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Distribution of potassium feldspar polymorphs in intrusive sequences

Published online by Cambridge University Press:  05 July 2018

Ian Parsons  and
Rognvald Boyd
Ian Parsons
Affiliation:
Department of Geology and Mineralogy, Marischal College, Aberdeen, AB9 1AS, Scotland
Rognvald Boyd
Affiliation:
Department of Geology and Mineralogy, Marischal College, Aberdeen, AB9 1AS, Scotland
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Summary

K-feldspar variation is described for four salic plutonic igneous complexes that show normative mineral variation in keeping with the course of fractionation predicted from experimental studies. These are a sodic syenite (Loch Ailsh), potassic syenites (Loch Loyal), a peralkaline syenite and soda granite complex (Puklen), and a calc-alkaline pluton (Foyers). Additional data are given for examples from the literature. Microcline becomes more abundant relative to orthoclase as members of these intrusive sequences approach thermal minima on the liquidus in the system Ab-Or-An-Qz. This relation exists irrespective of the compositional range of the rock suites, the bulk composition of the alkali feldspars, grain size, or field relations. Early orthoclase-bearing rocks retain orthoclase when enclosed as xenoliths in microcline-bearing rocks, or when cut by minor bodies of microcline-bearing rocks. The K-feldspar variation may reflect increasing water content or the peralkaline or peraluminous character of the fractionating magmas. These factors affect the feldspar structure at the time of initial crystal growth and dictate whether microcline will develop on cooling.

Type
Research Article
Information
Mineralogical Magazine , Volume 38 , Issue 295 , September 1971 , pp. 295 - 311
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1971

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