Massive pyroxene compositional oscillations on a metre scale in the Pyroxenite Marker, northern limb, Bushveld Complex, South Africa

Autor: R. Grant Cawthorn
Rok vydání: 2020
Předmět:
Zdroj: Lithos. :105392
ISSN: 0024-4937
DOI: 10.1016/j.lithos.2020.105392
Popis: In the Main Zone of the Bushveld Complex, South Africa, there is an horizon known as the Pyroxenite Marker that shows major reversals in mineral compositions, demonstrating significant addition of more primitive magma into the chamber. This layer has been well-studied in the eastern limb, documented in one borehole from the western limb, but considered to be absent in the northern limb. A re-examination of the mineral compositions from a borehole in the northern limb suggests that a layer equivalent to the Pyroxenite Marker is present. Differences between the northern limb and the other two limbs result from the addition of a relatively much smaller volume of primitive magma in the northern limb. In this interpretation, the added magma stratified above the instantaneous floor of the chamber, and mainly primitive pyroxene grains settled through the underlying layer of more evolved resident magma (that crystallized pyroxene with an mg# of 60) to produce thin pyroxene-rich layers with an mg# value of over 70. The associated plagioclase that formed from the more evolved liquid had lower (An60) compositions. Four such extremely thin pyroxene-rich layers formed within a 4 m-thick vertical section. Three of those thin layers contain a single grain of plagioclase with An values 5 to 10 An units higher than all the other plagioclase grains, and they may also have formed and sank with the pyroxene from the primitive magma. Adjacent samples
Databáze: OpenAIRE
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