The growth and decay of a Paleogene Intrusive Complex, Isle of Skye

Overview

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.

REFERENCES:

– 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

Click on an image to expand

Image Captions

Cullins, Isle of Skye

Methodology

The project will utilise both high precision zircon U-Pb geochronology and Ar/Ar dating under the supervision of Mark and Ickert. The isotope geochemistry will be performed at NIGL (LA-ICPMS), EIMF (ion probe) and SUERC/Durham under the supervision of Ellam and Macpherson. The different phases of magmatic activity in the Paleogene Igneous Complex, Isle of Skye will be sampled from secure stratigraphic settings and the required mineral phases for geochronology and geochemistry be extracted by the student. The fieldwork will be approx. 6 weeks over 2 years and rocks will be initially characterised using optical microscopy.

Project Timeline

Year 1

– Literature review of radioisotope geochronology (Ar/Ar and U-Pb)
– Literature review of Paleogene Igneous Complex and granite emplacement
– Review of field sites and development of sampling plan
– Fieldwork to sample rocks from secure stratigraphic settings
– Petrographic descriptions of rocks and elemental analyses
– Preparation of samples for high-precision geochronology (Ar/Ar and U-Pb)
– Preparation (acid cleaning, many cycles) clean room Teflon for U-Pb geochronology

Year 2

– Ar/Ar dating of minerals to reconstruct cooling paths
– U-Pb geochronology of intrusive rocks
– U-Pb dating of key samples at Purdue University
– Assessment of gaps in data and failed experiments and finalise final field sampling
– Fieldwork to collect final samples from secure stratigraphic settings
– Petrographic descriptions of rocks and elemental analyses
– Prepare materials for geochronology and geochemistry
– Conduct geochemistry on zircon (Hf and O isotopes)
– Presentation of data at UK conference

Year 3

– Complete geochronological work on samples collected/prepared year 2
– Complete geochemistry
– Mathematical modelling of emplacement history of intrusive complex
– Begin construction of thesis and publication of data
– Presentation of data at international conference

Year 3.5

– Complete thesis for submission and publication of data

Training
& Skills

The student will be trained in a variety of geochronological and geochemical techniques, including mass spectrometry (e.g., noble gas, thermal ionisation). The student will also work with Hf isotopes (LA-ICPMS) and O isotopes (ion probe). It is expected the student will become an emerging leader in geochemistry/geochronology at the completion of this PhD. The project demands the student spend many hours working in laboratories. There will also be extensive fieldwork including mapping of igneous units and rocks descriptions throughout the Isle of Skye (Figure 2). The student will present data at both UK and International conferences and prepare data/manuscripts for publication. The student will also engage with the global EARTHTIME community (http://www.earthtimetestsite.com), which includes attendance at bi-annual Gordon Research Conferences dedicated to geochronology: https://www.grc.org/geochronology-conference/2021/.

The project requires a student from a geological background who has a desire to combine field observations with precise and accurate laboratory measurements (geochemistry and geochronology). Experience of field mapping, numerically literate and demonstrable ability to work with isotope data are key. The student will be immersed in the lively high precision geochronology group at the Scottish Universities Environmental Research Centre (SUERC) led by Professor Mark, which currently includes 6 PhD students, 2 post-doctoral researchers and 4 research technicians. Working with CASE partners Thermo Scientific will allow the student to get direct experience in the mass spectrometer industry, including high precision design of cutting-edge instrumentation.

References & further reading

U-Th-Pb Geochronology: https://www.sciencedirect.com/science/article/pii/B9780080959757003107?via%3Dihub

Ar/Ar geochronology:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article/doi/10.1130/B35560.1/587798/Interpreting-and-reporting-40Ar-39Ar

Further Information

For further information please contact: Darren Mark (SUERC, University of Glasgow)
Darren.Mark@glasgow.ac.uk.
The project student will join the Caledonian Geochronology & Geochemistry Research Group (www.scottishisotopes.co.uk) and be based fulltime at SUERC (https://www.gla.ac.uk/research/az/suerc/)

To apply for this position, use the following link www.gla.ac.uk/ScholarshipApp

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