Mineralogy, geochemistry and the development of ductile shear zones in the Grampian Slide zone of the Scottish Central Highlands

Abstract

The Grampian Slide zone comprises a system of shear zones characterized by strain-induced, syn-tectonic muscovitization, growth of porphyroblasts of plagioclase, garnet and muscovite, concordant quartz veinlets, and by later-tectonic sillimanite and late- to post-tectonic kyanite. Traverses across shear zones in the Central Highland division show progressive modification of original gneissic fabrics, demonstrating that metamorphism and gneissification preceded the shearing event. Mylonitization involved bulk mobilization of alkalies (Na, Ca, K) and trace elements (Ba, Rb, Sr, Ni, Cr, Zr), accompanied by decreasing Fe oxidation ratios; and increasing shearing progressively changed the mineral chemistry of reversely zoned plagioclase, zoned garnet and micas. Advanced syn-shearing blastesis produced a suite of lenticular pegmatitic segregation veinlets in the mylonites. A coeval suite of larger, intrusive pegmatites represents more mobile material that has migrated through the shear zones. The pegmatites and the mylonite matrices yield U-Pb monazite ages of c. 806 Ma, corresponding to the Precambrian Knoydartian event in the Moine rocks of the Northern Highlands. The shear zones record compressional(?) transport directed towards the NNE, and in the northern sector of the Central Highlands were reactivated by later (Caledonian?) extensional SE-directed movement. The presence of the c. 806 Ma event in the Grampian Group challenges recent proposals that early deformation across the entire Dalradian Supergroup can be correlated with the c. 590 Ma Tay Nappe.