Lithological fabric as a proxy for Rb–Sr isotopic complexity

Abstract

In situ Rb–Sr dating, made possible by recent analytical advancement, is a powerful chronometer that enables the timing of crystal growth or cooling to be determined for fabric forming minerals (e.g. alkali feldspar, muscovite, biotite). In contrast with accessory phase geochronology, Rb–Sr geochronology can be deployed on minerals that constitute a significant component of a wide range of rock types. Here we report laser ablation inductively coupled plasma tandem mass spectrometry (LA-ICP-MS/MS) biotite Rb–Sr ratios in metaigneous and metasedimentary samples from across Western Australia. These biotite samples span a diversity of structural, metamorphic, and temporal states, yet highlight common salient features of this chronometer. Biotite in these samples received its isotopic compositions from completely to partially homogenized Sr reservoirs, as evident from isochronous to more complex distributions of isotopic ratios. The utility of the classic isochron approach is contrasted with biotite model ages for the geologic interpretation of Rb–Sr results for samples with and without constrained initial 87Sr/86Sr ratio. Model ages calculated from individual analyses aid in identifying multiple discrete events that may have only partially equilibrated isotopic systems. Thus, single-analysis model ages can enable the extraction of thermochronological information from complex Rb–Sr data sets, if interpreted under consideration of their geologic context. Textural analysis of thin sections via Fourier component image analysis quantifies the preferred orientation of biotite fabrics. This analysis reveals a correlation between diversity of grain orientation and biotite age complexity. Such a simple texture-to-age relationship highlights a potential pre-analysis screening tool to facilitate sample selection where Rb–Sr isochrons are sought.Results reveal a thermal event at ca. 550–500 Ma in the Lamboo Province, West Kimberley, and multiple contact metamorphic events at ca. 633 Ma and 619–615 Ma that occurred in the Yeneena Basin, Paterson Orogen. Biotite from the Yilgarn Craton yields a strong E-W trend in cooling ages, from 2477 Ma in the central Yilgarn Craton to 580 Ma at the western margin of the craton. These ages support slow cooling and thermo-tectonic quiescence after cratonisation in the central Yilgarn Craton, while the western margin was subject to Neoproterozoic reworking. Placed into a regional geologic context, these results highlight the sensitivity of the in situ Rb–Sr dating method to constrain the timing of geologic events that are not commonly captured by other isotopic systems.