The origin and evolution of granites: an in-situ study of zircons from Scottish Caledonian intrusions
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Date
2008Author
Appleby, Sarah Kristina
Metadata
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.