HR: 16:45h
AN: V54A-04    [Abstracts]
TI: Structural and Compositional Constraints on the Emplacement of the Bushveld Complex, South Africa
AU: * Clarke, B
EM: bmclarke@geoscience.org.za
AF: Council for Geoscience, PO Box 900, Pietermaritzburg, 3200, South Africa
AU: Uken, R
EM: uken@ukzn.ac.za
AF: School of Geological Sciences, University of KwaZulu-Natal, Westville Campus PBag X54001, Durban, 4000, South Africa
AU: Reinhardt, J
EM: reinhardtj@ukzn.ac.za
AF: School of Geological Sciences, University of KwaZulu-Natal, Westville Campus PBag X54001, Durban, 4000, South Africa
AB: Despite a plethora of petrological studies, the emplacement mechanics of the world's largest layered intrusion, the 2.06 Ga Bushveld Complex in South Africa, are still poorly understood. Early models considered the intrusion to comprise separate, lopolithic intrusions or even concentric cone sheets, but recently, overwhelming support for a sill-like intrusional form has emerged [1,2]. Examination of the contact aureole reveals three groups of emplacement related structures. Interfinger deformation zones and bridges, formed between intruding and dilating magma fingers, are preserved in both the western and eastern parts of the Complex. Two magma conduits are also known – both of these conduits are associated with strongly deformed wall-rocks and at least one of them was subjected to high magma fluxes and hosts economically significant Ni deposits. The final group of structures are diapiric domes that characterize the eastern contact aureole. These domes formed by diapiric amplification of initial interfinger deformation zones associated with the earliest mafic-ultramafic pulse of the Bushveld Complex [3,4]. The diapiric domes, interfinger deformation zones and magma conduits exhibit a strong NW-SE preferred orientation, while longitudinal conduit terminations and divergence of the conduits away from their source horizons indicates magma emplacement towards the SE. This emplacement direction is supported by numerous and varied petrological data, from bulk lithological facies variations in the layered sequence, to mineral chemical variations within specific horizons. The thickest and most chemically primitive accumulations of the lower zone of the Complex are found adjacent to the Thabazimbi-Murchison lineament (TML), a crustal scale lineament which has undergone polyphase reactivation from at least 2.7 Ga [5], and these accumulations thin towards the NW and SE away from the lineament. The Bushveld Complex was most likely fed by a feeder dyke that utilized the TML and spread laterally from the dyke-axis to form its current sill-like geometry. This implies the stress field at 2.06 Ga was suitably oriented to allow for dilation of the ENE-trending TML, and suggests the Kaapvaal craton was subject to a component of NW-SE extension. The Limpopo belt, however, records NW-SE directed transpressional collision of the Kaapvaal and Zimbabwe cratons at this time [6], suggesting that Bushveld emplacement occurred under conditions of far field extensional stress within this transpressional setting, and may add support to a subduction origin for the Bushveld Complex. References: [1] Cawthorn RG et al., S. Afr. J. Geol., 101, 291-298, 1998; [2] Kruger FJ, Econ. Geol. Res. Inst. Univ. Witwatersrand, 377, 26 p, 2004; [3] Uken R and MK Watkeys, Geology, 25, 723-726, 1997; [4] Gerya TV et al., Geology, 31, 753-756, 2003; [5] Good N and MJ de Wit, J. Geol. Soc. London, 154, 93-97, 1997; [6] Holzer L et al., J. Afr. Earth Sci., 28, 383-402, 1999.
DE: 1036 Magma chamber processes (3618)
DE: 3642 Intrusive structures and rocks
DE: 8035 Pluton emplacement
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting