BULK ROCK COMPOSITIONAL DEPENDENCE OF METAMORPHIC MINERAL ASSEMBLAGES: EXAMPLES FROM PELITE PSEUDOSECTIONS

Pseudosections (MnNaCaKFMASH system) were constructed for an average biotite zone pelite from the Waterville Formation, southcentral Maine using THERMOCALC and the thermodynamic data of Holland and Powell (1998). Activity models were those of Holland and Powell (1998) and those supplied with THERMOCALC, except that some solutions were extended to include Mn. Pseudosections indicate: i) the relevance of these diagrams for predicting parageneses and ii) the control that bulk rock composition exerts on the parageneses. Predicted mineral assemblages were compared with observed parageneses. Most results from pseudosections are in good agreement with natural assemblages. The biotite-in reaction calculated for the pseudosection indicates biotite growth at 420 °C and 3.5 kbars. Ferry (1984) estimated that biotite formed ca. 410 °C and 3.5 kbars, in very good agreement with the pseudosection. The most striking discrepancy is that observed staurolite+cordierite is not predicted by the pseudosection. The pseudosections predict staurolite stability above 4.3 kbars, higher than cordierite and/or andalusite stability. Two pseudosections were constructed by proportionally modifying the average biotite zone composition: 1) potash decreased from 3.8% to 2.15%, and 2) alumina increased from 17.83% to 20.17%. Comparisons of the three pseudosections show that small variations in bulk rock composition lead to significant changes in mineral assemblages. The biotite-in reaction shifts to higher T and lower P and the clinozoisite-out reaction shifts to lower T and higher P with increased alumina. Decrease in potash concentration drives paragonite stability to lower P (as low as 1 kbar), increases the P-T range of staurolite stability (mainly T range, P range remains above 4 kbars), and causes growth of aluminum silicate before biotite at P < 4 kbars. The average biotite zone pseudosection demonstrates the usefulness of pseudosections for predicting mineral assemblages with good (biotite stability) to moderate (staurolite) agreement between natural and predicted assemblages. Results from variation of alumina or potash concentration demonstrate the compositional dependence of metamorphic mineral assemblage.