MASS-TRANSFER DURING BARROVIAN METAMORPHISM OF PELITES, SOUTH-CENTRAL CONNECTICUT .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 Schist, Connecticut, is examined using a petrologic mass balance approach that takes full account of the closure problem and the multivariate nature of compositional data. Quartz veins are not included in the analysis so that mass and volume changes in aluminous pelite can be effectively isolated and quantified. Statistical analysis and regional mapping of chemistry and density variations strongly suggest that the physicochemical properties of the pelites are correlated with geographic position 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 ''immobile'' low-solubility elements (for example, Ti, Zr, and Nb) are consistent 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. Mass balance analysis strongly suggests that metamorphism led to significant mass and volume changes over minimum length scales on the order of typical hand sample dimensions. The average degree of physicochemical change increases in a general way with metamorphic grade. Average total mass change estimates, computed relative to the low-grade chlorite and biotite zone rocks using a Ti reference frame, are -10 +/- 9 percent, -19 +/- 6 percent, and -23 +/- 6 percent for the garnet, staurolite, and kyanite zone pelites, respectively (+/-2sigma; negative values indicate mass loss). A diverse spectrum of elements were apparently mobile during metamorphism. Si was lost from upper greenschist and amphibolite 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 from kyanite zone pelites, and Na was lost from the amphibolite facies pelites. On average, Mn and Zn were added to staurolite and kyanite zone pelites, and K and Ba were added to staurolite zone pelites. Amphibolite facies pelites may have lost some Ca and Sr, but the analysis is complicated by protolith heterogeneity. The grain density (rock density on a porosity-free basis) of the pelites increases systematically as metamorphic grade increases. The mass loss and grain density increases are interpreted to have caused significant decreases in volume. Estimates of average volume change for the garnet, staurolite, and kyanite zone pelites are -12 +/- 10 percent, -22 +/- 6 percent, and -28 +/- 6 percent, respectively (+/- 2sigma), relative to the chlorite and biotite zone rocks. The degree of mass transfer and volume change was variable within individual outcrops. The results of this study indicate that significant, heretofore unrecognized changes in the composition and volume of aluminous pelite may occur during Barrovian style metamorphism.