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1.
Coupling of anatectic reactions and dissolution of accessory phases and the Sr and Nd isotope systematics of anatectic melts from a metasedimentary source 总被引:10,自引:0,他引:10
Advances in field observations and experimental petrology on anatectic products have motivated us to investigate the geochemical consequences of accessory mineral dissolution and nonmodal partial melting processes. Incorporation of apatite and monazite dissolution into a muscovite dehydration melting model allows us to examine the coupling of the Rb-Sr and Sm-Nd isotope systems in anatectic melts from a muscovite-bearing metasedimentary source. Modeling results show that (1) the Sm/Nd ratios and Nd isotopic compositions of the melts depend on the amount of apatite and monazite dissolved into the melt, and (2) the relative proportion of micas (muscovite and biotite) and feldspars (plagioclase and K-feldspar) that enter the melt is a key parameter determining the Rb/Sr and 87Sr/86Sr ratios of the melt. Furthermore, these two factors are not, in practice, independent. In general, nonmodal partial melting of a pelitic source results in melts following one of two paths in εNd-87Sr/86Sr ratio space. A higher temperature, fluid-absent path (Path 1) represents those partial melting reactions in which muscovite/biotite dehydration and apatite but not monazite dissolution play a significant role; the melt will have elevated Rb/Sr, 87Sr/86Sr, Sm/Nd, and εNd values. In contrast, a lower temperature, fluid-fluxed path (Path 2) represents those partial melting reactions in which muscovite/biotite dehydration plays an insignificant role and apatite but not monazite stays in the residue; the melt will have lower Rb/Sr, 87Sr/86Sr, Sm/Nd, and εNd values than its source. The master variables controlling both accessory phase dissolution (and hence the Sm-Nd system), and melting reaction (and hence the Rb-Sr systematics) are temperature and water content. The complexity in Sr-Nd isotope systematics in metasediment-derived melts, as suggested in this study, will help us to better understand the petrogenesis for those granitic plutons that have a significant crustal source component. 相似文献
2.
Mixed Caledonian appinite magmas: implications for lamprophyre fractionation and high Ba-Sr granite genesis 总被引:12,自引:1,他引:12
Ach'Uaine Hybrid appinites represent a rare example of lamprophyric magmas that were demonstrably exactly contemporaneous
with felsic differentiates, preserved within a suite of minor, hypabyssal intrusions emplaced at the end of the Caledonian
orogeny in northern Scotland. Numerous small stocks, bosses and dykes show outcrop-scale relationships characteristic of mingling
between lamprophyric and syenitic magmas, and are commonly cut by sharp-sided granite veins. The mafic rocks are characterised
by Ni and Cr abundances and MgO sufficiently high to signal derivation from a mantle source within which radiogenic 87Sr/86Sr and nonradiogenic 143Nd/144Nd ratios require significant time-integrated incompatible element enrichment. This is manifest in high Ba, Sr and light REE
abundances and incompatible element ratios in the derived magmas directly comparable with those of high Ba-Sr granitoids and
related rocks. Quantitative major element, trace element, radiogenic and stable isotope modelling is consistent with early
fractionation of clinopyroxene and biotite, accompanied by minor crustal assimilation, having driven the evolving lamprophyric
magma to cogenetic syenite. Subsequent derivation of granite required a major change to feldspar-dominated crystal fractionation
with continued, still minor contamination. The elemental and isotopic characteristics of the granitic terminus are so similar
to high Ba-Sr granitoids both locally and worldwide, that these too may have had large mantle components and represent significant
juvenile additions to the crust.
Received: 26 September 1995 / Accepted: 5 June 1996 相似文献
3.
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
4.
James S. Beard Richard J. Abitz Gary E. Lofgren 《Contributions to Mineralogy and Petrology》1993,115(1):88-102
Experiments (P=6.9 kb; T=900–1000°C) on four crustal xenoliths from Kilbourne Hole demonstrate the varying melting behavior of relatively dry crustal lithologies in the region. Granodioritic gneisses (samples KH-8 and KH-11) yield little melt (<5–25%) by 925°C, but undergo extensive (30–50%) melting between 950 and 1000°C. A dioritic charnockite (KH-9) begins to melt, with the consumption of all modal K-feldspar, by 900°C. It is as fertile a melt source as the granodiorites at lower temperatures, but is outstripped in melt production by the granodiorite gneisses at high temperature, yielding only 26% melt by 1000°C. A pelitic granulite (KH-12) proved to be refractory (confirming earlier predictions based on geochemistry) and did not yield significant melt even at 1000°C. All melts have the composition of metaluminous to slightly peraluminous granites and are unlikely to be individually recognizable as magma contaminants on the basis of major element chemistry. However, the relative stability of K-feldspar during partial melting will produce recognizable signatures in Ba, Eu, K/Ba, and Ba/Rb. Melts of KH-11, which retains substantial K-feldspar throughout the melting interval, are generally low in Ba (<500–800 ppm), have high K/Ba and low Ba/Rb (est.) (62–124 and 1–3, respectively). Melts of KH-9, in which all K-feldspar disappears with the onset of melting, are Ba-rich [2000–2600 ppm, K/Ba=16–22; Ba/Rb (est.) =25–47]. Melts of KH-8 have variable Ba contents; <500 ppm Ba at low temperature but >900 ppm Ba in high-temperature melts coexisting with a K-feldspar-free restite. Although REE were not measured in either feldspar or melt, the high Kspar/melt Kds for Eu suggests that the melts coexisting with K-feldspar will have strong negative Eu anomalies. Isotopic and trace element models for magma contamination need to take into account the melting behavior of isotopic reservoirs. For example, the most radiogenic (and incompatible element-rich) sample examined here (the pelitic granulite,87Sr/86Sr=0.757) is refractory, while samples with far less radiogenic Sr (87Sr/86Sr=0.708-0.732) produced substantial melt. This suggests that, in this area, the isotopic signature of contamination may be more subtle than expected. The experimental results can be used to model the petrogenesis of Oligocene volcanic rocks exposed 150 km to the NW of Kilbourne Hole, in the Black Range in the Mogollon-Datil volcanic field. The experimental results suggest that a crustal melting origin for the Kneeling Nun and Caballo Blanco Tuffs is unlikely, even though such an interpretation is permitted by Sr isotopes. Curstal contamination of a mantle-derived magma best explains the chemical and isotopic characteristics of these tuffs. Both experimental and geochemical data suggest that the rhyolites of Moccasin John Canyon and Diamond Creek could represent direct melts of granodiorite basement similar, but not identical, to the Kilbourne Hole granodiorites, perhaps slightly modified by crystal fractionation. The absence of volcanic rocks having87Sr/86Sr>0.74 in the region is consistent with the refractory character of the pelitic granulite. 相似文献
5.
N. A. Zangana H. Downes M. F. Thirlwall E. Hegner 《Contributions to Mineralogy and Petrology》1997,127(1-2):187-203
Anhydrous spinel peridotite xenoliths from the Ray Pic Quaternary alkali basalt volcano (French Massif Central) show a wide
range of mineralogical and geochemical compositions, reflecting significant heterogeneities in the shallow sub-continental
lithospheric mantle. Variations in modal mineralogy, mineral chem istry, REE patterns and radiogenic isotope data suggest
that depletion by partial melting and enrichment by cryptic metasomatism were the major mantle processes which caused the
heterogeneity. The lithospheric mantle beneath Ray Pic contains two contrasting types of peridotite: (i) lherzolites with
LREE-depleted compositions, high 143Nd/144Nd, low 87Sr/86Sr and unradiogenic Pb isotope ratios; (ii) lherzolites, harzburgites and a wehrlite with LREE-enriched patterns, low 143Nd/144Nd, high 87Sr/86Sr and radiogenic Pb isotope ratios. The former closely resemble depleted MORB-source mantle. The latter are related to enrichment
by recent infiltration of small degree partial melts or fluids from the asthenospheric mantle, possibly related to the “low
velocity component” observed by Hoernle et al. (1995) in European Neogene alkaline magmas. Thus, the Ray Pic peridotite xenoliths
represent interaction between asthenospheric mantle-derived melts/fluids and depleted lithospheric mantle. This is probably
linked to the upwelling mantle plume imaged beneath the Massif Central (Granet et al. 1995). A relationship between textural
deformation, equilibration temperature and geochemistry of the xenoliths suggests that the hotter (> 900 °C) undeformed regions
are LREE-enriched and tend to have more enriched isotope ratios, whereas the cooler (< 900 °C) regions have undergone more
deformation and are more depleted both in LREE and in isotope compositions.
Received: 27 July 1996 / Accepted: 25 November 1996 相似文献
6.
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
7.
《Lithos》2007,93(1-2):17-38
A suite of schists, gneisses, migmatites, and biotite granitoids from the Puerto Edén Igneous and Metamorphic Complex (PEIMC) and biotite–hornblende granitoids of the South Patagonian batholith (southern Chile) has been studied. For that purpose, the chemistry of minerals and the bulk rock composition of major and trace elements including Rb–Sr and Sm–Nd isotopes were determined. Mineralogical observations and geothermobarometric calculations indicate high-temperature and low-pressure conditions (ca. 600–700 °C and 3 to 4.5 kbar) for an event of metamorphism and partial melting of metapelites in Late Jurassic times (previously determined by SHRIMP U–Pb zircon ages). Structures in schists, gneisses, migmatites and mylonites indicate non-coaxial deformation flow during and after peak metamorphic and anatectic conditions. Andalusite schists and sillimanite gneisses yield initial 87Sr/86Sr ratios of up to 0.7134 and εNd150 values as low as − 7.6. Contemporaneous biotite granitoids and a coarse-grained orthogneiss have initial 87Sr/86Sr ratios between 0.7073 and 0.7089, and εNd150 values in the range − 7.6 to − 4.4. This indicates that metamorphic rocks do not represent the natural isotopic variation in the migmatite source. Thus, a heterogeneous source with a least radiogenic component was involved in the production of the biotite granitoids. The PEIMC is considered as a segment of an evolving kilometre-sized and deep crustal shear zone in which partial melts were generated and segregated into a large reservoir of magmas forming composite plutons in Late Jurassic times. A biotite–hornblende granodiorite and a muscovite–garnet leucogranite show initial 87Sr/86Sr ratios of 0.7048 and 0.7061, and εNd100 values of − 2.6 and − 1.8, respectively, and are thus probably related to Early Cretaceous magmas not involved in the anatexis of the metasedimentary rocks. 相似文献
8.
Giovanna T. Sapienza William L. Griffin Suzanne Y. O’Reilly Lauro Morten 《Contributions to Mineralogy and Petrology》2009,157(1):1-22
In situ trace-element and isotopic (87Sr/86Sr) data and whole-rock Sr–Nd–Hf data on 12 gabbro xenoliths from the Hyblean Plateau (south-eastern Sicily) illustrate the complex petrogenetic evolution of this lithospheric segment. The gabbros formed by precipitation of plagioclase + clinopyroxene from a HIMU-type alkaline melt, then were cryptically metasomatized by a low-Rb, high-87Sr/86Sr fluid, and finally infiltrated by an exotic, late Fe–Ti-rich melt with 87Sr/86Sr ~ 0.7055, carrying high concentrations of Sr, Rb and HFSE. The geochemical and isotopic features of both the metasomatizing fluid and the Fe–Ti-rich melt are compatible with their common derivation by the progressive melting of an amphibole–phlogopite–ilmenite metasomatic domain (MARID-type?) that probably resided within the subcontinental lithospheric mantle. Therefore, both the astenosphere and the lithosphere underneath the Hyblean Plateau contributed to the petrogenesis of the gabbros. Sm–Nd dating yields an age of 253 ± 60 Ma for the cumulitic pile, roughly coinciding with a hydrothermal event recorded by crustal zircons in the area. We suggest that the Hyblean Plateau suffered a thermal event—probably related to lithospheric thinning and upwelling and melting of the asthenosphere—in Permo-Triassic time (the opening of the Ionian Basin?). The induced perturbation in the lithosphere caused consequent melting of some previously metasomatised portions. 相似文献
9.
The wadi Kid pluton of Iqna Granite, Southern Sinai, which was intruded during the last Precambrian magmatic phase, yields
a Rb-Sr total rock isocrhon age of 580±23 m.y., and an initial 87Sr/ 86Sr ratio of 0.7028±0.0028. The magma of the Iqna Granite was derived from a low Rb/Sr source shortly before its crystallization.
Partial resetting of biotite ages is detected by both Rb-Sr and K-Ar methods. Mineral isochrons yield higher initial values
(0.7045–0.7065) as a result of Sr isotopic redistribution within a closed total rock system.
The Rb-Sr resetting of the biotites is expressed by radiogenic Sr loss accompanied by a proportional enrichment of common
Sr. The Rb content was unaffected by this process. Oxidation of the iron within the biotite indicates the opening of the biotite
interlayer space, thus making the Sr exchange possible. These effects are attributed to a thermal event some 510–540 m.y.
ago. 相似文献
10.
The Urach volcanic field is unique within the Tertiary–Quaternary European volcanic province (EVP) due to more than 350 tuffaceous
diatremes and only sixteen localities with extremely undersaturated olivine melilitite. We report representative Pb-Sr-Nd
isotopic compositions and incompatible trace element data for twenty-two pristine augite, Cr-diopside, hornblende, and phlogopite
megacryst samples from the diatremes, and seven melilitite whole rocks. The Pb isotopic compositions for melilitites and comagmatic
megacrysts have very radiogenic 206Pb/204Pb ratios of 19.4 to 19.9 and plot on the northern hemisphere mantle reference line (NHRL). The data indicate absence of an
old crustal component as reflected in the high 207Pb/204Pb ratios of many basalts from the EVP. This inference is supported by 206Pb/204Pb ratios of ∼17.6 to 18.3 and ɛNd of ∼−7.8 to +1.6 for five phlogopite xenocryst samples reflecting a distinct and variably rejuvenated lower Hercynian basement.
The 87Sr/86Sr ratios of 0.7033 to 0.7035 in the comagmatic megacrysts are low relative to their moderately radiogenic Nd isotopic compositions
(ɛNd +2.2 to +5.1) and consistent with a long-term source evolution with a low Rb/Sr ratio and depletion in light rare-earth elements
(LREE). The melilitite whole-rock data show a similar range in Nd isotopic ratios as determined for the megacrysts but their
Sr isotopic compositions are often much more radiogenic due to surface alteration. The REE patterns and incompatible trace
element ratios of the melilitites (e.g. Nb/Th, Nb/U, Sr/Nd, P/Nd, Ba/Th, Zr/Hf) are similar to those in ocean island basalts
(OIB); negative anomalies for normalized K and Rb concentrations support a concept of melt evolution in the lithospheric mantle.
Highly variable Ce/Pb ratios of 29 to 66 are positively correlated with La/Lu, La/K2O, and Ba/Nd and interpreted to reflect melting in the presence of residual amphibole and phlogopite. The data suggest an
origin of the melilitites from a chemical boundary layer very recently enriched by melts from old OIB sources. We suggest
that the OIB-like mantle domains represent low-temperature melting heterogeneities in an upwelling asthenosphere under western
Europe.
Received: 9 March 1995/Accepted: 24 July 1995 相似文献
11.
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
12.
The Khawr Fakkan block of the Semail ophiolite (United Arab Emirates) exhibits a suite of 10–100 m scale metaluminous to
peraluminous granitic intrusions, ranging from cordierite-andalusite-biotite monzogranites to garnet-tourmaline leucogranites,
which intrude mantle sequence harzburgites and lower crustal sequence cumulate gabbros. Structural constraints suggest that
the subduction of continental sedimentary material beneath the hot proto-ophiolite in an intra-oceanic arc environment led
to granulite facies metamorphism at the subduction front and the generation of granitic melts which were emplaced up to the
level of the ophiolite Moho. Compositions indicate the analysed granitoids were largely minimum melts that crystallised at
variable a
H2O and pressures of 3 to 5 kbar. The LILE (Sr, Rb and Ba) covariation modelling suggests that the granitoids formed largely
by the dehydration melting of muscovite rich metasediments. Initial 87Sr/86Sr ratios of analysed dykes vary between 0.710 and 0.706 at initial ɛNd values of between −6.3 and −0.5. Cogenetic units of a composite sill from Ra's Dadnah yield a Sm-Nd isochron age of 98.8 ± 9.5 Ma
(MSWD = 1.18). Geochemical and isotopic characteristics of the analysed granitic intrusions indicate that the subducted continental
material was derived from oceanic trench fill (Haybi complex) sediments, preserved as greenschist (Asimah area) to granulite
facies (Bani Hamid area) ophiolitic metamorphic sole terranes. The Sr-Nd isotope systematics suggest that hybrid granitic
melts were derived from pre-magmatic mixing of two contrasting subduction zone sources.
Received: 17 December 1998 / Accepted: 19 July 1999 相似文献
13.
EL H. Talbi J. Honnorez N. Clauer F. Gauthier-Lafaye P. Stille 《Contributions to Mineralogy and Petrology》1999,137(3):246-266
Petrological and chemical variations, as well as oxygen and strontium isotopic data are presented for metagabbros from the
Romanche and Vema fracture zones. These rocks were affected by several types and degrees of alterations ranging from slight
hydrothermal alteration to complete amphibolitization. Five major kinds of alteration processes ranging from late-magmatic
deuteric alteration (stage I) to low temperature (<150 °C) alteration (stage V) were identified. Water-rock interactions between
300 and 650 °C are the most dominant interactions resulting in the most prevailing secondary mineralogical assemblages which
characterize the amphibolite and/or greenschist facies (amphibole ± plagioclase ± epidote ± titanite ± chlorite ± prehnite).
Hydrothermal alteration of these gabbroic rocks results in isotopic exchanges between rocks and seawater-derived fluids. These
exchanges lead to decrease of gabbroic δ18O toward values as low as +3.9‰, and larger Sr isotopic variations than other oceanic gabbroic rocks (87Sr/86Sr ratios shift to 0.7029–0.7051). Calculation of a chemical budget indicates that metagabbros are hydrated and enriched in
Fe and probably in Mg and Cl, while Si, Ca and Ti are released to the hydrothermal fluids. In addition to metamorphic recrystallization
and geochemical transformation, hydrothermal alteration of oceanic gabbros contributes to the control of the global ocean
geochemistry.
Received: 8 March 1999 / Accepted: 12 July 1999 相似文献
14.
Jian-Wei Li Xin-Fu Zhao Mei-Fu Zhou Paulo Vasconcelos Chang-Qian Ma Xiao-Dong Deng Zorano Sérgio de Souza Yong-Xin Zhao Gang Wu 《Mineralium Deposita》2008,43(3):315-336
The Tongshankou Cu–Mo deposit, located in the westernmost Daye district of the Late Mesozoic Metallogenic Belt along the Middle-Lower
reaches of the Yangtze River, eastern China, consists mainly of porphyry and skarn ores hosted in the Tongshankou granodiorite
and along the contact with the Lower Triassic marine carbonates, respectively. Sensitive high-resolution ion microprobe zircon
U–Pb dating constrains the crystallization of the granodiorite at 140.6 ± 2.4 Ma (1σ). Six molybdenite samples from the porphyry ores yield Re–Os isochron age of 143.8 ± 2.6 Ma (2σ), while a phlogopite sample from the skarn ores yields an 40Ar/39Ar plateau age of 143.0 ± 0.3 Ma and an isochron age of 143.8 ± 0.8 Ma (2σ), indicating an earliest Cretaceous mineralization event. The Tongshankou granodiorite has geochemical features resembling
slab-derived adakites, such as high Sr (740–1,300 ppm) and enrichment in light rare earth elements (REE), low Sc (<10 ppm),
Y (<13.3 ppm), and depletion in heavy REE (<1.2 ppm Yb), and resultant high Sr/Y (60–92) and La/Yb (26–75) ratios. However,
they differ from typical subduction-related adakites by high K, low MgO and Mg#, and radiogenic Sr–Nd–Hf isotopic compositions,
with (87Sr/86Sr)
t
= 0.7062–0.7067, ɛ
Nd(t) = −4.37 to −4.63, (176Hf/177Hf)
t
= 0.282469–0.282590, and ɛ
Hf(t) = −3.3 to −7.6. The geochemical and isotopic data, coupled with geological analysis, indicate that the Tongshankou granodiorite
was most likely generated by partial melting of enriched lithospheric mantle that was previously metasomitized by slab melts
related to an ancient subduction system. Magmas derived from such a source could have acquired a high oxidation state, as
indicated by the assemblage of quartz–magnetite–titanite–amphibole–Mg-rich biotite in the Tongshankou granodiorite and the
compositions of magmatic biotite that fall in the field between the NiNiO and magnetite–hematite buffers in the Fe3+–Fe2+–Mg diagram. Sulfur would have been present as sulfates in such highly oxidized magmas, so that chalcophile elements Cu and
Mo were retained as incompatible elements in the melt, contributing to subsequent mineralization. A compilation of existing
data reveals that porphyry and porphyry-related Cu–Fe–Au–Mo mineralization from Daye and other districts of the Metallogenic
Belt along the Middle-Lower reaches of the Yangtze River took place coevally in the Early Cretaceous and was related to an
intracontinental extensional environment, distinctly different from the arc-compressive setting of the Cenozoic age that has
been responsible for the emplacement of most porphyry Cu deposits of the Pacific Rim. 相似文献
15.
Summary The northwest-striking Pfahl zone, Bavarian Forest, is a mylonitic shear zone that is associated with brittle-ductile deformation
fabrics and a conspicuous hydrothermal quartz mineralization. Two granites from this shear zone yield U–Pb and Pb–Pb evaporation
ages between 321–329 Ma and two granodiorites give concordant 238U–206Pb and 235U–207Pb ages of 325±3 Ma and 326±3 Ma, respectively. Zircon populations of the granitoids show sub-types clustering around S20, S10 (granite) and S22 to S24 (granodiorite) testifying different magma affinity. Compositional and isotopic characteristics indicate that the granites
and granodiorites were coeval melts, but not differentiates of a single parent magma. The granodiorites were derived from
a source with higher time-integrated 87Rb/86Sr and lower 147Sm/144Nd ratios than the granites. One granite body is transected by the shear zone but the main mass of the granite is largely
undeformed. This finding suggests that granite intrusion predates the final stage of ductile deformation along the Pfahl shear
zone. 相似文献
16.
The significance of Cenozoic magmatism from the western margin of the eastern syntaxis, southeast Tibet 总被引:6,自引:0,他引:6
Hongfei Zhang Nigel Harris Liang Guo Wangchun Xu 《Contributions to Mineralogy and Petrology》2010,160(1):83-98
The major and trace-element geochemistry, Sr–Nd bulk-rock isotopes, U–Pb zircon chronology and Lu–Hf isotopic compositions
are described for three granitic bodies which intrude the Nyingchi gneisses (Lhasa terrane) along the western margin of the
eastern Himalayan syntaxis. The Bayi two-mica granite and Lunan granite–granodiorite were intruded at 22 ± 1 and 25.4 ± 0.3 Ma,
respectively, whereas the Confluence biotite granite was emplaced at 49.1 ± 0.4 Ma. All share strong depletions in Y and HREE
requiring a garnet-bearing source both during and following the Eocene collision of the Indian plate with the Lhasa terrane.
The isotope geochemistry of these intrusives (ε
Nd(t) = −3 to −5, 87Sr/86Sr(t) = 0.706–0.707) indicates a crustal source within the Lhasa terrane. Sr–Nd systematics of the garnet-bearing Nyingchi gneisses
together with the U–Pb and Lu–Hf isotopic ratios of detrital zircons recovered from this unit identifies it as a potential
melt source. The combined element and isotope geochemistry of the plutons indicate a mixed source; the gneisses provide the
older component whereas the Gangdese batholith provides a younger, siliceous component. The involvement of garnet-bearing
crustal material in melt sources from the Cretaceous (80 Ma) to the Miocene (20 Ma) is consistent with the presence of a thicker
continental crust in the eastern Lhasa terrane, as is the presence of magmatic epidote in several plutons which indicates
a regional deepening level of exposure eastwards. Post-collision crustal melting is synchronous with proposed slab break-off
during the early Miocene, suggesting advective heating by rising asthenospheric melts. 相似文献
17.
Rb-Sr study of Au-Ag Shkol'noe deposit (Kurama Mountains, north Tadjikistan): age of mineralization and time scale of hydrothermal processes 总被引:2,自引:0,他引:2
The epithermal Au-Ag Shkol'noe deposit is located in the Kandjol ore field, Kurama Mountains. This region is a part of the
east-west trending Late Hercynian Bel'tau-Kurama volcanic belt, an Andean-style collisional margin. The deposit comprises
a number of quartz-carbonate veins hosted by the syn-subductional Middle Carboniferous Karamazar granodiorites. The Au-Ag
mineralization is considered to be the result of the earliest hydrothermal event in the region. The Rb-Sr isochron age 296.3 ± 1.3 Ma
and an initial 87Sr/86Sr0=0.7071 ± 2 ratio were obtained for an adularia-sericite-quartz-calcite sample from Au-Ag mineralization. The 87Sr/86Sr ratio range from 0.70645 ± 10 to 0.70741 ± 10 was obtained for the calcites from the earlier and later mineral assemblages.
The Rb-Sr age is interpreted as a real geological age of the Au-Ag mineralization. It corresponds to the initial stage of
the Late Carboniferous – Early Permian collision following the main syn-subduction stage of Bel'tau-Kurama volcanic belt evolution.
The comparison of the Rb-Sr age with previously obtained 40Ar-39Ar and K-Ar data for adularia from the Au-Ag mineralization implies that gangue minerals of the Shkol'noe deposit bears the
fingerprint of at least three events in its history. They are (1) Au-Ag mineralization at 296.3 ± 1.3 Ma; and (2) two subsequent
thermal pulses at 277 ± 4 and 263–267 ± 8 Ma. The minimum time scale for the hydrothermal activity within the Shkol'noe deposit
is thus approximately 30 million years. A general uniformity of the strontium source during the hydrothermal processes within
the Au-Ag Shkol'noe deposit (87Sr/86Sr0=0.70645 ± 10 to 0.70741 ± 10) is suggested as well as within the Bel'tau-Kurama belt (87Sr/86Sr0=0.7051–0.707). The slight shift into a higher strontium isotope composition of the hydrothermal minerals of the Shkol'noe
deposit in comparison with other deposits and rocks of the Bel'tau-Kurama belt may be ascribed to the contribution of relatively
radiogenic strontium from the Karamazar-type granitoids. The mobilization of low radiogenic strontium during propylitic alteration
of diabase dikes emplaced after the Au-Ag mineralization could be responsible for comparatively low 87Sr/86Sr ratios in some of the latest post-dike carbonates.
Received: 4 August 1998 / Accepted: 25 August 1998 相似文献
18.
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
19.
A total of 17 alkali basalts (alkali olivine basalt, limburgite, olivine nephelinite) and quartz tholeiites, and of 10 peridotite xenoliths (or their clinopyroxenes) were analyzed for Nd and Sr isotopes. 143Nd/144Nd ratios and 87Sr/86Sr ratios of all basalts and of the majority of ultramafic xenoliths plot below the mantle array with a large variation in Nd isotopes and a smaller variation in Sr isotopes. The tholeiites were less radiogenic in Nd than the alkali basalts. Volcanics from the Eifel and Massif Central regions contain Nd and Sr, which is more radiogenic than that of the basalts from the Hessian Depression. Nd and Sr isotopic compositions of all rocks from the latter area, with the exception of one tholeiite and one peridotite plot in the same field of isotope ratios as the Ronda ultramafic tectonite (SW Spain), which ranges in composition from garnet to plagioclase peridotite. The alkali basaltic rocks are products of smaller degrees of partial melting of depleted peridotite, which has undergone a larger metasomatic alteration compared with the source rock of tholeiitic magmas. For the peridotite xenoliths such metasomatic alteration is indicated by the correlation of their K contents and isotopic compositions. We assume that the upper mantle locally can acquire isotopic signatures low in radiogenic Nd and Sr from the introduction of delaminated crust. Such granulites low in radiogenic Nd and Sr are products of early REE fractionation and granite (Rb) separation. 相似文献
20.
Stefanie S. Schmidberger Ernst Hegner 《Contributions to Mineralogy and Petrology》1999,135(4):373-385
Late Carboniferous (300–290 Ma) calc-alkaline basalts, andesites, and rhyolites typical of volcanic arc settings occur in
the intermontane Saar-Nahe basin (SW Germany) within the Variscan orogenic belt. The volcanic rock suite was emplaced under
a regime of tensional tectonics during orogenic collapse and its origin has been explained by melting of mantle and crust
in the course of limited lithospheric rifting. We report major, trace and rare-earth-element data (REE), and Nd-Pb-Sr-O isotope
ratios for a representative sample suite, which are fully consistent with an origin closely related to plate subduction. Major
and trace element data define continuous melt differentiation trends from a precursor basaltic magma involving fractional
crystallization of olivine, pyroxene, plagioclase, and magnetite typical of magma evolution in a volcanic arc. This finding
precludes an origin of the andesitic compositions by mixing of mafic and felsic melts as can be expected in anorogenic settings.
The mafic samples have high Mg numbers (Mg# = 65–73), and high Cr (up to 330 ppm) and Ni (up to 200 ppm) contents indicating
derivation from a primitive parental melt that was formed in equilibrium with mantle peridotite. We interpret the geochemical
characteristics of the near-primary basalts as reflecting their mantle source. The volcanic rocks are characterized by enrichment
in the large ion lithophile elements (LILE), negative Nb and Ti, and positive Pb anomalies relative to the neighboring REE,
suggesting melting of a subduction-modified mantle. Initial Nd values of −0.7 to −4.6, Pb, and 87Sr/86Sr(t) isotope ratios for mafic and felsic volcanics are similar and indicate partial melting of an isotopically heterogeneous and
enriched mantle reservoir. The enrichment in incompatible trace elements and radiogenic isotopes of a precursor depleted mantle
may be attributed to addition of an old sedimentary component. The geochemical characteristics of the Saar-Nahe volcanic rocks
are distinct from typical post-collisional rock suites and they may be interpreted as geochemical evidence for ongoing plate
subduction at the margin of the Variscan orogenic belt not obvious from the regional geologic context.
Received: 3 August 1998 / Accepted: 2 January 1999 相似文献