BEHAVIOR OF HAFNIUM AND NEODYMIUM ISOTOPES IN THE CRUST: CONSTRAINTS FROM PRECAMBRIAN CRUSTALLY DERIVED GRANITES

Abstract

Due to the large partition coefficients of garnet for Lu compared to Sm, Nd, and Hf, garnet residual from crustal melting events has a large potential for retaining Lu and, over time, producing high reservoirs in the lower crust. Therefore, melts derived from such garnet-bearing residual assemblages may be high in relative to . In order to determine the extent of potential hafnium isotopic heterogeneities in the lower crust from residual garnet, and to detail coupled HfNd isotopic behavior in older crustal rocks in general, we have determined the hafnium and neodymium isotopic compositions of thirty-two Precambrian granites and rhyolites from diverse suites of crustally derived magmatic suites. The majority of these rocks are known, from major-element and neodymium isotopic evidence, to have been derived from older (>300 m.y.) crust.Modeling of and partitioning during melting events in the lower crust indicates that anomalously high compositions should develop within 300–400 m.y. in residual assemblages provided melting was at least 25–30% and 10% or more garnet was left in the residue. In spite of the diverse older crustal sources for the granites and rhyolites in this study, and the potential for garnet to be present in their source regions, none of the granitoids have anomalously high initial compositions: all samples (with one exception) have initial Hf and Nd compositions that plot within a ±8 ϵHf unit wide band of the reference line (ϵHf = 2ϵNd + 2) for juvenile crust. The absence of high initial compositions in these granites and rhyolites most likely implies that either (1) garnet is not a common residual phase in the lower crust or (2) garnet-bearing restite is not easily incorporated into later melts or (3) not enough time was available to develop anomalous Hf compositions.