Sub-seafloor self-metasomatism and REE mobility in and around plagiogranites: Allanite and epidote of the Troodos ophiolite epidosites

Epidosides in the oceanic crust are rocks composed almost entirely of quartz and epidote that form by alteration of primary igneous minerals. They are common in the diabase of the sheeted dykes and form in regions that experienced high volumes of fluid flow. This hydrothermal fluid begins as seawater and is subsequently modified by reaction with heated rocks during circulation in the crust. The amount and nature of magmatic fluid input to this hydrothermal circulation systems is poorly constrained.
Plagiogranites also contain epidosites on occasion. Here we show examples from the Troodos Ophiolite in Cyprus of plagiogranite-hosted epidosides. Zoning in epidote and allanite (A REE-rich member of the epidote group) indicates that the epidotising fluids were initially derived from the magma and not from modified seawater and only later the fluid evolved to seawater-like characteristics. Similar magmatic signature is also observed in epidote–allanite veins observed in diabase-hosted epidosites, suggesting that magmatic fluids can migrate from the plagiogranites to other regions of the oceanic crust.

Further reading:

Anenburg, M., Katzir, Y., Rhede, D., Jöns, N., and Bach, W., 2015, Rare earth element evolution and migration in plagiogranites: a record preserved in epidote and allanite of the Troodos ophiolite: Contributions to Mineralogy and Petrology, v. 169, p. 25.