Lead isotopic evidence for deep crustal-scale fluid transport during granite petrogenesis

Abstract

Lead isotopic compositions are reported for K-feldspars from the Bega and Berridale batholiths in the Paleozoic Lachlan Fold Belt (LFB) of southeastern Australia. In marked contrast to the wide range in initial Nd ( ϵ Nd = +3 to −9.2), the feldspars exhibit an extremely limited variation in Pb isotopic composition with 206Pb 204Pb of 18.14 to 18.18, 207Pb 204Pb of 15.58 to 15.63 and 208Pb 204Pb of 38.04 to 38.21. This variability is less than that observed in modern intra-oceanic island arcs such as the Marianas. Despite the very limited range of Pb isotopic compositions, there are still good correlations with ϵ Nd values as well as between single-stage Pb-Pb and T Nd model ages. The Pb-Pb model ages, however, have a significantly reduced range from ~330 Ma to 440 Ma, compared to the older T Nd model ages which range from 810 Ma to 1770 Ma. The correlation, particularly of 207Pb 204Pb ratios with neodymium isotopic compositions, is attributed to limited late-stage mixing between mantle and crustal components. It is argued that this late-stage crust-mantle interaction was a relatively subtle feature, superimposed upon continental crust with an already homogenous Pb isotopic composition, probably via underplating and intrusion into the crust of mafic, mantle-derived magmas. The homogeneous crustal composition is most evident in the extremely limited range of 206Pb 204Pb ratios in the Bega Batholith, implying long-term variations in U Pb of the granite source rocks of < ±4%, despite their large range in T Nd model ages. Considering the differing geochemical properties of U and Pb, this very restricted range in U Pb ratios is thought to be an artefact of Pb isotopic homogenisation in the continental crust. The Pb isotopic composition in the granite source rocks was homogenised immediately prior to partial melting, probably as a result of mobility of Pb in deep, crustal-scale fluid advection systems. Lead mobility may be a consequence of the extremely high solubility of Pb chloride complexes in high chloride-sulphide environments at high temperatures. The fluid advection systems are envisaged to be a precursor to granite plutonism, and were probably most active at the elevated temperatures immediately prior to the deep (20 to 40 km) crustal melting responsible for production of the large granitic batholiths. It is proposed that these fluid systems affected large crustal segments, operating over horizontal length scales of ~10 3 km. Thus, fluids may play an important role in the redistribution of soluble elements and the convective transfer of heat within the continental crust.