EXSOLUTION AND ALTERATION MICROTEXTURES IN ALKALI FELDSPAR PHENOCRYSTS FROM THE SHAP GRANITE

Alkali feldspar phenocrysts (bulk composition Or(75.0)Ab(24.6)An(0.4)) in the subsolvus Shap granite comprise a line-scale mixture of subregular pristine crypto- and micro-perthites with altered, micropore-rich feldspar with irregular microstructures. The regular perthites are strain-controlled intergrowths of Albite and/or Pericline-twinned albite exsolution lamellae within tweed orthoclase. The microperthites formed at less than or equal to 590 degrees C by heterogeneous nucleation of thin albite films which coarsened to > 1 mu m length. Cryptoperthites developed at < 400 degrees C by homogeneous nucleation of sub-mu m long platelets between films. Platelets are coherent, but the coarser microperthite lamellae are semi-coherent, with pairs of misfit dislocations sub-regularly spaced along the albite-orthoclase interface. As much as 30% of any one feldspar crystal is turbid, a result of the formation of numerous mu m to sub-mu m sized micropores during deuteric alteration. In some areas, deuteric fluids gained access to the interior of feldspar crystals by exploiting semi-coherent film lamellae. Albite was selectively dissolved and micropore-rich irregular microcline was reprecipitated in its place. In other parts of the feldspars deuteric recrystallization completely cross-cuts the pristine microtextures and patch perthites have formed. These are coarse, incoherent to semi-coherent intergrowths of irregular microcline (replacing tweed orthoclase) and Albite-twinned albite. The deuteric reactions occurred at < 400 degrees C; the main driving force for dissolution and reprecipitation was decrease in the elastic strain energy at the coherent interfaces of crypto- and micro-perthite lamellae, and the recrystallization of tweed orthoclase to irregular microcline.