Origin of the basal lherzolite of the Muslim Bagh Ophiolite, Pakistan, deduced from the trace element characteristics of clinopyroxene

The origin of basal lherzolites in the mantle section of harzburgite-dominated ophiolites is enigmatic. The basal part of the mantle section is well exposed in the Muslim Bagh Ophiolite, Pakistan, which is one of the harzburgite-dominated ophiolites of the Tethys Ophiolite Belt. In this contribution, we describe the basal lherzolite of the Muslim Bagh Ophiolite, Pakistan, and discuss its origin based on the trace-element characteristics of its clinopyroxenes. The basal lherzolite exhibits porphyroclastic to mylonitic textures. Primitive mantle-normalized trace-element patterns of the porphyroclastic clinopyroxenes were characterized by low ratios of light rare-earth elements (LREEs) to heavy rare-earth elements (HREEs), low abundances of HREEs, and positive Sr anomalies. These geochemical characteristics are not consistent with a lherzolite and its clinopyroxenes, which is formed by a residue after a low degree of partial melting and melt extraction from peridotites in a mid-ocean ridge setting. Instead, the compositions of the clinopyroxenes are consistent with open-system melting induced by the infiltration of slab-derived fluids into residual peridotites that had been depleted in REEs. The compositions of chromian spinels in the chromitites of the basal peridotite sequence are also consistent with their formation in an arc setting. We conclude that the basal lherzolites of the Muslim Bagh Ophiolite represent a residue after a relatively high degree of partial melting, and that the clinopyroxenes were added as metasomatic crystallization from slab-derived arc-related melts to this residual depleted peridotite in a subduction setting.