Isotope disequilibrium during anatexis: a case study of contact melting, Sierra Nevada, California

The geochemical consequences of contact melting were investigated in the Sierra Nevada Batholith, where trachyandesite magmas have intruded and melted the host granitic rocks. Partial melting of the granite varies from 10–20% at contact regions, to 50–70% in a granite block within the trachyandesite. Thermal modelling suggests that the temperature of the granite block exceeded its solidus within 3 months of trachyandesite intrusion, reached a maximum of ∼ 1000°C after ca. 1.5 yr, and remained above 500°C for ca. 40 yr. and Pb isotope data of granitic melts record marked Sr isotope, and to a lesser extent Pb isotope, disequilibrium both within different glass fractions and between glasses and their source. Rb/Sr isochron calculations on residual mineral-glass pairs fail to yield the age of melting obtained by40Ar-39Ar dating (11.9 Ma). Sr and Pb isotope data of glasses establish that major rock-forming minerals of the granite had not attained isotope equilibrium before the onset of melting. Consequently, the isotopic composition of the successive liquid fractions originating from granite melting depended on the relative contribution of each mineral to the elemental budget of the melts. trapolation 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 characteristic of many geologic settings. If future studies substantiate these results, some geochemical models of crustal melting and assimilation may need revision. In addition, marked Sr isotope disequilibrium in the residual mineral phases within the partially melted granite raises concerns about the reliability of radiometric dating in metamorphic terranes.