The Paleogene Igneous Complex, Isle of Skye has fascinated geologists for over 200 years, encouraging multiple field expeditions despite the steep, jagged peaks (figure 1). The story of these hills begins more than 61 million years ago with volcanic activity during the initial opening stages of the North Atlantic Ocean. As North America and Europe ripped apart, large volumes of basalt lava were erupted from long narrow fissures on what is now the West coast of Scotland. As time went on, the volcanism became focused at several specific locations, creating large, central volcanoes. The remnants of these volcanoes can be observed today as either thick lava flows or large intrusive complexes and nested plutons (e.g., Cullins).
The emplacement and construction of such large plutons is a widely debated topic and controversies centre on (1) the origin of voluminous granites, and (2) how they are emplaced. These can be summarised as the source and the space problem. Models that have been hypothesised to address the source issue range from mostly partial melting of the crust to fractionation from basaltic sources, whereas models addressing emplacement mechanisms vary from km-scale diapiric rise to incremental emplacement of small magma batches.
Precise zircon U-Pb geochronology, however, suggests plutons are accreted by addition of small magma batches to the middle-upper crust over 10,000 to 100,000 year timescales. Subsequently, a complex of nested plutons can accumulate to form batholiths over 1,000,000 to 10,000,000 years (e.g., Coleman et al., 2004). During these periods pulses of high magma flux can be followed by periods of relative quiescence with magmatic flare-up periods proposed to explain the non-linear growth of large batholiths (e.g., Martinez et al., 2018).
In the field the surface (topographic) exposure of different phases of magmatic activity within such nested plutons may hinder the relative reconstruction of emplacement, however U-Pb geochronology applied to zircon offers a high-precision method to reconstruct the timescales of magmatic activity on the pluton, igneous intrusive suite, or bathoith scale. Ar/Ar geochronology on mineral phases from the same rocks can track cooling rates and periods of high heat flow as new magma batches are emplaced into the crust.
This PhD project aims:
(1) Use zircon U-Pb geochronology and isotopic variability (Hf and O isotopes) to precisely determine the timescales for emplacement of the Paleogene Igneous Complex, Isle of Skye.
(2) Document the development of a crustal thermal anomaly leading to increased crustal assimilation in the magmas and use Ar/Ar geochronology to detail the eventual cessation of activity and cooling of the intrusive bodies.
(3) Use different modelling techniques (e.g., Carrichi et al., 2014) to quantify emplacement rates for the different phases of the Paleogene Igneous Complex, Isle of Skye.
The development of improved techniques for the in situ analysis of Lu–Hf (LA-ICPMS) and O isotopes (ion probe) and of trace element contents (ICPMS) has revolutionized approaches to modeling the origins and evolution of the continental crust, and the petrogenesis of granite. The key has been to obtain representative samples, and it is increasingly accepted that these are best provided by zircons. In magmatic rocks they may be inherited or have crystallized in the evolution of the host magma. In situ Hf isotope ratios are now routinely measured with sub-epsilon unit precision by laser ablation ICP-MS. Trace element contents broadly constrain the tectonic setting of the magmas from which the zircons crystallized, Hf isotopes reflect when new crust was generated from the mantle, and oxygen isotopes have been widely used in petrogenetic evolution studies of granitic rocks to constrain the relative contributions of mantle and crust and the roles of fractional crystallisation, assimilation and magma mixing. The oxygen isotope compositions of granites and their constituent minerals are sensitive to modification by assimilation of 18O-enriched crustal rocks, magma mixing and/or hydrothermal alteration by 18O-depleted fluids.
The potential of an integrated approach of coupling these geochemical approaches to high precision chemical abrasion ID-TIMS U-Pb dating and Ar/Ar geochronology provides a novel approach to reconstructing the emplacement and construction history of the Paleogene Igneous Complex, Isle of Skye.
To push the limits of the geochronology the project has support from Thermo Scientific (CASE Partner) and the student will get access to the latest state-of-the-art mass spectrometer technology and access to the Thermo Scientific personnell.
– Coleman et al., 2004, Geology, 32, 433-436
– Martinez et al., 2018, Lithos, 326-327, 19-27
– Carrichi et al., 2014, Nature, 511, 457-461
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Cullins, Isle of Skye