Publication Date

2009

Document Type

Honors Thesis

Department

Geosciences

Abstract

Graphitic schists found on the northern end of the island of Syros, in the Cyclades, underwent high pressure/low temperature metamorphism as evidenced by the presence of glaucophane-bearing mineral assemblages. Many samples contain clusters of minerals that weather in relief and appear to be pseudomorphs after one or more protocrysts. Most of the pseudomorphs are believed to be after lawsonite because of their shape and the presence of rare, remnant lawsonite found in metabasalts on Syros (Brady et al., 2001), though pseudomorphs after other minerals are possible. The pseudomorphs range in size (from 2mm to 2 cm) and abundance, but are present in most samples. The typical matrix assemblage for the graphitic schists is phengite + quartz + calcite + sphene + graphite ± paragonite ± albite ± garnet ± clinozoisite ± glaucophane ± chlorite ± opaques. The pseudomorphs contain assemblages of phengite + quartz ± paragonite ± albite ± garnet ± clinozoisite ± chlorite ± sphene. The mineralogic composition within the boundaries of the pseudomorphs is inconsistent with the composition of lawsonite as the micas are K-rich and lawsonite is a Ca-Al silicate; this study attempts to reconcile the composition of the pseudomorph with the composition of lawsonite (or another protocryst) and the reactions that it could have undergone. It is hypothesized here that there are two types of protocrysts to the pseudomorphs in the graphitic schists, one not yet identified and the second after lawsonite. SEM analyses and microprobe element mapping provided chemical compositions and detailed images of the minerals within the pseudomorphs. Through mapping, two types of pseudomorphs were revealed, a Ca-rich (clinozoisite-rich) pseudomorph and a Ca-poor pseudomorph. The Ca and Al are believed to have remained largely immobile throughout the reactions, thus the original v amounts of Ca and Al are still present within the boundaries of the pseudomorphs. Mass balance equations yielded 20 to 50% of the area of the pseudomorph as original lawsonite. This indicates that the lawsonite protocryst overgrew a number of inclusions, which is evidenced by the presence of a pervasive fabric preserved in many of the pseudomorphs that is at an angle to the fabric in the matrix. Plotting the bulk compositions of the rocks on ACFN and ACF diagrams shows that the rocks fall into two distinct areas, the more Ca-rich bulk composition corresponds to the rocks that have Ca-poor pseudomorphs and the Ca-rich (clinozoisite-rich) pseudomorphs correspond to the more aluminous bulk compositions. This distribution supports the assertion that there are two different protocrysts because the bulk compositions of the rocks vary. Geothermobarometry was attempted using a garnet-phengite thermometer last calibrated by Hynes and Forest (1988), the results were low, but in a similar range as to what would be expected; the values ranged from 340 to 410 °C.

Comments

vi, 69 p. : ill. (chiefly col.), col map. Honors project-Smith College, Northampton, Mass., 2009. Includes bibliographical references.

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