Chromite-Rich and Chromite-Poor Ophiolites: The Oman Case

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Abstract

Chroinite deposits in Oman belong dominantly to the concordant structural type, meaning that these have been intensely deformed by plastic flow and tectonically rotated to become parallel to the peridotite foliation. This occurred soon after their formation within the transition zone below the ridge of origin. Subconcordant and discordant pods are also present. The latter have preserved delicate magmatic structures showing that they have only been little deformed after their formation in melt-carrying conduits. Regionally, the chromite deposits have been dominantly found, so far, in restricted areas whereas large areas of Oman seem to be devoid of deposits. Maqsad, one of the largest chromite districts, was also an area of mantle diapirism below the ridge of origin. This association is well explained if it is considered that most of mantle melt feeding the crust at ridges is expected to be delivered through such diapirs. Although Oman is the largest and best exposed ophiolite in the world, it seems to be comparatively poor in chromite due to the spreading situation. In the Lherzolite Ophiolite Type (LOT), thought to be derived from slow spreading ridges, the chromite deposits are absent as a result of chromium being retained in mantle diopside during partial melting. The chromite deposits are restricted to the Harzburgite Ophiolite Type (HOT) in which chromium has passed into the melt. Although Oman belongs to the HOT group, it seems to have been a particularly fast spreading HOT. It is suggested that in such a situation the transition zone, which is the level where the chromite normally precipitates from the melt, due to temperature drop, could have remained too hot to allow for abundant chromite formation.