New SHRIMP U-Pb zircon ages and CHIME monazite ages from South Harris granulites, Lewisian Complex, NW Scotland: Implications for two stages of zircon formation during Palaeoproterozoic UHT metamorphism

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon age dating was applied to several types of granulites and gneisses from the Lewisian Complex in South Harris, NW Scotland, to clarify the timing of Palaeoproterozoic tectono-thermal events. For comparison, monazite was dated using the chemical Th-U-total Pb isochron method (CHIME). The obtained U-Pb zircon age data were evaluated under the constrains of the modal amount of constituent minerals and whole-rock compositions of analysed samples, the zircon internal texture based on Th/U ratios, and the monazite CHIME ages. Zircons in three samples, characterised by zoned overgrowth rims, sector-zoned grains, and structureless high-U cores, yielded concordant ages of approximately 1900 Ma (1909 +/- 3 Ma, 1899 +/- 10, and 1888 +/- 15 Ma, respectively). The age of 1899 +/- 10 Ma, obtained from melanocratic granulite (garnet and kyanite/sillimanite: 76-91 vol.%), is interpreted to represent the timing of zircon crystallisation prior to the formation of restitic garnet and kyanite/sillimanite. Therefore, the age of similar to 1900 Ma is proposed to represent the timing of early metamorphism, close to the thermal peak (M1). In the leucocratic gneiss (sample 93921-7), the age population at 1868 +/- 35 Ma obtained from low-U and low-Th overgrowth rims represents a second stage of zircon growth from crystallising anatectic melt (M3), whereas the high-U cores represent the timing of zircon growth and recrystallisation prior to anatexis. Monazites from two leucocratic gneisses yield a relatively narrow range of CHIME ages: 1858 +/- 7 Ma and 1840 +/- 7 Ma. These ages probably represent the timing of zircon and monazite crystallisation from partial melt and resetting of the U-Th-Pb system. These results suggest that the timing of zircon formation was controlled by metamorphic processes, such as restite formation and crystallisation from partial melt. (C) 2012 Elsevier B.V. All rights reserved.