Browsing by Author "Williams, I. S."

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    Dampier Ridge, Tasman Sea, as a stranded continental fragment
    (Taylor and Francis, 1994) Mcdougall, Ian; Maboko, Makenya A. H.; Symonds, Philip; McCulloch, Malcolm T.; Williams, I. S.; Kudrass, H. R.
    Dredging of the Dampier Ridge recovered small fragments of granite, gabbro and sandstone. Dating of the igneous samples by the U-Pb, K-Ar and Rb-Sr methods yielded precise ages mainly in the range 250 to 270 Ma, mid-Permian. An imprecise Sm-Nd mineral age of approximately 310 Ma might reflect slightly earlier emplacement of the gabbro. A granitic fragment has a composition approximating that a manimum melt. Taken together with the Late Palaeozoic emplacement ages and other geochemical and geophysical data, the evidence strongly supports the conclusion that the Dampier Ridge is a continental fragment, formerly part of eastern Australia, with its present location a consequence of continential rifting and opening of the Tasman Basin by sea-floor spreading.
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    Discussion and Reply: Dampier Ridge, Tasman Sea, as a Stranded Continental Fragment
    (Taylor and Francis, 1995) Anfiloff, V.; Mcdougall, Ian; Maboko, Makenya A. H.; Symonds, Philip; McCulloch, M. T.; Williams, I. S.; Kudrass, H. R.
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    Geochronological Evidence for ∼ 530–550 Ma Juxtaposition ofTtwo Proterozoic Metamorphic Terranes in the Musgrave Ranges, Central Australia
    (Taylor and Francis, 1992) Maboko, Makenya A. H.; Mcdougall, Ian; Zeitler, P. K.; Williams, I. S.
    Contrasting temperature‐time paths from two Proterozoic metamorphic belts in the Musgrave Ranges, central Australia suggest that they represent district tectonothermal terranes. The thermal histories of these terranes converged about 530–550 Ma ago, as they were juxtaposed along the Woodroffe Thrust. Amphibolite fades metamorphism in the gneissic terrane north of the thrust is dated by zircon U‐Pb ages at ∼ 1600 Ma. Subsequently, the gneiss was intruded by granitoid at ∼ 1500 Ma, followed by a second metamorphic event which may be dated by a few zircon ages at ∼ 1400 Ma. Age maxima of ∼ 1300 Ma and ∼ 1260 Ma from steadily rising Ar‐Ar spectra date cooling below closure temperatures of hornblende and K‐feldspar respectively, following the latter metamorphism. In contrast, zircon U‐Pb ages date granulite facies metamorphism in the terrane south of the Woodroffe Thrust at ∼ 1200 Ma. Post‐metamorphic cooling in the granulite terrane was slow with hornblende and biotite remaining open to argon diffusion until after ∼ 930 and ∼ 690 Ma respectively. The thermal histories of the two terranes converged ∼ 530–550 Ma ago, when metamorphism associated with burial of the gneiss under an overriding ∼ 15 km thick thrust sheet of granulite led to the growth of syn‐tectonic muscovite in both terranes and caused near total resetting of Ar‐Ar ages in pre‐existing biotite and K‐feldspar in the gneiss. Cooling from this event was slow with the gneiss dropping below temperatures of ∼ 260° C at ∼ 350 Ma and the granulite cooling below ∼ 200°C at ∼ 400 Ma.
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    Zircon UPb Chronometry of the Pressure and Temperature History of Granulites in the Musgrave Ranges, Central Australia
    (1991) Maboko, Makenya A. H.; Williams, I. S.; Compston, William
    Ion microprobe U-Pb measurements of zircon in the Musgrave Ranges of central Australia date the emplacement of a charnockite, which is cut by a shear zone containing a garnet-clinopyroxene assemblage, M2 (PT conditions {approximately} 11 kb and 720C), at {approximately} 1200 Ma. Veinlets of a post-M2 adamellite cut an M4 assemblage (PT conditions {approximately} 6 kb and 670C) contained in a metadolerite inclusion, suggesting that the adamellite was emplaced during or after M4. Igneous and metamorphic zircons in the adamellite, however, grew at about 1200 Ma, indicating that M2 and M4 were synchronous. At the level of precision of the data, the time interval between the growth of the two assemblages can not have been more than 20 Ma. The authors interpret the pressure, temperature and time (PTt) path obtained by combining the thermobarometric data, and zircon U-Pb ages in terms of magmatic heat input in a thickened crustal segment followed by rapid, possibly tectonically driven, uplift of the lower part of the thickened crust to a depth of 15-20 km