An investigation into the formation of the lower Main Zone in the eastern limb of the Bushveld complex, South Africa

Abstract

The Main Zone is dominated by medium – grained, homogeneous gabbronorite, and anorthosites. The plagioclase compositions of the core is ((Na(0.227 – 0.353), K (0.012 – 0.046), Ca (0.651 – 0.777)) Al (1.630 – 1.752) Si (2.183 – 2.345) O8) and at the rim is ((Na (0.189 – 0.371), K (0.005 – 0.108), Ca (0.651 – 0.777)) Al (1.630 – 1.752) Si (2.183 – 2.345) O8). The composition of orthopyroxene is ((Mg (0.660 – 0.808), Fe (0.206 – 0.309), Ca (0.007 – 0.081)) Si (0.960 – 1.037) O3) and the compositions of clinopyroxene is ((Mg (0.229 – 0.678), Fe (0.092 – 0.427), Ca (0.012 – 0.475) Si (0.776 – 1.012) O3. The Mg# and An# varies with depth, where plagioclase increase in concentration the An# increases and the Mg# decreases. The variations in magma compositions are attributed to interlayering of different lithologies which are the result of fractional crystallisation in the magma chamber. This is supported by linear trends of the major and trace element bivariant plots. The magmatic event forming the Main Zone resulted in lateral expansion of the sheet – like magma chamber. The Main Zone formed through two magmas; first magma forming the lower Main Zone and the second magma, intruded the Main Zone at the level of the Pyroxenite Marker, forming the upper Main Zone. Copyright