Strontium Partitioning between Coexisting K-Feldspar and Plagioclase as an Indicator of Metamorphic Grade in Southwestern New Hampshire

Abstract

To test the validity of the use of strontium distribution coefficients determined for coexisting feldspars in metamorphic rocks as indicators of metamorphic grade, specimens collected from the metamorphic terrain of southern New Hampshire have been studied. Twenty pairs of concentrates of plagioclase and K-feldspars were obtained from samples representative of the kyanite-staurolite, sillimanite, and K-feldspar zones. The abundances of Na, K, and Ca. as well as Sr, were determined by atomic absorption analysis to allow a correction to be made in the Sr values after calculation of the feldspar contents of the samples. A preferential partitioning of Sr in the K-feldspar with increasing metamorphic grade is indicated. When points representing the molecular percents; of Sr-feldspar in K and plagioclase feldspars in the same rock sample were plotted on a Roozeboom diagram, all but three samples plotted along straight regression lines representing the distribution coefficient characteristic of one of the three metamorphic zones. The K d values so determined are: staurolite-kyanite zone, 0.71; sillimanite zone, 0.95; and K-feldspar zone, 1.28. The concentration of Sr does not affect the values of these distribution coefficients, nor do they appear influenced by the presence of non-feldspar phases. The variation in Sr concentration among the 40 feldspars analyzed is independent of the Ca variation. The pattern of metamorphic grace previously established by paragenetic studies in the southern New Hampshire area seems to be confirmed.