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1.
This paper presents and discusses the isotopic data from the hydrothermal studies of the Poços de Caldas Natural Analogue Project. The purpose of these studies was to elucidate the mass transport of relevant elements and isotopes associated with hydrothermal mineralization and alteration at the Osamu Utsumi uranium mine, as applicable to high-temperature radwaste isolation (particularly in the U.S. nuclear waste program). Research efforts were focused on studying the thermal, chemical and hydrologic nature of the palaeohydrothermal regime associated with a breccia pipe at the Osamu Utsumi mine, and related to the geochemical, geochronological and petrological characterization studies of unaltered regional nepheline syenite and phonolite.The regional rocks studies have a vertically elongated δD, δ18O pattern, which possibly indicates meteoric water/rock interaction. Regression of Rb---Sr whole-rock isotopic data for the regional nepheline syenite and phonolite samples did not produce isochrons. An internal, mineral-separate isochron regression from a nepheline syenite sample, considered representative of unaltered nepheline syenite of the Poços de Caldas plateau, yields an age of 78 Ma, and an initial ratio of approximately 0.7051. The initial ratios of the regional nepheline syenites are possibly indicative of a mantle source for the alkaline magmatism, with some incorporation of old, high Rb/Sr crustal material. The greater-than-mantle values of δ18O, if not due solely to surficial processes, also appear to require some assimilation of crustal material. Sm---Nd isotopic data for the regional rocks do not define any isochrons, although the nepheline syenite samples conform very well to a calculated reference isochron for 78 Ma and a fixed initial 143Nd/144Nd of 0.512359. The regional phonolite samples lie markedly off this isochron. This is probably due to the phonolite samples having different initial 143Nd/144Nd values. All regional samples lie within the “Mantle Array” trend. Their location within Nd–Sr space indicates as asthenospheric Mid Ocean Ridge Basalt (MORB)-type source magma also contaminated by continental igneous and metamorphic rocks (e.g. the Precambrian gneiss surrounding the Poços de Caldas plateau).The rocks studied at the Osamu Utsumi mine from the F4 drillcore have experienced varying degrees of hydrothermal mineralization and metasomatism, and deep weathering. The hydrothermally altered rocks have a quite pronounced δD shift, with only a slight δ18O shift. The δD-δ18O trend of the hydrothermally altered F4 samples most likely reflects the variability of temperature, hydrologic flow, mineralogical alteration and, therefore, water/rock interaction and isotopic exchange in the palaeohydrothermal regime.Regression of Rb---Sr whole-rock isotopic data for subsamples from a nepheline syenite xenolith sample yields an age of 76 Ma and an initial ratio of approximately 0.7053. Due to the marked hydrothermal alteration and metasomatism of this sample, the Rb---Sr isotopic system is interpreted as being re-equilibrated and thus the regressed age is the age of the hydrothermal event. Using a versus 1/Sr mixing diagram, distinct trends are seen for hydrothermal alteration, mineralization and weathering. Again, the F4 nepheline syenite samples do not define an Sm---Nd isochron, but conform very well to a calculated model isochron for 78 Ma and an initial 143Nd/144Nd of 0.512365. The Sm---Nd isotopic data also exhibit a possible disturbance by the hydrothermal, metasomatic alteration. A lamproite dyke which crosscuts the hydrothermal alteration in the Osamu Utsumi mine gives an age of 76 Ma, which is essentially the same as the Rb---Sr age of the hydrothermally altered nepheline syenite subsamples.  相似文献   

2.
The chemical, isotopic and mineralogical alteration which occurred during primary uranium ore deposition at the breccia pipe-hosted Osamu Utsumi mine, Poços de Caldas, Brazil was studied as a natural analogue for near field radionuclide migration. Chemical and isotopic alteration models were combined with finite difference models of the convective cooling of caldera intrusives. The modeling indicates that the intense chemical, isotopic, and mineralogical alteration of the Osamu Utsumi breccia pipe requires the circulation of > 105 kg/cm2 of boiling hydrothermal fluid > 200°C through each square centimeter cross-section of the pipe. This circulation could be driven by heat from a 6 km diameter intrusive extending to 10 km depth. Even with this large amount of circulation concentrated in the permeable breccia pipe, uranium solubilities must be orders of magnitude greater than indicated in the most recent experiments (and more in line with previous estimates) to produce the primary uranium mineralization at the Osamu Utsumi mine.The same models applied to a hypothetical high temperature waste repository show that heat from radioactive decay will produce a hydrothermal circulation system remarkably similar to that studied at the natural analogue site at Poços de Caldas. The depth of fluid convection induced by the hypothetical repository would be 5 to 10 km, the maximum temperature would be 300°C, the lifetime of the high temperature phase would be a few thousand years, and boiling would occur and cause most of the alteration within the hypothetical waste repository. This physical analysis emphasizes the importance of permeability on a 10 × 10 × 10 km scale in controlling the potential amount of circulation through the hypothetical repository.Application of the chemical models successfully used to interpret mineralization and alteration at the Poços de Caldas Osamu Utsumi mine to the hypothetical waste repository shows that even in a worst case scenario (waste implaced in a permeable host rock with no measures taken to inhibit flow though the repository) the amount of hydrothermal alteration in the hypothetical repository will be 0.1% of that in the breccia pipe at Osamu Utsumi. Assuming no barriers to uranium mobility, uranium precipitation above the hypothetical repository would be 0.04 ppm (rather than 40 ppm), hydrothermal alteration 0.03 wt% (rather than 30 wt%), etc.Our analysis indicates that modeled mineralogical alteration is sensitive to the thermodynamic data base used. Prediction of mineralogical alteration (which may be necessary to predict the migration of radionuclides other than uranium, for example) probably cannot be based directly on even very carefully collected laboratory thermodynamic data. Mineralogical complexities of the system, as well as data base uncertainties will require calibration of the thermodynamic framework against mineralogical alteration observed in the laboratory or field.  相似文献   

3.
Surface and ground waters, collected over a period of three years from the Osamu Utsumi uranium mine and the Morro do Ferro thorium/rare-earth element (Th/REE) deposits, were analyzed and interpreted to identify the major hydrogeochemical processes. These results provided information on the current geochemical evolution of ground waters for two study sites within the Poços de Caldas Natural Analogue Project.The ground waters are a K---Fe---SO4---F type, a highly unusual composition related to intense weathering of a hydrothermally altered and mineralized complex of phonolites. Tritium and stable isotope data indicate that ground waters are of meteoric origin and are not affected significantly by evaporation or water-rock interactions. Recharging ground waters at both study sites demonstrate water of less than about 35 years in age, whereas deeper, more evolved ground waters are below 1 TU but still contain in most cases detectable tritium. These deeper ground waters may be interpreted as being of 35 to 60 or more years in age, resulting mainly from an admixture of younger with older ground waters and/or indicating the influence of subsurface produced tritium.Geochemical processes involving water-rock-gas interactions have been modeled using ground water compositions, mineralogic data, ion plots and computations of speciation, non-thermodynamic mass balance and thermodynamic mass transfer. The geochemical reaction models can reproduce the water chemistry and mineral occurrences and they were validated by comparing the results of thermodynamic mass transfer calculations (using the PHREEQE program, Parkhurst et al., 1980). The results from the geochemical reaction models reveal that the dominant processes are production of CO2 in the soil zone through aerobic decay of organic matter, dissolution of fluorite, calcite, K-feldspar, albite, chlorite and manganese oxides, oxidation of pyrite and sphalerite, and precipitation of ferric oxides, silica and kaolinite. Gibbsite precipitation can be modeled for the shallow (recharge) water chemistry at Morro do Ferro, consistent with known mineralogy. Recharge waters are undersaturated with respect to barite and discharging waters and deeper ground waters are saturated to supersaturated with respect to barite demonstrating a strong solubility control. Strontium isotope data demonstrate that sources other than calcium-bearing minerals are required to account for the dissolved strontium in the ground waters. These may include K-feldspar, smectite-chlorite mixed-layer clays and goyazite [SrAl3(PO4)2(OH)5·H2O].  相似文献   

4.
Seven pyroxene varieties were identified in nepheline syenites and foidolites of the Khibiny pluton: enstatite, ferrosilite, diopside, hedenbergite, augite, aegirine-augite, and aegirine. Enstatite and augite are typical of alkaline and ultramafic rocks of dike series. Ferrosilite was found in country quartzitic hornfels. Diopside is a rock-forming mineral in alkaline and ultramafic rocks, alkali gabbroids, hornfels in xenoliths of volcanic and sedimentary rocks and foyaite, melteigite-urtite that assimilate them, and certain hydrothermal pegmatite veins. Hedenbergite was noted in hornfels from xenoliths of volcanic and sedimentary rocks and in a hydrothermal pegmatite vein at Mount Eveslogchorr. Aegirine-augite is the predominant pyroxene in all types of nepheline syenites, phonolites and tinguaites, foidolites, alkaline and ultramafic rocks of dike series, fenitized wall rocks surrounding the pluton, and xenoliths of Devonian volcanic and sedimentary rocks. Aegirine is an abundant primary or, more often, secondary mineral in nepheline syenites, foidolites, and hydrothermal pegmatite veins. It occurs as separate crystals, outer zones of diopside and aegirine-augite crystals, and homoaxial pseudomorphs after Na-Ca amphiboles. Microprobe analyses of 265 pyroxenes samples allowed us to distinguish ten principal trends of isomorphic replacement and corresponding typomorphic features of pyroxenes. Compositional variations in clinopyroxenes along the sampled 35-km profile from the margin of the Khibiny pluton to its center confirm the symmetric zoning of the foyaite pluton relative to semicircular faults of the Minor Arc and the Main (Central) Ring marked by Devonian volcanic and sedimentary rocks, foidolites, and related metasomatic rocks (rischorrite, albitite, and aegirinite). Changes in the composition of pyroxenes are explained mainly by the redistribution of elements between coexisting minerals of foyaites in the process of their intense differentiation under the effect of foidolite melts that have intruded into the circular fault zones.  相似文献   

5.
Felsic alkalic rocks are a minor component of many ocean island volcanic suites, and include trachyte and phonolite as well as various types of alkaline and peralkaline rhyolite. However, there is considerable debate on the nature of their formation; for example, are they formed by partial melting of anomalous mantle or the final products of fractional crystallization of mafic magmas. The phonolites and foidal phonolites on Rarotonga were formed by low pressure crystal fractionation of two chemically distinct parental magmas. Low silica and high silica mafic magmas produced a basanite-foidal phonolite series and an alkali basalt-phonolite series, respectively. The foidal phonolite composition evolved from the low silica mafic magmas by approximately 60% fractionation of titanaugite + leucite + nepheline + magnetite + apatite. Fractionation continued with the crystallization of aegirine-augite + nepheline + kaersutite + magnetite + apatite. The phonolites formed from the alkali basalts by approximately 40% fractionation of kaersutite + titanaugite + Fe-Ti oxide + plagioclase + apatite and continued to evolve further by fractionation of anorthoclase + nepheline + aegerine-augite + Fe-Ti oxides. As the magmas fractionated in both suites, their overall viscosities (solid + liquid) increased until a point was reached whereby viscosity inhibited the eruption of magmas with compositions intermediate between the mafic rocks and the felsic rocks. However, the magmas continued to fractionate under static conditions with the residual fluid becoming foidal phonolitic in the low silica suite or phonolitic in the high silica suite. These phonolitic liquids, as a result of an increase in volatiles and enrichment of alkalis over aluminum, would actually have a lower viscosity than the intermediate liquids. This decrease in viscosity and the switch from a magma chamber being predominantly a liquid with suspended solids to a solid crystalline network with an interstitial liquid enabled phonolitic liquids to migrate, pool, and eventually erupt on the surface.  相似文献   

6.
The alkaline igneous centres of eastern Uganda comprise a youngerTertiary group represented by dissected volcanoes and an olderCretaceous (?) group composed entirely of equivalent intrusiveformations. Both the extrusive and the intrusive suites are characterizedby extreme silica undersaturation and richness in alkalis, especiallysoda; they largely conform to a nephelinite series and an ijolite-carbonatiteseries, respectively. From field, petrographic, and chemical data it is argued thatthe immediately parental magma was of nephelinite/melteigitecomposition. The courses of evolution of the volcanic and plutonicrocks followed odivergent trends, owing to differences in theirphysico-chemical environments. The volcanic series evolved towardsa phonolitic or trachytic residium, whereas the ijolite seriesmaintained silica undersaturation and tended towards final enrichmentin lime, alkalis, and volatiles, including carbon dioxide. Thefinal residuum from this course of evolution was a carbonatiteor alkali carbonatite. The evidence suggests that the felspar-bearing rocks (trachytes,phonolites, syenites, and nepheline syenites) have had a numberof different modes of origin. Apart from evolution in the volcanicseries of crystallization differentiation, they appear to haveformed by fenitization of Basement rocks and by felspathizationof felspathoidal rocks, such as ijolites. Extrusive equivalentsmay then have arisen by mobilization.  相似文献   

7.
The Sivamalai alkaline complex comprises ferro-, pyroxene-hornblende- and nepheline-syenites. Field relations show that the nepheline syenites followed the emplacement of non-feldspathoidal syenites. Mineralogical data on the syenite suite have been reviewed. The Sivamalai alkaline rocks are not strongly enriched in rare-earth elements like most miaskites. Rb-Sr isotopic analyses of a suite of six samples from the various members of the complex define an isochron corresponding to an age of 623 ±21 Ma (2σ) and initial Sr ratio of 0.70376 ±14 (2σ). This is consistent with a model of fractional crystallization of a parent magma derived from an upper mantle source with apparently no isotopic evidence for more than one magma source for the complex. The Sivamalai alkaline complex represents a Pan-African alkaline magmatic event in the southern granulite terrane of Peninsular India.  相似文献   

8.
In order to test the geochemical models used in repository performance assessment, modelling groups were provided with selected major element analyses of Poços de Caldas groundwaters and asked to predict “blind” the solubility, speciation and limiting solid for a number of trace elements. This paper documents these predictions and compares them to field analyses. These tests illustrate particular stengths and weaknesses in current models/databases and allow recommendations for amendments/improvements to be made.  相似文献   

9.
The thorium and rare-earth element (Th-REE) deposit at Morro do Ferro formed under supergene lateritic weathering conditions. The ore body consists of shallow NW-SE elongated argillaceous lenses that extend from the top of the hill downwards along its south-eastern slope. The deposit is capped by a network of magnetite layers which protected the underlying highly weathered, argillaceous host rock from excessive erosion. The surrounding country rocks comprise a sequence of subvolcanic phonolite intrusions that have been strongly altered by hydrothermal and supergene processes.From petrological, mineralogical and geochemical studies, and mass balance calculations, it is inferred that the highly weathered host rock was originally carbonatitic in composition, initially enriched in Th and REEs compared to the surrounding silicate rocks. The intrusion of the carbonatite caused fenitic alteration in the surrounding phonolites, consisting of early potassic alteration followed by a vein-type Th-REE mineralization with associated fluorite, carbonate, pyrite and zircon. Subsequent weathering has completely decomposed the carbonatite forming a residual supergene enrichment of Th and REEs.Initial weathering of the carbonatite has created a chemical environment that might have been conductive to carbonate and phosphate complexing of the REEs in groundwaters. This may have appreciably restricted the dissolution of primary REE phases. Strongly oxidic weathering has resulted in a fractionation between Ce and the other light rare earth elements (LREEs). Ce3+ is oxidized to Ce4+ and retained together with Th by secondary mineral formation (cerianite, thorianite), and by adsorption on poorly crystalline iron- and aluminium-hydroxides. In contrast, the trivalent LREEs are retained to a lesser degree and are thus more available for secondary mineral formation (Nd-lanthanite) and adsorption at greater depths down the weathering column. Seasonally controlled fluctuations of recharge waters into the weathering column may help to explain the observed repetition of Th-Ce enriched zones underlain by trivalent LREE enriched zones.  相似文献   

10.
Malignites from the Poohbah Lake complex of northwestern Ontario, Canada are melanocratic cumulates. Cumulus pyroxene and apatite are poikilitically enclosed in a groundmass of large plates of intercumulus orthoclase and nepheline. Nepheline-feldspar fingerprint-like intergrowths occur. Nephelines are commonly zeolitized and pyroxenes altered to aggregates of biotite and/or garnet by deuteric alteration. Pyroxenes are weakly zoned from Di71 Hd18Ac11 to Di63Hd22Ac15, and are similar to the least evolved pyroxenes of other alkaline rocks. Nephelines all have compositions within the Morozewicz-Buerger convergence field and feldspars have a limited compositional range from Or88 to Or95. Perthites are absent.Inconsistancies in the usage of the terms malignite and juvite are discussed and it is considered that a non-genetic petrographic classification of nepheline syenites leads to the obscuration of a group of potassic nepheline syenites, characterized by the presence of nepheline plus orthoclase which are typically associated with saturated to over-saturated alkaline rocks, contain pseudo-leucite or nepheline-orthoclase intergrowths, are emplaced in mobile belts and are not associated with rocks of the ijolite-carbonatite suite.A genetic classification of nepheline syenites is suggested and it is proposed that; (1) mafic-rich nepheline syenites be referred to as mela-nepheline syenites (sensu lato) rather than as malignites; (2) the term malignite be used for magmatic potassic nepheline syenites characterised by the presence of nepheline plus a single potassium-rich feldspar (orthoclase or microcline) and devoid of exsolution perthite under subsolvus conditions; (3) the metasomatic malignites and juvites of ijolite-carbonatite complexes be referred to as varieties of fenites.  相似文献   

11.
对青藏高原西南部当惹雍错地区的中新世超钾质粗面岩及共存的富钠质方沸石(霞石)响岩进行了详细的岩石学、矿物学研究,揭示出两套岩石具有不同的矿物学特征.钠质方沸石(霞石)响岩中橄榄石斑晶的熔体包裹体与钾质火山岩的成分相当,应属于早期残余的钾质岩浆,表明了钠质方沸石(霞石)响岩的形成晚于超钾质粗面岩.两类岩石的全岩化学成分、...  相似文献   

12.
王玉生  李文君 《铀矿地质》1995,11(3):140-146
中新生代时期，华南东部地壳经历了挤压－松驰拉张－局部裂解的演化过程。相应地发育３期火山活动，构成初火山族回、主火山旋回和破火山旋回。破火山使回出现双峰式岩套，标志着已进入胚胎型裂谷演化阶段。火山岩型铀矿具有破火山族回成矿、破火山旋回活动带控矿、二次迭加富集特性，可称其为破火山系列新不整合脉型铀矿。划分为体型、层型和脉型３种形态类型和相应的勘查模式。华南东部破火山系列新不整合脉型铀矿具有巨大的找矿潜力，在花岗岩基底上发育的破火山旋回构造－岩浆杂岩区，是今后寻找大型铀矿的有利靶区。  相似文献   

13.
In the Sierras Pampeanas of San Luis, Argentina, Late Tertiary volcanic rocks extend along a 80-km NW-SE-trending belt, between La Carolina and Sierra del Morro. Several gold deposits, among which those in the western end of the belt are better known, are genetically related to the volcanic rocks, formed during a volcanic episode that occurred between 9.5 Ma and 1.9 Ma. Located 600 km from the Peru-Chile trench, the volcanic belt represents the easternmost and youngest mineralized magmatic manifestation associated with the shallowing of the Nazca plate in the flat-slab Andean segment extending from 28° to 33° S Lat.

The volcanic complex includes lavas and volcaniclastic rocks. Small-volume lavas were emplaced as domes, flows, and dikes. Pyroclastic deposits are associated with them in certain areas, such as at La Carolina, Cerro Tiporco, and Sierra del Morro. At La Carolina, phreatomagmatic breccias and base-surge deposits define a maar-diatreme volcanic setting. At Cerro Tiporco and Sierra del Morro, the volcaniclastic units are related to the formation of calderas. Mesosilicic magmas (SiO2 = 59% to 68%) belong to normal to high-K calc-alkaline and shoshonitic magma types. At both local and regional scales, K enrichment accompanies progressively decreasing age. Although the volcanic rocks differ from the typical Andean series, some geochemical features, such as Ta and Ti depletion, high large-ion-lithophile-element (LILE) contents, and arc-like Ba/La and La/Ta ratios, indicate an arc signature.

In the La Carolina zone, the most important mineralization is the La Carolina volcanic-hosted, low-sulfidation, epithermal gold deposit. Here, several gold and base-metal-bearing epithermal veins cut basement rocks. In the Canada Honda district, the most important mineral deposits are the Diente Verde gold-rich porphyry copper deposit and low-sulfidation epithermal gold and base-metal veins hosted by both basement and coeval volcanics.

There is no strong evidence of gold-bearing mineral deposits on the eastern side of the volcanic belt. However, there are hydrothermal alteration zones at Cerros del Rosario and El Morro as well as traces of gold at the Santa Isabel calcareous onyx deposit and inside the Sierra del Morro caldera. In addition, favorable volcanic structures, such as the calderas at Tiporco, Cerro Lomita, and El Morro, make the eastern side of the belt an interesting target for mineral exploration.  相似文献   

14.
The precision dating (U-Pb local by zircons, SHRIMP-II) of volcanic rocks in the unique uranium-bearing structure of Transbaikalia is performed for the first time. The basic conclusions are as follows. The volcanic activity in the Tulukuev caldera covers the period of not less than 30–35 mln years, within the period from (not later than) 162 to 128 mln years. Two stages of caldera evolution are established: the early (trachydacite-basalt) stage up to 154 mln years and the late (trachybasalt-rhyolite) stage from 142 to 128 mln years, with a 10 mln year break, which caused the deep erosion of the lower layer. Three phases of rhyolite magmatism are substantiated. The first one, 142 mln years, is the ejection of ignimbrites (microfelsitic rhyolites); the second one, 137–135 mln years, is the outflow of lavas of sanidine-morion rhyolites and subvolcanic and ring dyke intrusions. The third phase, 128 mln years, is connected with the occurrence of cesium-bearing perlites in the southwestern part of the caldera. The age of the granite-porphyries of the Krasnokamensk stock almost coincides with the precision data of the age of the uranium ores [4]. It is found that zircons from the granite-porphyries within the ore field of the Argunsk deposit have an anomalously high content of uranium. This fact can additionally testify to the time-and-spatial closeness of magmatism and processes of ore formation.  相似文献   

15.
The Abu Rumeil syenitic rocks represent the inner ring dyke of the Katherina Ring complex, southern Sinai, Egypt. They are divided petrologically into two types, alkali feldspar syenite and quartz syenite. The mineralogy and geochemistry of the syenites indicate an alkaline nature with a shoshonitic affinity. Although rare mafic xenocrysts overgrown by primary K-feldspars and overlapping rare earth element (REE) patterns indicate some role for crustal contamination, the trace element chemistry shows a dominant mantle contribution. The geochronology and field relations imply that the Abu Rumeil syenites were emplaced in a post-collisional, within-plate tectonic setting, yet they express the enrichments in large-ion lithophile elements relative to high field strength elements generally characteristic of subduction influence. We suggest that this signature is inherited from partial melting of a lithospheric mantle source previously affected by subduction during assembly of the Arabian-Nubian Shield. Little evidence of the early evolution of the suite is preserved; there are no associated mafic rocks. We therefore restrict our attention to a petrogenetic model that can explain the relations among the observed felsic composition. The REE patterns of all samples are enriched in light REE and fractionated, but it is notable that there are small positive Eu anomalies in the alkali-feldspar syenites contrasting with small negative Eu anomalies in the quartz syenites. Positive Eu anomalies suggest a cumulate nature for the alkali-feldspar syenites; there are also breaks in the slopes of most variation trends between the alkali-feldspar syenites and the quartz syenites. The general trends in all major oxides and trace elements within the suite can be modeled by fractional crystallization of feldspars—with smaller roles for pyroxene, biotite, apatite, and Fe-Ti oxides—from an intermediate liquid to form the quartz syenites and by assimilation of the near-liquidus phases into the same starting liquid to form the alkali feldspar syenites. The geothermobarometry of pyroxenes and amphiboles suggests shallow emplacement (<10 km depth) and crystallization temperatures ranging from 1100 °C down to 800 °C.  相似文献   

16.
Feldspar phenocrysts, microphenocrysts, groundmass feldspar, interstitial material of feldspar composition, and residual SiO2-K2O-rich glass in 24 rocks of the tholeiitic, alkalic, and nephelinic suites from Haleakala and West Maui volcanoes, Maui, Hawaii, were analyzed quantitatively with the electron microprobe. Rocks studied include tholeiite, olivine tholeiite, oceanite, alkalic olivine basalt, alkalic basalt, hawaiite, mugearite, trachyte, basanite, and basanitoid. Results and conclusions: i) In all rocks studied, An decreases and Or increases from phenocrysts to microphenocrysts to groundmass feldspar to interstitial material of feldspar composition. ii) Phenocrysts occur in rocks of the tholeiitic and alkalic suites and, in spite of differences in bulk rock compositions, overlap in composition. iii) Groundmass feldspar in rocks of the tholeiitic suite are nearly identical in composition; the same is true for rocks of the nephelinic suite. However, in the highly differentiated alkalic suite, groundmass feldspar composition ranges from labradorite to sanidine; i.e. the higher the bulk rock CaO, the higher is the An content, and the higher the bulk K2O, the higher is the Or content. iv) In general, rocks with phenocrysts have groundmass feldspar less An-rich than those without phenocrysts. v) In rocks of the tholeiitic suite, normative feldspar approaches modal feldspar. However, in rocks of the alkalic and nephelinic suites, normative feldspar, because of the presence of highly alkalic interstitial material and the absence of nepheline in the mode but its presence in the norm, is drastically different from modal feldspar. vi) Hawaiites contain labradorite and not andesine, as per definition, and mugearite contains andesine and not oligoclase, as groundmass feldspar. In fact, when considering phenocrysts and interstitial material of feldspar composition, hawaiites range from bytownite to sanidine and mugearite from andesine to sodic sanidine, but normative feldspar plots in the andesine field for hawaiites and the oligoclase field for mugearite. vii) Rocks of the three suites can be distinguished on the basis of Or and An in groundmass feldspar, the presence of thin rims of groundmass composition of phenocrysts of rocks of the alkalic suite, and the presence of interstitial material of anorthoclase to sanidine composition in rocks of the alkalic and nephelinic suites. iix) Rocks transitional between the tholeiitic and alkalic suites are observed and are characterized by transitional mineral compositions.This paper was first presented as a talk before the 68. Annual Meeting of the Cordilleran Section of the Geological Society of America, Honolulu, Hawaii, March 29–April 1, 1972.  相似文献   

17.
The paper describes a suite of volcanic and intrusive rocks from Ua Pu, one of the Marquesas Islands, situated in the central Pacific Ocean. The rocks comprise alkali olivine basalts, hawaiites, mugearites, trachytes, and phonolites. Their petrographic characters are briefly described and 24 new chemical analyses presented. The rocks fall into a sodic and a potassic series, since some rocks are richer in K2O than Na2O, whilst the phonolites have exceptionally high total alkali contents.The differentiation of the series is discussed and special attention is paid to the phonolites in which icositetrahedral pseudomorphs occur. It is concluded that these pseudomorphs are probably after analcime rather than leucite.  相似文献   

18.
An early Cretaceous alkaline ultramafic-mafic complex is emplaced within the Proterozoic rocks of Shillong plateau at Jasra, Karbi Anglong district of Assam. It is associated to the fracture system of Barapani-Tyrsad shear zone, Kopali faults, and Um Ngot lineaments and mainly comprises pyroxenite, gabbro and nepheline syenite. Few small mafic dykes, emplaced within pyroxenitic and granitic plutons, are also reported. No such dyke is reported to cut gabbros or nepheline syenites. Nepheline syenites occur either in the form of small dykes in pyroxenites or as differentiated bodies in the gabbros. Mineralogical and chemical composition of pyroxenite and gabbro clearly indicate their affinity to the alkaline magmatism. Syenitic samples show miaskitic character (agpaitic index <1), also indicates affinity with alkaline-carbonatite magmatism. Calcite is encountered in a number of pyroxenite samples. From the presented petrological and geochemical data it is difficult to establish any significant genetic relationship through simple differentiation process between these rocks. These data probably suggest that these rocks are derived from a primary carbonatite magma, generated by the low-degree melting of a metasomatized mantle peridotite. CO2 released by this process also progressively metasomatizes the lherzolite to an alkaline wehrlite and melts derived from alkaline wehrlite (ultrabasic alkaline silicate magma) may be responsible for crystallization of Jasra alkaline ultramafic-mafic rocks.  相似文献   

19.
Major, trace element and Sr isotopic compositions have been determined on 21 lava samples from Vico volcano, Roman Province, Central Italy. The rocks investigated range from leucite tephritic phonolites to leucite phonolites and trachytes. Trace element compositions are characterized by high enrichments of incompatible elements which display strong variations in rocks with a similar degree of evolution. Well-defined linear trends are observed between pairs of incompatible trace elements such as Th-Ta, Th-La, Th-Hf. A decrease of Large Ion Lithophile (LIL) elements abundance contemporaneously with the formation of a large central caldera is one of the most prominent characteristics of trace element distribution. Sr isotope ratios range from 0.71147 to 0.71037 in the pre-caldera lavas and decreases to values of 0.70974–0.70910 in the lavas erupted after the caldera collapse. Theoretical modelling of geochemical and Sr isotopic variations indicates that, while fractional crystallization was an important evolutionary process, AFC and mixing also played key roles during the evolution of Vico volcano. AFC appears to have dominated during the early stages of the volcanic history when evolved trachytes with the highest Sr isotope ratios were erupted. Mixing processes are particularly evident in volcanites emplaced during the late stages of Vico evolution. According to the model proposed, the evolution of potassic magmas emplaced in a shallow-level reservoir was dominated by crystal fractionation plus wall rock assimilation and mixing with ascending fresh mafic magma. This process generated a range of geochemical and isotopic compositions in the mafic magmas which evolved by both AFC and simple crystal liquid fractionation, producing evolved trachytes and phonolites with variable trace element and Sr isotopic compositions.  相似文献   

20.
Seven spinel-group minerals in various geological settings have been revealed in the rocks of the Khibiny pluton. Hercynite, gahnite, and vuorelainenite occur only in xenoliths of hornfels after volcanic and sedimentary rocks, whereas spinel and magnesiochromite occur in alkaline ultramafic rocks of dike series. Franklinite has been discovered in a low-temperature hydrothermal vein. Ubiquitous magnetite is abundant in foyaite, foidolites, alkaline ultrabasic rocks, and pegmatite and hydrothermal veins and may even be the main mineral in some foidolite varieties. The spinel-group minerals are characterized by various chemical compositions due to the fractionation of nepheline syenites resulting in formation of the Main ring of foidolites and apatite-nepheline ore. Like most other minerals found throughout the pluton, magnetite is characterized by variation in the chemical composition along the radial line from the contact with country Proterozoic volcanic rocks to the geometric center of the pluton. Toward the center, the total Ti and Mn contents in magnetite increase from 5–15 up to 40 at %.  相似文献   

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