward movement of kyanite-sillimanite facies series rocks to shallower depths with uplift rates faster than cooling rates. 相似文献
144.
145.
Robert Thunell Alan Federman Stephen Sparks Douglas Williams 《Quaternary Research》1979,12(2):241-253
The Y-5 ash is the most widespread layer in deep-sea sediments from the eastern Mediterranean. This ash layer was previously correlated with the Citara-Serrara tuff on Ischia Island and dated at approximately 25,000 yr B.P. New data on the glass chemistry of the Y-5 ash and pyroclastic deposits from the Neopolitan volcanic province suggest that the layer is correlative with the large-volume Campanian ignimbrite and not with the deposit from Ischia Island. The volume of the Y-5 ash is approximately 65 km3 which is comparable in magnitude to the volume of the Campanian ignimbrite. An interpolated age of approximately 38,000 yr B.P. is estimated based on sedimentation rates derived from δ18O stratigraphy. There is a discrepancy between this estimate and previously reported radiocarbon ages which range from 24,000 to 35,000 yr B.P. We propose that the “Campanian tuff ash layer” should be adopted as the full stratigraphic name for the Y-5 ash. The deep-sea ash layer is divisible into two units in proximal localities, probably correlating with two major phases of the eruption: plinian and ignimbrite. 相似文献
146.
147.
Compositional profiles of a series of garnets, extracted from a pelitic schist occurring in the Moinian rocks of Northern Sutherland, have been established. The profiles are complex and do not conform to a simple fractionation model. A model to explain them is suggested. 相似文献
148.
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
149.
150.
J. Alan Chalmers 《Pure and Applied Geophysics》1964,57(1):189-192
Summary The definition of a generator in atmospheric electricity is considered, and various phenomena are discussed as to whether they can be described as generators. 相似文献