Insights into the origin of the Scourie Dykes from geochemistry and geochronology.

• Author / Creator

  Davies, Joshua

• A multi-technique geochemical study of the Scourie dykes in the Lewisian gneiss complex (LGC) of northwest Scotland is presented. The Scourie dykes are a set of northwest-southeast trending mafic and ultramafic dykes that intrude the LGC between two periods of high-grade regional metamorphism. New U-Pb radiometric ages from zircon and baddeleyite indicate dyke emplacement occurred during two periods separated by ~400 m.y. The majority of the dykes were emplaced at ~2.4 Ga, with a minor intrusion event at ~2.0 Ga. The distribution of ages in the ~2.4 Ga event is consistent with dyke emplacement during post collisional extension after the collision of the Assynt and Rhiconich terranes in the northern LGC. The oxygen isotopic composition (δ18O) of zircon from the Scourie dykes indicates that the source region of the dykes contained a low δ18O component. The zircons have primary δ18O values down to -3.7 ‰. δ 18O values below 5.5 ‰ can only be created through interaction with meteoric water at the Earth’s surface. Therefore, the Scourie dyke magmas likely contain a recycled component that has previously interacted with surface waters. The magmatic low δ18O signature in the zircons is obscured by post-emplacement fluid alteration during the Caledonian orogeny at ~430 Ma. A new technique involving Raman spectroscopy and trace element analysis combined with ion probe oxygen isotopic analysis is presented which distinguishes areas of zircon grains that still record primary δ18O signatures from areas in grains which have been reset. Geochemical modeling of the Scourie dykes is also presented. The modeling indicates that melting of an enriched mantle source cannot produce the dyke trace element chemistry. Melting of a Primitive Mantle source followed by assimilation iii of crustal material can explain the enriched trace-element compositions found in the dykes. The geochemical model is consistent with the oxygen isotopic information from the zircons in the dykes. An initial study investigating the possibility of oxygen isotopic analysis of baddeleyite by ion probe is also presented. In mafic rocks baddeleyite is more common than zircon and commonly records concordant or close to concordant U-Pb ages. Oxygen isotopic analysis of zircon has been instrumental in understanding U-Pb systematics and fluid-rock interactions. Unfortunately, baddeleyite oxygen isotopic analysis by ion probe is currently hindered by heterogeneous reference materials. Analysis of baddeleyite from the Duck Lake sill (DLS) in northern Canada, which has co-crystallized with zircon suggests that the baddeleyite can be in isotopic equilibrium with zircon and therefore record the oxygen isotopic composition of the parent magma. The DLS baddeleyite oxygen isotopic compositions also indicate that baddeleyite may exchange oxygen more readily than zircon with meteoric water without losing significant lead.

• Subjects / Keywords

  • Raman
  • Geochemistry
  • U-Pb

• Graduation date

  Fall 2014

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3JT2V

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.