Isochron Dating Research Articles

Sm–Nd and Zircon U–Pb ages from garnet-bearing eclogites, NE Oman: constraints on High-P metamorphism

Poly-deformed and poly-metamorphosed glaucophane-eclogite mega-boudins beneath the Samail Ophiolite, Oman record an early subduction-related high-P metamorphism as well as subsequent overprinting deformation and metamorphism related to exhumation. Previously published Rb/Sr ages of 78 Ma and 40Ar/39Ar ages of 82–79 Ma record the major NE-directed shearing event that partially exhumed the eclogites to a shallower crustal level. New Sm/Nd garnet–garnet leachate–whole rock isochron data from garnet-bearing eclogite assemblages in the As Sifah subwindow in NE Oman are 110±9 Ma (DG02-87D); 5-point isochron) and 109±13 Ma (DG02-86E; 3-point isochron). On the basis of microfabric and field structural relationships these ages are interpreted to reflect the timing of prograde, peak high-P metamorphism in the rocks structurally beneath the Samail Ophiolite. This metamorphism clearly predates the age of formation of the obducted Samail oceanic lithosphere (97–94 Ma) as well as the subsequent obduction onto the margin (80–70 Ma). A U–Pb SHRIMP zircon age from small (<200 μm in length) zircons with herring-bone textured zoning from DG02-87D indicate that rapid zircon growth associated with high-Si phengites occurred at 82±1 Ma. Zircon growth is possibly related to liberation of Zr on garnet breakdown during decompression metamorphism under high-P conditions with exhumation. These data require that crustal stacking models attendant with ophiolite obduction are inappropriate to explain the Oman high-P metamorphism.

Tectonic evolution of the Antarctic–Phoenix plate system since 15 Ma

Joint inversion of magnetic isochron and fracture zone data from the extinct Antarctic–Phoenix spreading system in SW Drake Passage yields seven new finite reconstruction poles. The inversion results are very well constrained for such a short length of plate boundary. Although this is partly because the finite poles are located close to the reconstructed region, the optimum use of fracture zone identifications from satellite-derived free-air gravity data is also important – as the stability of stage poles throughout the short intervals in the model affirms. The model results describe a well-organised spreading system since magnetic anomaly chron C5AD (∼15 Ma) in which the Phoenix plate rotated about stage poles nearby to the southwest. Stage pole locations are broadly consistent with a hypothesis of pivoting subduction as the driving force of Phoenix plate movement, and there is some evidence in the progression of stage poles for late stage movement of the subduction pivot in response to the changing azimuth of the subduction zone at which the Phoenix plate was being consumed. The model kinematics alone provide no unequivocal support for previous interpretations of disruption of the subducted part of the Phoenix plate. The very latest stages of spreading saw falling spreading rates between magnetic anomaly chrons C4 (∼8.1 Ma) and C2A (∼3.3 Ma) when the Antarctic–Phoenix Ridge became extinct. This is consistent with an increase in shear stress across the plate bounding Shackleton Fracture Zone due to a plate reorganisation in the neighbouring Scotia Sea following the cessation of spreading on the West Scotia Ridge.

Sm–Nd studies at Mina III gold deposit, Crixás greenstone belt, central Brazil: implications for the depositional age of the upper metasedimentary rocks and associated Au mineralization

The Crixás greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultramafic and mafic metavolcanic rocks at the base and detrital and chemical metasedimentary rocks at the top. The sequence is part of the Archean sialic block exposed in the central part of the Neoproterozoic Brası́lia belt and therefore is strongly affected by the Brasiliano orogeny. Important gold deposits, such as Mina III, are mainly associated with carbonaceous and Fe- and Al-rich metasedimentary rocks of the upper unit. Sm–Nd isochron data for the basal metakomatiites and metabasalts indicate an age of 3.00±0.07 Ga. However, Nd isotopes for the upper metasedimentary rocks show T DM model ages of mostly 2.33–2.49 Ga, which indicates that neither they nor the associated Au mineralization are Archean. Also, garnet whole-rock dating for a Au-bearing chlorite–garnet schist yields an age of 505±7 Ma, suggesting that the mineralization is Neoproterozoic. The Nd model ages suggest that the metasedimentary sequence of Crixás represents a thrust sheet of Proterozoic rocks emplaced over the Archean terrains during the Neoproterozoic Brasiliano orogeny.

Geochronological and geochemical constraints on Mesozoic suturing in east central Tibet

This paper reports isotopic, major and minor element geochemistry of igneous and metamorphic rocks from the Kokoxili and Yushu regions of central and eastern Tibet. The first region lies along the Kunlun suture, which separates the Bayan Har‐Songpan Ganze (Songpan) terrane from the Tarim and Qaidam blocks. Two Kokoxili granitoids yield U‐Pb zircon dates of 217 ± 10 and 207 ± 3 Ma (Late Triassic), which represent the time of emplacement, and Rb‐Sr isochron dates of 195 ± 3 and 190 ± 3 Ma (Early Jurassic), which are interpreted as cooling ages. The geochemical signatures of these granitoids suggest that they are related to subduction continuing into the Late Triassic. In the Yushu area, three samples help constrain the age of the Jinsha suture, which separates the Songpan terranes from the Qiangtang blocks. A leucocratic granite and an orthogneiss in the suture zone yield U‐Pb zircon dates of 206 ± 7 and 204 ± 1 Ma, respectively, and a paragneiss south of it, a U‐Pb monazite date of 244 ± 4 Ma. The existence of coeval magmatism in both the Jinsha and Kunlun sutures suggests that the two subduction zones were simultaneously active. Combining isotopic dating with structural evidence on subduction polarity and paleomagnetic reconstructions, we propose that the Kunlun and Qinling block boundaries, which were distinct in the Permian, subsequently formed a continuous, Late Triassic, northward subducting plate margin. Our data suggest that the Jinsha suture correlates with the Benzilan and Nan‐Uttaradit sutures, which together belong to a major Late Triassic subduction zone.

40Ar/39Ar and Rb-Sr isochron dating of the gold deposits on northern margin of the Jiaolai Basin, Shandong, China

The Pengjiakuang, Dazhuangzi and Fayunkuang gold deposits, located on the northern margin of the Mesozoic Jiaolai Basin, east of Shandong Province, are controlled by a low-angle normal fault. Gold ores are typically brecciated, veinlet and disseminated. The Ar-Ar and Rb-Sr isochron dating methods were adopted to date ores and lamprophyre dike. The results indicate that the age of the Pengjiakuang gold deposit is 117.33—118.42 Ma, that of the Dazhuangzi gold deposit is 117.39 Ma, and that of the Fayunkuang gold deposit is (128.49±7.2) Ma. The consistency in metallogenic age between the gold deposits on the margin of the Jiaolai Basin and the gold deposits (115—126 Ma) of the northern uplift area suggests that both were formed in the same metallogenic period. That is to say, the large-scale metallogeny of the Jiaodong region took place in late-Yanshannian ((120±10) Ma).

New radiometric dating of the dykes from the Hurd Peninsula, Livingston Island, South Shetland Islands

Fifteen new K–Ar ages in the range of 79–31 Ma are partially confirmed by three 40Ar/ 39Ar plateaus and isochron data of 64.9±0.4, 55.5±0.1 and 52.8±0.6 Ma. The new geochronological data reveal a much more detailed picture of the subduction imprint in the Hurd Peninsula. Using cutting relationships, the dyke emplacement history is divided into four episodes. The Late Cretaceous–Paleocene dykes in the range of 80–60 Ma are related to the main magmatism in Livingston Island and most likely reflect the final stages of subduction of the proto-Pacific oceanic crust. The Early Eocene dykes (56–52 Ma) fill the gap in volcanic activity 70–50 Ma ago. They are the only magmatic event manifested at this time in the region. The 45–42 Ma dykes may be related to the intrusion of the Barnard Point tonalite. Three samples of Oligocene age appear to represent the last igneous activities on the Hurd Peninsula prior to the opening of the Bransfield Strait.

Formation of HP–LT rocks and their tectonic implications in the western Tianshan Orogen, NW China: geochemical and age constraints

The Western Tianshan high-pressure/low-temperature metamorphic belt in northwestern China formed within a Paleozoic accretionary wedge on the south side of the Yili–Central Tianshan plate. It comprises various lithologies, including mafic metavolcanic rocks, metavolcaniclastic rocks, metagreywacke, marble and serpentinite. The metavolcanics comprise eclogite, omphacitite, epidosite and blueschist. The trace and rare-earth element characteristics of these rocks indicate MORB to OIB affinities suggesting a seamount-like setting for the protoliths of these rocks. The ε Nd value of the eclogite (+8.9) indicates that the protolith was derived from a long-term depleted mantle, i.e. an N-MORB source, supporting the trace and rare-earth element characteristics of these samples. Sm–Nd isochron dating of 343±44 Ma (Omp–Gln–Grt–WR) and an age of 346±3 Ma (Grt–Gln) were determined using an eclogite boudin within blueschist. A well-defined 40Ar/ 39Ar plateau age of 344±1 Ma was obtained for crossite from an omphacite–phengite-bearing blueschist and is concordant with the Sm–Nd isochron age for the eclogite, suggesting that neither argon loss nor excess argon affected the crossite. A well-defined 40Ar/ 39Ar plateau age of 331±2 Ma was determined for phengite from the same sample. The high-pressure metamorphic rocks were formed by the B-type subduction of a Paleozoic ‘South Tianshan Ocean’ about 344 Ma ago. They were, however, exhumed to higher crustal levels during the collision of the Tarim and Yili–Central Tianshan plates about 331 Ma ago, similar to an ‘Alpine-type’ tectonic regime.

Paleomagnetic and 40Ar/39Ar geochronologic data bearing on the structural evolution of the Silver Peak extensional complex, west-central Nevada

Abstract The Silver Peak extensional complex, located in the Silver Peak Range of west- central Nevada, is a displacement-transfer system linking the Furnace Creek–Fish Lake Valley fault system and transcurrent faults of the central Walker Lane. Late Neogene, northwest-directed motion of an upper plate, composed of lower Paleozoic sedimentary rocks and late Tertiary volcanic and volcaniclastic strata, exhumed a lower-plate assemblage of metamorphic tectonites with Proterozoic and Mesozoic protoliths. Paleomagnetic investigation of Miocene–Pliocene pyroclastic and sedimentary rocks of the upper plate and Miocene mafic dikes in the lower plate reveals modest horizontal- axis tilting (northwest-side-up) and vertical-axis rotation (clockwise) within the extensional complex. Eight to ten samples from each of 123 sites were demagnetized; 95 sites yielded interpretable results. Dual- polarity results from one population of mafic dikes in the lower-plate assemblage indicate moderate, northwest-side-up tilting (declination D = 329°, inclination I = 37°, α95 = 4.3°, number N = 30 sites; in situ) (α95 = the confidence limit for the calculated mean direction expressed as an angular radius from the calculated mean direction). Some dikes yield exclusively normal-polarity results that are interpreted to indicate modest clockwise vertical-axis rotation (D = 021°, I = 57°, α95 = 4.3°, N = 19 sites; in situ) concurrent with uplift of the lower-plate rocks, and nine sites yield magnetization directions that are north-directed with positive inclinations of moderate steepness, similar to an expected Miocene field. Late Miocene pyroclastic rocks in the upper plate yield normal-polarity magnetizations suggestive of moderate, clockwise, vertical-axis rotation (D = 032°, I = 53°, α95 = 8.8°, N = 10 sites). The apparent clockwise rotation is unlikely to result from incomplete sampling of the geomagnetic field, because the overall dispersion of the VGP (virtual geomagnetic pole) positions is high for the latitude of the site location. Middle Miocene sedimentary rocks probably were remagnetized shortly after deposition. Of eight 40Ar/39Ar determinations from mafic dikes in the lower plate, five groundmass concentrates yield saddle-shaped age spectra, and one separate provided a plateau date of low confidence. Isochron analysis reveals that all six groundmass concentrates contain excess Ar. If rapid cooling and Ar retention below ∼250 °C are assumed, the preferred age estimate for mafic intrusions is provided by isochron dates and suggests emplacement between 12 and 10.5 Ma. The 40Ar/39Ar age-spectrum data are consistent with existing fission-track cooling and K-Ar isotopic age information from lower-plate granitic rocks and indicate rapid cooling of the lower-plate assemblage from well above 300 °C to 100 °C between 13 and 5 Ma. Rapid cooling may explain the overall distribution of paleomagnetic results from lower-plate intrusions such that the earliest acquired magnetizations reflect both northwest-side-up tilt and clockwise rotation and the younger magnetizations reflect northwest-side-up tilt. Overall, the paleomagnetic data from the Silver Peak extensional complex are interpreted to suggest that vertical-axis rotation of crustal-scale blocks, associated with displacement transfer in the central Walker Lane, may play an integral part in accommodating strain within a continental displacement-transfer system.

Luminescence isochron dating: a new approach using different grain sizes.

A new approach to isochron dating is described using different sizes of quartz and K-feldspar grains. The technique can be applied to sites with time-dependent external dose rates. It is assumed that any underestimation of the equivalent dose (De) using K-feldspar is by a factor F, which is independent of grain size (90-350 microm) for a given sample. Calibration of the beta source for different grain sizes is discussed, and then the sample ages are calculated using the differences between quartz and K-feldspar De from grains of similar size. Two aeolian sediment samples from north-eastern China are used to illustrate the application of the new method. It is confirmed that the observed values of De derived using K-feldspar underestimate the expected doses (based on the quartz De) but, nevertheless, these K-feldspar De values correlate linearly with the calculated internal dose rate contribution, supporting the assumption that the underestimation factor F is independent of grain size. The isochron ages are also compared with the results obtained using quartz De and the measured external dose rates.

Are buoyancy forces important during the formation of rifted margins?

Profiles of crustal thickness across rifted continental margins are examined in an attempt to understand the key observations and controlling parameters. Crustal stretching factor profiles from rifted continental margins supplemented by isochron data for early seafloor spreading have been used to determine a correlation between strain-rate () and stretching factor (β). Despite the different methods, assumptions and data sources, our –β relationship for rifted margins is consistent with that observed by Newman and White for intracontinental rift basins. The –β relationship we derive is also consistent with the dynamic models of Newman and White which include thermorheological strain-hardening and strain-softening, but omit crustal buoyancy forces generated by lateral crustal thickness variations. Whilst crustal buoyancy forces are not included in the above dynamic models, the –β data do not necessarily preclude their importance. Simple numerical models of buoyancy force evolution show that for the first ∼30 Myr after rifting the thermally-derived buoyancy forces within the lithosphere that assist extension are larger than the crustal buoyancy forces that oppose extension. This ‘rift push’ force acts as a positive feedback mechanism, is of the order of 3 × 1012 N m−1 and dominates over the opposing crustal buoyancy forces immediately after rifting. It is therefore clear that the delocalising effects of the crustal buoyancy force are dominant over a restricted range of conditions, namely at low strain-rate and at long times after rifting. Histograms of the lateral pressure gradients derived from crustal thinning factors along rifted margins show a dominant peak at 0.05 ± 0.3 kPa m−1 and a significant secondary peak at 0.8 ± 0.3 kPa m−1. The lower lateral pressure gradient peak corresponds to thinned parts of the continental crust which is adjacent to unstretched continental crust and may define the edge of a zone of thermal strain-softening. Independent observations show that narrow margins are associated with rapid strain-rates and are consistent with thermal strain-softening predicted by thermorheological models. However the dominant near-zero pressure gradient peak is consistent with the operation of crustal buoyancy force processes during rifting, which attempt to remove variations in crustal thickness.

Rb- Sr isochron dating of granitoids from the Kazaure schist belt, Nigeria

The results of Rb-Sr isochron dating of granites from the crystalline basement complex of Kazaure Schist Belt are presented in this paper. Whole rock Rb-Sr radiometric age determination of five samples of a syntectonic coarse-grained porphyritic granite define a 4-point whole-rock Rb-Sr isochron (MSWD=1.2) corresponding to an age of 592 + 14 Ma, the initial 87Sr/86Sr ratio being 0.7097 + 0.0004. This is distinctly a Pan-African age and is taken as the time of emplacement and crystallization of a granitic magma formed from a reworking of a pre-existing continental crust. The initial 87Sr/86Sr ratio indicates a significant contribution of older crustal materials by partial melting to the granitic magma. Key Words: Kazaure Belt, Geochronology, Isochron, Pan-African, Crust. (Global Journal of Pure and Applied Sciences: 2002 8(3): 319-324)

Precise two chronometer dating of Pleistocene travertine: The 230Th/ 234U and 226Ra ex/ 226Ra(0) approach

In order to determine the geochemical evolution of a freshwater limestone cave system located in central Switzerland (Hell Grottoes at Baar/Zug,) young postglacial tufaceous limestone and travertine precipitates were investigated using the 230Th/ 234U ingrowth system. Additional analyses of further radionuclides within the 238U decay chain, i.e. 226Ra and 210Pb, showed that the Th/U chronometer started with insignificant inherited 230Th over the entire formation period of the travertine setting (i.e. 230Th(0)=0). A contribution from detrital impurities with 230Th/ 234U in secular equilibrium could be precisely subtracted by applying isochron dating of cogenetic phases and recently formed travertine. The resulting precise 230Th/ 234U formation ages were found to be consistent with the geological stratigraphy and were furthermore used to demonstrate the applicability of the next geologically important chronometer in the 238U-decay series, based on decay of excess 226Ra normalized to the initial, i.e. 226Ra ex/ 226Ra(0). This system is suitable for dating phases younger than 7000 yr when the correction of a detritus component increasingly limits the precision of the 230Th/ 234U chronometer. Analytical solutions of the coupled 234U/ 230Th/ 226Ra radionuclide system predicted that the 226Ra ex/ 226Ra(0) chronometer is independent of the actual 230Th activity build up from decay of 234U, if the systems starts with zero inherited 230Th(0). The data set confirmed this hypothesis and showed furthermore that the initially incorporated 226Ra excess must have remained almost uniform in all limestone over a period of at least 7000 yr, i.e. 4–5 half-lives of 226Ra. This is concluded because (i) the 226Ra ex/ 226Ra(0) ages agreed well with those derived from 230Th/ 234U, (ii) all data plot within uncertainty on the 226Ra ex/ 226Ra(0) decay curve and (iii) the atomic Ba/Ca ratio was found to be constant in the travertine material independent of the sample ages. Provided that such boundary conditions hold, 226Ra ex/ 226Ra(0) should be applicable to materials which are suitable for 230Th/ 234U dating in sedimentology and oceanography, i.e. travertine, corals, phosphorites, etc., and should strongly support 230Th/ 234U for samples that have been formed a few thousand years ago.

U-Th dating of carbonate platform and slope sediments

Absolute chronology of marine sediment beyond the 14C age range provides a test for models of climate change and has many other applications. U-Th techniques have been used for such chronology by dating corals, but extending these techniques to marine sediment is complicated by the presence of significant initial 230Th—both in detrital material and scavenged from seawater. In this study, we investigate four methods of solving the initial 230Th problem for a particular type of marine sediment—the aragonite-rich sediments of carbonate platforms and slopes. Bulk sediment U-Th analyses can be corrected for initial Th to yield ages with ≈2 to 3 kyr precision for highstand periods when sediment aragonite contents are particularly high. Uncertainty on the corrections causes inadequate precision for sediment from other periods, however. Removal of scavenged Th before analysis would enable a dramatic increase in this precision but has not proved successful despite a range of chemical leach approaches. Using heavy liquids to separate the various carbonate minerals found in Bahamas sediment enables an isochron approach to correct for initial Th, but the presence of initial Th from two sources requires correction or removal of one source of initial Th before the other is deconvolved by the isochron. Quantitative removal of detrital material before isochron analysis proves a successful approach. Such isochron data demonstrate that, although sediment remains closed to U-Th on a centimetre scale, nuclides are moved from grain to grain by α-recoil. Such intergrain exchange is expected to be observed in all sediments containing mineral grains with different U concentrations. Measured 234U/238U allows the recoil movement to be corrected and results in isochron ages with precision sometimes as low as 3 kyr. The accuracy of this approach has been proved by dating samples within the 14C age range. Sediments spanning the penultimate deglaciation have also been dated. After a small correction for bioturbation, the age for this event is found to be 135.2 ± 3.5 ka. This date is ≈8 kyr before the peak in northern hemisphere insolation and suggests that deglaciation is initiated by a mechanism in the southern hemisphere or tropics. This isochron approach shows considerable promise for dating of sediments older than this event, which will provide further information about the timing and mechanisms of global climate change.

Some remarks on U–Th mineral ages from igneous rocks with prolonged crystallisation histories

Mineral isochron dating is a frequently used geochronological tool. One of its assumptions is that the minerals grow over a time period that is small compared to the half-life of the radiogenic isotope system used. In recent years, increasing analytical precision has promoted the use of the short-lived U-series isotope system in order to date young crystallisation events. Three whole-rock zircon U–Th isochrons from the 26.5 ka Oruanui eruption in the Taupo Volcanic Zone, New Zealand, yield pre-eruptive model ages of 5.5±0.8 ka, 9.7±1.7 ka and 12.3±0.8 ka for the sub-63 μm, 63–125 μm and 125–250 μm zircon size fractions, respectively. This suggests that in this case the assumption of instantaneous crystal growth breaks down. Instead, the U–Th data may be explained by continuous zircon growth over a period of ∼90 ka. However, cathodoluminescence shows that crystals are typically composed of an euhedral core surrounded by a sector-zoned euhedral rim, and the U–Th data can also be modelled by mixing an older (∼27 ka model age) population of zircon crystals with a young zircon rim that formed shortly prior to eruption of the Oruanui rhyolite. This indicates that detailed petrographic studies are critical for deciphering the histories of prolonged crystallisation in the magmatic environment. It is concluded that conventional U-series mineral isochrons may underestimate the age of the onset of crystallisation by more than an order of magnitude. In future, microanalytical techniques will lead to significant advances in the understanding of crystallisation processes and timescales.

New Zealand's Darran Complex and Mackay Intrusives; Rb-Sr whole-rock isochrons in the Median Tectonic Zone

The Cretaceous opening of the Tasman Sea, followed by uplift east of the Alpine Fault, has given access to the roots of a Mesozoic convergent Gondwana margin in western New Zealand. This long term margin contains the complex Median Tectonic Zone (MTZ) of recent authors and is pefectly exposed between Milford Sound and the Darran Range. In the western part of this profile, in the Milford Gneiss Complex, earlier MTZ history is obscured by Cretaceous metamorphism and structural transposition. However, the Pembroke Granulite is well preserved, and the metamorphic curtain gradually lifts toward the east. Rb-Sr whole-rock isochron dates and initial 87 Sr/ 86 Sr (I o )values are presented. The Darran Complex yields a whole rock isochron of 133 + or - 17 Ma, concordant with published zircon U-Pb and hornblende K-Ar results. The Mackay Intrusives (Mistake Diorite and Hut Suite) yield a whole rock isochron of 224 + or - 4 Ma (MSWD = 1.21) in agreement with published U-Pb dates of Mistake Diorite. Rafts of Christina Gneiss in Mackay Intrusives were subject to partial melting at the time of the Mackay Intrusive activity. However, the Darran Complex also appears spectacularly intruded by the Christina-Crosscut irruptives of Mackay granitoids, which plot precisely on the 224 Ma Mackay Rb-Sr whole rock isochron. Obvious ways of resolving this paradox are (1) that the Darran Complex has undergone thermal resetting after its intrusion by Mackay magma, and (2) the Christina-Crosscut irruptives are late, back-intrusive Darran differentiates containing assimilated Mackay material. These solutions fail on the grounds of heat transfer (Mackay isochron was not re-set) or of heat balance (surmised irruptives excessively contaminated for the available heat). A way past the constraints is the passive tectonic emplacement of the Mt. Crosscut irruptive as a dike-like roof pendant. The Mt. Christina irruptive is dated less reliably, is tectonically less modified, and may represent a late Darran liquid. A much older age and lower I o of the Darran correlative Rotoroa Complex may be inherited from a common Rotoroa/Darran protolith. I o increases systematically from Rotoroa to Western Darran (Milford/Pembroke), to Darran rocks, and finally to Western Fiordland Orthogneiss (WFO), which is chemically distinct from the Pembroke Granulite and Darran Complex.

Numerical dating of the Eckfeld maar fossil site, Eifel, Germany: a calibration mark for the Eocene time scale.

Sediments of the Eckfeld maar (Eifel, Germany) bear a well-preserved Eocene fauna and flora. Biostratigraphically, Eckfeld corresponds to the Middle Eocene mammal reference level MP (Mammals Paleogene) 13 of the ELMA (European Land Mammal Age) Geiseltalian. In the maar crater, basalt fragments were drilled, representing explosion crater eruption products. By 40Ar/39Ar dating of the basalt, for the first time a direct numerical calibration mark for an Eocene European mammal locality has been established. The Eckfeld basalt inverse isochron date of 44.3 +/- 0.4 Ma suggests an age for the Geiseltalian/Robiacian boundary at 44 Ma and, together with the 1995 time scale of Berggren et al., a time span ranging from 49 to 44 Ma for the Geiseltalian and from 44 to 37 Ma for the Robiacian, respectively. Additional 40Ar/39Ar dating on a genetically related basalt occurrence close to the maar confirms a period of volcanism of ca. 0.6 m.y. in the Eckfeld area, matching the oldest Eocene volcanic activity of the Hocheifel volcanic field.

Pb Isotope Study of Some Nonferrous Metallic Deposits in China

AbstractBased on Pb‐Pb isochron data of more than 40 Precambrian polymetallic deposits, the authors consider that there are four mineralization periods for the Precambrian copper deposits in China, and the major copper deposits were formed at about 1800 Ma; there are three mineralization periods for gold deposits formed from Archaean to Proterozoic. By studying hundreds of lead isotope data from some Mesozoic continental subvolcanic Cu and Ag polymetallic deposits and fine‐disseminated gold deposits, the authors found that the calculation based on the lead single‐stage evolution model or two‐stage evolution model cannot give the true ore‐forming ages but can provide more information about mineralization and material sources of the deposits.

Time-scales of magma formation, ascent and storage beneath subduction-zone volcanoes

There is now sucient information to attempt an integrated model for melt gener- ation, transfer and storage beneath subduction-zone volcanoes. Fluid release from the subducting oceanic crust into the mantle wedge may occur over a period ranging from a few hundred kyr, to as little as less than 1 kyr, before eruption. This supports models in which ®uid addition is closely linked to partial melting, though there may also be evidence for a component of decompression melting. The timing of the onset of ®uid addition may be linked to the rate of subduction (i.e. water supply rate) and the angle of subduction, and, consequently, the thermal structure of the mantle wedge. In contrast, contributions from subducted sediments to subduction-zone lava sources appear to occur some 350 kyr{4 Myr before eruption. Evidence for partial melting of the sediment component, combined with the short ®uid transfer times, phenocryst equilibration temperatures and other observations all point to quite high mantle wedge temperatures close to the interface with the subducting plate. New 226 Ra data permit only a short period of time between ®uid addition and eruption. This requires rapid melt segregation, magma ascent by channelled ®ow and minimal residence time within the lithosphere. Typically, the evolution from basalt to andesite occurs rapidly during ascent or in magma reservoirs, inferred from some geophysical data to lie within the lithospheric mantle. Mineral isochron data suggest that some andesitic magmas subsequently stall in more shallow crustal level magma chambers, where they can evolve to dacitic compositions via fractionation, typically combined with assimilation, on time-scales of a few thousand years or less.

New Sm–Nd ages for the Ivrea–Verbano Zone, Sesia and Sessera valleys (Northern-Italy)

The Ivrea–Verbano Zone (IVZ) is a slice of South-Alpine lower crust intruded by mantle magma. The main episode of magmatic underplating occurred during the Permian time and was related to continental rifting. We have carried out mineral Sm–Nd isochron dating of samples from the IVZ, including a gabbro dyke on which apparent Oligocene zircon ages have been reported, and a metapelite. The gabbro dyke sampled yielded three isochrons (cpx, opx, plag, amph and whole rocks) with the same initial 143Nd / 144Nd and an age of 274 ± 11 Ma (95% c.l., MSWD = 1.31) for the last major thermal event. Since this dyke is an embayment of the surrounding layered gabbro in the Balmuccia Peridotite, this age also represents the minimum age for the cooling of mafic–ultramafic sequence and for the emplacement of the Balmuccia peridotite in the IVZ. These conclusions are in contrast with those of Gebauer et al. (1992), who reported Oligocene U–Pb ages for magmatic zircons extracted from the same rocks. In order to evaluate the effect of Alpine intrusions on the IVZ, a sample of amphibole gabbro from the upper Sessera Valley was collected near Oligocene magmatic rocks. An internal Sm–Nd isochron (amph + cpx + plag + wr) from this sample yielded an age of 267 ± 21 Ma (95% c.l., MSWD = 0.716). Thus, despite the vicinity of the Oligocene magmatic activity, no thermal overprint (at T>650°C) occurred in this part of the IVZ more recently than 267 ± 21 Ma. Furthermore, a paragneiss septum within a shallower part of the intrusion in the lower Sessera Valley yielded a middle Triassic Sm–Nd garnet, biotite, plagioclase and whole rock age of 244 ± 4 Ma (95% c.l).

Age and isotopic characterisation of geological terranes in Marlborough Schist, Nelson/Marlborough, New Zealand

K‐Ar and Rb‐Sr ages of Marlborough Schist and Pelorus Group metasediments show coherent regional metamorphic/cooling age patterns, irrespective of their terrane protolith, in which ages are primarily inversely related to metamorphic grade and, more locally, with distance from the Picton Fault Zone. The oldest ages, from the lowest grade (prehnite‐pumpellyite facies, textural grade 1'IIA) Pelorus Group are mainly in the range 175–200 Ma, indicating regional metamorphism occurred at least in earliest Jurassic or latest Triassic times. Progressive cooling of Marlborough Schist resulted in a spectrum of cooling ages, mainly 110–180 Ma, which reflects slow regional uplift during the Jurassic and Early Cretaceous. Some disruption of this pattern occurred at the Picton Fault Zone where post‐Early Cretaceous movement has juxtaposed high and low structural levels. Initial 87Sr/86Sr isotopic ratios (i) at the time of metamorphism (t), derived from Rb‐Sr whole‐rock isochron data, further characterise terrane protoliths identified by field, petrographic, and geochemical evidence. The (t)‐(i) data largely confirm these prior regional groupings but do not entirely confirm the terrane identifications. A less radiogenic group, (i) < 0.7055, termed Western Marlborough Schist, comprises the Pelorus Group, and schist equivalents, west of a postulated suture/terrane boundary in the schists, and is correlated with Caples Terrane rocks in Otago. “Torlesse"‐type schists are more radiogenic, (i) > 0.7062, but do not match Torlesse (Rakaia) Terrane (t)‐(i) data from Wellington or Otago. They correlate better with Kaimanawa Schist of the central North Island. An intermediate group, (i) = 0.7042–0.7062, is separate from the “Torlesse” group, but partly overlaps the Caples Terrane group. This correlates well with Waipapa Terrane of the central North Island and Haast Schist of uncertain terrane affinity in Otago. The combined radiogenic and intermediate groups, termed Eastern Marlborough Schist, define a lineage which thus includes Kaimanawa Schist, Pahau Subterrane rocks, and both Waipapa‐type and Torlesse‐type schists in Marlborough. It is possible that the Moonlight Fault Zone of Otago has a continuation in Eastern Marlborough Schist as the Picton Fault Zone.