ZIRCON BEHAVIOR DURING PROGRESSIVE METAMORPHISM, BARROVIAN ZONES, SCOTLAND

Precambrian detrital zircons from the Barrovian zones in the Scottish Highlands were studied using secondary ion mass spectrometry (SIMS) analysis and backscattered electron (BSE) imaging to investigate the effects of peak metamorphism and later tectonic activity on the grains. SIMS analysis was performed at UCLA on the CAMECA ims 1270 using two techniques: depth profiling into the rims of whole grains to analyze nanometer-scale features (e.g., Carson et al., 2002; Mojzsis and Harrison, 2002; Breeding et al., 2004) and conventional spot analysis on sectioned and polished grains. Fifteen samples recording progressive metamorphism from the chlorite through the sillimanite−K-feldspar zones were investigated.

Spot analyses detected Grampian U-Pb ages (~470 Ma) in near-rim domains of sectioned crystals from the sillimanite (n=2) and sillimanite−K-feldspar (n=5) zones. Examination of BSE images reveals that spot analysis locations are on distinct, nonporous areas between 10 and 30 μm wide. Spot analyses failed to detect post-peak ages.

Most depth profiles yielded Grampian ages (n=22) in all zones from chlorite through sillimanite−K-feldspar. Concordant ages are identical, within error, across all geographical regions. The next most common rim age correlates with the intrusion of the Lochnagar granite and/or the Glen Doll diorite at ~420 Ma (n=8). Furthermore, some rim ages extend at least through the Permian, indicating that the rocks continued to record alteration associated with later thermal and fluid flow activity following peak metamorphism. Importantly, the length scale of rim alteration at lower metamorphic grades resolved by depth profiling (as little as ~80 nm) is much shorter than in the upper amphibolite facies (up to tens of μm). Our results show that progressive metamorphism from the chlorite through the kyanite zones affected only the outer ~80 nm to ~1 µm of detrital zircon grains. Thicker, 10 to 30 µm rims were produced only at high grades in the sillimanite and sillimanite−K-feldspar zones, probably in the presence of partial melt.