Fractionation of rare-earth elements during magmatic differentiation and weathering of calc-alkaline granites in southern Myanmar

Geochemical characteristics and rare-earth element (<em>REE</em>)<em>-</em>bearing minerals of calc-alkaline granites in southern Myanmar were investigated to identify the minerals controlling fractionation between light and heavy <em>REE</em> (<em>LREE</em> and <em>HREE</em>) during magmatic differentiation and weathering. The granites were classified on the basis of the mineral assemblages into two contrasting groups: allanite-(Ce)- and/or titanite-bearing granites; and more <em>HREE</em>-enriched granites characterized by hydrothermal minerals including synchysite-(Y), parisite-(Ce), bastnäsite-(Ce), xenotime-(Y), monazite-(Ce), Y-Ca silicate, waimirite-(Y) and fluorite. This suggests that allanite-(Ce) and titanite are not stable in differentiated magma and <em>HREE</em> are eventually preferentially incorporated into the hydrothermal minerals. The occurrence of the <em>REE</em>-bearing minerals is constrained by the degree of magmatic differentiation and the boundary of two contrasting granite groups is indicated by SiO₂ contents of ∼74 wt.% or Rb/Sr ratios of ∼3–8. Fractionation between <em>LREE</em> and <em>HREE</em> during weathering of the granites is influenced by weathering resistance of the <em>REE</em>-bearing minerals, i.e. allanite-(Ce), titanite, the <em>REE</em> fluorocarbonates and waimirite-(Y) are probably more susceptible to weathering, whereas zircon, monazite-(Ce) and xenotime-(Y) are resistant to weathering. Ion-exchangeable <em>REE</em> in weathered granites tend to be depleted in <em>HREE</em> relative to the whole-rock compositions, suggesting that <em>HREE</em> are more strongly adsorbed on weathering products or that <em>HREE</em> remain in residual minerals.