Source: Dissertation Abstracts International, Volume: 74-11(E), Section: B.

Adviser: Jay J. Ague.

After more than a century of investigation, questions still remain regarding the pressure-temperature (P-T) conditions, pressure-temperature-time (P-T-t) paths, mineralogical changes, and sources of heat in the Barrovian zones of the Scottish Highlands. Previous models for the regional metamorphism predict metamorphic conditions developing as the result of thermal relaxation of overthickened crust. This study, however, demonstrates repeatedly that the conditions require an additional heat source and that the mechanisms of metamorphism vary across the mountain belt. The goal of this study is to more completely characterize the ∼470 Ma Grampian Orogeny in this classical metamorphic area. Herein, chemical analyses of minerals, garnet chemical profiles and mapping, isotopic analysis of zircon, and examination of mineral textures are used to determine P-T conditions, determine timescales of peak temperature and cooling, document zircon behavior in different mineral zones, and constrain the timing and mineralogical effects of metamorphism in the different regions.

Thermobarometry and analysis of garnet chemical profiles and maps in samples from across the Highlands has demonstrated that there are different heat sources and mechanisms of metamorphism in different regions within the study area. Pressures at peak T generally increase from ∼0.4 GPa on the east coast to ∼1.1 GPa in the western half. Peak temperatures increase with distance from the Highland Boundary Fault to a maximum of >800 °C nearest to the synmetamorphic igneous intrusions in the northeast. In and around the Barrovian type locality of Glen Clova peak T conditions were driven by a brief thermal pulse or pulses during exhumation. The heat for these pulses, which lasted less than 106 yr in total, was likely supplied by nearby syn-metamorphic magmas. In the western half of the Highlands, however, burial and exhumation occurred with no additional heat input.

In Glen Muick, to the north of Glen Clova, peak granulite facies conditions of ∼1 GPa and >800 °C were also driven by syn-orogenic magmas. Diffusion modeling of retrograde exchange of Mg and Fe between garnet and biotite reveals rapid cooling following peak T. Over the first 200-300 °C following peak T the rocks cooled at an average rate of at least 30 °C/Ma, but with uncertainties in the initial T the rate may have been as fast as 800 °C/Ma. The cooling rates, however, start out even faster (either 300-425 °C/Ma or 2000-2500 °C/Ma, depending on the initial T used). In the first <1 Ma of cooling, these rapid initial cooling rates decrease to a constant rate of 20-50 °C/Ma (or 250-600 °C/Ma for a higher initial T).

The effects of peak metamorphism and later tectonic activity on detrital zircons from the Barrovian zones were investigated using secondary ion mass spectrometry (SEWS) and backscattered electron (BSE) imaging. The length scale of zircon growth and/or recrystallization is much shorter in the chlorite through kyanite zones (less than 1 mum) than in the sillimanite and sillimanite--K-feldspar zones (tens of mum). Sub-micrometer-scale alteration records the age of peak Grampian metamorphism as well as ages related to tectonic and/or magmatic events following the metamorphism.