The P1 layer of the Great Dyke is an ∼ 200 m thick pyroxenite succession in Cyclic Unit 1 and, as the topmost lithology of the Ultramafic Sequence, represents the transition from ultramafic to mafic rocks. Of critical importance to this part of the stratigraphy is the strong lateral environmental change from axis to margin as a result of the flared structure of the Great Dyke. During the formation of the P1 layer the axial zone was underlain by a great thickness of hot ultramafic cumulates whereas the same layer in the marginal zone progressively offiaps the lower ultramafic layers and is in close proximity to the underlying wall/floor rocks. Heat loss through the floor was therefore much greater in the marginal zone than in the axis. Major lateral variations are observed, with all lithological units and layers thinning towards the margins of the subchamber together with a progressive change in the form of the cumulates. Discordant relationships towards the margin between layer types (modal, cryptic, and form) are a feature of the P1 unit which has also been recognized in other parts of the Great Dyke (Prendergast, 1991). Pyroxene compositions show significant variations within an overall fractionation trend and decoupling occurs between major and minor element components of bronzite, suggesting strong compositional heterogeneity of the magma. This type of cryptic layering has not previously been described and is informally called ‘cryptorhythmic’ layering. Pyroxene compositional variation is related to reaction and modification by trapped intercumulus liquid, and few minerals preserve liquidus compositions. A similar situation must exist for most layered intrusions. The strong dependence of pyroxene compositions on incompatible element content in the whole-rock shows that the original liquidus compositions were modified by postcumu-lus overgrowth and reaction with the trapped intercumulus liquid. Well-constrained data arrays indicate that most cumulates in the P1 layer behaved as a closed system with little or no migration of intercumulus liquid. Liquidus compositions can therefore be deduced and the residual porosity and degree of postcumulus formation were modelled using a computer program. Residual porosity is shown to be between 1 and 13% (by mass). Rocks in the marginal facies have a relatively large proportion of discrete postcumulus phases but instead of representing crystallization of trapped liquid these are shown to be mainly heteradcumulus phases, i. e., interstitial minerals that have grown largely by adcumulus processes in equilibrium with the main body of magma. The heteradcumulus component can be as high as 27%. These phases occur as oikocrysts which give rise to a well-developed nodular pyroxenite (the ‘potato’ reef). The formation of the nodules caused local redistribution of primary sulphide liquid. The liquid layers which gave rise to cumulates in the marginal facies are shown to be enriched in iron and incompatible elements compared with the axial zone, indicating that the P1 pyroxenite layer formed by crystallization of a magma which was either compositionally stratified or exhibited a strong lateral compositional gradient.
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A. H. Wilson (1992) studied this question.