The origin and evolution of granites: an in-situ study of zircons from Scottish Caledonian intrusions

Abstract

Granitic magmatism in collision belts is widely regarded as a major mechanism for generating continental crust. This hypothesis can be tested by identifying the source rocks of granitic magmas, and in particular the contribution by pristine mantle material. The complexity of granites, and their susceptibility to post-crystallisation alteration, has until recently provided a major obstacle to progress. Zircon, a common and chemically robust accessory mineral in granitoid rocks, retains a record of the composition of the magma it grew from. Recent developments in microanalysis (ion microprobe and laser ablation ICP-MS) now enable in-situ analysis of zircon crystals at high spatial resolution and precision, providing access to this record at the previously inaccessible intra-crystal scale. The resulting data have enormous potential to provide new insights into the nature and age of source rocks and the processes driving magma evolution. This project used an integrated in-situ O, U-Pb and Hf isotope, trace and rare earth element study of zircon to identify the sources and chart the evolution of two ‘I-type’ (igneous/infracrustal precursor) Scottish late Caledonian (~430-400 Ma) granite plutons. I have constrained models of magma generation, the relative contributions of mantle and crust, the ages and identities of their lower crustal sources, and have shown that the plutons played, at most, a minor role in crustal growth. In addition, I have been able to resolve the extent to which open-system changes like magma mixing affected the magma compositions. The same approach was used in a pilot study of three Caledonian (~460 Ma) ‘S-type’ (sedimentary/supracrustal precursor) granite plutons, which theoretically represent magmas formed by melting of a purely supracrustal source. The data confirm that Dalradian country rocks were the primary source, but reveal remarkable isotopic diversity within and amongst the three plutons. The most important general conclusion from this PhD study is that the complexity and scale of isotopic heterogeneity between plutons, amongst samples of the same pluton, in single samples and within individual crystals is far greater than previously recognised, consistent with the incremental assembly of plutons from multiple melt batches of differing composition, sources and petrogenetic evolution.