Abstract

The Square Top intrusion, near Nundle, New South Wales, reveals in its mineralogy and chemistry the transition of analcime-olivine theralite to analcime tinguaite. Chemical, X-ray, and optical data are presented on 2 plagioclases, 9 alkali feldspars, 2 nephelines and 3 analcimes, from various members of the differentiation sequence. With differentiation, plagio-clase becomes enriched in Ab and, to a lesser extent, in Or. The coexisting alkali feldspars initially reveal enrichment in Ab and trend from sanidine to lime anorthoclase; subsequently in the tinguaites, the sanidines become enriched in Or. Nephelines in the lower theralites have highly sodic compositions and become slightly enriched in K in the later differentiates. Replacement of Si by NaAl in the analcimes decreases with progressive differentiation. The rapid cooling of this relatively small intrusion assisted fractional crystallization, and resulted in the preservation of disordered plagioclase and alkali feldspars. The Square Top feldspar crystallization trends are compared with trends presented by Tuttle and Bowen (1958). The location of the feldspar boundary curve in the Ab–An–Or system and some general relationships in the undersaturated portion of the Ne–Ks–Qz system, including the heteromorphic felsic assemblages developed from the lowest temperature liquids, are also discussed.

This content is only available as a PDF.
You do not currently have access to this article.