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1.
Åke Fagereng Chris Harris Mandy La Grange Gary Stevens 《Contributions to Mineralogy and Petrology》2008,155(1):63-78
The Vredefort dome in the Kaapvaal Craton was formed as a result of the impact of a large meteorite at 2.02 Ga. The central
core of Archaean granitic basement rocks is surrounded by a collar of uplifted and overturned strata of the Witwatersrand
Supergroup, exposing a substantial depth section of the Archaean crust. Orthogneisses of the core show little variation in
whole-rock δ
18O value, with the majority being between 8 and 10‰, with a mean of 9.2‰ (n = 35). Quartz and feldspar have per mil differences that are consistent with O-isotope equilibrium at high temperatures,
suggesting minimal interaction with fluids during subsequent cooling. These data refute previous suggestions that the Outer
Granite Gneiss (OGG) and Inlandsee Leucogranofels (ILG) of the core represent middle and lower crust, respectively. Granulite-facies
greenstone remnants from the ILG have δ
18O values that are on average 1.5‰ higher than the ILG host rocks and are unlikely, therefore, to represent the residuum from
the partial melting event that formed the host rock. Witwatersrand Supergroup sedimentary rocks of the collar, which were
metamorphosed at greenschist-to amphibolite-facies conditions, generally have lower δ
18O values than the core rocks with a mean value for metapelites of 7.7‰ (n = 45). Overall, through an ∼20 km thick section of crust, there is a general increase in whole-rock δ
18O value with increasing depth. This is the reverse of what is normal in the crust, largely because the collar rocks have δ
18O values that are unusually low in comparison with metamorphosed sedimentary rocks worldwide. The collar rocks have δD values ranging from −35 to −115‰ (average −62‰, n = 29), which are consistent with interaction with water of meteoric origin, having a δD of about −25 to −45‰. We suggest that fluid movement through the collar rocks was enhanced by impact-induced secondary permeability
in the dome structure.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
2.
Sukanta Roy Labani Ray Anurup Bhattacharya R. Srinivasan 《International Journal of Earth Sciences》2008,97(2):245-256
The Late Archaean Closepet Granite batholith in south India is exposed at different crustal levels grading from greenschist
facies in the north through amphibolite and granulite facies in the south along a ∼400 km long segment in the Dharwar craton.
Two areas, Pavagada and Magadi, located in the Main Mass of the batholith, best represent the granitoid of the greenschist
and amphibolite facies crustal levels respectively. Heat flow estimates of 38 mW m−2 from Pavagada and 25 mW m−2 from Magadi have been obtained through measurements in deep (430 and 445 m) and carefully sited boreholes. Measurements made
in four boreholes of opportunity in Pavagada area yield a mean heat flow of 39 ± 4 (s.d.) mW m−2, which is in good agreement with the estimate from deep borehole. The study, therefore, demonstrates a clear-cut heat flow
variation concomitant with the crustal levels exposed in the two areas. The mean heat production estimates for the greenschist
facies and amphibolite facies layers constituting the Main Mass of the batholith are 2.9 and 1.8 μW m−3, respectively. The enhanced heat flow in the Pavagada area is consistent with the occurrence of a radioelement-enriched 2-km-thick
greenschist facies layer granitoid overlying the granitoid of the amphibolite facies layer which is twice as thick as represented
in the Magadi area. The crustal heat production models indicate similar mantle heat flow estimates in the range 12–14 mW m−2, consistent with the other parts of the greenstone-granite-gneiss terrain of the Dharwar craton. 相似文献
3.
A portion of the aeromagnetic anomaly map of India, from 170 to 200 N and 78o to 84o E has been analysed to understand the tectonics of the region. The distribution of magnetic sources in the study region are clearly brought out in the analytic signal map and found to be associated with charnockitic rocks, iron formation and trap flows. The Godavari Graben is devoid of any magnetic sources. High-grade charnockitic rocks on surface and sub-surface, flank the shoulders of the Godavari Graben on either side. From the analysis of magnetic data, Sileru Shear Zone (SSZ) is identified as the contact of the Bastar craton and the Eastern Ghat Mobile Belt (EGMB). The Eastern Ghat is divided into two blocks: Block-N north of Srikakulam is devoid of magnetic sources while the charnockitic rocks are the main magnetic carriers in Block-S. The difference in magnetic characteristics of the two blocks has been attributed to the difference in metamorphic history. Block-N has an over print of amphibolite facies metamorphism while Block-S to the south depicts granulite facies metamorphism. The Euler solutions within the EGMB shows that the magnetic sources along SSZ is shallower than the south east implying that the exhumation process in the EGMB has a differential rate. 相似文献
4.
In the Dharwar tectonic province, the Peninsular Gneiss was considered to mark an event separating the deposition of the older
supracrustal Sargur Group and the younger supracrustal Dharwar Supergroup. Compelling evidence for the evolution of the Peninsular
Gneiss, a polyphase migmatite, spanning over almost a billion years from 3500 Ma to 2500 Ma negates a stratigraphic status
for this complex, so that the decisive argument for separating the older and younger supracrustal groups loses its basis.
Correlatable sequence of superposed folding in all the supracrustal rocks, the Peninsular Gneiss and the banded granulites,
indicate that the gneiss ‘basement’ deformed in a ductile manner along with the cover rocks. An angular unconformity between
the Sargur Group and the Dharwar Super-group, suggested from some areas in recent years, has been shown to be untenable on
the basis of detailed studies, A number of small enclaves distributed throughout the gneissic terrane, with an earlier deformational,
metamorphic and migmatitic history, provide the only clue to the oldest component which has now been extensively reworked. 相似文献
5.
Geological setting and chemistry of kimberlite clan rocks in the Dharwar Craton, India 总被引:2,自引:0,他引:2
Diamond exploration in India over the past decade has led to the discovery of over 80 kimberlite-inferred and lamproite-related intrusions in three of the four major Archean cratons that dominate the subcontinent. These intrusions are Proterozoic (1.1 Ga), and are structurally controlled: locally (at the intersections of faults); regionally (in a 200 km wide, 1000 km long diamond corridor); and globally (in the reconstructed supercontinent of Rodinia). The geochemistry of 57 samples from 13 intrusions in the southern Dharwar Craton of Andhra Pradesh has been determined by XRF spectrometry. The bodies are iron-rich with mg#=50–70 and are neither archetypal kimberlites nor ideal lamproites; this may be the underlying reason that conventional exploration techniques have thus far failed to locate the primary sources of India's historically famous diamonds. The two major fields of kimberlite-clan rocks (KCR) in the Dharwar Craton, Wajrakur and Narayanpet, are separated by a NW–SE trending, transcontinental (Mumbai-Chennai) gravity lineament. About 80% of intrusions in Wajrakur are diamondiferous, but diamonds have not yet been reported in Narayanpet. The gravity anomaly may mark the boundary of an architectural modification in the keel of the sub-continental lithosphere, a suggestion that is supported by differences in kimberlite mineralogy, chemistry, mantle xenoliths, structural setting and crustal host rocks. 相似文献
6.
H. N. Bhattacharya Abhijit Chakraborty Biplab Bhattacharya 《Journal of Earth System Science》2005,114(3):275-286
Basal part of the Gondwana Supergroup represented by Talchir and Karharbari Formations (Permo-Carboniferous) records an abrupt
change-over from glacio-marine to terrestrial fluviolacustrine depositional environment. The contact between the two is an
unconformity. Facies analysis of the glacio-marine Talchir Formation reveals that basal glaciogenic and reworked glaciogenic
sediments are buried under storm influenced inner and outer shelf sediments. Facies associations of the Karharbari Formation
suggest deposition as fluvio-lacustrine deposits in fault-controlled troughs. An attempt has been made in this paper to explain
the sedimentation pattern in Talchir and Karharbari basins, and the abrupt change-over from glacio-marine to terrestrial fluviolacustrine
depositional environment in terms of glacio-isostacy. 相似文献
7.
The earliest decipherable record of the Dharwar tectonic province is left in the 3.3 Ga old gneissic pebbles in some conglomerates
of the Dharwar Group, in addition to the 3.3–3.4 Ga old gneisses in some areas. A sialic crust as the basement for Dharwar
sedimentation is also indicated by the presence of quartz schists and quartzites throughout the Dharwar succession. Clean
quartzites and orthoquartzite-carbonate association in the lower part of the Dharwar sequence point to relatively stable platform
and shelf conditions. This is succeeded by sedimentation in a rapidly subsiding trough as indicated by the turbidite-volcanic
rock association. Although conglomerates in some places point to an erosional surface at the contact between the gneisses
and the Dharwar supracrustal rocks, extensive remobilization of the basement during the deformation of the cover rocks has
largely blurred this interface. This has also resulted in accordant style and sequence of structures in the basement and cover
rocks in a major part of the Dharwar tectonic province. Isoclinal folds with attendant axial planar schistosity, coaxial open
folds, followed in turn by non-coaxial upright folds on axial planes striking nearly N-S, are decipherable both in the “basement”
gneisses and the schistose cover rocks. The imprint of this sequence of superposed deformation is registered in some of the
charnockitic terranes also, particularly in the Biligirirangan Hills, Shivasamudram and Arakalgud areas. The Closepet Granite,
with alignment of feldspar megacrysts parallel to the axial planes of the latest folds in the adjacent schistose rocks, together
with discrete veins of Closepet Granite affinity emplaced parallel to the axial planes of late folds in the Peninsular Gneiss
enclaves, suggest that this granite is late-tectonic with reference to the last deformation in the Dharwar tectonic province.
Enclaves of tonalite and migmatized amphibolite a few metres across, with a fabric athwart to and overprinted by the earliest
structures traceable in the supracrustal rocks as well as in a major part of the Peninsular Gneiss, point to at least one
deformation, an episode of migmatization and one metamorphic event preceding the first folding in the Dharwar sequence. This
record of pre-Dharwar deformation and metamorphism is corroborated also by the pebbles of gneisses and schists in the conglomerates
of the Dharwar Group.
Volcanic rocks within the Dharwar succession as well as some of the components of the Peninsular Gneiss give ages of about
3.0 Ga. A still younger age of about 2.6 Ga is recorded in some volcanic rocks of the Dharwar sequence, a part of the Peninsular
Gneiss, Closepet Granite and some charnockites. These, together with the 3.3 Ga old gneisses and 3.4 Ga old ages of zircons
in some charnockites, furnish evidence for three major thermal events during the 700 million year history of the Archaean
Dharwar tectonic province. 相似文献
8.
Gregory T. Spanski 《Natural Resources Research》1992,1(2):163-183
Faced with an ever-increasing diversity of demand for the use of public lands, managers and planners are turning more often to a multiple-use approach to meet those demands. This approach requires the uses to be mutually compatible and to utilize the more valuable attributes or resource values of the land. Therefore, it is imperative that planners be provided with all available information on attribute and resource values in a timely fashion and in a format that facilitates a comparative evaluation.The Kootenai National Forest administration enlisted the U.S. Geological Survey and U.S. Bureau of Mines to perform a quantitative assessment of future copper/silver production potential within the forest from sediment-hosted copper deposits in the Revett Formation that are similar to those being mined at the Troy Mine near Spar Lake. The U.S. Geological Survey employed a quantitative assessment technique that compared the favorable host terrane in the Kootenai area with worldwide examples of known sediment-hosted copper deposits. The assessment produced probabilistic estimates of the number of undiscovered deposits that may be present in the area and of the copper and silver endowment that might be contained in them.Results of the assessment suggest that the copper/silver deposit potential is highest in the southwestern one-third of the forest. In this area there is an estimated 50 percent probability of at least 50 additional deposits occurring mostly within approximately 260,000 acres where the Revett Formation is thought to be present in the subsurface at depths of less than 1,500 meters. A Monte Carlo type simulation using data on the grade and tonnage characteristics of other known silver-rich, sediment-hosted copper deposits predicts a 50 percent probability that these undiscovered deposits will contain at least 19 million tonnes of copper and 100,000 tonnes of silver. Combined with endowments estimated for identified, but not thoroughly explored deposits, and deposits that might also occur in the remaining area of the forest, the endowment potential increases to 23 million tonnes of copper and 190,000 tonnes of silver. 相似文献
9.
E. A. Jarzembowski 《Cretaceous Research》1995,16(6)
The first fossil caddis cases from the Early Cretaceous (Wealden) of SE England are described and named Conchindusia rasnitsyni ichnosp. nov., Piscindusia sukachevae ichnogen. et isp. nov., ?Ostracindusia vyalovi ichnosp. nov. and Pelindusia percealleni ichnosp. nov. In addition, the necrotauliid caddisfly Paratrichopteridium purbeckianum (Handlirsch) comb. nov. from the earliest Cretaceous (Purbeck Group) of Dorset is redescribed. 相似文献
10.
The Upper Kaimur Group of the Vindhyan Supergroup in Central India, primarily consists of three rock types-DhandraulSandstone,
Scarp Sandstone and Bijaigarh Shale. Mineralogically and geochemically, they are quartz arenite, sublitharenite to litharenite
and litharenite to shale in composition, respectively. The A-CN-K ternary plot and CIA and ICV values suggest that the similar
source rocks suffered severe chemical weathering, under a hot-humid climate in an acidic environment with higher P
CO
2, which facilitated high sediment influx in the absence of land plants. Various geochemical discriminants, elemental ratios
like K2O/Na2O, Al2O3/TiO2, SiO2/MgO, La/Sc, Th/Sc, Th/Cr, GdN/YbN and pronounced negative Eu anomalies indicate the rocks to be of post-Archean Proterozoic granitic source, with a minor contribution
of granodioritic input, in a passive margin setting. The sediments of the Upper Kaimur Group were probably deposited in the
interglacial period in between the Paleoproterozoic and Neoproterozoic glacial epochs. 相似文献