Xenotime/zircon geochronology in the Archaean Witwatersrand Basin
N KositcinGavin L. EnglandBirger RasmussenI. R. FletcherN.J McNaughtonB. KrapežDavid I. GrovesBrendan Griffin
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Research Article| July 26, 2017 Detrital zircon geochronology of sandstones of the 3.6–3.2 Ga Barberton greenstone belt: No evidence for older continental crust Nadja Drabon; Nadja Drabon 1Department of Geological Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305, USA Search for other works by this author on: GSW Google Scholar Donald R. Lowe; Donald R. Lowe 1Department of Geological Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305, USA Search for other works by this author on: GSW Google Scholar Gary R. Byerly; Gary R. Byerly 2Department of Geology and Geophysics, Louisiana State University, 354 Howe-Russell, Baton Rouge, Louisiana 70803, USA Search for other works by this author on: GSW Google Scholar Jacob A. Harrington Jacob A. Harrington 1Department of Geological Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Nadja Drabon 1Department of Geological Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305, USA Donald R. Lowe 1Department of Geological Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305, USA Gary R. Byerly 2Department of Geology and Geophysics, Louisiana State University, 354 Howe-Russell, Baton Rouge, Louisiana 70803, USA Jacob A. Harrington 1Department of Geological Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305, USA Publisher: Geological Society of America Received: 20 Apr 2017 Revision Received: 05 May 2017 Accepted: 08 May 2017 First Online: 26 Jul 2017 Online Issn: 1943-2682 Print Issn: 0091-7613 © 2017 Geological Society of America Geology (2017) 45 (9): 803–806. https://doi.org/10.1130/G39255.1 Article history Received: 20 Apr 2017 Revision Received: 05 May 2017 Accepted: 08 May 2017 First Online: 26 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Nadja Drabon, Donald R. Lowe, Gary R. Byerly, Jacob A. Harrington; Detrital zircon geochronology of sandstones of the 3.6–3.2 Ga Barberton greenstone belt: No evidence for older continental crust. Geology 2017;; 45 (9): 803–806. doi: https://doi.org/10.1130/G39255.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The crustal setting of early Archean greenstone belts and whether they formed on or associated with blocks of older continental crust or in more oceanic settings remains a major issue in Archean geology. Here we report detrital zircon U-Pb age data from sandstones of the 3.26–3.20 Ga Fig Tree and Moodies Groups and from 3.47 to 3.23 Ga meteorite impact–related deposits in the 3.55–3.20 Ga Barberton greenstone belt (BGB), South Africa. The provenance signatures of these sediments are characterized by zircon age peaks at 3.54, 3.46, 3.40, 3.30, and 3.25 Ga. These clusters are coincident either with the ages of major episodes of felsic to intermediate igneous activity within and around the belt or with the ages of thin felsic tuffs reflecting distant volcanic activity. Only 15 of the reported 3410 grains (<0.5%) pre-date the age of the oldest rocks in the BGB. The extreme rarity of zircons older than the felsic components of the BGB itself, even after widespread deformation, uplift, and deep erosion of the BGB, implies that an older continental substrate is unlikely to have existed beneath or adjacent to the BGB. Ten of the 15 pre-BGB zircons were recovered from a single meteorite impact–related layer and may have been derived from far beyond the BGB by impact-related processes. The remaining old zircons could represent felsic rocks in older, unexposed parts of the BGB sequence, but are too few to provide evidence for a continental source. This finding offers further evidence that the large, thick, high-standing, highly evolved blocks of continental crust with an andesitic bulk composition that characterize the Earth during younger geologic times were scarce in the early Archean. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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This paper presents new U–Pb zircon analyses from garnet–sillimanite paragneisses from the Gweta borehole in northeast Botswana. Concordant to near-concordant analyses of zircon from these rocks reveal a billion year history from 3015 ± 21 Ma for the oldest detrital grain measured, to the age of high-grade metamorphism, 2027 ± 8 Ma. The maximum age of sedimentation in the Magondi belt is constrained by the age of the youngest concordant detrital zircon at 2125 ± 6 Ma. This contrasts with the age of sedimentation in the Central Zone of the Limpopo belt which is Archaean. The comparison of our results with U–Pb zircon data from the Magondi belt in Zimbabwe suggests that the granulite-facies metamorphism in this belt extended between c . 2027–1960 Ma. Granulite-facies rocks with U–Pb zircon ages in this interval are also known in the Ubendian belt and lend support to the correlation of these two segments of Palaeoproterozoic belts in southern and central–eastern Africa. The granulite facies metamorphism in the Magondi belt is coeval with the high-grade metamorphism and granitoids documented further south in the Central Zone of the Limpopo Belt.
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SHRIMP dating of xenotime overgrowths on detrital zircon grains can constrain maximum durations since diagenesis and therefore provide minimum dates of sediment deposition. Thus, xenotime dating has significant economic application to Precambrian sediment‐hosted ore deposits, such as Witwatersrand Au–U, for which there are no precise depositional ages. The growth history of xenotime in the Witwatersrand Supergroup is texturally complex, with several phases evident. The oldest authigenic xenotime 207 Pb/ 206 Pb age obtained in sandstone underlying the Vaal Reef is 2764 ± 5 Myr (1 σ), and most likely represents a mixture of diagenetic and hydrothermal growth. Nevertheless, this represents the oldest authigenic mineral age yet recorded in the sequence and provides a minimum age of deposition. Other xenotime data record a spread of ages that correspond to numerous post‐diagenetic thermotectonic events (including a Ventersdorp event at ≈ 2720 Ma) up to the ≈2020 Ma Vredefort event.
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Zircon U–Pb ages, εHf(t), and δ18O isotopic data together with geochemistry and limited Sm–Nd results from magmatic rocks sampled in deep-basement drill cores from undercover parts of the Thomson Orogen provide strong temporal links with outcropping regions of the orogen and important clues to its evolution and relationship with the Lachlan Orogen. SHRIMP U–Pb zircon ages show that magmatism of Early Ordovician age is widespread across the central, undercover regions of the Thomson Orogen and occurred in a narrow time-window between 480 and 470 Ma. These rocks have evolved εHf(t)zrn (−12.18 to −6.26) and εNd (−11.3 to −7.1), and supracrustal δ18Ozrn (7.01–8.50‰), which is in stark contrast to Early Ordovician magmatic rocks in the Lachlan Orogen that are isotopically juvenile. Two samples have late Silurian ages (425–420 Ma), and four have Devonian ages (408–382 Ma). The late Silurian rocks have evolved εHf(t)zrn (−6.42 to −4.62) and supracrustal δ18Ozrn (9.26–10.29‰) values, while the younger Devonian rocks show a shift toward more juvenile εHf(t)zrn, a trend that is also seen in rocks of this age in the Lachlan Orogen. Interestingly, two early Late Devonian samples have juvenile εHf(t)zrn (0.01–1.92) but supracrustal δ18Ozrn (7.45–8.77‰) indicating rapid recycling of juvenile material. Two distinct Hf–O isotopic mixing trends are observed for magmatic rocks of the Thomson Orogen. One trend appears to have incorporated a more evolved supracrustal component and is defined by samples from the northern two-thirds of the Thomson Orogen, while the other trend is generally less evolved and from samples in the southern third of the Thomson Orogen and matches the isotopic character of rocks from the Lachlan Orogen. The spatial association of the Early Ordovician magmatism with the more evolved metasedimentary signature suggests that at least the northern part of the Thomson Orogen is underlain by older pre-Delamerian metasedimentary rocks.
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In reconstructions of the Gondwana supercontinent, correlations of Archean domains between Madagascar and India remain debated. In this paper, we aim to establish correlations among these Archean domains using whole–rock geochemistry and U–Pb zircon geochronology of meta–granitoids from the Masora and the Antananarivo domains, central–eastern Madagascar. A meta–granitoid from the central part of Masora domain is dated at 3277 Ma and shows a typical Archean tonalite–trondhjemite–granodiorite composition, whereas a tonalitic gneiss from the southeastern part of the Antananarivo domain gives an age of 2744 Ma. The geochemical signature of this tonalitic gneiss differs from that of the ~ 2500 Ma granitoids of the northwestern part of Antananarivo domain. In addition, the geochemical composition of the ~ 760 Ma granitic gneisses is consistent with a volcanic–arc origin for the protolith. Based on the geochemical and geochronological results, along with existing data, we identified three episodes of granitic magmatism in central–eastern Madagascar at ~ 3300, 2700, and 2500 Ma. These three magmatic events are consistent with those reported for the Dharwar Craton in India, suggesting that the Archean Masora and Antananarivo domains in Madagascar were part of the Greater Dharwar Craton during the period of 3300–2500 Ma. The 700–800 Ma volcanic arc granites identified in eastern Madagascar have not been reported in India. Therefore, the subduction of the oceanic plate that led to the formation of these granites likely took place at the western margin of the Greater Dharwar Craton, which included part of eastern Madagascar.
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SHRIMP (Sensitive High‐Resolution Ion MicroProbe) analytical procedures have been developed to enable dating of the small, early diagenetic xenotime overgrowths that commonly occur on zircons in siliciclastic sedimentary rocks. The method will be particularly useful in Precambrian terranes, where diagenetic xenotime dating could play a role equivalent to biostratigraphic dating in the Phanerozoic. Reliable 207Pb/206Pb data are more readily obtained than 206Pb/238U, which also favours application to the Precambrian. However, it is demonstrated that 206Pb/238U dating of larger overgrowths (>10 μm) is also viable and applicable to Phanerozoic samples. SHRIMP Pb/Pb geochronology of authigenic xenotime in an unmetamorphosed Palaeoproterozoic sandstone in the Kimberley Basin has constrained diagenesis to a precision of ± 7 Ma. In contrast, greenschist‐facies metasediments of the Archaean Witwatersrand Basin, South Africa, contain both authigenic and alteration xenotime that record a complex history of growth from early diagenesis to the last major thermal event to affect the basin. Keywords: diagenesisgeochronologySHRIMPuranium‐thorium‐lead datingxenotime
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