Abstract
Layered intrusions represent part of the plumbing systems that deliver vast quantities of magma through the Earth's crust during the formation of large igneous provinces, which disrupt global ecosystems and host most of the Earth's endowment of Pt, Ni and Cr deposits. The Rustenburg Layered Suite of the enormous Bushveld Igneous Complex of South Africa has been presumed to have formed by deposition of crystals at the floor of a subterranean sea of magma several km deep and hundreds of km wide called a magma chamber. Here we show, using U–Pb isotopic dating of zircon and baddeleyite, that individual chromitite layers of the Rustenburg Layered Suite formed within a stack of discrete sheet-like intrusions emplaced and solidified as separate bodies beneath older layers. Our U–Pb ages and modelling necessitate reassessment of the genesis of layered intrusions and their ore deposits, and challenge even the venerable concept of the magma chamber itself.
Highlights
We address the statistical significance of the U–Pb zircon age differences between our samples using the Tu Student’s t statistic
Using the age of 2055.86±0.15 Myr (n 1⁄4 5; MSWD 1⁄4 0.78) for the Main Zone sample BV204, we determine with 98% confidence that the Merensky Reef is 0.32 Myr younger than the overlying Main Zone
If we include Z6 in the estimate of the age of the Main Zone its weighted mean age decreases to 2055.75±0.30 Myr, and confidence that it is older than the underlying Merensky Reef falls to 84%, which does not quite meet the conventional 90% bar for rejection of the null hypothesis but is strongly suggestive that the Merensky Reef is younger than the Main Zone and might be hosted by a sill
Summary
The resemblance of model restites to the observed anorthosites confirms that the anorthosite occurring both above and below each of the pyroxenite macro-layers in the Critical Zone could have been generated by partial melting of pre-existing noritic cumulate rocks.
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