Lead isotopes track intracrustal fractionation with implications for crustal thickness

Abstract

Lead isotopes provide valuable insight into the geological processes involved in the formation of continental crust, revealing information that may be difficult to access through other isotopic systems. While Lu-Hf (or Sm-Nd) and oxygen isotopes conceptually track mantle extraction or interaction with the hydrosphere, respectively, Pb isotopes are more sensitive to intracrustal processes. Here, we present new in situ LA-MC-ICP-MS K-feldspar and TIMS whole-rock Pb isotope data from I-, S-, and A-type granites covering an east-west transect across major lithotectonic boundaries in the Delamerian and the Lachlan Fold Belts, southeastern Australia. Initial Pb isotopic ratios determined from U- and Th-poor minerals (e.g., K-feldspar) and granite whole-rock samples (corrected for radiogenic Pb ingrowth) show no correlation with granite type nor have a statistically significant relationship with most whole-rock geochemistry and show little correlation with Nd isotopes. However, Pb isotope ratios reveal similar spatial patterns to various proxies of crustal thickness, such as regional gravity free-air anomalies, topographic elevation, and Moho depth, as well as changes in Sr/Y and Eu anomalies, which may be linked to melt extraction depths. These results support an interpretation in which Pb isotope signatures are set in the magma source region within the crustal column, essentially unaffected by later-stage fractionation. Lead isotopes thus effectively distinguish lower crustal, U- and Th-depleted sources from upper crustal, enriched sources. An enigmatic lower crustal Selwyn Block has previously been inferred in central Lachlan Fold Belt. Lead isotope data may provide insight into this block, resolving a U and Th depleted zone aligning with its areal extent, suggesting a previous high-grade melt depletion event. We conclude that granite Pb isotope signatures ultimately track the degree of intracrustal fractionation prior to the final melt extraction, which contributes to building a thickened crust.