Spinel composition as a petrogenetic indicator of the mantle section in the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco

The Neoproterozic Bou Azzer ophiolite, Anti-Atlas, Morocco, is one of the best preserved ophiolites of Neoproterozic age. However, due to severe serpentinization, almost all primary minerals have been converted to secondary minerals except chromian spinel which has survived alteration in ultramafic rocks and is used here as the only reliable petrogenetic indicator. Relics of the primary textures indicate that the mantle section at Bou Azzer is composed mainly of harzburgite with dunite lenses and veins and small scale chromitite pods. Wehrlite is also found as late-stage intrusions into the mantle section and still has preserved the primary silicates, olivine and clinopyroxene. The chromian spinel chemistry in both peridotite rocks and chromitite pods exhibits a very limited compositional range. The Cr# [= Cr/(Cr + Al) atomic ratio] of spinel is about 0.8 in all serpentinized peridotites and ranges from 0.65 to 0.85 in chromitites, while it is slightly lower, about 0.6, in wehrlite. This suggests the upper mantle from which the Bou Azzer peridotite had been derived was highly refractory. The frequent occurrence of the highly refractory mantle in Precambrian ophiolites may indicate that the high-degree partial melting was much more common in the Precambrian era.