Mineral/leucosome trace-element partitioning in a peraluminous migmatite (a laser ablation-ICP-MS study)

Abstract

Low-pressure peraluminous migmatites from the Peña Negra Complex (central Spain) stayed partially molten for 5–10 Ma, until the latest Hercynian deformations allowed the segregation of melt as discordant leucosome veins. Due to long residence within its source, the melt had time enough to equilibrate with residual phases other than accessories included within major, refractory minerals. We estimated crystal/melt partition coefficients as the concentration ratios between leucosome samples appearing to be pure melts and mesosome minerals, which were analyzed for trace elements with a laser probe coupled to an ICP mass spectrometer. Our data reveal that when biotite is stable Li, Rb, Cs, Tl, 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 strongly 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 REE partition coefficients for ferromagnesian silicates increase with the atomic number, this effect being progressively more important through biotite, cordierite and garnet. K-feldspar strongly fractionates Ba and Pb, but plays a secondary role to biotite for Rb and Cs. K-feldspar 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 produces a dramatic depletion in REE, Th and U, as well as a decrease in the LREE/HREE and Th/U ratios. Apatite also fractionates the REE although, in contrast with monazite, it increases the LREE/HREE ratio and does not affect the Th/U ratio. Zircon fractionation, like apatite and garnet, produces a strong depletion in the HREE and a concomitant increase in the LREE/HREE ratio. In contrast with monazite, zircon fractionation causes the Th/U ratio to increase.