COMPOSITIONAL DEPENDENCE OF LAWSONITE EPIDOTE BLUESCHIST PSEUDOSECTIONS FROM WESTERN CRETE, GREECE

Crete is located in the position of a fore-arc high in the accretionary wedge resulting from the continued subduction of the African lithosphere under the thin Aegean crust. Low-temperature blueschist facies metabasalts from the phyllite-quartzite unit in the area of Palea Roumata in Western Crete, Greece contain the mineral assemblage Ab + Chl + Ep + Sph + Gl ± Law ± Pmp ± Qz ± Om ± Tr. Estimates for the PT conditions of formation of these Cretan blueschists made from coincidence of individual mineral equilibria are 305 ± 35 ˚C and 7.5 ± 1.5 kbar (1 sigma errors). The coexistence of lawsonite, epidote and pumpellyite in these rocks is of fundamental importance for determining temperatures, as they are limited by several different lawsonite and pumpellyite-out reactions. For the T and P dependent reaction Gl + Law = Cz + Ab + Chl + Qz + H2O, glaucophane activity exerts a primary control over the phase boundary. Coexisting sodic and calcic amphiboles, typically Gl and Tr, show little chemical variation despite differences in overall bulk chemistry.

The difference in observed mineral chemistry and parageneses, mainly the presence or abasence of pumpellyite and lawsonite, between samples from the same area have been interpreted as differences in initial bulk chemistry, rather than metamorphic conditions. To investigate this assumption, and the compositional dependence of mineral reactions, pseudosections are constructed in the system NCFMASH using Perplex and the internally consistent database of Holland & Powell, 2011. TX pseudosections are computed at 7.5 kbar that illustrate the sensitivity of lawsonite-out reactions to a variety of chemical components, and in turn suggest bulk compositions that will further limit the stability of lawsonite in blueschists. This contribution also addresses the degree to which amphiboles and their associated activity models may fundamentally destabilize the calculation of pseudosections in blueschists, even for rocks of known PT. In addition, the ability of several amphibole activity models (e.g. Diener et al. 2007) to replicate compositions of coexisting Gl and Tr is assessed.