Magmatic and hydrothermal R.E.E. fractionation in the Xihuashan granites (SE China)

The Xihuashan stock (South Jiangxi, China) is composed of cogenetic granitic units (granites Xe, a, c, d and b) and emplaced during the Yanshanian orogeny (153±0.2 Ma). They are two feldspars, Fe-rich biotite±garnet and slightly peraluminous granites. Primary accessory minerals are apatite 1, monazite, zircon, uranothorite±xenotime in granites Xe and a, zircon, uranothorite, uraninite, betafite, xenotime 1; hydrothermal minerals are monazite altered into parisite and apatite 2, Y-rich parisite, yttroparisite, Y-rich fluorite and xenotime 2 in granites c and b. Petrographic observations, major element, REE, Y and Rb–Sr isotropic data point to a magmatic suite (granites Xe and a granites c and d granite b) distinct from hydrothermal Na-or K-alteration of b. From granite Xe to granite b, LREE, Eu, Th and Zr content are strongly depleted, while HREE, Y and U content increase. During K-alteration of b, these variations are of minor importance. Major and accessory mineral evidences, geochemical and fluid inclusion results indicate two successive alteration fluids interacting with b, (1) a late-magmatic F^– and CO₂–rich fluid and (2) a post-magmatic, aqueous and slightly saline fluid. The depletion of LREE and Th content and the increase in HREE, Y and U content correspond, in the magmatic suite to the early fractionation of monazite in the granites where there is no hydrothermal alteration (granites Xe and e) and to the hydrothermal alteration of monazite into parisite and secondary apatite, intense new formation of yttroparisite, Y enrichment and U loss in the uranothorite and late crystallization of uraninite in the granites c and b. Moreover, simulated crystallization of monazite and temperature of monazite saturation show early fractionation of monazite from the magma in the less evolved granites (Xe and e) and prevailing hydrothermal leaching of monazite in the most evolved granites (c-d and b) related to a late-magmetic event. The slight variations of REE, Y, Th and U content in the K-altered granites compared to granite b emphazes the distinct chemical nature of the successive hydrothermal fluids. Rb–Sr and Sm–Nd isotopic results point to a 30 Ma period of time between the late-magmatic and the post-magmatic fluid circulation.