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991.
992.
Andrea Borgia Clark Poore Michael J. Carr William G. Melson Guillermo E. Alvarado 《Bulletin of Volcanology》1988,50(2):86-105
Geologic mapping on a scale of 1:10000 and detailed stratigraphic studies of lava flows and tephra deposits of the Arenal-Chato volcanic system reveal a complex and cyclic volcanic history. This cyclicity provides insight into the evolution of magma batches during the growth of the andesitic volcanic system. The Arenal and Chato volcanoes have a central zone comprised of a lava armor and a distal zone comprised of a tephra apron. During Arenal's last two eruptive periods major craters formed near intersections of regional fractures at the lava armortephra apron transition. We suggest that such intersections are potential sites for future major explosions. The earliest rocks, i.e., the Chato lava flows, range in composition from basaltic andesite to andesite. These rocks, except for the andesitic domes of Chatito and La Espina, appear to have evolved from a common parental magma. The last active period of Chato volcano occurred 3550 B. P. The earliest known activity of Arenal volcano is 2900 B. P. Arenal lava flows have 54–56 wt% SiO2 and may be subdivided into a high-alumina group (HAG, Al2O3 = 20 wt%) and a low-alumina group (LAG, Al2O3 = 19 wt%). Compared to the HAG, the LAG also has smaller amounts of incompatible elements and higher amounts of FeO and MgO. Arenal tephra deposits were emplaced by Plinian-Sub-Plinian explosions occurring at 300±150-yr intervals. These deposits are compositionally zoned and alternate between dacite and basalt. The stratigraphy reveals an apparent magmatic cycle consisting of (a) dacitic-andesitic tephra, (b) HAG lava flows, (c) LAG lava flows, and (d) andesitic-basaltic tephra. This magmatic cycle is repeated four times during Arenal's history and is interpreted to have developed by the crystal fractionation and crystal redistribution of a single magma batch. The period of this cycle, and consequently the life of a magma batch, is about 800 years. If the cyclic pattern continues, a basaltic explosive phase may occur in the next 250 years. 相似文献
993.
Nd and Sr isotopic data are reported for a granulite terrain in the Proterozoic Arunta Block of Central Australia. Sm-Nd data from a wide range of rock types define a crust formation age of 2,070±125 Ma and provide further evidence for voluminous crustal growth in the Proterozoic. An
Nd value of +1.5±0.8 indicates a depleted mantle source for this crustal segment and there is no evidence for a large component of significantly older sialic crust. Field relationships, geochemistry and Rb-Sr data for mafic and felsic granulites indicate that intracrustal differentiation and polyphase deformation were followed by granulite facies metamorphism (Rb depletion) at 1,800 Ma. Rb-Sr data for strongly retrogressed granulites define an age of 1,700 Ma which is interpreted as the time of retrograde biotite growth. Partial melting at the presently exposed crustal level and anatexis at deeper crustal levels were broadly coeval with the retrograde metamorphism. Sm-Nd and Rb-Sr isotopic systematics of minerals indicate that the terrain cooled slowly, did not experience significant uplift until 1,000 Ma and remained at temperatures above 320° C until the late Palaeozoic. The mineral data are consistent with geological relationships and petrological evidence for a prolonged period of isobaric cooling followed by uplift late in the metamorphic evolution of the terrain. The granulite protoliths appear to have formed in a rift which closed within 280 Ma of initial separation. Deformation and granulite facies metamorphism at 1,800 Ma are interpreted to be a consequence of collision between the continental blocks which defined the rift. Regional retrogression and granitoid magmatism at 1,700 Ma are attributed to underthrusting of the granulites by lower grade rocks in the final stages of collision. Subsequent events in the cooling and uplift history appear to have been controlled by the presence of long-lived major faults in the crust and a prolonged history of episodic compression in the continental lithosphere. The results of this study suggest that granulite terrains, in general, cannot be equated with lower continental crust but instead represent assemblages of (mainly) supracrustal rocks which in some instances have been involved in major collision events.Abbreviations
Opx
orthopyroxene
-
Cpx
clinopyroxene
-
Plag
plagioclase
-
Hb
hornblende
-
Ox
opaque oxide
-
Bi
biotite
-
Ap
apatite
-
Zir
zircon
-
Gt
garnet
-
2°CA
secondary clinoamphibole
-
Qtz
quartz
-
Ol
olivine
-
Sp
spinel
-
Se
serpentine
-
Ep
epidote
-
Kf
alkali feldspar
-
Mz
monazite
-
Sph
sphene
-
Sill
sillimanite
-
Rut
rutile
-
Cd
cordierite
-
Sphal
sphalerite
-
Ms
muscovite
-
Act
actinolite
-
Cc
calcite
-
Scap
scapolite
-
Cor
corundum
-
Xen
xenotime
-
Sapph
sapphirine
-
Ged
gedrite
-
All
allanite
-
Clh
clinohumite
-
Dol
dolomite
-
Mt
magnetite
-
Ghn
gahnite
-
Gal
galena 相似文献
994.
Stephen C. Komor Don Elthon John F. Casey 《Contributions to Mineralogy and Petrology》1987,95(3):278-300
Data from detailed sample traverses in the layered gabbro unit of the North Arm Mauntain massif, Bay of Islands ophiolite, allow meter-scale resolution of magmatic processes in spreading ridge magma chambers. One suite of 46 samples from a 195 m interval near the base of the layered gabbro unit contains cumulus plagioclase (An73.7–87.5; average modal abundance=75%), clinopyroxene (Mg#=80.3–86.0; 18%), and olivine (Fo76.6–82.1; 6%), with intergranular orthopyroxene (Mg#=78.0–83.3; 1%), and accessory Cr-Al spinel (Cr#=32.3–41.4). Ilmenite rims spinel in one sample. Whole rock Zr contents range from <6 to 15 ppm. Plots of stratigraphic height in the traverse versus petrogenetic indicators (e.g. Mg#'s of mafic phases and An in plagioclase) reveal both normal and reverse cryptic variation patterns; the patterns for all indices are generally correlated. The normal portions of the patterns formed during fractional crystallization of basalt batches. Ranges of mineral compositions in the normal trends suggest that 29–38% crystallization of each batch of basalt occurred before magmatic replenishment. The reverse cryptic trends formed by crystallization of hybrid magmas produced during periods of magma mixing. Other evidence for magma mixing is the systematic association of spinel and reversely zoned plagioclase with the reverse trends. Experiments and observations of natural assemblages indicate that 55% modal plagioclase crystallizes from basalts at the olivine+plagioclase+clinopyroxene+liquid piercing point. The average plagioclase content of this suite of leucogabbros from North Arm Mountain is too high to have formed from simple crystallization at the piercing point. Petrologic modeling indicates the leucogabbros may have formed from basalts into which a small amount (<10%) of plagioclase was resorbed during mixing; the initial compositions of these hybrid basalts lie in the plagioclase primary phase volume. Other suites of layered gabbros from North Arm Mountain are not so plagioclase-rich as the leucogabbros described above. Crystallization of basalts in the plagioclase primary phase volume and the consequent formation of plagioclaserich gabbros may occur in restricted portions of zoned magma chambers underlying oceanic spreading centers, or may occur episodically in the overall lifetimes of the magma chambers. 相似文献
995.
Dr. A. Embey-Isztin G. Dobosi Dr. G. Noske-Fazekas E. Árva-Sós 《Mineralogy and Petrology》1989,40(3):183-196
Summary A new occurrence of basalt (minimum K/Ar age 57.9 ± 2.2 m.y.) is reported from Budaliget, near Budapest. Major and minor element concentrations show that the basalts are alkaline and potassic in nature ranging from olivine tholeiite to moderately undersaturated basanite. High mg-values and concentrations of Ni and Cr indicate that some of the samples may represent primary compositions. High pressure accidental xenoliths, xenocrysts and cognate megacrysts are frequent and the chemical zoning patterns of olivine and pyroxene phenocrysts indicate a complex evolution and polibaric conditions for the crystallization. The discovery of the new basalt occurrence is important from a geodynamical point of view: prior to the last two major geodynamical events (Plio-Pleistocene rifting with associated alkali basaltic volcanism and collision of microplates with associated Miocene calk-alkaline volcanism) the continental lithosphere below NE Transdanubia may have experienced another rifting period in the Paleocene or Upper Cretaceous.With 7 Figures 相似文献
996.
Lawrence M. Anovitz Allan H. Treiman Eric J. Essene Bruce S. Hemingway Edgar F. Westrum Victor J. Wall Ramón Burriel Steven R. Bohlen 《Geochimica et cosmochimica acta》1985,49(10):2027-2040
Low temperature adiabatic calorimetry and high temperature differential scanning calorimetry have been used to measure the heat-capacity of ilmenite (FeTiO3) from 5 to 1000 K. These measurements yield S2980 = 108.9 J/(mol · K). Calculations from published experimental data on the reduction of ilmenite yield Δ2980(I1) = ?1153.9 kJ/(mol · K). These new data, combined with available experimental and thermodynamic data for other phases, have been used to calculate phase equilibria in the system Fe-Ti-O. Calculations for the subsystem Ti-O show that extremely low values of are necessary to stabilize TiO, the mineral hongquiite reported from the Tao district in China. This mineral may not be TiO, and it should be re-examined for substitution of other elements such as N or C. Consideration of solid-solution models for phases in the system Fe-Ti-O allows derivation of a new thermometer/oxybarometer for assemblages of ferropseudobrookite-pseudobrookitess and hematite-ilmenitess. Preliminary application of this new thermometer/oxybarometer to lunar and terrestrial lavas gives reasonable estimates of oxygen fugacities, but generally yields subsolidus temperatures, suggesting re-equilibration of one or more phases during cooling. 相似文献
997.
J. L. García Casquero N. A. I. M. Boelrijk J. Chacón H. N. A. Priem 《International Journal of Earth Sciences》1985,74(2):379-384
A Rb-Sr whole-rock investigation of suites of samples from two orthogneiss bodies in the Almendralejo area of the Oporto-Portalegre-Badajoz-Córdoba belt points to an Ordovician age for the granitic magmatism: about 470 Ma for the Almendralejo Gneiss (peralkaline metagranite) and about 425 Ma for the Ribera del Fresno Gneiss (leucocratic subaluminous metagranite), both with high initial87Sr/86Sr ratios. This points to a post-Ordovician (Hercynian) age for the metamorphism and tectonic deformation that affected the area, including the gneisses, and rules out the alleged Cadomian or even older age.
Zusammenfassung Die Rb-Sr-Gesamtgesteinsanalysen von Probeserien zweier orthogneisischer Körper der Almendralejo-Region des Oporto-Portalegre-Badajoz-Córdoba-Gürtels weisen auf ein ordovizisches Alter des granitischen Magmas hin: rund 470 Ma für den Almendralejo-Gneis (peralkaliner Metagranit) und rund 425 Ma für den Ribera del Fresno-Gneis (leukokratischer Metagranit), beide bei einem hohen initialen87Sr/86Sr. Diese Ergebnisse deuten auf ein post-ordovizisches (herzynisches) Alter der Metamorphose und der tektonischen Deformation hin, die das Gebiet, einschließ-lich des Gneises, beeinflußt haben. Sie schließen ein vermeintliches kadomisches oder älteres Alter aus.
Résumé Une étude par Rb-Sr sur roche totale de séries d'échantillons de deux massifs d'orthogneiss de la région d'Almendralejo, dans la ceinture Oporto-Portalegre-Badajoz-Cordoue, indique un âge ordovicien pour le magmatisme granitique: environ 470 Ma pour le gneiss d'Almendralejo (métagranite peralcalin) et environ 425 Ma pour le gneiss de Ribera del Fresno (métagranite leucocrate subalumineux), tous deux avec des rapports initiaux87Sr/86Sr élevés. Ces résultats indiquent un âge post-ordovicien (hercynien) pour le métamorphisme et la déformation tectonique qui ont affecté la région, y compris les gneiss, et éliminent l'hypothèse proposée par ailleurs d'un âge cadomien ou même plus ancien.
— — — Rb/Sr : 425 -- ( ) 470 ( ) 87Sr/86Sr . - ( ) , , . , .相似文献
998.
The Devonian-Carboniferous contact in southern South America, characterized by a sharp unconformity, has been related to the Late Devonian-Early Carboniferous Eo-Hercynian orogeny. The Calingasta-Uspallata basin of western Argentina and the Sauce-Grande basin (Ventana Foldbelt) of eastern Argentina have been selected to characterize this unconformity. The Eo-Hercynian movements were accompanied in western Argentina by igneous activity related to a Late Devonian—Early Carboniferous magmatic arc mainly exposed today along the Andean Cordillera. This magmatic activity is partly reflected also in eastern Argentina (Ventana Foldbelt), where isotopic dates suggest a thermal event also related to the intrusions present to the west in the North Patagonian Massif and Sierras Pampeanas. The scarcity of Lower Carboniferous deposits in the stratigraphic record of southern South America suggests that the Early Carboniferous was a time interval dominated by uplift and erosion followed by widespread subsidence during the Middle and Late Carboniferous. The origin of the Eo-Hercynian orogeny can be linked with the convergence between the Arequipa Massif, and its southern extension, and the South American continent. Its effects are best represented along the Palaeo-Pacific margin, although distant effects are discernible in the cratonic areas of eastern South America.
Correspondence to: O. R. López-Gamundí 相似文献
999.
Peter Haase Susanne Lohse Steffen Pauls Karin Schindehütte Andrea Sundermann Peter Rolauffs Daniel Hering 《Limnologica》2004,34(4):349-365
In the past, no single standardised method for sampling and sorting benthic macroinvertebrates has been implemented in Germany. Therefore, we tested the suitability of two common sorting protocols, RIVPACS and AQEM/STAR, by taking samples with each protocol at 44 sampling sites. Our results reveal that different methods deliver slightly different assessment results. Moreover these two methods differ in costs. Although the AQEM/STAR protocol takes longer than the RIVPACS protocol, we favoured the AQEM/STAR protocol because of its higher level of standardisation. In order to limit costs to an acceptable level, a modification of the AQEM/STAR protocol (MAS method) is developed. This method is highly standardised, gives stable assessment results and is relatively inexpensive (€ 224.00 for processing of an average sample). A detailed protocol of the newly developed method is given. 相似文献
1000.
T. Gombosi J. Kóta A. J. Somogyi V. G. Kurt B. M. Kuzhevskii Yu. I. Logachev 《Solar physics》1977,54(2):441-456
Based on the data of the high-apogee satellite Prognoz-3, the April 29–30, 1973 solar particle event is analysed. The event's complex energetic particle, interplanetary magnetic field and solar wind plasma properties are discussed. The unusual behaviour of solar particles up to energies 100 MeV can well be explained in terms of the interaction with an interplanetary shock wave system passing the Earth. Assuming that the structure of the interplanetary shock wave system is similar to that considered first by Parker (1961) and Gold (1959) and reviewed later by Hundhausen (1972) and Dryer (1974, 1975), the main characteristics of the energetic particle fluxes, solar wind and interplanetary magnetic field can be understood. 相似文献