Rb-Sr Whole-rock Isochron Age Research Articles

Geochemistry, Rb-Sr whole rock age and Sr-Nd isotopic constraints on the Variscan A1-type granite from Azegour area in the Marrakech High Atlas (Moroccan Meseta) and their geodynamic implications

In the northern part of the Marrakech High Atlas (MHA), along the southern Variscan segment of the Western Meseta, a Variscan granitic intrusion crops out, intruding metasediments and meta-volcanosedimentary rocks of Early Cambrian to Ordovician age. A new whole-rock Rb-Sr isochron age of 268 ± 9 Ma for the granite, combined with a previously published whole-rock Rb-Sr radiometric dating (271 ± 3 Ma), reveals a post-kinematic (tectonic) character with regard to the main Variscan deformational event, belonging within the tectonic context of the Moroccan Variscan orogenic belt. Geochemically, the Azegour intrusion is metaluminous to peraluminous and exhibits a calc-alkaline affinity with a ferruginous composition. The massif shows an extremely differentiated character (SiO2 = 77.53–78.14 per cent), K2O and high total alkali contents, FeOt/(FeOt + MgO) and Ga/Al ratios, which have typical characteristics of an A-type granite. In addition, the granite contains high concentrations of LREE (LaN/SmN= 7.9–13.67) relative to HREE (LaN/YbN= 4.81–11.61) and a well-defined Eu negative anomaly (Eu/Eu* = 0.44–0.75). The granitic samples exhibit a strong enrichment of the most incompatible elements (RbN/YbN = 69.84–159.98) and a strong depletion of Ba, Sr, Eu, Nb, P and Ti. These characteristics are similar to those of A1-type granites. The absence of mineralogy typical of an S-type granite, combined with its weakly peraluminous character [A/CNK (molar Al2O3/CaO+Na2O+K2O) = 1,013–1,045], suggest that there is little or no significant involvement of supracrustal sources in the petrogenesis of the intrusion studied. Despite the strongly differentiated character of Azegour granitic rocks samples, their multi-element patterns shows many similarities to those of I-type granitoids, which has led to postulate that the parental liquids of A1-type were derived from partial melting of mafic magmas. The representative samples studied show less depleted εNd(t = 270 Ma)values of –0.94 to –4.85 and lower positive to slightly negative εSr(t = 270 Ma) values of –1.45 to 9.32. The isotopic data suggest that the Azegour granite was emplaced 270 myr ago, apparently generated by partial melting of a mafic/intermediate magma source in the lower crust as a result of the underplating of the asthenosphere mantle-derived Oceanic Island Basalt-like magmas. Alternatively, their isotopic signatures also can be attributed to the interaction and/or hybridisation of basaltic liquids derived from the mantle with these lower crust materials. The generated parental magma probably occurred at deep structural levels and involved fractional crystallisation processes by the separation of a mineralogical association composed of plagioclase + potassium feldspar ± biotite ± amphibole ± sphene ± apatite. The whole-rock Rb-Sr age of 268 ± 9 Ma, whole-rock geochemistry and Sr-Nd isotopic compositions of εNd(t = 270 Ma) and εSr(t = 270 Ma), combined with fieldwork data, suggest that the Azegour granite was emplaced.

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.

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.

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.

The basement of Calabria (southern Italy) within the context of the Southern European Variscides: LA-ICPMS and SIMS U–Pb zircon study

U–Pb spot zircon ages from metagabbros and metasediments of the Variscan lower crust in the Serre massif (southern Calabria) have been determined using LA-ICPMS and SIMS. The results, together with the literature data deriving from conventional and spot zircon ages and from Rb–Sr whole-rock isochrons, allow the geological history of the crustal block to be more tightly constrained from the Late Neoproterozoic to the Triassic, with particular emphasis on Variscan evolution. In a peri-Gondwana Pan-African/Cadomian basement, preserving memory of recycled older terrains with West African Craton affinities, mafic and felsic magmas emplaced at ca. 584 Ma (protoliths of metagabbros) and in the range 575–537 Ma (protoliths of felsic orthogneisses). Partially or completely recrystallized domains of zircons from metaigneous rocks yield ages ranging from 500 to 440 Ma. These ages (consistent with those of recrystallized cores of magmatic zircon from felsic orthogneisses and with Rb–Sr isochron data for the lower crust metasediments) suggest tectonothermal Ordovician activity, which is likely to be connected to the opening of Palaeozoic basin(s) separating the future Variscan blocks from Gondwana. Most zircon ages in the metasediments cluster in the 340–260 Ma range, which probably corresponds to two major events: (i) crustal thickening between 340 and 300 Ma (Barrowian metamorphic event) and (ii) lower-P/high-T metamorphism, Bt-dehydration melting, crustal extension between 300 and 260 Ma. Triassic ages ranging from 249 ± 4 Ma to 231 ± 5 Ma, together with whole-rock Rb–Sr isochron age at 234 Ma for the lowermost metapelites of the section, suggest tectonothermal activity, which prepared the opening of the Jurassic Neotethys. Similarities and differences relative to some circum-Mediterranean Variscan lower crust sections are also discussed, which contribute to the knowledge of south European Variscides evolution.

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.

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.

Temporal relationship between granite cooling and hydrothermal uranium mineralization at Dalongshan in China: a combined radiometric and oxygen isotopic study

A combined study using multi-radiometric dating and oxygen isotopic geothermometry was carried out for Mesozoic quartz syenite, alkali-feldspar granite and associated hydrothermal uranium mineralization at Dalongshan in the Middle-Lower Yangtze valley of east-central China. Radiometric dating of the quartz syenite yields a whole-rock Rb–Sr isochron age of 135.6±4.3 Ma, a zircon U–Pb isochron age of 132.9±2.2 Ma, and K–Ar ages of 126±2, 118±3 and 94±4 Ma for hornblende, biotite and orthoclase, respectively. The alkali-feldspar granite yields a whole-rock Rb–Sr isochron age of 117.3±3.3 Ma, a zircon U–Pb isochron age of 114.7±2.1 Ma, and K–Ar ages of 112±2, 109±3 and 88±4 Ma for hornblende, biotite and orthoclase, respectively. Oxygen isotope thermometry for both granites gives temperatures of 685 to 720, 555 to 580, 435 to 460 and 320 to 330 °C, for hornblende, magnetite, biotite and orthoclase respectively, when paired with quartz. The systematic differences among the ages by the different techniques on the different minerals are used to reconstruct the cooling history of the granite. The results yield rapid cooling rates of 27.4 to 58.6 °C/Ma from 800 to 300 °C in the early stage, but slow cooling rates of 6.3 to 7.2 °C/Ma from 300 to 150 °C in the late stage. The regular sequence of oxygen isotope temperatures for the different quartz–mineral pairs demonstrates that diffusion is a dominant factor controlling the closure of both radiometric and O isotopic systems during granite cooling. Pitchblende U–Pb isochron dating yields an uranium mineralization age of 106.4±2.9 Ma, which is younger than the age of the granite emplacement and thus considerably postdates the time of magma crystallization, but is close to the closure time of the K–Ar system in the biotite. This points to a close relationship between granite cooling and ore-forming process. It appears that hydrothermal mineralization took place in the stage of slow cooling of the granite, whereas the rapid cooling of the granite was concurrent with the migration of hydrothermal fluids along fault structures. Therefore, the activity of the ore-forming hydrothermal system is temporally dictated by the cooling rates of the granite and may lag about 25 to 30 Ma behind the crystallization timing of associated granite.

Heterogeneity of initial 87Sr/86Sr ratios within a single pluton: evidence from apatite strontium isotopic study

Magmatic apatite in samples of the unmetamorphosed undeformed Cretaceous pluton of the Habu Granodiorite, southwestern Japan, was examined to determine its viability as an indicator of initial 87Sr/86Sr heterogeneity in magmatic bodies. The Habu Granodiorite is a zoned pluton in the Ryoke metamorphic belt of southwestern Japan. A published Rb–Sr whole-rock isochron age of 124.2±10.8 Ma is significantly older than Th–U–total Pb monazite ages of ca. 86 Ma. A Rb–Sr mineral isochron of 90.0±2.5 Ma for a sample of granodiorite from the margin of the pluton has an initial 87Sr/86Sr (SrI) of 0.7056±0.0001, similar to a 87Sr/86Sr of 0.70539 for coexisting euhedral apatite. Similarly, a Rb–Sr mineral isochron of 91.4±2.5 Ma for a sample of adamellite from the center of the pluton has an SrI of 0.7068±0.0001, corresponding to a 87Sr/86Sr ratio of 0.70673–0.70684 for three fractions of coexisting euhedral apatite. The uniformity of 87Sr/86Sr values of apatite in individual samples and accordance of 87Sr/86Sr values of apatite and SrI from the mineral isochron suggest that magmatic apatite can be considered as a reliable indicator of SrI in granitic rocks. The Sr isotopic compositions of euhedral apatite, coupled with those determined through the mineral isochrons and whole-rock data, indicate that SrI varies between samples from 0.7054 to 0.7074. This provides direct evidence of SrI heterogeneity within a compositionally zoned pluton. Such heterogeneity can produce spurious whole-rock age estimates in granitic plutons, even where the true age of a pluton is uniform within measurable error. Assimilation cannot account for chemical differences between granodiorite and adamellite as well as Sr-isotopic heterogeneity. It is likely that the heterogeneity of SrI is caused by magma source materials.

Geochemical and geochronological constraints on the suture location between the North and South China blocks in the Dabie Orogen, Central China

The Dabie orogen is a collision zone between the North China block (NCB) and South China block (SCB), and a famous ultrahigh pressure metamorphic belt in the world. Location of the suture between the NCB and SCB in the Dabie orogen is still a controversial issue. We identify that the metabasaltic volcanic rocks from the Dingyuan Formation of the Beihuaiyang zone located in the northernmost Dabie orogen have typical geochemical feature of island arc basalt, e.g. negative Nb, Ti and P anomalies in trace elemental spidergram and high LREE, Ba and Pb abundance. Their Sm-Nd and Rb-Sr whole-rock isochron ages are 446±23Ma and 444±31Ma, respectively. These ages are consistent with the age of the island arc volcanics in the North Qinling belt that was the south active continental margin of the NCB in the Paleozoic. These data, together with the similarity in metamorphic history between the Dingyuan Formation and North Qinling belt, suggest that the Dingyuan formation is a Paleozoic magmatic arc on the south margin of the NCB. The Nd isotopic model ages of the Devonian metasediments from the Foziling flysch formation of the Beihuaiyang zone (TDM = 1.7 to 1.9Ga) are younger than those of the northern Paleozoic sediment cover of the SCB (TDM = 1.9 to 2.1Ga). It suggests that more Phanerozoic volcanic material, which could be derived from the active continental margin of the NCB, was involved into the flysch formation. The Huwan tectonic melange zone located to the south of the Dingyuan Formation was formed in Triassic. The eclogites involved in this melange zone were formed in the Carboniferous (301 ± 13 Ma). Their protoliths could be middle Paleozoic oceanic crust or late Proterozoic to early Paleozoic island arc basalt. All these data together with the recent discovery of the Triassic eclogites in the Northern Dabie Zone place important constraints on the suture location between the NCB and SCB. It should follow the Huwan tectonic melange zone in the west and the boundary between the Northern Dabie zone and the Beihuaiyang zone in the east.

Mineralogical and geochemical constraints on the genesis of the granite-hosted Huangao uranium deposit, SE China

The Huangao uranium deposit, located in Jiangxi Province, SE China, is hosted mainly by the Indosinian medium- to coarse-grained porphyritic biotite granite as a batholith (Sanerer pluton) and to a lesser extent by the Yanshanian fine-grained porphyritic biotite granite as stock. The Indosinian granite has a whole-rock Rb–Sr isochron age of 215±6 Ma, and the Yanshanian granite has a whole-rock Rb–Sr isochron age of 155.7±6.4 Ma. Both granites may be formed by anatexis of uranium-rich Precambrian and Cambrian metasedimentary rocks. Deuteric alteration of primary accessory minerals (zircon, sphene, apatite, allanite, monazite and xenotime) in the granites, and erosion of the granites under a hot, dry palaeoclimate resulted in the partial liberation of uranium. These processes were critical for the formation of the mesothermal to epithermal U deposit. Uranium in the deposit occurs as pitchblende along tectonic fractures, interstitially in granular minerals, and as cements of breccias. The host-rock alteration comprises pre-ore alkali-metasomatism and ore-stage hydrothermal chloritization, pyritization, hematitization, silicification, damouritization, and carbonatization. Other minerals present in the ore include pyrite, galena, sphalerite, chalcopyrite, hematite, microcrystalline quartz, chlorite, calcite and fluorite. Fluid inclusion studies on quartz, fluorite and calcite reveal that temperatures of the mineralizing fluids range from 110° to 260°C and give salinity values ranging from 11.7 to 21.6 wt.% NaCl equivalent. The δ 18 O and δD values of the fluids range from −4.0 to 3.6 and from −81.3 to −56.6‰, respectively, indicating meteoric water. Mineralogical, fluid inclusion and isotopic data are consistent with a mesothermal to epithermal origin for the Huangao deposit. Thermal activity at the Huangao deposit was maintained from the emplacement of the Indosinian granite, at 215 Ma, until the intrusion of the lamprophyre dike, at 105 Ma. Magmatism in the district as well as radioactive decay of K, U and Th in the granite may have provided an essential or additional heat source for the formation of the hydrothermal solutions. The U-bearing hydrothermal solution was also heated and deep convective circulation was mainly driven by the regional tectonism. This is supported by the U–Pb isochron age of four pitchblende samples at 48 Ma which is in the time range of the late Yanshanian regional tectonism that affected South China.

Sr and Nd isotopic compositions of late Paleozoic Youngju and Andong granites in the northeastern Yeongnam Massif, Korea.

Rb-Sr whole-rock isochron ages and Sr and Nd isotopic ratios have been determined for Youngju and Andong granites in the northeastern Yeongnam Massif, Korea. Six samples of Youngju granites yield an isochron age of 267 ± 27 Ma with an initial Sr ratio of 0.71505 ± 0.00026. Seven samples of Andong granites give an isochron age of 361 ± 41 Ma and an initial Sr ratio of 0.70944 ± 0.00011. The isochron ages indicate that Youngju and Andong granites were emplaced in middle Permian and late Devonian of late Paleozoic, respectively. Calculated εSr (T) and εNd (T) values at 267 Ma for Youngju granites are 152.2∼156.4 and -19.7∼-18.0, and those values at 361 Ma for Andong granites are 73.8∼79.7 and -11.4∼-8.0. Model ages of Youngju and Andong granites are 2.4∼3.3 Ga and 1.4∼1.8 Ga, respectively, suggesting that the Youngju granitic magma was derived from highly LREE-enriched lower crust formed in Late Archean, whereas the Andong granitic magma from slightly LREE-enriched lower crust formed in Proterozoic. Initial 87Sr/86Sr ratios of the granites in Ogcheon Belt and Yeongnam Massif display a broad range, but have two peaks around 0.709 and 0.715 regardless of emplacement age. It is likely that these two kinds of sources which are isotopically different have widely existed in the lower crust of Ogcheon Belt and Yeongnam Massif.

The proto-Andean margin of Gondwana: an introduction

Abstract A basic background is presented for the discussion of the Early Palaeozoic geology of western Argentina covered by this book. This includes the definition and terminology of orogenic cycles on this part of the Gondwana margin, represented by the Eastern Sierras Pampeanas. The Pampean orogeny (Early Cambrian) relates to an intense but short-lived period of terrane collision predating the rifting of the Precordillera terrane from Laurentia. The Famatinian cycle is predominantly represented by intense subductionrelated magmatism of Early-Middle Ordovician age, developed on the continental margin of Gondwana during the rifting and drifting of the Precordillera terrane. The Grenvillian basement of the latter is further exemplified by a new Rb-Sr whole-rock isochron age of 1021 ± 12 Ma for orthogneisses from the Western Sierras Pampeanas. A mid-Ordovician granite in this area (dated at 481 ± 6 Ma by U-Pb ion microprobe data) may be related to rifting while the Precordillera terrane was still attached to Laurentia. A divergence of opinion is pointed out between some authors in this book who favour mid-Ordovician collision of the Precordillera with Gondwana, and others who place it much latter, in Silurian or Devonian times.

The Moorby Microgranite: a deformed high level intrusion of Ordovician age in the concealed Caledonian basement of Lincolnshire

SUMMARY A granophyric microgranite has been proved in the sub-Carboniferous basement at Moorby, near Horncastle in south Lincolnshire. The microgranite is interpreted as a high level intrusion which has suffered strong alteration and deformation subsequent to emplacement, including the development of a spaced pressure-solution foliation and brecciation. The microgranite has yielded a U-Pb zircon age of 457 ± 20 Ma (late Ordovician) interpreted as the age of emplacement. A Rb-Sr whole-rock isochron age of 400 ± 9 Ma (early Devonian) is strongly discordant with the U-Pb age, and is interpreted to reflect thorough post-emplacement hydrothermal disturbance of the Sr isotope system, either associated with the development of the tectonic foliation during the Acadian phase of the Caledonian Orogeny or subsequent uplift. Geochemical data suggest that the affinities of the Moorby Microgranite lie with the arc-related Ordovician intrusions of the Lake District (e.g. the Ennerdale and Eskdale plutons) rather than with the early Devonian high heat production (HHP) granites such as Shap, Skiddaw and Weardale. While the physical properties of the Moorby Microgranite are comparable to those of the HHP granites, gravity modelling suggests that the microgranite is unlikely to be a major component of the inferred Caledonide Wash - north Norfolk Batholith.

Geochemistry and Sr- and Nd-isotopic study on rare-metal-bearing granitic rocks, central Eastern Desert, Egypt

Three post-orogenic granitic plutons exposed in the Egyptian basement complex, namely Homr Akarem, Homrit Mukbid and Nuweibi were selected for geochemical and NdSr isotopic studies. Based on petrographic and geochemical considerations, the granitic rocks are divided into medium-grained pink granites and albite-rich granites. Both types exhibit peraluminous character with many chemical similarities to the high Y/Nb A-subtype granites. Owing to the extreme fractionated nature and complex magma source of these granitic rocks in addition to the high volatile activity, their compositions resemble within-plate magma originated in attenuated crustal lithosphere. Whole-rock RbSr isochron ages of the medium-grained pink granites from Homr Akarem and Homrit Mukbid plutons are 541 ± 4 Ma and 527 ± 26 Ma, respectively. The Nuweibi albite granite (NAG) represents the youngest granite phase and yields a RbSr age of 490 ± 27 Ma. The medium-grained pink granites show a moderate and narrow range of initial 87Sr/ 86Sr ratio (0.7064−0.7081) and negative ϵ t Nd values (−1.31 to −6.68). The NAG has an initial 87Sr/ 86Sr (0.7071) and ϵ t Nd (−3.26 to −4.72), comparable to those of the medium-grained pink granites. The SmNd data yield model ages (T DM) of 0.96 to 1.70 Ga. Similar Nd model ages (1.2−1.8 Ga) have been reported for other post-orogenic granites from southwest Egypt and northern Sudan. The Nd and Sr isotopic compositions of the studied mineralized granites suggest the involvement of old, LREE-enriched continental crust of Pre-Pan-African age and lower crust- or mantle-derived magmas. The Nd and Sr isotopic variations can be explained by a mixing hypothesis.

Nd and Sr isotopic mapping of the Archaean-Proterozoic boundary in southeastern Tanzania using granites as probes for crustal growth

Granitoids of two different generations occur across the Archaean-Proterozoic boundary in southeastern Tanzania. The first generation, which is confined to the Archaean Tanzania Craton, yields RbSr whole-rock isochron ages of 2600 Ma and low, mantle-like, initial 87Sr/ 86Sr ratios (0.702–0.704). These coupled with positive or near-zero ε Nd values (−0.2 to 2.0) suggest that these granitoids represent juvenile mantle material which was added to the continental crust at about 2600 Ma. The second generation of granitoids intrudes the Palaeoproterozoic Usagaran Belt and is characterised by RbSr whole-rock isochron ages of about 1900 Ma. These granitoids show SmNd crustal formation ages which are 200 to 600 Ma older than their emplacement ages and negative ε Nd values (−2.2 to −6.2). Mixing calculations suggest that the Proterozoic granitoids may have formed by partial melting of 2000 Ma (Usagaran) mantle-derived material which had incorporated between 15 and 45% of an Archaean component from the adjacent Tanzania Craton.

Dating the transition of smectite to illite in Palaeozoic mudrocks using the Rb–Sr whole-rock technique

The interpretation of mudrock Rb–Sr whole-rock regression ages using stratigraphically well-constrained samples from the Ordovician and Silurian of central Wales is re-examined. Two new Rb–Sr whole-rock regressions are presented along with K–Ar and Rb–Sr data from the <2 μm illite fraction of samples from these and published whole-rock suites. Eight K–Ar ages from the <2μm fractions give ages which show a normal distribution, with a mean and quadratically combined 2σ error indicating a Devonian age of 399 ± 3 Ma. Rb–Sr whole-rock ages are less consistent, giving 431 ± 10, 428 ± 14, 414 ± 6 and 403 ± 15 Ma. Rb–Sr data from the illite fractions do not generate a linear array. The Rb–Sr whole-rock isochron ages are interpreted as dating the time of passage of the rock through the smectite–illite transition because the radiometric ages correspond well to the estimated time of this reaction. Other explanations of the ages, such as the possibility that they were generated by mixing, or that they date sediment provenance ages, peak metamorphism, or cleavage, are rejected on geological grounds.

Tectonic Interpretation of40Ar/39Ar Ages on Country Rocks from the Central Sector of the Río Negro-Juruena Province, Southwest Amazonian Craton

Five 10Ar/39Ar mineral age determinations were performed on two gneissic rocks in the state of Rondonia, Brazil, in the southwestern part of Amazonian Craton. These samples were taken from the crystalline basement of the Rio Negro-Juruena province, which is characterized by medium- to high-grade metamorphic rocks overprinted by polyphase deformation and retrograde metamorphism. This was intruded later by rapakivi granites and associated mafic rocks, as welt as by late tin-bearing granite phases over the time interval 1.57 to 1.0 Ga. Reconnaissance Rb-Sr studies on the country rocks from Rondonia yielded a whole-rock Rb-Sr isochron age of 1539 ± 36 Ma, which is interpreted as the timing of the regional metamorphism. Granitoid gneisses from the area dated by the SHRIMP U-Pb method on zircons reveal that their crystallization occurred at 1750 ± 17 Ma and 1570 + 17 Ma, The Younger Granites of Rondonia and associated mafic rocks in the area have Rb-Sr and U-Pb dates that range from 1.08 to 1.0 Ga. The new 40Ar...

Constraints on the stratigraphic age of metasedimentary rocks from the Larsemann Hills, East Antarctica: possible implications for Neoproterozoic tectonics

In this study we investigate crystallization ages of detrital zircon from paragneiss from the Larsemann Hills, East Antarctica, using the single zircon PbPb thermal evaporation technique. The 207Pb/ 206Pb ages of eleven detrital zircons range from 1200 ± 6(σ) Ma to 766 ± 12(σ) Ma. Although these ages must be regarded as minimum crystallization ages, investigation of the U content and damage state of the internal crystal structure using fission track analysis suggests that they may approach crystallization ages. This is confirmed by zircons from a granitic orthogneiss (W20401), which show consistent radiogenic Pb ratios released during the final evaporation steps. The 207Pb/ 206Pb age of this orthogneiss is 940 ± 6(σ) Ma, similar to its whole-rock RbSr isochron age of 1024 ± 45(σ) Ma (IR = 0.7241 ± 40). Its SmNd model age ( T DM) is about 2.2 Ga. Further Sm Nd model ages ( T DM) include 2.1 Ga for the paragneiss, and 1.6–1.7 Ga for a 0.55 Ga granite. The presented field and isotopic chronological data suggest that: (1) the 1-Ga event represents emplacement of the orthogneiss protolith, but does not necessarily involve high-grade metamorphism; (2) deposition of the paragneiss protoliths occurred during the Neoproterozoic, which suggests that accretion of the East Antarctic craton had not been completed until the early Palaeozoic; (3) deformation and granulite facies metamorphism in the region can be attributed to the ∼ 0.5-Ga (‘Pan-African’) event.