Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Schmitz, M. D. Articles by Bowring, S. A. Search for Related Content GeoRef GeoRef Citation

Ultrahigh-temperature metamorphism in the lower crust during Neoarchean Ventersdorp rifting and magmatism, Kaapvaal Craton, southern Africa

Mark D. Schmitz^,1 and Samuel A. Bowring^,1

¹ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

A diverse suite of ultrahigh-temperature (UHT) granulite xenoliths from kimberlites of the central Kaapvaal craton (South Africa) record the deep crustal manifestation of Neoarchean Ventersdorp extensional tectonics and magmatism. U-Pb geochronological data for texturally characterized metamorphic zircon and monazite from a number of metasedimentary (garnet + quartz ± sapphirine ± sillimanite ± antiperthite) UHT granulites are combined with published geothermobarometry to elucidate the pressure-temperature-time (P-T-t) path of metamorphism. Prograde metamorphic growth of zircon and monazite indicates ramping of lower crustal temperatures 25 Ma prior to peak UHT metamorphic conditions at ca. 2720–2715 Ma. The subsequent cooling path is established by dating of 2690 Ma zircon in retrograde kyanite-rich leucosomes, indicating initial cooling rates of 10 °C/Ma. Preservation of peak and prograde monazite dates through the UHT conditions support a very high volume diffusion closure temperature for this mineral, similar to that of zircon (>1000 °C).

The 5–10 Ma duration of UHT metamorphism in the lower crust is directly correlative with the rapid eruption of the Ventersdorp flood basalts through the central Kaapvaal craton at 2714 ± 8 Ma, and associated crustal melting, plutonism, and widespread extensional tectonics. UHT metamorphism is postulated to have occurred in response to nonuniform intracratonic lithospheric thinning and superimposed magmatic heat advection. The links between upper and lower crustal manifestations of 2.7 Ga magmatism and extension support the hypothesis that the present-day flat seismic Moho of the central and western Kaapvaal craton is a lower crustal rheological response to Ventersdorp tectonomagmatism. The lithospheric thermal gradient necessitated by the UHT lower crustal granulites demands a transient removal of the thermal lithosphere localized along the north-northeast–trending axis of the Ventersdorp rift at 2.72 Ga. However, contemporaneous maintenance of a coherent mantle keel with "cratonic" thermal gradients 200 km distal to the rift is required in order to preserve ancient (3.2–2.9 Ga) diamondiferous lithosphere. The contrast between the craton-wide geography of crustal extension and the more restricted localization of thermal effects in the lithospheric mantle embodies a profound crust-mantle decoupling in the Kaapvaal craton during intracratonic rifting, and bears upon our understanding of the generation and preservation of cratonic lithospheric mantle keels.

Key Words: U-Pb geochronology • zircon • monazite • UHT metamorphism • sapphirine • granulite • lower crust • Ventersdorp Supergroup • Kaapvaal craton • rifting

This article has been cited by other articles:

				H. E. Frimmel, A. Zeh, B. Lehrmann, D. Hallbauer, and W. Frank Geochemical and Geochronological Constraints on the Nature of the Immediate Basement next to the Mesoarchaean Auriferous Witwatersrand Basin, South Africa J. Petrology, November 4, 2009; (2009) egp073v1. [Abstract] [Full Text] [PDF]

				A. B. Kelts, M. Ren, and E. Y. Anthony Monazite occurrence, chemistry, and chronology in the granitoid rocks of the Lachlan Fold Belt, Australia: An electron microprobe study American Mineralogist, February 1, 2008; 93(2-3): 373 - 383. [Abstract] [Full Text] [PDF]

				J.A. Baldwin, M. Brown, and M.D. Schmitz First application of titanium-in-zircon thermometry to ultrahigh-temperature metamorphism Geology, April 1, 2007; 35(4): 295 - 298. [Abstract] [Full Text] [PDF]

				D. Y. Sumner and N. J. Beukes Sequence Stratigraphic Development of the Neoarchean Transvaal carbonate platform, Kaapvaal Craton, South Africa South African Journal of Geology, June 1, 2006; 109(1-2): 11 - 22. [Abstract] [Full Text] [PDF]

				B. M. Eglington and R. A. Armstrong The Kaapvaal Craton and adjacent orogens, southern Africa: a geochronological database and overview of the geological development of the craton South African Journal of Geology, June 1, 2004; 107(1-2): 13 - 32. [Abstract] [Full Text] [PDF]

				K.J. Westerlund, J.J. Gurney, R.W. Carlson, S.B. Shirey, E.H. Hauri, and S.H. Richardson A metasomatic origin for late Archean eclogitic diamonds: Implications from internal morphology of diamonds and Re-Os and S isotope characteristics of their sulfide inclusions from the late Jurassic Klipspringer kimberlites South African Journal of Geology, June 1, 2004; 107(1-2): 119 - 130. [Abstract] [Full Text] [PDF]

				S. J. Webb, R. G. Cawthorn, T. Nguuri, and D. James Gravity modeling of Bushveld Complex connectivity supported by Southern African Seismic Experiment results South African Journal of Geology, June 1, 2004; 107(1-2): 207 - 218. [Abstract] [Full Text] [PDF]

				M. D. Schmitz, J. D. Vervoort, S. A. Bowring, and P. J. Patchett Decoupling of the Lu-Hf and Sm-Nd isotope systems during the evolution of granulitic lower crust beneath southern Africa Geology, May 1, 2004; 32(5): 405 - 408. [Abstract] [Full Text] [PDF]