The effect of accessory minerals on the redistribution of lead isotopes during crustal anatexis: A model

Abstract

The initial Pb isotopic composition of anatectic granites is modelled as a mixture of Pb derived from major minerals that have characteristically low 238U 204Pb and 232Th 204Pb ratios (i.e., feldspars, biopyroboles, etc.) with Pb contributed from dissolved accessory minerals with characteristically high 238U 204Pb and 232Th 204Pb ratios (i.e., zircon, monazite, etc.). The initial Pb isotopic composition of anatectic granites is a function of (a) the age, type, modal distribution and heterogeneity in the initial U and Th content of the accessory minerals, (b) variation in melt composition and temperature during anatexis (c) the fraction of the source melted, and (d) the extent to which the melt was homogenized prior to crystallization. The differential behavior of accessory minerals during anatexis can explain the common yet enigmatic decoupling of the Pb isotopes from other chemical and isotopic systems in granite rocks. Granitic plutons derived by anatexis of the same source material are not required to have the same initial Pb isotopic composition. Low temperature, peraluminous granites are more likely to exhibit homogeneous initial Pb isotopic compositions dominated by the Pb contained within the major mineral assemblage of the source material. In contrast, the initial Pb isotopic composition of higher temperature, metaluminous or peralkaline anatectic granites is more likely to be heterogeneous, unless the melts are homogenized during emplacement, and will record a larger fraction of the radiogenic Pb component contained in the accessory mineral assemblage of the source material. Heterogeneity in the initial Pb isotopic composition of individual granites or granitic suites can preserve the weighted average age of the radiogenic Pb present in the accessory mineral assemblage of the source region even if these minerals are entirely dissolved during anatexis. The use of isotopic tracer studies in terrane analysis will be optimized by the isotopic characterization of granitic suites.