PRESSURE–TEMPERATURE CONDITIONS AND PRESSURE–TEMPERATURE–TIME PATHS, BARROVIAN METAMORPHIC ZONES, SCOTLAND

We measured mineral compositions and garnet zoning profiles to constrain P-T conditions and P-T-t paths in samples from the Barrovian metamorphic zones in Scotland. The area of Dalradian metasediments sampled ranges from the southwestern to the northeastern coasts and includes the type locality of Glen Clova. Twenty-five samples were analyzed from the garnet through the sillimanite zones. Thermobarometry results calculated using winTWQ and Thermocalc show three distinct baric regimes across the Highlands. Along the east coast equilibration pressures were between ~0.4 and ~0.5 GPa. In and around Glen Clova pressures were higher, between ~0.5 and ~0.8 GPa. The region of highest pressure, ~1.0 GPa, extends from the Glen Muick area into the SW Highlands (this study; Baker, 1985). A considerable range in “peak” temperatures is also evident. Along the east coast temperatures ranged from 535 °C to 620 °C through the garnet, staurolite, and sillimanite zones. The temperatures in the Glen Clova area ranged from ~500 °C in the garnet zone to ~660 °C in the sillimanite–muscovite zone. Most of the ~1.0 GPa rocks are in the garnet zone and record temperatures between ~500 °C and ~630 °C. However, the rocks in the Glen Muick area reached ~800 °C in the sillimanite–K-feldspar zone. We propose P-T-t paths for the different regions based on garnet zoning profiles and analysis of pseudosections. In the beginning stages of the metamorphism there was widespread crustal thickening across the region such that pressures as great as ~1 GPa were reached. Garnet growth during this stage is characterized by high grossular mole fractions (~0.2). Garnet cores from Glen Clova and the east coast often preserve these high grossular mole fractions, consistent with early high pressure metamorphism in these areas. The high pressure stage was followed by exhumation, during which there were brief thermal pulses at lower pressures in the eastern half of the Highlands while, at the same time, the rocks to the west continued to be exhumed with little or no pulsed heating. We conclude that these thermal pulses were related to magmatism and were most likely the result of the intrusion of the Newer Gabbros in the east (cf. Ague and Baxter, 2007). These thermal pulses were responsible for producing the classic Barrovian sequence (chlorite through sillimanite zone) ca. 470 Ma.