Incoming magma composition and style of recharge below the pyroxenite marker, eastern bushveld complex, south africa

Abstract

The Pyroxenite Marker in the eastern Bushveld Complex marks the termination of a pronounced upward trend to more primitive major and trace element mineral compositions. This geochemical reversal begins c. 350 m below the Pyroxenite Marker and suggests the input of a hotter, more primitive magma into the resident chamber. Here we present in situ major and trace element data for plagioclase, clinopyroxene, and low Ca-pyroxene to document the reversal interval. The data indicate that new magma batches were injected over time and that delivery rate was sufficiently slow that the main magma body was able to homogenize, cool, and partially crystallize between successive injections, even as it grew and became hotter and more primitive with each new injection. From the mineral data, we calculate the proportions and compositions of the incoming and resident magmas. The calculated incoming magma composition is similar to that of the tholeiitic B2 magma thought to have been the parent of the Upper Critical Zone. The similarity between the rocks of the Upper Critical Zone and the Upper Zone is a result of their similar source magma compositions rather than prior connectivity, as has been proposed elsewhere ©The Author 2013.