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

Abstract

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 600-700oC from Na-K-Fe-Ca bearing brines(45-60wt.%NaCl eq.) assuming pressures of 280-640 bars. Allanite and amphibole formed at much lower temperatures (230-250oC) 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 over-growth. 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.