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The Canadian Mineralogist; October 1996; v. 34; no. 5; p. 967-975
© 1996 Mineralogical Association of Canada
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Barium and titanium-rich biotite and phlogopite from the western and eastern gabbro, Coldwell alkaline complex, northwestern Ontario

Cliff S. J. Shaw, and Robert S. Penczak

University of Western Ontario, Department of Earth Sciences, London, ON, Canada
Queen's University, Canada

Barium- and titanium-enriched phlogopite and biotite are found in the subalkaline Eastern and Western Gabbro intrusions that are cut by syenite and syenite pegmatite in the Coldwell alkaline complex, northwestern Ontario. The micas contain up to 6.1 wt% BaO and 8.1 wt% TiO 2 . Mica occurs as a fringe on Fe-Ti oxides, an overgrowth on cumulus and intercumulus clinopyroxene, and coarse grains that enclose earlier-formed minerals. Ba-Ti-rich micas are normally found in potassic igneous rocks like lamproites, whereas the samples in this study occur in subalkaline gabbro. In the Western Gabbro, the mica is richer in Ba. This suite shows a trend of increasing Ba with decreasing mineral Mgnot present in the less Ba-rich samples of the Eastern Gabbro. Most of the grains analyzed have apparent deficiencies in their tetrahedral, octahedral and interlayer sites. Deficiencies in the octahedral sites are due to a combination of Ti-vacancy and Ti-Tschermak substitution. The interlayer-site deficiencies are due to the replacement of K by Ba. Coupled substitutions such as XII Ba+ IV Al = XII K+ IV Si also account for the accommodation of Ba in the structure. Moderately Ba-enriched micas in the Eastern and Western Gabbro are a result of crystallization of Ba-enriched residual liquids trapped in the cumulus framework of the gabbros. In addition, the latest Ba-Ti-rich mica in the Western Gabbro formed in response to infiltration of fluids derived from adjacent bodies of Ba-rich syenite pegmatite.

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[Abstract] [Full Text] [PDF]


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