COMPOSITIONAL AND MICROTEXTURAL ZONING IN ALKALI FELDSPARS FROM THE SHAP GRANITE AND ITS GEOCHEMICAL IMPLICATIONS

Alkali feldspar phenocrysts in the Lower Devonian Shap granite are com positionally and microtexturally zoned. Most of each phenocryst has ex solved to form microperthite with an approximate bulk composition of O r(72)Ab(28)An(<1), but sub-millimetre wide zones within the phenocryst s with an approximate bulk composition Or(69)Ab(30)An(1), have exsolve d to cryptoperthite. The sub-optical scale of exsolution microtextures in the narrow An-rich zones is due to Ca-Al coupling, which slows rat es of interdiffusion during exsolution relative to An-poor volumes of feldspar. This form of coupled compositional-microtextural zoning has not been described previously from Or-rich alkali Feldspars using elec tron microscopy but we have seen other examples and predict that it is common. Cryptoperthite zones are of particular importance because the y will have trace element and isotopic compositions least altered, sub sequent to growth in the magma, of any microtextures in alkali feldspa rs and so will be the most rewarding targets for chemical analysis.