Abstract
Study of K-feldspars from high-grade migmatitic rocks in northwest Maine reveals distinct interrelations among paragenesis, composition, and structural state of the K-feldspars. Structural state has beencharacterized in the manner suggested by Wright and Stewart (1968) and using the Δbc plot of Crosby(1971).
Petrographically the rocks have been subdivided into eight groups. These are:
| Group . | Rock Type . | Ab%in k-feldspar . | Δbc . |
|---|---|---|---|
| I | Coarse mica schist with groundmass K-feldspar and sillimanite | 14 | -765 |
| II | Biotite granulite with groundmass K-feldspar | 9.5 | -86 |
| III | Calc-silicate granulite with groundmass K-feldspar | 6.5 | -86 |
| IV | K-feldspar from sillimanite-bearing, coarse, leucocratic bands and megacrysts | 14.5 | -80 |
| IV' | K-feldspar from sillimanite-free, coarse, leucocratic bands and megacrysts | 10 | -83 |
| V | Retrograded specimens | 8.5 | -88 |
| VI | Specimens with resorbed sillimanite | 9 | -85 |
| VII | Specimens with retrograding concentrated along well defined veinlets | 2 | -70 |
| VIII | K-feldspar from adamellite | - | -912 |
| K-feldspar from diorite | - | -827 |
| Group . | Rock Type . | Ab%in k-feldspar . | Δbc . |
|---|---|---|---|
| I | Coarse mica schist with groundmass K-feldspar and sillimanite | 14 | -765 |
| II | Biotite granulite with groundmass K-feldspar | 9.5 | -86 |
| III | Calc-silicate granulite with groundmass K-feldspar | 6.5 | -86 |
| IV | K-feldspar from sillimanite-bearing, coarse, leucocratic bands and megacrysts | 14.5 | -80 |
| IV' | K-feldspar from sillimanite-free, coarse, leucocratic bands and megacrysts | 10 | -83 |
| V | Retrograded specimens | 8.5 | -88 |
| VI | Specimens with resorbed sillimanite | 9 | -85 |
| VII | Specimens with retrograding concentrated along well defined veinlets | 2 | -70 |
| VIII | K-feldspar from adamellite | - | -912 |
| K-feldspar from diorite | - | -827 |
K-feldspar compositions for specimens from Groups I, II, and III are believed to represent the original, high temperature, homogeneous feldspars. Several models explain the observed differences in structural state andtheir obvious relation to Ab content. The favored model involves disequilibrium ordering interrelated with the Al/(K + Na) ratio in the surrounding pore fluid.
