Shonkin Sag laccolith,Montana |
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| Authors: | W P Nash J F G Wilkinson |
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| Institution: | (1) Department of Geological and Geophysical Sciences, University of Utah, 84112 Salt Lake City, Utah;(2) School of Earth Sciences, Macquarie University, 2113 North Ryde, New South Wales |
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| Abstract: | The Shonkin Sag differentiation sequence displays systematic variations in major and trace elements with evolution. The accumulative more mafic shonkinites are enriched in Co, Ni, Cr; La, Ce, Zr, and Nb are concentrated in the most evolved differentiate, chemically a nepheline syenite. Maximum abundances of K, Rb, Ba, Pb, and Sr occur in the intermediate syenitic differentiates (differentiation index=64.0–68.8). A change in the variation trends for K, Rb, Pb, Sr, Na, Ce, and Zr coincided with a change in the rate of decrease of oxygen fugacity with falling temperature. |
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