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1.
The article describes the thermal metamorphism of siliceous carbonate rocks near the dolerite intrusive body in Eastern Siberia. The mineral associations at the immediate contact with dolerite are the following: wollastonite+rankinite, rankinite+spurrite (+melilite?), spurrite+melilite+merwinite+calcite and merwinite+monticellite+melilite+calcite. The melilite in these associations is usually unzoned; its composition being essentially gehlenitic. During the regressive stage of contact metamorphism new akermanite-rich melilite and calcite were formed by replacement of merwinite and earlier gehlenitic melilite through participation of CO2. The newly forming melilite grains have sharp compositional zoning. The origin of zoning was connected with the fall of temperature and decrease of the mole fraction of CO2 in the fluid equilibrated with the minerals.  相似文献   

2.
Late Cretaceous Chalk sedimentation history across the British Isles included (i) fault controlled uplift and subsidence in Northern Ireland and the Inner Hebrides and (ii) uplift along the lines of en echelon folds in Southern Britain and northern France. Synsedimentary slump folds and downslope displacement structures are compared with penecontemporaneous interbed slides and later tectonic folds and faults. Compressional strike-slip tectonic processes at Flamborough Head, Yorkshire, illustrate intra-Chalk slump beds in a half-graben setting. Progressive ‘growth’ of structures characterises early downslope slump folding, interbed sliding and some listric faulting. Sheet-flints replacing slide shear planes and early fractures provide evidence for early movements. Availability of open-slopes or the depth of burial under which the range of structures developed is reflected in the degree of disruption and fragmentation of chalk and flint. Fragmentation provides clues to the timing of events and origin of the Late Campanian Altachuile Breccia (Northern Ireland) and the Coniacian Hope Gap slides (Sussex). Fragmentation and formation of sheet flints together help distinguish intra-Chalk tectonics from Quaternary glacitectonic structures.The role of marl seams, high porosity chalk beds and hardgrounds on bed-sliding, décollement zones and disruption of chalk blocks from bedrock in glacitectonics is discussed. Chalk formations with marl seams develop a special style of fracturing related to early interbed sliding and pore-fluid escape structures. Marl-seams are shown to be primary sedimentary features and not the products of post depositional pressure-solution. More than any other formation the Late Santonian – Early Campanian Newhaven Chalk contains extensive sheet-flints and shows great lateral variation in thickness and lithology across the fold belts of southern England and northern France.  相似文献   

3.
 During the Cenozoic, in the western Paris Basin, atmospheric weathering of the chalks with flints of the Upper Cretaceous led to the creation of clay with flints. A reconstitution of the chalks lost to dissolution is proposed and is based on the determination of the age of the parent chalks of the clay with flints and the quantification of the thickness of dissolved chalk. The chalks affected by weathering range in age from Turonian to Maastrichtian, thus confirming the deposition of calcareous sediments in the western Paris Basin up to the Maastrichtian. Chalk weathering took place in situ, as indicated by the preservation of the stratigraphic succession of the chalk in the clay with flints profiles. Weathering led to the dissolution of 20–200 m of chalk, with regional variations. The weathering rate varies between 2.1 and 14.5 m/Ma. Received: 20 July 1998 / Accepted: 1 July 1999  相似文献   

4.
Strontium isotopes and other geochemical signatures are used to determine the relationships between CO2-rich thermal (Chaves: 76 °C) and mineral (Vilarelho da Raia, Vidago and Pedras Salgadas: 17 °C) waters discharging along one of the major NNE–SSW trending faults in the northern part of mainland Portugal. The regional geology consists of Hercynian granites (syn-tectonic-310 Ma and post-tectonic-290 Ma) intruding Silurian metasediments (quartzites, phyllites and carbonaceous slates). Thermal and mineral waters have 87Sr/86Sr isotopic ratios between 0.716713 and 0.728035. 87Sr/86Sr vs. 1/Sr define three end-members (Vilarelho da Raia/Chaves, Vidago and Pedras Salgadas thermal and mineral waters) trending from rainfall composition towards that of the CO2-rich thermal and mineral waters, indicating different underground flow paths. Local granitic rocks have 87Sr/86Sr ratios of 0.735697–0.789683. There is no indication that equilibrium was reached between the CO2-rich thermal and mineral waters and the granitic rocks. The mean 87Sr/86Sr ratio of the thermal and mineral waters (0.722419) is similar to the Sr isotopic ratios of the plagioclases of the granitic rocks (0.71261–0.72087). The spatial distribution of Sr isotope and geochemical signatures of waters and the host rocks suggests that the thermal and mineral waters circulate in similar but not the same hydrogeological system. Results from this study could be used to evaluate the applicability of this isotope approach in other hydrogeologic investigations.  相似文献   

5.
Germanium and silicon, dissolved in seawater, are considered to be incorporated into biogenic opal with no or little fractionation, which permitted to use diatoms as reliable recorders of seawater Ge/Si. Does some fractionation occur during diagenesis, preventing the use of Ge/Si in ancient sediments? We examined the Ge/Si ratio of fossil sponges and flint nodules of the Cretaceous Chalk Formation of northern France. Though disputed, silica in this formation is considered to originate from sponges. No fractionation is observed between sponges and diagenetic flints, which allows us to observe whether Ge/Si bears a biogenic or detrital signature. We may thus confirm that sponges were the main silica supplier during the chalk deposition. The Ge/Si ratio may be used to identify a biogenic signature in cherts where the origin of silica is dubious.  相似文献   

6.
《Applied Geochemistry》1998,13(4):463-475
Strontium isotope ratios were measured on 13 rock, 18 leachate and 28 pore-water samples from the Milk River aquifer, the confining argillaceous formations, and the glacial till mantling the recharge area. Strontium isotope ratios (87Sr/86Sr) of pore waters from the aquifer, confining units, and the glacial till ranged from 0.7069 to 0.7082. The 87Sr/86Sr ratios in aquifer pore waters decrease with increasing distance from the aquifer recharge area, and this is interpreted to be the result of mixing and water–rock interaction within the aquifer.The solute composition of the recharging groundwater is modified by the local lithology, causing distinct geochemical patterns along different flow paths within the aquifer. Whole-rock 87Sr/86Sr ratios indicate that the shales and till are generally more radiogenic than the aquifer sandstone. The authigenic carbonate cements and rock-forming minerals comprising the major lithologic units had little apparent influence on the pore-water Sr chemistry. Carbonate cement leachates from the till and the aquifer sandstone are more radiogenic than those from the confining shale formations. Feldspar separates from the aquifer sandstone have relatively radiogenic Sr isotope ratios, whereas bentonites from the Milk River and Colorado Shale Formations have whole-rock and leachate Sr isotope ratios that are relatively unradiogenic. Ratios of most Milk River aquifer pore waters are lower than those of any leachates or whole rocks analyzed, except the bentonites.The 87Sr/86Sr ratios of exchangeable Sr in the bentonites are similar to ratios found in the more evolved pore waters. Simple rock–water interaction models calculated for the whole-rock, leachate, and exchangeable-ion/pore-water pairs indicate that ion exchange with bentonite clays within the Milk River and Colorado Shale Formations appears to influence the isotopic evolution of the pore-water Sr in each of these units.  相似文献   

7.
The Vandfaldsdalen macrodike is a layered and differentiatedgabbroic dike approximately 3?5 km long and from 200 to 500m wide. It appears to cut the eastern margin of the Skaergaardintrusion and may have served as a feeder for the Basistoppensill. The macrodike can be divided into three series of rocks:a marginal series of differentiated gabbros adjacent to thewalls of the dike; a central series of differentiated and subhorizontallylayered gabbros and ferrodiorites in the interior of the dike;and an upper felsic series of granophyric rocks with abundantquartzo-feldspathic xenoliths. The mineral and bulk-rock compositionsthrough both the marginal series and central series show progressiveiron enrichment. The most Ca-rich plagioclase (An69) and mostmagnesian pyroxene (Wo42 En46 Fs12) occur in olivine-bearingrocks of the marginal series about 5 m from the contact withwall rocks. The most Na-rich plagioclase (An39) and Fe-richpyroxene (Wo38 En24 Fs38) are in olivine-free ferrodiorite ofthe central series, about 20 m below the contact with the felsicseries. Evidence from field observations, bulk-rock chemical compositions,and Sr and Nd isotopic data indicate the felsic series formedas a mixture of the initial macrodike magma and granitic countryrock. 87Sr/86Sr ratios of specimens from the felsic series rangebetween 0?7129 and 0?7294. 143Nd/144Nd ratios vary between 0?51208and 0?51118. Both ratios vary serially with the SiO2 contentsof the specimens. We suggest that the felsic series evolvedas a separate body of low density liquid which floated on thedenser gabbroic magma of the central series. Heat from crystallizationof the gabbroic magma must have diffused into the felsic layer,enabling extensive assimilation of the granitic xenoliths, butour data indicate there was very little exchange of chemicalcomponents between the two liquids.  相似文献   

8.
Forty-one metric tons of the mineral wollastonite (CaSiO3) was applied to an 11.8 hectare watershed at the Hubbard Brook Experimental Forest (HBEF; White Mountains, New Hampshire, USA) with the goal of restoring the Ca estimated to have been depleted from the soil exchange complex by acid deposition. This experiment provided an opportunity to gain qualitative information on whole watershed hydrologic flow paths by studying the response of stream water chemistry to the addition of Ca. Because the Ca/Sr and 87Sr/86Sr ratios of wollastonite strongly contrast that of other Ca sources in the watershed, the wollastonite-derived Ca can be identified and its amount estimated in various ecosystem components. Stream water chemistry at the HBEF varies seasonally due to shifts in the proportion of base flow and interflow. Prior to the wollastonite application, seasonal variations in 87Sr/86Sr ratios indicated that 87Sr/86Sr was higher during base flow than interflow, due largely to greater amounts of biotite weathering along deeper flow paths. After the application, Ca/Sr and 87Sr/86Sr changed markedly as the high Ca/Sr and low 87Sr/86Sr wollastonite dissolved and mixed with stream water. The Ca addition provided information on the response times of various flow paths and ion exchange processes to Ca addition in this small upland watershed. During the first year after the addition, wollastonite applied to the near stream zone dissolved and was partially immobilized by cation exchange sites in the hyporheic zone. In the second and third years after the addition we infer that much of this Ca and Sr was subsequently desorbed from the hyporheic zone and was exported from the watershed in stream flow. In the fourth through ninth years after the addition, Ca and Sr from wollastonite that had dissolved in upland soils was transported to the stream by interflow during wet periods when the ground water table was elevated. Between years three and nine the minimum annual Ca/Sr ratio (in late summer base flow) increased, providing evidence that Ca and Sr had increasingly infiltrated to the deepest flow paths. Strong seasonal variations in Ca/Sr and 87Sr/86Sr ratios of stream water resulted from the wollastonite addition to upland forest soils, and these ratios have become sensitive to changing flow paths during the annual cycle. Most notably, high flow events now produce large excursions in stream geochemistry toward the high Ca/Sr and low 87Sr/86Sr ratios of wollastonite. Nine years after the application we estimate that ∼360 kg of Ca from wollastonite has been exported from the watershed in stream flow. The rate of export of Ca from wollastonite dissolution has stabilized at about 11 kg of Ca per year, which accounts for ∼30% of the dissolved Ca in the stream water. Given that 19 metric tons of Ca were applied to the watershed, and assuming this current rate of loss, it should take over 1000 years for this added Ca to be transported from the watershed.  相似文献   

9.
Chemical and Sr-Nd isotopic data are presented for 14 Proterozoicdolerite intrusions from Sweden. The dolerites are unmetamorphosedand belong to two age groups at 1.26 and 0.95 Ga. The analysedrocks were selected from over 80 samples characterized for initial87Sr/86Sr ratio, and represent those from each intrusion thathave little to no crustal contamination effect on Sr isolopes.In these tholeiitic to mildly alkaline basalts, MgO varies from4 to 14%, and TiO2 from 1.0 to 4.7%. Incompatible-element abundancepatterns show similar shapes at widely varying overall enrichmentlevels; La varies from eight to 64 times primitive mantle. Manysamples have apparent Sr anomalies; positive for the less incompatible-elementenriched rocks, negative for those with the highest abundances,Hence Sr varies by a factor of only two, whereas neighboringelements vary by a factor of seven. Small Eu anomalies correlatewith the larger Sr variations. All samples have negative Nbanomalies, correlated with Nd 1 values that vary from –0.5to + 3.5 for both dolerite age groups. 87Sr/86Sr1 values aresystematically different in the two groups: 1.26-Ga doleritesvary from 0.7026 to 0.7035, and 0.95-Ga dolerites from 0.7041to 0.7051. Rather than urging any particular model of subcontinental mantleevolution, we use the dolerite results to deconvolute the effectsof crustal contamination, differentiation including crypticdifferentiation effects, and mantle-lithosphere interactionson possibly asthenosphere-derived magmas. Problems related toaddition of Rb and 87Sr giving false isochrons, as well as raised87Sr/86Sr from syn- or post-crystallization fluid effects, wereeliminated by the sample selection criteria. In the selecteddolerites, crustal contamination is not a major control on geochemistry,because all incompatible elements, including those of both highand low crustal abundance, are positively and similarly correlated.Overall abundances have been affected by migrations of residualmagma after much olivine and plagioclase crystallization. OnlySr is an approximate indication of the parent magma's enrichment,whereas all other incompatible elements may be depleted or enrichedby factors greater than two. On entering the crust, the magmasvaried by a factor of only two in overall incompatible-elementenrichment. They had Nb anomalies correlated with Nd1, and the0.95-Ga dolerites possessed higher 87Sr/86Sr than had characterizedthe 1.26-Ga dolerites. All mantle effects could be interpretedas resulting from a lithospheric magma source that experiencedaddition of subduction-related components 1.9–1.7 Ga ago,and maybe locally 1.6–1.0 Ga ago. We prefer a model whereasthenosphere-derived melts interact with lithospheric wallrocks, acquiring variable Nd down to –0.5 and Nb anomalies.Up to two-thirds of the budget of incompatible elements, butnot much Nb, were added to the magmas. Untimately, the contributionof any mantle-source is unconstrained, except that significantlithosphere has to have been involved. The mantle-related geochemicaleffects, which result in only weak constraints on process andsource, are very difficult to isolate in an objective approachto intracontinental basalts, because of superimposed differentiationand contamination effects.  相似文献   

10.
《Geodinamica Acta》1998,11(4):171-181
During the past century, the autochthonous and allochthonous origin of clay with flints from underlying chalk rocks has been the subject of many discussions. In order to have a better understanding of the origin of this unit, a weathering balance between chalk and clay with flints was made. This balance was estimated by an accurate knowledge of mother chalk ages and by comparison of grain size distribution and geochemistry between chalk and clay with flints. These comparisons are based on the quantities of flints in chalk and in clay with flints, because flints are autochthonous and their weathering is calculated. Results provide several new elements on the Cretaceous evolution of the Paris-London Basin: chalk of Maastrichtian age existed in the Paris Basin, and we now have good estimations of the quantities of chalk that disappeared through weathering. Results further show that the stratigraphic succession of chalk is maintained in the clay with flints. Additionally, flints (autochthonous elements of clay with flints) are only slightly weathered. Clay with flints matrix is composed of a significant allochthonous fraction in the first 10 m, which diminishes with depth and disappears below a depth of 20 m. Pure clay with flints issued from chalk decarbonatation only exists at a depth greater than 20 m.  相似文献   

11.
Strontium isotope compositions of ancient sulphate deposits not only provide chemostratigraphic information but also offer insight into the system in which the evaporites precipitated. Primary gypsum from two Middle Miocene (Badenian) sections in southern Poland shows steadily higher 87Sr/86Sr ratios than those expected from a marine‐derived formation. The ratios are interpreted as the result of increasing inflow into the basin at the time of gypsum precipitation. Palaeogeographic reconstructions suggest that riverine runoff sources were situated in the West and East European platforms (to the north and east, respectively) and the Carpathians (to the south), which are mostly composed of Mesozoic sedimentary rocks; their dissolution cannot be responsible for the higher 87Sr/86Sr ratios recorded. We conclude that Archaean and Palaeoproterozoic igneous and supracrustal rocks of the Ukrainian Shield were the source of the higher 87Sr/86Sr ratios recorded in the Badenian primary gypsum. A distinctive decreasing trend of 87Sr/86Sr ratios from western Ukraine to southern Poland is explained by a consistent direction of brine inflow during gypsum crystallization (typical cyclonic circulation controlled by the Coriolis effect).  相似文献   

12.
The isotopic composition of Sr and the abundances of Rb andSr have been determined in the alkaline rocks of the Fen Complex,South Norway. The 87Sr/88Sr ratios range from as low as 0.703in carbonatite (s?vite) to as high as 0.710 in rauhaugite. Thewhole rock analyses do not plot on a Nicolaysen diagram as anisochron. Calculation of the initial isotopic composition ofSr in the rocks at the time of intrusion of the complex, approximately550 m.y. ago, shows that the rocks are not simply related asthe differentiation products of a single magma, since they donot possess similar initial 87Sr/86Sr ratios. The lowest initialratio is seen in the s?vites and it is concluded that this rockrepresents the magma from which the other rocks of the complexwere derived. Production of hybrid rocks by bulk assimilationof granite gneiss in carbonatite is inadequate to account forthe observed Rb/Sr ratios. It is envisaged that selective concentrationof granitic Sr of high 87Sr/86Sr ratio together with loss orgain of variable amounts of Rb has taken place. The processis closely connected with the fenitization observed at the marginsof the complex and is of a metasomatic nature. Thus, the genesisof rocks of the melteigite-ijolite-urtite series may be consideredto be a rheomorphic process. The 87Sr/86Sr ratios observed inr?dberg and rauhaugite can be explained in terms of the metasomaticalteration of damtjernite.  相似文献   

13.
Compositionally heterogeneous basaltic centers from a varietyof tectonic environments, including Pisgah Crater in the MojaveDesert region of California, exhibit secular changes in theirchemistry that might be explained by the sequential meltingof ultramafic to mafic mantle sources. We have analyzed phenocrystsfrom alkali basalts and hawaiites erupted at Pisgah Crater toinvestigate the effects of open-system modifications imposedon basaltic systems. We present 87Sr/86Sr data for individualphenocrysts of amphibole and clinopyroxene and the first publishedresults of single olivine grains, in addition to plagioclase.Each mineral phase exhibits a range in Sr isotope compositionthat may only partially overlap the isotopic composition ofthe other mineral phases, suggesting an interplay between twomagmatic end-members that continued up to the time of eruption.Limited 87Sr/86Sr variability in minerals from early and intermediatelavas indicates only moderate syn-crystallization open-systemmodification, whereas minerals in late-erupted lavas have muchhigher 87Sr/86Sr, consistent with extensive open-system modification.Rimward increases in 87Sr/86Sr of plagioclase confirm that thesechanges occurred within the stability field of plagioclase and,therefore, at crustal or near-crustal depths. The major elementcompositions of olivine-hosted melt inclusions indicate thatan Al-rich component of andesitic composition (87Sr/86Sr 0·7056),possibly derived from plagioclase-rich cumulates or pelites,was assimilated by magma generated from asthenosphere or younglithosphere with 87Sr/86Sr 0·7038. The results clearlydemonstrate the utility of measuring the 87Sr/86Sr of individualminerals and indicate that Pisgah Crater basalts probably acquiredisotopically enriched geochemical signatures from crustal contamination,rather than from mixing of heterogeneous mantle melts. KEY WORDS: assimilation; basalts; melt inclusions; minerals; Sr isotopes  相似文献   

14.
The volcanic rocks of the Bearpaw Mountains are part of theMontana high-potassium province, emplaced through Archaean rocksof the Wyoming Craton between 54 and 50 Ma ago. Extrusive rocks,dominated by minettes and latites, have a volume of 825 km3.The minettes range in composition from 20 to 6% MgO. The moremagnesian varieties contain the phenocryst assemblage forsterite+ Cr-spinel + diopside phlogopite. More evolved rocks areolivine-free, with an assemblage of either salite + phlogopite+ pseudoleucite or salite + phlogopite + analcime. The analcimeis thought to be secondary after leucite, produced by loss ofpotassium from the minettes. Mineral chemistry and textures,especially of clinopyroxenes, indicate that mixing between minettemagmas of varying degrees of evolution was commonplace. Compositionalvariation was further extended by accumulation of olivine +spinel + clinopyroxene phenocrysts, and by the preservationof mantle xenocrysts in the minettes. The primary minette magmasare inferred to have had 12–14% MgO and to have been generatedat 30 kb from an olivine + diopside + phlogopite-bearing source.The primary magmas evolved dominantly by fractionation of olivine+ diopside. The minettes have high contents of large ion lithophile elements(LILE) and light rare earth elements (LREE), with K2O up to6.18%, Ba 5491 ppm, Sr 2291 ppm, and Ce 99 ppm. (87Sr/86Sr)0ranges from 0.707 to 0.710 and Nd varies from –10 to–16. These data, plus high LILE/HFSE (high field strengthelements) values, are interpreted to show that the minettescontain at least three different mantle components. The lithospherewas initially depleted in Archaean times, but was metasomaticallyenriched in the Proterozoic and in the late Cretaceous and earlyTertiary. The latites have many chemical features in common with the minettes,such as potassic character and high LILE/HFSE values. They formedby fractional crystallization of minette magma in combinationwith assimilation of crustal rocks; this process enriched themagmas in SiO2 and raised Na2O/K2O and 87Sr/86Sr values. Chemicaldata for phenocrysts and bulk rocks in minettes suggest mixingbetween minette and latite magmas.  相似文献   

15.
The processes operating in the development of chemical zonationin silicic magma chambers have been addressed with a Sr–Nd–Pb–Hf–Thisotope study of the chemically zoned trachyte pumice depositof the Fogo A eruption, Fogo volcano, Azores. Sr isotopic variationis observed in whole rocks, glass separates and sanidine phenocrysts(whole-rock 87Sr/86Sr: 0·7049–0·7061; glass87Sr/86Sr: 0·7048–0·7052; sanidine 87Sr/86Sr:0·7048–0·7062). Thorium isotopic variationis observed in glass separates, with (230Th/232Th)o rangingfrom 0·8737 to 0·8841, and exhibiting a negativecorrelation with Sr isotopes. The Nd, Pb and Hf isotopic compositionsof the whole-rock trachytic pumices are invariant and indistinguishablefrom basalts flanking the volcano. The Sr isotope variationsin the whole rocks are proposed to be the result of three distinctprocesses: contamination of the Fogo A magma by assimilationof radiogenic seawater-altered syenite wall rock, to explainthe Sr and Th isotopic compositions of the glass separates;incorporation of xenocrysts into the trachytic magma, requiredto explain the range in feldspar Sr isotopic compositions; andpost-eruptive surface alteration. This study emphasizes theimportance of determining the isotopic composition of glassand mineral separates rather than whole rocks when pre-eruptivemagmatic processes are being investigated. KEY WORDS: Azores; open-system processes; Sr isotopes; trachytic pumices; zoned magma chambers  相似文献   

16.
SIMONETTI  A.; BELL  K. 《Journal of Petrology》1994,35(6):1597-1621
Initial Nd, Pb, and Sr isotopic data from carbonatites and associatedintrusive silica-undersaturated rocks from the early Jurassic,Chilwa Island complex, located in southern Malawi, central Africa,suggest melt derivation from a Rb/Sr- and Nd/Sm-depleted butTh/Pb- and U/Pb-enriched mantle source. Initial 143Nd/144Nd(0.51265–0.51270) isotope ratios from the Chilwa Islandcarbonatites are relatively constant, but their initial 87Sr/86Sr(0.70319–0.70361) ratios are variable. The 18Osmow (9.53–14.15%0)and 13CPDB (–3.27 to –1.50%0) isotope ratios ofthe carbonates are enriched relative to the range of mantlevalues, and there is a negative correlation between 18O andSr isotope ratios. The variations in Sr, C, and O isotopic ratiosfrom the carbonatites suggest secondary processes, such as interactionwith meteoric groundwater during late-stage carbonatite activity.The initial 143Nd/144Nd (0.51246 0.51269) and initial 87Sr/86Sr(0.70344–0.70383) isotope ratios from the intrusive silicaterocks are more variable, and the Sr more radiogenic than thosefrom the carbonatites. Most of the Pb isotope data from Chilwa Island plot to the rightof the geochron and close to the oceanic regression line definedby MORBs and OIBs. Initial Pb isotopic ratios from both carbonatites(207Pb/204Pb 15.63–15.71; 206Pb/204Pb 19.13–19.78)and silicate rocks (207Pb/204Pb 15.61–15.72; 206Pb/204Pb18.18–20.12) show pronounced variations, and form twogroups in Pb-Pb plots. The isotopic variations shown by Nd, Pb, and Sr for the ChilwaIsland carbonatites and intrusive silicates suggest that thesemelts underwent different evolutionary histories. The chemicaldata, including isotopic ratios, from the carbonatites and olivinenephelinites are consistent with magmatic differentiation ofa carbonated-nephelinite magma. A model is proposed in whichdifferentiation of the carbonatite magma was accompanied byfenitization (metasomatic alteration) of the country rocks bycarbonatite-derived fluids, and subsequent alteration of thecarbonatite by hydrothermal activity. The chemical and isotopicdata from the non-nephelinitic intrusive silicate rocks reveala more complex evolutionary history, involving either selectivebinary mixing of lower-crustal granulites and a nephelinitemagma, or incremental batch melting of a depleted source andsubsequent crustal contamination.  相似文献   

17.
Chalk is a variable material, the properties of which are dependent upon its composition, textural features and diagenetic history. With the exception of certain horizons in the Lower Chalk that contain appreciable amounts of clayey material, the English Chalk is a remarkably pure micritic carbonate rock that generally can be divided into coarse and fine fractions. The latter comprises 70–80% of chalk. Cementation took place more or less contemporaneously with deposition so that the sediment was able to support relatively high overburden pressures. Hence, high values of porosity were retained. Chalk varies appreciably in density and hardness. The harder chalks are the result of diagenetic processes and bioturbation that brought about densification. In soft chalks the grains are only bound together at the points of contact by thin films of calcite.

The latest classification of chalk is based on an assessment of intact dry density, discontinuity aperture and discontinuity spacing. Chalk tends to vary from moderately weak to moderately strong and its strength is significantly reduced on saturation. Under triaxial loading conditions diagonal shear failure tends to occur at lower confining pressures but at higher confining pressures barrel-shaped failure occurs indicating plastic deformation and textural disaggregation. Similarly, at low loading, chalk exhibits low volume compressibility but much more significant consolidation occurs if the yield stress is exceeded.

Chalk undergoes dissolution and so solution features are found throughout its outcrop.

Mineworkings in the Chalk extend back into the distant past, the most ancient being those excavated in the Neolithic Age for flint. Several types of workings exist. Collapse of old mineworkings, most of which are unrecorded, is difficult to predict. The potential for subsidence, caused by the collapse of both mineworkings and dissolution features, affects development and its occurrence can lead to the abandonment of property or, worse, the loss of lives.  相似文献   


18.
《Applied Geochemistry》2005,20(4):749-766
A synthesis of Sr isotope data from shallow and deep groundwaters, and brines from the Fennoscandian and Canadian Shields is presented. A salinity gradient is evident in the water with concentrations varying from approximately 1–75 g L−1 below 1500 m depth in the Fennoscandian Shield and from 10 up to 300 g L−1 below 650 m depth in the Canadian Shield. Strontium isotope ratios were measured to assess the origin of the salinity and evaluate the degree of water–rock interaction in the systems. In both shields, the Sr concentrations are enriched relative to Cl, defining a positive trend parallel to the seawater dilution line and indicative of Sr addition through weathering processes. The depth distribution for Sr concentration increases strongly with increasing depth in both shields although the variation in Sr-isotope composition does not mirror that of Sr concentrations. Strontium-isotope compositions are presented for surface waters, and groundwaters in several sites in the Fennoscandian and Canadian Shields. Numerous mixing lines can be drawn reflecting water–rock interaction. A series of calculated lines links the surface end-members (surface water and shallow groundwater) and the deep brines; these mixing lines define a range of 87Sr/86Sr ratios for the deep brines in different selected sites. All sites show a specific 87Sr/86Sr signature and the occurrence of large 87Sr/86Sr variations is site specific in both shields. In Canadian Shield brines, the Sr isotope ratios clearly highlight large water rock interaction that increases the 87Sr/86Sr ratio from water that could have been of marine origin. In contrast to the Canadian Shield, groundwater does not occur in closed pockets in the Fennoscandian, and the well-constrained 87Sr/86Sr signatures in deep brines should correspond to a large, well-mixed and homogeneous water reservoir, whose Sr isotope signature results from water–rock interaction.  相似文献   

19.
EWART  A. 《Journal of Petrology》1982,23(3):344-382
The magmas of the Tertiary volcanic province of S. Queenslandare chemically bimodal, and occur in numerous volcanic centres,at least three representing original shield volcanoes. The maficlavas are dominantly hawaiites and tholeiitic andesites, whereasthe silicic magmas comprise mainly trachytes, rhyolites, andcomendites. The silicic rocks exhibit variable trace element abundance patterns.There is a progressive depletion of Sr, Ba, V, Mg, Ni, Cr, Mn,and P, through the trachytes to the rhyolites and comenditeswhile the behaviour of Zr, Nb, LREE, Y and Zn is very variable.Rb, Th, and to a lesser extent Pb exhibit a more regular behaviour,becoming most generally concentrated in the comendites and rhyolites.These trace element patterns are modelled by application ofthe Rayleigh distillation model, using partition coefficientsbased on analysed phenocrysts from the S. Queensland siliciclavas. Trace mineral phases, namely zircon, chevkinite, andallanite, are shown to be important in the probable controlof LREE, Zr, and Th abundances, while Nb and Zn are probablycontrolled during fractionation by magnetite. Trace elementdata for the hawaiites and tholeiitic andesites also indicateextensive although variable levels of fractional crystallizationof these magmas. The Sr and O isotopic compositions of the mafic lavas, trachytes,comendites and rhyolites are as follows: initial 87Sr/86Sr ratios;0.70357–0.70456, 0.70432–0.70589, 0.70495–0.70917,and 0.70708–0.70863 respectively. 18O range between 5.6–7.0(mafic lavas), 4.9–8.7 (trachytes), 5.0–7.6 (comendites)and 8.1–10.4 per mil (rhyolites). Pb isotopic compositionsare variable, showing a variation of 6.7 per cent for 206Pb/204Pbratios through the range of volcanic compositions. The rhyolitesexhibit a much greater divergence in their O, Sr, and Pb isotopiccompositions compared with those of associated mafic lavas,than is found in the trachytes and comendites. Within the silicicvolcanics, positive correlations exist between 18O and initialSr ratios, and between Pb isotopic compositions and initialSr ratios (with one group of trachytes providing a noteworthyexception). These correlations are not so clearly defined forthe mafic lavas, although these do exhibit positive correlationsbetween differentiation index, 18O, and initial Sr isotope ratios. The development of the silicic magmas, excepting two groups,is interpreted in terms of a model in which assimilation andfractional crystallization occur concurrently, involving a basaltor hawaiite magma component and a crustal component (modelledon the analysed Carboniferous basement greywackes outeroppingin the region); the data indicate, however, that differentiationcontinued in isotopically closed systems (i. e. isolated fromthe wallrocks). The highly depleted Sr and Ba abundances ofthe rhyolites and comendites suggest that contamination didnot occur after differentiation had ceased. The rhyolites havethe highest isotopic input of the crustal components and areinterpreted as crustal anatectic melts, produced locally withinthe crust in response to basalt/hawaiite magma intrusion, whereasmost of the trachytes and comendites are interpreted as primarilythe differentiated products from original mafic parental magmas,with variable assimilation of crustal wallrock components. Theisotopic data suggest that only the Minerva Hills trachyticlavas, and a Glass House comendite, have not been significantlymodified by wallrock assimilation processes. The erpted maficmagmas were also evidently modified by isotopic crustal wallrockinteractions, which independent petrological data suggest hasoccurred at intermediate to lower crustal depths.  相似文献   

20.
Most of the flows in the Palaeocene lava pile remnant of Skyeare members of the Skye Main Lava Series (SMLS), comprisingtransitional basalts and two associated suites of evolved lavas.The first suite evolves through Fe-rich hawaiites and mugearitesto benmoreites, and the second suite evolves through Fe-poorintermediates to trachytes. Ca-rich, alkali-poor olivine tholeiites(the Preshal Mhor magma type) occur as sparse flows in the stratigraphicallyhighest parts of the lava pile remnant and are abundant in thedyke swarm transecting it. Initial 87Sr/86Sr ratios rangingfrom 0.70308 to 0.70571 in 45 SMLS samples show no significantcorrelation with degree of zeolitization (H2O+), silica saturation,or 87Rb/86Sr. A moderately good negative correlation with totalSr confirms published Pb-isotope evidence of interaction withancient, sialic crust. Details of the (87Sr/86Sr)l versus Srpattern are consistent with previous hypotheses that the SMLSbasalt-benmoreite suite evolved at a depth near the Moho, whilstthe low-Fe trend to trachyte resulted from near-surface basaltfractionation. (87Sr/86Sr)l values ranging from 0.70307 to 0.70621 for PreshalMhor basalts show a strong positive correlation with total Sr,consistent with a model of extensive fractionation within theupper crust of a mantle-derived low 87Sr/86Sr-low Sr magma,which became progressively contaminated with comparatively radiogeniccrustal Sr. The lowest measured (87Sr/86Sr)l values of 0.70307and 0.70308, for a Preshal Mhor basalt and for an SMLS basaltrespectively, are consistent with the hypothesis that thesetwo magma types were produced by successive phases of partialmelting from a single volume of upper mantle. (87Sr/86Sr)l values for additional miscellaneous basaltic lavas,dykes and major intrusives from Skye and from nearby Isle ofMull exhibit considerable variability within the range 0.7038to 0.7072, whilst three basaltic dykes from Northern Englandare in the range 0.7089 to 0.7123. The latter values overlapwith published (87Sr/86Sr)l values for some of the granitesin the Tertiary Province of northwest Scotland and indirectlyremove objections based on Sr-isotopic arguments to the genesisof the granites by fractionation of basalt contaminated withcrustal Sr, but neither prove this nor disprove large-scalecrustal partial fusion.  相似文献   

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