Rb-Sr Data Research Articles

Rb–Sr and Sm–Nd isotopic study of the Puquı́ complex, Colombian Andes

The Puquı́ Complex is exposed in the northwestern part of the Colombian Central Cordillera and consists of a complex association of metamorphic rocks (amphibolite facies gneisses, migmatites, granulites, and amphibolites) intruded by the Puquı́ Tonalite, a large and mostly undeformed intrusion. The Puquı́ Tonalite is calc-alkaline and displays a mild peraluminous character, whereas the gneissic country rocks are strongly peraluminous (and interpreted as derived from a sedimentary protolith). A peculiar feature of this tonalitic body is the common presence of garnet (up to 7% modal proportion in some localities). The Rb–Sr data for the Puquı́ Gneiss yielded an isochron age of 306±11Ma with a high initial 87Sr/86Sr ratio (ca. 0.70980). This is interpreted as the result of Late Paleozoic metamorphic resetting of the Rb–Sr isotopic system of the sedimentary/metasedimentary protolith. This metamorphic event is also recognized in other parts of the Colombian Andes. Sm–Nd garnet–whole rock ages, however, indicate that these rocks have also been involved in a much younger metamorphic event, between ca. 140 and 120Ma. Sm–Nd model ages reveal a Precambrian age for the source of the original sediments (TDM model ages are in the range 1.33–1.53Ga). The Puquı́ Tonalite was emplaced at ca. 248±17Ma (whole-rock Rb–Sr isochron) with an initial 87Sr/86Sr ratio of ca. 0.70748, which suggests some contribution from older sialic material in the original magma. This is confirmed by the Sm–Nd isotopic data, which show εNd(248) between −5 and −6.15 and TDM model ages between 1.17 and 2.13Ga. These results suggest that the original tonalitic magma is strongly contaminated by the Puquı́ Gneiss. These preliminary Nd–Sr isotopic data suggest the presence of Precambrian crustal material in this part of the Colombian Central Cordillera. We also postulate that the Puquı́ Gneisses derived from Precambrian peraluminous crustal sources served as the contaminant of the tonalitic magma.

Petrology, geochronology, and tectonics of shear zones in the Zermatt–Saas and Combin zones of the Western Alps

AbstractMetre to tens‐of‐metre wide, steeply dipping, greenschist facies shear zones that cut blueschists and eclogites of the Combin and Zermatt–Saas Zones at Täschalp and in adjacent areas of the western Alps were sites of extensive recrystallization driven by fluid flow and deformation. Rb–Sr data imply that these shear zones formed at 42–37 Ma with a systematic younging of structures northward toward, and into, the hangingwall of the Mischabel Structure. Shearing commenced at 400–475 °C and 400–500 MPa and continued as pressures and temperatures fell to 300–350 °C and 300–350 MPa. Individual shear zones were active for 2–3 Myr with later lower grade stages of shearing concentrated into narrow zones. Fluids that infiltrated the shear zones were water rich (XH2O > 0.9). Alteration zones around albite veins and at the margins of serpentinite bodies are penecontemporaneous with these shear zones and formed at approximately the same conditions. The eclogites were exhumed from c. 64 km at 44 Ma to 14–16 km at 42–41 Ma implying exhumation rates of 2–5 cm yr−1. Rapid exhumation was probably achieved by extension aided by buoyancy, following subduction of continental crust, and rapid erosion. The shear zones form part of a regional‐scale extensional system responsible for a significant portion of the exhumation of the subducted oceanic crust.

Post-crystallization reheating and partial melting of eucrite EET90020 by impact into the hot crust of asteroid 4Vesta ∼4.50 Ga ago

We performed petrologic, radiometric (Ar-Ar, Sm-Nd, and Mn-Cr ages), and ion microprobe studies of the basaltic eucrite, EET90020. This is one of the few rare basaltic eucrites whose 39Ar-40Ar age has not been reset during impact bombardment on the HED parent body ≤4 Ga ago and, thus, should provide a unique opportunity to study the nature of the early thermal events on its parent body (presumably asteroid 4Vesta). Hand specimen inspection shows that the rock consists of a fine-grained and a coarse-grained lithology. Microscopy indicates that the fine-grained lithology has a granulitic texture, with a coarser-grained area and a large opaque assemblage embedded in the granulitic matrix. The coarse-grained lithology has an igneous, subophitic texture. The rock has pyroxenes similar to those in type 5 eucrites (type 5 pyroxene) and experienced prolonged thermal metamorphism after rapid crystallization from a near-surface melt. However, minor mineral assemblages are unusual and suggest a complex thermal history. Tridymite occurs as large laths, irregular crystals (<1.5 mm), and in a large assemblage of tridymite-plagioclase-pyroxene about 4 mm in size. One large opaque assemblage (∼1.4 mm in size) is a decomposition product of Cr-ulvöspinel and consists of zoned Ti-chromite, ilmenite and minor Fe-metal. The opaques are often rimmed by Fe-rich olivine and pigeonite with some chemical variations, indicating disequilibrium with the surrounding type 5 pyroxenes. Ca-phosphate with low REE abundances, and the LREE signatures observed in plagioclase suggest that melt was present during metamorphism. This indicates that EET90020 was reheated above the subsolidus temperature of eucrites of ∼1060°C, causing partial melting. Tridymite, Cr-ulvöspinel, and some pyroxene and plagioclase crystallized from the melt. The presence of unequilibrated phases related to opaques suggests cooling rates greater than several °C/day. The reheating event was too short to destroy the exsolution textures of the type 5 pyroxenes. The temperature of the rock just before the reheating could have been ∼870°C, based on the two-pyroxene temperature of the type 5 pyroxenes. The absence of shock effects in plagioclases suggests that EET90020 did not experience shock events >1–5 GPa after the reheating event. EET90020 seems to have experienced the following thermal history; (1) crystallization during rapid cooling near the surface; (2) some brecciation by impact; (3) thermal metamorphism that produced type 5 pyroxene; and (4) short reheating that caused partial melting and rapid cooling. 39Ar-40Ar measurements show a relatively flat pattern and an age of 4.49±0.01 Ga, which is consistent with rapid cooling from high temperature (event 4). Resetting of the Sm-Nd ages at 4.51 ± 0.04 Ga appears to be closely related to the remelting of Ca-phosphates. Rb-Sr data suggest Rb-loss from tridymite during partial melting. The resetting of the Mn-Cr age may have been related to the formation of Cr-ulvöspinels (event 4). We suggest that all these ages were reset by partial melting (event 4). We further suggest that the partial melting event (event 4) that reset the ages ∼4.50 Ga ago was caused by an impact into EET90020 which was part of the hot crust of 4Vesta and resulted in an increase in the temperature from the ambient temperature of ∼ 870°C to above the subsolidus temperature of eucrites of ∼1060°C.

Rb–Sr dating of diagenetic illite in Neoproterozoic shales, Varanger Peninsula, northern Norway

Rb–Sr dating of clay subfractions from three sedimentary formations of Late Riphean to Vendian age from Varanger Peninsula, northern Norway, has helped to refine the geochronology of the Late Neoproterozoic–Cambrian time interval in this region. Eighteen subfractions in six narrow size-ranges from 1–2 μm to &lt;0.1 μm were separated from shales of the Stangenes, Nyborg and Stappogiedde formations. The coarser subfractions include some 2M1 illite as well as the 1M polymorph. The finer subfractions contain only the 1Md illite. After leaching with ammonium acetate, Rb–Sr ‘leachochrons’ for the untreated subfraction, leachate and residue were derived for all subfractions. As the clay particle size decreases from 1–2 to 0.1–0.2 μm: (1) the proportion of chlorite, where present, is reduced; (2) the Crystallinity Index Standard (CIS) increases; (3) the 87Rb/86SR ratio in the residues increases; and (4) the apparent Rb–Sr age decreases. In addition, the data points of the residues are aligned linearly in both the 87Rb/86Sr–87Sr/86Sr and the 1/Sr–87Sr/86Sr diagrams. A similar pattern in the 87Rb/86Sr–87Sr/86Sr coordinates is observed for the data points of leachates. This suggests that mixtures of at least two, non-cogenetic, illite generations are present in all the shales, and that these illites crystallized in environments with dissimilar 87Sr/86Sr ratios. The minimum Rb–Sr ages of early burial diagenesis are c. 650 Ma for the Stangenes, and 560–530 Ma for the Nyborg and Stappogiedde formations. These results indicate that: (1) the age of the Riphean–Vendian boundary is &lt;630 Ma; (2) the age of the Varangerian glaciation on the Varanger Peninsula is bracketed between 630 and 560 Ma; (3) a c. 560 Ma burial diagenesis age for the intra-tillite Nyborg Formation and post-tillite Stappogiedde Formation may result from subsidence and sedimentation associated with the Late Vendian Timanian (Baikalian) deformation in adjacent areas; and (4) the ages of the finer, authigenic illite subfractions range from 440 to 390 Ma and thus appear to reflect phases of Scandian deformation and uplift.

Tertiary age and paleostructural inferences of the eclogitic imprint in the Austroalpine outliers and Zermatt–Saas ophiolite, western Alps

The Austroalpine Sesia–Lanzo inlier and upper Austroalpine Dent Blanche, Mt. Mary and Pillonet outliers occur on top of the western-Alpine orogenic wedge and, as a whole, override the structurally composite ophiolitic Piemonte zone. Instead, the Mt. Emilius, Glacier–Rafray, Etirol–Levaz and other lower Austroalpine eclogitic outliers are inserted within the Piemonte zone, between its upper (Combin) and lower (Zermatt–Saas) tectonic elements, or within the latter. Rb–Sr dating on phengitic micas show that the eclogitic imprint in the lower Austroalpine outliers, conventionally regarded as Late Cretaceous by comparison with the Sesia–Lanzo inlier, is of Eocene age (49–40 Ma), like the underlying Zermatt–Saas ophiolite (45–42 Ma) between the Aosta valley and Gran Paradiso massif. 40Ar–39Ar plateau ages on the same mica concentrates of the ophiolitic Zermatt–Saas nappe (46–43 Ma) are consistent with Rb–Sr dating, whereas that on the Austroalpine Glacier–Rafray klippe (92 Ma) is influenced by argon excess. The lower Austroalpine outliers underwent the subduction metamorphism concurrently with the Zermatt–Saas nappe, 20–25 Ma later than the eclogitic Sesia–Lanzo inlier and blueschist Pillonet klippe. The temporal gap and present intra-ophiolitic position mean that the lower Austroalpine outliers were probably derived from an intraoceanic extensional allochthon (Mt. Emilius domain) stranded inside the Piemonte–Ligurian ocean far from the Dent Blanche–Sesia domain and Adriatic margin.

The DAV and Periadriatic fault systems in the Eastern Alps south of the Tauern window

Alpine deformation of Austroalpine units south of the Tauern window is dominated by two kinematic regimes. Prior to intrusion of the main Periadriatic plutons at ~30 Ma, the shear sense was sinistral in the current orientation, with a minor north-side-up component. Sinistral shearing locally overprints contact metamorphic porphyroblasts and early Periadriatic dykes. Direct Rb–Sr dating of microsampled synkinematic muscovite gave ages in the range 33–30 Ma, whereas pseudotachylyte locally crosscutting the mylonitic foliation gave an interpreted 40Ar–39Ar age of ~46 Ma. The transition from sinistral to dextral (transpressive) kinematics related to the Periadriatic fault occurred rapidly, between solidification of the earlier dykes and of the main plutons. Subsequent brittle–ductile to brittle faults are compatible with N–S to NNW–SSE shortening and orogen-parallel extension. Antithetic Riedel shears are distinguished from the previous sinistral fabric by their fine-grained quartz microstructures, with local pseudotachylyte formation. One such pseudotachylyte from Speikboden gave a 40Ar–39Ar age of 20 Ma, consistent with pseudotachylyte ages related to the Periadriatic fault. The magnitude of dextral offset on the Periadriatic fault cannot be directly estimated. However, the jump in zircon and apatite fission-track ages establishes that the relative vertical displacement was ~4–5 km since 24 Ma, and that movement continued until at least 13 Ma.

Sm–Nd and Rb–Sr dating of amphibolite from the Nellore–Khammam schist belt, SE India: constraints on the collision of the Eastern Ghats terrane and Dharwar–Bastar craton

The Nellore–Khammam schist belt, SE India, is sandwiched in between the Proterozoic Eastern Ghats terrane and the Archaean Dharwar–Bastar craton. We report Sm–Nd and Rb–Sr mineral isochron ages of amphibolite from the schist belt. The Sm–Nd and Rb–Sr ages are 824±43 Ma and 481±16 Ma, respectively. The Sm–Nd age indicates the timing of peak metamorphism, whereas the Rb–Sr age indicates the Pan-African thermal overprint. The peak metamorphism was related to collision of the Eastern Ghats terrane with the Dharwar-Bastar craton, which occurred during early Neoproterozoic time.

Sm–Nd and Sr isotope systematics of scheelite from the giant Au(–W) deposit Muruntau (Uzbekistan): implications for the age and sources of Au mineralization

The application of the Sm–Nd isotope system of scheelite to dating of low-sulfide, quartz-vein hosted Au mineralization is still under discussion. In the present work, new Sm–Nd and Rb–Sr data for scheelite from the giant Muruntau/Myutenbai Au deposit (Kyzylkum, Western Uzbekistan) are discussed. Based on the geological relationship, mineralogical properties, and trace element characteristics, two types of scheelite can be distinguished within the deposit. The first one is represented by early bluish luminescent and weakly coloured scheelite (generation 1) found within strongly deformed flat quartz veins. The apparent isochron defined by this scheelite (351±22 Ma) is interpreted as a mixing line. Typically brownish to orange and yellowish luminescent scheelite from steeply dipping veins (generation 2) defines a Sm–Nd isochron age of 279±18 Ma (eNd=–9.5±0.3; MSWD: 1.5). No evidence for mixing or disturbance by late alteration were found for these scheelites. This Sm–Nd isochron age agrees with the Rb–Sr and K–Ar age range for wall rock alteration in this deposit reported previously. The age of 280 Ma is interpreted to date the high-grade ore formation in the Muruntau deposit. There are currently no reliable age data available on the magmatic events in the Muruntau region. Probably, there is some overlap in time of the Hercynian gold deposition with the intrusion of lamprophyric dykes. The Nd and Sr isotopic signatures of scheelite define the wall rocks (mainly metasiltstones and metasandstones) as the most probable sources for these elements in scheelite.

The Giant Muruntau Gold Deposit: Geologic, Geochronologic, and Fluid Inclusion Constraints on Ore Genesis

We reinterpret the regional geologic setting of the giant Muruntau gold deposit and report new 40 Ar/ 39 Ar isotope age determinations and a laser Raman microprobe analysis of fluid inclusions. New 40 Ar/ 39 Ar isotope age determinations of hydrothermal sericite selvages to gold-stage quartz veins are in excellent agreement with older Rb-Sr dates on auriferous quartz veins. They suggest Triassic sericite formation at 245 and 220 Ma, some 30 m.y. later than subjacent felsic intrusions as defined by Rb-Sr data. These dates call into question the role of magmatic fluid, metal, and heat input. Indeed, the Rb-Sr pluton ages may be a reflection of hydrothermal activity rather than cooling from magmatic conditions. The Muruntau deposit is situated adjacent to a major rift, which has been periodically active from the Devonian to the Tertiary and has accumulated a substantial thickness of chemical, volcaniclastic, and clastic sediments, including red beds and evaporites. We infer that the Muruntau deposit is so large because of the presence of a major source of reduced sulfur in these nearby evaporitic sediments, via thermochemical sulfate reduction. Some support for the role of reduced sulfur is provided by the presence of detectable H2S in fluid inclusions from late-stage auriferous quartz veins. Reduced sulfur-rich fluids could have been focused into the depositional site along major northeast-trending structures, which have influenced rift-basin architecture as recently as the Jurassic. Oxidation could, therefore, be an important ore-precipitating mechanism. A change from early CH4- to later syngold CO2dominant fluid inclusions may be an indication of this process.

Isotopic constraints on the genesis and age of autochthonous glaucony in the Oligo‐Miocene Torquay Group, south‐eastern Australia

The Oligo‐Miocene Torquay Group at Bird Rock in south‐eastern Australia comprises a sequence of fine‐grained skeletal carbonates and argillaceous and glauconitic sandstones, deposited in a cool‐water, mid‐shelf environment. The Bird Rock glaucony is autochthonous and consists predominantly of randomly interstratified glauconitic smectite, which constitutes bioclast infills and faecal pellet replacements. The results of Rb–Sr and oxygen isotopic analysis of samples taken from a single glauconitic horizon (the BW horizon) indicate that the glaucony developed through a series of simultaneous dissolution–crystallization reactions, which occurred during very early diagenesis in a closed or isochemical system, isolated from the ambient marine environment. The constituent ions of the glaucony were derived primarily from terrigenous clay minerals, but considerable potassium may have been sourced indirectly from sea water, through potassium enrichment of clay precursors. The pore fluids associated with glauconitization were marine derived, but progressively modified by the dissolution–crystallization of detrital clay minerals and autochthonous glaucony. Rb–Sr data for the BW horizon indicate that dating glauconies may be somewhat problematic, as co‐genetic glauconitic minerals can show a range of initial strontium compositions, which reflect the incorporation of strontium derived from mineralogical precursors and/or contemporaneous sea water. Rb–Sr isochrons indicate that the glaucony of the BW horizon formed at 23 ± 3 Ma. This age is in good agreement with both the established biostratigraphy and a 87Sr/86Sr age for the horizon (23 ± 1 Ma), but could only be determined using the independent age constraint and the estimate of the 87Sr/86Sr ratio of contemporaneous sea water provided by analysis of associated biogenic carbonate.

Rb-Sr, Sm-Nd, and Pb isotope systematics of pyrite: Implications for the age and genesis of lode gold deposits

Lode gold deposits commonly consist of pyrite and lesser amounts of lead and zinc sulfides accompanied by quartz and calcite. Their exact origin remains controversial mainly because of the scarcity of reliable geochronological data. Here we present direct Rb-Sr dating of pyrite from a lode gold deposit and propose the method as a useful geochronological technique for gold mineralization for which age data are poor. A positive correlation between present-day 87 Sr/ 86 Sr and 87 Rb/ 86 Sr ratios of pyrite (FeS 2 ) subsamples from the Linglong gold mine of China corresponds to an age of 122–123 Ma, which dates the age of gold mineralization. The Sr, Nd, and Pb isotopic compositions of pyrites and the associated rocks suggest that the hydrothermal fluids responsible for the pyrite and gold mineralization were probably derived from a mixed source (i.e., degassing of mafic magmas and meteoric water that had leached the country rocks).

U-Pb zircon ages from a craton-margin archaean orogenic belt in northern Zimbabwe

In northern Zimbabwe, the Archaean Pfunzi Orogen comprises an east-west-trending belt of migmatitic gneisses separating the Zimbabwe Craton from the Pan-African Zambezi Belt farther north. Previously available Rb-Sr dates for the Pfunzi Orogenic Belt have been interpreted to record formation of granulites at ca 3.0 Ga and subsequent amphibolite-facies metamorphism at ca 2.6 Ga. Here the first conventional IDTIMS U-Pb zircon dates for the Pfunzi Belt are reported, from orthogneisses in the eastern part of the belt in northeastern Zimbabwe. Two intrusive masses of granitic and tonalitic gneiss, together with a granitic leucogneiss inferred to have formed by leucosome segregation during migmatisation, yield crystallisation ages of ca 2.62 Ga. These results are interpreted to date a major tectonothermal event along the northern margin of the Zimbabwe Craton, involving regional amphibolite-facies metamorphism and migmatisation, as well as emplacement of compositionally diverse granitoid plutons and retrogression of older granulites. This event is roughly coeval with orogenesis in the Limpopo Belt along the southern craton margin and with emplacement of the craton-wide ca 2.6 Ga Chilimanzi Granite Suite.

IDADE DO MAGMATISMO ALCALINO DE PARIQUERA AÇU (SP)

This paper presents Rb-Sr isotopic data concerning the understanding of the origin, age and emplacement of the Pariqüela Açu massif, a small ocurrence (3,5 x 2,5 km) of mafic-ultramafic alkaline rocks discovered by aeromagnetic interpretation in 1979. The massif is situated in São Paulo - southeastem Brazil - near the Jacupiranga and Juquiá alkaline complexes in the low Ribeira de Iguape river valley. Rb-Sr data and the K/Ar available ages in the literature of nearby complexes are discusssed. It has also shown and discussed the characteristic aeromagnetic pattern of Pariqüera Açu body and related intrusives, as well as its geology, rock types, distribution and age and its relationships to the nearby complexes.

The Rb-Sr isochron of ore and pyrite sub-samples from Linglong gold deposit, Jiaodong Peninsula, eastern China and their geological significance

Here we report the first direct Rb-Sr dating of pyrites and ores using sub-sampling from lode gold deposits in Linglong, Jiaodong Peninsula, which is a supra-large lode gold deposit and propose this as a useful geochronological technique for gold mineralization with poor age constraint. The Rb-Sr data of pyrites yield an isochron age of (121.6±8.1) Ma, whereas those of ore and ore-pyrite spread in two ranges from 120.0 to 121.8 Ma and from 110.0 to 111.7 Ma. Studies of characteristic of gold deposit and microscopy of pyrite and quartz indicate that the apparent ages of ore and ore-pyrite are not isochron ages, only mixed by two end members, i.e. the primitive hydrothermal fluids and wall rocks, whereas the isochron age of pyrite sub-samples constrains the age of gold mineralization (121.6±8.1) Ma, i.e. early Cretaceous, which is in good agreement with the published SHRIMP zircon U-Pb ages.

K–Ar and Rb–Sr dating of authigenic illite–smectite in Late Permian coal measures, Queensland, Australia: implication for thermal history

K–Ar and Rb–Sr isotopic studies were carried out on authigenic illitic clay minerals in Late Permian coal measures from the Bowen Basin (Australia), in order to determine the timing of maximum paleotemperatures, which were responsible for coal maturation and coal seam gas generation. The results indicate two major thermal events affected the Bowen Basin region, at 205–215 and 140–155 Ma. The narrow range of age data from different size fractions, lithologies and stratigraphic depths indicates episodic, short-lived thermal events, rather than gradual temperature increase due to progressive burial as previously believed. The earlier, thermal event in the latest Triassic postdates maximum burial of the Bowen Basin strata, which occurred during the Middle to Late Triassic. The ages of 205–215 Ma correspond with the published evidence for regional Late Triassic extension in eastern Australia. The younger (140–155 Ma), thermal phase during the Late Jurassic–Early Cretaceous is related to the initial rifting and associated widespread igneous activities prior to break-up of Gondwana. The K–Ar dates in conjunction with vitrinite reflectance data as paleotemperature indicators indicate that the younger thermal event occurred at lower temperatures than the earlier one, except in the shallow part of the southern Bowen Basin. This event is recorded exclusively in less illitic, R=0 illite–smectite (I–S) mixed-layer samples mainly in the shallow part of the southern Bowen Basin (Baralaba Coal Measures).

Rb–Sr microchrons of synkinematic mica in mylonites: an example from the DAV fault of the Eastern Alps

A technique for texturally controlled microsampling from rock thick sections and subsequent Rb–Sr dating of μg-sized samples has been developed. It allows direct dating of minerals unequivocally related to deformation. The technique has been applied to strictly synkinematically grown white mica prepared from mylonites related to the Defereggen–Antholz–Vals (DAV) fault south of the Tauern window (Eastern Alps). Control of deformation ages is provided by field relationships with independently dated plutons and dykes dated here by single mineral Rb–Sr. The field observations and isotopic ages are fully consistent and demonstrate that this is a reliable method for sub-mm-scale geochronology. The ages obtained establish that sinistral strike slip shearing associated with the DAV fault lasted from at least 33 Ma to 30 Ma, followed by an abrupt change to dextral transpression related to the adjacent Periadriatic fault.

Geochronological constraints on the timing of granitoid magmatism, metamorphism and post‐metamorphic cooling in the Hercynian crustal cross‐section of Calabria

Exposed cross‐sections of the continental crust are a unique geological situation for crustal evolution studies, providing the possibility of deciphering the time relationships between magmatic and metamorphic events at all levels of the crust. In the cross‐section of southern and northern Calabria, U–Pb, Rb–Sr and K–Ar mineral ages of granulite facies metapelitic migmatites, peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late‐Hercynian peak metamorphism and granitoid magmatism as well as on the post‐metamorphic cooling. Monazite from upper crustal amphibolite facies paragneisses from southern Calabria yields similar U–Pb ages (295–293±4 Ma) to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust (300±10 Ma). Monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303–302±0.6 Ma. Zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290±2 Ma, which may be interpreted as the minimum age for metamorphism or intrusion. U–Pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern Calabria (Sila) also point to a near‐synchronism of peak metamorphism and intrusion at 304–300±0.4 Ma. At the end of the granulite facies metamorphism, the lower crustal rocks were uplifted into mid‐crustal levels (10–15 km) followed by nearly isobaric slow cooling (c. 3 °C Ma−1) as indicated by muscovite and biotite K–Ar and Rb–Sr data between 210±4 and 123±1 Ma. The thermal history is therefore similar to that of the lower crust of southern Calabria. In combination with previous petrological studies addressing metamorphic textures and P–T conditions of rocks from all crustal levels, the new geochronological results are used to suggest that the thermal evolution and heat distribution in the Calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late‐Hercynian orogeny.

Domainal variations of U–Pb monazite ages and Rb–Sr whole‐rock dates in polymetamorphic paragneisses (KTB Drill Core, Germany): influence of strain and deformation mechanisms on isotope systems

Polyphase metamorphic paragneisses from the drill core of the continental deep drilling project (KTB; NW Bohemian Massif) are characterized by peak pressures of about 8 kbar (medium‐P metamorphism) followed by strain accumulation at T &gt;650 °C, initially by dislocation creep and subsequently by diffusion creep. U–Pb monazite ages and Rb–Sr whole‐rock data vary in the dm‐scale, indicating Ordovician and Mid‐Devonian metamorphic events. Such age variations are closely interconnected with dm‐scale domainal variations of microfabrics that indicate different predominant deformation mechanisms. U–Pb monazite age variations dependent on microfabric domains exceed grain‐size‐dependent age variations. In ‘mylonitic domains’ recording high magnitudes of plastic strain, dislocation creep and minor static annealing, monazite yields concordant and near concordant Lower Ordovician U–Pb ages, and the Rb–Sr whole‐rock system shows isotopic disequilibrium at an mm‐scale. In ‘mineral growth/mobilisate domains’, in which diffusive mass transfer was a major strain‐producing mechanism promoting diffusion creep of quartz and feldspar, and in which static recrystallization (annealing) reduced the internal free energy of the strained mineral aggregates, concordant U–Pb ages are Mid‐Devonian. Locally, in such domains, Rb–Sr dates among mm3‐sized whole‐rock slabs reflect post‐Ordovician resetting. In ‘transitional domains’, the U–Pb‐ages are discordant. We conclude that medium‐P metamorphism occurred at 484±2 Ma, and a second metamorphic event at 380–370 Ma (Mid‐Devonian) caused progressive strain in the rocks. Dislocation creep at high rates, even at high temperatures, does not reset the Rb–Sr whole‐rock system, while diffusion creep at low rates and stresses (i.e. low ε/Deff ratios), static annealing and the presence of intergranular fluids locally assist resetting. At temperatures above 650 °C, diffusive Pb loss did not reset Ordovician U–Pb monazite ages, and in domains of overall high imposed strain rates and stresses, resetting was not assisted by dynamic recrystallization/crystal plasticity. However, during diffusion creep at low rates, Pb loss by dissolution and precipitation (‘recrystallization’) of monazite produces discordance and Devonian‐concordant U–Pb monazite ages. Hence, resetting of these isotope systems reflects neither changes of temperature nor, directly, the presence or absence of strain.

Dust production and deposition in Asia and the north Pacific Ocean over the past 12 Myr

The silicate fractions of recent pelagic sediments in the central north Pacific Ocean are dominated by eolian dust derived from central Asia. An 11 Myr sedimentary record at ODP Sites 885/886 at 44.7°N, 168.3°W allows the evaluation of how such dust and its sources have changed in response to late Cenozoic climate and tectonics. The extracted eolian fraction contains variable amounts (>70%) of clay minerals with subordinate quartz and plagioclase. Uniform Nd isotopic compositions ( ϵ Nd=−8.6 to −10.5) and Sm/Nd ratios (0.170–0.192) for most of the 11 Myr record demonstrate a well-mixed provenance in the basins north of the Tibetan Plateau and the Gobi Desert that was a source of dust long before the oldest preserved Asian loess formed. ϵ Nd values of up to −6.5 for samples <2.9 Ma indicate ≤35 wt% admixture of a young, Kamchatka-like volcanic arc component. The coherence of Pb and Nd in the erosional cycle allows us to constrain the Pb isotopic composition of Asian loess devoid of anthropogenic contamination to 206Pb/ 204Pb=18.97±0.06, 207Pb/ 204Pb=15.67±0.02, 208Pb/ 204Pb=39.19±0.11. 87Sr/ 86Sr (0.711–0.721) and Rb/Sr ratios (0.39–1.1) vary with dust mineralogy and provide an age indication of ∼250 Ma. 40Ar/ 39Ar ages of six dust samples are uniform around 200 Ma and match the K–Ar ages of modern dust deposited on Hawaii. These data reflect the weighted age average of illite formation. Changes from illite≥smectite with significant kaolinite to illite- and chlorite-rich, kaolinite-free assemblages since the late Pliocene document changes in the intensity of chemical weathering in the source region. Such weathering evidently did not disturb the K–Ar systematics, and only induced scatter in the Rb–Sr data. We propose that when smectite forms at the expense of illite, K and Ar are quantitatively lost from what becomes smectite, but are quantitatively retained in adjacent illite layers. 40Ar/ 39Ar age data, therefore, are insensitive to smectite formation during chemical weathering but date the diagenetic growth of illite, the major K-bearing phase in the dust. Over the past 12 Myr, the dust flux to the north Pacific increased by more than an order of magnitude, documenting a substantial drying of central Asia. This climatic change, however, did not alter the ultimate source of the dust, and neoformational products of chemical weathering always remained subordinate to assemblages reworked by mechanical erosion in dust deposited in eastern Asia and the Pacific Ocean.

Direct Isotope Dating of W(−Y) Mineralization at Kyzyltau (Mongolian Altai): Preliminary Results

Absolute ages of granite magmatism, as well as of tungsten mineralization, are poorly constrained in the Mongolian Altai and adjacent areas. There are no reports focusing on special isotopic investigations of the tungsten deposits. For the deposits in the Achit nuur and the Zagaan-Shibetin tectonic zones, two concepts that are discussed in the literature assume Paleozoic or Mesozoic ages for mineralization and related granite magmatism. We report the first results of a combined Sm-Nd and Rb-Sr isotope investigation of rocks and vein minerals of the Ulaan uul tungsten deposit at Kyzyltau; the results suggest Paleozoic ages for vein mineralization and for albitization of the host granite. The Sm-Nd isotope system, and the structure of vein minerals used for isotopic dating, were only slightly affected by late alteration processes. Sm-Nd mineral isochrons for wolframite and fluorite from the veins define an age of 303 ± 17 Ma (MSWD = 1.8, ϵNd = +0.9 ± 0.2). The Rb-Sr isotope system of the vein-hosting granite was strongly influenced by alteration processes. The Rb-Sr whole-rock isochron (282.2 ± 2.4 Ma, Sri = 0.70667 ± 0.00032, MSWD = 0.53) is interpreted as a mixing line and the age so defined has no direct geological meaning. Nevertheless, using Rb-Sr model ages and data on the degree of alteration of the samples, an age somewhat below 316 Ma can be estimated for albitization of the vein-hosting granite. This age estimation is in good agreement with the Sm-Nd isochron age for the vein mineralization. High ϵNd values obtained for wolframite and fluorite from the ore veins indicate an important role for material derived from the upper mantle in the ore formation processes.