Structural State and Composition of Alkali Feldspars in Granites of the St. George Pluton, South-Western New Brunswick

Michael E. Cherry and Lowell T. Trembath
Department of Geology, University of New Brunswick, Fredericton, New Brunswick E3B 5A3

Summary: The variation in the structural state of potassium feldspars from the St. George pluton, New Brunswick, Canada, is discussed. Monoclinic potassium feldspars occur near the eastern end of the pluton whereas the granites at the western end contain triclinic potassium feldspars. Although the bulk compositions of the host rocks are all very near the granite minimum melt composition there is a systematic increase in the degree of ordering for both the triclinic and monoclinic suites of feldspars with the more ordered variants tending to occur within rocks nearest the minimum melt composition. There is more abundant textural evidence for the presence of a late-stage fluid phase in the granites containing the monoclinic potassium feldspars. It is proposed that a slower post-crystallization cooling rate was also a factor in the nucleation of triclinic domains in the potassium feldspars at the western end of the pluton.

Mineralogical Magazine; September 1978 v. 42; no. 323; p. 391-399; DOI: 10.1180/minmag.1978.042.323.11
© 1978, The Mineralogical Society
Mineralogical Society (