Some current frontiers in LIP research include:
- (1) Testing various mantle plume and alternative hypotheses for the origin for LIPs.
(2) Characterizing individual LIPs in terms of (a) original volume and areal extent of their combined extrusive and intrusive components, (b) melt production rates, (c) plumbing system geometry, (d) nature of the mantle source region, and (e) links with ore deposits.
(3) Determining the distribution of LIPs in time (from Archean to Present) and in space (after continental reconstruction). This will allow assessment of proposed links between LIPs and supercontinent breakup, juvenile crust production, climatic excursions, and mass extinctions. It will also allow an evaluation of periodicity in the LIP record, the identification of clusters of LIPs, and postulated links with the reversal frequency of the Earth's magnetic field.
(4) Comparing the characteristics, origin and distribution of LIPs on Earth with planets lacking plate tectonics, such as Venus and Mars. Interplanetary comparison may also provide a better understanding of convective processes in the mantles of the inner planets.
In order to achieve rapid progress in these frontier areas, a global campaign is proposed, which would focus on high-precision geochronology, integrated with paleomagnetism and geochemistry. Most fundamentally, such a campaign could help hasten the determination of continental configurations in the Precambrian back to 2.5 Ga or greater. Such reconstructions are vital for the proper assessment of the LIP record, as well as providing first-order information related to all geodynamic processes. 相似文献
A consistent plate-tectonic model is proposed for the area, consisting of six main stages:
1. (1) development during Ordovician times of a calc-alkalic island-arc in the vicinity of a continental margin;
2. (2) extrusion during Late Ordovician to Silurian times of a primitive basalt-andesite intraoceanic island-arc suite, which terminated with boninites, the latter suggest rifting in the fore-arc, followed by the breakup of the arc;
3. (3) opening and development of the Trinity back-arc basin around 430–480 Ma ago;
4. (4) eruption of the Balaklala Rhyolite either in the arc or in the fore-arc, ending in Early Devonian time with intrusion of the 400 Ma Mule Mountain stock;
5. (5) break in volcanic activity from the Early Devonian to the Early Permian; and
6. (6) development of a mature island-arc from the Early Permian to the Late Triassic.
The eastern Klamath Mountains island-arc formations and ophiolitic suite are part of the “Cordilleran suspect terranes”, considered to be Gondwana margin fragments, that have undergone large northward translations before final collision with the North American craton during Late Mesozoic or Cenozoic times. These eastern Klamath Mountains island-arcs could be associated with the paleo-Pacific oceanic plate that led to accretion of these allochthonous terranes to the American margin. 相似文献
1. (a) cumulate minerals corresponding to eroded magmatic surface and subsurface rocks
2. (b) intercumulus material, and
3. (c) magmas crystallized to completion in the deep crust.
Estimates based exclusively on geophysical data tend to underestimate the true transfer of mass into the lower crust as gabbroic cumulate rocks, and melts crystallizing to completion in the lower crust have densities and seismic velocities similar to those of lower crustal wallrocks. 相似文献
1. (1) On the outer wall: faults, formed either by reactivation of the structural grain of the oceanic plate, when the latter is slightly oblique to the trench, or by a new fault network parallel to the trench, or both. The width of the faulted zone is about 50 miles.
2. (2) On the inner wall: either an accretionary prism or an extensional fault network, or both; collapsed structures and slumps are often associated, sometimes creating confusion with the accretionary structures.
3. (3) The overall structure of the trench itself is determined by the shape of the edge of the continental crust or of the island arc. Its detailed structure, however, is related to the oceanic plate, namely when the structural grain of the latter is slightly oblique to the trench, which then takes an “en echelon” form. Collapsed units can fill up the trench which is, in that case, restricted to an irregular narrow depression; the tectonic framework of the trench can be buried under a sedimentary blanket when the sedimentation rate is high and the trench bottom is a large, flat area.
Two extreme types of active margins can be distinguished: convergent compressive margins, when the accretionary mechanism is strongly active; and convergent extensional margins where the accretionary mechanism is absent or only weakly active.
The status of a given margin between these two extreme types is related to the convergence rate of the plates, the dip of the subduction zone, the sedimentation activity and the presence of a continental obstacle, because oceanic seamounts and aseismic ridges are easily subducted.
Examples are taken from the Barbados, Middle America, Peru, Kuril, Japan, Nankai, Marianna, Manila, New Hebredes and Tonga trenches. 相似文献
A review of the published interpretation leads to the general conclusion that:
1. (1) Pn-velocities vary from a value possibly as low as 7.7 km/sec under Vancouver Island to 8.6 km/sec and higher in the extreme eastern part of the shield and some parts of the Atlantic coast.
2. (2) Large areas of Canada have a crustal thickness of 30–40 km, with Vancouver Island, the southwestern Prairies, the Lake Superior basin and parts of the eastern shield of Quebec being thicker. No continental area in Canada is known to have a crust thinner than 29 km.
3. (3) The Riel discontinuity — a deep intra-crustal reflector and sometime refractor, is widely reported in the Prairies and Manitoba. It is not seen to the north in the vicinity of Great Slave Lake, nor in the Hudson Bay, Lake Superior and Maritime regions, nor in the interior of British Columbia. It may be present in some areas of the eastern shield.
4. (4) As experiments have become more detailed, crustal structures of greater complexity have been revealed. The concept that crustal structure becomes simpler with increasing depth is apparently unfounded.
Long-range refraction studies suggest that the Gutenberg P-wave low-velocity channel is poorly developed under the Canadian Shield. The analysis of the dispersion of surface waves, however, suggests that the channel is better developed for S-waves, and is present throughout the country. The lid of the channel is deepest under the central shield and shallowest under the Cordillera. 相似文献
A large laterite bound peat deposit in Sri Lanka was studied for its distribution of metals. Most metals, in particular Be, Zr, Li and the majority of the transition series metals, exhibit a strong positive correlation with K, Al, Fe and Mn. A noteworthy feature observed was the poor correlation of the metals with organic carbon, as contrasted with a very good correlation with elemental components of clay. The laterites found around the peat deposit appear to be the possible source materials for the metals in the peat deposit. The geochemical distribution of metals in the peat is governed by:
1. (1) the nature of the organic matter;
2. (2) clay component;
3. (3) pH and Eh changes;
4. (4) marine-continental sediment mixing;
5. (5) nature of source materials for the metals.
Most of the metals appeared to be bound to the clay matrix, coated perhaps with hydrous Fe- and Mn-oxides. The existence of brine-rich marine-based sediments in the peat deposit had an important bearing on the geochemical distribution of the metals, as evidenced by the fact that in saline conditions most metals form chloride complexes, particularly under the prevailing acidic conditions. The metals thus mobilized interact with humic acids. The peat deposit under investigation provides an ideal setting for the study of the distribution of metals in a laterite-clay-peat mixture. 相似文献
1. 1) pervasive greenschist faciés regional metamorphism (least-altered andesite)
2. 2) epidotization-silicification
3. 3) albitization-silicification
4. 4) chloritization
5. 5) sericitization-silicification. Alteration is localized on permeable zones such as amygdules, fractures, flow tops, discordant breccia dikes, and conformable breccia horizons.
Epidotized-silicified andesite is enriched in Ca-Sr-Eu and depleted in Mg and first transition series metals (FTSM) relative to least-altered andesite. Albitized-silicified andesite is significantly enriched in Na and depleted in most FTSM relative to least-altered andesite. The abundances and inter-element ratios of the rare-earth elements (REE) and most high field-strength elements (HFS: Y, Zr, Th, U, Hf, Ta) are similar in least-altered, epidotized-silicified and albitized-silicified andesites. The most silicified andesites are strongly enriched in Na-Si, strongly depleted in Mg and divalent FTSM and slightly but systematically depleted in REE and most HFS elements. Serialized andesites were previously silicified; they are very strongly enriched in K-Rb-Cs-Ba, very strongly depleted in Na-Ca-Sr-Eu and slightly depleted in light REE relative to silicified andesite. Chloritized andesitic rocks exhibit heavy REE and HFS element ratios similar to those of leastaltered andesite, but are relatively strongly enriched in Mg and divalent FTSM, strongly depleted in Si and large ion lithophile (LIL) elements and slightly depleted in light REE.
The coupled behavior of the heavy REE and most HFS elements during epidotization, albitization, silicification, chloritization and serialization suggests that they were inert during hydrothermal modification of the andesite. Mass balance calculations suggest that volume was conserved during epidotization-silicification and albitization-silicification, but that intense silicification was accompanied by volume increases up to 30 percent. 相似文献
1. (1) 12,500-10,200 yrs B.P. (lake stage): very rapid rise of the groundwater table of about 2 to 3 m, resulting in a shallow lake, followed by a more gradual rise of about 2.5 to 1.5 m.
2. (2) 10,200-7000 yrs B.P. (freshwater marsh, stage 1): slow overall rise of the water table interrupted by a drop of at least 1 m between about 9500 and 9000 yrs B.P. and of at least 0.8 m between about 8000 and 7500 yrs B.P., each event leading to oxidation and maceration of organic material.
3. (3) 7000-6400 yrs B.P. (complete desiccation of the swamp): rapid fall of the water table of at least 3.9 m. causing large-scale down-wasting of the accumulated peat.
4. (4) After 6400 yrs B.P. (freshwater marsh, stage 2): rapid rise of the water table.
The water-table rise of period 1 and the lowering of period 3 are attributed to predominantly local causes, while the groundwater fluctuations during period 2 are probably climate-related. The final water-level increase reflects the influence of Holocene relative sea-level rise. 相似文献
1. (1) pyroclastic rocks (breccias, tuffs) associated with coarse-grained sandstone, suggesting explosive eruptions in grabens
2. (2) basaltic or andesitic pillowed flows, interbedded first with fine-grained sandstone and small Halobia limestone strata, then with pelagic limestones and radiolarian red cherts
3. (3) columnar trachyte flows.
The whole volcanic series belongs to a very differenciated sodic suite with high titanium contents. The Mamonia lavas are very similar to the Afar volcanics and can be considered as belonging to an interplate volcanism in a rift system. This alkaline basaltic suite is found in many places of the East Mediterranean Alpine orogenic domain, especially in the Antalya nappes (South Turkey) and in the Baer-Bassit (Syria). In Greece, a similar volcanism has been noticed (Othrys—Pindos). In Italy there exists a Middle or Late Triassic volcanism with alkaline affinities. Therefore, this Late Triassic magmatism, which is widespread in the whole Mediterranean Alpine region and always in tectonic association with ophiolites, has a very great paleogeographic significance. We thus propose the existence of a rift system associated with an alkaline basaltic suite along the northern edge of the African plate during Norian—Carnian times. Afterwards a mid-oceanic ridge would have been formed during the Jurassic and Cretaceous. To explain this evolution two hypotheses can be proposed:
1. (1) A single mid-Tethysian ridge existed and all the ophiolites (Greece, Turkey, Cyprus, the ‘croissant ophiolitique peri-arabe’) have been thrust from the same area.
2. (2) A marginal sea existed along the mid-Tethysian ridge north of the African plate but separated from the Tethys by a carbonate shelf, where, after the Triassic events, oceanization began with slightly different ophiolites (a large sheeted complex, low-K tholeiites with some calc-alkaline affinities).
Therefore, Troodos, Hatay, Zagros and Oman would not have come from the main ophiolite zone present further north, but from a marginal ocean, now obducted on the African plate. We think that the second hypothesis is more reasonable because the Upper Cretaceous sedimentary cover (Kannaviou Formation) of the Troodos is very similar to the detritic formation present in South Turkey (Kastel Formation) which is known to grade to shelf carbonates belonging to the Arabian plate towards the south. 相似文献
1. (1) The length and position (relative to a defined coordinate system) of the principal axes of the strain ellipsoid at any stage of the progressive deformation.
2. (2) The position and length of the principal axes in any plane intersecting the strain ellipsoid, also at any stage of the deformation.
3. (3) The position and length of passive markers which initially coincided with the principal axes in an intersecting plane. This is of consequence for the distinction between passively rotating structures and actively forming structures.
4. (4) The shear strain parallel to an intersecting plane or sheet, as indicated by the angular difference between the normal to an intersecting plane at any time and the marker at the same time which initially, however, was parallel to the normal. This layer-parallel shear causes boudins to rotate and the axial plane of buckles to tilt.
The relationships have been expressed quantitatively in the bulk of the paper and illustrated in diagrams. The analysis presented is basic for the study of the deformational behavior of competent sheets of rocks embedded in less competent ones. 相似文献
1. (1) Incremental strain data from pressure shadow fibres. The significance of pressure shadow fibres for the determination of the deformation history of a region is discussed. Pressure shadows are used to quantify the amount of, and to describe the distribution of fold-axis parallel extension occurring in the Helvetic nappes.
2. (2) The extension directions of conjugate systems of en échelon veins. It is shown that an analysis of the geometry of conjugate vein systems can reveal a regional deformation pattern. The relative age of the conjugate en échelon vein systems in the Helvetic deformation history can be assessed, the geometry of the conjugate sets relative to the local anisotropy plane is described, and the significance of the preferred orientation of their extension directions is discussed.
3. (3) Fold-axis parallel sections. A comparison of the regional distribution of the fold-axis parallel strain with the shape of the Helvetic nappes in fold-axis parallel sections shows that the fold-axis parallel strain cannot be related to the footwall topography of the nappes.
It is concluded that the fold-axis parallel extension in the Helvetic nappes was induced by a change of direction of overthrust shear. This change occurred late in the deformation history and was superposed on the already formed nappes. The changing direction of overthrust shear is the expression of an overall anticlockwise rotation going on in the overthrusting Alpine nappe pile, relative to the European plate, a rotation which lead to the arcuate shape of the Western Alps. 相似文献
- (1) Older E-, SE- and S-flowing rivers, which cross the Carpathians, radiate towards the foreland and were probably established during the Middle Miocene (Sarmatian) collision event.
(2) A more recent drainage system related to the contemporaneous development of the volcanoes and intramontaneous basins, which generally drains westward into the Transylvanian Basin since late Miocene time and has been capturing the older river system.
The older river drainage system has also been modified by Late Pliocene–Quaternary folding, thrusting and monoclinal tilting along the Pericarpathian orogenic front and by reactivated transverse high angle basement faults, which cross the Eastern Carpathian foreland. 相似文献
During a first stage of metasomatism the mantle xenoliths were affected by high-temperature reactions at 600 to 925°C. They resulted from the interaction between solid mantle lherzolites and basic melts. The reactions are:
1. (1)those forming orthopyroxene-magnetite simplectite
2. (2)those forming plagioclase-magnetite corona
3. (3)clinopyroxene+spinel I→pargasitic hornblende+spinel II.
Chemical interactions between the upper mantle and oceanic magma chambers occurred as soon as the basic magmas had ascended through the upper mantle. The chemically modified magmas, within oceanic magma chambers, were depleted in Ti, Fe and Na. This could partly explain regional variations of the chemical compositions of primary magmas produced beneath slow-spreading ridges. The breakdown of olivine to orthopyroxene and magnetite participates in the control of the partition of magnetic Fe---Ti oxides between oceanic crust and mantle.
During the second stage, the serpentinization of olivine and the production of talc were superimposed on the products of the first stage. These reactions require large amounts of H2O. The hydrothermal fluid was probably seawater. It circulated in the brecciated area along the walls and floors of the magma chambers located at shallow depths. Such structural discontinuities thus played the role of penetration channels favoring seawater circulation in the oceanic crust.
All the chemical reactions examined suggest a significant open-system element transfer by infiltrating melts or circulating fluids. The results of this study suggest that caution is required in the interpretation of mineralogical and chemical information provided by mantle xenoliths carried to the surface by ascending magmas. 相似文献
Relevant petrographical data and the geochemical analysis lead us to consider that:
1. (a) the basic lavas can be linked to tholeitic basalts.
2. (b) the acid lavas are associated to metarhyolites.
3. (c) the two magmatic flows do not originate from the same parental magma.
The geochemical results obtained, compared to those from other deposits of nearly the same age outcropping around the Mediterranean, indicate that the latest Hercynian volcanism has been affected by intra-plate distension phenomena preceding the great Lower Mesozoic break-up which caused the formation of the Tethys Ocean.
It is remarkable that in the Mesogean Basin the sedimentary environment is nearly similar to that described in this paper. The volcanic flows end up in a sialic sedimentary basin containing coarse detritic formations that resulted from erosion of the Variscan chain along the southern margin of the European continent.
Abstract
Dans la zone pélagonienne appartenant aux zones internes helléniques, des coulées volcaniques d'âge paléozoïque supérieur sont étudiées, pour la première fois, pétrographiquement et géochimiquement. Les différents agencements pétrographiques et les analyses géochimiques nous apprennent que:
1. (a) les laves basiques peuvent être rattachées á la famille des basaltes tholéitiques.
2. (b) les laves acides s'apparentent aux métarhyolites.
La quantification des différents éléments analyses nous amène á considérer que les deux lignées magmatiques ne sont pas issues d'un même magma parental.
La comparaison des résultats géochimiques avec d'autres gisements, sensiblement contemporains affleurant autour de la Méditerranée actuelle, nous indique que le volcanisme fini-hercynien est assujetti á des phénomènes de distension intraplaques, préparatoires á la grande fracturation mésozoïque inférieure ayant donné naissance á l'océan téthysien.
Ce qui est remarquable dans le bassin mésogéen, c'est que l'environnement sédimentaire est toujours, á peu près, comparable á celui que nous décrivons ici. En effet, les coulées volcaniques s'épanchent dans un bassin sédimentaire á fond sialique au sein de formations détritiques grossières nées de la destruction de la chaîne varisque sur la bordure méridionale du continent europeen. 相似文献
Recent technological advances have placed new instruments and systems at the disposal of the geodesist, so that is now feasible to measure and analyze these micro and macro crustal movements within the accuracies required.
The paper describes three such systems:
1. (1) The Wild Electronic Theodolite T-2000 with a highly precise distance-measurement instrument, the DI-4S, together with a data collector, the GRE-3, which are connected to a computer and a plotter to measure and analyze both micro and macro crustal movements.
2. (2) The Wild NAK-2 level with an antimagnetic compensator which increases the accuracy in the height/velocity monitoring of vertical crustal movements by virtual elimination of the influence of natural or man-made magnetic fields on the automatic level.
3. (3) The use of analytical photogrammetry employing both terrestrial and aerial photography to monitor crustal movements.
By taking advantage of these new instruments and systems, the scientists capability to provide crustal movement data for use in the analysis and prediction of micro or macro crustal movement is greatly enhanced. 相似文献
1. (1) phengite±quartz+K,Mg-rich siliceous fluid=feldspar+phologopite+H2O
2. (2) phengite+talc+K,Mg-rich siliceous fluid=phlogopite+quartz/coesite+H2O
at temperatures between 400 and 700°C. The ultrapotassic fluid appearing at pressures above 15 kbar on the low-temperature sides of the corresponding reaction curves, which show positive dP/dT slopes, is probably supercritical. The P-T positions of the reactions are compatible with KMASH mineral reactions studied previously and with melting investigations in the KMASH system undertaken at temperatures higher than 700°C.
It is possible that natural rocks, chiefly K-rich metasediments subducted as minor portions of the oceanic crust, could give rise to low-temperature ultrapotassic fluids, mainly at temperatures between 300° to 600°C and pressures between 15 and 30 kbar. The ascending K-rich fluids would penetrate the overlying mantle to metasomatize it. After termination of the subduction process, heating of this mantle material, previously cooled by the subducted lithosphere, could lead to the formation of high-temperature K-rich magmas. 相似文献