2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 16
Presentation Time: 5:15 PM


HANSON, R.1, PANCAKE, J.1, CROWLEY, J.2, RAMEZANI, J.2, BOWRING, S.2, DALZIEL, I.3, GOSE, W.3, BLENKINSOP, T.4 and MUKWAKWAMI, J.5, (1)Geology Dept, TCU, Fort Worth, TX 76129, (2)Dept. Earth, Atmos. & Planet. Sci, MIT, Cambridge, MA 02139, (3)Geosci. Dept. & Inst. for Geophysics, Univ. of Texas, Austin, TX 78759, (4)Earth Sci, James Cook Univ, Townsville, QLD4811, Australia, (5)Geol. Dept, Univ. Zimbabwe, Harare, Zimbabwe, r.hanson@tcu.edu

Synchronous emplacement of large igneous provinces on the Laurentia and Kalahari cratons at ca. 1.1 Ga provides an important constraint on relative positions of the two cratons within the Rodinia supercontinent. The Laurentian province is best represented by the Midcontinent rift (MCR), where published data (including high-precision U-Pb geochronology) indicate that 75% of the total volume of basaltic magma was emplaced at 1108-1105 Ma, during an interval characterized by generally uniform magnetic polarity with sporadic, brief reversals. Traditionally, a reversed polarity has been assigned to this interval, to distinguish it from a succeeding long interval of “normal” polarity.

In southern Africa, the Umkondo igneous province on the Kalahari craton includes widespread mafic intrusions in eastern Botswana, Zimbabwe, and South Africa. Eleven separate intrusions have now yielded conventional U-Pb zircon or baddeleyite ages of ca. 1111-1105 Ma. Bimodal magmatic rocks within the province in western Botswana and Namibia have identical ages. Based on published paleomagnetic data and our new work, 60 sites from Umkondo intrusions and lavas in southern Africa have yielded statistically indistinguishable directions of magnetization. Forty eight of these sites have the same polarity. We infer that the majority of the Umkondo province was emplaced in a narrow time frame during the “reversed” interval documented from the MCR. The close similarity in timing of these major magmatic events makes it highly unlikely that they developed on unrelated cratons. Correlation of the Umkondo igneous event with 1108-1105 Ma magmatism in the MCR removes the ambiguity in polarity assignment inherent in Precambrian paleomagnetic studies and supports a Rodinia reconstruction in which Kalahari is located south of Laurentia, aligning the two igneous provinces behind a single convergent margin defined by the Grenville and Namaqua-Natal orogenic belts. However, this configuration does not align the Umkondo and MCR poles, and further work is required to produce a Rodinia model that can resolve this discrepancy.