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Interpretation of SHRIMP and isotope dilution zircon ages for the Palaeozoic time-scale: II. Silurian to Devonian

Published online by Cambridge University Press:  05 July 2018

W. Compston
W. Compston*
Affiliation:
Research School of Earth Sciences, Australian National University, Canberra 0200, A.C.T., Australia
*
*E-mail: [email protected]
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Abstract

Ion probe data are documented for zircons from tuffs within the early Llandovery, the mid-Caradoc and the Ludlow. 206Pb/238U ages for tuff magmatism have been interpreted using mixture-modelling to distinguish inheritance and Pb loss. Comparisons with the reference zircon SL13 have been improved through a direct determination of the component of secondary ion discrimination caused by changes in target potential.

Interpretation of the SHRIMP data for the Birkhill ash (Scotland, Llandovery) is ambiguous. The more conservative possibility is that most zircons are 439 Ma, in close agreement with the previous isotope dilution ages for the same zircon concentrate. The other is that the 439 Ma group should be split into an inherited population at ˜447 Ma, with a minority at ˜434 Ma that corresponds with the ash volcanism. Although imprecise, the latter is detectably younger than the multi-grain MSID age, which itself might be a composite of the same two ages.

Most zircon analyses from the mid Caradoc Pont-y-ceunant Ash, Wales, fit an age-group at 452.5 Ma, similar to its published 206Pb/238U age by MSID, with a definite older age group at ˜476 Ma but none showing Pb loss. By contrast, those from the Millbrig bentonite (Virginia) of the same age mainly fall in a well-defined post-eruption age group at 435 Ma, while the remainder give 456 Ma. Most zircon analyses from the Kinnekulle bentonite, Sweden, fall into an apparent 464 Ma group which exceeds other estimates for the mid-Caradoc magmatism. It is interpreted to be a composite age, caused by an inability to resolve it into a younger magmatic and older inherited group owing to the larger analytical errors of the Kinnekulle data. The best SHRIMP estimate for the mid-Caradoc volcanism is 452.6±1.7 Ma found by combining the ages for the three volcanic units. During unmixing of the combined ages, the Kinnekulle ages are redistributed and the 464 Ma ‘group’ vanishes. Precambrian grains are present in all the above volcanics.

The original and new zircon analyses from the Laidlaw Volcanics (Canberra, Australia) of Ludlow age, are dominated by three groups of inherited zircons at ˜436 Ma, ˜450 Ma and ˜476 Ma, which makes it unfavourable for time-scale definition using zircons. The youngest zircon age group is 417.5 Ma (˜30%), but this is not associated with overgrowths on older grains or with wholly younger grains. Instead, it is composed of sporadic low ages within older grains suggestive of Pb loss rather than magmatic zircon growth. Nevertheless, the age for volcanism is 420.7±1.1 Ma based on published Rb-Sr and K-Ar dating, so that the youngest zircon group does appear to be associated with volcanism.

One zircon U-Pb age for the Frasnian by MSID is much older than a precise age by other decay schemes, and another for the Lochkovian is significantly older than a recent SHRIMP age for the same Stage. By small changes in the common Pb composition, both MSID ages can be changed from single volcanic ages affected by Pb loss to an inherited and younger volcanic age, which removes the conflict with the other determinations.

A zircon-based geological time-scale is constructed from the Ordovician to the Carboniferous using the time-points presented and discussed in Parts I and II of this paper.

Keywords

SHRIMPisotope dilution zircon agesgeological time-scaleSilurianDevonian
Type
Research Article
Information
Mineralogical Magazine , Volume 64 , Issue 6 , December 2000 , pp. 1127 - 1171
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2000

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References

Chappell, B.W., Bryant, C.J., Wyborn, D.,White, A.J.R. and Williams, I.S. (1998) High- and low-temperature I-type granites. Resource Geol., 48, 225–35.CrossRefGoogle Scholar
Claoué-Long, J.C., Jones, P.J., Roberts, J. and Maxwell, S. (1992) The numerical age of the Devonian Carboniferous boundary. Geol. Mag., 129, 281–91.CrossRefGoogle Scholar
Claoué-Long, J.C., Compston, W., Roberts, J. and Fanning, C.M. (1995) Two Carboniferous ages: a comparison of SHRIMP Zircon dating with conventional zircon ages and 40Ar/39Ar analysis. Geochronology Time Scales and Global Stratigraphic Correlation.SEPM Spec. Publ., 54, 3-21.CrossRefGoogle Scholar
Compston, W. (1999) Geological age by instrumental analysis: the 29th Hallimond Lecture. Mineral. Mag., 63, 297-311.CrossRefGoogle Scholar
Compston, W. (2000) Interpretations of SHRIMP and Isotope Dilution zircon ages for the geological timescale: I. the early Ordovician and late Cambrian. Mineral. Mag., 64, 43-57.CrossRefGoogle Scholar
Compston, W. and Williams, I.S. (1992) Ion probe ages from the British Ordovician and Silurian stratotypes. Pp. 59-67 in: Global Perspectives on Ordovician Geology (Webby, B.D. and Laurie, J.R., editors). Balkerma, Rotterdam.Google Scholar
Davidek, K., Landing, E., Bowring, S.A., Westrop, S.R., Rushton, A.W.A., Fortey, R.A. and Adrain, J.M. (1998) New uppermost Cambrian U-Pb date from Avalonian Wales and age of the Cambrian-Ordovician boundary. Geol. Mag., 135, 305–9.CrossRefGoogle Scholar
Harland, W.B., Armstrong, R.L., Cox, A.V., Craig, L.E. and Smith, A.G. (1990). A Geologic Time Scale 1989.Cambridge University Press, Cambridge, UK.Google Scholar
Jagodzinski, E.A. and Black, L.P. (1999) U-Pb dating of silicic lavas, sills and syneruptive resedimented volcaniclastic deposits of the lower DevonianCrudine Group, Hill End Trough, New South Wales. Austral. J. Earth Sci., 46, 749–64.CrossRefGoogle Scholar
Keay, S., Steele, D. and Compston, W. (1999) Identifying granite sources by SHRIMP U-Pb zircon geochronology: an application to the Lachlan fold belt. Contrib. Mineral. Petrol., 137, 323–41.CrossRefGoogle Scholar
Landing, E., Bowring, S.A., Davidek, K.L., Westrop, S.R., Geyer, G. and Heldmaier, W. (1998) Duration of the Early Cambrian: U-Pb ages of volcanic ashes from Avalon and Gondwana. Canad. J. Earth Sci., 35, 329–38.CrossRefGoogle Scholar
Mortimer, G.E. (1984) Early to Middle Proterozoic granitoids, basaltic dykes and associated layered rocks of S.E. Eyre Peninsula, South Australia. PhD thesis (unpubl. ), Univ. Adelaide, Australia.Google Scholar
Roddick, J.C. and Bevier, M.L. (1995) U-Pb dating of granites with inherited zircon: Conventional and ion microprobe results from two Paleozoic plutons, Canadian Appalachians. Chem. Geol. (Isotope Geosci. Sect.), 119, 307–29.Google Scholar
Ross, R.J. Jr, Naeser, C.W., Izett, G.A., Obradovich, J.D., Bassett, M.G., Hughes, C.P., Cocks, L.R., Dean, W.T., Ingham, J.K., Jenkins, C.J., Richards, R.B., Sheldon, P.R., Toghill, P., Whittington, B.H. and Zalasiewicz, J. (1982) Fission-track dating of British Ordovician and Silurian stratotypes. Geol. Mag., 119, 135–53.CrossRefGoogle Scholar
Sambridge, M.S. and Compston, W. (1994) Mixture modelling of multi-component data sets with application to ion-probe zircon ages. Earth Planet. Sci. Lett., 128, 373–90.CrossRefGoogle Scholar
Stacey, J.S. and Kramers, J.D. (1975) Approximation of terrestrial lead isotope evolution by a two stage model. Earth Planet. Sci. Lett., 27, 207–21.CrossRefGoogle Scholar
Tucker, R.D. and McKerrow, W.S. (1995) Early Paleozoic chronology: a review in light of new U-Pb zircon ages from Newfoundland and Britain. Canad. J. Earth Sci., 32, 368–79.CrossRefGoogle Scholar
Tucker, R.D., Krogh, T.E., Ross, R.J. and Williams, S.H. (1990) Time-scale calibration by high-precision U-Pb zircon dating of interstrati. ed volcanic ashes in the Ordovician and Lower Silurian stratotypes of Britain. Earth Planet. Sci. Lett., 100, 51–8.CrossRefGoogle Scholar
Tucker, R.D., Bradley, D.C., Ver Straeten, C.A., Harris, A.G., Ebert, J.R. and McCutcheon, S.R. (1998) New U-Pb zircon ages and the duration and division of Devonian time. Earth Planet. Sci. Lett., 158, 175–86.CrossRefGoogle Scholar
Watson, E.B. and Harrison, T.M. (1983). Zircon saturation revisited:temperature and composition effects in a variety of crustal magma types. Earth Planet. Sci. Lett., 64, 295-304.CrossRefGoogle Scholar
Williams, I.S., Tetley, N.W., Compston, W. and McDougall, I. (1982) A comparison of K-Ar and Rb-Sr ages of two rapidly cooled igneous rocks: two points on the Palaeozoic time scale re-evaluated. J. Geol. Soc., London, 139, 557–68.CrossRefGoogle Scholar
Wyborn, D., Owen, M. Compston, W. and McDougall, I. (1982) The Laidlaw Volcanics: a Late Silurian point on the geological time scale. Earth Planet. Sci. Lett., 59, 90-100.CrossRefGoogle Scholar