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Ophiolites and Oceanic Crust |
1 Department of Geology, The University, Highfield, Southampton SO9 5NH, UK
2 Natural Environment Research Council, Polaris House, North Star Avenue, Swindon SN2 1EU, UK
Chromite deposits comprise an integral part of the mantle sequences observed within many ophiolite complexes; in particular, those where a harzburgite is the dominant mantle sequence lithology. The chromite segregations are invariably contained within a dunite envelope and show a wide variety of ore textures, some of which are directly analogous to those observed within major layered intrusions. The majority of chromite segregations contain chromite and olivine of variable modal proportions although in certain instances a wide variety of interstitial silicates may be preserved including plagioclase, orthopyroxene, clinopyroxene and amphibole. Evidence of deformation linked to increasingly lower temperatures is invariably present which can be linked either to high temperature ‘flow fabrics’ away from the spreading axis, the obduction of the ophiolite or post-obduction regional deformation events.
Analysis of chromite grains from a variety of ophiolitic chromite segregations shows them to be chrome-spinels with variable Cr* values (Cr/(Cr+Al)), typically between 0.4 and 0.85, with Mg* values (Mg/(Mg+Fe2+)) between (0.5 and 0.8) and invariably restricted Fe3+ contents. By way of a simple contrast to major layered intrusions the ophiolitic chromite segregations tend to range to lower Cr* values and show higher and more restricted Mg* ratios. The variation of the Cr* ratio from chromite segregations within an individual ophiolite can be related to the position of the segregation within the mantle sequence. The more aluminous deposits being located towards the petrological moho, whereas the more chrome-rich horizons tend to occur deeper within the sequence, in particular toward the harzburgite lherzolite transition.
A model is described which considers the chromite deposits to represent the early fractionates of ascending magmas within highly depleted mantle, likely to have developed above subduction zones, or at fast spreading centres. Transport of the mantle away from the spreading zone results in deformation of the chromite deposits within a falling P-T regime involving both ridge-axis and post-obduction deformation.
