Abstract

The writers studied thin sections and core chips of South Norwegian hyperites by optical methods and made modal analyses for each of the rocks studied. They used X-ray and spectro-chemical procedures to supplement optical mineralogical data.

The hyperites are slightly altered olivine gabbros. The parent gabbro differentiated by a separation of olivine from a mixture of diallage and labradorite.

Well-developed reaction rims (coronas) are present about primary olivine, magnetite, and diallage. The coronas about olivine consist of an inner bronzite rim and an outer symplectite of hornblende and pleonaste. The bronzite has formed from olivine by the addition of silica to the rock. Iron and magnesium from the reactions that formed bronzite have contributed to the development of the symplectite from adjacent plagioclase. The complete conversion of olivine to bronzite in a typical rock required the addition of about 10 percent by weight of silica. At the termination of the olivine-bronzite transformation, further additions of silica caused the formation of garnet from pleonaste and hornblende. Contemporaneously, carbon dioxide and chlorine caused the formation of scapolite from plagioclase. All the reactions followed the rules of volume-for-volume replacement.

The appearance of garnet and scapolite has not been governed solely by specific temperatures and pressures. The stabilization of these phases and the persistence of the original mineral assemblages have been determined by the original composition of the rock together with the amount of introduced water and silica. The final mineral assemblages in the altered hyperites cannot be ascribed to the operation of the usual variables referred to in the concepts of metamorphic facies.

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