Journal of Petrology | Volume 1 | Number 1 | Pages 286-303 | 1960
© Oxford University Press 1960
research-article |
Diabase-Granophyre Relations in the Endion Sill, Duluth, Minnesota
Geophysical Laboratory, Carnegie Institution of Washington Washington, D.C., U.S.A.
ABSTRACT
The gently dipping 1,500-ft thick Endion sill intrudes Keweenawan flows at Duluth, Minnesota. In a general way, from the bottom to the top of the body, there is a gradual transition from diabase through intermediate rock (granodiorite) to granophyre (adamellite). These latter two rock types together constitute about 40 per cent of the exposed mass. Fractional crystallization of basaltic magma produced a great thickness of basic and intermediate rock types which accumulated predominantly in the lower portions of the sill, and probably in the end-stage production of an aqueous, salic, alkalic liquid. It is proposed that, due to initial inclination of the sill, part of this salic fraction migrated up dip, accumulated and reacted with portions of the diabase and intermediate rock, and completed crystallization at the presently exposed level. Alternatively, much or all of the Endion sill granophyre may represent a separate intrusion unrelated to differentiation of the earlier diabase. In either case the sill is composite. Bulk compositions of cryptoperthites indicate both granophyre and intermediate rock crystallized at magmatic temperatures.
Compositional uniformity of the clinopyroxene and lack of iron enrichment in felsic portions of the Endion sill may be the result of accumulation of H2O and the presumed maintenance of nearly constant partial oxygen pressure during crystallization. This mechanism would furthermore account for inferred late magmatic solid solution between alkali feldspar and KFe3³Si3O8, with subsequent subsolidus exsolution of haematite.