SOLUTION-REDEPOSITION AND THE ORTHOCLASE-MICROCLINE TRANSFORMATION - EVIDENCE FROM GRANULITES AND RELEVANCE TO O-18 EXCHANGE

The Or-rich part of optically blebby to lamellar mesoperthite crystals from an Adirondack granulite has been shown by TEM to be a lamellar c ryptoperthite, composed dominantly of tweed orthoclase. A fluid-absent , two-stage thermal history is proposed to explain the coarse and fine textures, with the cryptoperthite forming by coherent exsolution belo w similar to 350 degrees C, probably during uplift. The mechanism was most probably homogeneous coherent nucleation rather than spinodal dec omposition. However, cutting the orthoclase cryptoperthite are thin (< 1 mu m) seams of tartan microcline with sharp boundaries, often define d locally by {110} planes, and micropores. The microcline has replaced orthoclase by solution-redeposition along narrow planes infiltrated b y fluid during minor retrogression at T < 350 degrees C. Solution-rede position is a common process in feldspars at T < 500 degrees C, potent ially accompanied by O-18 exchange, because release of elastic strain energy in coherent perthite lamellar boundaries and twin-domain walls, followed by Si, Al ordering, provide driving forces for dissolution a nd reprecipitation of unstrained, more ordered phases.