U-Th-Pb and Rb-Sr systematics of Allende and U-Th-Pb systematics of Orgueil

U-Th-Pb systematics study of Allende inclusions showed that U, Th and Sr concentrations in Ca, Al (pyroxene)-rich chondrules and white and pinkish-white aggregate separates of Allende are five to ten times higher than those of the matrix, whereas Mg (olivine)-rich chondrules have U and Th concentrations about twice as high as the matrix. Th concentrations are extremely high in white aggregates and in pinkish-white (spinel-rich) aggregates while U and Sr concentrations in white aggregates are more than twice as high as those in pinkish-white aggregates. Large enrichment of these refractory elements in the white aggregates indicates that they contain high-temperature condensates from the solar nebula. The Pb concentrations in the inclusions are less than half of those in the whole rock and matrix, indicating that the matrix is a lower-temperature condensate. The isotopic composition of lead in the matrix is less radiogenic than that of the whole meteorite, whereas lead in Ca- and Al-rich chondrules and aggregates is extremely radiogenic. The ²⁰⁶Pb/²⁰⁴Pb ratio reaches as high as 55.9 in a white aggregate separate. The lead of Mg-rich chondrules is moderately radiogenic and the ²⁰⁶Pb/²⁰⁴Pb ratio ranges from 18 to 26. A striking linear relationship exists among leads in the chondrules, aggregates and matrix on the ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁴Pb/²⁰⁴Pb plot. The slope of the best fit line is 0.6188 ± 0.0016, yielding an isochron age of 4553 ± 4 m.y. The regression line passes through primordial lead values obtained from Canyon Diablo troilite. The data, when corrected for Canyon Diablo troilite Pb and plotted on a U-Pb concordia diagram, show that the pink and white aggregates and the Ca-Al-rich and Mg-rich inclusions have excess Pb and define a chord which intersects the concordia curve at 4548 ± 25 m.y. and 107 ± 70 m.y. The intercepts might correspond to the agglomeration age of the meteorite and a time of probably later disturbance, respectively. The matrix and some chondrules which contain less radiogenic lead did, however, not fit on the chord. The Rb-Sr data of Allende did not define an isochron suggesting that the Rb-Sr system was also disturbed by a later event, as suggested by the U-Pb concordia data. The lowest observed ⁸⁷Sr/⁸⁶Sr ratio in Allende inclusions is similar to the initial ratio of the Angra dos Reis achondrite (Papanastassiou, Thesis, 1970).The initial Pb isotopic composition of Orgueil calculated by a single-stage evolution model is more radiogenic than that of Canyon Diablo troilite. To reconcile the U-Pb data of Orgueil and Allende, we propose that the initial lead isotopic composition of the carbonaceous chondrites was slightly different from that of Canyon Diablo troilite Pb.     

U-Th-Pb systematics of precambrian carbonate rocks: Dating of the formation and transformation of carbonate sediments

The current state in the Pb-Pb dating of the formation and epigenesis of Precambrian carbonate sediments has been reviewed. The geochemistry of admixture elements (U, Th, Pb, Rb, Sr, Mn, and Fe) constituting carbonate minerals and the redistribution of the elements in the course of early diagenesis and epigenesis have been considered. The advantages of choosing samples for Pb-Pb dating on the basis of geochemical criteria similar to those applied for Sr-isotope stratigraphy and the potentialities of applying different methods of decontamination of specimens, analyzed from surface contamination and possible epigenetic carbonate phases, were illustrated. By the example of a series of carbonate formations, distinctions in susceptibility of U-Pb and Rb-Sr systems of carbonate minerals to secondary transformations have been demonstrated. The prospects for using the method of step solution allowing noncogenetic carbonate generations to be separated and thus the accuracy and reliability of Pb-Pb dates to be upgraded were considered.     

Rb-Sr isotope systematics in metamorphic rocks,Kongsberg sector,south Norway

Rb-Sr isotope data are presented for gneisses, migmatite neosome material and granitic and gabbroic intrusive rocks from the southern part of the Kongsberg sector, south Norway. The maximum age of the crust in this area appears to be ～1.6 AE. Two metamorphic episodes at ～1.5–1.6 AE and at ～1.1–1.2 AE are recognized. Initial ⁸⁷Sr/⁸⁶Sr ratios for the granitic rocks give evidence for reworking of sialic crust and indicate that approximately 1.6 AE old crust repeatedly acted as a source for granitic magmas for a timespan of ～0.5 AE.     

U-Th-Pb isotope systematics related to igneous rocks and ore Pb,Mount Isa,Queensland

This study is a search for a genetic relationship between Pb sulphide ore and igneous rocks in the region of Mount Isa, Queensland. The approach involves derivation of Pb isotope initial ratios by the whole-rock isochron method, and comparison of the initial ratios (Pb²⁰⁶/Pb²⁰⁴, Pb²⁰⁷/Pb²⁰⁴ and Pb²⁰⁸/Pb²⁰⁴) with the isotopic composition of the ore Pb. Data are reported for four igneous units; Kalkadoon granodiorite, Kalkadoon adamellite, Sybella granite and Eastern Creek volcanics. The results display considerable scatter for each of the units, and reveal the effects of recent surficial loss of U. The positioning of isochrons is aided by previous Rb-Sr geochronological data wherever possible. Comparison of initial ratios and ore Pb suggests that none of the igneous rock units is co-genetic with the ore deposit. Both phases of the Sybella Granite are more radiogenic and are apparently younger than the ore Pb. The Kalkadoon Granite is possibly related to the ore through some post-emplacement process of extraction and transport of Pb (e.g. by erosion or by anatectic magma generation) to the present site of the orebodies.     

U-Th-Pb systematics in hydrothermally altered granites from the Granite Mountains,Wyoming

U-Th-Pb systematics were investigated in 15 samples representing two types of deuterically altered Archean granite (albitized and silicified-epidotized granite) from the Granite Mountains, Wyoming. The loss of K-feldspar during both types of deuteric alteration was accompanied by an extreme reduction of Pb content from roughly 40 ppm to less than 12 ppm in the most altered samples. Nine of the 15 samples yield anomalously young whole-rock Pb-Pb and Th-Pb ages compared to concordia ages for zircons and to whole-rock Pb-Pb and Th-Pb ages for samples of unaltered granite. The young ages are interpreted to be the result of radiogenic Pb loss during a middle Proterozoic metamorphism that disturbed several isotopic systems in the unaltered granite. The loss of radiogenic Pb from the whole-rock systems of the deuterically altered granites is most likely due to the absence of microcline. In many granitic rocks, potassium feldspar tends to act as a receptor for Pb that has been mobilized, and this effect may account for the closed-system behavior of Pb in whole-rock samples of the unaltered granite. It may also account for the apparent gain of radiogenic Pb during the Proterozoic by one sample which was collected at the edge of an alteration zone. The deuterically altered granites may have also lost U during the Proterozoic, but, except for two samples, the dominant U loss occurred relatively recently, probably during Tertiary uplift and erosion.The low common-Pb contents and rather high U and Th contents of the hydrothermally altered granites might seem to indicate that these rocks are well suited for geochronologic studies in the U-Th-Pb system because such rocks should have a high percentage of radiogenic Pb. Unfortunately, open-system behavior in response to post-crystallization metamorphism shows that apparent ages for these types of rocks must be interpreted with caution.     

Rb-Sr systematics on paragneiss series from the Bavarian Moldanubicum,Germany

The Nicolaysen-diagram is a means to present mineral or whole rock isochrons. The age values derived from mineral isochrons are normally interpreted as cooling ages, those given by whole rock isochrons as the age of an intrusion or a metamorphic event. Mineral isochrons reflect samples in the order of few mm to cm, whole rock isochrons those of a few decimeter to meter.The investigation presented here deals with samples representing regions of about 100 m to some kilometers in diameter. These regions we are going to name areals.Five areals of the Moldanubicum of Eastern Bavaria (West Germany) consisting of similar paragneisses are yielding mineral ages of about 315 Ma and whole rock ages of about 450 Ma. We calculated mean ⁸⁷Sr/ ⁸⁶Sr- and ⁸⁷Rb/⁸⁶Sr-values of these areals and displayed them in a Nicolaysen-diagram. The areal values define a straight line, yielding an age of 544±29 Ma with a ⁸⁷Sr/⁸⁶Sr-intercept at 0.7048±0.0014.Discussion arises whether or not this line can be interpreted as an isochron. We favour the interpretation of it as an isochron reflecting the possible age of sedimentation or of a metamorphic event which the paragneiss series has undergone. At present it can not be unequivocally decided which of the two possibilities will prove right. The areal isochron, however, appears to indicate a petrogenetic event which is older than the last Sr-isotope equilibration in the whole rocks within an individual areal of this Moldanuvian polymetamorphic region.     

Rb-Sr and U-Pb systematics of metasedimentary carbonate rocks: The Paleoproterozoic Kuetsjarvi Formation of the Pechenga Greenstone Belt, Kola Peninsula

The Rb-Sr and U-Pb systematics have been studied in the metasedimentary carbonate rocks from the Paleoproterozoic Kuetsjarvi Formation. Samples were taken from the borehole drilled in the northern zone of the Pechenga Greenstone Belt in the northwestern Kola Peninsula. The carbonate section of the formation is made up of three units (from the bottom to top): (I) dolomite (68 m), (II) calcareous-dolomite (9 m), and (III) clayey calcareous (1 m) ones. Dolomites (Mg/Ca = 0.55–0.61) from the lowermost unit I contain 70.3–111 ppm Sr. Initial ⁸⁷Sr/⁸⁶Sr ratio in them varies within 0.70560–0.70623 and characterizes the primary continental-lacustrine carbonate sediments. Calcareous dolomites (Mg/Ca = 0.39–0.59) and dolomitic limestones of units II and III (Mg/Ca = 0.02–0.36) are enriched in Sr (285–745 and 550–1750 ppm, respectively). Initial ⁸⁷Sr/⁸⁶Sr ratios in these rocks (0.70406–0.70486 and 0.70407–0.70431, respectively) fall within the range typical of the Jatulian seawater, which indicates that the carbonate sediments of two upper units were formed in an open marine basin. Study of dolomites from unit I showed that the Svecofennian metamorphism more significantly affected the U-Pb systems of carbonate rocks as compared to their Rb-Sr systems. In the ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb diagram, most data points corresponding to the carbonate constituent of dolomites define isochron with an age of 1900 ± 25 Ma (MSWD = 0.5). The same samples define a positive correlation in the ²⁰⁸Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb plot. Since sedimentary carbonates usually do not contain Th, this correlation points to secondary enrichment of the studied dolomites in Th or thorogenic ²⁰⁸Pb. Hence, the obtained Pb-Pb dating can be regarded as the age of the Svecofennian metamorphic event. Three samples from dolomites of unit I lack any disturbance of the initial U-Th-Pb systematics, but their trend in the ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb diagram deviates from the 1900 Ma isochron. Based on these samples, the model U-Pb premetamorphic age of the Kuetsjarvi carbonate sediments is 2075–2100 Ma. This interval is consistent with the age range of the Lomagundi-Jatulian event, which was responsible for the formation of carbonate sediments with high positive δ¹³C values.     

Isotope-geochemical systematics of kimberlites and related rocks from the Siberian Platform

Using the ICP-MS method we have studied the isotope systematics of Sr and Nd as well as trace element composition of a representative collection of kimberlites and related rocks from the Siberian Platform. The summarized literature and our own data suggest that the kimberlites developed within the platform can be divided into several petrochemical and geochemical types, whose origin is related to different mantle sources. The petrochemical classification of kimberlites is based on persistent differences of their composition in mg# and in contents of indicator oxides such as FeO_tot, TiO₂, and K₂O. The recognized geochemical types of kimberlites differ from one another in the level of concentration of incompatible elements as well as in their ratios.Most of isotope characteristics of kimberlites and related rocks of the Siberian Platform correspond to the earlier studied Type 1 basaltoid kimberlites from different provinces of the world: Points of isotopic compositions are in the field of primitive and weakly depleted mantle. An exception is one sample of the rocks from veins of the Ingashi field (Sayan area), which is characterized by the Sr and Nd isotopic composition corresponding to Type 2 micaceous kimberlites (orangeites).The most important feature of distribution of isotopic and trace-element compositions (incompatible elements) is their independence of the chemical rock composition. It is shown that the kimberlite formation is connected with, at least, two independent sources, fluid and melt, responsible for the trace-element and chemical compositions of the rock. It is supposed that, when rising through the heterogeneous lithosphere of the mantle, a powerful flow of an asthenosphere-derived fluid provoked the formation of local kimberlite chambers there. Thus, the partial melting of the lithosphere mantle led to the formation of contrasting petrochemical types of kimberlites, while the geochemical specialization of kimberlites is due to the mantle fluid of asthenosphere origin, which drastically dominated in the rare-metal balance of a hybrid magma of the chamber.     

Sm-Nd and Rb-Sr isotopic systematics of the East Antarctic Manning Massif alkaline trachybasalts and the development of the mantle beneath the Lambert-Amery rift

Summary Rb-Sr and Sm-Nd isotopic investigations have been carried out on samples forming a sequence from the bottom to the top of a suite of K-rich olivine trachybasalts from the Manning Massif in the northern Prince Charles Mountains, East Antarctica. Several separate flows, each 3.5-6.5 m thick, occur as individual small outcrops and are related to the Lambert-Amery rift. Nd-Sr isotopic features of the volcanics are similar to those for lherzolite xenoliths from the Mesozoic alkaline ultramafic rocks of the adjacent Jetty Peninsula area, but the trace element abundances and patterns are consistent with the occurrence of minor phases such as apatite, amphibole, ilmenite and perovskite in the source region. According to Nd-Sr isotopic characteristics (¹⁴³Nd/¹⁴⁴Nd = 0.512026-0.512269;⁸⁷Sr/⁸⁶Sr = 0.707765-0.708046), the trachybasalts were generated from an enriched mantle source. These features are suggested here to result from isotopic mixing between peridotite wall-rocks similar to the Jetty Peninsula xenolith samples and melts derived from ultramafic veins in the lithosphere. A synthesis from isochron and model ages on volcanics and xenoliths indicates formation of the lithosphere at about 1.2 Ga, followed by two episodes of subsequent enrichment at 0.91.0 Ga and ca. 0.6 Ga. The¹⁴³Nd/¹⁴⁴Nd values for the volcanics show a wide range, but are more enriched for the bottom part of the suite, becoming more depleted towards the top. This may be explained by a steadily decreasing vein/wall-rock ratio from 35% to 10% due to progressive heating of the source region. This variation in ENd is not accompanied by a corresponding increase in⁸⁷Sr/⁸⁶Sr, which constrains both vein and wall-rock characteristics quite closely: the wall-rock was an enriched peridotite, whereas the vein assemblage comprised dominantly clinopyroxene and amphibole with a lesser, but essential, amount of phlogopite. REE-rich accessory phases such as apatite and perovskite were present in the vein, but were rare, so that the vein as a whole had high Sr/Nd. Rb-Sr age determination on Manning Massif bulk rocks gives an apparent isochron age of 40±1.2 Ma (I_Sr = 0.70762), although the age is probably falsified by the mixing processes during melting.

---

Sm-Nd und Rb-Sr Isotopenuntersuchungen an alkalischen Trachybasalten des ostantarktischen Manning Massivs: Entwicklung des Mantels unter dem Lambert-Amery Rift

Zusammenfassung Sm-Nd- und Rb-Sr-Isotope wurden in einer Abfolge von K-reichen OlivinTrachybasalten im Manning Massiv in den nördlichen Prince Charles Mountains, Ost-Antarktis, untersucht. Proben wurden systematisch von mehreren sich überlagernden Lavaströmen genommen, die Mächtigkeiten von 3,5 bis 6,5 m erreichen und dem Lambert-Amery Rift zugeordnet werden. Nd-Sr Isotopenzusammensetzung der Vulkanite sind ähnlich wie in Iherzolithischen Mantelxenolithen aus alkalischen Ultramafiten der Jetty Halbinsel. Allerdings deuten die Spurenelementgehalte sowie die Spurenelementverteilungen auf die Anwesenheit von Apatit, Amphibol, Ilmenit und Perowskit in der Quellenregionhin. Nd-Sr Isotope der Trachybasalte (¹⁴³Nd/¹⁴⁴Nd= 0.512026–0.512269;⁸⁷Sr/⁸⁶Sr = 0.707765–0.708046) weisen auf eine angereicherte Mantelzusammensetzung hin. Für die hier gefundenen Werte wird ein Mischungsmodell zwischen einem Peridotit-Nebengestein ähnlich wie in den Jetty Halbinsel Xenolithen und einer Schmelze vorgeschlagen, die aus ultramafischen Gängen innerhalb der Lithosphäre stammte. Die Kombination aus Isochronenalter und Modellalter der Vulkanite und der Xenolithe zeigt, daß nach der Bildung vor 1.2 Ga die Lithosphäre zwei Anreicherungsereignisse vor 0.9-1.0 Ga und 0.6 Ga erfahren hatte. Trotz der hohen Variation der¹⁴³Nd/¹⁴⁴Nd Verhältnisse ergeben sich für die unteren Lavaströme höhere Werte, während die oberen Bereiche an¹⁴³Nd verarmt sind. Dies kann auf eine stetige Abnahme des Gang/Peridotit Verhältnisses von 35% auf 10% zurückgeführt werden, das auf eine fortschreitende Aufheizung der Quellenregion hinweist. Die Variation im¹⁴³Nd/¹⁴⁴Nd System wird aber nicht von einem Anstieg des⁸⁷Sr/⁸⁶Sr Verhältnisses begleitet, was mit ähnlichen Werten sowohl für die Gänge als auch für den angrenzenden Peridotit erklärt werden kann: das Nebengestein bestand aus einem angereicherten Peridotit, während die Gänge hauptsächlich aus Klinopyroxen und Amphibol zusammengesetzt waren. SEE-reiche Phasen wie Apatit und Perowskit kamen in so geringen Mengen vor, daß die Gesamtzusammensetzung der Gänge hohe Sr/Nd-Verhältnisse hatte. Rb-Sr Altersbestimmungen an Gesamtgesteinen der Manning Massiv-Vulkanite ergeben eine scheinbare Isochrone von 40±1.2 Ma (I_Sr = 0.70762), obwohl diese Altersinformation vermutlich durch die Mischungsprozesse während des partiellen Schmelzens verfälscht ist.

     

Rare earth abundances and Rb-Sr systematics of basalts,gabbro, anorthosite and minor granitic rocks from the Indian Ocean Ridge System,Western Indian Ocean

Basalts dredged from the Mid-Indian Ocean Ridge System have rare earth, Rb, and Sr concentrations like those from other mid-ocean ridges, but have slightly higher Sr⁸⁷/Sr⁸⁶ ratios. Underlying gabbroic complexes are similar to the basalts in Sr⁸⁷/Sr⁸⁶, but are poorer K, Rb, and in rare earths. The chemical and isotopic data, as well as the geologic relations suggest a cumulate origin for the bulk of the gabbroic complexes.     

Isotopic evidence for genetic relations between acid and alkaline rocks in Italy

Pb isotopic composition and concentrations were determined for whole-rock samples from the Pliocene-Pleistocene, acid, Tuscan Igneous Province, Italy, in order to clarify their relation to the Cenozoic alkaline volcanism of Central Italy. The Pb of the Tuscan rocks is fairly uniform in its isotopic composition, despite variable and high ₈₇Sr/₈₆Sr ratios (0.711–0.725; Barberi et al, 1971).A comparison of the Pb isotopic composition in the Tuscan and the potassic alkaline rocks strongly suggests a continuous mixing relation between both rock series, with partial melts of mantle material and argillaceous sedimentary rocks as likely end-members. Such an interrelation could also provide a simple explanation for the range in the Sr isotopic ratio within the Tuscan rocks. The favoured model envisages a similar history for both rock-series; it is suggested that their main petrographic differences result from a lower portion of mantle-derived material in the parental material for the Tuscan rocks, and from the mineral phases present in the residue during partial melting of the source region.     

Rb-Sr isotope systematics of muscovite from Pan-African granitic pegmatites of Western and Northeastern Africa

Summary Rb-Sr investigations have been carried out on early-formed muscovite from three pegmatite fields of the late Proterozoic to early Phanerozoic Pan-African Belt. The individual mineral ages obtained are highly discordant for each pegmatite field. Using Best Isochron Diagrams, isochron construction of selected muscovite samples yielded geologically realistic ages of pegmatite formation: around 670 Ma for the Bayuda Desert pegmatites of northern Sudan, around 550 Ma for the Wamba pegmatites of central Nigeria, and around 465 Ma for the Majayahan pegmatites of northeastern Somalia. Initial Sr ratios obtained from isochron calculations have unrealistic values and cannot be used for petrogenetic interpretations.The geologically unrealistic young model ages of some of the muscovite samples are most probably attributed to open-system behaviour and post-crystallization loss of⁸⁷Sr^* from the respective minerals. The amounts of⁸⁷Sr^* losses have been approximated from the discrepancies between isotopically measured and theoretically calculated (from decay of Rb)⁸⁷Sr^* concentrations. The loss of⁸⁷Sr^* from the micas is variable in each pegmatite field. In none of the three cases can this unsystematic, post-emplacement, open-system behaviour be directly related to a particular, temporally confined, geologic event.

---

Rb-Sr Isotopen-Systematik von Muskovit aus panafrikanischen Granit-Pegmatiten West-und Nordost-Afrikas

Zusammenfassung Rb-Sr Isotopen-Untersuchungen wurden an frühgebildetem Muskovit dreier Pegmatitfelder der spätproterozoischen bis frühphanerozoischen, panafrikanischen Mobilzone durchgeführt. Die jeweilig erhaltenen Muskovit-Modellalter sind für jedes Pegmatitfeld stark diskordant. Unter Benutzung von Best Isochron Diagrams ergaben Isochronen ausgesuchter Muskovit-Proben jedoch geologisch relevante Pegmatit-Bildungsalter: um 670 Ma für die Pegmatite der Bayuda Wüste im Nord-Sudan, um 550 Ma für die Pegmatite von Wamba in Zentral-Nigeria, und um 465 Ma für die Pegmatite von Majayahan in Nordost-Somalia. Die aufgrund der Isochronen-Berechnungen erhaltenen Sr-Initialverhältnisse haben allerdings unrealistische Werte und können nicht für petrogenetische Interpretationen herangezogen werden.Die geologisch ebenfalls unrealistischen, jungen Alter einiger der Muskovit-Proben sind sehr wahrscheinlich in offenen System-Verhältnissen und Verlust von⁸⁷Sr^* nach der Kristallisation der jeweiligen Minerale bedingt. Die Menge des Verlustes an⁸⁷Sr^* wurde annäherungsweise aufgrund der Unterschiede zwischen isotopisch gemessenen und anhand des Rb-Zerfalls theoretisch errechneten Gehalten an⁸⁷Sr^* bestimmt. Der Verlust der Glimmer an⁸⁷Sr^* ist innerhalb eines jeden Pegmatitfeldes variabel. Dieses unsystematische Vorkommen von post-kristallinen, offenen System-Verhältnissen kann in keinem der drei Fälle in direkte Beziehung gesetzt werden zu einem bestimmten, zeitlich begrenzten, geologischen Ereignis.

---

---

With 6 Figures     

Rb-Sr ages of the Archean rocks from the Vermilion district,northeastern Minnesota

The Vermilion district of northerneastern Minnesota is a classic example of a lower Precambrian greenstone-granite terrane. It is a complex volcanic-sedimentary pile, characterized by repeated periods of volcanism and the presence of intercalated pyroclastic, volcanoclastic and epiclastic rocks. The volcanic-sedimentary pile is surrounded and intruded by contemporaneous granitic batholiths. Several rock units from the district have been dated by the whole-rock Rb-Sr method. The isochron ages and the corresponding initial Sr⁸⁷/Sr⁸⁶ ratios (= I) are:

     

15.

New data on the age of the Guli intrusion and implications for the relationships between alkaline magmatism in the Maymecha-Kotuy province and the Siberian superplume: U-Th-Pb isotopic systematics     

L. N. Kogarko  R. E. Zartman 《Geochemistry International》2011,49(5):439-448

The Siberian flood basalts (SFB) make up one of the world’s largest known magmatic province and cover a vast area of the northern Siberian platform. The northeastern flank of the SFB province is occupied by the Maymecha-Kotuy ultramafic-alkaline complex consisting of enormous volumes of alkaline lavas, numerous dikes, and the Guli massif comprising many smaller alkaline plutons. The genetic relationship between SFB and the Maymecha-Kotuy complex continues to be a matter of considerable debate. Despite the fact that the rocks of both blocks have almost the same age, close to the Permian-Triassic boundary, there are still major uncertainties in the relative succession of emplacement and the source material for each lithology present. On the basis of previous petrological, geochemical, and isotopic results, this study attempted to put tighter limits on possible relationships between SFB and alkaline rocks.     

16.

Rb-Sr systematics of Permian volcanites in the Schwarzwald (SW-Germany)     

Helmut Schleicher  Hans Joachim Lippolt  Ingrid Raczek 《Contributions to Mineralogy and Petrology》1983,84(2-3):272-280

Rb-Sr mineral ages and Sr isotopic ratios were determined for several Permian rhyolites of the Schwarzwald, mainly by using mica-apatite pairs. Extrusion ages between_307+5 and 286+7 Ma were found ( _Rb=1.42· 10^–11 a^–1, 2). The initial Sr isotopic ratios range from 0,710 to 0.7125, with one exception (St. Märgen: 0.706). The Rb-Sr age of the granite porphyry Schweighof in the Southern Schwarzwald is 323+4 Ma.Thus the subsequent volcanic activity of the Hercynian orogeny lasted nearly 20–30 Ma, starting in the Late Carboniferous and culminating in the Early Permian. The volcanic activity moved in time from South to North. In the Southern as well as the Northern Schwarzwald a hiatus of about 20 Ma exists between late orogenic plutonism and subsequent volcanism. The initial ratios lie on the crustal evolution line of Schwarzwald basement and favour, with the St. Märgen porphyry as exception, a crustal provenance of the volcanic magmas.     

17.

Lead isotope systematics of some igneous rocks from the Egyptian shield     

J.G. Gillespie  T.H. Dixon 《Precambrian Research》1983,20(1):63-77

Lead isotope data on late Precambrian igneous rocks from the eastern desert of Egypt are presented. Previous work has indicated that this igneous suite is characterized by uniformly low initial ratios of ${}^{37}\text{Sr}{}^{86}\text{Sr}$ (< 0.7035). The Pb data define three groups, loosely corresponding to age. An older tonalite to granodiorite (OTG) suite, with ages in the range 610–710 Ma, has Pb isotope characteristics similar to modern, mantle-derived oceanic mafic rocks. The age-corrected initial Pb isotope ratios of the OTG group lie near the “ocean regression line” in correlation diagrams of ${}^{206}\text{Pb}{}^{204}\text{Pb}$ vs. ${}^{207}\text{Pb}{}^{204}\text{Pb}$ and ${}^{203}\text{Pb}{}^{204}\text{Pb}$. The isotope data imply an origin for the OTG group analogous to calc-alkalic igneous rocks in modern intra-oceanic island arcs.A younger suite of K-rich plutonic rocks (570–595 Ma) has similar ${}^{206}\text{Pb}{}^{204}\text{Pb}$ and ${}^{208}\text{Pb}{}^{204}\text{Pb}$ initial ratios relative to the OTG suite, but have higher ${}^{207}\text{Pb}{}^{204}\text{Pb}$ initial ratios when calculated from measured whole-rock U and Pb contents. However, K-feldspar and galena, associated with this suite, indicate initial Pb isotope ratios close to the OTG suite. We interpret this to indicate that the younger granites could have originated by anatexis of OTG-type material, but they probably experienced an episode of metasomatism after emplacement.A suite of volcanic rocks (the Dokhan Volcanics) from the central eastern desert are intermediate between the younger granites and OTG in both age (～ 610 Ma) and Pb isotope characteristics.Limited data on initial ${}^{208}\text{Pb}{}^{204}\text{Pb}$ ratios suggest that all the Egyptian samples originated from a source with anomalously low $\text{Th}\text{Pb}$ ratios.Although the relative abundance of granitic rocks in the Egyptian Shield decreases to the south, no evidence of north-south heterogeneity in the Pb isotope composition of the respective source regions is apparent in the data. However, the westernmost sample studied, from the Aswan area, contains distinctly more radiogenic Pb relative to the mean. This location may mark the boundary between the Late Precambrian ensimatic orogen now outcropping in the Egyptian and Saudi Arabian Shields, and an older sialic craton to the west.     

18.

Age of the Moncha Tundra fault, Kola Peninsula: Evidence from the Sm-Nd and Rb-Sr isotopic systematics of metamorphic assemblages     

E. V. Sharkov  V. F. Smol’kin  V. B. Belyatskii  A. V. Chistyakov  Zh. A. Fedotov 《Geochemistry International》2006,44(4):317-326

The first Sm-Nd and Rb-Sr dates were obtained for the dynamometamorphic processes associated with the origin and evolution of the Moncha Tundra fault, Kola Peninsula, which separates two large Early Paleoproterozoic layered intrusions: the Monchegorsk Ni-bearing mafic-ultramafic intrusion and the Main Range massif of predominantly mafic composition. The fault belongs to the regional Central Kola fault system, whose age was unknown. The material for the dating included metamorphic minerals from blastomylonitic rocks recovered by structural borehole M-1. Mineralogical thermobarometry suggests that the metamorphism occurred at 6.9–7.6 kbar and 620–640°C, which correspond to the amphibolite facies. The Sr and Nd isotopic systems were re-equilibrated, and their study allowed us to date the dynamometamorphic processes using mineral isochrons. It was established that the Moncha Tundra fault, and, respectively, the whole Central Kola fault system appeared in the middle of Paleoproterozoic ～2.0–1.9 Ga, simultaneously with the Svecofennian orogen in the central part of the region and the Lapland-Kola orogen in its northeastern part. Another episode of dynamometamorphism that occurred at 1.60–1.65 Ga is envisaged.     

19.

Sm-Nd and Rb-Sr isotopic and geochemical systematics in Phanerozoic granulites from Fiordland,southwest New Zealand   总被引：2，自引：0，他引：2  

M. T. McCulloch  J. Y. Bradshaw  S. R. Taylor 《Contributions to Mineralogy and Petrology》1987,97(2):183-195

Sm-Nd and Rb-Sr isotopic analyses are reported for granulite facies orthogneisses from Fiordland southwest New Zealand. Whole-rock samples define a Rb-Sr isochron age of 120±15 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70391±4. _Nd values (at 120 Ma) show a relatively wide range of from –0.4 to 2.7 indicating decoupling of Sr-Nd isotope systems. Associated ultramafic rocks have initial ⁸⁷Sr/⁸⁶Sr ratios of from 0.70380 to 0.70430 and _Nd values of from 0.1 to 3.0. The different initial ratios suggest that the various intrusions, although contemporaneous, were not derived through fractionation of a single parent magma. A metasedimentary enclave incorporated during emplacement of the granulitic rocks preserves a Proterozoic isotopic signature with a measured _Nd(0) value of –10.2, ⁸⁷Sr/⁸⁶Sr ratio of 0.73679 and a T ^Nd provenance age of 1490 Ma. The Rb-Sr whole rock age of the granulites is the same as obtained from recent U-Pb zircon dating (Mattinson et al. 1986) and is interpreted as the time of magmatic emplacement and essentially contemporaneous granulite facies metamorphism. Rb-Sr and Sm-Nd analyses of mineral systems indicate that the terrain had cooled below 300° C by 100 Ma providing further evidence that high grade metamorphism was of exceptionally short duration.Unmetamorphosed leucogabbros from the Early Cretaceous Darran Complex of eastern Fiordland have significantly higher _Nd values (3.9 to 4.6) and slightly lower ⁸⁷Sr/ ⁸⁶Sr (0.70373 to 0.70386) than the western Fiordland granulites. This indicates that the western and eastern Fiordland complexes are not correlative although both have geochemical similarities to Phanerozoic calc-alkaline island-arc suites. The Fiordland granulites are LREE enriched (La_N/ Yb_N=12 to 40) and have trace element characteristics (e.g. high K/Rb and low Rb/Sr ratios) typical of many Rb-depleted Precambrian granulite terrains. The Fiordland trace element trends, however are attributed to magmatic, not metamorphic processes, reflecting the character of the Early Cretaceous magma sources. The range of _Nd values, but uniform initial ⁸⁷Sr/⁸⁶Sr of the western Fiordland granulites is consistent with derivation of the parent Early Cretaceous magmas at least in part from a LREE enriched, low Rb/Sr protoliths of mid-to late-Paleozoic age. Partial melting of this protolith occurred during or immediately preceding a period of great crustal thickening culminating in rapid thickening of existing crust by 20 km following emplacement of the granulitic rocks. The rapid crustal thickening was probably a consequence of a collisional event in which an Early Cretaceous magmatic arc was over-ridden by one or more thrust sheets.     

20.

The U-Th-Pb systematics of zircons from the type Nûk gneisses,Godthåbsfjord,West Greenland     

H. Baadsgaard  V.R. McGregor 《Geochimica et cosmochimica acta》1981,45(7):1099-1109

U-Pb analyses were made on zircons separated from different lithologies within the Nûk gneisses taken from Godthåb (Nuuk) townsite and from the south-west shore of Bjørneøen. Size fractions of the zircons from each rock yield linear discordia which give apparent crystallisation ages for the parents of the Nûk gneisses of 2890–3065 Myr. This spread of ages is not a result of multiple intrusion, but more likely results from different degrees of ‘diffusion’ and metamorphic episodic lead loss by the zircons. Since U-Pb and Th-Pb discordance are similar, loss of radiogenic lead (and not U or Th-loss/gain) by radiation-damage diffusion and through metamorphism is the main factor causing discordance of the U-Pb dates. The results show that the discordia line given by the analyses of fractions of a single zircon population may not give a true crystallisation age when the rock has been strongly metamorphosed. This interpretation is supported by Rb-Sr whole rock analyses of the same rocks, in which the Nûk townsite rocks scatter about a reference isochron of 2840 Myr, while the Bjørneøen samples give an isochron (MSWD = 0.55) of 3076 ± 27 Myr and the initial ratio at 0.70205 ± 0.00009. The zircon results do not rule out the possibility that more than one metamorphic episode affected the original Nûk gneiss parents prior to 2520 Myr (Qôrqut granite intrusion). A date of 2660 Myr was obtained from an aplite intruded during a period of plutonic activity that included the formation of a linear belt of intense ductile deformation that extends through south-western Bjørneøen and Godthåb town.     

	$\text{t(}\text{in b.y.}\text{) ± 2σ}$	$\text{1 ± 2σ}$
Ely Greenstone	$\text{2.69 ± 0.08}$	$\text{0.70056 ± 0.00026}$
Newton Lake Formation	$\text{2.65 ±0.11}$	$\text{0.70086 ± 0.00024}$
Granitic pebbles	$\text{2.69 ± 0.28}$	$\text{0.70078 ± 0.00058}$

New data on the age of the Guli intrusion and implications for the relationships between alkaline magmatism in the Maymecha-Kotuy province and the Siberian superplume: U-Th-Pb isotopic systematics

The Siberian flood basalts (SFB) make up one of the world’s largest known magmatic province and cover a vast area of the northern Siberian platform. The northeastern flank of the SFB province is occupied by the Maymecha-Kotuy ultramafic-alkaline complex consisting of enormous volumes of alkaline lavas, numerous dikes, and the Guli massif comprising many smaller alkaline plutons. The genetic relationship between SFB and the Maymecha-Kotuy complex continues to be a matter of considerable debate. Despite the fact that the rocks of both blocks have almost the same age, close to the Permian-Triassic boundary, there are still major uncertainties in the relative succession of emplacement and the source material for each lithology present. On the basis of previous petrological, geochemical, and isotopic results, this study attempted to put tighter limits on possible relationships between SFB and alkaline rocks.     

Rb-Sr systematics of Permian volcanites in the Schwarzwald (SW-Germany)

Rb-Sr mineral ages and Sr isotopic ratios were determined for several Permian rhyolites of the Schwarzwald, mainly by using mica-apatite pairs. Extrusion ages between_307+5 and 286+7 Ma were found ( _Rb=1.42· 10^–11 a^–1, 2). The initial Sr isotopic ratios range from 0,710 to 0.7125, with one exception (St. Märgen: 0.706). The Rb-Sr age of the granite porphyry Schweighof in the Southern Schwarzwald is 323+4 Ma.Thus the subsequent volcanic activity of the Hercynian orogeny lasted nearly 20–30 Ma, starting in the Late Carboniferous and culminating in the Early Permian. The volcanic activity moved in time from South to North. In the Southern as well as the Northern Schwarzwald a hiatus of about 20 Ma exists between late orogenic plutonism and subsequent volcanism. The initial ratios lie on the crustal evolution line of Schwarzwald basement and favour, with the St. Märgen porphyry as exception, a crustal provenance of the volcanic magmas.     

Lead isotope systematics of some igneous rocks from the Egyptian shield

Lead isotope data on late Precambrian igneous rocks from the eastern desert of Egypt are presented. Previous work has indicated that this igneous suite is characterized by uniformly low initial ratios of ${}^{37}\text{Sr}{}^{86}\text{Sr}$ (< 0.7035). The Pb data define three groups, loosely corresponding to age. An older tonalite to granodiorite (OTG) suite, with ages in the range 610–710 Ma, has Pb isotope characteristics similar to modern, mantle-derived oceanic mafic rocks. The age-corrected initial Pb isotope ratios of the OTG group lie near the “ocean regression line” in correlation diagrams of ${}^{206}\text{Pb}{}^{204}\text{Pb}$ vs. ${}^{207}\text{Pb}{}^{204}\text{Pb}$ and ${}^{203}\text{Pb}{}^{204}\text{Pb}$. The isotope data imply an origin for the OTG group analogous to calc-alkalic igneous rocks in modern intra-oceanic island arcs.A younger suite of K-rich plutonic rocks (570–595 Ma) has similar ${}^{206}\text{Pb}{}^{204}\text{Pb}$ and ${}^{208}\text{Pb}{}^{204}\text{Pb}$ initial ratios relative to the OTG suite, but have higher ${}^{207}\text{Pb}{}^{204}\text{Pb}$ initial ratios when calculated from measured whole-rock U and Pb contents. However, K-feldspar and galena, associated with this suite, indicate initial Pb isotope ratios close to the OTG suite. We interpret this to indicate that the younger granites could have originated by anatexis of OTG-type material, but they probably experienced an episode of metasomatism after emplacement.A suite of volcanic rocks (the Dokhan Volcanics) from the central eastern desert are intermediate between the younger granites and OTG in both age (～ 610 Ma) and Pb isotope characteristics.Limited data on initial ${}^{208}\text{Pb}{}^{204}\text{Pb}$ ratios suggest that all the Egyptian samples originated from a source with anomalously low $\text{Th}\text{Pb}$ ratios.Although the relative abundance of granitic rocks in the Egyptian Shield decreases to the south, no evidence of north-south heterogeneity in the Pb isotope composition of the respective source regions is apparent in the data. However, the westernmost sample studied, from the Aswan area, contains distinctly more radiogenic Pb relative to the mean. This location may mark the boundary between the Late Precambrian ensimatic orogen now outcropping in the Egyptian and Saudi Arabian Shields, and an older sialic craton to the west.     

Age of the Moncha Tundra fault, Kola Peninsula: Evidence from the Sm-Nd and Rb-Sr isotopic systematics of metamorphic assemblages

The first Sm-Nd and Rb-Sr dates were obtained for the dynamometamorphic processes associated with the origin and evolution of the Moncha Tundra fault, Kola Peninsula, which separates two large Early Paleoproterozoic layered intrusions: the Monchegorsk Ni-bearing mafic-ultramafic intrusion and the Main Range massif of predominantly mafic composition. The fault belongs to the regional Central Kola fault system, whose age was unknown. The material for the dating included metamorphic minerals from blastomylonitic rocks recovered by structural borehole M-1. Mineralogical thermobarometry suggests that the metamorphism occurred at 6.9–7.6 kbar and 620–640°C, which correspond to the amphibolite facies. The Sr and Nd isotopic systems were re-equilibrated, and their study allowed us to date the dynamometamorphic processes using mineral isochrons. It was established that the Moncha Tundra fault, and, respectively, the whole Central Kola fault system appeared in the middle of Paleoproterozoic ～2.0–1.9 Ga, simultaneously with the Svecofennian orogen in the central part of the region and the Lapland-Kola orogen in its northeastern part. Another episode of dynamometamorphism that occurred at 1.60–1.65 Ga is envisaged.     

Sm-Nd and Rb-Sr isotopic and geochemical systematics in Phanerozoic granulites from Fiordland,southwest New Zealand

Sm-Nd and Rb-Sr isotopic analyses are reported for granulite facies orthogneisses from Fiordland southwest New Zealand. Whole-rock samples define a Rb-Sr isochron age of 120±15 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70391±4. _Nd values (at 120 Ma) show a relatively wide range of from –0.4 to 2.7 indicating decoupling of Sr-Nd isotope systems. Associated ultramafic rocks have initial ⁸⁷Sr/⁸⁶Sr ratios of from 0.70380 to 0.70430 and _Nd values of from 0.1 to 3.0. The different initial ratios suggest that the various intrusions, although contemporaneous, were not derived through fractionation of a single parent magma. A metasedimentary enclave incorporated during emplacement of the granulitic rocks preserves a Proterozoic isotopic signature with a measured _Nd(0) value of –10.2, ⁸⁷Sr/⁸⁶Sr ratio of 0.73679 and a T ^Nd provenance age of 1490 Ma. The Rb-Sr whole rock age of the granulites is the same as obtained from recent U-Pb zircon dating (Mattinson et al. 1986) and is interpreted as the time of magmatic emplacement and essentially contemporaneous granulite facies metamorphism. Rb-Sr and Sm-Nd analyses of mineral systems indicate that the terrain had cooled below 300° C by 100 Ma providing further evidence that high grade metamorphism was of exceptionally short duration.Unmetamorphosed leucogabbros from the Early Cretaceous Darran Complex of eastern Fiordland have significantly higher _Nd values (3.9 to 4.6) and slightly lower ⁸⁷Sr/ ⁸⁶Sr (0.70373 to 0.70386) than the western Fiordland granulites. This indicates that the western and eastern Fiordland complexes are not correlative although both have geochemical similarities to Phanerozoic calc-alkaline island-arc suites. The Fiordland granulites are LREE enriched (La_N/ Yb_N=12 to 40) and have trace element characteristics (e.g. high K/Rb and low Rb/Sr ratios) typical of many Rb-depleted Precambrian granulite terrains. The Fiordland trace element trends, however are attributed to magmatic, not metamorphic processes, reflecting the character of the Early Cretaceous magma sources. The range of _Nd values, but uniform initial ⁸⁷Sr/⁸⁶Sr of the western Fiordland granulites is consistent with derivation of the parent Early Cretaceous magmas at least in part from a LREE enriched, low Rb/Sr protoliths of mid-to late-Paleozoic age. Partial melting of this protolith occurred during or immediately preceding a period of great crustal thickening culminating in rapid thickening of existing crust by 20 km following emplacement of the granulitic rocks. The rapid crustal thickening was probably a consequence of a collisional event in which an Early Cretaceous magmatic arc was over-ridden by one or more thrust sheets.     

The U-Th-Pb systematics of zircons from the type Nûk gneisses,Godthåbsfjord,West Greenland

U-Pb analyses were made on zircons separated from different lithologies within the Nûk gneisses taken from Godthåb (Nuuk) townsite and from the south-west shore of Bjørneøen. Size fractions of the zircons from each rock yield linear discordia which give apparent crystallisation ages for the parents of the Nûk gneisses of 2890–3065 Myr. This spread of ages is not a result of multiple intrusion, but more likely results from different degrees of ‘diffusion’ and metamorphic episodic lead loss by the zircons. Since U-Pb and Th-Pb discordance are similar, loss of radiogenic lead (and not U or Th-loss/gain) by radiation-damage diffusion and through metamorphism is the main factor causing discordance of the U-Pb dates. The results show that the discordia line given by the analyses of fractions of a single zircon population may not give a true crystallisation age when the rock has been strongly metamorphosed. This interpretation is supported by Rb-Sr whole rock analyses of the same rocks, in which the Nûk townsite rocks scatter about a reference isochron of 2840 Myr, while the Bjørneøen samples give an isochron (MSWD = 0.55) of 3076 ± 27 Myr and the initial ratio at 0.70205 ± 0.00009. The zircon results do not rule out the possibility that more than one metamorphic episode affected the original Nûk gneiss parents prior to 2520 Myr (Qôrqut granite intrusion). A date of 2660 Myr was obtained from an aplite intruded during a period of plutonic activity that included the formation of a linear belt of intense ductile deformation that extends through south-western Bjørneøen and Godthåb town.