Heterogeneous Excess Argon and Neoproterozoic Heating in the Usagaran Orogen, Tanzania, Revealed by Single Grain 40 Ar/39 Ar Thermochronology

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dc.creatorReddy, Steven M.
dc.creatorCollins, Alan S.
dc.creatorBuchana, Craig
dc.creatorMruma, Abdulkarim H.
dc.date2016-09-21T17:08:47Z
dc.date2016-09-21T17:08:47Z
dc.date2004
dc.date.accessioned2018-03-27T08:57:43Z
dc.date.available2018-03-27T08:57:43Z
dc.descriptionFull text can be accessed at http://www.sciencedirect.com/science/article/pii/S0899536204000910
dc.descriptionExisting 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.
dc.identifierReddy, S.M., Collins, A.S., Buchan, C. and Mruma, A.H., 2004. Heterogeneous excess argon and Neoproterozoic heating in the Usagaran Orogen, Tanzania, revealed by single grain 40 Ar/39 Ar thermochronology. Journal of African Earth Sciences, 39(3), pp.165-176.
dc.identifierhttp://hdl.handle.net/20.500.11810/4116
dc.identifier10.1016/j.jafrearsci.2004.07.052
dc.identifier.urihttp://hdl.handle.net/20.500.11810/4116
dc.languageen
dc.publisherElsevier
dc.subjectPalaeoproterozoic
dc.subjectUsagaran orogen
dc.subjectEast African orogen
dc.subjectAr laser microprobe
dc.subject40Ar/39Ar geochronology
dc.subjectDeformation
dc.titleHeterogeneous Excess Argon and Neoproterozoic Heating in the Usagaran Orogen, Tanzania, Revealed by Single Grain 40 Ar/39 Ar Thermochronology
dc.typeJournal Article, Peer Reviewed

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