首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 250 毫秒
1.
The Wolfe Creek Meteorite Crater is an impact structure 880 m in diameter, located in the Tanami Desert near Halls Creek, Western Australia. The crater formed?<?300 000 years ago, and is the second largest crater from which fragments of the impacting meteorite (a medium octahedrite) have been recovered. We present the results of new ground-based geophysical (magnetics and gravity) surveys conducted over the structure in July?–?August 2003. The results highlight the simple structure of the crater under the infilling sediments, and forward modelling is consistent with the true crater floor being 120 m beneath the present surface. The variations in the dip of the foliations around the crater rim confirm that the meteorite approached from the east-northeast, as is also deduced from the ejecta distribution. Crater scaling arguments suggest a projectile diameter of?>?12.0 m, a crater formation time of 3.34 s, and an energy of impact of ～0.235 Mt of TNT. We also use the distribution of shocked quartz in the target rock (Devonian sandstones) to reconstruct the shock loading conditions of the impact. The estimated maximum pressures at the crater rim were between 5.59 and 5.81 GPa. We also use a Simplified Arbitrary Langrangian–Eulerian hydrocode (SALE 2) to simulate the propagation of shock waves through a material described by a Tillotson equation of state. Using the deformational and PT constraints of the Wolfe Creek crater, we estimate the maximum pressures, and the shock-wave attenuation, of this medium-sized impact.  相似文献   

2.
A newly discovered, morphologically well-preserved crater with a mean diameter of 260 m is reported from the Ophthalmia Range, Western Australia. The crater is located in hilly terrain ～36 km north of Newman, and is situated in the Paleoproterozoic Woongarra Rhyolite and the overlying Boolgeeda Iron Formation. The morphometry of the crater is consistent with features characteristic of small meteorite impact craters. The rhyolite of the crater's rim exhibits widespread shatter features injected by veins of goethite bound by sharply defined zones of hydrous alteration. The alteration zones contain micro-fractures injected by goethite, which also fills cavities in the rhyolite. The goethite veins are interpreted in terms of forceful injection of aqueous iron-rich solutions, probably reflecting high-pressure hydrothermal activity by heated iron-rich ground water. None of these features are present in the Woongarra Rhyolite outside the immediate area of the crater. Petrography of the rhyolite indicates possible incipient intracrystalline dislocations in quartz. The Boolgeeda Iron Formation, which crops out only on the southern rim of the crater, displays brecciation and mega-brecciation superposed on fold structures typical of the banded iron-formations in the region. Geochemical analysis of two goethite veins discloses no siderophile element (Ni and PGE) anomalies, negating any contribution of material from an exploding meteorite. Instead, the strong iron-enrichment of the fractured rhyolite is attributed to a hydrothermal system affecting both the Boolgeeda Iron Formation and the Woongarra Rhyolite, and localised to the area of the crater. An absence of young fragmental volcanic material younger than the Woongarra Rhyolite is inconsistent with an explosive diatreme, leading us to a preferred interpretation in terms of an original impact crater about 80 m deep excavated by a ～10 m-diameter projectile and accompanied by hydrothermal activity. A minor north–south asymmetry of the crater, and an abundance of ejecta north, up to about 300 m northwest and northeast of the crater, suggest high-angle impact from the south. A youthful age of the structure, probably Late Pleistocene (104–105 years old), is indicated by damming of the drainage of a south-southeast-flowing creek by the southern crater rim.  相似文献   

3.
The Australian continent has one of the best-preserved impact-cratering records on Earth, closely rivalling that of North America and parts of northern Europe, and the rate of new discoveries remains high. In this review 26 impact sites are described, including five small meteorite craters or crater fields associated with actual meteorite fragments (Boxhole, Dalgaranga, Henbury, Veevers, Wolfe Creek) and 21 variably eroded or buried impact structures (Acraman, Amelia Creek, Connolly Basin, Foelsche, Glikson, Goat Paddock, Gosses Bluff, Goyder, Kelly West, Lawn Hill, Liverpool, Matt Wilson, Mt Toondina, Piccaninny, Shoemaker, Spider, Strangways, Tookoonooka, Woodleigh, Yallalie, Yarrabubba). In addition a number of possible impact structures have been proposed and a short list of 22 is detailed herein. The Australian cratering record is anomalously biased towards old structures, and includes the Earth's best record of Proterozoic impact sites. This is likely to be a direct result of aspects of the continent's unique geological evolution. The Australian impact record also includes distal ejecta in the form of two tektite strewn fields (Australasian strewn field, ‘high-soda’ tektites), a single report of 12.1?–?4.6 Ma microtektites, ejecta from the ca 580 Ma Acraman impact structure, and a number of Archaean to Early Palaeoproterozoic impact spherule layers. Possible impact related layers near the Eocene?–?Oligocene and the Permian?–?Triassic boundaries have been described in the literature, but remain unconfirmed. The global K?–?T boundary impact horizon has not been recognised onshore in Australia but is present in nearby deep-sea cores.  相似文献   

4.
Here we present detailed geological maps and cross-sections of Liverpool, Wolfe Creek, Boxhole, Veevers and Dalgaranga craters. Liverpool crater and Wolfe Creek Meteorite Crater are classic bowl-shaped, Barringer-type craters. Liverpool was likely formed during the Neoproterozoic and was filled and covered with sediments soon thereafter. In the Cenozoic, this cover was exhumed exposing the crater's brecciated wall rocks. Wolfe Creek Meteorite Crater displays many striking features, including well-bedded ejecta units, crater-floor faults and sinkholes, a ringed aeromagnetic anomaly, rim-skirting dunes, and numerous iron-rich shale balls. Boxhole Meteorite Crater, Veevers Meteorite Crater and Dalgaranga crater are smaller, Odessa-type craters without fully developed, steep, overturned rims. Boxhole and Dalgaranga craters are developed in highly foliated Precambrian basement rocks with a veneer of Holocene colluvium. The pre-existing structure at these two sites complicates structural analyses of the craters, and may have influenced target deformation during impact. Veevers Meteorite Crater is formed in Cenozoic laterites, and is one of the best-preserved impact craters on Earth. The craters discussed herein were formed in different target materials, ranging from crystalline rocks to loosely consolidated sediments, containing evidence that the impactors struck at an array of angles and velocities. This facilitates a comparative study of the influence of these factors on the structural and topographic form of small impact craters.  相似文献   

5.
The Gulpuliyul structure is the eroded remains of a possible impact structure of Mesoproterozoic age, in western Arnhem Land, Northern Territory, on the Arnhem Shelf of the northwestern McArthur Basin. Enigmatic, highly deformed and brecciated strata, within the roughly circular or pentagonal feature about 8.5 km across, contrast with mildly deformed rocks of the surrounding Arnhem Shelf. Shock-metamorphic features have yet to be observed. Other features of the Gulpuliyul Structure are: (i) sharp and faulted outer boundaries; (ii) strata within the structure are younger than adjacent country rocks; i.e., the rocks have been emplaced downwards into the structure; (iii) outcrops display an overall concentric or tangential pattern, the stratigraphy is essentially coherent, and there is an overall younging from the centre outwards; and (iv) strata are commonly overturned by southward-directed thrusting and recumbent folding. It is suggested that the projectile impacted at a shallow angle from the north, to produce a southward-deepening crater about 8.5 km across. The depth of the transient crater was probably between ～500?–?700 m (minimum) and ～800 m (maximum). The central uplift probably rebounded only about 300?–?400 m. The present erosion level is thought to lie near the top of the low central uplift, at about or just below the floor of the final crater. The age of the possible impact is Mesoproterozoic (ca 1600?–?1325 Ma); it is most likely to have occurred very early in the Mesoproterozoic (1600?–?1500 Ma).  相似文献   

6.
Based on a detailed statistical analysis of chemical data published in the scientific literature, estimates were made of the minimum amounts of recoverable Ge contained within sulphidic zinc ores and coals, given current processing technologies. It is expected that at least 119 kt (～7 kt in zinc ores and ～112 kt in coal) of recoverable germanium exist within proven reserves (at present stage of knowledge) at grades in excess of 100 ppm in sphalerite and 200 ppm in coal, while at least 440 kt (～50 kt in zinc ores and ～390 kt in coal) should become recoverable in the future, being associated to coal reserves at 8–200 ppm Ge and zinc resources containing in excess of 100 ppm Ge in sphalerite. Mississippi Valley Type (MVT) deposits are expected to be the most important hosts of germanium-rich sphalerite, while both brown and hard coals are expected to be equally important as hosts of germanium. The approach taken in this publication shows that reliable minimum estimates for the availability of by-product metals lacking suitable reserve/resource data may be attained by using robust statistical methods and geochemical data published in the scientific literature  相似文献   

7.
The centre of the 13?×?11 km Spider impact structure, Western Australia, displays an unusual system of eroded folds and imbricated thrusts surrounding a sandstone dome. As inferred from GIS-integrated remote sensing, geological and digital elevation data, the structural setting of the original crater was influenced by, and hence post-dates, the formation of the Mt Barnett Syncline, the east?–?west-oriented axis of which runs through the Spider structure. The syncline formed during the regional Yampi Orogeny (ca 900 Ma), thus constraining the maximum age of the impact event. The sandstone dome in the centre of Spider formed prior to the imbrication, as interpreted from the present setting that indicates a deflection of the southward moving material during the crater collapse. Two modes of formation are discussed in order to explain the south-directed shortening in the Spider impact structure: (i) impact into the bottom of a syncline-controlled palaeovalley leading to uplift of the central crater floor followed by gravity-driven asymmetric sliding preferentially from the northern crater wall and valley slope, respectively; and (ii) moderately oblique (～10?–?30°) impact from the north onto the axis of the syncline, producing a central uplift under the influence of downrange residual momentum and, thus, asymmetric deformation inside the uplift and farther downrange. Neither model alone explains all the observations, and only a combination of both may provide a satisfactory solution.  相似文献   

8.
Goat Paddock in northern Western Australia is a ～5 km-diameter impact crater of Eocene age excavated in gently dipping Proterozoic sandstones. Roughly radial gorges formed by post-impact erosion provide cross-sectional views of the wall and rim zone. The predominant structural theme is one of synclinal rim folding with broad zones in which bedrock strata were deformed by impact to steep, vertical and overturned attitudes. Impact breccia is found craterward of deformed bedrock, on top of it, and downdropped into fault troughs roughly concentric to the crater. The bedrock?–?breccia contact is sharp in some places and gradational in others. In at least one section, the entire mass of upturned bedrock and breccia was displaced radially over essentially undisturbed bedrock, as indicated by slickensides on the horizontal contact. Talus deposits are similar to breccia, but show rough size sorting and clast orientation that dips steeply craterward, indicating that the talus formed as slides down the oversteepened crater wall immediately after crater formation. Shatter cones in some clasts indicate that allogenic material is incorporated in these deposits. Suevite, characterised by ropy flow textures, and by microclasts of quartz with planar deformation features, planar fractures, and of vesiculated silica glass, was found overlying deformed bedrock at a point where the surface of the bedrock forms a nearly horizontal bench midway up the crater wall. The crater was at least partially filled by later sediments, represented by bedded conglomerate close to the crater wall grading inward to sand, silt and mudstone recovered by drillholes on the crater floor. Some of the talus and conglomerate occupy re-entrants in the crater walls, suggesting an original scalloped outline to the crater. Two drillholes, one central and one halfway to the wall, both reached brecciated sandstone after penetrating 210 m of lake sediments. Goat Paddock has a flat floor with no indication of a central uplift and a depth/diameter ratio of ～0.073. This crater form, coupled with the modification of the crater walls by slumping and the scalloped outline of the crater rim suggests that Goat Paddock bridges the two traditional classes of impact crater: simple and complex.  相似文献   

9.
Soil moisture plays a very important role in hydrological processes. It has been found in many studies that the surface soil moisture (SSM) is highly related to the diurnal change of the surface soil temperature (?SST) at the same soil depth. However, some studies contradict this common belief with findings of a much stronger correlation between the SSM and the SST. In order to investigate this further, we have carried out for the first time a comparative assessment of the in-situ measured SST and ΔSST for SSM estimations, over two catchments with contrasting climate types and land uses (i.e. one in the UK and the other in Australia). In both catchments, the time point for the highest relationship between the SST and the SSM is explored. As a result, it is found the SST is more suitable to monitor the variability of the SSM than the ΔSST in both catchments. Moreover the proposed seasonal-based classification method further improves the SSM simulation results in both catchments, with a superior performance observed in the UK catchment (NSE = 0.900 and RMSE = 0.030). In the Australian catchment, a relatively weaker correlation is observed and some potential reasons are explained. The potential applications of the findings for remote sensing soil moisture retrievals are also discussed.  相似文献   

10.
Mapping carried out in the northern Murchison Terrane of the Archaean Yilgarn Craton, Western Australia, shows that correlation of units between isolated greenstone belts is very difficult and an informal stratigraphic subdivision is proposed where the greenstone sequences have been divided into a number of assemblages. The assemblages may not necessarily be time equivalent throughout the region. The lower units (Assemblages 1–3) consist of ultramafic, mafic and intermediate volcanic rocks deposited without significant breaks in volcanism. Felsic volcanic packages (Assemblage 4) are conformable with underlying units, but are spatially restricted. Discordant units of graphitic sedimentary rocks are developed along major crustal structures (Assemblage 5). SHRIMP and conventional U–Pb study of zircons reveal that felsic volcanic rocks of Assemblage 4 in the Dalgaranga Greenstone Belt were emplaced at 2747 ± 5 Ma, whereas those in the adjacent Meekatharra — Mt Magnet Greenstone Belt range in age from 2762 ± 6 to 2716 ± 4 Ma. The age of emplacement of a differentiated gabbro sill in the Dalgaranga Greenstone Belt at 2719 ± 6 Ma places a maximum age on major folding in the belt. The presence of 2.9–3.0 Ga inherited zircons in some of the felsic volcanic rocks indicates contamination with, or reworking of, underlying 3 Ga sialic crust. This distinguishes the Murchison Terrane from the central parts of the Eastern Goldfields terranes to the south, where there is no evidence for a 3 Ga imprint in zircons from volcanic or granitic rocks, and also from the Narryer Gneiss Terrane to the north and west, which is composed of older gneisses and granitoids. The ca 2.76–2.71 Ga felsic volcanism in the Murchison Terrane is significantly older than 2.71–2.67 Ga felsic volcanism in the Eastern Goldfields lending support to models advocating assemblage of the craton by terrane accretion.  相似文献   

11.
The Coompana Block is an essentially unknown basement province that separates the Gawler Craton of South Australia from the Yilgarn Craton of Western Australia. Previously unstudied granitic gneiss intersected by deep drilling in the Coompana Block represents an important period of within-plate magmatism during a time of relative magmatic quiescence in the Australian Proterozoic. Granitic gneiss intersected at ～1500 m depth in Mallabie 1 diamond drillhole is metaluminous and dominantly granodioritic in composition. The granodiorites have distinctive A-type chemistry characterised by high contents of Zr, Nb, Y, Ga, LREE with low Mg#, Sr, CaO and HREE. U – Pb LA-ICPMS dating of magmatic zircons provides an age of 1505 ± 7 Ma, interpreted as the crystallisation age of the granite protolith. ? Nd values are high with respect to exposed crust of the Musgrave Province and Gawler Craton, and range from +1.2 to +3.3 at 1.5 Ga. The granitic gneiss is interpreted to be a fractionated melt of a mantle-derived parental melt. The tectonic environment into which the precursor granite was emplaced is not clear. One possibility is emplacement within an extensional environment. Regardless, the granitic gneiss intersected in Mallabie 1 represents magmatic activity during the ‘Australian Mesoproterozoic magmatic gap’ of ca 1.50 – 1.35 Ga, and is a possible source for ca 1.50 detrital zircons found in sedimentary rocks of Tasmania and Antarctica, and metasedimentary rocks of the eastern Musgrave Province.  相似文献   

12.
Behavior of diorite under impact by variously-shaped projectiles   总被引：1，自引：0，他引：1  
SummaryBehavior of Diorite Under Impact by Variously-Shaped Projectiles The effects of striker diameter and tip geometry on the crater and crack network produced in diorite by normal projectile impact in the energy range from 4–30 J was investigated. Ejecta kinematics were determined by high-speed photography; elastic strain wave propagation was measured by embedded gages in a composite specimen; and the damage pattern was ascertained from an examination of the sectioned specimen. It was found that the projectile nose shape exerts a strong influence on the shape of the elastic transient, on the crater geometry, on the extent of the crack network and on the average size of the ejecta. The crater depth was found to be the most repeatable parameter in identical tests using the same striker and initial kinetic energy.With 7 Figures  相似文献   

13.
A new paleomagnetic pole position is obtained from the well-dated (636.3 ± 4.9 Ma) Nantuo Formation in the Guzhang section, western Hunan Province, and the correlative Long’e section in eastern Guizhou Province, South China. Remagnetization of the recent geomagnetic field was identified and removed for both sections. The hard dual-polarity, interpreted as primary, component of the Nantuo Formation, directs east–westward with medium inclinations, yielding an average pole of 9.3°N, 165°E, A95 = 4.3° that, for the first time, passed a strata-bound reversals test. The new data are consistent with previously published paleomagnetic data of the Nantuo Formation from Malong county, central Yunnan Province, which passed a positive syn-sedimentary fold test. Together, these sites represent shallow- to deep-water sections across a shelf-to-basin transect centered at ∼33° paleolatitude. The sedimentary basin may have faced an expansive ocean toward the paleo-East. In the ∼750 Ma and ∼635 Ma global reconstructions, the South China Block (SCB) was best fitted in the northern hemisphere close to northwestern Australia. However, a direct SCB-northwestern Australia connection, inferred to have existed during the Early Cambrian–Early Devonian, had not formed by the time of ∼635 Ma.  相似文献   

14.
Maar eruptions form small initially steep‐walled basins that contain important archives for the climatic and palaeoenvironmental history in continental areas. The two Ukinrek Maars in south‐western Alaska erupted between 30 March and 9 April 1977 and are the best‐documented maars that have erupted in historical time. This study presents a preliminary analysis of geomorphology, hydrology, magnetic susceptibility, geochemistry and sedimentology data of a field study in August 2004. These results, photographs and topographic surveys are combined for reconstructing the post‐eruptive evolution of Ukinrek East Maar. Within less than 30 years the initially polygonal shape of the crater with nearly vertical crater walls has developed into an almost elliptic form with slopes of 35° inclination on debris fans between a few escarpments. The water table and the crater floor have risen significantly and the crater diameter:depth ratio increased from 3·4 to 5·7, whereas the average height of the crater rim remained almost constant. The main sub‐aerial resedimentation process is formation of rock falls, rock slumps and scree resulting in debris flows and turbidites within the lake that is ice‐covered throughout about half of the year. Distal lake sediments consist of laminated minerogenic clayey‐sandy silts that document frequent turbidity currents. From the linear sedimentation rate of only ca 5 mm year?1 in 17 cm long cores it may be concluded that the largest portion of the crater sediments formed within the first few months of the maar history, however, this has to be confirmed by future studies.  相似文献   

15.
This research on an alluvial plain oasis in the middle reaches of the Heihe River used the trend estimation model of principal component analysis through fixed position experiments to investigate and analyze changes in saline meadow soil characteristics after reclamation of the oasis. The conclusion is that after cultivation for a number of years, clay content increased from 9.18% (before cultivation) to 12.93% (after 30 years of cultivation) and soil nutrient content increased as well, but available potassium decreased from 1,315.50 mg/kg (before cultivation) to 240.84 mg/kg (after 30 years of cultivation). As a result of engineering, biological, and agricultural methods, total salt content significantly decreased by 97.15% from 70.28 g/kg (before cultivation) to 2.00 g/kg (after 30 years of cultivation). Through the trend estimation model of principal components analysis (PCA), it was found that soil quality improved over time. Synthetic analysis of various indexes of soil quality revealed that meadow saline soil could be used for agricultural production with responsible tillage.  相似文献   

16.
Using orbital imaging radar, we detected a double circular structure, located in the southeastern part of the Libyan Desert, which is partially hidden under sandy sediments. Fieldwork confirmed it to be an unknown double impact crater, each crater having a diameter of about 10 km, younger than 140 Ma. Sampling on the site enabled the observation of quantities of shatter cone structures and impact breccias containing planar fractures. To cite this article: P. Paillou et al., C. R. Geoscience 335 (2003).  相似文献   

17.
The Holocene carbonate sequence of perennial North Stromatolite Lake, located adjacent to the Coorong Lagoon near Salt Creek, South Australia, includes a prominent sapropelic unit (7 – 12% total organic carbon), in places more than 2 m thick, that was sampled for the purpose of radiocarbon dating and documenting its diatom and ostracod biostratigraphy. The recovered ostracods were also subjected to carbon and oxygen isotopic analysis. The bulk organic matter at the base of the sapropel yielded an uncalibrated 14C age of 6080 ± 60 y BP. Diatoms, where preserved, are almost exclusively benthic. Stratigraphic variation of the proportions of key indicator species in diatom assemblages records a marked oscillation between oligosaline and eusaline conditions in the hypolimnion during deposition of the sapropel. Ostracod carbon isotope data indicate that the lake at this time was eutrophic, thereby enriching the dissolved inorganic carbon of the hypolimnion in 13C. However, the observed secular variation in δ13C implies a mid-sapropel drop in productivity, caused by a freshening of the lake. Ostracod δ18O values display an overall increase through the sapropel consistent with the rising salinity of the hypolimnion. The existence of a flourishing benthic ostracod community, together with the valve ornamentation of Osticythere baragwanathi, indicates that the bottom waters were well oxygenated. Thus, anoxia was not a prerequisite for sapropel accumulation. The biostratigraphy and chemostratigraphy of the sapropel concur in suggesting a lack of climatic uniformity during its deposition, a period of ～1200 years. This study therefore highlights the potential of diatoms and ostracods in shallow perennial alkaline lakes along the Coorong coastal plain as proxies for short-term (102 – 103 years) Holocene palaeoenvironmental change in southeastern Australia.  相似文献   

18.
This study analyzes the mechanism of the landslide event at Hsiaolin Village during Typhoon Morakot in 2009. This landslide event resulted in 400 deaths. The extremely high intensity and accumulative rainfall events may cause large-scale and complex landslide disasters. To study and understand a landslide event, a combination of field investigations and numerical models is used. The landslide area is determined by comparing topographic information from before and after the event. Physiographic parameters are determined from field investigations. These parameters are applied to a numerical model to simulate the landslide process. Due to the high intensity of the rainfall event, 1,675 mm during the 80 h before the landslide event, the water content of soil was rapidly increased causing a landslide to occur. According to the survivors, the total duration of the landslide run out was less than 3 min. Simulation results indicated that the total duration was about 150 s. After the landslide occurrence, the landslide mass separated into two parts by a spur at EL 590 in about 30 to 50 s. One part passed the spur in about 30 to 60 s. One part inundated the Hsiaolin Village and the other deposited at a local river channel and formed a landslide dam. The landslide dam had height between 50 and 60 m and length between 800 and 900 m. The simulation result shows that the proposed model can be used to evaluate the potential areas of landslides induced by extremely high intensity rainfall events.  相似文献   

19.
A partial record of the positions of Australia during Middle to Late Neoproterozoic time is provided by palaeomagnetic results for samples from the Lancer 1 stratigraphic drillhole in Western Australia. Lancer 1 was drilled vertically to 1501 m, through essentially horizontal Neoproterozoic strata of the western Officer Basin. We studied 123 samples from 28 intervals of drillcore which were oriented by matching features (fractures, cross-beds, etc.) in the core with oriented acoustic scanner images of the drillhole walls. Three new palaeopoles are reported for red mudstones and sandstones (redbeds) of the Browne (44.5°N, 141.7°E, dp = 5.1°, dm = 9.0°), Hussar (62.2°N, 85.8°E, dp = 7.3°, dm = 14.6°), and Kanpa (74.0°N, 128.8°E, dp = 7.7°, dm = 14.8°) Formations of the ca 830 – 720 Ma Buldya Group (Supersequence 1), which exhibit stable, two-polarity magnetisations carried by fine-grained hematite and magnetite. The overlying ca 610 – 590 Ma Wahlgu Formation glaciogenic diamictite (Supersequence 3) yielded dispersed directions and an imprecise palaeopole that overlaps results from the glaciogenic Elatina Formation and other Late Neoproterozoic rock units. The results help to elaborate the Middle to Late Neoproterozoic apparent polar wander path for Australia and indicate, in agreement with palaeoclimatic data and previous palaeomagnetic studies, that the continent was slow-moving and occupied low latitudes at this time. Assuming that Australia and Laurentia were still joined at ca 780 Ma, comparison of the new Hussar Formation palaeopole with coeval Laurentian data favours AUSMEX, rather than SWEAT or AUSWUS, as the most likely configuration of these two continents in Rodinia. This preliminary study of Lancer 1 demonstrates the utility of acoustic scanner logs for orienting drillcores, as well as the scope for additional sampling and palaeomagnetic studies of Lancer 1, and other oriented drillcores, to yield a more continuous record of Australia's past motions and to provide magnetostratigraphic data for enhancing inter-basin correlations.  相似文献   

20.
The alkali-feldspar and biotite in the sillimanite-biotite-garnet gneiss from East Antarctica preserves characteristic microstructural evidence of multi-stage H2O supplement during the retrograde metamorphism. The first microstructural evidence is the “zoned feldspar,” in which the mesoperthitic zone, the anti-perthitic zone, and lamella-free plagioclase zone coexist within a single crystal. They are occasionally found next to biotite, and are always depleted in orthoclase (Or) component toward the biotite. The formation process of this microstructure could be explained by the diffusion that oversteps the solvus. The second microstructural evidence is the serrate boundary between alkali-feldspar and biotite. The projections of biotite are selectively developed next to Or lamellae of alkali-feldspar every 3–5 μm. These two microstructures would have formed as the biotite grew by consuming potash in alkali-feldspar when H2O-bearing fluid locally passed through the grain boundaries. The former microstructure was formed at 825–900 °C before lamella formation, and the latter microstructure was formed after the lamella formation. These microstructures are the indicators of fluid pathways formed under two different temperature conditions. The common coexistence of these microstructures implies that the fluid used similar pathways during the retrograde metamorphism.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号