Enigmatic orthopyroxene-rutile-spinel intergrowth in the mantle xenoliths from Kutch, India

Several olivine nephelinite and alkali olivine basalt plugs intrude the flat lying gently dipping Mesozoic sediments of Kutch. Most of these plugs contain ultramafic xenoliths of mantle origin. The ultramafic xenoliths are mostly spinel-lherzolites with Cr-diopsides as the clinopyroxene phase. The orthopyroxenes in the spinel lherzolite from Mt. Sayala Devi contain numerous platelets of spinel. Besides the spinels, these also contain highly birefringent acicular inclusions with inclined extinction. Partial microprobe studies have identified the inclusions as rutile. The anomalous habit of the rutile indicates probable exsolution. On the basis of petrographic observations and the geochemical characteristics, the formation of such rutile has been discussed. Such intergrowths have been described previously as exsolution feature during cooling at high pressure. However, in the present instance it seems to be a low-pressure, low-temperature phenomenon with the onset of mantle metasomatism.