Investigating the metamorphic evolution of reworked terrains.

Abstract

This project is an integrated monazite and zircon LA-ICPMS geochronology and metamorphic phase equilibria study of regions which have experienced multiple metamorphic events. The aim of using this combination of techniques is to construct P-T-t evolution histories of both older and younger overprinting events in a terrane. The data presented in this thesis can be used as constraints for models of supercontinent evolution Two different regions are targeted in this study. The first region is the Palaeo- to Mesoproterozoic rocks of the Gawler Craton. These rocks have experienced extensive granulite facies metamorphism at c. 1690 - 1665 and 1590 Ma and are poorly exposed. An understanding of the tectonothermal history of this region is important for resolving the evolution of the supercontinent Columbia, which was proposed to have amalgamated from 2.1-1.8 Ga and broken up from 1.6-1.2 Ga. Based on arguments that link the formation of UHT belts to super-continental cycles, UHT granulite facies rocks from the Gawler Craton indicate that Columbia amalgamation may still have been ongoing at c. 1700 to 1500 Ma or breakup started earlier at c. 1700 Ma. The second region investigated in this study is the Neoproterozoic rocks of the north Atlantic region. The metapelitic rocks of the Moine Supergroup in Scotland and equivalents on the Shetland Islands have been found to have experienced several amphibolite facies metamorphic events at c. 930 Ma (Shetland), 830 Ma, 780 Ma and 725 Ma (Scotland). All of these regions have been extensively reworked by the Ordovician to Silurian Caledonian orogeny such that all the preserved structures and in many cases the metamorphic mineral assemblages are Caledonian in age. Fortunately, large garnet porphyroblasts have preserved the older events and in situ geochronology allows us to target monazite grains within these garnets providing the age of the older metamorphic events. The garnets can also be used to determine the P-T evolution of each event. The results of this study provide age and P-T constraints for each of the Neoproterozoic events. The data suggest that these events were part of an accretionary orogenic system termed the Valhalla orogeny which existed on a margin of the supercontinent Rodinia. As an additional result of this study, the P-T evolution of the Grampian event (c. 460 Ma) of the Caledonian Orogeny in both Shetland and Scotland has been well constrained.