Whole-rock Isochron Age Research Articles

Geochemical Study of the Osumi Granodiorite, Southwestern Japan

The Osumi Granodiorite, located on the Osumi Peninsula in southwest Japan, is an example of outer zone granites that were formed during a limited period (13–15 Ma) in response to the subduction of the Philippine Sea Plate. This event, which is linked to the separation of southwest Japan from continental Asia, resulted in unique igneous activity. The Osumi Granodiorite is the largest Miocene granite body in the region. It intrudes into the Mesozoic to Paleogene accretionary complex of the Shimanto Belt and affects contact metamorphism. Despite considerable research on the Osumi Granodiorite, limited geochemical studies, especially on trace and rare earth element (REE) analyses, have been conducted. Furthermore, there are insufficient data on the Rb–Sr isotopic system, leaving the formation process unclear. This study presents whole-rock geochemical and Rb-Sr isotopic data to investigate the petrogenesis of the Osumi Granodiorite. The results suggest a common magma origin for this pluton, as indicated by linear trends on the Harker diagrams and similar REE patterns. The presence of a relatively large Eu anomaly implies formation under a reducing environment. The AKF diagram indicates predominant contamination by pelitic rocks of the Shimanto Belt during magma formation. The Rb–Sr whole-rock isochron diagram and SrI–1000/Sr diagram suggest that the Osumi Granodiorite body was formed by heterogeneous assimilation of magma into the Shimanto Belt. Furthermore, the whole-rock isochron age is 64.3 Ma, which differs by approximately 50 My from the previously reported biotite K–Ar age (14–22 Ma). This age is considered to be a pseudo-isochron age, rather than the consolidation age. During the middle Miocene, the compressive stress field in the outer zone south of the Butsuzo Tectonic Line made it difficult for magma to rise. As a result, it reacted with the sedimentary rocks of the Shimanto Belt to various degrees. The Osumi Granodiorite underwent magma differentiation upon intrusion into the Shimanto Belt. It subsequently ascended, cooled, and interacted with pelitic rocks under stable geological conditions.

Use of high-U hydrothermal apatite containing excess 206Pb to constrain the age of uranium mineralization at the Coles Hill deposit, Virginia, USA

High-U hydrothermal apatite with complex UPb systematics is closely spatially associated with mineralization at the Coles Hill deposit, the largest unmined uranium deposit known in the United States. The deposit is hosted in metasomatized rocks of the 450- to 430-Ma-old Martinsville Intrusive Complex in south-central Virginia. Direct dating of metamict uranium-ore minerals, mostly coffinite, is not possible due to open-system radon loss. Instead, UPb isotopes in cogenetic apatite were investigated as a means of evaluating the age of mineralization. Here we report in situ electron probe microanalyses (EPMA) of coffinite, isotope-dilution thermal-ionization mass spectrometry (ID-TIMS) UPb data for mineralized whole rock samples, and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) UPb isotope data for apatite in both unmineralized and U-mineralized host rocks.Massive deficits in radiogenic Pb preclude reliable UPb “chemical ages” calculated from EPMA data obtained from coffinite. In contrast, LA-ICPMS data for secondary apatite in unmineralized rocks indicate low-U concentrations (100–102 ppm), “normal” (consistent with models of terrestrial Pb isotopic evolution) initial Pb isotope compositions, and UPb age estimates of ~330 Ma, which is consistent with dates previously proposed for the regional Paleozoic shear zone that hosts the deposit. Ore-stage apatite associated with coffinite has high-U concentrations (typically 102–103 ppm but up to 2.4 wt% U) and large excesses of 206Pb (207Pb/206Pb < 0.01) unsupported by in situ U decay. Data show that initial Pb had variable isotopic compositions including both “normal” Pb derived from host rocks and 206Pb-enriched Pb introduced by secondary metasomatic fluids. Evaluation of the complex evolution and mixing of Pb sources has broader implications for UPb dating of hydrothermal apatite.Excess 206Pb in apatite is derived from decay products of 222Rn lost from coffinite and mobilized by Na-, P-, and U-enriched metasomatic fluids during the main mineralizing event at ~230 Ma. Ore-stage alteration did not uniformly reset the UPb systematics in host rocks precluding a well-constrained whole-rock isochron age. However, whole-rock isotope data imply U mobility at ~200–220 Ma and support a Triassic age for the final stages of mineralization. Results also indicate that apatite with up to several weight percent uranium is able to retain U and its decay products for hundreds of millions of years; an important consideration when assessing this mineral as a potential matrix for long-term storage of radioactive waste.

Isotope-Geochemical Evidence of the Nature of the Protoliths of Diamondiferous Rocks of the Kokchetav Subduction–Collision Zone (Northern Kazakhstan)

Abstract —The isotope-geochemical features of diamondiferous metamorphic rocks of the Kokchetav subduction–collision zone (KSCZ) show that both the basement rocks and the sediments of the Kokchetav massif were their protoliths. A whole-rock Sm–Nd isochron from the diamondiferous calc-silicate, garnet–pyroxene rocks and migmatized granite-gneisses of the western block of the KSCZ yielded an age of 1116 ± 14 Ma, while an age of 1.2–1.1 Ga was obtained by U–Pb dating of zircons from the granite-gneiss basement of the Kokchetav microcontinent. Based on these data, we assume that the protoliths of the calc-silicate, garnet–pyroxene rocks and the granite-gneisses of the KSCZ were the basement rocks sharing an initially single Nd source, which was not influenced by high- to ultrahigh-pressure metamorphism (~530 Ma). Therefore, their geochemical features are probably not directly related to ultrahigh-pressure metamorphism. The corresponding rock associations lack isotope-geochemical evidence of partial melting that would occur during ultrahigh-pressure metamorphism, which suggesting that they were metamorphosed under granulite-facies conditions. At the same time, the high-alumina diamondiferous rocks of the Barchi area (garnet–kyanite–mica schists and granofelses), which were depleted to different degrees in light rare-earth elements (REE) and K, have yielded a Sm–Nd whole-rock isochron age of 507 ± 10 Ma indicating partial melting of these rocks during their exhumation stage. The close ɛNd (1100) values of the basement rocks and garnet–kyanite–mica schist with geochemical characteristics arguing against its depletion during high-pressure metamorphism indicate that the basement rocks were a crustal source for high-alumina sediments.

Chemical stratification in the post-magma ocean Earth inferred from coupled 146,147Sm–142,143Nd systematics in ultramafic rocks of the Saglek block (3.25–3.9 Ga; northern Labrador, Canada)

The coupled 146,147Sm–142,143Nd chronometer has the potential to provide precise constraints on both the age and the composition of silicate reservoirs generated by magma ocean processes on accreting planets. Application of this chronometer to early Earth differentiation, however, is made difficult by the poor preservation and complex geological history of Eoarchean rocks hosting 142Nd anomalies, which often prevents accurate determination of their initial 143Nd/144Nd ratios. In order to better constrain the chronological aspects of Earth's formation, we investigated the 146,147Sm–142,143Nd systematics of well-preserved mafic/ultramafic enclaves of the Archean Saglek block of northern Labrador (3.25–3.9 Ga). Our results show that two distinct ultramafic suites are present within the Hebron/Saglek fjords region. The first group of samples, with μ142Nd=1.6±2.8 and ε143Ndi=0.4±0.4, yields a whole-rock isochron age of 3365 ± 100 Ma and is tentatively suggested to be associated with the Mesoarchean Upernavik formation. The second group, with μ142Nd=8.6±3.3 ppm and ε143Ndi=1.4±0.6, yields an Eoarchean date of 3782 ± 93 Ma, and is assigned to the Nulliak assemblage. Application of coupled 146,147Sm–142,143Nd chronometry to the Nulliak suite yields a model age of differentiation of 4.40−0.06+0.05 Ga, and a corresponding (147Sm/144Nd)source ratio of 0.211±0.007 for the early depleted mantle. These estimates are remarkably similar to those obtained for a tholeiitic lava of the Abitibi greenstone belt (Theo's flow, 2.7 Ga) based on the 142,143Nd dataset of Debaille et al. (2013). Viewed in conjunction with previous 142,143Nd data, our results provide a precise estimate of the age of primordial differentiation of Earth's mantle, 160−20+30 Myr after formation of the solar system. This chronological constraint, combined with evidence for late solidification of the lunar magma ocean, strongly supports a young age for the giant impact and the Earth–Moon system. Further, the similarity of 146,147Sm–142,143Nd model ages and (Sm/Nd)source ratios inferred for Nulliak, Isua and Theo's flow suggests that their parent magmas were derived from a common mantle reservoir. This early depleted domain appears to have evolved as a closed-system on a multi-billion year timescale despite efficient mixing in the hot Hadean/Archean mantle. We thus propose that the occurrence of positive 142Nd anomalies in the Archean rock record reflects episodic melting of a depleted reservoir otherwise isolated from the convective system, rather than progressive homogenization of a highly depleted Hadean mantle.

Chemical separation of Nd from geological samples for chronological studies using 146Sm–142Nd and 147Sm–143Nd systematics

Sm–Nd dating, which involves long-lived 147Sm–143Nd and short-lived 146Sm–142Nd systematics, has been widely used in the field of geosciences. To obtain precise and accurate ages of geological samples, the determination of highly precise Nd isotope ratios with nearly complete removal of Ce and Sm is indispensable to avoid mass spectral interference. In this study, we developed a three-step column chemistry procedure for separating Nd from geological samples that includes cation exchange chromatography for separating major elements from rare earth elements (REEs), oxidative extraction chromatography using Ln Resin coupled with HNO3 + KBrO3 for separating tetravalent Ce from the remaining REEs, and final purification of Nd using Ln Resin. This method enables high recovery of Nd (>91%) with effective separation of Nd from Ce and Sm (Ce/Nd < 1.2 × 10−5 and Sm/Nd < 5.2 × 10−6). In addition, we devised a new method for determining Sm/Nd ratios with the isotope dilution inductively coupled plasma mass spectrometry method using 145Nd- and 149Sm-enriched spikes coupled with a group separation of REEs using TRU Resin. Applying the techniques developed in this study, we determined the Sm–Nd whole-rock isochron age of basaltic eucrites, yielding 4577 − 88+ 55 Ma and 4558 ± 300 Ma for 146Sm–142Nd and 147Sm–143Nd systematics, respectively.

Rb–Sr and Sm–Nd study of granite–charnockite association in the Pudukkottai region and the link between metamorphism and magmatism in the Madurai Block

Pudukkottai region in the northeastern part of the Madurai Block exposes the garnetiferous pink granite that intruded the biotite gneiss. Charnockite patches are associated with both the rock types. Rb–Sr biotite and Sm–Nd whole-rock isochron ages indicate a regional uplift and cooling at ∼550 Ma. The initial Nd isotope ratios (\(\varepsilon _{\text {Nd}}^{\mathrm {t}}=-20\) to −22) and Nd depleted-mantle model ages (TDM = 2.25 to 2.79 Ga) indicate a common crustal source for the pink-granite and associated charnockite, while the biotite gneiss and the charnockite within it represent an older crustal source (\(\varepsilon _{\text {Nd}}^{\mathrm {t}}= -29\) and TDM = > 3.2 Ga). The Rb–Sr whole-rock data and initial Sr–Nd isotope ratios also help demonstrate the partial but systematic equilibration of Sr isotope and Rb/Sr ratios during metamorphic mineral-reactions resulting in an ‘apparent whole-rock isochron’. The available geochronological results from the Madurai Block indicate four major periods of magmatism and metamorphism: Neoarchaean–Paleoproterozoic, Mesoproterozoic, mid-Neoproterozoic and late-Neoproterozoic. We suggest that the high-grade and ultrahigh-temperature metamorphism was preceded by magmatism which ‘prepared’ the residual crust to sustain the high P–T conditions. There also appears to be cyclicity in the tectono-magmatic events and an evolutionary model for the Madurai Block should account for the cyclicity in the preserved records.

Rb–Sr and Sm–Nd isotope systematics and geochemical studies on metavolcanic rocks from Peddavura greenstone belt: Evidence for presence of Mesoarchean continental crust in easternmost part of Dharwar Craton, India

Linear, north–south trending Peddavura greenstone belt occurs in easternmost part of the Dharwar Craton. It consists of pillowed basalts, basaltic andesites, andesites (BBA) and rhyolites interlayered with ferruginous chert that were formed under submarine condition. Rhyolites were divided into type-I and II based on their REE abundances and HREE fractionation. Rb–Sr and Sm–Nd isotope studies were carried out on the rock types to understand the evolution of the Dharwar Craton. Due to source heterogeneity Sm–Nd isotope system has not yielded any precise age. Rb–Sr whole-rock isochron age of 2551 ± 19 (MSWD = 1.16) Ma for BBA group could represent time of seafloor metamorphism after the formation of basaltic rocks. Magmas representing BBA group of samples do not show evidence for crustal contamination while magmas representing type-II rhyolites had undergone variable extents of assimilation of Mesoarchean continental crust (>3.3 Ga) as evident from their initial ε Nd isotope values. Trace element and Nd isotope characteristics of type I rhyolites are consistent with model of generation of their magmas by partial melting of mixed sources consisting of basalt and oceanic sediments with continental crustal components. Thus this study shows evidence for presence of Mesoarchean continental crust in Peddavura area in eastern part of Dharwar Craton.

Neoproterozoic oceanic crust remnants in northeast Brazil

Research Article| May 01, 2014 Neoproterozoic oceanic crust remnants in northeast Brazil Fabrício Caxito; Fabrício Caxito 1Instituto de Geociências, Universidade Federal de Minas Gerais, Belo Horizonte 31270-901, Brazil2Geotop, Université du Québec à Montréal, Montréal H3C 3P8, Canada Search for other works by this author on: GSW Google Scholar Alexandre Uhlein; Alexandre Uhlein 1Instituto de Geociências, Universidade Federal de Minas Gerais, Belo Horizonte 31270-901, Brazil Search for other works by this author on: GSW Google Scholar Ross Stevenson; Ross Stevenson 2Geotop, Université du Québec à Montréal, Montréal H3C 3P8, Canada Search for other works by this author on: GSW Google Scholar Gabriel J. Uhlein Gabriel J. Uhlein 1Instituto de Geociências, Universidade Federal de Minas Gerais, Belo Horizonte 31270-901, Brazil Search for other works by this author on: GSW Google Scholar Geology (2014) 42 (5): 387–390. https://doi.org/10.1130/G35479.1 Article history received: 16 Jan 2014 rev-recd: 27 Feb 2014 accepted: 05 Mar 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Fabrício Caxito, Alexandre Uhlein, Ross Stevenson, Gabriel J. Uhlein; Neoproterozoic oceanic crust remnants in northeast Brazil. Geology 2014;; 42 (5): 387–390. doi: https://doi.org/10.1130/G35479.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Borborema Province of northeast Brazil occupies a strategic position in the central portion of West Gondwana, linking three of its major cratonic constituents: the São Francisco–Congo, Amazon, and West Africa cratons. The southern portion of the province, the Riacho do Pontal fold belt, comprises an association of exhalative rocks with metabasalts having transitional mid-oceanic ridge basalt geochemistry (Monte Orebe Complex) that suggests the preservation of remnants of Neoproterozoic oceanic crust. This view is supported by geophysical data: the Riacho do Pontal fold belt corresponds to the inflexion of a paired positive-negative Bouguer anomaly similar to other Precambrian suture zones. Sm-Nd isotope data for the metabasalts yield a whole-rock isochron age of 819 ± 120 Ma with an initial εNd(t) = +4.4, indicating derivation from a depleted mantle source. The onset of subduction within the Borborema Province (ca. 630 Ma) caused the inversion of basins and obduction of slices of oceanic crust. In this scenario, the Riacho do Pontal fold belt represents a complete late Neoproterozoic plate tectonics cycle involving the collision of the São Francisco craton (lower plate) with the Pernambuco-Alagoas block (upper plate). This interpretation challenges current views that the Borborema Province has acted as a coherent block since Paleoproterozoic time (part of the Atlantica supercontinent), suggesting instead a dynamic setting, where multiple plates interacted during the Proterozoic. The Monte Orebe ophiolite provides a link with other Cryogenian oceanic crust occurrences in central Brazil and West Africa, indicating the preservation of a transcontinental Neoproterozoic suture zone in the heart of West Gondwana. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

山陽帯東部，土岐花崗岩体の冷却史

The cooling history of the Crateceous Toki granite, exposed in the eastern Sanyo Belt, Central Japan, was constructed from various geochronological data. The granite yields K-Ar biotite ages of 74±2 to 72±2 Ma (±1σ) and K-Ar hornblende ages of 75±4 to 74±4 Ma, which are concordant with a previously reported Rb-Sr whole-rock isochron age. Fission-track ages of zircon and apatite in the granite give ages 73±3 to 64±3 Ma and 40±4 to 37±4 Ma, respectively. These above thermochronological data suggests two distinctive cooling stages for the Toki granite; 1) a first rapid cooling stage in which the granitic magma was cooled to the temperature of host rock soon after intrusion at depths of 5-7 km in the upper crust, and 2) second slow cooling stage (7-9 °C/m.y.) associated with the uplift and subsequent erosion of the granite intrusion after ∼ 70 Ma.

The trace element geochemistry of clinopyroxenes from pyroxenites in the Lewisian of NW Scotland: insights into light rare earth element mobility during granulite facies metamorphism

Clinopyroxenes from layered pyroxenites and from pyroxenite pods in felsic gneisses of the Lewisian granulite complex, NW Scotland, have distinctive chemistries suggestive of different origins. Clinopyroxenes in the layered pyroxenites crystallised from mafic melts in a magma chamber located in the middle to shallow crust, whereas clinopyroxenes in pods in the felsic gneisses crystallised from the tonalitic protolith to the felsic gneisses. In detail clinopyroxenes in the layered pyroxenites are variably enriched in the light REE. Inversion modelling shows that this is not a primary feature inherited from their parent magmas. Rather selective light rare earth element enrichment took place through reaction with a felsic melt generated by the localised partial melting of the hornblende pyroxenites during granulite facies metamorphism. Published isotopic evidence suggests that the light REE mobilisation took place at ca 2.7 Ga, about 200 Ma after the time of crust formation. This observation provides an explanation for the scattered pattern of whole-rock isochron ages from the Lewsian granulites.

Crustal accretion and metamorphism of Mesoarchean granulites in Palghat-Cauvery Shear Zone, Southern India

This work provides unequivocal evidence of the existence of Mesoarchean granulite facies metamorphic event in the Palghat-Cauvery Shear Zone (PCSZ) of South India. Charnockite samples from two prominent hills at Kollaimalai (KM) and Pachchaimalai (PM) as well as from two quarries within the Bhavani Shear Zone (BSZ) have been analyzed for their Sm-Nd and Rb-Sr ages to investigate the existence or otherwise of the Archean granulite facies events within the PCSZ. The Rb-Sr whole-rock isochron ages for massive charnockites from both the hills appear to be contemporaneous at 2.9 Ga with the initial Sr isotopic ratios of 0.7012 and 0.7014, respectively. However, the Rb-Sr data for whole-rock samples of basic granulites from one of the quarries within the BSZ indicate open system behavior, while the charnockites from the other quarry have insufficient spread in 87Rb/86Sr ratios and do not yield any isochron. The Sm-Nd data, on the other hand, do not distinguish between the massive charnockite and the lowland charnockite and yield Depleted Mantle model ages in the range 2.98±0.3 Ga for all of them. The ɛTCHUR for all of these rocks are highly positive. Both the Sr isotopic ratios and positive ɛTCHUR values for these rocks strongly suggest a mantle source for all of them. An upper age limit of ∼3.28 Ga may be assigned to the crustal accretion of the protolith of all these rocks on the basis of their Nd model ages. The Rb-Sr isochron ages of 2.9 Ga for the two massifs could be the age of granulite facies metamorphism. Thus, the metamorphism in the KM and PM Hills took place within ∼100 Ma of crustal accretion of these rocks and probably was part of the same geological event of crust formation and metamorphism. The open system behavior with respect to Rb-Sr isotopes in the basic granulite from Bhavani is possibly due to the migration of Sr isotopes, triggered during the later shearing of these rocks.

Rb-Sr whole-rock isochron age of the Nissho Toge granite complex, northern Hidaka Mountains, central Hokkaido, Japan

We obtained a Rb-Sr whole-rock isochron age of 17.7±0.6 Ma from the Nissho Toge granite complex, which intruded the upper portion of a ∼20-km-thick crustal section exposed in the northern Hidaka Mountains, central Hokkaido, Japan. The initial 87Sr/86Sr ratio is 0.70422±0.00009. This age is similar to U-Pb zircon ages reported previously for the Pankenushi gabbro and Magarisawa tonalite (18.5±0.3 Ma and 18.7±0.5 Ma, respectively), which are both located in the lower portion of the crustal section examined in the present study. Our result suggests that almost all of the magmatic bodies in the crustal section were emplaced contemporaneously, during the Early Miocene.

Eclogites and Garnet Pyroxenites: Problems Resolving Provenance Using Lu-Hf, Sm-Nd and Rb-Sr Isotope Systems

Cratonic eclogites and garnet pyroxenites from the Kaapvaal craton have heterogeneous Hf^Nd^Sr^(O) isotope ratios that define a positive Hf^Nd isotope array and a negative Nd^Sr isotope array. Isotopic variability encompasses depleted (mid-ocean ridge basalt and ocean-island basalt) to enriched mantle compositions (Group I and II kimberlites) and overlaps with that of the Kaapvaal craton garnet peridotite xenoliths. Isotopic heterogeneity at Roberts Victor is less extreme than previously reported and ranges from eclogites with a highly depleted MORB-like signature to enriched eclogites similar to Group II and transitional kimberlites and Group II megacrysts (eHf1⁄4 ^32·8). Much of this similarity may well be due to partial or complete resetting during entrainment. For the majority of eclogites and garnet pyroxenites the Lu^Hf system records ‘older’ mantle events than the Sm^Nd system, but neither necessarily records the protolith age. Both the Lu^Hf and the Sm^Nd systems are prone to being reset by entrainment in high-temperature kimberlite and/or basaltic magmas (e.g. Kaapvaal) and emplacement in orogenic belts (e.g. Beni Bousera). In the case of one eclogite from Roberts Victor the Sm^Nd cpx^gt mineral isochron age (963·1 42·3Ma) differs from the Lu^Hf cpx^gt mineral isochron age (1953 13Ma) by 1Ga and the Rb^Sr clinopyroxene model age (3·15 Ga) is 1Ga older than the Lu^Hf age and the reconstructed whole-rock isochron age. Ironically, it may be that, in this instance, the Rb^Sr system gives a better indication of protolith age than Sm^Nd or Lu^Hf. Overall variable resetting of isotope systems between protolith formation in the Archaean (42·5 Ga) and kimberlite and/or basalt entrainment ( 0·2 Ga) masks our understanding of the exact protolith age of eclogites.

The Grenvillian Songshugou ophiolite in the Qinling Mountains, Central China: Implications for the tectonic evolution of the Qinling orogenic belt

The Qinling Mountains in Central China mark a gigantic composite orogenic belt with a complex tectonic evolution involving multiple phases of rifting and convergence. This belt separates the North China and South China Blocks and consists of the South and North Qinling terranes separated by the Shangdan suture. The suture is marked by the Grenvillian Songshugou ophiolite along the southern margin of the North Qinling terrane, which is key to understanding the Proterozoic tectonic evolution of the belt. The ophiolite consists of highly metamorphosed ultramafic and mafic rocks. Three groups of meta-basalts are present: group 1 rocks are LREE depleted and have a MORB compositional affinity. Their low Ta/Yb ratios (<0.1) are consistent with high degrees of partial melting of a depleted asthenospheric mantle. Rocks of group 2 have higher TiO 2 (1.63–2.08 wt%) and Ta/Yb ratios (>0.12), and display slight enrichment of LREE, suggesting that the original magmas were derived from a depleted mantle source mixed with some enriched material. Samples from group 3 are enriched in LREE and other incompatible elements (Ti, Zr, Ta, Nb), suggesting derivation from an enriched mantle source, possibly a plume. All the basalts have high εNd( t) (+4.2 to +6.9), variable εSr( t) and high 207Pb/ 204Pb and 208Pb/ 204Pb ratios for given 206Pb/ 204Pb ratios. These characteristics are compatible with formation at a mid-ocean ridge system above an anomalous Dupal mantle region. The mafic rocks have a Sm–Nd whole-rock isochron age of 1030 ± 46 Ma. The Songshugou ophiolite was emplaced onto the southern margin of the North Qinling terrane, an active continental margin from the Meso-Proterozoic to Neo-Proterozoic.

Os–Pb–Nd isotope and highly siderophile and lithophile trace element systematics of komatiitic rocks from the Volotsk suite, SE Baltic Shield

Highly siderophile element (HSE; Ru, Pd, Re, Os, Ir, and Pt) abundances and Re–Os isotope systematics in combination with lithophile trace element and Sm–Nd and Pb–Pb isotopic data are reported for komatiites and basalts from the Volotsk suite in the SE Baltic (Fennoscandian) Shield. The new Sm–Nd and Pb–Pb whole-rock isochron ages of 2850 ± 84 and 2861 ± 150 Ma, respectively, define the emplacement age of the volcanic sequence. This age is statistically indistinguishable from the emplacement age of the lower mafic-ultramafic sequences of the adjacent Sumozero-Kenozero greenstone belt. The Re–Os internal isochron derived from the analysis of a whole rock B 2–4 cumulate and three chromite separates yields an age of 2878 ± 81 Ma, which is consistent with the ages obtained from the lithophile element isotopic systems. The komatiites of the Volotsk suite were derived from a mantle source characterized by depletions in highly incompatible lithophile trace elements, with the magnitude of the time-integrated LREE depletion (initial ɛNd = +2.7 ± 0.2) typical of this period in the Earth's mantle history. Yet this source had an initial γ 187Os = −0.4 ± 0.4 and, thus, evolved with a time-integrated Re/Os that was identical, within uncertainty, to that of the carbonaceous chondritic reference. In addition, the calculated relative abundances of Os, Ir, Ru, and Pt in the komatiite source are similar to chondrites and may be explained by late (post-core segregation) accretion to the Earth of materials with high absolute and chondritic relative abundances of HSE or by re-entrainment of the core. If the former interpretation is correct, the late accreted materials were well homogenized within the upper mantle by 2.9 Ga. Like many other Archean komatiites, the studied rocks have radiogenic Nd isotopic composition, indicative of their long-term relative depletions in highly incompatible lithophile trace elements, yet the long-term Re/Os in their source remained near-chondritic. Modeling indicates that the komatiite source must have experienced only a small degree of partial melting (2–3%) and extraction of basaltic melts ca. 150 Ma prior to emplacement of the komatiitic sequences. Larger degrees of melting would otherwise have led to loss of a substantial fractionation of Re in the source, which would then have evolved to a subchondritic 187Os/ 188Os by the time of komatiite formation.

In situ U-Pb (Shrimp) dating of zircons from granosyenite of the Troitsk pluton, Kvarkush-Kamennogorsk Anticlinorium, central Urals

The Troitsk granosyenite pluton is located among the Neoproterozoic sedimentary sequences of the Kvarkush‐Kamennogorsk Anticlinorium on the western slope of the Urals. The sedimentary sequences comprise the Kedrovka and Basegi groups of the Upper Riphean and the Serebryanka and Sylvitsa groups of the Vendian [1, 2]. In the mid-1980s, the Troitsk pluton was used as a reference intrusive body for timing the lower boundary(620 ± 15 Ma) of the Upper Vendian [3]. However, later on, the validity of the Rb‐Sr wholerock isochron age of carbonated granosyenites was questioned [4]. In addition to the Troitsk pluton, a number of other igneous complexes (Shpalorezov, Dvoretsk, Blagodatny, Kus’ya, Zhuravlik, and others) are localized in the Vendian sedimentary rocks of the Kvarkush‐Kamennogorsk Anticlinorium. Their petrology, geochemistry, and age are discussed in many publications. The results of geological mapping under the supervision of Suslov [5] made an important contribution to recognition of the geological position of some igneous complexes. The most reliable correlation of the Late Riphean and Vendian igneous complexes in the Kvarkush‐Kamennogorsk Anticlinorium was given in [6‐8]. Karpukhina et al. [9] made an attempt to reconstruct geodynamic settings of the Dvoretsk, Kus’ya, and Blagodatny complexes and estimated their Sm‐Nd and Rb‐Sr ages. The modern data on geochemistry, geodynamics, and geochronology of the Neoproterozoic igneous complexes on the western slope of the central Urals are summarized in [10].

A mid-Ordovician Rb–Sr isochron age from the Ingleton Group, North Yorkshire: metamorphism or diagenesis?

SUMMARY An Rb–Sr whole-rock isochron age of 465 ± 10 Ma (95% confidence level) has been obtained for slates from the Ingleton Group sampled in the Ingleton area. This result is thought to be close to the age of metamorphism, which reached high anchizone grade, though it is impossible to rule out a diagenetic interpretation. The co-linearity of the data points implies a high degree of isotopic homogenization of Sr isotopes within the sample set caused by migration of aqueous fluids expelled from the clay minerals during either the smectite-illite transition or low-grade metamorphism. A previously published isochron age of 494 ± 18 Ma was derived from samples collected in the Horton area. They are coarser-grained and less metamorphosed (low anchizone to high diagenetic grade) than the Ingleton samples, and their older age may reflect their less complete isotopic homogenization. The new Rb–Sr age is consistent with a possible Arenig age of deposition. If interpreted as a metamorphic age, it implies that the Ingleton Group was metamorphosed much earlier than the Devonian (Acadian) metamorphism of the overlying Windermere Supergroup. Whether of metamorphic or diagenetic origin, the survival of the Rb–Sr isochron through the Acadian metamorphism implies that the latter event did not cause significant movement of aqueous fluids through the fine-grained components that were analysed.

Elemental geochemistry and Nd isotopic characteristics of the metasedimentary rocks from the metamorphic belt in central Jiangxi: Provenance and tectonically environmental constraints

The metamorphic belt in central Jiangxi, located in the compound terrain within the Cathaysia, Yangtze Block and Caledonian fold zone of South China, is composed dominantly of meta-argillo-are-naceous rocks, with minor amphibolite. These rocks underwent amphibolite-facies metamorphism. The meta-argillo-arenaceous rocks show large variations in major element composition, but have similar REE patterns and trace element composition, incompatible element and LIE enrichments [high Th/Sc (0.57−3.59), La/Sc (1.46−12.4), La/Yb (5.84−19.0)] and variable Th/U ratios, with ΣREE=129−296µg/g, δEu=0.51−0.86, and (La/Yb)N=3.95−12.9. The Nd isotopic model agest DM of these rocks vary from 1597 to 2124 Ma. Their143Nd/144Nd values are low [ɛNd(0)=−11.4 to −15.8]. Some conclusions have been drawn as follows: (1) The metamorphic rocks in central Jiangxi Province are likely formed in a tectonic environment at the passive continental margin of the Cathaysia massif. (2) The metamorphosed argillo-arenaceous rocks are composed dominantly of upper crustal-source rocks (Al- and K-rich granitic or/and sedimentary rocks of Early Proterozoic), which experienced good sorting, slow deposition and more intense chemical weathering. (3) According to the whole-rock Sm-Nd isochron ages (1113±49 to 1199±26 Ma) of plagioclase-amphibole (schist) and Nd isotopic model aget DM (1597−2124 Ma) of meta-argillo-arenaceous rocks, the metamorphic belt in central Jiangxi Province was formed during the Middle Proterozoic (1100−1600 Ma).

Age, geochemical and Sr–Nd–Pb isotopic constraints for mantle source characteristics and petrogenesis of Teru Volcanics, Northern Kohistan Terrane, Pakistan

This paper presents new geochemical and geochronology data for the Teru Volcanic Formation (previously known as the Shamran Volcanics) exposed west of Gilgit in the Kohistan terrane of the Pakistani Himalayas. The Teru Volcanic Formation ranges from basalt through andesite to rhyolite and has subalkaline and midalkaline affinities. Trace-element compositions and isotopic characteristics suggest these magmas were formed in a subduction zone setting; isotopic studies also support this conclusion. It is suggested that these lavas originated from a depleted mantle source, which experienced contamination by variable subduction components. Model mixing calculations using 87Sr/ 86Sr and 143Nd/ 144Nd data suggest that addition of 0.2–0.6% of Indus margin sediments and/or 2–4% of fluids derived from Indus margin sediment can generate the compositional variation of the Teru Volcanic Formation. Two samples from the Teru Volcanic Formation yielded 40Ar/ 39Ar ages of 43.8+0.5 and 32.5+0.4 Ma. These ages make the volcanic rocks of the Teru Volcanic Formation the youngest reported in the Kohistan terrane. These volcanic rocks unconformably overly the Shunji Pluton, which has a 65 Ma Rb–Sr whole-rock isochron age. The results of this research suggest that subduction-related volcanism was active until 33 Ma in the India–Asia collision zone.

Geochemical Characteristics and LA‐ICP‐MS Zircon U‐Pb Dating of Amphibolites in the Songshugou Ophiolite in the Eastern Qinling

Abstract Geochemical studies on the amphibolites in the Songshugou ophiolite from Shangnan County, Shaanxi Province demonstrate that the protolith of the amphibolites is tholeiitic. The amphibolites can be classified into two groups according to their REE patterns and trace element features. Rocks of the first group are depleted in LREE while rocks of the second group are slightly depleted in LREE or flat from LREE to HREE without significant Eu anomaly. The first group of rocks have (La/Yb)N = 0.33–0.55, (La/Sm)N = 0.45–0.65, and their La/Nb, Ce/Zr, Zr/Nb, Zr/Y and Ti/Y ratios are averaged at 1.20, 0.12, 31.02, 2.92 and 198, respectively, close to those of typical N‐MORB. The second group of rocks have (La/Yb)N = 0.63–0.95, (La/Sm)N = 0.69–0.90, and their average La/Nb, Ce/Zr, Zr/Nb, Zr/Y and Ti/Y ratios are 0.82, 0.83, 1.15, 0.16, 19.00, 2.58 and 225, respectively, which lie between those of typical N‐MORB and E‐MORB but closer to the former. The two groups of rocks both exhibit flat patterns from Th to Yb in the highly incompatible elements spider diagram, but the first group of rocks have lower element abundances than the modern N‐MORB, indicating a derivation of their mantle source from more depleted mantle source than the present N‐MORB. The abundances of Th, Ta, Nb, La and Ce in the second group of rocks are slightly higher than those of the present N‐MORB, and other elements, such as Hf, Zr, Sm, Ti, Y and Yb, are close to those of the N‐MORB, indicating that the original magma was derived from depleted mantle but mixed with the enriched mantle. These characteristics, combined with the regional geology and previous studies, provide further evidence that the mafic‐ultramafic rocks have the features of a typical ophiolite.Zircon grains from the amphibolite are generally rounded, and in most of them a distinguishable core‐mantle texture is preserved as shown in the cathodoluminescence (CL) images. The core or core‐mantle parts of the zircon grains are also rounded, same as those in basalts from other regions of the world. The LA‐ICP‐MS trace element and U‐Pb isotopic analyses show that the zircon grains from the amphibolites are similar to the typical magmatic zircon in terms of their very low U and Th contents (62.36–0.10 μg/g and 78.47–0.003 μg/g, respectively). Seven pits from the core and core‐mantle parts of the zircon grains yielded an average weighted 206Pb/238U age of 973±35 (2s̀) Ma with the Th/U ratios range from 0.01 to 8.38 and mostly greater than 0.23. This age is consistent within the error range with the whole‐rock Sm‐Nd isochron age of 1030±46 Ma for the same kind of rocks reported by Dong et al. (1997a). In a combined analysis with the zircon positions on the CL images and the corresponding Th/U ratios, the age of 973±35 Ma is probably the formation age of tholeiite, the protolith of the Songshugou amphibolite. The geochronological determination gives further evidence that the Songshugou ophiolite was formed during the Neoproterozoic. In addition, there is one pit from the rim of a zircon grain giving a 206Pb/238U age of 572±199 (1s̀) Ma with a Th/U ratio of 0.08. It may represent the age of the accretionary zircon in the amphibolite‐facies metamorphism.