K-Rb-Tl relationship in some gneissic rocks

Abstract

<p> Granitic and associated metamorphic rocks of Precambrian age in southeastern Ontario, Canada have been studied in order to determine the relationship of K, Rb and Tl and their behaviour during the process of partial melting and magmatic differentiation and metasomatism. </p> <p> Analytical results for major elements and rubidium and thallium abundances for 52 samples are given. </p> <p> From the present data, it appears possible that the series of migmatite and granodiorite gneiss originated by partial melting of paragneiss, together with pink granite which developed by late magmatic processes from a granodiorite melt, as was proposed by Chesworth (1967). </p> <p> Consideration of the geochemical data shows that the principal aspects of the distribution of Rb and Tl during partial melting are controlled by the mineralogical composition of the residuum which remains after partial melting. If mica is present in the residuum a newly-formed melt could have higher or the same K/Rb, K/Tl and Rb/Tl ratios as the parent rock and thus the "classical rule" of lower K/Rb and K/Tl in the anatectic melt is not valid. </p> <p> Enrichment of Tl in rocks affected by metasomatic processes is demonstrated. The marked decrease of K/Tl and Rb/Tl in the contact rocks (calc-silicate marbles) is caused by stronger migration of Tl, relative to K and Rb, from the intruded pink granite. </p> <p> The high mobility of thallium compounds is an important factor in the distribution of Tl during metasomatic and magmatogenic processes. </p>