A detective duo of apatite and zircon geochronology for East Avalonia, Johnston Complex, Wales

Abstract

The Johnston Complex represents a rare inlier of the Neoproterozoic basement of southern Britain and offers a window into the tectonomagmatic regime of East Avalonia during the assembly of Gondwana. This work presents in-situ zircon (U-Pb, Lu-Hf), apatite (U-Pb), and trace element chemistry for both minerals from the Complex. Zircon and apatite yield a coeval crystallisation age of 570 ± 3 Ma, and a minor antecrystic zircon core component is identified at 615 ± 11 Ma. Zircon Lu-Hf data imply a broadly chondritic source, comparable to Nd data from East Avalonia, and TDM 2 model ages of ca. 1.5 Ga indicate source extraction during the Mesoproterozoic. Zircon trace element chemistry is consistent with an ensialic calcalkaline continental arc setting and demonstrates that magmatism was ongoing prior to terrane dispersal at 570 Ma. Apatite trace element chemistry implies a sedimentary component within the melt consistent with voluminous S-type granite production during the formation of Gondwana. The similarity of ɛHf and geochemistry between both zircon age populations suggest derivation from a uniform source that did not undergo significant modification between 615 – 570 Ma. Time-constrained apatite-zircon chemistry addresses complexities in dating S-type granitoids (zircon inheritance) and permits inferences on post-magmatic thermal histories.