The Northern Highlands Terrane (NHT) in Scotland preserves a long record of metamorphism and convergent deformation related to several orogenic events that occurred from Neoproterozoic to Devonian time. Deconvolving the signatures of multiple tectonic events and determining the rates of metamorphism in settings like the NHT are important parts of better understanding the thermal and mechanical processes controlling convergent tectonics. I have used monazite-xenotime thermometry and geochronology, in conjunction with metamorphic petrology and additional accessory phase geochronology, to place constraints on the timing and rates of thermal metamorphism in a variety of structural settings throughout the NHT. Our data show that the ductile thrust nappes of northernmost Scotland preserve a record of Scandian (435-410 Ma) orogenesis. High grade metamorphism in the hinterland Naver nappe likely resulted from the widespread infiltration of granitic magmas at c. 425 Ma, which coincided with peak metamorphism. The timing of metamorphism in the hinterland Scandian thrust nappes is apparently younger than at least some deformation in the foreland Moine thrust zone, suggesting this orogenic wedge experienced large-scale out-of-sequence deformation and metamorphism. In contrast to the Scandian nappes, the Sgurr Beag nappe records primarily Precambrian metamorphism related to the Knoydartian orogeny (780-725 Ma). Additionally, monazite in the Sgurr Beag nappe preserves a record of widespread metasomatism and metamorphism at c. 600 Ma, possibly related to the break-up of Rodinia at that time. A potentially important heat source in orogenic systems, like those preserved in Scotland, is the thermal energy dissipated during deformation, otherwise known as shear heating. It is important to consider to how shear heating may contribute to metamorphism during orogenesis. This is challenging because there are few, if any, methods of relating observations from typical orogenic systems to magnitudes of shear heating. We have developed a model that is adaptable to a wide range of parameters that can be measured from naturally deformed rocks and places first-order constraints on magnitudes of shear heating. While our models suggest that shear heating is not particularly important in the NHT, in lower initial temperature mylonite zones shear heating could be more significant. / Doctor of Philosophy / The Northern Highlands Terrane (NHT) in Scotland preserves a long record of metamorphism and convergent deformation related to several orogenic events that occurred from Neoproterozoic to Devonian time. Understanding the record of each of these events and the rates at which metamorphic changes occurred is important for improving our understanding of the processes at work in continental collisions. The work presented in this thesis involves determining the temperatures recorded by metamorphic minerals and the ages of those minerals in order to reconstruct the temperature-time evolution of samples in a variety of positions within the NHT. Our data show that the collision and thermal metamorphism at 435-410 Ma is well preserved in northernmost Scotland. We argue that metamorphism in this area resulted from the widespread intrusion of hot magmas, which coincided in time with peak metamorphism. The timing of metamorphism in the core (hinterland) of this mountain belt is apparently younger than shallower deformation at the edges (foreland) of the mountain belt, suggesting active deformation and metamorphism retreated toward the hinterland during crustal shortening. In another part of the NHT, known as the Sgurr Beag nappe, a much older metamorphic event that occurred at 780-725 Ma is better preserved. In this area, the mineral monazite appears to record evidence of widespread fluid alteration at ~600 Ma, which has not previously been widely recognized in Scotland. A potentially important heat source in the Earth’s crust is shear heating associated with the thermal energy produced during deformation. It is important to consider what contribution shear heating may have made to the preserved metamorphic record in orogenic belts. This is challenging because there are few, if any, methods of relating observations from typical metamorphic rocks to estimated magnitudes of shear heating. We have developed a numerical model that is adaptable to a wide range of realistic natural scenarios and places first-order constraints on potential magnitudes of shear heating. While our models suggest that shear heating is not particularly important in the NHT, in some lower temperature fault zones shear heating could be more significant.

The Scandinavian Caledonides formed as a result of collision between the continents Baltica and Laurentia, in Silurian and Early Devonian time. The evolution of the orogen has been a topic of research since before the turn of the last century. However, there are still uncertainties regarding the character and timing of the orogenic processes involved in the formation of the Caledonian orogen. Identification and study of high-pressure terranes are a key to understanding the processes involved, and such terrains are found in Jämtland, central Sweden. The most well-known location is Mt. Åreskutan. This study focuses on the Helags Mountain, a locality potentially equivalent to Mt. Åreskutan. It has combined structural and mineralogical studies, pressure and temperature esti-mates, and monazite geochronology, in an attempt to obtain an overview of the metamorphic his-tory.The Helags Mt. geology, as on Åreskutan, is dominated by a klippe of high grade gneisses, overlying lower grade schists and amphibolites, both typical of the middle and lower part of the Seve Nappe Complex in the Swedish Caledonides. The gneisses are dominantly felsic and contain garnet. Two episodes of garnet growth, likely separated in time, are observed in the gneisses. The first episode probably took place in the presence of melt, as is evident from the presence of inclusion of so called nanogranites. This is further supported, but not fully confirmed, by observed homo-genization of the garnet core chemistry. Such processes take place at high temperature, above 700°C. Pressure estimates are less well defined and indicate about 1 GPa during this first garnet growth event. This event may be related to the observed migmatisation. The second garnet growth episode took place at lower pressure and temperature conditions, and similarities with garnet observed in studies elsewhere indicate a connection with shearing and emplacement of the Middle Seve unit. However, no garnets were observed in the studied shear zone, and it is with the available data not possible to confirm a relation to a specific event. Monazite geochronology has contributed Caledonian ages (400-480 Ma) but has not yielded any precise results with regard to the timing of the migmatisation and thrusting. / Den svenska fjällkedjan har en lång historia. Dess nuvarande utformning är ett resultat av att Iapetus-havet, en föregångare till dagens Atlanten, slöts och de tidigare kontinenterna Baltica och Laurentia kolliderade. Trots att fjällkedjan studerats flitigt sedan före sekelskiftet är det mycket som är okänt om de geologiska processerna som varit en del av bergens utveckling. För att bättre förstå fjällkedjans ut-veckling studeras bergarter från områden som varit särskilt kraftigt påverkade. Flera sådana områden påträffas i Jämtlandsfjällen. Denna studie har fokuserat på Helagsfjällets område, beläget i södra delen av Jämtlands län. Studier av det särskilt motståndskraftiga mineralet granat från områdets bergarter har tillsammans med dateringar av mineralet monazit givit ny information om områdets geologiska historia.Helagsfjällets geologi, liksom den välstuderade Åreskutan, utgörs av en enhet av granatförande gneisser, vilken överlagrar en undre enhet av lägre omvandlingsgrad. Detta är typiskt för den mellersta och lägre delen av det så kallade Sevekomplexet. Två generationer granater tyder på att minst två geologiska processer, skilda åt i tid, påverkat områdets bergarter. Den första av dessa granatgenerationer uppvisar bland annat inneslutningar vilka tolkas som bevarade delar av en tidigare smälta. Det är möjligt att denna granatgeneration är relaterad till den tidiga händelse som orsakat uppsmältning, av vilken spår kan studeras på flera platser i området. Tryck- och temperaturberäkningar visar att detta hände under tryck omkring 1 GPa, och temperaturer på över minst 600°C, kanske över 700°C. Den andra granat-generationen är mer svårtolkad. Tryck och temperatur var lägre, och likheter med granater observerade på andra platser tyder på att denna andra granattillväxt skedde i samband med skjuvning av den övre enheten, över den underliggande enheten. Försök att datera dessa två perioder av granattillväxt gav åldrar mellan 400 och 480 miljoner år. Liksom på Åreskutan tyder detta på en tektonisk historia som sträcker sig från Ordovicium till tidig Devon.

The Mazatzal orogeny (1.66-1.60 Ga) is a key element of the tectonic evolution of the North American continent during the Proterozoic (Whitmeyer and Karlstrom, 2007). Recently, Mesoproterozoic detrital zircon grains (1.55-1.45 Ga) have been found in metasedimentary rocks that were thought to have been deformed during the Paleoproterozoic Mazatzal orogeny (Jones et al. 2011; Doe et al. 2012, 2013; Daniel et al. 2013). Some type examples Mazatzal deformation now seem to be too young to have been deformed in the accepted time of that orogeny (1.66-1.60 Ga) and may have been deformed in the younger, newly defined, Picuris orogeny. This leads to questions regarding the timing and nature of the Mazatzal orogeny and its importance in the evolution of the North American continent. The object of this research is to constrain the timing of deformation related to the Mazatzal and Picuris orogenies and clarify the Proterozoic history of the North American continent. The Four Peaks area in central Arizona has been selected as an ideal location to tightly constrain the timing of deformation. The area hosts a package of Proterozoic metasedimentary rocks that are folded into a kilometer-scale syncline, surrounded by vi Mesoproterozoic to Paleoproterozoic granitoids. The Four Peaks syncline has been considered a type example of Mazatzal-age deformation (Karlstrom and Bowring, 1988). Zircon and monazite geochronology are presented along with structural and petrologic data in order to understand the geologic history of the Four Peaks area. The evidence suggests that three deformation events occurred at ~1675 Ma, 1665-1655 Ma and 1490-1450 Ma. Sedimentary deposition occurred 1665-1655 Ma and 1520-1490 Ma with a significant disconformity in between these episodes. Both the Mazatzal and Picuris orogenies can be associated with periods of deformation, sedimentary deposition and pluton emplacement. The most significant shortening event, which formed the Four Peaks syncline, occurred during Mesoproterozoic time and was related to the Picuris orogeny.