Uranium-rich accessory minerals in the peraluminous and perphosphorous Belvís de Monroy pluton (Iberian Variscan belt)

Abstract

The strongly peraluminous, perphosphorous (<0.85 wt% P2O5) and low-Ca granites from the Belvís de Monroy pluton contain the most U-rich monazite-(Ce) and xenotime known in igneous rocks. Along with these accessory minerals, P-rich zircon occurs, reaching uncommon compositions particularly in the more fractionated units of this zoned pluton. Monazite displays a wide compositional variation of UO2 (<23.13 wt%) and ThO2 (<19.58 wt%), positively correlated with Ca, Si, P, Y and REE. Xenotime shows a high UO2 content (2.37–13.34 wt%) with parallel increases of LREE, Ca and Si. Zircon contains comparatively much lower UO2 (<1.53 wt%) but high P2O5 (<14.91 wt%), Al2O3 (<6.96 wt%), FeO (<2.93 wt%) and CaO (<2.24 wt%) contents. The main mechanism of incorporating large U and Th amounts in studied monazite and U in xenotime is the cheralite-type [(Th,U)4+ + Ca2+ = 2(Y,REE)3+] substitution. Zircon requires several coupled mechanisms to charge balance the P substitution, resulting in non-stoichiometric compositions with low analytical totals. Compositional variations in the studied accessory phases indicate that the substitution mechanisms during crystal growth depend on the availability of non-formula elements. The strong P-rich character of the studied granites increases monazite crystallization, triggering a progressive impoverishment in Th and LREE in the residual melts, and consequently increasing extraordinarily the U content in monazite and xenotime. This is in marked contrast to other peraluminous (I-type or P-poor S-type) granite series. The P-rich and low-Ca peraluminous melt inhibits uraninite crystallization, so contributing to the U availability for monazite and xenotime.