Browsing by Author "Zeitler, P. K."
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Item Dating Late Pan-African Cooling in the Uluguru Granulite Complex Of Eastern Tanzania Using the 40Ar_39Ar Technique(Elsevier, 1989) Maboko, Makenya A. H.; Mcdougall, Ian; Zeitler, P. K.Age spectra measured by the 40Ar39Ar technique on hornblende, muscovite and K-feldspar from the Uluguru granulite complex of Eastern Tanzania indicate that following granulite facies metamorphism at ∼ 715 Ma the terrane cooled slowly, reaching a temperature of ∼475°C about 630 Ma ago. Subsequent cooling was even slower, reaching temperatures of ∼170°C about 420 Ma ago. Assuming a simple relationship between cooling rate and thermal gradient, the cooling history translates into an uplift path characterized by a phase of rapid uplift soon after granulite facies metamorphism followed by a period of slow uplift which began about 630 Ma ago. Such a history is consistent with model thermal histories of crustal segments undergoing thermal relaxation and isostatically-driven uplift following tectonic thickening events.Item Discordant ArAr ages from the Musgrave Ranges, Central Australia: Implications for the Significance of Hornblende ArAr Spectra(1991) Maboko, Makenya A. H.; Mcdougall, Ian; Zeitler, P. K.; Fitzgerald, J. D.Hornblende and biotite from granulites in the Musgrave Ranges, central Australia, yield variable 40Ar39Ar bulk ages even at the scale of a single outcrop. There is no obvious relationship between age, chemical composition, sample location, or textural characteristics of the minerals. We interpret the age variation as due to the incorporation of variable quantities of excess Ar. However, the age spectra of the minerals do not show the typical gain profiles commonly associated with excess Ar and isotope correlation diagrams do not indicate trapped Ar of non-atmospheric composition. Instead, some of the hornblendes and all the biotites show relatively simple age spectra similar to those commonly interpreted in terms of volume diffusion. These observations lead us to argue that, as is commonly the case with biotites, not all hornblende age spectra with simple patterns, similar to those predicted by diffusion theory, can be interpreted unambiguously in terms of geologically significant Ar concentration profiles. More specifically, we conclude that a relatively simple hornblende age spectrum does not necessarily indicate the absence of excess Ar.Despite their complexity, our results show that post-metamorphic cooling of the granulites was very slow. Although the incorporated excess Ar hinders the reconstruction of a more precise thermal history, the data show that following granulite-facies metamorphism ≈ 1200 Ma ago, the terrane did not cool below temperatures at which hornblende and biotite close to Ar diffusion until after ≈ 930 and ≈ 690 Ma, respectively.Item 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.Item Metamorphic PT Path of Granulites in the Musgrave Ranges, Central Australia(Elsevier, 1989) Maboko, Makenya A. H.; Mcdougall, Ian; Zeitler, P. K.