共查询到20条相似文献,搜索用时 46 毫秒
1.
A. I. Grabezhev 《Geology of Ore Deposits》2010,52(2):138-153
The Early Devonian Gumeshevo deposit is one of the largest ore objects pertaining to the dioritic model of the porphyry copper
system paragenetically related to the low-K quartz diorite island-arc complex. The (87Sr/86Sr)t and (ɛNd)t of quartz diorite calculated for t = 390 Ma are 0.7038–0.7045 and 5.0–5.1, respectively, testifying to a large contribution of the mantle component to the composition
of this rock. The contents of typomorphic trace elements (ppm) are as follows: 30–48 REE sum, 5–10 Rb, 9–15 Y, and 1–2 Nb.
The REE pattern is devoid of Eu anomaly. Endoskarn of low-temperature and highly oxidized amphibole-epidote-garnet facies
is surrounded by the outer epidosite zone. Widespread retrograde metasomatism is expressed in replacement of exoskarn and
marble with silicate (chlorite, talc, tremolite)-magnetite-quartz-carbonate mineral assemblage. The 87Sr/86Sr ratios of epidote in endoskarn and carbonate in retrograde metasomatic rocks (0.7054–0.7058 and 0.7053–0.7065, respectively)
are intermediate between the Sr isotope ratios of quartz dioritic rocks and marble (87Sr/86Sr = 0.70784 ± 2). Isotopic parameters of the fluid equilibrated with silicates of skarn and retrograde metasomatic rocks
replacing exoskarn at 400°C are δ18O = +7.4 to +8.5‰ and δD = −49 to −61‰ (relative to SMOW). The δ13C and δ18O of carbonates in retrograde metasomatic rocks after marble are −5.3 to +0.6 (relative to PDB) and +13.0 to +20.2% (relative
to SMOW), respectively. Sulfidation completes metasomatism, nonuniformly superimposed on all metasomatic rocks and marbles
with formation of orebodies, including massive sulfide ore. The δ34S of sulfides is 0 to 2‰ (relative to CDT);87Sr/86Sr of calcite from the late calcite-pyrite assemblage replacing marble is 0.704134 ± 6. The δ13C and 87Sr/86Sr of postore veined carbonates correlate positively (r = 0.98; n = 6). The regression line extends to the marble field. Its opposite end corresponds to magmatic (in terms of Bowman, 1998b)
calcite with minimal δ13C, δ18O, and 87Sr/86Sr values (−6.9 ‰, +6.7‰, and 0.70378 ± 4, respectively). The aforementioned isotopic data show that magmatic fluid was supplied
during all stages of mineral formation and interacted with marble and other rocks, changing its Sr, C, and O isotopic compositions.
This confirms the earlier established redistribution of major elements and REE in the process of metasomatism. A contribution
of meteoric and metamorphic water is often established in quartz from postore veins. 相似文献
2.
Joachim Hoefs Gunter Faure David H. Elliot 《Contributions to Mineralogy and Petrology》1981,75(3):199-203
The
18O (SMOW) values of the Kirkpatrick Basalt (Jurassic) on Mt. Falla, Queen Alexandra Range, vary between +6.3 and +8.6 The apparent enrichment of these rocks in18O excludes the possibility that they were altered by interaction with aqueous solutions of meteoric origin. The
18O values of the flows correlate significantly with the initial87Sr/86Sr ratios and all major elements. These correlations confirm the hypothesis that the basalt magma was contaminated by rocks of the continental crust through which it was extruded. Estimates of the chemical composition of the basalt magma and the contaminant, based on extrapolations of the new oxygen data, generally confirm earlier estimates based on extrapolations of initial87Sr/86Sr ratios. The87Sr/86Sr ratio of the uncontaminated basalt was 0.7093 which indicates that magma may have originated by melting either in old Rb-enriched lithospheric mantle under Antarctica or in the overlying crust, or both. 相似文献
3.
R. Altherr U. Henes-Klaiber E. Hegner M. Satir C. Langer 《International Journal of Earth Sciences》1999,88(3):422-443
Latest Devonian to early Carboniferous plutonic rocks from the Odenwald accretionary complex reflect the transition from
a subduction to a collisional setting. For ∼362 Ma old gabbroic rocks from the northern tectonometamorphic unit I, initial
isotopic compositions (εNd=+3.4 to +3.8;87Sr/86Sr =0.7035–0.7053;δ18O=6.8–8.0‰) and chemical signatures (e.g., low Nb/Th, Nb/U, Ce/Pb, Th/U, Rb/Cs) indicate a subduction-related origin by partial
melting of a shallow depleted mantle source metasomatized by water-rich, large ion lithophile element-loaded fluids. In the
central (unit II) and southern (unit III) Odenwald, syncollisional mafic to felsic granitoids were emplaced in a transtensional
setting at approximately 340–335 Ma B.P. Unit II comprises a mafic and a felsic suite that are genetically unrelated. Both
suites are intermediate between the medium-K and high-K series and have similar initial Nd and Sr signatures (εNd=0.0 to –2.5;87Sr/86Sr=0.7044–0.7056) but different oxygen isotopic compositions (δ18O=7.3–8.7‰ in mafic vs 9.3–9.5‰ in felsic rocks). These characteristics, in conjunction with the chemical signatures, suggest
an enriched mantle source for the mafic magmas and a shallow metaluminous crustal source for the felsic magmas. Younger intrusives
of unit II have higher Sr/Y, Zr/Y, and Tb/Yb ratios suggesting magma segregation at greater depths. Mafic high-K to shoshonitic
intrusives of the southern unit III have initial isotopic compositions (εNd=–1.1 to –1.8;87Sr/86Sr =0.7054–0.7062;δ18O=7.2–7.6‰) and chemical characteristics (e.g., high Sr/Y, Zr/Y, Tb/Yb) that are strongly indicative of a deep-seated enriched
mantle source. Spatially associated felsic high-K to shoshonitic rocks of unit III may be derived by dehydration melting of
garnet-rich metaluminous crustal source rocks or may represent hybrid magmas.
Received: 7 December 1998 / Accepted: 27 April 1999 相似文献
4.
Thomas H. Fleming Kenneth A. Foland David H. Elliot 《Contributions to Mineralogy and Petrology》1995,121(3):217-236
Isotopic (Nd and Sr) and chemical compositions of the 177 Ma Kirkpatrick Basalt and Ferrar Dolerite from north Victoria Land,
Antarctica, are examined in order to address the role of crustal assimilation and the characteristics of their mantle source.
Results for the Scarab Peak chemical type (SPCT) that constitutes the flow unit capping the lava sequence [Mg-number, Mg/(Mg+Fe+2)=24, MgO=2.4%, SiO2=57.1%, initial87Sr/86Sr=0.7087–0.7097, (εNd=−4.3) conform previous reports that attribute variations in the concentrations of the more mobile elements and calculated
initial87Sr/86Sr to mid-Cretaceous alteration and elevated δ18O to low-temperature interaction with meteoric water. The underlying lavas and the sills that are of the Mt. Fazio chemical
type (MFCT) display a much wider range of both chemical and isotopic compositions (Mg-number=40–65, MgO=3.7 7.5%; SiO2=52.6–58.3%, initial87Sr/86Sr=0.7087–0.7117, εNd=−5.6 to −4.8). The effects of rock alteration on apparent initial87Sr/86Sr are demonstrated by large differences between the initial ratio of mineral separates or leached fractions and whole rocks.
Cretaceous alteration produced Rb and Sr redistribution within the lava sequence that results in erroneous calculated initial87Sr/86Sr ratios. These effects are responsible for the large initial87Sr/86Sr variations previousl7 proposed which, combined with the large range in whole-rock δ18O, were purported to show very large degrees of crustal assimilation. The variations in εNd are restricted and indicate much smaller degrees of assimilation. The least altered of the MFCT rocks show good chemical
and isotopic correlations that can be integrated into a model involving fractionation of pyroxene and plagioclase coupled
with assimilation of material similar to early Paleozoic basement. The lower87Sr/86Sr and higher εNd of the SPCT suggest that they were derived by extensive fractionation of a more primitive, less contaminated, precursor of
the MFCT. The most isotopically primitive Ferrar rocks from the region still have a high initial87Sr/86Sr and low initial143Nd/144Nd; this may reflect either earlier assimilation or an enriched source. The chemical and isotopic similarities, as well as
the close geographic correspondence of the Ferrar Group to granitoids produced during the early Paleozoic Ross Orogeny suggest
that in either case Ross-type material may have been involved in the development of the enriched isotopic signature.
Editorial responsibility: I. Parsons 相似文献
5.
Summary The eastern Pyrenees host a large number of talc-chlorite mineralizations of Albian age (112–97 Ma), the largest of which
occur in the St. Barthelemy massif. There talc develops by hydrothermal replacement of dolostones, which were formed by alteration
of calcite marbles. This alteration is progressive. Unaltered calcite marbles have oxygen isotope composition of about 25‰
(V-SMOW). The δ18O values decrease down to values of 12‰ towards the contact with dolostones. This 18O depletion is accompanied by Mg enrichment, LREE fractionation and systematic shifts in the Sr isotope compositions, which
vary from 87Sr/86Sr = 0.7087–0.7092 in unaltered calcite marbles to slightly more radiogenic compositions with 87Sr/86Sr = 0.7094 near dolomitization fronts. Dolostones have δ18O values (about 9‰) lower than calcitic marbles, higher REE content and more radiogenic Sr isotope composition (87Sr/86Sr = 0.7109 to 0.7130). Hydrothermal calcites have δ18O values close to dolostones but substantially lower δ13C values, down to −6.5‰, which is indicative of the contribution of organic matter. The REE content of hydrothermal calcite
is one order of magnitude higher than that of calcitic marbles. Its highly radiogenic Sr composition with 87Sr/86Sr = 0.7091 to 0.7132 suggests that these elements were derived from silicate rocks, which experienced intense chlorite alteration
during mineralization. The chemical and isotopic compositions of the calcite marbles, the dolostones and the hydrothermal
calcites are interpreted as products of successive stages of fluid-rock interaction with increasing fluid-rock ratios. The
hydrothermal quartz, calcite, talc and chlorite are in global mutual isotopic equilibrium. This allows the calculation of
the O isotope composition of the infiltrating water at 300 °C, which is in the δ18O
= 2–4.5‰ range. Hydrogen isotope compositions of talc and chlorite indicate a δD
= 0 to −20‰. This water probably derived from seawater, with minor contribution of evolved continental water. 相似文献
6.
K. S. Viljoen 《Contributions to Mineralogy and Petrology》1995,121(4):414-423
One diamond-bearing and eight graphite-bearing eclogite xenoliths are described from the Bellsbank kimberlites, Cape Province,
South Africa. Graphite mostly occurs as discrete grains which are commonly in the form of tabular prisms. Diamond is octahedral.
Both Group I and Group II eclogite varieties are represented by the graphite-bearing specimens, while the single diamond-bearing
eclogite is of the Group I variety. The carbon isotopic composition of the graphite varies from δ13C=−7‰ to δ13C=−2.8‰. This is within the range of carbon isotopic compositions for inclusion-free diamonds in kimberlite from this locality,
suggesting that the carbon for the eclogites as well as some of the kimberlite diamonds are derived from the same source.
The present day Nd isotopic compositions of clinopyroxene from three graphite-bearing xenoliths are slightly higher than the
bulk earth estimate. Sr isotopic compositions of the clinopyroxene in these xenoliths vary from 87Sr/86Sr=0.703 to 87Sr/86Sr=0.706. This could be due to derivation of the xenoliths from a protolith with variable 87Sr/86Sr isotopic composition or could be the result of mixing between a low-Sr, high 87Sr/86Sr component and a high Sr, low 87Sr/86Sr component.
Received: 1 June 1994/Accepted: 6 March 1995 相似文献
7.
Joachim Hoefs Gunter Faure David H. Elliot 《Contributions to Mineralogy and Petrology》1980,75(3):199-203
Theδ18O (SMOW) values of the Kirkpatrick Basalt (Jurassic) on Mt. Falla, Queen Alexandra Range, vary between +6.3‰ and +8.6‰ The apparent enrichment of these rocks in18O excludes the possibility that they were altered by interaction with aqueous solutions of meteoric origin. Theδ18O values of the flows correlate significantly with the initial87Sr/86Sr ratios and all major elements. These correlations confirm the hypothesis that the basalt magma was contaminated by rocks of the continental crust through which it was extruded. Estimates of the chemical composition of the basalt magma and the contaminant, based on extrapolations of the new oxygen data, generally confirm earlier estimates based on extrapolations of initial87Sr/86Sr ratios. The87Sr/86Sr ratio of the uncontaminated basalt was 0.7093 which indicates that magma may have originated by melting either in old Rb-enriched lithospheric mantle under Antarctica or in the overlying crust, or both. 相似文献
8.
Yan-Jie Tang Hong-Fu Zhang Eizo Nakamura Ji-Feng Ying 《Contributions to Mineralogy and Petrology》2011,161(6):845-861
Elemental and Li–Sr–Nd isotopic data of minerals in spinel peridotites hosted by Cenozoic basalts allow us to refine the existing
models for Li isotopic fractionation in mantle peridotites and constrain the melt/fluid-peridotite interaction in the lithospheric
mantle beneath the North China Craton. Highly elevated Li concentrations in cpx (up to 24 ppm) relative to coexisting opx
and olivine (<4 ppm) indicate that the peridotites experienced metasomatism by mafic silicate melts and/or fluids. The mineral
δ7Li vary greatly, with olivine (+0.7 to +5.4‰) being isotopically heavier than coexisting opx (−4.4 to −25.9‰) and cpx (−3.3
to −21.4‰) in most samples. The δ7Li in pyroxenes are considerably lower than the normal mantle values and show negative correlation with their Li abundances,
likely due to recent Li ingress attended by diffusive fractionation of Li isotopes. Two exceptional samples have olivine δ7Li of −3.0 and −7.9‰, indicating the existence of low δ7Li domains in the mantle, which could be transient and generated by meter-scale diffusion of Li during melt/fluid-peridotite
interaction. The 143Nd/144Nd (0.5123–0.5139) and 87Sr/86Sr (0.7018–0.7062) in the pyroxenes also show a large variation, in which the cpx are apparently lower in 87Sr/86Sr and slightly higher in 143Nd/144Nd than coexisting opx, implying an intermineral Sr–Nd isotopic disequilibrium. This is observed more apparently in peridotites
having low 87Sr/86Sr and high 143Nd/144Nd ratios than in those with high 87Sr/86Sr and low 143Nd/144Nd, suggesting that a relatively recent interaction existed between an ancient metasomatized lithospheric mantle and asthenospheric
melt, which transformed the refractory peridotites with highly radiogenic Sr and unradiogenic Nd isotopic compositions to
the fertile lherzolites with unradiogenic Sr and radiogenic Nd isotopic compositions. Therefore, we argue that the lithospheric
mantle represented by the peridotites has been heterogeneously refertilized by multistage melt/fluid-peridotite interactions. 相似文献
9.
Interaction between crustal-derived felsic and mantle-derived mafic magmas in the Oberkirch Pluton (European Variscides, Schwarzwald, Germany) 总被引:5,自引:1,他引:4
R. Altherr F. Henjes-Kunst C. Langer J. Otto 《Contributions to Mineralogy and Petrology》1999,137(4):304-322
The composite Oberkirch pluton consists of three compositionally different units of peraluminous biotite granite. The northern
unit is relatively mafic (SiO2∼64%) and lacks cordierite. The more felsic central and southern units (SiO2=67.8 to 70.4%) can only be distinguished from each other by the occurrence of cordierite in the former. Mafic microgranular
enclaves of variable composition, texture and size occur in each of these units and are concentrated in their central domains.
Most abundant are large (dm to m) hornblende-bearing enclaves with dioritic to tonalitic compositions (SiO2=50.8 to 56.3 wt%; Mg#=63 to 41) and fine grained doleritic textures that suggest chilling against the host granite magma.
Some of these enclaves are mantled by hybrid zones. Less common are microtonalitic enclaves containing biotite as the only
primary mafic phase (SiO2=53.7 to 64.4%) and small hybrid tonalitic to granodioritic enclaves and schlieren. Synplutonic dioritic dikes (up to 6 m
thick) with hybrid transition zones to the host granite occur in the southern unit of the pluton. In chemical variation diagrams,
samples from unmodified hornblende-bearing mafic enclaves and dikes form continuous trends that are compatible with an origin
by fractionation of olivine, clinopyroxene, hornblende and plagioclase. Chemical and initial isotopic signatures (e.g. high
Mg#, low Na2O, ɛNd=−1.2 to −5.1, 87Sr/86Sr=0.7055 to 0.7080, δ18O=8.0 to 8.8‰) exclude an origin by partial melting from a mafic meta-igneous source but favour derivation from a heterogeneous
enriched lithospheric mantle. Samples from the granitic host rocks do not follow the chemical variation trends defined by
the diorites but display large scatter. In addition, their initial isotopic characteristics (ɛNd=−4.5 to −6.8, 87Sr/86Sr=0.7071 to 0.7115, δ18O=9.9 to 11.9‰) show little overlap with those of the diorites. Most probably, the granitic magmas were derived from metapelitic
sources characterized by variable amounts of garnet and plagioclase. This is suggested by relatively high molar ratios of
Al2O3/(MgO+FeOtot) and K2O/Na2O, in combination with low ratios of CaO/(MgO+FeOtot), variable values of Sr/Nd, Eu/Eu*[=Eucn/(Smcn × Gdcn)0.5] and (Tb/Yb)cn (cn=chondrite-normalized) as well as variable abundances of Sc and Y. Whole-rock initial isotopic signatures of mafic microtonalitic
enclaves (ɛNd=−4.6 to −5.2; 87Sr/86Sr=0.7060 to 0.7073; δ18O ∼8.1‰) are similar to those of the low ɛNd diorites. Plagioclase concentrates from a granite sample and a mafic microtonalitic enclave are characterized by initial
87Sr/86Sr ratios that are significantly higher than those of their bulk rock systems suggesting incorporation of high 87Sr/86Sr crustal material into the magmas. Field relationships and petrographic evidence suggest that the Oberkirch pluton originated
by at least three pulses of granitic magma containing mafic magma globules. In-situ hybridization between the different magmas
was limited. Late injection of dioritic magma into the almost solidified granitic southern unit resulted in the formation
of more or less continuous synplutonic dikes surrounded by relatively thin hybrid zones.
Received: 30 April 1999 / Accepted: 6 August 1999 相似文献
10.
Stephanos P. Kilias Manuel Pozo Manuel Bustillo Michael G. Stamatakis José P. Calvo 《Mineralium Deposita》2006,41(7):713-733
The Rubian magnesite deposit (West Asturian—Leonese Zone, Iberian Variscan belt) is hosted by a 100-m-thick folded and metamorphosed Lower Cambrian carbonate/siliciclastic metasedimentary sequence—the Cándana Limestone Formation. It comprises upper (20-m thickness) and lower (17-m thickness) lens-shaped ore bodies separated by 55 m of slates and micaceous schists. The main (lower) magnesite ore body comprises a package of magnesite beds with dolomite-rich intercalations, sandwiched between slates and micaceous schists. In the upper ore body, the magnesite beds are thinner (centimetre scale mainly) and occur between slate beds. Mafic dolerite dykes intrude the mineralisation. The mineralisation passes eastwards into sequence of bedded dolostone (Buxan) and laminated to banded calcitic marble (Mao). These show significant Variscan extensional shearing or fold-related deformation, whereas neither Rubian dolomite nor magnesite show evidence of tectonic disturbance. This suggests that the dolomitisation and magnesite formation postdate the main Variscan deformation. In addition, the morphology of magnesite crystals and primary fluid inclusions indicate that magnesite is a neoformed hydrothermal mineral. Magnesite contains irregularly distributed dolomite inclusions (<50 μm) and these are interpreted as relics of a metasomatically replaced dolostone precursor. The total rare earth element (REE) contents of magnesite are very similar to those of Buxan dolostone but are depleted in light rare earth elements (LREE); heavy rare earth element concentrations are comparable. However, magnesite REE chondrite normalised profiles lack any characteristic anomaly indicative of marine environment. Compared with Mao calcite, magnesite is distinct in terms of both REE concentrations and patterns. Fluid inclusion studies show that the mineralising fluids were MgCl2–NaCl–CaCl2–H2O aqueous brines exhibiting highly variable salinities (3.3 to 29.5 wt.% salts). This may be the result of a combination of fluid mixing, migration of pulses of variable-salinity brines and/or local dissolution and replacement processes of the host dolostone. Fluid inclusion data and comparison with other N Iberian dolostone-hosted metasomatic deposits suggest that Rubian magnesite probably formed at temperatures between 160 and 200°C. This corresponds, at hydrostatic pressure (500 bar), to a depth of formation of ~~5 km. Mineralisation-related Rubian dolomite yields δ
18O values (δ
18O: 12.0–15.4‰, mean: 14.4±1.1‰) depleted by around 5‰ compared with barren Buxan dolomite (δ
18O: 17.1–20.2‰, mean: 19.4±1.0‰). This was interpreted to reflect an influx of 18O-depleted waters accompanied by a temperature increase in a fluid-dominated system. Overlapping calculated δ
18Ofluid values (~+5‰ at 200°C) for fluids in equilibrium with Rubian dolomite and magnesite show that they were formed by the same hydrothermal system at different temperatures. In terms of δ
13C values, Rubian dolomite (δ
13C: −1.4 to 1.9‰, mean: 0.4±1.3‰) and magnesite (δ
13C: −2.3 to 2.4‰, mean: 0.60±1.0‰) generally exhibit more negative δ
13C values compared with Buxan dolomite (δ
13C: −0.2 to 1.9‰, mean: 0.8±0.6‰) and Mao calcite (δ
13C: −0.3 to 1.5‰, mean: 0.6±0.6‰), indicating progressive modification to lower δ
13C values through interaction with hydrothermal fluids. 87Sr/86Sr ratios, calculated at 290 Ma, vary from 0.70849 to 0.70976 for the Mao calcite and from 0.70538 to 0.70880 for the Buxan dolostone. The 87Sr/86Sr ratios in Rubian magnesite are more radiogenic and range from 0.71123 to 0.71494. The combined δ
18O–δ
13C and 87Sr/86Sr data indicate that the magnesite-related fluids were modified basinal brines that have reacted and equilibrated with intercalated siliciclastic rocks. Magnesite formation is genetically linked to regional hydrothermal dolomitisation associated with lithospheric delamination, late-Variscan high heat flow and extensional tectonics in the NW Iberian Belt. A comparison with genetic models for the Puebla de Lillo talc deposits suggests that the formation of hydrothermal replacive magnesite at Rubian resulted from a metasomatic column with magnesite forming at higher fluid/rock ratios than dolomite. In this study, magnesite generation took place via the local reaction of hydrothermal dolostone with the same hydrothermal fluids in very high permeability zones at high fluid/rock ratios (e.g. faults). It was also possibly aided by additional heat from intrusive dykes or sub-cropping igneous bodies. This would locally raise isotherms enabling a transition from the dolomite stability field to that of magnesite.Editorial handling: F. Tornos 相似文献
11.
Isotopic compositions of C, O, and Sr in carbonates, as well as Rb-Sr systems in the silicate material from Upper Precambrian
and Lower Cambrian rocks exposed by the Chapa River in the northern Yenisei Ridge, are studied. The Late Precambrian part
of the section includes the following formations (from the bottom to top): Lopatinskaya (hereafter, Lopatino), Vandadykskaya
(hereafter, Vandadyk) or Kar’ernaya, Chivida, Suvorovskaya (hereafter, Suvorovo), Pod”emskaya (hereafter, Podyom), and Nemchanka.
They are characterized by alternation of horizons with anomalously high and low δ13C values (such alternation is typical of the ∼700–550 Ma interval). The lower, relatively thin (20 m), positive excursion
(δ13C up to 4.3‰) was established in dolomites from the lower subformation of the Vandadyk (Kar’ernaya) Formation (hereafter,
lower Vandadyk subformation). The upper positive excursion (δ13C = 2.2 ± 0.6‰) was recorded in the 3-km-thick Nemchanka Formation enriched in terrigenous rocks. The lower negative excursion
stands out as uniform, moderately low δ13C values (−2 ± 1‰) and significant thickness. It comprises the upper part of the Vandadyk Formation, as well as Chivida and
Podyom formations. The upper negative excursion is related to a thin (∼20 m) marker carbonate horizon of the upper Nemchanka
subformation, in which δ13C values fall down to −8.3‰. The lower part of the Lebyazhinskaya (hereafter, Lebyazhino) Formation, which overlies the Nemchanka
Formation, shows a step-by-step increase in δ13C from −2.2 to 2.5‰ typical of the Vendianto-Cambrian (Nemakit-Daldyn Horizon/Stage) transitional sequences. The absence of
relationships between the carbon and oxygen isotope compositions and other parameters of postsedimentary alterations suggests
that the excursions characterized above could form at the sedimentation stage and coincide in general with δ13C fluctuations in seawater. The value of 87Sr/86Sr = 0.7076−0.7078 in limestones of the Podyom Formation points to their early Ediacaran age. Values of 87Sr/86Sr = 0.70841 and 0.70845 in dolomites of the lower Lebyazhino subformation correspond to the Early Cambrian. The Rb-Sr systems
of the clay material from the Vandadyk and Chivida formations are approximated by a straight line, parameters of which correspond
to the age of 695 ± 20 Ma (87Sr/86Sr0 = 0.7200 ± 0.0013) and probably characterize the epigenetic stage of older sedimentary rocks, which were subjected to very
rapid exhumation and “polar” sulfuric acid weathering in the course of glacioeustatic regression. 相似文献
12.
Craig R. McClung Jens Gutzmer Nicolas J. Beukes Klaus Mezger Harald Strauss Ellen Gertloff 《Mineralium Deposita》2007,42(5):537-549
Stratiform and stratabound barite ± magnetite beds are intimately associated with the polymetallic Broken Hill-type (BHT)
massive sulfide deposits of the Aggeneys-Gamsberg Pb–Zn–Cu ± Ag–Ba district in the Northern Cape Province, South Africa. Barite
samples were collected and studied from four localities in the district. Although metamorphic water–rock interaction processes
have partially altered the chemical and to a lesser degree the isotopic composition of barite, samples identified as being
the least altered display distinctly different isotopic compositions that are thought to reflect different modes of origin.
All barite samples are marked by low concentrations of SrO (0.5 ± 0.2 wt%), highly radiogenic 87Sr/86Sr ratios, elevated δ
34S and δ
18O values compared to contemporaneous Mesoproterozoic seawater. Radiogenic 87Sr/86Sr signatures (0.7164 ± 0.0028) point to an evolved continental crustal source for Sr and Ba, while elevated δ
34S values (27.3 ± 4.9‰) indicate that contemporaneous seawater sulfate, modified by bacterial sulfate reduction, was the single
most important sulfur reservoir for barite deposition. Most importantly, δ
18O values suggest a lower temperature of formation for the Gamsberg deposit compared with the occurrences in the Aggeneys area,
i.e. Swartberg-Tank Hill and Big Syncline. The obvious differences in temperature of formation are in good agreement with
the Cu-rich, Ba-poor nature of the sulfide mineralization of the Aggeneys deposits vs the Cu-poor, Ba-rich character of the
Gamsberg deposit. In conjunction with this, isotopic and petrographic arguments favor a sub-seafloor replacement model for
the stratabound barite occurrences of the Aggeneys deposits, while at Gamsberg, deposition at the sediment–water interface
as a true sedimentary exhalite appears more likely. 相似文献
13.
An array of samples from the eastern Upper Basin Member of the Plateau Rhyolite (EUBM) in the Yellowstone Plateau, Wyoming, were collected and analyzed to evaluate styles of deposition, geochemical variation, and plausible sources for low δ18O rhyolites. Similar depositional styles and geochemistry suggest that the Tuff of Sulphur Creek and Tuff of Uncle Tom’s Trail were both deposited from pyroclastic density currents and are most likely part of the same unit. The middle unit of the EUBM, the Canyon flow, may be composed of multiple flows based on a wide range of Pb isotopic ratios (e.g., 206Pb/204Pb ranges from 17.54 to 17.86). The youngest EUBM, the Dunraven Road flow, appears to be a ring fracture dome and contains isotopic ratios and sparse phenocrysts that are similar to extra-caldera rhyolites of the younger Roaring Mountain Member. Petrologic textures, more radiogenic 87Sr/86Sr in plagioclase phenocrysts (0.7134–0.7185) than groundmass and whole-rock ratios (0.7099–0.7161), and δ18O depletions on the order of 5‰ found in the Tuff of Sulphur Creek and Canyon flow indicate at least a two-stage petrogenesis involving an initial source rock formed by assimilation and fractional crystallization processes, which cooled and was hydrothermally altered. The source rock was then lowered to melting depth by caldera collapse and remelted and erupted. The presence of a low δ18O extra-caldera rhyolite indicates that country rock may have been hydrothermally altered at depth and then assimilated to form the Dunraven Road flow. 相似文献
14.
A critical study of 311 published WR chemical analyses, isotopic and mineral chemistry of anorthosites and associated rocks
from eight Proterozoic massif anorthosite complexes of India, North America and Norway indicates marked similarities in mineralogy
and chemistry among similar rock types. The anorthosite and mafic-leucomafic rocks (e.g., leuconorite, leucogabbro, leucotroctolite,
anorthositic gabbro, gabbroic anorthosite, etc.) constituting the major part of the massifs are characterized by higher Na2O + K2O, Al2O3, SiO2, Mg# and Sr contents, low in plagioclase incompatible elements and REE with positive Eu anomalies. Their δ
18O‰ (5.7–7.5), initial 87Sr/86Sr (0.7034–0.7066) and ɛ
Nd values (+1.14 to +5.5) suggest a depleted mantle origin. The Fe-rich dioritic rocks occurring at the margin of massifs have
isotopic, chemical and mineral composition more close to anorthosite-mafic-leucomafic rocks. However, there is a gradual decrease
in plagioclase content, An content of plagioclase and XMg of orthopyroxene, and an increase in mafic silicates, oxide minerals content, plagioclase incompatible elements and REE from
anorthosite-mafic-leucomafic rocks to Fe-rich dioritic rocks. The Fe-rich dioritic rocks are interpreted as residual melt
from mantle derived high-Al gabbro melt, which produced the anorthosite and mafic-leucomafic rocks. Mineralogically and chemically,
the K-rich felsic rocks are distinct from anorthosite-mafic-leucomafic-Fe-rich dioritic suite. They have higher δ
18O values (6.8–10.8‰) and initial 87Sr/86Sr (0.7067–0.7104). By contrast, the K-rich felsic suites are products of melting of crustal precursors. 相似文献
15.
Zhang Zhaochong Chai Fengmei Yan Shenghao Chen Bailin He Lixin He Yongsheng Zhou Gang 《Frontiers of Earth Science》2007,1(1):44-48
The Sr, Nd and O isotopic compositions of the Kalatongke and Xibodu mafic-ultramafic complexes from the southern margin of
the Altay orogenic belt show that they have similar isotopic compositions, characterized by low (87Sr/86Sr), and high ε
Nd(t) values (6.3–9.1). It suggests that they were derived from a depleted asthenospheric mantle. However, most of the samples
have δ
18O values >6‰ (5.4‰–10.2‰), indicating crustal contamination. A combination of Sr and O isotopic data shows the incorporation
of crustal materials into the depleted mantle. They were produced by the melting of depleted mantle by the incorporation of
subducted oceanic crust, and this incorporation might have occurred in the Early Paleozoic in the light of their Nd model
ages and regional tectonics. The Kalatongke complex might have undertaken the contamination of the upper crust whereas the
Xibodu complex does not have any signature of contamination of the upper crust. In addition, the similarities of the sources
of the two complexes, which were located at the northern and southern sides of the regional Irtysh fault zone respectively,
suggest that this fault might not be the boundary between the Altay and Junggar orogenic belts.
Translated from Geological Review, 2006, 52(1): 38–42 [译自: 地质论评] 相似文献
16.
This paper is dedicated to the interpretation of isotope-geochemical data on high Sr-Ba granitoids exemplified by their typical
representatives—Late Miocene laccoliths of the Caucasian Mineral Waters (CMW). These massifs are made up of amphibole granites,
granosyenites, syenites, and leucogranites, which show particular behavior of Sr, Nd, and O isotope systems. The syenites
and granosyenites are characterized by δ18O variations from 8 to 10‰. As compared to them, the amphibole granites have lowered (up to 7‰), while leucogranites, elevated
(up to 12.5–13.7‰) oxygen isotope compositions. The (87Sr/86Sr)8.3 ratio in the granitoids accounts for 0.7083–0.7086, whereas σNd(T) varies from −4.2 to −2.1. 相似文献
17.
The isotope-geochemical study of the Eocene-Oligocene magmatic rocks from the Western Kamchatka-Koryak volcanogenic belt revealed
a lateral heterogeneity of mantle magma sources in its segments: Western Kamchatka, Central Koryak, and Northern Koryak ones.
In the Western Kamchatka segment, magmatic melts were generated from isotopically heterogeneous (depleted and/or insignificantly
enriched) mantle sources significantly contaminated by quartz-feldspathic sialic sediments; higher 87Sr/86Sr (0.70429–0.70564) and lower 143Nd/144Nd(ɛNd(T) = 0.06–2.9) ratios in the volcanic rocks from the Central Koryak segment presumably reflect the contribution of enriched
mantle source; the high positive ɛNd(T) and low 87Sr/86Sr ratios in the magmatic rocks from the Northern Koryak segment area indicate their derivation from isotopically depleted
mantle source without significant contamination by sialic or mantle material enriched in radiogenic Sr and Nd. Significantly
different contamination histories of the Eocene-Oligocene mantle magmas in Kamchatka and Koryakia are related to their different
thermal regimes: the higher heat flow beneath Kamchatka led to the crustal melting and contamination of mantle suprasubduction
magmas by crustal melts. The cessation of suprasubduction volcanism in the Western Kamchatka segment of the continentalmargin
belt was possibly related to the accretion of the Achaivayam-Valagin terrane 40 Ma ago, whereas suprasubduction activity in
the Koryak segment stopped due to the closure of the Ukelayat basin in the Oligocene time. 相似文献
18.
R. Tribuzio M. F. Thirlwall B. Messiga 《Contributions to Mineralogy and Petrology》1999,136(1-2):48-62
The origin of the Sondalo gabbroic complex has been unravelled by means of a petrological study of the least evolved rocks,
troctolites to norites containing up to 20% of anhedral clinopyroxene and titanian pargasite. Pyroxenes and titanian pargasite
from the troctolites have higher Mg, Al and Cr, and lower Mn than those from the norites, whereas plagioclase does not show
systematic compositional variations (An ca. 65 mol%). The variation trend of anorthite content of plagioclase versus the forsterite
content of olivine differs from that of arc-related gabbroic rocks. Plagioclase, clinopyroxene, orthopyroxene and titanian
pargasite were analyzed for REE and selected trace elements by ion microprobe. Application of crystal/liquid partition coefficients
to trace element mineral compositions suggests that the parental liquids of both troctolites and norites had tholeiitic affinity
and were slightly LREE and LILE enriched relative to N-MORB. A troctolite and a norite give Sm-Nd mineral isochron ages of
300 ± 12 Ma and 280 ± 10 Ma. Plagioclase-amphibole Rb-Sr isochron ages are 266 ± 10 Ma and 269 ± 16 Ma for the same rock samples,
and they are interpreted to represent cooling ages. The Nd-Sr-O isotopic compositions indicate that a substantial crustal
contribution was involved in the petrogenesis of the norite, which has low ɛNd(290 Ma), high 87Sr/86Sr(290 Ma) and high δ18OPx (−2.6, 0.7057 and +7.9‰, respectively) compared with the troctolites. We thus conclude that the troctolite/norite association
formed by concomitant fractional crystallization and crustal assimilation. The somewhat elevated δ18OCpx (+6.4‰) and the relatively low forsterite contents in olivine suggest that the parental liquids of the troctolites had already
been evolved through an AFC type process. The ɛNd(290 Ma) and 87Sr/86Sr(290 Ma) of these rocks (ranging from +2.8 to +4.4 and from 0.7037 to 0.7040, respectively) probably do not reflect the
ɛNd and Sr isotopic compositions of their mantle source, and it is thus unclear whether the primary melts were derived from a
slightly enriched or from a depleted mantle source. The Sondalo gabbroic complex was most likely associated with the post-Variscan
gabbroic complexes of the Alpine belt. These gabbroic complexes can be ascribed to the intrusion at different crustal levels
of tholeiitic mantle-derived melts and were emplaced in the time span of 300–270 Ma.
Received: 14 September 1998 / Accepted: 4 January 1999 相似文献
19.
Evaluating crustal contamination in continental basalts: the isotopic composition of the Picture Gorge Basalt of the Columbia River Basalt Group 总被引:4,自引:0,他引:4
Alan D. Brandon Peter R. Hooper Gordon G. Goles Richard St J. Lambert 《Contributions to Mineralogy and Petrology》1993,114(4):452-464
Crustal contamination of basalts located in the western United States has been generally under-emphasized, and much of their isotopic variation has been ascribed to multiple and heterogeneous mantle sources. Basalts of the Miocene Columbia River Basalt Group in the Pacific Northwest have passed through crust ranging from Precambrian to Tertiary in age. These flows are voluminous, homogenous, and underwent rapid effusion, all of which are disadvantages for crustal contamination while en route to the surface. The Picture Gorge Basalt of the Columbia River Basalt Group erupted through Paleozoic and Mesozoic oceanic accreted terranes in central Oregon, and earlier studies on these basalts provided no isotopic evidence for crustal contamination. New Sr, Nd, Pb, and O isotopic data presented here indicate that the isotopic variation of the Picture Gorge Basalt is very small, 87Sr/86Sr=0.70307–0.70371, Nd=+7.7-+4.8, 18O=+5.6±6.1, and 206Pb/204Pb=18.80–18.91. Evaluation of the Picture Gorge compositional variation supports a model where two isotopic components contributed to Picture Gorge Basalt genesis. The first component (C1) is reflected by low 87Sr/86Sr, high Nd, and nonradiogenic Pb isotopic compositions. Basalts with C1 isotopic compositions have large MgO, Ni, and Cr contents and mantle-like 18O=+5.6. C1 basalts have enrichments in Ba coupled with depletions in Nb and Ta. These characteristics are best explained by derivation from a depleted mantle source which has undergone a recent enrichment by fluids coming from a subducted slab. This C1 mantle component is prevalent throughout the Pacific Northwest. The second isotopic component has higher 87Sr/ 86Sr and 18O, lower Nd, and more radiogenic Pb isotopic compositions than C1. There is a correlation in the Picture Gorge data of Sr, Nd, and Pb isotopes with differentiation indicators such as decreasing Mg#, and increasing K2O/TiO2, Ba, Ba/Zr, Rb/Sr, La/Sm, and La/Yb. Phase equilibrium and mineralogical constraints indicate that these compositional characteristics were inherited in the Picture Gorge magmas at crustal pressures, and thus the second isotopic component is most likely crustal in origin. Mixing and open-system calculations can produce the isotopic composition of the most evolved Picture Gorge flows from the most primitive compositions by 8 to 21% contamination of isotopic compositions similar to accreted terrane crust found in the Pacific Northwest. Therefore, in spite of the disadvantages for crustal contamination and their narrow range in isotopic compositions, the process controlling isotopic variation within the Picture Gorge Basalt is primarily crustal contamination. We suggest that comprehensive analyses for basaltic suites and careful consideration of these data must be made to test for crustal contamination, before variation resulting from mantle heterogeneity can be assessed.Deceased 相似文献
20.
Patterns of Nd and Sr isotopic ratios produced by magmatic and post-magmatic processes in the Shiant Isles Main Sill, Scotland 总被引:3,自引:0,他引:3
K. A. Foland F. G. F. Gibb C. M. B. Henderson 《Contributions to Mineralogy and Petrology》2000,139(6):655-671
The Shiant Isles Main Sill of the British Tertiary Igneous Province is a classic example of a differentiated, alkaline basic
sill. Four separate intrusions, each emplaced internally in rapid succession, form a 165-m-thick sill hosted by Lower Jurassic
sedimentary rocks. Extensive Nd and Sr isotopic studies were conducted on samples from a vertical section through the sill
where the relationships of samples to one another are well defined. The results illuminate patterns of modification of isotopic
ratios and clarify the petrogenesis (magma sources, crustal contamination), magmatic processes (bulk mixing, interstitial
liquid mixing), and post-magmatic alteration (hydrothermal effects on Sr and Nd). Overall, the whole-rock initial 87Sr/86Sr ratios range from ∼0.7037 to 0.7061 while initial 143Nd/144Nd ratios vary from ∼0.51243 to 0.51286 (ɛNd∼−0.7 to +5.7) – values that contrast markedly with those of the country rock. Acid leaching (HCl) of the whole-rock samples
that removes analcime indicates that most of the scatter in the 87Sr/86Sr is caused by the ubiquitous sub-solidus, aqueous alteration during which more-radiogenic Sr was introduced into the sill,
especially along the margins, and also reveals magmatic isotopic ratios. In contrast, Nd was immobile during fluid interaction
so that the sill 143Nd/144Nd ratios were not affected, even <1 m from the country-rock contact. Using leached rock values, 87Sr/86Sr and 143Nd/144Nd ratios are inversely correlated from magmatic processes. Magmas with two distinct isotopic compositions were involved:
a more primitive one with 143Nd/144Nd ∼0.51285 and 87Sr/86Sr ∼0.7035 that produced the first two intrusions and a more evolved one (with 0.51252 and 0.7048) that produced the third
intrusion. Mixing of the two magmas was very limited, restricted to near contacts between units, and apparently occurred by
interstitial melt migration. The more evolved crinanitic magma was probably produced from a batch of the more primitive picritic
melt by a small degree of crustal contamination and crystal fractionation during a short crustal residence prior to ascent
and emplacement.
Received: 20 December 1999 / Accepted: 5 May 2000 相似文献