Feldspar–fluid interactions in braid microperthites: pleated rims and vein microperthites |
| |
| Authors: | Martin R Lee Kim A Waldron I Parsons William L Brown |
| |
| Institution: | (1) Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW, U.K., GB;(2) Department of Geology, Colgate University, 13 Oak Drive, Hamilton, NY 13346-1398, U.S.A., US;(3) CNRS-CRPG, BP 20, F-54501, Vandœuvre-les-Nancy Cedex, France, FR |
| |
| Abstract: | Braid microperthitic alkali feldspars in the Klokken, South Greenland and Coldwell, Ontario syenite intrusions have bulk-compositional
variations along grain boundaries called pleated rims. These, together with vein microperthites in aplites which cross-cut the syenites, have been investigated by SEM and TEM.
We distinguish two main types of pleated rims, “arched ” and “parallel-sided ”, consisting of alternating Ab- and Or-rich areas on (001), which are 0.5–300 μm in length normal to (010) and 0.2–20 μm
in width along (010). The smallest pleats, which occur on intracrystalline boundaries in Klokken feldspars, are fully coherent
and composed of low albite and low microcline. Above the heads of some of the coarser pleats, braid microperthite grades into
a film crypto- and micro-perthite and antiperthite microtexture called a “transitional zone” containing roughly planar lamellae of low albite and tweed orthoclase. During pleat development, local alternating volumes
form in which the proportions of the phases differ ( phase separation) and the morphology of the intergrowths changes from braided to straight in response to this change in local bulk composition.
Straightening is also accompanied by transformation of low microcline to tweed orthoclase. The coarsest pleats, which occur
along grain boundaries in feldspars from the Coldwell syenite, are semi- or in-coherent and have a thick coherent and semicoherent
transitional zone. Coarsening of pleats and development of the transitional zone has been facilitated by diffusion of “water”
into grain interiors. In many cases, pleated rims have suffered deuteric alteration, by dissolution–reprecipitation processes,
through the action of a water-rich fluid from the grain boundary, in which tweed orthoclase was transformed into irregular
microcline and micropores developed. Vein microperthites in aplites from Klokken, and by extension the vein microperthites
almost universal in most alkali granites, are interpreted to have formed by propagation of pleat heads across entire crystals
during pervasive interaction with water.
Received: 10 June 1996 / Accepted: 12 December 1996 |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
