Phase Equilibria and Thermobarometry of Lawsonite and Pumpellyite-Bearing Metabasalts From Crete

High-pressure, blueschist facies rocks associated with the subduction and subsequent exhumation of sedimentary rocks along the Hellenic subduction zone now outcrop over a large portion of the Greek island of Crete. They host the rare pelitic index mineral, carpholite, and has one of the few occurrences of metamorphic aragonite in marbles. Though largely composed of pelites, many small pods of metabasalts contain index minerals such as lawsonite and glaucophane. The assemblage lawsonite + pumpellyite has been found, providing an excellent opportunity to unravel the pressures and temperatures to which these rocks were subjected during subduction. Blueschists from the central part of Crete are typically glaucophane-bearing, with plentiful epidote, sphene and chlorite. Millimeter scale domains within some of the rocks contain the assemblage, quartz + lawsonite + albite + chlorite + epidote ± pumpellyite ± glaucophane. The presence of lawsonite and epidote roughly locates the rocks between the lawsonite and epidote blueschist facies as defined by Evans (1990). Chemically the lawsonite is fairly pure, with only small amounts of Fe (less than 0.4 wt %). Although some pumpellyite has almost no iron, when in equilibrium with the lawsonite it typically contains subequal amounts of Fe and Mg, (Fe/(Fe+Mg) is on average ~0.45). Chlorite is also roughly halfway between clinochlore and chamosite. The glaucophane is crossitic in composition. Clinozoisite has around 4 weight percent Fe2O3 (0.25 atoms Fe per formula unit). The albite and quartz are pure. The reaction albite + pumpellyite + lawsonite = glaucophane + clinozoisite + quartz + fluid has a somewhat steep slope (74 bar/°C) and can be used to constrain temperatures. Another reaction between lawsonite and pumpellyite is glaucophane + lawsonite = pumpellyite + chlorite + albite + quartz + fluid. The slope of this reaction is (24 bar/°C) and so the intersection of the two provides a reasonable estimate of the P-T conditions of this assemblage. A more accurate activity calculation will allow this estimate to be further refined. Further work on the mineral compositions and assemblages in metabasalts change from west to east across the island investigates the proposed westward increase in metamorphic grade in blueschist outcrops. Other reactions which can be used from this assemblage, and indicate roughly similar conditions of formation include tremolite + clinozoisite + clinochlore + fluid = pumpellyite + quartz or lawsonite + glaucophane = clinozoisite + quartz + albite + clinochlore + H2O and other limits in the absence of calcic-amphibole, or sphene. Initial calculations in the presence of lawsonite and pumpellyite suggest these rocks formed around 350°C at pressures near 7 kbar. Lawsonite + pumpellyite breaks down when the activity of H2O is greater than 0.95 at 6 kbar, suggesting a relatively high water activity. This greatly simplifies P-T calculations. These temperatures are in good agreement with those reported for carpholite-bearing metapelites described from the same unit by Theye & Seidel (1983). Their reported assemblage included quartz + phengite ± sudoite ± carpholite ± chloritoid. Our current estimate is slightly lower in pressure than is required to form metamorphic aragonite (~9 kbar at 350°C), which we have also found in this unit, although not in the metabasalts. The westward increase in grade has been reported as largely a temperature difference, so pressure gradients may explain the infrequency of aragonite occurrences at Crete.