Sr and Nd isotope systematics of Francistown plutonic rocks, Botswana: implications for Neoarchaean crustal evolution of the Zimbabwe craton

The Francistown plutonic rocks at the south-western margin of the Zimbabwe craton consist of three igneous suites: Sanukitoid, Tonalite–Trondhjemite–Granite (TTG) suites and High-K granites. The TTG suite is subdivided into High Aluminum-TTG (HA-TTG) and Low Aluminum-TTG (LA-TTG) sub-suites. Their Rb–Sr isotope systems were partially homogenized by post-crystallization thermo–tectonic events, in which hydrothermal solutions and migmatization played an important role. Therefore, the Rb–Sr isochron age of 2427±54 Ma can only be regarded as a lower limit to the Francistown plutonic rock age. The large errors in the Sm–Nd isochron dates of Francistown granitoids indicate that these dates are not really constrained. In this study we compared the rock types of Francistown and adjacent areas, adopting the precise U, Th–Pb single zircon SHRIMP ages from the Vumba area as references. For TTG and Sanukitoid suites, the age we adopted is ca. 2.7 Ga, which is close to their depleted-mantle Sm–Nd model ages (T _DM). For High-K granites, the age adopted is ca. 2.65 Ga, which is also close to their Sm–Nd isochron age. The highest ε _Nd ^t values of Sanukitoids and TTG are +2.1 and +2.3, respectively. The positive ε _Nd ^t values and trace element geochemistry support partial melting of a depleted mantle and young oceanic crust for the genesis of Sanukitoid and the TTG suites respectively. The lowest ε _Nd ^t values of Sanukitoids and TTGs are −1.0 and −1.1, respectively, indicating contamination by continental crust, up to 10 and 14%, respectively. The ε _Nd ^t values of TTG decrease with decreasing Al₂O₃ and Sr contents and increasing Eu negative anomalies (Eu*–Eu), suggesting that the TTG magmas underwent a coupled fractionation crystallization and crustal contamination, and that the LA-TTG was the product of the fractionation and contamination of the HA-TTG sub-suite. In contrast, negative ε _Nd ^t values for the High-K granites (from −0.4 to −3.5) indicate the involvement of LA-TTG and some materials from an old continental crust in their genesis. The products of partial melting of both oceanic and continental crusts at the south-western margin of the Zimbabwe craton occurred within a short time interval (from 2.7 to 2.65 Ga ago) suggesting that the Francistown plutonic rocks were formed in a active continental margin environment, where a young ocean plate (Limpopo oceanic plate) subducted underneath an old continental plate (Zimbabwe craton).     

Computer simulation of Pb and Sr isotope evolution of the Earth's crust and upper mantle

Isotope evolution in a differentiated (crust and upper mantle) and chemically heterogeneous Earth has been computed for a model with isotopic exchange between crust and mantle at an exponentially decreasing rate. To simulate the effects of subduction, cells of crust and mantle are selected at random, their contents mixed and then redistributed into the cells from which they came—to give new chemically heterogeneous but momentarily isotopically homogeneous systems. Daughter isotopes in each cell grow according to equations appropriate for closed chemical systems. Rock age distributions and isotopic data created by the computer calculation mimic nature. Pb isotope data changes through geologic time are illustrated to demonstrate that two-stage interpretations applied to Pb data for rocks with complex histories may be misleading. The intercept of the Pb ore growth curve and regressions fit to Pb data gives minimum values only for the duration of heterogeneous $\text{U}\text{Pb}$ systems, not the time when heterogeneous distribution first occurred. An intercept derived time of about 3 b.y., in the Pb system is shown to be quantitatively compatible with an average $\text{Rb}\text{Sr}$ age of crustal rocks of 1.5 b.y. and with a constant degree of chemical heterogeneity for all of Earth history.     

He,Pb, Sr and Nd isotope constraints on magma genesis and mantle heterogeneity beneath young Pacific seamounts

Pb, Sr and Nd isotope variations are correlated in diverse lavas erupted at small seamounts near the East Pacific Rise. Tholeiites are isotopically indistinguishable from MORB (²⁰⁶Pb/²⁰⁴Pb=18.1–18.5; ⁸⁷Sr/⁸⁶Sr=0.7023–0.7028; ¹⁴³Nd/¹⁴⁴Nd=0.51326-0.51308); associated alkali basalts always show more radiogenic Pb and Sr signatures (²⁰⁶Pb/²⁰⁴Pb=18.8–19.2; ⁸⁷Sr/⁸⁶Sr=0.7029–0.7031) and less radiogenic Nd (¹⁴³Nd/¹⁴⁴Nd=0.51289–0.51301). The isotopic variability covers 80% of the variability for Pacific MORB, due to the presence of small-scale heterogeneity in the underlying mantle. Isotope compositions also correlate with trace element ratios such as La/Sm. Tholeiites at these seamounts have ³He/⁴He between 7.8–8.7 R _A(R _A= atmospheric ratio), also indistinguishable from MORB. He trapped in vesicles of alkali basalts, released by crushing in vacuo, has low ³He/⁴He (1.2–2.6 R)_Ain conjunction with low helium concentrations ([He]<5×10^–8 ccSTP/g). In many cases post-eruptive radiogenic ingrowth has produced He isotope disequilibrium between vesicles and glass in the alkali basalts; subatmospheric ³He/⁴He ratios characterize the He dissolved in the glass which is released by melting the crushed powders. The narrow range of ³He/⁴He in the vesicles of the alkali basalts suggests that low ³He/⁴He is a source characteristic, but given their low [He] and high (U + Th), pre-eruptive radiogenic ingrowth cannot be excluded as a cause for low inherited ³He/⁴He ratios. Pb, Sr and Nd isotope compositions in lavas erupted at Shimada Seamount, an isolated volcano on 20 m.y. old seafloor at 17°N, are distinctly different from other seamounts in the East Pacific (²⁰⁶Pb/²⁰⁴Pb=18.8–19.0, ⁸⁷Sr/ ⁸⁶Sr0.7048 and ¹⁴³Nd/¹⁴⁴Nd0.51266). Relatively high ²⁰⁷Pb/²⁰⁴Pb (15.6–15.7) indicates ancient (>2 Ga) isolation of the source from the depleted upper mantle, similar to Dupal components which are more prevalent in the southern hemisphere mantle. ³He/⁴He at Shimada Seamount is between 3.9–4.8 R _A. Because the helium concentrations range up to 1.5×10^–6, the low ³He/⁴He can not be due to radiogenic accumulation of ⁴He in the magma for reasonable volcanic evolution times. The low ³He/⁴He may be due to the presence of enriched domains within the lithosphere with high (U + Th)/He ratios, possibly formed during its accretion near the ridge. Alternatively, the low ³He/⁴He may be an inherent characteristic of an enriched component in the mantle beneath the East Pacific. Collectively, the He-Pb-Sr-Nd isotope systematics at East Pacific seamounts suggest that the range of isotope compositions present in the mantle is more readily sampled by seamount and island volcanism than by axial volcanism. Beneath thicker lithosphere away from the ridge axis, smaller degrees of melting in the source regions are less efficient in averaging the chemical characteristics of small-scale heterogeneities.     

Rare earth element behavior and Pb,Sr, Nd isotope systematics in a heavy metal contaminated soil

The aim of this study is to characterize the processes and phases which control migration and retention of rare earth elements (REE) in a heavy metal contaminated soil. In addition to concentration data, we used Pb, Sr and Nd isotopic compositions in order to distinguish between natural and anthropogenic trace metals and to characterize the phases leached away during the sequential extraction procedure.The samples were sequentially extracted in 3 steps with 1 N acetic acid (HAc), 1 N HCl and 1 N HNO₃. The Pb isotope data showed that anthropogenic Pb had mainly been retained in the uppermost 10 cm by the organic matter of the topsoil. The⁸⁷Sr/⁸⁶Sr ratios of the HAc extracts are almost constant and indicate that soil carbonate is derived from regionally outcropping carbonate-rich sediments. Most HCl and HNO₃ extracts have more radiogenic Sr isotopic compositions, but it is unclear whether this reflects a growing influence of anthropogenic or silicate-derived Sr.The depth distribution of the REE is mainly controlled by two different parameters: soil pH for the HAc extractable REE and FeMn oxides for the REE in the HCl and HNO₃ extracts. A part of the HNO₃ extractable REE was also bound to the organic matter of the topsoil. The REE concentrations in the HAc extractable phase increase with depth and increasing soil pH, which indicates that they are derived from the surface and hence are of anthropogenic origin. This is confirmed by¹⁴³Nd/¹⁴⁴Nd isotope ratios which show a mixing between a natural end-member at the top and an anthropogenic end-member at the base of the profile. We assume that the anthropogenic REE were transported in dissolved form as carbonate complexes and then precipitated during downward migration as soil pH increased.     

Neodymium isotope systematics of the mantle beneath the Baltic shield: Evidence for depleted mantle evolution since the Archaean

More than 300 published and unpublished Nd isotopic analyses of mantle derived rocks from the Baltic shield have been compiled. The rocks range in age from Archaean to Phanerozoic. Within any given age-interval, the mantle derived rocks range in ε_Nd(t) from depleted mantle values at or above the growth curves of the global depleted mantle reservoirs of DePaolo (1981) and DePaolo et al. (1991) to negative values. Initial neodymium isotopic compositions below the De Paolo curve are best explained by interaction between depleted mantle derived magmas and local crustal contaminants. The data now available lend no support to the existence of isolated, less depleted or undepleted mantle domains beneath the Baltic Shield, as was suggested by Mearns et al. (1986) and Valbracht (1991a, b).     

Nd, Sr and Os isotope systematics in young, fertile spinel peridotite xenoliths from northern Queensland, Australia: A unique view of depleted MORB mantle?

Northeastern Queensland, a part of the Phanerozoic composite Tasman Fold Belt of eastern Australia, has a Paleozoic to Mesozoic history dominated by subduction zone processes. A suite of 13 peridotite xenoliths from the <3 Ma Atherton Tablelands Volcanic Province, predominantly from Mount Quincan, comprise fertile (1.8-3.4 wt.% Al₂O₃ and 38.7-41.9 wt.% MgO) spinel lherzolites free from secondary volatile-bearing phases and with only weak metasomatic enrichment of incompatible trace elements (Sm_N/Yb_N = 0.23-1.1; La_N/Yb_N = 0.11-4.9). The suite is isotopically heterogeneous, with measured Sr (⁸⁷Sr/⁸⁶Sr = 0.7027-07047), Nd (¹⁴³Nd/¹⁴⁴Nd = 0.51249-0.51362), and to a lesser extent, Os (¹⁸⁷Os/¹⁸⁸Os = 0.1228-0.1292) compositions broadly overlapping MORB source mantle (DMM) and extending to more depleted compositions, reflecting evolution in a time-integrated depleted reservoir. Major and rare earth element systematics are consistent with mantle that is residual after low to moderate degrees of melt extraction predominantly in the spinel facies, but with a few samples requiring partial melting at greater pressures in the garnet field or near the garnet-spinel transition. In contrast to most previously studied suites of continental lithospheric mantle samples, the incompatible trace element contents and Sr and Nd isotopic systematics of these samples suggest only minimal modification of the sampled lithosphere by metasomatic processes.Five of six Mount Quincan xenoliths preserving depleted middle to heavy REE patterns form a whole rock Sm-Nd isochron with an age of ∼275 Ma (ε_Ndi = +9), coincident with widespread granitoid emplacement in the overlying region. This isochron is interpreted to indicate the timing of partial melting of a DMM-like source. Xenoliths from other Atherton localities scatter about the isochron, suggesting that the sampled mantle represents addition of DMM mantle to the lithosphere in the Permian, when the region may have broadly been within a subduction zone setting. A sixth middle to heavy REE-depleted Mount Quincan xenolith has a distinct Nd and Os isotopic composition consistent either with an earlier, possibly Precambrian melt extraction event, or with Permian derivation from a mantle source with a less depleted (time-averaged lower Sm/Nd) Nd isotopic composition, but a more depleted (low Re/Os) Os isotopic composition.The range in measured whole rock Os isotopic compositions cannot solely be the result of time-integrated effects of variable melt extraction, especially considering the coherent Sm-Nd systematics of the suite. The Os heterogeneity more likely reflects either a heterogeneous ∼275 Ma DMM source that would have a present-day Os composition (¹⁸⁷Os/¹⁸⁸Os ∼ 0.1265-0.1287) overlapping both abyssal peridotites and chondrites, or significant and variable enrichment within the lithospheric mantle by secondary sulfides carrying radiogenic Os in a cryptic chalcophile enrichment event. Regardless of the origin of the Os isotopic variability, these data highlight the mantle Re-Os isotopic heterogeneity that may be present over small length scales where the lithophile Sm-Nd system may be relatively homogeneous.     

Plume-related mantle source of super-large rare metal deposits from the Lovozero and Khibina massifs on the Kola Peninsula, Eastern part of Baltic Shield: Sr, Nd and Hf isotope systematics

The two world’s largest complexes of highly alkaline nepheline syenites and related rare metal loparite and eudialyte deposits, the Khibina and Lovozero massifs, occur in the central part of the Kola Peninsula. We measured for the first time in situ the trace element concentrations and the Sr, Nd and Hf isotope ratios by LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometer) in loparite, eudialyte an in some other pegmatitic minerals. The results are in aggreement with the whole rock Sr and Nd isotope which suggests the formation of these superlarge rare metal deposits in a magmatic closed system. The initial Hf, Sr, Nd isotope ratios are similar to the isotopic signatures of OIB indicating depleted mantle as a source. This leads to the suggestion that the origin of these gigantic alkaline intrusions is connected to a deep seated mantle source—possibly to a lower mantle plume. The required combination of a depleted mantle and high rare metal enrichment in the source can be explained by the input of incompatible elements by metasomatising melts/fluids into the zones of alkaline magma generation shortly before the partial melting event (to avoid ingrowth of radiogenic isotopes). The minerals belovite and pyrochlore from the pegmatites are abnormally high in ⁸⁷Sr /⁸⁶Sr ratios. This may be explained by closed system isotope evolution as a result of a significant increase in Rb/Sr during the evolution of the peralkaline magma.     

U-Th-Pb and Lu-Hf isotopic constraints on the evolution of sub-continental lithospheric mantle, French Massif Central

We have carried out a Pb double-spike and Lu-Hf isotope study of clinopyroxenes from spinel-facies mantle xenoliths entrained in Cenozoic intraplate continental volcanism of the French Massif Central (FMC). U-Th-Pb and Lu-Hf isotope systematics verify the existence of different lithospheric domains beneath the northern and southern FMC. Northern FMC clinopyroxenes have extreme Lu/Hf ratios and ultra-radiogenic Hf (ε_Hf = +39.6 to +2586) that reflect ∼15-25% partial melting in Variscan times (depleted mantle model ages ∼360 Ma). Zr, Hf and Th abundances in these clinopyroxenes are low and unaffected by hydrous/carbonatitic metasomatism that overprinted LILE and light REE abundances and caused decoupling of Lu/Hf-Sm/Nd ratios and Nd-Hf isotopes (ε_Nd = +2.1 to +91.2). Pb isotopes of northern FMC clinopyroxenes are radiogenic (²⁰⁶Pb/²⁰⁴Pb > 19), and typically more so than the host intraplate volcanic rocks. ²³⁸U/²⁰⁴Pb ratios range from 17 to 68, and most samples have distinctively low ²³²Th/²³⁸U (<1) and ²³²Th/²⁰⁴Pb (3-22). Clinopyroxenes from southern FMC lherzolites are generally marked by overall incompatible trace element enrichment including Zr, Hf and Th abundances, and have Pb isotopes that are similar to or less radiogenic than the host volcanic rocks. Hf isotope ratios are less radiogenic (ε_Hf = +5.4 to +41.5) than northern FMC mantle and have been overprinted by silicate-melt-dominated metasomatism that affected this part of FMC mantle. Major element and Lu concentrations of clinopyroxenes from southern FMC harzburgites are broadly similar to northern FMC clinopyroxenes and suggest they experienced similar degrees of melt extraction as northern FMC mantle. ²³⁸U/²⁰⁴Pb (53-111) and ²³²Th/²⁰⁴Pb ratios (157-355) of enriched clinopyroxenes from the southern FMC are extreme and significantly higher than the intraplate volcanic rocks. In summary, mantle peridotites from different parts of the FMC record depletion at ∼360 Ma during Variscan subduction, followed by differing styles of enrichment. Northern FMC mantle was overprinted by a fluid/carbonatitic metasomatic agent that carried elements like U, Pb, Sr and light REE. In contrast, much of the southern FMC mantle was metasomatised by a small-degree partial silicate melt resulting in enrichment of all incompatible trace elements. The extreme mantle ²³⁸U/²⁰⁴Pb (northern and southern FMC), ²³²Th/²³⁸U (northern FMC) and ²³²Th/²⁰⁴Pb ratios (southern FMC), coupled with unremarkable present-day Pb isotope ratios, constrain the timing of enrichment. Mantle metasomatism is a young feature related to melting of the upwelling mantle responsible for Cenozoic FMC volcanism, rather than subduction-related metasomatism intimately associated with mantle depletion during the Variscan orogeny. The varying metasomatic styles relate to pre-existing variations in the thickness of the continental lithospheric lid, which controlled the extent to which upwelling mantle could ascend and melt. In the northern FMC, a thicker and more refractory lithospheric lid (?80 km) only allowed incipient degrees of melting resulting in fluid/carbonatitic metasomatism of the overlying sub-continental lithospheric mantle. The thinner lithospheric lid of the southern FMC (?70 km) allowed larger degrees of melting and resulted in silicate-melt-dominated metasomatism, and also focused the location of the volcanic fields of the FMC above this region.     

Late Archaean mantle metasomatism below eastern Indian craton: Evidence from trace elements,REE geochemistry and Sr—Nd—O isotope systematics of ultramafic dykes

Trace, rare earth elements (REE), Rb-Sr, Sm-Nd and O isotope studies have been carried out on ultramafic (harzburgite and lherzolite) dykes belonging to the newer dolerite dyke swarms of eastern Indian craton. The dyke swarms were earlier considered to be the youngest mafic magmatic activity in this region having ages not older than middle to late Proterozoic. The study indicates that the ultramafic members of these swarms are in fact of late Archaean age (Rb-Sr isochron age 2613 ± 177 Ma, Sri ∼ 0.702 ± 0.004) which attests that out of all the cratonic blocks of India, eastern Indian craton experienced earliest stabilization event. Primitive mantle normalized trace element plots of these dykes display enrichment in large ion lithophile elements (LILE), pronounced Ba, Nb and Sr depletions but very high concentrations of Cr and Ni. Chondrite normalised REE plots exhibit light REE (LREE) enrichment with nearly flat heavy REE (HREE; (ΣHREE)_N ∼ 2–3 times chondrite, (Gd/Yb)_N ∼ 1). The ε^Nd(t) values vary from +1.23 to -3.27 whereas δ¹⁸O values vary from +3.16‰ to +5.29‰ (average +3.97‰±0.75‰) which is lighter than the average mantle value. Isotopic, trace and REE data together indicate that during 2.6 Ga the nearly primitive mantle below the eastern Indian Craton was metasomatised by the fluid (± silicate melt) coming out from the subducting early crust resulting in LILE and LREE enriched, Nb depleted, variable ε^Nd, low Sr_i(0.702) and low δ¹⁸O bearing EMI type mantle. Magmatic blobs of this metasomatised mantle were subsequently emplaced in deeper levels of the granitic crust which possibly originated due to the same thermal pulse.     

A periclase-dolomite-calcite carbonatite from the Oka complex,Quebec, and its calculated volatile composition

The eutectic mineral assemblage calcite-dolomite-periclase-apatite-forsterite-magnesioferrite-pyrrhotite-alabandite in a carbonatite dike within the Oka complex, Quebec, buffers the fugacities (and partial pressures) of all gas species in C-O-H-S-F, assuming vapor saturation. At the inferred eutectic (640° C, 1 kbar), the most important gas species and their partial pressures (bars) were: H₂O, 882; CO₂, 110; H₂, 4.6; H₂S, 2.7; CO, 0.5; and CH₄, 0.1. Oxygen fugacity was near the QFM buffer, logf(O₂)=–18.6, and sulfur fugacity was near the QFM-pyrrhotite buffer, logf(S₂)=–5.9. Fluorine fugacity was low, logf(F₂)=–43.9, consistent with the absence of fluoride minerals other than apatite. Presence of a water-rich gas phase is consistent with experiments on synthetic carbonatite systems (e.g. Fanelli et al. 1981), although compositions of the gas phase in published experiments cannot be determined exactly.Contribution no. 390 from the Mineralogical Laboratory, The University of Michigan     

In search of a hidden long-term isolated sub-chondritic Nd/Nd reservoir in the deep mantle: Implications for the Nd isotope systematics of the Earth

Here we search for evidence of the existence of a sub-chondritic ¹⁴²Nd/¹⁴⁴Nd reservoir that balances the Nd isotope chemistry of the Earth relative to chondrites. If present, it may reside in the source region of deeply sourced mantle plume material. We suggest that lavas from Hawai’i with coupled elevations in ¹⁸⁶Os/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os, from Iceland that represent mixing of upper mantle and lower mantle components, and from Gough with sub-chondritic ¹⁴³Nd/¹⁴⁴Nd and high ²⁰⁷Pb/²⁰⁶Pb, are favorable samples that could reflect mantle sources that have interacted with an Early-Enriched Reservoir (EER) with sub-chondritic ¹⁴²Nd/¹⁴⁴Nd.High-precision Nd isotope analyses of basalts from Hawai’i, Iceland and Gough demonstrate no discernable ¹⁴²Nd/¹⁴⁴Nd deviation from terrestrial standards. These data are consistent with previous high-precision Nd isotope analysis of recent mantle-derived samples and demonstrate that no mantle-derived material to date provides evidence for the existence of an EER in the mantle.We then evaluate mass balance in the Earth with respect to both ¹⁴²Nd/¹⁴⁴Nd and ¹⁴³Nd/¹⁴⁴Nd. The Nd isotope systematics of EERs are modeled for different sizes and timing of formation relative to ε¹⁴³Nd estimates of the reservoirs in the μ¹⁴²Nd = 0 Earth, where μ¹⁴²Nd is ((measured ¹⁴²Nd/¹⁴⁴Nd/terrestrial standard ¹⁴²Nd/¹⁴⁴Nd)−1 * 10⁻⁶) and the μ¹⁴²Nd = 0 Earth is the proportion of the silicate Earth with ¹⁴²Nd/¹⁴⁴Nd indistinguishable from the terrestrial standard. The models indicate that it is not possible to balance the Earth with respect to both ¹⁴²Nd/¹⁴⁴Nd and ¹⁴³Nd/¹⁴⁴Nd unless the μ¹⁴²Nd = 0 Earth has a ε¹⁴³Nd within error of the present-day Depleted Mid-ocean ridge basalt Mantle source (DMM). The 4567 Myr age ¹⁴²Nd-¹⁴³Nd isochron for the Earth intersects μ¹⁴²Nd = 0 at ε¹⁴³Nd of +8 ± 2 providing a minimum ε¹⁴³Nd for the μ¹⁴²Nd = 0 Earth. The high ε¹⁴³Nd of the μ¹⁴²Nd = 0 Earth is confirmed by the Nd isotope systematics of Archean mantle-derived rocks that consistently have positive ε¹⁴³Nd.If the EER formed early after solar system formation (0-70 Ma) continental crust and DMM can be complementary reservoirs with respect to Nd isotopes, with no requirement for significant additional reservoirs. If the EER formed after 70 Ma then the μ¹⁴²Nd = 0 Earth must have a bulk ε¹⁴³Nd more radiogenic than DMM and additional high ε¹⁴³Nd material is required to balance the Nd isotope systematics of the Earth.     

稳定Sr-Nd同位素体系及其对传统放射成因锶钕同位素组成的影响

稳定Sr、Nd同位素是全新的体系,其重点关注自然过程中稳定Sr和Nd同位素分馏特征与控制机制。这方面的知识可以修正传统Rb Sr和Sm-Nd同位素体系中恒定的稳定Sr、Nd同位素组成的前提假设所带来的认知偏差,补充和完善其知识体系。介绍了稳定Sr、Nd同位素体系的基本概念,综述并点评了最新的分析技术进展,并对稳定Sr、Nd同位素分馏对传统的放射成因Sr、Nd同位素组成的影响进行评估,同时对这两个新的稳定同位素体系的潜在研究应用进行展望。     

Lead isotope evolution of the Central European upper mantle:Constraints from the Bohemian Massif

The Pb isotope composition of the upper mantle beneath Central Europe is heterogeneous due to the subduction of regionally contrasting material during the Variscan and Alpine orogenies.Late Variscan to Cenozoic mantlederived melts allow mapping this heterogeneity on a regional scale for the last ca.340 Myr.Late Cretaceous and Cenozoic anorogenic magmatic rocks of the Bohemian Massif(lamprophyres,volcanic rocks of basanite/tephrite and trachyte/phonolite series) concentrate mostly in the Eger Rift.Cretaceous ultramafic lamprophyres yielded the most radiogenic Pb isotope signatures reflecting a maximum contribution from metasomatised lithospheric mantle,whereas Tertiary alkaline lamprophyres originated from mantle with less radiogenic ~(206)Pb/~(204)b ratios suggesting a more substantial modification of lithospheric source by interaction with asthenosphericderived melts.Cenozoic volcanic rocks of the basanite/tephrite and trachyte/phonolite series define a linear mixing trend between these components,indicating dilution of the initial lithospheric mantle signature by upwelling asthenosphere during rifting.The Pb isotope composition of Late Cretaceous and Cenozoic magmatic rocks of the Bohemian Massif follows the same Pb growth curve as Variscan orogenic lamprophyres and lamproites that formed during the collision between Laurussia,Gondwana,and associated terranes.This implies that the crustal Pb signature in the post-Variscan mantle is repeatedly sampled by younger anorogenic melts.Most Cenozoic mantle-derived rocks of Central Europe show similar Pb isotope ranges as the Bohemian Massif.     

东蒙地区燕山期花岗岩Nd、Sr、Pb同位素及其岩石成因

由于缺乏系统的同位素分析研究工作，过去对东蒙地区燕山期岩浆岩的成因探讨，主要集中在大量岩石地球化学方面的分析研究，因此，其成因观点和岩浆起源的认识也各持己见，主要有3种认识：1)本区中生代壳源和幔源共生的“双模式”观点，认为锡多金属成矿与这种“双模式”的岩浆岩有成因联系；2)中生代花岗岩属于引张环境下，地幔上隆所引发的亚碱性一碱性非造山岩浆作用的产物；3)中生代岩浆岩是中生代大陆内部伸展造山环境下底侵作用形成的一套壳幔混熔岩浆的产物。总之，研究者多认为燕山期岩浆岩具有壳幔混合起源的特征。笔者对燕山期花岗质岩石的钕、锶、铅同位素进行了分析研究。其εNd(t)全为正值，变化范围为 0．75～ 8．12，平均值为 3．07，说明该区燕山期花岗岩的物质来源与亏损地幔有成因联系。其初始锶比值比较集中，变化于0．7028～0.7096，平均为0．7063，介于现代大洋玄武岩(0．702～0．706)和大陆地壳(0．706～0．718)之间，更接近大洋玄武岩。该区燕山期花岗岩的初始铅同位素的3个比值^206Pb/^204Pb、^207Pb／^204Pb、^308b／^204Pb都较高，平均值分别为18．3742，15．5500，38．1810。由钾长石的铅同位素比值计算出来的μ值介于9．51～8．91之间，低于μ=9．74的陆壳演化线。结合邻区兴蒙—北疆一带的岩浆岩同位素研究成果，笔者认为东蒙地区的燕山期花岗岩岩浆起源于亏损地幔的部分熔融作用和亏损地幔起源的晚华力西期古蒙古洋壳的部分熔融作用，即燕山期花岗岩浆最终起源于亏损地幔。并且提出了亏损地幔—古蒙古洋壳—边缘陆块活化的演化模式。     

The petrogenesis of leucogranitic dykes intruding the northern Semail ophiolite, United Arab Emirates: field relationships, geochemistry and Sr/Nd isotope systematics

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 ɛ_Nd 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     

Sm／Nd Evolution of Upper Mantle and Continental Crust：Constraints on Gowth Rates of the Continental Crust

A new approach to the investigation of the Sm/Nd evolution of the upper mantle directly from the data on lherzolite xenoliths is described in this paper.It is demonstrated that the model age TCHUR of an unmetasomatic iherzolite zenolith ca represent the mean depletion age of its mantle source, thus presenting a correlation trend between f^Sm/Nd and the mean depletion age of the upper mantle from the data on xenoliths.This correlation trend can also be derived from the data on river suspended loads as well as from granitoids.Based on the correlation trend mentioned above and mean depletion ages of the upper mantle at various geological times, an evolution curve for the mean f^Sm/Nd value of the upper mantle through geological time has been established.It is suggested that the upwilling of lower mantle material into the upper mantle and the recycling of continental crust material during the Archean were more active ,thus maintaining fairly constantf^Sm/Nd and εNd values during this time period. Similarly ,an evolution curve for the mean f^Sm/Nd value of the continental crust through geological time has also been established from the data of continental crust material.In the light of both evolution curves for the upper mantle and continental crust ,a growth curve for the continental crust has been worked out ,suggesting that :(1)about 30%(in volume )of the present crust was present as the continental crust at 3.8 Ga ago ;(2)the growth rate was much lower during the Archean ;and (3)the Proterozoic is another major period of time during which the continental crust wsa built up .