Isotopic and chemical constraints on the building of the deep Scottish lithosphere

Synopsis The Scottish late Caledonian granites define a chemical province characterised by high Na, Ba and Sr abundances, particularly towards the north and west where they also have higher Zn, REEs and La/Y and lower Rb and Th. A comparison with data for inclusions found in younger volcanics and hypabyssal intrusions and thought to have been excavated from the mantle and lower crust indicates that the source of at least some of the chemical variation in the granitic magmas lies in the lithospheric mantle. The asthenosphere or postulated subducted slab material from the Iapetus ocean floor cannot have directly contributed significant amounts of these elements to most of the granitoids. Further, isotopic data clearly indicate that lower crust-derived materials are essential components of the more evolved granites, implying that considerable melting took place near the Moho. The age of the lower crust parallels the changes in chemistry in the lithospheric mantle which suggests that the Moho is not a major tectonic discontinuity. The Mid-Grampian line probably represents the rifted continental edge during Dalradian deposition. There is no isotopic evidence for primary crust of Grenville age under Scotland; rather the lower crust appears to be a complicated mixture including some components which are very young (probably Lower Palaeozoic) under the Midland Valley. The only location where clearly enriched mantle has been identified is under the Lewisian of the Outer Hebrides. The lower crust in this region is also Lewisian, apparently metasomatised. We tentatively suggest that the chemical enrichments and depletions in the lithosphere north of the Mid Grampian line are most simply explained by a volatile flushing mechanism.