The Feldspathic Syenites of the Loch Ailsh Intrusion, Assynt, Scotland

The highly feldspathic syenites are divided on field evidence into three phases, S1, S2, and S3. S3 overlies S1 and S2, and encloses xenoliths and xenocrysts from the underlying and earlier syenites. The feldspars show a range of forms and coarseness of perthite, and show enrichment in potassium from Or20 to Or40, and increase in 2V, in successive syenites. X-ray diffractometer and single-crystal methods show that SI has a dominantly monoclinic K-phase, S2 a microcline with some monoclinic material, and S3 a highly oblique microcline, exclusively. The earlier feldspars preserve these features even when enclosed by later S3 material. There is a suggestion of systematic regional distribution of obliquity values. The compositional range of the feldspars is related to the experimentally determined minimum in the Ab-Or-An system. The feldspars crystallized as sodium-rich sanidines. Temperature-pressure limits are deduced, and the unmixing history of the perthites discussed. A mechanism for the preservation of orthoclase in S1 and S2 is suggested.