Metamorphism of Precambrian Granitic Xenoliths in a Mica Peridotite at Rose Dome, Woodson County, Kansas: Part 2, Petrologic and Mineralogic Studies

Abstract

Field, drill-core, petrographic, and mineralogic studies show that the Precambrian granitic rocks exposed on Rose Dome, Woodson County, Kansas, were emplaced as inclusions of basement rock in a mica peridotite magma that intruded the Pennsylvanian section of Rose Dome during Late Cretaceous time. High temperatures of the alkaline ultramafic magma (probably greater than 800° C) led to metamorphism of the granitic inclusions and the formation of high sanidine and high albite from original microcline and albite. Contact metamorphic effects on country rock include the development of buchite-like sanidine-magnesian biotite hornfels from Weston Shale (Pennsylvanian) that was intruded by the mica peridotite. The high temperatures of the peridotite magma also led to partial melting of the granitic rocks; a quartzofeldspathic matrix shows volcanic textures and binds mineral and rock fragments together to produce the varied range of textures and structures of the granitic xenoliths. Partial melting of the granitic rocks accounts for those features that led earlier workers to conclude that the “granite” on Rose Dome had intruded the Pennsylvanian section. Those features included xenoliths of hornfels enclosed by granitic material and apparently intrusive relationships between the granitic rocks and metamorphosed shale. The lack of concordance in the Rb-Sr ages of the granitic rocks (Part 1) may stem partly from melting of the “granite” and exchange of ions with the mica peridotite magma, from the “sampling” of different levels of granitic crust by the upwelling peridotite magma, or from weathering.