“Understanding the origins of granite rocks in Donegal”

September 4, 2019

Dobbin Atlantic Scholarship Report 2018/19:

Name:                            Dr Donnelly B Archibald

Home University:    St. Francis Xavier University, Nova Scotia

Host University:       University College Dublin and County Donegal

Dates of Visit:            18th May – 22nd June 2019

Title of Research:     Understanding the origins of granite rocks in Donegal

Research conducted, experimental procedures & results:
Ireland’s cultural connection to Atlantic Canada developed over the past 300 years but the geological connection spans more than a billion years. Before the Atlantic Ocean opened, Ireland and Atlantic Canada were neighbors that shared a similar geological history. Nestled in the rugged scenery of Donegal are classic exposures of granite rock that record the stabilization of this continental crust about 400 million years ago. Granite is an igneous rock, the product of cooling molten magma. Its mineral and chemical compositions are inherited from the melting of deeper layers of the Earth. When these magmas form, their buoyancy facilitates upward transport until they cool and intrude, effectively stitching together the rocks of the Earth’s crust. Granites are exquisitely preserved in County Donegal and the area has long been recognized as a superb natural laboratory for understanding granite formation.

However, despite more than a century of research into their origin, the duration of magma formation, the magma source(s), and even the processes that initiated and sustained the granite magmatism are poorly understood. My research project involves sampling of the various rocks in County Donegal followed by careful selection of samples for laboratory work to test hypotheses about the age, origin and isotopic signature (crustal inheritance) of the granite. Classical investigations in the 1960s and 1970s focused on describing the field geology and their findings form a solid foundation for this study. However, limited laboratory work was performed and so these classic rock exposures have the potential to provide fresh insights.

For the first two weeks of my visit to Ireland, I collected data from rock samples at University College Dublin (UCD). This work involved investigating the lead isotopic composition of a mineral named feldspar to understand the compositional variation of the magmas through time. Feldspar is an ubiquitous mineral in granite and the lead isotope data can inform us about the magma sources and the age of the material that melted to form the granite magmas. I collaborated with Dr. Stephen Daly who is an expert in isotope geology and igneous petrology, including mineral-scale studies of magmatic processes. He has researched extensively on lead isotopes and he is an expert in granite intrusions, especially the granitic rocks in Ireland. Furthermore, Dr. Daly has an innovative research lab with experienced researchers. Dr. Eszter Badenski is a postdoctoral research fellow in Dr. Daly’s lab whom I worked closely with during my visit.

Dr. Stephen Daly (left) and Dr. Donnelly Archibald standing in front of a classic geological map of County Donegal. The colors on the map represent different rock units.

The analyzed samples were collected in July 2018. Thin slices (approx. 250 micrometers) of the rock samples were mounted on a glass slide. Upon arrival at UCD, we took photographs of the samples using a scanning electron microscope. The imaging is essential because it can reveal complex micro-zonation in the feldspar crystals and/or inclusions of other minerals. We then collected the lead isotopic data using laser-ablation inductively coupled plasma multiple-collector mass spectrometry (LA-ICP-MC-MS). This technique was chosen because it is fast, provides high spatial resolution data, and requires only a small sample volume leaving the sample essentially undamaged. Other lead isotope data collection techniques require total dissolution and destruction of the feldspar crystal. The rock slices were placed in the laser ablation instrument and shot with a laser. The laser drills a small hole in the sample and the ablated material is transported to the mass spectrometer that records the isotopic composition of the samples.

Our interpretation of the data is ongoing but a preliminary analysis shows that that the granite rocks originated from a similar magma source deep in Earth’s crust despite having different crystallization ages between about 435 and 400 million years ago. In light of these data, Dr. Daly and I discussed the origin of the rocks in County Donegal and their significance in understanding the origin of continents and earth processes.

During the second half of my visit, I travelled to County Donegal. The purpose of the trip was to collect additional rock samples to fill-in sampling gaps remaining after my visit in July 2018. I also reexamined critical rock outcrop locations to investigate the field relationships among the various rock units. The rocks of interest are located throughout County Donegal in coastal sections, road cuts, rock quarries, and on exposed mountain tops. I visited many places near and including Ardara, Portnoo, Dungloe, Annagry, Dunlewey, Bloody Foreland, Falcarragh, Kilmacrenan, Fanad Head, Rosguill, Dunaff, and Malin Head, just to name a few. At each locality, the rocks contained important information regarding the geological history of County Donegal. Resembling pages in a book, each rock contains the information required to complete the geological story. At each locality, I collected field data, photographs, measurements, and rock samples for further study back in Canada.

Photograph of the “Poisoned Glen” near Dunlewey. These mountains are an excellent example of one of the many granite intrusions present in County Donegal.

Contacts established and plans for future collaboration
Learning from local experts is a strategy that expedited my research. This opportunity fostered international research collaborations in Ireland that will continue into the future. On my visit, I worked with:

  • Dr. Stephen Daly (University College Dublin)
  • Dr. Eszter Badenski (University College Dublin)

The lab work at UCD stimulated fruitful discussions on the procedures for collecting lead isotopic data and the geological history of northwestern Ireland. We also discussed plans for future collaboration including using the same method to collect data from rocks in Atlantic Canada. For example, we have plans to collect lead isotope data from granite rocks from Nova Scotia.

My time spent in Ireland was an extremely rewarding and fruitful experience both personally and professionally. The Dobbin Atlantic Scholarship provided an opportunity to work with accomplished geoscientists in Ireland. This was a valuable experience for my development as an early career geoscientist and it permitted the cultivation of productive international research collaborations. The scholarship also allowed me to revisit County Donegal and re-examine important rock exposures. These rocks represent an excellent laboratory to investigate and understand the igneous rock processes operating during the growth of the continental crust and the timescales over which these processes are active. I am truly thankful to the Ireland Canada University Foundation for their support and my colleagues at University College Dublin for facilitating my visit. I am also grateful for the cultural experience and the friendly and welcoming people whom I met and shared stories with during my time in Ireland.