Proposed Bushveld Scenario: Impact, Mantle Upwelling, Meltdown, Collapse W
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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, [email protected]. 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. 6 3 known of its kind (Vt=10 km ): 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 -13C excursion and a significant increase in atmosג signment to the Transvaal Supergroup [2]. Subsidence of the global 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