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

Abstract

A combination of detailed petrography and in situ SIMS 207Pb/206Pb micro-chronometry of monazite has been carried out on samples from the Lewisian high-grade metamorphic terrain, northwest Scotland. Two episodes of granulite-facies metamorphism have been recognised, the thermal maxima of which are estimated at or before 2760 +/- 12 Ma (2σ) and at 2526 +/- 8 Ma, respectively. Pegmatites at Scourie crystallised at ~ 2480 Ma. The amphibolite-facies metamorphism of the main Laxfordian event is estimated at 1752 +/- 12 Ma and the formation of kyanite and staurolite is associated with this event.The oldest ages of ~ 2760 Ma have been obtained from micro-inclusions of monazite in garnet. It appears that garnet, which has a high closure temperature for the U-Pb system, plays an important role in the behaviour of the monazite U-Pb system. It serves to shield monazite from reaction with the other phases and toarmour monazite from diffusive Pb loss or from resetting.