Sr-isotope evolution of granitoid source rocks.

The hypothesis of restite control for the geochemical variation in granitoids (Chappell, 1966) in which linear variation lines are interpreted as mixing lines between a melt end-member and refractory source residue termed 'restite', has been developed further to estimate Rb/Sr for the individual granitoid source regions. The chemistry of (model) restite is indentified when normative quartz disappears as the variation lines of the major elements are extrapolated towards lower Si0₂. The melt end-members have known SiO₂ contents in the range 70-76%. The Rb and Sr contents of the restites and melts are determined by combining known Rb, Sr distribution coefficients for the restite minerals with their observed abundances in the granitoids. The source Rb, Sr is then expressed as the fraction F of melt component plus (1-F) of the restite, and Rb/Sr evaluated for each granitoid source as a function of F. Adapting an analysis of melt extraction by Turcotte and Ahern (1978), F may be constrained as greater than c15%, and by an arguement based on the very low mechanical strength of rocks containing 25% or more liquid, as less than 25% for non-minimum melts. For granitoids containing minimum melts, F may be 25% or greater and a maximum constraint is available only by modelling the composition of the source rocks. Regional averages of modern island-arc volcanics remote from continental areas indicate that Si0₂ would not exceed c65% over the source volumes of most granitoids, which is equivalent to 61% for F. -Authors