LEWISIAN CRUSTAL HISTORY FROM IN-SITU SIMS MINERAL CHRONOMETRY AND RELATED METAMORPHIC TEXTURES

A combination of detailed petrography and in situ SIMS (207)pb/(206)pb micro-chronometry of monazite has been carried out on samples from th e Lewisian high-grade metamorphic terrain, northwest Scotland. Two epi sodes of granulite-facies metamorphism have been recognised, the therm al maxima of which are estimated at or before 2760 +/- 12 Ma (2 sigma) and at 2526 +/- 8 Ma, respectively. Pegmatites at Scourie crystallise d at similar to 2480 Ma. The amphibolite-facies metamorphism of the ma in Laxfordian event is estimated at 1752 +/- 12 Ma and the formation o f kyanite and staurolite is associated with this event, The oldest age s of similar to 2760 Ma have been obtained from micro-inclusions of mo nazite in garnet. It appears that garnet, which has a high closure tem perature for the U-Pb system, plays an important role in the behaviour of the monazite U-Pb system. It serves to shield monazite from reacti on with the other phases and to armour monazite from diffusive Pb loss or from resetting.