More than 97 separate occurrences of ultramafic rocks, some of which are included in a north west-trending zone of alpine-type ultramafic rocks, have been mapped in the Eagle quadrangle, east-central Alaska. They are divided into three groups primarily on the basis of degree of serpentinization. Group I consists of lens-shaped bodies of serpentinite 1 m2 (10 ft2) to several 100 m2 (1,000 ft2) in area. Relict textures and presence of bastite indicate that the original rock was harzburgite and dunite. Group II consists of bodies composed of partially serpentinized harzburgite and dunite and includes the large Mount Sorenson and American Creek bodies. Group III is dominantly hornblendite and pyroxenite, probably intrusive and not genetically related to groups I and II. The authors believe that the ultramafic bodies of groups I and II are alpine-type peridotites and may include dismembered ophiolitc. The Tintina fault system could have provided a zone of weakness along which mantle material was tectonically emplaced or it may have been a plate boundary in late Paleozoic time. If it represents a plate boundary, the metamorphic terrane which lies between the Tintina and Denali fault systems would have to be allochthonous, perhaps originating as a northward-moving slice of continental crustal material. During the course of the movement as the two continental masses approached and perhaps collided, mantle peridotite and oceanic crustal material were squeezed up along the continental margin onto the continental slice.
