Perthitic alkali feldspar as indicator mineral in the Central Swiss Alps. Dip and extension of the surface of the microcline/sanidine transition isograd

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Granitoid rocks have been probed along 20 traverses through the Central Swiss Alps across the zoning of the late-Alpine metamorphism. The K-feldspar and plagioclase components of their microperthites have been studied by X-ray powder methods. With respect to the structural state of K-feldspar two regions can be distinguished from the north to the south: (1) Pre-Alpine structural states are preserved in the northern (marginal) region where maximum temperatures did not exceed T max ≈ 350°C. The southern boundary of this region coincides fairly well with the stilpnomelane-out boundary. (2) The southern (central) region is subdivided into two zones: (2a) In the northern zone temperatures ranged between ≈ 350°C and the temperature of the microcline/sanidine transition T diff ≈ 450–480°C. Here, the pre-Alpine structural states have all been homogenized to stable low microcline. (2b) In the southern zone, T max exceeded 450–480°C so that microcline states were transformed into sanidine which on subsequent cooling back-reacted to various kinetically stranded microcline states as well as to stable low microcline. Thus, from the north to the south the first occurrence of “X-ray orthoclase” or intermediate microcline is indicative of the microcline/sanidine transition isograd. The isograd manifests a discontinuity in the spread of K-feldspar structural states. It has been located in the traverses as a function of altitude. From this information the three-dimensional extension of the isograd surface can be delineated over a distance of nearly 140 km. The An-content of the microperthitic plagioclase allows one to distinguish two main zones: (1) a northern zone extending to the south beyond the K-feldspar discontinuity where the plagioclases are more or less albitic, and (2) a southern zone — cut by the Insubric fault line — which is separated from the northern zone by a discontinuous increase of the An-content to oligoclase (-andesine) composition. The northern boundary of the oligoclase zone closely coincides with the staurolite-in boundary (≈ 550°C).