Monazite geochronology and pressure-temperature constraints on Early Neoproterozoic and Caledonian-aged metamorphism in the Shetland Islands.

• Main Author: Cutts, K. A.
• Other Authors: School of Physical Sciences
• Format: Thesis
• Language: English
• Published: 2006
• Subjects: Honours; Geology; monazite; geochronology; metamorphism; mineral chemistry; Greenland; East Greenland
• Online Access: http://hdl.handle.net/2440/98533

This item is only available electronically. EMPA U-Th-Pb and LA-ICPMS U-Pb monazite data and pressure-temperature calculations on lower to upper amphibolite facies rocks in the Shetland Islands constrain the evolution of Caledonian metamorphism and have identified an early Neoproterozoic metamorphic basement to the Dalradian Supergroup. EMPA monazite and LA-ICPMS data suggest that peak metamorphism occurred at c. 460 Ma in the central and northern Shetland Islands, and at c. 910 Ma in the basement sequences. These Neoproterozoic age samples have no evidence of Caledonian aged monazite and have been interpreted to be a metamorphic basement to the Dalradian, possibly forming part of the Moine Supergroup. Low-grade rocks immediately beneath the Shetland ophiolite on the island of Unst are characterised by garnet-staurolite­chloritoid-chlorite assemblages with up-P prograde paths culminating in peak conditions of c. 550°C and 7-8 kbars. In the slightly higher-grade rocks on Yell, prograde staurolite-biotite­muscovite assemblages have been replaced by peak garnet-kyanite assemblages that formed at c. 650°C and 8 kbar. At Lunna Ness on North-eastern Mainland, metapelites had peak assemblages of garnet-sillimanite-biotite-rutile-quartz, which replaced the earlier garnet-biotite-muscovite­kyanite-quartz assemblages. In mafic lithologies, the peak assemblages are defined by garnet­hornblende-plagioclase-quartz. Peak assemblages were associated with partial melting and local granite emplacement and formed at around 700-800°C and 9-10 kbar. Based on available data, the thickness of the Shetland ophiolite is insufficient to have generated the peak metamorphic pressures via abduction. It is therefore likely that peak Caledonian metamorphism predates the ophiolite obduction event. The Caledonian Orogen (Fig. 1A) marks the collision of Baltica and Laurentia in an event that produced an orogenic belt of Himalayan scale. The products of this orogenic event are preserved in east Greenland (Kalsbeek et al. 2000; Watt et al. ...