Age of the Ballachulish and Glencoe Igneous Complexes (Scottish Highlands), and paragenesis of zircon, monazite and baddeleyite in the Ballachulish Aureole

摘要

U–Pb zircon ages are presented for the Ballachulish Igneous Complex (207Pb–206Pb age 427 ± 1 Ma; 206Pb–238U age 423 ± 0.3 Ma) and Glencoe Volcanic Complex (207Pb–206Pb age 406 ± 6 Ma) of the Scottish Highlands. These ages are significantly more precise than pre-existing age constraints, and discriminate a previously unresolved age difference of c. 20 Ma between the two complexes. This difference in age provides an explanation for the c. 10 km difference in crustal level between the two magmatic events, and constrains exhumation rates for the Argyll region to be on average c. 0.4 km Ma–1 over the period c. 425–405 Ma. With improved age constraints on the Ballachulish Igneous Complex in place, the associated metamorphic aureole is used as a type locality to investigate the metamorphic behaviour of the U-bearing accessory minerals baddeleyite, zircon and monazite. Baddeleyite formed in impure dolomites by the reaction of detrital zircon with dolomite and gives the same U–Pb age as the intrusive complex, consistent with its contact metamorphic origin. Detrital zircon appears to be inert throughout much of the metamorphic aureole, with contact metamorphic zircon growth being restricted to migmatite-grade rocks. A possible exception is the development of enigmatic, very narrow overgrowths at temperatures between c. 500 and 600 °C. Monazite exhibits a variety of textures in lower-grade parts of the aureole (<550 °C), but occurs as distinctive clusters and trails of tiny ovoid grains at temperatures above about 560–600 °C. Monazite thus appears sensitive to metamorphism at lower temperatures than zircon and may therefore be a better target for metamorphic age measurements in rocks that reach mid-amphibolite facies but do not experience partial melting.