ISOTOPE DISEQUILIBRIUM DURING ANATEXIS - A CASE-STUDY OF CONTACT MELTING, SIERRA-NEVADA, CALIFORNIA

The geochemical consequences of contact melting were investigated in t he Sierra Nevada Batholith, where trachyandesite magmas have intruded and melted the host granitic rocks. Partial melting of the granite var ies from 10-20% at contact regions, to 50-70% in a granite block withi n the trachyandesite. Thermal modelling suggests that the temperature of the granite block exceeded its solidus within 3 months of trachyand esite intrusion, reached a maximum of similar to 1000 degrees C after ca. 1.5 yr, and remained above 500 degrees C for ca. 40 yr. Sr, Nd and Pb isotope data of granitic melts record marked Sr isotope, and to a lesser extent Pb isotope, disequilibrium both within different glass f ractions and between glasses and their source. Rb/Sr isochron calculat ions on residual mineral-glass pairs fail to yield the age of melting obtained by Ar-40-Ar-39 dating (11.9 Ma). Sr and Pb isotope data of gl asses establish that major rock-forming minerals of the granite had no t attained isotope equilibrium before the onset of melting. Consequent ly, the isotopic composition of the successive liquid fractions origin ating from granite melting depended on the relative contribution of ea ch mineral to the elemental budget of the melts. The extrapolation of these results to the petrogenesis of crustal melts and more generally to studies of crustal evolution, suggests that isotope disequilibrium during anatexis and high-grade metamorphism may be a common characteri stic of many geologic settings. If future studies substantiate these r esults, some geochemical models of crustal melting and assimilation ma y need revision. In addition, marked Sr isotope disequilibrium in the residual mineral phases within the partially melted granite raises con cerns about the reliability of radiometric dating in metamorphic terra nes.