Fingerprinting feldspar phenocrysts using crystal isotopic composition stratigraphy: implications for crystal transfer and magma mingling in S-type granites

Microsampling of cm-scale feldspar crystals within an S-type granite from t he Lachlan Fold Belt of southeastern Australia has revealed complex interna l Sr and Nd isotopic variations. The observed isotopic zonations are in par t interpreted as recording feldspar crystallisation in a dynamically mixing magma system, the isotopic composition of which was varying in response to the influx of more mafic and isotopically more mantle-like magmas, the lat ter stages of which are now represented in modified form by microgranular e nclaves. Similar core to rim isotopic variations in feldspar megacrysts fro m a microgranular enclave and the adjacent host granite strongly suggest me gacrysts in the enclave were transferred from the granitic magma during cry stallisation. Feldspar rims have higher Sr-87/Sr-86(i) and lower epsilon(Nd (i)) than adjacent whole rock analyses, but match those of mineral separate s from the surrounding enclave matrix. This suggests that the final stages of megacryst growth occurred in the presence of a component that had previo usly interacted with a high Sr-87/Sr-86, low epsilon(Nd(i)) component such as metasedimentary wall rocks. Isotopic heterogeneities are also presererve d within different mineral phases in the enclave matrix, suggesting that di ffering phases grew at differing stages of equilibration between the enclav e magma and its host granitic magma. Our results reveal major isotopic hete rogeneities on a single crystal and also inter-mineral scale in a pluton wh ich shows well constrained evidence for magma mingling. These results indic ate the suitability of feldspars as recorders of isotopic change in magmati c systems, even those which have cooled slowly in the plutonic environment and suggest that much heterogeneity in plutonic systems may be overlooked o n a whole rock scale.