Magnetic susceptibility and AMS of the Bushveld alkaline granites, South Africa

The Bushveld Complex in South Africa includes one of the world's largest an orogenic alkaline granite intrusions (66,000 km(2)). The granite forms a co mposite laccolith, of 350 x 250 km in area and about 2 km in thickness, whi ch was emplaced at about 5 km depth into sediments overlying the Kaapvaal c raton, at 2054 Ma. The Bushveld granite and its roof-rocks have long been m ined for Sn, W and F. The Bushveld granites have high magnetic susceptibili ties (K-m from 1000 to 4000 mu SI), and a quantitative model is presented, suggesting that susceptibility fabrics are primarily carried by ferromagnet ic minerals. The measured AMS foliations coincide with observed subhorizont al mineral lineations and compositional layering. Magnetic lineation trends vary considerably within the horizontal plane. The existence of a weak pla nar fabric and, an almost absent linear component may reflect (a) laccolith ic emplacement by roof uplift, causing flattening magmatic fabrics, or (b) emplacement of largely crystal-free magma crystallizing in-situ and develop ing horizontal compositional layering from thermal chemical diffusion front s and gravity-driven mechanisms. Weak magnetic fabrics, like those identifi ed in the Bushveld granites require specific sampling schemes and procedure s, in addition to rigorous constraint of magnetic mineralogy and crystalliz ation sequence. (C) 1999 Elsevier Science B.V. All rights reserved.