TY - BOOK

T1 - The upper zone of the Bushveld Complex, South Africa

T2 - Parental magma and crystallization processes

AU - Fischer, Lennart A

N1 - Doctoral thesis

PY - 2018

Y1 - 2018

N2 - The Bushveld Complex in South Africa is the largest layered intrusion on Earth. Its upper part is known for huge resources of iron, titanium, vanadium and phosphorus. Associated with the layered character of the rocks, these economically valuable elements are enriched at certain levels of the intrusion. Thus it is important to understand the formation processes of those layers. Using samples from the Bierkraal drill cores, representing the entire Upper and Upper Main Zone in the Western Limb, this thesis gives detailed insights into magmatic processes and the parental magma as well as the prevailing conditions forming the Upper and Upper Main Zone of the Bushveld Complex. A detailed study of modal proportions of the Bierkraal drill cores, contributed to a better understanding of the top part of the intrusion. The observed compositional cyclicity of the Upper and Upper Main Zone was recently explained by several magma injections with a plagioclase-laden magma. Anorthosite layers are produced by crystal settling of the transported plagioclase crystals and magnetitite layers crystallized from a hybrid melt, produced by mixing of resident and injected magma. However, further discussion of the prevailing magma conditions affecting the mineral compositions as well as detailed description of the sub-magmatic system is missing. In this study, the investigation of melt inclusions in apatite from the Upper Zone showed a compositional range from Fe-rich to Si-rich liquids best explained by silicate liquid immiscibility. During late-stage magmatic evolution the liquid line of descent of the Upper Zone reaches the two-liquid field and immiscible melts start to segregate. The continuous range of melt inclusion compositions observed in this study is the result of cooling. Thus, the liquid evolves along the binodal of the two-liquid field, producing more and more contrasting compositions. The dense, low-viscous, Fe-rich liquid percolates downwards and the cumulates from this Fe- and P-rich immiscible melt form the nelsonite layers observed in the Upper Zone. Trace-element distribution in titanomagnetite and clinopyroxene as well as major-element composition of clinopyroxene revealed two major compositional shifts towards more evolved signatures within the Upper Zone stratigraphy. These distinct changes in mineral composition are in correlation with bulk-rock vanadium concentrations. They can be explained by the injection of a more evolved magma. Compared to previously proposed cyclical shifts incomposition, these two events are much more pronounced, arguing against a continuously evolving staging chamber. In fact, this supports the hypothesis of a sub-compartmentalized Bushveld staging chamber as a source for the magma injections. Prevailing oxygen fugacity (fO2) conditions in the Upper Zone were estimated using the V partitioning between titanomagnetite and clinopyroxene. The results show that fO2 variations are minor in the Upper Zone and are approximately around FMQ - 2, which is 1.5 - 2 log units lower than previously assumed. To discuss potential parental magmas of the Upper and Upper Main Zone, crystallization experiments were performed in an internally heated pressure vessel, aiming to reproduce the mineral assemblage above the Pyroxenite Marker. Previously proposed and new calculated compositions were tested at conditions relevant to the Bushveld Complex (2 kbar; 1080°C - 1140°C; ~FMQ -2). In contrast to previously proposed basaltic compositions, the experimental results show that the parental magma to the Upper and Upper Main Zone of the Bushveld complex must be andesitic. Moreover, experimentally produced mineral compositions revealed, that the Upper and Upper Main Zone parental magma contained a residual liquid from the underlying zones high in Mg and Ca also small amounts of H2O (

AB - The Bushveld Complex in South Africa is the largest layered intrusion on Earth. Its upper part is known for huge resources of iron, titanium, vanadium and phosphorus. Associated with the layered character of the rocks, these economically valuable elements are enriched at certain levels of the intrusion. Thus it is important to understand the formation processes of those layers. Using samples from the Bierkraal drill cores, representing the entire Upper and Upper Main Zone in the Western Limb, this thesis gives detailed insights into magmatic processes and the parental magma as well as the prevailing conditions forming the Upper and Upper Main Zone of the Bushveld Complex. A detailed study of modal proportions of the Bierkraal drill cores, contributed to a better understanding of the top part of the intrusion. The observed compositional cyclicity of the Upper and Upper Main Zone was recently explained by several magma injections with a plagioclase-laden magma. Anorthosite layers are produced by crystal settling of the transported plagioclase crystals and magnetitite layers crystallized from a hybrid melt, produced by mixing of resident and injected magma. However, further discussion of the prevailing magma conditions affecting the mineral compositions as well as detailed description of the sub-magmatic system is missing. In this study, the investigation of melt inclusions in apatite from the Upper Zone showed a compositional range from Fe-rich to Si-rich liquids best explained by silicate liquid immiscibility. During late-stage magmatic evolution the liquid line of descent of the Upper Zone reaches the two-liquid field and immiscible melts start to segregate. The continuous range of melt inclusion compositions observed in this study is the result of cooling. Thus, the liquid evolves along the binodal of the two-liquid field, producing more and more contrasting compositions. The dense, low-viscous, Fe-rich liquid percolates downwards and the cumulates from this Fe- and P-rich immiscible melt form the nelsonite layers observed in the Upper Zone. Trace-element distribution in titanomagnetite and clinopyroxene as well as major-element composition of clinopyroxene revealed two major compositional shifts towards more evolved signatures within the Upper Zone stratigraphy. These distinct changes in mineral composition are in correlation with bulk-rock vanadium concentrations. They can be explained by the injection of a more evolved magma. Compared to previously proposed cyclical shifts incomposition, these two events are much more pronounced, arguing against a continuously evolving staging chamber. In fact, this supports the hypothesis of a sub-compartmentalized Bushveld staging chamber as a source for the magma injections. Prevailing oxygen fugacity (fO2) conditions in the Upper Zone were estimated using the V partitioning between titanomagnetite and clinopyroxene. The results show that fO2 variations are minor in the Upper Zone and are approximately around FMQ - 2, which is 1.5 - 2 log units lower than previously assumed. To discuss potential parental magmas of the Upper and Upper Main Zone, crystallization experiments were performed in an internally heated pressure vessel, aiming to reproduce the mineral assemblage above the Pyroxenite Marker. Previously proposed and new calculated compositions were tested at conditions relevant to the Bushveld Complex (2 kbar; 1080°C - 1140°C; ~FMQ -2). In contrast to previously proposed basaltic compositions, the experimental results show that the parental magma to the Upper and Upper Main Zone of the Bushveld complex must be andesitic. Moreover, experimentally produced mineral compositions revealed, that the Upper and Upper Main Zone parental magma contained a residual liquid from the underlying zones high in Mg and Ca also small amounts of H2O (

U2 - 10.15488/3414

DO - 10.15488/3414

M3 - Doctoral thesis

CY - Hannover

ER -