The role of igneous processes in the formation of ophiolitic chromitite.

Roberts, Stephen (1986). The role of igneous processes in the formation of ophiolitic chromitite. PhD thesis The Open University.



The ultramafic portions of the Semail ophiolite, Oman, Vourinos Complex, Greece and the Tiebaghi Massif, New Caledonia, all contain significant chromite concentrations. The chromite deposits and host rocks of all three complexes have been examined in order to elucidate the role of igneous processes in the genesis of ophiolitic chromitite.

The outcrop of all three ophiolites is dominated by Mantle Sequence lithologies; more than 50% in each case. The dominant rock type is harzburgite with subordinate dunite, lherzolite and chromitite. Melting models indicate that the most residual Mantle Sequences (harzburgite > lherzolite) are associated with chromium-rich magmas (1200-1600 ppm Cr3+) for reasonable degrees of partial melting (10-30%) of a fertile spinel peridotite. These residual sequences are, in turn, shown to contain significant concentrations of chromite and to correspond with the supra-subduction zone (S.S.Z.) ophiolites of Pearce et al. (1985). Within the S.S.Z. ophiolites chromite formation is enhanced by the high Cr3+ content and expanded olivine-chromite phase volume of the hydrous primary magma.

The recognition of relict igneous textures within the chromite deposits, including channel structures, occluded silicates and euhedral chromite grains poikilitically enclosed in pyroxene laths, reaffirms their "primary magmatic nature." Chromitite formation occurs in small open-system magma chambers where "magma mixing" or fluctuations in "P total" associated with the release of a H-O-S-Cl-rich vapour phase dramatically reduce Cr3+ solubility in the magma. Major deposits arise via continued replenishment of the chambers by variably fractioned basaltic/picritic melts.

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