Occurrence of Th, U, Y, Zr and REE-bearing accessory minerals in late-Variscan granitic rocks from the Sierra de Guadarrama (Spain)

The paragenesis of Y, Th, U, Zr and REE accessory minerals in four different late-Variscan granitic associations of the Sierra de Guadarrama are discussed and EPMA results are tabulated for zircons, monazites, allanites, xenotimes, thorites and bastnaesites. Characteristic paragenetic sequences of accessory minerals are found in each association. The occurrence of different minerals is controlled by the initial trace-element contents of the melt, the aluminous index, the Ca content and volatiles, as well as contents of F and P, the fO (sub 2) , and the LREE/HREE, Th/REE, U/REE and U/Th ratios of the melts. The accessory mineral assemblage can be changed by post-magmatic hydrothermal processes. The differences between the accessory mineralogical paragenesis and the individual mineral compositions in these granitic rocks are discussed in terms of these parameters, which play a key role in determining the crystallization of accessory REE, Th and U-bearing phases. The geochemical behaviour of these elements during granite petrogenesis is also discussed.