Rare earth element evolution and migration in plagiogranites: a record preserved in epidote and allanite of the Troodos ophiolite

Abstract

Plagiogranites from the Troodos ophiolite in Cyprus are occasionally epidotised, either partially or completely. Epidotisation phenomena include replacement of pre-existing minerals and filling of miarolitic cavities. In addition to epidote, miarolites in one plagiogranite body (located near the village of Spilia) contain coexisting ferriallanite-(Ce) and allanite-(Y). Textural and geochemical evidence indicates that late-stage REE-enriched granitic melt facilitated crystallisation of magmatic ferriallanite- (Ce). High REE contents persisted after fluid exsolution, causing crystallisation of allanite-(Y) from hydrothermal fluids in the miarolites. The REE pattern of the hydrothermal allanite-(Y) is characterised by LREE and Eu depletion, similar to the parent plagiogranitic magma. As allanite had sequestered most of the REE in the fluid, epidote took over as the principle hydrothermal mineral. Epidote in Troodos plagiogranites records a fluid evolutionary trend beginning with REE-rich–Eu-depleted similar to allanite- (Y) and gradually transforming into the REE-depleted– Eu-enriched pattern prevalent throughout ‘conventional’ sub-seafloor fluids. A comparison of allanite-bearing and allanite-absent plagiogranites from the same locality suggests that REE-bearing fluids migrated from the plagiogranites. Similar fluid evolution trends observed in diabase-hosted epidote, located adjacent to a large plagiogranite body, suggest influx of plagiogranite-derived REEbearing fluids. Epidotisation in oceanic settings is usually considered to be the result of alteration by high fluxes of seawater-derived hydrothermal fluids. Although epidotisation by magmatic fluids has been suggested to occur in plagiogranites, our study shows that this autometasomatic process is the dominant mechanism by which epidosites form in plagiogranites. Furthermore, epidotisation of diabase has been attributed solely to seawater-derived fluids, but we show that it is possible for diabase-hosted epidosites to form by migration of plagiogranite-derived fluids.