A lateral cryptic variation is recognized in layered ultramafic rocks of the Hartley Complex, the largest of the four lopoliths that constitute the Great Dyke. Over a lateral distance of some 120 km, the En content of orthopyroxene from thick, continuous bronzitite layers is up to 6% less at the southern extremity of the Complex than in the central area. This is interpreted to be a result of more rapidly falling temperatures in a thinner body of basic magma that gave rise to a condensed sequence in marginal areas of the lopolith compared to the central area.
Following a well-marked period of addition of parental basic magma which appears to have had the incidental effect of linking up the four lopoliths, this compositional relationship in composition is reversed, i.e. the orthopyroxene from the base of an upper bronzitite layer of the southern marginal area of the Hartley Complex is more magnesian than that from the equivalent level in the central area. This is interpreted to arise from the proportion of fresh added magma to previously emplaced and partly crystallized magma being relatively greater in the marginal area than in the central area of the lopolith.
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