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71.
The Archean greenstone belts of the Nyanzian System in western Kenya are composed principally of andesite with minor tholeiitic basalt and siliceous volcanics. The Nyanzian tholeiite is an intermediate-K tholeiite with a flat REE pattern. There are two chemically-distinct andesites: a low-K andesite (Andesite I) and a high-K andesite (Andesite II). The REE pattern of the Andesite II is enriched in light REE and depleted in heavy REE relative to Andesite I.Major and trace element calculations indicate an origin for the Nyanzian tholeiite by 35–40% equilibrium melting of a lherzolite source followed by 10% shallow fractional crystallization. Similar calculations best explain Andesite I and Andesite II by 20 and 5% melting, respectively, of an ecologite or garnet amphibolite source of Nyanzian tholeiite composition. The rhyolite may have formed either by 20–30% partial melting of a siliceous granulite or by 20–30% fractional crystallization of a granodiorite parent magma.With respect to total exposure areas, the Nyanzian volcanics have significantly less tholeiite and more Andesite and siliceous volcanics than other Archean greenstone belts. If these abundances are representative, two models are proposed to explain the anomalous abundances of Andesite and siliceous volcanics. The first model involves an Archaen upper mantle with a relatively low geothermal gradient beneath Kenya, while the second model involves a relatively cool mantle plume. Both models inhibit ascent of a significant amount of primary tholeiite to the surface and prevent formation of secondary tholeiite. Other Archean greenstone terranes with higher mantle geotherms or hotter mantle plumes would receive higher proportions of mafic and ultramafic magmas. 相似文献
72.
D. W. Davis E. S. Schandl H. A. Wasteneys 《Contributions to Mineralogy and Petrology》1994,115(4):427-437
U-Pb geochronology of igneous zircon from rhyolitic host rocks to the Archean Kidd Creek, Geco and Winston Lake massive sulfide deposits, in the Superior Province of Ontario, shows that volcanism, which accompanied mineralization, occupied a narrow time span (2717±2 Ma, 2720±2 Ma and 2723±2 Ma, respectively). Precise ages of hydrothermal monazite, allanite and rutile from alteration zones surrounding the above deposits indicate that these minerals crystallized 40–70 million years after volcanism. Monazite from Kidd Creek mine is 2659±3 Ma old, in agreement with spatially associated 2664±25 Ma old rutile. Monazite from a biotite schist at Geoco mine gives a similar age of 2661±1 Ma. However, monazite from a sericite schist, which hosts the ore at Geco mine, is 2675±2 Ma old. Abraded large monazite grains from three units in the Winston Lake deposit are coeval with biotite crystallization and record an age of 2677±2 Ma, approximately the same as monazite in the sericite schist at Geco. Data points from allanite fractions from both the Winston Lake and Geco deposits fall on a Pb-Pb isochron that gives an age of 2672±5 Ma. Rutile from Winston Lake gives a younger age of 2651±6/-2 Ma and may date retrograde alteration of biotite to chlorite. The ca. 2676 Ma age of monazite from Winston Lake and in the sericite schist at Geco mine probably dates a regional metamorphic event that affected most of the southern Superior Province. The ca. 2660 Ma old monazite in the biotite schist at Geco mine and in the chlorite-sericite alteration at Kidd Creek may date later K-metasomatism caused by metamorphically derived fluids that were focussed along old fault structures. Such fluids were also responsible for local sulfide remobilization. Monazite and rutile are spatially associated with chlorite and sericite alterations at Kidd Creek. Their young ages indicate that these originally syngenetic mineral assemblages may have been significantly affected by regional metamorphism. Formation of monazite at all three deposits studied was a result of significant REE remobilization during metamorphism. The discrete character of syn-metamorphic hydrothermal activity in different units of the same deposit, as well as its synchroneity among different, widely separated deposits, requires a mechanism for episodic injection of heat and fluid into the crust on a regional scale. These activities are broadly coeval with, and probably related to, plutonism within adjacent metasedimentary subprovinces and middle to lower crustal metamorphism in the Superior Province. 相似文献
73.
Michael G. Davis Michael O. Garcia Paul Wallace 《Contributions to Mineralogy and Petrology》2003,144(5):570-591
Glasses from Mauna Loa pillow basalts, recent subaerial vents, and inclusions in olivine were analyzed for S, Cl, F, and major elements by electron microprobe. Select submarine glasses were also analyzed for H2O and CO2 by infrared spectroscopy. The compositional variation of these tholeiitic glasses is dominantly controlled by crystal fractionation and they indicate quenching temperatures of 1,115-1,196 °C. Submarine rift zone glasses have higher volatile abundances (except F) than nearly all other submarine and subaerial glasses with the maximum concentrations increasing with water depth. The overwhelming dominance of degassed glasses on the submarine flanks of Mauna Loa implies that much of volcano's recent submarine growth involved subaerially erupted lava that reached great water depths (up to 3.1 km) via lava tubes. Anomalously high F and Cl in some submarine glasses and glass inclusions indicate contamination possibly by fumarolic deposits in ephemeral rift zone magma chambers. The relatively high CO2 but variable H2O/K2O and S/K2O in some submarine rift zone glasses indicates pre-eruptive mixing between degassed and undegassed magma within Mauna Loa's rift system. Volatile compositions for Mauna Loa magmas are similar to other active Hawaiian volcanoes in S and F, but are less Cl-rich than Ll'ihi glasses. However, Cl/K2O ratios are similar. Mauna Loa and Ll'ihi magmas have comparable, but lower H2O than those from Kilauea. Thus, Kilauea's source may be more H2O-rich. The dissimilar volatile distribution in glasses from active Hawaiian volcanoes is inconsistent with predictions for a simple, concentrically zoned plume model. 相似文献
74.
Basil Tikoff Matthew R. Davis Christian Teyssier Michel de St. Blanquat Guillaume Habert Sven Morgan 《Tectonophysics》2005,400(1-4):209-226
The Cascade Lake shear zone occurs on the eastern margin of the Tuolumne Intrusive Suite, Sierra Nevada Batholith, California. Foliation in the zone is NNW trending and subvertical, and lineation is moderately south plunging. Deformation is syn-tectonic with emplacement of the Cathedral Peak granodiorite. A deformation gradient exists toward the NE margin of this pluton, with higher strains and lower temperatures of deformation found near the contact. We compare fabric data collected very densely in this shear zone using several techniques: field fabrics, 3D orientation of K-feldspar megacrysts, and AMS (anisotropy of magnetic susceptibility) analysis. In general, the results from the three different methods are in agreement. Deformation in this shear zone is part of a larger pattern of deformation within the Cathedral Peak granodiorite, as recorded by AMS analysis, and dextral shearing associated within the last stage of plutonism within the Sierra Nevada magmatic arc. 相似文献
75.
76.
印度板块和亚洲大陆在何时何地碰撞 总被引:1,自引:0,他引:1
印度板块和亚洲大陆的初始碰撞时间是所有相关的喜马拉雅-西藏造山体系演化模式的主控条件,并严重影响到对众多与青藏高原隆升和东亚大陆挤出相关的地质过程速率的解释,以及对新生代全球气候变化的理解。尽管印度板块和亚洲大陆汇聚的速率在55Ma突然减缓被广泛地认为是初始碰撞的标志,但这次碰撞所造成的主要构造效应直到20多个百万年以后才显现出来。对印度板块和亚洲大陆相对位置的重新估算,表明它们在55Ma时并没有达到可以彼此发生碰撞的距离。基于来自西藏新的野外证据和对已有数据的重新评估,认为初始碰撞发生在始新世—渐新世之交(约34Ma),并对55Ma时发生的地质事件提出了另一种解释 相似文献
77.
78.
Relationships between gold concentration and structure in quartz veins from the Hodgkinson Province, northeastern Australia 总被引:1,自引:0,他引:1
The Hodgkinson Province is a tract of␣multiply deformed Silurian-Devonian rocks in north␣Queensland, Australia. Gold-bearing
quartz veins from the West Normanby Goldfield in the northern Hodgkinson Province were emplaced during the Permian D4 event, broadly coeval with regional granite emplacement. Taylors Fault, a major structure that formed during D2, hosts the veins which infill dilatational jogs opened during sinistral-normal reactivation of the fault in D4. Veins contain graphitic laminations that formed when fault planes segmented wallrocks adjacent to the veins, producing tabular
clasts that were tectonically sliced into the reefs. Laminations are the result of progressive shear strain, associated with
continued movement on the faults, which caused strain-enhanced dissolution of silicate minerals and residual graphite enrichment
in the clasts. This process produced graphite-coated shear planes that delimit zones of grain size reduction in the veins.
Laminations commonly contain stylolites, which nucleated on pronounced sinuosities of the shear planes due to progressive
shortening during D4. Gold particles have preferentially nucleated in zones of relatively coarser-grained quartz adjacent to the shear planes,
where shortening strain caused microfracturing and allowed fluid access. Gold may have been introduced with the quartz, but
was redistributed within the reefs and localized along the laminations by the effects of synchronous, progressive deformation.
Regionally, gold deposits show close spatial relationships with granite plutons of the Permian Whypalla Supersuite. Relationships
in the West Normanby Gold Field support a regional model of reef emplacement and gold mineralization during the Permian D4 event.
Received: 24 August 1997 / Accepted: 14 October 1997 相似文献
79.
Stephen?MoyseyEmail author Stanley?N.?Davis Marek?Zreda L.?DeWayne?Cecil 《Hydrogeology Journal》2003,11(6):615-627
The natural distribution of 36Cl/Cl in groundwater across the continental United States has recently been reported by Davis et al. (2003). In this paper, the large-scale processes and atmospheric sources of 36Cl and chloride responsible for controlling the observed 36Cl/Cl distribution are discussed.The dominant process that affects 36Cl/Cl in meteoric groundwater at the continental scale is the fallout of stable chloride from the atmosphere, which is mainly derived from oceanic sources. Atmospheric circulation transports marine chloride to the continental interior, where distance from the coast, topography, and wind patterns define the chloride distribution. The only major deviation from this pattern is observed in northern Utah and southern Idaho where it is inferred that a continental source of chloride exists in the Bonneville Salt Flats, Utah.In contrast to previous studies, the atmospheric flux of 36Cl to the land surface was found to be approximately constant over the United States, without a strong correlation between local 36Cl fallout and annual precipitation. However, the correlation between these variables was significantly improved (R
2=0.15 to R
2=0.55) when data from the southeastern USA, which presumably have lower than average atmospheric 36Cl concentrations, were excluded. The total mean flux of 36Cl over the continental United States and total global mean flux of 36Cl are calculated to be 30.5±7.0 and 19.6±4.5 atoms m–2 s–1, respectively.The 36Cl/Cl distribution calculated by Bentley et al. (1986) underestimates the magnitude and variability observed for the measured 36Cl/Cl distribution across the continental United States. The model proposed by Hainsworth (1994) provides the best overall fit to the observed 36Cl/Cl distribution in this study. A process-oriented model by Phillips (2000) generally overestimates 36Cl/Cl in most parts of the country and has several significant local departures from the empirical data.
Resumen Davis et al. (2003) han informado de la distribución natural de la proporción 36Cl/Cl en las aguas subterráneas de la parte continental de los Estados Unidos de América [EUA]. En este artículo, se discute cuáles son los procesos a gran escala y las fuentes atmosféricas del 36Cl y del cloruro que dan lugar a la distribución observada de 36Cl/Cl.El proceso dominante que afecta a la relación 36Cl/Cl en las aguas subterráneas de origen meteórico a escala continental es el aporte de cloruro estable desde la atmósfera, que procede principalmente de los océanos. La circulación atmosférica transporta el cloruro marino hacia el interior, donde la distancia a la costa, topografía y corrientes del viento definen la distribución del cloruro. La única desviación principal de este esquema tiene lugar al norte de Utah y en el sur de Idaho, donde se deduce que existe una fuente continental de cloruro en los Rellanos Salados de Bonneville (Salt Flats).En contraste con estudios previos (Knies et al. 1994; Phillips 2000), se ha descubierto que el flujo atmosférico de 36Cl hacia la superficie terrestre es aproximadamente constante en todos los estados, sin deducirse una correlación fuerte entre el aporte de 36Cl y la precipitación anual. Sin embargo, la correlación entre estas variables se ve mejorada de forma significativa, con coeficientes de regresión comprendidos entre 0,15 y 0,55, cuando se excluyen los datos recogidos en el sudeste de los EUA, que tienen concentraciones de 36Cl atmosférico presuntamente inferiores a la media. El flujo medio total de 36Cl calculado en la zona continental de los Estados Unidos vale 30,5±7,0 átomos por metro cuadrado y segundo, mientras que el flujo total global de 36Cl es de 19,6±4,5 átomos por metro cuadrado y segundo.La distribución de 36Cl/Cl calculada por Bentley et al. (1986) infravalora la magnitud y variabilidad observada en los valores medidos a lo largo de los Estados Unidos. El modelo propuesto por Hainsworth (1994) proporciona el mejor ajuste conjunto a la distribución observada de 36Cl/Cl en este estudio. El modelo orientado a procesos de Phillips (2000) sobreestima por lo general la distribución de 36Cl/Cl en la mayoría del país y difiere significativamente de algunos valores locales empíricos.
Résumé La distribution naturelle du rapport 36Cl/Cl dans les eaux souterraines des États-Unis a été récemment présentée par Davis et al. (2003). Dans ce travail, les processus à grande échelle et les sources atmosphériques de 36Cl et de chlorure responsables du contrôle de la distribution observée du rapport 36Cl/Cl sont discutés. Le processus dominant qui affecte le rapport 36Cl/Cl dans les eaux souterraines dorigine météorique à léchelle continentale est lapport atmosphérique de chlorure stable, qui provient pour lessentiel de sources océaniques. La circulation atmosphérique transporte des chlorures marins vers lintérieur des continents, où la distribution de chlorure est définie par la distance à la côte, la topographie et les régimes des vents. La seule exception majeure à ce schéma est observée dans le nord de lUtah et le sud de lIdaho où lon suppose quil existe une source continentale de chlorure dans les bas-fonds salés de Bonneville. Au contraire de précédentes études (Knies et al. 1994; Phillips 2000), on trouve que le flux atmosphérique de 36Cl vers le sol est approximativement constant sur lensemble des États-Unis, sans forte corrélation entre la retombée locale de 36Cl et les précipitations annuelles. Cependant, la corrélation entre ces variables devient significative (R 2=0.15 à 0.55) lorsquon supprime les données du sud-est des États-Unis, dont on pense quelles présentent des concentrations en 36Cl atmosphérique inférieures à la moyenne. Le flux total moyen de 36Cl sur les États-Unis continentaux et le flux moyen global de 36Cl sont respectivement évalués à 30.5 ± 7.0 et 19.6 ± 4.5 atomes.m–2.s–1. La distribution du rapport 36Cl/Cl calculée par Bentley et al. (1986) sous-estime lordre de grandeur et la variabilité observés pour la distribution mesurée du rapport 36Cl/Cl sur les États-Unis continentaux. Le modèle proposé par Hainsworth (1994) fournit le meilleur ajustement densemble à la distribution du rapport 36Cl/Cl observée dans cette étude. Un modèle orienté vers les processus proposé par Phillips (2000) surestime dans lensemble le rapport 36Cl/Cl dans la plupart des régions du pays et présente plusieurs désaccords locaux avec les données empiriques.相似文献
80.
The ca. 2.7–2.5 Ga Slave Province is a granitegreenstone terrane comprising deformed sedimentary and subordinate volcanic belts extensively intruded by granitoid rocks. The Nd isotopic data are reported for 58 samples of supracrustal and granitoid rocks exposed along a 400 km, east-west, transect at 65°N across the structural grain of the province. Initial
Nd values reveal distinctly different crustal sources in the eastern compared to the western parts of the province, as expected from tectonic assembly of the province through accretion of juvenile crust to older continental crust. Supracrustal sequences (ca. 2.71–2.65 Ga) from the central and eastern parts of the province have positive
Nd(1) values (+0.3 to +3.6), consistent with juvenile sources and formation remote from significantly older crust. Syn to late-deformation (ca. 2.63–2.60 Ga), mantle-derived diorites and related tonalites (type I) from the central and eastern parts of the province have similar initial
Nd values (-0.1 to +2.7). In contrast, samples from the westernmost plutons, which intrude exposed pre-3.1 Ga crust, have much lower
Nd(1) values (-1.0 to4.6) suggesting contamination of these magmas by older crust. The
Nd(1) values of post-deformation granites (s.s.) (type II) also vary systematically across the province: values for granites west of longitude 110°30W range from-0.2 to -5.3; those to the east range from +0.6 to +3.7. These data suggest mixed crustal sources dominated by Mid to Early Archean material (
Nd-2.6 to- 17 at 2.6 Ga) for the western granitoid rocks and juvenile sources for the eastern granites. The Nd isotopic data are consistent with the geology of the province in that exposures of Mid to Early Archean crustal rocks, predating the principal 2.7–2.5 Ga orogenic event are restricted to the western part of the province. The asymmetric pattern defined by the Nd isotopic data indicates the presence of distinct crustal rocks beneath the Slave Province. Similar isotopic variations observed across Phanerozoic collisional orogens have been interpreted to reflect tectonic assembly of crust by accretion of juvenile crustal terranes to an older continental margin. This process may also have been an important mechanism in the cratonization of the Slave Province. 相似文献