Solution--Redeposition and the orthoclase-microcline transformation: Evidence from Adirondack granulites

Feldspar microtextures in Adirondack granulites suggest that the transformation of orthoclase to microcline has been accomplished by solution--redeposition. The orthoclase (Or)-rich part of an optically mesoperthitic alkali feldspar was found to be cryptoperthitic when examined by transmission electron microscopy. A two-stage thermal history is proposed to explain the coarse and fine textures, with the cryptoperthite forming by coherent exsolution below 350--400 C. Of particular interest are areas within the Or-rich cryptoperthite where the predominant tweed microtexture is cut by seams'' of microcline parallel to b and the cryptoperthite lamellae. The seams contain polygonal microcline subgrains and micropores (0.1--1.5 [mu]m in diameter). Micropores have been liked with fluid- assisted subgrain formation and turbidity in feldspars. Subgrains, micropores and the tweed/microcline interfaces within the seams often have simple crystallographic indices characteristic of the Adularia habit. The Adularia habit is characteristic of feldspar that has grown in low temperature veins or in sediments and appears to develop during in situ solution--redeposition of alkali feldspar. The crystal habit of the microcline subgrains, the presence of micropores within the seams and the textural relationship between the seams and cryptoperthite lamellae suggest that low temperature fluid infiltration resulted in solution--redeposition of the alkali feldspar on amore » remarkably fine scale. Subsolidus recrystallization of feldspars in this fashion may have important geochemical implications, particularly for [sup 18]O/[sup 16]O exchange.« less