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1.
Sixty-nine analyses are given for NH4 in minerals of metamorphic and granitic rocks mostly from the Ryoke belt, Japan. The distribution of NH4 in coexisting minerals is quite systematic, suggesting that NH4 is one of the stable geochemical components in high temperature processes.Biotite has the highest content of NH4, followed by muscovite, K-feldspar and plagioclase. Pure quartz is almost free from NH4. Calcic plagioclase contains less NH4 than does sodic plagioclase. The partition coefficients , and are, on the average, 0.11, 0.38 and 0.43 respectively. The fractionation of NH4 in these minerals is quite similar to that of Rb but much smaller than that of Cs.Distribution of NH4 as well as those of Rb and Cs appears to be explained by its ionic radius and the shortest cation-O distances in alkali positions of minerals. 相似文献
2.
The Archean Yellowknife Supergroup (Slave Structural Province. Canada) is composed of a thick sequence of supracrustal rocks, which differs from most Archean greenstone belts in that it contains a large proportion ( ~ 80%) of sedimentary rocks. Felsic volcanics of the Banting Formation are characterized by HREE depletion without Eu-anomalies, indicating an origin by small degrees of partial melting of a mafic source, with minor garnet in the residua. Granitic rocks include synkinematic granites [HREE-depleted; low ()I], post-kinematic granites [negative Eu-anomalies, high ()I] and granitic gneisses with REE patterns similar to the post-kinematic granites. Sedimentary rocks (turbidites) of the Burwash and Walsh Formations have similar chemical compositions and were derived from 20% mafic-intermediate volcanics, 55% felsic volcanics and 25% granitic rocks. Jackson Lake Formation lithic wackes can be divided into two groups with Group A derived from 50% mafic-intermediate volcanics and 50% felsic volcanics and Group B, characterized by HREE depletion, derived almost exclusively from felsic volcanics.REE patterns of Yellowknife sedimentary rocks are similar to other Archean sedimentary REE patterns, although they have higher . These patterns differ significantly from typical post-Archean sedimentary REE patterns, supporting the idea that Archean exposed crust had a different composition than the present day exposed crust. 相似文献
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Structural control of the chlorine content of OH-bearing silicates (micas and amphiboles) 总被引：1，自引：0，他引：1
M. Volfinger J.-L. Robert D. Vielzeuf A.M.R. Neiva 《Geochimica et cosmochimica acta》1985,49(1):37-48
Experimental studies of the incorporation of chlorine in trioctahedral biotite-like micas, belonging to the series phlogopite-annite, phlogopite-KCo3AlSi3O10(OH)2 and phlogopite-KNi3AlSi3O10(OH)2, were performed at 600°C and 2 kbars, with a duration of two weeks.The results confirm for the incorporation of an anion in a crystal structure, the fundamental role of the dimension of the anion site, as has been established for cations in previous works. In biotites, the dimension of (OH-Cl) site is mainly controlled by the rotation angle α of the tetrahedra around a direction approximately parallel to .The experiments were performed using hydrothermal solutions with KCl? 0.5 M; under these conditions, the quantity of incorporated chlorine does not exceed ?0300 ppm in the most receptive mica (annite) and is twenty times less in the less receptive ones (phlogopite, for example).These results are applied to natural biotites in porphyry copper deposits, metamorphic rocks and mafic rocks. We conclude that most natural biotites which have a chlorine content of 1000 ppm or more crystallized in equilibrium with a fluid phase with chloride contents of several molar (minimum 3 M).The consideration of micas applies in the same way to amphiboles. A clear correlation between the Cl content and XFe is observed which can be interpreted in terms of local structure of the minerals. The structural factors which favour the fixation of chlorine, a large anion are the same which favour the fixation of large alkali cations (replacement of Na by K). This explains the observed correlations between Cl and K in natural amphiboles. 相似文献
6.
The Hidra Massif (Rogaland complex, S.W. Norway) is a massif-type anorthositic-charnockitic body. It consists of undeformed anorthosites and leuconorites, grading into fine-grained jotunites at the contact with the granulite facies gneisses of the metamorphic envelope. A stockwork of charnockitic dykes cross-cuts the massif. The Pb isotopic compositions of the anorthosites and leuconorites are comparable or slightly less radiogenic than those of the jotunites ( from 18.079 to 19.307,( from 15.568 to 15.657 and from 37.617 to 38.493). These values are compatible with an upper mantle origin for the parental magma of jotunitic composition and for the plagioclasic cumulates, but show the incorporation of lower crustal material (U-depleted and thus less radiogenic). The charnockitic dykes have significantly less radiogenic Pb isotopic compositions ( from 17.472 to 19.171, from 15.489 to 15.620and from 36.991 to 40.922) which can be explained by a larger proportion of lower crustal contamination material. The contaminant could be the granulite facies gneisses of the metamorphic envelope. This interpretation is compatible with the K-Rb relationships of these rocks and with the O and Sr isotopic geochemistry.The proportion of contaminating lead in the charnockitic dykes can be estimated at 55 ± 15% considering the border facies jotunite as the uncontaminated parental magma and the least radiogenic gneiss of the metamorphic envelope as the contaminant. 相似文献
7.
To better understand the process of crustal contamination/assimilation, 23 Pb isotopic compositions and 12 concentrations have been measured on lavas and basement rocks from the Edgecumbe volcanic field, SE Alaska. Measured isotopic ratios have the following ranges: ; ; . While the data form well-constrained linear arrays on Pb-Pb diagrams, no simple correlation exists with major element composition. Basaltic lavas (≤ 51 wt% SiO2) are characterized by two isotopic groups. The olivine basalt (≤ 48% SiO2) is more radiogenic than the plagioclase basalt (48–51%) which also shows more heterogeneity. In the silica range 52–55%, Pb isotopic ratios increase significantly but remain fairly constant in the range 55–70% SiO2. Lead concentrations vary from 1 ppm in the basalts to 7 ppm in the rhyodacites. Analyzed basement rocks are more radiogenic than any of the lavas ; ; . The Pb isotopic data are qualitatively consistent with the contamination process described by Myerset al. (1984). However, because of fundamental differences in the mixing relations between the Sr system studied earlier and the Pb system, the new Pb data have revealed details of the process not apparent from the Sr data alone. In particular, it has been shown that the parent magma was more primitive than originally assumed, and that two contamination events are recorded in the lavas. The first event, involving a mafic parent and different crustal contaminants, produced the intermediate and siliceous hybrids in cupolas located above the main basaltic chamber. The types of country rock intruded as well as the degree of partial fusion achieved in individual cupolas controlled the range of hybrid compositions produced while the eruption sequence was determined by the order in which the cupolas were tapped. The second contamination event produced the plagioclase basalt, the most voluminous basaltic unit, by mixing the mafic parent with the olivine basalt, an independent, primary magma. Our results suggest crustal contamination models that assume bulk assimilation of crustal end members may be too simplistic. 相似文献
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Initial 87Sr/86Sr ratios, major and trace element compositions have been determined for the Paleogene granitic rocks in the Tsukuba district, Japan. Isotopic ages strongly suggest that the granitic rocks (seven units) were continuously emplaced and solidified during a short time interval. Initial 87Sr/86Sr ratios for seven granitic units vary from 0.7082 to 0.7132, while sedimentary and metasedimentary country rocks have ratios at the time of granitic magma emplacement ranging from 0.7149 to 0.7298. Continuous linear arrays for the granitic rocks in the diagrams of initial 87Sr/86Sr ratios versus some chemical parameters can be explained by either of following two processes. One is the assimilation — fractional crystallization (AFC) process between the parental magma (SiO2 of 68% and initial ratio of 0.7078) and sedimentary country rocks, and the other is magma mixing process between above parental magma and sediment derived acidic magma (melt) (SiO2 of 75%). The high initial ratios (0.7078–0.7098) for basic rocks such as gabbro or diorite in the Tsukuba district and the similar characteristics observed in the rocks of Ryoke belt (SW Japan) suggest that the parental magma had the same source region as the basic rocks, probably the lower crustal source. 相似文献
9.
Gerard Gruau Herve Martin Bernard Leveque Raymond Capdevila Alain Marot 《Precambrian Research》1985,30(1):63-80
Nine samples of metavolcanic rock from the lower parts of greenstone belts in central French Guiana (the Paramaca series) and 14 granitic samples from the intrusive gneisses (the Degrad Roche and Arawa gneisses) were selected for Sm—Nd and Rb—Sr analysis.The Sm—Nd results from the metavolcanic series (including two tholeiites, five peridotitic komatiites and two andesites) yield an isochron age of 2.11±0.09 (2 σ) Ga with an initial ratio (INd) of 0.51002±9 (2 σ), corresponding to ?Nd(T) = + 2.1 ± 1.8. This isochron is interpreted as representing the age of initial volcanism of the Paramaca series. Acid intrusives were dated by the Rb—Sr method. A whole rock Rb—Sr isochron, including data points from both the Degrad Roche and Arawa gneisses, yields an age of 2.00±0.07 (2 σ) Ga with initial ratio (ISr value) of 0.7019±4 (2 σ). This result is considered to be the time of emplacement of the orthogneiss protoliths.The positive εNd value (+ 2.1 ± 1.8) obtained from the metavolcanic rocks of French Guiana suggests that their mantle sources have evolved in reservoirs slightly depleted in Light Rare Earth Elements (LREE). This result confirms the possible existence of ancient LREE-depleted reservoirs within the lower Proterozoic mantle. Moreover, the high εNd(T) value for these rocks excludes any significant crustal contamination during magma genesis.The French Guianese orthogneisses yield a low ISr value (0.7019±4 (2 σ)) which, together with geochemical considerations, suggests that their granitic protoliths could have originated by partial melting of short-lived crustal precursors of basaltic to granodioritic composition.The present geochronological and isotopic study suggests that the Guiana Shield may represent a major continental accretion event during the lower Proterozoic. 相似文献
10.
High precision mass spectrometric determination of calcium isotope ratios allows the 40K → 40Ca radioactive decay to be used for dating a much broader range of geologic materials than is suggested by previous work. is used to monitor enrichments in 40Ca and can be measured to ±0.01% (2σ) using an exponential mass discrimination correction (Russell et al., 1978) and large ion currents. The earth's mantle has such a low (～0.01) that it has retained “primordial” ± 0.011 (normalized to ), as determined by measurements on two meteorites, pyroxene from an ultramafic nodule, metabasalt, and carbonatite. ratios can be conveniently expressed relative to this value as ?Ca in units of 10?4. To test the method for age dating, a mineral isochron has been obtained on a sample of Pikes Peak granite, which has been shown to have concordant KAr, RbSr, and UPb ages. Plagioclase, K-feldspar, biotite, and whole rock yield an age of 1041 ± 32 m.y. (2σ) in agreement with previous age determinations (λK = 0.5543 b.y.?1, , 40K = 0.01167%). The initial of 151.024 ± 0.016 (?Ca = +0.5 ± 1.0), indicates that assimilation of high K/Ca crust was insufficient to affect calcium isotopes. Measurements on two-mica granite from eastern Nevada indicate that the magma sources had K/Ca ≈ 1, similar to intermediate-composition crustal rocks. These results show that the KCa system can be used as a precise geochromometer for common felsic igneous and metamorphic rocks, and may prove applicable to sedimentary rocks containing authigenic K minerals. The relatively short half-life of 40K, the non-volatile daughter, and the fact that potassium and calcium are stoichiometric constituents of many minerals, make the KCa system complementary to other dating methods, and potentially applicable to a variety of geologic problems. 相似文献
11.
Robert L. Cullers Subramanian Ramakrishnan Pieter Berendsen Tom Griffin 《Geochimica et cosmochimica acta》1985,49(6):1383-1402
Lamproite sills and their associated sedimentary and contact metamorphic rocks from Woodson County, Kansas have been analyzed for major elements, selected trace elements, and strontium isotopic composition. These lamproites, like lamproites elsewhere, are alkalic (molecular ), are ultrapotassic , are enriched in incompatible elements (LREE or light rare-earth elements, Ba, Th, Hf, Ta, Sr, Rb), and have moderate to high initial strontium isotopic compositions (0.7042 and 0.7102). The silica-saturated magma (olivine-hypersthene normative) of the Silver City lamproite could have formed by about 2 percent melting of a phlogopite-garnet lherzolite under high ratios in which the Iherzolite was enriched before melting in the incompatible elements by metasomatism. The Rose Dome lamproite probably formed in a similar fashion although the extreme alteration due to addition of carbonate presumably from the underlying limestone makes its origin less certain. Significant fractional crystallization of phases that occur as phenocrysts (diopside, olivine, K-richterite, and phlogopite) in the Silver City magma and that concentrate Co, Cr, and Sc are precluded as the magma moved from the source toward the surface due to the high abundances of Co, Cr, and Sc in the magma similar to that predicted by direct melting of the metasomatized Iherzolite.Ba and, to a lesser extent, K and Rb and have been transported from the intrusions at shallow depth into the surrounding contact metamorphic zone. The Silver City lamproite has vertical fractionation of some elements due either to volatile transport or to variations in the abundance of phenocrysts relative to groundmass most probably due to flow differentiation although multiple injection or fractional crystallization cannot be conclusively rejected. 相似文献
12.
and ratios have been measured for whole-rock samples and mineral separates from the mafic and ultramatic rocks of the Cambro-Ordovician Highland Border Suite. The H- and O- isotopic compositions of these rocks record individual stages in a relatively complex 500 Myr old hydrothermal/metamorphic history. Lizardite serpentinites (δD ～ ? 105‰; δ18O ～ + 6.2‰) record a premetamorphic history and indicate that parent harzburgites, dunites, and pyroxenites were serpentinized through low-temperature interaction with meteoric waters during cooling. The other rocks of the Highland Border Suite record subsequent interaction with metamorphic fluids. Amphibolite facies hornblende schists were produced through thrust-related (dynamothermal) metamorphism of spilitic pillow lavas. During dehydration, D-enriched fluids were driven off from the spilites thus leaving the hornblende schists to equilibrate with a relatively D-depleted internal fluid reservoir (δD ～ ? 45‰). The expelled D-enriched fluids may have mixed with more typical Dalradian metamorphic waters which then exchanged with the remaining mafic rocks and lizardite serpentinites during greenschist facies regional metamorphism to produce antigorite serpentinites (δD ～ ? 62‰; δ18O ～ + 8‰) and greenschist metaspilites (δD ～ ? 57‰; δ18O ～ + 7.3‰) with similar H- and O-isotopic compositions. Serpentinites which have been only partially metamorphosed show intermediate H-isotopic compositions between that of metamorphic antigorite (δD ～ ? 62‰) and non-metamorphic lizardite δD ～ ? 105‰) end members. 相似文献
13.
Archean metasedimentary rocks occur as components of the Isua supracrustals, Akilia association and Malene supracrustals of southern West Greenland. Primary structures in these rocks have been destroyed by metamorphism and deformation. Their chemistry and mineralogy is consistent with a sedimentary origin, but other possible parents (e.g. acid volcanics, altered pyroclastic rocks) cannot be excluded for some of them. There is little difference in the composition of metasedimentary rocks from the early Archean Isua supracrustals and probable correlative Akilia association. Both have a wide range in rare earth element (REE) patterns with ranging from 0.61?5.8. The REE pattern of one Akilia sample, with low , compares favourably with that of associated tholeiites and it is likely that such samples were derived almost exclusively from basaltic sources. Other samples with very steep REE patterns are similar to felsic volcanic boulders found in a conglomeratic unit in the Isua supracrustals. Samples with intermediate REE patterns are best explained by mixing of basaltic and felsic end members. Metasedimentary rocks from the Malene supracrustals can be divided into low silica (≤55% SiO2) and high silica (>77% SiO2) varieties. These rocks also show much variation in (0.46?14.0) and their origin is explained by derivation from a mixture of mafic volcanics and felsic igneous rocks. The wide range in trace element characteristics of these metasedimentary rocks argues for inefficient mixing of the various source lithologies during sedimentation. Accordingly, these data do not rigorously test models of early Archean crustal composition and evolution. The systematic variability in trace element geochemistry provides evidence for the bimodal nature of the early Archean crust. 相似文献
14.
RbSr ( and U-Pb () measurements were undertaken in the Sudbury area, Sudbury, Ontario to determine the ages of the Sudbury Nickel Irruptive, Superior Province granites north of Sudbury, Sudbury Breccia and subsequent metamorphism. The Sudbury Nickel Irruptive norite whole rock Rb-Sr data yield an age of (; all results quoted at 2π level) while the Nickel Irruptive micropegmatite Rb-Sr system has been disturbed and does not yield an isochron. A plagioclase-whole rock pair from the norite near the norite-micropegmatite transition yields an age of 1866 Myr, which when taken in conjunction with field (Stevenson and Colgrove, 1968) and geochemical (Naldrettet al., 1970, 1972) data does not support the conclusion of gibbins and McNurr (1972) that the micropegmatite is a later intrusion rather than a differentiate of the magma which produced the norite. Rb-Sr studies of the Superior Province granites north of Sudbury yield an age of (). U-Pb zircon studies of these granites and granitic clasts within the Sudbury Breccia yield an age of Byr and suggest the breccia granitic clasts were derived from the Superior Province granites. The granitic rocks ~150 km north of Sudbury have been undisturbed for ~ 2.6 Byr based on Rb-Sr mineral studies, whereas the granites and Sudbury Breccia within ~ 15 km of the Nickel Irruptive, as well as the Sudbury norite at the perimeter of the Irruptive have been disturbed by the Penokean Orogeny 1.7–1.75 Byr ago. The Penokean event appears to have overprinted isotopic evidence of the Sudbury impact event at least in the area studied. 相似文献
15.
The solubility of rutile has been determined in a series of compositions in the K2O-Al2O3-SiO2 system ( + Al2O3) = 0.38–0.90), and the CaO-Al2O3-SiO2 system (). Isothermal results in the KAS system at 1325°C, 1400°C, and 1475°C show rutile solubility to be a strong function of the ratio. For example, at 1475°C the amount of TiO2 required for rutile saturation varies from 9.5 wt% () to 11.5 wt% () to 41.2 wt% (). In the CAS system at 1475°C, rutile solubility is not a strong function of . The amount of TiO2 required for saturation varies from 14 wt% () to 16.2 wt% ().The solubility changes in KAS melts are interpreted to be due to the formation of strong complexes between Ti and K+ in excess of that needed to charge balance Al3+. The suggested stoichiometry of this complex is K2Ti2O5 or K2Ti3O7. In CAS melts, the data suggest that Ca2+ in excess of A13+ is not as effective at complexing with Ti as is K+. The greater solubility of rutile in CAS melts when is less than 0.54 compared to KAS melts of equal ratio results primarily from competition between Ti and Al for complexing cations (Ca vs. K).TiKβ x-ray emission spectra of KAS glasses () with 7 mole% added TiO2, rutile, and Ba2TiO4, demonstrate that the average Ti-O bond length in these glasses is equal to that of rutile rather than Ba2TiO4, implying that Ti in these compositions is 6-fold rather than 4-fold coordinated. Re-examination of published spectroscopic data in light of these results and the solubility data, suggests that the 6-fold coordination polyhedron of Ti is highly distorted, with at least one Ti-O bond grossly undersatisfied in terms of Pauling's rules. 相似文献
16.
Garrett W. Brass 《Geochimica et cosmochimica acta》1976,40(7):721-730
The ratio in sea water has varied over geologic time due to the addition of strontium to the sea from rocks with a variety of ratios. The measurements by Petermanet al. (Geochim. Cosmochim. Acta34, 105–120, 1970) of the value of the marine ratio have been confirmed by several other workers and by some new measurements on JOIDES samples. They form the basis of a model calculation of the relative proportions of ‘basaltic’ () and ‘granitic’ () strontium being supplied to the sea. For the last 200 million years, the proportions of these two sources appear to reflect the history of global tectonics; ‘basaltic’ during rifting and increasingly ‘granitic’ during the present episodes of uplift and continental collision 相似文献
17.
J.R Holloway 《Geochimica et cosmochimica acta》1973,37(3):651-666
The stability of the amphibole pargasite [NaCa2Mg4Al(Al2Si6))O22(OH)2] in the melting range has been determined at total pressures (P) of 1.2 to 8 kbar. The activity of H2O was controlled independently of P by using mixtures of H2O + CO2 in the fluid phase. The mole fraction of H2O in the fluid () ranged from 1.0 to 0.2.At P < 4 kbar the stability temperature (T) of pargasite decreases with decreasing at constant P. Above P ? 4 kbar stability T increases as is decreased below one, passes through a T maximum and then decreases with a further decrease in . This behavior is due to a decrease in the H2O content of the silicate liquid as decreases. The magnitude of the T maximum increases from about 10°C (relative to the stability T for ) at P = 5 kbar to about 30°C at P = 8 kbar, and the position of the maximum shifts from at P = 5 kbar to at P = 8 kbar.The H2O content of liquid coexisting with pargasite has been estimated as a function of at 5 and 8 kbar P, and can be used to estimate the H2O content of magmas. Because pargasite is stable at low values of at high P and T, hornblende can be an important phase in igneous processes even at relatively low H2O fugacities. 相似文献
18.
Large amounts of diorite—tonalite magma were intruded into the island-arc successions of the southern Arabian shield between ca. 900 and 700 Ma ago. Major oxide, trace element, rare earth (REE) and isotopic data are presented for two plutons exemplifying older and younger members of this plutonic phase. The Thurrat pluton, which was emplaced into virtually unmetamorphosed volcanics of sequence B, has yielded a 10-point Rb-Sr isochron indicating emplacement 744 ± 22 Ma ago and an initial ratio of 0.70281. It consists of gabbros, diorites, quartz diorites, tonalites and low-Al2O3 trondhjemites which are compositionally primitive, with depleted lithophile element contents and flat REE patterns with a negative Eu anomaly in the trondhjemites. The magma was most probably mantle-derived, and analogies with other trondhjemitic plutons suggest that it was probably emplaced in an island-arc setting. The Bidah pluton, which was emplaced into a compositionally very immature succession of metamorphosed volcanics and volcaniclastics of sequence C, has yielded a nine-point near-isochron (MSWD = 2.86) indicating an age of 901 ± 37Ma and an initial ratio of 0.70246. This date is accepted as the age of emplacement. The rocks are compositionally primitive gabbros, diorites, quartz diorites, tonalites and granodiorites with depleted lithophile element contents, and flat REE patterns with a negative Eu anomaly in the more siliceous components. The country-rock volcanics were formed in an immature island-arc environment, and the composition of the Bidah pluton is suggestive of a mainly mantle-derived magma emplaced into that arc. The data for these two plutons therefore provide further evidence that most of the rock material added to the Arabian shield between 900 and 700 Ma ago was derived from the mantle. 相似文献
19.
Kodjopa Attoh 《Lithos》1976,9(2):75-84
Metamorphic mineral-forming reactions are used to predict the volume proportions (percentages) of minerals in metapelitic rocks. Only minerals whose composition can be expressed in an idealised pelitic system (SiO2Al2O2FeOK2OH2O) are considered in postulating the chemical reactions. The volume proportions predicted from the products of the chemical reactions are compared with the volume proportions of the minerals actually present in selected samples of metapelitic rocks from the Michigamme Formation (Michigan, U.S.A.).Evidence is presented to show that the reaction: 4 produces staurolite poikiloblasts in which the estimated quartz to staurolite ratio is 1:2.28. 相似文献
20.
Thomas W. Dougan 《Precambrian Research》1976,3(4):317-342
Biotite—quartz—oligoclase gneisses constitute the dominant lithology in a 3-b.y.-old metamorphic assemblage in the Venezuelan Guyana Shield. The assemblage includes basaltic amphibolites as well as granitic gneisses, iron formation, and other metasedimentary lithologies.Consideration of major- and trace-element compositions indicates that both the biotite gneisses and basaltic amphibolites are meta-igneous. The amphibolites have oceanic tholeiite compositions.Two groups of biotite gneisses can be differentiated by chemical criteria. Both groups have major and trace-element compositions which allow their derivation as partial melts of tholeiite compositions at mantle depth, as has been suggested for similar rocks in other areas. However, the compositional correspondence of the gneisses with low variance liquidus loci in low-PT synthetic systems, and their relatively oxidized character, appear more compatible with an origin by partial melting of graywackes at crustal levels. For both groups of gneisses, primary melts with relatively low ratios can be postulated which are credibly derived by melting of graywackes; reasonable fractionation processes can be hypothesized to explain the compositional variations within each group.Although stratigraphic relations in the Venezuelan Guyana Shield are uncertain, geologic relations in and near the map area allow, as valid working hypotheses, stratigraphic sequences which parallel those which have been recognized in better-known Archean terranes. 相似文献