Redefining the emplacement age of the Cornubian Batholith: applying Bayesian statistics to zircon U-Pb LA-ICP-MS data

Understanding the emplacement timescales of the Cornubian Batholith is important due to the association of widespread mineralisation that is spatially and temporally linked to the magmatism. Historically, dating has been focussed on xenotime and monazite which established the magmatism to have occurred from ~295-275 Ma (Chen et al., 1993, Chesley et al., 1993). However, these minerals are less precise for dating due to issues with low U/Pb ratios and inability to perform chemical abrasion. Our project aims to build a temporal framework for the formation of nested plutons by utilising high precision zircon U-Pb dating on the Variscan S-type Cornubian Batholith of southwest England.   

To minimise issues of inheritance, cathodoluminescence scanning electron microscope imaging (CL-SEM) was used to identify inherited cores and fractures/inclusions within zircon. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was then completed on cores and rims of zircon, with the aim of selecting the best zircon crystals for further chemical-abrasion isotope-dilution thermal-ionisation mass spectrometry (CA-ID-TIMS) analysis. The LA-ICP-MS data confirmed the complicated geochronological history, with issues of inheritance and Pb-loss ubiquitous across all sampled plutons. Whilst an accurate and precise emplacement age could not be identified based on the spread of data, most sampled plutons did show promising trends that could be related to peaks in magmatism within the currently established period of activity.  Zircon rims were isolated in preparation for CA-ID-TIMS to minimise issues with inheritance.  

Applying CA-ID-TIMS, ²⁰⁴Pb and ²³⁰Th corrections, and Bayesian modelling will improve precision with the aim of filtering out complicating effects to produce reliable emplacement ages for the main phases of Cornubian magmatism. Achieving accurate zircon U-Pb ages from S-type granites is both challenging and significant as it would open avenues for reinterpreting ages from difficult-to-date plutonic bodies, a potential milestone for U-Pb geochronology.  

Bibliography
 
Chen, Y., Clark, A.H., Farrar, E., Wasteneys, H.A.H.P., Hodgson, M.J., Bromley, A.V., 1993. Diachronous and independent histories of plutonism and mineralization in the Cornubian Batholith, southwest England, Journal of the Geological Society. 

Chesley, I., J.T., Halliday, A.N., Snee, L.W., Mezger, K., Shepherd, T.J., Scrivener, R.C., 1993. Thermochronoloy of the Cornubian batholith in southwest England: Implications for pluton emplacement and protracted hydrothermal mineralization, Geochimica et Cosmochimica Acta.