MASS-TRANSFER DURING BARROVIAN METAMORPHISM OF PELITES, SOUTH-CENTRALCONNECTICUT .1. EVIDENCE FOR CHANGES IN COMPOSITION AND VOLUME

Evidence for mass and volume changes attending the Barrovian (chlorite to kyanite zone) metamorphism of aluminous pelites of the Wepawaug Sc hist, Connecticut, is examined using a petrologic mass balance approac h that takes full account of the closure problem and the multivariate nature of compositional data. Quartz veins are not included in the ana lysis so that mass and volume changes in aluminous pelite can be effec tively isolated and quantified. Statistical analysis and regional mapp ing of chemistry and density variations strongly suggest that the phys icochemical properties of the pelites are correlated with geographic p osition and metamorphic grade. The nature and degree of the variations indicate that they are the result of metamorphism, not sedimentary or diagenetic processes. The concentration systematics of relatively ''i mmobile'' low-solubility elements (for example, Ti, Zr, and Nb) are co nsistent with the hypothesis that the garnet, staurolite, and kyanite zone pelites had initial (protolith) compositions that were comparable to those of the presently exposed chlorite and biotite zone rocks. Ma ss balance analysis strongly suggests that metamorphism led to signifi cant mass and volume changes over minimum length scales on the order o f typical hand sample dimensions. The average degree of physicochemica l change increases in a general way with metamorphic grade. Average to tal mass change estimates, computed relative to the low-grade chlorite and biotite zone rocks using a Ti reference frame, are -10 +/- 9 perc ent, -19 +/- 6 percent, and -23 +/- 6 percent for the garnet, stauroli te, and kyanite zone pelites, respectively (+/-2sigma; negative values indicate mass loss). A diverse spectrum of elements were apparently m obile during metamorphism. Si was lost from upper greenschist and amph ibolite facies pelites; average silica mass change values are -15(14)17 percent, -32(-9)+11 percent, and -38(-8)+9 percent for the garnet, staurolite, and kyanite zones, respectively (+/- 2sigma). P was lost f rom kyanite zone pelites, and Na was lost from the amphibolite facies pelites. On average, Mn and Zn were added to staurolite and kyanite zo ne pelites, and K and Ba were added to staurolite zone pelites. Amphib olite facies pelites may have lost some Ca and Sr, but the analysis is complicated by protolith heterogeneity. The grain density (rock densi ty on a porosity-free basis) of the pelites increases systematically a s metamorphic grade increases. The mass loss and grain density increas es are interpreted to have caused significant decreases in volume. Est imates of average volume change for the garnet, staurolite, and kyanit e zone pelites are -12 +/- 10 percent, -22 +/- 6 percent, and -28 +/- 6 percent, respectively (+/- 2sigma), relative to the chlorite and bio tite zone rocks. The degree of mass transfer and volume change was var iable within individual outcrops. The results of this study indicate t hat significant, heretofore unrecognized changes in the composition an d volume of aluminous pelite may occur during Barrovian style metamorp hism.