In-situ Hf isotope measurements of complex zircons from Irish granitoids reveal hidden Palaeoproterozoic and Archaean sources at depth [abstract]

Minor c. 470 Ma intrusions that cut the Slishwood Division (SLD) and Tyrone Central Inlier (TCI) in northwest Ireland, contain complex zircons with core and rim structures. The advantage of in situ Hf isotopic measurements in zircon compared to whole grain solution Hf analyses (or Sm-Nd whole rock isotopic measurements) is that Hf mantle extraction ages for the melts (as deduced from the zircon rim Hf isotopic composition) can be determined without any contribution from inherited material present in the zircon cores. Zircon rims from intrusions cutting the SLD have weighted mean Hf470 values of - 7.7 ± 1.1 and tDM model ages of c. 1380 Ma, which is more juvenile than the weighted mean of the zircon core population and Sm-Nd tDM model ages for the intrusions. Zircon textures demonstrate that the inherited cores were not dissolving nor contributing their Hf isotope signature to the melt. Cores were scavenged from the host metasediments during intrusion, and yield ages and Hf isotope signatures similar to detrital grains from the SLD metasediments. Therefore they do not reveal any information about the melt source(s). The modelled Lu-Hf evolution of the Rhinns Complex and Annagh Gneiss Complex suggests that either of these protoliths (or sediments derived from them) could be melt sources for the intrusions. Juvenile Grenville rocks (Doolough gneiss) within the Annagh Gneiss Complex were not important contributors to the intrusions cutting the SLD, possibly suggesting that the Grenville Front runs between the SLD and the Annagh Gneiss Complex. Although parts of the Dalradian Supergroup cannot be discounted as a source, we prefer to interpret the data as recording Palaeoproterozoic Rhinns Complex-like crust at depth. Zircon rims from granitic pegmatites cutting the TCI have Hf470 values of c. –40 and Hf tDM model ages of c. 3200 Ma, which are much more evolved than the cores. As is the case for the SLD, the cores were likely incorporated into the melt from local metasedimentary rocks during intrusion and demonstrate the TCI is itself not a source for the melt. The melt source is therefore Archaean and we suggest that Lewisian Complex basement (or sediments wholly derived from such basement) were melted and are present beneath the TCI at depth. Given the evidence for Palaeoproterozoic and Archaean elements on the Laurentian margin in western Ireland, it is possible that the present-day disposition of basement rocks may not represent its pre-Grampian architecture. Both the SLD and TCI are thought to represent outboard Laurentian microcontinents, which may have been translated along strike during the Grampian Orogeny.