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PROPOSED BUSHVELD SCENARIO IMPACT_ MANTLE UPWELLING_ MELTDOWN

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					Large Meteorite Impacts and Planetary Evolution IV (2008)                                                                           3015.pdf



         PROPOSED BUSHVELD SCENARIO: IMPACT, MANTLE UPWELLING, MELTDOWN, COLLAPSE
         W. E. Elston, Department of Earth and Planetary Sciences, MSC03 2040, University of New Mexico, Albuquerque,
         NM 87131-0001, USA, welston@unm.edu.

              Unique Bushveld Rocks: The unique Bushveld is            “missing” Magaliesberg Formation, detached from the
         the largest continental igneous complex and in a class        collapsing central uplift. Impact timing is bracketed
         by itself among proposed terrestrial impact structures.       between Pretoria sedimentation and Rooiberg coming-
         Each component (~85% endogenic) is the largest                to-rest.
         known of its kind (Vt=106km3): mafic cumulates                     Mantle Upwelling, Lithosphere Melting: Impacts
         (Rustenburg Layered Suite), A-type Lebowa Granite             triggered an upwelling of the upper mantle [7]. De-
         Suite, multi-origin Rashoop Granophyre, and the               compression partial melting created the Rustenburg
         Rooiberg Group of unique high-T pseudovolcanic                Layered Suite, induced crustal melting the Lebowa
         meltrocks with major sedimentary components [1],              Granite Suite. Rising through inward-dipping fractures
         commonly misinterpreted as felsite or “classical exam-        [8] into the first outer ring,, successive mafic and gran-
         ple” of rhyolite [2]. Rooiberg rocks share some charac-       itic melts spread below Rooiberg accumulations, as
         teristics with the Onaping Fm. (Sudbury) but rarely           km-thick sills. In most places, evidence for impact in
         preserve shock features in an environment dominated           basal Rooiberg zones was obliterated by contact
         by heat. They span the entire history of the Bushveld         metamorphism and metasomatism [9]. Rustenburg
         Complex [3] and record its proposed stages: impact,           cumulate zones, traceable over hundreds of kilometers
         mantle upwelling (or plume [3]) with melting in mantle        on the Bushveld perimeter, imply extraordinary quies-
         and crust, quiescence interrupted by catastrophic cal-        cence. Meanwhile, explosive overflows from the
         dera-like collapse, and the beginning of a >100 m.y.          meltpool into the outer ring continued, triggered by
         period of adjustment.                                         influxes of water.
              Impact: A catastrophe at ~2.06 Ga, interpreted as             Models suggest that “an impact big enough to lift
         a cluster of quasi-simultaneous impacts, abruptly             hot mantle rocks close to the surface” with “a final
         ended epicontinental sedimentation (Pretoria Group,           crater 400-500 km” is “possible, but…highly improb-
         Transvaal Supergroup). A multiring [4] four-lobed             able…in the last 3.3 b.y years of terrestrial… history”
         transient crater (or overlapping crater cluster) was          [10]. It is proposed that the improbable happened.
         modified by collapse of its central uplift(s) and peak             Caldera-Like Collapse: Generation of Lebowa
         ring. The Rooiberg Group began to accumulate in the           Granite by crustal melting resulted in instability (e.g.,
         first outer ring (eventual depth 4-5 km [2]) from re-         crustal diapirs, also known as “cold fingers in a hot
         peated overflows of a superheated Sudbury-type melt-          magma” [11]). It culminated in catastrophic caldera-
         pool [5].                                                     like collapse, creating the present lobate Bushveld ba-
               Evidence for Crater Collapse: Segments of the           sin. Paleomagnetic evidence [12] shows Rustenburg
         collapsed central uplift and peak ring are exposed in         sills to have been horizontal until at least the em-
         paired inliers of Pretoria Group rocks (diam. tens of         placement of the upper zones. They acquired their pre-
         km) inside the western and eastern lobes [6]. A promi-        sent basinward dip when collapse enlarged the Bush-
         nent Pretoria Group unit, 500-m Magaliesberg Quartz-          veld basin beyond the limit of the outer ring. Much
         ite, is absent. In each pair, brecciated, faulted, but oth-   reduced be erosion, it still covers 67,349 km2. By com-
         erwise mildly deformed post-Magaliesberg units are            parison, Sudbury, also a remnant, covers only 1,341
         overlain by undeformed basal Rooiberg meltrocks and           km2 [13]. The resulting Bushveld structure is compati-
         juxtaposed by a strike-slip fault against intensely de-       ble with a dipping sheets geophysical model [14],
         formed pre-Magaliesberg units. The deformed units             modified by replacing some Lebowa Granite in the
         are attenuated, metamorphosed to pyroxene hornfels            center with the geophysically indistinguishable
         facies, and highly contorted [6]. Such deformation of         Rooiberg meltpool. In this model, unlike the classic
         Pretoria-age rocks is unknown in the RSA beyond the           lopolith, Rustenburg sills do not extend into the center
         Bushveld.                                                     of the Bushveld basin.
              Interpretation: Mildly deformed inliers are seg-              Evidence for Caldera-Like Collapse: A succes-
         ments of the collapsed peak ring; deformed inliers            sion of tuff breccia, megabreccia (with clasts up to tens
         parts of the collapsed central uplift. In the juxtaposing     of meters), and ash-fall tuff (Union Tin “shale”) in the
         fault of the eastern pair, quartzite slabs from the unde-     upper part of the Rooiberg Group preserves evidence
         formed inlier, up to hundreds of m long, were engulfed        for caldera-like collapse. It resembles collapse breccias
         by basal Rooiberg debris flows of recrystallized-to-          of large ignimbrite calderas [15] in all but vastly
         melted sediments. The source of the sediments was the         greater scale. As a marker around the Bushveld pe-
Large Meteorite Impacts and Planetary Evolution IV (2008)                                                                                            3015.pdf



         riphery [16], it shows the Rooiberg Group to be a sin-           Fe, Ti, and P appears. The remainder of the Damwal Forma-
         gle stratigraphic package. Rhyolite provinces of simi-           tion consists of siliceous rocks of highly complex petrogra-
         lar 105km3 volume resolve into ignimbrite sheets radi-           phy. Interlayered high-energy siliciclastic sedimentary de-
         ating from numerous calderas. Remarkably, no                     posits (unsorted matrix-supported exotic clasts [25]) record
         Rooiberg source is known, other than a meltpool                  cold water influxes into the meltpool, causing explosive
         largely obscured by invading granite. Only a segment             eruptions.
         of its massive red granophyric upper zone (cf., Sud-                 The mafic-siliceous succession with high Fe-Ti-P inter-
         bury) survived, at the Rooiberg          (the atypical           face mimics the Sudbury meltpool [5]. Textures suggestive
         Rooiberg Group type locality).                                   of Sudbury-style immiscible superheated emulsion droplets
             The same granite invasions obliterated all in situ           [5] include spherules. A 1-m “cool” Rosetta Stone bed (in a
         evidence for impact. In the meltpool, they triggered             3,500-m section) revealed diaplectic quartz, maskelynite in
         further Rooiberg eruptions and generated a type of               plagioclase, kink bands, and features that remain unex-
         Rashoop Granophyre by reaction between granite                   plained.
         magma and hot melt [17]. Late Rooiberg flows equili-                 Over much of the Bushveld, granite intrudes the basal
         brated with granophyre and granite [3]. Physically and           Kwaggaskop Fm. [16]; it is unknown whether Dullstroom
         chemically, they superficially resemble rhyolite.                and Damwal exist in the subsurface. Kwaggaskop and
             Aftermath: The last Rooiberg flows intertongue               Schrikkloof Fms. resemble conventional rhyolite; the col-
         with sedimentary crater fill (Loskop Formation [18]);            lapse megabreccia is at their contact. Interbedded sediments
         deformation reached the Waterberg Supergroup [19].               have local origin.
              The Extrusive Bushveld: It has frequently been ignored          Regional and Global Implications: Space permits only
         that the Bushveld Complex is extrusive. Its members intrude      brief hints: From field evidence, both Bushveld and Vrede-
         each other, with low-density Rooiberg rocks as roof, but         fort impacts preceded Rustenburg magmatism. A multiple
         there is “no continuous sedimentary roof at all” [20]. Confu-    Bushveld-Vredefort impact disturbed paleomagnetic orienta-
         sion arose from a misguided ruling that “the Rooiberg is not     tions and isotopic systems, remobilized Wits gold, and left
         part of the Bushveld Complex” [21)] and its mistaken as-         anomalous lithosphere to this day. There is evidence for a
         signment to the Transvaal Supergroup [2]. Subsidence of the      global ‫31ג‬C excursion and a significant increase in atmos-
         first outer ring, concomitant with central upwelling [22],       pheric oxygen, with biological and sedimentological implica-
         allowed >10 km of magma to accumulate on the surface,            tions [31].
         without collapse.                                                     Acknowledgments: This study would have been impossible
              Rooiberg Evidence: Exposures of all four Rooiberg for-      without generous (if skeptical) field guides (David Twist, Jochen
                                                                          Schweitzer, Frik Hartzer) and field assistants (E. G. Deal, M. Caress,
         mations (Dullstroom, Damwaal, Kwaggasnek, Schrikkloof)           J. M. de Moor, T. Manyeruke), and support by the University of
         are confined to the southeastern Bushveld [16]. The scoured      Pretoria (Professors von Gruenewaldt, De Waal, Eriksson). My
         and locally polished Pretoria-Rooiberg contact was pre-          sincere thanks to them all!
         served from later intrusions in only in only three places: the        References: [1] Coetzee, G. L. (1970) Geol. Soc. S. Africa Spec.
         two undeformed inliers (not reached by dipping sheets) and       Pub. 1, 312-325. [2] Eales, H. V. (2001) Pop. Geoscience Ser. 2,
                                                                          Council for Geoscience. [3] Schweitzer et al. (1997) J. African Earth
         distal paleochannels at the base of the type-Dullstroom For-     Sci. 24, 95-104 [4] Rhodes, R. C. (1975) Geology 3, 549-554. [5]
         mation, a 1,200-m sliver of basal Rooiberg beneath the east-     Zieg, M. J. & Marsh, B. D. (2005) Geol. Soc. America Bull. 117,
         ern Rustenburg limb [16]. Critical exposures of impact ejecta    1427-1450. [6] Hartzer, F. J. (2000) Geol. Surv. S. Africa Mem. 88.
         (as distinguished from later overflows) are confined to a 200-   [7] Jones, A. P. et al. (2003) E&PSL 202, 551-561. [8] Sharpe, M. R.
         m basal Dullstroom Fm. section in the same three locations.      et al. (1981) Geol. Soc. S. Africa Trans. 84, 139-244. [9] von Grue-
                                                                          newaldt, G. (1972) Geol. Soc. S. Africa Trans. 75, 121-134. [10]
         They consist of inflated debris avalanches of recrystallized
                                                                          Ivanov, B. A. & Melosh, H. J. (2003) Geology 31, 869-872. [11]
         cm-to-m quartzite clasts in a variable matrix of average         Gerya, T. V. et al. (2003) Geology 31, 753-756. [12] Hattingh, P. J.
         crust and siliciclastic sediments, in every stage toward su-     (1998) Southern African Geophys. Rev. 2, 75-77. [13] Hunter, D. R.
         perheated melting and quenching, with residues of sedimen-       (1975) Bushveld Map, Econ. Geol. Res. Unit, Univ. Witwatersrand.
         tary quartz but no phenocrysts (Basal Rhyolite [16]). Quartz     [14] Meyer, R. and de Beer, J, H. (1987) Nature 325, 610-612. [15]
                                                                          Lipman, P. W. (1976) Geol. Soc. America Bull. 87, 1397-1410. [16]
         paramorphs after three high-T SiO2 polymorphs (“tridymite”
                                                                          Schweitzer, J. K. et al. (1995) S. African J. Geology 98, 245-255.
         [23]), unknown in volcanic rocks, are typical; the lowest-       [17] De Bruiyn, H. (1975) Geol. Soc. S. Africa Trans. 78, 185-190.
         temperature form (swallow-tailed needles) is known from          [18] Martini, J. E. J., 1998, J. African Earth Sci. 27, 193-222. [19] de
         partially melted basal Onaping breccia [24]. Though not yet      Bruiyn, H. (1971-72) Annals Geol. Survey 9, 91-94. [20] Daly, R. A.
         systematically sampled, these basal zones have yielded ex-       & Molengraaff, G. A. F. (1924) J. Geology 32, 1-25. [21] South
                                                                          African Committee for Stratigraphy (1980), Geol Surv. Handbook 8.
         amples of medium-level shock (cataclasis, mosaicism, de-
                                                                          [22] Marsh, B. D. (1982) Am. J. Sci .282, 908-955. [23] von Grue-
         formation twins in quartz).                                      newaldt, G. (1968) Geol .Soc S. Africa Trans. 71, 153-176. [24]
              The remainder of the Dullstroom Formation is a hetero-      Stevenson, J. S. (1963) Canadian. Mineralogist. 7, 413-419. [25]
         geneous succession of mainly mafic rocks. At the base of the     Eriksson et al. 1994, J. Sedimentary Research 64, 836-846. [26]
         Damwal Fm., a glassy rock with quench needles, enriched in       Melezhik, V. A. et al. (2005) Geology Today 15, no. 11, 4-11.

				
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