Rutile and sphene in blueschist and related high-pressure facies rocks

Sphene and rutile are characteristic accessory minerals of blueschist facies metamorphic rocks. However, only sphene is present in the lowerr grade blueschist assemblages. In many areas of the world these blueschists pass into, or are in fault contact with, higher grade glaucophane schists, rutile-bearing amphibolites, and eclogites. The origin of the rutile may be related to a prograde metamorphic reaction that involves the alteration of sphene to rutile, but few data are available to support this hypothesis. It seems more probable that the rutile crystallized directly from ilmenite or titanium-rich clinopyroxene and magnetite in basalts and gabbros during the conversion of these rocks to amphibolites and eclogites containing omphacite, amphibolite, and garnet. Rutile in the higher grade blueschists, regardless of its origin, is a potential titanium resource and is currently being mined from eclogite and glaucophane-eclogite rocks in the Soviet Union. Resources are likely to be up to millions of tons contained Ti02.