5
.
7
. Accumulation, differentiation and contamination
As discussed, the evolution of the Shaw Dome komatiitic rocks can be attributed to variable
amounts of olivine accumulation followed by liq- uid differentiation and or crustal contamination
Fig. 8. The massive dunite and wehrlite sills and dykes and massive komatiitic basalt flows can be
explained by fractional olivine accumulation. In contrast, the differentiated wehrlite – pyroxenite –
gabbro and olivine cumulate to spinifex textured to aphyric komatiite units can be explained by
differentiation and or A-FC. Differentiation can be distinguished from A-FC on the basis of geo-
chemical and mineralogical parameters as inferred from different rock types. For example, A-FC
processes are represented by LREE enrichment and, perhaps, by the presence of igneous amphi-
bole in the amphibole gabbro lithology.
6. Evolution of the Shaw Dome complex
Field mapping and petrographic and geochemi- cal variation studies of the Shaw Dome komatiitic
rocks indicate that the thick, undifferentiated LKH wehrlite sills and the thick, undifferentiated
UKH komatiitic flows strongly resemble each other in geochemical and mineralogical composi-
tion. The amphibole gabbros and the komatiitic basalts exhibit many similar geochemical charac-
teristics. In consequence, the LKH and UKH are inferred to be genetically related and to constitute
a
single intrusive – extrusive
stratigraphic se-
quence. The sequence is interpreted to represent a contaminated dyke – sill lava complex and con-
strains facies models for komatiitic magmatism – volcanism.
A model for the evolution of the Shaw Dome dyke – sill – lava complex is presented in Fig. 9. In
this model, the adcumulate dunite sills represent flow differentiation texture formed by fractional
accumulation of olivine in dynamic flow-through conduits. Ponding of komatiitic magma in adja-
cent, more distal magma chambers, where it un- derwent olivine fractional crystallisation under
more static conditions, and formed cogenetic wehrlite sills. Eruption of the initial magma re-
sulted in emplacement of cogenetic, channelised komatiite flows. Eventually, heated crustal mate-
rial adjacent to the proximal magma conduits was thermally eroded and assimilated to produce con-
taminated komatiite magmas. Ponding of this magma and fractional crystallisation of olivine 9
clinopyroxene produced the wehrlite – amphibole gabbro units. Eruption of the contaminated
magma resulted in emplacement of the cogenetic komatiitic basalt flows. In this model, the dunite
and the non-brecciated komatiite represent more ‘proximal’ channel facies. The wehrlite sills, differ-
entiated wehrlite – gabbro sills, brecciated flows and komatiitic basalt flows represent more ‘distal’
sheet flow facies Table 7.
The reason for the lack of contamination in the komatiite flows remains to be determined. Magma
ascent could have been more rapid and eruption rates been greater than estimated by Huppert et
al. 1984 and Huppert and Sparks 1985a,b, and or the crust may have been very thin. The
viscosity variation in cooler boundary zones might have inhibited convection at the margins of
dykes and flows, and substantially reduced erosive capability cf. Bickle et al., 1993. Alternatively,
contaminated komatiite lava might have flowed downstream away from the study area.
7. Discussion and implications
