The thermal structure of collisional orogens – insights from the Grenville orogen in Canada and Scotland

Exhumed collisional orogens provide important time-integrated records of metamorphic, magmatic and deformation processes occurring over timescales of up to hundreds of millions of years. When combined with observations and principles from present-day deformed continental regions, research of deeply-exhumed terrains enables the conditions and timescales of metamorphism to be more fully understood across large regions of exposed orogenic crust. This is essential to examining the thermal structure of orogens and their co-evolution with crustal deformation patterns. In this thesis, I present a multi-method approach to the study of the Mesoproterozoic (c. 1.1–1.0 Ga) Grenville orogen, integrating P–T results from phase equilibrium modelling, age constraints from U-Pb zircon and monazite geochronology, structural observations and measurements, and regional geological and geophysical datasets. My research focuses on two study regions: (1) the eclogite-facies Glenelg inlier in northwest Scotland, and (2) a 130 km across-strike transect through the type-locality in eastern Canada, encompassing the orogenic foreland and two overlying thrust sheets. Together, these areas span a wide range of metamorphic conditions, within distinctive tectonic settings undergoing crustal thickening at a similar time (c. 1.1–1.0 Ga). My study of the Glenelg inlier combined phase-equilibrium modelling, hornblende-plagioclase thermometry, Zr-in-rutile thermometry and U-Pb zircon geochronology to constrain eclogite formation at 19 kbar and 700–800 °C between 1.1 and 1.0 Ga. Consideration of the distribution, architecture, age and depositional setting of the surrounding sedimentary rocks, deposited at the time of this metamorphism, imply eclogite formation within a thick-skinned foreland setting beyond the range-front of the Grenville orogen. The proposed setting is analogous to ‘pop-up’ structures seen in present-day orogens, beneath which crustal thickening is occurring. One-dimensional thermal models produced in this study show that this setting is thermally viable. I therefore provide an example region in which an eclogite is not associated with a suture zone, deeply-exhumed orogenic interior or lithospheric mantle, as is typically the case. My study of the Grenville type-locality in Canada integrated structural mapping, petrography and phase equilibrium modelling, and revealed a consistent record of amphibolite-facies metamorphic conditions across the 130 km transect, with peak conditions of 8–11 kbar and 700–800 °C. I find from in-situ U–Pb–REE monazite geochronology that the overall duration for peak-thermal conditions was ~60 Myr, increasing in age from c. 1040 Ma to 1100 Ma towards structurally higher levels. Overall, the data further constrain the timing and conditions of metamorphism across the crustal section and show that melt generation was more modest than previously thought. A compilation of P–T data from across the orogen shows that the majority of analysed rocks recording syn- to post-collisional metamorphism (i.e. c. 1.1–1.0 Ga) yield temperatures of up to ~800 °C—the constrained upper limit across the study transect. Metamorphic temperatures between ~800 and 1000 °C are generally recorded in four localities across the Grenville Province, across a range of crustal depths (8–20 kbar), implying lower thermal conditions for the orogen as a whole than is typically thought. Two-dimensional thermal modelling shows that mid-crustal temperatures of up to ~800 °C can be produced for generic values of radiogenic heating, convergence rate and erosion intensity as encountered in orogens. Taken together, my research contributes new insights into the conditions and timing of metamorphism in the Grenville orogen and demonstrate that 700–800 °C temperatures, across a range of depths, are common to the type-locality in Canada and the Glenelg inlier in Scotland—an isolated eclogite fragment formed beyond the orogen's range-front. The results highlight how integrating metamorphic, structural, chronological and thermal modelling approaches in exhumed orogenic terrains can elucidate links between temperature structure, rheology and deformation, which together govern the long-term evolution of continental crust.

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