Abstract

Rb-Sr isotopic analyses of whole-rock and mineral-separate samples obtained from drill core and surface boulders of the granitic rocks exposed near the apex of Rose Dome, Wood-son County, Kansas, show that the “granite” is of Precambrian age and originally crystallized about 1,200 m.y. ago. The granitic rocks include: (1) coarse-grained, apparently plutonic material, (2) recrystallized granitic breccia, and (3) granitic breccia showing pronounced alignment of quartz and feldspar grains that is suggestive of flow. Eight whole-rock analyses of type (1) “granite” yield an isochron indicating an age of 1,190 ± 100 m.y. with an initial Sr87/Sr86 ratio of about 0.706. Eight samples of granite types (2) and (3) yield analyses that do not plot on the whole-rock isochron. Although the granitic rocks appear to have intruded the Pennsylvanian sedimentary section of Rose Dome, it seems likely that they were emplaced in their present stratigraphic position passively when mica peridotite that occurs in the shallow subsurface was intruded during Late Cretaceous time. The Rb-Sr isotope system of some of the granitic material may have been disturbed during emplacement, but evidently the minerals were not re-equilibrated.

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