APPLICATION OF QUIG BAROMETRY TO THE BARROVIAN ZONES, SCOTLAND: A RE-EVALUATION OF CLASSICAL GEOBAROMETRY IN A CLASSIC LOCALITY

Samples from the garnet, staurolite, kyanite and sillimanite zones from the type locality of Barrovian metamorphism in Glen Esk, Scotland, have been selected for comparison of QuiG barometry with classical thermobarometers and mineral assemblage diagrams (MADs). Peak conditions inferred from MADs for the garnet zone are 480–580 ˚C and 2–10 kbar and for the staurolite zone are 570–680 ˚C at 8–10 kbar. MADs for the sillimanite zone suggest peak conditions of 600–670 ˚C at 5.5–6.5 kbar but early kyanite + rutile parageneses in one sample indicates a maximum pressure 8–12 kbar at 650–730 ˚C. Mineral composition isopleths calculated with the MADs show that the P–T conditions of the garnet core composition isopleths for all samples lie above but within 50 degrees of the equilibrium garnet-in reaction.

Pressures calculated from the garnet–plagioclase–muscovite–biotite barometer (GPMB) for samples from the garnet and staurolite zones range from 3–5 kbar at 500–550 ˚C and 4–6 kbar at 570–600 ˚C, respectively. Samples from the sillimanite zone yield pressures of 6–7 kbar at 590–630 ˚C.

All entrapment pressures (P_ent) measured using QuiG were higher by 1–4 kbar than pressures inferred from conventional geobarometry. For the garnet zone, P_ent is around 6.5–7.5 kbar and for the staurolite zone, P_ent is 8.5–9.5 kbar. Assuming P_ent reflects the pressure of garnet nucleation, comparing the P_ent with the intersecting isopleths from the garnet core compositions indicates significant overstepping of the garnet-in reaction of 1–5 kbar, depending on the metamorphic zone. Additionally, QuiG barometry from one sillimanite zone sample is consistent with the occurrence of early kyanite and trace rutile in the sample and also the MADs, but inconsistent with the peak P–T conditions inferred from thermobarometry.

In summary, the pressures determined from QuiG are more consistent with the peak conditions suggested by the observed mineral assemblages combined with the calculated MADs than with other barometers.