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

The highly feldspathic syenites are divided on field evidenceinto three phases, S1, S2, and S3. S3 overlies S1 and S2, andencloses xenoliths and xenocrysts from the underlying and earliersyenites. The feldspars show a range of forms and coarseness of perthite,and show enrichment in potassium from Or₂₀ to Or₄₀, and increasein 2V, in successive syenites. X-ray diffractometer and single-crystalmethods show that SI has a dominantly monoclinic K-phase, S2a microcline with some monoclinic material, and S3 a highlyoblique microcline, exclusively. The earlier feldspars preservethese features even when enclosed by later S3 material. Thereis a suggestion of systematic regional distribution of obliquityvalues. The compositional range of the feldspars is related to the experimentallydetermined minimum in the Ab-Or-An system. The feldspars crystallizedas sodium-rich sanidines. Temperature-pressure limits are deduced,and the unmixing history of the perthites discussed. A mechanismfor the preservation of orthoclase in S1 and S2 is suggested.