Rb-Sr Analyses Research Articles

Metamorphic P–T conditions, in‐situ LA–ICP–MS biotite–muscovite Rb–Sr geochronology and tectonic implications of two‐mica–quartz schist from the Kuanping Group in the North Qinling Orogenic Belt

The Kuanping Group is an important lithotectonic unit in the North Qinling Tectonic Belt, which has experienced metamorphism from greenschist to high amphibolite facies and is of great significance in defining the orogenic process of the Qinling Orogenic Belt. This study presents the results of petrology, geothermobarometry, phase equilibrium modelling, in‐situ laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) biotite–muscovite Rb–Sr analysis, for the two‐mica–quartz schist of the Kuanping Group in the Hongtuling and Heihe areas. These results are employed to better understand their metamorphic P–T conditions and ages, as well as geological implications. Garnet‐bearing two‐mica–quartz schist in the Hongtuling area records a clockwise P–T trajectory on the basis of phase equilibrium modelling, with peak P–T conditions of ~600°C and ~0.85 GPa. In addition, the Ti‐in‐biotite thermometry yielded metamorphic temperature conditions of ~578°C–580°C and ~590°C–596°C for the garnet‐bearing two‐mica–quartz schist from the Hongtuling area and the two‐mica–quartz schist from the Heihe area, respectively. These results indicate that metamorphic temperature conditions in both areas are higher than the closure temperature of the Rb–Sr isotopic system of muscovite and biotite. In‐situ LA–ICP–MS biotite and muscovite Rb–Sr analyses yielded three groups of isochron age, i.e., ca. 410 Ma, ca. 360 Ma and ca. 320–300 Ma. On the basis of previously published data and the data in this study, the Kuanping Group underwent ca. 440 Ma regional metamorphism in the NQB, retrograde cooling at ca. 410–360 Ma due to exhumation and ca. 320–300 Ma tectonothermal overprinting.

A comparison of 87Rb/87Sr and 40Ar/39Ar dates: Evaluating the problem of excess 40Ar in Himalayan mica

The problem of excess 40Ar (ArE) affecting 40Ar/39Ar dates, particularly in biotite, is a long-standing issue in Himalayan geology. The development of in situ Rb-Sr dating presents a convenient alternative method to extract geochronological data from the same material and avoids potential ArE complications. Herein we present a comparison between previously published 40Ar/39Ar data, including many interpreted to reflect ArE, and new in situ Rb-Sr analyses for the same specimens. This work demonstrates that Rb-Sr mica dates from across the exhumed Himalayan metamorphic core typically overlap 40Ar/39Ar dates from the same rocks that are unaffected by ArE. Where 40Ar/39Ar dates have been interpreted to reflect ArE, the Rb-Sr method invariably yields dates that are younger than the ArE date and consistent with other mineral chronometer systems. Finally, while further refinement is required, we propose a technique to extract spot ages for high Rb/low Sr biotite analyses, by correcting for common 87Sr based on the present known ratio of 87Sr/88Sr, removing the need to assume an initial 87Sr/86Sr.

Late Neoproterozoic-to-recent long-term t–T-evolution of the Kaoko and Damara belts in NW Namibia

This research aims to reconstruct the Late Neoproterozoic-to-recent long-term time–temperature-evolution of the NW Namibian Kaoko and Damara belts combining numerical modeling of new thermochronological data with previously published geochronological data, i.e., U–Pb, Sm–Nd, and Rb–Sr analyses, and K/Ar, 40Ar/39Ar low-temperature thermochronology. Consequently, we retrieve a coherent long-term time–temperature-evolution of the NW Namibian Neoproterozoic basement rocks including rates of exhumation and subsidence periods over the last ~ 500 Myr. Neoproterozoic basement rocks indicate fast post-Pan African/Brasiliano cooling and exhumation, reheating, or rather subsidence during the development of the Paleozoic-to-Mesozoic SW Gondwana intraplate environment and a significant thermal overprint of the rocks during South Atlantic syn- to post-rift processes, and therefore, resemble the opponent SE Brazilian time–temperature-evolution. We provide an overview of thermochronological data including new apatite and zircon fission-track data derived from Neoproterozoic, Late Paleozoic, and Lower Cretaceous rocks. Apatite fission-track ages range from 390.9 ± 17.9 Ma to 80.8 ± 6.0 Ma in the NW Kaoko Belt with youngest ages confined to the coastal area and significant age increase towards the inland. New zircon apatite fission-track data reveal ages between 429.5 ± 47.8 and 313.9 ± 53.4 Ma for the rocks of the Kaoko Belt. In the central Damara Belt, new apatite fission-track ages range between 138.5 ± 25.3 Ma to 63.8 ± 4.8 Ma. Combined apatite fission-track age distributions from Angola to Namibia and SE Brazil correlate for both sides of the South Atlantic passive continental margin and the reset AFT ages overlap with the lateral Parana–Etendeka dike swarm distribution.

Isotope Systematics and the Age of Authigenic Minerals in Shales of the Inzer Formation, the Southern Urals

The 2–5, 0.6–2, 0.3–0.6, 0.2–0.3 and 0.1–0.2 µm clay subfractions (SFs) separated from two shale samples of the Upper Riphean Inzer Formation, the southern Urals, were studied by the TEM, XRD, and U–Pb, Sm–Nd, Rb–Sr and K–Ar isotopic methods. All the SFs consist of the low-temperature 1Md illite; admixtures of quartz, chlorite and 2M1 illite occur only in the coarsest SFs. The clay particles are isometric, regardless of their size. The CIS (Crystallinity Index Standard) illite values for the all SFs are typical for the dia(kata)genetic zone. As the size of particles in the SF decreases from 2–5 to 0.1–0.2 µm, the CIS rises, the I002/I001 ratio on the XRD diagrams decreases, and the K content and the K/Rb ratio increase.
 Leaching with 1N HCl and 1N ammonium acetate (NH4OAc) and subsequent U–Pb, Sm–Nd and Rb–Sr analyses of the untreated SF, acid (acetate) leachate and residue made possible to study the mixing systematics in mobile and silicate materials of the shales. The 238U/204Pb and 87Rb/86Sr ratios in the acid and acetate leachates are below, and the 147Sm/144Nd ratio is above those in the residues. Less radiogenic Pb and Sr and more radiogenic Nd are also common for the leachates compared to the residues. As the size of particles in the SFs decreases, the U, Pb, Sm, Nd and Sr contents in the residues are smoothly reduced, whereas the Rb content shows an increase. The 87Rb/86Sr and 87Sr/86Sr values in the residues for fine-grained SFs are well above, and the 238U/204Pb value is well below those for coarse-grained SFs. What is more, in the 87Rb/86Sr–87Sr/86Sr and 1/86Sr–87Sr/86Sr diagrams, data points for the residues of variable size are arranged on the mixing lines. The data points of respective acid and acetate leachates also form linear trends in the 238U/204Pb–206Pb/204Pb, 206Pb/204Pb–207Pb/204Pb, 147Sm/144Nd–143Nd/144Nd, and 87Rb/86Sr–87Sr/86Sr coordinates. The apparent Rb–Sr age values, calculated from the slopes of “inner isochrons” (“leachochrons”), along with the K–Ar ages are smoothly lowered from 835–836 and 721–773 m.y. for the 2–5 µm SF to 572–580 and 555–580 m.y. for the 0.1–0.2 µm SF. Hence the XRD and isotopic data testify that the clay as well as the mobile material of the shale represent the mixtures of at least two components, the silicate phase containing authigenic illites of different ages. The first illite generation enriched in coarse-grained 2–5 and 0.6–2 µm SFs was formed shortly after deposition of the Inzer sediments, and its age of 803–836 m.y. is in agreement with the stratigraphic age of the formation. Simple lithostatic burial or intensive lateral fluid flow induced by tectonic inversion in the eastern regions of the Urals paleobasin may be considered as the geological processes responsible for the forming of this illite. The second illite generation was formed 572–580 m.y. ago. As the starting points for its formation, alternatively, may be concerned either vertical tectonics or renewal of pore fluid compositions during deformations and metamorphism on the southern Urals area related to evolution of the Beloretsk metamorphic complex.

Multi-chronometer thermochronological modelling of the Late Neoproterozoic to recent t-T-evolution of the SE coastal region of Brazil

South-eastern Brazil is as an important geological archive for understanding and reconstructing various plate tectonic stages of the Wilson Cycle. In the Neoproterozoic, the area of the today's South Atlantic passive continental margin (SAPCM: e.g. between São Paulo and Laguna) of south-eastern Brazil underwent subduction, followed by the collision of the contemporary plates of South America and Africa creating a Neoproterozoic orogeny within the supercontinent Gondwana. During the Palaeozoic and Lower Mesozoic (stage 1), the future SAPCM, as an intracratonic area, experienced erosion, denudation of the Neoproterozoic mobile belts (Pan African/Brasiliano orogeny), and large basin formation (Paraná Basin) (stage 2). Possibly plume-driven pre-to syn-rift (embryonic), ocean spreading (juvenile), and post-break up (mature) processes led to the recent evolution of the SAPCM since the Upper Mesozoic (stage 3).For the first time, this research aims to reconstruct the syn-to post-orogenic t-T-evolution of Neoproterozoic basement rocks of the SE coastal region of Brazil covering the entire geological evolution since the Late Neoproterozoic. Therefore, this study uses geochronological and thermochronological data combined with numerical modelling. This includes published geochronological data of Neoproterozoic basement samples such as U-Pb, Sm-Nd and Rb-Sr analyses, and low temperature thermochronology (LTT) data revealed by K/Ar, 40Ar/39Ar analyses. To this existing LTT data set, we report new apatite (AFT) and zircon (ZFT) fission-track, and (U-Th-Sm)/He (AHe, ZHe) data. Numerical modelling of that LTT data attached to the existing geochronological data indicates the following evolution:- Stage 1: In the central part of the future SAPCM, the Pan African/Brasiliano post-orogenic cooling and exhumation (uplift and erosion of Neoproterozoic rocks to the surface) history occurs in three phases: (i) rapid Late Neoproterozoic exhumation, (ii) a period of relative thermal stability (temperatures of about 200–300 °C) in which rocks reside at upper crust levels during the Early Cambrian to Devonian, and (iii) a second rapid exhumation phase moving the Neoproterozoic basement rocks to the surface during the Devonian. The northern and southern parts indicate a distinct post-orogenic exhumation suggesting faster cooling and exhumation from the Late Neoproterozoic to Devonian/Carboniferous than in the central section.- Stage 2: A phase of subsidence leading to the formation of the Paraná Basin followed by pre-to syn-rift processes and the emplacement of the Paraná-Etendeka flood basalts.- Stage 3: Post-South Atlantic break up processes, such as erosion and exhumation.

Rb–Sr Dating of Gold‐Bearing Pyrite in the Jinchang Porphyry Cu–Au Deposit, Heilongjiang Province, NE China

AbstractThe Jinchang Cu–Au deposit in Heilongjiang Province, NE China, is located in the easternmost part of the Central Asian Orogenic Belt. Rb–Sr analyses of auriferous pyrite from the deposit yielded an isochron age of 113.7 ±2.5 Ma, consistent with previously reported Re–Os ages. Both sets of ages represent the timing of Cu–Au mineralization because (i) the pyrite was separated from quartz–sulfide veins of the mineralization stage in granite porphyry; (ii) fluid inclusions have relatively high Rb, Sr, and Os content, allowing precise measurement; (iii) there are no other mineral inclusions or secondary fluids in pyrite to disturb the Rb–Sr or Re–Os decay systems; and (iv) the closure temperatures of the two decay systems are ≥500°C (compared with the homogenization temperatures of fluid inclusions of 230–510°C). It is proposed that ore‐forming components were derived from mantle–crust mixing, with ore‐forming fluids being mainly exsolved from magmas with minor amounts of meteoric water. The age of mineralization at Jinchang and in the adjacent regions, combined with the tectonic evolution of the northeast China epicontinental region, indicates that the formation of the Jinchang porphyry Cu–Au deposit was associated with Early Cretaceous subduction of the paleo‐Pacific Plate.

The geological significance of 40Ar/39Ar and Rb–Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re)crystallization during exhumation?

AbstractThe Attic‐Cycladic crystalline belt in the central Aegean region records a complex structural and metamorphic evolution that documents Cenozoic subduction zone processes and exhumation. A prerequisite to develop an improved tectono‐metamorphic understanding of this area is dating of distinct P–T–D stages. To evaluate the geological significance of phengite ages of variably overprinted rocks, 40Ar/39Ar and Rb–Sr analyses were undertaken on transitional blueschist–greenschist and greenschist facies samples from the islands of Syros and Sifnos. White mica geochronology indicates a large age variability (40Ar/39Ar: 41–27 Ma; Rb–Sr: 34–20 Ma). Petrologically similar samples have either experienced greenschist facies overprinting at different times or variations in ages record variable degrees of greenschist facies retrogression and incomplete resetting of isotopic systematics. The 40Ar/39Ar and Rb–Sr data for metamorphic rocks from both islands record only minor, localized evidence for Miocene ages (c. 21 Ma) that are well documented elsewhere in the Cyclades and interpreted to result from retrogression of high‐pressure mineral assemblages during lower pressure metamorphism. Field and textural evidence suggests that heterogeneous overprinting may be due to a lack of permeability and/or limited availability of fluids in some bulk compositions and that retrogression was more or less parallel to lithological layering and/or foliation as a result of, possibly deformation‐enhanced, channelized fluid ingress. Published and new 40Ar/39Ar and Rb–Sr data for both islands indicate apparent age variations that can be broadly linked to mineral assemblages documenting transitional blueschist‐to‐greenschist‐ and/or greenschist facies metamorphism. The data do not record the timing of peak HP metamorphism, but may accurately record continuous (partial) resetting of isotopic systematics and/or (re)crystallization of white mica during exhumation and greenschist facies retrogression. The form of 40Ar/39Ar phengite age spectra are complex with the lowest temperature steps yielding Middle to Late Miocene ages. The youngest Rb–Sr ages suggest maximum ages of 20.6 ± 0.8 Ma (Syros) and 22.5 ± 0.6 Ma (Sifnos) for the timing of greenschist facies overprinting. The results of this study further accentuate the challenges of interpreting isotopic data for white mica from polymetamorphic terranes, particularly when mixing of populations and/or incomplete resetting of isotopic systematics occurs during exhumation. These data capture the full range of isotopic age variations in retrogressed HP rocks documented in previous isotopic studies, and can be interpreted in terms of the geodynamic evolution of the Aegean.

Petrology and geochronology of ‘muscovite age standard’ B4M

Abstract Muscovite B4M, distributed in 1961 as an age standard, was ground under ethanol. Five grain size fractions were obtained and characterized by X-ray diffraction. They display a mixing trend between a phengitic (enriched in the fraction <0.2 µm) and a muscovitic component (predominant in the fraction >20 µm). High-pressure phengite is preserved as a relict in retrograde muscovite. Electron microprobe analyses of the distributed mineral separate reveal at least four white mica populations based on Si, Al, Mg, Na, Fe and F. Rb/K ratios vary by one order of magnitude. Rb–Sr analyses link the mineralogical heterogeneity to variable Rb/Sr and 87 Sr/ 86 Sr ratios. The grain size fractions define no internal isochron. Relict fine-grained phengite gives older ages than coarse-grained retrograde greenschist facies muscovite. The inverse grain size–age relationship also characterizes 39 Ar/ 40 Ar analyses. Cl/K anticorrelates with step ages: Cl-rich coarse muscovite is younger than Cl-poor fine relict phengite. Sr and Ar preserve a similar isotopic inheritance despite peak metamorphism reaching 635±20 °C. A suitable mineral standard requires that its petrological equilibrium first be demonstrated. Relicts and retrograde reaction textures are a guarantee of isotopic disequilibrium and heterogeneous ages within single crystal at the micrometre scale. Supplementary material: Electron microprobe results on two grain mounts of the unprocessed B4M separate as distributed and on a whole-rock thin section of Brione gneiss from the teaching collection of the Universität Bern are available at: www.geolsoc.org.uk/SUP18590 .

Bracketing the Age of Magmatic-Hydrothermal Activity at the Cerro de Pasco Epithermal Polymetallic Deposit, Central Peru: A U-Pb and 40Ar/39Ar Study

The Cerro de Pasco epithermal Cordilleran base metal deposit in central Peru consists of two mineralization stages which are spatially and genetically related to a mid-Miocene diatreme-dome complex. In order to constrain the duration of the magmatic-hydrothermal system at Cerro de Pasco, U-Ph age determinations and Hf isotope determinations have been conducted on single zircon grains. Biotite from intrusive rocks, and sericite and alunite related to the first and second mineralization stages, respectively, have been used for stepwise IR-CO2 laser Ar-40/Ar-39 age determinations. This study has been complemented by Rb-Sr analyses of sericite and associated pyrite. Single crystal zircon U-Pb analyses yield ages of 15.36 +/- 0.03 Ma for an accretionary lapilli tuff from a collapsed block within the diatreme, 15.40 +/- 0.07 Ma for it dacite porphyry dome in the diatreme, 15.35 +/- 0.05 and 15.16 +/- 0.04 Ma for two quartz monzonite porphyry dikes emplaced into the diatreme-dome complex that are within a time span of 350,000 yr. There is no field evidence for any magmatic activity predating the formation of the diatreme-dome complex. Stepwise IR-CO2 laser Ar-40/Ar-39 ages have been determined on sericite and alunite. The latter formed during the second mineralization stage in advanced argillically altered domes and from halos of oxidized enargite-pyrite veins in the Venecocha and Santa Rosa area, located on the northwestern margin of the diatreme-dome complex and on the southern part of the Cerro de Pasco open pit, respectively, Ar-40/Ar-39 alunite ages cluster between 14.54 +/- 0.08 and 14.41 +/- 0.07 Ma. This age consistency suggests that advanced argillic alteration related to the second mineralization stage at Cerro de Pasco formed during a period of 100,000 yr. Three significantly younger alunite ages of 1.2.39 +/- 0.06, 12.13 +/- 0.07 Ma, and 10.94 +/- 0.10 M, probably reflect partial resetting by late circulating fluids. Therefore, it appears that magmatic-hydrothermal activity at Cerro de Pasco Pasco lasted about 1 m.y., from 15.4 to.14.4 Ma, similar to the duration in the nearby mid-Miocene Colquijirca district. An attempt to more precisely date the first mineralization stage, bracketed by the quartz monzonite. dikes (15.1 Ma) and the second mineralization stage (14.5 Ma) failed: the Ar-40/Ar-39 ages of sericite in the alteration halo of the pyrite-quartz body are not reliable, Probably due to inherited argon derived from Precursor micas in Paleozoic clasts of the diatreme breccia. This is indicated by Rb-Sr data on sericite which indicate binary mixing between Miocene magmatic mid Paleozoic basement sources and do not carry any age, information.

The geochemistry, petrogenesis and age of an unusual alkaline intrusion in the western Pilbara craton, Western Australia

The recently discovered Yanyare dykes are located approximately 80 km southwest of Karratha in the north-western Pilbara craton, Western Australia. The dykes intrude intercalated sedimentary and volcanic units of the PalaeoProterozoic Fortescue Group. The Yanyare-02 dyke has an inequigranular texture and is comprised of intensely altered olivine macrocrysts set in a matrix of phlogopite (55%), sanidine (20%) and apatite (5%). Replacement of phlogopite by rutile and chlorite is common. Sanidine occurs as euhedral grains and poikilitic plates enclosing apatite and is variably altered to muscovite and calcite. Aside from fluorapatite, no accessory minerals thought to characterise lamproites were observed. Yanyare-02 is classified as an olivine–phlogopite sanidine lamproite. Average initial (1.72 Ga) ratios for Yanyare-02 are: 87Sr/ 86Sr i = 0.708; 143Nd/ 144Nd i = 0.510; and 176Hf/ 177Hf i = 0.281 ( n = 6). Trace element data reveal extreme LREE enrichment, with La N/Lu N ≈ 100. The petrogenesis of Yanyare-02 is proposed to have involved melting of depleted lithosphere veined with ancient, enriched material of MARID composition. 40Ar/ 39Ar laser step-heating analyses of phlogopite grains produced a weighted mean age of 1724 ± 8 Ma (2σ). Rb–Sr analyses of whole-rock and phlogopite separates yielded an 8-point isochron age of 1681.6 ± 8.3 Ma (95% CL). An age of ca. 1.7 Ga makes Yanyare-02 the oldest lamproite known.

(U–Th)/He dating of kimberlites—A case study from north-eastern Kansas

Dating kimberlite intrusive rocks by radiogenic isotope geochronology often is a difficult task, complicated by both the lack of dateable minerals within kimberlite as well as significant sample alteration that can degrade samples and alter parent–daughter ratios. This study presents a new geochronologic tool for timing the emplacement of kimberlites using the (U–Th)/He system to date the cooling of common kimberlite phenocrystic and xenocrystic minerals. To demonstrate the use of this technique, new apatite, titanite, zircon, magnetite and garnet (U–Th)/He ages constrain the timing of emplacement for the Stockdale, Tuttle, Baldwin Creek, Bala, and Leonardville kimberlite pipes, located in Riley County, Kansas. Zircon from the Tuttle pipe and titanite from the Stockdale pipe yield (U–Th)/He ages of 108.6 ± 9.6 Ma and 106.4 ± 3.1 Ma, respectively. These data are consistent with new Tuttle kimberlite Rb–Sr analyses of phlogopite megacrysts that give a five point isochron age of 106.6 ± 1.0 Ma. Similarly, an apatite (U–Th)/He age of 85.3 ± 2.3 Ma from the Baldwin Creek kimberlite is in agreement with a Rb–Sr phlogopite age of 88.4 ± 2.7 Ma. These dates demonstrate that (U–Th)/He thermochronometry provides reliable timing constraints on the cooling of common kimberlite xenocrystic phases, thereby timing kimberlite emplacement. In addition to the use of more commonly used apatite and zircon (U–Th)/He thermochronometers, we produced reliable emplacement ages of 103.0 ± 7.5 Ma for the Bala kimberlite using (U–Th)/He dating of phenocrystic magnetite and an age of 98.8 ± 8.9 Ma for the Tuttle kimberlite using (U–Th)/He dating of megacrystic garnet. In contrast, kimberlitic apatite (U–Th)/He ages from the Stockdale, Bala, Tuttle, and Leonardville kimberlites yield ages ranging from 67.3 ± 4.4 Ma to 64.3 ± 5.6 Ma, suggesting a local, possibly hydrothermal reheating event resulting in resetting of the apatite (U–Th)/He clock in latest Cretaceous to earliest Tertiary time. Additional (U–Th)/He analyses of apatite from nearby sandstone and basement rocks suggest regional cooling below ~ 70 °C at ~ 165 Ma. These (U–Th)/He and Rb–Sr age data imply that the kimberlites were emplaced over a period of time from ~ 85–110 Ma with several pipes subjected to local reheating at ~ 65 Ma.

A two-stage exhumation of the Variscan crust: U-Pb LA-ICP-MS and Rb-Sr ages from Greater Kabylia, Maghrebides

The significance and role of major shear zones are paramount to understanding continental deformation and the exhumation of deep crustal levels. LA-ICPMS U–Pb dating of monazites, combined with Rb–Sr analyses of biotites, from an anatectic metapelite from Greater Kabylia (Algeria) highlights the history of shear zone development and the subsequent exhumation of deep crustal levels in the internal zones of the Maghrebides. Monazites give an age of 275.4 ± 4.1 Ma (2σ) dating the thermal peak coeval with anatexis. This age is identical to those recorded in other crystalline terranes from south-easternmost Europe (i.e. South Alpine and Austro-Alpine domains) that suffered crustal thinning during the continental rifting predating the Tethys opening. Rb–Sr analyses of biotites yield a cooling age of 23.7 ± 1.1 Ma related to the exhumation of the buried Variscan crust during the Miocene as an extrusive slice, roughly coeval with the emplacement of nappes, and shortly followed by lithospheric extension leading to the opening of the Alboran sea.

Geochronology and isotope transport systematics in a subsurface granite from the Larderello–Travale geothermal system (Italy)

A monzogranite, here referred to as the Montieri pluton, was encountered along two depth profiles from ca. 2 to 4 km depth in wells of the Larderello–Travale geothermal system, Italy. We obtained high-resolution petrographic and microchemical data on seven fresh samples of the Montieri pluton. These data are required to correctly interpret the combined isotope data for the Rb–Sr and K–Ar systems. Biotite and K-feldspar from all samples were analyzed by Rb–Sr and 39Ar– 40Ar. Rb–Sr analyses gave extremely variable two-point Rb–Sr apparent ages and Sr initials, indicating that biotite did not equilibrate with feldspar. Bulk dissolution of K-feldspars does not define a Rb–Sr alignment that could have proved a common origin of all K-feldspar samples from a single magma batch. The leachable fractions of K-feldspar separates have much more homogeneous 87Sr/ 86Sr ratios around 0.712, which may reflect the isotopic composition of a late-stage circulating fluid. Because of the anatectic origin of the granites from the Larderello–Travale geothermal system, it is likely that every K-feldspar separate contains at least three isotopically distinct feldspar generations: relics of the Hercynian gneiss basement, Pliocene magmatic minerals, and hydrothermal retrogression products. Such heterogeneity can be confirmed and quantified by petrographical observations and electron microprobe analyses. The 39Ar– 40Ar age spectra of the biotite samples show some internal discordance. Because of deviations from stoichiometry, biotite discordance can be attributed both to chlorite intergrowths, as predicted from the active fluid circulation, and to the presence of multiple biotite generations, such as have already been documented from other micas from the Larderello–Travale geothermal system and as indeed confirmed by high-resolution petrography. Total K–Ar biotite ages on cluster around 3 Ma and broadly agree with a Rb–Sr age obtained by regressing only the biotite analyses. This suggests that the Montieri pluton, whose known volume is at least 25 km 3, was emplaced at that time. The K-feldspar spectra are strongly discordant; no step age approaches the zero-age which would be predicted by Ar diffusivity modelling. The extraneous Ar in the feldspars is not excess Ar, but inherited Ar instead. Ar inheritance in the feldspar separates correlates with inheritance of Sr. Arrhenius trajectories of the apparent Ar diffusivities for all seven feldspars are astonishingly identical, despite the heterochemism amongst samples. However, the apparent diffusivity calculated from the step-heating experiments led to irreproducible models for the thermal history along the depth profile, as well as time-scales shorter than the historical record. This indicates that Ar diffusivities calculated from laboratory experiments must not be extrapolated to geological conditions.

Age of the Pueblo Viejo Gold-Silver Deposit and Its Significance to Modelsfor High-Sulfidation Epithermal Mineralization

Isotopic analyses were undertaken to clarify age relationships between mineralization in the Pueblo Viejo high-sulfidation epithermal deposit and its host Los Ranchos Formation. This study is important to models for high-sulfidation epithermal deposits because the Los Ranchos Formation is a largely submarine, island-arc tholeiite sequence, very unlike the subaerial, calc-alkaline volcanic sequences that host most high-sulfidation epithermal deposits. Previous U-Pb analyses of zircons from the Los Ranchos Formation show that it formed between about 118 and 111 Ma and was intruded by the Cotui stock at about 112 Ma. New U-Pb isotope analyses of zircons from quartz porphyries at two different levels in the ore-hosting, upper Los Ranchos Formation indicate an age of 111.4 ± 0.5 Ma (MSWD = 1.92), essentially the same as that of the Cotui stock, confirming that the stock was probably the source of the quartz porphyry magmas and mineralizing fluids. Rb-Sr analyses of sphalerite from gold-bearing veins reflect mixing between magmatic and seawater Sr reservoirs but yield no age constraints. Ar-Ar analyses of illite from advanced argillic alteration in the Moore orebody yield a plateau age of 58 Ma interpreted to represent complete resetting or neocrystallization, probably related to the thermal effect of small diorite intrusions immediately south and west of the district. These measurements, combined with geologic relationships, confirm that high-sulfidation mineralization at Pueblo Viejo was coeval with deposition of its host Los Ranchos Formation, part of the island-arc tholeiite sequence that forms the base of the Greater Antilles arc. This greatly expands the types of volcanic terranes that might be prospective for high-sulfidation mineralization beyond subaerial calc-alkaline sequences to more mafic, dominantly submarine, island-arc tholeiite sequences. At present, relatively few high-sulfidation deposits are known in island-arc tholeiite sequences. This could reflect the greater difficulty of developing large advanced argillic alteration zones in these dominantly mafic rocks or simply a failure to recognize island-arc tholeiite sequences in volcanic arcs that contain mineralization. Exploration might also have been discouraged by the fact that most island-arc tholeiite sequences are dominantly submarine and contain exhalative mineralization. That both types of mineralization might form in these arcs is suggested by relationships in the Tonga-Kermadec ridge and adjacent areas, where dacitic calderas in island-arc tholeiite sequences host exhalative and high-sulfidation mineralization. The presence of an unusually large deposit such as Pueblo Viejo in an island-arc volcanic sequence is attributable to the iron-rich nature of the wall rocks, which promoted early deposition of gold by sulfidation, and to the presence of carbonaceous sedimentary rocks that isolated the hydrothermal system and promoted ore deposition. The age and isotopic data show further that these carbonaceous sedimentary rocks were deposited during ocean anoxic event (OAE) 1b, encouraging speculation that other island-arc tholeiite volcanic sequences containing ocean anoxic event sediments might also be favorable for high-sulfidation epithermal mineralization.

Single-grain zircon dating of the metamorphic and granitic rocks from the Biscayarhalv�ya-Holtedahlfonna zone, north-west Spitsbergen

The Biscayarhalvoya-Holtedahlfonna zone (BHZ) in north-western Spitsbergen is a north-south trending, narrow horst, with crystalline basement rocks exposed under a Devonian unconformity. Previous K-Ar, 40Ar/39Ar and Rb-Sr analyses have confirmed the occurrence of Caledonian thermal events, and Grenvillian ages have been obtained by conventional zircon U-Pb and single-zircon Pb evaporation methods. A total of 55 zircon grains from three samples (an augen metagranite, a micaceous schist and a granitic neosome of migmatite) have been analysed by the single-zircon Pb evaporation method. The grains with the age range of ca. 950-1100 My (million years) are the major component in all three samples, suggesting tectono-thermal activity in that period. The detrital versus resorption orgin of the rounded shapes of these grains from the granitic neosome is not clear yet. Therefore, the ages of the migmatization and of the sedimentary protoliths are not concluded. The youngest presumed detrital grain from the granitic neosome is 1060 My old. The metagranite, cutting the Richarddalen unit, yielded grains with an age of ca. 950 Mya. A granite dyke with an age range of 955-968 My cuts the Biscayarhuken unit in the northern Liefdefjorden area. These indicate the sedimentary protoliths of the Richarddalen and Biscayarhuken units are pre-Neoproterozoic. The youngest detrital zircon ages of ca. 940 My indicate Neoproterozoic sedimentary protoliths of the Solanderfjellet micaceous schists. A significant population of zircon grains with an age range of 1600-1900 My in all three samples suggests a wide exposure of these rocks in the source areas during Meso- and Neoproterozoic times. Several Archean ages have also been obtained. The results are generally conformable with those obtained from north-western Spitsbergen.

Rb-sr dating of epithermal vein mineralization stages in the eastern Harz Mountains (Germany) by paleomixing lines

A method is evaluated that enables the dating of binary isotopic paleomixing in hydrothermal alteration assemblages by iterative reconstruction and regression of corresponding two-component mixing lines from conventional Rb-Sr data. The model has been applied to illite-bearing granulometric fractions of hydrothermally altered sedimentary rocks associated with epithermal vein mineralization in the eastern Harz Mountains, Germany. These veins are characterized by a principal quartz-sulfide stage containing quartz, base metal sulfides, and siderite and a subsequent dominant calcite-fluorite-quartz stage. Rb-Sr analyses were performed on untreated granulometric fractions, HCl-treated residues, corresponding HCl leachates, and cogenetic vein minerals obtained from two silicified rock samples related to the quartz-sulfide and the calcite-fluorite-quartz mineralization, respectively. Neither the untreated nor the HCl-leached fractions of these rock samples yield statistically robust and geochronologically meaningful isochrons. When applying the binary mixing model, however, two well-defined paleomixing lines can be reconstructed for the HCl-treated fractions of the rock samples and associated vein minerals at model ages of 226 ± 1 Ma (quartz-sulfide alteration) and 209 ± 2 Ma (calcite-fluorite-quartz alteration). We examine the significance and reliability of these model ages and the corresponding paleomixing lines by means of statistical and geological criteria and discuss the effects of leaching experiments performed on the granulometric fractions using 1 N HCl as well as the nature and origin of the components dissolved in the acid leachates. Our results suggest that the mixing model provides a successful dating concept for isotopic disequilibrium processes during low-temperature hydrothermal events that produce binary mixtures among newly formed alteration phases.

Single-grain zircon dating of the metamorphic and granitic rocks from the Biscayarhalvøya-Holtedahlfonna zone, north-west Spitsbergen

The Biscayarhalvøya-Holtedahlfonna zone (BHZ) in north-western Spitsbergen is a north-south trending, narrow horst, with crystalline basement rocks exposed under a Devonian unconformity. Previous K-Ar, 40Ar/39Ar and Rb-Sr analyses have confirmed the occurrence of Caledonian thermal events, and Grenvillian ages have been obtained by conventional zircon U-Pb and single-zircon Pb evaporation methods. A total of 55 zircon grains from three samples (an augen metagranite, a micaceous schist and a granitic neosome of migmatite) have been analysed by the single-zircon Pb evaporation method. The grains with the age range of ca. 950-1100 My (million years) are the major component in all three samples, suggesting tectono-thermal activity in that period. The detrital versus resorption orgin of the rounded shapes of these grains from the granitic neosome is not clear yet. Therefore, the ages of the migmatization and of the sedimentary protoliths are not concluded. The youngest presumed detrital grain from the granitic neosome is 1060 My old. The metagranite, cutting the Richarddalen unit, yielded grains with an age of ca. 950 Mya. A granite dyke with an age range of 955-968 My cuts the Biscayarhuken unit in the northern Liefdefjorden area. These indicate the sedimentary protoliths of the Richarddalen and Biscayarhuken units are pre-Neoproterozoic. The youngest detrital zircon ages of ca. 940 My indicate Neoproterozoic sedimentary protoliths of the Solanderfjellet micaceous schists. A significant population of zircon grains with an age range of 1600-1900 My in all three samples suggests a wide exposure of these rocks in the source areas during Meso- and Neoproterozoic times. Several Archean ages have also been obtained. The results are generally conformable with those obtained from north-western Spitsbergen.

Carbonate-Hosted Zinc-Lead Deposits in the Lower Cambrian of Hunan, South China: A Radiogenic (Pb, Sr) Isotope Study

Zinc-lead deposits hosted by Lower Cambrian carbonate rocks near Huayuan in the Hunan-Guizhou orogenic belt, southern China, have been studied by means of Pb and Sr isotopes. The deposits are characterized by two mineralization stages: (1) a main economic stage comprising cockade-style sphalerite, galena, pyrite-marcasite, fluorite, barite, and calcite, and (2) a subsequent noneconomic stage with nodular (rosette) sphalerite, saddle dolomite, and bitumen. Both ore stages are postdated by a generation of late calcite. Main-stage sulfides (206Pb/204Pb = 18.162–18.225) display a steep linear trend on conventional Pb-Pb diagrams which indicates binary mixing of lead components bearing the imprint of ancient, 207Pb-dominated crustal rocks and reservoirs with elevated Th/U ratios, possibly located in the lower crust. Nodular rosette sphalerites fall outside this line but reflect a second mixing trend between mineralizing fluids characterized by 206Pb/204Pb > 18.3 and lead isotopically similar to specific main-stage cockade sphalerites, suggesting either remobilization of Pb from main-stage ores or contemporaneous mixing of three distinct lead components in the course of an evolving hydrothermal event. The 87Sr/86Sr ratios of diagenetic and hydrothermal carbonate cements and barites range from 0.70905 to 0.71006 and increase systematically with the paragenetic sequence, indicating progressive mixing of marine Sr derived from the Lower Cambrian carbonate host rocks with radiogenic Sr supplied by external mineralizing fluids. In order to identify possible local metal sources, U-Pb and Rb-Sr analyses were performed on six whole- rock samples from the Lower Cambrian sequence and their time-integrated Pb and Sr isotope evolution was compared to the isotopic signature of the mineralization. Whereas shales and black shales of the Cambrian Balang and Niutitang Formations underlying the ore-bearing Qingxudong carbonate unit appear to be possible external sources for the radiogenic Sr found in hydrothermal carbonates, the U-Pb data do not permit these shales to be considered as primary source rocks for the ore lead. Both the Pb and the Sr isotopes indicate mineralizing fluids that had interacted with various external metal sources, some of them most probably being located in sedimentary or crystalline basement rocks. This refutes prevailing genetic models favoring the hydrocarbon- and metal-rich Lower Cambrian Niutitang black shales as the most likely source reservoir for the Zn and Pb in the deposits of the Huayuan district. The Pb and Sr isotope mixing trends observed for the ore and gangue preciptates reflect small-scale heterogeneous fluid systems that were not capable of producing large deposits. This may indicate that the Huayuan district is not a favorable target for future exploration of economic ore deposits. It also limits the prospects of successful radiometric dating of the mineralization, since conventional geochronologic models require initial isotopic homogeneity.

Dating deformation and cooling in the Caledonian thrust nappes of north Sutherland, Scotland: insights from 40 Ar/ 39 Ar and Rb–Sr chronology

40 Ar/ 39 Ar and Rb–Sr mineral ages have been determined from various lithologies exposed in the Caledonian foreland and structurally overlying thrust nappes of north Sutherland, Scotland. Rb–Sr muscovite ages of c . 428, c . 421 and c . 413 Ma obtained from Moine Thrust Zone mylonites are interpreted to date closely regional thrusting during the Late Silurian to Early Devonian. 40 Ar/ 39 Ar muscovite ages within the lower parts of the Moine nappe are mostly anomalously old with respect to Rb–Sr analyses of muscovites from the same samples; it is likely that this discrepancy results from a component of extraneous or ‘excess’ argon. 40 Ar/ 39 Ar hornblende ages and Rb–Sr and 40 Ar/ 39 Ar muscovite ages obtained from structurally higher metamorphic units in the Caledonian thrust nappes generally range between c . 440 Ma and c . 410 Ma. These ages are interpreted to date cooling during and following ‘D 2 ’ regional thrusting and folding within internal sectors of the nappe sequence. A possible tectonic model involves the Silurian collision of Baltica with Scottish segments of Laurentia resulting in the Scandian orogeny and broadly coeval Moine Thrust Zone. D 2 structures were superimposed on structures and metamorphic fabrics formed during a regional Mid-Ordovician tectonothermal event dated previously at c . 470–460 Ma. Syn-D 2 temperatures were generally >600°C and sufficient to achieve more or less complete thermal rejuvenation of Rb–Sr and 40 Ar/ 39 Ar isotopic systems in muscovite and hornblende, even in areas of low D 2 strain.

Oligo-Miocene granitic magmatism in central Vietnam and implications for continental deformation in Indochina

Two granitoid intrusions within the Bu Khang extensional complex in central Vietnam have been dated by U–Pb and Rb–Sr geochronology. A monazite U–Pb age of 26.0 ± 0.2 (2σ) Myr was obtained for the Bu Khang pluton and 23.7 + 1.6/–1.7 Myr for monazite, allanite and zircon from the Dai Loc intrusion. These ages date crystallization of magmas previously assigned Precambrian to Devonian. Rb–Sr analyses of K-feldspar and biotite fractions from the samples yield ages of 19.8 ± 0.6 (2σ) Myr and 19.6 ± 0.5 Myr, respectively. The thermal history recorded by the different geochronometers implies an average exhumation rate of ∼2 mm yr−1 corresponding to ∼9 km of unroofing. Magmatism was either (i) induced passively by lithospheric thinning driven by changes in regional tectonic stresses, or (ii) triggered actively by an ascending plume. Tertiary exhumation and magmatism documented elsewhere in Indochina (e.g. Ailao Shan-Red River and Wang Chao shear zones) favours a regional tectonic cause for extension and granitoid magmatism in the Bu Khang complex. On the other hand, the presence of an upwelling thermal anomaly since at least 35 Ma, causing mantle melting below Indochina, is supported by shear-wave velocity variations in the mantle, and source geochemistry of both the Bu Khang plutons and the Red River belt intrusions. In either case, Tertiary exhumation of the Bu Khang complex can account for previously undocumented NE–SW-directed extension, which is required in northern Vietnam to account for structural changes related to the opening of the South China Sea.