Compositional and microtextural zoning in alkali feldspars from the Shap granite and its geochemical implications

Abstract

Alkali feldspar phenocrysts in the Lower Devonian Shap granite are compositionally and microtexturally zoned. Most of each phenocryst has exsolved to form microperthite with an approximate bulk composition of Or 72 Ab 28 An <1 , but sub-millimetre wide zones within the phenocrysts with an approximate bulk composition Or 69 Ab 30 An 1 , have exsolved to cryptoperthite. The sub-optical scale of exsolution microtextures in the narrow An-rich zones is due to Ca–Al coupling, which slows rates 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 electron microscopy but we have seen other examples and predict that it is common. Cryptoperthite zones are of particular importance because they will have trace element and isotopic compositions least altered, subsequent to growth in the magma, of any microtextures in alkali feldspars and so will be the most rewarding targets for chemical analysis.