Sm-Nd Isotopic Compositions Research Articles

U-Pb Ages of Zircons in Western Qinling Shan, China, and Their Tectonic Implications

The Western Qinling Shan is the westward extension of the Eastern Qinling Shan, which connects the North China craton, the Qilian and Qaidam, and the Songpan-Ganzi block on the north, on the west, and on the south, respectively. Clastic sedimentary rocks of Precambrian to Triassic age from this region are studied in this article. Detrital zircons were extracted from the sedimentary rocks for U-Pb dating through LA-ICPMS. Nd depleted mantle model ages resemble those of the Yangtze craton, with a statistical population of 1.55–1.98 Ga, peaking approximately at 1.81 Ga, in contrast to Paleoproterozoic and Archean population of the North China craton. Devonian detrital zircon U-Pb ages of 930 to 730 Ma indicate their affinity to the Yangtze craton, which was the intensive crustal growth stage in the provenance region. The majority of the Permo-Triassic clastic sedimentary rocks are characterized by ages older than 1600 Ma. Detrital zircon U-Pb ages and Sm-Nd isotopic compositions indicate that the basement rocks, on the southern margin of the North China craton, became an important component of the Permo-Triassic clastic sedimentary rocks. The Yangtze craton conjoined the North China craton during the Triassic time period. Up to 50% of the Permo-Triassic clastic sedimentary rocks of the west Qinling segment contain material from the basement rocks on the southern margin of the North China craton.

Zircon LA-ICP-MS U-Pb dating and Sr-Nd isotope study of the Guposhan granite complex, Guangxi, China

Zircon U-Pb dating by the LA-ICP-MS method was applied to determining the ages of different units of the Guposhan granite complex, among which the East Guposhan unit is 160.8±1.6 Ma, the West Guposhan unit is 165.0±1.9 Ma, and the Lisong unit is 163.0±1.3 Ma in age. Much similarity in ages of the three units has thus proved that the whole Guposhan granite complex was formed in the same period of time. They were the products of large-scale granitic magmatism through crust-remelting in the first stage of the Middle Yanshanian in South China. However, the three units have differences both in petrology and in geochemistry. Besides the differences in major, trace and rare-earth elements, they are distinct in their Rb-Sr and Sm-Nd isotopic compositions. The East Guposhan unit and Lisong unit and its enclaves have a similar (87Sr/86Sr)i value of 0.7064 with an average of ɛNd(t)=−3.03, indicating that more mantle material was evolved in the magma derivation; whereas the West Guposhan unit has a higher (87Sr/86Sr)i value of 0.7173 but a lower ɛNd(t) value of −5.00, and is characterized by strong negative Eu anomalies and higher Rb/Sr ratios, suggesting that its source materials were composed of relatively old crust components and new mantle-derived components. In addition, an inherited zircon grain in the East Guposhan unit (GP-1) yielded a 206Pb/238U age of 806.4 Ma, which is similar to the ages of the Jiulin cordierite granite in northern Jiangxi and of the Yinqiao migmatic granite in Guangxi in the HZH granite zone. All this may provide new evidence for Late Proterozoic magmatism in the HZH granite zone.

Precambrian terranes in the southwestern framing of the Siberian craton: isotopic provinces, stages of crustal evolution and accretion-collision events

We studied geology and main rock assemblages of the Precambrian Kan, Arzybei, and Derba terranes of the Central Asian Fold Belt which border the Siberian craton in the southwest. The Precambrian terranes include three isotopic provinces (Paleoproterozoic, Mesoproterozoic, and Neoproterozoic) distinguished from the Sm-Nd isotope compositions of granitoids, felsic metavolcanics, and metasediments. The terranes formed in three stages of crustal evolution: 2.3–2.5, 0.9–1.1, and 0.8–0.9 Ga. Proterozoic juvenile crust was produced by subduction-related magmatism; it was originally of transitional composition and transformed into continental crust by potassic plutonism as late as the Late Vendian-Cambrian. Terrigenous sediments in the Arzybei and Derba terranes vary in T(DM) Nd model ages from 1.0 to 2.0 Ga. The Nd ages of the underlying metavolcanics and lowest T(DM) of metasediments indicate that terrigenous sedimentation started in the Neoproterozoic. It was maintained by erosion of Mesoproterozoic-Neoproterozoic crust and, to a lesser extent, of Early Precambrian rocks on the craton margin or in Paleoproterozoic terranes. Ar-Ar dating of amphiboles and biotites from metamorphic rocks and U-Pb dating of zircons from granitoids yielded 600–555 and 500–440 Ma, respectively, corresponding to the Vendian and Early Paleozoic stages of nearly synchronous metamorphism and plutonism. Accretion and collision events caused amalgamation of the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic terranes in the Vendian and their collision with the Siberian craton. The lateral growth of the paleocontinent completed in the Late Ordovician.

Composition and structure of the lower crust of the Belomorian Mobile Belt, Baltic Shield

The lower crust of the Belomorian Mobile Belt consists predominantly of garnet peridotites with subordinate amounts of pyroxenites and spinel peridotites, which occur as xenoliths in Devonian diatremes and dikes in the southern part of the Kola Peninsula. When transported to the surface by ultrabasic melts, the xenoliths were affected by fluids from the host ultrabasic lamprophyres with the introduction of Ca, Mg, and such trace elements as Ba, Nb, Sr, and P. The concentrations of trace elements (Sm, Nd, Y, Ti, Zr, Ni, Cr, and others) and the Sm-Nd isotopic composition were not significantly modified, which makes it possible to use them to compare the xenoliths with the near-surface complexes and to reproduce the composition of the protoliths. The Paleoproterozoic lower crust was produced during the emplacement of mantle magmas into metabasites in the Neoarchean lower crust, a process that was accompanied by the contamination of the melts and the origin of rocks showing characteristics of mantle and crust material. The emplacement of significant melt volumes into the Neoarchean lower crust caused its heating and enabled its viscous-plastic flow. This flow could likely also affect the material of the upper mantle, as follows from the occurrence of spinel peridotite nodules among the garnet granulites with an increase in the amount of mantle xenoliths from the roof to bottom of the lower crust. The overall amount of ultrabasic rocks in the lower crust was evaluated at 8–10%.

Multi-isotopic system geochronology of low temperature eclogite from Huangzhen, Southern Dabie Terrain

Zircon U-Pb thermal ionization mass-spectrometer (TIMS) and secondary ion mass-spectrometer (SIMS) dating, mica and amphibole 40 Ar- 39 Ar dating and mineral Sm-Nd isotopic compositions of Huangzhen low temperature eclogite and country granitic gneiss are carried out. The zircon U-Pb weighted average SIMS age is (231.6±9.7) Ma for one eclogite. The mica 40 Ar- 39 Ar isochron age is (232.6±2.1) Ma and the lowest plateau age is (221.7±2.4) Ma from same sample. U-Pb TIMS concordant ages from other eclogite zircons are from (221.3±1.4) Ma to (222.5±2.3) Ma. U-Pb SIMS low intercept age from country granitic gneiss is (221±35) Ma. The retrograde amphibole 40 Ar- 39 Ar isochron age is (205.9±1.0) Ma. Except for mica, which may contain excess 40 Ar, all the ages represent peak and retrograde metamorphism of low temperature eclogites. It is indicated that the Huangzhen low temperature eclogites differ from Xiongdian low temperature eclogites of north of the Northern Dabie Terrain in metamorphic ages. Huangzhen low temperature eclogites share one coherent HP-UHP terrain with high temperature eclogites from Southern Dabie Terrain and they may have differences in subduction depth and cooling rates during exhumation.

Precise Isotope Determination of Trace Amounts of Nd in Magnesium-rich Samples

We developed a technique combining mass spectrometry with chemical separation to analyze Sm-Nd isotopic compositions of ultramafic rock samples, which have extremely high Mg and low Nd concentrations and are always difficult to analyze. In the case of ultramafic rocks, abundant Mg forms fluorides during decomposition by HF, and minor amounts of Sm and Nd are strongly partitioned into fluorides, which would be a significant reason of low recovery yields of these elements and obstructed to determine a reliable isotopic ratio of Nd and Sm. We introduced pre-decomposition by HCl prior to the conventional method by HF and HClO4 that made it possible to elevate recovery yield to 85%. On the other hand, replicate measurements of Nd standard solution (La Jolla) by changing the ion current of Nd+ and the amount of Nd indicate that reliable isotopic ratios are measured under the condition of >0.04×10-11 A of 144Nd+ ion current and >2 ng of Nd. By applying our method, we analyzed 143Nd/144Nd ratio and Sm and Nd concentrations of PCC-1, which is an ultramafic standard rock sample of United States Geological Survey. The results of our analysis were 0.512226±0.000026, 0.00551±0.00041, and 0.0278±0.0011 ppm, respectively. These results are the first report of 143Nd/144Nd ratio for PCC-1.

Constraints on the petrogenesis of Martian meteorites from the Rb-Sr and Sm-Nd isotopic systematics of the lherzolitic shergottites ALH77005 and LEW88516

Detailed Rb-Sr and Sm-Nd isotopic analyses have been completed on the lherzolitic shergottites ALH77005 and LEW88516. ALH77005 yields a Rb-Sr age of 185 ± 11 Ma and a Sm-Nd age of 173 ± 6 Ma, whereas the Rb-Sr and Sm-Nd ages of LEW88516 are 183 ± 10 and 166 ± 16 Ma, respectively. The initial Sr isotopic composition of ALH77005 is 0.71026 ± 4, and the initial εNd value is +11.1 ± 0.2. These values are distinct from those of LEW88516, which has an initial Sr isotopic composition of 0.71052 ± 4 and an initial εNd value of +8.2 ± 0.6. Several of the mineral and whole rock leachates lie off the Rb-Sr and Sm-Nd isochrons, indicating that the isotopic systematics of the meteorites have been disturbed. The Sm-Nd isotopic compositions of the leachates appear to be mixtures of primary igneous phosphates and an alteration component with a low 143Nd/144Nd ratio that was probably added to the meteorites on Mars. Tie lines between leachate-residue pairs from LEW88516 mineral fractions and whole rocks have nearly identical slopes that correspond to Rb-Sr ages of 90 ± 1 Ma. This age may record a major shock event that fractionated Rb/Sr from lattice sites located on mineral grain boundaries. On the other hand, the leachates could contain secondary alteration products, and the parallel slopes of the tie lines could be coincidental.Nearly identical mineral modes, compositions, and ages suggest that these meteorites are very closely related. Nevertheless, their initial Sr and Nd isotopic compositions differ outside analytical uncertainty, requiring derivation from unique sources. Assimilation-fractional-crystallization models indicate that these two lherzolitic meteorites can only be related to a common parental magma, if the assimilant has a Sr/Nd ratio near 1 and a radiogenic Sr isotopic composition. Further constraints placed on the evolved component by the geochemical and isotopic systematics of the shergottite meteorite suite suggest that it (a) formed at ∼4.5 Ga, (b) has a high La/Yb ratio, (c) is an oxidant, and (d) is basaltic in composition or is strongly enriched in incompatible elements. The composition and isotopic systematics of the evolved component are unlike any evolved lunar or terrestrial igneous rocks. Its unusual geochemical and isotopic characteristics could reflect hydrous alteration of an evolved Martian crustal component or hydrous metasomatism within the Martian mantle.

Rb-Sr and Sm-Nd isotopes in garnet pyroxenite xenoliths from Siberian kimberlites: an insight into lithospheric mantle.

Rb-Sr and Sm-Nd isotope compositions of seven garnet pyroxenite xenoliths from Udachnaya and Mir kimberlites, Siberia, are presented. Five of them are websterites, and the other two are clinopyroxenites. The websterite suite is divided into two groups (Group A and Group B) according to their mineral chemistry. Group B websterites are characterized by presence of jadeite-rich clinopyroxene and pyrope-poor garnet. The Garnet-orthopyroxene geobarometry indicates that Group A were produced in deep positions of the cratonic mantle (up to diamond stability field) and Group B websterites are formed at lower pressure (ca. 19 kb). Probably, the Rb-Sr isotope system was re-equilibrated at the time of kimberlite emplacement at 367 Ma. The initial Sr isotope compositions differs from one group to the other. The initial ratio 87Sr/86Sr (0.70272-0.70338) for Group A is lower than that of Group B (0.70485-0.70737). The Sm-Nd isotope system in all samples gives apparent isochron ages considerably older than that of the emplacement of host kimberlites. Group A websterites and clinopyroxenites are apparently younger (1223-582 Ma) than those of Group B (1550-1465 Ma). The origin of garnet pyroxenites is suggested as crystal accumulation from LREE-enriched melts which episodically added into peridotite-dominated lithospheric mantle. These melts played also a role of metasomatic agent to the host peridotites. A delaminated lower crustal protolith has probably been involved in Group B origin. The Sm-Nd isotope study reveals a systematic increase of initial 143Nd/144Nd with time. A vairable Sm-Nd isotope data suggest that peridotites and pyroxenites in lithospheric mantle beneath the Udachnaya area were in isotopic equilibrium, but not beneath the Mir area where peridotites depleted in initial Nd isotope composition.

Gold mineralization in the Mazowe area, Harare-Bindura-Shamva greenstone belt, Zimbabwe: II. Genetic relationships deduced from mineralogical, fluid inclusion and stable isotope studies, and the Sm-Nd isotopic composition of scheelites

In the Mazowe area some 40 km NW of Harare in Zimbabwe, gold mineralization is hosted in a variety of lithologies of the Archean Harare-Bindura-Shamva greenstone belt, in structures related to the late Archean regional D2/3 event. Conspicuous mineralzogical differences exist between the mines; the mainly granodiorite-hosted workings at Mazowe mine are on pyrite-rich reefs, mines of the Bernheim group have metabasalt host rocks and are characterized by arsenopyrite-rich ores, and Stori's Golden Shaft and Alice mine, both in metabasalts, work sulfide-poor quartz veins. In contrast to the mineralogical diversity, near-identical fluid inventories were found at the different mines. Both H2O-CO2-CH4 fluids of low salinity, and highly saline fluids are present and are regarded to indicate fluid mixing during the formation of the deposits. Notably, these fluid compositions in the Mazowe gold field markedly contrast to ore fluids “typical” of Archean mesothermal gold deposits on other cratons. Stable isotope compositions of quartz from the various deposits (δ18O=10.8 to 13.2‰ SMOW), calcite (δ18O=9.5 to 11.9‰ SMOW and δ13C=−3.2 to −8.0‰ PDB), inclusion water (δD=−28 to −40‰ SMOW) and sulfides (δ34S=1.3 to 3.2‰ CDT) are uniform within the range typical for Archean lode gold deposits worldwide. The fluid and stable isotope compositions support the statement that the mineralization in the Mazowe gold field formed from relatively reduced fluids with a “metamorphic” signature during a single event of gold mineralization. Microthermometric data further indicate that the deposits formed in the PT range of 1.65–2.3 kbar and 250–380 °C. Ages obtained by using the Sm/Nd and Rb/Sr isotope systems on scheelites are 2604 ± 84 Ma for the mineralization at Stori's Golden Shaft mine, and 2.40 ± 0.20 Ga for Mazowe mine. The Archean age at Stori's is regarded as close to the true age of gold mineralization in the area, whereas the Proterozoic age at Mazowe mine probably reflects later resetting.

Nd model ages of sedimentary profile from the northwest Yangtze Craton, Guangyuan, Sichuan province, China and their geological implication.

Sm-Nd isotopic compositions of Late Neoproterozoic (Sinian), Early Cambrian, Early Ordovician, Late Triassic, and Jurassic sedimentary rocks from the northwestern Yangtze Craton, Guangyuan, Sichuan province, China show that the averaged Nd model age is 1.9 Ga. Based on Nd model ages (1.0 Ga to 3.3 Ga) combining geochronological data, three end-members are identified, which are represented by ca. 3.3 Ga, and 1.0 Ga, and 2.0 Ga respectively. The 2.0 Ga is postulated from geographical distribution of Nd model ages and partly geochronological data. Because the highest εNd(t) values only occurred in Cambrian and Ordovician sedimentary rocks, Early Paleozoic was a very important stage of orogen, which led to newly formed crustal end-member elevated and denudated widely. The 1.0 Ga growth episode occurred in the circum-Yangtze Craton principally.

Isotopic studies of ferroan anorthosite 62236: a young lunar crustal rock from a light rare-earth-element-depleted source

Isotopic analyses of mineral fractions and whole rocks from the ferroan anorthosite 62236 yield a Sm-Nd isochron with an age of 4.29 ± 0.06 Ga and an initial εNd143 value of +3.1 ± 0.9. We have also measured εNd142 anomalies of +0.25 on two fractions of 62236. These values are higher than the value of −0.1 predicted if 62236 was derived from a chondritic source at 4.29 Ga, but are consistent with the positive initial εNd143 value. The Sm-Nd isotopic composition of 62236 has been modified by the capture of thermal neutrons such that the 147Sm/144Nd, 143Nd/144Nd, and 142Nd/144Nd ratios measured on the mineral fractions and whole rocks must be corrected. The corrections do not significantly alter the Sm-Nd isotopic results determined on 62236. Despite the fact that the Ar-Ar and Rb-Sr isotopic systematics of 62236 have been reset by impact metamorphism at 3.93 ± 0.04 Ga, the Sm-Nd systematics appear to have been unaffected. The Sm-Nd isotopic systematics of 62236 provide several constrains on models of lunar crustal differentiation provided they have not been reset since crystallization. First, the relatively young age of 62236, as well as the old ages determined on several crustal plutonic rocks of the Mg-suite, require multiple sources of magmas on the Moon very early in its history. Second, positive εNd143 values determined on all analyzed ferroan anorthosites suggest that they were derived from sources depleted in light rare earth elements. And third, models based on initial εNd143 and present-day εNd142 values suggest that the source of 62236 was depleted in light rare earth elements at ∼4.46 Ga. In order to reconcile these observations with the lunar magma ocean model (1) the magma ocean must have existed for a very short period of time, and may have had a sub-chondritic Nd/Sm ratio, and (2) the youngest ferroan anorthosites, such as 62236, cannot be cumulates from the magma ocean, but must form by other processes.

The Nd-Sr isotope study of Mesozoic granitoids in Jiangxi Province

Nd-Sr isotopic compositions of 12 Mesozoic granitoids from Jiangxi Province have been reported. They show low-∈ND(−13.8–−8.3), high-/Sr(0.71069–0.73981) and oldtDM (2087–1635 Ma). Comparison between the Sm-Nd isotopic compositions of the granitoids and those of the basement metamorphic racks suggests that these granitoids were mainly derived from the metasedimentary rocks. There is an east-west trend Mesozoic granitoid zone with low-tDM from S. Jiangxi to S. Fujian. The origin of these granites was preliminarily discussed.

Contrasting geochemical and Sm-Nd isotopic compositions of Archean metasediments from the Kongling high-grade terrain of the Yangtze craton: evidence for cratonic evolution and redistribution of REE during crustal anatexis

Twenty-three clastic metasediments from the Kongling high-grade terrain of the Yangtze craton, South China were analyzed for major, trace and rare earth elements and Sm-Nd isotopic ratios. Associated dioritic-tonalitic-trondhjemitic (DTT) and granitic gneisses as well as amphibolites were also analyzed in order to constrain provenance. The results show that the clastic metasediments can be classified into 3 distinct groups in terms of mineralogical, geochemical and Sm-Nd isotopic compositions. Group A is characterized by having no to slight negative Eu anomalies (Eu/Eu∗ = 0.82–1.07), being high in Cr (191–396 ppm) and Ni (68–137 ppm), and low in Th (3.3–7.8 ppm) and REE (ΣREE = 99–156 ppm). These characteristics are similar to those of metasediments from Archean greenstone belts. In addition, the Group A metasediments have the value of the Chemical Index of Alteration (CIW) close to felsic gneisses. Their Sm-Nd isotopic, REE and trace element compositions can be interpreted by mixtures of the DTT gneisses and amphibolites. Dating of detrital zircons from 2 Group A samples by SHRIMP reveals a major concordant age group of 2.87–3.0 Ga, which is identical to the age of the trondhjemitic gneiss. These results strongly suggest that Group A was principally the first-cycle erosion product of the local Kongling DTT gneiss and amphibolite. Moreover, the higher than amphibolite Cr content and slight Eu depletion exhibited by some samples from this group infer that ultramafic rocks like komatiite and granite of probably 3.0–3.3 Ga in age also played a role. Group B is characterized by the presence of graphite and shows a more evolved composition similar to post-Archean shales with a prominent negative Eu anomaly (Eu/Eu∗ = 0.48–0.77) and high CIW. On paired Cr/Th vs La/Co and Co/Th plots, Group B samples conform to a two-end member mixing line of the Kongling granitic gneiss and amphibolite. However, data on Nd model age and CIW suggest that the granite component should be younger than the sampled granitic gneiss and derived from a distal source. Both Groups A and B exhibit a clear positive correlation between CIW and T DM and a negative one between CIW and Eu/Eu∗. These correlations point to the crustal evolution of the Yangtze craton towards coupled increasing CIW and Eu depletion with decreasing age. This in turn reflects the change of granitoid magmatism from local Na-rich dioritic-tonalitic-trondhjemitic rocks to widespread K-feldspar granite. The change led to the intracrustal differentiation, stabilization and growth of the craton. Group C is restite and contains abundant sillimanite and garnet and unusually high ilmenite (7–11vol%), which can be seen to be dehydration melting products of biotite under the microscope. This group shows extremely varied REE distributions from LREE enriched to depleted and from negative to strong positive (Eu/Eu∗ = 1.63) Eu anomalies. Compared to Groups A and B, Group C is severely depleted in Na 2O, K 2O, LREE, Rb and Ba, whereas TiO 2, Co, V, Sc and HREE and Y are considerably enriched. This is accompanied by anomalous high Sm/Nd (0.21–0.28), 147Sm/ 144Nd (0.1361–0.1738) and 143Nd/ 144Nd (0.511589–0.511958) ratios. T DM correlates clearly with Sm/Nd ratio and 2 out of 3 samples give significantly older to unrealistic T DM (3.9–4.9 Ga). The results document redistribution of REE and an open behavior of the Sm-Nd isotope system during the biotite dehydration melting of metasediments.

U-Pb ages and tectonomagmatic relationships of early Ordovician low-Ti tholeiites, boninites and related plutonic rocks in central Newfoundland, Canada

The Wild Bight Group (WBG) and South Lake Igneous Complex (SLIC) together comprise one of the Ordovician accreted oceanic terranes of the central mobile belt of the Newfoundland Appalachians. Combined detailed mapping, geochemistry, Sm-Nd isotopic studies and U-Pb geochronology have shown that sheeted dykes and hornblende diorite and tonalite plutons of the SLIC are genetically related to a discrete package of volcanic rocks in the WBG. These igneous rocks are geochemically, isotopically and temporally distinct from volcanic rocks in the rest of the WBG. Plutonic rocks of the SLIC range in age from 486 ± 3 Ma to 489 ± 3 Ma, and a cross-cutting gabbro dyke gives a minimum age of 486 ± 4 Ma for the related volcanic sequence. Volcanic rocks in the rest of the WBG sequence are predominantly younger than 472 ± 3 Ma. The older volcanic sequence of the WBG and the SLIC occur as fault-bounded packages interleaved within the younger WBG sequence. A conformable stratigraphic relationship between the older and younger sequences of the WBG has not been demonstrated. The mafic rocks of the older package include boninites and low-Ti, high-Mg tholeiitic island arc basalts which are interpreted to be genetically related, and normal island arc tholeiites (IAT). The high-Mg mafic rocks are interpreted to have formed in an extensional setting during subduction zone initiation, and the normal IAT are thought to represent stabilisation of the volcanic front. The associated high-Si, low-K rhyolite and tonalite are interpreted to be the products of secondary melting at the base of thickened early arc crust. Sm-Nd isotopic compositions indicate that the characteristic trace element signature of the boninites developed at or near their time of generation and was not a long lived characteristic of the source region. The boninites and low-Ti tholeiites are interpreted to have originated from a similar source, which was metasomatized by different subduction-related components. Apparent decoupling of Sm-Nd geochemical and isotopic compositions suggests that these very depleted rocks may be recording the effect of subduction zone processes not yet fully understood.

An Sm-Nd isotopic dating study of the Archean Kongling Complex in the Huangling area of the Yangtze Craton

Sm-Nd isotopic compositions of associated amphibolites, TTG gneisses and granitic gneisses from the Archean Kongling Complex of the Yangtze Craton, South China are systematically analyzed. The results show that the amphibolites and TTG gneisses have a similar age of ca. 2 700 Ma, which well agrees with zircon U-Pb dating of previous studies. However, their eND(t) are different, suggestive of possible dissimilarity in source composition.

Crust-mantle interaction in the evolution of the Ilímaussaq Complex, South Greenland: Nd isotopic studies

Sm-Nd isotopic compositions were determined for the peralkaline Ilímaussaq Complex of the Gardar Province of southern Greenland. The majority of the samples in the agpaitic and augite syenitic units have near chondritic initial ɛ Nd(≈ 0), whereas a few samples trend towards ɛ Nd values as low as − 6 at the time of intrusion (1143 Ma). This latter value, from a sample taken from the margin of the complex, lying on the evolutionary trend for Ketilidian country-rock granitoids, suggests that large-scale contamination took place only at the margins of the complex. The similarity of the Nd isotopic compositions of the augite syenite and agpaitic units suggests that their parental magmas were derived from the same reservoir. A comparison of the Nd with existing Sr and Hf isotopic data for the complex suggests an origin by combined assimilation fractionation processes. Assimilation-fractional crystallization modeling of the isotopic compositions indicates that the Ilímaussaq magmas could have formed through fractional crystallization of a basaltic melt while assimilating granitic crust. The model requires initially higher assimilation rates from basalt to augite syenite composition with subsequent decreasing assimilation rates from augite syenite to agpaitic compositions. Alkali granites, which formed after the intrusion of the augite syenites, have isotopic compositions intermediate between those of the augite syenites and the surrounding Ketilidian basement. This implies even greater amounts of assimilation and is interpreted as evidence for an origin through fractionation of a basaltic or augite syenite magma with concurrent assimilation of Ketilidian crust.

The geochemical evolution of the proterozoic continental crust in northwestern Jiangxi and western Zhejiang, China

The petrochemistry and Sm-Nd isotopic compositions and- the geochemical characteristics of REE, U,Th, etc. in the Late Proterozoic and Early Palaeozoic strata in northwestern Jiangxi and western Zhejiang provinces are described in this paper. It seems to be sure that the middle Proterozoic strata of southeastern China are low in the degree of maturation. The strata contain much mantle-derived material. At the end of Late Proterozoic there was an abrupt turn with respect to the crustal geochemical evolution of the eastern part of South China. Since then, the geochemical environment has undergone a change from a simple reducing environment to a complex oxidizing-reducing environment, which would be geochemically beneficial to the formation of Late Sinian to Early Cambrian U-bearing formations.

Isotope systematics of the Kiruna magnetite ores, Sweden; Part 1, Age of the ore

U-Pb isotope dating of zircons from a granophyric dike complex which crosscuts the Kiirunavaara magnetite-apatite body demonstrates that the ore was emplaced before 1.880 + or - 0.003 Ga. Whole-rock Sm-Nd isotope compositions for five out of six samples from the footwall and hanging-wall complexes lie on a 1.89 + or - 0.09-Ga isochron. The initial ratio of 0.50991 + or - 0.00007 corresponds to epsilon Nd (T) = -5.6 and suggests a substantial late Archean component in the igneous rocks.Since the ore cannot have been emplaced after the granophyre nor before the host rocks, ore formation is limited to the period between 1.90 and 1.88 Ga. However, the granophyre remains at present in the form of a crosscutting dike, largely unaffected by the ductile and brittle deformation that has affected both the ore and host rocks. This implies that ore formation, igneous activity, and deformation were all confined to a short time interval during this period and suggests that the Kiruna magnetite-apatite ores were generated during orogenesis.This association of orogenesis, emplacement temperatures up to 600 degrees C, abundant evidence of hydraulic processes during ore formation, and extensive porphyries is reminiscent of other types of intrusion-related, high-temperature, ore-forming processes.

飛騨山地久婦須川上流地域の灰色花こう岩の同位体年代

Rb-Sr and Sm-Nd isotopic compositions and REE abundances were measured on the Gray granite in the upper Kubusu River area, Hida mountains. The Gray granite is grouped into two based on the Rb-Sr age. The majority are plotted along an isochron giving an age of about 500 Ma, which is assumed to be the age of emplacement for the Gray granite. The remaining samples of the Gray granite give a Rb-Sr age of about 180 Ma, and they may be correlated to the Funatsu granitic rocks. The Nd model ages for the Gray granite are also divided into two; about 180 Ma and 1300-1500 Ma in harmony with Rb-Sr data. The older Gray granite is probably originated in the late Precambrian. REE patterns of the Gray granite are characterized by a marked enrichment in light REE, although there is a difference in abundance between the older and younger samples.

Neodymium isotope stratigraphy of gullfaks oilfield

Samples collected from over 500 m of stratigraphy from 4 wells, which represent the complete reservoir interval of Gullfaks Oilfield, have been analysed for Sm-Nd isotopic composition. The data are used to examine variations in the provenance of the various reservoir units, to assess the utility of the Sm-Nd method as a correlation tool and to illuminate palaeogeography.