2006 Philadelphia Annual Meeting (22–25 October 2006)
Paper No. 119-16
Presentation Time: 1:30 PM-5:30 PM


ELSTON, Wolfgang E., Earth and Planetary Sciences, University of New Mexico, MSC03 2040, Albuquerque, NM 87131-0001, welston@unm.edu

Hypothesis: The 2.06 Ga Bushveld Complex (BC) occupies a heat-dominated multiring impact structure. In 1925, Daly & Molengraaff (J. Geol. 32, 1-35) recognized it as surficial; persistent myths of an intrusion (classic: lopolith) trace back to misinterpretations of its supposed roof, the Rooiberg “felsite” Group (part of the BC, though the S.A. Committee for Stratigraphy officially ruled otherwise in 1980). Hatton & Schweitzer (J. Af. Earth Sci. 21, 579-594. 1995) documented a Rooiberg mafic-to-felsic suite encompassing the entire BC, but Eales (Council Geosci. Ser. 2, 25-44, 2001) still called it “a classic example” of rhyolite and part of the pre-BC Transvaal Supergroup. Buchanan & Reimold (E&PSL 155, 149-165, 1998) and Buchanan et al., (Contr. Min. Petrol. 137, 133-146, 1999) modeled Rooiberg meltrocks as conventional volcanics with sedimentary interbeds, but textures and minerals differ from known volcanic rocks and no eruptive centers are known. Textural evidence favors impact-triggered superheated and quenched emulsions, as at Sudbury (Zieg & Marsh, GSA Bull. 117, 1427-1450, 2005). Tsunami-type interbeds suggest that water influxes triggered explosive overflows from a Sudbury-type meltpool (only upper granophyre is exposed). Inflated overflows overrode the collapsing peak ring and pooled (up to 4 km) in the outer ring. Above partly melted quartzose debris (detached from collapsed central uplift), the transition from mafic to felsic meltrocks is enriched in Fe, Ti, P (Twist, Econ Geol. 80, 1153-1165, 1885), as at Sudbury.

Meanwhile, decompression melts from an impact-induced upwelling (Jones et al., E&PSL 202, 551-556) formed massive mafic and granitic sills (> 10 km; horizontal from paleomagnetic evidence; Hattingh, S. Af. Geophys. Rev. 2, 75-77, 1998) beneath accumulating Rooiberg melts in the outer ring. Simultaneous subsidence explains their survival without collapse. Crustal melting eventually led to caldera-like collapse of the entire BIC, forming a lobate 400-km basin ringed by megabreccia. Sills in the outer ring, tilted basinward by collapse, became the dipping sheets modeled by geophysics (Meyer and de Beer, Nature 325, 610-612, 1987). As the meltpool equilibrated with the crust, the upper zones of Rooiberg overflow (easily accessible to visitors) came to resemble conventional rhyolite.

2006 Philadelphia Annual Meeting (22–25 October 2006)
General Information for this Meeting
Session No. 119--Booth# 132
Impact Craters: Structures, Drilling, Ages, and Geophysics (Posters)
Pennsylvania Convention Center: Exhibit Hall C
1:30 PM-5:30 PM, Monday, 23 October 2006

Geological Society of America Abstracts with Programs, Vol. 38, No. 7, p. 299

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