Pseudomorphs after Lawsonite from Syros, Greece

Syros is part of the Attic-Cycladic blueschist belt and consists principally of marbles, schists, and metabasites that have been metamorphosed to the blueschist- to eclogite-facies at about 52 Ma, with a greenschist-facies overprint beginning at about 25 Ma. Distinct, whitish clusters of submillimeter-sized grains organized into orthorhombic polyhedral shapes occur in a variety of the blueschists. These clusters are interpreted to be pseudomorphs after lawsonite based on their shape, chemistry, and the presence of rare relict lawsonite in a few of the clusters. A considerable range in observed pseudomorph modes reflects both variety in rock compositions and the extent of greenschist overprinting. Most pseudomorphs contain epidote-group minerals (clinozoisite/epidote), similar to the composition of ideal lawsonite, except for added Fe and lost water. Many contain white mica (phengite, paragonite, muscovite), which requires the addition of K and/or Na to the original lawsonite. In some cases, the pseudomorph is largely mica, so that Ca must have been lost to an epidote mineral, amphibole, or calcite in the matrix. Phengite in the pseudomorphs typically has a lower celadonite content than phengite in the matrix. It appears that the mica has changed its composition by breakdown in the matrix, K diffusion, and regrowth in the pseudomorph. We propose that lawsonite breakdown must be in part driven by the reaction of lawsonite with the celadonite content of phengitic mica in the matrix, to form celadonite-poor (muscovite-rich) white mica in the pseudomorph. The preservation of the lawsonite shape in the pseudomorphs in weakly deformed rocks appears to be due to the relative immobility of Al during the pseudomorphing reactions, with K, Na, and Fe moving to the pseudomorphs and Ca and H2O leaving. The only source of K in these rocks is phengite, which should become less rich in celadonite with an increase in temperature, needed to drive lawsonite breakdown.