Abstract

Systematic examination of published sedimentary and metamorphosed pelite analyses has revealed evidence of significant mass transfer and volume strain during regional metamorphism. Statistical analysis of the data shows that Barrovian zone metamorphism of pelitic schist generally causes increases in the whole-rock concentrations of the low-solubility elements Ti and All The observed increases in Ti and Al contents as functions of metamorphic grade are almost certainly due to residual enrichment caused by the removal of other more soluble species. Application of mass-balance principles to the petite compositional trends indicates that the average pelite may lose as much as 30% of its mass and volume during progressive metamorphism from subgreenschist to amphibolite facies conditions. The bulk of the lost mass is silica, not volatiles. In addition, other elements, particularly Ca, Na, and K, appear to be highly mobile in deep-crustal metamorphism. Contrary to conventional interpretations, it is concluded that the regional metamorphism of pelites is not an isochemical process.

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