Browsing by Author "Collins, Alan S."

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    Complex high-strain Deformation in the Usagaran Orogen, Tanzania:Structural Setting of Palaeoproterozoic Eclogites
    (2003-11) Reddy, S.M; Collins, Alan S.; Mruma, Abdulkarim H.
    The Palaeoproterozoic Usagaran Orogen of Tanzania contains the Earth's oldest reported examples of subduction-related eclogite facies rocks. Detailed field mapping of gneisses exposed in the high-grade, eclogite-bearing part of the orogen (the Isimani Suite) indicates a complex deformation and thermal history. Deformation in the Isimani Suite can be broadly subdivided into five events. The first of these (D1), associated with formation of eclogite facies metamorphism, is strongly overprinted by a pervasive deformation (D2) at amphibolite facies conditions, which resulted in the accumulation of high strains throughout all of the exposed Isimani rocks. The geometry of foliations and lineations developed during D2 deformation are variable and have different shear directions that enable five D2 domains to be identified. Analysis of these domains indicates a geometrical and kinematic pattern that is interpreted to have formed by strain and kinematic partitioning during sinistral transpression. U–Pb SHRIMP zircon ages from a post-D2 granite and previously published geochronological data from the Usagaran eclogites indicate this deformation took place between 2000 ± 1 Ma and 1877 ± 7 Ma (at 1σ error). Subsequent greenschist facies deformation, localised as shear zones on boundaries separating D2 domains, have both contractional and extensional geometries that indicate post-1877 Ma reactivation of the Isimani Suite. This reactivation may have taken place during Palaeoproterozoic exhumation of the Usagaran Orogen or may be the result of deformation associated with the Neoproterozoic East African Orogen. U–Th–Pb SHRIMP zircon ages from an Isimani gneiss sample and xenocrysts in a “post-tectonic” granite yield ∼2.7 Ga ages and are similar to published Nd model ages from both the Tanzanian Craton and gneiss exposed east of the Usagaran belt in the East African Orogen. These age data indicate that the Isimani Suite of the Usagaran Orogen reflects reworking of Archaean continental crust. The extensive distribution of ∼2.7 Ga crust in both the footwall and hangingwall of the Usagaran Orogen can only be explained by the collision of two continents if the continents fortuitously had the same protolith ages. We propose that a more likely scenario is that the protoliths of the mafic eclogites were erupted in a marginal basin setting as either oceanic crust, or as limited extrusions along the rifted margin of the Tanzanian Craton. The Usagaran Orogen may therefore reflect the mid-Palaeoproterozoic reassembly of a continental ribbon partially or completely rifted off the craton and separated from it by a marginal basin.
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    Heterogeneous Excess Argon and Neoproterozoic Heating in the Usagaran Orogen, Tanzania, Revealed by Single Grain 40 Ar/39 Ar Thermochronology
    (Elsevier, 2004) Reddy, Steven M.; Collins, Alan S.; Buchana, Craig; Mruma, Abdulkarim H.
    Existing tectonic models for the evolution of the Usagaran Orogen place much significance on Palaeoproterozoic K–Ar and Rb–Sr ages. Laser 40Ar/39Ar data from single mica grains from the Isimani Suite near the western margin of the orogen indicate that excess 40Ar is common in micas and this casts considerable doubt on tectonic models that are based on previously published K–Ar ages. Biotites lying within a well-developed S2 foliation (previously constrained at 1999–1991 Ma) yield ages up to 3.3 Ga and contain a significant excess 40Ar component that is variable at an intra- and inter-sample scale. Textural evidence indicates that muscovite grew or recrystallized after the synkinematic biotites and they also record younger 40Ar/39Ar ages with individual steps from 524 to 1055 Ma. It is shown that the mica age variation does not reflect different periods of growth but the preferential partitioning of excess 40Ar into the biotite. The muscovite data also have a component of excess 40Ar and the youngest muscovite 40Ar/39Ar date (535.4 ± 2.3 Ma) indicates a maximum age for greenschist facies metamorphism. This date corresponds to thermal activity associated with the East African Orogen. Greenschist facies deformation (D4 and D5) is interpreted to have been coincident with this thermal event and indicates localized tectonic activity associated with Gondwanan amalgamation. The data are also consistent with greenschist facies deformation, metamorphism and deposition of the Usagaran Konse Group being of Neoproterozoic–Early Palaeozoic age. These new data therefore preclude a solely Palaeoproterozoic tectonic history for the Usagaran Orogen and indicate a complex thermal-tectonic reworking in the Neoproterozoic–Early Palaeozoic.
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    Heterogeneous Excess Argon and Neoproterozoic Heating in the Usagaran Orogen, Tanzania, Revealed by Single grain 40Ar/39Ar Thermochronology
    (2004-09) Reddya, Steven M.; Collins, Alan S.; Buchana, Craig; Mruma, Abdulkarim H.
    Existing tectonic models for the evolution of the Usagaran Orogen place much significance on Palaeoproterozoic K–Ar and Rb–Sr ages. Laser 40Ar/39Ar data from single mica grains from the Isimani Suite near the western margin of the orogen indicate that excess 40Ar is common in micas and this casts considerable doubt on tectonic models that are based on previously published K–Ar ages. Biotites lying within a well-developed S2 foliation (previously constrained at 1999–1991 Ma) yield ages up to 3.3 Ga and contain a significant excess 40Ar component that is variable at an intra- and inter-sample scale. Textural evidence indicates that muscovite grew or recrystallized after the synkinematic biotites and they also record younger 40Ar/39Ar ages with individual steps from 524 to 1055 Ma. It is shown that the mica age variation does not reflect different periods of growth but the preferential partitioning of excess 40Ar into the biotite. The muscovite data also have a component of excess 40Ar and the youngest muscovite 40Ar/39Ar date (535.4 ± 2.3 Ma) indicates a maximum age for greenschist facies metamorphism. This date corresponds to thermal activity associated with the East African Orogen. Greenschist facies deformation (D4 and D5) is interpreted to have been coincident with this thermal event and indicates localized tectonic activity associated with Gondwanan amalgamation. The data are also consistent with greenschist facies deformation, metamorphism and deposition of the Usagaran Konse Group being of Neoproterozoic–Early Palaeozoic age. These new data therefore preclude a solely Palaeoproterozoic tectonic history for the Usagaran Orogen and indicate a complex thermal-tectonic reworking in the Neoproterozoic–Early Palaeozoic.
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    Temporal Constraints on Palaeoproterozoic Eclogite Formation and Exhumation (Usagaran Orogen, Tanzania
    (Elsevier, 2004) Collins, Alan S.; Reddy, Steven M.; Buchan, Craig; Mruma, Abdulkarim H.
    In-situ rock suites that preserve assemblages consistent with metamorphism to eclogite-facies conditions are absent from the Archaean. Their Palaeoproterozoic appearance is one of the markers of the onset of tectonic processes similar to those seen in the Phanerozoic Earth. We report new U–Pb Secondary Ion Mass Spectrometry (SIMS) zircon data from the oldest known eclogites (the ∼2.0 Ga Usagaran eclogites, Tanzania) that constrain the timing of high-grade metamorphism, deformation and exhumation of these eclogites and constrain the speed of these processes at this critical period in Earth history. Direct dating of metamorphic zircon from mafic eclogites, and isostructurally recrystallised zircon rims from pelites and felsic gneisses indicates that high-grade metamorphism occurred at 1999.1±1.1 Ma. The rocks were quickly cooled and exhumed at rates of ∼25 °C/Ma and 0.06–0.22 GPa/Ma, respectively, at least in part, by amphibolite-facies sinistral transpression constrained by a 1991±2 Ma pegmatite dyke that crosscuts foliation. Detrital zircons in metasedimentary gneiss protoliths were dominantly derived from two sources: (1) the Tanzanian craton, and (2) a 2400–2640 Ma source region that is compatible with a belt of reworked Archaean and Palaeoproterozoic rocks that lie in the East African Orogen.
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    Temporal Constraints on Palaeoproterozoic Eclogite Formation and Exhumation (Usagaran Orogen, Tanzania)
    (2004-06) Collins, Alan S.; Reddy, Steven M.; Buchan, Craig; Mruma, Abdulkarim H.
    In-situ rock suites that preserve assemblages consistent with metamorphism to eclogite-facies conditions are absent from the Archaean. Their Palaeoproterozoic appearance is one of the markers of the onset of tectonic processes similar to those seen in the Phanerozoic Earth. We report new U–Pb Secondary Ion Mass Spectrometry (SIMS) zircon data from the oldest known eclogites (the ∼2.0 Ga Usagaran eclogites, Tanzania) that constrain the timing of high-grade metamorphism, deformation and exhumation of these eclogites and constrain the speed of these processes at this critical period in Earth history. Direct dating of metamorphic zircon from mafic eclogites, and isostructurally recrystallised zircon rims from pelites and felsic gneisses indicates that high-grade metamorphism occurred at 1999.1±1.1 Ma. The rocks were quickly cooled and exhumed at rates of ∼25 °C/Ma and 0.06–0.22 GPa/Ma, respectively, at least in part, by amphibolite-facies sinistral transpression constrained by a 1991±2 Ma pegmatite dyke that crosscuts foliation. Detrital zircons in metasedimentary gneiss protoliths were dominantly derived from two sources: (1) the Tanzanian craton, and (2) a 2400–2640 Ma source region that is compatible with a belt of reworked Archaean and Palaeoproterozoic rocks that lie in the East African Orogen.
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    Temporal Constraints on Palaeoproterozoic Eclogite formation and exhumation (Usagaran Orogen, Tanzania)
    (2004-06) Collins, Alan S.; Reddya, Steven M.; Buchana, Craig; Mruma, Abdulkarim H.
    In-situ rock suites that preserve assemblages consistent with metamorphism to eclogite-facies conditions are absent from the Archaean. Their Palaeoproterozoic appearance is one of the markers of the onset of tectonic processes similar to those seen in the Phanerozoic Earth. We report new U–Pb Secondary Ion Mass Spectrometry (SIMS) zircon data from the oldest known eclogites (the ∼2.0 Ga Usagaran eclogites, Tanzania) that constrain the timing of high-grade metamorphism, deformation and exhumation of these eclogites and constrain the speed of these processes at this critical period in Earth history. Direct dating of metamorphic zircon from mafic eclogites, and isostructurally recrystallised zircon rims from pelites and felsic gneisses indicates that high-grade metamorphism occurred at 1999.1±1.1 Ma. The rocks were quickly cooled and exhumed at rates of ∼25 °C/Ma and 0.06–0.22 GPa/Ma, respectively, at least in part, by amphibolite-facies sinistral transpression constrained by a 1991±2 Ma pegmatite dyke that crosscuts foliation. Detrital zircons in metasedimentary gneiss protoliths were dominantly derived from two sources: (1) the Tanzanian craton, and (2) a 2400–2640 Ma source region that is compatible with a belt of reworked Archaean and Palaeoproterozoic rocks that lie in the East African Orogen.