Silicic Magmas Derived by Fractional Crystallization from Miocene Minette, Elkhead Mountains, Colorado

P. T. Leat*, R. N. Thompson*, M. A. Morrison, G. L. Hendry and A. P. Dickin
Department of Geology, Imperial College of Science and Technology, London SW7 2BP, U.K.
Department of Geological Sciences, University of Birmingham, P.O. Box 363, Birmingham B15 2TT
Department of Geology, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L85 4M1
*Present Address: Department of Geological Sciences, University of Durham, South Road, Durham DH1 3LE, U.K.

Abstract: The rock association of minette with silicic lavas and intrusions (granites, syenites, dacites) is a common geologic feature in both collisional and extensional tectonic settings. Considerable doubt exists as to whether a genetic link exists between these mafic and silicic rocks. We describe a Miocene sill from NW Colorado which is a clear example of a mixed magma consisting of originally-liquid inclusions of minette in a silicic trachydacite host. Chemical and Sr, Nd and Pb isotopic data are consistent with derivation of the silicic host magma of the sill dominantly by fractional crystallization of the minette magma. Correlations between the elemental compositions of the rock types and their Sr and Nd isotopic ratios imply minor assimilation of continental crust with relatively low values of both 87Sr/86Sr and 143Nd/144Nd, concomitantly with fractional crystallization. The parental minette magma was probably derived by partial melting of subcontinental lithospheric mantle. While the sill was emplaced in a rift-like tectonic setting, the chemical and isotopic composition of the lithosphere-derived minette magmas (and hence the silicic fractionates) was largely independent of this setting, but dependent upon the composition and age of the lithospheric mantle and crust.

Keywords: minette • trachydacite • mixed rock • silicic residue • isotopes • Rio Grande rift • Colorado

Mineralogical Magazine; December 1988 v. 52; no. 368; p. 577-585; DOI: 10.1180/minmag.1988.052.368.03
© 1988, The Mineralogical Society
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