Characterization and origin of the alkali feldspar of granitoid rocks from the Variscan anatectic Tormes Dome (West-Central Spain)

The alkali feldspars from anatectic peraluminous granites in this region are more albitic (<< Or (sub 83) ), poorer in P, rich in Rb and have a maximally ordered structural state. By contrast, feldspars from the calc-alkaline series are richer in Or (> Or (sub 85) ) and Ba (> 2000 ppm), and their structural state is variable (triclinicity 0 << Delta << 0.97). Both optical-structural (2V (sub alpha ) vs Sigma t1) and chemical-structural (% Or + Cn)/Ab vs Sigma t1 correlations can be observed in the analysed feldspars. The thermometric values obtained suggest: a) 730-650 degrees C as minimum crystallization T, b) 560-530 degrees C as maximum unmixing T, c) 420-360 degrees C as minimum T according to the coherent solvus of the granitic rocks of the calc-alkaline series, and d) approximately 300 degrees C for the albite-low microcline association of the partial deuteric unmixing field of the most peraluminous granites. The stability of micas in anatectic magmas could be explained by higher f (sub H) (sub 2) (sub O) , hence the ordering of the alkali feldspar of the S-type leucogranites, whereas disordered feldspar is characteristic of the granitic rocks with calc-alkaline affinities; the more disordered, the higher the Ba and K contents.