Parental Magma Composition of the Main Zone of the Bushveld Complex: Evidence from in situ LA-ICP-MS Trace Element Analysis of Silicate Minerals in the Cumulate Rocks

摘要

In situ trace element analysis of cumulus minerals may provide a clue to the parental magma from which the minerals crystallized. However, this is hampered by effects of the trapped liquid shift (TLS). In the Main Zone (MZ) of the Bushveld Complex, the Ti content in plagioclase grains shows a clear increase from core to rim, whereas most other elements [e.g. rare earth elements (REE), Zr, Hf, Pb] do not. This is different from the prominent intra-grain variation of all trace elements in silicate minerals in mafic dikes, which have a faster cooling rate. We suggest that crystal fractionation of trapped liquid occurred in the MZ of Bushveld and the TLS may have modified the original composition of the cumulus minerals for most trace elements except Ti during slow cooling. Quantitative model calculations suggest that the influence of the TLS depends on the bulk partition coefficient of the element. The effect on highly incompatible elements is clearly more prominent than that on moderately incompatible and compatible elements because of different concentration gradients between cores and rims of cumulate minerals. This is supported by the following observations in the MZ of Bushveld: (1) positive correlation between Cr, Ni and Mg# of clinopyroxene and orthopyroxene; (2) negative correlation between moderately incompatible elements (e.g. Mn and Sc in clinopyroxene and orthopyroxene; Sr, Ba and Eu in plagioclase); but (3) poor correlation between highly incompatible elements and Mg# of clinopyroxene and orthopyroxene or An# of plagioclase. Modeling suggests that the extent of the TLS for a trace element is also dependent on the initial fraction of the primary trapped liquid, with strong TLS occurring if the primary trapped liquid fraction is high. This is supported by the positive correlation between highly incompatible trace element abundances in cumulus minerals and whole-rock Zr contents. We have calculated the composition of the parental magma of the MZ of the Bushveld Complex. The compatible and moderately incompatible element contents of the calculated parental liquid are generally similar to those of the B3 marginal rocks, but different from those of the B1 and B2 marginal rocks. For the highly incompatible elements, we suggest that the use of the sample with the lowest whole-rock Zr content and the least degree of TLS is the best approach to obtain the parental magma composition. The heavy REE contents of the magma calculated from orthopyroxene are similar to those of B3 rocks and lower than those of B2 rocks. The calculated REE contents from clinopyroxene are generally significantly higher than for B2 or B3 rocks, and those from plagioclase are in the lower level of B2, but slightly higher than for B3. However, the calculated REE patterns for both clinopyroxene and plagioclase show strong negative Eu anomalies, which are at the lower level of the B2 field and within the B3 field, respectively. We suggest that Eu may be less affected by TLS than other REE owing to its higher bulk compatibility. Based on this and the fact that the calculated REE contents of the parental magma should be higher than the real magma composition owing to some degree of crystal fractionation and TLS, even for the sample with the lowest amount of trapped liquid, we propose that a B3 type liquid is the most likely parental magma to the MZ of the Bushveld Complex. In the lowermost part of the MZ, there is involvement of the Upper Critical Zone (UCZ) magma.