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The formation and alteration of allanite in skarn from the Beinn an Dubhaich granite aureole, Skye

Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom

^* Current and correspondence address: School of the Environment, University of Brighton, Cockcroft Building, Lewes Road, Brighton, BN2 4GJ, U.K. e-mail: martin.smith{at}brighton.ac.uk

Allanite-(Ce) occurs as a metasomatic phase in diopside-hedenbergite-anorthite-hastingsite endoskarn from the Beinn an Dubhaich granite aureole, Skye. Fluid-inclusion studies indicate formation of pyroxene at temperatures in the range 600–700°C from Na-K-Fe-Ca bearing brines (45–60 wt. % NaCl eq.) assuming pressures of 280–640 bars. Allanite and amphibole formed from similar brines at 500–600°C (mean 550°C). Fluorite formed during cooling and dilution of these brines. Vesuvianite formed at much lower temperatures (230–250°C) from low- to moderate-salinity fluids ( 0–15wt % NaCl eq.). High F contents in both amphibole and vesuvianite indicate that the fluids were F bearing at all stages in the paragenesis.

Allanite has textures resulting from growth and alteration. Britholite-(Y) occurs as either a discrete phase, or as an alteration product of allanite. LA-ICP-MS analyses demonstrate that whilst Th remained relatively constant in individual allanite grains, U was leached during alteration. The Th content does vary with zonation, however, and may be altered by processes such as overgrowth. The hydrothermal mobility of the REE and Th was almost certainly a result of high F contents in the hydrothermal fluids. The hydrothermal formation and alteration of allanite may have important consequences for geochronological studies of other allanite occurrences.

Key-words: allanite, Rare Earth Elements, skarn, fluid inclusions.

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