A detective duo; an apatite–zircon case study of the Johnston Complex, Wales

Determining the crystallization of S-type granitic material can be challenging due to a lack of neoblastic zircon growth (e.g. thin overgrowths) and the potential of large inherited zircon cargos. Coupled apatite–zircon geochronology can help address such complexities and also clarify post-magmatic thermal history, given the disparate Pb closure temperatures in these minerals. Here we present a case study on the Johnston Complex, a rare outcrop of the Precambrian basement in southern Britain, representing a window into the tectonic regime of Avalonia. Zircon and apatite yield identical U-Pb ages, within uncertainty, of 569 ± 2 Ma and 576 ± 11 Ma, respectively. A minor antecrystic zircon core component is identified at 615 ± 11 Ma. Given the previously reported zircon U-Pb age of 643 Ma, these results demonstrate that the Complex represents a composite suite of plutons along its ca. 20 km length. Zircon Lu-Hf data imply a broadly chondritic source, with model ages consistent with crustal extraction during Rodinia formation. Zircon trace elements are consistent with a calc-alkaline continental magmatic arc setting. Whilst, apatite trace elements demonstrate a sedimentary component within the melt. Combined, these results support arc granite production within the peri-Gondwanan realm during amalgamation of Eastern Avalonia and associates the Johnston Complex to the Cymru subterrane. Importantly, congruent zircon–apatite ages imply rapid cooling after crystallisation, and that subsequent thermal heating did not exceed the apatite Pb closure temperature.