Metamorphic zircon formation in gabbroic rocks – the tale of microtextures

Sammanfattning: Dating of metamorphic events is crucial for the understanding and reconstruction of large-scale geological processes such as orogenesis. Zircon is one of the most commonly used minerals for dating of igneous and metamorphic events. Zircon incorporates uranium and excludes lead during crystallization, and with time the uranium decays to lead. The diffusion rates of both elements are slow, making zircon resilient to isotopic resetting. However, in order to date geological events, it is imperative to know exactly by which process the dated zircon formed. For example, regional metamorphism is a dynamic process taking place over millions of years. During tectonic burial and heating the rock gradually responds to the increasing temperature and pressure, giving rise to prograde mineral assemblages, whereas retrograde metamorphism takes place during cooling and exhumation. So, in a regionally metamorphosed rock, does the zircon age date the tectonic burial or the exhumation? The interpretation of how zircon formed has direct influence on the tectonic interpretation. Zircon can form or recrystallize within a wide range of metamorphic pressures and temperatures and by several different processes. This means that, for meaningful interpretation of a metamorphic zircon age, the zircon growth needs to be linked to the mineral reactions in the rock. Due to the high closure temperature of zircon (the temperature below which zircon will not undergo isotope diffusion), zircon ages have traditionally been assigned to date the peak of metamorphism (the highest temperature). On the other hand, mass balance models suggest that, in mafic rocks, zircon dissolves during prograde and grows during retrograde mineral reactions and therefore generally dates cooling and exhumation.If hydrous fluids are not present, mafic igneous rocks may remain largely unaffected during a metamorphic event. Coarse-grained mafic rocks such as gabbro are the least permeable, and may record the gradual transition from pristine gabbro to its completely metamorphic recrystallized equivalent. Such metamorphic transitions zones provide information about how metamorphic zircon formed. Two different metamorphic transition zones have been investigated in detail in this thesis: a) a gabbro to eclogite transition at Vinddøldalen in south-central Norway and, b) a gabbro to garnet amphibolite transition at Herrestad in South-central Sweden. The aim has been to link reaction textures to zircon growth and to obtain a direct U-Pb age of the metamorphic process. A third study investigates and reviews the zircon-forming textures in a number of metagabbro and metadolerite bodies metamorphosed at different pressures and temperatures. The results in this thesis show that zircon formation is remarkably similar in all of the investigated metagabbroic rocks, and that zircon is mainly produced by the breakdown of igneous baddeleyite during prograde mineral reactions. The metamorphic mineral reactions and the associated zircon formation in gabbroic rocks are tightly linked to deformation and infiltration of hydrous fluids, and to a lesser extent dependent of variations in pressure and temperature. Therefore, in most gabbroic rocks, zircon formation will take place at the earliest stage of metamorphic recrystallization.