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251.
William Ian Ridley 《Contributions to Mineralogy and Petrology》1970,26(2):124-160
Tenerife is the largest of the seven Tertiary to Recent volcanic islands that make up the Canary Archipelago. The island is composed of volcanics belonging to the basanitetrachyte-phonolite assemblage that characterises many Atlantic islands. The most voluminous development of intermediate and salic volcanics has been in the centre of the island where the Las Canadas volcanoes arose upon a basement shield composed mainly of basanite and ankaramite flows, tuffs and agglomerates. The initial post-shield activity built the Vilaflor volcanic complex (Lower and Upper Canadas Series) that originally covered much of the underlying shield volcanics. A vast collapse of the complex, probably during post-Pleistocene times, in the centre of the island has left a large semi-circular wall, and provides an excellent vertical section through the complex. Quaternary volcanism within the collapsed area has built the twin, central-type volcanoes, Viejo and Teide, both of which have attendant satellite vents. That part of the Vilaflor Complex exposed in Las Canadas, together with the Viejo and Teide volcanoes, comprise the Las Canadas volcanoes.Four distinct rock types can be recognised in these volcanoes, basanite, trachybasanite, plagioclase phonolite, and phonolite. Each rock type can be recognised chemically and mineralogically, but there is essentially a gradational series from basanite to phonolite that includes both aphyric and glomerophyric rocks. The volcanics are strongly undersaturated and sodic, and some of the phonolites are mildly peralkaline. Variations in degree of undersaturation, and trace element abundances indicate a number of cycles of activity which would be consistent with the known field relations.Forsteritic olivine occurs in the basanites and trachybasanites but is not a stable phase in the more salic volcanics. Clinopyroxene is ubiquitous, varying in composition from titanaugite in the basanites to slightly sodic augite in the phonolites. Strongly sodic pyroxene is restricted to the groundmass of the microcrystalline phonolites along with aenigmatite and a kataphoritic amphibole. Plagioclase is found only in the groundmass of the basanites, but andesine and potash-oligoclase are common phenocryst minerals in the trachybasanites and plagioclase phonolites respectively, whereas the characteristic feldspar of the phonolites is anorthoclase.The relatively smooth curves of major and trace element variation, the presence of accumulative volcanics at all stages of differentiation, zoning of the mineral phases, and the clustering of the phonolites around the low temperature trough in Petrogeny's Residua System, all indicate that the descent from basanite to phonolite has resulted from fractional crystallisation of a basanite parent magma. The trend of pyroxene crystallisation, and the fairly constant FeO/Fe2O3 ratio during fractionation indicate crystallisation under low PO2 conditions. 相似文献
252.
253.
William S. Wise 《Contributions to Mineralogy and Petrology》1969,23(1):53-64
The basaltic lavas erupted throughout the Mojave Desert are basanites (SiO2<46%, normative nepheline>5%, and K2O>1.5%), alkali-olivine basalts (SiO2=46–48%; ne=0–5%; and K2O=1.0–1.5%), and low-alumina, sub-alkaline basalts (SiO2=48–51%; ne=0; K2O<1.0%). One volcano, Pisgah Crater, erupted five times, with lava from each successive phase containing more silica and less potash than the one proceeding it. This compositional trend is the reverse of that expected from differentiation of a single alkalic magma, and therefore, may represent a succession of magmas tapped from a zone of continuing partial melting in the mantle.These lava compositions suggest that first melting was under high water pressure and was followed by relatively dry partial melting of gamet-orthopyroxene-clinopyroxene-olivine assemblages. The successive increase in silica and alkali decrease also requires that the partial melting zone move to shallower levels.All lavas sampled in the Mojave Desert area have compositions that can best be explained by the extraction of magma from such a rising melting zone, analogous to the mantle diapirs suggested by Green and Ringwood. 相似文献
254.
Morteza Janghorbani David E Gillum William D Ehmann 《Geochimica et cosmochimica acta》1973,37(4):905-908
Abundances of O, Si, Al and Mn have been determined in Luna 20 fines sample 22001,9 by instrumental neutron activation analysis. The abundances of O, Si and Al are among the highest we have observed in lunar samples and reflect a highlands origin for much of this regolith sample. The Luna 20 abundances reported here most closely resemble those we have determined in four samples of two Apollo 16 fines, rock 14310, and a clast from breccia 15459. The Luna 20 ratio of 1.96 ± 0.05 is similar to that in most other lunar samples, but the ratio of 0.532 ± 0.024 is exceeded only by our data on the Apollo 16 fines. This ratio is in agreement with the value of 0.55 ± 0.06 determined by the remote X-ray fluorescence experiment for the highlands between Mare Crisium and Mare Smythii which lie near the Luna 20 site (Adleret al., 1972). 相似文献
255.
The paleomagnetism of the Late Cretaceous Poços de Caldas alkaline complex (46.6°W, 21.9°S) was investigated through 42 oriented cores from seven sites. Six sites, reversed relative to the present magnetic field of the Earth, yield a pole at 127°W, 82°S (dp = 8°,dm = 13°). This pole is located close to other Late Cretaceous poles for South America obtained by Creer [1] from untreated paleomagnetic samples. The results are significantly different from those for the nearby Early Cretaceous Serra Geral basalt but close to the Triassic pole for South America. The polar wandering path for South America for the Mesozoic seems to be more complicated than anticipated. The available paleomagnetic information may not yet be precise enough to determine the time of opening of the Atlantic. 相似文献
256.
257.
Navy Fan is a Late Pleistocene sand-rich fan prograding into an irregularly shaped basin in the southern California Borderland.
The middle fan, characterized by one active and two abandoned “distributary” channels and associated lobe deposits, at present
onlaps part of the basin slope directly opposite from the upper-fan valley, thus dividing the lower-fan/basin-plain regions
into two separate parts of different depths. Fine-scale mesotopographic relief on the fan surface and correlation of individual
turbidite beds through nearly 40 cores on the middle and lower fan provide data for evaluating the Late Pleistocene and Holocene
depositional processes.
Margin setting represents fan and/or source area 相似文献
258.
A general analysis of the absorption of the Schumann-Runge bands of molecular oxygen has been made in order to compare the various experimental and theoretical results which have been obtained for an application to the O2 atmospheric absorption and its photodissociation in the mesosphere and stratosphere. The different values of the oscillator strengths deduced from the laboratory absorption spectra and of the predissociation linewidths used for the calculation of the absorption have been compared.Calculations based on a Voight profile of the O2 rotational lines have led to simple formulas for atmospheric applications taking into account that the total photodissociation rate in the stratosphere depends strongly on the absorption of solar radiation in the spectral range of the O2 Herzberg continuum. Specific examples are given. 相似文献
259.
General conservation equations for multi-phase systems: 3. Constitutive theory for porous media flow
Equations which describe single phase fluid flow and transport through an elastic porous media are obtained by applying constitutive theory to a set of general multiphase mass, momentum, energy, and entropy equations. Linearization of these equations yields a set of equations solvable upon specification of the material coefficients which arise. Further restriction of the flow to small velocities proves that Darcy's law is a special case of the general momentum balance. 相似文献
260.
A mechanism capable of accounting for the large mean eccentricity (0.175) and inclination (7°.2) of Mercury is discussed. Provided the gravitational field of the rapidly rotating primordial Sun had a sufficiently large second degree harmonic (i.e., J2 ? order 10?3), subsequent solar spin down would drive the orbit of Mercury through two secular resonances with Venus, one involving the precession of the line of apsides, the other one involving the regression of the nodal line. Resonance passage generates contributions to the eccentricity and inclination that are proportional to the square root of the characteristic solar spin down time. We find that an initial solar rotation l period of guarantees passage through resonance and that a spin down time of of order 106 years could have produced the observed eccentricity and inclination. Such a primordial rotation rate is comparable to the measured rotations of very young stars and the spin down time appears consistent with the time scale derived for magnetic braking of the Sun's rotation by an intense solar wind during a T-Tauri stage of solar evolution. 相似文献