MINERAL LEUCOSOME TRACE-ELEMENT PARTITIONING IN A PERALUMINOUS MIGMATITE (A LASER ABLATION-ICP-MS STUDY)

Low-pressure peraluminous migmatites from the Pena Negra Complex (cent ral Spain) stayed partially molten for 5-10 Ma, until the latest Hercy nian deformations allowed the segregation of melt as discordant leucos ome veins. Due to long residence within its source, the melt had time enough to equilibrate with residual phases other than accessories incl uded within major, refractory minerals. We estimated crystal/melt part ition coefficients as the concentration ratios between leucosome sampl es appearing to be pure melts and mesosome minerals, which were analyz ed for trace elements with a laser probe coupled to an ICP mass spectr ometer. Our data reveal that when biotite is stable Li, Rb, Cs, TI, Sc , V, Cr, Ni, Nb and Ta become strongly compatible. The role of biotite in fractionating Y, Th, U and the REE is insignificant. Cordierite st rongly fractionates Li and Be and also has some effect on the HREE and U. Garnet produces extreme fractionation of Sc, Y and the HREE. The R EE partition coefficients for ferromagnesian silicates increase with t he atomic number, this effect being progressively more important throu gh biotite, cordierite and garnet. K-feldspar strongly fractionates Ba and Pb, but plays a secondary role to biotite for Rb and Cs. K-feldsp ar fractionation does not change the LREE/HREE ratio, but plagioclase fractionation produces a significant decrease in the LREE/HREE ratio. Both feldspars greatly fractionate Sr and Eu. Monazite fractionation p roduces a dramatic depletion in REE, Th and U, as well as a decrease i n the LREE/HREE and Th/U ratios. Apatite also fractionates the REE alt hough, in contrast with monazite, it increases the LREE/HREE ratio and does not affect the Th/U ratio. Zircon fractionation, like apatite an d garnet, produces a strong depletion in the HREE and a concomitant in crease in the LREE/HREE ratio. In contrast with monazite, zircon fract ionation causes the Th/U ratio to increase.