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

Abstract

Geochemical characteristics and rare-earth element (REE)-bearing minerals of calc-alkaline granites in southern Myanmar were investigated to identify the minerals controlling fractionation between light and heavy REE (LREE and HREE) 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 HREE-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 HREE are eventually preferentially incorporated into the hydrothermal minerals. The occurrence of the REE-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 LREE and HREE during weathering of the granites is influenced by weathering resistance of the REE-bearing minerals, i.e. allanite-(Ce), titanite, the REE fluorocarbonates and waimirite-(Y) are probably more susceptible to weathering, whereas zircon, monazite-(Ce) and xenotime-(Y) are resistant to weathering. Ion-exchangeable REE in weathered granites tend to be depleted in HREE relative to the whole-rock compositions, suggesting that HREE are more strongly adsorbed on weathering products or that HREE remain in residual minerals.

Hydrofluoric Acid‐Free Digestion of Geological Samples for the Quantification of Rare Earth Elements by Inductively Coupled Plasma‐Optical Emission Spectrometry

“…The mineralogical compositions of all RMs and samples are shown in Table . The potential REE‐bearing minerals (Zaitsev et al , Zaitsev et al , Ali , Jordens et al , Sanematsu et al ) are marked with asterisks.…”

Petrogenesis and mode of emplacement of a Neoarchean tonalite–trondhjemite–diorite suite: the Eau Jaune Complex, Abitibi greenstone belt

“…For these units, depletion in HREE is likely due to the presence of residual garnet and/or amphibole in the source ) and, to a lesser extent, to the fractional crystallization of amphiboles (Prowatke and Klemme 2006;Szopa et al 2013). Residual titanates (e.g., rutile, and also titanite; Briqueu et al 1984) are likely responsible for the negative Nb, Ta, and Ti anomalies (Sanematsu et al 2016). Trondhjemite 3a, which is also silica-rich (> 64 wt% SiO 2 ), is slightly less depleted in HREE, less fractionated, and has less pronounced negative Nb, Ta, and Ti anomalies than the felsic phases mentioned previously.…”

Ferrihydrite Transformation Impacted by Adsorption and Structural Incorporation of Rare Earth Elements

“…Because of their reactive surface, they are effective in scavenging REEs, often resulting in the enrichment of REEs. Regolith-hosted REE deposits are dominantly located in the weathering crusts of granites and soils in South China, with a few deposits, and potential prospects recently discovered and explored in Southeast Asia, − Madagascar, and the Southeastern United States . The ore grades range from 140 to 6500 mg kg –1 (typically ∼800 mg kg –1 ) of REEs, and some of the deposits are remarkably enriched in HREEs .…”