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1.
Simulation of chloride migration rates in Paleo Pennar Delta Region, Coastal Andhra Pradesh, India 总被引:1,自引:0,他引:1
Quaternary alluvial aquifers in the paleo Pennar delta region of Andhra Pradesh (Long. 80°0′ and 80°12′; Lat. 14°40′ and
14°20′) constitute an important coastal strip, with potential fresh groundwater resources and several well fields in operation.
The lineament patterns and traces of paleo channels provide basic information on the configuration and boundaries of paleo
delta and the possible neotectonic movements in the region. The paleo delta region is essentially characterized by freshwater
aquifer systems at the near-surface depths, (up to 50 m from ground level) with transmissivity values in the system ranging
from 1200 to 2500 m2/day. This groundwater system has been subjected to heavy draft situation over the last two decades, parlicularly near the
outer rim of the delta where an unlined brackish-water canal runs parallel to the coast imparting chloride contamination to
the adjoining fresh aquifers. A two-dimensional solute transport model solution was applied to assess chloride migration rates
inland under different hydraulic stresses, combining finite difference solution of flow equation and the method of characteristic
solution of solute transport equation. Groundwater flow and chloride migration patterns/rates were obtained for different
simulated stress events in the delta system, and measures required to protect the freshwater resources ara outlined.
Received: 2 January 1997 · Accepted: 4 November 1997 相似文献
2.
The major chemical components of bauxite deposits of Belgaum (76° 24′E : 15° 54′N) and Yercaud (78° 14′E : 11° 48′N) areas
have been determined. A chemical continuity between parent rocks (zone I) to bauxites (zone IV) via clay (zone II) and laterites
(zone III) clearly indicates that bauxites have been derived byin situ weathering of the respective parent rocks. 相似文献
3.
Abhishek Saha Sohini Ganguly Jyotisankar Ray Nilanjan Chaterjee 《Journal of Earth System Science》2010,119(5):675-699
The Samchampi-Samteran alkaline complex occurs as a plug-like pluton within the Precambrian granite gneisses of Mikir Hills,
Assam, northeastern India and it is genetically related to Sylhet Traps. The intrusive complex is marked by dominant development
of syenite within which ijolitemelteigite suite of rocks is emplaced with an arcuate outcrop pattern. Inliers of alkali pyroxenite
and alkali gabbro occur within this ijolite-melteigite suite of rocks. The pluton is also traversed by younger intrusives
of nepheline syenite and carbonatite. Development of sporadic, lumpy magnetite ore bodies is also recorded within the pluton.
Petrographic details of the constituent lithomembers of the pluton have been presented following standard nomenclatorial rules.
Overall pyroxene compositions range from diopside to aegirine augite while alkali feldspars are typically orthoclase and plagioclase
in syenite corresponds to oligoclase species. Phase chemistry of nepheline is suggestive of Na-rich alkaline character of
the complex. Biotite compositions are typically restricted to a uniform compositional range and they belong to ‘biotite’ field
in the relevant classification scheme. Garnets (developed in syenite and melteigite) typically tend to be Ti-rich andradite,
which on a closer scan can be further designated as melanites. Opaque minerals mostly correspond to magnetite. Use of Lindsley’s
pyroxene thermometric method suggests an equilibration temperature from ∼450°–600°C for melteigite/alkali gabbro and ∼400°C
for syenite. Critical assessment of other thermometric methods reveals a temperature of equilibration of ∼700°–1350°C for
ijolite-melteigite suite of rocks in contrast to a relatively lower equilibration temperature of ∼600°C for syenite. Geobarometric
data based on pyroxene chemistry yield an equilibration pressure of 5.32–7.72 kb for ijolite, melteigite, alkali pyroxenite,
alkali gabbro and nepheline syenite. The dominant syenite member of the intrusive plug records a much higher (∼11 kb) equilibration
pressure indicating a deeper level of intrusion. Major oxide variations of constituent lithomembers with respect to differentiation
index (D.I.) corroborate a normal magmatic differentiation. A prominent role of liquid immiscibility is envisaged from field
geological, petrographic and petrochemical evidences. Tectonic discrimination diagrams involving clinopyroxene chemistry strongly
suggest within plate alkaline affinity for the parental magma which is in conformity with the regional plume tectonics. 相似文献
4.
The Wittichen Co–Ag–Bi–U mining area (Schwarzwald ore district, SW Germany) hosts several unconformity-related vein-type mineralizations
within Variscan leucogranite and Permian to Triassic redbeds. The multistage mineralization formed at the intersection of
two fault systems in the last 250 Ma. A Permo-Triassic ore stage I with minor U–Bi–quartz–fluorite mineralization is followed
by a Jurassic to Cretaceous ore stage II with the main Ag and Co mineralization consisting of several generations of gangue
minerals that host the sub-stages of U–Bi, Bi–Ag, Ni–As–Bi and Co–As–Bi. Important ore minerals are native elements, Co and
Ni arsenides, and pitchblende; sulphides are absent. The Miocene ore stage III comprises barite with the Cu–Bi sulfosalts
emplectite, wittichenite and aikinite, and the sulphides anilite and djurleite besides native Bi, chalcopyrite, sphalerite,
galena and tennantite. The mineral-forming fluid system changed from low salinity (<5 wt.% NaCl) at high temperature (around
300°C) in Permian to highly saline (around 25 wt.% NaCl + CaCl2) at lower temperatures (50–150°C) in Triassic to Cretaceous times. Thermodynamic calculations and comparison with similar
mineralizations worldwide show that the Mesozoic ore-forming fluid was alkaline with redox conditions above the hematite–magnetite
buffer. We suggest that the precipitation mechanism for native elements, pitchblende and arsenides is a decrease in pH during
fluid mixing processes. REE patterns in fluorite and the occurrence of Bi in all stages suggest a granitic source of some
ore-forming elements, whereas, e.g. Ag, Co and Ni probably have been leached from the redbeds. The greater importance of Cu
and isotope data indicates that the Miocene ore stage III is more influenced by fluids from the overlying redbeds and limestones
than the earlier mineralization stages. 相似文献
5.
Sampa Hazra Pritam Saha Jyotisankar Ray Avijit Podder 《Journal of the Geological Society of India》2010,75(5):760-768
The porphyritic granite body (∼ 600 Ma) around Mylliem (25°32′N: 91°52′E), east Khasi hills district, Meghalaya occurs as
a distinct intrusive body into the host rock of low- grade meta-sediments belonging to Proterozoic Shillong Group. The porphyritic
granite body manifests prominent lath- shaped feldspar phenocrysts giving rise to dominant porphyritic texture. Further, this
porphyritic granite body is characterized by primary foliation (defined by parallely arranged feldspar laths), tongues and
appophyses and xenoliths of older metamorphics. Petrographically, the Mylliem porphyritic granite is grey leucocratic, coarse
grained, phanerocrystalline with modal variants ranging from granite to granodiorite tending to tonalite. Simple statistical
studies based on correlation coefficient values involving modal variables of the Mylliem porphyritic granite bodies appear
significant in majority of the cases indicating magmatic crystallization. Use of relevant thermometric method indicates temperature
of equilibration of the body in the range of 369°C to 507°C. The crystallization of the Mylliem porphyritic granite pluton
initiated at an average lithostatic pressure in the tune of ∼11 Kb followed by dominant PH2O controlled milieu. Spatially projected mineralogical parameters suggest a general trend of the magmatic cooling of the body
from margin inward with sudden influx of volatile matters that occasionally offsets mineralogical trends. 相似文献
6.
The portion of the FeSO system including pyrrbotite,wüsite, magnetite, and iron has been studied between 900and 1080 °C by modifed silica-tube techniques. At 900 °C,tie lines extend from pyrrhotite containing between 63.53 and62.8±0.2 wt. per cent Fe to wüstite solid solutionand form pyrrhotite containing between 62.8 and 60.0 wt. percent Fe to magnetite. A ternary eutectic, troilite-wüstite-iron-liquidoccurs at 915±2 °C. A ternary invariant point, wherepyrrhotite (composition 62.8±0.2 wt. per cent Fe)+wüstite magnetite+liquid occurs at 934 °C. Pyrrohotite compositionstrongly influences the temperature of thee magnetite-pyrrhotitesolidus. Magnetite-pyrrhotite assemblages begin to melt at 934°C when the pyrrhotite contains 62.8 wt. lper cent Fe, at1010 °C when it contains 62.5 wt. per cent Fe, at 1030 °Cwhen it contains 62.0 wt. per cent Fe, and at 1050 °C whenit contains 60.5 wt. per cent Fe. Craig & Naldrett (1967) have shown that up to 20 wt. percent nickel substituting for iron in pyrrhotite solid solutionon a weight per cent basis has little effect on magnetite-pyrrhotitesolidus temperature and that up to 2 wt. per cent copper substitutingin a sunukar way lowers the solidus less than 20 °C. Byredetermining the solidus in the presence of H2O at 2 kb totalpressure Naldrett & Ricahrdson (1967) have show that, withinexperimental accuracy (± 10 °C), water has no effecton melting temperature. since natural iron-sulfide magmas rarelycrystallize pyrrhotites containing more than 62.5 wt. per centtotal metal, the temperature range of from 1010 to 1050 °Cdetermined in this study is probably within 20 °C of theminimum temperature of introduction of a large number of magnaticsulfide ores. Comparison of the melting temperatures of ores with those ofthe rocks with which they are associated suggest that crystallizationunder different water pressures is responsible for the presenceof sulfides disseminated as rounded buck-shottype spherules in some rocks ans as an interstittial fillingin others. The composition of an iron sufide-oxide ore magma settling fromits associated silicate magma is controlled by the sulfur andoxygen fugacities of the silicate magma at the moment when equilibrationbetween the two ceases. In the case of large bodies of massivesulfide ore, equilibration probably ceased when the ore settledout of its host; the sulfide to magnetite ratio of such orewill depen on how far below its liquidus temperature the sulfide-oxideliquid was at the moment of separation. In the case of sulfide-richdroplets remaining disseminated throughtot the plutonic hostrock, equilibration probably continued to subsolidus temperatures;under these conditions it is possible that the droplets maylose all of their oxygen to the host rock. Finally in the caseof sulfife-rich droplets trapped within rapidly cooled volcanicrocks complete re-equilibration was probably prevented by therate of cooling and consequently these droplets retain muchof their original oxygen as magnetite. 相似文献
7.
马兰庄铁矿属鞍山式沉积变质铁矿。以往的工作证实,矿体受倒转向斜构造控制,向斜走向30°,轴面倾向NW,倾角70°~80°;向斜的NE端翘起,倾伏方向SW,倾角18°。近几年,为探测深部矿体赋存情况,投入了地、物研究工作,并进行钻探验证。各项工作表明沙河山矿段深部赋存隐伏矿体,控矿构造形态与先前的认识相比略有变化。综合新获得信息和前人成果,对该区控矿构造进行再认识,初步认为沙河山段矿体受叠加倒转向斜控制,隐伏矿体与浅部矿体间的关系可用"同一矿层叠加两期褶皱作用"来解释。基本褶皱形态——倒转向斜的转折端位置应在原有认识基础上向西偏移,向斜深部转折端矿体与原来认识相比增厚。若此控矿构造新认识经探矿工程证实,预期将为矿山增加资源量约3 000×104 t。 相似文献
8.
At Malanjkhand, Central India, lode-type copper (-molybdenum) mineralization occurs within calcalkaline tonalite-granodiorite
plutonic rocks of early Proterozoic age. The bulk of the mineralization occurs in sheeted quartz-sulfide veins, and K-silicate
alteration assemblages, defined by alkali feldspar (K-feldspar ≫ albite) + dusty hematite in feldspar ± biotite ± muscovite,
are prominent within the ore zone and the adjacent host rock. Weak propylitic alteration, defined by albite + biotite + epidote/zoisite,
surrounds the K-silicate alteration zone. The mineralized zone is approximately 2 km in strike length, has a maximum thickness
of 200 m and dips 65°–75°, along which low-grade mineralization has been traced up to a depth of about 1 km. The ore reserve
has been conservatively estimated to be 92 million tonnes with an average Cu-content of 1.30%. Supergene oxidation, accompanied
by limited copper enrichment, is observed down to a depth of 100m or more from the surface. Primary ores consist essentially
of chalcopyrite and pyrite with minor magnetite and molybdenite. δ34S (‰) values in pyrite and chalcopyrite (−0.38 to +2.90) fall within the range characteristic of granitoid-hosted copper deposits.
δ18O (‰) values for vein quartz (+ 6.99 to +8.80) suggest exclusive involvement of juvenile water. Annealed fabrics are common
in the ore. The sequence of events that led to the present state of hypogene mineralization is suggested to be as follows:
fracturing of the host rock, emplacement of barren vein quartz, pronounced wall-rock alteration accompanied by disseminated
mineralization and the ultimate stage of intense silicification accompanied by copper mineralization. Fragments of vein quartz
and altered wall rocks and striae in the ore suggest post-mineralization deformation. The recrystallization fabric, particularly
in chalcopyrite and sphalerite, is a product of dynamic recrystallization associated with the post-mineralization shearing.
The petrology of the host rocks, hydrothermal alteration assemblages, ore mineral associations, fluid inclusions and the sulfur
and oxygen isotopes of ores are comparable to those in Phanerozoic (and reported Precambrian) porphyry-copper systems, and
the Malanjkhand deposit has important implications for both metallogenic models for, and mineral exploration in, Precambrian
terrains. 相似文献
9.
The thermal stabilities and decay kinetics of three peroxy radicals (Centers #1, B and B′) and three other radiation-induced
defects (#3, C′ and E1′) in natural quartz from the high-grade McArthur River uranium deposit (Athabasca basin, Canada) have been investigated by
isochronal and isothermal annealing experiments and electron paramagnetic resonance (EPR) spectroscopy. Single-crystal EPR
spectra of isochronally (2 h) annealed quartz show that these centers all grow in intensity to 280°C and then decay with further
increase in temperature, but their disappearance temperatures differ markedly and depend on the initial concentrations (e.g.,
Center #1 in a dark smoky quartz is annealed out at 380°C, B and B′ at 420°C and #3 and C′ at 580°C). The isothermal decay
processes of these centers are all of the second order type. The calculated activation energies for the peroxy radicals [#1
and B + B′ at 0.36 (9) and 0.83 (8) eV, respectively] are smaller than those of Centers #3, C′ and E1′ [1.09 (8), 1.24 (8) and 1.45 (7) eV, respectively]. Gamma-ray irradiations of thermally bleached quartz restore a fraction
of the peroxy radicals, suggesting that their diamagnetic precursors are stable up to at least 800°C. The unusual decay characteristics
of “peroxy radicals” in quartz reported in the literature are shown to most likely arise from multiple radiation-induced defects.
These results have implications for not only applications of peroxy radicals in quartz for EPR dating but also better understanding
of thermoluminescence and cathodoluminescence spectra of this mineral. 相似文献
10.
The effects of metamorphism on O and Fe isotope compositions in the Biwabik Iron Formation, northern Minnesota 总被引:3,自引:0,他引:3
Elizabeth Valaas Hyslop John W. Valley Clark M. Johnson Brian L. Beard 《Contributions to Mineralogy and Petrology》2008,155(3):313-328
The Biwabik Iron Formation of Minnesota (1.9 Ga) underwent contact metamorphism by intrusion of the Duluth Complex (1.1 Ga).
Apparent quartz–magnetite oxygen isotope temperatures decrease from ∼700°C at the contact to ∼375°C at 2.6 km distance (normal
to the contact in 3D). Metamorphic pigeonite at the contact, however, indicates that peak temperatures were greater than 825°C.
The apparent O isotope temperatures, therefore, reflect cooling, and not peak metamorphic conditions. Magnetite was reset
in δ18O as a function of grain size, indicating that isotopic exchange was controlled by diffusion of oxygen in magnetite for samples
from above the grunerite isograd. Apparent quartz–magnetite O isotope temperatures are similar to calculated closure temperatures
for oxygen diffusion in magnetite at a cooling rate of ∼5.6°C/kyr, which suggests that the Biwabik Iron Formation cooled from
∼825 to 400°C in ∼75 kyr at the contact with the Duluth Complex. Isotopic exchange during metamorphism also occurred for Fe,
where magnetite–Fe silicate fractionations decrease with increasing metamorphic grade. Correlations between quartz–magnetite
O isotope fractionations and magnetite–iron silicate Fe isotope fractionations suggest that both reflect cooling, where the
closure temperature for Fe was higher than for O. The net effect of metamorphism on δ18O–δ56Fe variations in magnetite is a strong increase in δ18OMt and a mild decrease in δ56Fe with increasing metamorphic grade, relative to the isotopic compositions that are expected at the low temperatures of initial
magnetite formation. If metamorphism of Iron Formations occurs in a closed system, bulk O and Fe isotope compositions may
be preserved, although re-equilibration among the minerals may occur for both O and Fe isotopes.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
11.
Most skarn deposits are closely related to granitoids that intruded into carbonate rocks. The Cihai (>100 Mt at 45% Fe) is a deposit with mineral assemblages and hydrothermal features similar to many other typical skarn deposits of the world. However, the iron orebodies of Cihai are mainly hosted within the diabase and not in contact with carbonate rocks. In addition, some magnetite grains exhibit unusual relatively high TiO2 content. These features are not consistent with the typical skarn iron deposit. Different hydrothermal and/or magmatic processes are being actively investigated for its origin. Because of a lack of systematic studies of geology, mineral compositions, fluid inclusions, and isotopes, the genetic type, ore genesis, and hydrothermal evolution of this deposit are still poorly understood and remain controversial.The skarn mineral assemblages are the alteration products of diabase. Three main paragenetic stages of skarn formation and ore deposition have been recognized based on petrographic observations, which show a prograde skarn stage (garnet-clinopyroxene-disseminated magnetite), a retrograde skarn stage (main iron ore stage, massive magnetite-amphibole-epidote ± ilvaite), and a quartz-sulfide stage (quartz-calcite-pyrite-pyrrhotite-cobaltite).Overall, the compositions of garnet, clinpyroxene, and amphibole are consistent with those of typical skarn Fe deposits worldwide. In the disseminated ores, some magnetite grains exhibit relatively high TiO2 content (>1 wt.%), which may be inherited from the diabase protoliths. Some distinct chemical zoning in magnetite grains were observed in this study, wherein cores are enriched in Ti, and magnetite rims show a pronounced depletion in Ti. The textural and compositional data of magnetite confirm that the Cihai Fe deposit is of hydrothermal origin, rather than associated with iron rich melts as previously suggested.Fluid inclusions study reveal that, the prograde skarn (garnet and pyroxene) formed from high temperature (520–600 °C), moderate- to high-salinity (8.1–23.1 wt.% NaCl equiv, and >46 wt.% NaCl equiv) fluids. Massive iron ore and retrograde skarn assemblages (amphibole-epidote ± ilvaite) formed under hydrostatic condition after the fracturing of early skarn. Fluids in this stage had lower temperature (220°–456 °C) and salinity (8.4–16.3 wt.% NaCl equiv). Fluid inclusions in quartz-sulfide stage quartz and calcite also record similar conditions, with temperature range from 128° to 367 °C and salinity range from 0.2 to 22.9 wt.% NaCl equiv. Oxygen and hydrogen isotopic data of garnet and quartz suggest that mixing and dilution of early magmatic fluids with external fluids (e.g., meteoric waters) caused a decrease in fluid temperature and salinity in the later stages of the skarn formation and massive iron precipitation. The δ18O values of magnetite from iron ores vary between 4.1 and 8.5‰, which are similar to values reported in other skarn Fe deposits. Such values are distinct from those of other iron ore deposits such as Kiruna-type and magmatic Fe-Ti-V deposits worldwide. Taken together, these geologic, geochemical, and isotopic data confirm that Cihai is a diabase-hosted skarn deposit related to the granitoids at depth. 相似文献
12.
The Kalahari Goldridge Mine is located within the Archaean Kraaipan Greenstone Belt, about 60 km southwest of Mafikeng in the North West Province, South Africa. The ore body thickness varies from 15 to 45 m along a strike length of about 1.5 km within approximately N–S striking banded iron formation (BIF). The stratabound ore body is hosted primarily by BIF, which consists of alternating chert and magnetite–chlorite–stilpnomelane–sulphide–carbonate bands of millimetre- to centimetre scale. A footwall of sericite–carbonate–chlorite schist underlain by mafic amphibolite occurs to the west and carbonaceous metapelites in the hanging wall to the east. Overlying the hanging wall, carbonaceous metapelites, units of coarse-grained metagreywackes fining upwards, become increasingly conglomeratic up the stratigraphy. Small-scale isoclinal folds, brecciation, extension fractures and boudinage of cherty BIF units reflect brittle-ductile deformation. Fold axial planes have foliation, with subvertical plunges parallel to prominent rodding and mineral lineation in the footwall rocks. Gold mineralisation is associated with two generations of quartz–carbonate veins, dipping approximately 20° to 40° W. The first generation consists of ladder-vein sets (group IIA) preferentially developed in centimetre-scale Fe-rich mesobands, whereas the second generation consists of large quartz–carbonate veins (group IIB), which locally crosscut the entire ore body and extend into the footwall and hanging wall. The ore body is controlled by mesoscale isoclinal folds approximately 67° E, orthogonal to the plane of mineralised, gently dipping veins, defining the principal stretching direction and development of fluid-focussing conduits. The intersections of the mineralised veins and foliation planes of the host rock plunges approximately 08° to the north. Pervasive hydrothermal alteration is characterised by chloritisation, carbonatisation, sulphidation and K-metasomatism. Gold is closely associated with sulphides, mainly pyrite and pyrrhotite, and to a lesser extent, with bismuth tellurides and carbonate minerals. Mass balance transfer calculations indicate that hydrothermal alteration of BIF involved enrichment of Au, Ag, Bi, Te, S and CO2 (LOI), MgO, Ba, K and Rb, but significant depletion of SiO2 and, to a lesser extent, Fe2O3. Extensive replacement of magnetite and chlorite in BIF and other pelitic sedimentary rocks by sulphide and carbonate minerals, both on mesoscopic and microscopic scales, is evidence of interaction of CO2- and H2S-bearing fluids with the Fe-rich host rocks. The fineness of gold grains ranges from 823 to 921, similar to that of other epigenetic Archaean BIF-hosted gold deposits, worldwide. 相似文献
13.
Dipak Ghosh Argha Deb Swarup Ranjan Sahoo Subrata Haldar Rosalima Sengupta 《Natural Hazards》2011,58(3):877-889
This paper reports the measurement of radon concentration in well water at the site of Jalpaiguri (26°32′N, 88°46′E) near
the active fault zone of West Bengal, India. Radon concentration has been measured in well water with the help of solid-state
nuclear track detectors (SSNTD). The study indicates a positive correlation between radon anomaly and earthquake. The data
of radon content in well water have been compared with that in soil gas at the same site. 相似文献
14.
A. Kontny R. Engelmann J. C. Grimmer R. O. Greiling A. Hirt 《International Journal of Earth Sciences》2012,101(3):671-692
Magnetite-bearing mylonitic garnet–micaschists close to the major suture between the Baltica and Iapetus terranes (Seve Nappe
Complex, Scandinavian Caledonides) show very high anisotropy of magnetic susceptibility (AMS) with corrected degree of anisotropy
(P′) up to 4.8. Three different magnetic fabric types can be distinguished. They correspond to protomylonite (type I, P′ < 2), mylonite (type II, 2 < P′ < 3), and ultramylonite (type III, P′ > 3), respectively. The orientation of the ellipsoid axes from all applied magnetic fabric methods in this study is similar
with shallow dips of the metamorphic foliation toward WSW and subhorizontal, mostly NW–SE trending mineral lineation. Differences
between subfabrics were minimized under high shear strain as all markers tend to align parallel with the shear plane. The
very high anisotropies and mostly oblate ellipsoid shapes of type III correlate with high magnetic susceptibility (k
mean up to 55 × 10−3 SI units) and are related to the concentration of magnetite aggregates with shape-preferred orientation. They show a distinct
field dependence of magnetic susceptibility of up to 10% in the k
max-direction. We attribute this field dependence to a “memory” of high strains in the domain walls of the crystals acquired
during synkinematic magnetite growth during shear zone fabric development at temperatures of 550–570°C. 相似文献
15.
Abhishek Saha Avik Dhang Jyotisankar Ray Suvankar Chakraborty David Moecher 《Journal of Earth System Science》2010,119(3):365-381
Field studies supplemented by petrographic analyses clearly reveal complete preservation of ophiolite suite from Port Blair
(11°39′N: 92°45′E) to Chiriyatapu (11°30′24″N: 92°42′30″E) stretch of South Andaman. The ophiolite suite reveals serpentinite
at the base which is overlain unconformably by cumulate ultramafic-mafic members with discernible cumulus texture and igneous
layering. Basaltic dykes are found to cut across the cumulate ultramafic-mafic members. The succession is capped by well exposed
pillow basalts interlayered with arkosic sediments. Olivine from the basal serpentinite unit are highly magnesian (Fo80.1–86.2). All clinopyroxene analyses from cumulate pyroxenite, cumulate gabbro and basaltic dyke are discriminated to be ‘Quad’ and
are uniformly restricted to the diopside field. Composition of plagioclase in different lithomembers is systematically varying
from calcic to sodic endmembers progressively from cumulate pyroxenite to pillow basalt through cumulate gabbro and basaltic
dyke. Plagioclase phenocrysts from basaltic dyke are found to be distinctly zoned (An60.7-An35.3) whereas groundmass plagioclase are relatively sodic (An33-An23.5). Deduced thermobarometric data from different lithomembers clearly correspond to the observed preservation of complete ophiolite
suite. 相似文献
16.
VHF amplitude scintillations recorded during the daytime period from January 1991 to December 1993, April 1998 to December
1999 and January 2008 to December 2008 at low latitude station Varanasi (geographic lat. = 25°15′N; long. = 82°59′E; geomagnetic
lat. = 14°55′N, long. = 154°E, dip angle = 37.3°, sub-ionospheric dip = 34°) have been analyzed to study the behaviour of
ionospheric E-region irregularities during the active solar and magnetic periods. The autocorrelation functions, power spectral
densities, signal de-correlation times are computed to study the temporal features of ionospheric E-region irregularities
linked with daytime scintillations. Derived spectral index ranges between −2 and −9. Assuming velocity of irregularities,
the characteristic lengths of the E-region irregularities are estimated. We have estimated the minimum and maximum range of
scale length of sporadic-E (E
s
) irregularities to be observed over Varanasi. These results are in close agreement with those reported from this latitude
region. 相似文献
17.
Summary Augite and diopside crystals from the Oligo-Miocene high-Mg and high-Al basalts, basaltic andesites and andesites of Montresta
(Sardinia, Italy) have been analysed by means of X-ray single crystal diffraction and electron microprobe. It has been found
that crystallization or equilibration processes took place within the crust at pressures below 5 kbar. In particular, samples
from high-Al basalts, basaltic andesites and andesites crystallised at pressures below one kbar while those from high-Mg basalts
crystallised in a range from about 1.5 to 4.5 kbar. The intracrystalline temperatures range between about 550 °C for high-Mg
basalts and 750 °C for andesites. The M2′ site is occupied in each of the studied crystals, which implies slow cooling rates. 相似文献
18.
Stephanos P. Kilias Jon Naden Ioannis Cheliotis Thomas J. Shepherd Heleni Constandinidou John Crossing Ioannis Simos 《Mineralium Deposita》2001,36(1):32-44
The Profitis Ilias gold deposit, located on the western part of Milos Island, Greece, is the first epithermal gold deposit
discovered in the Pliocene–Pleistocene Aegean volcanic arc. Estimated ore reserves are 5 million tonnes grading 4.4 g/tonne
Au and 43 g/tonne Ag. The deposit is closely associated with a horst and graben structure, and occurs in a series of steep
interconnected crustiform-banded quartz veins up to 3 m wide, extending to depths of at least 300 m. The mineralisation occurs
in three stages and is hosted by 3.5–2.5 Ma old silicified and sericitised rhyolitic lapilli-tuffs and ignimbrites. It consists
of pyrite, galena, chalcopyrite, electrum and native gold. Additionally, adularia occurs with quartz mainly in veins. Homogenisation
temperatures of primary liquid-rich inclusions vary from 145 to 399 °C for the ore stage, and 112 to 263 °C for the post-ore
stage. Salinities range between 0.1 and 11.4 wt% NaCl equiv. and 0.93 to 8.5 wt% NaCl equiv. for the ore stage and the post-ore
stage, respectively. Rare vapour-rich inclusions in ore stage quartz homogenise between 368 and 399 °C and estimates of eutectic
melting (−25 to −38 °C) indicate the presence of Ca and Mg in the ore fluids. Sample elevation versus fluid inclusion Th–salinity relationships show (1) a high-salinity trend, where moderate-temperature (300–250 °C) and moderate-salinity brines
(∼3 wt% NaCl equiv.) trend to high-salinity (up to 15 wt% NaCl equiv.) fluids with lower (∼25–50 °C) homogenisation temperatures,
and (2) a high-Th trend where moderate-salinity and moderate-temperature brines (200–250 °C; 3 wt% NaCl equiv.) develop into low-salinity (<1 wt%
NaCl equiv.), high-temperature (>350 °C) fluids. These trends are best explained by extreme boiling and vapourisation phenomena
between 200 and 250 °C. The 430–450 m asl (metres above sea level) level marks the transition between a lower liquid-dominated
segment of the system where only the steep high-salinity trend is seen, and an upper vapour-dominated segment where the high-Th trend or a combination of both are seen. There is a close spatial association between mineable gold grades and the upper
segment of the system. Depth-to-boiling curves suggest that the paleo-surface was ∼200 m above the present summit of Profitis
Ilias. Comparison of the mineralisation and fluid geochemistry at Profitis Ilias with that of the nearby modern geothermal
system indicates that the processes of metal mineralisation have probably been continuous since the Late Pliocene.
Received: 24 February 2000 / Accepted: 15 July 2000 相似文献
19.
M. Radha Krishna 《Journal of Earth System Science》1995,104(4):693-706
A detailed seismicity map of the Central Indian Ridge for the period 1912–1993 is presented, and the earthquakes pertaining
to four major transforms offsetting the ridge are utilized to study the moment release pattern. The scalar moment release
for the period 1912–1993, and the summed moment rate tensors for both short period (1977–1993) and long period (1912–1993)
bring out a unified picture of moment release pattern.
The fraction of seismic slip calculated based on depths of 100°C and 400°C limiting temperatures suggests that the Marie-Celeste
transform requires a slip almost to a depth of 400°C isotherm to account for the observed moment, and the Argo transform requires
depth of faulting much above the 400°C isotherm. A very small fraction of slip is accounted seismically for Vema (53%) and
12° 12′S (23%) even to depths of 100°C isotherm, suggesting a very low order of moment release along these transforms.
The horizontal plate velocities and the corresponding strain rates obtained from moment tensor summation of long period data
(82 years) give rise to (V
y
y
; V
y
x
mm. yr−1) of 6.0 and 6.1 along Marie-Celeste, 1.3 and 0.50 along Argo, 0.06 and 0.06 along 12° 12′S, 1.6 and 0.25 along Vema transforms.
The corresponding strain rates (ε
y
y
;ε
y
x
× 10−15 S−1) are 12.7 and 6.8 along MarieCeleste, 6.9 and 1.4 along Argo, 0.27 and 0.14 along 12° 12′S, 7.3 and 0.58 along Vema transforms.
These results suggest that the strain rates were highest and almost all predicted motion is taken up seismically along the
Marie-Celeste transform. The strain rates are lower along Argo transform and the observed moment release require shallower
depth of faulting in order to slip to be accounted seismically. The Vema and 12° 12′S transforms are characterized by low
strain rates and less than 15 per cent of motion is accommodated seismically within the seismogenic layer. It is proposed
that the deficiency of moment release along the Vema and 12° 12′S multiple transform system may be due to most of the plate
motion occurring aseismically. 相似文献
20.
In Bahia Samborombon (SW Atlantic; 35°30′–36°22′S; 56°45′–57°23′W) migratory American golden plover (Pluvialis dominica), black-billed plover (P. squatarola), ruddy turnstone (Arenaria interpres), and whimbrel (Numenius phaeopus) forage on fiddler crabs (Uca uruguayensis). These shorebirds have distinct patterns of feeding behavior and capture different proportions of each sex and size. P. dominica made short runs, capturing mostly juvenile crabs, which are eaten whole. P. squatarola use a similar feeding strategy but capture females primarily. A. interpres walks continuously, capturing almost exclusively large male crabs (71%). N. phaeopus walk in the Uca patch and probe burrows by inserting the bill; they primarily capture females. The feeding rate of A. interpres is higher than that of P. dominica, P. squatarola, and N. phaeopus. Handling time shown by P. dominica and A. interpres was greater than the other two species for all categories of crabs. In all cases, handling time of male crabs was greater than those of either females or juveniles. All the evidences suggest that U. uruguayensis is an important food source for all these species and should be accounted in any conservation endeavor. *** DIRECT SUPPORT *** A01BY085 00006 相似文献