F. Bea

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

Abstract Low-pressure peraluminous migmatites from the Pena 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.

Behavior of accessory phases and redistribution of Zr, REE, Y, Th, and U during metamorphism and partial melting of metapelites in the lower crust: an example from the Kinzigite Formation of Ivrea-Verbano, NW Italy

Abstract This study is aimed at understanding the behavior of monazite, xenotime, apatite and zircon, and the redistribution of Zr, REE, Y, Th, and U among melt, rock-forming and accessory phases in a prograde metamorphic sequence, the Kinzigite Formation of Ivrea-Verbano, NW Italy, that may represent a section from the middle to lower continental crust. Metamorphism ranges from middle amphibolite to granulite facies and metapelites show evidence of intense partial melting and melt extraction. The appearance of melt controls the grain size, fraction of inclusions and redistribution of REE, Y, Th, and U among accessories and major minerals. The textural evolution of zircon and monazite follows, in general, the model of Watson et al. (1989) . Apatite is extracted from the system dissolved into partial melts. Xenotime is consumed in garnet-forming reactions and is the first source for the elevated Y and HREE contents of garnet. Once xenotime is exhausted, monazite, apatite, zircon, K-feldspar, and plagioclase are progressively depleted in Y, HREE, and MREE as the modal abundance of garnet increases. Monazite is severely affected by two retrograde reactions, which may have consequences for U-Pb dating of this mineral. Granulite-grade metapelites (stronalites) are significantly richer in Ti, Al, Fe, Mg, Sc, V, Cr, Zn, Y, and HREE, and poorer in Li, Na, K, Rb, Cs, Tl, U, and P, but have roughly the same average concentration of Cu, Sr, Pb, Zr, Ba, LREE, and Th as amphibolite-grade metapelites (kinzigites). The kinzigite-stronalite transition is marked by the sudden change of Th/U from 5–6 to 14–15, the progressive increase of Nb/Ta, and the decoupling of Ho from Y. Leucosomes were saturated in zircon, apatite, and (except at the lowest degree of partial melting) monazite. Their REE patterns, especially the magnitude of the Eu anomaly, depend on the relative proportion of feldspars and monazite incorporated into the melt. The presence of monazite in the source causes an excellent correlation of LREE and Th, with nearly constant Nd/Th ≈ 2.5–3. The U depletion and increase in Th/U characteristic of granulite facies only happens in monazite-bearing rocks. It is attributed to enhancement of the U partitioning in the melt due to elevated Cl activity followed by the release of a Cl-rich F-poor aqueous fluid at the end of the crystallization of leucosomes. Halide activity in partial melts was buffered by monazite and apatite. Since the U (and K) depletion does not substantially affect the heat-production of metapelites, and mafic granulites maintain similar Th/U and abundance of U and Th as their unmetamorphosed equivalents, it seems that geochemical changes associated to granulitization have only a minor influence on heat-production in the lower crust.

Tracking magmatic processes through Zr/Hf ratios in rocks and Hf and Ti zoning in zircons: An example from the Spirit Mountain batholith, Nevada

Abstract Zirconium and Hf are nearly identical geochemically, and therefore most of the crust maintains near-chondritic Zr/Hf ratios of ~35-40. By contrast, many high-silica rhyolites and granites have anomalously low Zr/Hf (15-30). As zircon is the primary reservoir for both Zr and Hf and preferentially incorporates Zr, crystallization of zircon controls Zr/Hf, imprinting low Zr/Hf on coexisting melt. Thus, low Zr/Hf is a unique fingerprint of effective magmatic fractionation in the crust. Age and compositional zonation in zircons themselves provide a record of the thermal and compositional histories of magmatic systems. High Hf (low Zr/Hf) in zircon zones demonstrates growth from fractionated melt, and Ti provides an estimate of temperature of crystallization (TTiZ) (Watson and Harrison, 2005). Whole-rock Zr/Hf and zircon zonation in the Spirit Mountain batholith, Nevada, document repeated fractionation and thermal fluctuations. Ratios of Zr/Hf are ~30-40 for cumulates and 18-30 for high-SiO2granites. In zircons, Hf (and U) are inversely correlated with Ti, and concentrations indicate large fluctuations in melt composition and TTiZ (>100°C) for individual zircons. Such variations are consistent with field relations and ion-probe zircon geochronology that indicate a >1 million year history of repeated replenishment, fractionation, and extraction of melt from crystal mush to form the low Zr/Hf high-SiO2 zone.

MAFIC PRECURSORS, PERALUMINOUS GRANITOIDS, AND LATE LAMPROPHYRES IN THE AVILA BATHOLITH : A MODEL FOR THE GENERATION OF VARISCAN BATHOLITHS IN IBERIA

The Avila batholith of central Spain is composed of upper Carboniferous peraluminous granitoids that were preceded by volumetrically insignificant bodies of mafic‐ultramafic hybrid magmas and postdated by several dike swarms of camptonitic lamprophyres. Rb‐Sr dating indicates continuous magmatic activity from \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Controls on the trace element composition of crustal melts

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Geochemical variation in peridotite xenoliths and their constituent clinopyroxenes from Ray Pic (French Massif Central): implications for the composition of the shallow lithospheric mantle

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The Eocene bimodal Piranshahr massif of the Sanandaj–Sirjan Zone, NW Iran: a marker of the end of the collision in the Zagros orogen

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