Pb Isotopic Ages Research Articles

Cyclicities in Triassic platform carbonates: synchronizing radio‐isotopic and orbital clocks

ABSTRACT Sedimentary cycles recorded in young sediments are often attributed to fluctuations of the Earth's climate on a 104−106‐year scale which in turn is governed by periodic variations in solar insolation linked to orbital (Milankovitch) parameters. A spectacular example of cyclic stratal patterns in ancient deposits is the Middle Triassic Latemar carbonate platform (W Dolomites, N Italy). Based on spectral analyses from previous studies, a superimposition of precession (∼20 ka) and eccentricity (∼100 ka) controlled sea‐level fluctuations has been suggested to account for the stacking hierarchy at Latemar, with ∼20 ka being assigned to each highest‐order depositional cycle. Zircon U–Pb isotopic ages from volcanic‐ash layers within the cyclic succession, corroborated by biostratigraphic constraints, suggest that the average time interval for every individual cycle is significantly smaller than the shortest Milankovitch period and therefore challenge previously published interpretations relating distinct spectral peaks to the above mentioned hierarchy. However, our new spectral data indicate that cyclicities resembling Milankovitch characteristics might exist, but on an entirely different scale. Our findings show that frequency spectra should only be interpreted in combination with robust age control. They also encourage the search for complementary mechanisms controlling carbonate deposition.

U–Pb geochronology of Archean metasedimentary rocks in the Pontiac and Abitibi subprovinces, Quebec, constraints on timing, provenance and regional tectonics

U–Pb isotopic ages of single detrital zircons have been obtained from metasandstones in the northern part of the Pontiac subprovince and from the Cadillac, Kewagama and Lac Caste Groups within the southern Abitibi subprovince. Youngest detrital zircon ages from these sequences are 2685, 2686, 2687, and 2693 Ma, respectively. The Lac Fournière pluton, which intrudes Pontiac Group metasedimentary rocks, is dated at 2682±1 Ma. A similar age was previously measured from a pluton that cuts the Kewagama Group. These data bracket deposition of greywacke in the Pontiac and Abitibi subprovinces to 2685±3 Ma. In contrast, the youngest zircon found in a volcaniclastic unit interbedded with Timiskaming Group metawacke is 2673±2 Ma, while the age of a pluton that intrudes the Timiskaming is 2672±1 Ma. Thus, the Timiskaming Group is distinctly younger than the other metasedimentary rocks. The normalized probability distributions of detrital zircon ages from the Pontiac and the Abitibi metasedimentary rocks, as well as of dated igneous rocks from the southern Abitibi subprovince, all show that their major zircon populations formed in the age range 2685–2710 Ma. The distributions for the Abitibi metaturbidites and dated igneous rocks are very similar, suggesting that metasedimentary rocks in the Abitibi belt were mostly derived from the surrounding granite–greenstone terrain. The largest peak of the Pontiac distribution is a few million years younger than that from the Abitibi metasedimentary rocks but there is a correlation between major peaks and both groups show similar age ranges for older, pre-Abitibi components (2750–2850 and 3020–3030 Ma). Deposition of the ca. 2685 Ma sediments was virtually syn-deformational. Based on previous field, geochemical and isotopic studies, as well as the detrital zircon age data, the Pontiac Group may represent a foreland basin that may have developed from an accretionary prism. The source of much of the sediment may have been an uplifted extinct continental arc to the north. The arc was mostly eroded into the sediment basin and perhaps tectonically overridden by a southward advancing nappe complex comprised of its back arc basin, now partly preserved in the southern volcanic zone of the Abitibi subprovince. Metaturbidites exposed between metavolcanic assemblages within the greenstone belt may represent overturned unconformities at the structural base of the greenstones. These were brought up by high angle faults, which also localized deposition of non-marine (Timiskaming-type) sediments. It is tentatively suggested that Pontiac-type metasedimentary rocks extend most of the way beneath the Abitibi subprovince, the oldest part of the basin emerging as the Quetico Group in the western Superior province.

Metamorphism and exhumation of the NW Himalaya constrained by U–Th–Pb analyses of detrital monazite grains from early foreland basin sediments

Single detrital monazite grains from the Dharamsala and Lower Siwalik Formations (early to mid-Miocene continental foreland basin sediments in NW India) have been dated by two techniques; isotope dilution thermal ionization multicollector mass spectrometry (ID-TIMS) and laser ablation plasma ionization multicollector mass spectrometry (LA-PIMMS). The results give U–Th–Pb isotopic ages of c. 400–1300 Ma and 28–37 Ma and suggest that the source of detritus shed from the uplifting Himalayan mountains and captured in the foreland basin included (1) the protolith to the High Himalayan Crystalline Series (HHCS), i.e. rocks unaffected by the Himalayan metamorphism, (2) Cambro-Ordovician granites and (3) HHCS affected by the M 1 phase of Barrovian metamorphism (Eo-Himalayan) related to the Indo-Asian collision. Deposition of the Dharamsala Formation was coeval with M 2 sillimanite grade Himalayan metamorphism and crustal melting. The youngest monazite ( c. 28–37 Ma) ages imply that Indian plate rocks, having experienced the earliest Himalayan metamorphic event which occurred within 10–20 Ma of collision were exhumed, eroded and deposited within c. 10–20 Ma of metamorphism. This indicates a minimum cooling rate of between 60 and 40°C Ma −1 for the period 30–20 Ma. After 20 Ma our study suggests no change in source area and that this same sequence, comprising both metamorphosed and unmetamorphosed rocks, was supplying detritus and being progressively incised by erosion for at least a further 8 million years.

Metamorphic zircon formation by solid‐state recrystallization of protolith igneous zircon

Protolith zircon in high‐grade metagranitoids from Queensland, Australia, partially recrystallized during granulite‐grade metamorphism. We describe the zircon in detail using integrated cathodoluminescence, U–Pb isotope, trace element and electron backscatter diffraction pattern (EBSP) analyses. Primary igneous oscillatory zoning is partially modified or obliterated in areas within single crystals, but is well preserved in other areas. A variety of secondary internal structures are observed, with large areas of transgressive recrystallized zircon usually dominant. Associated with these areas are recrystallization margins, interpreted to be recrystallization fronts, that have conformable boundaries with transgressive recrystallized areas, but contrasting cathodoluminescence and trace element chemistry. Trace element analyses of primary and secondary structures provide compelling evidence for closed‐system solid‐state recrystallization. By this process, trace elements in the protolith zircon are purged during recrystallization and partitioned between the enriched recrystallization front and depleted recrystallized areas. However, recrystallization is not always efficient, often leaving a ‘memory’ of the protolith trace element and isotopic composition. This results in the measurement of ‘mixed’ U–Pb isotope ages. Nonetheless, the age of metamorphism has been determined. A correlation between apparent age and Th/U ratio is indicative of incomplete re‐setting by partial recrystallization. Recrystallization is shown to probably not significantly affect Lu–Hf ages. Recrystallization has been determined by textural and trace element analysis and EBSP data not to have proceeded by sub‐grain rotation or local dissolution/re‐precipitation, but probably by grain‐boundary migration and defect diffusion. The formation of metamorphic zircon by solid‐state recrystallization is probably common to high‐grade terranes worldwide. The recognition of this process of formation is essential for correct interpretation of zircon‐derived U–Pb ages and subsequent tectonic models.

Pb and Nd isotopic constraints on Paleoproterozoic crustal evolution of the northeastern Yeongnam massif, South Korea

We report Pb isotopic ages and Nd isotopic signatures of Paleoproterozoic basement rocks from the Pyeonghae area, northeastern Yeongnam massif, South Korea. The PbSL (lead step-leaching) garnet data of the Wonnam group (Precambrian metasediments) yield a 207Pb/ 206Pb age of 1840±26 Ma, which can be regarded as the timing of amphibolite to upper amphibolite facies metamorphism and associated garnet growth. Whole rock data for the Pyeonghae gneiss intruding the Wonnam group give a 207Pb/ 206Pb age of 2093±86 Ma, denying the possibility of a direct link between the intrusion of the Pyeonghae gneiss and the regional metamorphism of the Wonnam group. Our results confirm the significance of the 2.1 Ga and 1.8 Ga episodes that have been broadly constrained in the Yeongnam massif. The depleted mantle Nd model ages of metasedimentary rocks from the Wonnam group (2.63–2.47 Ga) are slightly younger than those of the Pyeonghae gneiss samples (2.71–2.57 Ga). This Nd isotopic signature also precludes a direct derivation of the Pyeonghae gneiss from the Wonnam Group, instead implying the presence and involvement of the older, probably late Archean crustal materials during the 2.1 Ga magmatism in the northeastern Yeongnam massif. Compiled Pb and Nd isotope data from the Yeongnam and Gyeonggi massifs suggest a similar geologic history for them, arguing against the conventional idea that the Gyeonggi and Yeongnam massifs are separate continental blocks respectively correlated to the South and North China blocks. The whole rock Pb isotope data of basement rocks from the two massifs form a well defined 207Pb/ 206Pb linearity of around 2.0 Ga, suggesting their common crustal evolution process for the past two billion years. A broad coincidence of major tectonic episodes in the two massifs is confirmed by reviewed geochronological data. The Nd model ages of basement rocks from the two massifs support a probable existence of Archean crusts in South Korea. The Nd model ages, both Archean and Proterozoic, of the Gyeonggi and Yeongnam massifs agree with neither those of the North China block (predominantly Archean) nor those of the South China block (predominantly Proterozoic). Our compiled isotope data together with recent estimation for the age of the Honam shear zone appear to refute the presence of suture zone between the two South Korean Precambrian massifs, which leaves the Imjingang belt as the possible suture zone.

Zircon U–Pb sensitive high mass‐resolution ion microprobe dating of granitoids in the Ryoke metamorphic belt, Kinki District, Southwest Japan

Abstract Zircon U–Pb sensitive high mass‐resolution ion microprobe dating was carried out on three types of granitic rock (gneissose biotite granodiorite, biotite granite and two‐mica granite) from the Cretaceous Ryoke belt of the Kinki district, Southwest Japan. The results give the ages of granitic magmatism in the Shigi‐san area of between 87 and 78 Ma and suggest extensive melting of the Cretaceous Ryoke granitic crust to form the two‐mica granite, probably at ca 80 Ma. Discrimination into older and younger granites based on development of gneissosity does not appear to represent the sequence of magma generation, although there is some scope in the interpretation of the zircon U–Pb data that would allow all three granites to form at 83 Ma. Compilation of chemical Th‐U‐total Pb isochron dating method ages, whole rock Rb–Sr isotope ages and U–Pb isotope ages indicates that most Ryoke plutonism occurred from ca 70 Ma to ca 100 Ma. Younger (85 Ma–70 Ma) plutonism with the formation of two‐mica granite occurred only in the eastern sector of the Ryoke belt, including the Kinki District.

Powder coating: U.S. Patent 5,728,789. Mar. 17, 1998 C. Wamprecht et al., assignors to Bayer AG, Leverkusen, Germany

We present new U–Pb isotopic age data for detrital zircons from 16 deformed sandstones of the Ross Supergroup in north Victoria Land, Antarctica. Zircon U/Th ratios primarily point to dominantly igneous parent rocks with subordinate contributions from metamorphic sources. Comparative analysis of detrital zircon age populations indicates that inboard stratigraphic successions (Wilson Terrane) and those located outboard of the East Antarctic craton (the Bowers and Robertson Bay terranes) have similar ~ 1200–950 Ma (Mesoproterozoic–Neoproterozoic) and ~ 700–490 Ma (late Neoproterozoic–Cambrian, Furongian) age populations. The affinity of the age populations of the sandstones to each other, as well as Gondwana sources and Pacific-Gondwana marginal stratigraphic belts, challenges the notion that the outboard successions form exotic terranes that docked with Gondwana during the Ross orogeny and instead places the terranes in proximity to each other and within the peri-Gondwana realm during the late Neoproterozoic to Cambrian. The cumulative zircon age suite from north Victoria Land yields a polymodal age spectra with a younger, primary 700–480 Ma age population that peaks at ~ 580 Ma. Cumulative analysis of zircons with elevated U/Th ratios (> 20) indicating metamorphic heritage yield ~ 657–532 Ma age probability peaks, which overlap with the younger dominantly igneous zircon population. The data are interpreted to give important new evidence that is consistent with ongoing convergent arc magmatism by ~ 626 Ma, which provided the dominant zircon-rich igneous rocks and subordinate metamorphic rocks. Maximum depositional ages as young as ~ 493–481 Ma yielded by deformed sequences in the outboard Bowers and Robertson Bay terrane samples provide new support for late Cambrian to Ordovician deformation in outboard sectors of the orogen, consistent with tectonic models that call for cyclic phases of contraction along the north Victoria Land sector of the Ross–Delamerian orogen.

Timing of high-pressure metamorphism in the Yukon – Tanana terrane, Canadian Cordillera: constraints from U – Pb zircon dating of eclogite from the Teslin tectonic zone

Zircon from eclogite near Last Peak in the Teslin tectonic zone yielded a U–Pb isotopic age of 269 + 2 Ma (2σ), the first precise age for such a rock in the Yukon –Tanana terrane of the Canadian Cordillera. Both the morphology and geochemistry of the eclogitic zircons indicate a metamorphic origin, and the U – Pb age therefore constrains the timing of peak high-pressure metamorphism in this rock. The U – Pb age demonstrates for the first time that an Early Permian high-pressure metamorphic event occurred in rocks now making up the Teslin tectonic zone, and possibly elsewhere in the Yukon – Tanana terrane. This U – Pb age provides a new geochronologic "pin" in the evolution of the Yukon – Tanana terrane prior to its Mesozoic accretion to the North American continental margin and, combined with recent 40Ar/39Ar muscovite data, indicates that high-pressure metamorphism at this time was a relatively short-lived event.

An extended episode of early Mesoproterozoic metamorphic fluid flow in the Reynolds Range, central Australia*

ABSTRACT The products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the Upper Calcsilicate Unit in the granulite portion of the Late Palaeoproterozoic Reynolds Range Group, northern Arunta Block, central Australia. The zones of retrogression, characterized by minerals such as wollastonite, grossular and clinohumite, local resetting of oxygen isotopic compositions and local major element metasomatism, were channelways for water‐rich fluids derived from granulite facies metapelites. U–Th–Pb isotopic ages measured by the SHRIMP ion microprobe on zircon and monazite from a granulite facies semipelite, an early semiconcordant aluminous quartz‐rich fluid‐flow segregation and a late discordant quartz‐rich segregation record some of the extended thermal history of the area. Zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 ± 11 Ma old, itself derived from a source containing zircon as old as 2.2 Ga. Low‐Th/U overgrowths on the zircon grew during granulite facies metamorphism at 1594 ± 6 Ma. Monazite cooled to its blocking temperature at 1576 ± 8 Ma. Zircon cores from the semiconcordant segregation are dominantly >2.3 Ga old, indicating that the source of the fluids was not the particular metamorphosed semipelite studied. Two generations of low‐Th/U overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 ± 8 and 1582 ± 8 Ma, respectively. The monazite age is the same, 1576 ± 12 Ma. Zircon from the late discordant segregation gave 1568 ± 4 Ma. Fluid flow occurred for at least 18 ± 3 (σ) Ma and ended 26 ± 3 (σ) Ma after the peak of metamorphism, suggesting a very slow cooling rate of ∼3°C Ma–1. The last regional high‐grade metamorphism in the Reynolds Range occurred at ∼1.6 Ga, not ∼1.78 Ga as previously thought. The high‐grade event at ∼1.78 Ga is a separate event that affected only the basement to the Reynolds Range Group.

Quaternary geochronology using the UThPb method

We describe a method of uranium-thorium-lead (UThPb) isotopic age dating for Quaternary rocks. The approach uses an instrumental mass discrimination correction for lead isotope ratios, which allows small enrichments of radiogenic 206Ph and 208Ph to be detected at the level of 0.001%. Igneous rocks hosting minerals with a range in 238 U 204 Pb values of 100 can be dated with uncertainties of approximately ±15–20 kyr. A Quaternary rhyolite dated at 1.19 Ma by KAr yields a 238U 206Ph age of 1.03 ± 0.10 Ma. A Holocene dacite (9.5 ka) has uniform 206 Pb 207 Pb to within 0.0015% in groundmass phases, but 1 mm plagioclase phenocrysts have lower 206 Pb 207 Pb by 0.105 ± 0.002% indicating contamination of the magma after plagioclase crystallization. High precision 206 Pb 207 Ph ratios may be a useful new tool for petrogenetic studies.

Tectonic and metamorphic events in the westernmost Grenville Province, central Ontario: new results from high-precision U–Pb zircon geochronology

The U–Pb isotopic ages of metamorphic zircons from deformed amphibolitized metadiabase dykes and associated unstrained anatectic pegmatites have been determined to constrain the times of postdyke ductile flow related recrystallization and deformation. Samples from the Central Gneiss Belt of the Grenville Province in the southern Georgian Bay region provide a means of linking areas formed during common episodes of ductile flow. Three zircon fractions from one attenuated dyke in the Moon River domain gave identical concordant ages of 1062, 1065, and 1064 ± 2 Ma, while those from another gave identical concordant ages of 1052, 1051, and 1050 ± 2 Ma for the time of metamorphism. Zircons from a folded dyke from the Go Home domain gave identical concordant ages of 1049, 1047, and 1046 ± 2 Ma, while a single overgrowth fragment from an unstrained anatectic pegmatite in the fold hinge gave a concordant age of 1047 ± 2 Ma, as if folding, metamorphic zircon growth and postfolding pegmatite are part of the same process. An anatectic pegmatite in a metadiabase boudin neck in the Ahmic domain gave two zircon fractions with identical concordant ages of 1079 and 1080 ± 2 Ma that match the concordant 1078 ± 2 Ma age of a single overgrowth type grain from the host migmatite. Zircons from the metadiabase have similar but less precise ages due to a low level of uranium and a low abundance. U–Pb results for the Love Lake granite situated at the Parry Sound–Ahmic domain boundary are equivocal but may imply folding of this structure after 1068 Ma, the age of contained titanite. Either the upper or lower end of the ca. 1050–1372 Ma zircon discordia may correspond to granite emplacement. Zircons from a dyke that postdates a granulite-grade migmatization in the Parry Sound domain record a metamorphism at 1114 ± 2 Ma but older inherited grains are present in other fractions. Ductile flow in the Go Home and Moon River domains occurred in the interval 1047–1064 Ma during which the latter was probably emplaced, whereas earlier (1080 Ma) ductile flow in the Ahmic domain may relate to the time of overthrusting of the Parry Sound domain. Parry Sound domain dyke metamorphic ages imply a lack of post 1114 Ma migmatization, and a predyke migmatization consistent with a more exotic derivation of this domain. A [Formula: see text] Ma age for a postmigmatite crosscutting pegmatite dyke sets an upper limit on the age of a late buckling event in the Ahmic domain.

Carbonate Pb[sbnd]Pb ages of the Wittenoom Formation and Carawine Dolomite, Hamersley Basin, Western Australia (with implications for their correlation with the Transvaal Dolomite of South Africa)

Pb isotopic analyses of carbonates from the Hamersley Group, Western Australia, yield the following results. (1) A late Archean (minimum) age of ⩾ 2.5 Ga is definitely established for the deposition of both the deeper-water basinal Wittenoom Formation and the shallow-water Carawine Dolomite. Stratigraphic correlation of the Transvaal Dolomite (South Africa) with these two formations is supported by the present Pb isotope age data. (2) A PbPb isochron age of 2505 ± 37 Ma for the Paraburdoo Member (lower part) of the Wittenoom Formation is best interpreted as a late diagenetic event, which took place ∼ 100 Ma after the initial deposition. (3) A PbPb isochron age of 2346 ± 38 Ma for the Bee Gorge Member most likely represents a still later local event of calcite replacement in fractures and veins. (4) A PbPb isochron age of 2541 ± 32 Ma for Carawine Dolomite could represent a regional diagenesis slightly earlier than the Paraburdoo carbonates if the Carawine carbonates were deposited contemporaneously with the Wittenoom Formation. Alternatively, it may represent the time of early diagenesis which closely followed the initial sedimentation. A study of trace element abundances and stable isotope compositions has revealed a major difference in Fe and Mn contents and δ18O values between the Wittenoom and Carawine dolomite samples. This suggests that the two suites of dolomites have undergone separate diagenetic histories with different chemical and oxygen isotopic modifications.

Application of PB Isotopes to the Absolute Timing of Regional Exposure Events in Carbonate Rocks: An Example From U-Rich Dolostones from the Wahoo Formation (Pennsylvanian), Prudhoe Bay, Alaska

ABSTRACT Pb isotope data from U-rich dolostones from the Wahoo Formation (Pennsylvanian) from the subsurface at Prodhoe Bay, Alaska, demonstrate that the U-Th-Pb system can be a powerful geochemical and geochronological tool in understanding carbonate diagenesis. These U-rich dolostones are developed beneath a major, Late Permian to Early Triassic truncational unconformity. U enrichment is uniquely associated with the mineral dolomite, but anomalously high concentrations of U are not present within the dolomite crystal lattice. Major mineral or fluid phases can be ruled out as U hosts. SEM analyses indicate that U anomalies are present in an unknown mineral phase associated with authigenic clays and are commonly concentrated along stylolites. The Pb isotopic composition of the U-rich dolostones shows considerable spread in 206Pb/204Pb (32-79) and 207Pb/204Pb (16.3-18.7) but little spread in 208Pb/204Pb (38.4-39.5). The high U/Pb and low Th/U ratios implied from these ratios reflect the greater mobility of U relative to Th and Pb under near-surface oxidizing conditions. The spread in Pb isotopic composition of 16 samples from a single core chip yields a Pb-Pb age of 255 ± 47 Ma (2s), which is consistent with the Pb-Pb age of 263 ± 41 Ma for the entire data set (n = 28). A subset of pyrolized samples yields 238U-206Pb and 235U-207Pb ages of 26 ± 31 Ma and 274 ± 28 Ma. The Pb-Pb and U-Pb ages are in excellent agreement with the timing of the development of the overlying unconformity at about 270 Ma. The relatively large uncertainties in Pb-Pb and U-Pb ages are the result of scatter of data outside that expected from analytical uncertainties. The scatter can be attributed to a prolonged period of exposure (30-40 million years) associated with the development of the unconformity. Initial isotopic heterogeneity and multicomponent mixing may also contribute to this scatter. Scatter on U-Pb isochron plots has been enhanced by recent U loss. Regardless of the age uncertainties, the data preserve evidence of a U enrichment event during the Permian. We suggest that U was introduced into the sequence by oxidized groundwat r and enrichment occurred in response to local redox conditions at or near the unconformity surface. The Pb isotopic ages, coupled with the observation that anomalous U concentrations are restricted to dolomitic zones within the Upper Wahoo Formation, suggests that U enrichment was concurrent with much of the dolomitization. The U-Pb system was not reset during uplift and exposure in the Late Cretaceous and subsequent burial in the Tertiary, revealing that Pb isotopic ages can be preserved in complex disgenetic systems. This observation may indicate that this part of the upper Wahoo Formation was completely cemented during Mesozoic burial and/or that the youngest phase of porosity development took place under reducing conditions in which U, Th, and Pb were not mobilized. Thus, redox cond tions appear to have been important to both the U enrichment process and the preservation of the Pb isotopic ages.

Preliminary results of a palaeomagnetic and rock magnetic study of the Proterozoic Tsuomasvarri intrusions, northern Fennoscandia

Preliminary palaeomagnetic results are presented for the Proterozoic Tsuomasvarri central ultramafic intrusion and surrounding, older gabbro-diorite intrusion in the northeastern Fennoscandian shield. In the ultramafic intrusion, five remanence components, reflecting different geological processes, were isolated in the alternating field (AF) and thermal demagnetization of 55 samples from ten sites. A reversed polarity remanence component C (pole: Plat=26°N, Plon=245°E, dp=5°, dm=9°, n =32 samples) dominates in the ultramafic intrusion. The normal polarity pole A (Plat=44°N, Plon=232°E, dp=4°, dm=6°, n =9 samples) corresponds to an age of ∼ 1.88 Ga on the Apparent Polar Wander Path (APWP), and is probably a thermochemical remanence acquired during the Svecokarelian orogeny or in the late stages of magma cooling. Pole B (Plat=35°N, Plon=158°E, dp=9°, dm=15°, n =5 samples, normal polarity) corresponds to the ∼ 1.75 Ga old post-orogenic activity in the area, and is also seen in results of KAr (whole rock) isotopic age determinations from the vicinity. The normal polarity pole D (pole: Plat=25°S, Plon=285°E, dp=12°, dm=19°, n =11 samples) has a palaeomagnetic age of ∼ 2.44 Ga which is in contrast with the isotopic age determination of 1.93 Ga (UPb age on zircon) of the gabbro-diorite intrusion. Pole F (Plat=3°N, Plon=212°E, dp=10°, dm=18°, n =5 samples) indicates an age of 0.5-0.4 Ga on the Palaeozoic APWP segment of Fennoscandia and probably reflects a weak Caledonian overprint in northeastern Finland. In the gabbro-diorite intrusion, two remanence components were isolated in 46 samples from eight sites. The palaeomagnetic age, ∼ 1.93 Ga, of the dominant normal polarity component, A′ (pole: Plat=40°N, Plon=247°E, dp=5°, dm=8°, n =28 samples) is consistent with the isotopic age of 1.93 Ga (UPb determination on zircon). The normal polarity component D (pole: Plat=20°S, Plon=285°E, dp=14°, dm=23°, n =5 samples), which was also isolated in the ultramafic intrusion, contradicts the UPb isotopic age determination.

Formation of massive sulfide lenses by replacement of folds; the Hercules Pb-Zn mine, Tasmania

The Hercules Pb-Zn deposit consists of a number of isolated, ellipsoidal massive sulfide pods hosted in a tuffaceous slate unit of Middle Cambrian age. These pods are aligned roughly parallel to the main cleavage and have completely discordant to moderately folded bedding. Mineralization is localized in the short limb zones of parasitic F 2 folds from the micro- to the macroscopic scale. Antithetic shearing induced along the bedding during D 3 caused localized dilation and fracturing of some of the short limb zones. This resulted in alteration and replacement late in D 3 such that Sa was overprinted by the mineralization. The following paragenetic replacement sequence exists: pyrite I --> quartz --> chlorite --> pyrite II --> (sphalerite-galena-chalcopyrite). Pb isotope age data are consistent with the derivation of the deposit from dispersed primary sulfides in the Mount Read Volcanics by synmetamorphic mobilization into structural traps at higher levels in the crust.

Age of emplacement and basement character of the Cache Creek terrane as constrained by new isotopic and geochemical data

New U–Pb and Rb–Sr isotopic and major and trace element geochemical data are reported for Late Triassic to Eocene granite bodies that intrude the Cache Creek and Stikine terranes in the Atlin–Bennett area of northwestern British Columbia. The U–Pb isotopic age data help constrain closure of the Cache Creek ocean and obduction of the Cache Creek terrane to before 172 Ma (Middle Jurassic). Low 87Sr/86Sr initial ratios (0.7037–0.7046) and lack of evidence for inheritance in zircons suggest that rocks underlying the Cache Creek terrane are largely primitive in nature, with derivation of intrusive rocks predominantly from unevolved sources.A genetic link between Late Triassic granitic plutons and Stuhini arc volcanics of Stikinia is supported by trace and rare-earth element data which indicate generation of magma in a volcanic-arc setting. Geochemical patterns for postaccretion Middle Jurassic and Late Cretaceous to early Tertiary age plutons are similar to those of the Late Triassic, indicating that Middle Jurassic and younger plutons could be derived from the same source area as the Late Triassic plutons.These data do not support recent theories proposing that the Cache Creek and Stikine terranes are klippe overlying a thick section of deformed, Proterozoic and lower Paleozoic continental-margin strata and attenuated cratonal basement. Rather, they are consistent with models in which remnants of Cache Creek ocean basin are placed over the Stikine arc in the Early to Middle Jurassic. Both terranes in turn overlie mainly late Paleozoic to early Mesozoic juvenile crustal material or the upper mantle.

Cover gneisses of the Monashee Terrane: a record of synsedimentary rifting in the North American Cordillera

The Monashee Terrane of southeastern British Columbia is composed of Lower Proterozoic basement gneisses unconformably overlain by cover gneisses. The latter constitute a thick (> 2000 m) and laterally extensive (> 150 km) upper-amphibolite-grade succession of metasedimentary rocks, locally intercalated with minor intrusive and extrusive units. This succession is interpreted as reflecting initial broad, amagmatic subsidence and sedimentation on a cratonic platform (basement gneisses), most likely of North American affinity. Throughout most of the terrane, syndepositional magmatism is first marked by a laterally extensive (> 100 km) stratiform pyroclastic carbonatite, which is part of intermittent (long-lived?) alkaline magmatism. One alkaline body was intruded at ca. 740 ± 36 Ma (U–Pb zircon), suggesting that it may be part of initial Windermere rifting. Post-pyroclastic-carbonatite syndepositional extensional tectonism is further evidenced at the north end of the terrane by interlayered mature and immature siliciclastic sediments, with rapid facies changes, intercalated with ultramafic and mafic sills and flows, plus minor felsic pyroclastic deposits. All of these later deposits lie above stratigraphy correlated with strata hosting a stratiform Pb–Zn deposit with an Early Cambrian galena Pb-isotope age and, therefore, may be correlative with a rift–drift transition recorded in Hamill Group strata to the east. Rift tectonism recorded in cover gneisses may reflect one or more documented rift and rift–drift events recorded in Upper Proterozoic to Lower Cambrian strata of the western North American continental terrace prism.

Evidence for the stability of the potential nuclear waste host, sphene, over geological time, from uranium-lead ages and uranium-series measurements

A sphene-based, glass-ceramic material is being investigated as a potential host for Canadian nuclear fuel recycle waste. The ability of sphene to retain certain radionuclides over geological time is examined using observed properties and age measurements of natural sphenes. The long-term stability of sphene is determined by comparing U Th Pb isotopic age data for sphene, from the literature, with ages determined for zircon and other minerals. Better internal agreement is found between U Th Pb sphene ages than for zircon ages indicating the resistance of sphene to daughter radionuclide loss over periods as long as 3000 Ma. Excellent agreement was observed between sphene ages and the estimated age of crystallization of the host rocks or vein infillings, except where subsequent metamorphim had reset the sphene age. Complete lead loss was generally found to accompany high-grade metamorphic conditions when temperatures exceeded ∼ 500°C. Stability over shorter periods of geological time was found for vein infilling and mineral separates of sphene using uranium-series disequilibrium analysis. Conditions of secular equilibrium in the decay chain 238U- 234U- 230Th- 226Ra were observed for most of the 21 sphenes studied indicating that no radionuclide migration had occurred over periods as long as the last 1 Ma. Exceptions to this were four sphenes that had 234U deficiencies between 6 and 24% and two sphenes that showed 226Ra/ 230Rh disequilibrium. No correlation was found between the extent of radiation damage suffered by the sphenes and the presence of uranium-series disequilibrium. The presence of alteration products in three sphenes did correlate, however, with measured disequilibrium, indicating that some 234U and 226Ra had been lost or gained by these samples in the last 1 Ma and 8 ka, respectively.

U–Pb isotopic ages from a granulite-facies xenolith from Partan Craig in the Midland Valley of Scotland

ABSTRACTU–Pb isotopie data for zircons from a granulite-facies xenolith from a volcanic vent at Partan Craig in the southeastern part of the Midland Valley of Scotland display evidence of an early Proterozoic (>2·0 Ga) component. The xenolith is probably relict metasediment and the data therefore indicate old detritus in the sedimentary precursor. The exact ages of sedimentation and metamorphism are uncertain but these must be compatible with Sm–Nd data on similar xenolithic fragments from the same locality which constrain the ages to c. 1–0 Ga or younger.

Pb isotopic dating of some Precambrian rocks from North China—An ensuing discussion on Precambrian Geochronological Scale of China

Pb isotopic ages of some Precambrian rocks from North China were determined. New data from the Chuanlinggou Formation of the Great Wall Group in the Yanshan region, together with published data, show that their whole-rock Pb-Pb isochron age is 1,922±92 (δ) m.y. Thus, 2,000±100 m.y. as the lowest age of the Sinian Geochronological Scale of China is important. In view of the fact that the whole-rock Pb-Pb isochron age from the Liaohe Group in Liaoning Province is estimated to be 1,977±49 (δ) m.y., which is in agreement with the possible maximum deposition age calculated from Rb−Sr data, we prefer to consider the Liaohe Group to be Sinian rather than pre-Sinian in age. Zircon, apatite and other minerals from the Tiejiashan granitic gneiss at Anshan, Liaoning Province, lie on two U-Pb discordant lines with ca. 3,300 and 2,800 m.y. respectively. Zircon, on the discordia with 3,300 m.y., is non-magnetic and free from dark inclusions. Apatite with common Pb isotopic composition, lying on the line of older age, is close to the concordia, indicating that it has not undergone any geological thermal events since it was formed. Zircon and apatite with inclusions and magnetism lie on the discordia of younger age. A U-Pb age of ca. 3,300 m.y. for the Tiejiashan granitic gneiss seems to be possible. However, it is necessary to examine the ages of these rocks with Rb−Sr and other methods in the future. A precursory Precambrian Geochronological Scale of China is propsed based on the Sinian Geochronological Scale of China published in 1977[1] as well as on new data from this study.