Tracking volcanic, plutonic, and pegmatitic sources in sediments: implications for the Early Earth history

Understanding magmatic activity on the Early Earth remains a challenge for geoscientists, as most of its rock record has been destroyed or altered. The oldest exposed rocks belong to the Tonalite-Trondhjemite-Granodiorite (TTG) plutonic suite, only rarely associated with volcanic units of the same age. For this reason, TTGs are often interpreted as magmas that have not erupted, and their compositions thought to represent melts. However, if TTGs are the left-overs from shallow magma reservoirs that have lost some melt to the now-eroded volcanic record, their bulk composition would be at least partly biased towards crystal cumulates. As post-emplacement metamorphism typically overprints many of the chemical characteristics of the initial magmatic minerals, the more resistant magmatic minerals (quartz and zircons) within sedimentary successions derived from these systems provide the best chance of identifying volcanic lithologies that have been completely eroded. Here we use a novel approach to show that Ti-in-quartz and Ti-in-zircon thermometers can be used to recognise different magmatic sources in sedimentary rocks. In quartz, Ti thermometry calibrated against blue cathodoluminescence obtained from scanning electron microscopy allows for fast and statistically meaningful Ti quantification in hundreds of sedimentary quartz grains. This imaging-derived Ti distribution matches well with the distribution of Ti concentrations obtained by LA-ICP-MS spot measurements of individual crystals. We compare this quartz record to Ti distributions in zircons, which have the benefit of also providing a crystallisation age. We applied these techniques to the Pikes Peak Batholith (CO, USA), a 1.1 Ga A-type granite hosting several pegmatites, and the Tava sandstone, a series of Cryogenian intra-granite sedimentary dikes that represents the oldest terrestrial sediments in the Front Ranges of Colorado. Our data successfully separates plutonic from pegmatitic crystals and shows that quartz and zircon crystals in the Tava Sandstone crystallised at statistically higher temperatures than the ones observed in the Pikes Peak Batholith, implying potential contribution from a volcanic source that is no longer available on the surface. The proposed techniques can therefore be used to identify eroded magmatic lithologies and to estimate proportions of different magmatic components (volcanic, plutonic, pegmatitic) in sediments.