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in:Bushveld Complex
M. Knoper¹, Thomas H. Jordan², Lewis D. Ashwal¹
¹Dept. of Geology, R.A.U.,
Auckland Park 2006 Johannesburg R.S.A.
²Dept. of Earth, Atmospheric, and Planetary Sciences, M.I.T., Cambridge Mass.
02139 U.S.A.
The origin of the Bushveld Complex has been attributed most commonly to a deep-seated mantle plume [1] or to a catastrophic bolide impact [2]. The lack of evidence supporting an impact origin [3] has led to wide acceptance of models invoking a mantle plume to generate the Bushveld magmas. Although a mantle plume can successfully explain most magmatic features of the Bushveld Complex, a plume origin is inconsistent with development of the thick clastic sedimentary sequence that precedes intrusion of the Bushveld magmas. To account for the magmatic features of the Bushveld Complex and yet explain pre-Bushveld clastic sedimentation resulting from thermal subsidence, we reject the mantle plume hypothesis and instead propose that the Bushveld Complex originated from an unstable region of "asthenospheric" mantle (i.e., fertile, oceanic mantle that can participate in convection) trapped under a thinspot in the Kaapvaal continental lithosphere.
The Bushveld Complex was emplaced at 2.06 Ga [4] along or near the unconformity between sedimentary rocks of the Pretoria Group and overlying Rooiberg Group volcanic rocks [5]. Prior to Bushveld emplacement, deposition of alternating mudstone and sandstone, interbedded volcanic rocks, and subordinate conglomerates and carbonate rocks of the 7 km thick Pretoria Group [6] are thought to have occurred in a thermal subsidence basin [7] over a 400 m.y. interval of time starting at about 2.4 Ga. Eruption of the dominantly felsic lavas of the Rooiberg Group occurred at about 2.06 Ga synchronous with [8] or immediately prior to Bushveld intrusion. Early workers observed the close association between deposition of the Pretoria Group, eruption of the Rooiberg felsites, and intrusion of the Bushveld magmas and perceived a link [9,10]. Alex du Toit suggested that the ...crust beneath the centre of the Transvaal, weakened by the enormous amount of subsidence while the Transvaal System and Rooiberg Series were being laid down - during which time lavas were repeatedly erupted - sagged, … actually seems to have been torn asunder and so allowed an immense volume of basic magma to well-up from below and to spread in sheet-like masses within the collapsing strata, particularly along a horizon somewhere near the top of the Pretoria Series, chiefly … between the latter and its covering of the Rooiberg Series. [10]. Alex du Toit's original perception may be correct, and we elaborate upon his interpretation linking the origin of the Bushveld Complex and Rooiberg felsites to that of the Pretoria Group sedimentary basin.
One set of difficulties with a plume model involves the expected interaction between a mantle plume and continental lithosphere. When plumes impinge on lithosphere, the crust first domes upwards, volcanism closely follows, and then active rather than passive rifting occurs [11]. When considering a plume origin for the Bushveld Complex, there is no clear evidence supporting active rifting to form the Pretoria Group basin in the appropriate time interval prior to intrusion of the Bushveld Complex at 2.06 Ga. Sagging of the Kaapvaal crust by thermal subsidence to accumulate sediment has instead been advocated [7,12], and volcanic rocks erupted intermittently throughout the 400 m.y. duration of Pretoria Group sedimentation [13]. These events are more characteristic of passive rifting, that is, crustal extension and subsidence driven by the continental lithosphere (i.e., tectosphere [14]), rather than by dominately or entirely asthenospherically driven processes. Additional features inconsistent with a plume origin include the lack of any clearly discernable Paleoproterozoic plume track on the Kaapvaal craton, and the perhaps detrimental effect plumes would have on diamond preservation within the Kaapvaal tectosphere.
We suggest that the Bushveld Complex originated from an unstable region of asthenospheric mantle trapped under a thinspot in the Kaapvaal lithosphere. Such a thinspot possibly formed after the Kaapvaal and Zimbabwe cratons sutured during collisional tectonics at about 2.7 Ga [15] or more likely at about 2.47 Ga [16]; entrapment of the asthenospheric mantle was facilitated by the thick (>200 km) sub-cratonic mantle (i.e., tectosphere [14]) of the Kaapvaal craton [17]. The thinspot model predicts that the trapped mantle conductively cooled through the surface as well as into the sidewalls of the cratons, causing surface subsidence and sedimentary infilling of the Pretoria Group basin. Because the cratonic sidewalls are thick, this subsidence proceeded for a long period (approximately 400 m.y.) but eventually the density increase from mantle cooling initiated a convective instability. In response to sinking of cool, dense mantle material along the sidewalls, hot, buoyant mantle passively upwelled in the centre of the trapped region of asthenospheric mantle. Adiabatic decompression melting of upwelling mantle and contamination of these melts with crust were the source of the Bushveld magmas. Early thermal subsidence to form the Pretoria Group sedimentary basin resulted from conductive cooling of trapped mantle (Fig. 1), and the Bushveld and Rooiberg magmas originated later during upwelling of hot, buoyant mantle (Fig. 2).
The thinspot model dynamically links thermal subsidence of the Pretoria Group sedimentary basin to later production of Bushveld magmas and eruption of Rooiberg felsites. By analogy to oceanic lithosphere [18], a thinspot model implies (a) thermal subsidence spanning 400 m.y. corresponds to development of a 200 km thick thermal boundary layer in the mantle trapped under the lithospheric thinspot, (b) conductive cooling of 200 km of mantle results in about 3 km of thermal subsidence after 400 m.y., (c) mantle instability is shortlived and convective overturn of trapped mantle likely spans less than 5 m.y., (d) adiabatic decompression melting of 1.2×107 km3 source mantle produces over 1×106 km3 of mafic magma, and (e) Bushveld mafic magmas were generated from asthenospheric mantle that was "not anomalously hot" (i.e., not anomalously above 1280° C).
This research is a collaborative effort of part of the Bushveld working group associated with the N.S.F.-, N.R.F.-, and industry-funded "Anatomy of an Archean Craton" research project awarded in part to T.H.J., and of the N.R.F.- and industry-funded "African Lithospheric Evolution" research project awarded to L.D.A.
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in:
Bushveld Complex
·Web publication date: July 15, 1999·
"Rockscapes" 1996-2001 M Knoper
