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911.
Petrogenesis and tectonic setting of late Precambrian ensimatic volcanic rocks,central eastern desert of Egypt 总被引:1,自引:0,他引:1
Robert James Stern 《Precambrian Research》1981,16(3):195-230
Early stages in the geologic evolution of the central eastern desert of Egypt (CED) reflect an intense episode of ensimatic volcanic activity similar to modern magmatism of the ocean floors and island arcs. This paper reports results from studies of the petrology and petrogenesis, and interprets the significance of these Late Precambrian volcanic rocks.A three-fold stratigraphy is preserved in the basement of the CED. A basal section of oceanic crust includes ultramafics, gabbros and pillowed basalts. These older metavolcanics (OMV) are conformably succeeded by dominantly volcanogenic metasediments, which are in turn succeeded by a dominantly andesitic, calc-alkaline sequence of younger metavolcanics (YMV). The OMV and YMV are largely restricted to the CED in Egypt, but analogous terranes are found in northern Arabia. (40–400 ppm) and Ni (30–260 ppm). They are poor in K2O (0.05–0.92%), Rb (0.3–5.0 ppm) and Ba (11–89 ppm). On Ti-Zr-Cr-V-Ni-P discriminant diagrams, the OMV plot in the field of modern abyssal tholeiites. High K/Rb (450–1800) and light REE depletions support this inference, although K/Ba (25–45) is lower than modern mid-ocean ridge basalts (MORB). The sum of OMV geochemical characteristics requires that these magmas were derived by the fractional fusion of the mantle. It is suggested that the OMV were generated by 20–25% fractional melting of previously depleted mantle at depths of less than 60 km. Relatively little fractionation accompanied ascent to the surface, where the OMV were erupted in a primitive crustal environment, either a small oceanic rift or a back-arc basin.Metamorphism of the YMV resulted in little elemental redistribution. These andesites have sub-alkaline clinopyroxenes and major-element geochemical characteristics indistinguishable from modern calc-alkaline andesites. YMV andesites in the central and western CED have K/Rb = 400–600, K/Ba = 20–40 and are light REE-enriched and heavy REE depleted. High concentrations of Cr (50–150 ppm) and Ni (20–100 ppm) and low initial 87Sr/86Sr ratios (0.7028–0.7030) indicate that these magmas were generated by melting in the mantle. Modelling studies and consideration of experimental data indicate that these andesites were formed by 2–10% fractional fusion of hydrous, undepleted, garnet therzolite at depths of 65 km or more in the mantle.The data show that an intense episode of instability, convection, and widespread melting occurred in the mantle beneath Afro-Arabia at the end of the Precambrian. 相似文献
912.
Michael J. Carr Darrell G. Mayfield James A. Walker 《Journal of Volcanology and Geothermal Research》1981,10(1-3)
Most of the lavas at the nine volcanic centers along the volcanic front of El Salvador are basalts, basaltic andesites and andesites. The compositional variation within and among these centers can be explained by fractionation processes within the crust. Cognate gabbroic inclusions found in the lavas have appropriate mineralogy (plagioclase, olivine, magnetite and augite) to be cumulates formed by fractional crystallization. Two main variation trends occur, depending on the proportion of plagioclase removal. The more common, or normal, trend has a high (> 55%) proportion of plagioclase being removed. A less common, Al-rich, trend has a low (40%) proportion of plagioclase being removed. The Al-rich trend is found only at volcanoes that lack large negative Bouguer gravity anomalies. These volcanoes are unlikely to have large shallow magma chambers and fractionation probably occurs deeper in the crust where plagioclase removal is inhibited.The incompatible element (Na2O, K2O, Rb, Ba) contents of lavas vary systematically with the volume of the volcanic centers. At the same level of SiO2, large volcanic centers have higher incompatible element contents than small volcanic centers. This suggests that open system fractionation in a periodically refilled chamber is the controlling factor. The large difference in Ba contents of lavas between eastern (low) and western (high) El Salvador suggests a difference in the mantle source region. 相似文献
913.
Martin Frey Johannes C. Hunziker James R. O'Neil Hans W. Schwander 《Contributions to Mineralogy and Petrology》1976,55(2):147-179
Nine samples from the Monte Rosa Granite have been investigated by microscopic, X-ray, wet chemical, electron microprobe, stable isotope and Rb-Sr and K-Ar methods. Two mineral assemblages have been distinguished by optical methods and dated as Permian and mid-Tertiary by means of Rb-Sr age determinations. The Permian assemblage comprises quartz, orthoclase, oligoclase, biotite, and muscovite whereas the Alpine assemblage comprises quartz, microcline, albite+epidote or oligoclase, biotite, and phengite.
Disequilibrium between the Permian and Alpine mineral assemblages is documented by the following facts: (i) Two texturally distinguishable generations of white K-mica are 2 M muscovite (Si=3.1–3.2) and 2 M or 3 T phengite (Si=3.3–3.4). Five muscovites show Permian Rb-Sr ages and oxygen isotope fractionations indicating temperatures between 520 and 560 ° C; however, K-Ar ages are mixed or rejuvenated. Phengite always shows mid-Tertiary Rb-Sr ages, (ii) Two biotite generations can be recognized, although textural evidence is often ambiguous. Three out of four texturally old biotites show mid-Tertiary Rb-Sr cooling ages while the oxygen isotopic fractionations point to Permian, mixed or Alpine temperatures, (iii) Comparison of radiogenic and stable isotope relations indicates that the radiogenic isotopes in the interlayer positions of the micas were mobilized during Alpine time without recrystallization, that is, without breaking Al-O or Si-O bonds. High Ti contents in young muscovites and biotites also indicate that the octahedral (and tetrahedral) sites remained undisturbed during rejuvenation. (iv) Isotopic reversals in the order of O18 enrichment between K-feldspar and albite exist.
Arguments for equilibrium during Permian time are meagre because of Alpine overprinting effects. Texturally old muscovites show high temperatures and Permian Rb-Sr ages in concordancy with Rb-Sr whole rock ages. For the tectonically least affected samples, excellent concordance between quartz-muscovite and quartz-biotite Permian temperatures implies oxygen isotope equilibrium in Permian time which was undisturbed during Alpine metamorphism.
Arguments for equilibrium during the mid-Tertiary metamorphism are as follows: (i) Mid-Tertiary Rb-Sr mineral isochrons of up to six minerals exist, (ii) Oxygen isotope temperatures of coexisting Alpine phengites and biotites are concordant.The major factor for the adjustment of the Permian assemblages to Alpine conditions was the degree of Alpine tectonic overprinting rather than the maximum temperatures reached during the mid-Tertiary Alpine metamorphism. The lack of exchange with externally introduced fluid phases in the samples least affected by tectonism indicates that the Monte Rosa Granite stewed in its own juices. This seems to be the major cause for the persistence of Permian ages and corresponding temperatures. 相似文献
914.
James M. Stroh 《Contributions to Mineralogy and Petrology》1976,54(3):173-188
The addition of Fe and Cr to the simple system MgO-SiO2-Al2O3 markedly affects the activities of phases involved in the equilibrium
\textMg\text2 \textSiO\text4 \text + MgAl\text2 \textSiO\text6 \text = MgAl\text2 \textO\text4 \text + Mg\text2 \textSi\text2 \textO\text6 \textOlivine + Opx\textsolid solution \text = Spinel + Opx\textsolid solution \begin{gathered} {\text{Mg}}_{\text{2}} {\text{SiO}}_{\text{4}} {\text{ + MgAl}}_{\text{2}} {\text{SiO}}_{\text{6}} {\text{ = MgAl}}_{\text{2}} {\text{O}}_{\text{4}} {\text{ + Mg}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{\text{6}} \hfill \\ {\text{Olivine + Opx}}_{{\text{solid solution}}} {\text{ = Spinel + Opx}}_{{\text{solid solution}}} \hfill \\ \end{gathered} 相似文献
915.
The stability and phase relations of iron chlorite below 8.5 kb P_{{\text{H}}_{\text{2}} {\text{O}}}
Iron chlorites with compositions intermediate between the two end-members daphnite (Fe5Al2Si3O10(OH)8) and pseudothuringite (Fe4Al4Si2O10(OH)8) were synthesized from mixtures of reagent chemicals. The polymorph with a 7 Å basal spacing initially crystallized from these mixtures at 300 °C and 2 kb
after two weeks. Conversion to a 14 Å chlorite required a further 6 weeks at 550 °C. Shorter conversion times were required at higher water pressures. The products contained up to 20% impurities.The maximum equilibrium decomposition temperature for iron chlorite, approximately 550 °C at 2kb, is at an
between assemblages (1) and (2) listed below. Synthetic iron chlorite will break down by various reactions with variable P, T, and fugacity of oxygen. For the composition FeAlSi = 523, the sequence of high temperature breakdown products with increasing
traversing the magnetite field for P
total
=
=2kb is: (1) corierite+ fayalite+hercynite; (2) cordierite+fay alite+magnetite; (3) cordierite+magnetite+quartz; (4) magnetite+mullite+quartz. Almandine should replace cordierite in assemblages (1) and (2) but it did not nucleate. The significance of the relationship between iron cordierite and almandine in this system is discussed.At water pressures from 4 to 8.5 kb and
at the nickel-bunsite buffer, iron chlorite+quartz break down to iron gedrite+magnetite with temperature 550 to 640 °C along the curve. At temperatures 50 °C greater and along a parallel curve, almandine replaces iron gedrite. For
on this buffer curve, almandine is unstable below approximately 4 kb for temperatures to approximately 750 °C. 相似文献
916.
Evaporative process plays a dominant role in determining the water chemistry of the springs at Teels Marsh, a closed basin in western Nevada. Analysis of the spring waters indicates that calcium, magnesium, sulfate, and silica are removed from solution during dry periods, even though groundwater is undersaturated with respect to gypsum, amorphous silica, and sepiolite. The removal mechanism is precipitation of authigenic phases such as gypsum above the water table, in the vadose zone.In episodes of rain and snowfall in which none of the waters enters the phreatic zone, ions in the rain and snow accumulate near the ground surface. This accumulation of material, together with the sparse rain and snowfall, inhibits chemical weathering of silicate minerals. Only at high elevations in the basin is there sufficient fluxing of water through the alluvium for silicate weathering to make a significant contribution to the sodium content of the springs. When a sufficiently heavy rainfall occurs, salts are partially dissolved and the ions transported to the permanent groundwater. The kinetics of dissolution of secondary phases in the vadose zone exert an important control on the composition of the springs. 相似文献
917.
Knudsen cell-quadrupole mass spectrometry was used to study the high-temperature vaporization of Hawaiian basalts, plagioclase, tektites, and samples from the Allende meteorite. Procedures are described by which mass loss rates and vapor pressures of Na and K were measured quantitatively.Gas-rich glassy basalts were observed to vesiculate under vacuum over the 900–1000°C region and simultaneously evaporate alkalis in nonequilibrium fashion at rates (units of μg/g/hr) of approximately 200–300 Na and 75–250 K. Degassed residues of the same basalts demonstrated equilibrium evaporation rates (over the same temperature range) of 60–120 Na and 30–60 K. The gas-deficient plagioclase and tektite sample showed only equilibrium vaporization with rates of 60 Na, 10 K (plagioclase) and 10 Na, 5K (tektites) at 900–1000°C. The Allende meteorite vaporized at rates of 2400 Na and 200 K at 900–1000°C, possibly by the reaction of Na2O and K2O with C or S2, or by the thermal decomposition of nepheline or sodalite.The nonequilibrium vaporization of alkalis from the gas-rich basalts is attributed to vigorous agitation of the melt during its vesiculation by a gas phase composed principally of SO2, CO2, H2O, CO, and H2S. The major gases released from the Allende meteorite at 900–1000°C are, in order of decreasing abundance, CO, S2, CO2, H2O, SO2, and H2S.It is proposed that nonequilibrium vaporization of alkalis during the vesiculation of lunar lavas was responsible for the production of alkali-rich vapors which subsequently deposited plagioclase crystals in the vugs of lunar rocks. The vesiculative, nonequilibrium vaporization of Na and K during a lunar volcanic eruption should be expected to occur at a high rate upon initial extrusion of the lava into vacuum but then decrease by a factor of approximately three when degassing is nearing completion. Vaporization losses remain inadequate to explain the uniformly low alkali concentrations in lunar basalts. 相似文献
918.
The extent of hydrogen and oxygen isotope exchange between clay minerals and water has been measured in the temperature range 100–350° for bomb runs of up to almost 2 years. Hydrogen isotope exchange between water and the clays was demonstrable at 100°. Exchange rates were 3–5 times greater for montmorillonite than for kaolinite or illite and this is attributed to the presence of interlayer water in the montmorillonite structure.Negligible oxygen isotope exchange occurred at these low temperatures. The great disparity in D and O18 exchange rates observed in every experiment demonstrates that hydrogen isotope exchange occurred by a mechanism of proton exchange independent of the slower process of O18 exchange.At 350° kaolinite reacted to form pyrophyllite and diaspore. This was accompanied by essentially complete D exchange but minor O18 exchange and implies that intact structural units in the pyrophyllite were inherited from the kaolinite precursor. 相似文献
919.
James W. Hawkins 《Earth and Planetary Science Letters》1976,28(3):283-297
The Lau Basin is a marginal sea, located between the Tonga and Lau Ridges, in the southwestern Pacific. The basin is on the “inner” or concave side of the Tonga Trench-Arc system and is situated above the deep seismic zone dipping westward from the Tonga Trench. The Tonga Trench-Arc system is undoubtedly located above a zone of crustal shortening as evidenced by the deep seismicity and vulcanism. However, the geological and geophysical data give strong support to the contention that the Lau Basin has formed by crustal dilation.Rocks dredged from ridges and seamounts in the basin are sub-alkaline basalt. The average major element composition of least altered samples is: SiO2 48.8%, TiO2 1.2%, K2O 0.18%, P2O5 0.08%, H2O+ 0.30%, FeIII/FeII = 0.26,CaO/Al2O3 = 0.77. The data for Lau Basin basalt (LBB) show close similarity to data of typical oceanic ridge basalt (ORB). Trace element abundances (ppm): Ni 160, Cr 390, Sr 100, Ba < 31, Rb < 1 also resemble ORB values. K/Rb in a least altered and unfractionated sample is 860, Ba/Sr is 0.1, Ba/Rb is 8. Strontium isotope data show the only marked variance from ORB chemistry with LBB values ranging from 87Sr/86Sr=0.7020 to 0.7051. The low Sr abundances in the samples suggest the possibility of crustal Sr contamination to explain the radiogenic Sr enrichment. An alternate possibility is that the mantle source rocks were enriched in 87Sr. Variation within dredge hauls and between dredge sites may be explained by low-pressure fractional crystallization of magmas separated from the mantle at about 50 km depth.The basin probably began to open in middle to late Miocene time either by the disruption of a single andesitic island arc by splitting along its axis or by dilation of the area between two closely spaced concentric arcs. Mantle counterflow in the asthenosphere above the downgoing oceanic lithosphere slab is the probable driving force for dilation and has provided a continuous supply of parent material for the basalt of the basin floor. 相似文献
920.
Norman K. Grant James L. Powell Flora R. Burkholder John V. Walther Max L. Coleman 《Earth and Planetary Science Letters》1976,31(2):209-223
Strontium and oxygen isotope measurements on the alkali basalt-trachyte-phonolite suite of St. Helena show that some of the late-fractionated rocks are enriched in 87Sr and depleted in 18O relative to the older basalts. The data rule out both the formation of the late-fractionated rocks by the partial melting of hydrothermally altered oceanic crust and the contamination of the volcanic rocks by oceanic sediment. It also appears to be incompatible with models based either on the melting of previously fractionated and crystallized liquids in the volcanic pile, or the long-term fractionation of lavas over several millions of years in a sub-volcanic magma chamber.It is concluded that hydrothermal interaction with meteoric water is the most important cause of the 18O depletion. If the interaction occurred at widely differing temperatures, and involved meteoric and seawaters, it might conceivably have caused both the oxygen and strontium isotope heterogeneities. 相似文献
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